Imaging of the Knee: Techniques and Applications [2 ed.] 303129730X, 9783031297304

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Medical Radiology · Diagnostic Imaging Series Editors: Hans-Ulrich Kauczor · Paul M. Parizel · Wilfred C.G. Peh

Mark Davies Steven James Rajesh Botchu   Editors

Imaging of the Knee Techniques and Applications Second Edition

Medical Radiology

Diagnostic Imaging Series Editors Hans-Ulrich Kauczor Paul M. Parizel Wilfred C. G. Peh

The book series Medical Radiology  – Diagnostic Imaging provides accurate and up-to-date overviews about the latest advances in the rapidly evolving field of diagnostic imaging and interventional radiology. Each volume is conceived as a practical and clinically useful reference book and is developed under the direction of experienced editors, who are world-renowned specialists in the field. Book chapters are written by expert authors in the field and are richly illustrated with high quality figures, tables and graphs. Editors and authors are committed to provide detailed and coherent information in a readily accessible and easy-to-understand format, directly applicable to daily practice. Medical Radiology  – Diagnostic Imaging covers all organ systems and addresses all modern imaging techniques and image-guided treatment modalities, as well as hot topics in management, workflow, and quality and safety issues in radiology and imaging. The judicious choice of relevant topics, the careful selection of expert editors and authors, and the emphasis on providing practically useful information, contribute to the wide appeal and ongoing success of the series. The series is indexed in Scopus.

Mark Davies  •  Steven James Rajesh Botchu Editors

Imaging of the Knee Techniques and Applications Second Edition

Editors Mark Davies Department of Musculoskeletal Radiology Royal Orthopaedic Hospital Birmingham, UK

Steven James Department of Musculoskeletal Radiology Royal Orthopaedic Hospital Birmingham, UK

Rajesh Botchu Department of Musculoskeletal Radiology Royal Orthopaedic Hospital Birmingham, UK

ISSN 0942-5373     ISSN 2197-4187 (electronic) Medical Radiology ISSN 2731-4677     ISSN 2731-4685 (electronic) Diagnostic Imaging ISBN 978-3-031-29730-4    ISBN 978-3-031-29731-1 (eBook) https://doi.org/10.1007/978-3-031-29731-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2003, 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Musculoskeletal imaging continues to evolve since the first edition of this book published 20 years ago. The knee is arguably one of the commonest anatomical sites imaged by orthopedic surgeons, rheumatologists, and other musculoskeletal clinicians. It is necessary to continually update the knowledge of all working in this field. As before, the book takes a dual approach with the first section dealing with the full range of techniques available for imaging knee pathologies including contribution on radiographs, ultrasound, computed tomography, and MR imaging. The second section comprising 12 chapters discusses the optimal application of these techniques to specific pathologies highlighting practical solutions to everyday clinical problem. The editors are grateful to the international panel of authors for their contribution to this second edition that aims to provide a comprehensive overview of current imaging of the knee.   

Mark Davies Steven James Rajesh Botchu

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Contents

Part I Imaging Techniques Radiography ��������������������������������������������������������������������������������������������   3 Yaron Berkowitz and Oliver Czarnecki  Computed Tomography (CT) and CT Arthrography��������������������������  29 Nuttaya Pattamapaspong and Wilfred C. G. Peh Magnetic Resonance Imaging ����������������������������������������������������������������  65 Manickam Subramanian, Michael S. M. Chin, and Wilfred C. G. Peh Ultrasound������������������������������������������������������������������������������������������������ 109 Stefano Bianchi, Viviane Créteur, Antoine Moraux, and Giorgio Tamborrini Part II Clinical Applications The Pediatric Knee���������������������������������������������������������������������������������� 141 Timothy Shao Ern Tan and Eu-Leong Harvey James Teo The Knee: Bone Trauma ������������������������������������������������������������������������ 171 Simranjeet Kaur, Prudencia N. M. Tyrrell, and Victor N. Cassar-Pullicino Stress Injuries������������������������������������������������������������������������������������������ 201 Jennifer Murphy, Emily Smith, Steven L. James, and Rajesh Botchu The Knee: The Menisci���������������������������������������������������������������������������� 215 Hema N. Choudur and Samir M. Paruthikunnan  The Cruciate and Collateral Ligaments������������������������������������������������ 239 Nikola Tomanovic and Andoni P. Toms Postoperative Meniscus �������������������������������������������������������������������������� 275 Haron Obaid  The Postoperative Knee: Cruciate and Other Ligaments�������������������� 293 Yildiz Sengul, Kurt P. Spindler, and Carl S. Winalski

