Imaging Anatomy: Text and Atlas Volume 3: Bones, Joints, Muscles, Vessels, and Nerves 9781626239845, 9781626239852, 9781638536123

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Imaging Anatomy Text and Atlas Volume 3 Bones, Joints, Muscles, Vessels, and Nerves

Farhood Saremi, MD

Professor of Radiology and Medicine Department of Radiology University of Southern California Keck Medicine of USC Los Angeles, California, USA Associate Editors: Dakshesh B. Patel, MD Associate Professor of Clinical Radiology Department of Radiology University of Southern California Keck Medicine of USC Los Angeles, California, USA Damián Sánchez-Quintana, MD, PhD Professor of Human Anatomy Faculty of Medicine Department of Anatomy and Cell Biology University of Extremadura Badajoz, Spain Hiro Kiyosue, MD, PhD Professor of Radiology Department of Diagnostic Imaging and Analysis Faculty of Life Sciences Kumamoto University Kumamoto, Japan 2175 illustrations

Thieme New York • Stuttgart • Delhi • Rio de Janeiro

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Meng Law, MD Professor of Radiology, Neurology, and Neurological Surgery University of Southern California; Biomedical Engineering Viterbi School of Engineering; Director of Neuroradiology and the Neuroradiology Fellowship Program Keck School of Medicine Los Angeles, California, USA R. Shane Tubbs, MD Professor of Neurosurgery and Structural & Cellular Biology; Director of Surgical Anatomy Tulane University School of Medicine New Orleans, Louisiana, USA; Chief Scientific Officer Vice President Seattle Science Foundation Seattle, Washington, USA

Library of Congress Cataloging-in-Publication Data is available from the publisher

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© 2024. Thieme. All rights reserved. Thieme Medical Publishers, Inc. 333 Seventh Avenue, 18th Floor, New York, NY 10001, USA www.thieme.com +1 800 782 3488, [email protected] Cover design: © Thieme Cover images source: © Thieme Typesetting by Thomson Digital, India Printed in India by Replika Press Pvt. Ltd.

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DOI: 10.1055/b000000328 ISBN: 978-1-62623-984-5 Also available as an e-book: eISBN (PDF): 978-1-62623-985-2 eISBN (epub): 978-1-63853-612-3

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Contents Preface........................................................................................................................................................................................................................................... xiii Contributors.............................................................................................................................................................................................................................  xiv 1.

Bones, Muscles, Tendons, Joints, and Cartilage................................................................................................................................. 1 Introduction.............................................................................. 1 Bones............................................................................................ 1 Gross Appearance. ................................................................... 1 Development and Ossification............................................ 1 Extracellular Matrix Mineralization. ................................... 2 Bone Remodeling..................................................................... 3 Vasculature and Innervations.............................................. 5 Bone Marrow............................................................................ 8 Imaging of Bone Marrow....................................................... 8 Bone Marrow Maturation...................................................... 8 Conversion of Red Marrow. .................................................. 8 Senile Osteoporosis................................................................. 9 Treatment-Related Changes of Bones............................11 Periosteum. .............................................................................12 Muscles......................................................................................14 Muscle Architecture. .............................................................15 Vascular Supply, Lymphatic Drainage, and Innervation.......................................................................17 Injury and Pathology.............................................................18 Tendons, Ligaments, and Fasciae.................................19 Morphology..............................................................................19 Microstructure.........................................................................21 Macrostructure........................................................................22 Tendon Junctions....................................................................23

2.

Development and Ossification. .....................................48 Shoulder Girdle........................................................................48 Elbow Region............................................................................53 Shoulder Girdle......................................................................54 Scapula. ......................................................................................54 Clavicle........................................................................................58 Shoulder Girdle Muscles. .....................................................59 Shoulder Girdle Ligaments. ................................................59

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Joints and Cartilage.............................................................29 Classification.............................................................................29 Fibrous Joints............................................................................30 Cartilaginous Joints................................................................30 Synovial Joints..........................................................................31 Synovium..................................................................................36 Hyaline Cartilage..................................................................37 Components.............................................................................37 Zonal Layers..............................................................................37 Growth and Expansion.........................................................38 Mechanical Properties..........................................................39 Imaging. .....................................................................................39 Meniscus...................................................................................40 Structure....................................................................................40 Function. ....................................................................................42 Lesions. .......................................................................................42 Imaging. .....................................................................................42 Joint Capsule...........................................................................42 Bursae. .......................................................................................44

Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm................................................................................................48 Introduction............................................................................48

3.

Vascularity..................................................................................24 Fat Pads........................................................................................25 Injury. ..........................................................................................26 Tendinopathy...........................................................................28 Tendon and Ligament Imaging.........................................29

Shoulder Girdle Bursae.........................................................69 Neurovascular Structures....................................................70 Shoulder Girdle Mechanics.................................................70 Common Pathology. .............................................................71 Arm and Forearm.................................................................73 Humerus. ...................................................................................73 Radius..........................................................................................75 Ulna..............................................................................................78 Biomechanics of the Arm and Forearm.........................80 Common Pathologies...........................................................80

Muscles of Shoulder Girdle, Arm, and Forearm...............................................................................................................................82 Introduction............................................................................82 Shoulder Muscles. ................................................................82

Deltoid Muscle..........................................................................82 Latissimus Dorsi Muscle........................................................82 Pectoralis Major Muscle.........................................................84

v

Contents Supraspinatus and Infraspinatus Muscles........................85 Teres Minor Muscle.................................................................88 Teres Major Muscle..................................................................89 Subscapularis Muscle..............................................................89

Arm Muscles. ..........................................................................90 Anterior Muscle Compartment of the Arm..................90 Posterior Muscle Compartment of the Arm................95 Forearm Muscles...................................................................98

4.

Aortic Arch............................................................................ 129 Shape of the Aortic Arch. ................................................. 132 Embryology.......................................................................... 132 Anatomy. ............................................................................... 133 Subclavian Artery. ............................................................... 133

Superficial Veins................................................................. 156 Basilic Vein.............................................................................. 156 Cephalic Vein......................................................................... 157 Median Vein........................................................................... 158

Imaging of the Nerves.................................................... 167 Anatomy. ............................................................................... 167 Spinal Cord Level.................................................................. 167 Neck and Shoulder Levels................................................. 167 Ventral Rami, Upper Trunk, and Divisions................. 171

Lymphatic Vessels. ............................................................ 197 Superficial Lymphatic Vessels. ........................................ 197 Deep Lymphatic Vessels.................................................... 198 Communication of the Superficial and Deep Lymphatic Vessels................................................................ 198 Lymph Nodes of the Upper Extremities................. 199

Development and Ossification. .................................. 203 Ossification of the Pelvis................................................... 203 Ossification of the Hip, Knee, and Ankle.................... 204

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Deep Venous Thrombosis. ............................................ 165

Lateral Cord and Its Branches......................................... 173 Medial Cord and Its Branches......................................... 174 Median Nerve........................................................................ 180 Posterior Cord and Its Branches..................................... 184 Brachial Plexus Blood Supply. ..................................... 193 Common Pathologies...................................................... 193

Superficial Lymph Nodes. ................................................. 199 Deep Lymph Nodes............................................................. 199 Axillary Lymph Nodes. ....................................................... 200 Diseases Related to the Lymphatic System of the Upper Extremities. ............................. 201 Axillary Lymph Node Metastasis and Dissection. ........ 201 Lymphedema. ....................................................................... 201

Lower Extremity Bones: Pelvis, Femur, Tibia, Fibula................................................................................................................. 203 Introduction......................................................................... 203

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Deep Veins............................................................................ 159 Perforating Veins. ................................................................ 160 Hand Veins............................................................................. 160 Axillary and Subclavian Veins.......................................... 161

Lymphatic System of the Upper Extremity. ..................................................................................................................................... 197 Introduction......................................................................... 197

8.

Arterial Supply of the Bones........................................ 154

Brachial Plexus and Its Branches.............................................................................................................................................................. 167 Introduction......................................................................... 167

7.

Axillary Artery....................................................................... 136 Brachial Artery...................................................................... 141 Radial and Ulnar Arteries.................................................. 142 Interosseous Artery............................................................ 146 Persistent Median Artery.................................................. 148 Wrist and Hands................................................................... 148

Upper Extremity and Shoulder Venous System............................................................................................................................ 156 Introduction......................................................................... 156

6.

Upper Extremity Compartments............................... 118 Compartments..................................................................... 120

Upper Extremity Arteries.............................................................................................................................................................................. 129 Introduction......................................................................... 129

5.

Muscle Compartments of the Forearm. ........................98 Antecubital Fossa. ..................................................................98 Common Flexor Tendon. .....................................................99 Common Extensor Tendon.................................................99 Volar (Anterior) Compartment. ........................................99 Mobile Wad (Radial) Compartment............................. 111 Dorsal Compartment......................................................... 114

Pelvis........................................................................................ 209 Iliac Bones (Greater Pelvis)............................................... 210 Lesser Pelvis........................................................................... 216 Pubic Symphysis................................................................... 216 Inlet and Outlet of Pelvis. ................................................. 218

Contents Hip............................................................................................. 218 Function of the Pelvis......................................................... 218 Pelvic Bursae.......................................................................... 220 Femur and Patella............................................................. 220 Patella....................................................................................... 223

9.

Tibia and Fibula.................................................................. 223 Tibia. ......................................................................................... 223 Fibula........................................................................................ 226 Proximal Tibiofibular Joint................................................ 226 Distal Tibiofibular Joint...................................................... 228 Interosseous Membrane................................................... 231 Long Bone Function............................................................ 231

Lower Extremity Muscles: Pelvic Girdle, Thigh, and Leg........................................................................................................ 233 Introduction......................................................................... 233 Pelvic Girdle and Thigh Muscles. ............................... 233 Gluteus Muscles................................................................... 233 Fascia Lata/Tensor Fascia Lata/Iliotibial Band............ 236 Iliopsoas Muscle................................................................... 240 Quadratus Lumborum Muscle........................................ 242 Short External Rotators of the Hip................................ 242 Adductor and Medial Thigh Muscles............................ 252

Anterior Thigh Muscles (Quadriceps Femoris Muscle)......................................... 259 Posterior Thigh Muscles (Hamstring Muscles)......... 263 Leg Muscles.......................................................................... 274 Superficial Posterior Compartment of the Leg........ 274 Deep Posterior Compartment Muscles. ..................... 278 Muscles of the Anterior Compartment....................... 286 Muscles of the Lateral Compartment.......................... 288 Muscle Compartments................................................... 295

10.

Accessory Muscles. ............................................................................................................................................................................................ 301 Introduction......................................................................... 301 Thorax..................................................................................... 301 Pectoralis Major Variation................................................ 301 Pectoralis Minimus.............................................................. 302 Latissimus Dorsi variations. ............................................. 302 Accessory Subscapularis................................................... 302 Upper Limb........................................................................... 302 Coracobrachialis................................................................... 302 Biceps Brachii. ....................................................................... 302 Brachialis................................................................................. 302 Triceps Brachii....................................................................... 303 Anconeus Epitrochlearis. .................................................. 303 Brachioradialis....................................................................... 303 Flexor Carpi Ulnaris............................................................. 303 Palmaris Longus................................................................... 303 Flexor Carpi Radialis............................................................ 303 Flexor Digitorum Superficialis......................................... 303 Gantzer’s Muscle. ................................................................ 304

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Extensor Carpi Radialis. ..................................................... 304 Accessory Extensor Digiti Minimi.................................. 305 Extensor Indicis and Extensor Medii............................. 306 Hypothenar Variants.......................................................... 307 Gluteal Region..................................................................... 310 Thigh........................................................................................ 310 Knee Region......................................................................... 310 Tensor Fasciae Suralis......................................................... 310 Accessory Plantaris/Lateral Segmental Origin of the Lateral Gastrocnemius............................ 311 Third Head of the Gastrocnemius................................. 311 Medial Gastrocnemius....................................................... 312 Accessory Popliteus............................................................ 312 Ankle........................................................................................ 313 Peroneal Muscles and Associated Variations............ 313 Accessory Muscles of the Medial Ankle...................... 315

Lower Extremity Arteries.............................................................................................................................................................................. 322 Introduction......................................................................... 322 Imaging.................................................................................. 322 Pelvic Arteries..................................................................... 322 Common Iliac Arteries....................................................... 322 Internal Iliac Arteries.......................................................... 322 Posterior Division of Internal Iliac Artery. .................. 322 Anterior Division of Internal Iliac Artery..................... 325 External Iliac Arteries.......................................................... 331 Variant Arterial Anatomy of the Pelvis........................ 335 Arterial Collateral Network of the Pelvis. ................... 340 Arteries of the Thigh........................................................ 341 Common Femoral Artery. ................................................ 341

Deep Femoral Artery.......................................................... 345 Superficial Femoral Artery. .............................................. 347 Variant Arterial Anatomy of the Thigh........................ 348 Collateral Pathways in the Thigh................................... 349 Arteries of the Knee and Lower Leg. ....................... 350 Popliteal Artery..................................................................... 350 Anterior Tibial Artery. ........................................................ 353 Tibioperoneal Trunk. .......................................................... 356 Peroneal (Fibular) Artery.................................................. 356 Posterior Tibial Artery........................................................ 357 Variant Arterial Anatomy of the Lower Leg............... 358 Collateral Pathways in the Knee and Leg. .................. 358 Arterial Supply of the Foot........................................... 359

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Contents Dorsalis Pedis Artery.......................................................... 359 Posterior Tibial Artery........................................................ 359 Variant Arterial Anatomy of the Foot. ......................... 361

12.

Classification........................................................................ 365 Superficial Venous System............................................... 365 Deep Veins............................................................................. 369

Imaging of the Nerves.................................................... 380 Lumbar Plexus. ................................................................... 380 Iliohypogastric Nerve (T12, L1)...................................... 381 Ilioinguinal Nerve (L1). ...................................................... 381 Genitofemoral Nerve (L1, L2)......................................... 381 Lateral Cutaneous Nerve of the Thigh (L2, L3)........ 386 Obturator Nerve (L2, L3, L4)........................................... 386 Femoral Nerve (L2, L3, L4)............................................... 390 Sacral Plexus........................................................................ 395 Sciatic Nerve (L4, L5, S1, S2, S3)................................... 395 Tibial Nerve (L4, L5, S1–S3)............................................. 402

Superficial Lymphatic Vessels. .................................... 422 Medial Group......................................................................... 422 Lateral Group. ....................................................................... 424 Deep Lymphatic Vessels................................................. 424 Communication between the Superficial and Deep Lymphatic Systems. .................................... 425

Imaging Techniques......................................................... 434 Articular Anatomy. ........................................................... 437 Glenohumeral Joint............................................................. 437 Acromioclavicular Joint. .................................................... 437 Osseous Anatomy. ............................................................ 437 Proximal Humerus............................................................... 437 Scapula. ................................................................................... 458 Clavicle..................................................................................... 461 Soft Tissue Structures..................................................... 462

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Common Fibular Nerve (L4, L5, S1, S2)...................... 408 Deep Fibular Nerve............................................................. 409 Superficial Fibular Nerve................................................... 410 Superior Gluteal Nerve (L4, L5, S1). ............................. 417 Inferior Gluteal Nerve (L5, S1, S2). ............................... 418 Pudendal Nerve (S2, S3, S4)............................................ 418 Posterior Cutaneous Nerve of Thigh (S1, S2, S3)................................................................ 420 Perforating Cutaneous Nerve (S2, S3)........................ 421 Nerve to Obturator Internus (L5, S1, S2)............................................................................. 421 Nerve to Quadratus Femoris (L4, L5, S1).............................................................................. 421 Nerve to Piriformis (S1, S2)............................................. 421

Lymph Nodes in the Lower Extremity. ................... 425 Lymph Nodes in the Leg. .................................................. 426 Popliteal Lymph Nodes...................................................... 427 Thigh Lymph Nodes............................................................ 427 Inguinal Lymph Nodes....................................................... 427 Diseases Related to the Lymphatic System of the Lower Extremity.................................. 430 Lymphedema. ....................................................................... 430 Emerging Noninvasive Imaging Method.................... 431

Anatomy of the Shoulder Joint................................................................................................................................................................. 434 Introduction......................................................................... 434

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Common Pathologic Observations. ......................... 376

Lymphatics of the Lower Extremities................................................................................................................................................... 422 Introduction......................................................................... 422

15.

Foot Veins............................................................................... 372 Perforating Veins. ................................................................ 372 Pelvic Veins............................................................................. 376

Lower Extremity Nerves................................................................................................................................................................................ 380 Introduction......................................................................... 380

14.

Postsurgical Anatomy..................................................... 361

Lower Extremity and Pelvic Venous System................................................................................................................................... 365 Introduction......................................................................... 365

13.

Common Pathologies...................................................... 361

Glenoid Labrum. .................................................................. 462 Ligaments............................................................................... 463 Joint Capsule, Recesses, and Joint Stabilizers........... 470 Rotator Cuff and Tendons. ............................................... 470 Rotator Cable........................................................................ 472 Long Head of Biceps Tendon, Bicipital Anchor, and Pulley System. ............................ 473 Rotator Interval. ................................................................... 473 Bursae. ..................................................................................... 474 Neurovascular Structures................................................. 474 Biomechanics. ..................................................................... 475 Conclusion............................................................................. 477

Contents

16.

Elbow Joint.............................................................................................................................................................................................................. 479 Introduction......................................................................... 479

Muscular Variants................................................................ 522

Normal Anatomy............................................................... 479 Bones........................................................................................ 479 Ligaments............................................................................... 481 Muscle and Tendons........................................................... 483 Joint Capsule, Cartilage, and Bursa. ............................. 485

Normal Joint Biomechanics.......................................... 523

Anatomic Variations. ....................................................... 521 Osseous Variants. ................................................................ 521 Soft-Tissue Variants............................................................ 522

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Common Pathologies...................................................... 525 Osseous Trauma................................................................... 525 Tendon and Ligament Injuries........................................ 525 Nerves...................................................................................... 526 Bursae. .................................................................................... 528

Wrist............................................................................................................................................................................................................................ 530 Introduction......................................................................... 530

Muscle and Tendon........................................................... 585

Osseous Anatomy. ............................................................ 530 Distal Radius and Ulna....................................................... 530 Carpus...................................................................................... 531 Ossification............................................................................. 540

Tunnels.................................................................................... 587 Carpal Tunnel. ....................................................................... 587 Guyon’s Canal....................................................................... 587

Joints........................................................................................ 567 Distal Radioulnar Joint....................................................... 567 Radiocarpal Joint.................................................................. 574 Midcarpal Joint...................................................................... 575 Pisotriquetral Joint. ............................................................. 575

18.

Imaging Techniques......................................................... 524

Biomechanics. ..................................................................... 587 Vascular Supply.................................................................. 591 Nerves..................................................................................... 591 Surface Anatomy............................................................... 594

Ligaments.............................................................................. 575

Imaging.................................................................................. 595

Retinacula of the Wrist................................................... 582 Extensor Retinaculum........................................................ 582 Flexor Retinaculum............................................................. 585

Pathologies........................................................................... 596 Summary............................................................................... 598

Hand............................................................................................................................................................................................................................. 600 Introduction......................................................................... 600 Skin, Nails, and Fascia..................................................... 600 Skin............................................................................................ 600 Fingertip and Nail................................................................ 600 Fibrous Skeleton of the Hand and Palmar Aponeurosis........................................................................... 600 Osseous Anatomy. ............................................................ 603 Metacarpal Bones................................................................ 603 Phalangeal Bones................................................................. 604 Sesamoid Bones................................................................... 604 Ossification............................................................................. 604 Joints, Ligaments, and Cartilage. .............................. 605 Carpometacarpal Joints. ................................................... 605 Metacarpophalangeal Joints. .......................................... 623 Interphalangeal Joints........................................................ 625 Muscles and Tendons. ..................................................... 627 Long Tendons of the Hand............................................... 627 Synovial Sheaths and Bursa. ............................................ 635 Anatomic Variation of Flexor Tendons................................................................................... 635

Anatomic Variation of Extensor Tendons................... 635 Volar Deep Spaces and Dorsal Spaces......................... 638 Intrinsic Muscles and Compartments.......................... 639 Anatomic Variation of the Intrinsic Muscles of the Hand............................................................................ 643 Arterial Supply.................................................................... 643 Radial Artery and Its Branches. ...................................... 644 Ulnar Artery and Its Branches......................................... 648 Anatomic Variation............................................................. 648 Venous Drainage. .............................................................. 649 Lymphatic Drainage......................................................... 649 Innervation........................................................................... 650 Ulnar Nerve............................................................................ 650 Median Nerve........................................................................ 651 Radial Nerve........................................................................... 651 Cutaneous Innervation...................................................... 652 Nerve Anastomoses in the Hand................................... 652 Surface Anatomy............................................................... 652 Volar Surface Anatomy...................................................... 652

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Contents Structure and Biomechanics........................................ 653 Axis of the Hand................................................................... 653 Arches. ..................................................................................... 653 Movements............................................................................ 653

19.

Sacrum.................................................................................... 662 Shape, Size, and Orientation........................................... 662 Surfaces................................................................................... 663 Sacral Canal and Vertebral Foramina........................... 668 Structure................................................................................. 668 Development and Ossification....................................... 670 Sex Difference and Variation........................................... 670 Posterior Iliac Bones. ....................................................... 672 Coccyx..................................................................................... 673 Shape, Size, and Orientation........................................... 673 Surfaces................................................................................... 673 Development and Ossification....................................... 673 Sacroiliac Joint. ................................................................... 675

Radiology and Common Pathology......................... 688 Conclusion............................................................................. 691

Synovium, Plicae, and Pectinofoveal Fold. ........... 723

Osseous Anatomy. ............................................................ 708 Acetabulum. .......................................................................... 708 Proximal Femur. ................................................................... 712

Vascular Supply.................................................................. 723

Cartilage................................................................................. 715 Supraacetabular Fossa....................................................... 715 Stellate Crease (or Lesion)................................................ 715

Muscles and Tendons. ..................................................... 725 Flexors...................................................................................... 725 Adductors............................................................................... 725 Abductors............................................................................... 725 Extensors................................................................................. 725 External (Lateral) Rotators. .............................................. 726 Iliocapsularis Muscle........................................................... 726

Innervation........................................................................... 724

Bursae. .................................................................................... 727 Iliopsoas Bursa...................................................................... 727 Obturator Externus Bursa................................................. 727 Obturator Internus Bursa. ................................................ 727 Trochanteric Bursae............................................................ 727 Ischiogluteal Bursa.............................................................. 728

Knee............................................................................................................................................................................................................................. 733 Introduction and Biomechanics................................. 733 Imaging Modalities. ......................................................... 733 Osseous and Cartilaginous Anatomy...................... 733 Distal Femur. ......................................................................... 733 Proximal Tibia........................................................................ 755 Patella....................................................................................... 755 Central Structures............................................................. 758 Anterior Cruciate Ligament............................................. 758

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Anatomical Variation....................................................... 687

Introduction......................................................................... 695

Ligaments.............................................................................. 719 Transverse Acetabular Ligament................................... 719 Ligamentum Teres. ............................................................. 719 Capsular (or Extracapsular) Ligaments. ...................... 720

x

Articulating Surfaces, Shape, and Size........................ 675 Location and Orientation. ................................................ 675 Development and Age-Related Changes................... 677 Joint Capsule.......................................................................... 677 Ligaments............................................................................... 677 Sacrococcygeal Joint. ......................................................... 680 Blood Supply and Lymphatic Drainage....................... 682 Innervation............................................................................. 682 Relations.................................................................................. 682 Surface Anatomy................................................................. 684 Biomechanics and Movement........................................ 684

Hip................................................................................................................................................................................................................................. 695

Labrum.................................................................................... 716 Intrasubstance Signal Variation. .................................... 717 Labroligamentous Sulcus................................................. 717 Perilabral Recess................................................................... 717 Sublabral Sulcus (Labrocartilaginous Cleft).............. 718

21.

Conclusion............................................................................. 658

Sacrum, Coccyx, and Sacroiliac Joints. ................................................................................................................................................ 662 Introduction......................................................................... 662

20.

Radiological Evaluation.................................................. 654

Posterior Cruciate Ligament............................................ 758 Meniscofemoral Ligaments............................................. 760 Menisci..................................................................................... 762 Meniscomeniscal (Intermeniscal) Ligaments........... 765 Anterior Compartment. ................................................. 765 Quadriceps and Patellar Tendons.................................. 765 Fat Pads and Bursae............................................................ 765 Plicae. ....................................................................................... 768 Medial Patellar Retinaculum............................................ 771 Lateral Patellar Retinaculum............................................ 772

Contents Medial Compartment...................................................... 774 Medial Collateral Ligament Complex........................... 774 Posterior Oblique Ligament. ........................................... 774 Semimembranosus Tendon and Expansions............ 775 Oblique Popliteal Ligament............................................. 776 Pes Anserine Tendons........................................................ 776 Bursae. ..................................................................................... 776 Posterior Compartment................................................. 776 Vessels, Nerves, and Lymph Nodes. ............................. 776

22.

Ankle............................................................................................................................................................................................................................ 794 Introduction......................................................................... 794 Joints........................................................................................ 794 Distal Tibiofibular Joint...................................................... 794 Talocrural Joint...................................................................... 800 Tendons.................................................................................. 811 Tendons of the Lateral Region of the Ankle.............. 811 Tendons of the Anterior Region..................................... 817 Tendons of the Medial Region of the Ankle.............. 820 Tendon of the Posterior Region of the Ankle........... 824 Nerves and Vessels. .......................................................... 827 Nerves and Vessel of the Lateral Region of the Ankle. .......................................................................... 827

23.

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Lateral Compartment...................................................... 781 Iliotibial Tract......................................................................... 781 Mid-Third Lateral Capsular Ligament/Anterolateral Ligament. ............................... 782 Fibular Collateral Ligament and Biceps Femoris Tendon.................................................................... 782 Popliteus Tendon, Popliteofibular Ligament, and Popliteomeniscal Fascicles...................................... 782 Fabellofibular Ligament..................................................... 787 Arcuate Ligament................................................................ 787

Nerves and Vessels of the Anterior Region of the Ankle. .......................................................................... 829 Nerves and Vessels of the Medial Region of the Ankle. .......................................................................... 831 Surroundings Joints.......................................................... 832 Subtalar Joint......................................................................... 832 Talocalcaneonavicular Joint............................................. 832 Tarsal Sinus (Sinus Tarsi) and Tarsal Canal................. 833 Calcaneocuboid Joint......................................................... 836 Transverse Tarsal Joint or Midtarsal Joint (Chopart Joint)........................................................... 837

Foot............................................................................................................................................................................................................................... 842 Introduction......................................................................... 842 Skin........................................................................................... 842 Fibrous Skeleton................................................................ 842 Plantar Aponeurosis (Plantar Fascia)............................ 842 Osseous Anatomy. ............................................................ 845 Talus.......................................................................................... 845 Calcaneus. .............................................................................. 850 Navicular...............................................................851 Cuneiforms and Cuboid.................................................... 854 Metatarsals and Phalanges.............................................. 855 Sesamoids. ............................................................................. 856 Accessory Bones................................................................... 857 Ossification............................................................................. 859 Joints and Ligaments....................................................... 860 Lisfranc Joint.......................................................................... 860 Lesser Metatarsophalangeal Joints............................... 862 First Metatarsophalangeal Joint..................................... 865 Muscles and Tendons. ..................................................... 868 Long Tendons of the Foot. ............................................... 868 Intrinsic Muscles of the Foot........................................... 872 Synovial Sheaths. ................................................................. 901 Synovial Bursa....................................................................... 901 Compartments.................................................................... 903 Dorsal Compartments....................................................... 903

Plantar Compartments...................................................... 903 Fat Pads.................................................................................... 904 Arterial Supply.................................................................... 904 Dorsalis Pedis Artery and Its Branches........................ 906 Posterior Tibial Artery and Its Branches. .................... 907 Venous Drainage. .............................................................. 907 Lymphatics............................................................................ 908 Innervation........................................................................... 908 Sural Nerve............................................................................. 908 Superficial Peroneal Nerve............................................... 908 Deep Peroneal Nerve......................................................... 908 Posterior Tibial Nerve......................................................... 908 Saphenous Nerve. ............................................................... 910 Cutaneous Innervation of the Foot.............................. 911 Surface Anatomy............................................................... 912 Structure and Biomechanics........................................ 912 Gait. .......................................................................................... 914 Radiological Evaluation.................................................. 914 Bone Pathologies............................................................... 915 Conclusion............................................................................. 917

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Contents

24.

Temporomandibular Joint............................................................................................................................................................................ 920 Introduction......................................................................... 920

Anatomic Variations. ....................................................... 924

Normal Anatomy............................................................... 920 Osseous Structures............................................................. 920 Articular Disk......................................................................... 920 Supporting Structures....................................................... 922

Imaging Techniques......................................................... 925

Normal Joint Mechanics................................................. 923 Normal Mechanics of Osseous Structures ................ 923 Normal Mechanics of Soft Tissues ............................... 924

Common Pathology......................................................... 928 Disk Abnormalities.............................................................. 928 Nondisk Abnormalities...................................................... 929 Conclusion............................................................................. 931

Index.............................................................................................................................................................................................................................................. 933

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Preface In the preface of his anatomy text collection “de Humani Corporis Fabrica” published in 1543, Andreas Vesalius wrote “… anatomy should rightly be regarded as the strong foundation of the whole art of medicine …,” acknowledging the fundamental role the study of anatomy has in the practice of medicine. Furthermore, one of the major lasting successes of his influential anatomy series was the ability to reproduce many superb illustrations of human anatomy dissection which was made possible by technical developments during his time. Likewise, recent advancement in imaging technology and the development of newer postprocessing software have increased our ability to image and show anatomic detail of the human organs. At the same time, over the past decade, the literature is filled with many precious articles on anatomy and related t­ opics. Unfortunately, there is an increasing gap between our fundamental core anatomy reference texts and our understanding and depiction of human anatomy using modern imaging and postprocessing methods. The idea for this project arose from the ­necessity for a text to fill this gap and describe anatomy in the context of current advances in imaging technology and science. Computed tomography (CT) and magnetic resonance (MR) have been the traditional methods in performing noninvasive studies, and both have immensely contributed to our ability to deliver accurate diagnostic information. Current generation MR and CT as well as dedicated peripheral and intraluminal ultrasound provide the spatial and contrast resolutions required to demonstrate anatomic details with unprecedented accuracy. When it comes to the assessment of details, there is no imaging modality that can compete with the speed, accuracy, and spatial resolution of new CT scanners. Volume rendering and mutliplanar reformations facilitate understanding of the complex structures such as heart, vessels, and bones. With current scanners, the entire anatomic span of the major organs can be covered in a few seconds with spatial resolution of less than 0.5 mm³. In this series, this technology along with state-of-the-art postprocessing methods have been used to create volumetric color-coded images. On the other hand, MRI provides high levels of contrast resolution that would be difficult to obtain with CT. High-field MRI provides superb resolution of brain anatomy. Taking advantage of a 7 Tesla MR scanner at our facility, high-quality images were obtained to complete the chapters of the neuroanatomy volume (volume 4). Using MR tractography, it is possible to depict the ­anatomic course of the white tracts in detail, and with new postprocessing software, it is further possible to map areas of the brain cortex and subfield regions of small structures such hippocampus. These technologies have ushered in a new era for investigation of brain anatomy and are being used for the planning of sophisticated brain surgeries and for localization of small regions of interest.

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This imaging anatomy series is a reference atlas and text with a practical discussion of specific topics along with examples of anatomical variants. Divided into four major volumes, each volume takes the instructive format of a classic text but is infused with a large number of images and illustrations to teach readers anatomy via state-of-the-art cross-sectional and volumetric ­imaging. The volumes are divided into lung, mediastinum, and heart; abdomen and pelvis; musculoskeletal; and head, neck, and spine imaging. Peripheral vasculature and nerves have been included in the musculoskeletal volume. The length of each chapter and number of images vary in accordance with the complexity of the topic. In all instances, effort has been made to provide a concise yet comprehensive review of the topic. Each chapter contains an in-depth review of the anatomy and anatomical variants. Pertinent embryology, microanatomy, and a brief description of physiology are discussed in each chapter. Postsurgical anatomy and important gross and surgical pathology images are included. The text is supported by high-quality cross-sectional images with correlative three-dimensional and color-coded CT and MR views. Up-to-date references are used to support the text. Many new topics in radiology and surgery have been gathered from the recent 10-year literature. Surgical and clinical applications in each anatomy topic are presented with relevant images. In order to make each topic understandable, difficult anatomical concepts are supported by sketches as well as cross-sectional and topographic cadaveric views provided by internationally known anatomists. Superb cadaveric views are provided by Professors Damián Sánchez-Quintana, R. Shane Tubbs, and the late Professor Albert L. Rhoton Jr. Images of high-resolution axial cadaveric cuts have been provided by the University of Auckland, New Zealand, thanks to efforts of Professor Ali Mirjalili. As the author and chief editor of this textbook, I was fortunate to have assistance of other editors who have shared their e ­ xperiences for specific parts of this compilation. Countless contributors from high-ranking academic institutions, with extensive experience in imaging, surgery, human anatomy and embryology, have also provided substantial work and shared their e ­ xpertise. Together over more than 5 years of constant work, we have ­created a comprehensive textbook and atlas on imaging and surgical anatomy that aims to serve a broad spectrum of the medical community including the medial students, radiologists, surgeons, clinicians, and research scholars. Finally, this collaborative effort would have remained unfinished without the unreserved assistance and expert guidance of the entire team at Thieme Publishers. Farhood Saremi, MD Los Angeles, California, USA

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Contributors Jay Acharya, MD Assistant Professor of Radiology Department of Radiology Division of Neuroradiology University of Southern California Keck Medical Center of USC Los Angeles, California, USA

Emilie Dodre, MD Department of Musculoskeletal Imaging CCIAL, CHRU of Lille; Laboratory of Anatomy Faculty of Medicine University of Lille 2 Lille, France

Nihal Apaydin, MD Department of Anatomy and Brain Research Center Faculty of Medicine Ankara University Ankara, Turkey

Nastaran Fatemi, MD Adjunct Associate Clinical Professor Department of Radiology Keck School of Medicine University of Southern California Los Angeles, California, USA

José Acosta Batlle, MD Department of Radiology Hospital Universitario Ramón y Cajal University of Alcalá de Henares Madrid, Spain Javier Sánchez Blazquez, MD Chief Department of Radiology Hospital Universitario Ramón y Cajal University of Alcalá de Henares Madrid, Spain Noah Brauner, MD Department of Radiology University of Southern California USC University Hospital Los Angeles, California, USA Taylor Choy, MD Department of Radiology Ronald Reagan UCLA Medical Center Los Angeles, California, USA Christine B. Chung, MD Department of Radiology University of California San Diego Medical Center; VA San Diego Healthcare System San Diego, California, USA Anne Cotten, MD Department of Musculoskeletal Imaging CCIAL, CHRU of Lille Lille, France Xavier Demondion, MD Department of Musculoskeletal Imaging CCIAL, CHRU of Lille; Laboratory of Anatomy Faculty of Medicine University of Lille 2 Lille, France

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Madeleine J. Gust, MD Kaiser Permanente Physician Plastic and Hand Surgery Southern California Permanente Medical Group; Clinical Instructor Plastic and Reconstructive Surgery Loma Linda University Loma Linda, California, USA Luke Hiller, MD Associate Professor Department of Radiology University of California San Diego, California, USA Thibaut Jacques, MD Department of Musculoskeletal Imaging CCIAL, CHRU of Lille Lille, France Ali Jahed, MD Division of Musculoskeletal Radiology Joint Department of Medical Imaging Toronto Western Hospital University Health Network University of Toronto Toronto, Ontario, Canada Redouane Kadi, MD Saint Jean Clinic Brussels; University Hospital of Brussels; Vrije Universiteit Brussel (VUB) Brussels, Belgium Neal Larkman, MBBS, FRCR Department of Radiology Harrogate District General Hospital Harrogate, North Yorkshire, UK

Contributors Jason D. Lather, MD Clinical Instructor The Ohio State University Wexner Medical Center Columbus, Ohio, USA Sulabha Masih, MD Department of Radiology VA Greater Los Angeles Healthcare System Los Angeles, California, USA George R. Matcuk Jr., MD Associate Professor of Clinical Radiology Fellowship Director Department of Musculoskeletal Radiology Keck School of Medicine University of Southern California Los Angeles, California, USA Christopher L. McCrum, MD Assistant Professor Sports Medicine Department of Orthopedic Surgery University of Texas Southwestern Medical Center Dallas, Texas, USA Aramsadat Meraji, MD Department of Radiology University of Southern California USC University Hospital Los Angeles, California, USA S. Ali Mirjalili, MD Associate Professor Senior Lecturer Department of Anatomy and Medical Imaging Faculty of Medical and Health Sciences; Director of BMedSci (Hon) Programme Faculty of Medical and Health Sciences University of Auckland Grafton, Auckland, New Zealand Rakesh Mohankumar, MD Division of Musculoskeletal Radiology Joint Department of Medical Imaging University Health Network Mount Sinai Hospital and Women’s College Hospital Toronto, Ontario, Canada Brandon H. Murti, MD

Department of Radiology

University of Southern California USC University Hospital Los Angeles, California, USA Ali Naraghi, MD Division of Musculoskeletal Radiology Joint Department of Medical Imaging Toronto Western Hospital University Health Network University of Toronto Toronto, Ontario, Canada

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Fumio Ohnishi, MD Assistant Professor Department of Plastic Surgery Saitama Medical University Kamoda Kawagoe, Saitama, Japan María Dolores López Parra, MD Department of Radiology Hospital Universitario Ramón y Cajal University of Alcalá de Henares Madrid, Spain Dakshesh B. Patel, MD Associate Professor of Clinical Radiology Department of Radiology University of Southern California Keck Medicine of USC Los Angeles, California, USA Luis Cerezal Pesquera, MD Chief Department of Radiology Diagnóstico Médico Cantabria Santander, Spain Anandh Rajamohan, MD Assistant Professor of Radiology Department of Radiology Division of Neuroradiology University of Southern California Keck Medical Center of USC Los Angeles, California, USA Farhood Saremi, MD Professor of Radiology and Medicine Department of Radiology University of Southern California USC University Hospital Los Angeles, California, USA Aaron Schein, MD Radiologist Los Robles Radiology Associates Thousand Oaks, California, USA Maryam Shahabpour, MD Saint Jean Clinic Brussels; University Hospital of Brussels; Vrije Universiteit Brussel (VUB) Brussels, Belgium James Shi, MD Department of Radiology Ronald Reagan UCLA Medical Center Los Angeles, California, USA Shigeyoshi Soga, MD Assistant Professor Department of Radiology National Defense Medical College Tokorozawa, Saitama, Japan

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Contributors Milan Stevanovic, MD, PhD Professor of Orthopedics and Surgery Exceptional Clinical Scholar Program Director of Joseph H. Boyes Hand Fellowship Program Keck School of Medicine University of Southern California Los Angeles, California, USA Monica Tafur, MD Department of Radiology University of California San Diego Medical Center San Diego, California, USA Eric W. Tan, MD Assistant Professor Foot and Ankle Surgery Department of Orthopedic Surgery Keck School of Medicine of USC Los Angeles, California, USA Ramon Ter-Oganesyan, MD Assistant Professor of Clinical Radiology Department of Radiology University of Southern California USC University Hospital Los Angeles, California, USA R. Shane Tubbs, MD Professor of Neurosurgery and Structural & Cellular Biology Director of Surgical Anatomy Tulane University School of Medicine New Orleans, Louisiana, USA; Chief Scientific Officer and Vice President Seattle Science Foundation Seattle, Washington, USA

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Felipe Urdaneta, MD Department of Radiology University of Southern California USC University Hospital Los Angeles, California, USA Quentin Vannod-Michel, MD Department of Neuroradiology Lille University Hospital Lille, France Koichi Watanabe, MD Assistant Professor Department of Anatomy Kurume University School of Medicine Fukuoka, Japan Lynda J. S. Yang, MD, PhD Department of Neurosurgery University of Michigan Ann Arbor, Michigan, USA

1 Bones, Muscles, Tendons, Joints, and Cartilage

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Farhood Saremi

◆◆ Introduction

This chapter is an overview of the development, structural anatomy, morphologic categories, physiology, biomechanics, and imaging of the bones, skeletal muscles, tendons, joints and cartilage, and related structures.

◆◆ Bones

The adult skeleton is comprised of 206 bones with the largest being the femur and the smallest being the stapes of the middle ear. There are 126 bones in the extremities or appendicular skeleton, 74 bones in the axial skeleton, and 6 auditory ossicles. Additionally, there are many small sesamoid bones in different locations near the joints.1,2 The bones have been divided into four general morphologic categories: long bones (i.e., femur, metacarpals, and phalanges), short bones (i.e., carpal and tarsal), flat bones (i.e., skull and scapula), and irregular bones (i.e., vertebrae and hyoid). The skeleton serves as the structural support for the body which protects the internal organs and allows movement and activity. The bones also provide the environment for blood production within the marrow spaces, hold reserve of calcium and phosphate needed for the maintenance of serum homeostasis and acid–base balance, and finally serve as a reservoir of growth factors and cytokines.1 Bone is a mineralized structure made up of collagenous matrix, cells, vessels, and crystals of calcium compounds (hydroxyapatite). In contrast to cartilages, bones are highly vascular structures with the capability of repair and remodeling. Remodeling is a constant and ongoing process during life that is accomplished by osteoblasts and osteoclasts to reshape the bone by removing old damaged bone and replacing it with new bone. This process helps the bone to adapt to changing biomechanical forces and achieve the strength against mechanical forces.

Gross Appearance The long bones are composed of a tubular shaft or diaphysis with two cone-shaped metaphyses at each side of it. Each metaphysis connects to one or more rounded epiphyses with their interface being called the growth plates. Each bone is surrounded by a shell of compacted bone called cortex, which comprises 80% of the skeletal mass. The cortex of the diaphysis is thick and surrounds the medullary space that contains bone marrow. The metaphysis and epiphysis are composed primarily of a honeycomblike meshwork of trabecular bone also known as spongy or cancellous bone filled with bone marrow and hematopoietic cells and all surrounded by a relatively thin shell of cortical bone. The cancellous bone is highly vascular and the major site of hematopoiesis. The vertebra

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is composed of one-third of cortical bone and two-thirds of trabecular bone, whereas the femoral head cortex forms 50% of the bone and the radial bone diaphysis cortex forms 95% of the bone.1 The trabecular bone is composed of plates and rods. Within long bones such as the femoral neck, the trabeculae are aligned along the mechanical forces that weight-bearing bones experience (see Chapter 20 “Hip”). The trabecular bone contributes to major mechanical support in the vertebral bodies. The functional units of the cortical and trabecular bones are microscopic columns called osteons. In the cortex, the osteons are called haversian systems and the osteons of the trabecular bone are called packets. Haversian systems are cylindrical in shape, approximately 4-cm long and 2-cm wide at their bases, and form a branching network along the long axis of the cortical bone. Each column consists of multiple concentric lamellae, each containing osteoblasts and osteocytes around a central canal called the haversian canal where vessels and nerves pass (▶Fig. 1.1). The haversian canals connect the lacunae (the space around osteocytes) by tiny channels called canaliculi. The haversian canals are also connected with one another by Volkmann’s canals. All haversian columns are surrounded by outer circumferential lamellae and separated from each other by interstitial lamellae (▶Fig. 1.1). The interstitial lamellae are deformed and partially resorbed haversian columns. The cortical trabecular osteons are normally formed in a lamellar pattern, in which collagen fibrils are laid down in alternating orientations. The lamellar pattern, just like a plywood, increases the bone strength. Disorganized deposition of the collagen fibrils results in a weak bone called woven bone. The woven bone is normally produced during formation of the primary bone. It is also seen in the callus of the repaired fractures and in pathological conditions such as in bone lesions of hyperparathyroidism and Paget’s disease. The outer cortical bone is covered by a fibrous connective tissue called the periosteum, and the inner cortex and the trabecular bone and Volkmann’s canals are covered by a membranous layer called endosteum (▶Fig.  1.1). The periosteum does not extend into the joints where bone is lined by articular cartilage. It is attached tightly to the cortex by thick collagenous fibers called Sharpey’s fibers. The endosteum consists of a single layer of flat cells supported by a thin layer of reticular connective tissue. Both the periosteum and the endosteum contain blood vessels, nerve fibers, and osteoblasts and osteoclasts. The endosteal surface has higher bone formation activity than the periosteal surface.

Development and Ossification In general, the flat bones are formed by membranous bone formation, whereas the long bones are formed by a combination of

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Bones, Muscles, Tendons, Joints, and Cartilage

1

Spongy bone (Red marrow)

Epiphysis

Nutrient vessels

Periosteal artery

Cortical bone

Metaphysis

Cortex

Trabecular bone

Venous sinosoids Central vessels Radial arteries

Yellow marrow Diaphysis

Endosteum Central vessels

Nutrient vessels Periosteum

Mesenchymal stromal cells

Periosteal artery

Outer lamellae Osteon Haversian canal Periosteum Perforating fibers

Spongy bone

Lacuna

Perforating canal

Osteon Haversian canal

Sinusoid

HSC

Adipocytes

Trabeculae

Macrophage

Outer circumferential lamellae

Vessels and nerve

Blood vessel

Schwann-like cell

Megakaryocyte

Interstitial lamellae Canaliculi Osteocyte

White blood Endothelial cells cells Perivascular CAR cell

Platelets Extracellular matrix

Osteoclast

Red blood cells Osteoblast

Sympathetic neuron

Fig. 1.1 Bone and bone marrow anatomy and vascular supply. The normal bone marrow anatomy is composed of bone cells, blood vessels, and red and yellow marrow. Normal hematopoietic stem/progenitor cell (HSPC) reside in the red marrow where they differentiate into red blood cells, white blood cells, and platelets via different progenitor stages. Yellow marrow represents largely fat cells with minimal hematopoiesis. The interface of bone and bone marrow, also called the endosteum, is covered by bone-forming osteoblasts and bone-resorbing osteoclasts. The endosteal surfaces have a rich network of arterioles and sinusoids. Arteries enter the sinusoids, which coalesce to form the venous circulation. Sinusoids are specialized venules that form a reticular network of fenestrated vessels that allow cells to pass in and out of circulation (CAR cell, CXCL12-abundant reticular cell; HSC, hematopoietic stem cells).

endochondral and membranous bone formation that occurs in a centrifugal pattern spreading outward in both directions from the center of the bone (▶Fig. 1.2). The cartilage, bone, and skeletal muscles are mainly formed by the mesoderm with some neural crest contribution.3,4,5 The intraembryonic mesoderm, located on either side of the neural groove between the endoderm and the ectoderm, is divided into the paraxial, intermediate, and lateral mesoderm. During the third week of gestation, the paraxial mesoderm forms blocks of cells called somites3 (▶Fig. 1.3). Each somite has two components: the dermamyotome and the sclerotome. The sclerotome of the somites contributes to the development of the craniofacial skeleton and most of the axial bone and cartilage. The cells in the lateral plate mesoderm contribute to the bones and cartilage of the limbs. The cranial neural crest contributes to the development of the craniofacial bones and cartilage. The dermamyotomes are the precursors of the dermis of the dorsal skin, the skeletal muscles of the back and body. The myoblasts close to the neural tube form the epaxial muscles (the deep muscles of the back), whereas the myoblasts remote from the neural tube produce the hypaxial muscles of the body wall and limbs. The cells in the middle of the dermamyotome are called the dermatomes that contribute to generation of the dermis and the mesenchymal connective tissue of the skin. The mesenchymal cells of the lateral plate mesoderm condense and differentiate into two groups of bone-forming cells, either osteoblasts or chondrocytes. This differentiation is regulated by

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a process known as canonical Wnt signaling.4 Bone formation by direct differentiation of the osteoblasts from the preexisting mesenchymal cells is called intramembranous (i.e., calvaria of the skull) and bone formation by initial transformation of the mesenchymal cells into chondrocytes, which secrete the cartilaginous matrix, followed by ossification of the cartilage is called endochondral ossification (i.e., most long bones6; ▶Fig. 1.2).

Extracellular Matrix Mineralization The majority of the bone structure is formed by mineralized matrix (70–90%). Two-thirds of the mineralized matrix is mineral and one-third is organic matrix. The rest of bone is water (5–10%) and lipids (3%). Hydroxyapatite, Ca10(PO4)6(OH)2, is the most abundant mineral, with small amounts of carbonate, magnesium, and acid phosphate. The organic matrix is composed of 85 to 90% collagenous proteins, with type I collagen being the most common. The rest of organic matrix is noncollagenous proteins, with osteocalcin, osteopontin, and bone sialoprotein being the common ones. Osteonectin plays a role in calcium binding, stabilization of hydroxyapatite crystals in the matrix, and regulation of bone formation.7 Matrix maturation is associated with expression of alkaline phosphatase. Phosphoprotein kinases and alkaline phosphatase regulate the mineralization process. Vitamin D plays an indirect role in stimulating mineralization of unmineralized bone matrix.

Bones, Muscles, Tendons, Joints, and Cartilage

Mesenchyme

Cartilage

Hypertrophic chondrocytes

Osteoblasts (bone)

Proliferating chondrocytes

1

Epiphyseal cartilage Growth plate

Bone marrow

Bone

Vessel Growth plate

a

Secondary ossification center

Osteoid matrix Osteoblasts Osteoblasts

Loose mesenchyme

b

Calcified bone

Blood vessel

Bone cell (osteocyte)

Fig. 1.2 (a) Endochondral ossification involves proliferation, hypertrophy, and mineralizing of chondrocytes. First, the mesenchymal cells condense and differentiate into chondrocytes to form the cartilaginous matrix. Later, the chondrocytes in the center of the shaft undergo hypertrophy and apoptosis while they change and mineralize their extracellular matrix. Following death of chondrocytes, blood vessels grow and bring in osteoblasts, which bind to the degenerating cartilaginous matrix and deposit bone matrix. Secondary ossification centers also form as blood vessels enter near the bone ends. At the same time, osteoclasts are derived from the blood macrophages and dissolve the bone matrix and contributes to the final shape of bones. (b) Intramembranous ossification. Osteoblasts are formed by condensation of mesenchymal cells and deposit osteoid matrix. These osteoblasts are arranged along the calcified margin of the matrix. Osteoblasts that are trapped within the bone matrix become osteocytes. No cartilage is seen to precede the formation of bone.

Production of organic matrix and mineralization of matrix are performed by osteoblasts. The osteoblasts are formed by osteoprogenitor cells, which in turn are derived from pluripotent stem cells. Bone marrow contains a small population of mesenchymal stem cells that are capable of differentiating into bone, muscle, or adipocytes. At the completion of bone formation, approximately 50 to 70% of osteoblasts undergo apoptosis. Osteocytes represent terminally differentiated flattened osteoblasts. These cells are located in lacunae within mineralized bone and have extensive microfilaments that pass within the canaliculi in mineralized bone and interconnect all cells. Osteocytes support bone structure

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and become active during osteolysis by secreting lysosomes. It is shown that osteocytes may also undergo apoptosis. Increased osteocyte apoptosis due to estrogen deficiency or chronic glucocorticoid administration is harmful to bone structure.8,9 Administration of estrogen and treatment with bisphosphonate and exercise may prevent osteoblast and osteocyte apoptosis.

Bone Remodeling Bones undergo longitudinal and radial growth, modeling, and remodeling during life.10,11,12 Longitudinal growth occurs at the growth plates, and radial growth occurs by activity of the

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Bones, Muscles, Tendons, Joints, and Cartilage Neural groove

1

Amniotic cavity Ectoderm

Epaxial myotome

Notochord

Somite

Lateral mesoderm

Intermediate mesoderm Intraembryonic coelom

Dorsal aorta

Neural tube Endoderm

c

Notocord Myotome

Hypaxial dermamyotome lip Notochord

Sclerotome cell Dorsal aorta

Late 4-day embryo Central Somite Epidermis dermamyotome region Vertebrae Epaxial dermamyotome lip Dorsal root ganglia

Epidermis

Hypaxial myotome

Neural tube

Neural tube Sclerotome

Notochord Yolk sac

Central dermamyotome region

Epidermis Epaxial dermamyotome lip

Dermamyotome

Dorsal aorta

b

4-day embryo

Epidermis

Visceral mesoderm

Paraxial mesoderm

a

3-day embryo

Parietal mesoderm

Amnion

Intercostal (hypaxial) muscle

Back (epaxial) muscle

Rib region derived from sclerotome Epaxial muscle region Hypaxial muscle region Rib region derived from epaxial myotome

Rib region derived from hypaxial myotome

Fig. 1.3 (a–c) Development of the paraxial mesoderm. Transverse sections through the trunk of a chick embryo on days 2 to 4. The paraxial mesoderms form the somites and each somite contributes to development of the sclerotome cells and dermamyotome cells. Soon the sclerotome cells migrate toward the neural tube. The sclerotomes contribute to the development of the craniofacial skeleton and part of the axial bone and cartilage. On day 4, the dermamyotome cells divide. A layer of muscle cell precursors (the myotome) forms beneath the epithelial dermamyotome. The dorsomedial cells form an epaxial myotome beneath the dermamyotome, which develops into the deep muscles of the back, whereas the ventrolateral cells form a hypaxial myotome, which contributes to the development of the muscles of the body wall and limbs. Lateral plate mesoderms are located on either side of the intermediate mesoderms that contribute to development of the bones and cartilage of the limbs. Each lateral plate mesoderm splits into a parietal (somatic) mesoderm, which forms the future body wall, and a visceral (splanchnic) mesoderm, which forms the circulatory system. The space between the two layers becomes the coelom—which will be subdivided into the pleural, pericardial, and peritoneal cavities.

periosteum. Bone remodeling is a continuous repairing process that allows maintenance of shape, quality, and size of the skeleton, thereby increasing bone strength. In this process, the osteoclasts remove aged or damaged bone, leaving space for osteoblasts to make a new bone matrix that subsequently becomes mineralized. Remodeling is characterized by the coordinated work of osteoclasts and osteoblasts that follow a preprogrammed sequence of events. The remodeling cycle consists of three consecutive phases: 1. Bone resorption during which the osteoclasts digest old bone. This phase continues for 2 weeks. 2. Reversal phase during which mononuclear cells appear on the bone surface. This phase may last 4 to 5 weeks. 3. Bone formation, in which osteoblasts form new bone until the resorbed bone is completely replaced and the osteoid is fully mineralized.11 This phase will continue for 4 to 6 months. Osteoclasts are the only cells that are capable of resorbing bone (▶Fig.  1.1). These giant multinucleated cells are derived mainly from bone marrow monocyte–macrophage precursor cells.13 The osteoclasts are found in contact with a calcified bone surface and within a lacuna that is formed by their resorptive activity (Howship’s lacunae). Lysosomal enzymes such as tartrate-resistant acid phosphatase and cathepsin K are actively synthesized

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by the osteoclast.13 Osteoclasts resorb bone by proteolysis of the bone matrix and unbound the hydroxyapatite crystals by digestion of the collagen. During each cycle, 10 osteoclasts create a cylindrical tunnel in the dominant loading direction that will be followed by activity of many osteoblasts to fill the tunnel. This process allows remodeling of 2 to 5% of cortical bone each year. Remodeling of the trabecular bone is more active than cortical bone due to its larger surface area. The regulation of bone remodeling is both systemic and local. The major systemic regulators include parathyroid hormone, calcitriol, and other hormones such as growth hormone, glucocorticoids, thyroid hormones, and sex hormones.9 Receptors for classical hormones are located in osteoblasts. Parathyroid hormone is the most important regulator of calcium homeostasis. A number of cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-10 (IL-10) modulate this system. Major local factors include insulinlike growth factors, prostaglandins, tumor growth factor-beta, bone morphogenetic proteins, and cytokines.14 Radial growth and bone widening normally occur with aging in response to periosteal apposition of new bone and endosteal resorption of old bone. Bone remodeling increases in perimenopausal and early postmenopausal women. Bone modeling may be increased in hypoparathyroidism, renal osteodystrophy, or treatment with anabolic agents.

Bones, Muscles, Tendons, Joints, and Cartilage

Vasculature and Innervations Vessels and nerves enter or exit bones via nutrient canals (▶Fig. 1.4). The number of nutrients canals varies.15,16 Most long bones have one or two nutrient canals passing obliquely through the cortex. In the majority of bones, the nutrient canal is directed away from the growing end, and their location and length are variable and may alter during growth (▶Fig.  1.4). Knowing the anatomic location of these foramina is useful in certain operative procedures to preserve the circulation especially in growing bones. The external opening of the nutrient canal is called the nutrient foramen. It is located at the site of the original center of ossification. On the upper posterior surface of the proximal tibia, there is a single nutrient foramen, which is probably the largest

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in long bones (▶Fig.  1.4). The femur usually has two nutrient foramina near the linea aspera in the middle third of the bone. The absence of nutrient foramina in long bones is not uncommon. Each nutrient canal contains a nutrient artery and one or two nutrient veins. The nutrient artery splits into ascending and descending branches and each travels along the shaft in the central part of the marrow cavity and gives rise to small radial arteries that enter the venous sinusoids, the haversian canals, and/ or interconnect with periosteal artery branches and interconnect each other (▶Fig. 1.1). The radial arteries branch into many small arterioles and capillaries that extend outwardly toward the cortical bone. At the endosteal level, the arterioles anastomose with the venous sinuses. These venous sinuses drain via collecting venules into the central longitudinal vein that finally exit the

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Fig. 1.4 Nutrient canal of the tibial bone. (a) Plain film, (b) computed tomography (CT), and (c) magnetic resonance imaging (MRI) showing a large nutrient canal in the mid-diaphysis of the tibia. Vessels and nerves enter or exit bones via nutrient canals. MRI shows a large vein within the canal (arrow).

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Bones, Muscles, Tendons, Joints, and Cartilage

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Fig. 1.5 Benign-appearing focal cortical thickening at the nutrient foramen of the distal femur (arrow) which was mistakenly called an osteoid osteoma. (a) Plain film. (b) Coronal and (c) axial magnetic resonance (MR) imaging showing the nutrient vein (arrows).

bone via the nutrient veins (▶Fig. 1.1, ▶Fig. 1.4). Mild thickening of the cortex at the nutrient foramen is not unusual and should not be mistaken with infection, fracture, or osteoid osteoma (▶Fig. 1.5). The bone and bone marrow have multiple blood vessels that interconnect through an endosteal network of vessels (▶Fig. 1.1). Blood circles within the marrow cavity, from its center toward the periphery and then back toward the center of the marrow cavity. In flat bones, the marrow is supplied by numerous blood vessels of various sizes entering the bone via large and

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small nutrient canals.15 The marrow does not have lymphatic drainage. Innervation of bone marrow is provided by myelinated and nonmyelinated nerves that enter through the nutrient canals. Some innervations also occur through epiphyseal and metaphyseal foramina. The venous sinuses, along with collecting venules, form an extensive interconnecting vascular network within the bone marrow, which finally drains into the systemic vein via the central longitudinal vein. This interconnecting vascular system could

Bones, Muscles, Tendons, Joints, and Cartilage be the cause of remote spread of cancer or microorganisms. This is specially the case in the vertebral bodies in which the bone is surrounded by valveless venous plexus. This intercommunicating vertebral venous system consists of basivertebral vein, external vertebral venous plexus, and internal vertebral venous plexus, traditionally referred to as Batson’s plexus that drains into the systemic vein of the skull, chest, abdomen, and subcutaneous tissue.17,18 For diagnostic and treatment purposes, intraosseous venography and fluid administration using a bone marrow needle have been used as a rapid method for obtaining vascular access in children and adult populations, particularly in the

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presence of failed peripheral intravenous line placement and in patients with cardiac arrest. Several anatomic locations (sternum, proximal humerus, and tibia) are available for intraosseous needle insertions19,20 (▶Fig. 1.6). Percutaneous vertebroplasty is a procedure consisting of the injection of polymethyl methacrylate into the bone marrow that commonly is used to treat compression fractures especially in patients with osteoporosis. Leakage of polymethyl methacrylate into the systemic veins due to communications with vertebral venous system is a potential complication of this procedure and in some cases results in symptomatic pulmonary embolism.21

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Fig. 1.6 Intravenous contrast computed tomography (CT) study of the chest showing opacification of the intraosseous venous channels through collateral channels. (a) Coronal image showing proximal humerus intraosseous veins (arrow). (b) Coronal image showing manubrium intraosseous veins (arrow). (c) Sagittal image showing communicating vein in the body of the sternum (arrow). As seen, the intraosseous veins communicate directly with the extraosseous veins, and form a network with potential for spread of infection and neoplasia between bone and remote organs. In patients with no peripheral venous access, intraosseous injection using a bone marrow needle can be used as an alternative method for fluid administration.

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Bones, Muscles, Tendons, Joints, and Cartilage

◆◆ Bone Marrow

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Normal bone marrow contains fat and hematopoietic cells surrounded by vascular sinuses, and is a supported by fine mesh of reticulin within the trabecular bone scaffold22,23,24,25 (▶Fig.  1.1, ▶Fig. 1.7). In the fetus, hematopoietic stem cell activity for hematopoiesis begins in the yolk sac before transferring to the placenta, liver, and spleen during the second trimester. Along with development of the medullary cavity in the long bones, hematopoiesis shifts into this space. After birth, the bone marrow is the primary site of red blood cell formation.26,27 For hematopoiesis to occur, a microenvironment is required to support proliferation, differentiation, and maturation of stem cells. This microenvironment (also known as stroma) consists of adventitial reticular cells (barrier cells), endothelial cells, macrophages, adipocytes, possibly osteoblasts, and the extracellular matrix. The matured hematopoietic stem cells of bone marrow form the three basic groups of blood cells in blood circulation for oxygenation (red blood cells), cellular immunity (white blood cells), and coagulation (platelets). These cells traverse the wall of the venous sinuses to enter the bloodstream.

Bone Marrow Maturation

Imaging of Bone Marrow Because of high contrast resolution and lack of ionizing radiation, magnetic resonance imaging (MRI) is the best imaging technique for evaluation of normal and pathologic bone marrow. The MRI appearance of bone marrow depends on the proportion of yellow and red marrow (▶Fig. 1.7). T1-weighted (T1W) images are more sensitive to evaluate the cellular content and pathologies in bone marrow because of high signal fat on this sequence that allows

Infancy Cartilage

Childhood Red marrow

Adolescence

Early adulthood >25 years

Yellow marrow

Fig. 1.7 Red to yellow marrow conversion in the long bones, femur in these sketches. At birth, the medullary cavity is filled with hematopoietic red marrow only. The red marrow of the secondary ossification centers converts to yellow marrow within 6 months. The rest of bone marrow converts later, starting in the diaphysis first and progresses bidirectionally toward the metaphyses. The diaphyseal conversion is faster distally than proximally, so that residual red marrow is mainly found only within the proximal metaphysis during adulthood.

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optimal tissue contrast to see small lesions that appear low signal on T1W images. On T1W images, fatty marrow appears bright, whereas hematopoietic red marrow or other cells (i.e., cancer cells) appear as relatively low signal (▶Fig. 1.8, ▶Fig. 1.9, ▶Fig. 1.10). The greater the cellularity of the marrow, the lower the T1W signal. On T2-weighted (T2W) images, yellow marrow shows intermediate to high signal intensity and red marrow shows slightly lower signal (▶Fig.  1.8). Metastatic lesions may appear brighter than normal bone marrow on T2W MRI due to their high water content. Fatsuppressed sequences can increase contrast to see pathologies. Fat saturation can be applied to T2W and gadolinium-enhanced T1W images (▶Fig.  1.10). Gadolinium-enhanced T1W images may be helpful in detecting some marrow lesions.28,29 Computed tomography (CT) scan is a great technique for assessing the marrow cavity of bones including bone pathologies (▶Fig. 1.11). However, focal medullary lesions without lytic bone could be challenging to detect. Although MRI and fluorodeoxyglucose positron emission tomography (PET/CT) are of higher sensitivity to detect lytic bone and bone marrow lesions, both techniques are expensive and time-consuming compared with CT.

The red marrow appears red due to the presence of hemoglobin. A newborn’s marrow consists mainly of hematopoietically active “red” marrow. There is a progressive conversion toward “yellow” marrow with age. For example, the fat fraction of bone marrow increases from 40% in infants to 60% by age of 70 years.

Conversion of Red Marrow Replacement of red marrow with yellow marrow occurs early in life. This physiological phenomenon is called conversion (▶Fig. 1.7, ▶Fig. 1.8, ▶Fig. 1.9, ▶Fig. 1.10). It begins in the extremities and progresses toward the axial skeleton. The proportions of red and yellow marrow in the axial skeleton vary by age, sex, and environmental factors. Yellow marrow first appears in the terminal phalanges before birth and progresses centrally during the first two decades of life.27 In long bones, bone marrow conversion starts in the epiphysis and apophysis, followed by the diaphysis, distal metaphysis, and proximal metaphysis (▶Fig. 1.7, ▶Fig. 1.8). In most young children, the diaphysis of long bones still contains hematopoietic marrow. An adult pattern is finally reached at approximately 25 years of age. It is characterized by the presence of red marrow predominately in the central skeleton, such as the pelvis, sternum, cranium, ribs, vertebrae and scapulae, and variable amount in the proximal ends of the femur and humerus.

Variants of Marrow Conversion In both flat and long bones, marrow may become heterogeneous due to nonuniform marrow conversion (▶Fig. 1.10). Small islands of yellow or red marrow may be found in normal bone marrow. This heterogeneous appearance should not be confused with pathologic marrow replacement (▶Fig. 1.10). A “bull’s-eye” appearance, referring to a red marrow island with a central focus of yellow marrow, is believed to be a benign process in most examples.27,28

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Fig. 1.8 Red to yellow marrow conversion in the epiphyseal ossification centers. (a) Coronal T1-weighted (T1W) and T2-weighted with fat suppression (FS), in a 13-year-old boy. Note the high signal intensity of the fibrocartilage disk on T2-weighted with fat suppression. (b) Left shoulder in a 16-year-old. In the long bones, the red marrow of the secondary ossification centers converts to yellow marrow in the first year of life and much earlier than diaphyseal and metaphyseal marrow. (c) Lucency of the right humeral head (arrow) in a 46-year-old patient with shoulder pain. T1W magnetic resonance imaging (MRI) shows normal fatty marrow (arrow).

In young adults, the MR signal of the vertebral bodies remains low due to active hematopoietic marrow (▶Fig. 1.9). After the age of 40 years, the vertebral bone marrow is increasingly replaced with fatty marrow. In the vertebral bodies, conversion begins around the vascular axis (▶Fig. 1.9). Three imaging patterns have been described: a bandlike fatty replacement along the endplates, small foci of fatty marrow replacement, or larger globular areas of fat replacement giving a patchy or mottled appearance to bone marrow (▶Fig.  1.10). Some elderly patients and patients with malnutrition or osteoporosis may have near-complete replacement of vertebral marrow by fat (▶Fig. 1.9).

Reconversion of Fatty Marrow Reconversion of fatty marrow to normal hematopoietic red marrow occurs in response to dietary changes, anemia, chronic hypoxia, chemotherapy, and other medications, through various cytokines.30,31 Benign medullary hyperplasia is a common

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phenomenon in times of increased hematopoietic demand such as chronic hypoxia (e.g., smokers, high altitude, cyanotic congenital heart disease), anemia (e.g., pregnancy, sickle cell disease), or increased oxygen demand and mechanical hemolysis (e.g., runners) causing decreased T1 signal or increased CT density ­especially in the metaphysis of the long bones and vertebral bodies (▶Fig. 1.10). In patients who received extensive radiation therapy, the reconversion phenomenon can occur in the nonirradiated skeleton.32 Reconversion occurs in a reverse order of conversion. It starts from the axial skeleton to the long bones and involving the proximal metaphysis, distal metaphysis, and diaphysis, in this order. In severe cases (e.g., thalassemia), epiphyseal reconversion, bone expansion, or extramedullary hematopoiesis can occur.

Senile Osteoporosis Women have a larger amount of hematopoietic marrow in early adulthood, with a decline after 60 years, whereas in men, the

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Fig. 1.9 Red to yellow marrow conversion in the vertebral body at different ages shown with sagittal T1-weighted (T1W) magnetic resonance imaging (MRI). Yellow marrow contains fat and appears bright (white) on T1W MRI. At the first few months after birth, the medullary cavity is predominantly filled with hematopoietic red marrow. Soon, the central part of the vertebral body where the basivertebral vein exists (arrow) ­converts to yellow marrow and later progresses toward the vertebral endplates. By the age of 61 years, the entire red marrow is converted to yellow marrow, indicating decreased hematopoiesis and myelofibrosis.

Fig. 1.10 (a) Patchy red marrow conversion/reconversion in vertebral bodies with chronic cirrhosis, anemia, and hypoxemia. Sagittal images showing mottled bone marrow appearance in a 59-year-old man. T1-weighted (T1W) and T2-weighted (T2W) with fat suppression and postcontrast T1W with fat suppression are shown. This patchy marrow reflects red marrow reconversion and increased marrow blood flow. Remaining bone marrow fat islands are best shown on T1W by multiple hyperintense foci in the red marrow. These foci disappeared on fat suppression images and do not show enhancement on postcontrast study indicating a benign nature of heterogeneous bone marrow. These findings are favored to be reflective of red marrow reconversion and increased marrow blood flow. (b) Computed tomography (CT) shows similar mottled bone marrow, but differentiation from metastasis is difficult without magnetic resonance imaging (MRI).

decline occurs in the first 25 years and continues steadily.33,34,35 Progressive deposition of fat in the marrow occurs in postmenopausal females that may accelerate osteoporosis (▶Fig.  1.11). Although abnormal bone mineral metabolism has been the focus of attention on the pathogenesis of senile osteoporosis, recent investigations have shifted that focus toward the relationship of

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marrow fat and functionality hematopoietic tissue. It is shown that increased bone marrow fat cells may have toxic effect on bone metabolism and hematopoietic differentiation.36,37 It is shown that rapid decline in estrogen levels in menopausal females leads to a higher circulating androgen, which promotes an “android” pattern of fat deposition in postmenopausal females. The periosteal

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Fig. 1.11 Sagittal computed tomography (CT) images showing striated trabeculation and decreased bone density in a 60-year old and an 80-year old with osteoporosis, and extensive osteoblastic bone metastasis in a patient with prostate cancer.

osteogenic activity partly compensates for the endocortical bone loss seen in premenopausal women by deposition of new bone onto the cortical bone surface.

Treatment-Related Changes of Bones Radiation, chemotherapy, granulocyte colony-stimulating factor, and bone marrow transplant are routinely used in the treatment of neoplasms and lead to temporary or permanent changes in marrow composition and MRI signal intensity.32 Early and late bone marrow changes occur during and after radiation therapy. Radiation therapy is often limited to the radiation field, whereas chemotherapy changes are often diffuse35 (▶Fig. 1.12). During the first 2 weeks after irradiation, bone marrow may show vascular congestion, edema (decreased signal on T1W), hemorrhage, and contrast enhancement. After 3 to 6 weeks, replacement of the red marrow with yellow marrow occurs due to cytotoxic effect of radiation (▶Fig. 1.12). In radiation for treatment of bone metastasis, a fatty halo may appear around the site of disease. In young patients, bone growth disturbances may be observed after irradiation of the immature and growing skeleton. Deleterious effects are greater in younger patients or at puberty and with high doses. In children, epiphyseal changes can occur with doses as low as 400 cGy. At higher doses, avascular necrosis

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Fig. 1.12 Acceleration of bone marrow fatty replacement due to decreased hematopoiesis after bone marrow radiation in a 28-year-old patient with history of intracerebral germinoma. Sagittal T1-weighted (T1W) magnetic resonance imaging (MRI) and computed tomography (CT) show fatty conversion throughout the vertebral bodies. Months or years after radiation, due to reactivation of bone marrow, the vertebral body reconversion to red marrow can occur.

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Bones, Muscles, Tendons, Joints, and Cartilage and even pathologic fracture could be seen within 2 to 3 years after treatment. Within 3 to 4 weeks following successful chemotherapy, red marrow hyperplasia begins. Bone complications after chemotherapy are rarely seen, and include osteopenia, insufficiency fractures, cortical hyperostosis, and bone infarction.

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◆◆ Periosteum

The periosteum is an external membrane that covers all sides of bones except the intra-articular surfaces, tendon insertions, and sesamoid bones. This structure has the potential to form bones particularly in children and plays an important role in adults for bone remodeling and fracture repair.38,39 The periosteum is composed of an outer fibrous layer and an inner proliferative layer, also known as the cambium. The outer layer is continuous with the joint capsule, tendons, and muscle fasciae and epimysium. The cambium is characterized by abundant blood vessels and mesenchymal stem cells that have considerable osteogenic and

Fig. 1.13 Normal periosteum, growth plates, and epiphyseal ossification center in a 5-year-old ankle. In childhood, the periosteum is contiguous with a ring of perichondrium at the physeal growth plate level, until it transforms into the periosteum with physeal closure (pink arrows). In children, the cambium is thick, vascular, and active with considerable osteoblastic potential. Note the enhancement of the cambium and growth plate on postcontrast with fat suppression (FS) image (yellow arrows). Note the high signal intensity of the fibrocartilage disk and growth plate on T2-weighted with fat suppression (green arrows).

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chondrogenic potential. In childhood, the periosteum is contiguous with a ring of perichondrium at the physeal level, until it transforms into the periosteum with physeal closure (▶Fig. 1.13). In children, the cambium is thick, vascular, and active with considerable osteoblastic potential (▶Fig.  1.13). In adults, the periosteum is thin, inactive, closely attached to the cortex, and less vascular, and the cambium layer is barely visible. The periosteum is attached to the underlying bone by collagenous Sharpey’s fibers, which arise from the outer layer and penetrate the cortex. The attachment is rather loose in young individuals and becomes tighter with increasing age. Periosteal attachment is tight in flat bones including the skull. In children, the role of periosteum in the pathogenesis of acute osteomyelitis has been emphasized. Infection can spread from the medullary cavity and through the haversian and Volkmann canals into the subperiosteal space and periosteum causing reactive bone formation.

Fig. 1.14 Adductor insertion avulsion syndrome. Solid broad-based periosteal new bone formation (arrow) along the posterior medial left distal femur corresponding to the distal attachment of the adductor magnus muscle. This imaging finding may be seen in the setting of thigh splints.

Bones, Muscles, Tendons, Joints, and Cartilage

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Fig. 1.15 Lamellar periosteal (onion skin) reaction seen on sagittal and axial images of the fibula (arrows) due to aneurysmal bone cyst.

Fig. 1.16 Spiculated periosteal reaction in a 43-year-old woman with metastatic colon cancer to the right scapula. Axial and sagittal computed tomography (CT) images showing patchy sclerosis of the right scapula with associated spiculated periosteal reaction infiltrating into the surrounding musculature.

The periosteum is richly supplied by numerous vessels, sensory and vasomotor nerves, and lymphatics. It is the sensitive part of the bone, and its numerous nociceptors result in severe pain after injury. The periosteal vasculature provides a crucial blood supply to the bone via musculoperiosteal and corticoperiosteal anastomoses (▶Fig. 1.1). Normal periosteum is barely visible in healthy adults, but in children, it could be seen by MRI or ultrasound as a thin band

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located over the bone cortex especially in children40,41 (▶Fig. 1.13). An elevated periosteum is not seen on a radiograph unless it is mineralized. A wide variety of benign, malignant, and systemic conditions can stimulate the bone-forming potential of the periosteum. This effect is called a periosteal reaction40,41 (▶Fig. 1.14, ▶Fig. 1.15, ▶Fig. 1.16). Mineralization requires 10 days to 3 weeks following the insult to be seen. Using CT, the process of mineralization can be detected earlier. MRI can demonstrate both the

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Bones, Muscles, Tendons, Joints, and Cartilage mineralized and nonmineralized parts of the periosteal reaction. On MRI, the mineralized periosteal reaction appears as lines of signal void on all pulse sequences. Subperiosteal tissue is usually isointense on T1W, hyperintense on T2W, and can enhance following intravenous gadolinium injection. Periosteal reaction presents is different forms depending on type and aggressiveness of pathologies. Solid periosteal reaction is usually a response to a chronic, nonaggressive, and benign process. It is due to successive and slow deposition of compact lamellar bone to the surface. Over time, the periosteal layers fuse and become uniform and solid (▶Fig.  1.14). A solid periosteal reaction is common in stress fractures, medial tibial stress syndrome, and adductor insertion avulsion syndrome. Medial tibial stress syndrome, also known as shin splints, is a common cause of exercise-induced pain along the middle to distal posteromedial aspect of the tibia and has an unclear origin causing periosteal thickening on the posteromedial aspect of the tibia. Medial tibial stress syndrome, also known as thigh splints, represents a tendinous avulsion injury at the medial femoral shaft related to one or more of the adductor muscles that insert in this region (▶Fig. 1.14). Lamellar periosteal reaction (also known as onion-skin reaction) is a more aggressive form of periosteal reaction. It is seen in diseases of intermediate aggressiveness such as malignant processes, acute osteomyelitis, and some benign masses (▶Fig. 1.15).

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The presence of spiculated periosteal reaction implies faster growth and a more aggressive pathology (▶Fig. 1.16). In this type, the periosteum is elevated, and in some cases disrupted, by the pathologic process, creating a space under the periosteum where osteogenesis occurs perpendicular to the cortex.

◆◆ Muscles

Two types of striated muscles exist in the body: cardiac and skeletal muscles. These muscles provide whole-body oxygen supply, metabolic balance, and locomotion. Skeletal muscle tissue comprises 35 to 40% of the total body mass. Each skeletal muscle is composed of densely packed muscle fibers. The muscle fibers are cylindrical in shape and multinucleated (▶Fig. 1.17, ▶Fig. 1.18). The nuclei are located at the periphery of the muscle fibers. The cytoplasm of the muscle fiber is called sarcoplasm and its cell membrane is called the sarcolemma. The myofibers are separated from each other by a thin sheet of connective tissue called “endomysium.” The length of fibers varies from a few millimeters to up to 40 centimeters with a thickness of 20 and 90 μm.42 Approximately 20 to 200 of these longitudinal muscle fibers are stacked and surrounded by another sheet of connective tissue called “perimysium,” and together form a muscle fascicle or fiber bundle. Muscle fascicles are stacked to form the skeletal muscle. The epimysium envelopes the skeletal muscle and connects to

Sarcolemma Muscle fiber

Mitochondrion Microvascular unit

Myofibril

Nucleus Light I band

Fascicle

Muscle fiber

Feeding artery

a

Terminal arteriole

Collecting venule

Sarcolemma

b

Dark A band Sarcoplasm I band

H zone

T-tubule Sarcomere Z disk

H zone

Z disk

CapZ Titin Z-disk

Thick (myosin) filament

Myosin head Relaxed Contracted

Sarcoplasmic reticulum

c

Triad

I band

T-tubule Terminal cisternae

A band

I band

I band

Myosin tail Actin filament

M line

Thin (actin) filament

d

M-line

Fig. 1.17 General anatomical structures of skeletal muscle and its vascular supply. (a) Skeletal muscle is composed of muscle fascicles. The ­primary arteries give rise to feed arteries that give rise to branching arterioles that finally teminate in the perimysium and give rise to numerous capillaries. (b) Each skeletal muscle fascicle is composed of densely packed multinucleated muscle fibers. The nuclei are located at the periphery of the muscle fibers. (c) Individual muscle fibers are formed by a stack of longitudinally oriented protein filaments known as myofibrils. The myofibrils are surrounded by interconnecting tubules known as sarcoplasmic reticulum. At the Z lines, the sarcoplasmic reticulum connects a branched network of membrane invaginations called T-tubules. (d) Cytoplasmic proteins including actin (thick filaments) and myosin (thin ­filaments) are arranged in an alternating order along the myofibril, giving the skeletal muscle its characteristic striated pattern.