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The Postoperative Knee: Arthroplasty, Arthrodesis, Osteotomy ������������������������������������������������������������������������������������������������ 353 Winnie A. Mar, Joseph Albert Karam, Michael D. Miller, and Mihra S. Taljanovic  Patellar and Quadriceps Mechanism: Clinical, Imaging, and Surgical Considerations ������������������������������������������������������������������ 381 Breann K. Tisano, Jay P. Shah, and Avneesh Chhabra Infection���������������������������������������������������������������������������������������������������� 407 James Francis Griffith and Margaret Ip Arthritis���������������������������������������������������������������������������������������������������� 427 Holly W. Christopher, Emma Rowbotham, and Andrew J. Grainger Tumors and Tumorlike Lesions�������������������������������������������������������������� 459 Anish Patel, A. Mark Davies, and Daniel Vanel

Contents

Part I Imaging Techniques

Radiography Yaron Berkowitz and Oliver Czarnecki

1 Introduction

Contents 1    Introduction 

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2    Radiographic Techniques 

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3    Radiographic Projections  3.1  “Standard” Series  3.2  Trauma Series  3.3  Degenerative Disease 

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4    Radiographic Measurement Techniques  4.1  Assessment of Alignment  4.2  QPR or SKI  4.3  Roentgen Stereophotogrammetric Analysis  4.4  Femoral Condyle Configuration  4.5  Patellar Position 

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References 

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Y. Berkowitz (*) Department of Radiology, Nuffield Orthopaedic Centre, Oxford University Hospitals NHS Trust, Oxford, UK e-mail: [email protected] O. Czarnecki Department of Radiology, Oxford University Hospitals NHS Trust, Oxford, UK

The knee joint is one of the most vital joints in human bipedal mobility and is affected by a wide range of pathologies from trauma to inflammatory etiologies. Trauma can be chronic and repetitive low impact, acute high impact, or positional such as in twisting or pivot injuries. It occurs in older people with low bone mineral density, as well as younger populations partaking in recreational or professional sports activities, and the presentations vary based on the type of trauma and quality of the bone. Degenerative changes (osteoarthritis, OA) may develop with increasing age and a lifetime of repetitive microtrauma but may also be seen in younger people as a sequela of high-impact trauma to the cartilage or secondary to improper joint motion and dynamics after cruciate or collateral ligament damage leading to instability, for example. In this group of patients, premature OA often occurs many years prior to the would-be natural onset. The knee joint is often affected by arthropathies, both septic and nonseptic (inflammatory or crystal arthropathies for example). Rarer conditions may also present with pain in the knee, and osteosarcomas in children and adolescents are most common around the knee.

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_367, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 21 April 2023

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In all these conditions, radiographic examination to assess the joint and the bones should be the first step in the diagnostic workup, and in the vast majority of cases, it is sufficient for diagnosis and to guide management.

2 Radiographic Techniques Three different techniques are used to obtain a radiograph: the conventional analogue technique, computed radiography, and digital radiography. In the analogue technique, a cassette with film and intensifying screens is used. This technique is well known and used all over the world and is not discussed in detail here. In computed radiography, a cassette with an image storage phosphor plate (image plate, IP) is used. This cassette is of the same size as a conventional film cassette and is used in the same way during the examination. Energy proportional to the amount of radiation is stored in the IP when electrons excited by the X-ray photons become trapped at higher energy levels in the phosphor crystals. The stored image is read out in an IP reader where the IP is scanned with a laser beam, which allows the trapped electrons to return to a lower energy level. The light emitted during this transition is converted into a digital image, which is then processed according to the examination selected. The image processing is programmable for each examination and must be done with care in order to avoid image artifacts and to optimize visualization of the pathology. Image artifacts can appear as a radiolucent zone at high-contrast steps, for instance at the interface between metal, cement, and bone in a joint prosthesis. The image processing is automatically adjusted to give the desired image density regardless of the radiation dose within a large range. The advantage of computed radiography is the broad exposure range and the free choice of data processing. The dynamic range is more than 10,000:1, which is more than 100 times that of analogue film-screen systems. This reduces the need for retakes, reducing the dose burden to the patient due to faulty exposure factors. The radiation dose to the patient can be decreased without affecting