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Bones, Muscles, Tendons, Joints, and Cartilage the perimysium, the overlying fascia, and the tendons. The endo-, peri-, and epimysium consist of type I, II, and III collagens synthesized by fibroblasts. In normal skeletal muscle, fibroblast nuclei comprise 8 to 15% of all nuclei in the tissue42 (▶Fig. 1.18). Individual muscle fibers are formed by a stack of longitudinally oriented protein filaments known as myofibrils with multiple mitochondria located between the myofibrils (▶Fig.  1.17). The myofibrils contain contractile proteins (actin and myosin) and regulatory proteins (such as troponin and tropomyosin) that are involved in active muscle contractions. In addition to contractile and regulatory proteins, the myofibrils contain structural proteins (such as titin and nebulin) which contribute to passive elastic forces of muscles. Cytoplasmic proteins including actin (thick filaments) and myosin (thin filaments) are arranged in an alternating order along the myofibril, giving the skeletal muscle its characteristic striated pattern (▶Fig.  1.17, ▶Fig.  1.18). These alternating units are called a “sarcomere,” which appear as dark and light bands under the microscope and are bound together by the Z lines, a characteristic feature of skeletal muscles. The sarcomere is the smallest functional unit of muscle contraction and measures 2 to 3 μm in length. Each sarcomere contains a central myosin-rich dark band and two actin-dominated light bands The myofibrils are surrounded by interconnecting tubules known as sarcoplasmic reticulum. At the Z lines, the sarcoplasmic reticulum connects a branched network of membrane

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invaginations called T-tubules (▶Fig.  1.17). The two networks enable synchronous calcium release throughout the entire cell volume and play a crucial role in the activation and relaxation of the actin and myosin. Activation and contraction occur as a result of a transient elevation in the intracellular calcium concentration, causing the actin and myosin filaments to engage and slide with respect to each other (▶Fig.  1.17). Myosins are ATP dependent and responsible for actin-based motility.

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Muscle Architecture The muscle fibers are classified by their metabolic properties, speed of contraction, and ability to resist fatigue into three general phenotypes: type I or slow-twitch oxidative fibers; type II or fast-twitch, oxidative/glycolytic; and type III fast-twitch, glycolytic fibers.43,44 Type I slow-twitch fibers are resistant to fatigue and primarily rely on oxidative metabolism for energy, and therefore exhibit high mitochondrial numbers and oxidative enzyme content, and low glycogen levels and glycolytic enzyme activity. Characteristics of their fibers include thin fibers that are invested by a denser capillary network, and red appearance due to a higher oxygen-binding protein myoglobin. Similar to type I fibers, type II fibers are red and fatigue resistant, but they have larger fibers with both higher amount of glycogen and more mitochondria that help for accelerated rate of ATP hydrolysis in these fast-twitch fibers. On the other Cardiac

Structure

Skeletal

Striated muscle cell Fibroblast

a

Satellite cell Blood vessel

Intercalated disk Extracellular matrix

Axon branch Neuromuscular junction Presynaptic terminal

Capillary Muscle fiber

Terminal axon

Sarcolemma

Presynaptic terminal

Sarcoplasm

Synaptic cleft

Sarcoplasmic reticulum

Muscle fiber

Myofibrils

b

c

Postsynaptic membrane

Fig. 1.18 Structure of striated muscles and innervation. (a) Adult skeletal muscle contains longitudinally aligned multinucleated myofibers, blood vessels, and resident satellite cells, with fewer fibroblasts. Adult cardiac muscle consists of a branched network of shorter cardiomyocytes connected via intercalated disks and surrounded by blood vessels and extracellular matrix secreted primarily by fibroblasts. Individual skeletal muscle fibers arise from the fusion of many muscle cells (myofibers), producing multinucleated linear fibers, millimeters to centimeters in length. In contrast, the cardiac muscle consists of a cellular syncytium wherein individual cells are electromechanically interconnected in a branched pattern via specialized structures known as intercalated disks. (b) Neuromuscular junction and (c) motor endplate. Nerves travel through the connective tissue network of the muscle to reach the endomysium. At the muscle fiber level, the nerves become demyelinated to reach the neuromuscular junction.

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Bones, Muscles, Tendons, Joints, and Cartilage hand, type III fibers are white with little amount of myoglobin and fewer mitochondria. Therefore, type III fibers rely on the energy stored in glycogen and phosphocreatine, and therefore are easily fatigable. In general, slow-twitch fibers are more present in postural muscles, whereas fast-twitch fibers are primarily seen in the extremity muscles that require short bursts of muscle contraction. Gross skeletal muscle architecture is defined as the number and orientation of the muscle fibers within a muscle and relative to the axis of force generation of the muscle. Major parameters used in the analysis of force generation of the muscles include the length of muscle, the length of fibers, muscle volume, pennation angle (θ), anatomical cross-sectional area (ACSA), and physiological cross-sectional area (PCSA).45,46,47 These parameters have been studied by performing microdissection, high-resolution ultrasound, and MRI-based muscle diffusion tensor imaging techniques.48,49,50,51,52 The ACSA is simply the largest cross-section area across the muscle, whereas the PCSA is the sum of the cross-sectional area of all fibers. Therefore, for pennate muscles, the PCSA better predicts a muscle’s maximal force production than the ACSA. Large mass and short fiber length both contribute to a large PCSA. For example, the soleus muscle (multipennate) has a modest mass with very short fibers (4 cm), which results in its exceptionally large PCSA. In contrast, the vastus lateralis (unipennate) mass is 50% greater, but its fibers are much longer than the soleus (10 cm), which results in a smaller PCSA. Fascicle lengths and pennation angles are important determinants of the muscle’s contractile force, power, and shortening velocity. Muscle architecture can be altered by bed rest, training, or as the result of genetic defects (e.g., Duchenne’s muscular dystrophy) and infiltrative pathologies.53

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Muscle architecture varies between muscles. Common architectures include longitudinal and pennate architectures. These architectures, which span the length of the muscle, are seen in fusiform or spindle-shaped (e.g., biceps brachii, hamstrings), straplike (e.g., sartorius), and circular (e.g., orbicularis) muscles (▶Fig. 1.19). In pennate architectures, the muscles contain shorter fibers that extend at an angle along the length of the muscle. The pennate architectures can be unipennate (e.g., vastus lateralis), bipennate (e.g., gastrocnemius, dorsal foot interossei), or multipennate (e.g., soleus, deltoid, trapezius; ▶Fig.  1.19, ▶Fig.  1.20). Multipennate muscles have a complex pattern in which fibers are oriented at multiple angles. In general, most muscles have a structure that is a combination of these architectural patterns but dominated with one pattern. The pectoralis major muscle demonstrates a fanshaped configuration in which the muscle fibers converge toward a small distal twisted tendon inserting on the proximal humerus (▶Fig. 1.20). Hydrostat is a term used for complex muscles with unique anatomic features such as the trunk of an elephant or possibly the tongue in which muscles can move in different directions without bone attachments by coordinated action of muscle fibers parallel, perpendicular, and helical relative to the long axis of the muscle.54 In muscles with longitudinally oriented fibers, the sarcomeres act in series and along muscle’s line of action. The action of the individual sarcomeres in series are additive, which favor higher length changes and increased velocity of shortening. Leg muscles with the longest fiber lengths, therefore the greatest excursion, are the sartorius, gracilis, and semitendinosus (▶Fig. 1.19).

Fig. 1.19 Generalized picture of muscle architectural types. (a) Biceps and (b) gluteus maximus show longitudinal or parallel architecture in which the muscle fibers are oriented parallel to the muscle’s force-generating axis. The fibers span the entire length of the muscle. The sartorius muscle is a straplike muscle with parallel architecture. Muscles with longitudinally oriented fibers have higher length changes and increased velocity of shortening. (c) The vastus lateralis shows pennate architecture in which the muscle fibers are oriented at a fixed angle relative to the force-generating axis. (d) The gluteus medius, (e) deltoid, and (f) soleus have multipennate architecture in which the muscle fibers are oriented at multiple angles relative to the force-generating axis. The gastrocnemius muscle is a bipennate muscle. The anatomical cross-sectional area (ACSA) is the largest cross-section area across the muscle. The physiological cross-sectional area (PCSA) is the sum of the cross-sectional area of all fibers. Large mass and short fiber length both contribute to a large PCSA and maximal force production as seen in the pennate architecture. In the lower extremity, the three strongest muscles (based on PCSA) are the soleus, vastus lateralis, and gluteus medius. (FL, fiber length; ML, muscle length.)

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Fig. 1.20 Additional muscle architectural types. (a) The rectus abdominis is a parallel muscle divided by tendinous intersections into distinct muscle bellies. (b) The pectoralis major has a complex morphology with five subsegments. (c) Bipennate (V-shaped) and unipennate architectural patterns of the plantar foot muscles. In the bipennate pattern, the muscle fibers converge in opposite diagonal directions toward a central tendon.

In pennate muscles, the fibers run obliquely to the muscle’s line of action (the pennation angle) and act in parallel. When sarcomeres in parallel contract, their forces add. Therefore, pennate muscles are well designed for short-length but high-force contractions. In the lower extremity, the three strongest muscles (based on PCSA) are the soleus, vastus lateralis, and gluteus medius (▶Fig. 1.19).

Vascular Supply, Lymphatic Drainage, and Innervation At rest, skeletal muscles receive approximately 25% of cardiac output. The average blood flow to individual muscles ranges between 5 and 10 mL/min/100 g muscle.55,56 The vascular inflow to skeletal muscles is provided by primary arteries. The primary arteries enter the muscles through one or two dominant or several small vascular pedicles and travel through the connective tissue framework of the muscle (▶Fig. 1.17). For example, the tensor fasciae latae is supplied only by the ascending branch of the lateral circumflex femoral artery, whereas the rectus abdominis receives two pedicles from the superior and inferior epigastric arteries and the sartorius is supplied by several small arteries. The primary arteries give rise to feed arteries that enter the epimysium of the muscle. The feed arteries account for 30 to 50% of the total resistance to blood flow, thereby control blood flow to the arterioles and capillaries. The arterioles originate from the feed arteries and enter the perimysium and travel perpendicular to the muscle fibers until giving rise to terminal branches that travel in the perimysium and give rise to numerous capillaries. The capillaries are embedded in the endomysium surrounding each muscle fiber and interconnect with the capillaries around

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neighboring fibers. The microvascular unit of the muscles is referred to the group of capillaries perfused by a terminal arteriole and represents the smallest unit for autoregulation function of the muscles56 (▶Fig.  1.17). The density of the capillaries and the number of their interconnections are greater in slow-twitch (oxidative) red muscle fibers and fast-twitch white muscle fibers. The capillaries gradually enlarge as they approach the postcapillary venules. The arrangement of venules and veins is similar to that described for the arterioles and arteries. During dynamic exercise, contracting skeletal muscle has the vasodilatory capacity to increase blood flow by several folds.57,58,59,60 With advancing age, muscle mass decreases, blood flow may diminish, and rapid-onset vasodilatation will be impaired that can lead to exercise intolerance. In other words, autoregulation of skeletal muscle blood flow may be impaired with age. Frequent exercise training modifies blood flow capacity in the muscle tissue having the greatest increase in ­activity. Exercise training increases regional blood flow capacity to the muscles containing fibers that experience increased a ­ ctivity  during exercise.58 Aerobic exercise training can reverse age-related impairment of vascular reactivity in skeletal muscle and endothelial dysfunction of the muscle arterioles. Hypoperfusion and decreased microcirculation of the skeletal muscle occur in many chronic diseases, such as peripheral arterial disease, heart failure, diabetes, cachexia, and chronic obstructive pulmonary disease. An extensive network of lymphatic vessels runs in parallel to the blood vascular system. Lymphatic vessels originate as blind endothelial tubes near the postcapillary venules. These tubes connect each other and form larger lymphatics that pass through the perimysium and connect to larger lymphatics that travel with arterioles and venules. Intramuscular lymphatic vessels have no contractile power. Therefore, forward propulsion of the lymph

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Bones, Muscles, Tendons, Joints, and Cartilage within the muscle is primality facilitated by muscle contractions and arteriolar pulsation. However, contractile lymphatics exist on the surface of muscles, which facilitate transport of lymph.55 Synchronized contractions of lymphatic muscle cells are essential for active lymph propulsion. Weakening of contractile power of lymphatic muscle cells is probably an important contributor to the lymphatic pump dysfunction underlying many forms of lymphedema.61 Muscles of the extremities, face, and neck are usually innervated by a single nerve; however, abdominal muscles with origin from several embryonic segments are supplied by more than one nerve. Usually, nerves travel with vessels in a neurovascular bundle. In general, nerves of the muscles are called “motor nerves,” but they contain both motor and sensory myelinated axons. Nerves travel through the connective tissue network of the muscle to reach the endomysium. At the muscle fiber level, the nerves become demyelinated to reach the neuromuscular junction. The neuromuscular junction is the synapse between an alpha motor neuron and the muscle fiber. The motor endplate is a modified saucer-shaped area of the muscle fiber membrane at which a synapse occurs (▶Fig. 1.18). Each muscle fiber has one endplate, but long ones may have two. The motor unit consists of a motor neuron and the myofibers it innervates. With age, the motor unit undergoes profound changes and the neuromuscular junction morphology deteriorates. In normal adult life, muscle undergoes repeating cycles of denervation and reinnervation. These cycles of denervation–reinnervation involve

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a transient disconnect of an individual muscle fiber from its motor neuron, followed by reinnervation by the original motor axon. Very old age is characterized by increasing frequency of axonal degeneration and motor neuron death, which lead to muscle atrophy and increased susceptibility to falls.62

Injury and Pathology Following small tears and contusions, the skeletal muscle can regenerate due to the abundances of resident muscle stem cells called satellite cells. These cells activate after injury and proliferate and fuse to repair damaged fibers or form new ones. The function of the satellite cells decreases with age or in chronic degenerative diseases such as muscular dystrophy resulting in impaired muscle regeneration and chronic fibrosis.63 Pathophysiologic fibrosis, which is essentially an excessive accumulation of extracellular matrix components, particularly collagen, is the end result of a cascade of events proceeding from tissue injury via inflammation, resulting in permanent scar formation64 (▶Fig.  1.21). Fibrosis can impair tissue function and cause chronic diseases in a large variety of vital organs and tissues, including bone marrow. In skeletal muscle, fibrosis is most often associated with the muscular dystrophies, a clinically and molecularly heterogeneous group of diseases. In the most severe cases, such as Duchenne muscular dystrophy, which is caused by the lack of the dystrophin protein, muscle loss and fibrosis also cause premature death through respiratory and cardiac failure.

Fig. 1.21 Extracellular matrix deposition in acute and chronic muscle regeneration. Acute injury to healthy muscle produces rapid and controlled inflammation that removes dead and damaged myofibers and promotes regeneration of the injured muscle. However, in conditions of chronic injury, as occurs in the muscular dystrophies, chronic inflammatory events result in the excessive accumulation of extracellular matrix components, which inhibit myogenic repair and persistent collagen deposition that leads to muscle being replaced by fibrotic/scar tissue. Bars = 50 μm.

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Fig. 1.22 Muscle atrophy. (a) Coronal computed tomography (CT) images in an 89-year-old man showing extensive muscle atrophy and fatty deposition. Due to fatty infiltration, the epimysial muscle fascia is clearly shown (arrows) at multiple levels. (b) Axial T2-weighted (T2W) magnetic resonance (MR) images of the lower thigh and (c) axial lower pelvis in a 20-year-old woman with dermatomyositis showing extensive bilateral edema of the muscles of the pelvis and thigh involving multiple compartments (arrows).

Aging is also associated with loss of skeletal muscle mass and function with deposition of collagen and fibrosis. Muscle diseases are commonly characterized by inflammation, fat infiltration, fibrosis, and atrophy. Muscle atrophy and fatty replacement of muscles are common findings due to chronic inactivity, denervation, paralysis, and chronic muscular dystrophy (▶Fig. 1.22). Plain X-rays are not very useful for evaluation of muscle pathology, except in cases with calcifications and heterotopic bone formation. Muscle architecture can be determined by ultrasound, CT, or MRI. Normal skeletal muscle is striated and contains interlaced fat within and between major muscle bundles. Fatty infiltration can be assessed by CT or MRI (▶Fig. 1.22). Other muscle pathologies can be best detected with MRI.65,66 T1W images show the architecture of the muscle related to its interlaced high-signal-intensity fat, whereas T2W images show increased fluid, muscle edema, contusion, hematoma, and tumors. The best known inflammatory myopathies are polymyositis and dermatomyositis.66 Both initially involve the proximal lower limb muscles, before ascending to involve the pelvic and proximal upper limb muscles. MRI shows symmetric edema with preserved muscle architecture, typically associated with subcutaneous and fascial thickening and edema (▶Fig. 1.22).

◆◆ Tendons, Ligaments, and Fasciae

Tendons, ligaments, and supporting regional fasciae transmit mechanical forces between muscles and bones.67,68 These structures are made of collagenous fibers. Tendons are tubular

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in shape and connect muscle to bone and actively transfer the forces generated by muscles to the bony skeleton and guide the motion of the joints. The ligaments are dense bands that span between the bones to stabilize them (e.g., sacrotuberous ligament) or strengthen the joint integrity and limit their abnormal movements (e.g., knee collateral ligaments). The fasciae envelop the musculature and extend between muscles to form septa and other fibrous partitions that link muscles together and ultimately attach to bones.

Morphology Tendons vary in size, shape, orientation, and location. Tendon fibers can travel into muscles as intramuscular tendon, thereby allowing the pennate arrangement of muscle fibers (▶Fig. 1.24). Intermediate tendons are referred to small tendons that connect one muscle belly to another (▶Fig. 1.25). Aponeuroses are referred to flat thin sheets of collagenous material that emerge directly from the muscle bellies and gradually thickens to form tendon (e.g., the tendons of latissimus dorsi and pectoralis major) or form fibrous sheets on the surface of a muscle (modified deep fascia) or within it and connect muscles to parts they act upon (e.g., soleus, oblique muscles of abdomen, vastus intermedius, and gluteus minimus; ▶Fig. 1.26, ▶Fig. 1.27; see Chapter 9 “Lower Extremity Muscles: Pelvic Girdle, Thigh, and Leg”). Some tendons have a mixed paddle-shaped morphology, with the proximal side being rounded or oval, and the distal attachment end becoming more flattened and aponeurotic (e.g.,

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Bones, Muscles, Tendons, Joints, and Cartilage Mineral

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Collagen fiber

Epitenon Tertiary fiber bundle Secondary fiber bundle (fascicle) Primary fiber bundle (subfascicle)

a

Endotenon

b

Collagen fiber

Collagen fiber

Bone

Tendon

Osseofibrous tunnel (synovial cavity) Synovial covering of tendon (visceral) Synovial lining of tunnel (parietal)

Collagen fibril

Annular part of fibrous digital sheath

c

Aggrecan

Synovial sheath Tendon

Tendon Phalanx

Vein Artery Nerve

Fig. 1.23 (a) Tendon macrostructure. Tendons are composed of collagens. Collagen fibrils form fibers and fibers form bundles and fascicles. The endotenon is continuous with the tendon sheath or epitenon that surrounds each tendon. (b) Morphology of the tendon-to-bone enthesis. At enthesis, the collagenous tendon structure is modified to fibrocartilage, mineralized fibrocartilage zone, and finally bone. (c) The tendon sheath and mesotenon of a hand flexor. A tendon sheath, composed of a closed tube of double-layered synovium. The point of continuity between the two parietal and visceral layers of the tendon sheath, where blood vessels pass into the tendon, is called “mesotenon.” At the fingers, the tendon blood supply is provided directly by the vessels entering from the flexor synovial sheaths and through the vincula.

Fig. 1.24 Axial images showing pennate arrangement of muscle fibers in the subscapularis muscle. Also note the intramuscular course of tendons to form the major tendon.

pes anserinus; ▶Fig. 1.28). At the rotator cuff insertions, fusion of the tendon fibers occurs, thereby forming a common continuous insertion onto the lesser and greater tubercles of the humerus.69 The extensor tendons are linked to each other by fibrous bands known as juncturae tendinum (▶Fig.  1.29), which stabilize the spacing of the extensor tendons and contribute to the functional unity of the tendinous web on the dorsum of the hand.67 The

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largest tendon is the Achilles and the longest tendons are seen in the hands and feet. In general, the flexor tendons are relatively round, and the extensor tendons are flattened (▶Fig. 1.29, ▶Fig. 1.30). In the hands, the flattened shape of the extensor tendons is probably an adaptation to the convex shape of joint surfaces to reduce the risk of subluxation.

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Bones, Muscles, Tendons, Joints, and Cartilage Variation in morphology of tendons with limited functionality is common. Highly functional tendons that are in constant movement are less prone to morphological variation. For example, tendon variation is less on the radial side than on the ulnar side of the hand and even less in the index finger, which has the greatest degree of independent movement than other fingers.67

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Microstructure

Fig. 1.25 Deltoid muscle compartments. (a) Axial cadaveric cut. (b) Axial magnetic resonance imaging (MRI). (c) Sagittal MRI. (d) Volume-rendered view of the deltoid muscle showing multipennate muscle with several intermediate and intramuscular small tendons that connect one muscle belly to another. (c) and (d) also show muscle tendons within the deltoid converging toward the deltoid tuberosity. (HH, humeral head.)

Tendons and ligaments are dense fibrous connective tissue composed of collagens and proteoglycans. This unique extracellular matrix is synthesized by fibroblasts.70 The fibroblasts in tendons are called tenocyte. Tenocytes are responsible for matrix synthesis and they are relatively few in number. These cells are near the collagen fibrils and arranged in rows between longitudinal bundles of collagen fibers.71 With increasing age, the fibroblasts flatten and become less numerous. The extracellular matrix is dominated by the fibril-forming, type I collagen. Under polarized light, collagen bundles are aligned along the long axis of the ligament or tendon. At the molecular level, collagens are synthesized as procollagen molecules and then secreted into the extracellular space.70 Once outside the cell, the collagen molecules line up and begin to form fibrils and then fibers (▶Fig. 1.23). Specialized enzyme called lysyl oxidase provides the strength required for each ligament by crosslink formation between the molecules of the collagen fibers. Collagen fibrils are arranged in a helical fashion, similar to man-made ropes, and crosslinked to one another by specialized

Fig. 1.26 Aponeuroses of the latissimus dorsi (a, b) and oblique muscles of the abdomen (c, d). Aponeuroses are fibrous sheets on the surface of a muscle (modified deep fascia) that connect muscles to the bones or other muscles. (a, b) Axial images show latissimus dorsi aponeurosis (yellow arrows) that connects to the thoracolumbar fascia medially and the iliac crest inferiorly. (c, d) Axial images show the rectus sheath ­connecting to the anterior muscle aponeuroses (red arrows).

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Fig. 1.27 Soleus aponeurosis and gastrocnemius flat tendon shown by axial cadaveric and T1-weighted magnetic resonance (MR) images from top to bottom. On the sides of the soleus muscle, the medial and lateral aponeuroses exist which extend toward the midline of the muscle belly to form the anterior aponeurosis (yellow arrows). The distal tendon arises from the anterior aponeurosis of the soleus in the center of the muscle belly. Muscular fibers of the medial and lateral gastrocnemius heads converge into a broad and flat tendon (green arrows) which connects with the underlying aponeurosis of the soleus muscle to form the Achilles tendon.

enzyme called lysyl oxidase and wrapped in a tendon sheath.72 Collagen fibers display an underlying “waviness” or crimp along the length, allowing the tendon or ligament to elongate without sustaining damage. This architecture also contributes to their inherent flexibility.73 Tendons and ligaments are composed of two-thirds water and one-third solid. Type I collagen forms 75% of the solid part of the ligament, with the rest being made up by other collagens, proteoglycans, elastin, and other proteins such as actin, laminin, and the integrins. Collagen synthesis is proportional to exercise and loading stress and there is a rapid increase in collagen synthesis after strenuous exercise in human tendon and muscle. It is shown that collagen synthesis in the patellar tendon significantly increases after a single bout of active exercise and the effect may still be evident up to 3 days later.74

Macrostructure Tendons and ligaments are designed to work under tensional forces and vary in elasticity and strength.75 Some tendons, such as Achilles tendon, withstand strong tensile forces and some ligaments are highly elastic such as those of the cruciate ligament of the knee or flavum ligament of the spine.67 Some tendons mainly function to transmit loads (e.g., patellar and Achilles tendons) and others mainly transmit motions (e.g., flexor tendons). In tendons, collagen fibrils are grouped into fibers, fibers into fiber bundles, and fiber bundles into fascicles (▶Fig.  1.23,

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▶Fig. 1.30). The fascicles of tendons can slide against each other, allowing tendons to deform as their muscles contract.72 The presence of a thin film of loose connective tissue between fascicles and bundles known as endotenon also facilitates the sliding capability of the tendons. The endotenon is continuous with the tendon sheath or epitenon that surrounds each tendon (▶Fig. 1.23). A tendon sheath, composed of a closed tube of double-layered synovium, is seen around tendons that travel through fibro-osseous tunnels or around corners. The point of continuity between the two parietal and visceral layers of the tendon sheath, where blood vessels pass into the tendon, is called “mesotenon” or “mesotendon” (▶Fig.  1.23). The tendon sheath contains small amount of lubricating glycoprotein, called lubricin. Infection and inflammation of the tendon sheath can cause adhesion of the two opposing surfaces of the synovium (▶Fig. 1.31). Tendon sheaths do not exist in every tendon. The peroneal tendons share a synovial sheath in the retromalleolar groove posterior to the lateral malleolus with the peroneus brevis anteromedial to the peroneus longus. The extensor tendons of the fingers have no synovial sheaths (▶Fig. 1.29, ▶Fig. 1.30), whereas the flexor tendons of fingers and both flexors and extensors of the wrist are covered by synovial sheaths and retained to the bones by an outer retinacular or pulley layer to prevent them from buckling during movements.76 Depending on their location, the retaining structures holding tendons in position have different names: retinacula, fibrous pulleys, annular and cruciform pulleys, or fibrous sheaths (see Chapter 18 “Hand”). These supporting structures simply are formed by local fibrous tissue condensations of the outer layer of the tendon sheath. Damage to the flexor sheaths and pulleys of the fingers is common in rock climbers and can result in prominent bowstringing of the flexor tendons. Ultrasound is a great modality to confirm diagnosis of pulley rupture and evaluates degree of displacement of the flexor tendons.77 In addition to the pulleys, the deep flexor tendons of the fingers are fixed through a split in the superficial tendon known as “Camper’s chiasma” (see Chapter 18 “Hand”). Finally, some tendons are surrounded with a sheath known as the paratenon that is separate from the epitenon. This false sheath is formed by condensation of surrounding connective tissue. The best example of a tendon with a distinct paratenon is the Achilles tendon (▶Fig. 1.32). The Achilles paratenon merges with the deep fascia of the skin distally. Proximally, it connects with the crural fascia. The crural fascia is a connective tissue of the posterior region of the leg that interfaces and connects the epimysia of the gastrocnemius and soleus calf muscles78 (▶Fig. 1.27). Inflammation of this sheath is a common cause of Achilles tendon pain in runners. The role of the paratenon in healing of damaged tendon has been emphasized.79 Ligaments are organized into groups of parallel fibers known as bundles that are difficult to separate. The ligaments have higher proteoglycan and water and lower collagen content than tendons. In addition, their structure is less uniform, with the collagen bundles generally showing a less ordered, interlaced, weaving pattern.80 Ligaments are covered by a vascular layer termed the “epiligament.”81,82 This layer is adhered to the ligament and merges into the periosteum of the bone around the insertion sites

Bones, Muscles, Tendons, Joints, and Cartilage

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Fig. 1.28 (a) The pes anserinus long aponeurotic tendon attachments to the medial knee formed by the semimembranosus (SM), semitendinosus (ST), gracilis (G), and sartorius (S) muscle tendons (BF-L, biceps femoris long head). (b) Anterior dissection of the antecubital fossa showing the bicipital aponeurosis. The bicipital aponeurosis is a fascial expansion, which attaches to the medial border of the distal biceps tendon and fuses with the deep fascia of the forearm and the posterior border of the ulna. (c) Lateral knee dissection showing iliotibial band and biceps femoris tendon. The iliotibial band is an aponeurotic structure that passes over the lateral femoral epicondyle parallel to the biceps femoris tendon and attaches to Gerdy’s tubercle on the anterolateral aspect of the tibia.

of the ligament. Sensory and proprioceptive nerves travel near the blood vessels of the epiligament with more nerves located close to the bony insertions.

Tendon Junctions At the muscle–tendon junction (myotendinous junction), the muscle and tendon are bound together by the collagen network of the junctional plates of the perimysium. Individual muscle fibers attach to aponeurotic intramuscular tendinous extensions of tendon both proximally and distally (▶Fig. 1.24). At this end, the tendon sheath also merges with the fascia covering the muscle. Muscle strain injuries commonly occur at or near the myotendinous junctions. The distal myotendinous junction of the medial gastrocnemius is the typical site for muscle injuries to the calf. This particular injury is known as a “tennis leg” calf injury (see Chapter 9 “Lower Extremity Muscles: Pelvic Girdle, Thigh, and Leg”). The tendon’s composition may transform from complete fibrous to fibrocartilage as they pass around bony pulleys or underneath fibrous retinacula to change the direction of muscle pull. These tendons are called wraparound or gliding tendons.83

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Large amount of proteoglycan typical of fibrocartilage, at the concave side of the tendon facing the pulley, is typical of gliding tendons. This increases the strength of the tendons at these levels.83 Examples of the fibrocartilaginous tendons are the proximal tendon of biceps at it curves over the head of the humerus, the extensor tendons of the fingers where they cross the proximal interphalangeal joints, and the distal tendon of the tibialis posterior where it twists around the medial malleolus.84 Tendon injury at the level of pulleys due to compression and overuse may result in fatty degeneration, fragmentation, and hypercellularity of the fibrocartilage parts of the tendons. Tendons and ligaments attach to bone at the insertions or entheses.85,86 At the osteotendinous junction to the appendicular skeleton, bony protuberances known as “eminence” may develop (e.g., deltoid tuberosity on the humerus; ▶Fig. 1.25). At the insertion, tendons and ligaments merge with the bone periosteum. A characteristic structural feature of entheses is the presence of fibrocartilage, which is likely a modification of the periosteum. Four zones have been identified at direct insertion of tendon or ligament to bones: collagenous zone, fibrocartilage zone, mineralized fibrocartilage zone, and bone (▶Fig. 1.23).

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Fig. 1.29 Doral and palmar surfaces of the hand showing gross anatomy of tendons in the hand. (a) The extensor tendons on the dorsum of the hand are interconnected by films of connective tissue (yellow arrows) and obliquely orientated juncturae tendinum (red arrow) to form a weblike complex. Also seen are the extensor hoods over the metacarpophalangeal joints. (b) The flexor digitorum superficialis tendons are shown that pass beneath the flexor retinaculum. The tendons are rounded and at the digital level are covered by tendon sheaths.

Many tendons attach immediately beyond the joint and on their way blend with the joint capsule. Capsular insertion increases the efficiency of tendon to move the joint. Furthermore, tendon–capsule fusion reduces the risk of capsular entrapment during joint movements. A good example of capsular insertion is seen in the distal semimembranosus tendon. This complex tendon attaches to the structures around the knee including bone, ligaments, and joint capsule by way of five to eight tendinous arms (see Chapter 21 “Knee”). Capsular blend is also seen in the interphalangeal joints of the fingers and toes, where the extensor tendon replaces the capsule dorsally (see Chapter 18 “Hand”). It is also described in relation to the tendon of gluteus minimus and the hip joint capsule. Both muscles and tendons have widespread insertions to the fasciae. Examples include gluteus maximus attachment to the fascia lata of the thigh (▶Fig. 1.33), attachment of the tensor fasciae latae to the iliotibial tract (a thickening of the fascia lata), and also the palmaris longus attachment to the palmar fascia. Some tendons, especially in the lower extremity, end as flattened fascial expansions just below the knee that blend with the fasciae of the leg.85 Examples include the semimembranosus tendon and the pes anserinus tendons of the semitendinosus, gracilis, and sartorius. The distal quadriceps tendon inserts on the superior pole of the patella and sends fibers anterior to the patella that connect with the patellar tendon.87 Another great example of fascial connection is seen in the distal tendon of the biceps brachii. This tendon has a bony insertion on the radial tuberosity and a connection to the deep fascia of the medial arm via the bicipital aponeurosis (▶Fig. 1.28). Fascial connections not only link muscles together to form a mechanical chain but also help reduce the risk of tendon wear and tear by dissipating stress concentration at distal tendon insertions.

Vascularity The presence of normal vascular flow is important for the normal functioning of tendon cells and the ability of tendons to repair. Nerve fibers regulate blood flow within the tendon and ligaments,

Fig. 1.30 Morphology of the flexor and extensors of the hand in axial cross-section. The flexor tendons are relatively rounded and covered by tendon sheaths. The extensor tendons are flattened but not covered by tendon sheaths at the hand level. Note the fascicles within the flexor tendons. Tendon vessels are shown passing between the bone and the flexor tendon (red arrows).

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Fig. 1.31 (a) Normal flexor tendons shown by magnetic resonance imaging (MRI). On MRI, tendons look homogenously dark. (b) Longitudinal and (c) axial ultrasound images of the wrist showing synovitis of the extensor tendon sheath at the wrist level. Fluid accumulation is seen within the synovial sheath surrounding the extensor tendons or the wrist (arrow). Note the fascicular structure within the flexor tendon as multiple closely spaced echogenic parallel lines on longitudinal and several dots on the axial image.

and relay pain to the central nervous system. Numerous vessels enter the tendons at their myotendinous junctions. In tendons that are surrounded by true synovial sheaths, vessels enter the tendon via the mesotenon. At the wrist and ankle, mesotenons are sheetlike folds, but in the digits, they are reduced to small segments and through cordlike remnants of mesotenons known as the vincula (see Chapter 18 “Hand”). In the tendon itself, the vessels run longitudinally, parallel to the fascicles and within the endotenon. Poorly vascularized regions are seen in the tendons, especially where tendons wrap around bony pulleys (e.g., tibialis posterior and supraspinatus tendons). For example, the tibialis posterior tendon is supplied by two vessels entering the tendon approximately 4.5 cm proximal and 2.0 cm distal to the medial malleolus, and the area between the two vessels is relatively hypovascular.88 These watershed zones are prone to degeneration and chronic rupture, especially in older individuals. Chronic peroneal tendon tear is relatively common and reduced blood supply is suggested to play a role in tendon degeneration. Branches of the peroneal and anterior tibial arteries penetrate the peroneal tendons via one or two vincula from the posterolateral side to facilitate intratendinous blood supply.89 In the peroneus longus tendon, avascular zones are found where the tendon turns around the lateral malleolus and the cuboid.90 Normal fibrocartilage at the tendon–bone interface is avascular. Tendon sheaths, paratenon, and tendon-associated adipose tissue have a richer blood supply and sensory nerves than do the tendons (▶Fig. 1.32). Exercise is an important factor in normal blood supply to the tendons. It is shown that blood flow in the peritendinous tissues is increased with physical activity.91 Once damaged, vessels can grow into the tendon and fibrocartilaginous entheses.68 Structural changes along with neovascularization are shown in patients with jumper’s knee and Achilles

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tendinopathy.92,93 Sensory nerves can also grow into damaged tendons and ligaments and contribute to pain syndrome.

Fat Pads In some areas, fat pads are seen near the tendons. The fat pads deep to the patella and Achilles tendons are relatively large (▶Fig.  1.32, ▶Fig.  1.34). Theses fat pads are highly vascular and supplied by sensory nerves and mechanoreceptors. Kager’s fat pad, also described as the pre-Achilles fat pad, is a triangular area delineated by the flexor hallucis longus muscle and tendon anteriorly, the superior cortex of the calcaneus inferiorly, and the Achilles tendon posteriorly94 (▶Fig. 1.32). It supports the Achilles tendon–associated structures, reduces the risk of tendon adhesions to the superior tuberosity, minimizes pressure changes in the retrocalcaneal bursa during movements, and protects blood vessels passing through it to supply the Achilles tendon.95 Patients with Achilles tendinopathy often show an edema of fat pad. Another important fat pad is the infrapatellar fat pad, also known as Hoffa’s fat pad (▶Fig.  1.32). It is located behind the patella between the joint capsule anteriorly and the synovial membrane posteriorly. Hoffa’s fat pad penetrates into the deep surface of the patellar tendon as fingerlike extensions87 (▶Fig. 1.34). These fatty streaks improve distribution of synovial fluid and may help shock absorption and protect adjacent tissues. This fat pad contains inflammatory cells and is a source of adipokines, cytokines, and growth factors, which might impact disease. Hoffa’s fat pad can herniate through a defect in the lateral patellar retinaculum, presenting clinically as an anterolateral knee mass.96 In joint pathologies and in response to repetitive trauma, the infrapatellar fat pad may be involved by inflammation, hypertrophy, and fibrosis. It is characterized by anterior knee pain.97

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Fig. 1.32 Normal Achilles tendon and paratenon shown by (a) sagittal and (b) axial magnetic resonance (MR) images of the hindfoot. Kager’s fat pad is a triangular area between the flexor hallucis longus, calcaneus, and Achilles tendon. The distal tibiofibular syndesmosis is shown in (b). It is an amphiarthrosis fibrous joint with limited movement. The side walls of the bones at this joint are anchored by the interosseous ­ligament superiorly and three supporting ligaments at other sides.

Fig. 1.33 Examples of the muscle fascia and tendon attachments to bone or other muscles by deep fasciae. The Iliac bone, tensor fascia lata (TFL), and proximal portion of the gluteus maximus (Gmax) attachments to the iliotibial band (ITB)/fascia lata (red arrows) are shown on axial and coronal magnetic resonance (MR) images. The fascia lata covers a part of the gluteus medius (Gmed). It also extends to the lateral fibers of the vastus lateralis (VL).

Injury Tendon degeneration or injury is caused by aging, overuse, laceration, or interactions with neighboring structures. Common locations of tendon injury include the rotator cuff, flexor tendons of the fingers, patellar tendons, and Achilles tendons (▶Fig. 1.35). Tears are most common in the supraspinatus tendon, typically at

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the anterior attachment to the greater tuberosity of the humerus. The tendon of the tibialis posterior is most susceptible to degenerative tearing at or just distal to the medial malleolus (see Chapter 22 “Ankle”). The anterior talofibular ligament is the most commonly injured ankle ligament. At the pelvic level, the posterior tendon pathology is most commonly due to hamstring injury at the ischial tuberosity. Gluteal tendon disease is an important cause of greater trochanteric pain. Lateral epicondylitis, sometimes referred to as “tennis elbow,” occurs because of repetitive stress. It occurs at the common extensor tendon, most often involving the extensor carpi radialis brevis at its origin, where there is tendon degeneration and tearing. Tendon injuries are divided into acute and chronic. Lacerations and ruptures are common acute tendon injuries, and both often occur in athletic settings. Chronic tendon injury is often referred to as tendinopathy (▶Fig.  1.35). Under physiological loading conditions, tendons and ligaments are relatively compliant, due to recruitment of “crimped” collagen fibers and interactions of other matrix materials. Continued loading results in increasing ligament stiffness until a level beyond which tensile strength fails and ligament begins to disrupt. Tendon and ligaments also “creep,” which is defined as the deformation (or elongation) under a constant or cyclically repetitive load. Ligament creep is particularly important when considering joint injury or reconstructive surgery as excessive creep could result in laxity of the joint, thus predisposing it to further injury.98,99 Following tear, the disrupted ends of the tendon or ligament retract. The gap between the disrupted ends fills with blood clot and then it is replaced with cellular infiltrate. Increased blood

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Fig. 1.34 Magnetic resonance (MR) images showing fatty streaks (yellow arrows) in distal quadriceps tendon patella tendon, tendinous insertions of the triceps heads, and Achilles tendon.

Fig. 1.35 Achilles tendon contusion. Sagittal short tau inversion recovery (STIR) T2-weighted (T2W) magnetic resonance imaging (MRI), sagittal ultrasound, axial ultrasound, and axial T2W MRI show severe distal hypertrophic Achilles tendinopathy (arrows) without a discrete tear. There is also a mild-Achilles paratenonitis.

vascularity takes place, followed by cellular proliferation and production of “scar tissue” formed by hypertrophic fibroblastic cells and collagenous connective tissue matrix bridging the torn ends. The scar tissue is initially disorganized with more blood vessels, fat cells, and fibroblastic and inflammatory cells. After a few weeks of healing, the collagen fibers will be aligned with the long axis of the ligament. Scars formed at the site of injured tendons and ligaments are characterized by fewer type I collagen fibers, which are thinner, more type III collagen with inefficient

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tensile strength and increased vascularization.99,100,101,102 The healing of ligament and tendon injuries varies from tissue to tissue (▶Fig.  1.36). Tendinopathies can take up to 1 year for the pain to subside before one could return to normal activity. A partially ruptured ligament or tendon can generally heal spontaneously; however, its remodeling process takes years and its viscoelastic and creep properties never return to normal.99,100 Healing may result in a thick, elongated, lax, and weakened ligament that is prone to rupture.

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Tendinopathy

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Fig. 1.36 Hypertrophic chronic Achille’s tendinopathy. Sagittal T2-weighted (T2W) fat-suppressed magnetic resonance imaging (MRI) showing irregularly thickened tendon without significant edema or signal abnormality. Tendinopathies can take up to 1 year to heal before one could return to normal activity.

Tendinopathy is manifested by pain and inflammation. At advanced stages, it is associated with the formation of damaging lipids, proteoglycans, and calcified tissues in tendon lesions (▶Fig. 1.37). Tendinopathy involves a diverse group of tendon disorders that are caused either by genetic mutations in extracellular matrix or by mechanical stress that leads to tendon damage.103,104 There are several theories on the pathogenesis of tendinopathy. Chronic repetitive tendon overload is the most commonly proposed cause of tendinopathy (e.g., overuse Achilles tendinopathy). Chronic mechanical friction by adjacent structures such an enlarged peroneal tubercle or thickened inferior peroneal retinaculum can cause a peroneal tendon tear and/or tenosynovitis104 (▶Fig.  1.38). Ligament abnormalities typically occur after acute trauma; however, as in tendons, repetitive microtraumas may play a role. The stiffness of tendons varies with age, sex, and physical activity. Aging tendons undergo a decrease in water content and changes in collagen structure that predispose them to damage. Tendon disease often occurs at the hypovascular areas and vascularity also decreases with age. Metabolic or endocrine abnormalities may also increase the risk of tendon damage. Maximum range of motion, that is, flexibility, increases

Fig. 1.37 Ligament calcification and calcific enthesopathy (arrows). (a) Frontal pelvic x-ray showing bilateral calcifications of the sacrotuberous ligaments. (b) Right shoulder x-ray showing calcification of the supraspinatus tendon. (c) Lateral x-ray of the ankle showing calcification of the Achilles tendon at the calcaneal insertion consistent with chronic degenerative enthesopathy. Also noted is calcification of the plantar fascia. (d) Lateral x-ray of the knee showing ossification of the patellar tendon. An ossific density is also identified adjacent to the left tibial tubercle, a finding that may be seen with healed Osgood–Schlatter disease. Mild calcific enthesopathy of the quadriceps insertion is also noted.

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Bones, Muscles, Tendons, Joints, and Cartilage (▶Fig. 1.35). Normal tendons appear as a tubular low signal intensity on all conventional MRI sequences (▶Fig. 1.32). T2W images are best for identifying fluid signal in tendon or ligament tears as well as for demonstrating changes in the surrounding tissues80 (▶Fig.  1.35). With CT scan, tendons appear as medium-density soft-tissue structures. Calcification can be detected best with CT scan. Fatty degeneration, contusion, and rupture are optimally seen with MRI. On MRI, the first sign of tendon abnormality is often an increase in the signal intensity and thickening of the tendon (▶Fig. 1.35). Ultrasound shows hypoechogenicity of the tendon with fibrillar separation and neovascularization (▶Fig. 1.35). Ligament sprains have similar imaging characteristics. There is typically fluid adjacent to the injured ligament in the acute phase. The damaged ligament may be thickened, lax, and have increased signal. Scarring can lead to intermediate signal and leave the ligament or tendon thickened, thinned, or irregular.

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◆◆ Joints and Cartilage

Fig. 1.38 Peroneus tendon lesions. (a, b) Axial T2-weighted (T2W) magnetic resonance imaging (MRI) showing peroneus tenosynovitis (yellow arrows) and split tear of the peroneus brevis tendon. (c, d) Axial T2W MRI 3 years alter showing complex tear of the peroneus longus (red arrow) and severe injury of the peroneus brevis tendons.

after stretching training.105 Many ruptured Achilles tendons have clear degenerative changes before the rupture.101 Tendons are not usually calcified although calcification is common pathologically. The presence of sesamoid bones is particularly common in the foot tendons. Ligament or tendon ectopic ossifications are usually secondary degeneration and aging (▶Fig. 1.37). Inflammatory enthesitis can lead to ectopic ossification that can spread from the bone to the tendon or ligament (▶Fig.  1.37). As mentioned earlier, adipose tissue is a common feature of normal entheses and should not be regarded as a sign of degeneration.106

Tendon and Ligament Imaging Knowledge of the normal imaging appearance of tendons and ligaments is important to diagnose subtle pathological changes.80,107,108,109 Ultrasound is an easy and accessible way to assess tendon or ligament integrity. In most examples, a linear transducer with a frequency of 10 to 12 MHz has been recommended.107 On ultrasound, the fascicular structure is seen as multiple, closely spaced echogenic parallel lines on longitudinal scanning (▶Fig. 1.31), whereas in the transverse plane multiple echogenic dots or lines are visible. Ligaments also appear as echogenic fibrillar structures. The normal patellar and Achilles tendons have a typical fibrillar appearance on ultrasound with a thin surrounding paratenon (▶Fig. 1.35). Paratenonitis may be present with circumferential hypoechogenicity around the tendon and increased vascularity (▶Fig. 1.35). Paratenon changes will be best visualized on MRI as high signal on T2W fat-suppressed images

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Evaluation of the joints involves a major part of musculoskeletal imaging studies. Imaging is performed to evaluate the integrity of joint cartilage, disks, articulate cortex, joints capsule, ligaments, and tendons. Knowledge of the anatomy is integral for correct interpretation of the images and differentiate anatomical variants from pathologies. Specific anatomy of each joint is described in separate chapters. This chapter is dedicated to the classification of joints with specific attention to the structure of articular cartilage and menisci of the synovial joints.

Classification Joints are where two or three bones articulate with each other. Articulating bones are covered with cartilage or fibrous connective tissue and in most forms are supported by a joint capsule, ligaments, and tendon sheaths. Joints are classified based on their function and structure. Functionally joints are classified as immobile (synarthrosis), slightly mobile (amphiarthrosis), and freely mobile (diarthrosis or synovial joints). The main function of the immobile and slightly mobile joints is to protect internal organs. The diarthrotic joints allow a range of movements of the body and limbs in different directions. The diarthrosis is the most common type of joint (▶Fig. 1.39). Examples of synarthrosis include fibrous skull sutures or the sternomanubrial joint (▶Fig. 1.40). Examples of amphiarthrosis include the intervertebral disks that are separated by fibrocartilage disks or the pubic symphysis, a cartilaginous joint that connects the pubic bones together (▶Fig. 1.41). Structurally, the joints are classified into fibrous, cartilaginous, and synovial. In fibrous joints, the adjacent bones are united by fibrous connective tissue, whereas in cartilaginous joints the bones are joined by the hyaline cartilage or fibrocartilage. The fibrous and cartilaginous joints are functionally classified into a synarthrosis or amphiarthrosis. In contrast to the fibrous and cartilaginous joints, at a synovial joint, the bones do not unite; instead, they are separated from each other by the joint cavity, which contains synovial fluid.

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Fig. 1.39 Diarthrotic joint morphology. Magnetic resonance (MR) arthrogram of the elbow showing a hinge uniaxial articulation. The joint capsule is filled with contrast. Relative thickness of the articular cartilage layer at different levels are shown. Thinning and erosion of the cartilage is seen along the inferior aspect of the capitellum (arrows). The cartilage height at the coronoid tip is relatively large compared to the size of the coronoid tip. Note that the cartilage is very thin or absent in the mid-trochlear notch (red arrow).

Fibrous Joints There are three types of fibrous joints: sutures, syndesmosis, and gomphosis. The skull and facial bone sutures are functionally classified as a synarthrosis, although some sutures may allow for slight movements between the cranial bones (▶Fig.  1.40). These fibrous joints can normally fuse in later life and become completely ossified, and in some cases disappear. Fusion between bones is called a synostosis (▶Fig. 1.40). Early synostosis of skull sutures will result in different abnormality ion skull morphology (see ­Chapter 1 “Calvaria” of Volume 4). Syndesmosis is functionally classified as an amphiarthrosis with limited movement. It is a type of joint in which bones are joined by thick fibrous ligaments, or a sheet of connective tissue called an interosseous membrane. The interosseous membrane is seen in the forearm between the shafts of the radius and ulna and in the leg between the shafts of the tibia and fibula (▶Fig. 1.42). At the distal tibiofibular syndesmosis, the side walls of the bones are anchored by the interosseous ligament and three superficial supporting ligaments (▶Fig. 1.32). Injuries to the distal tibiofibular syndesmosis are common and occur as isolated ligamentous injuries or in association with ankle fractures. Gomphosis is referred to a specialized peg-and-socket synarthrosis that anchors the roots of a tooth into the bony socket of

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the maxillary or mandible bones. The gap is filled with short strands of dense connective tissue, called periodontal ligament.

Cartilaginous Joints Cartilaginous joints are divided into synchondrosis and symphysis. In a synchondrosis, bones are joined by the hyaline cartilage and subclassified into temporary and permanent. Synchondroses are functionally classified as a synarthrosis. Temporary synchondroses are the epiphyseal growth plates between the diaphysis and epiphysis of the long bone that finally will be replaced by bone (▶Fig.  1.43). Injury of the growth plate in growing bones will result in shortening or deformity of the involved bone (▶Fig. 1.44). Another example of the temporary synchondrosis is the Y-shaped hyaline cartilage that connects the ilium, ischium, and pubis parts of the hip bone. This cartilage will be completely ossified (synostosis) by the age of 25 years (▶Fig. 1.45). Examples of permanent synchondroses are costochondral junctions and the first sternocostal joint. Other sternocostal joints are synovial joints (▶Fig. 1.46). A symphysis is a cartilaginous joint where bones are united by a fibrocartilage disk and supported by periarticular ligaments (▶Fig. 1.41, ▶Fig. 1.46). Symphyses are located in the midline of the body (e.g., mandibular, pubic). Because of numerous fibers of thick collagen, fibrocartilage is much stronger than hyaline

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Fig. 1.40 Examples of a synarthrosis. (a) A 3-month-old infant versus (b) adult skull sutures. Note the synostosis of all cranial sutures in the adult skull.

Fig. 1.41 Examples of amphiarthrosis. Pubic symphysis. (a) Axial cadaveric cut and (b) axial magnetic resonance (MR) image. The pubic symphysis is reinforced by several ligaments.

cartilage. This allows symphyses to unite the adjacent bones efficiently. Most symphyses are functionally classified as an amphiarthrosis. Fibrocartilage of the symphysis is less likely to ossify and obliterate than hyaline cartilage of the synchondrosis. The pubic symphysis is a narrow fibrocartilage amphiarthrodial joint (▶Fig. 1.41). The ends of both pubic bones are covered by a thin layer of hyaline cartilage, which is attached to the fibrocartilage disk. The gap between the cartilages may be filled with a small amount of fluid. The pubic symphysis is surrounded and reinforced by several ligaments (▶Fig.  1.41). The manubriosternal joint, a synarthrotic joint at the sternal angle, is another example of a symphysis filled with fibrocartilage (▶Fig. 1.46). The intervertebral disks are fibrocartilaginous joints and classified as wide symphysis (▶Fig. 1.9). The sacrococcygeal articulation

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is a form of amphiarthrodial symphysis, homologous with the intervertebral disks, which will be partially or completely obliterated in old age (see Chapter 24 “Spine” of Volume 4).

Synovial Joints Synovial joints are freely mobile diarthrosis and are characterized by a fluid-filled joint cavity formed by a fibrous capsule and supported by overlying ligaments and tendons. The joint capsule is lined with a thin synovial membrane. In contrast to the fibrous or cartilaginous joints, the articulating bone surfaces of a synovial joint do not fuse with each other. Bones are separated by articular cartilage covering bone ends, synovial fluid, and in some examples an intervening fibrocartilage disk/meniscus (▶Fig.  1.44).

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Fig. 1.42 Interosseous membrane (arrows) shown by (a) axial magnetic resonance (MR) and (b) cadaveric cut in the forearm between the interosseous borders of the shafts of the radius and ulna. The interosseous membrane is a form of fibrous joint called syndesmosis.

Fig. 1.43 Fusion of the growth plates between the diaphysis and epiphysis of the long bone is shown. Epiphyseal growth plates are categorized as temporary synchondroses.

Supporting ligaments of the synovial joints are classified into intracapsular and extracapsular ligaments. The bursae are connective tissue sacs filled with small amount of lubricating liquid that are located outside of a synovial joint either subtendinously, submuscularly, or subcutaneously.

Types of Synovial Joints Functionally, diarthrotic joints are classified into uniaxial, biaxial, and multiaxial. A uniaxial diarthrosis (e.g., elbow) allows for movement within a single anatomical plane (▶Fig. 1.47). A biaxial diarthrosis (e.g., metacarpophalangeal) allows for movements in two planes (▶Fig. 1.48). Multiaxial joints, such as the shoulder or hip joint, allow for multidirectional movements (▶Fig. 1.8). Morphologically, synovial joints are divided into six types: pivot, hinge, plane, saddle, condyloid, and ball-and socket-joints (▶Fig. 1.8, ▶Fig. 1.47, ▶Fig. 1.48). The pivot joint is a uniaxial diarthrosis with best examples seen in the atlantoaxial and proximal radioulnar joints (▶Fig. 1.47). In a pivot joint, a portion of bone (axis or radial head) rotates within

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a ring formed by a bone (atlas) or a combination of bone and ligaments (ulna and annular ligament). Hinge joints are uniaxial articulations, and as seen in the elbow, ankle, knee, and interphalangeal joints, these joints permit movement in only one plane. Plane or gliding joints are characterized by flat or slightly curved articular surfaces allowing the bones to glide against each other (▶Fig. 1.47). These joints are more likely multiaxial in function. Examples are the intercarpal, intertarsal, acromioclavicular, and subtalar joints. The vertebral facet (zygapophysial) joints between the superior and inferior articular processes are also classified under this group. Saddle-shaped joints are classified as biaxial joints and are characterized by opposing articular surfaces with a reciprocal concave–convex shape. Examples are the sternoclavicular and the first carpometacarpal (between the trapezium and the first metacarpal) joints (▶Fig. 1.48, ▶Fig. 1.49). A condyloid joint (ellipsoid joint) is characterized by a shallow concavity at one end that articulates with a rounded surface formed by one or more bones at the other end, allowing flexion–extension as well as side-to-side movement. Examples are the metacarpophalangeal, metatarsophalangeal, and radiocarpal joints (▶Fig.  1.48). In the latter, the radius articulates with an elliptical surface formed by the scaphoid and lunate. Ball-andsocket joints have the greatest range of motions. The hip and glenohumeral joints are examples of this kind of articulation (▶Fig. 1.8). The shallow socket of the glenoid cavity in the glenohumeral joint allows a more extensive range of motion compared with the deep socket of the acetabulum in the hip. Many joints do not fit in one specific category. For example, the temporomandibular joint is a mixed hinge and gliding joint between the concave mandibular condyle and the convex temporal articular surface allowing both uniplanar and gliding movements (▶Fig. 1.50). Another example is the sacroiliac joint where the anterior part of the joint is formed by a synovial joint, whereas the posterior third is connected by fibrotic symphysis (▶Fig. 1.51). Many authors consider the anterior part to be a true diarthrodial joint with unique features in which the articular surface is covered by a combination of hyaline cartilage on the sacral side and by the fibrocartilage on the iliac surface.

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Fig. 1.44 Blount’s disease in a 7-year-old boy. In this disease, abnormal development and early fusion of the growth plate of the medial aspect of the proximal tibial physis (arrows) results in a lower limb curved deformity and angulation named a bowleg. Also note normal magnetic resonance (MR) signal of the remaining growth plates, articular cartilages, and menisci. (a) Coronal T1-weighted (T1W). (b) Coronal T2-weighted (T2W) with fat suppression. (c) Sagittal T1W. (d) Sagittal T2W with fat suppression.

Fig. 1.45 Temporary synchondrosis of the acetabulum shown at age 11 years and 42 years. The triradiate hyaline cartilage that connects the ilium, ischium, and pubis parts of the hip bone is seen at age 11 years (arrows). Synostosis usually occurs by the age of 25 years.

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Fig. 1.46 Anterior chest wall computed tomography (CT) images showing different types of joints at the sternal level. Examples of permanent synchondroses are seen at the costochondral junctions and the first sternocostal joint. Other sternocostal joints and the sternoclavicular joint are synovial joints. The sternoclavicular joint is a saddle joint that contains a disk. The manubriosternal joint, a synarthrotic joint at the sternal angle, is an example of a symphysis filled with fibrocartilage. (IMA, internal mammary artery.)

Fig. 1.47 (a) The pivot joint at the atlantoaxial articulation. (b) The plane or gliding joint at the articular facet of L5–S1 (arrow). (c) The hinge joint of the elbow (arrow). (d) Frontal x-ray and axial computed tomography (CT; inlay image) showing the pivot joint of the proximal radioulnar joint (arrow).

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Fig. 1.48 Osseous anatomy of the hand showing the types of synovial joints. The interphalangeal joints are examples of the hinge joints. The first carpometacarpal (between the trapezium and the first metacarpal) joint is a saddle-shaped biaxial joint. The intercarpal joints are examples of the plane or gliding joints. The metacarpophalangeal and radiocarpal joints are examples of the condyloid joint (ellipsoid joint). In the latter, the radius articulates with an elliptical surface formed by the scaphoid and lunate. (CMC, carpometacarpal joints; DIP, distal interphalangeal joint; MP, metacarpophalangeal joint; PIP, proximal interphalangeal joint.)

Fig. 1.49 Sternoclavicular joints shown by axial cadaveric cut. The sternoclavicular joint is a synovial saddle diarthrosis. The joint contains two hyaline cartilages on the articular cortices of the clavicle and sternum and one fibrocartilage in between.

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Fig. 1.50 The temporomandibular joint is a mixed hinge and gliding joint. (a) Sagittal computed tomography (CT) in the closed-mouth position. (b) Sagittal proton density magnetic resonance (MR) with fat suppression in the open-mouth position. In the open-mouth position, the low-signal articular disk (thin arrow) translates anteriorly and locates between the articular eminence (yellow arrows) and the condyle of the mandible. The articular cortex of the temporal bone is shown by orange arrows.

Fig. 1.51 Sacroiliac joints are shown by axial T1-weighted (T1W) magnetic resonance imaging (MRI). In the sacroiliac joint, the anterior part of the joint is formed by a synovial joint (yellow arrows) and the posterior third is connected by fibrotic symphysis (orange arrows).

◆◆ Synovium

The inner surfaces of the articular capsule, tendon sheaths, and bursae are lined by a thin synovial membrane that secretes the synovial fluid.110 Histologically, the synovium of diarthrodial joints consists of a 1- to 2-cell-thick intimal layer (synoviocytes) and an underlying subintima.111,112 Macrophages and fibroblasts are also seen in the intima. Unlike serosal surfaces, the synovium is derived from the ectoderm and does not contain a basal lamina. The most common type of subintima is the areolar type, which is 5 mm in thickness and contains scattered blood vessels, lymphatics, nerve fibers, fat cells, and fibroblasts, with scattered lymphocytes or macrophages.113,114 Adipose-type subintima is usually seen on fat pads and is also found within the villi. Fibrous-type subintima is seen in the ligament or tendon and could be indistinguishable from fibrocartilage.

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Aside from lubrication of the joint and facilitating joint movement, the synovial fluid has an important role in nourishment of the chondrocytes in avascular articular cartilage. The ability of synovial fluid to lubricate cartilage surfaces is dependent on a glycoprotein known as “lubricin.” Synovial surfaces must remain nonadherent to allow continued movement. Prevention of adhesion is facilitated by the production of hyaluronan by the intimal fibroblasts. Maintenance of the synovial fluid is provided by intimal macrophages and fibroblasts, which allow free exchange of crystalloids and proteins. The synovium is involved in many inflammatory joint diseases, including rheumatoid arthritis and spondyloarthritis. Normal synovium cannot be differentiated from the fibrous capsule by imaging methods. In patients with inflammatory arthritis, the synovium is markedly thickened due to edema and infiltration of lymphocytes, plasma cells, and macrophages, as

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Fig. 1.52 Axial proton density images with fat suppression at the (a) suprapatellar and (b) knee joint levels in a patient with inflammatory arthritis and synovitis, showing diffuse synovial thickening (arrows), mild knee effusion, and a distended popliteal bursa. The popliteal bursa is located between the medial gastrocnemius tendon (yellow arrow), semitendinosus tendon, and semimembranosus tendon.

well as proliferation of blood vessels, especially the lymphatics (▶Fig. 1.52). Failure of lymphatic drainage of synovial fluid may be a cause of villous proliferation in rheumatoid synovial tissue. In osteoarthritis, joint effusion is likely reactive and due to mechanical irritation by damaged bone and cartilage; therefore, the composition of synovial fluid remains normal.115 On the other hand, in inflammatory edema of the synovial tissue, the effusion is exudative and created by increased vascular permeability.

◆◆ Hyaline Cartilage

Elastin fibers

Chondrocytes

Chondrocytes are the cells in the articular hyaline cartilage. Each chondrocyte with its pericellular glycocalyx matrix is enclosed in a capsule and collectively forms a “chondron.” The chondron is the primary structural and functional unit of the articular cartilage that maintains metabolic balance of the articular cartilage.117 Collagens are the most abundant structural macromolecules in the extracellular matrix of the articular cartilage and account for approximately 10 to 20% of the weight of the articular cartilage. Type II collagen represents 90 to 95% of the collagens in the extracellular matrix of the articular cartilage. Proteoglycans are protein–polysaccharide macromolecules in the extracellular matrix and account for 10 to 15% of the articular cartilage.119 By attracting water molecules, proteoglycans provide hydration and swelling pressure to the tissue, enabling it to withstand compressive forces and deformations. Elastin fibers are macromolecules that are concentrated mainly in the articular cartilage surface and are aligned parallel to the articular surface. They are closely associated with adjacent collagen fibers and chondrocytes.120,121 Elastin fibers have structural, biomechanical, and protective roles for

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Radial zone

Collagen fibers

The articular cartilage of the synovial joints is formed by the hyaline cartilage. It is composed of chondrocytes and an extracellular matrix mainly containing water, collagen, proteoglycans. These elements are arranged in layers to protect the bones.116,117,118

Components

Superficial zone Transitional zone

Tide mark Calcified cartilage Subchondral bone

Fig. 1.53 Zonal organization of the articular hyaline cartilage (longitudinal view). The arrangements of collagen and elastin fibers within the articular cartilage and changes in the chondrocyte shape and arrangement with depth are shown.

chondrocytes and are mainly concentrated in the superficial layer. The rest of the articular cartilage is formed by fluid (70–75%). In general, the fluid component of the articular cartilage permits nutrient and waste product movement continuously between the chondrocytes and the surrounding nutrient-rich synovial fluid.122

Zonal Layers Mature articular cartilage is composed of four distinct zones116,117,118 (▶Fig. 1.53). The superficial layer or tangential zone makes up 10% of the cartilage thickness. This smooth gliding surface layer is primarily responsible for tensile resistance as the chondrocytes are flattened and the collagen fibers are oriented parallel to the joint surface. The second layer is the transitional

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Bones, Muscles, Tendons, Joints, and Cartilage zone. The chondrocytes are more rounded and collagen fibrils are packed loosely and aligned obliquely to the articular surface. The third layer is called the radial zone. In this layer, the chondrocytes and collagen fibers are oriented perpendicular to the joint surface and can resist the high compressive loads. The fourth zone is called calcified cartilage layer. This thin layer contains the tidemark or the boundary between the calcified and uncalcified cartilage. The chondrocytes are small and nonfunctional and, in some regions, the extracellular matrix is calcified. The subchondral bone plate lies just beneath the calcified layer. The calcified cartilage is involved in endochondral ossification.

1

Growth and Expansion Joint formation is a complex multistep process (▶Fig.  1.54). In growing children, the long bones increase in length and their epiphyses expand and enlarge markedly. The articular cartilage at birth is highly cellular with scant extracellular matrix. Later, the cartilage becomes thick and matrix rich and expands laterally to cover the growing epiphyses (▶Fig. 1.55). As the child grows, the articular cartilage will acquire its mature zonal organization. This zonal pattern is very important for hyaline cartilage durability and its normal biomechanical function.

Developmental time

Synovial capsule

Proximal end

Cartilage cap

Distal end

Future joint site

Mesenchymal condensation

a

Joint initiation

b

Interzone formation

c

Morphogenesis

d

Joint maturation

e

Fig. 1.54 Schematic representation of the major steps of synovial joint formation. (a,b) Following condensations of mesenchymal prechondrogenic tissue, several mechanisms, possibly involving the Hox genes, determine the exact location for joint initiation. (c) Soon, the interzone becomes recognizable as a thicker and more compact structure oriented perpendicularly to the long axis of the long bone anlagen. (d) The interzone, composed of mesenchymal cells connected by gap junctions, and its adjacent epiphyseal cartilaginous tissue initiate a morphogenetic process that transforms it to the distinct three-dimensional configuration of the synovial joint. (e) Eventually, all other components of a mature joint including articular cartilage and capsule develop.

Fig. 1.55 Normal growth plates and epiphyseal ossification centers of the knee joint in a 1-year-old knee. After birth, the articular cartilage of the synovial joints of the extremities is thick and matrix rich. It expands laterally to cover the growing epiphyses and reduce contact stress within the joint.

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Bones, Muscles, Tendons, Joints, and Cartilage The articular hyaline cartilage is a permanent structure and, under normal physical loads, remains functional through life. Therefore, the articular cartilage differs from transient cartilage that constitutes the embryonic skeleton and the growth plates in which the chondrocytes undergo proliferation, maturation, and hypertrophy, and are eventually replaced by the endochondral bone.118

Mechanical Properties The mechanical properties of the articular cartilage are determined by the three-dimensional collagen network, proteoglycans, and interstitial water.123,124 The interstitial fluid pressure contributes strongly to the tissue stiffness under instant loads. However, under prolonged loads, fluid flows out of the tissue and proteoglycans will be in charge for the compressive stiffness of the cartilage.122 Collagen fibers determine the tensile properties of the articular cartilage. The creep provided by the collagen fibers will reduce contact stress. Normal cartilage function is directly related to mechanical stimuli during routine physical activities.125 Increase in cartilage volume is found with increased physical activity in healthy children and with increased muscle cross-sectional area in healthy adults. Knee and shoulder cartilage thinning has been observed in paraplegic patients due to unloading.126 Initial cartilage deterioration induces loss of proteoglycans and increases in water content in combination with disorganization and loss of the collagen matrix. Natural aging is characterized by a progressive loss of extracellular matrix and resilience in articular cartilage, resulting in stiffness and reduced biomechanical properties.127 In osteoarthritis, reduction of proteoglycans and collagen, collagen fibrillation, and an increase in the fluid content lead to increased permeability and water content of the cartilage that alters dynamic mechanical stiffness of the articular cartilage. Overactivity of the calcified cartilage layer in osteoarthritis can lead to increased thickness of the subchondral bone and osteophyte formation.123 Since normal adult articular cartilage has a limited

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potential to regenerate, damage to the cartilage can lead to osteoarthritis (▶Fig. 1.56). Cartilage is also devoid of neural and vascular tissue and its nourishment relies on diffusion from the synovial fluid. Effective repair of damaged articular cartilage and its restoration with organized hyaline cartilage remains a challenge.123 Articular cartilage defects are very common and can cause excruciating pain, locking, swelling, and functional impairment124 (▶Fig. 1.56). Following injury, the process of healing of the chondral defects typically leads to the production of type I collagen and fibrocartilaginous tissue as opposed to normal hyaline cartilage. This fibrous tissue is not strong enough and does not prevent progression toward osteoarthritis. Marrow stimulation techniques such as microfracture are used to stimulate cartilage repair. In this technique, small holes are made to penetrate the subchondral bone, allowing marrow stem cells, capable of chondrogenesis, to migrate into the cartilage defect and stimulate cartilage formation. Reconstructive and implantation techniques involve filling of the cartilage defects with autologous or homologous chondrocyte or synthetic material.

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Imaging Current clinical imaging methods to evaluate cartilage and joints include radiography, CT, MRI, and nuclear scan. At research level, transmission electron microscopy is used to study the ultrastructure of the articular cartilage. Confocal laser scanning microscopy is one of the common techniques to study chondrocytes. MRI provides superior contrast resolution and is regarded as the modality of choice to evaluate the articular cartilage. Several techniques have been introduced.128,129,130 With MRI, evaluation of the internal structure of the cartilage is possible. Multilayered differentiation is possible in thicker cartilages such as in the patella using high-resolution and high-field strength MRI (▶Fig.  1.56). With T2 mapping, the collagen content within the extracellular matrix of cartilage can be measured. Disruption of the cartilage matrix results in enhanced water mobility that can be measured

Fig. 1.56 Normal articular cartilage and early damage at the patellofemoral joint. (a) Axial proton density fat-suppressed magnetic resonance (MR) image and (b) magnified view of a healthy articular cartilage of the patella. Note the three-layered cartilage appearance: deeper dark band (1), intermediate bright layer (2), and superficial dark layer of the lamina splendans (3) close to the cortex (4). (c) Axial T2-weighted (T2W) fat-suppressed MR image shows chondromalacia of the patellofemoral joint causing erosion of the cartilage (arrow). Normal articular cartilage has a limited potential to regenerate, and its damage and loss can lead to osteoarthritis.

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Bones, Muscles, Tendons, Joints, and Cartilage by diffusion-weighted imaging (DWI). DWI can also characterize variation in orientation of collagen fibrils.131 The most commonly used MRI sequences for evaluation of joint cartilage include T1W, proton density–weighted, and T2W imaging sequences with or without fat suppression. T1W images show details of hyaline cartilage contents, but do not provide good contrast between joint effusion and the cartilage surface (▶Fig. 1.44). T2W imaging provides good contrast between the cartilage surface and joint effusion and detects small lesions within the cartilage (▶Fig. 1.56). Proton density–weighted imaging and sequence with intermediate echo time (TE) of 33 to 60 ms as well as balanced steady-state free precession (bSSFP) sequence with fat suppression are best techniques for depicting surface cartilaginous defects as well as abnormalities of internal cartilage composition132 (▶Fig. 1.56). In routine T2W MRI, a three-layer pattern can be identified in the articular cartilage: a low-signal layer adjacent to the subchondral bone, a thicker intermediate-signal layer in the middle related to the radial zone, and a low-signal layer related to the superficial and middle zones (▶Fig. 1.56). This regional variation in the signal intensity is largely due to T2 variations caused by the orientation of the collagen fibrils relative to the magnetic field.128 This normal trilaminar pattern is difficult to image in small joints and in location where the articular cartilage is relatively thin. Abnormal signal within the damaged articular cartilage is a common finding in chondromalacia, fibrocartilage formation after injury, chondrocalcinosis, and cartilage defects. The temporomandibular joint is an exceptional synovial articulation in which the articular surfaces are covered by dense fibrocartilage.133 A fibrocartilaginous disk exists between the temporal and mandibular articular surfaces (▶Fig.  1.50). The articular ­cartilage measures 5 mm in thickness and is divided into four zones: fibrous, proliferative, mature, and hypertrophic (▶Fig. 1.56). The surface of the cartilage is covered by a layer of fibrous tissue with abundant type I collagen (▶Fig. 1.59). Other

1

layers are fibrocartilage. The proliferative zone contains mesenchymal cells and a chondroitin sulfate-based proteoglycan with a matrix rich in type I collagen. The orientation of collagen fibers in the first two zones is anisotropic in the anteroposterior direction. The hypertrophic zone is rich in chondrocytes and collagen type II. The temporomandibular joint condylar fibrocartilage contains less glycosaminoglycans than the hyaline cartilage.133

◆◆ Meniscus

Menisci are fibrocartilaginous tissues that play a critical role for the transmission and distribution of loads in the joints. Menisci are present in the knee, wrist, sternoclavicular, acromioclavicular, and temporomandibular joints (▶Fig. 1.50, ▶Fig. 1.58).

Structure The fibrocartilaginous tissue of menisci is composed of a highly hydrated (72% water) extracellular matrix composed largely of collagen, which varies in type from predominantly type II collagen in the more cartilaginous inner zone to primarily type I collagen in the vascularized, fibrous outer zone (▶Fig. 1.57).134,135 Their complex architecture and anisotropic composition allow for an optimal resistance to compressive, circumferential, shear forces. Three zones have been assigned to the meniscus structure (▶Fig. 1.59). The meniscus cells are called by different names such as fibrocytes, fibroblasts, fibrochondrocytes, and chondrocytes.136 In the outer zone, the majority of cells are fusiform-shaped fibroblasts. The hoop strength of extracellular matrix in the outer and middle zones is provided predominantly by type I collagen (80%).134 The collagen fibers in these two zones run circumferentially and parallel to the long axis of the meniscus and are further supported by radially oriented fibers. Together they form a unique heterogeneous and anisotropic resistant network (▶Fig.  1.59). In the inner zone, similar to the hyaline articular, the cartilage

Collagen fibers Fibroblast Mesenchymal cells

Fibrous zone Proliferative zone Mature zone

Chondrocytes

Hypertrophic zone

Bone

Fig. 1.57 Zonal organization of articular fibrocartilage of the temporomandibular joint.

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Fig. 1.58 Normal versus abnormal meniscus of the knee joint. Sagittal (a) T1-weighted (T1W) and (b) T2-weighted (T2W) fat-suppressed magnetic resonance (MR) images of a healthy medial knee meniscus. The meniscus appears homogeneously dark on both sequences, whereas the articular cartilage (green arrows) appears brighter on both T1W and T2W sequences. (c) Coronal T2W fat-suppressed MR image shows torn lateral meniscus. Also note complete erosion of the articular cartilage of the lateral knee compartment (red arrows). The medial compartment shows healthy articular cartilage (green arrows) and mild mucoid degeneration in the center of the medial meniscus (blue arrow). The healing capacity of the meniscus is directly related to blood circulation, leaving the inner region susceptible to permanent posttraumatic and degenerative lesions. Meniscal tears in the middle-aged and elderly patients usually result from long-term degeneration.

Red–red zone

Periphery Free edge

Red–white Vessels zone White–white zone

a

Fibroblast like cells

Chondrocyte Cells of the superficial like cells zone

Collagen bundles

Radial fiber

b

Fig. 1.59 Regional variations in vascularization and cell populations, and supporting matrix of the knee meniscus. (a) In adulthood, the outer (red–red region) zone contains the majority of blood vessels and cells are spindle-shaped fibroblast. The cells in the middle zone (red–white region) and inner zone (white–white region) are more chondrocytelike. Cells in the superficial layer of the meniscus are small and round. (b) Circumferentially oriented collagen bundles provide hoop strength and course parallel to the long axis of the meniscus, whereas the radial fibers form a lattice and provide additional structural support. The meniscal matrix is composed largely of type II collagen in the more cartilaginous inner zone, and type I collagen in the vascularized, fibrous outer zone.

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Bones, Muscles, Tendons, Joints, and Cartilage cells are round and are surrounded by a matrix comprised largely of type II collagens (60%) that are less organized and a higher concentration of proteoglycans.134 The inner zone function is to reduce cartilage compressive loads. Proteoglycans are mainly chondoitin-6-sulfate (60%) and the rest include dermatan sulfate, chondroitin-4-sulfate, and keratin sulfate. Proteoglycans enable the meniscus to absorb water, and support the tissue under compression. Vascularization of the menisci changes with age. In the prenatal period, the meniscus is fully vascularized. Over time, vascularization subsides and shifts toward the periphery of the meniscus.137 In a mature meniscus of the knee, vessels and nerves are arranged in three zonal layers: an outer vascular/neural zone along the periphery of the joint, a middle-mixed zone, and an inner avascular/aneural zone facing the joint cavity134,138 (▶Fig.  1.59). The healing capacity of the meniscus is directly related to blood circulation, leaving the inner region susceptible to permanent posttraumatic and degenerative lesions. Therefore, the menisci show little capacity for repair, except for certain types of injuries occurring in the outer vascularized region. The temporomandibular joint also contains a disk. A fibrocartilaginous biconcave disk exists between the temporal and mandibular articular surfaces. The high collagen content of this disk provides great rigidity and durability. The disk is attached to the condyle and temporal bone by fibrous connective tissue (▶Fig.  1.50). Orientation of collagen fibers is circumferential in the periphery and anteroposterior in the center of the disk. Type I collagen predominates and similar to the knee disks, the peripheral attachments of the temporomandibular joint disk are well vascularized, but its central zone remains hypovascular.133

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Function The primary function of the meniscus is to protect the articular cartilage of the joint by transfer of weight and absorbing shock during dynamic activity. Meniscal structure and function are influenced by physical activity, age, and degree of degeneration. The meniscus of the knee absorbs >50% of the total axial load applied in the joint. Therefore, when the knee is in 90 degrees of flexion, the axial load in the joint is 85% greater than the standing position. Following total meniscectomy, the tibiofemoral contact area decreases by approximately 50%. This leads to an overall increase in axial forces to the bone by two to three times.139,140 Even partial meniscectomy has been shown to lead to markedly increase in contact forces on the articular cartilage.140 Longitudinal tears can be repaired, whereas horizontal and radial tears usually require partial meniscectomy. Obese individuals have four to five times increased risk of knee osteoarthritis when compared with normal-weight individuals due to increased joint loading.141 Acceleration of cartilage degeneration and increased cartilage and meniscal defects are associated with obesity.

Lesions Tear or degeneration of the meniscus is associated with pain and joint dysfunction. Loss of meniscal function or surgical meniscectomy leads to relatively rapid and progressive osteoarthritis.

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In general, knee meniscal lesions occur frequently in the middle-aged and elderly patients. Tears encountered in these groups usually result from long-term degeneration (▶Fig.  1.58). The reported prevalence of meniscal lesions in patients with clinical and radiographic findings of osteoarthritis is 68 to 90%.142 Degenerated menisci show areas of hyperintensity in MRI and mucoid changes in the histology (▶Fig. 1.58). Temporomandibular joint disorders are also very common.143 Osteoarthritis of the joint can cause destruction of the articular fibrocartilage, thickening or perforation of the disk, and bone remodeling with osteophyte formation144 (see Chapter 24 “Temporomandibular Joint”).