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the diagnostic accuracy, especially when measurements of lines and angles are needed (Jónsson et al. 1996; Sanfridsson et al. 1998). With the analogue technique, it is possible to overexpose a film, rendering the pathology less obvious. The broad exposure range with CR may, however, be a disadvantage. A heavily overexposed picture does not show on the image obtained and appears as a normally exposed film. This is a threat, because the patient may receive a heavy radiation dose, which is not easily detected. This can be checked by looking at the sensitivity value (S-value) that is available for every image. The S-value can be used for selection of the correct exposure settings on the X-ray generator. A high S-value corresponds to a low dose, and a low S-value indicates a high exposure. The S-value is related to the speed of a screenfilm system, and it indicates which speed a screen-film system should have to produce the correct density on the film. Images with a low S-value have low noise, while those with a high S-value are grainier due to high noise. The image can be transferred to a laser image printer for film output, although it is more usually uploaded to a picture archiving and communication system (PACS), where the image is reviewed on a workstation and stored in a digital archive. It is possible to perform a complete reprocessing of the images if the IP reader transfers the image raw data to a dedicated workstation. The reprocessing capabilities can also be incorporated in the IP reader in some models. A PACS workstation allows image processing (windowing, zoom, image rotation, angle measurements, CAD overlays, etc.) depending on the software. The free choice of data processing means that from a single exposure, it is possible to obtain the image that is best suited for detecting and displaying the pathology looked for in each individual case. In knee examinations, this is advantageous for the diagnosis of suprapatellar pouch distension (Fig. 1) and flake fragments (Fig. 2) and for the disclosure of small intra-articular calcifications, for example. It is possible to process the images with a higher degree of edge enhancement to increase the degree of contrast for the desired spatial resolution. This can produce adjacent near-identical images on a single film or on PACS by processing

Radiography

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Fig. 1 (a, b) The suprapatellar pouch. (a) Normal; CR technique. (b) Suprapatellar effusion

the images independently: one “conventional” and the other with “edge enhancement.” This is most commonly applied in chest radiography assessing for line positioning but may be helpful in knee imaging for fracture fragments. Computed radiography has a lower spatial resolution than conventional radiography, but this has not proven to be a limiting factor with modern systems (Jónsson et al. 1995; Scott Jr et al. 1993). Digital radiography is performed making use of a flat panel detector. The size of the detectors varies and is approximately the size of a large cassette (43  ×  43  cm). The detector converts the X-ray image into a digital signal, which is transferred to a computer for image processing and then to PACS or less commonly a laser printer. The detector is usually mounted on a stand, and some stands allow the detector to be moved for different X-ray projections. There are also portable detectors available for mobile radiography, removing the need

for fixed systems and allowing for easier positioning of patients in obtaining different views. This system has the advantage of providing an immediate digital readout of the image without needing to transport the cassette to a separate readout unit. The conversion of X-ray photons into an electrical signal can be done with two different processes, either an indirect or a direct process. Flat panels using an indirect process convert the X-ray photons into light, with a scintillator, often a layer of cesium iodide (CsI) or gadolinium oxysulfide (GOS or Gadox), covering the entrance side of the detector. The light is detected with an amorphous silicon photodetector ­ (photodiode) array placed against the scintillator, converted into an electrical signal, read out with a thin-film transistor array, and converted into a digital signal. Each photodiode represents a pixel. Flat panels using a direct process convert the X-ray photons into an electrical charge in an

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Fig. 2 (a) Horizontal lateral view of a knee with a single fluid-fluid level (arrowhead). No obvious fracture is seen. (b) AP view of the same knee. A flake fragment is seen (arrow)

amorphous selenium layer. A bias voltage is applied between a surface electrode on the detector and an electrode array below the selenium layer in order to collect the electrical charge. The charge on the electrode array is read out with a thin-film transistor array and converted into a digital signal. Each electrode in the electrode array represents a pixel. The advantage of the indirect process is the possibility of achieving a high detective quantum efficiency because of the high X-ray absorption of

CsI. The advantage of the direct process is the possibility of obtaining a higher resolution because the X-ray photons are directly converted into electrical charge without the conversion to light, which might get scattered, thereby reducing resolution. The characteristics of flat panel detectors continue to change owing to new technology, and digital radiography is thought to be the future of radiographic imaging by many (Körner et al. 2007). The pixel size and resolution are generally accepted as being approximately 150 μm, although some manufacturers claim resolutions as high as 50  μm although this is yet to be validated. It must be stressed that radiography, analogue or digital, is excellent for the evaluation of bone detail, whereas the soft tissues are poorly visualized. A large fluid effusion of the knee is seen as a distension of the suprapatellar pouch (Fig. 1), but the radiographic appearances are the same whether the fluid is blood in hemarthrosis, effusion in active arthritis, or pus in septic arthritis. Conventional tomography is used less frequently since the advent of computed tomography (CT) with multiplanar reconstructions, but as a technique, it may be used with analogue, computed, or digital radiography and can still be a useful tool when CT is unavailable. The conventional X-ray tube and the film-screen combination (or image plate) are used to define a predetermined plane in the body, while the structures above and below this plane are eliminated or blurred. This is achieved by moving the X-ray tube and the filmscreen combination in a defined manner in relation to each other, while the examined part of the body remains stationary. The motion of the X-ray tube and the film may be either unidirectional (linear tomography) or pluridirectional (circular, elliptical, spiral, or hypocycloidal tomography). The more complex the movement, the better the quality of the image with less longitudinal streaking, which may be seen in unidirectional tomography. The disadvantage with tomography is the long examination time and the high radiation exposure to the patient. Conventional tomography is most useful in demonstrating the degree of depression of a tibial plateau fracture (Fig. 3), to assess premature growth plate fusion, to evaluate healing of bony pseudarthrosis, or to identify sequestra in chronic osteomyelitis.

Radiography

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Fig. 3 (a) AP view of the knee after trauma. There appears to be compression of the lateral tibial condyle (arrow). (b) Conventional hypocycloidal tomography in the AP projection. There is moderate depression of a fragment from the

lateral tibial condyle. (c) Conventional hypocycloidal tomography in the lateral projection. The fragment is located posteriorly. With tomography, it is possible to outline the size and degree of depression of a fragment

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3 Radiographic Projections The technique and radiographic projections in the examination of the knee depend on the clinical indications as alluded to later. A large number of textbooks on radiographic positioning have been published over the years, for instance Ballinger (1991), Bernau (1995), Bontrager (1997), Pavlov et  al. (1999), and Whitley et  al. (2015). It is important to use projections that are easily reproducible for both speed and accurate comparison with future studies. Different techniques are possible, and choice may depend on the available equipment and different views on the clinical scenario. The minimum views are anteroposterior (AP) and lateral views. These radiographs may be taken with the patient supine (usually following trauma) or upright and weight bearing. The AP is usually with the knee straight, and the lateral with the knee slightly flexed. A tunnel view (also known as “einblick” view) can be taken with knee flexion of 40°–50° and with the X-ray beam angulated to the same degree to visualize the intercondylar notch and tibial spines; ideally, the beam will be tangential to the tibial plateau. This is useful for visualization of loose osseous bodies in this region. In the evaluation of the joint space and OA, radiographs should be taken with the patient weight bearing and with the knee in slight flexion. Rosenberg et al. (1988) described a PA view where the knee is flexed to 45°, which is more sensitive for detection of joint space narrowing. The protocol employed locally is to perform a weight-bearing AP view with straight legs, bearing weight on both legs to show physiologic functional alignment, a Rosenberg view, and a lateral view. Prior to a total knee arthroplasty with previously established OA, Rosenberg and lateral views are all that is needed. With DR, the lateral view is obtained after fluoroscopic monitoring or test views so that the dorsal aspects of the femoral condyles are superimposed (Fig. 4c). The AP view is obtained with the X-ray beam centered over the joint space and tangential to the tibia plateau, with the tibial spines centered in the femoral notch. Boegård et al. (1997) described a similar technique with the PA (posteroanterior) projection, where the patella and