Imaging Both T2W and proton density–weighted images are equally diagnostic for the assessment of meniscal tears (▶Fig. 1.58). The use of high-field MRI often improves the spatial resolution of the image to detect small meniscal lesions. In MRI, the menisci appear as low-signal-intensity structures (▶Fig.  1.58). The menisci of the knee are semilunar and wedge shaped in coronal images and the meniscus of the temporomandibular joint demonstrates a biconcave appearance in sagittal views (▶Fig. 1.50). The MR criteria for diagnosing a tear include meniscal distortion, discontinuity, and increased signal intensity within the disk that extends to the margin of the disk (▶Fig. 1.58). Meniscal calcification (chondrocalcinosis) and meniscal ossicles may increase signal intensity of the meniscus and should not be mistaken with tear.145 Calcifications are better distinguished with CT scan (▶Fig.  1.60). Globular or linear areas of increased signal intensity within the meniscus can be seen in children due to normal vascularity. In adults, internal mucinous degeneration and posttrauma contusion can cause areas of increased signal intensity146 (▶Fig. 1.58).

◆◆ Joint Capsule

The joint capsule consists of dense fibrous connective tissue, lined with synovium, that adheres firmly to the bones.147,148 Arteries, veins, and proprioceptive and sensory nerves travel through the capsule. The synovium may protrude and form a pouch through the gaps where the vasculature penetrates the capsule. In most joints, there are two or more localized thickenings of the capsule, forming ligaments (▶Fig. 1.61); there are at least four distinguishable capsular ligament systems in the human hip (see Chapter 20 “Hip”). Joint capsules may be further supported by accessory ligaments, inside or outside the capsule, which also restrict motion (e.g., anterior cruciate of the knee). Similar to the tendon–bone insertion, the capsular attachment to bone is distinguished by four zones.147 Parts of the capsule that articulate with the joint itself become fibrocartilaginous and pressure resistant. In many sites, tendons attach to the capsule, and in some cases replace it. For example, the quadriceps and patellar tendons form the anterior part of the knee joint capsule, and the extensor tendons form the dorsal part of the capsule of interphalangeal joints. The shoulder joint capsule has a complex anatomic structure that is supported by the tendons of the supraspinatus, teres minor, infraspinatus, and subscapularis. It is unusually large to allow

Bones, Muscles, Tendons, Joints, and Cartilage

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Fig. 1.60 Posttraumatic fractures of the distal femur in a patient with calcium pyrophosphate dihydrate deposition disease and osteopenia. (a) Coronal and (b) axial computed tomography (CT) scans show chondrocalcinosis of the medial and lateral menisci and the articular ­cartilage (green arrows). Also seen are calcifications of the capsule, ligaments, and tendon (yellow arrows) due to calcium crystal deposition.

Fig. 1.61 (a) Anatomical boundary of the elbow joint capsule (yellow arrows) in a cadaveric dissection. (b) Axial section through the first ­metatarsophalangeal joint showing structure of this synovial joint.

the wide range of movement of this ball-and-socket joint. In the elbow, the capsule forms pouches around the head of the radius and the olecranon (▶Fig. 1.39, ▶Fig. 1.61). Pockets of fat are seen between the capsule and the synovial membrane with the largest over the olecranon fossa (see Chapter 16 “Elbow Joint”). Synovial folds or synovial plicae (▶Fig. 1.39) are thin folds of the synovial layer of the joint capsule that project into the joint space and are

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filled with extrasynovial fat (see Chapter 16 “Elbow Joint”). The joint capsule may participate in structural support of the joint disk. Examples include menisci of the knee and the articular disk of the temporomandibular joint. Capsular injuries more commonly occur by avulsion of a bone fragment beneath the attachment site of the capsule or by tearing of the tendon, ligament, or capsule above it (see Chapter 18

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Fig. 1.62 (a) Sagittal T2-weighted (T2W) fat-suppressed magnetic resonance (MR) image of the ankle region showing a moderate retrocalcaneal bursitis (arrow) and hypertrophic insertional Achilles tendinopathy. (b) Sagittal T2W fat-suppressed magnetic resonance (MR) image of the knee showing a small prepatellar bursitis (arrow).

“Hand”). Laxity of the capsule is a common cause of shoulder dislocation and congenital dislocation of the hip that is often required to be corrected surgically. Contraction and thickening of the joint capsule following trauma or immobilization of a limb result in “frozen shoulder” in which the mobility of the joint is significantly limited. In this condition, surgical capsulotomy is performed to improve joint mobility. Deposition of calcium can occur in calcium pyrophosphate dihydrate crystal deposition disease.

◆◆ Bursae

Anatomical bursae are synovial-lined, potential spaces between bony prominences and where tendons move against each other or glide over a bony surface. Most anatomical bursae are subtendinous at the insertion site of the tendon.149,150 Examples of the subtendinous bursae include the bicipital bursa at the insertion of the tendon of the biceps brachii, the retrocalcaneal bursa at the insertion of the Achilles tendon, and deep infrapatellar bursae at the insertion of patellar tendon on tibia (▶Fig. 1.62). Other anatomical bursae are superficial and usually lie between a tendon and superficial structures (▶Fig.  1.62). The bursa superficial to the distal Achilles tendon is an example of superficial bursae. The bursae are typically located around large joints, and some may communicate directly with articular joints (▶Fig. 1.52). Examples of communicating bursae are the iliopsoas bursa anterior to the hip and the gastrocnemius–semimembranosus bursa posteromedial to the knee (▶Fig. 1.52). In certain locations, communication between the joint and the bursa is abnormal. An example is the subacromial–subdeltoid bursa that lies superior to the rotator cuff and inferior to the acromion. Another group of bursae are called adventitious. These bursae are acquired, nonendothelial-lined fluid collections that develop in areas of chronic friction, as a response to stress.

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Most of the bursae are potential spaces and are not normally visualized on imaging. Inflammation and pathological enlargement of the bursa is called bursitis (▶Fig. 1.52). Bursitis occurs as a result of excessive local friction, infection, arthritides, or direct trauma.149,150 In this case, fluid and debris collect within the bursa or fluid extends into the bursa from the adjacent joint. The walls of the bursa thicken, and the fluid-filled sac can grow into adjacent structures. Deep-seated fluid-filled bursae are best visualized as a high-signal T2 structure on MRI and hypodense structures with an enhancing thickened wall on CT (▶Fig. 1.52, ▶Fig. 1.62).

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cise in ageing humans. J Physiol 2016;594(8):2261–2273 61. Zhang R, Taucer AI, Gashev AA, Muthuchamy M, Zawieja DC, Davis MJ. Maximum shortening velocity of lymphatic muscle approaches that of striated muscle. Am J Physiol Heart Circ Physiol 2013;305(10):H1494–H1507 62. Hepple RT, Rice CL. Innervation and neuromuscular control in ageing skeletal muscle. J Physiol 2016;594(8):1965–1978 63. Dumont NA, Wang YX, Rudnicki MA. Intrinsic and extrinsic mechanisms regulating satellite cell function. Development 2015;142(9):1572–1581 64. Mann CJ, Perdiguero E, Kharraz Y, et al. Aberrant repair and fibrosis development in skeletal muscle. Skelet Muscle 2011;1(1):21 65. Smitaman E, Flores DV, Mejía Gómez C, Pathria MN. MR imaging of atraumatic muscle disorders. Radiographics 2018;38(2):500–522 66. Flores DV, Mejía Gómez C, Estrada-Castrillón M, Smitaman E, Pathria MN. MR imaging of muscle trauma: anatomy, biomechanics, pathophysiology, and imaging appearance. Radiographics 2018;38(1):124–148 67. Benjamin M, Ralphs JR. Tendons and ligaments: an overview. Histol Histopathol 1997;12(4):1135–1144 68. Benjamin M, Kaiser E, Milz S. Structure-function relationships in tendons: a review. J Anat 2008;212(3):211–228 69. Vosloo M, Keough N, De Beer MA. The clinical anatomy of the insertion of the rotator cuff tendons. Eur J Orthop Surg Traumatol 2017;27(3):359–366

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71. Subramanian A, Schilling TF. Tendon development and musculoskele-

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2015;142(24):4191–4204 72. Fallon J, Blevins FT, Vogel K, Trotter J. Functional morphology of the supraspinatus tendon. J Orthop Res 2002;20(5):920–926 73. Lo IK, Chi S, Ivie T, Frank CB, Rattner JB. The cellular matrix: a feature of tensile bearing dense soft connective tissues. Histol Histopathol 2002;17(2):523–537 74. Miller BF, Olesen JL, Hansen M, et al. Coordinated collagen and muscle protein synthesis in human patella tendon and quadriceps muscle after exercise. J Physiol 2005;567(Pt 3):1021–1033 75. Zitnay JL, Weiss JA. Load transfer, damage, and failure in ligaments and tendons. J Orthop Res 2018;36(12):3093–3104 76. Doyle JR. Palmar and digital flexor tendon pulleys. Clin Orthop Relat Res 2001;(383):84–96 77. King EA, Lien JR. Flexor tendon pulley injuries in rock climbers. Hand Clin 2017;33(1):141–148 78. Webborn N, Morrissey D, Sarvananthan K, Chan O. Acute tear of the fascia cruris at the attachment to the Achilles tendon: a new diagnosis. Br J Sports Med 2015;49(21):1398–1403 79. Müller SA, Evans CH, Heisterbach PE, Majewski M. The role of the paratenon in achilles tendon healing: a study in rats. Am J Sports Med 2018;46(5): 1214–1219 80. Hodgson RJ, O’Connor PJ, Grainger AJ. Tendon and ligament imaging. Br J Radiol 2012;85(1016):1157–1172 81. Chowdhury P, Matyas JR, Frank CB. The “epiligament” of the rabbit medial collateral ligament: a quantitative morphological study. Connect Tissue Res 1991;27(1):33–50 82. Bray RC. Blood supply of ligaments: a brief overview. Orthopaedics 1995;3:39–48 83. Benjamin M, Ralphs JR. Fibrocartilage in tendons and ligaments: an adaptation to compressive load. J Anat 1998;193(Pt 4):481–494 84. Vogel KG, Koob TJ. Structural specialization in tendons under compression. Int Rev Cytol 1989;115:267–293 85. Benjamin M, McGonagle D. Entheses: tendon and ligament attachment sites. Scand J Med Sci Sports 2009;19(4):520–527

1991;73(10):1507–1525 99. Jung HJ, Fisher MB, Woo SL. Role of biomechanics in the understanding of normal, injured, and healing ligaments and tendons. Sports Med Arthrosc Rehabil Ther Technol 2009;1(1):9 100. Wang JH, Guo Q, Li B. Tendon biomechanics and mechanobiology - a mini-review of basic concepts and recent advancements. J Hand Ther 2012;25(2):133–141 101. Magnusson SP, Qvortrup K, Larsen JO, et al. Collagen fibril size and crimp morphology in ruptured and intact Achilles tendons. Matrix Biol 2002;21(4):369–377 102. Järvinen TA, Järvinen TL, Kannus P, Józsa L, Järvinen M. Collagen fibres of the spontaneously ruptured human tendons display decreased thickness and crimp angle. J Orthop Res 2004;22(6):1303–1309 103. Järvinen TA, Kannus P, Maffulli N, Khan KM. Achilles tendon disorders: etiology and epidemiology. Foot Ankle Clin 2005;10(2):255–266 104. Celikyay F, Yuksekkaya R, Almus F, Bilgic E. Tenosynovitis of the peroneal tendons associated with a hypertrophic peroneal tubercle: radiography and MRI findings. BMJ Case Rep 2014;2014:bcr2013200204 105. Kubo K, Kanehisa H, Fukunaga T. Effects of resistance and stretching training programmes on the viscoelastic properties of human tendon structures in vivo. J Physiol 2002;538(Pt 1):219–226 106. Benjamin M, Redman S, Milz S, et al. Adipose tissue at entheses: the rheumatological implications of its distribution. A potential site of pain and stress dissipation? Ann Rheum Dis 2004;63(12):1549–1555 107. Lee JC, Healy JC. Normal sonographic anatomy of the wrist and hand. Radiographics 2005;25(6):1577–1590 108. Pierre-Jerome C, Moncayo V, Terk MR. MRI of the Achilles tendon: a comprehensive review of the anatomy, biomechanics, and imaging of overuse tendinopathies. Acta Radiol 2010;51(4):438–454 109. Benjamin M, Milz S, Bydder GM. Magnetic resonance imaging of entheses. Part 1. Clin Radiol 2008;63(6):691–703 110. Smith MD. The normal synovium. Open Rheumatol J 2011;5:100–106

86. Thomopoulos S, Genin GM, Galatz LM. The development and morphogenesis of

111. Iwanaga T, Shikichi M, Kitamura H, Yanase H, Nozawa-Inoue K. Morphology

the tendon-to-bone insertion: what development can teach us about healing.

and functional roles of synoviocytes in the joint. Arch Histol Cytol

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87. Toumi H, Higashiyama I, Suzuki D, et al. Regional variations in human patellar

112. Smith MD, Barg E, Weedon H, et al. Microarchitecture and protective mech-

trabecular architecture and the structure of the proximal patellar tendon enthe-

anisms in synovial tissue from clinically and arthroscopically normal knee

sis. J Anat 2006;208(1):47–57 88. Manske MC, McKeon KE, Johnson JE, McCormick JJ, Klein SE. Arterial anatomy of the tibialis posterior tendon. Foot Ankle Int 2015;36(4):436–443 89. van Dijk PA, Madirolas FX, Carrera A, Kerkhoffs GM, Reina F. Peroneal tendons well vascularized: results from a cadaveric study. Knee Surg Sports Traumatol Arthrosc 2016;24(4):1140–1147 90. Petersen W, Bobka T, Stein V, Tillmann B. Blood supply of the peroneal tendons: injection and immunohistochemical studies of cadaver tendons. Acta Orthop Scand 2000;71(2):168–174 91. Cook JL, Kiss ZS, Ptasznik R, Malliaras P. Is vascularity more evident after exercise? Implications for tendon imaging. AJR Am J Roentgenol 2005;185(5):1138–1140

joints. Ann Rheum Dis 2003;62(4):303–307 113. Wilkinson LS, Edwards JC. Microvascular distribution in normal human synovium. J Anat 1989;167:129–136 114. Mapp PI. Innervation of the synovium. Ann Rheum Dis 1995;54(5):398–403 115. Loeser RF. Osteoarthritis year in review 2013: biology. Osteoarthritis Cartilage 2013;21(10):1436–1442 116. Carballo CB, Nakagawa Y, Sekiya I, Rodeo SA. Basic science of articular cartilage. Clin Sports Med 2017;36(3):413–425 117. He B, Wu JP, Kirk TB, Carrino JA, Xiang C, Xu J. High-resolution measurements of the multilayer ultra-structure of articular cartilage and their translational potential. Arthritis Res Ther 2014;16(2):205

92. Hoksrud A, Ohberg L, Alfredson H, Bahr R. Color Doppler ultrasound findings in

118. Decker RS, Koyama E, Pacifici M. Articular cartilage: structural and devel-

patellar tendinopathy (jumper’s knee). Am J Sports Med 2008;36(9):1813–1820

opmental intricacies and questions. Curr Osteoporos Rep 2015;13(6):

93. Klos K, Gueorguiev B, Carow JB, et al. Soft tissue microcirculation around the healthy Achilles tendon: a cross-sectional study focusing on the Achilles tendon and dorsal surgical approaches to the hindfoot. J Orthop Surg Res 2018;13(1):142 94. Theobald P, Bydder G, Dent C, Nokes L, Pugh N, Benjamin M. The functional anatomy of Kager’s fat pad in relation to retrocalcaneal problems and other hindfoot disorders. J Anat 2006;208(1):91–97 95. Pingel J, Petersen MC, Fredberg U, et al. Inflammatory and metabolic alterations of Kager’s fat pad in chronic Achilles tendinopathy. PLoS One 2015;10(5):e0127811 96. Chauvin NA, Khwaja A, Epelman M, Callahan MJ. Imaging findings of Hoffa’s fat pad herniation. Pediatr Radiol 2016;46(4):508–512

407–414 119. Oldberg A, Antonsson P, Hedbom E, Heinegård D. Structure and function of extracellular matrix proteoglycans. Biochem Soc Trans 1990;18(5):789–792 120. Mansfield J, Yu J, Attenburrow D, et al. The elastin network: its relationship with collagen and cells in articular cartilage as visualized by multiphoton microscopy. J Anat 2009;215(6):682–691 121. Mansfield JC, Bell JS, Winlove CP. The micromechanics of the superficial zone of articular cartilage. Osteoarthritis Cartilage 2015;23(10):1806–1816 122. Linn FC, Sokoloff L. Movement and composition of interstitial fluid of cartilage. Arthritis Rheum 1965;8:481–494 123. Arokoski JP, Jurvelin JS, Väätäinen U, Helminen HJ. Normal and pathological adaptations of articular cartilage to joint loading. Scand J Med Sci Sports 2000;10(4):186–198

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Bones, Muscles, Tendons, Joints, and Cartilage 124. Mäkelä JT, Huttu MR, Korhonen RK. Structure-function relationships in osteoarthritic human hip joint articular cartilage. Osteoarthritis Cartilage 2012;20(11):1268–1277 125. Koo S, Rylander JH, Andriacchi TP. Knee joint kinematics during walking influences the spatial cartilage thickness distribution in the knee. J Biomech 2011;44(7):1405–1409 126. Vanwanseele B, Eckstein F, Knecht H, Spaepen A, Stüssi E. Longitudinal analysis of cartilage atrophy in the knees of patients with spinal cord injury. Arthritis Rheum 2003;48(12):3377–3381 127. Pacifici M, Koyama E, Shibukawa Y, et al. Cellular and molecular mechanisms of synovial joint and articular cartilage formation. Ann N Y Acad Sci 2006;1068:74–86 128. Recht MP, Goodwin DW, Winalski CS, White LM. MRI of articular cartilage: revisiting current status and future directions. AJR Am J Roentgenol 2005;185(4):899–914 129. Liu F, Choi KW, Samsonov A, et al. Articular cartilage of the human knee joint: in vivo multicomponent T2 analysis at 3.0 T. Radiology 2015;277(2): 477–488 130. Kim T, Min BH, Yoon SH, et al. An in vitro comparative study of T2 and T2* mappings of human articular cartilage at 3-Tesla MRI using histology as the standard of reference. Skeletal Radiol 2014;43(7):947–954 131. Wei H, Gibbs E, Zhao P, et al. Susceptibility tensor imaging and tractography of collagen fibrils in the articular cartilage. Magn Reson Med 2017;78(5): 1683–1690 132. Crema MD, Roemer FW, Marra MD, et al. Articular cartilage in the knee: current MR imaging techniques and applications in clinical practice and research. Radiographics 2011;31(1):37–61 133. Van Bellinghen X, Idoux-Gillet Y, Pugliano M, et al. Temporomandibular joint regenerative medicine. Int J Mol Sci 2018;19(2):E446 134. Makris EA, Hadidi P, Athanasiou KA. The knee meniscus: structure-function, pathophysiology, current repair techniques, and prospects for regeneration. Biomaterials 2011;32(30):7411–7431 135. Danso EK, Oinas JMT, Saarakkala S, Mikkonen S, Töyräs J, Korhonen RK. Structure-function relationships of human meniscus. J Mech Behav Biomed Mater 2017;67:51–60 136. Nakata K, Shino K, Hamada M, et al. Human meniscus cell: characterization

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of the primary culture and use for tissue engineering. Clin Orthop Relat Res

137. Arnoczky SP, Warren RF. Microvasculature of the human meniscus. Am J Sports Med 1982;10(2):90–95

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138. Nguyen JC, De Smet AA, Graf BK, Rosas HG. MR imaging-based diagnosis and classification of meniscal tears. Radiographics 2014;34(4):981–999 139. Fukubayashi T, Kurosawa H. The contact area and pressure distribution pattern of the knee. A study of normal and osteoarthrotic knee joints. Acta Orthop Scand 1980;51(6):871–879 140. Seedhom BB, Hargreaves DJ. Transmission of the load in the knee joint with special reference to the role of the menisci part II: experimental results, discussion, and conclusions. Eng Med Biol. 1979;8:220–228 141. Gersing AS, Schwaiger BJ, Nevitt MC, et al. Is weight loss associated with less progression of changes in knee articular cartilage among obese and overweight patients as assessed with MR imaging over 48 months? Data from the osteoarthritis initiative. Radiology 2017;284(2):508–520 142. Bhattacharyya T, Gale D, Dewire P, et al. The clinical importance of meniscal tears demonstrated by magnetic resonance imaging in osteoarthritis of the knee. J Bone Joint Surg Am 2003;85(1):4–9 143. Gopal SK, Shankar R, Vardhan BH. Prevalence of temporo-mandibular joint disorders in symptomatic and asymptomatic patients: a cross-sectional study. Int J Adv Health Sci 2014;1:14–20 144. Ingawalé S, Goswami T. Temporomandibular joint: disorders, treatments, and biomechanics. Ann Biomed Eng 2009;37(5):976–996 145. Kaushik S, Erickson JK, Palmer WE, Winalski CS, Kilpatrick SJ, Weissman BN. Effect of chondrocalcinosis on the MR imaging of knee menisci. AJR Am J Roentgenol 2001;177(4):905–909 146. Kaplan PA, Nelson NL, Garvin KL, Brown DE. MR of the knee: the significance of high signal in the meniscus that does not clearly extend to the surface. AJR Am J Roentgenol 1991;156(2):333–336 147. Ralphs JR, Benjamin M. The joint capsule: structure, composition, ageing and disease. J Anat 1994;184(Pt 3):503–509 148. Benjamin M, Ralphs JR, Shibu M, Irwin M. Capsular tissues of the proximal interphalangeal joint: normal composition and effects of Dupuytren’s disease and rheumatoid arthritis. J Hand Surg [Br] 1993;18(3):371–376 149. Friedman MV, Stensby JD, Long JR, Currie SA, Hillen TJ. Beyond the greater trochanter: a pictorial review of the pelvic bursae. Clin Imaging 2017;41:37–41 150. Hirji Z, Hunjun JS, Choudur HN. Imaging of the bursae. J Clin Imaging Sci 2011;1:22

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2 Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm

2

Felipe Urdaneta and Farhood Saremi

◆◆ Introduction

The upper extremity appendicular skeleton consists of the shoulder girdle, arm, forearm, and hand. The hand and wrist are described in separate chapters. The shoulder girdle consists of the clavicle and scapula which is connected to the axial skeleton by the sternoclavicular joint.

◆◆ Development and Ossification

Shoulder Girdle

The chondrogenic bed of the humerus and the medial border of the scapula can be observed from as early as Carnegie stage 17 (41 days) of the embryo, whereas that of the rest of the scapula appears at stage 18 (44 days). At stage 19 (46 days), the glenohumeral joint forms. The ossification of the shoulder girdle occurs in a sequential order1 (▶Fig. 2.1, ▶Fig. 2.2). The osteogenic process begins in week 10 of the fetus development for the humeral head and week 11 for the scapula.2 At birth, the only ossified structures of the shoulder include the humeral diaphysis, the midportion of the clavicle, and the body of the scapula (▶Fig. 2.3). The scapular

spine develops around the 3rd month after birth and the glenoid cavity ossifies later. Throughout osseous maturation, secondary ossification centers arise from the epiphyses and apophyses of the shoulder3 (▶Fig. 2.1, ▶Fig. 2.2, ▶Table 2.1, ▶Table 2.2). The scapula is ossified from eight or more centers: one large center forms the body. There are two or three centers for each coracoid process and the acromion. Single centers are located in the medial border and the inferior angle. One or several ossification centers exist for the glenoid rim. The coracoid begins its ossification at 3 months of age with the appearance of a central ossification center. A second secondary ossification center located at the base of the coracoid appears at 8 to 10 years of age. In some patients, a third ossification center may be seen at the tip of the coracoid that should not be confused with a fracture or ligamentous injury (▶Fig. 2.4). The acromion is cartilaginous at birth. Ossification occurs in multiple anterolateral ossifications and one posterior center. An ossicle at the tip of the acromion may remain unfused (▶Fig. 2.4). The failure of fusion of the multiple ossification centers can result in an os acromiale (▶Fig.  2.5). The ossification of the cartilaginous glenoid fossa begins in the upper one-third of the glenoid.

Fig. 2.1 Normal ossification of the shoulder girdle. The blue structures are cartilaginous areas at birth that subsequently develop ossification centers, which are depicted in green. The head of the humerus is formed from the humeral head ossification center, the greater tuberosity ­ossification center, and the lesser tuberosity ossification center. The glenoid and coracoid are ossified in a common cartilaginous base. The superior one-third of the glenoid ossifies from the subcoracoid ossification center, and the inferior two-thirds portion of the glenoid arises from multiple ossification centers. Two ossification centers account for the ossification of the coracoid: one at the center and the other at the base of the coracoid.

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Fig. 2.2 Drawing showing the sequential appearance of the ossification centers of the elbow, along with the usual age (in years) at their ­appearance. The blue structures are cartilaginous areas at birth that subsequently develop ossification centers, which are depicted in green. 

Fig. 2.3 Posterior (a, c) and anterior (b, d) views of the chest of an infant at 5 days (top row) versus 2 years of age (lower row). At birth, the only ossified structures of the shoulder include the humeral diaphysis, the midportion of the clavicle, and the body of the scapula. The scapular spine and mid center of the coracoid develop around the third month after birth and the glenoid cavity ossifies later. Note that after day 5 of birth only a tiny ossification center exists at the humeral head and the coracoid is not ossified. At year 2, the central coracoid is ossified but its base still remains unossified. The glenoid margin and humeral head are more developed.

49

Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm Table 2.1 Approximate age of appearance and fusion of the multiple ossification centers in the shoulder girdle Bone Structure and Ossification Center

Age of Child at Appearance

Age of Subject at Fusion (y)

Head of the humerus

2–4 mo

3-puberty

Greater tuberosity

7–10 mo

3-puberty

Lesser tuberosity

5y

6-puberty

Proximal humerus

2

Scapula: Glenoid Subcoracoid

8–10 y

14–17

Centers in the inferior two-thirds of the glenoid

14–15 y

17–18

Center of the coracoid process

3 mo

15–17

Base of the caracoid process

8–10 y

15–17

14–16 y

18–25

Scapula: Coracoid process

Scapula: Acromion Acromial secondary ossification centers

Table 2.2 CRITOE: Mnemonic for sequential appearance and fusion of the ossification centers in the elbow Letter of Mnemonic

Site of Ossification Center

Age of Child at Appearance (y)

Age of Child at Physeal Fusion (y)

C

Capitellum

1

14

R

Radial head

4–5

16

I

Medial epicondyle (Internal)

5–7

15

T

Trochlear

8–9

14

O

Olecranon

8–10

14

E

Lateral epicondyle (External)

11–12

16

Fig. 2.4 (a) Incomplete fusion of ossification centers at the tip of acromion and coronoid processes (red arrows) at the age of 14 years. (b) Computed tomography (CT) scan of a normal boy at the age of 16 years showing cartilaginous base of the coronoid process (yellow arrows). Note the tent-shaped ossification center of humeral head (green arrow).

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Fig. 2.5 Large os acromiale (arrows) shown by angled axial maximum intensity projection (MIP) and coronal computed tomography (CT) of the right shoulder. Note low position of the os acromiale on the coronal view causing rotator cuff impingement in this patient with chronic shoulder pain.

Fig. 2.6 (a) An 18-year-old male with Bankart fracture, an avulsion of the anteroinferior glenoid, with recurrent anterior shoulder dislocation (arrows). (b) Little leaguer’s shoulder in a 12-year-old male. Anteroposterior (AP) radiograph of the right shoulder showing physeal widening and mild metaphyseal irregularity and sclerosis in the lateral aspect of the proximal humeral physis (arrow), consistent with chronic repetitive stress injury (little leaguer’s shoulder).

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Fig. 2.7 Unfused medial clavicular epiphysis ossification center in a 22-year-old patient. The medial clavicular epiphysis ossification starts at 18 years of age and will be completed by 22 to 25 years of age.

The inferior two-thirds of the cartilaginous glenoid fossa ossifies from multiple ossification centers that coalesce to form a horseshoe-like structure that could be mistaken for an osseous Bankart lesion (▶Fig.  2.1, ▶Fig.  2.6). Ossification of the inferior angle of scapula occurs between 14 and 20 years of age. Ossification center in the lateral clavicular cartilaginous physis is not a consistent finding but when present it should not be confused with a fracture. This physis is weak relative to the acromioclavicular and coracoclavicular ligaments and can be the site of epiphyseal separation in childhood.4 Prior to complete ossification, the acromioclavicular joint appears wide on plain x-rays which simulates acromioclavicular separation. Magnetic resonance imaging (MRI) can help to distinguish from acromioclavicular separation by showing the normal nonossified cartilage. The medial clavicular epiphysis ossification starts at 18 years of age and will be completed by 22 to 25 years of age (▶Fig. 2.7). The proximal humeral epiphysis ossification centers are located on the head of the humerus, the greater tuberosity, and the lesser tuberosity. A secondary ossification center appears in the medial humeral head cartilage by 2 to 4 months of age, followed by one in the greater tuberosity by 7 to 10 months of age (▶Table  2.1, ▶Fig.  2.1, ▶Fig.  2.8). The cartilaginous part is best seen with MRI. Fusion of these two ossification centers starts by 3 years of age and will be completed by puberty. Ossification of the lesser tuberosity ossification is a matter of debate and may occur later

Fig. 2.8 (a) At 2 years of age. (b) At 3 years of age. At the age of 2 years, the central coronoid is ossified but its base remains unossified. The greater tuberosity and humeral head are more developed. These two ossification centers are fused together at 3 years of age. In general, an ossification center appears in the medial humeral head cartilage by 2 to 4 months of age, followed by one in the greater tuberosity by 7 to 10 months of age. 

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Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm at 5 years and being fused to other centers later and completed by about 13 years of age. The borders of the new single ossification center may remain irregular for some time and should not be confused with pathology. The proximal humeral physis appears flat at birth but it becomes tented in contour with the apex located in the lateral aspect of the humerus at 14 years of age, when physeal closure begins (▶Fig. 2.4). Irregularity of the physis with metaphyseal bone marrow edema could be a sign of little leaguer’s shoulder4 (▶Fig. 2.6).

2

Elbow Region At birth, the elbow joint is completely cartilaginous and cannot be reliably evaluated by plain radiography. MRI, computed tomography (CT), or ultrasound can be used instead. Maturation of the elbow occurs at six ossification centers within the distal humerus and proximal radius and ulna in a predictable chronologic order; however, it may vary from patient to patient (▶Table 2.2, ▶Fig. 2.2). Comparison with the contralateral side is always useful in identifying subtle abnormalities. The ossification order can be remembered with the mnemonic “CRITOE” (capitellum, radial head, medial, or internal epicondyle, trochlea, olecranon, and lateral or external epicondyle).1 Attention to this order of development is important for diagnosis of pathologies. For example, missing of one ossification center may indicate an avulsion fracture of that center. In general, ossifications occur 6 to 12 months earlier in girls than in boys (▶Fig. 2.2). Ossification of the capitellum starts at the age of 1 year and continues sequentially for other centers at about every 2 years thereafter (▶Fig. 2.9). The capitellum may appear as early as 3 months. Normally, the capitellum is anteverted forming an angle of 130 degrees with the humeral shaft. Its posterior side of the cartilaginous physis is wider than the anterior aspect. This widening should not be misdiagnosed as a fracture at this location (▶Fig.  2.10). The capitellum is an important landmark when evaluating pediatric arm on x-rays. For example, the radial head should align with the capitellum in all views in order to rule out dislocation (▶Fig. 2.10). The radial head ossifies at the age of 3 to 4 years. Incomplete ossification is common at the lateral side that can be mistaken for a fracture. The medial epicondyle ossifies between 5 and 7 years of age. Ossification of the trochlea usually occurs at multiple sites beginning around the age of 8 to 9 years. Its fragmented appearance should not be confused with a fracture or avascular necrosis. The olecranon begins to ossify around the age of 8 to 10 years at

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Fig. 2.9 Left hand radiographs at 1 and 2 years of age. At the age of 1 year, ossification has already started in the humeral head, greater tuberosity, capitellum, and radial end. 

two or more sites. Finally, the lateral epicondyle begins ossifying around the age of 11 to 12 years1 (▶Fig. 2.2). The capitellum fuses with the trochlea and lateral epicondyle of the humerus to form a single ossification center. This single center fuses with the distal humeral metaphysis between 14 and 16 years of age. The medial epicondyle fuses approximately 2 years later.

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2

Fig. 2.10 Nondisplaced supracondylar fracture in a 2-year-old boy. Note the normal alignment of the radial head with the capitellum in both views, an important finding to rule out dislocation. Normally, the capitellum is anteverted forming an angle of 130 degrees with the humeral shaft.

◆◆ Shoulder Girdle

Scapula

The scapula is a large, flat, triangular bone on the posterior lateral aspect of the thoracic wall.5 The glenohumeral joint connects the scapula to the humerus. It is connected to the sternum by the clavicle (▶Fig.  2.11). The scapula has dorsal and costal surfaces, three distinct borders, and three bony processes: the spine, the acromion, and the coracoid process (▶Fig.  2.12, ▶Fig.  2.13, ▶Fig. 2.14).

Surfaces •• Costal: The subscapularis muscle attaches to the subscapular fossa, a broad concavity in the costal (ventral) surface which is in close relation to the convexity of the adjacent posterolateral chest wall (▶Fig. 2.12). Fibrous intramuscular septa of the subscapularis muscle attach to several obliquely oriented ridges arising from the medial periphery of the costal surface. The serratus anterior muscle attaches to a narrow strip along the medial edge of the costal surface which broadens slightly into oval surfaces in its superior and inferior margins (▶Fig. 2.12). •• Dorsal: The spine of the scapula divides the dorsal surface into two asymmetric parts. Superior to the spine, the smaller, concave supraspinatus (supraspinous) fossa gives origin to

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the supraspinatous muscle. Inferiorly, the larger, infraspinatus (infraspinous) fossa gives origin to the infraspinatous muscle medially (▶Fig.  2.13). The osseous floor of the infraspinous fossa can become very thin with age and sometimes show perforations. The supraspinous and infraspinous fossae converge at the spinoglenoid notch between the dorsal aspect of the neck of the scapula and the lateral border of the spine of the scapula (▶Fig. 2.14). A bony ridge traversing the periphery of the dorsolateral surface from the glenoid to the inferior angle contains a fibrous septum which delineates and separates the borders of the infraspinatous muscle (medially), teres minor (superior lateral), and teres major (inferior lateral) (▶Fig. 2.13).

Borders •• The superior border is concave, extending inferior and lateral from the superior angle of the scapula to the base of the coracoid process. Medial to the base of the coracoid process is the suprascapular notch (scapular incisure), a notch that appears in various sizes and depths and is covered by the transverse scapular ligament (▶Fig.  2.13). The lateral aspect of the superior border, near the root of the coracoid, gives origin to the inferior belly of the omohyoid muscle. •• The medial border is flat, mildly convex, and somewhat thickened, particularly near the medial aspect of the spine of the scapula. It extends from the superior angle to the inferior angle

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Fig. 2.11 Anatomical position of the scapula in relation to the thorax. Associated superficial muscles are shown. (a, b) Posterior views. (c) Posterior view after partial removal of the trapezius. (d, e) Lateral views. 

of the scapula. The medial border is the largest of the three borders. Three muscles attach to the medial border: the levator scapulae to its upper margin, the rhomboid minor in the mid portion, and the rhomboid major to the inferior margin (▶Fig. 2.11, ▶Fig. 2.13). •• The lateral border runs obliquely, inferior and medial from the inferior margin of the glenoid to the inferior angle. The

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infraglenoid tubercle—a rough, slightly broadened surface in the superior lateral end of the border, just inferior to the glenoid—gives origin to the long head of the triceps muscle. The lateral border separates the attachments of the subscapularis, teres minor, and teres major (▶Fig. 2.13, ▶Fig. 2.15).

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Fig. 2.12 Anterior views of the left scapula and humerus and their muscular and tendinous attachments. Compared to the humerus, the scapula is magnified. 

Projections •• Glenoid cavity: The glenoid cavity is a socket at the superolateral aspect of the scapula near the base of the coracoid process (▶Fig. 2.12). The articular surface of the glenoid is one-third to one-fourth the size of the humeral head and measures 6 to 8 cm2 in adults. It appears retroverted about 4 to 8 degrees. The glenoid tubercle is an elevation just inferior to the center of the floor of the glenoid cavity. At this tubercle, the articular cartilage may change to fibrous cartilage. The glenoid notch, a pear-shaped depression, is seen at the ventral margin of the

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glenoid cavity in 55% of scapulas. Superior to the glenoid cavity, the supraglenoid tubercle is seen. It is the origin of the tendon of the long head of biceps and lies intraarticularly (▶Fig. 2.12). The infraglenoid tubercle is located in the inferior aspect of the scapular neck. This small irregular tuberosity is the site of origin of the long head of the triceps muscle and is located extraarticularly (▶Fig. 2.12). •• Spine: The spine of the scapula is a triangular, osseous, platelike projection on the superior aspect of the dorsal surface of the scapula (▶Fig.  2.13, ▶Fig.  2.14). The upper surface of the spine broadens and becomes more elevated as it courses

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Fig. 2.13 Posterior views of the left scapula and femur and their muscular and tendinous attachments.

laterally toward acromion. The trapezius attaches to the inner superior aspect of the spine. Fibers of the deltoid muscle attach to the outer inferior aspect of the spine (▶Fig. 2.13, ▶Fig. 2.15, ▶Fig. 2.16). •• Acromion: Osseous projection is continuous with the lateral edge of the spine of the scapula which extends laterally and superiorly, overhanging the glenoid. The medial border of the acromion articulates with lateral end of the clavicle. The lateral edge of the acromial end of the clavicle is the highest point of the shoulder (▶Fig. 2.13). A portion of the trapezius muscle attaches to the medial border. Its thick, irregular lateral border serves as

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origin for the middle fibers of the deltoid muscle (▶Fig. 2.16). The acromion is normally formed by the fusion of several ossification centers to form three centers. These three centers merge to form a triangular epiphyseal bone, which finally fuses with the basiacromion by the age of 25 years. Nonfusion to the basiacromion by the age of 25 years is called “os acromiale,” which is seen in 7 to 15% of cases (▶Fig.  2.5). The connection of the os acromiale with the basiacromion is diarthrosis and synchondrosis which can show signs of osteoarthrosis causing chronic shoulder pain. Os acromiale is usually large but it may be small at the tip of the acromion, also known as the preacromion. Os

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Fig. 2.14 Superior and lateral views of the left scapula.

acromiale should not be confused with an intercalated ossicle of the acromioclavicular joint. The slope of the acromion can be presented in two different ways; type 1 (flat) occurs in 10% of cases while type 2 (curved) occurs in 89%.6 Type 1 results in a narrowed supraspinatus outlet, a potential cause for impingement that may require surgical intervention. The inferior side of the acromion which is often uneven is covered with a thick periosteal layer, while the superior side is dense connective tissue layer. •• Coracoid process: Hook-like osseous projection arising from the superior border of the neck of the scapula with an angulated body at a nearly 90-degree angle which extends anteriorly and laterally over the glenoid. The infraclavicular brachial plexus and the axillary vessels lie inferior and medial to the coracoid. The acromial branch of the thoraco-acromial artery passes above it, and the sensory branch of the lateral pectoral nerve to the rotator interval capsule, together with accompanying vessels, lies immediately below. The coracoid apex serves as the attachment of the coracobrachialis muscle and short head of the biceps brachii muscle (▶Fig. 2.15, ▶Fig. 2.17, ▶Fig. 2.18, ▶Fig. 2.19, ▶Fig. 2.20). The pectoralis minor muscle attaches to the convex and irregular superior–medial surface of the coracoid. The lateral aspect of the base of the coracoid serves as the attachment for the coracohumeral ligament. The coracoacromial ligament attaches to the tip of the acromion. It extends from the inferior anterolateral surface of the acromion to insert onto the lateral edge of the coracoid process (▶Fig. 2.19, ▶Fig. 2.20). Calcification or ossification of the coracoacromial ligament can contribute to shoulder impingement syndrome. Two coracoclavicular ligaments (conoid and trapezoid) connect the superior surface of the coracoid process and the inferior

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surface of the clavicle (▶Fig. 2.19, ▶Fig. 2.20). Clavicular attachments of these ligaments are posterior to the deltoid, anterior to the trapezius, and lateral to the subclavius attachments (▶Fig. 2.21).

Clavicle The clavicle is the first bone in the human body to ossify, usually in the 5th gestational week. The clavicle is the only bone of the postcranial skeleton that is ossified mainly by desmal ossification. The acromial end ossification is intramembranous, and the medial end ossification is endochondral. Therefore, it has only one epiphysis located at its sternal end with complete ossification at 22 to 25 years of age, the last one in the body to fuse (▶Fig. 2.7). The clavicle is usually S-shaped but it may appear straight or extremely curved. The sternal end of the clavicle is thickened with excessive physical strain, while the acromial end is flattened and has a small, oval acromial facet (▶Fig. 2.20, ▶Fig. 2.21). In the shaft an obliquely directed nutrient canal exists, which travels toward the acromial extremity. In about 6 to 10% of cases, the shaft of the clavicle has a sagittally directed accessory canal for passage of the supraclavicular nerve.5,7 The clavicle is the site of attachments of several muscles and ligaments (▶Fig.  2.21). The lateral half of the shaft of the clavicle has irregular ridges or tubercles for the insertion of the deltoid (deltoid tubercle) or trapezoid (trapezoid tubercle) muscles. On the medial end, there is an oval impression in the inferior surface for the costoclavicular ligament. The medial half of the shaft is the site of attachments of the pectoralis major anteriorly, the sternohyoid muscle posteriorly, the sternocleidomastoid muscle superiorly, and the costoclavicular ligament inferiorly. The lateral half of the shaft is the site

Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm

Fig. 2.15 Scapulohumeral muscles. The coracobrachialis is seen originating from the tip of the coracoid process of the scapula where it blends with the short head of the biceps brachii and inserts into the medial surface of the shaft of the humerus. The long head of the triceps brachii is seen arising from the infraglenoid tubercle of the scapula and passing distally anterior to the teres minor and posterior to the teres major. AC, acromioclavicular. 

of attachments of the deltoid muscle anteriorly, the trapezius muscle posteriorly, and the costoclavicular ligament inferiorly (▶Fig. 2.18, ▶Fig. 2.19, ▶Fig. 2.21).

Shoulder Girdle Muscles The scapula has a total of 17 muscular attachments (▶Table 2.3). The axial muscles stabilize the scapula to the spine and chest wall and include the levator scapulae, pectoralis minor, rhomboid, trapezius, serratus anterior, and pectoralis major muscles

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(▶Fig.  2.15, ▶Fig.  2.16, ▶Fig.  2.17, ▶Fig.  2.18, ▶Fig.  2.19). The scapula is attached to the posterior aspect of the thorax via the trapezius and rhomboid muscles and to the anterolateral chest via the serratus anterior muscle (▶Fig. 2.17). The scapulohumeral muscles include the supraspinatus, infraspinatus, teres minor, teres major, deltoid, long and short heads of the biceps brachii, long head of the triceps brachii, and coracobrachialis. The trapezius is composed of descending (cervical), ascending, and transverse fibers. The descending fibers arise from the occipital protuberance and the nuchal ligament and attach to the lateral clavicle (▶Fig. 2.16, ▶Fig. 2.21). These cervical fibers elevate and rotate the scapula. The horizontal fibers originate from the C7–T3 spinous processes and attach on the acromion and lateral scapular spine. The horizontal fibers retract the scapula and will be antagonized by the pectoralis minor. The ascending fibers arise from the T3–T12 spinous processes to attach to the medial aspect of the scapular spine, working as scapular retractors and depressors. The levator scapula attaches to the transverse processes of C1– C4 and inserts on the superior angle of the scapula (▶Fig. 2.22). Its main function is elevation and rotation of the scapula.8 The minor and major rhomboid muscles originate from the C6–C7 and T1–T4 spinous processes, respectively, and attach to the medial aspect of the scapular spine (▶Fig.  2.11, ▶Fig.  2.13, ▶Fig.  2.17). They retract and elevate the scapula. The serratus anterior arises anteriorly from the upper 10 ribs and attaches along the anterior surface of the medial border of the scapula from the inferior angle to the superior angle (▶Fig.  2.11, ▶Fig.  2.13, ▶Fig.  2.17). The serratus anterior contraction is important for protraction and upward rotation. It also holds the medial scapula against the thoracic cage. Dysfunction of the serratus anterior muscle leads to scapular winging. The pectoralis minor originates from the anterior aspect of the third to fifth ribs and attaches to the medial aspect of the coracoid process (▶Fig. 2.16, ▶Fig. 2.17, ▶Fig. 2.18, ▶Fig. 2.19). It participates in protraction and depression of the scapula. Subclavius muscle: The subclavius is a small, elongated muscle running underneath the clavicle. It arises from the first costochondral junction, anterior to the costoclavicular ligament, and inserts laterally into the inferior clavicular groove (▶Fig. 2.18, ▶Fig. 2.19, ▶Fig. 2.20, ▶Fig. 2.23). It is supplied by the thoracoacromial artery and innervated by the C5 and C6 nerves. Duplicated or bifurcated subclavius with insertions into both clavicle and coracoid process is not uncommon. The subclavius posticus is a supernumerary muscle beneath the clavicle, extending between the sternal end of the first rib and the superior border of the scapula.9 It may play a role in compression of the suprascapular nerve.10 The subclavius posticus passes anterior to the subclavian vein and the brachial plexus (▶Fig. 2.19). This explains the possible occurrence of the thoracic outlet syndrome.9

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Shoulder Girdle Ligaments Three scapular ligaments have been described: The superior transverse ligament, the inferior transverse ligament, and the coracoacromial ligament.

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Fig. 2.16 Scapulohumeral muscles. The trapezius attaches to the inner superior aspect of the spine. Fibers of the deltoid muscle attach to the outer inferior aspect of the spine. The pectoralis minor tendon attaches to medial end of the coracoid.

Fig. 2.17 Axial computed tomography (CT) scans at the level of the left scapula from superior to inferior showing scapulohumeral muscles. Note that the supraspinatus muscle lies deep to the trapezius and the serratus anterior deep to the subscapularis. Also note insertion of the pectoralis minor tendon into medial end of the coracoid. AC, acromioclavicular. (Continued)

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Fig. 2.17 (Continued)

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Fig. 2.18 Coronal computed tomography (CT) scans at the level of the left scapula from posterior to anterior showing scapulohumeral muscles. Ligaments attaching to the coracoid include the coracoacromial ligament and the two coracoclavicular ligaments (conoid and trapezoid). (Continued)

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Fig. 2.18 (Continued) (SCM, sternocleidomastoideus.)

The superior transverse scapular ligament which runs from the suprascapular notch to the base of the coracoid forms a foramen through which the suprascapular nerve courses (the nerve may become impinged when the ligament is ossified) (▶Fig.  2.13). The suprascapular artery courses over the transverse scapular ligament to supply the infraspinatus and supraspinatus muscles, while the suprascapular nerve courses below the ligament to innervate the infraspinatus and supraspinatus. The ligament ossifies in 10% of individuals, resulting in a complete osseous passage. Narrowing of the suprascapular foramen can produce a

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complete peripheral compression syndrome of the suprascapular nerve (paresis of both supraspinatus and infraspinatus muscles). The inferior transverse ligament or spinoglenoid ligament extends between the lateral margin of the base of the scapular spine and the dorsal side of the glenoid cavity. This ligament fixes the neurovascular bundle (suprascapular nerve, artery, and vein) within the spinoglenoidal notch against the neck of the scapula. Following the exit from the suprascapular foramen, the suprascapular nerve passes through the spinoglenoid notch to enter the infraspinous fossa. Compression of the suprascapular nerve

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Fig. 2.19 Sagittal computed tomography (CT) scans at the level of the left scapula from lateral to medial showing scapulohumeral muscles. Ligaments attaching the coracoid including the coracoacromial ligament and the two coracoclavicular ligaments (conoid and trapezoid). The attachments of the subclavius muscle are shown. Lateral attachments to the first rib and medial attachments to both the clavicle and scapula are seen.  (Continued)

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Fig. 2.19 (Continued)

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Fig. 2.20 Coracoid and its muscular and ligamentous attachments. The coracoid apex serves as the attachment of the coracobrachialis muscle and short head of the biceps brachii muscle. The coracoacromial ligament attaches to the tip of the coracoid and acromion. Two coracoclavicular ligaments (conoid and trapezoid) connect the superior surface of the coracoid process and the inferior surface of the clavicle.

Fig. 2.21 Superior and inferior views of the left clavicle with muscular and ligamentous attachments. SCM, sternocleidomastoideus.

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Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm Table 2.3 Scapulohumeral muscles Muscle

Origin

Insertion

Innervation

Biceps brachii

Short head: Tip of the coracoid process of the scapula; long head: supraglenoid tubercle of the scapula

Tuberosity of the radius

Musculocutaneous nerve

2

Coracobrachialis

Coracoid process of the scapula

Medial aspect of midshaft of humerus

Musculocutaneous nerve

Deltoid

Lateral one-third of the clavicle, acromion, the lower lip of the crest of the spine of the scapula

Deltoid tuberosity of the humerus

Axillary nerve

Infraspinatus

Infraspinatus fossa

Greater tubercle of the humerus (middle facet)

Suprascapular nerve

Latissimus dorsi

Vertebral spines from T7 to the sacrum, posterior third of the iliac crest, lower 3 or 4 ribs sometimes from the inferior angle of the scapula

Floor of the intertubercular groove

Thoracodorsal nerve

Levator scapulae

Transverse processess of C1—C4 vertebrae

Medial border of the scapula from the superior angle to the spine

Dorsal scapular nerve

Omohyoid (inferior belly)

Upper border of scapula

Hyoid bone

Ansa cervicalis

Pectoralis minor

Ribs 3—5

Coracoid process

Medial pectoral nerve

Rhomboid major

Spines of vertebrae T2—T5

Medial border of the scapula inferior to the spine of the scapula

Dorsal scapular nerve

Rhomoid minor

Inferior end of the ligamentum nuchae, spines of vertebrae C7 and T1

Medial border of the scapula at the root of the spine of the scapula

Dorsal scapular nerve

Serratus anterior

Upper 10 ribs

Anterior aspect of medial border of scapula

Long thoracic nerve

Subscapularis

Medial two-third of the costal surface of the scapula (subscapular fossa)

Lesser tubercle of the humerus

Upper and lower subscapular nerves

Supraspinatus

Supraspinatus fossa

Greater tubercle of the humerus (highest facet)

Suprascapular nerve

Teres major

Dorsal surface of the inferior angle of the scapula

Crest of the lesser tubercle of the humerus

Lower subscapular nerve

Teres minor

Upper two-third of the lateral border of the scapula

Greater tubercle of the humerus (lowest facet)

Axillary nerve

Trapezius

Medial third of the superior nuchal line, external occipital protuberance, ligamentum nuchae, spinous processes of vertebrae C7—T12

Lateral third of the clavicle, medial side of the acromion, and the upper crest of the scapular spine, tubercle of the scapular spine

Spinal accessory nerve

Triceps brachii long head

Infraglenoid tubercle of the scapula

Olecranon process of the ulna

Radial nerve

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Fig. 2.22 Levator scapulae muscle. The levator scapulae attaches to the transverse processes of C1–C4 and inserts on the superior angle of the scapula. Its upper part lies deep to the sternocleidomastoideus (SCM), splenius capitis, and trapezius. It forms part of the floor of the posterior triangle of the neck. Its main function is elevation and rotation of the scapula.

Fig. 2.23 Subclavius muscle. The subclavius muscle arises from the first costochondral junction, anterior to the costoclavicular ligament, and inserts laterally into the inferior clavicular groove.

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Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm under the ligament can cause dysfunction of the infraspinatus muscle only. The coracoacromial ligament extends from the tip of the acromion to the posterolateral border of the coracoid. The ligament is continuous with the short head of the biceps brachii muscle inferiorly. The costoclavicular ligament is a small rhomboid-shaped ligament between the inferior surface of the sternal end of the clavicle and the superior surface of the first costochondral junction, and medial part of the cartilage of the first rib (▶Fig. 2.18). The clavicular attachment can be very deep (▶Fig. 2.24).

Shoulder Girdle Bursae A bursa is a synovial lined sac that reduces friction at bone– tendon, bone–muscle, or tendon–tendon interfaces.11 Kuhn et al described the locations of six bursae (two anatomic and four adventitial) participating in scapulothoracic articulation12 (▶Fig. 2.25). The anatomical bursae are the primary physiologic bursae that are found in the deep layers during arthroscopy, but the adventitial bursae are not consistent and may develop as a

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result of abnormal scapulothoracic articulation. The bursae, when distended with fluid, will be easily detected with MRI, CT, and occasionally ultrasound (▶Fig. 2.26). The anatomical bursae include the scapulothoracic (infraserratus) and subscapularis (supraserratus) bursae. The scapulothoracic bursa measures 9 × 7 cm and is located between the serratus anterior and the posterolateral chest wall near the superior angle of scapula. The subscapularis bursa is a 5 × 5 cm bursa between the subscapularis and serratus anterior muscles.13 The adventitial bursae are typically found along the inferior angle of the scapula, at the superomedial border of the scapula close to the serratus anterior, or between the trapezius muscle and the medial base of the scapular spine; the latter is scapulotrapezial or trapezoid. A superficial bursa is rarely seen between the latissimus dorsi and the inferomedial angle of the scapula. Abnormal motion of the scapula against the underlying thorax or repetitive trauma are the basis for the development of scapulothoracic bursitis (▶Fig.  2.26) which causes neck or shoulder pain associated with snapping or crepitus, the so-called snapping scapula syndrome.14

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Fig. 2.24 Irregular margin of the inferior clavicles at the costoclavicular ligament attachment.

Scapulothoracic bursa (infraserratus) Scapulotrapezial bursa (trapezoid)

Subscapularis bursa

Subscapularis bursa (supraserratus)

Scapulothoracic bursa (infraserratus)

a

Scapulothoracic bursa

b

Fig. 2.25 Scapular bursae. (a) Posterior and (b) axial illustrations of the scapula showing the locations of the adjacent bursae.

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Fig. 2.26 Scapulothoracic bursitis (arrows) shown by axial T1-weighted (T1W), axial T2-weighted (T2W) with fat suppression, and sagittal T2W magnetic resonance (MR) images. This enlarged bursa is 5 × 2 cm between the serratus anterior muscle and chest wall, adjacent to the superomedial tip of the left scapula.

Neurovascular Structures A number of neurovascular structures travel around the scapulothoracic articulation. These structures may be damaged by trauma and surgical or imaging interventional procedures. The scapulothoracic vessels are described in Chapter 1 “Thoracic Wall” of Volume 1. The suprascapular nerve originates from the upper trunk of the brachial plexus (C5) and courses toward the suprascapular notch with the suprascapular artery. The suprascapular nerve passes under the superior and inferior suprascapular ligaments whereas the suprascapular artery passes above the superior suprascapular and under the inferior suprascapular ligament. The dorsal scapular nerve (C5) and artery supply the levator scapulae and rhomboid muscles. They lie deep to the rhomboids and course approximately 1 to 2 cm medial to the vertebral border of the scapula. The spinal accessory nerve, which innervates the trapezius, travels with the superficial branch of the transverse cervical artery along the central aspect of the levator scapulae and deep to the trapezius. The long thoracic nerve (C5–C7) courses along the anterior margin of the serratus anterior to innervate it (see chapter 6, brachial plexus) (▶Fig.  2.11). Nerve damage during surgery causes winging of the scapula.

Shoulder Girdle Mechanics Normal scapulothoracic motion is essential for normal shoulder function. It is influenced by several muscle forces and simultaneous supportive mechanics of the sternoclavicular, acromioclavicular, and glenohumeral joints. Normal scapular motion with respect to the curvature of hemithorax consists of a combination of three movements using the scapular YXZ Euler sequence (▶Fig. 2.27, ▶Fig. 2.28)15,16,17,18,19: •• Internal (protraction) and external (retraction) rotations around a vertical axis (Y-axis) of the thorax/scapula. Protraction is produced by gliding the scapula around the curved chest away from the vertebral column. •• Upward (elevation, anterior tilt) and downward (depression, posterior tilt) movements around a horizontal axis (scapular Z-axis) parallel to the scapular spine. In upward and anterior

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Fig. 2.27 Normal scapulothoracic motion. Normal scapular motion with respect to the curvature of hemithorax consists of a combination of three movements using the scapular YXZ axes with the Z-axis being parallel to the acromial long axis. Internal (protraction) and external (retraction) rotations occur around a vertical axis (Y-axis) of the thorax. Upward (elevation, anterior tilt) and downward (depression, posterior tilt) movements occur around the Z-axis. Lateral (abduction) and medial (adduction) rotations occur around the X-axis (pointing forward perpendicular to the scapular surface).

tilt, the scapula moves superiorly, and the inferior angle of the scapula moves away from the ribs. •• Lateral (abduction) and medial (adduction) rotations around the X-axis (pointing forward perpendicular to the scapular surface). Lateral rotation is usually defined as the movement of the glenoid fossa superiorly while the inferior angle of the scapula moves laterally (abduction). Medial rotation includes inferior movement of the glenoid with medial translocation (adduction) of the inferior angle of the scapula. The clavicle acts as a strut for the shoulder complex movements, enabling upward/downward translation of the shoulder and

Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm retraction/protraction of the scapula around the thorax. As the scapula moves on the thoracic surface, it rotates to maintain congruity with the thoracic surface while under load. Compared to neutral, with hyperextension of the arm, the scapula elevates 10 to 12 cm and its inferior angle translates laterally up to 10 to 15 cm. The scapula will also be tilted 45 to 60 degrees. The scapula is required to tilt posteriorly and externally rotate to clear the acromion from the moving arm in elevation and abduction. In general, the rotations (Euler angles) and translations of the scapula are coupled together. Therefore, protraction or forward movement is a combination of internal rotation, anterior tilt, and abduction (lateral translation). The pectoralis minor and the serratus anterior are the prime movers. Retraction or winging is the combination of scapular external rotation, posterior tilt, and adduction (medial translation) (▶Fig.  2.28). The ascending and transverse fibers of the trapezius and the rhomboids are the prime movers. The coupling of upward translation, anterior tilt, and internal rotation is seen as a shrug. In hyperextended arms, the scapula rotates upward and the primary movers are the trapezius and the serrarus anterior (▶Fig. 2.29). In posterior–anterior plain chest X-rays, in order to move the scapulae away from the lung fields, the hands are placed on the hips, the shoulders are tilted forward, and the chest pressed against the image plate while holding breath in deep inspiration (▶Fig. 2.30). During CT of chest, to avoid shoulder artifacts, the patient lies supine, with the arms placed in hyperextended position or raised above the head (▶Fig. 2.27). The scapula is elevated by the cervical fibers of the trapezius and the levator scapulae. For lateral rotation (usually with upward translation), the cervical fibers of the trapezius and

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serratus anterior are the prime movers with the rhomboids to resist it. The medial rotation is passive but in forceful situation, the rhomboids and the levator scapulae are the prime workers. Scapular dyskinesis refers to abnormal motion of the scapula during shoulder movement. It may be the underlying cause or the accompanying result of many forms of chronic shoulder pain and dysfunction. Scapular dyskinesis is a common finding in athletes and is often associated with SICK scapula syndrome represented by scapular malposition, inferior medial border prominence, and coracoid pain.

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Common Pathology A wide variety of anatomic variants as well as traumatic, neoplastic, and infectious pathologies can involve the scapula.20,21 Detection of scapular lesions using radiography can be challenging because of the obscuration by the overlying structures. In most cases, MRI or CT would be preferential techniques. Sprengel’s deformity is a congenital deformity characterized by a high scapula as an isolated finding or as part of Klippel-Feil syndrome. The scapula is small, and the inferior edge is rotated internally (▶Fig. 2.31). Scapular fractures are one of the most difficult fractures to diagnose on radiographs. The most common cause is blunt trauma during a high-energy vehicular accident. Extraarticular fractures of the coracoid process, acromion process, neck, body, and spine account for most of scapular fractures. Intraarticular fractures involving the glenoid cavity constitute 10 to 30% of all scapular fractures and may be associated with shoulder dislocation (▶Fig. 2.32).

Elevation

Depression

Retraction

Protraction

Upward rotation

Downward rotation (return to anatomical position)

Fig. 2.28 Scapular movements.

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Fig. 2.29 Posterior and lateral views of the thorax in hyperextended arm position. In this position, the scapula rotates upwardly with mild protraction while its inferior angle moves laterally in abducted position. Prime rotators for this position are the upper (cervical) and lower trapezius with serratus anterior. The opposite action will be performed by the levator scapulae, rhomboids, and pectoralis minor.

Fig. 2.30 Posterior–anterior plain chest x-rays with the arm on the patient’s side (a) and moved laterally (b). The scapula has moved away from the lung field on b. In order to move the scapulae away from the lung fields, the hands are placed on the hips, the shoulders are tilted forward, and the chest pressed against the image plate while holding breath in deep inspiration.

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Fig. 2.31 Sprengel’s deformity. (a) Anteroposterior chest radiograph showing high position of the left scapula (arrows) consistent with Sprengel’s deformity. (b) Sagittal magnetic resonance imaging (MRI) of the spine showing congenital partial fusion of the C2, C3, and C4 vertebral bodies.

Fig. 2.32 Intraarticular fracture of the left scapula involving the inferior glenoid cavity and lateral scapular border without dislocation shown by computed tomography (CT) scan.

◆◆ Arm and Forearm

Humerus

The humerus serves as the skeletal support of the arm (region of the upper extremity between the shoulder and the elbow). It is the longest bone in the upper limb and displays a triangular or deltoid appearance in cross section with anterior, lateral, and medial borders conforming to each tip of the triangle and separate anterolateral, anteromedial, and posterior surfaces (▶Fig. 2.1).

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Head The proximal humeral end takes a spheroidal configuration which articulates with the glenoid fossa of the scapula. The smooth articular surface of the humeral head is covered with hyaline cartilage which thins out toward the periphery. At any given time, only a portion of the humeral head is in direct contact with the glenoid, as the articular surface of the humeral head is larger than the adjacent glenoid cavity.

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Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm Two main tuberosities arise from the proximal humeral end (▶Fig.  2.1). The lesser tubercle (tuberosity) projects from the anterior border of the proximal humerus, just distal to the anatomical neck, and in close proximity to the spheroidal humeral head whose inferior margin delineates the medial border of the lesser tubercle. The lateral border of the lesser tubercle is defined by the medial edge of the intertubercular sulcus, a vertically oriented groove running between the lesser and greater tubercles, containing an ascending branch from the anterior circumflex humeral artery, and the long tendon and synovial sheath of the biceps (▶Fig. 2.12). The greater tubercle forms the lateral edge of the proximal humerus, protruding laterally past the margin of the acromion. The attachments for the supraspinatus, infraspinatus, and teres minor stem from three flattened impressions on the posterior– superior aspect of the greater tubercle (▶Fig. 2.13, ▶Fig. 2.33). The medial border of the greater tubercle, formed by the rough lateral edge of the intertubercular sulcus, is marked by the attachment of the bilaminar tendon of the pectoralis major (▶Fig. 2.12).

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Shaft The proximal humeral end merges with the humeral shaft via the surgical neck, a constriction immediately below the greater and lesser tubercles, in close association to the axillary nerve and posterior circumflex humeral artery (▶Fig. 2.13, ▶Fig. 2.33). The proximal shaft of the humerus, proximal to the deltoid tuberosity, demonstrates a cylindrical configuration. Distal to the deltoid tuberosity, however, the humeral diaphysis is rotated medially with respect of the proximal head, thus achieving a spiroidal

morphology of the humeral diaphysis and a near-triangular or deltoid shape in cross section (▶Fig. 2.13). This peculiar morphology gives rise to three distinct surfaces and borders which become more apparent in the mid to distal portions of the humerus. The lateral border thickens distally to form the lateral supracondylar ridge. In the mid lateral humeral shaft, the lateral border is marked by a triangular area termed the deltoid tuberosity to which the deltoid muscle attaches (▶Fig. 2.13). The medial border broadens as it extends distally and becomes the medial supracondylar ridge. The lateral margin of the proximal medial border demarcates the medial aspect of the intertubercular sulcus (bicipital groove). The medial margin of the proximal medial border extends proximally, becoming the medial calcar of the humerus just below the anatomical humeral neck (▶Fig. 2.12). The anterior border originates anterior to the greater tubercle and extends distally. The proximal anterior border demarcates the lateral margin of the intertubercular sulcus. The mid anterior border delineates the anterior aspect of the deltoid tuberosity. In between the lateral and anterior borders lies the anterolateral surface. The deltoid muscle covers the proximal, nearly featureless aspect of the anterolateral surface (▶Fig. 2.12, ▶Fig. 2.34). The anteromedial surface lies between the anterior and medial borders of the humerus. The proximal portion of this surface, just below the intertubercular sulcus, is relatively smooth and lacks any major muscle attachment. The distal portion of the anteromedial border is covered by the medial aspect of the brachialis muscle and the coracobrachialis (▶Fig. 2.12, ▶Fig. 2.34). The posterior surface, between the lateral and medial borders, is broad and relatively flat. The surface, however, becomes somewhat convex as it descends distally (▶Fig. 2.13).

Fig. 2.33 Major nerves around the scapula. The quadrilateral and triangular spaces. The axillary nerve and the posterior circumflex humeral artery pass in the quadrilateral (quadrangular) space and can be damaged in athletes. The circumflex scapular artery passes in a small triangular space between the teres major and the lateral margin of the scapula. The radial nerve passes through the lateral triangular space between the long head of the triceps and teres major into the posterior compartment of the arm. The radial artery may be trapped in this space. The medial triangular space is between the long head of the triceps and teres muscles and contains the circumflex scapular vessels.

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Fig. 2.34 Tendon attachments to the upper humeral shaft. The deltoid attaches to its tuberosity at the anterolateral surface and the coracobrachialis medial to the anterior border. The pectoris major tendon attaches close to the radial side of the long head of biceps, and the latissimus dorsi tendon and teres major to the medial side.

Distal Humerus The distal humerus articulates with the radius and ulna at the elbow joint (see Chapter 16 “Elbow Joint”). The capitelo-trochlear sulcus is a small groove that divides the articular surface of the distal humerus into the capitulum laterally and the trochlea medially (▶Fig.  2.12). As the humeral shaft extends distally, its medial border turns slightly backwards forming a blunt projection in the medial aspect of the distal humerus, the medial epicondyle. The smooth posterior surface of the medial epicondyle carries a portion of the ulnar nerve in a shallow sulcus as it traverses into the elbow (▶Fig. 2.13). The lateral edge of the humerus terminates in the lateral epicondyle, which occupies the nonarticular aspect of the distal humeral condyle. The upper aspect of the lateral epicondyle gives rise to the lateral supracondylar ridge. The superficial group of extensor muscles of the forearm create a distinct osseous impression in the lateral and anterior surfaces of the lateral epicondyle, at their insertion site (▶Fig. 2.13). The capitulum is a hemi-spherical projection forming the anterior and inferior surfaces of the lateral aspect of the articular distal humerus. The capitulum articulates with the radial head. The trochlea is a grooved structure similar to a pulley, which forms

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the medial articular surface of the distal humerus. The trochlea forms the medial anterior, medial inferior, and medial posterior surfaces of the distal humeral condyle which articulate with the trochlear notch of the ulna. Immediately superior to the trochlea, on the posterior surface of the distal humeral condyle, a deep hollow region forms the olecranon fossa, which houses the olecranon when the elbow is extended. The coronoid fossa is a similar hollow region in the anterior surface of the distal humerus just above the trochlea. When the elbow is flexed, the coronoid fossa lodges the anterior aspect of the coronoid process.

Radius The radius is long, paired, asymmetrical bone constituting the framework for the lateral structures of the forearm (▶Fig.  2.35, ▶Fig. 2.36). The radial head is a broad, rounded cylindrical structure in the uppermost margin of the radius. The surface of the radial head demonstrates a central concavity termed the fovea articularis radii which articulates with the capitulum of the humerus (see Chapter 16 “Elbow Joint”). The radial head is commonly fractured in adults following trauma to the elbow, often identified on plain radiograph by direct visualization of the

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Fig. 2.35 Radius and ulna.

fracture line, or by displacement of the posterior fat pad. The radial head is supported by a narrow portion of the proximal radius, the radial neck. The radial tuberosity is a bony, ovoid, protuberance extruding from the anterior medial margin of the radial epiphysis just inferior to the radial neck. The posterior rough surface of the radial tuberosity serves as the tendinous insertion site for the biceps brachii muscle. Distally, the radius becomes increasingly voluminous and resembles a truncated quadrilateral pyramid with two palpable regions: the styloid process laterally and Lister’s tubercle dorsally. The distal radius articulates with the adjacent ulna, scaphoid, and lunate (▶Fig. 2.35, ▶Fig. 2.36). Laterally, the articular surface for the carpus displays a triangular and concave morphology. This surface further subdivides into a medial quadrilateral articular surface for the lunate, and a lateral triangular articular surface of the scaphoid (▶Fig. 2.36). Medially, the sigmoid notch, a narrow concave surface, makes the articular surface for the ulnar head. Three distinct nonarticular surfaces also arise from the distal end of the radius. A volar surface serves as attachment for the palmar radiocarpal ligament. The dorsal surface serves as attachment for the dorsal radiocarpal ligament. The lateral surface gives rise to the styloid process of the radius, which attaches the brachioradialis tendon at its base and the radial collateral ligament at its apex (▶Fig. 2.37).

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Fig. 2.36 Articular surface of the distal radius with the lunate and scaphoid. Medial, quadrilateral, and lateral triangular surfaces of the distal radius are seen. The sigmoid notch is a shallow articular surface in the medial side of the distal radius for the ulnar head, which is convex proximally and concave distally, where it meets the lunate facet.

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Fig. 2.37 Radius and ulna muscle attachments.

Similar to the humeral shaft, the radial shaft has a triangular morphology when seen in cross section, with three distinct surfaces and borders. An interosseous membrane originating along the distal threequarters of the radial shaft attaches the interosseous margins of the body of the radius to the adjacent ulna and separates the anterior and the posterior compartments of the forearm (▶Fig. 2.35). The membrane is not continuous at proximal and distal ends and is perforated by posterior and anterior interosseous vessels. The proximal fibers of the membrane form a strong oblique band known as oblique cord. The oblique cord arises from the tubercle of the radius and ends at the proximal ulna.

Borders •• The volar/anterior border separates the volar/anterior and lateral surfaces of the body of the radius. It extends from the inferior margin of the radial tuberosity to the superior–anterior aspect of the radial styloid process (▶Fig. 2.35). The flexor digitorum superficialis and flexor pollicis longus muscles originate from the superior half of the volar/anterior border of the radius (▶Fig. 2.37). •• The dorsal/posterior border separates the lateral and posterior surfaces. The dorsal/posterior border is obtuse and relatively indistinct, predominantly at its proximal and distal ends. It

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extends from the posterior aspect of the radial neck to the posterior margin of the base of the radial styloid. •• The interosseous border is relatively smooth proximally near the radial tuberosity and distally, but rather sharp in its mid portion.

Surfaces •• The volar/anterior surface is a relatively smooth, concave surface predominantly proximally and medially. The surface broadens and flattens distally. The upper half gives origin to the flexor digitorum superficialis and flexor pollicis longus muscles. The distal aspect serves as the insertion for the pronator quadratus muscle. •• The dorsal/posterior surface is predominantly flat with slight proximal and distal convexities. The supinator muscle attaches proximally. The more concave middle third gives rise to the abductor pollicis longus and extensor pollicis brevis muscles (▶Fig. 2.37, ▶Fig. 2.38). •• The lateral surface is convex along its course. Its upper segment is rounded and convex and serves as the insertion for the supinator muscle. The mid portion of the lateral surface is a ragged surface, where the pronator teres muscle inserts. The distal segment is narrow and smooth and in close approximation with tendons of the abductor pollicis longus and extensor pollicis brevis which course over its surface.

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Fig. 2.38 Axial cross-sectional views at the mid-arm level showing the triangular shape of the shaft of the ulna and radius. The interosseous membrane (arrows) attaches to the interosseous border.

Ulna The ulna is a long bone which parallels the radius. The broad hook-like proximal end of the ulna articulates with the distal humerus. Its distal narrow end articulates with the radius and carpal bones of the wrist (▶Fig. 2.35, ▶Fig. 2.36, ▶Fig. 2.37, ▶Fig. 2.38). The proximal ulna possesses two curved bony prominences: the olecranon and the coronoid process. The semilunar (or trochlear) notch and radial notch are two proximal cavities serving as articular surfaces for the distal humerus and radius, respectively (see Chapter 16 “Elbow Joint”).

Olecranon The olecranon is a voluminous apophysis in the posterior–superior aspect of the proximal ulna resembling a prism with a quadrilateral base. The posterior aspect of the olecranon is a triangular smooth surface covered by a bursa. Anteriorly, it demonstrates a

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smooth, concave depression merging into the upper segment of the semilunar notch. The rough, quadrilateral superior surface of the olecranon serves as the insertion point for the triceps brachii and attachment for part of the posterior ligament. The medial surface of the olecranon serves as attachment for the posterior and oblique bands of the ulnar collateral ligament, and as attachment for the flexor carpi ulnaris. The anconeous muscle attaches to its lateral surface (▶Fig. 2.37).

Coronoid Process It is a triangular bony projection extending anteriorly and slightly superiorly in relation to the ulnar shaft. Its smooth, concave superior surface forms the inferior portion of the semilunar notch. The coronoid process is composed of tip, body, anterolateral facet, and anteromedial facet. The brachialis muscle inserts into a rough bony imprint in the anterior–inferior surface of the coronoid process and further distally at the tuberosity of the ulna at the

Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm junction of the anterior surface of the coronoid process with anterior ulnar shaft. Its lateral surface presents the radial notch, a narrow oval articular depression for the medial aspect of the adjacent radial head. An occasionally present ulnar head of the flexor pollicis longus may attach to the lateral, or on rare occasions to the medial, surface (▶Fig. 2.37). The sublime tubercle, on the anteromedial facet, is the insertion for the anterior bundle of the medial collateral ligament. The flexor digitorum superficialis originates from the anterior margin of the medial surface of the coronoid process. The flexor digitorum profundus and pronator teres also originate from the medial surface of the coronoid process.

Shaft The ulnar shaft conforms to a triangular pyramidal shape for most of its proximal and mid portions. The distal end of the ulnar shaft forms an irregular cylindrical shape. The ulnar shaft possesses three surfaces and borders (▶Fig. 2.38). The distal ulna is slightly expanded consisting of the ulnar head and the ulnar styloid process. The lateral convex surface of the ulnar head articulates with the radial sigmoid notch, while a more downward directed surface articulates with an articular disc (triangular fibrocartilage complex), which separates the head of the ulna from the carpus. The styloid process of the ulna is a rounded osseous projection extending from the medial posterior aspect of the distal ulna where the ulnar collateral ligament attaches. Morphological variation of the styloid process of the ulna is common including elongated process, angulated, hypertrophic, and unfused separate ossification center (▶Fig.  2.38). Variation in the relative lengths of the forearm bones does exist. In normal ulnar-neutral variance, the distal cortical surface of the ulnar head is leveled with the cortical surface of the most proximal aspect of the lunate fossa. Shortening of the distal ulna by >2.5 mm is called negative ulnar variance and may be associated with avascular necrosis of the lunate (▶Fig.  2.39). In negative ulnar variance, the ulnar head is cone-shaped. Positive ulnar variance may predispose the joint to early degeneration. In a normal wrist with neutral ulnar variance, 82% of the stress is transferred

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to the radiocarpal joint and 18% to the ulnocarpal joint. If the ulna is 2.5 mm longer, the ulnocarpal joint receives 42% more stress. Ulnar shortening is the most widely used method for treating ulnar impaction syndrome.22

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Borders The interosseous border is a lateral crest which arises from the lateral edge of the radial notch and serves as the origin for the supinator. The interosseous membrane attaches to the interosseous border (▶Fig. 2.38). The anterior border extends from the medial aspect of the ­coronoid process to the base of the styloid process. The upper three-fourth and lower one-fourth of the anterior border serve as origins for the flexor digitorum profundus and pronator quadratus muscles, respectively (▶Fig. 2.37). The posterior border originates as a confluence of the medial and lateral borders of the olecranon and extends distally to the base of the styloid process. The aponeurosis for the flexor carpi ulnaris, extensor carpi ulnaris, and flexor digitorum profundus attach to the upper three-fourth of the posterior border.

Surfaces The anterior surface lies between the anterior and interosseous borders. It is concave along its course and narrows distally. An obliquely oriented ridge which serves as the origin for the pronator quadratus separates the lower one-fourth from the proximal three-fourth of the anterior surface which gives origin to the flexor digitorum profundus. An upward obliquely oriented nutrient canal is found proximal to its midpoint. The medial surface of the ulnar shaft is convex and smooth and lies between the anterior and posterior borders. One of the origins for the flexor digitorum profundus can be found in its upper three-fourths. The posterior surface lies between the interosseous and posterior borders and is divided into three distinct areas. In its most proximal portion, there is faint ridge stemming from posterior dorsal aspect of the radial notch and running obliquely toward

Fig. 2.39 (a) Unfused styloid process of the ulna (blue arrow). Note normal ulnar-neutral variance in which the distal cortical surface of the ulnar head is leveled with the cortical surface of the most proximal aspect of the lunate fossa. (b) Negative ulnar variance (arrow). In negative ulnar variance, the ulnar head is cone-shaped and small. (c) Fracture of the distal radius metaphysis (yellow arrow) and styloid process of the ulna (green arrow).

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Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm the posterior border of the ulnar shaft. The anconeus inserts on a surface above the aforementioned ridge. The supinator muscle attaches along the upper margin of the ridge. Below the ridge, the posterior surface of the shaft is further divided by a vertical bony ridge into a medial surface and a lateral surface. The lateral surface gives origin to the supinator, extensor pollicis longus, and abductor pollicis longus.

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Biomechanics of the Arm and Forearm Function of the glenohumeral, elbow, and wrist joints is described in Chapter 16 “Elbow Joint.” The human forearm is uniquely adapted to bear significant load. Pronation and supination are main rotational motions of the forearm.23,24 Pronation/supination motions provided by the pronator teres, pronator quadratus, and supinator muscles allow for approximately 180 degrees rotation. Rotation of the forearm accompanied by the rotation of the shoulder permits for 270 degrees of rotation of the upper limb. The mechanical bases for pronation/supination are the radius and ulna and two coupled trochoid joints—the proximal and distal radioulnar joints which function as a single “forearm joint,” resulting in pronation/supination. The stability of the proximal radioulnar joint is maintained by a strong osteoligamentous cavity that is made of the radial notch of the ulna and the annular ligament. The distal radioulnar joint consists of the radioulnar joint and the triangular fibrocartilage complex. The distal radioulnar articulation occurs at the sigmoid notch. The triangular fibrocartilage complex consists of the dorsal and palmar radioulnar ligaments, the meniscus homologue, and the articular disc. The interosseous membrane, which includes the body, a thicker central band, and the distal and proximal oblique cords, maintains the stability of the forearm by preventing proximal migration of the radius.25 The ulna is the primary “load-bearing” bone of the forearm and supports the distal radius as it rotates around the ulna. From pronation to supination, the radius rotates around the ulnar head about a longitudinal axis that extends from the center of the radial head to the fovea of the ulnar head. In neutral position, the styloid process of the ulna is aligned with the sigmoid notch of the radioulnar joint. In pronation, the ulnar head rotates to roll the styloid process to the palmar side. In supination, the styloid process moves to the dorsal side (▶Fig. 2.40). The radius migrates proximally with forearm pronation, which increases ulnar variance. In ulnar-positive variance, pronation increases the impaction of the ulnar head to the lunate and triquetrum. During pronation/supination, the axes of the proximal and distal radioulnar joints must be aligned coaxially; otherwise, the forearm rotation is blocked.

Common Pathologies Elbow fractures are common in children. The most common fractures are supracondylar humerus fractures and radial neck fractures (▶Fig.  2.10). Little leaguer’s elbow was initially described as an avulsion of the medial epicondyle, but the term is now expanded to describe several injuries incurred in various sports1 (▶Fig. 2.6). The distal radius and ulna are common sites of injury. In the distal metaphysis of the radius, buckle fractures (torus

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a

Neutral

Pronation

Palmar

Dorsal

Dorsal

Palmar

Palmar

b

Pronation

Dorsal Midrotation

Supination

Fig. 2.40 (a) The radius rotates around the ulnar head about a longitudinal axis between the center of the radial head and the fovea of the ulnar head. (b) In neutral position, the styloid process of the ulna is aligned with the sigmoid notch of the radioulnar joint. In pronation, the ulnar head rotates to roll the styloid process to the palmar side. In supination, the styloid process moves to the dorsal side.

fractures) and complete fractures are the most common fractures in children. In children, metaphyseal fractures occur at the transition of the dense lamellar bone of the diaphysis to the porous metaphyseal bone. This transition moves distally with age. Therefore, in older children the fractures occur closer to the physis. Fractures of the distal radius are commonly associated with fracture of the styloid process of the ulna. It is not uncommon to find an unfused styloid process ossification center that should not be mistaken for a fracture. Upper extremity arterial injuries after trauma are relatively uncommon.26,27 The axillary artery can be damaged in high-­ energy injuries causing fracture and dislocation of the shoulder girdle. In addition, the signs of ischemia are usually mild because there is good collateral flow to the upper limb. These patients usually have subtle signs such as reduced arterial pulse volumes and capillary refill, described as “pink pulseless” hands. Professional athletes, such as volleyball players, are at risk of distal arterial emboli arising from an aneurysmal and thrombosed

Upper Extremity Bones: Shoulder Girdle, Arm, and Forearm proximal posterior circumflex humeral artery in the dominant shoulder.28 The posterior circumflex humeral artery is a relatively small branch originating from the third part of the axillary artery and is usually damaged in the quadrilateral space (▶Fig. 2.33). Humeral shaft fractures account for 3 to 5% of all skeletal fractures and 20% of all humeral fractures. Radial nerve palsy associated with humeral shaft fractures is the most common nerve lesion complicating fractures of long bones and occurs in 11%.29,30 The symptoms vary from sensory loss of the dorsolateral aspect of the hand to complete paralysis of the brachioradialis muscle and the extensor muscles of the wrist and fingers.

14. Frank RM, Ramirez J, Chalmers PN, McCormick FM, Romeo AA. Scapulothoracic anatomy and snapping scapula syndrome. Anat Res Int 2013;2013:635628 15. Fayad F, Hoffmann G, Hanneton S, et al. 3-D scapular kinematics during arm elevation: effect of motion velocity. Clin Biomech (Bristol, Avon) 2006;21(9): 932–941

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16. Kibler WB, Sciascia A, Wilkes T. Scapular dyskinesis and its relation to shoulder injury. J Am Acad Orthop Surg 2012;20(6):364–372 17. Preziosi Standoli J, Fratalocchi F, Candela V, et al. Scapular dyskinesis in young, asymptomatic elite swimmers. Orthop J Sports Med 2018;6(1): 2325967117750814 18. Bolsterlee B, Veeger DHEJ, Chadwick EK. Clinical applications of musculoskeletal modelling for the shoulder and upper limb. Med Biol Eng Comput 2013;51(9):953–963 19. Wu G, van der Helm FC, Veeger HE, et al; International Society of Biomechanics.

References 1. Delgado J, Jaramillo D, Chauvin NA. Imaging the injured pediatric athlete: upper extremity. Radiographics 2016;36(6):1672–1687 2. Hita-Contreras F, Sánchez-Montesinos I, Martínez-Amat A, Cruz-Díaz D, Barranco RJ, Roda O. Development of the human shoulder joint during the embryonic and early fetal stages: anatomical considerations for clinical practice. J Anat 2018;232(3):422–430 3. Laor T, Jaramillo D. MR imaging insights into skeletal maturation: what is normal? Radiology 2009;250(1):28–38 4. Zember JS, Rosenberg ZS, Kwong S, Kothary SP, Bedoya MA. Normal skeletal maturation and imaging pitfalls in the pediatric shoulder. Radiographics 2015;35(4):1108–1122 5. Prescher A. Anatomical basics, variations, and degenerative changes of the shoulder joint and shoulder girdle. Eur J Radiol 2000;35(2):88–102 6. Bigliani LU, Ticker JB, Flatow EL, Soslowsky LJ, Mow VC. The relationship of acromial architecture to rotator cuff disease. Clin Sports Med 1991;10(4):823–838 7. dos Santos LF. The vascular anatomy and dissection of the free scapular flap. Plast Reconstr Surg 1984;73(4):599–604 8. Frank DK, Wenk E, Stern JC, Gottlieb RD, Moscatello AL. A cadaveric study of the motor nerves to the levator scapulae muscle. Otolaryngol Head Neck Surg 1997;117(6):671–680 9. Smayra T, Nabhane L, Tabet G, Menassa-Moussa L, Hachem K, Haddad-Zebouni S. The subclavius posticus muscle: an unusual cause of thoracic outlet syndrome. Surg Radiol Anat 2014;36(7):725–728 10. Cogar AC, Johnsen PH, Potter HG, Wolfe SW. Subclavius posticus: an anomalous muscle in association with suprascapular nerve compression in an athlete. Hand (NY) 2015;10(1):76–79 11. Hirji Z, Hunjun JS, Choudur HN. Imaging of the bursae. J Clin Imaging Sci 2011;1:22 12. Kuhn JE, Plancher KD, Hawkins RJ. Symptomatic scapulothoracic crepitus and bursitis. J Am Acad Orthop Surg 1998;6(5):267–273 13. Osias W, Matcuk GR Jr, Skalski MR, et al. Scapulothoracic pathology: review of

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ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion—Part II: shoulder, elbow, wrist and hand. J Biomech 2005;38(5):981–992 20. Mazaheri P, Fayad LM, Fishman EK, Demehri S. Advanced imaging of the scapula: what every radiologist needs to know. J Comput Assist Tomogr 2016;40(4):567–575 21. Ropp AM, Davis DL. Scapular fractures: what radiologists need to know. AJR Am J Roentgenol 2015;205(3):491–501 22. Cha SM, Shin HD, Kim KC. Positive or negative ulnar variance after ulnar shortening for ulnar impaction syndrome: a retrospective study. Clin Orthop Surg 2012;4(3):216–220 PubMed 23. Lees VC. The functional anatomy of forearm rotation. J Hand Microsurg 2009;1(2):92–99 24. Ibáñez-Gimeno P, Jordana X, Manyosa J, Malgosa A, Galtés I. 3D analysis of the forearm rotational efficiency variation in humans. Anat Rec (Hoboken) 2012;295(7):1092–1100 25. Matthias R, Wright TW. Interosseous membrane of the forearm. J Wrist Surg 2016;5(3):188–193 26. Franz RW, Skytta CK, Shah KJ, Hartman JF, Wright ML. A five-year review of management of upper-extremity arterial injuries at an urban level I trauma center. Ann Vasc Surg 2012;26(5):655–664 27. Hafiz S, Zubowicz EA, Abouassaly C, Ricotta JJ, Sava JA. Extremity vascular injury management: good outcomes using selective referral to vascular surgeons. Am Surg 2018;84(1):140–143 28. Rollo J, Rigberg D, Gelabert H. Vascular quadrilateral space syndrome in 3 overhead throwing athletes: an underdiagnosed cause of digital ischemia. Ann Vasc Surg 2017;42:63.e1–63.e6 29. Chang G, Ilyas AM. Radial nerve palsy after humeral shaft fractures: the case for early exploration and a new classification to guide treatment and prognosis. Hand Clin 2018;34(1):105–112 30. Theeuwes HP, van der Ende B, Potters JW, Kerver AJ, Bessems JHJM, Kleinrensink GJ. The course of the radial nerve in the distal humerus: a novel, anatomy based, radiographic assessment. PLoS One 2017;12(10):e0186890

anatomy, pathophysiology, imaging findings, and an approach to management. Skeletal Radiol 2018;47(2):161–171

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3 Muscles of Shoulder Girdle, Arm, and Forearm Farhood Saremi 3

◆◆ Introduction

Skeletal muscles connect one bone, across a joint (or joints), to another bone. Muscles stabilize bones and approximate their sites of attachment. All skeletal muscles have a distinct origin and a precise insertion site where the muscle is attached to the bones. The origin is usually broader than the insertion site. This design is believed to focus the force of muscle contraction at the tendon insertion rather than at the broad-based proximal origin.1 An aponeurotic plane of fascial tissue (epimysium) separates one muscle from another, allowing the muscle to move freely from other muscles. Each muscle has a distinct nerve innervation that allows it to be stimulated and contracted independently. A vascular network of arteries and veins also supplies each muscle. Muscular nerves and vessels frequently travel together in a neurovascular bundle in the fascial planes prior to entering the muscle belly and distributing their terminal branches to the muscle fibers and overlying skin. In the extremities, each skeletal muscle has a distinct function in producing a primary movement, and is paired with an antagonistic muscle that produces an opposing movement. All extremity joint and muscular forces are interconnected and weakness of any muscle or joint can affect the entire connection. Therefore, any underlying isolated muscular pathology needs to be addressed to prevent further injury.1 Variants of the extremity muscles are common and can be categorized into three groups, namely, progressive, retrogressive, and atavistic.2,3 The progressive type is defined for the muscle variants with a tendency to become increasingly complex. The deep flexor muscles of the forearm belong to the progressive group of variations. In the retrogressive type, muscles undergo degeneration and lose their functions. Examples of this type are the palmaris longus and the plantaris muscles. The atavistic muscles are the muscular elements which have disappeared in the course of evolution but may reappear. The axillary arch muscle, a remnant of the panniculus carnosus, is an example of an atavistic muscle.4 In radiology, muscles can be imaged using different modalities. Normal muscles are not well differentiated on plain X-ray. Computed tomography (CT) and magnetic resonance (MR) are the best modalities for the assessment of these muscles. Ultrasonography is commonly used for the assessment of superficial tendons. All tendons are best evaluated with magnetic resonance imaging (MRI). Using special MR sequences, tendons appear dark and can be easily differentiated from one another and from their parent muscles. In this chapter, an effort has been made to show the anatomy of the extremity muscles by color-coded CT images. MR views are presented to show details of the tendinous attachments.

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◆◆ Shoulder Muscles

Shoulder region muscles include muscles arising from the upper thorax and scapula and inserting onto the humeral head and proximal humeral shaft. The supraspinatus, infraspinatus, subscapularis, and teres minor muscle tendons form the rotator cuff (see Chapter 15 “Anatomy of the Shoulder Joint”). All rotator cuff muscles act together to give stability to the shoulder joint by securing the humeral head in the glenoid fossa. The teres major muscle is a scapulohumeral muscle. Other muscles of the shoulder include the deltoid, pectoralis major, and latissimus dorsi. The superior aspect of the shoulder is mainly formed by the deltoid muscle and the inferior aspect by the axilla. The axilla region is a pyramidal area between the upper thoracic wall and the arm. The posterior axillary border (fold) is formed by the latissimus dorsi and teres major muscles, and the anterior border is formed by the distal margin of the pectoralis major (▶Fig. 3.1). It contains axillary vessels, cords and branches of brachial plexus, lymph nodes, loose areolar tissue with fat, and axillary tail of breast in many instances. A number of accessory muscle slips (also known as “axillary arch”) may arise from the lateral border of pectoralis major or latissimus dorsi muscles with variable insertions to the pectoralis muscle, coracobrachialis, or biceps brachii tendons.4 These accessory slips may cross over the axillary vessels and nerves (▶Fig. 3.1).

Deltoid Muscle The deltoid muscle is divided into three anatomical parts: anterior, middle, and posterior.5 The anterior deltoid arises widely from the anterior border and superior surface of the lateral-third of the clavicle and the superior surface and the anterior-third of the lateral acromion. The middle deltoid is relatively narrow and originates from the mid-third of the lateral margin of the acromion. The posterior part is attached to the posterior-third of the lateral acromion and the scapular spine (▶Fig. 3.2, ▶Fig. 3.3, ▶Fig. 3.4). It is believed that the activation pattern of these three portions is different during shoulder motion.6,7 The deltoid muscle inserts on the deltoid tuberosity on the midshaft of the humerus. Tears of the deltoid muscle, or avulsion of the tendon from the acromial origin associated with rotator cuff tears, is rare. However, rupture or dehiscence of the deltoid muscle at the acromial origin is a well-recognized complication following open rotator cuff repair or open acromioplasty.8

Latissimus Dorsi Muscle The latissimus dorsi is a large, flat, dorsolateral muscle on the trunk, which extends lateral to the arm. Medially, it is partly covered by the trapezius. The latissimus dorsi originates from the

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Fig. 3.1 Shoulder region. The superior aspect of the shoulder is mainly formed by the deltoid muscle. The posterior axillary fold is formed by the latissimus dorsi and teres major muscles, and the anterior axillary fold is formed by the distal margin of the pectoralis major. Note the accessory muscle slip arising from the latissimus dorsi muscle extending toward the short biceps brachii tendon.

Fig. 3.2 Deltoid muscle. The deltoid muscle is divided into three anatomical portions: anterior, middle, and posterior. The anterior deltoid arises widely from the outer-third of the clavicle and the anterior aspect of the acromion. The middle deltoid is relatively narrow and originates from the mid-acromion. The posterior part is attached to the posterior margin of the acromion and the scapular spine.

spinous processes of thoracic T7–T12 vertebrae, thoracolumbar fascia, iliac crest and inferior three or four ribs, and inferior angle of scapula. The muscle tendon inserts on the floor of intertubercular groove of the humerus (▶Fig. 3.4). The posterior humeral circumflex artery may pass under or close to the tendon of the

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latissimus dorsi and may be injured by muscle contractions. Anterior and posterior muscle slips, found in 2% of cases, usually extend to pectoralis major or teres major9 (▶Fig. 3.1, ▶Fig. 3.5). The latissimus dorsi is involved in extension, adduction, horizontal abduction, flexion from an extended position, and internal

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Fig. 3.3 Deltoid muscle compartments. Axial (upper row, inferior to superior) and sagittal (lower row, lateral to medial), fat-suppressed T2-weighted images showing muscle tendons within the deltoid converging toward the acromioclavicular junction and scapular spine. H, humerus; HH, humeral head.

rotation of the shoulder joint. It also has a synergistic role in extension and lateral flexion of the lumbar spine. The latissimus dorsi has been used for rotator cuff repair. A tight latissimus dorsi may be one cause of chronic shoulder and back pain.

Pectoralis Major Muscle The pectoralis major is a broad and triangular muscle at the anterior superior chest wall. The pectoralis major has a complex morphology with two major segments. The motor nerves arise from the medial and lateral pectoral nerves. The proximal segment is subdivided into the clavicular and manubrial parts (lamina), and the distal segment into the sternocostal, costal, and abdominal parts10,11 (▶Fig. 3.5, ▶Fig. 3.6). The clavicular part arises from the medial half of the clavicle and the manubrial part from the manubrium sterni and the medial aspect of the first and second ribs. The sternocostal part originates from the sternal body and medial aspect of the third to sixth ribs. The abdominal part is a separate layer that is located dorsal to the costal part, but occasionally originates from the aponeurosis of the external oblique abdominal muscle.

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The insertion tendon has a bilaminar U shape and attaches to the anterior surface of the humeral diaphysis, just lateral to the biceps brachii long head tendon (▶Fig. 3.5).10,11 The length of the tendon footprint on the bone is approximately 4 to 6 cm. The clavicular and manubrial parts insert into the anterior limb of the U, the sternocostal part inserts into the inferior cup of the tendon, and the costal and abdominal parts insert into the posterior limb of the tendon.10 This configuration is analogous to the appearance of an unfolded hand-held fan11 (▶Fig. 3.6). A small fat pad is interposed between the two limbs of the tendon (▶Fig. 3.7). Each part of the muscle has a distinct vascular and nerve supply that run in the epimysium on the posterior surface of the pectoralis major muscle. The muscle is supplied by the thoracoacromial artery and the lateral thoracic artery (branches of the axillary artery). The main function of the pectoralis major is to adduct, internally rotate, and flex the humerus. Rupture of pectoralis major muscle is uncommon. The muscle is at greatest risk when the shoulder is in abducted, extended, and externally rotated position with maximal muscle tension.12 Poland’s syndrome is a rare congenital anomaly characterized by unilateral absence of the sternal portion of the pectoralis major muscle and sometimes ipsilateral symbrachydactyly (abnormally short and webbed fingers).13

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Fig. 3.4 Axial, color-coded computed tomography (CT) images from superior to inferior showing the relative anatomic position and insertions of the deltoid, pectoralis major, teres major, and latissimus dorsi muscles to the proximal humerus in an arm extended above shoulder. The insertion tendon of the pectoralis major attaches to the anterior surface of the humeral diaphysis, just lateral to the bicipital groove. Tendon insertions of the teres major and latissimus dorsi along the medial side of the bicipital groove are also shown. In this case an accessory muscle slip was seen extending from the axillary margin of the latissimus dorsi toward the short head of biceps. Deltoid muscle inserts on the deltoid tuberosity on the midshaft of the humerus.

Supraspinatus and Infraspinatus Muscles The superior and posterosuperior parts of the rotator cuff ­consist of the tendons of the supraspinatus and infraspinatus muscles as well as the underlying articular capsule (▶Fig. 3.8, ▶Fig. 3.9). The supraspinatus muscle originates from the

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supraspinous fossa and the superior surface of the scapular spine. It inserts onto the superiormost impression of the greater tuberosity of the humerus. The supraspinatus muscle consists of a large anterior and a smaller posterior part.14 The infraspinatus muscle originates from the infraspinous fossa and the inferior surface of the scapular spine, and inserts into the highest and middle impressions (facets) of the greater tuberosity

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Fig. 3.5 Pectoralis major attachments (extended arm views). The origins from the distal-third of clavicle and sternum are seen in the top image. The insertion tendon attaches to the anterior surface of the humeral diaphysis, just lateral to the bicipital groove. Tendon insertions of the teres major and latissimus dorsi along the medial aspect of the bicipital groove are also shown. An accessory muscle slip of the latissimus dorsi to the short head of biceps is seen.

Clavicular

Biceps, long tendon

Sternal

Posterior Humerus Biceps, short tendon

Anterior

Biceps, long tendon

Fig. 3.6 Normal left distal pectoralis major tendon. The clavicular and sternal heads of pectoralis major form anterior and posterior layers respectively. The two layers are fused inferiorly (U-shaped).

(▶Fig. 3.8, ▶Fig. 3.9). The anteriormost part of the infraspinatus tendon insertion is lateral to the insertion site of the supraspinatus muscle. Most of the time, the infraspinatus muscle fibers are blended with the teres minor muscle fibers, and together, they insert on the greater tuberosity of the humerus. In some cases, a fibrous septum arising from the dorsal surface of the scapula may separate the two muscles. Some describe the infraspinatus as being composed of two parts: a large oblique part, which originates from the infraspinous fossa, and a superior and transverse part, which originates from the inferior surface of the scapular spine.14,15 The infraspinatus muscle is also described as a muscle with two or three bellies with the superior one or two muscle

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bellies representing the infraspinatus muscle and the inferior belly representing the teres minor muscle.16 The supraspinatus and infraspinatus muscles are innervated by branches of the suprascapular nerve. The suprascapular nerve passes through the suprascapular foramen formed by the suprascapular notch and the superior transverse scapular ligament. The supraspinatus muscle performs about 60% of the elevation– abduction motion of the arm. The infraspinatus and teres minor play the main role in external rotation of the arm. Atrophy of the infraspinatus is very frequent in patients with rotator cuff tears. Its atrophy may also be seen in compression and/or entrapment of the suprascapular nerve by the superior transverse scapular ligament or a regional ganglion.

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Fig. 3.7 Axial computed tomography (CT) images at four levels from top to bottom showing the components of the pectoralis muscle and its tendon insertion to the humerus just lateral to the bicipital groove for long head of biceps tendon. The V-shaped connection of the pectoralis tendons is shown in the inlay images (dashed yellow lines). The two layers of the tendon are fused prior to insertion on the humerus. The insertion point is not separable from the long tendon of the biceps in this example. Pectoralis muscle is shown in yellow.

Fig. 3.8 Rotator cuff muscles. The supraspinatus, infraspinatus, subscapularis, and teres minor muscle tendons form the rotator cuff. The supraspinatus muscle originates from the supraspinous fossa and the superior surface of the scapular spine. It inserts into the superiormost impression (facet) of the greater tuberosity of the humerus. The infraspinatus muscle originates from the infraspinous fossa and the inferior surface of the scapular spine, and inserts onto the highest and middle impressions (facets) of the greater tuberosity. The infraspinatus muscle is usually blended with the teres minor muscle and together, they insert on the greater tuberosity of the humerus. The subscapularis muscle is the largest and most powerful rotator cuff muscle. It arises from the anterior surface of the scapula and inserts on the lesser tuberosity and surgical neck of the humerus. The teres major muscle is a scapulohumeral muscle. It originates from the lower third of the lateral margin of the scapula, near its inferior angle and inserts onto the medial aspect of the humeral metaphysis near the insertion of the latissimus dorsi.

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Fig. 3.9 Rotator cuff muscles and tendons. Axial, color-coded computed tomography (CT) images in an extended arm from superior to inferior showing the rotator cuff muscles. The supraspinatus muscle originates from the supraspinous fossa and inserts into the superior impression of the greater tuberosity of the humerus. The infraspinatus muscle originates from the infraspinous fossa of the scapula, and inserts onto the highest and middle impressions (facets) of the greater tuberosity. The infraspinatus muscle is usually blended with the teres minor muscle and together insert on the greater tuberosity. The subscapularis muscle is the largest rotator cuff muscle. It arises from the anterior surface of the scapula and inserts onto the lesser tuberosity and surgical neck of the humerus. The teres major muscle is a scapulohumeral muscle. It originates from the lower third of the lateral margin of the scapula, near its inferior angle, and inserts onto the medial aspect of the humeral metaphysis near insertion of the latissimus dorsi.

Teres Minor Muscle The teres minor muscle arises from the axillary border of scapula posteriorly and can be separated from the infraspinatus and teres major muscles by aponeurotic membranes. The superior fiber of the muscle inserts onto the lowest impressions of the greater tuberosity of the humerus and inferior fibers onto the humeral

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metaphysis immediately below the superior insertion (▶Fig. 3.8, ▶Fig. 3.9). The teres minor functions primarily to externally rotate the arm and secondarily to depress the humeral head. The teres minor muscle provides 20 to 45% of the external rotation power to the glenohumeral joint especially after severe rotator cuff tears. Isolated tear of the teres minor tendon is rare, and the tendon

Muscles of Shoulder Girdle, Arm, and Forearm usually remains intact even in severe rotator cuff tears. In the clinical evaluation and in the diagnosis of rotator cuff lesions, the teres minor and the infraspinatus are not easily differentiated. Compensatory hypertrophy of the teres minor can occur in rotator cuff, subscapularis, or infraspinatus tendon tears to restore external rotation strength and stabilize shoulder function.17,18

Teres Major Muscle The teres major muscle originates from the lower third of the lateral margin of the scapula, near its inferior angle, and inserts into the medial lip of bicipital groove of the humerus near insertion of the latissimus dorsi (▶Fig. 3.8, ▶Fig. 3.9). The teres major, in cooperation with latissimus dorsi, adducts and medially rotates arm, and assists in arm extension. The teres major is also important for stabilization of the humeral head in the glenoid cavity. Variation of teres major tendon insertion is very rare. It rarely fuses with latissimus dorsi muscle.19 In treating rotator cuff deficiency, the tendons of latissimus dorsi, teres major, pectoralis major, deltoid, and trapezius muscle are commonly considered for tendon transfer surgeries. The teres major muscle has been reported to have the most functional activation after transfer surgery.20 The lower subscapular nerve innervates the teres major in 86% of shoulders and the rest is innervated by the thoracodorsal nerve.21 The quadrilateral space is a small opening in medial aspect of the humeral neck where the axillary nerve and the posterior circumflex humeral artery pass. It is bounded by the teres minor superiorly, the upper border of teres major and in some cases latissimus dorsi inferiorly, the surgical neck of the humerus laterally, and the long head of triceps medially (▶Fig. 3.10). The axillary nerve gives off branches to the teres minor and part of deltoid. Compression of the nerve can be caused by any condition that reduces the area of this space.22 The fibrous band is described as a common cause for the clinical symptoms of pain and paresthesia also known as “quadrilateral space syndrome.”23

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Subscapularis Muscle The subscapularis muscle is the largest and most powerful rotator cuff muscle. It arises from the anterior surface of the scapula. The superior two-thirds of the subscapularis inserts along the lesser tuberosity and the lower one-third inserts along the humeral metaphysis24–26 (▶Fig. 3.8, ▶Fig. 3.9). The subscapularis has between four and six tendon slips ­originating within the muscle. These tendon slips converge superiorly and laterally to form a strong main tendon that inserts along the superior aspect of the lesser tuberosity. The upper fibers of the subscapularis tendon interdigitate with the anterior fibers of the supraspinatus tendon to contribute to the structures of the rotator cuff interval. The superiormost insertion of the subscapularis tendon extends as a thin tendinous slip to the fovea capitis of the humerus.26 The coracohumeral ligament and the superior glenohumeral ligament also contribute to the rotator cuff interval. These two ligaments insert onto the lesser tuberosity and form the “reflection pulley”, which is a ligamentous sling that stabilizes the long head of the biceps before it enters into the intertubercular groove. Tear of the reflection pulley results in subluxation of the long head of the biceps and may also be associated with subscapularis tear.27 The subscapularis muscle is innervated by the upper and lower subscapular nerves. The upper subscapular nerve originates from the posterior spinal cord (C5–C6) and usually innervates the bulk of the muscle. The lower subscapular nerve is a branch of the posterior cord and supplies the axillary portion of the subscapularis muscle and tendon and the teres major.27 The principal function of the subscapularis tendon is internal rotation of the humerus. The teres major, pectoralis major, and latissimus dorsi also act synergistically with the subscapularis tendon in performing this function. Depending on the position of the arm, the subscapularis tendon also contributes to flexion, extension, adduction, and abduction of the shoulder and overall glenohumeral stability.

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Fig. 3.10 The quadrilateral space is bounded by the teres minor superiorly, the surgical neck of the humerus laterally, the long head of triceps medially, and the upper border of teres major inferiorly. It contains the axillary nerve and the posterior circumflex humeral artery. Nerve compression occurs when the space is narrowed causing pain and paresthesia also known as “quadrilateral space syndrome.” Narrowing of the proximal brachial artery can be seen in this patient, which is trauma related.

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Muscles of Shoulder Girdle, Arm, and Forearm Rotator cuff tears typically occur in the supraspinatus tendon and extend posteriorly into the infraspinatus tendon.25 Tears involving the subscapularis tendon are common, but the diagnosis can be easily missed.26,28 Tears of the muscle tendons are generally characterized on an MRI as areas of disorganized tendon morphology and of abnormally high signal intensity on T2-weighted images. An injury and tear of the reflection pulley is seen as extra-articular collection of contrast material just anterior to the superior border of the subscapularis tendon on axial images.

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◆◆ Arm Muscles

Anterior Muscle Compartment of the Arm The brachialis, biceps, and coracobrachialis muscles reside in the anterior muscle compartment of the arm and act as the main flexors in the upper arm. Brachialis is the largest contributor to elbow flexion (▶Fig. 3.11).

Biceps Brachii Muscle The biceps brachii muscle is a biarticular muscle in the anterior aspect of the arm. It is a two-headed muscle that originates from the scapula and attaches distally on the bicipital or radial tuberosity of the radius, thus spanning two joints (▶Fig. 3.11, ▶Fig. 3.12). The long head of biceps tendon arises from the superior glenoid labrum and supraglenoid tubercle of the scapula, forming the bicipitolabral complex.29,30 It passes through the rotator interval between the supraspinatus and subscapularis tendons to enter the bicipital groove. The long head of biceps tendon curves over the anterosuperior portion of the humeral head, passing under the coracohumeral ligament. The biceps tendon will be stabilized on its course toward the bicipital groove by the biceps reflective pulley, an anterior sling formed by the superior glenohumeral and coracohumeral ligaments. At the level of intertubercular groove, the biceps tendon is kept in place by a complex network of fibers from the subscapularis tendon, supraspinatus tendon, and coracohumeral ligament, forming the transverse ligament, which spans the area between the lesser and greater tuberosities. The long head of biceps tendon is intra-articular as it passes through the glenohumeral joint, but due to normal reflections of the synovium, the tendon remains extrasynovial along its course.29,31 The short head of the biceps, along with coracobrachialis muscle, shares a thick, flattened common tendon arising from the tip of coracoid process.29,31 The two proximal tendons converge to form a conjoint tendon that merges with a common muscle belly at the level of the midshaft of the humerus, near the deltoid tuberosity. The short and long heads of the biceps muscle ­gradually coalesce as they pass distally with interdigitating of adjacent muscle and tendon fibers, described as a “goose quill appearance,” making it difficult to separate the myotendinous junction of each muscle belly. The distal biceps tendon is a flat, extrasynovial structure with no tendon sheath. It is formed by the conjoined short head and long head of the biceps muscle, approximately 7 cm above

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Fig. 3.11 Anterior muscle compartment of the arm. Brachialis, biceps, and coracobrachialis muscles are the anterior arm muscles. The biceps brachii muscle is a two-headed muscle that originates from the scapula and attaches distally on the bicipital or radial tuberosity of the radius. The short head of the biceps, along with coracobrachialis muscle, shares a thick, flattened common tendon arising from the tip of coracoid process. The long head of biceps tendon arises from the superior glenoid labrum and supraglenoid tubercle of the scapula. The coracobrachialis muscle extends from the coracoid process to the junction of the proximal and middle third of the humeral shaft. The brachialis mainly originates from the distal third of the humeral shaft and inserts onto the ulnar tuberosity distal to the coronoid process.

the elbow joint (▶Fig. 3.13). The anterior portion of the distal biceps tendon is formed by distinct fibers from the short head, and the posterior tendon receives fibers from the long head. These two parts are not distinguishable by imaging methods. The two ­tendons twist 90 degrees along the course of the distal tendon (▶Fig. 3.13). This twisting arrangement of the biceps tendon assists in supination of the forearm.32 The biceps tendon enters the antecubital fossa and inserts distally into the posterior or ulnar margin of the bicipital tuberosity of the radius and the bicipital aponeurosis (▶Fig. 3.13). The bicipital aponeurosis is a fascial expansion, which attaches to the medial border of the tendon and fuses with the deep fascia of the forearm and the posterior border of ulna30 (▶Fig. 3.14). This form of insertion also helps in efficient supination of the forearm.33 A bony ridge is seen along the ulnar margin of the bicipital tuberosity, and the distal biceps tendon passes over this ridge to insert on the bicipital tuberosity. A prominent ridge may erode the distal biceps tendon during pronation, leaving it susceptible to rupture. As the forearm moves from supination into pronation, the bicipital tuberosity of the radius moves from anterior to posterior over the ulna, which draws the distal biceps between the two bones. Two separate bursae, namely, the bicipitoradial

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Fig. 3.12 Muscles of the arm. Axial, color-coded computed tomography (CT) images (extended arm) from superior to inferior showing arm muscles. The brachialis, biceps, and coracobrachialis muscles are the anterior compartment muscles of the arm and triceps is located in the posterior compartment. The biceps originates from the scapula and attaches distally on the bicipital or radial tuberosity of the radius. The short head of the biceps and coracobrachialis muscle arise from the tip of coracoid process. The long head of biceps tendon arises from the supraglenoid tubercle of the scapula. The coracobrachialis muscle extends from the coracoid process to the junction of the proximal and middle third of the humeral shaft. The brachialis originates mainly from the distal third of the humeral shaft and inserts onto the ulnar tuberosity distal to the coronoid. The triceps brachii muscle is composed of three heads, long, lateral, and medial. The long head originates from the infraglenoid tubercle of the scapula, the lateral head from the humerus, superior to the radial groove, and the medial head from the humerus, inferior to the radial groove. The muscle inserts into the upper surface of olecranon process of the ulna. HH, humeral head. (Continued)

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Fig. 3.12 (Continued)

Fig. 3.13 Axial T1-wighted images from superior to inferior showing tendinous insertions of the biceps and brachialis muscles as well as two layers of the supinator muscle. Coronal T1-wighted images from anterior to posterior are also showing biceps and brachialis tendons. The biceps tendon enters the antecubital fossa and inserts distally into the posterior or ulnar margin of the bicipital tuberosity of the radius (blue arrows) and the bicipital aponeurosis (green arrow). Note the twisting biceps as it approaches the radial tuberosity. The distal biceps tendon is formed by the conjoined short head (anteriorly located) and long head (posteriorly located) of the biceps muscle, approximately 7 cm above the elbow joint. The two tendons twist 90 degrees along the course of the distal tendon to assist tendon in proper supination of the forearm. The brachialis has two heads. The superficial head inserts onto the ulnar tuberosity (pink arrows), and the deep head (red arrows) attaches to all but the tip of the coronoid. Insertion of the deep head is composed of three units: a medial aponeurosis, a lateral aponeurosis, and muscle fibers inserted directly into the ulna. The boundary of the brachialis muscle is shown by dashed white lines. Axial images also show the two layers of the supinator muscle, superficial and deep (yellow arrows). The posterior interosseous branch of the radial nerve passes the fibrous arcade of Frohse (proximal edge of the superficial layer) to run between the two layers of the muscle. BR, brachioradialis; ECR, extensor carpi radialis longus; PT, pronator teres. (Continued)

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Fig. 3.13 (Continued) (Continued)

bursa and interosseous bursa, are found in the antecubital fossa adjacent to the distal biceps tendon. The bicipitoradial bursa is interposed between the anterior part of the distal biceps tendon and bicipital tuberosity and can become inflamed and filled with fluid (see Chapter 16 “Elbow Joint”). The interosseous bursa is in contact with the interosseous membrane and is located medial to the bicipitoradial bursa. Anatomical variations of the biceps muscle are common, and some of these may have important clinical implications.34 Variability of the origin of the long head of the biceps tendon with attachments on the supraglenoid tubercle or posterior, superior,

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or anterior labrum may be seen causing problem during repair of injured tendons. Occurrence of a biceps third head is a relatively common variant seen in 7% of cases and usually arises from the anteromedial aspect of humerus, between the coracobrachialis insertion and the brachialis origin.33 Extra heads arising from the humeral head, bicipital groove, capsule of the shoulder joint, the shaft of humerus, or from coracoid process have been reported.33 Congenital absence of the long head of biceps tendon is rare. The distal biceps tendon may appear bifurcated (persistent division between the short and long heads) in 25% and can be characterized with MRI.32 Other variants of distal tendon

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Fig. 3.13 (Continued)

morphology include slips extending to the medial intermuscular septum, medial epicondyle, pronator teres, or extensor carpi radialis brevis. The biceps brachii muscle plays an essential role in movement of the upper extremity, acting as a powerful supinator and flexor of the elbow, and an important dynamic stabilizer of the shoulder, particularly with the arm in abduction and internal rotation. Athletes involved in overhead activities place extreme stress and biomechanical demands on their shoulders and the biceps tendon. Injuries of the bicipitolabral complex are also commonly seen in throwing athletes, particularly type 2 superior labrum

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anterior and posterior (SLAP) tears. Injuries to the distal biceps are less common and can develop as a result of chronic repetitive forces on a degenerate tendon (▶Fig. 3.15). Instability of the long biceps tendon is closely related to biceps pulley lesions that can be caused by degenerative changes and acute or chronic trauma.35

Coracobrachialis Muscle The coracobrachialis muscle is a part of the anterior muscle compartment of the arm and the only muscle that represents the adductor compartment of the arm. Normally it has two parts, the

Muscles of Shoulder Girdle, Arm, and Forearm lesion of the musculocutaneous nerve is a known complication of the coracoid bone block abutment procedure.39 The coracobrachialis may be injured during shoulder reconstructive surgery. The coracobrachialis has been used as a vascularized muscle for the treatment of long-standing facial palsy.

Brachialis Muscle

Fig. 3.14 Anterior dissection of the antecubital fossa showing the bicipital aponeurosis. The bicipital aponeurosis is a fascial expansion, which attaches to the medial border of the distal biceps tendon and fuses with the deep fascia of the forearm and the posterior border of ulna.

superficial head which originates from the proximal 10 cm of the short head of the biceps and the deep head which arises from the apex of the coracoid process along with the conjoint tendon of the short head of biceps (▶Fig. 3.11, ▶Fig. 3.12). These two heads may be separated from each other by the passage of the musculocutaneous nerve.36 Its muscle belly forms the inconspicuous rounded bulge on the upper medial side of the arm. The pulse of the brachial artery can often be palpated in the depression posterior to the muscle. The coracobrachialis muscle extends from the coracoid process in the distal direction and inserts into the middle 3 to 5 cm of the shaft of the humerus. The area of insertion is roughly at the junction of the proximal and middle third of the humerus between the attachment of the triceps and brachialis. The coracobrachialis muscle can have variation in its origin, insertion, and nerve supply.37,38 The presence of a third accessory head of coracobrachialis muscle, which originated from an abnormal site on coracoid process, has been reported.37 The musculocutaneous nerve is predominantly derived from the fibers of ventral rami of C5, C6, and C7, which innervate the coracobrachialis. A

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The brachialis muscle is located in the upper arm, deeper than the biceps brachii and partly in the cubital fossa region. It assists the biceps brachii as a synergist in flexing the elbow joint. The brachialis has two heads: the superficial head and the deep head.40,41 The larger superficial head originates from the humerus close to the deltoid tuberosity, whereas the deep head originates from the distal third of the humeral shaft. The superficial head inserts onto the ulnar tuberosity distal to the coronoid, and the deep head attaches to all but the tip of the coronoid (▶Fig. 3.11, ▶Fig. 3.12, ▶Fig. 3.13). Insertions of the deep head consist of three units: a medial aponeurosis, a lateral aponeurosis, and muscle fibers inserted directly into the ulna.40 The brachialis muscle is innervated by the musculocutaneous nerve that runs between brachialis and biceps brachii muscles. The nerve lies on the surface of the brachialis muscle. In 81% of cases, the inferolateral fibers of the deep head or middle third of the muscle receives its innervation from the radial nerve branches.42 The median nerve supplies the lower quarter of the brachialis muscle in 15%.43,44 The brachialis is a flexor of the elbow joint with the forearm either pronated or supinated. The larger, superficial head seems to have the mechanical advantage of a more proximal origin and a more distal insertion, which may enable the muscle to provide the bulk of flexion strength.41 The coronoid process is one of the anterior and medial constraints providing a buttress against posterior dislocation. The soft tissue attached to the coronoid process also provides stability to the elbow, such as the anterior joint capsule of the elbow, the medial collateral ligament, and the deep head of the brachialis muscle.40 The most commonly reported variation of the brachialis muscle is the division of the muscle into two or more parts.45 In these accessory muscles, insertion of the muscle can occur on various parts of the proximal end of the ulna and radius or fascia of the antebrachium. Additional muscular bundles of the accessory muscles in the arm can cause compression of nerves and vessels. These additional muscular bundles arise either from the brachialis, coracobrachialis, or biceps brachii muscles.46

Posterior Muscle Compartment of the Arm Triceps Brachii Muscle The triceps brachii muscle is a muscle of the posterior compartment of the arm and an extensor of the forearm at the elbow joint. It is composed of three heads, long, lateral, and medial (▶Fig. 3.16, ▶Fig. 3.17). The long head originates from the infraglenoid tubercle of the scapula, the lateral head from the humerus superior to the radial groove and lateral intermuscular septum, and the

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Fig. 3.15 Traumatic tear of distal biceps tendon shown by axial magnetic resonance (MR) images from superior to inferior.

Fig. 3.16 Triceps brachii muscle. The triceps brachii muscle is composed of three heads, long, lateral, and medial. The long head originates from the infraglenoid tubercle of the scapula, the lateral head from the humerus superior to the radial groove and lateral intermuscular septum, and the medial head from the humerus inferior to the radial groove and medial intermuscular septum. The muscle inserts into the upper surface of olecranon process of the ulna.

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Fig. 3.17 Triceps brachii tendon. Axial, fat-suppressed, T1-weighted magnetic resonance (MR) images (superior to inferior) of the elbow demonstrating the triceps brachii tendon and its insertion on olecranon (arrows). The medial head of the triceps has a smaller tendon and appears more muscular at the insertion compared to the common tendon of lateral and long heads. Coronal T1-weighted images from anterior to posterior showing tendinous insertions of the triceps heads.

medial head from the humerus inferior to the radial groove and medial intermuscular septum47,48 (see section on muscle compartments of the forearm). This entire muscle extends distally to insert into the upper surface of olecranon process of the ulna. The triceps brachii tendon has a bipartite appearance at the insertion site. The medial head is deeper and mainly muscular at

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its insertion (▶Fig. 3.17). In contrast, the common tendon formed by the long and lateral heads is superficial and tendinous.48 The triceps brachii muscle receives its entire motor innervation from the radial nerve, with the major contributions arising from C6 and C8 nerve roots. The branching pattern of the radial nerve to the triceps brachii muscle may vary.49

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Variations of triceps muscle are relatively uncommon. There are a few reports on additional heads of triceps brachii.47,50,51 Slips may be found connecting the tendons or muscle of the triceps to the shoulder capsule, coracoid process, anconeus, subscapularis, and extensor carpi ulnaris.50 The “Struthers arcade” is a thin ligamentous structure or a thickening of the brachial fascia or muscle fibers of the medial head of the triceps muscle that extend to the medial intermuscular septum.50 In most of individuals, the ulnar nerve passes under arcade of Struthers and may be kinked following triceps muscle transposition procedures. This arcade should not be mistaken with the ligament of Struthers which runs from the supracondylar process of the humerus to the medial epicondyle in less than 3% of population and is a rare cause of compression of the median nerve and brachial artery (supracondylar process syndrome).52

fibrosus and pronator teres proximally and the carpal tunnel distally. Knowledge of compartmental anatomy is relevant for surgical planning of space-occupying lesions, for assessment of nerve entrapment syndromes and nerve injuries, and for evaluation of compartment syndromes.54 The classic compartmental division may not be sufficient to explain distribution of soft tissue abnormalities.54 The compartments are generally interconnected and the interosseous membrane also does not seem to be a complete barrier between the flexor and extensor compartments.54 Another approach to compartmental division of the forearm muscles based on muscle innervation has been suggested by George and Smith.55 The authors have described five tissue planes that do not correspond to any of the classic anatomic regions.55

◆◆ Forearm Muscles

Antecubital Fossa

Muscle Compartments of the Forearm The anatomy of forearm is complex with a large number of muscles, vessels, and nerves running in various directions. Muscles of the forearm are surrounded by the antebrachial fascia. Septa ­arising from this fascia to the radius, ulna, and interosseous membrane divide the forearm muscles into compartments. Classically, three compartments for the forearm are described, namely, volar, mobile wad (radial), and dorsal compartments.53,54,55,56,57 The compartments of the forearm are bounded by the lacertus

The antecubital fossa is a triangular area anterior to the elbow joint formed by the brachioradialis laterally and the pronator teres medially (▶Fig. 3.18). The base of the triangle is a line connecting the humeral epicondyles. The floor of this fossa is formed by the brachialis and supinator muscles. The contents include the biceps brachii tendon, deep (posterior interosseous) and ­superficial divisions of the radial nerve, radial and ulnar arteries, and median nerve. The median antecubital vein interconnects with the cephalic and basilic veins in a fascia covering the fossa (▶Fig. 3.18).

Fig. 3.18 Antecubital fossa. The antecubital fossa is a triangular area anterior to the elbow joint formed by the brachioradialis laterally and the pronator teres medially. The base of the triangle is a line connecting the humeral epicondyles. The floor of this fossa is formed by the brachialis and supinator muscles. The contents include the biceps brachii tendon, the deep (posterior interosseous) and superficial divisions of the radial nerve, the radial and ulnar arteries, and the median nerve.

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Common Flexor Tendon

large muscular portion with thin short tendinous portion. Lateral epicondylitis, or “tennis elbow,” is a common musculotendinous degenerative disorder (enthesopathy) of the common origin of the wrist extensors at the lateral humeral epicondyle.59

The anterior common tendon is a conjoined tendon at medial elbow formed by convergence of the intermuscular fascia of the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis (▶Fig. 3.19). The anterior common tendon is attached to the medial epicondyle and the anterior joint capsule, just anterior and parallel to the anterior bundle of the medial ulnar collateral ligament.58 It is an important structure for elbow stability. The mean length of the anterior common tendon is 28 ± 4 mm. The intermuscular fascia between the flexor digitorum superficialis and flexor carpi ulnaris also forms a common tendon (posterior common tendon), which is attached to the inferior end of the medial epicondyle and medial joint capsule, just posterior to the anterior oblique ligament.58 The anterior oblique ligament along with posterior oblique ligament and transverse ligament form the medial ulnar collateral ligament of the elbow. This ligament also plays an important role in stabilizing the elbow joint mainly against valgus stress.

The volar compartment contains the flexor and pronator muscles of the forearm and is divided into superficial and deep groups (layers) by a transverse septum. The superficial layer consists of the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, and pronator teres53 (▶Fig. 3.21, ▶Fig. 3.22). The deep group originates on the radius, ulna, and interosseous membrane and includes the flexor pollicis longus, flexor digitorum profundus, and pronator quadratus muscles (▶Fig. 3.21, ▶Fig. 3.22). Major vasculature of the forearm, the median nerve, the anterior interosseous nerve, the ulnar nerve, and the deep branch of the radial nerve are within this compartment.53

Common Extensor Tendon

Flexor Carpi Radialis Muscle

The common extensor tendons include the extensor carpi radialis brevis, extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris (▶Fig. 3.20, ▶Fig. 3.21). The extensor carpi radialis brevis is part of the mobile wad and the rest of the common extensor tendons are located in the superficial dorsal compartment. These muscles are composed of a muscular portion and a thin membranous tendon. The brachioradialis and extensor carpi radialis longus arise from the lateral supracondylar ridge of the humerus above the common extensor tendon and contain a

The flexor carpi radialis is a bipennate and biarticular muscle in the anterior forearm compartment. It originates from the medial epicondyle of the humerus and inserts onto the base of second and to a lesser extent third metacarpal (▶Fig. 3.21, ▶Fig. 3.22). The flexor carpi radialis tendon descends deep to the antebrachial fascia and superficial to the flexor pollicis longus. The length of distal tendon is approximately 7 cm, but the myotendinous junction begins approximately 15 cm proximal to the radiocarpal joint.

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Volar (Anterior) Compartment 3

Fig. 3.19 The common flexor tendons include pronator teres (red), flexor carpi radialis (yellow), palmaris longus (pink), flexor digitorum superficialis (light green), and flexor carpi ulnaris (dark green). Proximal tendon origin from the medial epicondyle of the humerus.

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Fig. 3.20 Common extensor tendon. The four muscles of the superficial group of the dorsal compartment arise from the lateral humeral epicondyle through a strong and short common extensor tendon shared with extensor carpi radialis brevis from mobile wad compartment. These muscles include extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, and anconeus. The extensor carpi radialis longus usually originates from the lower third of the lateral supracondylar ridge and the extensor carpi radialis brevis (pink dashed area) from the deep surface of the common extensor tendon, which is attached to the lateral epicondyle. Brachioradialis (red dashed circle) originates from the upper part of the lateral supracondylar ridge of the humerus.

Fig. 3.21 Axial and coronal magnetic resonance (MR) images demonstrating the origins of the common flexor and extensor tendons from humeral epicondyles. FCR, flexor carpi radialis; FDS, flexor digitorum superficialis; LE, lateral epicondyle; ME, medial epicondyle.

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(Continued)

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Fig. 3.21 (Continued)

At the level of distal radius, the flexor carpi radialis tendon is bordered by the palmar cutaneous branch of the median nerve, ­palmaris longus tendon, and median nerve on the ulnar side (▶Fig. 3.22, ▶Fig. 3.23, ▶Fig. 3.24). The flexor pollicis longus tendon is located posteriorly. On the radial side, the flexor carpi radialis tendon is bordered by the radial artery, brachioradialis tendon, superficial radial nerve, and the first extensor compartment of the wrist. At the wrist level, the flexor carpi radialis tendon courses inside a fibro-osseous tunnel. Proximally, this tunnel is formed by the scaphoid/trapezium body, the flexor retinaculum, and a vertical retinacular septum originating from the distal pole of the scaphoid and the trapezoid and inserting into the flexor retinaculum. This vertical retinacular septum separates the flexor carpi radialis tendon from the carpal tunnel.60 In this tunnel some tendinous fibers insert to the trapezium tubercle.

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Flexor Carpi Radialis Brevis Muscle The flexor carpi radialis brevis is an accessory muscle with a fusiform muscle belly and relatively short tendon, seen in 2.6 to 8.6% of individuals. It typically arises from the volar surface of the distal radius usually between the origin of the flexor pollicis longus and the insertion of the pronator quadratus, and then courses between the radial artery and pronator quadratus radial to the flexor pollicis longus tendon (▶Fig. 3.25). The flexor carpi radialis brevis tendon courses through the flexor carpi radialis tunnel, and attaches to the second metacarpal or trapezium.61,62 Alternative attachments include the third metacarpal, scaphoid, and capitate.

Palmaris Longus The palmaris longus is a superficial spindle-shaped flexor muscle of the forearm that lies anteriorly between the flexor carpi ulnaris and the flexor carpi radialis muscles (▶Fig. 3.22, ▶Fig. 3.23).

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Fig. 3.22 Volar compartment of the forearm. The superficial group consists of the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, and pronator teres, all arising from the medial epicondyle of the humerus by the common flexor tendon. The flexor digitorum superficialis is the deepest part of the superficial group. The deep group includes the flexor pollicis longus, flexor digitorum profundus, and pronator quadratus muscles. The flexor carpi radialis inserts onto the base of second metacarpal. The palmaris longus is a superficial muscle that attaches superficially to the flexor retinaculum and fans out into the palmar aponeurosis. The flexor carpi ulnaris inserts distally at the pisiform, hamate, and fifth metacarpal bones. The flexor digitorum profundus arises from the ulna and adjacent interosseous membrane and inserts onto the palmar base of the distal phalanges. The flexor pollicis longus originates from the distal third of the radius and inserts into the base of the distal phalanx of the thumb. The flexor carpi radialis and ulnaris muscles are the flexor of the wrist. The palmaris longus also acts as a weak ­accessory flexor of the wrist.

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Fig. 3.23 Cross-sectional views of the volar compartment from proximal to distal. The superficial group arises from the medial epicondyle of the humerus by the common flexor tendon. The flexor digitorum superficialis (FDS) has two heads. The humero-ulnar head attaches to the medial epicondyle of humerus, and the radial head arises from proximal radius just distal to the radial tuberosity. There is a space between the two heads where the median nerve and ulnar artery pass, a known site of median nerve compression. The pronator teres inserts on the volar body of the proximal radius. The flexor digitorum profundus arises from the ulna and adjacent interosseous membrane and inserts onto the palmar base of the distal phalanges. The flexor pollicis longus originates from the distal third of the radius and inserts into the base of the distal phalanx of the thumb. (Continued)

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Fig. 3.23 (Continued)

Fig. 3.24 Volar wrist structures. Anatomy of the flexor carpi radialis tendon in the distal forearm and proximal wrist. At the level of the distal radius, the flexor carpi radialis tendon (yellow) is bordered by the palmaris longus tendon and median nerve (pink) on the ulnar side, by the flexor pollicis longus tendon (blue) posteriorly, and by the radial artery on the radial side. The flexor carpi radialis tendon courses inside a fibro-osseous tunnel at the level of the scaphoid separating it from the carpal tunnel. Note the ulnar nerve (blue) moving medial to the flexor carpi ulnaris. The ulnar nerve passes in the ulnar canal (Guyon’s canal) between the pisiform and hook of hamate. The flexor carpi radialis inserts onto the base of second metacarpal. The flexor carpi ulnaris inserts distally at the pisiform, hamate, and fifth metacarpal bones.

Phylogenetically, the palmaris longus is considered a degenerated flexor muscle of metacarpophalangeal joint. It originates from the medial epicondyle of the distal humerus within the common flexor tendon, and from adjacent intermuscular septa and deep antebrachial fascia.63 Its tendon attaches superficially to the flexor retinaculum in midline at the wrist level and then fans out as a flat sheet into the palmar aponeurosis of the hand. The palmaris longus acts as a weak accessory flexor of the wrist. It also opposes strong shearing forces on the skin of the palm

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during gripping. The palmaris longus muscle is one of the most variable muscles in the human body.63,64 Anatomic variations in number and morphology are frequent.64,65 The muscle belly may be doubled, reversed, central, bifid, two- or three-headed, and the tendon may be divided into two or three. The palmaris longus tendon has been reported to be absent in about 26% of Caucasian populations and in between 4.5 and 11% of Asian and African populations.66 The lowest rate of its absence is found in East and South-East African populations.66

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Fig. 3.25 Flexor carpi radialis brevis. The flexor carpi radialis brevis is an accessory muscle seen in 2.6 to 8.6% of individuals. It originates between the origin of the flexor pollicis longus and the insertion of the pronator quadratus, and then runs superficial to the pronator quadratus and enters into the osteofibrous tunnel of the flexor carpi radialis to attach to the second metacarpal, trapezium, or third metacarpal. The tendon of flexor pollicis longus is cut in the third image. Also note the accessory head of the flexor pollicis longus arising from the medial side of the coronoid process of the ulna. The radial side of the pronator quadratus appears hypoplastic.

The prevalence of its absence is reported to be higher in females than males, and more common on left side than on right side. Meta-analysis by Yammine showed predominant involvement of the right hand and no gender dominance.66 The importance of the palmaris longus in clinical practice is mainly for reconstructive hand surgery as a donor tendon for transfer or transplant. Dupuytren’s contracture is a progressive disorder characterized by localized thickening and contracture of palmar aponeurosis (see Chapter 18 “Hand”). This contracture results in flexion deformity of the interphalangeal joints and loss of hand function.

Flexor Carpi Ulnaris Muscle The flexor carpi ulnaris is a bipennate muscle arising from two heads.67 The humeral head shares the medial epicondyle of the humerus with the common flexor tendon origin, and the other head arises from the medial margin of the olecranon and an aponeurosis to posterior border of the ulna (▶Fig. 3.22, ▶Fig. 3.23). The flexor carpi ulnaris inserts distally at the pisiform, hamate, and fifth metacarpal bones (▶Fig. 3.24). Near its insertion at the pisiform, its tendon blends with deep fascia laterally by a transverse band known as the volar (palmar) carpal ligament. The volar carpal ligament bridges across the ulnar nerve and vessels on the medial aspect of the flexor retinaculum and in part forms the roof of the ulnar canal (also known as Guyon’s canal) between the pisiform and the hook of hamate68 (▶Fig. 3.24).

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The flexor carpi ulnaris is innervated by the ulnar nerve (C7, C8, and T1 fibers). The ulnar nerve passes between the two heads of the muscle and divides into proximal and distal branches at the level of the elbow joint (▶Fig. 3.24, ▶Fig. 3.26). The ulnar nerve leaves the forearm toward the hand via the ulnar canal. The flexor carpi ulnaris muscle is the dominant flexor and ulnar deviator of the wrist. The proximal muscle fibers overlying the medial collateral ligament of the elbow may also play an important role in dynamic stability of the elbow. The ulnar artery supplies the dominant pedicle in 86% of cases, whereas the recurrent ulnar artery supplies it in only 14%.67 The anatomy of the flexor carpi ulnaris, including its long muscle belly and vascularity, and its proximity to the posterior elbow make this a well-suited flap for small to moderate defects of the posterior elbow. Absence of the flexor carpi ulnaris muscle and presence of an accessory muscle are rarely reported.68

Flexor Digitorum Superficialis Muscle The flexor digitorum superficialis or flexor digitorum sublimis is a digastric muscle in the anterior compartment of the forearm and can be considered the deepest part of the superficial layer of this compartment.69 Like the flexor carpi ulnaris, the flexor digitorum superficialis muscle has two heads at the origin (▶Fig. 3.22, ▶Fig. 3.23). The humeroulnar head attaches to the medial epicondyle of humerus, ulnar collateral ligament, and coronoid process of ulna. The radial head arises from the volar aspect of the

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Fig. 3.26 Nerve branches above, at and below elbow levels shown by magnetic resonance imaging (MRI). Joint fluid is seen as white patches. The ulnar nerve (blue arrows) enters the forearm between the medial epicondyle (ME) and the ulnar head of the flexor carpi ulnaris (FCU) and travels between the two heads of the flexor carpi ulnaris in the superficial volar compartment. It then courses between the FCU and the flexor digitorum profundus (FDP), where it is joined by the ulnar artery. The radial nerve descends into the forearm via the radial tunnel (the space between the brachioradialis [BR] and the brachialis). At the level of the radiocapitellar joint, the nerve divides into two branches: the deep motor radial nerve (yellow arrows) and the superficial sensory radial nerve (red arrows). These branches initially lie adjacent to the brachioradialis and extensor carpi radialis (ECR) longus tendons of the mobile wad compartment. The superficial radial nerve travels with the radial artery as they pass the insertion of the pronator teres. The deep radial nerve (also known as the posterior interosseous nerve) usually penetrates between bellies of the supinator. Below the elbow, the median nerve passes between the pronator teres (PT) muscle and the brachialis muscle. The nerve then passes between the deep and superficial heads of the pronator teres. BR, brachioradialis; ECR, extensor carpi radialis; ECU, extensor carpi ulnaris; ED, extensor digitorum; FCU, flexor carpi ulnaris; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis; PL, palmaris longus; PR, pronator teres; SP, supinator.

proximal radius just distal to the radial tuberosity. There is a space between the two heads where the median nerve and ulnar artery pass. Tendons to the fingers arise from its muscle belly (▶Fig. 3.23). Central to flexor digitorum superficialis muscle mass in the forearm is a large flat common tendon that connects a single proximal muscle belly to two or three separate distal muscles, thus forming a complex digastric muscle. The muscle to middle finger tendon is totally independent. The tendons of the flexor digitorum superficialis and profundus muscles pass underneath the flexor retinaculum and are covered by a common flexor sheath. The distal tendons of the flexor digitorum superficialis attach to the sides of the middle phalanges of the second through fifth ­f ingers. The two slips of each tendon after the chiasm attaches to the sides of

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the middle phalanx with the tendon of flexor digitorum profundus passing superficial. The volar plate is the only structure that attaches to the volar base of middle phalanx. Muscle innervations are provided by the median nerve (C7, C8, T1). The primary function of flexor digitorum superficialis is flexion of the middle phalanges at the proximal interphalangeal joints. Anatomic variations of the flexor digitorum superficial muscle– tendon unit are common, including anomalous muscle bellies, connections between musculotendinous units, and absence of the small finger tendon.70 Muscle belly anomalies may present as a palmar mass or with carpal tunnel symptoms. Tendon anomalies, on the other hand, usually produce few clinical symptoms. The anatomical variations of the flexor digitorum superficialis to

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Fig. 3.27 The median nerve and the brachial artery enter the volar compartment in the antecubital fossa. The median nerve travels under the bicipital aponeurosis. The nerve passes between the deep and superficial heads of the pronator teres and descends distally. It passes in a space between the two heads of the flexor digitorum superficialis (FDS) arch and is closely bound to the deep surface of this muscle by its fascial sheath. The median nerve becomes more superficial and enters the carpal tunnel at the wrist. The median nerve innervates the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. It also supplies branches to proximal portions of the flexor pollicis longus and flexor digitorum profundus.

Brachial artery Median nerve Pronator teres, humeral head

Ligament of struthers Pronator teres (humeral head)

Median nerve

Radial artery

Ulnar head

FDS arch

Pronator teres ulnar head

a

b

Fig. 3.28 Median nerve. (a) The median nerve above the elbow as it travels medial to the brachial artery. The artery may be superficial to the ligament of Struthers (as shown here) or accompany the median nerve deep to it. (b) The median nerve as it travels beneath the proximal fibrous arch of the flexor digitorum superficialis in the forearm. FDS, flexor digitorum superficialis.

the small finger are well known. The tendon may be absent or hypoplastic or may rely on inter-tendinous links with the flexor digitorum superficialis of the ring finger to obtain flexion. The flexor digitorum superficialis arch (between the two heads) is a known site of median nerve compression in the proximal forearm71 (▶Fig. 3.27, ▶Fig. 3.28). Two variations of this arch are

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described: a distinct fibrous arch and an indistinct arch. In 42% of cases, a distinct fibrous transverse arch exists with the median nerve passing underneath.72 Olehnik et al found the flexor digitorum superficial arch to be the most common median nerve compression site, seen in 22 out of 39 forearms.73 Other areas of potential compression include the lacertus fibrosus (bicipital

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Muscles of Shoulder Girdle, Arm, and Forearm aponeurosis), the ligament of Struthers (extending from a supracondylar process to the medial epicondyle), between the two heads of the pronator teres muscle (pronator syndrome), and accessory head of the flexor pollicis longus (Gantzer’s muscle) (▶Fig. 3.27, ▶Fig. 3.28, ▶Fig. 3.29, ▶Fig. 3.30). The clinical picture can be confused with the more commonly diagnosed carpal tunnel syndrome. The absence of nighttime pain and decreased sensation in the distribution of the palmar cutaneous branch of the median nerve can be helpful in distinguishing between the two syndromes.

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Flexor Digitorum Profundus Muscle The flexor digitorum profundus is a deep flexor muscle of the forearm.3,74 It arises from the upper three-fourths of the anterior and the medial surfaces of the ulna, the adjacent interosseous membrane, and the upper three-fourths of the posterior border of the ulna. The flexor digitorum profundus lies deep to the flexor digitorum superficialis (▶Fig. 3.22, ▶Fig. 3.23). Along with the flexor digitorum superficialis, the muscle fans out into four tendons (second to fifth fingers) to the palmar base of the distal phalanges.

Fig. 3.29 Flexor pollicis longus. Cross-sectional views of the volar muscle compartment from proximal to distal. The deep group in this compartment includes pronator quadratus, flexor pollicis longus, and flexor digitorum profundus. The flexor pollicis longus originates from the anterior surface of the body of the distal third of the radius, the adjacent part of the interosseous membrane, and inserts into the base of the distal phalanx of the thumb.

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Fig. 3.30 Relationship of the median nerve to the radial and ulnar heads of the pronator teres and the humeroulnar and radial heads of the flexor digitorum superficialis (FDS).

The flexor digitorum profundus is a flexor of the wrist (midcarpal), metacarpophalangeal, and distal interphalangeal joints. The medial part of the muscle is innervated by the ulnar nerve and the lateral part by the anterior interosseous nerve, a branch of the median nerve. The anterior interosseous nerve always innervates the tendon of the index finger. Flexor indicis profundus is a part of the flexor digitorum profundus. The flexor indicis profundus originates from the anterior surface of shaft of the ulna and the adjoining interosseous membrane, along with the flexor digitorum profundus. It is located between the flexor digitorum profundus and the flexor pollicis longus. Its tendon inserts the palmar surface of the base of the distal phalanx of the index finger. Separation of the tendon for the index finger from the rest of the tendons of the flexor digitorum profundus muscle is a characteristic feature in human. The innervations are mainly provided by the ulnar nerve and the lateral half is innervated by the anterior interosseous branch of the median nerve, C8 and T1. Anomalous muscles are quite common in the flexor compartment. The flexor digitorum superficialis is likely the most common muscle, followed by the flexor digitorum profundus, to show anatomical variations. Most muscular variations are usually asymptomatic. The most common variation of the flexor digitorum profundus is at the level at which its tendons become independent. Congenital defects of the flexor digitorum profundus tendon of the little finger have been reported.76 The tendon for the index finger may be quite independent, forming a flexor indicis profundus. Connections occur between the flexor digitorum profundus and the flexor digitorum superficialis or flexor pollicis longus tendons in the form of muscular slips.77 An accessory tendon arising

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from the coronoid process that joins the index finger tendon of the flexor digitorum profundus is seen in about 20% of the cadavers.2

Flexor Pollicis Longus Muscle The flexor pollicis longus originates from the anterior surface of the body of the distal third of the radius, the adjacent part of the interosseous membrane, and frequently an accessory muscle slip from the medial border of the coronoid process of the ulna. The muscle inserts into the base of the distal phalanx of the thumb (▶Fig. 3.22, ▶Fig. 3.24, ▶Fig. 3.25, ▶Fig. 3.29). It belongs to the deep layer of the volar compartment muscles. The flexor pollicis longus tendon passes underneath the flexor retinaculum of the wrist, runs between the superficial and deep bellies of the flexor pollicis brevis muscle, and then passes through the osteofibrous canal of the thumb.79 At the level of the first metacarpal head, the tendon passes between two sesamoid bones. The lateral sesamoid is located in the combined tendon of the flexor pollicis brevis and abductor pollicis longus, and the medial sesamoid resides in the adductor pollicis tendon. The flexor pollicis longus is the flexor of the distal phalanx of the thumb and may assist in flexing the wrist when the thumb is fixed. This muscle receives innervations from the anterior interosseous nerve, a branch of the median nerve. Therefore, electrodiagnostic evaluation of this muscle is useful in patients suspicious for median nerve neuropathy in order to identify the site and severity of the abnormality. An accessory slip of the flexor pollicis longus, Gantzer’s muscle, may be seen in some individuals. Congenital absence of the flexor pollicis longus tendon with or without associated anomalies of

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Muscles of Shoulder Girdle, Arm, and Forearm the thumb and thenar muscles have been reported.79,80 Most of the reports have documented unilateral absence of flexor pollicis longus tendon; bilateral absence is extremely rare.81 Inability to flex the interphalangeal joint of the thumb may be due to ­congenital absence, anomalous insertion, or traumatic rupture of the flexor pollicis longus, as well as tenovaginitis of the flexor tendon sheath, and partial anterior interosseous nerve paralysis.

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Pronator Teres Muscle The pronator teres has two anatomical origins: the humeral head and the ulnar head (▶Fig. 3.22, ▶Fig. 3.23, ▶Fig. 3.28, ▶Fig. 3.30). The ulnar head of the pronator teres is seen in 78 to 92% of cases, just behind the humeral head.58,82 It is usually muscular and mainly originates from the anterior edge of the sublime tubercle (inner wall of the coronoid process). However, the upper part of ulnar head attaches directly to the medial epicondyle via a thickened joint capsule, just anterior to the anterior bundle of the medial ulnar collateral ligament.58 Therefore, muscle activation of the pronator teres can directly increase the strain of the medial elbow joint capsule. The humeral head is present in all cases and

originates directly from the anterosuperior aspect of the medial epicondyle of the humerus and medial intermuscular septum.82 The muscle inserts on the body of the proximal radius (▶Fig. 3.22, ▶Fig. 3.23, ▶Fig. 3.30). The brachial artery divides into the radial and ulnar arteries proximal to the pronator teres muscle, and ulnar artery passes obliquely downward, seated deeply and covered by the pronator teres.83 The pronator teres muscle is innervated by the median nerve from the lateral cords of the brachial plexus, originating from the ventral roots of C6 and C7. The median nerve usually traverses from the cubital fossa and gives off a branch to pronator teres muscle before entering between the two heads of the pronator teres muscle.84 Needle electromyography of this muscle is useful for evaluating brachial plexopathy, cervical radiculopathy, median neuropathy, or anterior interosseous nerve syndrome. Pronator teres syndrome and anterior interosseous nerve syndrome are proximal median neuropathies of the elbow and forearm.85,86 A fibrous arch (pronator arch) is formed between the two heads of the pronator teres (▶Fig. 3.28, ▶Fig. 3.29, ▶Fig. 3.30, ▶Fig. 3.31). The median nerve entrapment at this level can cause pronator teres syndrome. Predisposing factors for pronator teres

Fig. 3.31 Median nerve and pronator teres. Axial, T1-weighted magnetic resonance (MR) images of the left forearm distal to the elbow from proximal to distal. The median nerve passes between the deep (ulnar) and superficial (humeral) heads of the pronator teres and descends distally. It passes deep in relation to the fibrous arch formed by the flexor digitorum superficialis (FDS) and is closely bound to the deep surface of this muscle by its fascial sheath. A fibrous arch (pronator arch) is formed between the two heads of the pronator teres. The median nerve entrapment at this level can cause pronator teres syndrome. FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; FDP, flexor digitorum profundus; S, supinator.

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Muscles of Shoulder Girdle, Arm, and Forearm syndrome include short and tendinous ulnar head, conjoined ulnar head and the arch of the flexor digitorum superficialis muscle, and humeral head perforated by the median nerve.82 Similar to carpal tunnel syndrome, pronator teres syndrome typically presents with aching pain in the proximal, volar forearm with paresthesias radiating into the thumb, index finger, middle finger, and the radial half of the ring finger. Anterior interosseous nerve of median nerve is a motor-only nerve innervating the deep muscles of the forearm (flexor pollicis longus, flexor digitorum profundus to the index and middle fingers, and pronator quadratus). A patient with a complete anterior interosseous nerve palsy presents with motor dysfunction of these muscles.

Pronator Quadratus Muscle The pronator quadratus muscle is a quadrilateral muscle in deep volar compartment with attachments at the distal volar aspect of the ulna and radius87 (▶Fig. 3.32). The length of muscle is 3 to 4 cm. The pronator quadratus muscle is an important muscle for the normal function of the forearm. The superficial head of the pronator quadratus muscle acts mainly in forearm pronation, and the deep head works as a dynamic stabilizer of the distal radioulnar joint.88 The pronator quadratus muscle integrity is also essential to the blood supply of the distal radius during fracture healing. The anterior interosseous artery supplies blood to both pronator quadratus muscle and distal radial periosteum.

Mobile Wad (Radial) Compartment The mobile wad compartment is the radial group of forearm muscles innervated by the radial nerve and is composed of two wrist extensors and a forearm flexor, including the extensor carpi radialis longus, extensor carpi radialis brevis, and brachioradialis muscles (▶Fig. 3.33, ▶Fig. 3.34).

Extensor Carpi Radialis Longus and Brevis Muscles There are two extensor carpi radialis muscles: the extensor carpi radialis longus and the extensor carpi radialis brevis. The extensor carpi radialis longus usually originates from the lower third

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of the lateral supracondylar ridge, and the extensor carpi radialis brevis from the deep surface of the common extensor tendon, which is attached to the lateral epicondyle89 (▶Fig. 3.30, ▶Fig. 3.34). In many cases, it is not possible to separate the origin of the extensor carpi radialis brevis from the common extensor tendon. There are two anatomic variations of the extensor carpi radialis muscles. In the first variation, both the muscle bellies are fused into a single muscle belly, which can have two or three tendons that insert onto the base of second metacarpal. In the second variation, there is an additional muscle termed the extensor carpi radialis intermedius, which arises independently from the lateral epicondyle. Greenbaum et al reported no definitive separation of the extensor carpi radialis brevis and extensor digitorum communis at the osteotendinous junction.90 The extensor carpi radialis muscles extend and radially abduct the wrist. Lateral epicondylitis, or “tennis elbow,” is a common musculotendinous degenerative disorder (enthesopathy) of the common origin of the wrist extensors at the lateral humeral epicondyle.91 Repetitive occupational or athletic activities involving wrist extension and supination are the main causative factors. Other causes of lateral sided elbow pain include stenotic changes in the annular ligament, chondromalacia of the capitellum or the radial head, and synovial fringe in the radiocapitellar joint. The extensor carpi radialis brevis origin in the common extensor tendon has been considered as the major tendon for the pathological basis of lateral epicondylitis or tennis elbow. Compared to other tendons in the common extensor tendon complex, the extensor carpi radialis brevis origin is composed of a thick tendinous portion without any muscular portion. This large tendinous attachment is believed to be an initial factor leading to the development of lateral epicondylitis due to ischemia and incomplete healing capability.91

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Brachioradialis Muscle The brachioradialis originates from the upper part of the lateral supracondylar ridge of the humerus and inserts on the distal styloid process of the radius (▶Fig. 3.30, ▶Fig. 3.34). The proximal muscular portion of the brachioradialis forms the radial boundary of the antecubital fossa. The brachioradialis is a flexor of the forearm at the elbow. Variations in its origin are very rare. A

Fig. 3.32 Pronator quadratus muscle. Cross-sectional views of the volar compartment at the level of the pronator quadratus muscle. The pronator quadratus muscle is a quadrilateral muscle in deep volar compartment attaching at the distal volar aspect of the ulna and radius. It is an important muscle for the normal pronation of the forearm.

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Fig. 3.33 Superficial dorsal and mobile wad (radial) compartments of the forearm. The mobile wad compartment is the radial group of forearm muscles, including the extensor carpi radialis (ECR) longus, extensor carpi radialis brevis, and brachioradialis. The extensor carpi radialis longus originates from the lower part of lateral supracondylar ridge, and the extensor carpi radialis brevis from the lateral epicondyle (white dashed area). In many cases both the muscle bellies are fused into a single muscle belly with two or three tendons that insert onto the base of second metacarpal. The brachioradialis originates from the upper part of lateral supracondylar ridge of the humerus and inserts on the distal styloid process of the radius. The muscles of the superficial layer of the dorsal compartment arise from the lateral humeral epicondyle through a strong and short common extensor tendon shared with extensor carpi radialis brevis. The superficial dorsal muscles include extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, and anconeus. The anconeus muscle is a small, triangular-shaped muscle with its origin just posterior and superior to the lateral epicondyle of the humerus.

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Fig. 3.34 Superficial dorsal and mobile wad (radial) compartments of the forearm. The extensor carpi radialis longus and brevis insert onto the base of second metacarpal. The brachioradialis inserts on the distal styloid process of the radius. The extensor carpi ulnaris passes the sixth osteofibrous tunnel to insert to the base of the fifth metacarpal. The extensor tendons of extensor digitorum communis traverse the fourth dorsal osteofibrous tunnel. The extensor digiti minimi tendon passes the fifth extensor wrist compartment, the extensor carpi ulnaris passes the sixth osteofibrous tunnel, and the extensor carpi radialis tendons pass the second compartment. The anconeus muscle originates just posterior and superior to the lateral epicondyle of the humerus and inserts on the posterolateral surface of the proximal ulna. The deep surface of the muscle is connected to the supinator muscle. (Continued)

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Fig. 3.34 (Continued)

separate supernumerary muscle in the lateral cubital fossa originating from the humerus proximal to the brachioradialis origin or directly from brachioradialis and inserting into the radius, pronator teres, or supinator muscle has been considered as a variation of the brachioradialis muscle.92 The brachioradialis is supplied by branches of the radial or recurrent radial arteries. Although rare, radial nerve compression at the humeral origin of the brachioradialis muscle has been reported.93,94,95 Radial nerve compression usually occurs in the forearm and commonly at the arcade of Frohse near the supinator origin. Radial nerve entrapment above the elbow is infrequent. The teres major, triceps, and the space between the brachialis and brachioradialis muscles are rare sites of radial nerve compression above the elbow.96 The brachioradialis is commonly used for rotational muscle flap or musculocutaneous flap in the reconstruction of posterolateral, posterior, and anterior elbow wounds.93

Dorsal Compartment The dorsal compartment contains the wrist and finger extensors. The muscles in this compartment are arranged into superficial and deep groups. The four muscles of the superficial group arise from the lateral humeral epicondyle through a common tendon shared with extensor carpi radialis brevis. From lateral to medial direction, these muscles include extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris (▶Fig. 3.20). The anconeus muscle is also located in this compartment. The deep group consists of five muscles: the supinator, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. The extensors of this group originate from a position distal to the supinator. The posterior interosseous nerve and artery supply these muscles.

Extensor Digitorum Communis and Extensor Digiti Minimi Muscles The extensor digitorum communis originates from the posterior fascia of the lateral humeral epicondyle, the antebrachial fascia,

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and the fibrous septa that divides it from extensor carpi radialis brevis and extensor digiti minimi (▶Fig. 3.33, ▶Fig. 3.34). Fairbank et al reported that the extensor digitorum communis muscle is made up of four segments, one for each of the four digits. The index finger and middle finger segments are completely independent and separable from each other but the muscle segments to the ring and little fingers interdigitated proximally. The only segment of the muscle that attaches to the lateral epicondyle is the muscle segment for the middle finger, and the other segments attached more distally to surrounding muscles and fascia.97 The antebrachial fascia is superficial to the extensor digitorum communis, and the supinator muscles, extensor pollicis brevis, extensor pollicis longus, and abductor pollicis are located deep to the muscle. In the mid-forearm, the extensor digitorum communis divides into three fasciae: the lateral fascia from which two tendons arise, and the other two fasciae that give rise to a tendon each. The four extensor tendons traverse the fourth dorsal osteofibrous tunnel to reach the fingers. At the level of each proximal phalanx, the tendon is divided into three portions: the central portion inserts to the base of the middle phalanx and the lateral ones to the distal phalanx. The extensor digiti minimi arises from the posterior fascia of the lateral epicondyle, the antebrachial fascia, and the fibrous septa, which separate it from the adjacent muscles.97 Approximately half way down the forearm, a long tendon traverses the fifth osteofibrous tunnel, and distally, at the level of the metacarpal, it fuses with the extensor tendon of the fifth finger coming from the fourth osteofibrous tunnel. These muscles extend the phalanges and the wrist.

Extensor Carpi Ulnaris Muscle The extensor carpi ulnaris arises from the lateral humeral epicondyle, the radial collateral ligament of the elbow, the antebrachial fascia, the posterior margin of ulna, and the contiguous intermuscular fibrous septa (▶Fig. 3.33, ▶Fig. 3.34). At the lower third of the forearm, it continues into the homologous tendon that traverses the sixth osteofibrous tunnel to insert to the base of the

Muscles of Shoulder Girdle, Arm, and Forearm fifth metacarpal. The extensor carpi ulnaris extends the wrist and is innervated by the posterior interosseous branch of radial nerve. The muscle can be injured along with the short common tendon group in tennis elbow syndrome.

Anconeus Muscle The anconeus muscle is a small triangular-shaped muscle with its origin just posterior and superior to the lateral epicondyle of the humerus (▶Fig. 3.33, ▶Fig. 3.34). The muscle fibers fan out to a broad insertion spanning the posterolateral surface of the proximal ulna.98 The deep surface of the muscle is connected to the supinator muscle, and the elbow joint. Functionally, the anconeus muscle is a weak accessory extensor to the triceps brachii. The anconeus muscle has a close anatomical relationship to the lateral collateral ligamentous complex and may contribute to the posterolateral stability of the elbow during forearm abduction and extension. The anconeus has been frequently used as a model in neuromuscular and anatomical investigations.99

Supinator Muscle The supinator muscle exhibits a complex morphology. The supinator muscle consists of two layers, one superficial and one deep.100 They arise from the lateral epicondyle, radial collateral ligament of the elbow, radial annular ligament, and supinator crest of ulna.