Y. Berkowitz and O. Czarnecki

the big toe touch the upright examination table. To obtain reproducible projections, the medial border of the foot is parallel with the X-ray beam. Other protocols for examination of the weight-­ bearing knee have been reported, and some are described here. Different institutes use different systems, but the key is for the examination to be reproducible and therefore comparable for multiple examinations taken now and in the future. Buckland-Wright (1995) designed a protocol where the patient flexes the knee until the tibial plateau is horizontal and parallel to the central X-ray beam. This requires a knee flexion of 11°– 20°, depending on the inclination of the tibial plateau. The position is checked with fluoroscopy. The foot is rotated internally or externally until the tibial spines appear centrally placed relative to the femoral notch. The position of the foot is drawn and recorded on a piece of paper placed in a defined position on the floor. Reproducible views at follow-up examinations are then possible using the same position of the foot. In another protocol for flexed PA knee examination, the patient is positioned with the patella and the hip in contact with the surface of the upright examination table, with the feet pointing straight ahead vertically relative to the knee, and with the knee flexion of around 30°, the “schuss view” (Piperno et  al. 1998). Using fluoroscopy, the X-ray beam is adjusted to be tangential with the tibial plateau. One group has developed a technique for AP knee examination in extension (Ravaud et  al. 1996). The patient stands distributing his or her weight on both feet. The posterior aspect of the knee is placed as close to the X-ray film as possible. During fluoroscopy, the inclination of the X-ray beam (approximately 5° downward) is checked to be tangential to the medial tibia plateau. The foot is rotated until the tibial spines are centered beneath the femoral notch. The axial view of the patella is taken with weight bearing and 30°–60° knee flexion with a vertical X-ray beam to ensure that the middle portion of the articular surface of the patella is in contact with the articular surface of the femur. The central X-ray beam is tangential with the articular surface of the patella. The same tech-

Radiography

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Fig. 4 (a) Weight-bearing AP view of the knee. (b) Rosenberg view. (c) Lateral view

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Fig. 5 (a–f) Different radiographic techniques used to obtain axial non-weight-bearing views of the patella. [From Merchant et al. 1974. Reproduced with permission from the Journal of Bone and Joint Surgery (American edition)]

nique is used by Buckland-Wright (1995). One has to be aware that with this technique, patellar pathology seen with flexion below 30° is missed. In trauma patients, an axial view of the patella can be obtained by various techniques (Fig. 5). Correct alignment of the radiographic beam in relation to total or partial knee replacement is essential. In the Oxford Partial Knee replacement, radio-opaque bearings are located posteriorly and a horizontal bar is located anteriorly in the prosthesis, which are used to aid radiographic alignment; correct alignment of the radiographic

beam is achieved when the bearings and the bar are in line. Adjustments can be made by subtle craniocaudal tilt of the beam and by knee rotation to ensure correct alignment, which can reveal otherwise occult pathology (Fig. 6).

3.1 “Standard” Series The minimum projections are AP and lateral views. At our institution, these are done weight bearing where possible. A tunnel view can be

Radiography

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Fig. 6 (a) Bearings lying superior to the bar indicating that the beam needs to be angled up. (b) Bearings lying inferior to the bar indicating that the beam needs to be angled down. (c) Bearings aligned horizontally with the bar, indicating

that the beam is aligned with the joint line. This image is a repeat of image b, and the improved alignment reveals areas of lucency around the prosthesis (white arrows) which were not appreciated on the misaligned image

added but is not always necessary and may be helpful in identifying radio-opaque (i.e. ossified) loose bodies. Cartilage loose bodies are not visualized. The tunnel view can also add value in delineating osteochondritis dissecans, which in most cases is on the inner aspect of the medial femoral condyle (Fig. 7).