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Both muscle layers are directed distally, anteriorly, and laterally and wrap around the proximal third of the radius and insert on the anterior and lateral walls of the radius (▶Fig. 3.35, ▶Fig. 3.36). The belly of the supinator muscle is perforated by the posterior interosseous branch of the radial nerve through the fibrous arcade of Frohse (proximal edge of the superficial layer) that runs between the two layers of the muscle101 (▶Fig. 3.21, ▶Fig. 3.26, ▶Fig. 3.36). Repetitive pronation and supination of the forearm can predispose to compression of the posterior interosseous nerve branch of the radial nerve at the fibrous arcade of Frohse.101,102,103 This condition is called “radial tunnel syndrome” and clinically can be difficult to differentiate from symptoms of tennis elbow (i.e., lateral epicondylitis).

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Abductor Pollicis Longus Muscle The extrinsic thumb muscles include the abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and flexor pollicis longus (▶Fig. 3.37, ▶Fig. 3.38). The abductor pollicis longus originates from the posterior surface of the radius and ulna and from the interosseous membrane. It travels along the lateral surface of the distal radial diaphysis. At the distal third, it gives rise to a tendon. Together with extensor pollicis brevis, situated proximally in relation to the dorsal carpal ligament, it travels laterally in the superficial layer and crosses over the tendons of

Fig. 3.35 Supinator muscle. The supinator muscle is a part of the deep dorsal compartment muscles of the forearm and is the most proximal one. It arises from the lateral epicondyle, radial collateral ligament of the elbow, radial annular ligament, and supinator crest of the ulna. Muscle layers are directed distally, anteriorly, and laterally and wrap around the proximal third of the radius and are inserted to the anterior and lateral walls of the radius.

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Fig. 3.36 Radial nerve. Anterior view of a left elbow. (a) Dissection of the supinator muscle (S) showing its relationship with the radial nerve (RN). The proximal edge of the supinator is usually tendinous (yellow arrows). (b) Color-coded computed tomography (CT) image. The deep branch (db) of the RN gives off multiple muscular branches and the posterior interosseous nerve. The superficial branch (sb) of the radial nerve travels down along the lateral side of the radius toward the wrist. B, brachialis; BR, brachioradialis; ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; MCN, musculocutaneous nerve; MN, median nerve; PT, pronator teres.

Fig. 3.37 Dorsal compartment muscles of the forearm. Muscles are arranged into superficial and deep groups. The deep group (layer) consists of five muscles. The abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, extensor indicis, and supinator. The three extensors originate from a position distal to the supinator.

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(Continued)

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Fig. 3.37 (Continued)

Fig. 3.38 Distal radius grooves are formed by the tendons of the abductor pollicis longus, which abducts and extends the thumb, and the extensor pollicis longus, extensor pollicis brevis, and extensor indicis, which extend the phalanges.

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Muscles of Shoulder Girdle, Arm, and Forearm extensor carpi radialis longus and extensor carpi radialis brevis. Still accompanied by the tendon of extensor brevis, the tendon of abductor pollicis longus traverses the first osteofibrous tunnel and inserts to the side of the base of the first metacarpal. Absence, hypoplastic, supernumerary, and accessory tendons of the abductor pollicis longus have been reported.104

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Extensor Pollicis Brevis Muscle The extensor pollicis brevis lies on the medial side of abductor pollicis longus (▶Fig. 3.37, ▶Fig. 3.39). It arises from the posterior surface of the radius and from the interosseous membrane distal to the attachment of the abductor pollicis longus and travels parallel with abductor pollicis longus crossing over the extensor carpi radialis longus and extensor carpi radialis brevis or second compartment muscles. It passes in the first osteofibrous extensor wrist compartment along with the abductor pollicis longus ­tendon. It inserts on the posterior surface of the base of the proximal phalanx of the thumb after passing under the extensor retinaculum. The extensor pollicis brevis and abductor longus tendons within the first extensor compartment are separated in 30 to 60% of cases by either a complete or partial septum.105,106 Its primary function is to extend and stabilize the metacarpophalangeal joint of the thumb. It also contributes to abduction of the thumb and carpus. Absence of the extensor pollicis brevis muscle belly is seen in 6 to 23%; instead its tendon arises from the belly of the abductor pollicis longus.107 Variation in tendon insertion is also common.105 Accessory tendons of the extensor pollicis brevis and abductor pollicis longus are common and may be seen 30% of cases.108 De Quervain’s disease is stenosing tenosynovitis involving the abductor pollicis longus and extensor pollicis brevis tendons within the first dorsal extensor compartment of the wrist causing painful swollen tendons and retinaculum109 (see Chapter 18 “Hand”). Following radial nerve palsy, loss of the extensor pollicis longus, abductor pollicis longus, and extensor pollicis brevis tendons results in loss of thumb extension and radial abduction.

Extensor Pollicis Longus Muscle The extensor pollicis longus arises from the middle third of the posterior surface of the ulnar diaphysis and the interosseous membrane. It travels laterally and gives rise to a tendon that crosses over extensor carpi radialis brevis and extensor carpi radialis longus to insert to the base of the distal phalanx of the thumb (▶Fig. 3.37, ▶Fig. 3.38, ▶Fig. 3.39). It is the only tendon that forms the third osteofibrous tunnel (third extensor compartment) of the wrist (▶Fig. 3.39). The distal tendon of extensor pollicis longus delimits the dorsal wall of the anatomical snuffbox of the wrist (▶Fig. 3.38). Its palmar wall is delimited by the tendons of abductor pollicis longus and extensor pollicis brevis. The radial artery is situated at the floor of the snuffbox, which consists of extensor carpi radialis brevis and extensor carpi radialis longus (the second osteofibrous tunnel) (▶Fig. 3.38). This muscle extends the metacarpophalangeal and interphalangeal joints. The extensor pollicis longus is considered as the most consistent structure with the least variation among the muscles

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involved in movement of thumb.110 Delayed tendon rupture has been reported after both displaced and nondisplaced fractures of the distal radius. The extensor pollicis longus tendon can also be involved in tenosynovitis, ankylosing spondylitis, tophaceous gout infiltration, and Madelung’s deformity.

Extensor Indicis Muscle The extensor indicis arises from the posterior surface of the distal ulnar diaphysis and the interosseous membrane below the origin of the extensor pollicis longus (▶Fig. 3.37, ▶Fig. 3.38, ▶Fig. 3.39). At the proximal part of the fourth osteofibrous tunnel (fourth extensor wrist compartment), it gives rise to a tendon that travels medial (ulnar) and parallel with the tendons of extensor digitorum communis (▶Fig. 3.33). It then moves laterally to join the ulnar side of the second (index finger) tendon of the extensor digitorum communis. The extensor indicis acts to extend the index finger and the wrist. Anatomical variations of the extensor muscles of the hand, including the extensor indicis, are commonly seen in up to 25% of cases.110,111 A dual tendon of the extensor indicis to the second and third digits has been reported.112 Variations in tendon anatomy are important during surgeries involving repair or transfer of tendons.

◆◆ Upper Extremity Compartments

Upper and lower extremity muscle groups are separated from each other by fascial septa, forming compartments. Pathologies limited to a fascial compartment can cause a series of symptoms and complications collectively known as compartment syndrome.113 Compartment syndrome of the leg and forearm is a well-known pathology. It is most common in the anterior and deep posterior compartments of the leg and the volar compartment of the forearm. The pathologic process can be divided into acute, subacute, and chronic compartment syndrome. Acute compartment syndrome is usually a complication of fracture (60–70%) and in some cases, it occurs after burns and orthopedic, vascular, or other surgery of the limbs. The classical five Ps of clinical picture are: pain, paresthesia, pallor, paralysis, and pulselessness. Chronic compartment syndrome typically involves young athletes who are engaged in endurance sports and presents with chronic pain.114 An upper leg compartment syndrome occurs less frequently, and a compartment syndrome of the upper arm is even less common. Upper arm acute compartment syndrome has been reported after hemorrhage (i.e., following anticoagulation).114 Increased pressure in the involved compartment interrupts tissue perfusion that can lead to edema within the compartment, tissue necrosis, permanent functional impairment, and even renal failure and death due to acute rhabdomyolysis. MRI is the modality of choice to show compartmental muscle edema and necrosis (▶Fig. 3.40). Treatment of acute compartment syndrome usually involves emergent fasciotomy to relieve neurovascular symptoms and return full motor function.115,116

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Fig. 3.39 Dorsal compartment muscles of the forearm, deep layer. The deep group consists of five muscles. The supinator (not shown), abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. The abductor pollicis longus originates first from the posterior surface of the radius and ulna and from the interosseous membrane. Accompanied by the tendon of extensor pollicis brevis, the tendon of abductor pollicis longus traverses the first osteofibrous compartment tunnel and inserts to the side of the base of the first metacarpal. The extensor pollicis brevis lies on the medial side of abductor pollicis longus and arises from the posterior surface of the radius and from the interosseous membrane. It inserts on the posterior surface of the base of the proximal phalanx of the thumb. The extensor pollicis longus arises from the middle third of the posterior surface of the ulnar diaphysis and the interosseous membrane. It travels laterally and gives rise to a tendon, which crosses over extensor carpi radialis brevis and extensor carpi radialis longus to insert to the base of the distal phalanx of the thumb. The extensor pollicis longus is the only tendon that forms the third extensor compartment of the wrist. The extensor indicis arises from the posterior surface of the distal ulnar diaphysis and the interosseous membrane below the origin of the extensor pollicis longus. Its tendon passes under the fourth compartment to join the ulnar side of the second (index finger) tendon of the extensor digitorum communis.

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Fig. 3.40 Forearm acute compartment syndrome. Axial, fat-suppressed T2-weighted (T2W) (upper row) and T1-weighted (T1W) images are shown. T2W images showing extensive edema of the muscular compartments with the flexors being involved greater than extensors. Severe diffuse edema of the pronator quadratus (anterior superficial) and pronator teres muscles (anterior deep) are evident.

Compartments The upper extremity can be divided into three parts, namely, the shoulder and brachium (arm), the antebrachium (forearm), and the hand.115 Each part is subdivided into two to three myofascial compartments. Individual variation in number and extent of compartments is not uncommon. On many occasions, distinct and separate tissue subcompartments may be identified at the time of surgical exploration. Examples of isolated subcompartments include the pronator quadratus or deep flexor muscles within the volar forearm or the anconeus, extensor carpi ulnaris, and extensor pollicis longus within the dorsal forearm115 (▶Fig. 3.41). The lacertus fibrosus (bicipital aponeurosis) (▶Fig. 3.14) may act as a source of compression around the elbow. This fascia originates from the biceps tendon and spreads distally and medially to insert into the pronator fascia. The flexor retinaculum is also another structure that can cause nerve compression. The lacertus fibrosus and flexor retinaculum may be released in a complete volar forearm fasciotomy.

Shoulder and Arm The shoulder and arm (brachium) consist of three anatomic compartments: the deltoid, the anterior brachium, and the posterior brachium. The posterior compartment is separated from the anterior compartment by the humerus and the lateral and medial intermuscular septa (▶Fig. 3.42, ▶Fig. 3.43). The brachial fascia

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is a dense fibrous sheath that surrounds the muscles in each compartment. The deltoid compartment is subdivided into anterior, middle, and posterior subcompartments (▶Fig. 3.41). The deltoid fascia becomes continuous with the brachial fascia and the lateral intermuscular septum distally. The deltoid is innervated by the axillary nerve. The anterior compartment contains the biceps, the brachialis, and the coracobrachialis muscles (▶Fig. 3.43). The anterior compartment musculature is supplied by the brachial artery and mainly innervated by the musculocutaneous nerve with an inconsistent contribution from the radial nerve. In the upper arm, the musculocutaneous nerve may pass through the coracobrachialis muscle and then at the midarm level, it runs between the biceps and the brachialis muscles (▶Fig. 3.43). At the distal arm, the radial nerve course within the anterior compartment and is at risk of injury from elevated interstitial tissue pressure (▶Fig. 3.43). Having a marginal course, the median and ulnar nerves are less vulnerable to injury. The posterior compartment of the brachium contains the three heads of the triceps: medial, lateral, and long. The radial nerve innervates the triceps muscle and the profunda brachii is the arterial supply. The nerve passes through the lateral triangular space to enter the posterior compartment. In the lower arm, the radial nerve penetrates the lateral intermuscular septum to reach the anterior compartment of the arm (▶Fig. 3.43).

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Fig. 3.41 (a) Transverse cross section through the level of the humeral head demonstrating the deltoid compartment divided by fascial septa (arrows) into subcompartments. (b) Transverse cross section at the level of the distal forearm demonstrating the pronator quadratus and its investing fascia which may create a distinct osteofascial compartment.

Fig. 3.42 Anatomical distribution of the arm nerves in a hyperextended arm. The profunda brachii artery and radial nerve move together from the anteromedial side of the arm toward the posterolateral aspect of the humeral shaft. From there, the profunda brachii stays behind the lateral intermuscular septum within the posterior compartment but the radial nerve crosses the lateral intermuscular septum to enter the anterior ­compartment. In the midarm, the musculocutaneous nerve runs anterior to the brachialis muscles.

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Fig. 3.43 Anatomical distribution of the arm nerves. Color-coded, axial computed tomography (CT) images from the proximal to the distal arm are shown. The profunda brachii artery and the radial nerve move together from the anteromedial side of the arm toward the posterolateral aspect of the humeral shaft. From there, the profunda brachii stays behind the lateral intermuscular septum within the posterior compartment but the radial nerve crosses the lateral intermuscular septum (arrow) to enter the anterior compartment and moves between the brachialis ­muscle and the brachioradialis muscle. In the upper arm, the musculocutaneous nerve may pass through the coracobrachialis muscle and then at the midarm level, it runs between the biceps and the brachialis muscles. The ulnar nerve usually passes under the arcade of Struthers (thickening of the brachial fascia extending to the medial intermuscular septum) and may be kinked following triceps muscle transposition procedures.

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Forearm Three general compartments have been described: the anterior (volar or flexor) compartment, posterior (dorsal or extensor) compartment, and mobile wad (▶Fig. 3.44, ▶Fig. 3.45). These compartments may be interconnected.

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The volar and dorsal compartments are separated from each other by the radius, ulna, and interosseous membrane. The mobile wad is located along the dorsal and radial side of the forearm. It is separated from other compartments by a connective tissue septum extending from the antebrachial fascia (▶Fig. 3.44). The antebrachial fascia is the deep fascia of forearm that extends

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Fig. 3.44 (a–c) Upper forearm magnetic resonance (MR) images showing the anterior and posterior compartments. Three compartments of the forearm are the anterior (volar or flexor), the posterior (dorsal or extensor), and the mobile wad. The volar and dorsal groups also have been subdivided into the superficial and deep groups. (Continued)

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Fig. 3.44 (Continued)

Fig. 3.45 (a, b) Mid forearm magnetic resonance (MR) images showing the anterior and posterior compartments. Three compartments of the forearm are the anterior (volar or flexor), the posterior (dorsal or extensor), and the mobile wad. The volar and dorsal groups also are subdivided into the superficial and deep groups.

between the brachial fascia (deep fascia of the arm) and the volar carpal ligament. The volar and dorsal groups are subdivided into superficial and deep groups. The median, ulnar, and anterior interosseous nerves traverse the volar compartment. The superficial group of the volar compartment consists of the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, and pronator teres. The deep group includes the flexor pollicis longus, flexor digitorum profundus, and pronator quadratus muscles. In

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the dorsal compartment, the superficial group include extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, and anconeus muscles. The deep group consists of five muscles: the supinator, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis. The nerve supply to these muscles is the posterior interosseous nerve (▶Fig. 3.46). The mobile wad compartment is composed of the extensor carpi radialis longus, extensor carpi radialis brevis, and brachioradialis

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Fig. 3.46 Relationship of the radial and ulnar arteries with nerves and muscles. (a) Volume-rendered, color-coded views in different projections. (b) Axial computed tomography (CT) images of the forearm from proximal to distal.

muscles. These three muscles are innervated by the radial nerve. The superficial branch of the radial nerve courses within the compartment (▶Fig. 3.46).

Hand The musculotendinous structures of the hand are divided into six compartments: thenar, hypothenar, adductor, interosseous, carpal canal, and digit (see Chapter 18 “Hand”). The thenar compartment is enveloped by the thenar fascia and may be divided into two discrete fascial spaces. It is innervated by

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the recurrent branch of the median nerve and contributions from the ulnar nerve. The hypothenar compartment is invested by the hypothenar fascia and subdivided into two compartments in 75% of cases; both are innervated by the ulnar nerve. The adductor compartment, containing the adductor pollicis, is invested by volar and dorsal fascia and is innervated by the ulnar nerve. It is located between the anterior interosseous and the lumbrical muscles and in 30% of individuals is in continuity with interosseous compartments.115 There are four dorsal and three volar interosseous muscles of the hand, innervated by the ulnar nerve.

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Muscles of Shoulder Girdle, Arm, and Forearm The carpal canal is covered by the flexor retinaculum anteriorly and carpeted by the volar extrinsic radiocarpal ligaments posteriorly. Typically, the lumbrical muscles originate distal and external to the carpal canal, although variations exist. Post-traumatic edema and hemorrhage can cause acute carpal tunnel syndrome with resultant median nerve dysfunction. Severe flexor tenosynovitis may precipitate digital compartment syndrome that can compromise perfusion of the involved finger.

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50. Swamy R, Rao M, Somayaji S, Raghu J, Pamidi N. Bilateral additional slips of triceps brachii forming osseo-musculo-fibrous tunnels for ulnar nerves. Ann Med Health Sci Res 2013;3(3):450–452 51. Nayak SR, Krishnamurthy A, Kumar M, Prabhu LV, Saralaya V, Thomas MM. ­Four-headed biceps and triceps brachii muscles, with neurovascular variation. Anat Sci Int 2008;83(2):107–111 52. Natsis K. Supracondylar process of the humerus: study on 375 Caucasian subjects in Cologne, Germany. Clin Anat 2008;21(2):138–141 53. Boles CA, Kannam S, Cardwell AB. The forearm: anatomy of muscle compartments and nerves. AJR Am J Roentgenol 2000;174(1):151–159 54. Hodler J, Cotten A, Trudell D, Resnick D. Magnetic resonance imaging of the forearm: cross-sectional anatomy in a cadaveric model. Invest Radiol 1998;33(1):6–11 55. George V, Smith AG. Anatomic considerations of the peripheral nerve in compressive neuropathies of the upper extremity. Orthop Clin North Am 1996;27(2):211–218 56. Doyle JR. Anatomy of the upper extremity muscle compartments. Hand Clin 1998;14(3):343–364 57. Fröber R, Linss W. Anatomic bases of the forearm compartment syndrome. Surg Radiol Anat 1994;16(4):341–347 58. Otoshi K, Kikuchi S, Shishido H, Konno S. The proximal origins of the flexor-pronator muscles and their role in the dynamic stabilization of the elbow joint: an anatomical study. Surg Radiol Anat 2014;36(3):289–294 59. Ando R, Arai T, Beppu M, Hirata K, Takagi M. Anatomical study of arthroscopic surgery for lateral epicondylitis. Hand Surg 2008;13(2):85–91 60. Luong DH, Smith J, Bianchi S. Flexor carpi radialis tendon ultrasound pictorial essay. Skeletal Radiol 2014;43(6):745–760 61. Lee YM, Song SW, Sur YJ, Ahn CY. Flexor carpi radialis brevis: an unusual anomalous muscle of the wrist. Clin Orthop Surg 2014;6(3):361–364 62. Smith J, Kakar S. Combined flexor carpi radialis tear and flexor carpi radialis brevis tendinopathy identified by ultrasound: a case report. PM R 2014;6(10):956–959 63. Kumar V, George BM. An unusual palmaris longus tendon: variation in the insertion and orientation at the level of wrist joint. IJAV 2009;2:138–139 64. Regan PJ, Roberts JO, Bailey BN. Ulnar nerve compression caused by a reversed palmaris longus muscle. J Hand Surg [Br] 1988;13(4):406–407 65. Kumar N, Patil J, Swamy RS, et al. Presence of multiple tendinous insertions of palmaris longus: a unique variation of a retrogressive muscle. Ethiop J Health Sci 2014;24(2):175–178 66. Yammine K. Clinical prevalence of palmaris longus agenesis: a systematic review and meta-analysis. Clin Anat 2013;26(6):709–718 67. Sharpe F, Barry P, Lin SD, Stevanovic M. Anatomic study of the flexor carpi ulnaris muscle and its application to soft tissue coverage of the elbow with clinical correlation. J Shoulder Elbow Surg 2014;23(1):82–90 68. Ang GG, Rozen WM, Vally F, Eizenberg N, Grinsell D. Anomalies of the flexor carpi ulnaris: clinical case report and cadaveric study. Clin Anat 2010;23(4):427–430 69. Agee J, McCarroll HR, Hollister A. The anatomy of the flexor digitorum superficialis relevant to tendon transfers. J Hand Surg [Br] 1991;16(1):68–69 70. Elliot D, Khandwala AR, Kulkarni M. Anomalies of the flexor digitorum superficialis muscle. J Hand Surg [Br] 1999;24(5):570–574 (British volume) 71. Hartz CR, Linscheid RL, Gramse RR, Daube JR. The pronator teres syndrome: compressive neuropathy of the median nerve. J Bone Joint Surg Am 1981;63(6):885–890 72. Guo B, Wang A. Median nerve compression at the fibrous arch of the flexor digitorum superficialis: an anatomic study of the pronator syndrome. Hand (N Y) 2014;9(4):466–470 73. Olehnik WK, Manske PR, Szerzinski J. Median nerve compression in the proximal forearm. J Hand Surg Am 1994;19(1):121–126

MuscularSystem/Text/F/16Flexor.shtml 79. Arminio JA. Congenital anomaly of the thumb: absent flexor pollicis longus

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tendon. J Hand Surg Am 1979;4(5):487–488 80. Suresh SS, Abraham R, Kumar SS. Absent flexor pollicis longus: case report with review of literature. Hand (NY) 2009;4(1):74–77 81. Chaudhary V, Sehgal H, Bano S, Parmar PR, Kumar S. Bilateral congenital absence of flexor pollicis longus with thumb hypoplasia and thenar atrophy. Indian J Radiol Imaging 2014;24(3):268–270 82. Nebot-Cegarra J, Perez-Berruezo J, Reina de la Torre F. Variations of the pronator teres muscle: predispositional role to median nerve entrapment. Arch Anat Histol Embryol 1991;74:35–45 83. Rodríguez-Niedenführ M, Vázquez T, Nearn L, Ferreira B, Parkin I, Sañudo JR. Variations of the arterial pattern in the upper limb revisited: a morphological and statistical study, with a review of the literature. J Anat 2001;199(Pt 5):547–566 84. Canovas F, Mouilleron P, Bonnel F. Biometry of the muscular branches of the median nerve to the forearm. Clin Anat 1998;11(4):239–245 85. Tubbs RS, Beckman JM, Loukas M, Shoja MM, Cohen-Gadol AA. Median nerve branches to the pronator teres: cadaveric study with potential use in neurotization procedures to the radial nerve at the elbow. J Neurosurg 2011;114(1):253–255 86. Fuss FK, Wurzl GH. Median nerve entrapment. Pronator teres syndrome. Surgical anatomy and correlation with symptom patterns. Surg Radiol Anat 1990;12(4):267–271 87. Takada N, Otsuka T. Anatomical features of the pronator quadratus muscle related to minimally invasive plate osteosynthesis of distal radial fractures with a volar locking plate: a cadaver study. Eur Orthop Traumatol 2011;2:133–136 88. Stuart PR. Pronator quadratus revisited. J Hand Surg [Br] 1996;21(6):714–722 89. Mitsuyasu H, Yoshida R, Shah M, Patterson RM, Viegas SF. Unusual variant of the extensor carpi radialis brevis muscle: a case report. Clin Anat 2004;17(1):61–63 90. Greenbaum B, Itamura J, Vangsness CT, Tibone J, Atkinson R. Extensor carpi radialis brevis. An anatomical analysis of its origin. J Bone Joint Surg Br 1999;81(5):926–929 91. Nimura A, Fujishiro H, Wakabayashi Y, Imatani J, Sugaya H, Akita K. Joint capsule attachment to the extensor carpi radialis brevis origin: an anatomical study with possible implications regarding the etiology of lateral epicondylitis. J Hand Surg Am 2014;39(2):219–225 92. Rodríguez-Niedenführ M, Vázquez T, Parkin I, Nearn L, Sañudo JR. Incidence and morphology of the brachioradialis accessorius muscle. J Anat 2001;199(Pt 3):353–355 93. Leversedge FJ, Casey PJ, Payne SH, Seiler JG III. Vascular anatomy of the brachioradialis rotational musculocutaneous flap. J Hand Surg Am 2001;26(4):711–721 94. Cherchel A, Zirak C, De Mey A. The humeral origin of the brachioradialis muscle: an unusual site of high radial nerve compression. J Plast Reconstr Aesthet Surg 2013;66(11):e325–e327 95. Mehta V, Suri R, Arora J, Rath G, Das S. Anomalous constitution of the brachioradialis muscle: a potential site of radial nerve entrapment. Clin Ter 2010;161(1):59–61 96. Lee YK, Kim YI, Choy WS. Radial nerve compression between the brachialis and brachioradialis muscles in a manual worker: a case report. J Hand Surg Am 2006;31(5):744–746 97. Fairbank SM, Corlett RJ. The role of the extensor digitorum communis muscle in lateral epicondylitis. J Hand Surg [Br] 2002;27(5):405–409 98. Pereira BP. Revisiting the anatomy and biomechanics of the anconeus muscle and its role in elbow stability. Ann Anat 2013;195(4):365–370 99. Bergin MJ, Vicenzino B, Hodges PW. Functional differences between anatomical regions of the anconeus muscle in humans. J Electromyogr Kinesiol 2013;23(6):1391–1397

74. Chepla KJ, Goitz RJ, Fowler JR. Anatomy of the flexor digitorum profundus

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insertion. J Hand Surg Am 2015;40(2):240–244

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Muscles of Shoulder Girdle, Arm, and Forearm 100. Berton C, Wavreille G, Lecomte F, Miletic B, Kim HJ, Fontaine C. The supinator muscle: anatomical bases for deep branch of the radial nerve entrapment. Surg Radiol Anat 2013;35(3):217–224 101. Debouck C, Rooze M. The arcade of Fröhse: an anatomic study. Surg Radiol Anat 1995;17(3):245–248 102. Konjengbam M, Elangbam J. Radial nerve in the radial tunnel: anatomic sites of entrapment neuropathy. Clin Anat 2004;17(1):21–25 103. Ozturk A, Kutlu C, Taskara N, Kale AC, Bayraktar B, Cecen A. Anatomic and

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morphometric study of the arcade of Frohse in cadavers. Surg Radiol Anat 2005;27(3):171–175 104. Bravo E, Barco R, Bullón A. Anatomic study of the abductor pollicis longus: a source for grafting material of the hand. Clin Orthop Relat Res 2010;468(5):1305–1309 105. Nayak SR, Hussein M, Krishnamurthy A, et al. Variation and clinical significance of extensor pollicis brevis: a study in South Indian cadavers. Chang Gung Med J 2009;32(6):600–604 106. Brunelli GA, Brunelli GR. Anatomy of the extensor pollicis brevis muscle. J Hand Surg [Br] 1992;17(3):267–269 107. Zaino CJ, Mitgang JT, Rawat M, Patel MR. Anomalous muscles within the first dorsal extensor compartment of the wrist. Hand (N Y) 2014;9(4):551–553

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108. Fabrizio PA, Clemente FR. A variation in the organization of abductor pollicis longus. Clin Anat 1996;9(6):371–375 109. Motoura H, Shiozaki K, Kawasaki K. Anatomical variations in the tendon sheath of the first compartment. Anat Sci Int 2010;85(3):145–151 110. Türker T, Robertson GA, Thirkannad SM. A classification system for anomalies of the extensor pollicis longus. Hand (N Y) 2010;5(4):403–407 111. Kumka M. A variant extensor indicis muscle and the branching pattern of the deep radial nerve could explain hand functionality and clinical symptoms in the living patient. J Can Chiropr Assoc 2015;59(1):64–71 112. Talbot CE, Mollman KA, Perez NM, et al. Anomalies of the extensor pollicis longus and extensor indicis muscles in two cadaveric cases. Hand (NY) 2013;8(4):469–472 113. Thomas N, Cone B. Acute compartment syndrome in the upper arm. Am J Emerg Med 2017;35(3):525.e1–525.e2 114. Maeckelbergh L, Colen S, Anné L. Upper arm compartment syndrome: a case report and review of the literature. Orthop Surg 2013;5(3):229–232 115. Leversedge FJ, Moore TJ, Peterson BC, Seiler JG III. Compartment syndrome of the upper extremity. J Hand Surg Am 2011;36(3):544–559, quiz 560 116. Prasarn ML, Ouellette EA. Acute compartment syndrome of the upper extremity. J Am Acad Orthop Surg 2011;19(1):49–58

4  Upper Extremity Arteries Brandon H. Murti, Farhood Saremi, and Ramon Ter-Oganesyan

◆◆ Introduction

The arterial supply to the upper extremities begins at the arch of the aorta, which is the continuation of the ascending aorta. The aortic arch extends superiorly on the right side of the body, and courses posteriorly as it crosses the midline to end up on the left side of the body. The arch then continues down the left side of the thoracic spine as the descending thoracic aorta (▶Fig. 4.1).

◆◆ Aortic Arch

In conventional anatomy, the aortic arch gives rise to three main branches. The most central and largest is the brachiocephalic or innominate artery. This artery divides into the right subclavian artery to continue to supply the right upper extremity and the

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right common carotid artery to supply the right side of the head and neck. The second branch of the aorta is the left common carotid artery, which supplies the left side of the head and neck. The third and last branch of the aortic arch is the left subclavian artery, which supplies the left upper extremity. This standard anatomy occurs in approximately 70 to 80% of the population.1,2,3,4 Several variations of the above conventional anatomy of the arch have been observed (see Chapter 8 “Thoracic Aorta” of Volume 1). The most prevalent of these is a common origin of the brachiocephalic and left common carotid arteries from the aortic arch, which is seen in approximately 15 to 20% of people2,4 (▶Fig. 4.2). Less common, but similar to the variant above, is origination of the left common carotid artery from the brachiocephalic artery itself, also known as truncus bicaroticus, is seen in 9% of the population.1 These two variations have been historically described

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Fig. 4.1 Normal aorta and its major branches. (a) Anterior view and (b) left anterior oblique view of three-dimensional computed tomography (CT) angiography of the aorta and its branches with volume-rendered reconstruction. (c) Anterior view of aortography. AA, ascending aorta; Arch, aortic arch; BA, bronchial artery; BCA, brachiocephalic artery; CCA, common carotid artery; CCT, costocervical trunk; DA, descending aorta; DCA, deep cervical artery; DSA, dorsal scapular artery; ICA, intercostal artery; ITA, internal thoracic artery; R(L)CCA, right (left) common carotid artery; R(L)SCA, right (left) subclavian artery; R(L)VA, right (left) vertebral artery; SSA, suprascapular artery; TCA, transverse cervical artery; TCT, thyrocervical trunk.

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Fig. 4.2 Left anterior oblique view of the three-dimensional computed tomography (CT) angiography showing (a) common origin (arrow) of the brachiocephalic artery (BCA) and left common carotid artery (LCCA), which is seen in approximately 15 to 20% of people. (b) Direct origin of the left vertebral artery (arrow) from the aortic arch between the LCCA and left subclavian artery (LVA) in a patient with coarctation of the aorta. LSA, left subclavian artery; RCCA, right common carotid artery; RSA, right subclavian artery.

and commonly referred to in the medical literature as the “bovine arch.” This is a misnomer, as the arch system in the bovine family of animals is quite different, and demonstrates a brachiocephalic trunk which gives rise to bilateral subclavian arteries and a bicarotid trunk. Perhaps more like a true “bovine arch” is the rare common origin of the brachiocephalic artery, left common carotid artery, and left subclavian artery, or a single brachiocephalic trunk which then bifurcates into right and left brachiocephalic arteries, with the left similarly giving rise to the left common carotid artery and left subclavian atery.5 A relatively more common aortic arch variant, seen in approximately 1 to 6% of the population, involves the left vertebral artery.2,3,4 This usually arises from the left subclavian artery, but can originate directly from the aorta, typically as a third branch between the left common carotid artery and left subclavian artery (▶Fig. 4.2). Aortic origin of the left vertebral artery is also seen in association with common origin of the brachiocephalic and left common carotid arteries in approximately 1 to 2% of people.3 Rarely the right vertebral artery originates from the right common carotid artery associated with an aberrant right subclavian artery. The left vertebral artery may demonstrate a double origin from the left subclavian artery and aortic arch.6 Another rare but interesting variant is the aberrant origin of the right vertebral artery from the aortic arch distal to the origin of the left subclavian artery. This vessel usually has a retroesophageal course, hence named “vertebral arteria lusoria” (▶Fig. 4.3). Knowledge of this anomaly is important during esophagectomy and aortic surgery.7 Another variation to conventional arch anatomy is an aberrant right subclavian artery (also known as arteria lusoria) which typically arises as a fourth branch of the aortic arch after the left subclavian artery and is seen in approximately 0.5 to 2% of the population.2,3,4,8 There is variable relationship to the esophagus as it courses back to the right side of the body3 (▶Fig. 4.4). Most common is a course posterior to the esophagus (80%), followed by in-between the esophagus and trachea (15%), and least commonly anterior to the trachea (5%).7 Depending on its course, this variant

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Fig. 4.3 Vertebral arteria lusoria (yellow arrows). Aberrant origin of the right vertebral artery from the aortic arch distal to the origin of the left subclavian artery (LSCA). This vessel usually has a retroesophageal course, hence named “vertebral arteria lusoria.” Note the funnel-shaped origin of it from the posterior wall of the aortic arch (red arrow). Knowing this anomaly is important during esophagectomy or aortic surgery.

may cause symptoms related to the compression of the esophagus (dysphagia lusoria) and/or trachea (dyspnea). Additional consequences of an aberrant right subclavian artery include proximal aneurysmal dilatation (Kommerell diverticulum) and a nonrecurrent right laryngeal nerve. An aberrant left subclavian artery can

Upper Extremity Arteries

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Fig. 4.4 Aberrant right subclavian artery (ARSA) compressing the upper esophagus causing dysphasia. (a) Axial computed tomography (CT), (b) sagittal CT, and (c) superior, volume-rendered views of the aortic arch demonstrating ARSA (red arrows) coursing behind the upper ­esophagus (green arrow) and compressing it. LCCA, left common carotid artery; LSA, left subclavian artery.

Fig. 4.5 Thyroidea ima. (a) Brachiocephalic artery catheter angiogram showing the thyroidea ima branch arising from the superior wall (arrow) of the artery in this 69-year-old male with a history of resection of base of tongue tumor and recent tracheostomy. The patient presented with acute bleeding through his tracheostomy site and went into cardiac arrest but was resuscitated. (b) Selective catheterization of the thyroidea ima showing active bleeding which was successfully embolized.

also occur, but this only occurs in conjunction with a right-sided aortic arch, which demonstrates the reverse of normal anatomy. Other rare aortic arch variants have been observed including right and left brachiocephalic arteries arising separately off the arch, a bicarotid trunk with separate origins of the right and left subclavian arteries, which may include an aberrant right subclavian artery, common origin of the carotid arteries along with common origin of the subclavian arteries, absence of the brachiocephalic artery, and an additional artery originating from the aortic arch such as the thyroidea ima (thyroid ima) artery (see below). The brachiocephalic artery, which measures 4 to 5 cm, most often has no branches other than its terminal branches, the right subclavian and right common carotid arteries. An uncommon artery named the thyroidea ima artery has been observed in

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approximately 1.5 to 12.2% of people (▶Fig. 4.5). When present, it most often arises as a branch of the brachiocephalic artery (1.9– 10.5%).9 Less commonly, it can also originate from the common carotid artery, aortic arch, thyrocervical trunk, or internal thoracic artery. When an anomalous artery supplying the inferior thyroid gland arises directly off the subclavian artery, it is referred to as an accessory inferior thyroid artery. Thymic and bronchial branches can also rarely arise from the innominate artery. Thyroidea ima is 3 to 5 mm in diameter and ascends in front of the trachea to the bottom of the thyroid gland. It supplies the inferior thyroid gland, often in association with absent inferior thyroidal arteries but it can also perfuse the trachea and the parathyroid glands. Diagnosis of the thyroidea ima is important before surgical or percutaneous interventions,10 as injury to it can be a cause of severe bleeding during thyroidectomy and percutaneous tracheostomy (▶Fig. 4.5).

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Fig. 4.6 Variations to the shape of the aortic arch (angulation and tortuosity). Vertical distance from the (middle of the) origin of the innominate artery to the top of the arch (Inn-Top). Type I: 2 LCCA diameters.

Shape of the Aortic Arch Increasing tortuosity and angulation of the aortic arch may interfere with cannulation of the arch vessels during interventional studies of the upper extremity and cranial vessels. Knowing its variation has been suggested as a relevant anatomic feature as it increases the risk of endoleak and graft migration in patient with aortic dissection who have been scheduled for endovascular stent graft placement, probably due to reduced contact surface between the endograft and the native aorta.11 Variation in tortuosity and angulation of the aortic arch has been classified into type I, type II, or type III based on the distance between the origin of the brachiocephalic artery and the top of the arch (▶Fig. 4.6): Type I: 2 LCCA diameters.11 It has been shown that type II and type III arches have reduced stent–native aortic contact surface with a resultant higher rate of endoleak and stroke compared with type I.12

◆◆ Embryology

The mechanisms underlying the development and maintenance of the vasculature of the developing limb is not completely understood. Based on classic “sprouting” theory, the arteries of the developing upper limb arise successively from a single trunk of a primary axial artery.13 The dominant vessel of the upper limb in the early stages of embryonic life is the subclavian artery which is continuous with the primary axial artery. Later, the primary axial artery forms the axillary and brachial arteries in the arm and the common anterior and posterior interosseous arteries in the forearm. The ulnar and radial arteries appear in later stages. A superficial brachial artery from the axillary artery develops and continues as the radial artery. The definitive radial artery, however, develops following regression of the median artery and anastomosis between the distal part of the brachial artery and superficial brachial artery, with concurrent regression of proximal segment of superficial brachial artery. As the limb bud grows, the common interosseous artery becomes incorporated into the ulnar artery.

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Brachial artery

a

Radial artery

Brachial artery

Interosseous artery

b

Median artery Ulnar artery

Fig. 4.7 Arterial remodeling in the developing upper limb between Carnegie stages 17 and 18. (a) Stage 17. At this stage, the vascular remodeling occurs at the level of the elbow, where the brachial artery branches into the capillary network. Alternative pathways of blood flow resulting in arterial variations occur at this stage. (b) Stage 18. The definitive origin of the radial artery is established at this stage. The distal portion of the radial artery remains in a capillary state, while the interosseous, median, and ulnar arteries can be traced along their entire course to the hand.

According to recent theories,14 the definitive arterial pattern of the upper limb is formed from the primitive capillary plexus known as “vascular labyrinth.” In this capillary plexus, the dominant arterial channels gradually differentiate as a result of capillary remodeling (▶Fig. 4.7). It is hypothesized that this mechanism of arterial development along with additional gene regulators may give rise to variations of the definitive arterial pattern.15 Therefore, some typically seen vessels may disappear

Upper Extremity Arteries or incompletely develop, whereas remnants of primitive channels may persist.16 Examples include anastomotic connections between the brachial artery and the median or the anterior interosseous artery, the presence of a persistent median artery, a rudimentary radial artery connected by anastomosing branches with the axillary artery (“superior root”), the “deep” median artery (“middle root”) or the anterior interosseous artery (“inferior root”),16 and variation in palmar arches.17 Knowledge of variant branching pattern of upper extremity arteries is important in the reconstructive and vascular surgery as well as during angiographic procedures.

◆◆ Anatomy

Subclavian Artery The arterial supply to the upper extremity follows the subclavian artery, typically either as bifurcation of the innominate artery on the right or arising directly from the aorta on the left. The subclavian artery exits the thorax at the superior thoracic aperture and travels between the anterior and middle scalene muscles, although it can sometimes course superficial to or through the anterior scalene (▶Fig. 4.8). The subclavian artery is divided into three parts in relation to the anterior scalene muscle (▶Fig. 4.8, ▶Fig. 4.9). The first part begins at the bifurcation and extends to the medial margin of the anterior scalene, the second part is posterior to the anterior scalene, and the third part spans the lateral border of the anterior scalene to the lateral border of the first rib, after which point it is referred to as the axillary artery. As can be expected, the first portions of the right and left subclavian arteries are slightly different as they have different origins, but the second and third parts of the subclavian arteries are similar (▶Fig. 4.1). The first part of the left subclavian artery travels in

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the groove along the mediastinal margin of the left upper lobe. In some cases, the first part of the left subclavian artery may invaginate into the left upper lobe18 (▶Fig. 4.10). The first major branches of the subclavian artery include the vertebral and internal thoracic (mammary) arteries and the thyrocervical trunk (▶Fig. 4.11, ▶Fig. 4.12). On the left, the costocervical trunk usually arises from the first portion of the subclavian artery, but from the right, it arises from the second part (▶Fig. 4.12, ▶Fig. 4.13). The costocervical trunk usually bifurcates into the deep cervical and highest (supreme) intercostal arteries (▶Fig. 4.9). The first two posterior intercostal arteries originate from the supreme intercostal artery (▶Fig. 4.9). The deep cervical artery can occasionally arise directly from the subclavian artery. The dorsal scapular artery arises from either the second or third (more common) parts of the subclavian artery or as a branch of the transverse cervical artery (▶Fig. 4.11). Compression of the subclavian/axillary artery as it passes through the superior thoracic aperture can result in one of the three distinct thoracic outlet compression syndromes. The most common of the compression syndromes is neurogenic thoracic outlet syndrome, which results from compression of the brachial plexus, while the second most common results from compression of the subclavian vein. Arterial thoracic outlet syndrome is the least common form and can present with upper extremity ischemia causing pain, weakness, paresthesia, coolness, pallor, fatigue, claudication, decreased pulses, and nonhealing wounds or ulcers. There is usually an association with a cervical rib or other compressive abnormality, including but not limited to scalenus anticus syndrome which results from abnormal insertion of the anterior scalene on the first rib. Compression can occur in the scalene triangle (between the anterior and middle scalene muscles), costoclavicular space (between the clavicle and first rib), or subpectoral space (between the pectoralis minor and coracoid process). Passage of the subclavian artery anterior to the anterior scalene is also reported.19

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Fig. 4.8 Relationship of the scalene muscles with the subclavian vessels. The anterior scalene inserts on the tubercle on the anteromedial aspect of the first rib anterior to the subclavian artery. The middle scalene attaches to the upper surface of the first rib. The posterior scalene inserts to outer surface of the second rib. The lateral margin of the first rib is the border between the subclavian and the axillary arteries.

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Upper Extremity Arteries Vertebral artery

Anterior view

Deep cervical artery Transverse cervical artery

Internal carotid artery External carotid artery

1st posterior intercostal artery

Inferior thyroid artery

Suprascapular artery

Thyrocervical trunk

Acromion and acromial anastomosis

1

Dorsal scapular artery

2

Supreme intercostal artery

4

Ascending cervical artery

Subscapular artery Circumflex scapular

2nd posterior intercostal artery Subclavian artery Internal thoracic artery

Acromial artery Deltoid

Costocervical trunk

Superior thoracic artery

3

Thoraco-acromial artery

Anterior circumflex humeral artery

Clavicular branch

Posterior circumflex humeral artery Thoracodorsal artery

Pectoral branch Lateral thoracic artery

*1, 2, and 3 indicate 1st, 2nd, and 3rd parts of axillary artery

Fig. 4.9 Subclavian and axillary arteries and their branches.

decompression, stenting may be used to treat underlying residual stenosis or other luminal irregularities.

Thyrocervical Trunk

Fig. 4.10 Left subclavian artery invagination. Intrapulmonary course of the left subclavian artery is seen (arrow). Both the visceral and parietal pleurae surround the artery.

The compression of the subclavian artery can result in ulcerative lesions or aneurysms with subsequent associated mural thrombus formation, which can then result in distal embolic phenomena. Signs and symptoms or arterial thoracic outlet syndrome are exacerbated by certain arm positions and maneuvers, such as when raised (abducted) above the head. Definitive ­treatment involves thoracic outlet decompression, including possible rib resection. Endovascular therapy may be used initially, however, will likely only involve thrombolysis and/or thrombectomy of the compressed artery if thrombus is present. Primary stenting of this region is usually not performed prior to surgical decompression given unresolved strong extrinsic compression, which may result in the stent being crushed. Following surgical

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The thyrocervical trunk arises from the first segment of the subclavian artery immediately distal to the vertebral artery, and gives rise to branches supplying blood to the cervical region and upper limb (▶Fig. 4.11). Soon after takeoff, it branches into the inferior thyroid, suprascapular, ascending cervical, and transverse cervical arteries (▶Fig. 4.11). Anomalous origin of the vertebral artery or internal thoracic artery from the thyrocervical trunk is rarely seen.20,21 The suprascapular artery runs laterally along the superior border of the scapula to enter the supraspinatous fossa to travel between the scapular bone and the supraspinatus muscle (▶Fig. 4.12, ▶Fig. 4.14). The artery then passes through the great scapular notch (spinoglenoid notch) to the infraspinatous fossa, where it anastomoses with the circumflex scapular artery and the descending branch of the transverse cervical artery (▶Fig. 4.14). The transverse cervical artery runs laterally, parallel to the suprascapular artery (▶Fig. 4.12). The dorsal scapular artery most commonly arises directly from the subclavian artery, but in 38% of cases, it originates from the transverse cervical artery, which is a branch of the thyrocervical trunk22 (▶Fig. 4.11, ▶Fig. 4.15). It may originate as a common trunk with the transverse cervical and together form the cervicodorsal trunk. On rare occasion, it arises from the costocervical trunk (▶Fig. 4.12). It travels inferiorly, together with the dorsal scapular nerve, along the medial border of the scapula (▶Fig. 4.14). The dorsal scapular artery makes anastomoses with the subscapular artery branches (▶Fig. 4.14).

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Fig. 4.11 (a) Right subclavian angiography (frontal view). (b) Selective angiography of the right thyrocervical trunk. Typically, it branches into the inferior thyroid, suprascapular, ascending cervical, and transverse cervical arteries. Note the origin of the dorsal scapular artery (DSA) from the transverse cervical artery (TCA). The ascending cervical artery (ACA) also arises from the TCA. ACA, ascending cervical artery; DCA, deep cervical artery; ITA, internal thoracic artery; iThyA, inferior thyroidal artery; SupSA, suprascapular artery; VA, vertebral artery.

Fig. 4.12 Branches of the subclavian artery (SCA) shown by computed tomography (CT) angiography. The thyrocervical trunk is colored green and the costocervical trunk is red. The transverse cervical artery runs parallel to the suprascapular artery. The right costocervical trunk is arising from the second portion of the subclavian artery and splits into the deep cervical and superior (supreme) intercostal arteries. The dorsal scapular artery is arising from the costocervical trunk. Also seen is abnormal origin of the suprascapular from the internal mammary artery (IMA). CCA, common carotid artery. (Continued)

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Fig. 4.12 (Continued)

Axillary Artery

Fig. 4.13 Selective angiography of the left costocervical trunk arising from the first portion of the subclavian artery. Second ICA, second intercostal artery; DCA, deep cervical artery; HICA, highest intercostal artery; RA, radicular artery; VB, vertebral body branch.

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The subclavian artery continues as the axillary artery at the l­ ateral margin of the first rib to the lower border of teres major (▶Fig. 4.16). It is in intimate relation with the cords and branches of brachial plexus. Similar to the subclavian artery, there are three parts of the axillary artery defined in relation to the pectoralis minor muscle, which lies superficial to the artery (▶Fig. 4.9, ▶Fig. 4.16). The first part is proximal, second part posterior, and third part distal to the pectoralis minor. Generally, the axially artery gives rise to six branches, although the number of branches may vary as two or more branches often arise as a single trunk or smaller branches may arise individually. The major conventional branching patterns of the axillary artery include the origin of superior thoracic artery from the first part, the thoracoacromial and lateral thoracic arteries from the second part, and the subscapular, posterior circumflex humeral, and anterior circumflex humeral arteries from the third part (▶Fig. 4.9, ▶Fig. 4.17). The branching pattern of these arteries is quite variable, mainly involving the subscapular and posterior circumflex humeral arteries (66%), with the most common of these being high origin of the subscapular artery from the second part of the axillary artery (29–36%), or common origin of the subscapular and posterior circumflex humeral arteries (▶Fig. 4.17), either from the second or third part of the axillary artery (12–42%).23,24,25 The subscapular artery typically gives rise to the circumflex scapular artery and thoracodorsal artery, but axillary origin of the thoracodorsal artery has been observed in 3 to 10% of people.26,27 Another variation is the superior thoracic artery or lateral thoracic artery arising from the thoracoacromial artery. Early branching of the axillary artery into two brachial arteries is possible, in which case one runs a superficial course and is called the superficial brachial artery, and the other takes a deep course and is called the deep

Upper Extremity Arteries Vertebral artery Thyrocervical trunk Subclavian artery Transverse cervical artery

Suprascapular artery

Acromial branches

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Circumflex scapular artery

Axillary artery Anterior circumflex humeral artery

Dorsal scapular artery Posterior circumflex humeral artery Subscapular artery Thoracodorsal artery

Profunda brachii artery Brachial artery

Fig. 4.14 Scapular arterial network.

Fig. 4.15 (a) Dorsal scapular and subscapular arteries. The dorsal scapular artery arises from the third segment of the subclavian artery. (b) Common trunk of the subscapular and posterior circumflex humeral artery is seen in up to 42%. The subscapular artery arises from the ­second or third segment of the axillary artery and typically gives rise to the circumflex scapular artery and the thoracodorsal artery.

brachial artery.28 The axillary artery can also give rise to the radial and ulnar arteries as discussed below.29 The peri-scapular circulatory anastomoses provide collaterals between the first segment of the subclavian artery and the third segment of the axillary, especially important when there is axillary artery occlusion. These arteries include the transverse cervical, dorsal scapular, suprascapular, and subscapular arteries (▶Fig. 4.14).

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Certain chronic inflammatory conditions of the lungs, such as chronic bronchiectasis, pulmonary tuberculosis, and chronic obstructive pulmonary disease, can result in hypertrophy of the central upper extremity arteries supplying the chest and neck, and branches of the thoracic aorta supplying the chest and lungs (bronchial, intercostal, and inferior phrenic arteries). The hypertrophy of the bronchial arteries in chronic inflammatory lung conditions is self-explanatory. The pulmonary supply and subsequent

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Fig. 4.16 Axillary artery segments. The axillary artery extends between the lateral margin of the first rib to the lower border of teres major. The first part is proximal, second part posterior, and third part distal to the pectoralis minor. SCA, subclavian artery.

Fig. 4.17 Axillary artery branches. (a) Typical pattern. (b) Typical branching of the subscapular artery. (c) High origin of the subscapular artery from the second part of the axillary artery. Also seen is the superior thoracic artery and lateral thoracic artery arising from the subscapular artery. (d) Common origin of the subscapular and posterior circumflex humeral arteries. IMA, internal mammary artery.

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Upper Extremity Arteries hypertrophy of the neck and chest arteries is a result of the development of transpleural arterial anastomoses and collateral arterial supply recruitment. The upper extremity arterial branches that are most involved in transpleural collateralization are the internal mammary artery, branches of the thyrocervical and costocervical trunks, thoracoacromial artery, dorsal scapular artery, subscapular artery, the lateral thoracic artery, and the thoracodorsal artery. Knowledge of this collateral anatomy is especially important to the interventional radiologist when treating hemoptysis associated with chronic pulmonary inflammatory conditions with embolization. Hypertrophy of the axillary arteries is also seen in hypervascular soft tissue or osseous tumors in this region (▶Fig. 4.18).

Thoracoacromial Artery The thoracoacromial artery arises from the first or second segment of the axillary artery, proximal to the coracoid attachment of the pectoralis minor.23 Major branches include the pectoral, acromial, clavicular, and deltoid. The pectoral branch runs

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between the pectoralis major and minor. The deltoid branch moves laterally to run in the groove between the deltoid and pectoralis muscles along with the cephalic vein. Rarely the radial artery may arise directly from the thoracoacromial artery.30

Lateral Thoracic Artery The lateral thoracic artery originates from the axillary artery or one of its branches and travels along the lateral chest wall to supply the serratus anterior, pectoralis major, and subscapularis muscles (▶Fig. 4.19). Branches are sent to the axilla and breast soft tissues. The lateral thoracic artery may originate from the axillary, thoracoacromial, thoracodorsal, suprascapular, or subscapular arteries31,32 (▶Fig. 4.17, ▶Fig. 4.19). The most common variant origin of the lateral thoracic artery is from the thoracoacromial artery (67%) and the second most common is the axillary artery (17%). Origin of the thoracodorsal artery from the lateral thoracic artery is seen in 7% and the subscapular artery from the lateral thoracic artery in 5%.25

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Fig. 4.18 Hypervascular right scapular metastasis from renal cell carcinoma. (a) Before and (b) after embolization of the circumflex scapular artery.

Fig. 4.19 Right lateral and left lateral computed tomography (CT) angiograms in a patient with aortic coarctation causing enlargement of the thoracic wall arteries. On the right side, both the lateral thoracic and thoracodorsal arteries arise from a common trunk. This variant is seen in 5 to 7%. The subscapular artery divides into the circumflex scapular and thoracodorsal arteries. Note extensive periscapular collaterals in this patient with coarctation of the aorta.

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Subscapular Artery The subscapular artery, the largest branch of the axillary artery, arises at the lower border of the subscapularis muscle and ­lateral to the pectoralis minor, and flows to the inferior angle of the scapula, where it anastomoses with the lateral thoracic, intercostal arteries, and descending branch of the dorsal scapular artery.25 It supplies the serratus anterior, latissimus dorsi, and subscapular muscles.27 High origin of the subscapular artery is seen in 35% (▶Fig. 4.17). A common variant is common o ­ rigin with the posterior circumflex humeral artery in 42%23 (▶Fig. 4.15, ▶Fig. 4.17). The artery divides into two major branches, the circumflex scapular and thoracodorsal arteries (▶Fig. 4.17). The circumflex scapular branch enters the infraspinous fossa on the dorsal surface of the scapula. Duplicated circumflex scapular arteries are found in 4% of patients and in 3% the artery branches directly from the axillary artery.33 The thoracodorsal artery supplies branches to the latissimus dorsi and serratus anterior muscles. It travels inferiorly with

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the thoracodorsal nerve. In 10% it originates directly from the axillary artery.23 This artery has an important place in reconstructive surgery. The thoracodorsal artery pedicle is used in several different flaps, primarily latissimus dorsi and serratus anterior flaps.

Circumflex Humeral Artery The posterior and anterior circumflex humeral arteries are crucial for the blood supply of humeral head. Both arteries are typically final branches of the third part of axillary artery in up to 90% of individuals and run in close proximity to the proximal humerus (▶Fig. 4.20). The posterior circumflex humeral artery originates from a common trunk with the anterior circumflex humeral artery in 10 to 30% of individuals.34,35 Other variants include the origin of the posterior circumflex humeral artery from the deep brachial artery, the subscapular artery, or the lateral thoracic artery (▶Fig. 4.15, ▶Fig. 4.17, ▶Fig. 4.20).

Fig. 4.20 Anatomy of the circumflex humeral arteries. (a) Typical origin. (b) Selective injection showing the extent of branches toward the deltoid and posterior arm compartment muscles. (c) Catheter angiogram and (d) computed tomography (CT) angiogram (different patients) showing variant anatomy with unusual origin of the posterior circumflex humeral and profunda brachii arteries from the subscapular trunk.

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Fig. 4.21 Relationship of the brachial artery with the median nerve and the profunda brachii with the radial nerve in a hyperextended arm. In the arm, the median nerve crosses superficial to the brachial artery latero-medially and enters the cubital fossa.

Injury to axillary artery, especially the posterior circumflex where it passes in quadrilateral space in the setting of repeated trauma, proximal humeral fractures, and related surgeries, can result in avascular necrosis.34,36 Narrowing, aneurysm, and thrombosis of the proximal posterior circumflex humeral artery in the dominant shoulder may be a source of distal emboli and resultant inschemia of the digits in athletes, such as volleyball players.36 Aberrant posterior circumflex humeral artery passing under the tendon of the latissimus dorsi muscle may be injured by muscle contractions.37

Brachial Artery The brachial artery is the major arterial supply to the arm and is the continuation of the axillary artery at the inferior margin of the teres major, deep and medial to the median nerve (▶Fig. 4.21, ▶Fig. 4.22). The brachial artery travels down the arm and bifurcates into its terminal branches, the radial and ulnar arteries, at the antecubital fossa. The main branch of the brachial artery in the upper arm is the profunda brachii (▶Fig. 4.20, ▶Fig. 4.21, ▶Fig. 4.22, ▶Fig. 4.23). It divides into the radial collateral and the middle collateral arteries.38 Additional branches of the brachial artery include nutrient, superior and inferior ulnar collateral, and muscular arteries. The superior ulnar collateral artery runs along the ulnar nerve and anastomoses with the inferior ulnar collateral and posterior ulnar recurrent arteries (▶Fig. 4.24).

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Unilateral brachial artery variations are more common than bilateral. The most common variation in this situation is the superficial brachial artery (4–12%), which courses superficial, instead of deep to the median nerve. Because of its superficial course, this variant is vulnerable to injury. The superficial brachial artery is an important vessel in fetal life for replacing or supporting the definitive brachial and radial arteries. The superficial brachial artery first appears in the 21-mm-long embryo and descends from the medial side to the lateral side of the forearm to reach the posterior surface of the wrist joint. Following regression of the median artery, the proximal portion of the superficial brachial artery also disappears, and its distal part connects to the brachial artery to remain as the radial artery. The superficial brachial artery may replace the main brachial artery trunk completely or may be accompanied by a smaller or equal sized deep brachial artery.39 Rarely, the axillary artery bifurcates into superficial and deep branches to form a duplicated brachial artery in which the superficial division continues as the brachial artery and the deep division functions as the axillary artery by giving all related branches and then continues as the profunda brachii artery.40,41,42 Yang et al43 classified the superficial brachial artery into three types (▶Fig. 4.25). In type I, the superficial brachial artery is the dominant brachial artery and divides into the radial and ulnar arteries. In type II, it moves parallel to the deep brachial artery and continues in the forearm as the radial artery. Type II variant is also

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Fig. 4.22 Relationship of the arteries and nerves of the arm. The profunda brachii (deep brachial) is the largest branch of the brachial artery. It passes with the radial nerve, through the lateral triangular space to enter the posterior compartment. The brachial artery runs with the median nerve.

known as, a high origin of the radial artery, which is the most frequent variation in Western populations. Type III is a rare variant in which the superficial brachial artery is short and disappears in the upper arm into small cutaneous blood vessels. Nerve entrapment syndromes have been described due to unusual course of the arteries. The median nerve is formed lateral to the axillary artery by the union of its medial and lateral roots originating respectively from the medial and lateral cords of the brachial plexus. In the arm, the median nerve crosses superficial to the brachial artery from lateral to medial and enters the cubital fossa39,44 (▶Fig. 4.21). The median nerve or its roots may be entrapped between the superficial brachial artery and the axillary artery. The profunda brachii (deep brachial) is the largest branch of the brachial artery, arising as a single trunk at the level of the tendon of the teres major muscle in 55 to 70% of the cases. It travels in the lateral triangular interval with the radial nerve and along the radial groove of the humerus and continues as the radial collateral to anastomose with the radial recurrent artery (▶Fig. 4.22, ▶Fig. 4.23). The profunda brachii is rarely absent. In the triangular interval, the profunda brachii may communicate with the posterior circumflex humeral artery (▶Fig. 4.20). It may arise from the axillary artery in 22%, as a common trunk with the superior ulnar collateral artery in 22%, or less commonly as a branch of the circumflex humeral artery or subscapular artery (▶Fig. 4.20), or a single trunk with the common interosseous arteries. The deep

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Fig. 4.23 Deep and collateral branches of the brachial artery. Angiograms of two different patients are shown. The profunda brachii is the main branch of the brachial artery. It divides into the radial collateral and the middle collateral arteries. It continues as the radial collateral to anastomose with the radial recurrent artery.

brachial artery may give rise to the anterior circumflex humeral artery.

Radial and Ulnar Arteries The terminal branches of the brachial artery are the radial and ulnar arteries, which divide into multiple named branches to supply the forearm, wrist, and hand (▶Fig. 4.24, ▶Fig. 4.26). The ulnar artery is usually larger than the radial artery and is the principal source of blood supply to the proximal forearm, while the radial artery typically supplies the distal forearm and hand.45 Two venae comitantes accompany each artery (▶Fig. 4.24). Within the cubital fossa, both the radial and ulnar arteries send recurrent branches that anastomose with those of the brachial and profunda brachii arteries (▶Fig. 4.23, ▶Fig. 4.26). The radial artery courses along the lateral aspect of the forearm to the wrist. The radial recurrent artery is the first branch of the radial artery and originates just distal to the bifurcation of the brachial artery. It supplies the brachioradialis, extensor carpi radialis longus and brevis, and supinator muscles (▶Fig. 4.27). The radial recurrent artery extends superiorly into the arm anastomosing with the terminal branches of the profunda brachii. More distally in the forearm, the radial artery contributes to multiple muscular branches. Distally, the radial artery gives rise to the palmar and dorsal carpal branches, the first dorsal metacarpal artery, and the princeps pollicis artery.

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Fig. 4.24 Relationship of the arteries and nerves of the forearm. The posterior ulnar recurrent artery runs along with the ulnar nerve and anastomoses with the inferior ulnar collateral artery. The posterior compartment of the forearm is located posterior and lateral to the radial artery. The superficial branch of the radial nerve runs close to the radial artery in the midarm.

The ulnar artery passes medial to the median nerve, deep to pronator teres muscle, and anterior to the brachialis and flexor digitorum profundus muscles (▶Fig. 4.21, ▶Fig. 4.27). Distally, it runs in proximity to the ulnar nerve, and then pierces the deep fascia just proximal to the flexor retinaculum to form the palmar arches. The first branches of the ulnar artery are the anterior and

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posterior ulnar recurrent arteries, followed by the short common interosseous artery which quickly bifurcates into the anterior and posterior interosseous arteries at the upper border of the interosseous membrane. The terminal branches of the ulnar artery are the dorsal carpal branch, and the superficial and deep palmar arches.

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Lateral cord

Lateral cord

Lateral cord

Medial cord

Medial cord

Medial cord

Superficial brachial artery

Superficial brachial artery

Superficial brachial artery

Median nerve

Median nerve

Median nerve

Brachial artery

Brachial artery

Brachial artery

Ulnar artery

Ulnar artery

Radial artery

Radial artery

Ulnar artery Radial artery

Type I

Type II

Type III

Fig. 4.25 Different types of superficial brachial artery (SBA). Type I SBA: The SBA bifurcates into the radial and the ulnar arteries after giving muscular branches to the biceps brachii and brachialis muscles. Type II SBA: After giving muscular branches to the biceps brachii and brachialis muscles, the SBA continues as the radial artery in the forearm. This variant is also known as a high origin of the radial artery which is the most common of the three. Type III SBA: The slender SBA supplies the arm musculature and ends in the upper arm.

Fig. 4.26 Terminal branches of the brachial artery. (a, b) The radial and ulnar arteries and their branches around the elbow are shown. The radial recurrent artery is the first branch of the radial artery and anastomoses with the terminal branches of the profunda brachii. The anterior and posterior ulnar recurrent arteries are the first branches of the ulnar artery, followed by the short common interosseous artery which quickly bifurcates into the anterior and posterior interosseous arteries. AIA, anterior interosseous artery; CIA, common interosseous artery; PIA, posterior interosseous artery.

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Fig. 4.27 Relationship of the radial and ulnar arteries with nerves and muscles. The ulnar artery passes medial to the median nerve, deep to pronator teres, and anterior to the brachialis and flexor digitorum profundus muscles. It runs distally close to the ulnar nerve. The superficial branch of the radial nerve runs close to the radial artery in the midarm and stays under the tendon of brachioradialis.

Brachial artery Radial recurrent artery Ulnarrecurrent artery

Brachial artery

a

Radial recurrent artery

High radial artery High radial artery

Radial artery

Radial recurrent artery

Radial recurrent artery

Common interosseous artery

Ulnar artery

Brachial artery

Brachial artery

High radial artery

Anterior interosseous artery

b

Ulnarrecurrent artery

Common interosseous artery

Median artery Ulnar artery Anterior interosseous artery

c

Radial artery

Ulnar artery

d

Fig. 4.28 Select anatomical variations of the radial artery in the forearm. (a) Typical course of the radial artery. (b) High radial artery formed by three roots (anastomosing branches): the superior root from the axillary artery (not illustrated), the middle root from the median artery, and the inferior root from the anterior interosseous artery. (c) Radial artery replaced by an atypical branch of the anterior interosseous artery. (d) Low division and trifurcation of the brachial artery.

Several variations of the radial and ulnar arteries have been described, which like brachial artery variations occur more commonly unilaterally than bilaterally (▶Fig. 4.28, ▶Fig. 4.29). The radial artery usually arises from the brachial artery 3.5 to 4 cm distal to the intercondylar line of the humerus.46,47,48,49,50 Its most common variant is high origin of the radial artery, also known as a brachioradial artery, found in 15 to 20% of people in a cadaveric

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series, and when present most commonly arises from the upper third of the brachial artery (65%) followed by the axillary artery (32%) (▶Fig. 4.30). High origin of the radial artery is classified as a variant of the superficial brachial artery by some investigators (▶Fig. 4.25). The radial artery can also arise from the middle and lower thirds of the brachial artery. The ulnar artery, similarly to the radial arterial variant, can also rarely have a high origin, again

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Fig. 4.29 Variations of the radial and ulnar arteries. The radial artery usually arises from the brachial artery 3.5 to 4 cm distal to the intercondylar line of the humerus. (a) Trifurcation of the brachial artery. (b) Hypoplastic radial artery.

from either the upper third of the brachial artery, the axillary artery, or the superficial brachial artery. A high origin of the ulnar artery is termed the “superficial ulnar artery” and is seen in 1 to 9% of the population.46,47,48 This variant passes superficial to the superficial flexors of the forearm, and the palmaris longus muscle may be absent. Since this artery is closely related to the median cubital vein, it could be unintentionally punctured during venipuncture attempts of the median cubital vein. Combined high origin of the radial and ulnar arteries has also been rarely observed (1%), and is referred to as a brachioulnoradial artery, which usually arises from the axillary artery. If the radial artery at the wrist courses over the tendons and subcutaneously along the superficial branch of the radial nerve, it is referred to as a superficial radial artery, a very rare variant occurring in less than 1% of people. The radial and ulnar arteries can also be hypoplastic, absent, or duplicated48 (▶Fig. 4.29). Variation of the radial recurrent artery is not uncommon. The main radial recurrent artery originates from radial artery in 65% of the population. It may originate from the radioulnar division, the brachial artery, or the ulnar-interosseous trunk. A second or accessory radial recurrent artery is seen in up to 30% of the people, and usually arises from the brachial artery, proximal to the origin of the radial artery, and passes posterior to the bicipital tendon.49,50

Interosseous Artery The embryonic primary axial artery forms brachial artery in the arm and the common interosseous artery in the forearm. As the limb bud grows, the common interosseous artery becomes incorporated into the ulnar artery. The common interosseous artery is

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the principal branch of the ulnar artery in the forearm. It passes to the upper border of interosseous membrane and divides into the anterior and posterior interosseous arteries. In cases where the distal radial artery is hypoplastic, the anterior interosseous artery may supply the hand (▶Fig. 4.26). The posterior interosseous artery usually originates from the common interosseous artery, but in 20%, it originates from the ulnar artery.51 After passing between the chorda oblique and the interosseous membrane, the posterior interosseous artery enters the deep extensor compartment of the forearm, underneath the supinator. It then gives origin to the recurrent interosseous artery which anastomoses with the profunda brachii branches. The posterior interosseous artery continues distally in the intermuscular septum to reach the wrist, lateral to the ulnar head, where it anastomoses with the anterior interosseous artery. An interosseous recurrent artery originates from the anterior interosseous artery and connects at the junction of the proximal and distal half of the posterior interosseous artery (▶Fig. 4.24, ▶Fig. 4.26). The posterior interosseous artery distal to this anastomotic branch usually remains hypoplastic and is believed by some to represent the dorsal branch of the anterior interosseous artery. This distal segment forms a vascular arcade that connects to the dorsal carpal arch and supplies the ulnar head52 (▶Fig. 4.31). The posterior interosseous artery gives rise to fasciocutaneous perforators as well as branches that pass through the deep fascia to supply the underlying deep extensor muscles. The interosseous arteries, especially the posterior interosseous artery, provide the vascular basis for fasciocutaneous flaps, commonly used for reconstructive surgeries.52

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Fig. 4.30 (a, b) High origin of the radial artery, also known as a brachioradial artery, arising from the upper third of the brachial artery. High ­origin of the radial artery is classified as a variant of the superficial brachial artery. Also seen is severe stenosis (arrows) and mild aneurysmal ­dilation of the proximal part of the posterior circumflex humeral artery (PCHA) due compression by the superior edge of the latissimus dorsi muscle in the quadrilateral space. Both brachioradial and brachioulnar arteries send muscular branches. The profunda brachii (deep brachial) is arising from the PHCA. The PHCA itself is originating from the subscapular artery.

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Fig. 4.31 (a) The distal segment of the posterior interosseous artery usually remains hypoplastic but may continue by the dorsal branch of the anterior interosseous artery. This distal segment forms a vascular arcade that connects the dorsal carpal arch and supplies the ulnar head. (b) Complete deep palmar arch. The superficial palmar arch is formed by the ulnar artery and continues as the princeps pollicis artery of the thumb. The second palmar metacarpal artery arises from the deep palmar arch continues as the second common digital artery. Findings are consistent with Raynaud’s disease with apparent occlusions of some of the digital arteries.

Anatomical variation of the interosseous arteries is common. Absence or hypoplasia of the posterior interosseous artery in the distal part of the forearm and the absence of the anastomosis between the posterior and anterior interosseous arteries are not uncommon (▶Fig. 4.24, ▶Fig. 4.26, ▶Fig. 4.29).

Persistent Median Artery The median artery is a transitory fetal structure providing the main blood supply to the hand in the embryo before the 8th week of gestation. It may persist in up to 10% of people, accompanying the median nerve down the forearm, and reaches the palm through the carpal tunnel.53,54 The artery usually arises from the ulnar artery but may also originate from the radial, interosseous, brachial, or axillary arteries (▶Fig. 4.32). In the presence of persistent median artery, the palmar arch may not completely form.55,56

Wrist and Hands The radial and ulnar arteries supply blood to the wrist and hand (▶Fig. 4.24, ▶Fig. 4.31, ▶Fig. 4.33, ▶Fig. 4.34). Additional circulation may come from the median artery or the interosseous arterial system (▶Fig. 4.32). Both radial and ulnar arteries are more important to the blood supply of the hand than the posterior interosseous artery. Dominant vessel is generally larger with greater flow57,58 (▶Fig. 4.33, ▶Fig. 4.34). Radial artery p ­ rovides dominant supply to the thumb and index fingers (▶Fig. 4.35).

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Other fingers are supplied equally by both the radial and ulnar arteries.59 The radial and ulnar arteries interconnect, at four levels, via the palmar and dorsal carpal arches at the wrist, and the superficial and deep palmar arches at the hand. If the radial or ulnar artery becomes occluded, a single vessel runoff from the other artery is adequate to supply the hand the majority of the time (▶Fig. 4.36). To identify patients with inadequate collateral arterial supply to the hand, which puts them at higher risk of developing digital ischemia, a Barbeau test is recommended prior to any radial arteriotomy for endovascular procedures. (We prefer to use the Barbeau test vs modified Allen test as the Barbeau is more sensitive.) In patients with primary Raynaud’s phenomenon, the diameters of the radial and ulnar arteries and the flow volume of the d ­ igital arteries will diminish with cold provoca60 tion, and in severe cases, the digital arteries may be completely occluded (▶Fig. 4.31, ▶Fig. 4.37).

Wrist At the wrist, the radial artery crosses the scaphoid bone in the anatomical snuff box, and the ulnar artery passes through the ulnar (Guyon’s) canal, which is bounded by the pisiform medially and the hook of the hamate laterally (▶Fig. 4.33). There is a rich anastomotic network in the wrist and hand. At the wrist level, both radial and ulnar arteries give rise to the palmar and dorsal carpal branches, which join to form the palmar and dorsal carpal arches, respectively (▶Fig. 4.3). The dorsal carpal arch arises from the radial artery in 80% of the people. In the remaining, it arises from a

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4 Fig. 4.32 Persistent median artery (arrows). Axial magnetic resonance (MR) images showing a persistent median artery running along with the median nerve in the carpal tunnel.

Fig. 4.33 Complete superficial palmar arch. (a–c) Computed tomography (CT) images showing the radial artery giving rise to the lateral palmar digital artery of the thumb, which continues as the princeps pollicis. The first palmar metacarpal on the volar side of the first metacarpal divides into the medial palmar digital artery of the thumb and the radialis indicis artery along the radial aspect of the index finger, from which point ­connects to the superficial palmar branch. The superficial palmar branch of the ulnar artery travels radially and connects to the radialis indicis branch of the radial artery to complete the arch. In this case the superficial palmar branch of the radial artery remains proximal and does not participate in formation of the superficial palmar arch. (d) Catheter angiogram of another hand. In this case, the princeps pollicis continues as the palmar digital artery of the thumb. The radial artery also connects to the superficial palmar arch by a small artery. In contrast, the radial artery forms a relatively large deep palmar arch. The second palmar metacarpal of the arch forms the radialis indicis.

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Fig. 4.34 Incomplete superficial palmar arch. Ulnar arterial dominance to the hand is shown. The radialis indicis is supplied by the ulnar branches. The radial artery terminates in the palmar metacarpal (princeps pollicis) artery and the first palmar digital artery. Note the diminutive deep palmar arch. There are multiple areas of occlusion of the distal digital arteries (arrows) with distal reconstitution via small collaterals related to chronic autoimmune vasculitis.

Fig. 4.35 Angiogram of the hand demonstrating occluded ulnar artery with minimal blood supply of its territory. There is typical branching pattern of the radial artery supplying the thumb and index. The superficial palmar arch is partially supplied by the radial artery. The deep palmar arch is not clearly seen.

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Fig. 4.36 Occlusion of the radial artery with retrograde reconstitution via the deep (green) and superficial (yellow) palmar arches; both appear complete (red arrows). There is also anastomosis at the wrist level between the ulnar and radial artery through the interosseous branches. Patent ulnar artery and superficial palmar arch, and retrogradely reconstituting distal radial artery via deep arch.

Fig. 4.37 Angiogram of the hand before and following intra-arterial verapamil administration in a patient with Raynaud’s disease, ­demonstrating occlusions of several distal digital arteries, resulting in dry gangrene of the involved digits. Both palmar arches are better visualized after ­verapamil injection. The second palmar metacarpal artery is shown extending from the deep arch to the common digital arteries. The ulna also provides four common digital arteries with the one splitting into the princeps pollicis artery and the radialis indicis.

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Upper Extremity Arteries branch of the radial artery that also forms the second dorsal metacarpal artery and a communicating branch to the palmar system.61 Distal branches of the anterior and posterior interosseous arteries also connect to the palmar and dorsal carpal arches (▶Fig. 4.37). The superficial palmar branch of the radial artery is usually a small subcutaneous branch arising from the radial artery at the wrist level (▶Fig. 4.33, ▶Fig. 4.35). In many cases, this branch of the radial artery is absent or small, and only supplies a minor part of the thenar muscles.62 A well-developed superficial palmar branch is seen in only 30% of cases, which may connect with the ulnar artery to form the superficial palmar arch. It may also continue distally as the first common palmar digital artery and further bifurcate into the radialis indicis and princeps pollicis artery.63

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Terminal Radial and Ulnar Arteries The radial artery moves dorsally under the abductor pollicis longus and extensor pollicis brevis tendons to enter the anatomic snuffbox, and gives rise to the dorsal branch to the radial side of the thumb and the dorsal carpal branch (▶Fig. 4.33, ▶Fig. 4.38). The radial artery then continues to the surface of the first dorsal interosseous muscle where it gives rise to the first dorsal metacarpal artery before moving back to palmar side as the deep palmar arch. The deep palmar branch of the ulnar artery passes between the abductor digiti minimi and the flexor digiti minimu brevis muscles to complete the deep palmar arch from the ulnar side (▶Fig. 4.35, ▶Fig. 4.36). At the hand level, the ulnar artery becomes the superficial palmar branch/arch (▶Fig. 4.33, ▶Fig. 4.34).

Dorsal Side of the Hand The first dorsal metacarpal artery arises from the radial artery at the first intermetacarpal space and divides into the dorsal digital arteries on the ulnar side of the thumb and the radial side of the index finger (▶Fig. 4.33). The dorsal digital artery to the ulnar side of the thumb may arise directly from the radial artery or the princeps pollicis in 15% (▶Fig. 4.33). The dorsal carpal arch, located at the level of the distal carpal row deep to the third and the fourth dorsal extensor compartments, gives rise to the second, third, and fourth dorsal metacarpal arteries (▶Fig. 4.37). In 20%, the second dorsal metacarpal artery arises directly from the radial artery. The third and fourth dorsal metacarpal arteries may be small or absent in 10% of cases. At the level of the metacarpal heads, the dorsal metacarpal arteries divide into the distal palmar communicating branch and dorsal digital arteries on the radial and ulnar sides of opposing digits. Dorsal metacarpal arteries also give rise to small perforating branches to supply the dorsal skin of the hand. The anatomy of the dorsal metacarpal arteries is important for surgical planning for flaps taken from the dorsum of the hand.61 The dorsal digital arteries terminate over the middle phalanges, but the palmar digital arteries supply the dorsum of the distal phalanges. There is rich arterial anastomotic network between the dorsal metacarpal arteries and the palmar metacarpal arteries (see Chapter 18 “Hand”).

Palmar Side of the Hand At the palmar side, the terminal branches of the radial artery are the deep palmar branch and the first palmar metacarpal artery. Classically, the latter divides into the radialis indicis, which runs

Fig. 4.38 Anatomic snuffbox. The radial artery travels dorsally under the abductor pollicis longus and extensor pollicis brevis tendons to enter the anatomic snuffbox. At this location, the radial artery gives rise to the dorsal branch to the radial side of the thumb and the dorsal carpal branch.

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Upper Extremity Arteries along the radial aspect of the index finger, and the princeps pollicis, which runs on the volar side of the first metacarpal. The latter divides into the lateral and medial proper digital arteries of the thumb (▶Fig. 4.35), but variation in this pattern is very common (▶Fig. 4.33, ▶Fig. 4.34). The superficial palmar arch gives origin to the proper digital artery to the ulnar side of the fifth finger, and three common digital arteries (▶Fig. 4.33, ▶Fig. 4.34). The common digital arteries further divide into the proper digital arteries to the ulnar sides of the second, third, and fourth fingers, and the radial sides of the third, fourth, and fifth fingers (▶Fig. 4.33, ▶Fig. 4.34). The deep palmar arch gives rise to the second, third, and fourth palmar metacarpal arteries, which in turn give rise to the ­common digital arteries and form anastomoses with the dorsal metacarpal arteries (▶Fig. 4.31, ▶Fig. 4.33). Proper digital arteries are arranged on the radial (lateral) and ulnar (medial) sides of the palmar and dorsal surfaces. Several anastomoses exist between these vessels. Variation in arterial supply to the thumb is common (▶Fig. 4.33, ▶Fig. 4.34, ▶Fig. 4.35, ▶Fig. 4.36, ▶Fig. 4.37). Based on a recent systematic review the dominant suppliers of the thumb are the radial artery branches including the first palmar metacarpal artery (60%), the superficial palmar branch of the radial artery (8%), and first dorsal metacarpal artery (15%).64,65 The ulnar branches may participate in the arterial supply of the thumb in a minority of cases (▶Fig. 4.33, ▶Fig. 4.34). The princeps pollicis artery is the

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continuation of the first palmar metacarpal branch of the radial artery in 80% of hands, which also supplies the radialis indicis in 50%65 (▶Fig. 4.35, ▶Fig. 4.36). The term “princeps pollicis artery,” which is interchangeably used for the first palmar metacarpal artery and the first palmar digital artery, has been reconsidered by some anatomists.64 In 5 to 10% of hands, the princeps pollicis and radialis indicis arise from the second palmar metacarpal artery or the superficial palmar arterial arch of the ulnar artery.65,66 The princeps pollicis artery is the origin of the palmar radial, palmar ulnar, and dorsal ulnar digital arteries of the thumb in 73%.67 The dorsal radial artery of the thumb is a direct branch of the first dorsal metacarpal artery.

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Palmar Arch Variation Knowledge of the variations and respective locations of the palmar arches helps in the interpretation of abnormal angiograms and can facilitate microsurgery of the hand.68,69 Patients undergoing coronary artery bypass surgery should be screened before radial artery harvesting to confirm the presence of a viable collateral circulation.70 Similar to evaluation of the palmar arch prior to radial arteriotomy for endovascular procedures, this can be achieved by the Barbeau test. Localization of the deep palmar arch can be useful in identifying perforators used for microsurgical reconstruction of the hand and fingers.

Complete both

30%

30% Superficial palmar arch

Dorsal radial artery

Deep palmar arch

Radial artery

Ulnar artery

a

b

Incomplete superficial

Incomplete deep

60 degrees) and the lunocapitate angle (>30 degrees). In volar intercalated segmental instability (VISI), the lunate is tilted volarly with decrease of the scapholunate angle (30 degrees). (b, c) Lateral radiographs of the wrist show DISI and VISI patterns, respectively.

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Wrist

◆◆ Vascular Supply

The radial, ulnar, and the anterior interosseous arteries anastomose at the wrist to form four dorsal (supraretinacular, radiocarpal, intercarpal, and dorsal basal metacarpal) and three palmar (radiocarpal, intercarpal, and deep palmar) transverse arches that supply the carpal bones (▶Fig. 17.55). The frequency of the presence of these arches, the arterial source of these arches, and the relative contribution to the supply from the arteries is variable.78,79 The radial artery is the major blood source for the scaphoid (▶Fig. 17.56). An artery or arteries (one or two) entering through the dorsal ridge form the major supply with intraosseous branches running proximally in a retrograde fashion. In particular, this accounts for nearly all of the supply to the proximal pole.80 Interruption of these vessels have been implicated as a cause of avascular necrosis of the proximal pole of the scaphoid after scaphoid fracture (▶Fig. 17.57). The majority of the lunate is covered by articular cartilage and the blood vessels enter through the palmar and dorsal surfaces, through one or two nutrient vessels on each surface. The palmar vessels are the sole supply in 20% of cases. The dorsal and palmar vessels anastomose after entering the bone, but similar to the scaphoid, the proximal lunate receives its blood supply in a retrograde fashion. Chronic repetitive trauma leading to compression fracture and resultant interruption of the blood supply has been implicated as a cause of

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Kienböck’s disease81 (▶Fig. 17.58). The capitate also has intraosseous blood vessels that run in a retrograde fashion to supply the head.79 The distal radius and ulna have their blood supply from branches arising from the radial, anterior interosseous, posterior interosseous, and ulnar arteries. The distal radius is a common site for harvest of vascularized bone graft.82 The ulnar head is supplied by small branches from the ulnar artery, whereas the distal ulnar metaphysis is supplied by the terminal branches of the anterior interosseous artery.83

◆◆ Nerves

The median nerve courses along the volar wrist and passes through the carpal tunnel with the flexor tendons of the wrist (▶Fig. 17.50). The median nerve can traverse the wrist and carpal tunnel as a single unit; it can split before traversing the tunnel into two branches, making it bifid; or it can trifurcate making it trifid. The median nerve may bifurcate anywhere along the carpal tunnel, or distal to it. The most common location of bifurcation is within the distal carpal tunnel84 (▶Fig. 17.50). The position of the median nerve varies with respect to the degree of wrist flexion and extension, as does the shape of the median nerve.85 Embryologically, the median nerve is accompanied by the median artery, which typically regresses; however, there are reported

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Fig. 17.55 Vascular supply of the palmar and dorsal surfaces of the wrist. The radial, ulnar, and anterior interosseous arteries anastomose at the wrist to form four dorsal (supraretinacular, radiocarpal, and intercarpal, and dorsal basal metacarpal arch) and three palmar (radiocarpal, intercarpal, and deep palmar arch) transverse arches that supply the carpal bones.

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Fig. 17.56 The radial artery is the major blood source for the scaphoid. The scaphoid arteries entering through the volar or dorsal ridges form the major supply with intraosseous branches running from distal to proximal in a retrograde fashion.

Fig. 17.57 (a) Posteroanterior view showing scaphoid nonunion advanced collapse (SNAC wrist). This is a complication of scaphoid fracture when the scaphoid fracture does not unite. The proximal fragment rotates with the lunate due to intact scapholunate ligament while the distal fragment flexes. Untreated, this ultimately leads to radioscaphoid osteoarthritis. (b) Normal scaphoid view is shown for comparison. The scaphoid view is obtained with ulnar deviation of the wrist and elbow flexed 90 degrees, and the shoulder abducted 90 degrees.

17 cases of persistence of this vascular structure within the carpal tunnel54 (see Chapter 4 “Upper Extremity Arteries”). Enlargement and edema of the median nerve may be seen in carpal tunnel syndrome (▶Fig. 17.59). The median nerve may also be involved by intrinsic lesions such as a neurofibroma (▶Fig. 17.60). At the level of the wrist, the ulnar nerve travels superficially to the transverse carpal ligament and subsequently dives deep into its fibro-osseous tunnel (Guyon’s canal). Unlike the median nerve, the ulnar nerve has less variability in appearance. Dysfunction

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of the ulnar nerve at the level of the Guyon canal (ulnar tunnel syndrome) can occur due to intrinsic lesions such as traumatic injury, neuritis, or neurofibroma or due to extrinsic lesions such as ganglion cyst (▶Fig. 17.61). The palmar cutaneous branch of the median nerve, which is a sensory nerve, arises from the median nerve in the distal forearm and courses radially and distally to the thenar eminence, crossing the wrist between the superficial and deep layers of the flexor retinaculum.

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Wrist

Fig. 17.58 Kienböck’s disease. (a) Plain radiograph of the wrist shows increased density of the lunate (arrow) compared to the surrounding bones consistent with avascular necrosis. Corresponding coronal (b) T1 and (c) short tau inversion recovery (STIR) magnetic resonance (MR) images show decreased signal on T1-weighted images and minimally increased signal on T2-weighted images (arrows). Note the negative ulnar variance.

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Fig. 17.59 Carpal tunnel syndrome. Axial (a) proton density fat-saturated (PDFS) and (b) T1, (c) coronal STIR, and (d) sagittal T2 magnetic resonance (MR) images show focal increased signal of the median nerve (yellow arrows) in this patient with carpal tunnel syndrome. Note findings related to carpal tunnel release (green arrows).

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Fig. 17.60 Carpal tunnel syndrome due to neurofibroma of the median nerve. Axial proton density fat-saturated (PDFS) images at the level of the (a) proximal carpal tunnel and (b) distal carpal tunnel, and (c) Coronal PDFS and (d) sagittal PDFS images show focal hyperintense mass (arrows) in the median nerve consistent with a nerve sheath tumor in this patient with known neurofibromatosis.

Another sensory nerve, the superficial branch of the radial nerve, courses along the dorsolateral forearm, radial to the brachioradialis, to extend into the hand (see Chapter 6 “Brachial Plexus and Its Branches”).

◆◆ Surface Anatomy

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A number of structures are easily palpated or seen about the wrist. The radial styloid and the ulnar styloid can be palpated on the radial and ulnar aspects of the wrist, respectively. The pisiform can also be palpated easily on the palmar ulnar aspect of the wrist. Distal and radial to it the hook of hamate can be palpated. On the radial volar aspect, the scaphoid tubercle and the trapezium ridge are palpated (▶Fig. 17.17). Along the volar wrist, from radial to ulnar, the following can be palpated aided by maneuvers such as flexion of the wrist: the radial artery, the FCR tendon,

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the palmaris longus tendon, the ulnar artery, and the flexor carpi ulnaris tendon (▶Fig. 17.50). Dorsally, the ECRL, ECRB, and ECU can be palpated with extension of the wrist. Lister’s tubercle protrudes from the dorsal distal radius.86 The ulnar head is palpated on the dorsal ulnar aspect. On the radial aspect, the anatomical snuffbox is seen (▶Fig. 17.5) between the first (EPB and APL) and third (EPL) extensor compartment tendons with the radial artery palpated within it (see ▶Fig. 17.19, ▶Fig. 17.20, ▶Fig. 17.21, anatomical slices). The scaphoid waist lies in the floor of the snuffbox distal to the radial styloid, and tenderness is elicited by palpation with scaphoid fracture. The tuberosity of the scaphoid is felt at the distal flexor crease, whereas the pisiform is just distal to the crease.86 Apart from the above palpable structures, the proximal and the distal palmar creases are seen on the volar wrist. These are used to identify the location of various anatomical structures.87

Wrist

Fig. 17.61 Ulnar tunnel syndrome ganglion cyst. Axial T1 and proton density fat-saturated (PDFS) images at the level of the (a, c) radiocarpal joint and (b, d) at the level of the pisiform show a T1 hypointense and PDFS hyperintense mass (yellow arrows) in Guyon’s canal displacing the ulnar neurovascular structures volarly and radially. There was no enhancement following administration of intravenous gadolinium (not shown). This is consistent with a ganglion cyst. Note the ulnar artery and nerve just proximal to Guyon’s canal.

◆◆ Imaging

Imaging of the wrist is most commonly performed by means of radiography and often is the only modality necessary. The most common views are the posteroanterior (PA), oblique, and lateral (▶Fig. 17.1, ▶Fig. 17.41). Special views are obtained to improve detection of pathology, specifically scaphoid views when there is concern for scaphoid fracture (▶Fig. 17.57). The position of the styloid of ulna in relation to the head of the ulna changes in the PA versus anteroposterior (AP) view (▶Fig. 17.62). Tunnel view of the wrist can be obtained when there is concern for fractures of the osseous components of the volar wrist, including the pisiform, the hamate, and the triquetrum or for evaluation of carpal tunnel syndrome (▶Fig. 17.27). Lateral radiography allows assessment of the central column of the wrist, which is comprised of the capitate, the lunate, and the distal radius. In normal wrists, a line can connect the base of the third metacarpal, the capitate, the lunate, and the distal radius (▶Fig. 17.54). The lunocapitate angle can be measured on lateral radiography by bisecting the lunate through its long axis and bisecting the capitate and measuring its angle, which should be less than 30 degrees (▶Fig. 17.54). When it is greater than 30 degrees, carpal instability or scaphoid fracture is considered.88

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Another measurement obtained on lateral radiography is the scapholunate angle that is measured between the volar long axis of the scaphoid (obtained by connecting the most volar projections proximally and distally) and a line bisecting the lunate. This should be between 30 and 60 degrees. Measurements above 80 and below 20 degrees indicate instability of the ligament88 (▶Fig. 17.54). Using the scapholunate and lunocapitate measurements, common instabilities can be determined; dorsal intercalated segment instability is diagnosed when the lunocapitate angle measures over 30 degrees and/or the scapholunate angle is greater than 80 degrees. When the scapholunate angle is less than 20 degrees, volar intercalated segment instability is present; fractures need not be present for these type of instabilities.88,89 Additional measurements can be obtained in the setting of fracture including radial length, radial inclination, and volar tilt. Radial length is measured on the PA view using two lines perpendicular to the long axis of the radius, one at the level of the tip of the radial styloid and the other at the distal extent of the sigmoid notch. In normal wrists, this measures between 10 and 13 mm. Volar inclination of the radius can be measured on true lateral radiographs and is obtained as the angle between a line perpendicular to the long axis of the radius and a line parallel

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Wrist

Fig. 17.62 Posteroanterior (PA) versus anteroposterior (AP) view. Due to rotation of the radius around the ulna in pronation and supination, the position of the styloid of the ulna in relation to the head the of ulna changes. With the PA view, the styloid is medially position and seen in the profile. On anteroposterior view, the wrist is in supination and the styloid of the ulna is seen en face, projecting midway over the head of the ulna.

along the distal radial articular surface. The normal range is 3 to 15 degrees. Radial inclination is measured on the PA projection as the angle produced between a line perpendicular to the long axis of the radius and a line connecting the distal tip of the radial styloid with the radial portion of the DRUJ and should measure 21 to 25 degrees.89 Another deformity of the proximal wrist is widening of the scapholunate interval, which occurs in the setting of an SLL injury (▶Fig. 17.44). An injury to the SLL causes widening of the space between these structures. Instability results, and the scaphoid and lunate gradually separate further. The capitate collapses proximally with progressive instability and scapholunate widening, which eventually results in scapholunate advanced collapse (SLAC). The normal scapholunate space on radiography measures 2 mm.90 Many other measurements on plain radiographs have been described. Fluoroscopy is used to evaluate dynamic instabilities and to guide procedures. Conventional arthrography has largely been replaced by MRI and MR arthrography. CT is used for evaluation of the osseous structures, particularly in case of trauma. It is also a modality used to evaluate for DRUJ subluxation, although MRI is an alternative. For this, transverse images of the DRUJ are obtained in at least full pronation and supination (▶Fig. 17.42). Ultrasound (US) is used to evaluate the joint and synovial sheath effusion, and tendons. It can also be used to evaluate masses. Its role is increasing, particularly in the field of rheumatology since it can identify synovitis. MRI is the imaging modality of choice for evaluation of soft tissues and masses, and is also very sensitive for detection of osseous abnormalities such as fractures, bone contusions, and avascular necrosis (▶Fig. 17.58, ▶Fig. 17.59, ▶Fig. 17.60, ▶Fig. 17.61). The sensitivity and specificity of MRI for evaluation of carpal ligaments and TFCC are improved by MR arthrography

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(▶Fig. 17.47). Lesions of the TFC are a common cause of ulnarsided wrist pain.

◆◆ Pathologies

The wrist is a common joint affected by trauma and instabilities. It is also a common site of pain arising from the abnormalities affecting the many tendons and tendon sheaths. The tendons of the wrist are easily evaluated with routine MRI sequences included T1, proton density (PD), proton density fat-saturated (PDFS), and T2 fat-saturated (T2FS) images. The axial and coronal planes are most useful in evaluating the tendons for injury. Tenosynovial inflammation of the first extensor compartment, de Quervain’s tenosynovitis, is occasionally found in patients with radial side pain (▶Fig. 17.63). The swelling is often apparent clinically with bogginess to the lateral soft tissues. Radiographically, soft-tissue swelling may be apparent. MRI clearly demarcates the inflammatory process and also excludes coexisting tears of compartment 1 tendons. Tenosynovitis manifests as fluid within the sheath enveloping the tendon. The inflammatory process may cause injury to the tendons themselves and weaken their integrity. US will show the localized tendon sheath distention. This entity is occasionally found in mothers in the postpartum state, and is attributed to altered biomechanics related to cradling the newborn’s head.91 On the ulnar side, tendinosis of the ECU tendon, tenosynovitis of its tendon sheath, or subluxation of the tendon from the groove are causes of wrist pain (▶Fig. 17.64). Causes of dorsal wrist pain related to tendons include intersection syndromes, one at the site of crossing of the first extensor compartment tendons over the second extensor compartment tendons and one at the site of crossing of the EPL over the second extensor compartment tendons.92

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Fig. 17.63 de Quervain’s tenosynovitis. Axial (a) fat-saturated TIW and (b) fat-saturated proton density (PS) images showing intermediate signal intensity within the extensor pollicis brevis tendon (green arrows), consistent with tendinopathy. Tenosynovitis of the first ­compartment is noted. The findings are consistent with de Quervain’s tenosynovitis. Incidental small amount of fluid in extensor pollicis longus tendon sheath (orange arrow). Other extensor tendons are unremarkable.

Fig. 17.64 Extensor carpi ulnaris tendinosis and peritendinitis in a patient with ulnar-sided wrist pain. (a) Coronal and (b) axial proton density (PD) and (c) PD fat-saturated (PDFS) images show irregularly increased signal within the extensor carpi ulnaris tendon (arrows) and increased fluid-sensitive signal surrounding the tendon (arrowheads) consistent with tendinosis and peritendinitis.

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Wrist Carpal tunnel syndrome related to impingement of the median nerve in the carpal tunnel is also common and US or MRI is often used for its evaluation (▶Fig. 17.59). Ganglion cysts are common around the wrist and are often asymptomatic (▶Fig. 17.61). Other benign and malignant bone and soft-tissue tumors are also common around the wrist including giant cell tumor of the distal radius, giant cell tumors of the tendon sheath (tenosynovial giant cell tumor), and neurofibroma (▶Fig. 17.60). The wrist is also a joint commonly affected by rheumatological diseases such as rheumatoid arthritis and gout.

18. Timins ME. Osseous anatomic variants of the wrist: findings on MR imaging. AJR

◆◆ Summary

23. Sagerman SD, Zogby RG, Palmer AK, Werner FW, Fortino MD. Relative articular

The wrist is a complex joint comprised of many small osseous structures that are supported by many articulations and ligaments, many of which are not readily apparent on routine MRI. Recognition of common anatomical variants can aid in the interpretation of imaging studies of the wrist. Understanding the common pathophysiologies of a disease such as tenosynovitis aids in recognition of pathologic entities regardless of the anatomical location. A detailed knowledge of the clinically relevant anatomy of the wrist will better serve the interpreting physician.

References 1. Boyer MI, Korcek KJ, Gelberman RH, Gilula LA, Ditsios K, Evanoff BA. Anatomic tilt x-rays of the distal radius: an ex vivo analysis of surgical fixation. J Hand Surg Am 2004;29(1):116–122 2. Tolat AR, Stanley JK, Trail IA. A cadaveric study of the anatomy and stability of the distal radioulnar joint in the coronal and transverse planes. J Hand Surg [Br] 1996;21(5):587–594 3. Topper SM, Wood MB, Ruby LK. Ulnar styloid impaction syndrome. J Hand Surg Am 1997;22(4):699–704 4. Hulten O. Uber anatomische variationen der handgelenkknochen. Acta Radiol 1928;9:155–168 5. Jung JM, Baek GH, Kim JH, Lee YH, Chung MS. Changes in ulnar variance in relation to forearm rotation and grip. J Bone Joint Surg Br 2001;83(7):1029–1033 6. Jafari D, Shariatzadeh H, Najd Mazhar F, Ghahremani MH. Ulnar variance in scaphoid nonunion. Arch Iran Med 2013;16(5):301–302 7. Bonzar M, Firrell JC, Hainer M, Mah ET, McCabe SJ. Kienböck disease and negative ulnar variance. J Bone Joint Surg Am 1998;80(8):1154–1157 8. van Leeuwen WF, Oflazoglu K, Menendez ME, Ring D. Negative ulnar variance and Kienböck disease. J Hand Surg Am 2016;41(2):214–218 9. Kristensen SS, Thomassen E, Christensen F. Ulnar variance in Kienböck’s disease. J Hand Surg [Br] 1986;11(2):258–260 10. Bell MJ, Hill RJ, McMurtry RY. Ulnar impingement syndrome. J Bone Joint Surg Br 1985;67(1):126–129 11. Watson HK, Brown RE. Ulnar impingement syndrome after Darrach procedure: treatment by advancement lengthening osteotomy of the ulna. J Hand Surg Am 1989;14(2, Pt 1):302–306

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12. Cerezal L, del Piñal F, Abascal F, García-Valtuille R, Pereda T, Canga A. Imaging findings in ulnar-sided wrist impaction syndromes. Radiographics 2002;22(1):105–121 13. Gilula LA. Carpal injuries: analytic approach and case exercises. AJR Am J Roentgenol 1979;133(3):503–517 14. Watson HK, Yasuda M, Guidera PM. Lateral lunate morphology: an X-ray study. J Hand Surg Am 1996;21(5):759–763 15. Park JH, Jang WY, Kwak DH, Park JW. Lunate morphology as a risk factor of idiopathic ulnar impaction syndrome. Bone Joint J 2017;99-B(11):1508–1514 16. Yazaki N, Burns ST, Morris RP, Andersen CR, Patterson RM, Viegas SF. Variations of capitate morphology in the wrist. J Hand Surg Am 2008;33(5):660–666 17. Chow JC, Weiss MA, Gu Y. Anatomic variations of the hook of hamate and the relationship to carpal tunnel syndrome. J Hand Surg Am 2005;30(6):1242–1247

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Am J Roentgenol 1999;173(2):339–344 19. Freyschmidt J, Köhler A, Brossmann J, et al. Freyschmidt’s “Koehler/Zimmer” Borderlands of Normal and Early Pathologic Findings in Skeletal Radiography. New York, NY: Thieme; 2003 20. Mespreuve M, Vanhoenacker F, Verstraete K. Lunotriquetral coalition, a normal variant that may rarely cause ulnar sided wrist pain. JBR-BTR 2015;98(2):72–78 21. Daneshvar P, Willing R, Pahuta M, Grewal R, King GJ. Osseous anatomy of the distal radioulnar joint: an assessment using 3-dimensional modeling and clinical implications. J Hand Surg Am 2016;41(11):1071–1079 22. Tolat AR, Sanderson PL, De Smet L, Stanley JK. The gymnast’s wrist: acquired positive ulnar variance following chronic epiphyseal injury. J Hand Surg [Br] 1992;17(6):678–681 inclination of the distal radioulnar joint: a radiographic study. J Hand Surg Am 1995;20(4):597–601 24. Nakamura T, Yabe Y, Horiuchi Y. Functional anatomy of the triangular fibrocartilage complex. J Hand Surg [Br] 1996;21(5):581–586 25. Thiru RG, Ferlic DC, Clayton ML, McClure DC. Arterial anatomy of the triangular fibrocartilage of the wrist and its surgical significance. J Hand Surg Am 1986;11(2):258–263 26. Harley BJ, Pereria ML, Werner FW, Kinney DA, Sutton LG. Force variations in the distal radius and ulna: effect of ulnar variance and forearm motion. J Hand Surg Am 2015;40(2):211–216 27. Totterman SM, Miller RJ. Triangular fibrocartilage complex: normal appearance on coronal three-dimensional gradient-recalled-echo MR images. Radiology 1995;195(2):521–527 28. Kleinman WB. Stability of the distal radioulna joint: biomechanics, pathophysiology, physical diagnosis, and restoration of function what we have learned in 25 years. J Hand Surg Am 2007;32(7):1086–1106 29. Xu J, Tang JB. In vivo changes in lengths of the ligaments stabilizing the distal radioulnar joint. J Hand Surg Am 2009;34(1):40–45 30. Lees VC. Functional anatomy of the distal radioulnar joint in health and disease. Ann R Coll Surg Engl 2013;95(3):163–170 31. Buck FM, Gheno R, Nico MA, Haghighi P, Trudell DJ, Resnick D. Ulnomeniscal homologue of the wrist: correlation of anatomic and MR imaging findings. Radiology 2009;253(3):771–779 32. Ishii S, Palmer AK, Werner FW, Short WH, Fortino MD. An anatomic study of the ligamentous structure of the triangular fibrocartilage complex. J Hand Surg Am 1998;23(6):977–985 33. Berger RA. The anatomy of the ligaments of the wrist and distal radioulnar joints. Clin Orthop Relat Res 2001;(383):32–40 34. Theumann NH, Pfirrmann CW, Chung CB, Antonio GE, Trudell DJ, Resnick D. Pisotriquetral joint: assessment with MR imaging and MR arthrography. Radiology 2002;222(3):763–770 35. Yamaguchi S, Viegas SF, Patterson RM. Anatomic study of the pisotriquetral joint: ligament anatomy and cartilagenous change. J Hand Surg Am 1998;23(4):600–606 36. Viegas SF, Patterson RM, Hokanson JA, Davis J. Wrist anatomy: incidence, distribution, and correlation of anatomic variations, tears, and arthrosis. J Hand Surg Am 1993;18(3):463–475 37. Schmid MR, Schertler T, Pfirrmann CW, et al. Interosseous ligament tears of the wrist: comparison of multi-detector row CT arthrography and MR imaging. Radiology 2005;237(3):1008–1013 38. Berger RA. The gross and histologic anatomy of the scapholunate interosseous ligament. J Hand Surg Am 1996;21(2):170–178 39. Ritt MJ, Bishop AT, Berger RA, Linscheid RL, Berglund LJ, An KN. Lunotriquetral ligament properties: a comparison of three anatomic subregions. J Hand Surg Am 1998;23(3):425–431 40. Viegas SF, Patterson RM, Peterson PD, et al. Ulnar-sided perilunate instability: an anatomic and biomechanic study. J Hand Surg Am 1990;15(2):268–278 41. Short WH, Werner FW, Green JK, Sutton LG, Brutus JP. Biomechanical evaluation of the ligamentous stabilizers of the scaphoid and lunate: part III. J Hand Surg Am 2007;32(3):297–309 42. Berger RA, Landsmeer JM. The palmar radiocarpal ligaments: a study of adult and fetal human wrist joints. J Hand Surg Am 1990;15(6):847–854 43. Pulos N, Bozentka DJ. Carpal ligament anatomy and biomechanics. Hand Clin 2015;31(3):381–387

Wrist 44. Lee DJ, Elfar JC. Carpal ligament injuries, pathomechanics, and classification. Hand Clin 2015;31(3):389–398 45. Viegas SF, Yamaguchi S, Boyd NL, Patterson RM. The dorsal ligaments of the wrist: anatomy, mechanical properties, and function. J Hand Surg Am 1999;24(3):456–468 46. Taleisnik J, Gelberman RH, Miller BW, Szabo RM. The extensor retinaculum of the wrist. J Hand Surg Am 1984;9(4):495–501 47. Palmer AK, Skahen JR, Werner FW, Glisson RR. The extensor retinaculum of the wrist: an anatomical and biomechanical study. J Hand Surg [Br] 1985;10(1):11–16 48. Massaki AN, Tan J, Huang BK, Chang EY, Trudell DJ, Resnick DL. Extensor retinaculum of the wrist: gross anatomical correlation with MR imaging after

69. Crisco JJ, Heard WM, Rich RR, Paller DJ, Wolfe SW. The mechanical axes of the wrist are oriented obliquely to the anatomical axes. J Bone Joint Surg Am 2011;93(2):169–177 70. Kaufmann RA, Pfaeffle HJ, Blankenhorn BD, Stabile K, Robertson D, Goitz R. Kinematics of the midcarpal and radiocarpal joint in flexion and extension: an in vitro study. J Hand Surg Am 2006;31(7):1142–1148 71. Viegas SF, Patterson RM, Ward K. Extrinsic wrist ligaments in the pathomechanics of ulnar translation instability. J Hand Surg Am 1995;20(2):312–318 72. Berger RA. The ligaments of the wrist. A current overview of anatomy with ­considerations of their potential functions. Hand Clin 1997;13(1):63–82 73. Kamal RN, Starr A, Akelman E. Carpal kinematics and kinetics. J Hand Surg Am 2016;41(10):1011–1018

ultrasound-guided tenography with emphasis on anatomical features in

74. Mitsuyasu H, Patterson RM, Shah MA, Buford WL, Iwamoto Y, Viegas SF. The role

wrist dorsiflexion responsible for tendon impingement. Skeletal Radiol

of the dorsal intercarpal ligament in dynamic and static scapholunate instabil-

2013;42(12):1727–1737 49. Cobb TK, Dalley BK, Posteraro RH, Lewis RC. Anatomy of the flexor retinaculum. J Hand Surg Am 1993;18(1):91–99 50. Pacek CA, Chakan M, Goitz RJ, Kaufmann RA, Li ZM. Morphological analysis of the transverse carpal ligament. Hand (N Y) 2010;5(2):135–140

ity. J Hand Surg Am 2004;29(2):279–288 75. Huang JI, Hanel DP. Anatomy and biomechanics of the distal radioulnar joint. Hand Clin 2012;28(2):157–163 76. Palmer AK, Werner FW. Biomechanics of the distal radioulnar joint. Clin Orthop Relat Res 1984; (187):26–35

51. Stecco C, Macchi V, Lancerotto L, Tiengo C, Porzionato A, De Caro R. Comparison

77. Werner FW, Palmer AK, Fortino MD, Short WH. Force transmission through the

of transverse carpal ligament and flexor retinaculum terminology for the wrist.

distal ulna: effect of ulnar variance, lunate fossa angulation, and radial and pal-

J Hand Surg Am 2010;35(5):746–753 52. Resnick D. Roentgenographic anatomy of the tendon sheaths of the hand and wrist: tenography. Am J Roentgenol Radium Ther Nucl Med 1975;124(1):44–51 53. Goto S, Kojima T. An anomalous lumbrical muscle with an independent muscle belly associated with carpal tunnel syndrome. Handchir Mikrochir Plast Chir 1993;25(2):72–74 54. Zeiss J, Guilliam-Haidet L. MR demonstration of a persistent median artery in carpal tunnel syndrome. J Comput Assist Tomogr 1993;17(3):482–484 55. Zeiss J, Jakab E. MR demonstration of an anomalous muscle in a patient with coexistent carpal and ulnar tunnel syndrome. Case report and literature summary. Clin Imaging 1995;19(2):102–105 56. Olave E, Prates JC, Gabrielli C, Pardi P. Morphometric studies of the muscular branch of the median nerve. J Anat 1996;189(Pt 2):445–449 57. Lange H. Carpal tunnel syndrome caused by the palmaris profundus muscle. Case report. Scand J Plast Reconstr Surg Hand Surg 1999;33(2):251–252 58. Vanhees M, Verstreken F, van Riet R. What does the transverse carpal ligament contribute to carpal stability? J Wrist Surg 2015;4(1):31–34 59. Mesgarzadeh M, Schneck CD, Bonakdarpour A. Carpal tunnel: MR imaging. Part I. Normal anatomy. Radiology 1989;171(3):743–748 60. Cobb TK, Carmichael SW, Cooney WP. Guyon’s canal revisited: an anatomic study of the carpal ulnar neurovascular space. J Hand Surg Am 1996;21(5):861–869 61. Zeiss J, Jakab E, Khimji T, Imbriglia J. The ulnar tunnel at the wrist (Guyon’s canal):

mar tilt of the distal radius. J Hand Surg Am 1992;17(3):423–428 78. Freedman DM, Botte MJ, Gelberman RH. Vascularity of the carpus. Clin Orthop Relat Res 2001;(383):47–59 79. Gelberman RH, Gross MS. The vascularity of the wrist. Identification of arterial patterns at risk. Clin Orthop Relat Res 1986;(202):40–49 80. Gelberman RH, Menon J. The vascularity of the scaphoid bone. J Hand Surg Am 1980;5(5):508–513 81. Gelberman RH, Bauman TD, Menon J, Akeson WH. The vascularity of the lunate bone and Kienböck’s disease. J Hand Surg Am 1980;5(3):272–278 82. Shin AY, Bishop AT. Vascular anatomy of the distal radius: implications for vascularized bone grafts. Clin Orthop Relat Res 2001;(383):60–73 83. Wright TW, Glowczewskie F. Vascular anatomy of the ulna. J Hand Surg Am 1998;23(5):800–804 84. Pierre-Jerome C, Smitson RD Jr, Shah RK, Moncayo V, Abdelnoor M, Terk MR. MRI of the median nerve and median artery in the carpal tunnel: prevalence of their anatomical variations and clinical significance. Surg Radiol Anat 2010;32(3):315–322 85. Zeiss J, Skie M, Ebraheim N, Jackson WT. Anatomic relations between the median nerve and flexor tendons in the carpal tunnel: MR evaluation in normal volunteers. AJR Am J Roentgenol 1989;153(3):533–536 86. Srinivas Reddy R, Compson J. (i) Examination of the wrist—surface anatomy of the carpal bones. Curr Orthop 2005;19:171–179

normal MR anatomy and variants. AJR Am J Roentgenol 1992;158(5):1081–1085

87. Kwiatkowska M, Jakutowicz T, Ciszek B, Czubak J. Can palmar creases serve

62. Gil YC, Shin KJ, Lee JY, et al. Topographic anatomy of the ulnar tunnel. Surg

as landmarks for the deeper neuro-vascular structures? Surg Radiol Anat

Radiol Anat 2015;37(7):757–764 63. Fadel ZT, Samargandi OA, Tang DT. Variations in the anatomical structures of the Guyon canal. Plast Surg (Oakv) 2017;25(2):84–92 64. Ombaba J, Kuo M, Rayan G. Anatomy of the ulnar tunnel and the influence of wrist motion on its morphology. J Hand Surg Am 2010;35(5):760–768

2014;36(5):495–501 88. Smith DK, Gilula LA, Amadio PC. Dorsal lunate tilt (DISI configuration): sign of scaphoid fracture displacement. Radiology 1990;176(2):497–499 89. Goldfarb CA, Yin Y, Gilula LA, Fisher AJ, Boyer MI. Wrist fractures: what the ­clinician wants to know. Radiology 2001;219(1):11–28

65. Ruby LK, Cooney WP III, An KN, Linscheid RL, Chao EY. Relative motion of

90. Kaawach W, Ecklund K, Di Canzio J, Zurakowski D, Waters PM. Normal ranges

selected carpal bones: a kinematic analysis of the normal wrist. J Hand Surg Am

of scapholunate distance in children 6 to 14 years old. J Pediatr Orthop

1988;13(1):1–10 66. Kijima Y, Viegas SF. Wrist anatomy and biomechanics. J Hand Surg Am 2009;34(8):1555–1563 67. Moritomo H, Apergis EP, Garcia-Elias M, Werner FW, Wolfe SW. International Federation of Societies for Surgery of the Hand 2013 Committee’s report on wrist dart-throwing motion. J Hand Surg Am 2014;39(7):1433–1439

2001;21(4):464–467 91. Anderson SE, Steinbach LS, De Monaco D, Bonel HM, Hurtienne Y, Voegelin E. “Baby wrist”: MRI of an overuse syndrome in mothers. AJR Am J Roentgenol 2004;182(3):719–724 92. Plotkin B, Sampath SC, Sampath SC, Motamedi K. MR imaging and US of the

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wrist tendons. Radiographics 2016;36(6):1688–1700

68. Rainbow MJ, Wolff AL, Crisco JJ, Wolfe SW. Functional kinematics of the wrist. J

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18  Hand Madeleine J. Gust, Milan Stevanovic, and Dakshesh B. Patel

◆◆ Introduction

The superb sensitivity of the fingertip is comprised of several features. Fingerprints are epidermal ridges based on underlying dermal ridges.11 The full function of fingerprints is not clearly understood. Although it was previously thought to increase the ability to grip, these may be more related to tactile sensation and also prevention of blistering.12 Moisture of the fingertip has been found to be finely regulated to improve gripping.13 The glomus apparatus is a set of arteriovenous shunts in the dermis of the fingertips. These shunts function in temperature regulation, coming closer to the skin when hot or further away from the skin when cold.14,15 Meissner’s end organs respond to direct pressure. Their number is higher in the distal pulp skin compared to the proximal finger.6 Pacinian corpuscles sense vibration and pressure and have the highest concentration in the fingers.6

◆◆ Skin, Nails, and Fascia

Nail

The hand is a complex mechanism that allows us to interact accurately with the environment. Many bones, muscles, tendons, ligaments, and other smaller structures come together to give the hand its dexterity and power to accomplish simple and complex activities. In the practice of hand surgery, the digits of the hand are named rather than numbered to avoid confusion. They are called the thumb, index, middle (or long), ring, and small (or little) fingers. The finger ray starts just distal to the wrist and includes the metacarpal and phalangeal bones. The sides of the hand are labeled as radial and ulnar instead of lateral and medial, respectively. The front and back of the hand are labeled volar (or palmar) and dorsal.

Skin

The skin on the volar and dorsal surfaces of the hand is markedly different. Glabrous skin covers the volar side of the hand. Glabrous means bald; thus, this skin is without hair. In addition to this trait, it also lacks sebaceous glands. The glabrous skin of the palm has a much thicker and abrasion-resistant epidermis than the nonglabrous skin of the dorsum.1,2 It is tightly adherent to the underlying fascia, the deep extension of these fascial bands are called the septa of Legueu and Juvara.3,4 On the volar surface, the skin does not move with finger motion, but instead bunches up at the palmar and finger creases.5 The glabrous skin of the hand has four different mechanoreceptors: Pacinian corpuscles (vibration and pressure), Meissner’s corpuscles (light touch), Merkel’s disks (discrimination or shapes and textures), and Ruffini corpuscles (stretching of skin).6,7 Skin on the dorsum of the hand is thin hair bearing skin. It is very loosely adherent to the underlying tissue to allow for gliding as the hand flexes and extends.1 Skin creases form at the sites where the skin is adherent to the deep fascia. They serve as surface landmark for the underlying bones, joints, pulley and tendon system, and the vasculature.

Fingertip and Nail Pulp Space 18

The pulp spaces, along the volar aspect of the distal phalanx, are filled with fatty tissue. Multiple fascial septa connect the finger pulp skin to the bone and help hold the skin in place during gripping and pinching.8,9,10 The apical spaces are at the tip of the digit (▶Fig. 18.1).

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The whole region around the nail is called the perionychium. The hyponychium is the area where the sterile matrix meets the skin of the fingertip (▶Fig. 18.1). Beyond it, the nail plate is no longer attached to the underlying tissue. The paronychium is the border tissue around the nail. The proximal and lateral nail folds are cutaneous tissue that covers the proximal and lateral nail plate, respectively. The eponychium is the layer of skin over the nail along the roof of the nail fold.16 The nail bed is made up of the germinal matrix and the sterile matrix. The nail is produced by the germinal matrix and then adheres to the sterile matrix. The nail is made proximally and continues to get pushed distally as new nail plate is formed. The nail plate is made of onychin, which is similar to keratin.16

Fibrous Skeleton of the Hand and Palmar Aponeurosis The palmar aponeurosis is a sheet of fibrous connective tissue just deep to the subcutaneous fat of the palm.17 Multiple septa connect the palmar fascia to the deeper structures of the hand and divide the hand into compartments18 (▶Fig. 18.2). In severe infection, fracture, or injury that causes swelling, these fascial compartments may need to be surgically released to prevent compartment syndrome of the hand and digits.19 The palmar aponeurosis originates from the palmaris longus and fans out in a triangular shape with fibers toward each of the fingers. These fibers are longitudinal and superficial to the transverse carpal ligament.20 They terminate mainly by inserting into the palmar skin (superficial layer), with some fibers continuing into the fingers to merge into the lateral digital sheets (intermediate layer). Some fibers (deep layer) extend deep as vertical (sagittal) fibers to insert into the interosseous muscle fascia and

Hand

Fig. 18.1 (a) Anatomy of the distal finger as seen on the sagittal image of the thumb. Nail plate (arrowhead), eponychium (thick arrow), nail bed (thin arrow), and pulp space (asterisk). Note the extensor tendon (ET) inserting along the dorsal base of the distal phalanx (DP), whereas the flexor tendon (FT) attaches more distally on the volar aspect. (b) Image of the nail shows the nail plate (thin yellow arrow), lunula (asterisk), eponychium (thick yellow arrow), and lateral nail fold (yellow arrowheads). (c) Schematic of the nail showing components of the distal finger and nail. The nail bed is comprised of the germinal matrix and the sterile matrix. (d) Axial cadaveric cut through the distal phalanx of the thumb and two to five proximal phalanges shows vessels arborizing in the pulp space (distal plexus), and tendon vessels.

some traverse through the deep transverse metacarpal ligaments (TMLs) to insert onto the metacarpals and proximal phalanges. Transverse fibers are found superficial and deep to the longitudinal fibers in the area of the mid-palm (transverse fibers of palmar aponeurosis) and the web spaces: the superficial and deep TML and the natatory ligament.4,21,22 The superficial TML and the nat atory ligaments run just deep to the skin of the web spaces.4 Between the superficial and deep TML run the neurovascular bundles and the tendons of the lumbricals.22 The deep TML is attached to the volar plates of the index, middle, ring, and small fingers.4,23 The tendons of the palmar and dorsal interossei run deep or dorsal to the deep TML, while the lumbricals and the digital vessels and nerves run volar to it (▶Fig. 18.2b). The deep TML functions in maintaining the transverse metacarpal arch of the hand.4 Between the superficial and deep TMLs are the vertical fibers, which divide the palm into seven compartments holding the flexor tendons (flexor septal canals), and the lumbricals, vessels, and nerves (web space canals). These fascial bands are called the

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septa of Legueu and Juvara.3,20 The intermetacarpal fat pads are found in three of these compartments between the second, third, fourth, and fifth metacarpal heads.24 These fat pads function to allow gliding of the digital neurovascular structures as well as protecting these structures from compressive forces.24 In the fingers, sagittally oriented fascial layers extend distally, adjacent to the digital neurovascular bundle, from the coalescence of palmar fascia and natatory ligaments as lateral digital sheets. Cleland’s and Grayson’s ligaments are found dorsal and volar to the neurovascular bundles, respectively,25 extending between the phalanges and flexor tendon sheaths, and the lateral digital sheet. Cleland’s ligaments work to hold the skin in place with the bones. Cleland’s and Grayson’s ligaments work together to stabilize the neurovascular bundles during digital flexion.26,27 In Dupuytren’s disease, there is proliferation, thickening, and tightening of the fascia of the palm and fingers, which can lead to nodules and cords. The cords can cause contractures at the metacarpophalangeal (MP) joint and the proximal interphalangeal (PIP) joints25 (▶Fig. 18.3).

18

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Hand

3 2

4

5

Flexor tendon sheath Natatory ligaments 1

Distal interphalangeal crease Proximal interphalangeal crease Palmar-digital crease Transverse fibers

Commissural ligaments Abductor pollicis brevis muscle Radial artery Median nerve

a

Pretendinous bands Abductor digiti minimi muscle Palmaris brevis Ulnar nerve and artery Palmaris longus tendon

Fig. 18.2 (a) Fibrous skeleton of the hand and palmar aponeurosis. Transverse fibers are found superficial and deep to the longitudinal fibers in the area of the mid-palm (transverse fibers of palmar aponeurosis) and the web spaces: the superficial and deep transverse metacarpal ligaments (TML) and the natatory ligament. (b) Axial cadaveric cuts just proximal, at, and just distal to the metacarpophalangeal (MP) joints showing fibrous connective tissue, tendons, and ligaments. The palmar aponeurosis is a sheet of fibrous connective tissue just deep to the subcutaneous fat of the palm. Superficial and deep TML and vertical fascial bands of Legueu and Juvara are seen. In the fingers, sagittally oriented fascial layers extend distally, adjacent to the digital neurovascular bundle, from the coalescence of palmar fascia and natatory ligaments as lateral digital sheets. The tendons of the palmar and dorsal interossei run deep or dorsal to the deep TML, whereas the lumbricals and the digital vessels and nerves run volar to it. Numbers 1 to 5 indicate the digits.

18

Fig. 18.3 Dupuytren’s disease. (a) Coronal and (b) sagittal T1 images show thickened cord of the palmar fascia (arrows). Some prominence of the transverse fibers is also present (arrowhead). Compare the appearance of normal longitudinal fibers (thick arrow) in (c).

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Hand

◆◆ Osseous Anatomy

The skeletal anatomy of the hand includes the carpal and metacarpal bones, as well as the proximal, middle, and distal phalanges of the fingers and the proximal and distal phalanges of the thumb (▶Fig. 18.4). Chapter 17 “Wrist” contains a description of the carpal bones.

Metacarpal Bones The five metacarpal bones each consist of the base, shaft, neck, and head. The metacarpals diverge distally, and their volar surfaces are concave. The first metacarpal is rotated so that its volar surface faces toward the ulnar side. They articulate proximally

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with the carpals. The base also articulates with the adjacent metacarpal bases except for the bases of the first and second metacarpals, which do not articulate with each other. Distally, the convex head articulates with the concave base of the proximal phalanx. Two oval condyles sit below the head. The metacarpal bones form the transverse arches of the palm.28 In the axial projection of the hand, the thumb and small finger metacarpals are more volar than the metacarpals of the index, middle, and ring fingers. The proximal transverse arch near the carpus is rigid, whereas the distal transverse arch near the metacarpal heads is very mobile as the first, fourth, and fifth metacarpals rotate toward and away from the stable index and middle finger metacarpals. The index and middle finger metacarpals, along with the proximal carpal row, establish the longitudinal arch of the palm20 (▶Fig. 18.5).

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Fig. 18.4 Osseous anatomy of the hand. Top: volar and dorsal views of volume-rendered computed tomography (CT) images. Bottom row: Normal posteroanterior and lateral views of the hand. Note the zigzag pattern of the carpometacarpal articulation (blue). C, capitate; CMC, carpometacarpal joints; DIP, distal interphalangeal joint; DP, distal phalanx; H, hamate. The triquetrum is the larger bone dorsal to the pisiform; M, metacarpal; MP, middle phalanx; MP joint, metacarpophalangeal joint; L, lunate; P, pisiform; PIP, proximal interphalangeal joint; PP, proximal phalanx; S, scaphoid; Tp, trapezium; Tq, triquetrum; Tz, trapezoid.

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Fig. 18.5 Mechanical arches. The transverse arch (yellow) along the transverse axis through the metacarpal heads, the longitudinal arch (red) along the long axis of the metacarpals, and the oblique arch (blue).

Phalangeal Bones The index, middle, ring, and small fingers have three phalangeal bones: the proximal, middle, and distal phalanxes. The proximal phalanx contains a base, shaft, neck, and bicondylar head that articulates with the base of the middle phalanx. The middle phalanx also includes a base, shaft, neck, and bicondylar head. Between the two condyles arises a groove that interdigitates with the ridge in the base of the distal phalanx. This configuration gives stability to the joint.28 The distal phalanx consists of a base, shaft, and a flared tuft (ungual tuberosity). The thumb consists of only two phalanges: the proximal and distal phalanxes. The proximal phalanx of the thumb is similar to the proximal phalanx of the fingers; however, it is shorter in size, comparable to the length of the proximal phalanx of the small finger.28 The distal phalanx of the thumb is similar to the distal phalanx of the other fingers; however, it is larger in size.28 Fractures and dislocations of the metacarpal and phalanges are common injuries. While most fractures can be treated with closed reduction and splinting, irreducible or unstable fractures need percutaneous pinning or internal fixation.29 It is crucial to check rotational malalignment when evaluating fracture reduction.29

Sesamoid Bones Sesamoid bones are small bones that are within a tendon. They are called sesamoid because they are similar in appearance to sesame seeds. Most people have five sesamoid bones; however, there is a large amount of variation. The most common locations include the following: two at the MP joint of the thumb, one at the IP joint of the thumb, one at the MP joint of the index finger, and one at the MP joint of the small finger30,31 (▶Fig. 18.4). The function of sesamoid bones is not completely understood, but they play a role in transmitting tendon force across a joint and also helping the tendon glide across a joint.31 Injuries to these bones can include sesamoiditis (inflammation) and fractures of the sesamoid bones.31

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Ossification The shafts of the metacarpals and the phalanges ossify from a primary ossification center in the intrauterine period. Metacarpal

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Fig. 18.6 Ossification centers of the hand. The ossification centers are proximal in the phalanges, but distal in the metacarpals, except for the first metacarpal, which has a proximal ossification center.

secondary ossification centers appear distally in the second through fifth metacarpals but proximal in the first metacarpal (similar to the phalanges). The ossification centers of the phalanges are proximal. In general, they appear at about the age of 10 months to 3 years and fuse at about the age of 13 to 16 years, with

Hand the ossification centers appearing and fusing earlier in females32 (▶Fig. 18.6). Knowledge of the age of appearance of the ossification centers, their shape, and age of fusion is important for determining the bone age of a person.

◆◆ Joints, Ligaments, and Cartilage

The hand joints are synovial joints (see ▶Fig. 18.7, ▶Fig. 18.8, anatomical image series). Synovial joints contain an outer articular capsule, which holds the periosteum of the two bones together and an inner synovial membrane, which keeps the synovial fluid inside. All of the finger joints have a box-shaped configuration (▶Fig. 18.4). The volar plate, a substantial piece of connective tissue, stabilizes the floor of the joint. The flexor tendons and sheath are the secondary stabilizers of the volar side of the joint.33 Radial

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and ulnar collateral ligaments support the joint from medial and lateral translation. The extensor mechanism crosses the joint on the dorsal side.33

Carpometacarpal Joints The second through fifth metacarpal bases articulate with each other in addition to articulating with the carpal bones. The second through fifth carpometacarpal (CMC) joints form a single cavity that extends into joints between the metacarpal bases. The CMC joints of the hand have different shapes specialized to their various functions. The CMC joints of the thumb, ring, and small fingers allow a large degree of motion. The index and middle finger CMC joints are relatively fixed.1,34 The second and third CMC joints are gliding or plane joints.1,34 A CMC boss is a bony growth at the base of the second or third CMC joint2 (▶Fig. 18.9). These may be asymptomatic, but

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Fig. 18.7 Axial image series from proximal to distal. Asterisk, flexor pollicis longus tendon; C, capitate; D1-4, dorsal interossei 1-4; H, hamate; L, lunate; L, lumbricals; M1 to M5, metacarpals 1 to 5; MC, metacarpophalangeal joints; P, pisiform; R, radius; red dashed line, superficial palmar arch and deep palmar arch. The flexor tendons of the index, long, ring, and little fingers can be serially followed distally from the carpal tunnel; S, scaphoid; Tp, trapezium; Tq, triquetrum; Tz, trapezoid; U, ulna; V1-3, volar interossei 1-3. (Continued)

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18 Fig. 18.7 (Continued) (Continued)

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Hand

18 Fig. 18.7 (Continued) (Continued)

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18 Fig. 18.7 (Continued) (Continued)

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18 Fig. 18.7 (Continued) (Continued)

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Fig. 18.7 (Continued)

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Fig. 18.8 Coronal image series from palmar to dorsal surfaces. Asterisk, hook of hamate; C, capitate; H, hamate; L, lunate; P, pisiform; S, ­scaphoid; Tp, trapezium; Tq, triquetrum; Tz, trapezoid. (Continued)

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Fig. 18.8 (Continued) (Continued)

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Hand

Fig. 18.8 (Continued) (Continued)

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Fig. 18.8 (Continued) (Continued)

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Hand

Fig. 18.8 (Continued) (Continued)

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Fig. 18.8 (Continued) (Continued)

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Hand

Fig. 18.8 (Continued) (Continued)

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Fig. 18.8 (Continued)

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Fig. 18.9 Carpal boss (arrowhead).

are sometimes removed for aching pain. Pain may be caused by underlying arthritis, a ganglion cyst, or an irritated bursa.35

First Carpometacarpal Joint The thumb CMC (trapeziometacarpal joint) has unique anatomy (▶Fig. 18.4). The thumb metacarpal does not articulate with any of the other metacarpals. It is a saddle joint, which allows for two planes of movement. The joint allows flexion, extension, abduction, adduction, and opposition.36 Although initial studies believed that the axes of flexion-extension and abduction-­adduction were orthogonal to each other, newer studies find that they are oblique

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to each other.37 Rotation and translation of the metacarpal relative to the trapezium are also possible.37 The ligaments (▶Fig. 18.10, ▶Fig. 18.11) surrounding the thumb trapeziometacarpal joint have to serve the dual function of allowing multiple planes of motion while maintaining the stability of the joint.37 The anterior oblique ligament (volar beak ligament) originates from the palmar tubercle of the trapezium and inserts on the volar side of the thumb metacarpal. It is divided into a superficial component and a deep component. The ulnar collateral ligament originates from the flexor retinaculum and inserts volarly and ulnarly on the thumb metacarpal. The dorsal ­ligamentous complex, which includes the posterior oblique ligament, dorsal

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Hand

Palmar view

Lateral view Dorsal surface

Radial surface 2nd metacarpal

2nd metacarpal

1st metacarpal Intermetacarpal ligament Ulnar collateral ligament

Transverse carpal ligament Flexor carpi radialis

Posterior oblique ligament Anterior oblique ligament Abductor pollicis longus

c

Dorsal radial ligament Extensor carpi radialis longus

d

Fig. 18.10 Major ligaments of the trapeziometacarpal joint as viewed from (a) the front and (b) radial dorsal aspect. The dorsal ligament complex consisting of the dorsal radial ligament (brown), dorsal central ligament (yellow), and posterior oblique ligament (red) is the main stabilizer of the joint. The anterior oblique ligament (blue) was previously thought to be the most important ligament. Intermetacarpal ligament links the first and the second metacarpals. (c, d) Ligaments of the trapeziometacarpal region.

central ligament, and dorsal radial ligament, is the most important stabilizer of the joint. The posterior oblique ligament travels from the dorso-ulnar side of the trapezium to the ulnopalmar tubercle of the first metacarpal. The dorsal central ligament and dorsal radial ligament travel from the dorsal sides of the trapezium to the first metacarpal.38 In addition, the ­intermetacarpal ligament extending between the ulnopalmar tubercle to the dorsoradial aspect of the second metacarpal also stabilizes the first metacarpal (▶Fig. 18.12). Additional ligaments including the dorsal intermetacarpal ligament extending between the dorso-ulnar aspect of the first metacarpal and the dorsoradial aspect of the second metacarpal have also been described.39,40

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Clinically, the CMC joint of the thumb has special significance because it is the most common site of osteoarthritis. Due to the many planes of motion of this joint, the joint is dependent on the ligaments for stability.41 This joint is commonly affected by osteoarthritis (▶Fig. 18.13). A Bennett fracture (▶Fig. 18.14a,b) is a common fracture of the CMC joint of the thumb. This is an unstable fracture because the abductor pollicis longus (APL) muscle pulls the distal fragment dorsally and proximally, whereas the proximal fragment is held in place by the ligaments of the CMC joint.42,43,44 A Rolando fracture (▶Fig. 18.14c,d) is a comminuted intra-articular fracture of the base of the thumb metacarpal that often needs operative fixation.45

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Hand

Fig. 18.11 Ligaments of the first carpometacarpal joint. (a–c) Zoomed coronal sequential images of the hand from palmar to dorsal show the dorsal ligament complex (arrowheads) that consists of dorsal radial ligament, dorsal central ligament, and posterior oblique ligament from volar to dorsal. They appear in continuity on magnetic resonance imaging (MRI). Beak ligament (anterior oblique ligament, pink arrows) seen on coronal and sagittal (d) images.

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Fig. 18.12 First to second intermetacarpal ligament. Coronal image of another patient shows intermetacarpal ligament (arrow) extending between in the first (1) and second (2) metacarpals. S, scaphoid; Tp, Trapezium; Tz, trapezoid.

Fig. 18.13 Advanced first carpometacarpal joint osteoarthrosis with osteophytes, subchondral sclerosis, and severe joint space narrowing (arrow). There is also mild osteoarthritis of the trapezioscaphoid joint with subchondral sclerosis and mild joint space narrowing.

18 Fig. 18.14 Bennett’s and Rolando’s fractures. (a, b) Plain radiographs and (c, d) 3D volumetric reconstruction images show Bennett’s (thin arrows) and Rolando’s (thick arrows) fractures, respectively.

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Hand

Metacarpophalangeal Joints The MP joints are condyloid joints (▶Fig. 18.4). The joint is made up of an oval-shaped condyle, which fits into an elliptical cavity.46 Because of this configuration, these joints allow adduction, abduction, and circumduction in addition to flexion and extension. The MP joints have less bone contact than the PIP and DIP joints. The collateral ligaments, joint capsule, and volar plate provide stability to the MP joint.46 The cartilage extends more proximally on the volar surface compared to the dorsal surface. The volar plate stabilizes the volar side

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of the MP joint (▶Fig. 18.15, ▶Fig. 18,16, ▶Fig. 18.17, ▶Fig. 18.18). The volar plate is a thickened fibrocartilaginous structure reinforcing the capsule. It is firmly attached to the base of the phalanx, often with a central recess between it and the base that should not be construed as a tear on arthrography or magnetic resonance imaging (MRI). Proximally it thins and attaches to the neck of the metacarpal. The sagittal bands, the accessory collateral ligaments, the adjacent deep TMLs, and the flexor tendon sheath attach to the volar plate. The extensor mechanism and the dorsal fibrocartilage provide dorsal stability to the joint. The dorsal cartilage complex covers the dorsal 120 degrees of the joint.47 The collateral

Dorsolateral tubercle

a

Metacarpal head

b

Collateral ligament Volar plate

c

Tendon sheath

Deep transverse metacarpal ligament

Fig. 18.15 Collateral ligaments at the metacarpophalangeal joint. (a, b) metacarpophalangeal joint (orange, proper collateral ligament; blue, accessory collateral ligament; green, volar plate). (c) The volar plates of four palmar metacarpals are held together firmly by the deep transverse metacarpal ligament, which is continuous with the volar plate. Eaton calls this the “intervolar plate ligament.”

18 Fig. 18.16 Metacarpophalangeal joint and volar plate. (a) Sagittal and (b) axial magnetic resonance (MR) images. Note flexor (FT) and e ­ xtensor (ET) tendons, volar plate (thick yellow arrow), deep transverse metacarpal ligament (thin pink arrow), and the A1 pulley (yellow arrowheads). MC, metacarpal; PP, proximal phalanx. Note the recess between the volar plate and the proximal phalanx (green arrowhead).

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Hand ligaments of the MP joint include the deep and superficial portions of the radial and ulnar collateral ligaments (proper collateral ligaments) and the accessory collateral l­ igaments46 (▶Fig. 18.15, ▶Fig. 18.16, ▶Fig. 18.17, ▶Fig. 18.18, ▶Fig. 18.19). The proper collateral ligaments cover the joint capsule. They originate from the tubercles on the dorsoulnar and dorsoradial aspect of the metacarpal heads and extend to the volar aspect of the bases of the proximal phalanges (▶Fig. 18.17). The accessory collateral

ligaments originate in continuity, along the volar aspect of the proper collateral ligament and distally attach to the volar plates (▶Fig. 18.19). The proper collateral ligaments are taut (stretched) in flexion, whereas the accessory collateral ligaments are taut in extension48 (▶Fig. 18.17). The distance between the proximal and distal attachments of the collateral ligaments (the length of the ligaments) vary depending on the flexion and extension of the joint. The MP joint is often called a “cam,” which is an eccentric

Dorsolateral tubercle Hyperextension Middle phalanx

45° Position of rest

Central slip

Proximal phalanx

Maximum mobility Lateral slip Proper collateral ligament

Accessory collateral ligament

Check rein Volar plate

Flexion

Proper collateral ligament Accessory collateral ligament

Minimum mobility

a

b

Votar plate

Fig. 18.17 (a) Proper collateral ligaments of the metacarpophalangeal (MP) joints and range of motion. The shape of the metacarpal head is eccentric, resulting in a cam effect that makes the proper collateral ligaments more taut in flexion than in extension. The cam effect is not present in the proximal interphalangeal joint. The proper collateral ligaments are taut (stretched) in flexion, whereas the accessory collateral ligaments are taut in extension. The ligaments surrounding the proximal interphalangeal (PIP) (b) and distal interphalangeal (DIP) joints form a box in the same manner as the MP joint ligaments. The volar plate is the floor, the radial and ulnar collateral ligaments are the sides, and the extensor mechanism is the roof.

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Fig. 18.18 The metacarpophalangeal joint collateral ligaments. (a–e) Consecutive coronal images from dorsal to palmar aspect show the course of the proper collateral ligaments (arrowheads). The origins of the ligaments from the dorsolateral metacarpal heads are shown in (a) and the insertions on the proximal phalanx base are show in (c–e). The accessory collateral ligaments take their origin in continuity with the proper ­collateral ligaments and extend volarly in a fan-shaped fashion to insert on the volar plate.

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Hand wheel or cylinder (http://www.merriam-webster.com/dictionary/ cam). Thus, when the MP joint is fully flexed, the ligaments are most tight, and the least amount of adduction and abduction are possible. When the MP joint is extended, the ligaments transverse a shorter distance; thus, the ligaments become loose and allow for greater abduction and adduction46,49 (▶Fig. 18.17). The MP joints can achieve 90 degrees of flexion and 0 degrees of hyperextension.2 The safe position of splinting the hand (to prevent shortening of the ligaments) is based on this concept; the MPs must be splinted in 90 degrees of flexion and the IP joints straight.46 The understanding of the pathophysiology of the MP joint cartilage and destructive arthritis at the MP joint is evolving. The proximal MP joint contains a “bare area” that does not contain cartilage (▶Fig. 18.20). This area was initially thought to be the site where osteoclastic elements were able to break down bone in rheumatoid arthritis.50 However, there is new evidence that compression of the bone adjacent to the collateral ligaments may predispose to the bony erosion51 (▶Fig. 18.20).

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The thumb MP joint allows for flexion–extension and abduction–adduction52,53 (▶Fig. 18.17). Its collateral ligaments are covered superficially by the adductor aponeurosis on the ulnar and the abductor aponeurosis on the radial side (▶Fig. 18.21). Injury to the ulnar collateral ligament of the thumb is common.54 Acute injury to the ulnar collateral ligament of the thumb MP joint is termed a skier’s thumb, whereas chronic injury is termed a gamekeeper’s thumb55 (▶Fig. 18.22). A partial injury of the ligament can be treated nonoperatively. However, a full rupture often leads to a “Stener” lesion in which the adductor aponeurosis is interposed between the two torn ends of the ligament and prevents healing2,56 (▶Fig. 18.22).

Interphalangeal Joints The PIP and distal interphalangeal (DIP) joints are hinge joints.20 A hinge joint can only move in one plane.57 Tight collateral ligaments and the underlying bony anatomy limit the motion of the

Fig. 18.19 (a) Axial cadaveric cuts just proximal (upper image) and at (middle image) the metacarpophalangeal joint (MPJ), and proximal to the proximal interphalangeal joint (lower image) of the long finger showing collateral ligaments and volar plates. The proper collateral ligaments cover the joint capsule. They originate from the tubercles on the dorsolateral aspect of the metacarpal heads and extend to the volar aspect of the bases of the proximal phalanges. The accessory collateral ligaments originate in continuity, along the volar aspect of the proper collateral ligament and distally attach to the volar plates. (b) The MPJ on axial magnetic resonance (MR) view at the level of the metacarpal tubercle showing the origin of the accessory collateral ligaments. Accessory collateral ligaments (green arrowhead) extend from the dorsolateral aspects of the metacarpal head (pink arrowheads), where proper collateral ligaments are also attached, and run distally to insert in the volar plate (thick orange arrow). Note the sagittal bands (yellow arrows), which cover these ligaments and also insert in the volar plate as does the deep t­ ransverse metacarpal ligament (blue arrow) and flexor tendon sheaths/pulley (pink arrow). Blue arrowhead, interosseous muscle/tendon; ET, extensor ­tendon; FT, flexor tendons; L, lumbrical.

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Fig. 18.20 (a) Schematic drawing of the metacarpophalangeal joint showing bare areas (arrows), the area of intra-articular bone not covered by cartilage (blue) or synovium (green). M, metacarpal; PP, proximal phalanx. (b) Erosions of the bare area. Note erosions (arrowheads) of the radial aspect of the head of the second metacarpal and bases of the second and third proximal phalanges. Compare it to the normal appearance of the other metacarpophalangeal (MP) joints.

Fig. 18.21 Ulnar collateral ligament of the first metacarpophalangeal (MP) joint and adductor aponeurosis. (a) Coronal and (b) transverse fluid-sensitive magnetic resonance (MR) images show the ulnar collateral ligament (thin arrow) and the adductor aponeurosis (arrowheads). The thick arrow indicates radial collateral ligament.

PIP and DIP joints to flexion and extension.57,58 The surrounding ligaments of the PIP and DIP joints form a box in the same manner as the MP joint ligaments (▶Fig. 18.17). The volar plate is the floor, the radial and ulnar collateral ligaments are the sides, and the extensor mechanism is the roof59 (▶Fig. 18.19). Proximally, the lateral portions of the volar plate of the PIP joints attach to the side of the shaft of the proximal phalanx by thickened bands, called the “check reins” (▶Fig. 18.17, ▶Fig. 18.23). The volar plates at the DIP joints may not be well formed. The collateral ligaments

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similar to the MP joint extend from the dorsoulnar and dorsoradial head of the phalanx proximally to the base of the phalanx distally (proper collateral ligament) and the volar plate (accessory collateral ligament). The interdigitation of the articular surface of the distal to middle phalanx and the middle to proximal phalanx also contributes to the joint stability, disallowing medial and lateral motion. The DIP joint can achieve 90 degrees of flexion and 30 degrees of hyperextension. The PIP joint can attain 110 degrees of flexion and 5 degrees of hyperextension.20

Hand

Fig. 18.22 Injuries of collateral ligaments of the first metacarpophalangeal (MP) joint. (a) Acute injury to the ulnar collateral ligament or skier’s thumb (thin arrow). (b) Stener lesion. Note the thin adductor aponeurosis (thick green arrow) in (b) interposed between the torn ligament (curved arrow) and proximal phalanx. (c) Radial collateral ligament injury (arrowhead).

The flexor tendons are on the volar surface, and the extensor tendons are on the dorsal surface. The intrinsic muscles are within the hand. Tendon attachments are shown in ▶Fig. 18.24.

Long Tendons of the Hand Flexor Tendons

Fig. 18.23 Proximal interphalangeal joint: checkrein. Sagittal image of a finger to the side of the midline shows checkrein of the proximal interphalangeal joint volar plate (arrowheads). The thin arrow indicates the volar plate of the proximal interphalangeal joint, and the thick arrow indicates the volar plate of the metacarpophalangeal joint.

◆◆ Muscles and Tendons

The extrinsic and intrinsic muscles coordinate movement of the digits (see ▶Fig. 18.7, ▶Fig. 18.8, anatomical image series). The extrinsic muscles are in the forearm and exert their effect on the digits via their tendons.

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Tendons connect muscles to bones; they are made of tough connective tissue composed primarily of collagen. The superficial flexors of the forearm, the flexor carpi radialis, the palmaris longus, and the flexor carpi ulnaris, all insert in the hand, but have their mechanism of action at the wrist. These will be discussed in the section on the wrist and the forearm. The flexor tendons of the fingers lie on the volar side of the hand. They are responsible for flexion of the DIP and PIP joints of the fingers and the IP joint of the thumb. The flexor digitorum profundus (FDP) flexes the DIP joints of the fingers. The FDP originates in the forearm and inserts at the metaphysis of the distal phalanx. The terminal extensor tendon inserts on the epiphysis of the distal phalanx. In children, the flexor and extensor tendons insert on opposite sides of the growth plate.60 The flexor digitorum superficialis (FDS) flexes the PIP joints of the fingers. It originates in the forearm and inserts on the sides of the middle phalanx. The FDP and FDS tendons are enclosed by a common flexor sheath. The FDS starts superficial in the palm of the hand. At the level of the proximal part of the proximal

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Fig. 18.24 Volar and dorsal attachments (red: origins; blue: insertions). Please see text for the exact numbering of the volar interossei. Volar attachment: The insertion of the flexor carpi ulnaris to the hamate and the fifth metacarpal base is through the pisohamate and pisometacarpal ligaments, respectively. The palmar or volar interossei (interossei volares in older literature) are three small, unipennate muscles in the hand that lie between the metacarpal bones and are attached to the index, ring, and little metacarpals. They are smaller than the dorsal interossei of the hand. Dorsal attachment: The extensor indicis and the extensor digiti minimi contribute to the extensor mechanism in the index and little fingers, respectively.

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Hand phalanx, the FDS starts to split into two tendons through which the FDP tendon passes.61 The split tendons of the FDS decussate near the PIP where it is called Camper’s chiasm (▶Fig. 18.25, ▶Fig. 18.26, ▶Fig. 18.27). The flexor tendons receive nutrition both by the synovial fluid and by vincula.62,63 Vincula are connections between the flexor tendons and arterial arches in the fingers that provide blood supply to the tendons.62,63 There are many vincula described in the literature, but the most prominent are the vincula brevis to the distal FDS and distal FDP (▶Fig. 18.25). The vincula longus to the FDP may be considered an extension of the vincula brevis of the FDS.62,63 In the thumb, there is only one flexor, the flexor pollicis longus that flexes the IP joint of the thumb by inserting on the metaphysis of the distal phalanx.

Pulley System

The flexor tendons of the hand run through the carpal tunnel, deep to the palmar fascia of the hand, and enter the flexor tendon sheath at the level of the MP joint. In the palm, there is a pulley of the palmar aponeurosis formed by the transverse fibers (superficial TML) that attach by the vertical septa (intertendinous fibers, septa of Legueu and Juvara) to the deep TML. The flexor tendon sheath is composed of a set of pulleys.64 There are five annular pulleys and three cruciate (or cruciform) pulleys (▶Fig. 18.28, ▶Fig. 18.29). The annular pulleys are composed of thick connective tissue. The A1 pulley is over the MP joint, the A2 pulley is

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over the proximal part of the proximal phalanx, the A3 pulley is over the PIP joint, the A4 pulley is over the middle part of the middle phalanx, and the A5 pulley is over the DIP joint. The three cruciate pulleys are between the A2 and A3, the A3 and A4, and the A4 and A5. These pulleys are much thinner and collapse with finger flexion to allow flexion and approximation of the thicker annular pulleys.65 The function of the pulleys is to keep the flexor tendon closely apposed to the bone. The A2 and A4 pulleys are crucial to prevent bowstringing of the tendons and should be left in place or repaired whenever possible.66,67,68,69 The thumb has three pulleys. A1 is just proximal to the MP joint. The oblique pulley is on the proximal part of the proximal phalanx. It connects to the adductor pollicis (AdP). The A2 pulley is over the distal portion of the proximal phalanx.65

Flexor Tendon Zones

The flexor tendons of the hand are separated into zones (▶Fig. 18.30). Zone I is distal to the FDS insertion. Zone II is from the A1 pulley to the FDS insertion. Zone III is between the distal edge of the transverse carpal ligament and the proximal end of the A1 pulley. Zone IV is in the carpal tunnel, whereas zone V is proximal to it. There are three flexor zones of the thumb. TI is d ­ istal to the insertion of the flexor pollicis brevis (FPB). TII is between the A1 pulley and the distal insertion of the FPB. TIII is proximal to the A1 pulley and distal to the transverse carpal ligament.70,71

Fig. 18.25 Flexor and extensor tendons. (a) Flexor tendons. (b) Flexor tendons with A1–A5 annular pulleys (blue). (c) Flexor tendons, the long and short vincula. (d) Extensor tendons and mechanism. DTML, deep transverse metacarpal ligament; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis. The extensor tendon at the metacarpophalangeal (MP) joint level is held in place by the transverse lamina or sagittal band, which tethers and centers the extensor tendons over the joint. This sagittal band arises from the volar plate and the intermetacarpal ligaments at the neck of the metacarpals. Any injury to the extensor hood or expansion may result in subluxation or dislocation of the extensor tendon. The intrinsic tendons from the lumbrical and interosseous muscles join the extensor mechanism at approximately the level of the proximal and middle portion of the proximal phalanx and continue distally to the distal interphalangeal (DIP) joint of the finger. The extensor mechanism at the proximal interphalangeal (PIP) joint is best described as a trifurcation of the extensor tendon into the central slip, which attaches to the dorsal base of the middle phalanx and the two lateral bands. These lateral bands continue distally to insert at the dorsal base of the distal phalanx. The extensor mechanism is maintained in place over the PIP joint by the transverse retinacular ligaments. The tendons of the flexor digitorum superficialis and profundus are connected to each other, and to the phalanges, by tendinous bands known as vincula tendina. Four branches arise from the digital vessels and enter the vincula at the neck of the middle phalanx, the base of the middle phalanx, and the neck of the proximal phalanx, and the base of the proximal phalanx.

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Fig. 18.26 Camper’s chiasm. (a–f) Serial transverse images from proximal to distal show splitting of the flexor digitorum superficialis tendon (arrowheads) through which the flexor digitorum profundus tendon passes to become superficial. Note the insertion of the flexor digitorum superficialis slips on the middle phalanx (arrows).

Fig. 18.27 Coronal. Camper’s chiasm. Coronal images of the hand of (a) the previous patient and (b) that of another patient show Camper’s chiasm (arrowheads) of the fourth ray. Note splitting of the flexor digitorum superficialis (FDS) tendon in the third ray before the chiasm (yellow arrow) and extension of the tendon toward its insertion after the chiasm (blue arrow).

Extensor Tendons Dorsal to the dorsal interossei lie the extensor tendons, dorsal fascia, and the skin (▶Fig. 18.19). The extensor tendons are organized into six extensor compartments at the wrist as they pass under the extensor retinaculum (see Chapter 3 “Muscles of Shoulder Girdle, Arm, and Forearm”).

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Extensor Tendon Compartments

From radial to ulnar, the first compartment contains the APL, and the extensor pollicis brevis (EPB). The second compartment contains the extensor carpi radialis longus (ECRL) and extensor carpi

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radialis brevis (ECRB). The ECRL inserts on the base of the second metacarpal and the ECRB inserts on the base of the third metacarpal. These muscles extend the wrist. The third compartment contains the extensor pollicis longus (EPL). The EPL starts ulnar to the second extensor compartment and then crosses radially over the ECRL and ERCB to insert on the epiphysis of the distal phalanx of the thumb. The fourth extensor compartment contains the extensor indices proprius (EIP) and the extensor digitorum communis (EDC). The fifth extensor compartment contains the extensor digitorum minimi (EDM). The sixth extensor compartment contains the extensor carpi ulnaris (ECU). This tendon inserts on the base of the fifth metacarpal.

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Hand

Tendon Synovial covering of tendon (visceral) Synovial lining of tunnel (parietal) Annular part of fibrous digital sheath

Osseofibrous tunnel (synovial cavity) Phalanx Mesotendon (synovial fold or vincula)

Vein Synovial sheath

b

Tendon

Artery Nerve

Fig. 18.28 Flexor tendon pulleys and vasculature. (a) Tendon pulleys. There are five annular pulleys and three cruciate (or cruciform) pulleys. The A2 and A4 pulleys are crucial to prevent bowstringing of the tendons and should be left in place or repaired whenever possible. (b) Flexor tendon sheaths and vessels. (c) Axial cadaveric cuts of the second and third digits at four levels, from proximal to distal. Major blood supply to the flexor tendon is seen at the base of the proximal phalanx (yellow arrows), whereas at the mid-levels of the proximal and middle phalanges tendon vascularity is minimal. Tendon blood supply is provided directly by the vessels entering from the flexor synovial sheaths and through the vincula. The second source of nourishment is by diffusion through synovium. The blood supply is provided by four transverse branches of the digital arteries entering the short and long vincula (see Fig. 18.23). A volar avascular area (danger zone) of the profundus tendon exists at the mid proximal phalanx. FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis.

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Fig. 18.29 A1 and A2 pulleys (green arrowheads). Sagittal (a) and axial (b) images of the metacarpophalangeal joint. The pulley is a very thin structure and can be discerned by the sudden step-off seen just distal to the distal arrowhead in (a). ET, extensor tendon; FT, flexor tendons; MC, metacarpal; PP, proximal phalanx; thick yellow arrow, volar plate; thin blue arrow, deep transverse metacarpal ligament.

Extensor Mechanism of The Thumb

The abductor pollicis brevis (AbPB) and AdP coalesce with the EPL over the proximal phalanx of the thumb. The APL inserts on the base of the thumb metacarpal. The EPB inserts on the base of the proximal phalanx, and the EPL inserts on the base of the distal phalanx.73

Extensor Mechanisms of the Fingers

Fig. 18.30 Flexor and extensor tendon zones. The flexor (yellow) and extensor (white) tendon zones are important for communicating the level of injury. The extensor zones are easy to remember as the odd-numbered zones are over the joint, whereas the even-numbered zones are over the bones. See text for details.