In standard series, it is important not to overor underexpose analogue films, and to check the computed radiography picture on the monitor with different settings for window width and level. If this is not done, subtle changes such as small calcified loose bodies in osteochondromatosis may be missed. In tumors and osteomyeli-

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Fig. 7  Tunnel view of the knee. Osteochondritis dissecans lesion of the inner aspect of the medial femoral condyle (arrow)

tis, the first findings are often subtle, with a faint periosteal reaction such as Codman’s triangle that may be missed. Marginal osteophytes are important indicators of OA.  It has been shown that when small marginal osteophytes are present on the femoral or tibial condyles, there are always local degenerative changes of the cartilage, seen with magnetic resonance imaging (MRI) (Boegård et al. 1998a) or arthroscopy. However, in some instances of degenerative changes, there are no accompanying osteophytes. Osteophytes at the tibial spine and at the intercondylar fossa are not reliable signs of knee OA (Boegård et al. 1998a). In children with pain over the tibial tuberosity, Osgood-Schlatter disease is clinically suspected. Historically, this diagnosis could be confirmed with a coned-down view with soft tissue expo-

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sure of the tuberosity, revealing bone fragmentation at the patella tendon insertion (Fig.  8). Fragments may also be seen with a skyline view of the patella overlying the patellofemoral joint. A similar type of enthesitis is Sinding-Larsen-­ Johansson syndrome, where the fragmentation is at the distal pole of the patella, at the proximal insertion of the patellar tendon. These conditions are now more usually diagnosed clinically, or with ultrasound or MRI if there is diagnostic doubt or need for specialist input (according to the UK’s Royal College of Radiology Guidelines (2022)). A wide spectrum of normal variants of the knee can be seen on radiography and have been described in standard texts like Keats (1996) and Brossmann et al. (2001). Benign cortical defects are common around the knee joint in children, in both the distal femur and the proximal tibia. When these defects heal, they fill out with sclerosis, which will eventually resolve. Irregularities of the distal femoral epiphysis are common with spicula-like ossification of the cartilage. A not uncommon normal variation of ossification is a defect of the distal surface of the femur that looks like osteochondritis dissecans. These defects are seen prior to closure of the growth plate and will eventually fill out with normal ossification. A rounded defect of the dorsal, articular surface of the patella may look like osteochondritis dissecans but is a normal variation of ossification. Sometimes, accessory ossification centers occur around the knee, giving rise to accessory bones, as in bipartite patella or an accessory ossicle at the superior end of the proximal fibula. Relatively thin radiodense bands are sometimes seen in the proximal metaphysis of the tibia. These bands are often called arrest lines and are considered to be sequelae of previous fractures or attacks of arthritis or infection. These benign bands must be differentiated from sclerotic bands that are seen after heavy metal poisoning, such as lead poisoning. In lead poisoning, the sclerotic bands are thick and occur in the metaphyses of the femur, tibia, and fibula adjacent to the growth plate.

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Fig. 8 (a) Bone fragmentation seen at the distal patella insertion consistent with Osgood-Schlatter disease. (b) Bone fragmentation at the proximal patella tendon insertion consistent with Sinding-Larsen-Johansson syndrome

3.2 Trauma Series

represents separation of cellular elements from supernatant serum. Double fluid-fluid levels are a The minimum radiographic projections are AP more specific finding, indicating fat in the joint. and lateral views in the supine position with the As intra-articular fractures may not always be knee straight, with the latter view referred to as a apparent on standard films, extra projections may horizontal beam lateral (HBL) projection be necessary or the use of CT imaging. Fractures (Fig. 1a). Optional views for further information from subtle osteochondral injury may be easily may be obtained, for instance medial oblique to missed, especially on overexposed films, as the show the head of the fibula and the proximal tib- osseous part may be very small compared to the iofibular joint. cartilage component. It is often difficult to localA fracture is not always shown on the views, ize the origin of a visualized flake fragment on but the suprapatellar pouch is often distended due standard radiographs as they can migrate away to intra-articular bleeding. A fluid-fluid level is from the site of injury. only detectable on an HBL, commonly described Most fractures of the knee are apparent on the as a “lipohemarthrosis” as in cases of intra-­ standard trauma series. Sometimes, a finding may articular fracture and marrow fat leaks into the be subtle or equivocal, particularly in the case of joint creating a fat-fluid level (Fig.  2a). Lugo-­ tibial condylar fractures. There may be a subtle and Olivieri et  al. (1996) found that a single fluid-­ localized depression of the joint surface, but the fluid level is rarely due to fat in the joint, but just degree and extension are not clear on plain films. In

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such cases, the standard examination should be completed with CT and multiplanar reconstructions or conventional tomography (Fig. 3). Stress views of the knee can be valuable for indirect demonstration of capsular and ligamentous disruptions. Stress can be applied to the collateral ligaments and capsule via varus and valgus maneuvers with the knee straight and/or slightly flexed (20°) and to the cruciates by anterior or posterior draw tests. The method of applying stress can be by manual force with the aid of computerized arthrometers and hydraulic or spring devices, and there remains debate around the best method. Imaging is obtained by fluoroscopic screening or using radiographic methods already described. If stress radiographs are done in the acute stage after trauma, ideally the test should be done following good analgesia to avoid muscle spasm and reduce pain. Radiographs can also be done post-surgery to assess the laxity of reconstructions, and it is advised that comparisons are made with the contralateral knee. Strobel and Stedtfeld (1990) suggested that the examination should be limited to a

anterior and posterior drawer test post-trauma, although many authors (Burrus et  al. 2016; Gwathmey et al. 2012; de Faria et al. 2020) argue that examination of the degree of laxity in the capsule, posterolateral corner, and collateral ligaments is important in preoperative planning. The anterior and posterior drawer tests can be done supine, prone with the knee slightly flexed, or with the help of a chair. Prone radiographs, where gravity performs the anterior drawer, are more helpful than supine and avoid the need for external manipulation in assessing for anterior cruciate ligament laxity (Mae et al. 2018), while kneeling on a chair provides stress of the posterior cruciate ligament. MRI in combination with clinical examination has for the most part replaced stress testing in trauma, although stress radiography is inexpensive and objective and can quantify the degree of ligament instability. If there is any clinical suspicion of a patellar fracture, the lateral view is crucial to review as fractures may not be appreciated on the AP view (Fig. 9). A sunrise or skyline view of the patella b

Fig. 9 (a) AP view of the knee with no patella fracture appreciated. (b) Lateral view of the patella demonstrating a patella inferior pole fracture (arrow)

Radiography

should also be included in the examination (Fig.  10). Several techniques may be used to obtain this view in a trauma patient (Fig. 5). After lateral patellar dislocation, a small osseous fragment may be avulsed from the medial patella border, which can also be a helpful clue to the mechanism of trauma after relocation.

Fig. 10  Skyline view of the patella

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3.3 Degenerative Disease Osteoarthritis is a disorder that affects both the radiolucent hyaline cartilage and the subchondral bone. With plain radiography, OA of the tibiofemoral joint may be diagnosed by the presence of marginal osteophytes, while the degree of degeneration is indicated by the severity of joint space narrowing (Fig. 11). Ahlbäck (1968) and Leach et al. (1970) found that weight-bearing examination was superior to that obtained in the supine position for demonstration of joint space narrowing in tibiofemoral OA, and Kim et al. (2014) have also shown weightbearing Merchant or skyline views are superior in assessment of patellofemoral alignment and joint space narrowing. Rosenberg et al. (1988) and more recently Scott et al. (2007), meanwhile showed that this accuracy could be further improved upon by performing a weight-bearing posteroanterior (PA) b

Fig. 11 (a) AP view of a knee which demonstrates tibiofemoral OA with osteophytes (arrows) and joint space narrowing. (b) Lateral view of the knee which demonstrates patellofemoral OA