Extensor Tendon Zones

Just like the flexor tendons, the extensor tendons are also divided into zones proposed by Kleinert and Verdan.72 Zone I is over the DIP joint, zone II over the middle phalanx, zone III over the PIP joint, zone IV over the proximal phalanx, zone V over the MP joint, and zone VI over the metacarpals. Zones VII and VIII are more proximal, the former over the wrist and the latter proximal to it in the forearm. The thumb is divided into TI over the IP joint, TII over the proximal phalanx, TIII over the MP joint, and TIV over the thumb metacarpal.72

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The extrinsic extensors of the fingers include the EIP, EDC, and EDM. After passing beneath the extensor retinaculum at the wrist, the extensor tendons pass over the metacarpal of their respective fingers. The EIP travels ulnar to the EDC to the index finger, and the EDM travels ulnar to the EDC of the small finger. Variations in the number and type of extensor tendons are common. The most common configuration include one EIP, one EDC of the index finger, one EDC of the long finger, two EDCs of the ring finger, no EDC of the small finger, and two EDMs of the small finger.74 Over the metacarpals, the EDC also gives off juncturae tendinum. These are connections between the extensor tendons over the dorsal surface of the metacarpals.75,76 The most common arrangement of the juncturae tendinum includes a band between the index and middle EDC, middle and ring EDC, and ring and small finger EDM.75,76 These tendinous bands are believed to be important in coordinated extension of the fingers, redistribution of force, and stabilization of the MP joints.77 At the level of the MP joint, the extrinsic tendons form the dorsal expansion (or extensor hood, or dorsal hood, or dorsal aponeurosis; ▶Fig. 18.19, ▶Fig. 18.25, ▶Fig. 18.31). This is also the area of the sagittal bands. Distal to the level of the MP joint, the extrinsic extensor tendon divides into three slips: the central slip and two lateral slips. The central slip inserts on the epiphysis of the base of the middle phalanx.2,78,79,80,81 The lateral slips join with the tendons of the lumbricals on the radial side of the digit, and with the tendons of the dorsal interossei on the radial aspect of the index finger, both sides of the middle finger, and the ulnar aspect of the ring finger to form the lateral bands (conjoined tendon; ▶Fig. 18.25, ▶Fig. 18.31). The palmar interossei also contribute to the lateral bands inserting on the ulnar border of the index finger, and the radial border of the ring and small fingers. The lateral bands converge over the distal part of the middle phalanx to form the terminal tendon, and this inserts on the epiphysis

Hand

Fig. 18.31 Extensor mechanism. From proximal to distal at the level of the (a) distal metacarpal, (b) metacarpal head, (c) proximal phalanx base, (d–f) shaft of the proximal phalanx, (g) proximal phalanx head, and (h) middle phalanx base. The blue arrowheads progressively show extensor tendon (a,b), lateral slips (c), and lateral bands (d–h). In (b), the yellow arrowheads show sagittal bands, the red arrow shows the interosseous muscle/tendon, and the blue arrow shows the lumbrical muscle. In (c), the blue arrowheads show the division of the tendon into central and ­lateral slips, and the red arrow shows the interosseous tendon. In (g), the thick arrow shows the central slip.

of the base of the distal phalanx.2,78,79,80,81 At the level of the MP joint, the lumbricals are volar, and the interossei are dorsal to the deep TML (▶Fig. 18.19). However, they are all volar to the axis of flexion–extension of the MP joint and thus flex the MP joint. There are several bands of connective tissue that keep the extensor tendons in correct relationship to the bones. These are similar in function to the volar pulleys; however, their location and function changes depending on the position of the joint: the extensor hood, the sagittal bands, the transverse retinacular ligament, the oblique retinacular ligament, and triangular ligament. Although they are called ligaments, they are not true ligaments that connect bone to bone. Instead, they are connective tissue that helps stabilize the extensor mechanism.2,78,79,80,81 The sagittal bands originate from the volar plate and the intermetacarpal ligaments and insert onto the extensor hood (▶Fig. 18.19, ▶Fig. 18.25). The sagittal bands are the main connection between the extensor mechanism and the MP joint. The

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bands and the volar plate form a lasso around the MP joint and base of the proximal phalanx and with the pull of the extrinsic extensor muscles the lasso extends the MP joint.2,78,79,80,81 The bands also stabilize the extensor tendon and prevent bowstringing. Injury and tear of the sagittal band can be shown with MRI (▶Fig. 18.32). The transverse retinacular ligaments originate from the flexor tendon sheath volarly and insert onto the lateral bands dorsally. They prevent the lateral bands from subluxating dorsally. The oblique retinacular ligament of Landsmeer originates from the proximal phalanx and flexor tendon sheath at the PIP joint volarly and inserts onto the terminal tendon dorsally (▶Fig. 18.25). The triangular ligament stabilizes the lateral bands distal to the PIP joint and keeps them centered dorsally over the middle phalanx.2,78,79,80,81 At the level of the PIP joint, the lateral bands are dorsal to the axis of flexion–extension of the joint, whereas the oblique retinacular ligaments are volar to that axis.

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Fig. 18.32 Sagittal band injury. (a) axial T1W and (b) T2W with fat saturation (different case). (a) Injury of the radial sagittal band of the index finger seen as increased signal and thickening of the band (arrow). (b) Tear of the ulnar-sided sagittal band (arrow) of the long finger. Compare with the normal sagittal bands in the ring finger.

Swan-neck Extrinsic

PIP axis ORL Intrinsic

Boutonniere

PIP axis

Extrinsic

ORL

Intrinsic

b Fig. 18.33 (a) Lateral radiograph of the hand shows swan neck deformity of the ring finger (arrowhead) and boutonniere deformity of the little finger (arrow). In the former, there is flexion of the distal interphalangeal (DIP) joint with extension of the proximal interphalangeal (PIP) joint, whereas in the latter, it is the opposite. (b) Mechanism. In swan neck deformity, there is elongation or rupture of the terminal tendon resulting in flexion of the DIP joint, and attenuation of the volar plate at the hyperextended PIP joint. In boutonniere deformity, there is rupture of the central slip, which allows for the head of the proximal phalanx to buttonhole dorsally as the lateral slips sublux volarly. This leads the intrinsic muscles to act as flexors of the PIP joint and extensors of the DIP joint. ORL, oblique retinacular ligament of Landsmeer.

Imbalance of Extensor Tendons

Disruption of the balance between the flexors and extensors of the finger can happen through injury of various components of the extensor mechanism.82 Two common deformities include the boutonniere deformity and the swan neck deformity (▶Fig. 18.33). The boutonniere deformity consists of PIP joint flexion and DIP joint hyperextension.83 The name derives from the fact that the PIP joint “buttons” through the lateral bands. In this deformity, the lateral bands slide volar to the axis of the PIP joint and contraction

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of the extensor tendon and thus the lateral bands leads to flexion at the PIP joint and hyperextension at the DIP joint. This deformity can be caused by rupture of the central slip.2,83 The swan neck deformity has the opposite configuration: hyperextension of the PIP joint and flexion of the DIP joint.84,85 This deformity is primarily due to laxity at the volar plate of the PIP joint, which allows hyperextension at the PIP joint. Mallet finger injury, FDS rupture, intrinsic contracture, and MP joint subluxation can all lead to a swan neck deformity.2,84,85

Hand

Fig. 18.34 (a) Flexor synovial sheaths and radial and ulnar bursae. (b) Extensor bursae.

Synovial Sheaths and Bursa The flexor tendons, and, to a smaller extent, the extensor tendons, are enveloped in tendon sheaths. These sheaths are encapsulations that surround the tendons, help with gliding, and contain synovial fluid that helps nourish the tendons. The flexor tendon sheaths extend volarly from the A1 pulley at the MP joint to the A5 pulley at the DIP joint (▶Fig. 18.34). Each finger has an individual sheath. The synovial sheaths associated with the flexor tendons of thumb and little finger pass without interruption from the wrist to the fingers. Extensive development of the synovial sheaths at these levels is probably due to high mobility of the metacarpal bones of the thumb and little finger than those of the other fingers. Therefore, compared to other fingers, local infection in the thumb or little finger can spread more proximally toward the wrist through the synovial sheaths. The radial bursa is an extension of the thumb flexor tendon sheath that extends under the transverse carpal ligament. The ulnar bursa is a continuation of the small finger flexor tendon sheath, and it also extends under the transverse carpal ligament. In the palm, the ulnar bursa widens and covers the fourth and third flexor tendons. There is a connection between the index, middle, or ring finger bursa with the ulnar bursa in 15% of patients.86,87 There is also a link between the radial and ulnar bursa of the hand in 30 to 100% of patients.87,88 In the carpal tunnel, the flexor tendons are also within individual flexor tendon sheaths. Proximal to the transverse carpal ligament lies the potential space of Parona, a space superficial to the fascia of the pronator quadratus and deep to the FDP sheaths (▶Fig. 18.35). In some patients, there is a connection between the radial and ulnar bursa in this space.89

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Infections of the flexor tendon sheaths of the hand are a surgical emergency. The pressure within the sheath causes a compartment syndrome and cutting off the blood supply to the tendons causes tendon necrosis and death.90 In the case of infection, there may be spread of infection to the space of Parona via rupture of the ulnar or radial bursa in this location.91 A short area of synovial sheath under the extensor retinaculum encloses the extensor tendons92 (see Chapter 17 “Wrist”; ▶Fig. 18.34).

Anatomic Variation of Flexor Tendons The FDS to the small finger is missing in 6 to 16% of patients93 (▶Fig. 18.36). Lack of the FDS to other digits or the FDP is rare, but has been reported.94 Digastric flexor digitorum superficialis muscle to the index ­f inger: Instead of a tendinous portion in the hand, there is a second muscular portion of the FDS to the index finger either in the carpal tunnel or in the palm, which may be mistaken for a palmar mass or cause carpal tunnel syndrome.95,96

Anatomic Variation of Extensor Tendons Variations in the number and type of extensor tendons are common. The most common configuration include one EIP, one EDC index, one EDC long, two EDC ring, no EDC small, and two EDM small.74 The following variations were found: two EIP, two or three EDC long, one or three EDC ring, and one or two EDC small.74

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Fig. 18.35 Axial cadaveric sections from the wrist distally showing the potential space of Parona, and thenar and hypothenar muscles. Proximal to the transverse carpal ligament lies the potential space of Parona, a space superficial to the fascia of the pronator quadratus and deep to the flexor digitorum profundus sheaths. In some patients, there is a connection between the radial and ulnar bursae in this space. In the case of infection, there may be spread of infection to the space of Parona via rupture of the ulnar or radial bursa in this location. The thenar compartment contains three muscles that power the thumb. These include the abductor pollicis brevis (AbPB), the flexor pollicis brevis (FPB), and the opponens pollicis (OP). The OP arises from the flexor retinaculum and the tubercle of the trapezium and inserts into the radial side of the first metacarpal (M). The FPB muscle arises from the flexor retinaculum, the tubercle of the trapezium, and the capitate and inserts into the ulnar side of the base of the proximal phalanx of the thumb. The AbPB is located just under the skin. It originates from the flexor retinaculum of the hand, the tubercle of the scaphoid bone, and inserts into the radial side of the base of the proximal phalanx of the thumb. The oblique head of the adductor pollicis (AdP) arises by several slips from the capitate and the bases of the second and third metacarpals. The transverse head of the AdP arises from the lower ­t wo-thirds of the third metacarpal. The AdP tendon unites with the FPB tendon and together insert onto the ulnar side of the base of the proximal phalanx of the thumb. This tendon usually contains a sesamoid bone. The tendon of the flexor pollicis longus (FPL) passes through the carpal tunnel and then between the FPB and the AdP to the base of the first distal phalanx. The hypothenar compartment contains three muscles that power the small finger. These include the abductor digiti minimi (ADM), the flexor digiti minimi (FDM), and the opponens digiti minimi (ODM). The ADM forms the ulnar border of the palm. It is distinguished from the FDM by the branches of the ulnar artery and nerve, which pass between the two. Both muscles attach to the base of the fifth proximal phalanx. The ODM is seen beneath other hypothenar muscles and attaches to the metacarpal bone of the little finger. C, capitate; FCR, flexor carpi radialis; H, hamate; L, lumbricals; Lu, lunate; M, metacarpal; P, pisiform; PP, ­proximal phalanx; R, radius; Sc, scaphoid; Tp, trapezium; Tq, triquetrum; Tz, trapezoid; U, ulna.

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Hand

Fig. 18.35 (Continued)

Fig. 18.36 Absent flexor digitorum superficialis (FDS). Transverse T1 proton density fat-saturated (PDFS) images of the hand at the level of (a) metacarpals, and (b) proximal phalanx, show absent FDS tendon of the fifth finger (arrows). Clinically, the patient did not have any restriction of motion before or after the injury that the patient had sustained. Note only one tendon for the little finger. Compare with the well-seen FDS and flexor digitorum profundus (FDP) tendons of the adjacent ring finger.

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Hand Extensor medii proprius: In this variation, there in an independent second extensor, analogous to the EIP, which goes to the long finger (1–12%).74,80 Extensor indicis et medii communis: In this variation, the EIP inserts on the middle and index fingers (2–5%).74,97,98

Volar Deep Spaces and Dorsal Spaces In addition to the synovial spaces described earlier, subfascial spaces exist in the volar hand, separated by the midpalmar septum and the hypothenar septum, and where fluid can accumulate (▶Fig. 18.37). The hypothenar septum extends from the third metacarpal to the palmar aponeurosis, whereas the hypothenar septum extends from the fifth metacarpal to the ulnar aspect of palmar aponeurosis. Radial to the midpalmar septum is the thenar space bounded radially by the thenar eminence, and between the two septa is the midpalmar space. Dorsally these are limited by the

fascia covering the AdP muscle, and the fascia of the second and third volar interossei and periosteum of the third, fourth, and fifth metacarpals, respectively. The volar extent of these spaces is the palmar aponeurosis and the sheaths of the flexor tendons, that for the index finger for the thenar space, and those for the remaining finger for the midpalmar space. The hypothenar space is between the hypothenar septum, fascia of the superficial hypothenar muscles, and fascia of the deep hypothenar muscles and periosteum of the fifth metacarpal. Dorsally, there are two spaces separated by the aponeurosis of the extensor tendons, one (dorsal subcutaneous space) superficial to it and one (dorsal subaponeurotic space) between it and the fascia of the dorsal interossei and periosteum of the metacarpals (▶Fig. 18.37). Although the subcutaneous space does not have distinct margins, the subaponeurotic space is limited radially and ulnarly by the attachment of the aponeurosis to the fascia of the dorsal interossei and the periosteum of first and fifth metacarpals.89,99

Fig. 18.37 (a, b) Nonsynovial spaces in the hand where fluid can accumulate. Dorsally, there are two spaces separated by the aponeurosis of the extensor tendons: the dorsal subcutaneous (pink) and subaponeurotic (red) spaces. Volar spaces are separated by the midpalmar septum (light blue) and the hypothenar septum (dark blue) and include volar midpalmar (yellow), thenar (light green), and hypothenar (dark green) spaces. The arrows indicate palmar aponeurosis. (c, d) Dorsal subcutaneous and subaponeurotic abscess. A 42-year-old man with pain, redness, and swelling. Axial short tau inversion recovery (STIR; a) and postcontrast T1 fat-saturated (b) images show extensive near circumferential but predominantly dorsal cellulitis with nonenhancing abscesses in the dorsal subcutaneous (red arrows) and subaponeurotic (yellow arrow) spaces.

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Hand

Intrinsic Muscles and Compartments The intrinsic muscles are contained completely in the hand. The volar hand is subdivided into four compartments: the thenar compartment, the hypothenar compartment, the central (midpalmar) compartment, and the deep (adductor/interossei) compartment100 (▶Fig. 18.38). The deep (adductor/interossei) compartment can further be subdivided into four dorsal and three volar interossei compartments for each of the interossei. Proper knowledge of compartment anatomy is essential for adequate surgical decompression.

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Fig. 18.38 Compartments of the volar hand: thenar (pink), hypothenar (green), adductor (blue), central/mid-palmar (red), and interossei (yellow) compartments. The adductor and interossei compartments form the deep compartment.

The thenar compartment contains three muscles that power the thumb. These include the AbPB, the FPB, and the opponens pollicis (OP) (▶Fig. 18.35, Fig. 18.39, ▶Fig. 18.40). The hypothenar compartment contains three muscles that power the small finger. These include the abductor digiti minimi (ADM), the flexor digiti minimi (FDM), and the opponens digiti minimi (ODM). The palmaris brevis (PB) muscle overlies the hypothenar compartment (▶Fig. 18.39). Its origin is the transverse carpal ligament and the palmar aponeurosis, and it inserts into the skin on the ulnar side of the hand and the pisiform in some specimens.101 The central compartment contains the flexor tendons and the lumbricals (▶Fig. 18.35, ▶Fig. 18.41). The lumbricals flex the MP joint and extend the IP joints. There are four lumbricals in the hand, one in each web space. The lumbricals originate on the FDP tendons (▶Fig. 18.41, ▶Fig. 18.42). The first lumbrical is on the index finger, the second is on the middle finger, the third is bipennate from both the middle and ring finger FDPs, and the fourth, also bipennate, is on the ring and small finger FDPs (▶Fig. 18.24). The lumbricals transverse the radial side of the digits and become part of the lateral band of the finger extensor mechanism just distal to the MP joint (▶Fig. 18.25). The deep compartment contains the AdP, the palmar interossei, and the dorsal interossei (▶Fig. 18.38). The palmar interossei adduct the fingers (toward the middle finger) mnemonic—palmar interossei adduct (PAD). The dorsal interossei abduct the fingers (away from the middle finger) mnemonic dorsal interossei abduct (DAB).102 For a complete description of intrinsic muscle origins, insertions, and function, please see ▶Table 18.1.

18 Fig. 18.39 Intrinsic muscles in the thenar and hypothenar eminence. Transverse images from proximal to distal. ADM, abductor digiti minimi; AdP, adductor pollicis; AbPB, abductor pollicis brevis; FPB, flexor pollicis brevis; OP, opponens pollicis; arrowhead: flexor digiti minimi; yellow arrow: opponens digiti minimi; star: flexor pollicis longus tendon; green arrows: palmar aponeurosis.

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Fig. 18.40 Intrinsic muscles in the thenar and hypothenar eminences. Coronal magnetic resonance (MR) images from volar to dorsal. The ­abductor digiti minimi arises from the pisiform bone and the flexor retinaculum. It forms the ulnar margin of the hand and terminates into the proximal phalanx. ADM, abductor digiti minimi; APB, abductor pollicis brevis; FPB, flexor pollicis brevis; OP, opponens pollicis; yellow arrow, flexor digiti minimi; blue arrow, opponens digiti minimi; red arrows, palmaris brevis.

Fig. 18.41 Intrinsic muscles of the fingers: interossei and origin of the lumbricals. The three palmar interossei (orange areas) adduct the fingers. The four dorsal interossei (green areas) abduct the fingers. There is no palmar interossei attachment to the middle finger. There are four lumbricals in the hand, one in each web space. The lumbricals originate on the flexor digitorum profundus tendons.

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Fig. 18.42 Intrinsic muscles of the fingers: interossei and lumbricals. Coronal images along the volar aspect of the hand at level of the (a) flexor tendons and (b) volar aspect of the metacarpal heads and transverse images at the level of (c) the metacarpal shaft and (d) metacarpal heads show the interossei and the lumbricals. The dorsal (D1–D4) and volar (P1–P3) interossei are dorsal to the deep transverse metacarpal ligament (DTML, arrowhead), whereas the lumbricals are volar to the DTML. However, all are volar to the axis of rotation of the metacarpophalangeal (MP) joint, thus flexing the MP joint. Yellow arrows show the interossei tendons. Table 18.1 Compartments and intrinsic muscles of the hand Muscle

Origin

Insertion

Innervation

Action

Scaphoid, trapezium, and flexor retinaculum

Base of the proximal phalanx of the thumb

Median nerve

Abduction of the CMC joint of the thumb

Superficial head

Flexor retinaculum

Base of the proximal phalanx of the thumb

Median nerve

Flexion of the CMC joint of the thumb

Deep head

Capitate and trapezium

Base of the proximal phalanx of thumb

Ulnar nerve

Tubercle trapezium Flexor retinaculum

Radial border of the first metacarpal

Median nerve

Opposition of the CMC joint of the thumb

Variations

Thenar compartment Abductor pollicis brevis Flexor pollicis brevis

Opponens pollicis

Hypothenar compartment Palmaris brevis

Ulnar border of the palmar aponeurosis

Hypothenar eminence skin

Ulnar nerve (superficial branch)

Tenses skin on the ulnar side of the palm and deepens the palmar hollow

Abductor digiti minimi

Pisiform

Ulnar base of the small finger proximal phalanx and dorsal expansion of the small finger extensor mechanism

Ulnar nerve

Flexion and abduction of the MP joint of the small finger Extension of the PIP and DIP joints of the small finger

Flexor digiti minimi

Hook of the hamate and Base of the small finger the flexor retinaculum proximal phalanx

Ulnar nerve

Flexion of the MP joint of the small finger

Opponens digiti minimi

Hook of the hamate and Ulnar border of the small flexor retinaculum finger metacarpal

Ulnar nerve

Opposition of the small finger metacarpal

Median nerve

Flexion of the MP joints Extension of the PIP and DIP joints

Accessory abductor digiti minimi: present in 24% of the population, can originate from the transverse carpal ligament, the palmaris longus, or the antebrachial fascia

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Central compartment Lumbricals 1

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Radial side of the flexor digitorum profundus index

Lateral slip of the finger extensor mechanism just distal to the MP joint index

The lumbrical muscles originate from the FDP tendons distal to the carpal tunnel; however, in up to 22% of individuals the origin of the lumbricals is in the carpal tunnel when the fingers are extended (Continued)

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Hand Table 18.1 Compartments and intrinsic muscles of the hand (Continued) Muscle

Origin

Insertion

Innervation

2

Radial side of the flexor digitorum profundus of the middle finger

Lateral slip of the finger extensor mechanism just distal to the MP joint of the middle finger

Median nerve

3

Bipennate, flexor digitorum profundus of the middle and ring fingers

Lateral slip of the finger extensor mechanism just distal to the MP joint of the ring finger

Ulnar nerve

4

Bipennate, flexor digitorum profundus of the ring and small fingers

Lateral slip of the finger extensor mechanism just distal to the MP joint of the small finger

Ulnar nerve

Action

Variations

Deep compartment Adductor pollicis Transverse head

Palmar surface of the middle finger metacarpal

Base of the proximal phalanx of the thumb

Ulnar nerve

Oblique head

Capitate, base of index and middle finger metacarpals

Base of the proximal phalanx of thumb

Ulnar nerve

1

Ulnar side of the index metacarpal

Dorsal expansion and base of the proximal phalanx of the extensor mechanism of the index finger

Ulnar nerve

2

Radial side of the ring metacarpal

Dorsal expansion and base of the proximal phalanx of the extensor mechanism of the ring finger

Ulnar nerve

3

Radial side of the small metacarpal

Dorsal expansion and base of the proximal phalanx of the extensor mechanism of the small finger

Ulnar nerve

1

Dorsal shaft of the thumb and index metacarpals

Dorsal expansion and radial side of the proximal phalanx of index

Ulnar nerve

2

Dorsal shaft of the index and middle metacarpals

Dorsal expansion and radial side of the proximal phalanx of middle

Ulnar nerve

3

Dorsal shaft of the middle and ring metacarpals

Dorsal expansion and ulnar side of the proximal phalanx of the middle finger

Ulnar nerve

4

Dorsal shaft of the ring and small metacarpals

Dorsal expansion and ulnar side of the proximal phalanx of the ring finger

Ulnar nerve

Adduction of the CMC joint of thumb Flexion of the MP joint of the thumb

Palmar interossei Flexion of the MP joints Extension of the PIP and DIP joints Adduction toward the middle finger

Dorsal interossei

18

Flexion of the MP joints Extension of the PIP and DIP joints Abduction away from the middle finger

Extensor digitorum brevis manus muscle is found in 1–3% of the population and is commonly mistaken for a dorsal ganglion cyst. It originates from the dorsal carpal ligament or the distal radius and inserts of the extensor hood of the middle or long finger

Abbreviations: CMC, carpometacarpal joint; DIP, distal interphalangeal; FDP, flexor digitorum profundus; MP, metacarpophalangeal; PIP, proximal interphalangeal.

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Hand Two recent cadaver dissection studies have shown that there is only thin areolar tissue overlying the thenar muscles, the hypothenar muscles, and the AdP (▶Fig. 18.35). These muscles lack a thick fibrous fascial covering unlike previously postulated. However, the skin and subcutaneous tissue are closely adherent to the compartments; thus, the release of the skin is critical in the setting of compartment syndrome.100,102

Anatomic Variation of the Intrinsic Muscles of the Hand Variation in the origin or insertion of muscles and presence of accessory intrinsic muscles of the hand are not uncommon (▶Fig. 18.43). Accessory ADM muscle: This muscle, which is present in 24% of the population, can originate from the transverse carpal ligament, the palmaris longus, or the antebrachial fascia. It rarely can cause compression of the median or ulnar nerves.103,104,105 Extensor digitorum brevis manus muscle: This dorsal muscle, present in 1 to 3% of the population, is commonly mistaken for a dorsal ganglion cyst. It originates from the dorsal carpal ligament

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or the distal radius and inserts onto the extensor hood of the middle (long) finger.104,106,107 Proximal origin of lumbrical muscles: The lumbrical muscles originate from the FDP tendons distal to the carpal tunnel; however, in up to 22% of individuals, the origin of the lumbricals is in the carpal tunnel when the fingers are extended (▶Fig. 18.44). This may lead to carpal tunnel syndrome due to the increased volume inside the confines of the carpal tunnel.104,108,109

◆◆ Arterial Supply

The radial and ulnar arteries supply blood to the hand. The two arteries connect via the superficial and deep palmar arches (▶Fig. 18.45, ▶Fig. 19.46, ▶Fig. 18.47, ▶Fig. 18.48). Historically, the ulnar artery has been considered the dominant blood supply to the hand. However, in a study of older patients, the thumb, index, and small fingers lost pulsatile blood flow in 20% of patients with radial artery compression, compared to only 5% with ulnar artery compression. This study hypothesized that the hand acts as a “single vascular bed” with many interconnections between the vessels, not just the deep and superficial palmar arches.110

Fig. 18.43 Accessory muscle: extensor digitorum brevis manus. (a) Coronal and (b) transverse T1 magnetic resonance (MR) images show accessory muscle belly (arrows) along the dorsum of the wrist. Normally, the muscle bellies of the extensor tendons do not extend distal to the radiocarpal joint. M, third metacarpal base; P, pisiform.

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Fig. 18.44 Proximal origin of the lumbricals. Axial (a) T1 and (b) proton density fat-saturated (PDFS) magnetic resonance (MR) images show ­lumbricals (arrows) arising from the flexor digitorum profundus (FDP) tendons more proximal than usual, at the level of the carpal tunnel. This may lead to carpal tunnel syndrome. H, hook of hamate.

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Fig. 18.45 Volume-rendered computed tomography (CT) images showing the branches of the radial and ulnar arteries, and superficial veins. In this case, the superficial palmar branch of the radial artery remains proximal and does not participate in formation of the superficial palmar arch. Instead, the superficial palmar arch is mainly supplied by the ulnar artery. The ulnar artery forms the one to three common digital arteries that divide into the proper digital arteries. The radial artery gives off the lateral palmar digital artery of the thumb and continues as the princeps pollicis, which gives off radial indicis and the medial palmar digital artery of the thumb. It also joins the superficial palmar arch and completes it. The deep palmar arch is mainly formed by the radial artery.

According to one study, complete superficial palmar arches were seen in 84% of patients, whereas all specimens had a complete deep palmar arch.111 The hand is very well vascularized, and there are many interconnections between all the arteries. There have been reports of a vascularized hand with laceration of both the radial and ulnar arteries.112

Radial Artery and Its Branches The radial artery enters the hand between the flexor carpi radialis and the brachioradialis. It gives off a superficial palmar

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branch that connects to the superficial palmar arch (▶Fig. 18.45, ▶Fig. 18.46, ▶Fig. 18.47, ▶Fig. 18.48). Then the artery dives dorsally, through the anatomic snuffbox between the first and third dorsal extensor compartments (▶Fig. 18.45). It gives off the dorsal branch to the radial side of the thumb and then continues on the surface of the first dorsal interosseous muscle where it gives off the first dorsal metacarpal artery. The artery also gives rise to the dorsal carpal arch. The first dorsal metacarpal artery divides into the dorsal digital arteries to the ulnar side of the thumb and the radial aspect of the index finger (▶Fig. 18.45, ▶Fig. 18.47). The dorsal carpal arch gives off the second, third, and fourth dorsal metacarpal arteries that branch into the dorsal digital arteries on

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Fig. 18.46 Volume-rendered computed tomography (CT) images showing the deep and superficial palmar arches. The superficial palmar branch of the radial artery is small and does not show a major participation in superficial palmar arch. The superficial palmar arch supplies one to three common digital arteries. The first palmar common digital artery connects to the radial artery in this case. Therefore, arterial supply to the thumb in this case is provided by both ulnar and radial arteries. In this case, the superficial palmar branch of the radial artery remains superficial and does not participate in the formation of the superficial palmar arch. In general, the radial artery gives off the lateral palmar digital artery of the thumb and continues as the princeps pollicis, which divides into the radial indicis and the medial palmar digital artery of the thumb. (These images are provided courtesy of Farhood Saremi, MD.)

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Fig. 18.47 Axial computed tomography (CT) angiography from the wrist to the middle phalange showing arteries and veins of the hand. (Continued)

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18 Fig. 18.47 (Continued) (Continued)

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Fig. 18.47 (Continued)

the radial and ulnar sides of each digit. These digital arteries end over the middle phalanx and the volar digital arteries supply the dorsum of the distal phalanx. The dorsal metacarpal arteries also anastomose with the deep palmar arch and the volar metacarpal arteries (▶Fig. 18.48). After the dorsal branches, the radial artery continues between the two heads of the first dorsal interosseous muscle, around the base of the thumb metacarpal, and in the deep palm gives off the branch to the deep palmar arch that connects with the same branch from the ulnar artery. Finally, it ends into the radialis indicis, which runs along the radial aspect of the index finger, and the princeps pollicis on the volar side of the thumb metacarpal, which divides into the lateral and medial proper digital arteries of the thumb (▶Fig. 18.45). Variation in the terminal branches is common and some may arise from the ulnar artery (▶Fig. 18.47). The princeps pollicis and radialis indicis arteries may arise from the first palmar metacarpal artery.

Ulnar Artery and Its Branches The ulnar artery enters the hand volarly through Guyon’s canal between the pisiform and the hook of the hamate. It becomes the superficial palmar arch in the hand (▶Fig. 18.45, ▶Fig. 18.46, ▶Fig. 18.47). The arch gives off branches to the hypothenar musculature, the proper digital artery to the ulnar side of the small finger, and the common digital arteries for the second, third, and fourth web spaces. The later arteries further branch into the proper digital arteries to the small, ring, middle, and ulnar sides of the index finger (▶Fig. 18.46). In most cases, the superficial

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arch then travels radially and connects to the superficial palmar branch of the radial artery. In other cases, it may connect to the princeps pollicis artery, the radialis indicis artery, or the median artery. The anterior and posterior interosseous arteries do not enter the hand; however, they contribute to the overall vascularity of the hand. The anterior interosseous artery gives branches to the dorsal and volar carpal networks. The posterior interosseous artery anastomoses with the anterior interosseous artery and joins the dorsal carpal network.

Anatomic Variation A persistent median artery has been found in studies in 2 to 28% of the time.113 The median artery is present in the embryo and usually disappears by 8 weeks of gestation. When present, it runs with the median nerve in the carpal tunnel (▶Fig. 18.49). It arises from the ulnar, common or anterior interosseous, radial, or brachial arteries.114 Before drawing blood or putting an arterial line in the radial artery, it is important to ascertain the patency of the radial and ulnar arteries and to check their connection. This is done with Allen’s test. First, both arteries are occluded by putting pressure on the radial and ulnar sides of the wrist. Next, the fist is pumped several times to empty the hand of blood; finally, one artery is released and the time to reperfusion is recorded. This is repeated for the second artery.115 The superficial palmar arch and digital arteries can also be checked with a handheld Doppler if there is any concern for vascular injury.

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Fig. 18.48 Intercommunication between the dorsal and palmar arteries of the hand. (This image is provided courtesy of Farhood Saremi, MD.)

Fig. 18.49 Persistent median artery. Axial (a) T1-weighted and (b) proton density fat-saturated images of the wrist, at the level of the pisiform show median nerve (long arrows) with adjacent persistent median artery (short arrows). An enlarged persistent median artery can cause mass effect on the median nerve leading to carpal tunnel syndrome.

◆◆ Venous Drainage

The venous drainage of the fingers consists of subcutaneous veins that are larger dorsally, small volarly, with many anastomotic connections116,117 (see Chapter 5 “Upper Extremity and Shoulder Venous System”). Similarly, the thumb also contains multiple veins in the subcutaneous tissue.118,119 In the fingers and the thumb, the veins often do not always run with the digital arteries116,117,118,119 (▶Fig. 18.45, ▶Fig. 18.47). At the level of the metacarpals, the formation of a deep and superficial drainage system becomes more apparent. The dorsal digital veins drain to three dorsal metacarpal veins. These form a subcutaneous venous network, along with veins from the thumb, radial

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aspect of the index finger, and ulnar aspect of the little finger, and drain into the cephalic and basilic veins. The volar veins drain into the superficial and deep venous palmar arches, which accompany the arterial arches. The deep and superficial palmar arch empty into the venae comitantes of the radial and ulnar arteries.120

◆◆ Lymphatic Drainage

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The lymph from the fingers and hand drains into superficial and deep lymphatic vessels. These drain mainly to the lymph nodes in the axillary region, with a few draining into the supratrochlear nodes and some into the deltopectoral lymph nodes (see Chapter 7 “Lymphatic System of the Upper Extremity”).

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◆◆ Innervation

Three nerves innervate the hand: the ulnar nerve, the median nerve, and the radial nerve (see Chapter 6 “Brachial Plexus and Its Branches”; ▶Fig. 18.35, ▶Fig. 18.50).

Ulnar Nerve The ulnar nerve has three main divisions in the hand: the dorsal cutaneous, the motor, and the sensory branches.121,122 The ulnar nerve innervates the majority of muscles in the hand. Of the four compartments of the hand, the ulnar nerve supplies the hypothenar compartment, the deep (adductor pollicis and interosseous muscles) compartment, and the ulnar two of the four lumbricals (▶Fig. 18.35, ▶Fig. 18.50). The nerve supplies sensation to the ulnar side of the hand.121,122 At approximately 8 cm proximal to the wrist crease, the nerve gives off a dorsal cutaneous branch. This branch runs along the dorsal side of the hand in the subcutaneous tissue and gives off sensory branches to the ulnar/dorsal hand skin, and to the dorsum of the small finger and the ulnar half of the ring finger.123,124 Immediately proximal to the wrist crease, the ulnar palmar cutaneous nerve leaves the epineurium of the ulnar nerve. However, the anatomy of this nerve is highly variable and can be

one or multiple branches. This nerve or nerves supply the palmar skin of the hypothenar eminence ulnar to the ring finger metacarpal axis.125 In Guyon’s canal, the main trunk of the ulnar nerve divides into the deep motor branch and the superficial sensory branch (see Chapter 17 “Wrist”). Guyon’s canal has for its floor the transverse carpal ligament, ulnarly the pisiform, radially the hook of the hamate, and the pisohamate ligament as its roof. Both branches of the nerve can be compressed in zone 1, proximal to the hook of the hamate. The deep branch can be compressed in zone 2, as the deep motor branch dives deep to the hypothenar musculature ulnar to the hook of the hamate. Finally, the superficial branch can be compressed in zone 3 as the superficial sensory branch passes over the hypothenar eminence.126,127 Injury to the ulnar nerve can cause denervation atrophy in the distribution of the nerve (▶Fig. 18.51). The motor branch supplies the muscles of the hypothenar eminence (the ADM, the FDM brevis, and the ODM), then it follows the deep palmar arterial arch and innervates half of the superficial palmar compartment (the third and fourth lumbricals) and innervates all the muscles in the deep compartment of the hand (the palmar and volar interossei and the AdP). It ends on the radial side of the hand innervating the deep head of the FPB128,129 (▶Fig. 18.52) Surprisingly, the PB muscle is innervated by the sensory branch of the ulnar nerve.130,131,132 The sensory branch further divides into

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Fig. 18.50 Palmar hand nerves. The median nerve divides into the common and then proper digital nerves for the sensation of the volar side of the thumb, index finger, middle finger, and radial side of the ring finger. The ulnar nerve divides into the palmar cutaneous branch, common digital nerve to the fourth web space, and digital nerve to the ulnar aspect of the small finger. (This image is provided courtesy of Damián Sánchez-Quintana, MD, PhD.)

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Hand the palmar cutaneous branch, common digital nerve to the fourth web space, and digital nerve to the ulnar aspect of the small finger. The common digital nerve splits into the radial digital nerve of the small finger and the ulnar digital nerve of the ring finger.121,122

Median Nerve Due to the prevalence of carpal tunnel syndrome, the median nerve is the most well known of the hand nerves. The median nerve enters the hand through the carpal tunnel133 (▶Fig. 18.35,

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▶Fig. 18.50). The floor of the carpal tunnel consists of the wrist bones. The ulnar wall is made up of the pisiform and the hook of the hamate. The scaphoid tubercle and the ridge of the trapezium form the radial wall. The roof consists of the transverse carpal ligament. Just as for the ulnar nerve, the median nerve also gives off a palmar cutaneous branch proximal to the wrist.134 The palmar cutaneous branch provides sensory innervation to the thenar eminence and palm radial to the ring finger metacarpal. The exact course of this nerve varies between individuals.134 Once the nerve enters the carpal tunnel, it gives off a recurrent motor branch that innervates the muscles of the thenar eminence133,135: the OP, abductor pollicis brevis, and superficial part of FPB. The median nerve innervates the first and second lumbricals. A mnemonic to remember the median nerve–innervated muscles in the hand is “LOAF” for Lumbricals 1 & 2, OP, AbPB, and FPB. The median nerve then divides into the common and then proper digital nerves for the sensation of the volar side of the thumb, index finger, middle finger, and the radial side of the ring finger (▶Fig. 18.50). The median nerve also supplies sensation to the distal part of the dorsal aspect of the thumb, index, middle, and radial half of the ring finger.133

Radial Nerve

Fig. 18.51 Denervation muscle atrophy in the distribution of the ulnar nerve. Atrophy of the muscles supplied by the ulnar nerve, which includes muscles of the hypothenar eminence, muscles of the deep compartment, and the third and fourth lumbricals.

The radial nerve is on the dorsal side of the hand. There are no muscles innervated by the radial nerve in the hand. The radial nerve gives off a superficial branch, which travels in the subcutaneous tissue over the dorsum of the radial side of the hand. The superficial branch of the radial nerve provides dorsal digital branches to the thumb, index, middle, and radial side of the ring finger.136,137

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Fig. 18.52 Neurovascular bundle (circle and arrows): at the level of the (a) metacarpals, (b) metacarpophalangeal (MCP) joint, (c) distal proximal phalanx, and (d) distal middle phalanx. The arrowheads indicate Cleland’s ligament.

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Cutaneous Innervation The cutaneous innervation of the hand and fingers are supplied by the median, ulnar and radial nerves (▶Fig. 18.53). The median and ulnar nerves supply the sensation to the volar surface of the hand and fingers. The median nerve gives off the median palmar cutaneous nerve, which innervates the radial side of the palm. The ulnar nerve gives off the ulnar palmar cutaneous nerve, which innervates the ulnar side of the palm. The proper digital nerves provide sensation to the volar sides of the digits. They also contribute to the innervation of the distal dorsal skin. The radial and ulnar nerves provide sensation to the dorsal hand. The median, ulnar, and radial nerves supply sensation to the dorsal fingers. The superficial branch of the radial nerve supplies the dorsal/radial side of the hand, the thumb, the index, and the middle finger, and radial side of the ring finger up to the IP and PIP joints. The ulnar dorsal cutaneous nerve supplies the dorsal/ulnar side of the hand and the entire small finger and the ulnar side of the ring finger. The median nerve supplies the dorsal thumb distal to the IP joint, the index and middle fingers distal to the PIP joint and the radial side of the ring finger distal to the PIP joint.

Nerve Anastomoses in the Hand Four common nerve anastomoses in the upper extremity are known to change the innervation of muscles in the hand. Martin– Gruber (MG) anastomosis is a connection between the median and ulnar nerves in the forearm. Reported prevalence is 6 to 47%. Median nerve innervation of the thenar, hypothenar, and dorsal interosseous muscles has been found with the MG anastomosis. In a meta-analysis of all previous studies, it was found to be 19.5%.138

Fig. 18.53 Cutaneous innervation. Territories of the medial (red), ulnar (green), and radial (blue) nerves. The palmar aspect of the hand from the lateral aspect of the fourth digit through the first digit sends sensory fibers to the median nerve in addition to the dorsal distal aspects of the first, second, third, and lateral portion of the fourth digit. The ulnar nerve is also responsible for sensation of the medial hand including the fifth digit and the medial half of the fourth digit. The radial nerve and its branches receive sensation from the dorsal forearm and hand from the proximal two-thirds of the first through the lateral half of the fourth digit.

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The Marinacci anastomosis is from the ulnar to the median nerve in the forearm resulting in ulnar innervation of the thenar musculature. Prevalence is reported at 1 to 17%.138 In a meta-analysis, it was found to be 0.7%. The Riche-Cannieu anastomosis is from the deep motor branch of the ulnar nerve to the recurrent motor branch of the median nerve. This causes all the muscles of the thenar eminence to be innervated by the ulnar nerve. Prevalence has been reported at 1.4139 to 77%.138 The pooled prevalence was 55.5%.138 The Berrettini anastomosis is a sensory nerve connection between the ulnar and median nerve in the third web space. In a patient with this anastomosis, there is an overlap of innervation for the radial side of the ring finger and the ulnar aspect of the middle finger. Many studies report an incidence greater than 80%. Thus, this is considered normal anatomy rather than a variant by many researchers. The pooled prevalence was 60.9%.138 Branches of the dorsal ulnar sensory nerve and the superficial branch of radial nerve anastomose over the dorsum of the hand.

◆◆ Surface Anatomy

The surface anatomy of the hand is used for describing the pathology and for planning surgical incisions.

Volar Surface Anatomy On the volar surface of the hand, there are two longitudinal creases. The thenar crease defines the ulnar border of the thenar eminence and the ulnar palmar crease defines the hypothenar eminence. The thenar and hypothenar eminences correspond to the muscle bulk of the thenar and hypothenar muscles (▶Fig. 18.54). The transverse creases in the palm include the proximal and distal palmar creases. Kaplan’s cardinal line140 is an often-quoted landmark when performing open carpal tunnel release (▶Fig. 18.54). There are many descriptions of this line and also many underlying structures associated with this line. The original description from Dr. Kaplan was a line “drawn from the apex of the interdigital fold between the thumb and index finger toward the ulnar side of the hand, parallel with the middle crease of the hand.”141 In a cadaver study, the location of the superficial palmar arch was on average 10.4 to 11.8 mm distal to Kaplan’s line from the radial to the ulnar border of the ring finger ray140 (▶Fig. 18.54). Laceration of the superficial palmar arch is one risk of carpal tunnel release; thus, this is an important landmark. Alternately, Hoppenfeld and DeBoer described Kaplan’s cardinal line as directly overlying the deep palmar arch.142 The fingers contain the palmar–digital crease, the PIP crease, and the DIP crease. The thumb contains the palmar–digital crease and the IP crease. Trigger finger release depends on the accurate location of the A1 pulley. The distance from the palmar–digital crease to the PIP crease is measured. The same distance is measured proximally from the palmar–digital crease and this location marks the proximal end of the A1 pulley.143 Flexor tendon repair depends on the zone of injury. Surface landmarks can be used to accurately determine the locations of underlying flexor pulleys and flexor tendon anatomy. Gordon et al defined four zones of the finger: A—proximal extent of the A1 pulley to the palmar–digital crease; B—palmar–digital crease to PIP crease; C—PIP crease to

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Fig. 18.54 Palmar creases. (a) Hand schematic shows the major palmar creases. Blue star indicated the thenar eminence and the red star indicates the hypothenar eminence. The location of the superficial palmar arch (SPA) is 10 mm distal to Kaplan’s line. Laceration of the superficial palmar arch is one risk of carpal tunnel release. IP, interphalangeal. (b) Schematic showing the relationship of the creases to the underlying bones and joints.

the DIP crease; and D—DIP crease to the tip.61 The decussation of the FDS begins in the distal part of zone A, the A2 pulley is on the border between zones A and B, Camper’s chiasm lies in the distal part of zone B, the A4 pulley is in the middle of zone C, and the FDS insertion is also in the middle of zone C. On the dorsum of the hand, the bony prominences of the MP, PIP, and DIP (and IP for the thumb) joints are plainly visible. The extensor tendons are also visible crossing longitudinally from the wrist to the MP joints. The first dorsal interossei muscle can be palpated on the dorsum of the hand between the metacarpals of the thumb and index fingers.120 Wasting of the first dorsal interossei muscle can be found in ulnar nerve injury144 (▶Fig. 18.51). The anatomic snuffbox is the area between the APL/EPB (first dorsal compartment) and the EPL (third dorsal compartment).120 Tenderness in this area can be indicative of a scaphoid fracture145 and the dorsal branch of the radial artery can also be palpated in this area.120

◆◆ Structure and Biomechanics

Axis of the Hand

The functional axis of the hand is regarded to extend through the middle finger, and adduction and abduction of the other fingers are in relation to this axis.

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Arches The hand has a longitudinal arch, a transverse arch, and an oblique arch146,147,148,149,150 (▶Fig. 18.5). The hand is able to perform an extraordinary variety of movement from grasping a single human hair to a large heavy hammer. The shape of the arches of the hand changes during grasp to accommodate the object being grasped.151

Movements Motion of the Fingers Movements of the fingers are defined relative to the central axis of the hand (▶Fig. 18.55). Adduction is toward the middle finger, whereas abduction is away from the middle finger. Adduction and abduction are performed at the MP joint.120,152 Flexion is toward the palm. Extension is away from the palm. Flexion and extension are performed at the IP joints and the MP joint.120,152 In general, the motion in each finger is controlled by three extrinsic (two flexors and one extensor) and three intrinsic (two interossei and one lumbrical) muscles, with one additional extensor in the index and little fingers. Additional muscles result in additional motions of the little finger.

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Fig. 18.55 Central axis and motion. The central axis of the hand extends through the middle finger. Abduction (blue arrow) and adduction (brown arrow) are moving of the fingers away and toward this axis in the plane of the palm, respectively, whereas flexion and extension are movements of the finger toward the volar side (red arrow) and toward the dorsal side (green arrow) perpendicular to the plane of the palm, respectively.

Motion of the Thumb The thumb is able to flex and extend at the IP and MP joints. The thumb is also able to abduct and adduct at the MP joint.120,152 In addition to these movements, the thumb has significant mobility at the CMC joint. The thumb is able to abduct and adduct in the plane of the hand and perpendicular to the plane of the hand. Terminology for the movements of the thumb is not always consistent between different sources. For example, the thumb can perform radial abduction and palmar abduction.120,152 Opposition results from palmar abduction combined with pronation.152 Retropulsion is the movement of bringing the thumb up and back when the hand is placed flat on a table.120,153 Circumduction is the total circular motion of the thumb based at the CMC joint.120,152,153

◆◆ Radiological Evaluation

Knowing the patient’s history, clinical findings, and the expected pathology and structure of concern helps direct the imaging approach. Imaging of the hand usually starts with plain radiographs, and is usually the only imaging study required in cases of trauma. The standard radiographic series of the hand includes posteroanterior (PA), oblique, and lateral views (▶Fig. 18.4). On the lateral view, the fingers should be spread to prevent overlap. Often dedicated radiographs of the finger or thumb may be needed for better visualization. Several different x-ray views, such as Brewerton, Robert, Bett/ Gedda, and Eaton stress views, are used to evaluate the specific pathology. For imaging of rheumatoid arthritis, a “ball catcher’s” (Nørgaard projection) view is obtained, which provides better visualization of the MP joints for assessment of erosions.

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For evaluation of soft tissues, MRI and high-resolution ultrasound (US) are used. MRI with gadolinium contrast is used to evaluate soft-tissue masses of the hand. Rarely, it is used to evaluate ligament injuries, as these can usually be adequately evaluated on physical examination. Due to the superficial nature of the structures in the hand, radiologists and hand surgeons are increasingly utilizing ultrasound. It is used to evaluate soft-tissue masses, flexor tendon or pulley ruptures, and can easily demonstrate synovitis and erosions in rheumatoid arthritis. US evaluation has the advantage of dynamic imaging and comparison to the opposite side. Computed tomography (CT) scan with 3D reconstruction can be helpful for preoperative planning of complex fractures of the hand, especially those at the base of the metacarpals. Nuclear scintigraphy has largely been replaced by other modalities. Intervention in the hand is often for aspiration of the joint, tendon sheath, cysts, and abscess; biopsies of masses and synovium; and injection of steroids. The hand is a common site of injury, resulting from direct penetrating or nonpenetrating trauma commonly seen with falls, sports, and occupation-related activities. Foreign bodies are frequently seen in the hand. Although some are radiopaque, others may require ultrasound or MRI for localization. Common fractures include crush fractures of the terminal tuft of the distal phalanx, boxer’s fractures of the metacarpal, avulsion fractures of the volar middle phalanx and dorsal distal phalanx (mallet finger), and Bennett’s fracture of the base of the first metacarpal (▶Fig. 18.56). Bennett’s fracture is an intra-articular fracture of the base of the first metacarpal, usually associated with dorsal radial displacement of the distal fragment. Rolando’s fracture is the comminuted counterpart of Bennett’s fracture. Boxer’s fracture involves the neck of the metacarpal with volar angulation of the distal fragment.

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Fig. 18.56 Examples of fracture and dislocation/subluxation in the hand (arrows). (a) Game keeper’s fracture. (b) Boxer’s fracture. (c) Mallet finger. (d) Dorsal distal interphalangeal (DIP) dislocation. (e) Dorsal proximal interphalangeal (PIP) dislocation. (f) Volar plate avulsion fracture.

Dislocations of the PIP and DIP joints are common with sports activity. Although the PIP joint dislocations, which are the most common dislocations, may be volar or dorsal, the DIP joint commonly dislocates dorsally. Soft-tissue injuries are also common (▶Fig. 18.32). In general, the tendons and ligaments have low signal intensity on all MR pulse sequences. Ligament injury may manifest as thickening, discontinuity, or avulsion with or without associated bone fragment. Volar plate injuries are seen as an isolated injury or with other injuries. It usually avulses off from its distal attachment to the base of the middle phalanx, with or without a bony fragment. Injuries of the ulnar collateral (skier’s thumb or gamekeeper’s thumb) and radial collateral ligaments of the thumb are more common than other collateral ligaments (▶Fig. 18.22). When the ulnar collateral ligaments avulse from its distal attachment and retract proximally, the adductor aponeurosis may interpose between the retracted end and the bone, resulting in Stener’s lesion (▶Fig. 18.22). Soft-tissue injuries of the MP joint include injuries of the sagittal band (boxer’s knuckle), often resulting in instability of the extensor tendon. It commonly affects the third ray, and associated injuries of joint capsule and cartilage can occur. Tendon injuries can result from penetrating injuries or avulsion, the latter with or without bone fragment. Although they are diagnosed clinically, US and MRI are used in evaluation of tendon ruptures to determine the exact site of tear, degree of retraction, and position of tendon ends. Avulsion of the FDP from the volar base of the distal phalanx with or without bone fragment results in a jersey finger (▶Fig. 18.57). The pulleys that keep the flexor tendons in position may also be injured, leading

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Fig. 18.57 Flexor digitorum profundus tendon tear. (a) Ultrasound image shows normal attachment of the flexor digitorum profundus tendon of the left little finger. Normal tendon on ultrasound is echogenic with fibrillar appearance (yellow arrowheads). Compare this to the appearance of (b) the right little finger where the tendon is not seen and the sheath is filled with heterogeneously echogenic hemorrhage (yellow arrow). White arrowheads indicate echogenic surface of the bone and white arrow indicates the distal interphalangeal joint.

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Fig. 18.58 Osteomyelitis of the distal phalanx. (a, b) Coronal T1 and short tau inversion recovery (STIR) magnetic resonance (MR) images of the finger show decreased signal on T1 and increased signal on STIR images (arrows) in the distal phalanx, consistent with osteomyelitis.

to bowstringing of the tendons. It is common in rock climbers, and the A2 pulley is more commonly affected than the A4 pulley. Muscle injuries range from low-grade strain to lacerations. Acute and chronic infections of the hand are initially evaluated with radiographs to look for foreign bodies and gas in the soft tissues. Deeper infections may be evaluated with ultrasound or MRI. Infections may affect the soft tissue (e.g., cellulitis, felon, paronychia, superficial and deep abscess, infective tenosynovitis, and pyomyositis), bone (osteomyelitis), or joint (septic arthritis). The infection can spread far due to the extent of the many synovial

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spaces, and their inter communications (▶Fig. 18.58). Organisms range from the common Staphylococcus and Streptococcus to uncommon organisms such as atypical Mycobacterium and fungus. The hand is commonly affected by arthritis, with the imaging features (joint space narrowing, bone changes [erosions/osteophytes], soft-tissue swelling, and joint alignment) and distribution of involved joints useful for diagnosis and monitoring. Rheumatoid arthritis commonly involves the MP joints, whereas osteoarthritis favors the DIP joints (▶Fig. 18.59). The DIP joints are also affected by seronegative arthritis (such as psoriasis and

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Fig. 18.59 Example of arthritis. (a, b) Rheumatoid arthritis. The posteroanterior (PA) views of an elderly woman with a long-standing disease show features of rheumatoid arthritis including erosions and narrowing of the joint spaces predominantly affecting the wrist and the metacarpophalangeal joints. (c, d) Osteoarthritis. Plain radiographs and 3D shaded surface volumetric reconstruction images show osteoarthritis in this individual predominantly affecting the distal interphalangeal joints (arrows: osteophytes).

reactive arthritis) and erosive osteoarthritis, whereas the MP joints are also target joints for calcium pyrophosphate dihydrate deposition disease and hemochromatosis. Other crystal-induced arthritis such as gout and hydroxyapatite deposition also affects the hand. Connective tissue diseases also manifest in the hands with findings ranging from soft-tissue calcification and alignment changes to bone resorption. Certain bone and soft-tissue tumors are more common in the hand and wrist. The ganglion/synovial cysts and giant cell tumor of tendon sheath are the most common benign cystic and

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solid soft-tissue tumors, respectively. Malignant tumors include synovial sarcoma, liposarcomas, rhabdomyosarcoma, and malignant vascular and malignant peripheral nerve sheath tumors (▶Fig. 18.60). A vast variety of other benign tumors and tumorlike conditions specific to the hand include epidermoid, glomus tumor, vascular and nerve sheath tumors, synovial chondromatosis, fibromas, palmar fibromatosis, and foreign body granulomas. Benign bone tumors in the hand are not unusual, and mainly include the enchondromas, whereas malignant bone tumors are rare (▶Fig. 18.60).

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Hand

Fig. 18.60 (a) Ganglion cyst. An ultrasound (US) image of the hand for evaluation of palpable mass shows an anechoic lobulated lesion (arrowheads) with posterior acoustic enhancement. No internal flow was seen on color doppler study (not shown). (b) transverse short tau inversion recovery (STIR) and (c) sagittal T1 images of the thumb show a fairly well-demarcated mass (arrows) related to the flexor pollicis longus tendon, pathologically proven to be a giant cell tumor of tendon sheath (arrowhead, magnetic resonance imaging [MRI] compatible marker indicating the site of clinically palpable mass). (d) Enchondroma. The posteroanterior (PA) view of the hand shows an expansile lesion of the fourth proximal phalanx with internal mineralization compatible with an enchondroma (arrow).

◆◆ Conclusion

The complex anatomy of the hand allows for precise as well as powerful movements, which allow us to accomplish a variety of daily activities. Injury of any small component can result in a major loss of function. Knowledge of the anatomy of this complex body part is essential to understand the biomechanics, function, pathophysiology, and to evolve strategies to treat this spectrum of disease.

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19  Sacrum, Coccyx, and Sacroiliac Joints Dakshesh B. Patel and Farhood Saremi

◆◆ Introduction

The sacrum is a large wedge-shaped bone between the lumbar vertebra, the pelvic bones, and the coccyx, and is formed by fusion of five sacral vertebrae and their intervening intervertebral disks. The coccyx forms the terminal portion of the distal vertebral column (▶Fig. 19.1, ▶Fig. 19.2). Sacroiliac joints are strong weight-bearing joints consisting of an anterior synovial portion and a posterior syndesmosis supported by its associated ligaments and muscles. They are important in absorbing, dissipating, and transmitting the axial load of the upper body through the vertebral column to the legs. The weight of the upper body during standing is transferred through the sacroiliac joints to the lower extremities via the hips; and while sitting, the forces are transferred through the sacroiliac joints to the ischial tuberosities. Conversely, as in walking and jumping, the forces from the lower extremity are absorbed and transmitted through the sacroiliac joint to the vertebral column. The sacroiliac joints are important and under-recognized source of pain.1 In the early last century, the sacroiliac joint was considered to be an important source of pain. However, with the demonstration of disk herniation as important source of low back pain, attention was diverted away from the sacroiliac joints. More recently, the focus has been redirected to the sacroiliac joints and they are now considered to be an important source of low back pain, seen in 10 to 30% of patients.1,2,3,4,5 A number of conditions including congenital abnormalities, tumors, infection, osteoarthritis, and seronegative spondyloarthropathies affect the sacrum and sacroiliac joints.

◆◆ Sacrum

Five sacral vertebral segments (S1–S5) fuse to form the sacrum along the dorsal portion of the pelvic ring. The S1 vertebra is the largest with progressive decrease in size of the vertebral bodies caudally (▶Fig. 19.2). There has been considerable interest in the anatomy of the first and second sacral segments due to their importance in lumbosacral fixation, and fixation of sacral fractures.6,7,8,9,10 The S1 pedicles are the largest pedicles in the spine extending lateral to the sacral canal.8,11 The height of the S1 pedicle is approximately 26 to 30 mm and the anteroposterior dimension is approximately 28 mm.9,10

Shape, Size, and Orientation The sacrum appears like an inverted triangle, with a base superiorly and a flat apex inferiorly. The ventral (anterior) surface of the sacrum is concave, and the dorsal (posterior) surface appears convex. The sacrum is tilted horizontally in the sagittal plane such that the base of the sacrum is directed anteriorly. This forms an angle of about 130 to 160 degrees, open posteriorly, between the lower lumbar spine and the sacrum (▶Fig. 19.3). Hence, the ventral surface faces anteriorly and inferiorly, while the dorsal surface faces posteriorly and superiorly. The sacrum is relatively flat during the early years of life with the curvature developing with age in response to the upright posture.12 It measures about 10.4 to 11.4 cm in height and approximately 10.2 to 11.6 cm in width.13

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Fig. 19.1 (a) Anteroposterior and (b) lateral radiographs of the pelvis. SC, sacrococcygeal.

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Sacrum, Coccyx, and Sacroiliac Joints The superior surface of the S1 vertebral body slopes anteroinferiorly. The superior surface of the ala also slopes anteroinferiorly, more so than the vertebral body, with the two forming an angle of about 37 degrees.14 The sacral angle (sacral slope), an indicator of tilt of the base of the sacrum, is measured between the line drawn through the upper border of S1 and the true horizontal and measures about 43 degrees, without significant difference between the sexes15 (▶Fig. 19.3).

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Fig. 19.2 Anatomy of the sacrum. SC, sacrococcygeal.

Surfaces The upper sacrum is wide with the upper surface of the first sacral vertebra forming the base of the sacrum. The superior surface of the body of the first sacral vertebra articulates with the last lumbar vertebra, with an intervening disk. The prominent anterior margin of the first sacral body is called the promontory (▶Fig. 19.2). Posteriorly, the triangular opening of the sacral canal is seen

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Fig. 19.3 Angles. Sacral angle (black), sacrococcygeal angle (blue), sacrococcygeal joint angle (red), and intercoccygeal angle (yellow). Note the surface of the sacral ala (arrowheads). Sacrococcygeal angle is formed by the intersection of a line between the mid-point of the upper borders of the S1 and C1 and a line between the latter and the tip of the coccyx (males 107 ± 12 degrees and females 106 ± 14 degrees). Sacrococcygeal joint angle is between lines intersecting the middle of S5 and C1 (males 168 degrees and females 164 degrees).

bounded laterally by short thick pedicles. The superior articular facets usually face medially and posteriorly; but the plane of these processes varies considerably, and the two sides can be asymmetric in orientation (tropism) (▶Fig. 19.4). The processes can be rudimentary or hypoplastic.16,17 These superior articular processes form zygapophyseal joints with the inferior articular processes of the last lumbar vertebra. Bilaterally, the lateral aspect of the sacral base is expanded like a wing, with the pedicles lateral to the body and the most lateral aspect referred to as sacral ala (▶Fig. 19.2). The surface of the ala is sloped from posterosuperior to anteroinferior and sometimes the sacral ala may be recessed.14,18 Most of the sacral ala is related to the psoas major muscle but laterally some fibers of the iliacus muscle originate on the ala (▶Fig. 19.5). Caudally is the narrow apex that bears a facet through which the sacrum articulates with the coccyx with an intervening disk (▶Fig. 19.2). The ventral or pelvic surface is concave craniocaudally and from side to side, and has four transverse ridges (linea transversaria) representing the location of deeper rudimentary intervertebral disks at the site of fusion of the sacral vertebral bodies (▶Fig. 19.2). Along the lateral aspect of the ridges, between the adjacent vertebrae, are four sets of anterior sacral foramina that divide the sacrum into the central portion formed by the fusion of the vertebral bodies, and the lateral portion called the lateral masses representing the costal elements and transverse processes (▶Fig. 19.2). The foramina communicate through the intervertebral foramina with the sacral canal. The anterior surface of the lateral masses is undulating with the piriformis muscle originating from the elevations on this surface between S2 and S4, and the anterior divisions of the sacral nerves coursing in the grooves (▶Fig. 19.5, ▶Fig. 19.6). The roofs of the upper three (or sometimes two) anterior sacral foramina and the neural grooves project as arcs on anteroposterior radiograph of the pelvis and form radiographic sacral arcuate lines19 (▶Fig. 19.1).

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The convex dorsal surface is irregular and has five longitudinally oriented elevations called the crests on which tubercles are interspersed. In the midline is the median sacral crest formed by the fusion of the laminae. Three or four tubercles representing the spinous processes are seen on the crest, with the S1 tubercle usually being the most prominent (▶Fig. 19.2). The laminae of the fifth, usually the fourth, and occasionally also the third sacral vertebra do not fuse in midline resulting in an opening, the sacral hiatus, leading to the sacral canal.20,21 The size of sacral hiatus varies in different individuals (▶Fig. 19.7). Medial to the posterior sacral foramina and lateral to the midline crest are the intermediate sacral crests formed by the fusion of the articular processes from S2 to S5. Indistinct articular tubercles are present on these crests. The tubercles of the fifth sacral vertebra protrude inferiorly as rounded or horn-shaped processes called the sacral cornua, which articulate with the cornua of the coccyx (▶Fig. 19.2). The most lateral are the lateral sacral crests formed by fusion of the transverse processes on which a series of five transverse tubercles are visible.22 The region intervening between the median and the intermediate sacral crest are the shallow sacral grooves, formed by the fused laminae. Between the intermediate and lateral crests are the four sets of dorsal sacral foramina which communicate with the sacral canal through the intervertebral foramina, and through which the dorsal divisions of the sacral nerves emerge (▶Fig. 19.4). The dorsal surface along with the ligaments attached to this surface, and the posterior superior iliac spine (PSIS) and the posterior iliac crest are the site of attachment of many muscles. The multifidus (part of transversospinalis) attaches on the posterior aspect of the sacrum mainly to the sacral groove up to the fourth sacral segment (▶Fig. 19.4, ▶Fig. 19.5). Its attachment extends to the lateral part of the median sacral crest, the intermediate crest

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Fig. 19.4 Superior articular facets and L5–S1 facet joints (arrows). The superior articular facets of the sacrum usually face medially and posteriorly; but the plane of these processes varies considerably, and the two sides can be asymmetric in orientation.

Fig. 19.5 Muscle attachments to the sacrum and coccyx. (a) Posterior attachments. The gluteus maximus attaches along the inferolateral aspect, just below the sacroiliac joints. The multifidus attaches on the posterior aspect of the sacrum, medially to the sacral groove up to the fourth sacral segment, and laterally to the posterior sacroiliac ligament and the posteromedial aspect of the posterior superior iliac spine (PSIS). The thick and broad erector spinae aponeurosis attaches in a “U”-shaped fashion (yellow line) to the median sacral crest, turns laterally across the dorsal surface of the sacrum, and then extends superiorly along the dorsal aspect of the iliac crest. At or slightly above the level of the PSIS, the erector spinae aponeurosis fuses to the posterior layer of the thoracolumbar fascia to form the thoracolumbar composite. (b) Anterior attachments. The piriformis muscle originates from the anterior surface between S2 and S4. The coccygeus extends between the lower lateral sacrum and coccyx, and the ischial spine. SS, sacrospinous.

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Sacrum, Coccyx, and Sacroiliac Joints and its sacral tubercles from S1 to S3, the medial aspect of posterior sacroiliac ligament, and the posteromedial aspect of the PSIS. The attachment of the fascicle from L5 is the most medial and that from L1 is the most lateral. The insertion on the intermediate sacral crest and tubercles is through a fascia rather than direct muscle insertion creating a potential space where the medial branches of the sacral dorsal rami can entrap.23,24,25 The erector spinae (extensor spinae, sacrospinalis) has multiple components including the longissimus thoracic and the iliocostalis lumborum, each with a thoracic portion and lumbar portion. The longissimus is medial, while the iliocostalis is lateral (see Chapter 1 “Abdominopelvic Wall” of Volume 2 and Chapter 24 “Spine” of Volume 4). The thoracic fibers of the longissimus and iliocostalis form the thick and broad erector spinae aponeurosis and attach in a “U”-shaped fashion to the lumbar spinous processes, median sacral crest, and S1 to S3 spinous processes, turns laterally across

the dorsal surface of the sacrum, and then extends superiorly along the interspinous region of the posterior ileum, PSIS, and the dorsal aspect of the iliac crest. The junction of the pars thoracic of the longissimus thoracic and pars thoracic of the iliocostalis lumborum occurs at the caudal aspect of the PSIS. The aponeurosis partially surrounds the multifidus, with some fibers of the multifidus inserting onto the aponeurosis.23 The deep lamina of the posterior layer of the thoracolumbar (lumbodorsal) fascia attaches to the spinous processes of the sacrum26 (▶Fig. 19.8). Additional attachment of this fascia includes the sacrotuberous ligament, PSIS, iliac crest, and the long dorsal ligament.26 The horizontal part of the posterior sacroiliac ligament attaches to the transverse tubercles of the S1 and S2 sacral vertebrae; the oblique fascicle of the posterior sacroiliac ligament attaches to the S3 and S4 tubercles, and the sacrotuberous ligament attaches to the S4 and S5 tubercles. The gluteus maximus is attached along

Fig. 19.6 Axial cadaveric cuts from superior to inferior showing muscles and ligaments around the sacroiliac (SI) joints. The pudendal nerve (S2–S4) and artery pass anterior to the piriformis muscle, then enter the lesser sciatic foramen between the sacrospinous (SS) ligament/ ischio-coccygeus (IC) muscle and sacrotuberous ligament (red arrows) before entering the pudendal (Alcock) canal. The sacrospinous ligament passes behind the ischio-coccygeus (IC) muscle and may not be distinguishable from the muscle on the axial cuts. The sciatic nerve is seen anterior to the piriformis muscle formed by the L4, L5, and S1 nerves. The posterior layer of the thoracolumbar fascia covers the posterior surface of the erector spinae. Medially positioned multifidus lumborum (ML) and laterally located iliocostalis lumborum (ICL) are seen. The iliac tuberosity articulates with the middle of the sacral concavity with strong interosseous ligament extending between the two bones in this region. Associated SI joint ligaments including the anterior and posterior SI ligaments are seen. The posterior interosseous SI ligaments are shown which consist of many short ligamentous bands in a fatty background. The iliolumbar ligament (IL) bands are also seen extending from the tip of the L5 transverse process to the cranial part of the iliac tuberosity. Left iliac bone is colored in green for clarity. Gmax, gluteus maximus; OI, obturator internus; PSIS, posterior superior iliac spine.

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(Continued)

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Fig. 19.6 (Continued)

Fig. 19.7 Sacral hiatus and greater sciatic notch. The laminae of the fifth, usually the fourth, and occasionally also the third sacral vertebra do not fuse in midline resulting in a sacral hiatus of variable length. The greater sciatic notch is shown by green curved line. PIIS forms the dorsal aspect of the greater sciatic notch. PIIS, posterior inferior iliac spine; PSIS, posterior superior iliac spine; SS, sacrospinous; ST, sacrotuberous.

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Fig. 19.8 (a) Axial computed tomography (CT) (upper row) and magnetic resonance imaging (MRI) (lower row) images showing the aponeurosis of the erector spinae (ES) muscles separate from the posterior layer of the thoracolumbar fascia at the lumbar level. (b) These two fasciae fuse at or slightly above the level of the posterior superior iliac spine (PSIS) to form the thoracolumbar composite.

the inferolateral aspect of sacrum, just below the sacroiliac joints (▶Fig. 19.4, ▶Fig. 19.5). The lateral surface is formed by the auricular surface. It is triangular with the base above, and the upper three sacral vertebrae bear the auricular facet through which the sacrum articulates with the ileum (▶Fig. 19.2). Dorsal to the auricular surface is the rough surface of the sacral concavity where the interosseous sacroiliac ligaments attach. Inferiorly, distal to the auricular surface, the lateral surface narrows into a border which suddenly curves medially resulting in a protrusion, the inferior lateral angle (▶Fig. 19.2). The concave surface medial to this angle is often converted into a foramen by the upward projection of the transverse process of the first coccygeal segment and the lateral sacrococcygeal ligament, and through which the anterior division of the fifth sacral nerve passes (▶Fig. 19.2, ▶Fig. 19.7). The inferior lateral surface forms the medial margin of the greater sciatic notch. The sacrotuberous and the sacrospinous ligaments attach to the inferior half of the lateral surface (▶Fig. 19.6). Some of the fibers of the gluteus maximus (posterior) and the coccygeus (anterior) also originate in this region (▶Fig. 19.5).

Sacral Canal and Vertebral Foramina The sacral canal contains the nerve roots from the cauda equina, the meninges, the filum terminale, and fibrofatty tissue (see Chapter 25 “Spine” of Volume 4). It is triangular at S1 level becoming more elliptical distally27 (▶Fig. 19.4, ▶Fig. 19.6). The anterior nerves pass through the intervertebral foramina which are directed anteriorly, laterally, and distally, and emerge through

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the anterior sacral foramina (▶Fig. 19.9). Branches of the anastomosing median and lateral sacral vessels also travel through these foramina.27,28,29 The posterior nerves exit the sacrum through the posterior sacral foramina. These foramina are smaller than the anterior foramina and are more medial in relation to the anterior foramina.27,30

Structure The sacrum is primarily made of cancellous bone that is surrounded by a thin shell of compact bone31 (▶Fig. 19.10). The cancellous bone is denser ventrally. In the upper sacrum the main trabecular orientation is from the body laterally toward the anterior ala, and from the pedicles to the sacroiliac joint articular surface. These trabeculae intersect in the anterior portion of the sacrum in area that corresponds to the zone II described for the classification of sacral fracture. The mineral density in the body is highest near the superior end plate followed by the region near the inferior end plate with the lowest density in the mid body. In comparison, the mineral density in the ala decreases progressively from its superior aspect inferiorly, except in the most lateral portion where the density progressively increases. Transversely, the mineral density in the body is higher in the lateral portion corresponding to the region of trabecular intersection. Lateral to the body, the bone marrow density is higher in the region of the pedicle compared to the more lateral portion.32 Due to osteoporosis, there is generalized decrease in bone mineral density with the greatest decrease in the area lateral to the neural foramina, which has been described

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Fig. 19.9 Sacral foramina. (a) Transverse magnetic resonance imaging (MRI) image at level of first sacral segment showing anterior (arrowhead) and posterior (arrow) sacral foramina. Note S1 nerve root (N) traversing in the intervertebral foramen. (b, c) Axial contrast-enhanced computed tomography (CT) showing neural foramina and superior lateral sacral arteries (red arrows). The superior lateral sacral artery arises from the internal iliac artery and extends medially and inferiorly to enter the anterior sacral foramen and exit the posterior foramen. (d, e) Coronal CT views showing the cauda equina and sacral nerve roots (arrows). T, thecal sac in the sacral canal.

Fig. 19.10 Structure. (a) Transverse computed tomography (CT) of the upper S1 segment in a young adult shows structure of the sacrum. Arrows indicate anterior cortex formed by compact bone. Ca, cancellous bone in the sacral ala; note trabecular pattern. (b) With osteoporosis, there is generalized decrease in bone mineral density, with the greatest decrease in the sacral area also described as the area of “alar void.”

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Sacrum, Coccyx, and Sacroiliac Joints as the area of “alar void”33 (▶Fig. 19.10). The sacrum is not an uncommon site of insufficiency fracture in the elderly. The anterior sacral cortical thickness is greater than that of the posterior cortex. There is generalized decrease in cortical thickness in osteoporosis.33,34,35,36 Overall, the bone density in the sacrum is higher in the region above the S1 and S2 foramina, and the cortical thickness is highest in the region of the mid anterior cortex. Thus, placement of a screw such that it extends through the region above the foramina with its tip penetrating the mid anterior cortex is ideal.8,36,37,38 The orientation of the subchondral trabeculae on the sacral side is perpendicular to the articular surface indicating compressive forces, while that on the iliac side is obliquely oriented suggesting shearing forces.39

Development and Ossification The sacrum starts to develop in the first trimester but the development continues into the third decade. Early in the life the five sacral vertebral bodies are separated by rudimentary fibrocartilaginous intervertebral disks but later in life the disks are surrounded circumferentially by bone and are not usually visible from the surface. Overall, the sacrum is formed by enchondral ossification from more than 50 ossification centers. There are five primary ossification centers—one central, two neural arches, and two costal ossification centers22 (▶Fig. 19.11, ▶Fig. 19.12). The central ossification center forms the center of the vertebral body, the two neural ossification centers form the neural arches, while the two costal ossification centers form the lateral masses. Ossification of the centrum and the neural arches progresses in caudal direction. Additional small apophyses (secondary ossification centers) ossify later in life and fuse to various portions of the primary ossification centers (▶Fig. 19.13). Fusion of the ossification centers of the sacral vertebrae begins in the second decade beginning with fusion of the costal elements. Intervertebral fusion progresses from caudal to cranial and is

complete by about 18 to 25 years, occurring earlier in females compared to males.40 The lateral elements fuse by the age of 18 years.22

Sex Difference and Variation The female sacrum is shorter, wider, less curved, and more horizontally oriented with increased lumbosacral angle. The promontory is also less pronounced. Overall, the larger width, more horizontal orientation, and decreased concavity of the anterior surface result in a larger true pelvic cavity. The sacrococcygeal angle hints to the overall anterior curvature of the sacrum and the coccyx, and is defined by angle formed between a line from the midpoint of the S1 and C1 and a line from the latter to the tip of the coccyx (▶Fig. 19.3). The female sacroiliac joints have less pronounced ridges and depressions.13,41,42 Lumbosacral transitional vertebra is a common finding. The sacrum may consist of six or more segments, four segments infrequently. The former is called sacralization and may result from addition of the lumbar or a coccygeal segment. The degree of sacralization ranges from L5 vertebra with broadened elongated transverse processes to complete fusion to the sacrum (▶Fig. 19.14). With lumbarization, the first sacral segment fails to unite, partially or completely, with the remaining sacrum resulting in six “lumbar” vertebrae with a diminutive or well-formed disk present between the first and second sacral segments. More commonly, the first two sacral segments may fail to unite completely, with residual disk seen as a transverse cleft on the anterior surface. Nonfusion of the laminae of the first sacral segment is not uncommon, resulting in spina bifida occulta (▶Fig. 19.14). Some of the other laminae may also fail to fuse with the sacral canal remaining open over multiple segments.20 Rarely, none of the laminae fuse (▶Fig. 19.15). Fusion of the sacrum to coccyx can occur after birth.43 Sacral agenesis is rare.

19 Fig. 19.11 Sacrum and coccyx in a 5-year-old boy. Note incomplete fusion of the intervertebral disks, spinous processes, lamina, and lateral masses between the sacral vertebrae. Also seen is ossification of the C1 but the rest of the coccyx still remains cartilaginous. Sacral curvature is not formed.

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Fig. 19.12 Ossification centers. (a) Transverse computed tomography (CT) image showing ossification centers in a 27-month-old boy. B, C, and N represent the center for the body, costal element, and the neural arch, respectively. (b–d) Coronal reformat CT images of a 4-month-old (b), a 27-month-old (c), and an 8-year-old (d). Note progressive fusion of the ossification centers and other age-related changes.

Fig. 19.13 Secondary ossification centers (arrows) seen on transverse (a) and coronal (b) computed tomography (CT) images. Note iliac crest ossification center (arrowheads).

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Fig. 19.14 Transitional vertebra (partial sacralization). (a) Note elongated, broad, right transverse process of the transitional vertebra articulating with the sacral ala and the posterior iliac bone (arrow). This leads to variation in position and size of the sacroiliac articular surface. Compare with normal appearance on the left. (b) The left transverse process of the transitional vertebra articulates with the ileum (arrow) altering the shape of the corresponding sacroiliac joint.

Fig. 19.15 (a) Spina bifida occulta (arrow). (b, c) Open sacral canal. (b) Coronal maximum intensity projection (MIP) computed tomography (CT) showing open sacral canal (arrows) due to failure of fusion of the sacral laminae. Note changes related to previous septic arthritis and surgery in the left hip. (c) Transverse CT of the same patient at S2 level showing nonfusion of the sacral laminae (arrows).

◆◆ Posterior Iliac Bones

The iliac crests end posteriorly in the posterior superior iliac spines which are variable in size and contour.44 The portion of the iliac bone below the PSIS forms the lateral surface of the sacroiliac articulation (▶Fig. 19.5, ▶Fig. 19.7). The thickness of the iliac bone at the level of the sacroiliac joint is 8 to 22 mm; it is thickest anterosuperiorly and thinnest posteroinferiorly.45 The iliac crest is a common donor site for bone graft harvesting. Due to a large amount of red bone marrow, it is also a site for bone marrow ­harvest. Distal to the PSIS is the posterior inferior iliac spine (PIIS). The PIIS forms the dorsal aspect of the greater sciatic notch (▶Fig. 19.7). As previously mentioned, the attachment of the thoracic portion of the longissimus thoracic from the sacrum continues superiorly along the interspinous region of the posterior ileum to the caudal margin of the PSIS. The thoracic component of the iliocostalis

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lumborum attaches on the posterior iliac crest up to 5 cm from the PSIS (▶Fig. 19.5). The lumborum portion of the iliocostalis attaches to the lateral half of the PSIS and along the ventral aspect of the posterior iliac crest, while the lumborum portion of the longissimus attaches to ventral aspect of the PSIS and the ventromedial aspect of the posterior iliac crest.23 The latissimus dorsi, along with the thoracolumbar composite fascia, arises from the external lip of posterior iliac crest (see Chapter 1 “Abdominopelvic Wall” of Volume 2). The PSIS also serves as the attachment of the posterior sacroiliac ligament and the deep lamina of the posterior layer of the thoracolumbar fascia.46 The quadratus lumborum originates from the posterior iliac crest and sometimes the iliolumbar ligament.47 Often the posterior iliac bone overhangs the sacroiliac joint (iliac buttressing) which can prevent direct needle access to that region of the sacroiliac joint (▶Fig. 19.16).10,48

Sacrum, Coccyx, and Sacroiliac Joints scoliotic deformity can occur, often referred to as type VI. Types I and II were the most common morphology seen in some studies, while types II and III were more common in another study.15,49,52 In about one-fourth of cases, a bony spicule is seen on the last coccygeal segment15 (▶Fig. 19.18g). These are most commonly seen in nonmobile coccyges, and spicules have been implicated as one of the causes of coccydynia.53

Surfaces

Fig. 19.16 Iliac buttress. The overhanging posterior iliac bone (arrowhead) prevents needle access to the synovial portion of the sacroiliac joint (dashed arrow).

◆◆ Coccyx

The coccyx usually consists of three or four coccygeal segments in adults, but the number can vary from one to five. Fusion of the distal coccygeal segments is common, with variable presence of the intercoccygeal disks in adults.

Shape, Size, and Orientation The coccyx is a triangular-shaped bone with a proximal base and distal apex. It is curved anteriorly like a beak.15 The first coccygeal segment is the largest with progressively smaller distal segments. Although the first coccygeal segment resembles other vertebrae in the spinal column, successive coccygeal segments lose that resemblance. Coccygeal length is variable depending on the number of coccygeal segments (▶Fig. 19.17). On an average, the male and female coccyx measure 44 and 40 mm in curved length, respectively. The coccyges in women are shorter, straighter, and more prone to retroversion.15,49,50 The intercoccygeal angle, formed between the longitudinal axis of the first coccygeal segment and the longitudinal axis of the last coccygeal segment, is used as an index of coccygeal curvature (▶Fig. 19.8). The mean angle is about 138 and 147 degrees in men and women, respectively.15 Classically, depending on the ­curvature, it has been categorized into four types51 (▶Fig. 19.18). Type I has a mild anterior curvature, type II is more curved with the coccyx pointing straightforward, while type III has sharp anterior curvature. Type IV shows subluxation of the sacrococcygeal or intercoccygeal joints. The tip can rarely be pointing backwards known as retroverted coccyx in type V.52 A coccyx with

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The first coccygeal segment forms the base on which a facet is present that articulates with the facet at the apex of the sacrum. Transverse processes extend laterally from this coccygeal segment and may articulate or fuse with the lateral portion of the sacrum creating foramina through which the anterior division of the fifth sacral nerve passes. Dorsally, there are two cornua representing the articular processes (▶Fig. 19.2). They articulate with the cornua of the sacrum.51,54 The anterior surface is usually concave with transverse grooves marking the site of fusion of the coccygeal segments. Laterally, coccyx is thin forming the lateral border. The anterior surface gives attachment to the anterior sacrococcygeal ligament superiorly and the levator ani inferiorly. The anococcygeal ligament connects the tip of the coccyx to the external anal sphincter (see Chapter 5 “Colon” of Volume 2). The sacrotuberous and sacrospinous ligaments attach to the posterolateral margins of the coccyx, with the coccygeus attaching in front of them on the lateral aspect of the anterior surface and the gluteus maximus attaching posterior to them on the lateral aspect of the dorsal surface50,55 (▶Fig. 19.5, ▶Fig. 19.6, ▶Fig. 19.7). Dorsally the filum terminale externum fuses with the periosteum of the dorsal aspect of the coccyx or sacrum.56

Development and Ossification The coccygeal segments develop during the first three decades of life. Each coccygeal segment ossifies from one primary ossification center, that for the centrum. The first coccygeal segment has additional secondary ossification centers for the neural arch and transverse processes. The ossification centers for the first coccygeal segment appear from birth to 18 years of age and fuse between 6 and 30 years of age. The other coccygeal segments ossify from 16 months to 18 years of age and the ossification progresses distally22 (▶Fig. 19.11). Early in the life the coccygeal segments are separated by fibrocartilaginous disks that can disappear later in life, being replaced by bone. Fusion of the sacrococcygeal joint can be seen in nearly one half of the subjects on radiographs and computed tomography (CT) but the true incidence remains uncertain as it was not seen in any of the patients on a study using magnetic resonance imaging (MRI).15,51,57 Similarly, fusion of the intercoccygeal segment is also variable, although it is more common in the distal segments.15

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Fig. 19.17 Coccyx with varying number of segments. Coccyx with one (a), two (b), three (c), and four (d) segments.

Fig. 19.18 Types and variations of coccyx: (a) type I, (b) type II, (c) type III, (d) type IV (anterior subluxation), (e) type IV (posterior subluxation), (f) type V (retroverted, arrowhead), and variation such as (g) bony spicule (highlighted) and (h) scoliosis.

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◆◆ Sacroiliac Joint

There is disagreement regarding the classification of the sacroiliac joint. It has an anterior synovial articulation (one-third to one-half) and dorsal syndesmosis (two-thirds to one-half)29,58,59,60 (▶Fig. 19.19, ▶Fig. 19.20). Many authors consider it to be a true diarthrodial joint with unique features that vary from the other diarthrodial joints.1,61,62,63 These include lack of hyaline cartilage on both articulating surfaces—it has fibrocartilage in addition to hyaline cartilage, the posterior capsule is interrupted, and the joint surfaces are uneven.4,62,64,65 One relatively recent study with a small sample size has argued to classify the ventral articulation as symphysial rather than synovial, based on the finding that the synovial characteristics including an inner capsule with synovial cells is limited to the distal one-third of the joint.66

Articulating Surfaces, Shape, and Size The articular surface of the sacrum is called the auricular surface due to its resemblance to the shape of the ear (▶Fig. 19.2, ▶Fig. 19.19, ▶Fig. 19.20). It has also been variably described as L-shaped, C-shaped, Chevron-shaped (with the apex ventrally and inferiorly), and boomerang-shaped.37,62,64,67 It articulates with a corresponding shaped articular surface on the ileum.68 The shape of the auricular surfaces varies highly from infancy to adulthood.62 The superoinferior extent of the joint measures about 64 mm and the anteroposterior extent measures about 29 mm.69 The surface of the sacroiliac joint at birth is 1.5 cm2, at puberty it is 7 cm2, and in the adult it reaches 17.5 cm2.64

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In general, the sacral auricular surface is concave, while the iliac articular surface is convex (▶Fig. 19.19). An iliac convexity (iliac tuberosity) is present posterior to the auricular surface with corresponding sacral concavity or depression. This sacral concavity is often divided into three fossae from superior to inferior (▶Fig. 19.9). The iliac tuberosity articulates with the fossae of the sacral concavity by strong interosseous ligaments extending between the two bones (▶Fig. 19.5). Overall, the joint surfaces are irregular with elevations and depressions interlocking with corresponding depressions and elevations, respectively, on the opposing surface. These elevations and depressions increase with age. An “alar tuberosity” on the posterosuperior aspect of the sacral ala has also been described that articulates with a depression anterosuperior to the iliac tuberosity.70,71 In the adults, the auricular surface is covered with 2 to 4 mm hyaline cartilage on the sacral side and 1 to 2 mm fibrocartilaginous material on the iliac side37,40,72 (▶Fig. 19.6). The cartilage thickness on both sides of the joint decreases with age.37,40,58,59,62,64,66 Bowen and Cassidy found the sacral cartilage smooth and clear during the early decades with the iliac cartilage bluer, duller, and striated.62 In their study, Puhakka et al66 found hyaline cartilage in the central portion of the joints with fibrocartilage (more collagen fibers) at the articular periphery where the ligaments attach.

Location and Orientation The articular facet on the sacral side usually extends from the mid S1 vertebra to the S3 vertebra (▶Fig. 19.7, ▶Fig. 19.21). However, the position and size of the articular surface vary with the transitional

Fig. 19.19 Sacroiliac (SI) joint. The auricular surface of the sacrum in the anterior SI joint (yellow area and where marked with red brackets) is the true synovial joint that articulates with a corresponding shaped articular surface on the ileum. The posterior SI joint between the iliac tuberosity and the sacral concavity (blue area and blue brackets) is connected by the fibrous interosseous ligaments, hence called dorsal syndesmosis. Note the changing orientation of the joint plane (yellow lines) giving a propeller-like configuration in consecutive series.

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Fig. 19.20 Gross anatomy of the sacroiliac (SI) joint shown by color-coded, volume-rendered computed tomography (CT). The auricular surface of the left sacrum (yellow area) is the true synovial joint that articulates with a corresponding shaped articular surface on the ileum. The auricular facet on the sacral side usually extends from the mid S1 vertebra to the S3 vertebra. The nonsynovial posterior SI joint surface of the sacrum also known as sacral concavity (blue area) is connected to the iliac tuberosity by the fibrous interosseous ligaments. PSIS, posterior superior iliac spine.

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Fig. 19.21 Osteitis condensans ilii. Plain radiograph of the pelvis (a) showing bilateral sclerosis (arrows) on the iliac side of the sacroiliac joints. (b) Axial computed tomography (CT) in a different case showing similar findings. Note also mild sclerosis on the sacral side. This process is ­common with age and is seen more frequently in multiparous women.

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Sacrum, Coccyx, and Sacroiliac Joints anatomy seen at the lumbosacral articulation that occurs in 18 to 30% of the population.73,74,75 With lumbarization, the auricular surface is more cephalad, extending from cranial aspect of the first sacral segment of the fused four remaining sacral vertebrae to the caudal aspect of the next vertebra. With sacralization of the L5 vertebra which occurs in 6% of Americans,76 the joint surface is more caudal extending from the first sacral segment of the fused six sacral vertebrae to the caudal S3 vertebral body. Thus, with unilateral transitional anatomy, the size, shape, and location of the sacroiliac joint will be completely different from that on the opposite side.73,77 In general, the auricular surfaces of the sacroiliac joints are oriented dorsomedially–ventrolaterally. However, the orientation at the level of S1, S2, and S3 is different leading to a twisted appearance like that of a propeller78,79,80 (▶Fig. 19.19, ▶Fig. 19.20).

Development and Age-Related Changes The sacroiliac joint develops early in the fetal life with the joint cavity present during the second month of life.59,62 During the fetal life the articular surfaces are smooth, and septa may be seen transiently in the joint cavity which disappear before birth. The articular surfaces remain vertical, flat, and smooth during the first decade of life.81 Beginning in the second decade of life, around puberty, the joint changes shape with development of horizontal and vertical limbs, and the joint surface becomes more uneven with development of ridges and grooves. A convex central longitudinal ridge/tubercle in the auricular surface of ileum and a corresponding concave groove or depression on the sacral side develop, apparently as an adaptive response to increase in weight.82 Later the iliac bony tuberosity develops posterior to the auricular surface in the region of the interosseous sacroiliac ligament with corresponding fossa on the sacral side. The joint capsule forms during the fetal life. Early in life it is pliable but with growth it thickens and becomes progressively stiffer. The interosseous ligament becomes stronger during the third decade which along with the increasingly coarse texture of the cartilage surface and interlocking of the ridges and depression leads to increased joint stability.83 With increasing age degenerative changes begin to develop in the joint, in men as early as the third decade on the iliac side and in the fourth decade on the sacral side. These are predominantly seen as joint irregularity with surface fibrillation and deeper fissures in the cartilage. During the fourth and fifth decades, marginal osteophytes, thinning of cartilage, and fibrous adhesions between the two articular surfaces begin to develop. Subchondral sclerosis develops, which is more noticeable on the iliac side. The osteophytes may extend across the joint to the opposite margin with development of extra-articular bony ankyloses (▶Fig. 19.22). Fibrous ankyloses may develop62 and the joint can become completely immobile in the eight decade. Of note, intra-articular bony ankyloses is not part of the aging process and is considered pathologic.84

Joint Capsule The capsule attaches close to the articular margins. It has outer fibrous and an inner synovial layer. Anteriorly, the capsule is

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approximately 1.5-mm thick and blends with the ventral sacroiliac ligament. Posteriorly, the capsule blends with the interosseous ligament and cannot be distinguished or may be absent, with the joint limited posteriorly by the interosseous ligament.65,79 The joint capsule which may be perforated or discontinuous leads to extravasation of fluid injected from the joint. This can flow ventrally around the branches of the lumbosacral plexus, dorsally into the dorsal sacral foramina or superiorly to the fifth lumbar epiradicular sheath. Similar spread of inflammatory mediators from the joint may be the reason for the different pattern of pain distribution.62,65,85

Ligaments The bones are held closely by ligaments which have been divided into two groups: the main ligaments and the accessory ligaments. The former group consists of the ventral (anterior), the interosseous, and the dorsal (posterior) ligaments (▶Fig. 19.6, ▶Fig. 19.23), while the latter group consists of the iliolumbar, the sacrotuberous, and the sacrospinous ligaments.

Ventral Sacroiliac Ligament The ventral sacroiliac ligament is considered to be thickening of the anterior joint capsule although microscopically they can be seen separately. Together they measure about 2 mm in thickness (▶Fig. 19.5, ▶Fig. 19.23). It consists of many bands of tissue inserting on the periosteum close to the margins of the articular surfaces of the iliac and sacral bone and onto the cartilage. Anteroinferiorly it is well developed and reaches up to the arcuate line.66,85 The ligament stabilizes the joint anteriorly. It may become calcified (▶Fig. 19.22).

Interosseous Ligaments The interosseous ligament is dorsal and cephalad to the auricular portion of the joint and consists of many short, strong ligamentous bands extending between the convex tuberosity of the ilium and opposing concave surface of the sacrum.58 The interosseous ligaments are the largest and strongest sacroiliac ligaments providing stability to the joint5,86 (▶Fig. 19.6, ▶Fig. 19.23). The ligamentous space is rich in fatty tissue containing arteries and veins.40,66 In the center of the ligament lies the “axial” joint (discussed later), and ossification may be seen in this region.58 Its more superficial fibers blend with the posterior sacroiliac ligament.

Dorsal Sacroiliac Ligaments The strong dorsal sacroiliac ligament (more commonly referred to as posterior sacroiliac ligament) is superficial to interosseous ligament from which it is separated by the posterior rami of sacral spinal nerves.58 It has numerous fascicles that run in different directions. Its deeper portion, called the short posterior sacroiliac ligament, is relatively horizontally oriented extending between the intermediate and lateral sacral crest at the level of S1 and S2, and the PSIS and the medial aspect of the adjacent posterior iliac crest58 (▶Fig. 19.6, ▶Fig. 19.23). The superficial portion, the

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Fig. 19.22 (a) Transverse computed tomography (CT) image shows bilateral anterior sacroiliac joint bridging (extra-articular bony ankylosis) in a 55-year-old man (arrows) possibly due to calcification of the anterior sacroiliac ligaments. (b) Calcified iliolumbar ligament (arrowhead). (c) Sacrotuberous ligament ossifications (arrows). (d) Sacrospinous ligament ossifications (arrows).

Fig. 19.23 Sacroiliac ligaments. Transverse magnetic resonance (MR) images (a, b) of the sacrum showing anterior (arrowhead), interosseous (IO), and posterior (arrow) sacroiliac ligaments.

long posterior sacroiliac ligament, is obliquely oriented extending between the PSIS and adjacent internal lip of iliac crest, and the third and fourth and sometimes the fifth tubercles of the lateral sacral crest. The middle cluneal nerves (lateral branches of dorsal sacral rami) penetrate the ligament, with the branches of S2 and S3 doing so in nearly all cases, branches of S4 in more than half of the cases, and branches of S1 occasionally.44,87 Some of these nerve fibers innervate the ligament. Other fibers pass through or deep to the ligament where they can potentially get entrapped.

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This ligament is intimately related to a number of important surrounding structures (▶Fig. 19.6). Its medial aspect is attached to the erector spinae aponeurosis, the posterior layer of thoracolumbar fascia, and the fascial layer from the sacral attachment of the multifidus muscle, while the lateral aspect is continuous with the gluteus maximus aponeurosis and sacrotuberous ligament44,46,87 (▶Fig. 19.5). It tenses during counternutation, and relaxes with nutation and increased tension in the latissimus dorsi or the gluteus maximus.

Sacrum, Coccyx, and Sacroiliac Joints

Additional Ligaments An axial interosseous ligament has been described just behind the auricular surface in the region of the axial joint. It appears to be a part of and comprises approximately 14% of the interosseous ligament. It consists mainly of loose connective tissue rich in adipocytes. It has been presumed that the axis of rotation of the sacroiliac joint is in the region of axial joint putting the ligament under strain during motion. Many authors have not described this ligament separate from the interosseous ligament.69,88 An additional ligament called the superior intracapsular ligament or Illi’s ligament has also been described extending between the superior aspects of the sacral and iliac auricular surfaces. Its biomechanical significance is uncertain.89

Accessory Ligaments Iliolumbar Ligament

Iliolumbar ligament (also called lumbo-ilio-sacral ligament) is a complex ligament with several distinct bands extending laterally from the transverse process of the fifth lumbar vertebra. Its two main bands are the posterior band and the fan-like anterior band. The posterior band and the anterior band originate from the tip of the L5 transverse process and the anteroinferior aspect

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of the transverse process, respectively. Both the bands extend to the ileum with the posterior attaching to the cranial part of the iliac tuberosity on the internal surface and the anterior band anteroinferior to the attachment of the dorsal band90,91,92 (▶Fig. 19.6, ▶Fig. 19.24). The iliolumbar ligament is an important ligament for stability of the lumbosacral junction. It restricts the motion during flexion, extension, lateral flexion (lateral bending), and torsion (rotation) at this region and prevents diastasis of the superior aspect of the sacroiliac joint.91,93,94 Partial or complete ossification of the ligament is not uncommon (▶Fig. 19.22).

Sacrotuberous Ligament

The sacrotuberous ligament has a broad upper attachment to the PSIS, PIIS, lower (fourth and fifth) transverse sacral tubercles, and dorsolateral surface of the lower sacrum and upper coccyx (▶Fig. 19.6, ▶Fig. 19.25, ▶Fig. 19.26). Its fibers intermingle and blend with the lower fibers of the posterior sacroiliac ligament (▶Fig. 19.25, ▶Fig. 19.26). The fibers extend inferiorly and laterally to form a narrow band which then expands to insert onto the medial aspect of the ischial tuberosity. A portion of the ligament continues along the inferior ischial ramus for about 45 mm as the falciform process (seen in about 87% of the population) that inserts on the ischial ramus, and blends with the fascial sheath of the pudendal vessels and nerve and the obturator fascia, and at times extends along the ischioanal fossa to the lateral

Fig. 19.24 Iliolumbar ligament. Consecutive coronal (a, b) and transverse (c) magnetic resonance imaging (MRI) images of the pelvis and oblique transverse (d) image of the sacrum showing the course and attachment of the iliolumbar ligaments (arrows). I, posterior iliac bone; IO, interosseous ligaments; T, transverse processes of L5 vertebra.

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Fig. 19.25 Axial T2-weighted magnetic resonance imaging (MRI) from bottom to top showing the anatomic location and course of the sacrotuberous (ST, shown in green) and sacrospinous (SS, colored in pink) ligaments. The coccygeus muscle is attached to its inner surface of the SS ligament and is often called the muscular part of the ligament. The sacrotuberous ligament extends from the ischial tuberosity anterior to the gluteus maximus (Gmax) and has a broad upper attachment to the dorsolateral surface of the lower sacrum and upper coccyx. It extends superiorly to the posterior superior iliac spine. The internal pudendal vessels and nerve enter the lesser sciatic foramen (LSF) between the SS and the ST ligaments. The tendon of obturator internus (OI) traverses the lesser sciatic foramen. Note attachment of the gluteus maximus aponeurosis to the coccyx (arrow). GSF, greater sciatic foramen; PFC, posterior femoral cutaneous nerve; Red dots, internal pudendal artery.

anococcygeal ligament.95 The superficial fibers of the lower part of the sacrotuberous ligament continue into the biceps femoris tendon. The superficial fibers give origin to the gluteus maximus, while the deeper fibers are partly attached to the deeper sacrospinous ligament (▶Fig. 19.25). Some fibers of the piriformis originate from the ventral part of the ligament in most cases with the fascia over the piriformis continuous with the sacrotuberous ligament.96

Sacrospinous Ligament

The sacrospinous ligament is smaller than the sacrotuberous ligament. It has triangular appearance with the apex at the ischial spine and a broad base attached to the lateral sacrum and coccyx (▶Fig. 19.6, ▶Fig. 19.25, ▶Fig. 19.26). The coccygeus is attached to its inner surface and is often called the muscular part of the ligament (▶Fig. 19.25). The coccygeus extends between the lower lateral sacrum and coccyx, and the ischial spine. It forms the pelvic diaphragm with the levator ani. The sacrotuberous and the

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sacrospinous ligaments limit the forward rotation of the upper sacrum, that is, nutation.

Sacrococcygeal Joint The sacrococcygeal joint is formed between the sacral facet and the coccygeal facet, with the sacrococcygeal joint angle formed between the lines intersecting the middle of S5 and C1 of about 165 to 170 degrees (▶Fig. 19.3). The joint can be a symphysis with a disk, a true synovial joint, or a mixed type53 (▶Fig. 19.2, ▶Fig. 19.27). The anterior sacrococcygeal ligament is present anteriorly blending superiorly with the anterior longitudinal ligament. The posterior sacrococcygeal ligament extends from the coccyx to the sacrum dorsally (▶Fig. 19.2). It has a superficial component and a deep component. The superficial part attaches to the margin of the sacral hiatus, while the deep part blends into the posterior longitudinal ligament. Laterally, the lateral sacrococcygeal ligaments connect the coccygeal transverse processes to the inferolateral

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Sacrum, Coccyx, and Sacroiliac Joints

Fig. 19.26 Sacrotuberous (ST) and sacrospinous (SS) ligaments and the sciatic foramina. The sacrospinous (SS) ligament is the border between the upper greater and lower lesser sciatic foramen. The sacrotuberous ligament is shown in light blue extending between the ischial tuberosity (IT), and the lateral margin of the distal sacrum and the posterior superior iliac spine (PSIS). The piriformis muscle (green) occupies most of the greater sciatic foramen. Three branches of the superior gluteal (SG) are best shown on the posterior views. The inferior gluteal (IG) vessels (along with the sciatic nerve) exit the pelvis inferior to the piriformis and above the SS ligament. The internal pudendal vessels and nerve also course posteriorly between the piriformis and the SS ligament before entering the lesser sciatic foramen. The internal pudendal vessels and nerve, and the tendon of obturator internus (OI) traverse the lesser sciatic foramen. EIA, external iliac artery; IIA, internal iliac artery; PSIS, posterior superior iliac spine; QF, quadratus femoris.

19 Fig. 19.27 Sacrococcygeal joint (arrows) seen on plain anteroposterior (AP) (a) and lateral (b) radiographs of the pelvis. Note prominent coccygeal cornu (arrowhead).

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Sacrum, Coccyx, and Sacroiliac Joints aspect of the sacrum (▶Fig. 19.2). Dorsally the cornua of the sacrum articulate with the cornua of the coccyx forming zygapophyseal joints, and are connected by intercornual ligaments50 (▶Fig. 19.2).

Blood Supply and Lymphatic Drainage The blood supply of the sacroiliac joint is derived from the iliolumbar, superior gluteal, and the lateral sacral arteries. The lateral sacral arteries arise from the internal iliac artery or its posterior division and anastomose with the middle sacral artery. There are usually a superior and an inferior lateral sacral arteries that enter the anterior sacral foramen and exit the posterior foramen (▶Fig. 19.9). The veins and lymphatics course along with the arteries. The iliac and lumbar nodes drain the joint.42

Innervation The exact nerve supply of the sacroiliac joint is variable and has not yet been exactly determined. Histologically, myelinated and unmyelinated nerve fibers and paciniform and nonpaciniform mechanoreceptors have been found in the joint tissue and ligaments.97 Nerve fibers showing reactivity to calcitonin gene-related peptide (CGRP) and substance P are also seen in the joint cartilage.98 This suggests that pain and proprioceptive information may originate in these joints.99 Joint innervation is mainly from the first and second sacral nerves, with overall innervation from nerves from L3 to S2 (lumbosacral plexus) and the superior gluteal nerve.79,100,101,102 Fortin and colleagues showed that the pain referral map of the sacroiliac joint pain was in a stripe that is 3 cm wide and 10 cm long extending inferiorly and laterally from the PSIS which is the distribution of the sacral dorsal rami (▶Fig. 19.28).103 They concluded that the nerve supply to the sacroiliac joint was predominantly, if not entirely, from the dorsal divisions of the sacral rami.104 However, later study showed that the pain referral map is not limited to the buttock and lumbar region and that it is more variable and diffuse.105

Relations Many important vessels and nerves are in close relation to the sacroiliac joint, particularly along its anterior aspect. This puts them at risk for injury during anterior approach to the sacroiliac joint. The internal iliac artery and its two divisions are in close relation to the sacroiliac joint. The superior gluteal and iliolumbar arteries, and the bifurcation of the internal iliac artery are related to the sacroiliac joint (▶Fig. 19.26, ▶Fig. 19.29). The iliolumbar artery, a branch of the internal iliac artery or its posterior division, starts medial to the superior aspect of the sacroiliac joint. It later turns to travel laterally behind the common iliac artery crossing the upper sacroiliac joint.30 One of its branches, the nutrient artery of the ilium, also extends across the sacroiliac joint to supply the ilium.106,107 The lateral sacral artery, depending on the origin, may cross the sacroiliac joint. If there are two lateral sacral arteries, then the superior artery is seen at the level of first and second sacral foramen, while the inferior artery, if it is present, enters at the level

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Fig. 19.28 Composite pain referral map. PSIS, posterior superior iliac spine.

of lower anterior foramen. The artery extends medially and inferiorly coursing along the anterior surface of the sacrum lateral to the foramen (▶Fig. 19.9). The median sacral artery (also called the middle sacral artery), a branch of the abdominal aorta just before its bifurcation, travels caudally along the anterior aspect of the sacrum in the midline up to the coccyx.30 The median and the lateral sacral arteries anastomose sending branches into the anterior sacral foramina. The superior gluteal artery passes between the fourth and fifth lumbar anterior rami proximal to the formation of lumbosacral trunk, or between the lumbosacral trunk and the first sacral anterior ramus and exits the pelvis through the greater sciatic notch (▶Fig. 19.26, ▶Fig. 19.29). It extends superiorly, lateral to the PSIS, before dividing into superficial, deep superior, and deep inferior branches to supply the surrounding gluteal musculature108,109 (▶Fig. 19.29). The inferior gluteal artery also passes through the branches of the sacral plexus (between S1 and S2 nerves or less commonly between the S2 and S3 nerves) and along with the ­sciatic nerve exits the pelvis through the greater sciatic notch superior to the sacrospinous ligament, and then runs caudally110 (▶Fig. 19.26, ▶Fig. 19.29). The veins follow the arteries. The internal iliac veins lie on the anterolateral surface of the sacrum medial to the sacroiliac joints at about S1–S2.6,108 Both the iliac veins are dorsal to the corresponding arteries, but the vein lies medial to the artery on the left and lateral to the artery on the right.111 Anterior to the vessels the ureter courses toward the bladder.

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Fig. 19.29 Relationship of the arteries and the sacroiliac joint. Close relationship of the superior gluteal (SG) and iliolumbar (IL) arteries, and the bifurcation of the internal iliac artery to the joint is shown. The inferior gluteal (IG) artery and the pudendal artery usually arise from the anterior division (upper row), but occasionally with the SG artery from the posterior division (lower row). The IG artery then moves posteriorly along with the sciatic nerve to exit the pelvis.

The lumbosacral plexus as well as the dorsal sacral plexus are intimately related to the sacroiliac joint (▶Fig. 19.30). In general, the nerves lie between the vessels and the sacrum and sacroiliac joints (see Chapter 13 “Lower Extremity Nerves ”). The L4 and L5 rami lie along the anterior aspect of the proximal sacrum between the internal iliac vessels medially and the sacroiliac joint laterally.6 The L5 nerve travels on the sacral ala medial to the sacroiliac joint with the L4 nerve laterally between it and the sacroiliac joint112,113 (▶Fig. 19.6). The L4 nerve at this site is at about the middle of the sacral ala along the medial aspect of the psoas.30 The L4 and L5 nerves unite to form the lumbosacral trunk which is about 12 mm medial to the sacroiliac joint, along the medial aspect of the psoas major muscle. The trunk then extends inferolaterally coming within 10 mm of the sacroiliac joint as it crosses into the true pelvis113,114 (▶Fig. 19.6, ▶Fig. 19.30). It continues to descend on the anterolateral surface of the sacrum and along with the anterior branch of the first sacral nerve, and sometimes the second and the third sacral nerves, and form the sacral plexus over the anterior aspect of the piriformis and posterior to the internal iliac vein. The obturator nerve, along the medial border of the psoas, passes just cephalad to the sacroiliac joint as it courses laterally (see Chapter 1 “Abdominopelvic Wall” of Volume 2). The sacral nerves are medial to the sacroiliac joint and about 8 mm from the midline. The S1, S2, and S3 nerves are approximately 2, 17, and 24 mm medial to the inferior aspect of the

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sacroiliac joint.112 The major branch of the lumbosacral plexus is the sciatic nerve which exits the pelvis through the greater sciatic notch (▶Fig. 19.6, 19.30) with the inferior gluteal artery above the sacrospinous ligament.115,116 Variation in exit of the sciatic nerve from the pelvis in relation to the piriformis muscle has been described115 (see Chapter 9 “Lower Extremity Muscles: Pelvic Girdle, Thigh, and Leg”). The sacrotuberous and sacrospinous ligaments convert the sciatic notches into foramina, with the sacrospinous ligament dividing the foramen further into the upper greater and lower lesser sciatic foramen (▶Fig. 19.7, ▶Fig. 19.26). The greater sciatic foramen is bounded anterosuperiorly by the greater sciatic notch, superiorly by the anterior sacroiliac ligament, posteriorly by the sacrotuberous ligament, and inferiorly by the sacrospinous ligament and ischial spine. The piriformis muscle passes through the foramen occupying most of it toward its insertion (▶Fig. 19.26). It divides the foramen into two parts, the superior and the inferior. The superior gluteal vessels and nerve exit the pelvis superior to the piriformis, while the inferior gluteal vessels and nerves, the internal pudendal vessels and the pudendal nerve, the sciatic nerve, the posterior femoral cutaneous nerve, and the nerves to obturator internus and quadratus femoris exit the pelvis inferior to the piriformis (▶Fig. 19.25). The lesser sciatic foramen is bounded superiorly by the sacrospinous ligament and the ischial spine, anteriorly by the ischial

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Fig. 19.30 Relationship between the sciatic nerve and the sacroiliac joint. L4, L5, and S1 nerve roots unite to form the sciatic nerve inferior to the sacroiliac joint. The sciatic nerve passes anterior to the piriformis muscle belly without branching and posterior to the obturator internus tendon and gemellus muscles. Note the inferior gluteal artery coursing posteriorly along with the sciatic nerve to exit the pelvis.

body and tuberosity, and posteriorly by the sacrotuberous ligament (▶Fig. 19.7, ▶Fig. 19.26). The internal pudendal vessels and the pudendal nerve, the tendon of obturator internus, and the nerve to the obturator internus traverse the foramen. The internal pudendal vessels and nerve course posterior to the ischial spine or the adjacent sacrospinous ligament.117 The sympathetic chains, one on each side, are adherent to the anterior surface of the sacrum as they extend caudally medial to the anterior sacral foramina. They unite on the anterior surface of the coccyx forming the ganglion impar.30 The S5 and coccygeal nerves exit the sacral hiatus and pass between the apex of the sacrum and the intercornual ligaments. The anterior division of the S5 nerve passes above the transverse process of the first coccygeal segment, while that of the coccygeal nerve passes below it. They take part in the coccygeal plexus which forms on the ventral surface of the ischiococcygeus. The plexus supplies part of the sacrococcygeal joint. The dorsal rami separate and pass posterior to the transverse processes. The dorsal rami of the sacral nerve after emerging from the foramina divide into medial and lateral branches. The medial branches penetrate the multifidus. The lateral branches of the first three sacral dorsal rami, the middle cluneal nerves, extend laterally on the sacral grooves deep to the multifidus where they anastomose. The nerves then pass laterally through or deep to the long posterior sacroiliac ligament before turning inferiorly coursing on the sacrotuberous ligament.118

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Surface Anatomy The easily palpable iliac crests terminate posteriorly at the PSIS which can be felt in the depressions (sacral dimples) along the superior aspect of the buttocks (▶Fig. 19.7, ▶Fig. 19.28). A line

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drawn along the inferior aspects of the PSIS approximately corresponds to the S2 spinous tubercle and the middle of the sacroiliac joints.119,120,121 The thecal sac commonly terminates at this level. The PSIS forms an important anatomical landmark for percutaneous intervention.122 The S2 dorsal foramen is approximately 20 to 30 mm inferomedial to the middle of the PSIS with the lower margin of S1 and upper margin of S3 foramen about 14 and 13 mm from the S2 foramen (▶Fig. 19.7). The dorsal sacral rami are sometimes ablated for management of pain.120 The ischial tuberosities are palpable along the lower part of the buttocks. In standing position, they are covered by the lower part of the gluteus maximus (▶Fig. 19.5), while in sitting position, the gluteus maximus are displaced and the tuberosities become subcutaneous, bearing the weight of the body.

Biomechanics and Movement Stability of the Sacroiliac Joint The stability of the joint is dependent on a number of factors including the strong ligaments, the numerous ridges and grooves on the joint surfaces, coarse texture of the cartilage surface (increased friction) and orientation of the joint surfaces, and the muscles surrounding the joints. The sacroiliac articular surfaces are subject to tremendous forces from the weight of the upper body. The predominantly flat shape of the sacroiliac joint is suited for the transfer of large bending moments and compression. However, this also predisposes it to shear. The action of muscles such as erector spinae, quadratus lumborum, obliquus abdominis, rectus abdominis, latissimus dorsi, and psoas major increases the shear force that is opposed by the friction between the two joint surfaces and compression of the joint.

Sacrum, Coccyx, and Sacroiliac Joints A self-bracing model based on form closure and force closure helps understand the role of different factors in stability of the joint. Form closure refers to the situation where the joint surfaces fit closely with each other eliminating the need of external forces to maintain the stability of the joint. Force closure refers to the situation where the external forces compress the joint increasing the friction of the articulating surface to prevent movement. At the sacroiliac joint, the many closely fitting reciprocal elevations and depressions with contribution from the texture of the cartilage increases the friction between the joint surfaces and represents the force closure. The strong ligaments which keep the two iliac bones together, dynamically supported by the various muscles and aponeurotic expansions, represent the force closure. The combination of this form and force closure prevents the movement of sacrum and is called the self-bracing mechanism. Analogous to the apex key stone of the roman arch, the configuration (aided by the propeller shape) helps sacrum wedge between the two iliac bones. Forces of the upper body provide the loading force which compresses the sacrum against the iliac bones. The strong ligaments, most notably the interosseous ligament which has been described as the strongest ligament in the body, resist the tendency of diastasis.41 The posterior (the gluteus maximus, the erector spinae, the latissimus dorsi, and the biceps femoris through the sacrotuberous ligaments) and the anterior (the transverse and oblique abdominal) muscles dynamically compress the joint leading to more tighter fit of the two surfaces (screw home phenomena) to increase the friction. The gluteus maximus and piriformis because of the orientation of their fibers perpendicular to the joint surface are the main muscles in self-bracing. The caudal fibers of thoracodorsal fascia that cross the midline to the opposite side have fibers perpendicular to the joint surface. The latissimus dorsi acting through the thoracodorsal fascia hence works synergistically with the gluteus maximus of the opposite side in certain situations. The other ligaments including the sacrotuberous and sacrospinous ligaments contribute to the stability of the joint. The tension of the sacrotuberous ligament is increased by its attachment to the biceps femoris, the gluteus maximus, and the deep layer of the thoracodorsal fascia. There are many aponeurotic expansions that blend with the posterior sacroiliac ligaments strengthening the joint and increasing its stability.

Movement at the Sacroiliac Joint No specific muscle or muscles directly move the sacroiliac joint. However, a number of muscles involved in maintaining the posture as well as those involved in movement of the lumbar spine and lower extremities affect movement of the sacroiliac joint. The notable among these are erector spinae, quadratus lumborum, multifidus, iliopsoas, rectus abdominis, gluteus maximus, and pirifomis. When the muscles relax, a small degree of motion is possible, limited by the strong ligaments. There are physiologic situations which affect the degree of motion. In the young, where the articulating surfaces are relatively smooth, greater degree of motion is possible. In women, the ridges and depressions are less pronounced affecting the form closure, and allowing greater degree

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of movement compared to men.82,123,124 During pregnancy, under the influence of hormone relaxin produced by the corpus luteum and decidua of uterus, the ligaments and the symphysis pubis weakens.125 This affects force closure with increase in movement of the joints which helps the process of parturition. An appropriately placed pelvic belt, above the greater trochanters, used during the later stages of pregnancy and after pregnancy counteract the loose ligaments providing pain relief. External fixation frame can also decrease sacroiliac joint movement.126 The increased coarsening of the joint texture due to degeneration as well as formation of osteophytes in the elderly progressively limits sacroiliac joint motion. However, Tullberg et al demonstrated that manipulation of the joint did not alter the position of the sacrum in relation to the ilium.127 The movement of the sacroiliac joint is small and varies between individuals and according to the load applied (▶Fig. 19.31). Many studies have been performed looking at the degree of sacral motion. These include studies done with varied methodology, and those done in cadavers and live subjects. The three types of rotatory motions are along the sagittal plane (nutation and counternutation), in the axial plane (gapping of the anterior and posterior aspect of the joint), and in the coronal plane (gapping of the superior and inferior aspect of the joint). The primary motion is nutation/counternutation. In nutation the sacral promontory moves anteroinferiorly with simultaneous posterosuperior movement of the sacral apex rotating the sacrum around the x-axis. In nutation, the iliac crests move inwards while the ischial tuberosities move outwards. The opposite takes place in counternutation.123 Other motions include gliding/translation and a combination of movements. Sacral nutation occurs with extension of the spine.123 The maximum movement occurs when changing from the standing to lying prone with hyperextension of the leg (mimicking the toe off position).128 The overall motion is very small, believed to be less than 4 degrees of rotation and less than 3 mm of translation.123,125,129 The axis of rotation is 5 to 10 cm below the sacral promontory and at the interosseous sacroiliac joint.69,70 However, Wilder suggested that the rotation is accompanied by simultaneous translation.130 The concept of screw axis motion is used to explain the sacral motion where there is simultaneous sagittal plane rotation with translation akin to the movement of the screw.4 Due to the complex motion, there are different instantaneous centers of rotation, mostly located near the pubic symphysis.131

Movements of Coccyx Small and variable amount of flexion and extension occur at the sacrococcygeal joint (▶Fig. 19.32). Flexion is produced by levator ani while extension is passive due to relaxation of these muscles, facilitated by increased intra-abdominal pressure.57 The range of motion is the angle formed by the line extending from the center of the caudal part of the sacrum (or upper border of the coccyx if the joint is fused) and the tip of the coccyx in standing and in sitting positions. Flexion larger than 25 degrees represents hypermobility and slipping greater than 25% represents subluxation. Marked hypermobility (>35 degrees), subluxation, and extension (>15–20 degrees) are pathologic.53 However, in asymptomatic

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Fig. 19.31 Movement of the sacroiliac joint. (a–c) The primary motion is sacral nutation and counternutation, (a) and (b), respectively. The ­iliac bones approximate, whereas the ischial bone move apart with nutation. (d) Complex movement of the iliac and sacral bones (see text for details).

Fig. 19.32 Coccygeal mobility. Abnormal mobility of the coccyx between standing (a) and sitting (b). Note position of the tip of coccyx (arrows) and the curvature of coccyx.

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Sacrum, Coccyx, and Sacroiliac Joints patients the coccyx can be very mobile during defecation.57 Based on mobility, the coccyx can be divided into normal, immobile, hypermobile, or subdued.132

◆◆ Anatomical Variation

The sacroiliac joint anatomy is highly variable in size, shape, contour, and location of the articular surfaces, and plane of orientation of the joint space between individuals and between the two sides in the same individual; and no two joints are identical.70,133 Accessory sacroiliac articulations (▶Fig. 19.33) may be present in 8 to 40%, particularly posterior to the auricular surface.79,134,135,136 High frequency of the accessory joints in some reports may be related to misinterpretation of the interdigitation of the ridges and depressions as accessory joints.137 Accessory joint can also be seen in about 5% of children and in one series was seen exclusively in girls.40 These joints can be single or double, unilateral or bilateral, and if bilateral may vary from side to side. Accessory joints are more common in patients with obesity and their incidence increases with age. Although some joints are true diarthrodial, and present at birth, most are considered to be fibrocartilagenous joints acquired due to stress of weight-bearing.133,134

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Two types of accessory sacroiliac joints have been described—a superficial joint between the PSIS and the lateral crest of the sacrum at the level of second posterior sacral foramen, and the deeper joint between the iliac tuberosity and the opposing sacral concavity just posterior to the synovial portion of the sacroiliac joint, at the level of first sacral foramen. The deeper is also called the “axial” sacroiliac joint and is in the region of the interosseous ligament. The joint surface is covered by cartilage in as many as 50% of the cases, usually on the iliac side. The articulating facets are usually round or oval.69,134,138 The “iliosacral complex,” seen in about 6% of patients, is formed by an iliac projection inserting into a complementary sacral recess. This is most commonly seen posterosuperiorly at the junction of the synovial and the ligamentous portions of the joint, but can be seen posteroinferiorly in the synovial compartment. It is more commonly seen bilaterally and in women.133 An iliac bony cleft or channel is described along the dorsal aspect of the synovial portion of the joint at the level of proximal two-thirds and distal one-third66 (▶Fig. 19.33). Asymmetric fusion of primary and secondary ossification centers is not uncommon and should not be mistaken for a fracture line22 (▶Fig. 19.13). Persistent synchondrosis of primary sacral

19 Fig. 19.33 Anatomical variants of sacroiliac joints. (a) Iliosacral complex—note iliac bony projection (arrowheads); (b) channel in the iliac bone (arrow); (c) semicircular defects in the sacrum (arrowheads) and iliac bone (arrow c); (d) bipartite iliac bone plates (arrows); (e) crescent-shaped articular surface of ilium (arrowhead); and (f) accessory sacroiliac joint (arrow). Note true sacroiliac joint (thick arrow).

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Sacrum, Coccyx, and Sacroiliac Joints ossification centers in adults is rare but has been reported, and may mimic a fracture line.139 Secondary ossification centers in the articular cartilage of the lateral part of the sacrum are commonly seen appearing between the ages of 12 and 18 years and fusing around 18 to 25 years. They may fail to fuse.140 Unfused ossification centers for the sacral wings superoposteriorly have also been described133

◆◆ Radiology and Common Pathology

Evaluation of the sacrum, sacroiliac joint, and coccyx begins with plain radiographs although often the evaluation is limited by the overlying bowel gas and fecal material, and the tilt of the sacrum.141 Plain radiographs for general survey include ­anteroposterior (AP), inlet, outlet, and lateral views (▶Fig. 19.1, ▶Fig. 19.34). Of note, the AP view of the sacrum and AP view of the coccyx are slightly different with the tube angle superiorly (15 degrees cephalad) for the former and inferiorly (10 degrees caudad) for the latter. The inlet view shows the anterior cortex of the S1 and sacral ala, sacral canal, sacroiliac joint, and anterior margins of the foramina. The outlet view demonstrates most of the sacrum en face including the foramina, depicts the height of the sacral bodies, and shows the sacroiliac joint in long axis.11 The lateral view, which is often the most useful projection when bowel gas and fecal material limit evaluation, shows the anterior surface of the vertebral bodies (▶Fig. 19.1). It also shows the lumbosacral and sacrococcygeal junction as well as the curvature of the coccyx.

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The lateral mass above the projection of the S1 anterior foramen, and the mass between the S1 and S2 anterior foramina projections are the safe zones for dorsal screw placement, while the lateral sacral mass in the region posterior to the synovial portion of the sacroiliac joint is commonly used for iliosacral screw placement.8,9,10 The inlet and outlet radiographs are useful in evaluation of dorsal sacral screw or iliosacral screw placement. A screw violating the anterior foramen can be best appreciated on an outlet radiograph, while the screw penetrating the anterior cortex of the sacrum, sacral canal, or sacroiliac joint can be best detected on an inlet radiograph.11 The projection of the auricular surfaces of the sacroiliac joints varies with the radiographic projection and the inclination of the pelvis. On the AP view, the medial border of the projected joint represented the anterior border of the joint and posterior border of the joint in equal number of cases. The posteroinferior part of the joint projects inferiorly, medial to the other parts of the sacroiliac joint.78 On inlet view, the pelvic brim is seen with the sacroiliac joint projected perpendicular to its long axis, with the most anterior portion of the projected joint representing the middle portion of the joint. So, while the outlet view provides a good overview of alignment of all parts of the joint and vertical relationship of the sacroiliac joint, the inlet view is a good view for diagnosis of subtle anterior sacral diastasis. AP view with cephalad angulation of the tube by 30 to 35 degrees (▶Fig. 19.34b) and oblique views (obtained by raising the side of interest by 25 to 30 degrees away from the table top) (▶Fig. 19.34) of the sacroiliac joints are more specific than AP view of the pelvis for evaluation of the sacroiliac joints, particularly for detection of erosions and joint space narrowing of

Fig. 19.34 (a) Anteroposterior (AP) view of the pelvis showing the sacrum and the sacroiliac joints which are foreshortened due to the sacral tilt. The anterior aspect of the sacroiliac joints (arrow) projects lateral to the posterior aspect (arrowhead). (b) Ferguson view. This is performed with the tube angled 30 to 35 degrees cephalad to better show the sacroiliac joints. This view is similar to outlet view of the pelvis. (c, d) Oblique projections of sacroiliac joints. The left posterior oblique (LPO) view shows the right sacroiliac joint (arrow) in profile and vice versa. Note sharp, well-defined white margin of the subchondral bone plate.

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Fig. 19.35 (a, b) Bilateral sacroiliitis in a 51-year-old woman with ankylosing spondylitis. Note ill-defined and irregular subchondral bone plates (arrows) denoting erosions, particularly on the iliac side. (c, d) Coronal T2 with fat saturation (T2FS) image along the plane of the sacrum (c) and transverse T1 with fat saturation (T1FS) image of the pelvis following intravenous administration of gadolinium contrast (d) showing subchondral edema and enhancement (arrows) secondary to sacroiliitis.

spondyloarthropathies (▶Fig. 19.35). Although in general, the oblique views are tangential to the anterior and the posterior part of the sacroiliac joint, the degree of overlap depends on the angulation of the plane of the sacroiliac joint. Sacroiliac joint intervention is often needed for diagnosis of infection and to evaluate low back pain. For joint aspiration/injection, under fluoroscopy, the most inferior portion of the joint is assessed with the patient in prone position. The nuclear scintigraphy, CT and MRI are the more advanced modalities for evaluation of pathology of the sacrum and the sacroiliac joints. CT and MRI are more sensitive and specific, with CT better for evaluation of calcifications. The common pathology involving the sacrum, the sacroiliac joints, and the coccyx include sacroiliac joint pain, coccydynia, trauma, fracture, neoplasm (frequently metastasis), and infection and inflammation (▶Fig. 19.36, ▶Fig. 19.37, ▶Fig. 19.38). Sacroiliac joint is a common source of chronic nonradicular low back pain. It is commonly secondary to sacroiliac joint dysfunction

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with predisposing factors that include true and apparent leg length discrepancy, older age, inflammatory arthritis, previous spine surgery, pregnancy, and trauma. It is usually a clinical diagnosis. When three or more pain provocation tests are positive it has sensitivity and specificity of 91 and 78%, respectively, with the specificity increasing if the pain is in typical distribution and is not present in the midline.142 Intra-articular injections, under fluoroscopic guidance, may be useful for diagnosis (local anesthetic) and management (corticosteroid injection, phenol ablation). Coccydynia is pain in the coccygeal region without significant radiation, often aggravated by pressure.132 While one-third of coccydynia cases are idiopathic, most are due to sacrococcygeal or intercoccygeal instability.15,50,52,53 Instability may be secondary to trauma, which could be acute such as in a fall or during childbirth, or chronic and repetitive.132 Female gender, obesity, abnormal coccygeal morphology including increased anterior curvature, retroversion, deviation to one side, presence of bony spicule, and coccygeal subluxation predispose to coccydynia.51,143,144,145

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Fig. 19.36 Fused sacroiliac joints. Zoomed anteroposterior (AP) radiograph of the pelvis (a) and axial T1 magnetic resonance (MR) (b) images showing fused bilateral sacroiliac joints in this person with known ankylosing spondylitis.

Fig. 19.37 (a, b) Sacral fracture. Transverse and coronal computed tomography (CT) images of the sacrum showing fracture of the right side of the sacrum with fracture line (arrows) extending to involve the right S1 neural foramen. Note incidental accessory articulation on the left in a. Sacral insufficiency fracture. (c) Axial CT showing vertical band of fractures with sclerosis in the sacral ala bilaterally (arrowheads). (d) Bone scan image (flipped horizontally to match the plain radiograph) showing corresponding increase in delayed phase activity (arrowsheads). The patient also had insufficiency fractures of the right superior and inferior pubic rami with mild activity (not shown).

Coccygeal tumors are rare and include sacrococcygeal teratoma, chordoma, benign notochordal tumor, carcinoid tumor, and metastatic disease (▶Fig. 19.38). The seronegative spondyloarthropathies include the ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and arthritis related to inflammatory bowel disease (▶Fig. 19.35, ▶Fig. 19.36). Conventional radiography is commonly used as initial modality for the evaluation, but CT shows the findings better. However, MRI is the most sensitive test for detection of sacroiliitis showing marrow edema (fluid-sensitive sequences with fat saturation), and cortical erosions and subchondral sclerosis (T1-weighted sequences) (▶Fig. 19.36).

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Congenital lesions include neural tube defects. Open neural tube defects include myeloceles and meningomyeloceles, while closed neural tube defects include the meningocele, myelocystocele, lipomyelomeningoceles, dermal sinus, neuroenteric cyst, and lipoma. Sacral fractures usually occur with other pelvic fractures and are usually seen with high impact trauma such as motor vehicle accidents and fall from height. These fractures (commonly vertical) are difficult to identify and evaluate without a CT scan. Stress fractures, commonly insufficiency fractures and uncommonly fatigue fractures, occur in the sacrum (▶Fig. 19.38). Insufficiency fractures are commonly seen in the elderly women who are usually osteoporotic.

Sacrum, Coccyx, and Sacroiliac Joints

Fig. 19.38 (a, b) Metastasis. Transverse computed tomography (CT) image (a) and corresponding attenuation-corrected positron emission tomography (PET) image (b) showing widespread sclerotic metastasis from breast cancer. Areas of metabolically active regions are seen as areas of brightness on PET imaging. (c, d) Sacral chordoma. Transverse CT image (a) showing mildly expansile lesion of the lower sacrum with bone destruction. On fused positron emission tomography–computed tomography (PET-CT) image, there is increased metabolic activity in the tumor. This was pathologically proven to be a chordoma.

These fractures, often bilateral, are longitudinally oriented within the sacral alae lateral to the foramina.146 MRI and scintigraphy are sensitive for diagnosis, but CT is considered to be more specific.

◆◆ Conclusion

The sacrum, sacroiliac joints, and coccyx are a common source of pain and pathology. Particularly, the sacroiliac joint is a common source of sacroiliac pain, and idiopathic coccydynia is not uncommon. The sacrum and sacroiliac joints are frequently involved in injury, particularly those related to motor vehicular accidents. An understanding of the complex anatomy of the sacroiliac joint and associated structures, their biomechanics, radiological appearance, and knowledge of common pathology is important for the diagnosis and management of these traumatic and nontraumatic conditions.

5. Eichenseer PH, Sybert DR, Cotton JR. A finite element analysis of sacroiliac joint ligaments in response to different loading conditions. Spine 2011;36(22):E1446–E1452 6. Mirkovic S, Abitbol JJ, Steinman J, et al. Anatomic consideration for sacral screw placement. Spine 1991;16(6, Suppl):S289–S294 7. Cecil ML, Rollins JR Jr, Ebraheim NA, Yeasting RA. Projection of the S2 pedicle onto the posterolateral surface of the ilium. A technique for lag screw fixation of sacral fractures or sacroiliac joint dislocations. Spine 1996;21(7):875–878 8. Xu R, Ebraheim NA, Yeasting RA, Wong FY, Jackson WT. Morphometric evaluation of the first sacral vertebra and the projection of its pedicle on the posterior aspect of the sacrum. Spine 1995;20(8):936–940 9. Ebraheim NA, Xu R, Biyani A, Nadaud MC. Morphologic considerations of the first sacral pedicle for iliosacral screw placement. Spine 1997;22(8):841–846 10. Ebraheim NA, Lu J, Yang H, Heck BE, Yeasting RA. Anatomic considerations of the second sacral vertebra and dorsal screw placement. Surg Radiol Anat 1997;19(6):353–357 11. Xu R, Ebraheim NA, Gove NK. Surgical anatomy of the sacrum. Am J Orthop 2008;37(10):E177–E181 12. Abitbol MM. Sacral curvature and supine posture. Am J Phys Anthropol 1989;80(3):379–389

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37. McLauchlan GJ, Gardner DL. Sacral and iliac articular cartilage thickness and

66. Puhakka KB, Melsen F, Jurik AG, Boel LW, Vesterby A, Egund N. MR imaging

cellularity: relationship to subchondral bone end-plate thickness and cancel-

of the normal sacroiliac joint with correlation to histology. Skeletal Radiol

lous bone density. Rheumatology (Oxford) 2002;41(4):375–380 38. Sabry FF, Xu R, Nadim Y, Ebraheim NA. Bone density of the first sacral vertebra in relation to sacral screw placement: a computed tomography study. Orthopedics 2001;24(5):475–477 39. Kampen WU, Tillmann B. Age-related changes in the articular cartilage of human sacroiliac joint. Anat Embryol (Berl) 1998;198(6):505–513

2004;33(1):15–28 67. Alderink GJ. The sacroiliac joint: review of anatomy, mechanics, and function. J Orthop Sports Phys Ther 1991;13(2):71–84 68. Waldrop JT, Ebraheim NA, Yeasting RA, Jackson WT. The location of the sacroiliac joint on the outer table of the posterior ilium. J Orthop Trauma 1993;7(6):510–513

40. Bollow M, Braun J, Kannenberg J, et al. Normal morphology of sacroiliac joints

69. Bakland O, Hansen JH. The “axial sacroiliac joint.” Anat Clin 1984;6(1):29–36

in children: magnetic resonance studies related to age and sex. Skeletal Radiol

70. Weisl H. The articular surfaces of the sacro-iliac joint and their relation to the

1997;26(12):697–704 41. Vleeming A, Schuenke MD, Masi AT, Carreiro JE, Danneels L, Willard FH. The sacroiliac joint: an overview of its anatomy, function and potential clinical implications. J Anat 2012;221(6):537–567 42. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. New York: Elsevier Limited; 2016 43. Tague RG. Fusion of coccyx to sacrum in humans: prevalence, correlates, and effect on pelvic size, with obstetrical and evolutionary implications. Am J Phys Anthropol 2011;145(3):426–437

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for understanding low back pain. Spine 1996;21(5):556–562 47. Phillips S, Mercer S, Bogduk N. Anatomy and biomechanics of quadratus lum-

44. McGrath MC, Zhang M. Lateral branches of dorsal sacral nerve plexus and the long posterior sacroiliac ligament. Surg Radiol Anat 2005;27(4):327–330 45. Ebraheim NA, Lu J, Biyani A, Yeasting RA. Anatomic considerations for posterior approach to the sacroiliac joint. Spine 1996;21(23):2709–2712

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movements of the sacrum. Acta Anat (Basel) 1954;22(1):1–14 71. Cramer GD, Darby SA. Clinical Anatomy of the Spine, Spinal Cord, and ANS. 3rd ed. St. Louis: Elsevier; 2014:xv, 672 p. 72. Diel J, Ortiz O, Losada RA, Price DB, Hayt MW, Katz DS. The sacrum: pathologic spectrum, multimodality imaging, and subspecialty approach. Radiographics 2001;21(1):83–104 73. Mahato NK. Relationship of sacral articular surfaces and gender with occurrence of lumbosacral transitional vertebrae. Spine J 2011;11(10):961–965 74. Nardo L, Alizai H, Virayavanich W, et al. Lumbosacral transitional vertebrae: association with low back pain. Radiology 2012;265(2):497–503 75. Delport EG, Cucuzzella TR, Kim N, Marley J, Pruitt C, Delport AG. Lumbosacral transitional vertebrae: incidence in a consecutive patient series. Pain Physician 2006;9(1):53–56

Sacrum, Coccyx, and Sacroiliac Joints 76. Tague RG. High assimilation of the sacrum in a sample of American skeletons: prevalence, pelvic size, and obstetrical and evolutionary implications. Am J Phys Anthropol 2009;138(4):429–438 77. Mahato NK. Variable positions of the sacral auricular surface: classification and importance. Neurosurg Focus 2010;28(3):E12 78. Ebraheim NA, Mekhail AO, Wiley WF, Jackson WT, Yeasting RA. Radiology of the sacroiliac joint. Spine 1997;22(8):869–876 79. Solonen KA. The sacroiliac joint in the light of anatomical, roentgenological and clinical studies. Acta Orthop Scand Suppl 1957;27:1–127

102. Grob KR, Neuhuber WL, Kissling RO. [Innervation of the sacroiliac joint of the human] [Article in German]. Z Rheumatol 1995;54(2):117–122 103. Fortin JD, Dwyer AP, West S, Pier J. Sacroiliac joint: pain referral maps upon applying a new injection/arthrography technique. Part I: Asymptomatic volunteers. Spine 1994;19(13):1475–1482 104. Fortin JD, Kissling RO, O’Connor BL, Vilensky JA. Sacroiliac joint innervation and pain. Am J Orthop 1999;28(12):687–690 105. Slipman CW, Jackson HB, Lipetz JS, Chan KT, Lenrow D, Vresilovic EJ. Sacroiliac joint pain referral zones. Arch Phys Med Rehabil 2000;81(3):334–338

80. Snijders CJ, Vleeming A, Stoeckart R. Transfer of lumbosacral load to iliac bones

106. Ebraheim NA, Lu J, Biyani A, Yang H. Anatomic considerations of the principal

and legs. Part 2: Loading of the sacroiliac joints when lifting in a stooped pos-

nutrient foramen and artery on internal surface of the ilium. Surg Radiol Anat

ture. Clin Biomech (Bristol, Avon) 1993;8(6):295–301 81. Beal MC. The sacroiliac problem: review of anatomy, mechanics, and diagnosis. J Am Osteopath Assoc 1982;81(10):667–679

1997;19(4):237–239 107. Alla SR, Roberts CS, Ojike NI. Vascular risk reduction during anterior surgical approach sacroiliac joint plating. Injury 2013;44(2):175–177

82. Vleeming A, Stoeckart R, Volkers AC, Snijders CJ. Relation between form and

108. Ebraheim NA, Olexa TA, Xu R, Georgiadis G, Yeasting RA. The quantita-

function in the sacroiliac joint. Part I: Clinical anatomical aspects. Spine

tive anatomy of the superior gluteal artery and its location. Am J Orthop

1990;15(2):130–132 83. Vleeming A, Volkers AC, Snijders CJ, Stoeckart R. Relation between form and function in the sacroiliac joint. Part II: Biomechanical aspects. Spine 1990;15(2):133–136 84. Resnick D, Niwayama G, Goergen TG. Comparison of radiographic abnormalities of the sacroiliac joint in degenerative disease and ankylosing spondylitis. AJR Am J Roentgenol 1977;128(2):189–196 85. Jaovisidha S, Ryu KN, De Maeseneer M, et al. Ventral sacroiliac ligament. Anatomic and pathologic considerations. Invest Radiol 1996;31(8):532–541 86. Steinke H, Hammer N, Slowik V, et al. Novel insights into the sacroiliac joint ligaments. Spine 2010;35(3):257–263

1998;27(6):427–431 109. Xu R, Ebraheim NA, Yeasting RA, Jackson WT. Anatomic considerations for posterior iliac bone harvesting. Spine 1996;21(9):1017–1020 110. Thompson JR, Gibb JS, Genadry R, Burrows L, Lambrou N, Buller JL. Anatomy of pelvic arteries adjacent to the sacrospinous ligament: importance of the coccygeal branch of the inferior gluteal artery. Obstet Gynecol 1999;94(6):973–977 111. Ebraheim NA, Xu R, Farooq A, Yeasting RA. The quantitative anatomy of the iliac vessels and their relation to anterior lumbosacral approach. J Spinal Disord 1996;9(5):414–417 112. Waikakul S, Chandraphak S, Sangthongsil P. Anatomy of L4 to S3 nerve roots. J Orthop Surg (Hong Kong) 2010;18(3):352–355

87. McGrath C, Nicholson H, Hurst P. The long posterior sacroiliac ligament: a his-

113. Atlihan D, Tekdemir I, Ateŝ Y, Elhan A. Anatomy of the anterior sacro-

tological study of morphological relations in the posterior sacroiliac region.

iliac joint with reference to lumbosacral nerves. Clin Orthop Relat Res

Joint Bone Spine 2009;76(1):57–62 88. Bechtel R. Physical characteristics of the axial interosseous ligament of the human sacroiliac joint. Spine J 2001;1(4):255–259

2000;(376):236–241 114. Ebraheim NA, Padanilam TG, Waldrop JT, Yeasting RA. Anatomic consideration in the anterior approach to the sacro-iliac joint. Spine 1994;19(6):721–725

89. Freeman MD, Fox D, Richards T. The superior intracapsular ligament of the

115. Benzon HT, Katz JA, Benzon HA, Iqbal MS. Piriformis syndrome: anatomic

sacroiliac joint: presumptive evidence for confirmation of Illi’s ligament. J

considerations, a new injection technique, and a review of the literature.

Manipulative Physiol Ther 1990;13(7):384–390 90. Pool-Goudzwaard A, Hoek van Dijke G, Mulder P, Spoor C, Snijders C, Stoeckart R. The iliolumbar ligament: its influence on stability of the sacroiliac joint. Clin Biomech (Bristol, Avon) 2003;18(2):99–105 91. Sims JA, Moorman SJ. The role of the iliolumbar ligament in low back pain. Med Hypotheses 1996;46(6):511–515 92. Pool-Goudzwaard AL, Kleinrensink GJ, Snijders CJ, Entius C, Stoeckart R. The sacroiliac part of the iliolumbar ligament. J Anat 2001;199(Pt 4):457–463 93. Leong JC, Luk KD, Chow DH, Woo CW. The biomechanical functions of the iliolumbar ligament in maintaining stability of the lumbosacral junction. Spine 1987;12(7):669–674 94. Yamamoto I, Panjabi MM, Oxland TR, Crisco JJ. The role of the iliolumbar ligament in the lumbosacral junction. Spine 1990;15(11):1138–1141 95. Loukas M, Louis RG Jr, Van der Wall B, et al. Iliolumbar membrane, a newly recognised structure in the back. Folia Morphol (Warsz) 2006;65(1):15–21 96. Vleeming A, Stoeckart R, Snijders CJ. The sacrotuberous ligament: a conceptual approach to its dynamic role in stabilizing the sacroiliac joint. Clin Biomech (Bristol, Avon) 1989;4(4):201–203 97. Vilensky JA, O’Connor BL, Fortin JD, et al. Histologic analysis of neural elements in the human sacroiliac joint. Spine 2002;27(11):1202–1207

Anesthesiology 2003;98(6):1442–1448 116. Lanzieri CF, Hilal SK. Computed tomography of the sacral plexus and sciatic nerve in the greater sciatic foramen. AJR Am J Roentgenol 1984;143(1):165–168 117. Roshanravan SM, Wieslander CK, Schaffer JI, Corton MM. Neurovascular anatomy of the sacrospinous ligament region in female cadavers: implications in sacrospinous ligament fixation. Am J Obstet Gynecol 2007;197(6):660. e1–660.e6 118. Willard F, Carreiro J, Manko W. The long posterior interosseous ligament and the sacrococcygeal plexus. In Third Interdisciplinary World Congress on Low Back and Pelvic Pain. Vienna, Austria: 1998. 119. McGaugh JM, Brismée JM, Dedrick GS, Jones EA, Sizer PS. Comparing the anatomical consistency of the posterior superior iliac spine to the iliac crest as reference landmarks for the lumbopelvic spine: a retrospective radiological study. Clin Anat 2007;20(7):819–825 120. McGrath MC, Stringer MD. Bony landmarks in the sacral region: the posterior superior iliac spine and the second dorsal sacral foramina: a potential guide for sonography. Surg Radiol Anat 2011;33(3):279–286 121. Chakraverty R, Dias R. Audit of conservative management of chronic low back pain in a secondary care setting—part I: facet joint and sacroiliac joint interventions. Acupunct Med 2004;22(4):207–213

98. Szadek KM, Hoogland PV, Zuurmond WW, de Lange JJ, Perez RS. Nociceptive

122. Yin W, Willard F, Carreiro J, Dreyfuss P. Sensory stimulation-guided sacroiliac

nerve fibers in the sacroiliac joint in humans. Reg Anesth Pain Med

joint radiofrequency neurotomy: technique based on neuroanatomy of the

2008;33(1):36–43 99. Szadek KM, Hoogland PV, Zuurmond WW, De Lange JJ, Perez RS. Possible nociceptive structures in the sacroiliac joint cartilage: an immunohistochemical study. Clin Anat 2010;23(2):192–198 100. Nakagawa T. [Study on the distribution of nerve filaments over the iliosacral joint and its adjacent region in the Japanese] [Article in Japanese]. Nihon Seikeigeka Gakkai Zasshi 1966;40(4):419–430 101. Ikeda R. [Innervation of the sacroiliac joint. Macroscopical and histological

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studies] [Article in Japanese]. Nihon Ika Daigaku Zasshi 1991;58(5):587–596

dorsal sacral plexus. Spine 2003;28(20):2419–2425 123. Sturesson B, Selvik G, Udén A. Movements of the sacroiliac joints. A roentgen stereophotogrammetric analysis. Spine 1989;14(2):162–165 124. Bussey MD, Bell ML, Milosavljevic S. The influence of hip abduction and external rotation on sacroiliac motion. Man Ther 2009;14(5):520–525 125. Walker JM. The sacroiliac joint: a critical review. Phys Ther 1992;72(12): 903–916

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126. Sturesson B, Udén A, Onsten I. Can an external frame fixation reduce the movements in the sacroiliac joint? A radiostereometric analysis of 10 patients. Acta Orthop Scand 1999;70(1):42–46

693

Sacrum, Coccyx, and Sacroiliac Joints 127. Tullberg T, Blomberg S, Branth B, Johnsson R. Manipulation does not alter the position of the sacroiliac joint. A roentgen stereophotogrammetric analysis. Spine 1998;23(10):1124–1128, discussion 1129 128. Rahl MD. Anatomy and biomechanics. In: Dall EB, Eden VS, Rahl DM, eds. Surgery for the Painful, Dysfunctional Sacroiliac Joint: A Clinical Guide. Cham: Springer International Publishing; 2015:15–35

137. Fortin JD, Ballard KE. The frequency of accessory sacroiliac joints. Clin Anat 2009;22(8):876–877 138. Hadley LA. Accessory sacroiliac articulations with arthritic changes. Radiology 1950;55(3):403–409 139. Green BN, Schultz G, Stanley M. Persistent synchondrosis of a primary sacral ossification center in an adult with low back pain. Spine J 2008;8(6):1037–1041

129. Egund N, Olsson TH, Schmid H, Selvik G. Movements in the sacroiliac joints

140. Götz W, Funke M, Fischer G, Grabbe E, Herken R. Epiphysial ossification cen-

demonstrated with roentgen stereophotogrammetry. Acta Radiol Diagn

tres in iliosacral joints: anatomy and computed tomography. Surg Radiol Anat

(Stockh) 1978;19(5):833–846 130. Wilder DG, Pope MH, Frymoyer JW. The functional topography of the sacroiliac joint. Spine 1980;5(6):575–579 131. Lavignolle B, Vital JM, Senegas J, et al. An approach to the functional anatomy of the sacroiliac joints in vivo. Anat Clin 1983;5(3):169–176 132. Nathan ST, Fisher BE, Roberts CS. Coccydynia: a review of pathoanatomy, aetiology, treatment and outcome. J Bone Joint Surg Br 2010;92(12):1622–1627 133. Prassopoulos PK, Faflia CP, Voloudaki AE, Gourtsoyiannis NC. Sacroiliac joints: anatomical variants on CT. J Comput Assist Tomogr 1999;23(2):323–327 134. Ehara S, el-Khoury GY, Bergman RA. The accessory sacroiliac joint: a common anatomic variant. AJR Am J Roentgenol 1988;150(4):857–859 135. Wu L-P, Li YK, Li YM, Zhang YQ, Zhong SZ. Variable morphology of the sacrum in a Chinese population. Clin Anat 2009;22(5):619–626 136. Trotter M. Accessory sacro-iliac articulations. Am J Phys Anthropol 1937;22(2):247–261

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1993;15(2):131–137 141. Disler DG, Miklic D. Imaging findings in tumors of the sacrum. AJR Am J Roentgenol 1999;173(6):1699–1706 142. Laslett M. Evidence-based diagnosis and treatment of the painful sacroiliac joint. J Manual Manip Ther 2008;16(3):142–152 143. Karadimas EJ, Trypsiannis G, Giannoudis PV. Surgical treatment of coccygodynia: an analytic review of the literature. Eur Spine J 2011;20(5):698–705 144. Woon JT, Maigne JY, Perumal V, Stringer MD. Magnetic resonance imaging morphology and morphometry of the coccyx in coccydynia. Spine 2013;38(23):E1437–E1445 145. Maigne JY, Lagauche D, Doursounian L. Instability of the coccyx in coccydynia. J Bone Joint Surg Br 2000;82(7):1038–1041 146. Johnson AW, Weiss CB Jr, Stento K, Wheeler DL. Stress fractures of the sacrum. An atypical cause of low back pain in the female athlete. Am J Sports Med 2001;29(4):498–508

20  Hip George R. Matcuk Jr.

◆◆ Introduction

The hip or femoroacetabular joint is functionally and structurally complex, comprised of bone, cartilage, labrum, ligaments, synovial-lined capsule, and supporting soft tissues and muscles.1,2,3,4,5,6,7,8,9,10 It is the primary link between the trunk and lower limb and plays a crucial biomechanical role in standing, gait, and athletic activities.11,12,13 Evaluation of the patient with hip pain begins with a thorough clinical examination. Pain that localizes clinically to the hip may arise due to problems originating in the back or knee, so evaluation of these regions should also be considered. The first-line imaging evaluation of the hip remains radiography, with an anteroposterior (AP) view of the pelvis and AP and frog-leg lateral views of the affected hip routinely obtained. Dual-energy x-ray absorptiometry (DEXA) scans can be used to quantitatively measure bone mineral density to assess for osteopenia or osteoporosis. Computed tomography (CT) is usually obtained in the setting of trauma, particularly to better evaluate fracture patterns and extent. Ultrasound (US) has the advantage of providing a real-time dynamic examination of the hip without ionizing radiation, but can be technically challenging and user-dependent and can only assess the surface of the osseous structures. Magnetic resonance imaging (MRI) is the current imaging gold standard for evaluation of the hip, offering assessment of the bone marrow, cartilage, acetabular labrum, and soft tissues. Arthrography with the injection of a dilute gadolinium solution prior to the MRI (MR arthrography) can provide a more sensitive evaluation of the hip, particularly the acetabular labrum, cartilaginous surfaces, and capsular structures

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(see ▶Fig. 20.1, ▶Fig. 20.2, ▶Fig. 20.3 showing MR arthrogram views of the hip). MR arthrography demonstrates a sensitivity of 69 to 81% for labral tears compared to 50% for conventional MRI, and MR arthrography shows a sensitivity of 71 to 92% for cartilage defects compared to 58 to 83% for conventional MRI.14 Nuclear medicine studies such as technetium-99m-methylene-­disphosphonate (99mTc-MDP) or sodium 18F-fluoride (18F-NaF) bone scans or 18F-fluorodeoxyglucose (18F-FDG) PET or positron emission tomography–computed tomography (PET/CT) scans can provide additional information regarding metastatic disease or primary bone tumors. Musculoskeletal disorders affecting the hip are common and can be a major source of morbidity and disability. Hip osteoarthritis (OA) increases consistently with age with a global prevalence of 0.85%.15 Inflammatory arthritides, such as rheumatoid arthritis, and deposition disease, such as gout, may also affect the hip. Osteoporosis also places large number of individuals (particularly females and the elderly) at risk for fragility fractures of the hip with an estimated incidence of 2% of people over the age of 50 years, and with the worldwide incidence expected to double by 2040.16,17 Femoroacetabular impingement (FAI) has also become a well-­ recognized clinical entity affecting up to 10 to 15% of young adults with an increased risk for early-onset osteoarthrosis.18,19,20 The identification of imaging features concerning for FAI and the development of arthroscopic surgical techniques for minimally-invasive treatment of FAI have helped spur renewed interest in hip anatomy, imaging techniques, and identification of normal variants. This chapter will review the essential anatomy of the hip joint with imaging examples and discuss normal measurements

Fig. 20.1 Axial oblique hip: From superior to inferior. AIIS, anterior inferior iliac spine; IBILFL, inferior band of the iliofemoral ligament; IBISFL, inferior band of the ischiofemoral ligament; IG, inferior gemellus; OE, obturator externus; OI, obturator internus; PFL, pubofemoral ligament; QF, quadratus femoris; SBILFL, superior band of the iliofemoral ligament; SBISFL, superior band of the ischiofemoral ligament; SG, superior gemellus; ZO, zona orbicularis.

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(Continued)

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Fig. 20.1 (Continued) (Continued)

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Fig. 20.1 (Continued) (Continued)

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Fig. 20.1 (Continued) (Continued)

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Fig. 20.1 (Continued)

Fig. 20.2 Coronal hip: Anterior to posterior. AIIS, anterior inferior iliac spine; IBILFL, inferior band of the iliofemoral ligament; IBISFL, inferior band of the ischiofemoral ligament; IG, inferior gemellus; OE, obturator externus; OI, obturator internus; PFL, pubofemoral ligament; QF, quadratus femoris; SBILFL, superior band of the iliofemoral ligament; SBISFL, superior band of the ischiofemoral ligament; SG, superior gemellus; ZO, zona orbicularis. (Continued)

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Fig. 20.2 (Continued) (Continued)

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Fig. 20.2 (Continued) (Continued)

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Fig. 20.2 (Continued) (Continued)

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Fig. 20.2 (Continued)

Fig. 20.3 Sagittal hip: Lateral to medial order. AB, adductor brevis; AIIS, anterior inferior iliac spine; AL, adductor longus; AM, adductor magnus; IBILFL, inferior band of the iliofemoral ligament; IBISFL, inferior band of the ischiofemoral ligament; IC, iliocapsularis; IG, inferior gemellus; OE, obturator externus; OI, obturator internus; PFL, pubofemoral ligament; QF, quadratus femoris; SBILFL, superior band of the iliofemoral ligament; SBISFL, superior band of the ischiofemoral ligament; SG, superior gemellus; ZO, zona orbicularis. (Continued)

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Fig. 20.3 (Continued) (Continued)

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Fig. 20.3 (Continued) (Continued)

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Fig. 20.3 (Continued) (Continued)

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Fig. 20.3 (Continued)

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Hip and relationships, variants, and potential pitfalls. ▶Fig. 20.1, ▶Fig. 20.2, and ▶Fig. 20.3 show MR arthrography views of a normal hip joint.

◆◆ Osseous Anatomy

The osseous anatomy of the lower extremity, including the pelvis and hip, is discussed in Chapter 8 “Lower Extremity Bones.” However, the relevant anatomy to the hip and normal relationships and measurements will be reviewed here. The hip is a balland-socket joint, with the femoral head articulating with the acetabulum of the pelvis. The hip joint capsule extends from the bony margins of the acetabulum to the base of the femoral neck.

Acetabulum The acetabulum is formed by the junction of the ilium, ischium, and pubis, with the surface forming approximately two-thirds of a sphere (incomplete at the inferior aspect) to accommodate the femoral head3,21,22 (▶Fig. 20.4, ▶Fig. 20.5). The acetabulum can

also be subdivided into anterior and posterior walls (or rims) laterally (the portions covering the femoral head), and anterior and posterior columns (the portions not covering the femoral head) medially23,24,25 (▶Fig. 20.6). The superior portion of the acetabulum is also referred to as the acetabular roof (or superior rim), and the thin medial portion between the more robust portions of the anterior and posterior columns is sometimes referred to as the medial wall. The open portion of the acetabulum inferiorly is the acetabular notch (▶Fig. 20.7). The extent and pattern of involvement of these portions of the acetabulum form the basis for the Judet–Letournel classification of acetabular fractures.26,27,28 On AP view of the pelvis or hip, there are several radiographic lines that serve as important osseous landmarks (▶Fig. 20.8). The iliopectineal line (also referred to as the iliopubic line) is a continuous arcuate line formed by the superomedial border of the superior pubic ramus and anterior column of the acetabulum continuing superiorly along the medial border of the ilium, including the iliopectineal eminence.29,30,31 A line extending from the medial border of the ischium superiorly along the posterior column of the acetabulum and medial border of the iliac wing is

Fig. 20.4 Anteroposterior (AP) radiograph of the right hip demonstrating how the acetabulum is formed by contributions from the ilium (red), ischium (yellow), and pubis (blue).

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Fig. 20.5 Volume rendered computed tomography (CT) images of hemipelvis demonstrating how the acetabulum is formed by contributions from the ilium (red), ischium (yellow), and pubis (blue). Views from a 6-year-old pelvis showing the three components of the hemipelvis before fusion.

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Hip

Fig. 20.6 Axial computed tomography (CT) image through the acetabula demonstrating differentiation of the anterior columns (blue) and walls (red) from the posterior columns (yellow) and walls (green) based on a line through the midpoint of medial walls (dotted line) and lines abutting the medial aspect of the femoral heads and parallel to the medial wall (black lines).

Fig. 20.7 Surface-shaded, three-dimensional computed tomography (CT) reconstruction of the hemipelvis showing key osseous and anatomic landmarks.

Fig. 20.8 Anteroposterior (AP) radiograph of the right hip demonstrating important pelvic lines: iliopectineal (or iliopubic) line (yellow), ilioischial line (red), acetabular teardrop (black), obturator ring (white), superior acetabular rim (blue), anterior acetabular rim (green), and posterior acetabular rim (orange).

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Hip the ilioischial line. The obturator ring is formed by pubis medially, superior pubic ramus superiorly, inferior pubic ramus inferiorly, and ischium laterally. The anterior and posterior rims of the acetabulum are seen as smooth arc-shaped lines representing the lateral margins of the acetabulum, with the anterior rim the more medial of the two, except in cases of acetabular retroversion. The teardrop is seen along the medial aspect of the acetabulum and represents the summation of shadows of the medial acetabular wall.32 The teardrop distance, measured from the lateral edge of the teardrop shadow to the medial aspect of the femoral head, can be useful for evaluation for hip joint effusion or hip dysplasia in the pediatric patient, particularly in comparison to the opposite side. Tubular tracking of contrast can be seen within the acetabular fossa on MR arthrography in 16% of hips.33 This is hypothesized to be due to pumping of joint fluid through nutrient foramina at the margin of the acetabular fossa, more commonly in the posterior fossa than anterior. Radiographs also demonstrate several fat planes or stripes that can be useful when evaluating potential abnormalities29,34 (▶Fig. 20.9). Laterally, the gluteal fat stripe represents normal fat between the gluteus medius and minimus tendons. There is also a more subtle capsular fat stripe between the gluteus minimus tendon and ischiofemoral ligament, paralleling the superior aspect of the femoral neck. Both can bulge superiorly with a hip effusion. The iliopsoas fat stripe represents normal fat between the iliopsoas tendon and medial femoral neck and the obturator internus fat stripe is medial to the obturator muscle paralleling the iliopectineal line. Both can be displaced by a fracture, hematoma, or mass. The superior acetabulum usually covers approximately 75% of the femoral head, with undercoverage seen in developmental dysplasia of the hip and overcoverage seen in pincer-type FAI.4 There are numerous measurements that can be performed to assess acetabular coverage, but the most commonly applied are the lateral center-edge (LCE) angle of Wiberg, femoral head extrusion index (FHEI), and acetabular depth. The LCE angle is measured on AP radiographs as the angle between a line passing through the

center of the femoral head perpendicular to the pelvic horizontal (interischial) line and a second line from the center of the femoral head to the lateral margin of the acetabulum35 (▶Fig. 20.10a). An angle of 25 to 40 degrees is considered normal, while 20 to 25 degrees is considered borderline. FHEI is the length of the femoral head extending beyond the lateral margin of the acetabulum (A) divided by the horizontal width of the femoral head (B) × 100, with