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view in 45° of knee flexion. Locally, Rosenberg and lateral radiographs are obtained as standard views when investigating for OA.  With general degeneration of the joint cartilage, joint space narrowing is observed under weight bearing, but focal chondral lesions are unlikely to be appreciated and MRI or CT arthrography is superior. A number of factors influence the possibility of achieving reproducible and optimal views of the joint space. One factor is that the slope of the tibial plateau differs between the medial and lateral condyles. The shape of the tibial condyles differs in that the medial condyle is slightly concave and a tangential view shows the superimposition of the anterior and posterior margins over the true joint surface. The true joint surface was defined by Buckland-Wright (1995) as “the line between the tibial spine and the medial or outer margin, across the center of the floor of the articular fossa in the midcoronal plane of the joint.” This line is defined by the superior margin of the bright radiodense band of the subchondral cortex and appears below the anterior and posterior articular margins of the tibial plateau. The joint space is measured between this line and the distal convex margin of the femoral condyle in the medial compartment. The lateral tibial plateau is slightly less concave as compared to the medial tibial condyle, and sometimes convex. BucklandWright (1995) defined the measuring line of the lateral tibial condyle as “the proximal margin of the articular surface, defined by the superior margin of the bright radiodense band of the subchondral cortex extending from near the tibial spines to the lateral or outer margin.” Thus, there is a difference between the medial and lateral compartments in that it is easy to define the joint surface of the lateral tibial plateau when the X-ray beam is tangential to the surface. In the medial compartment, the anterior and posterior margins of the joint surface superimpose the true joint surface. The slope of the tibial plateau is 5°–20°. This means that a horizontal X-ray beam will not run tangential to the plateau and the inclination of the X-ray beam has in most cases to be adjusted by fluoroscopy. Previous studies have shown that degeneration of the joint cartilage of the femoral condyles starts at a position posterior to the apex of the condyle and affects the medial side more commonly than the lateral side (Boegård et  al. 1997; Boegård

Y. Berkowitz and O. Czarnecki

1998). Therefore, a patient with nonspecific knee pain should be examined with weight bearing and 20°–30° flexion of the tibiofemoral joint, especially if he or she is over the age of 30, in order to reveal joint space narrowing. A Rosenberg view also has superior sensitivity for detecting medial osteophytes and joint space narrowing. In patients who are not able to stand and weight bear, a tunnel view of the knee may be valuable. Resnick and Vint (1980) described six patients in whom the tunnel view showed the most striking evidence of cartilage damage when compared with the weightbearing AP view. Various projections and their advantages/disadvantages are shown in (Fig. 12). Weight bearing can be done by placing all the weight on a single leg, or more commonly by standing on both legs with equal load on both knees. Boegård et  al. (1998b) compared the two techniques and found that there was a reduction of the minimal joint space width in the medial compartment when the patient stood with the weight equally distributed on both legs. With weight load on one leg, the joint space reduction was most prominent in the lateral compartment. Because OA is up to ten times more common in the medial compartment than in the lateral compartment, examination with equal weight on both legs is preferred and considered closer to functional. One must be aware that the slope of the tibial plateau differs between the medial and the lateral side, and if an examination is aimed at the medial compartment, there may be difficulties in assessing the lateral joint space. Different grading systems for knee OA have been proposed. Those most commonly employed are the systems suggested by Kellgren and Lawrence (1957) and Ahlbäck (1968). The two grading systems are compared in Table  1. Ahlbäck (1968) proposed that narrowing, as a sign of cartilage loss, exists if the minimal joint space width is less than 3 mm, and most authors have accepted this limit. This limit has been verified by Boegård et al. (1997). As mentioned previously, the presence of osteophytes is an important indicator of OA in the early stage of the disease. In the advanced stage, osteophytes are often extensive. Several atlases of radiographs separately define and record individual radiographic features of knee OA, such as joint space narrowing, osteophytes,

Radiography

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a

b

c

d

e

Fig. 12 (a–e) Line drawing of examination technique in OA. (a) Normal joint. The central beam from the X-ray tube (R) is tangential to the subchondral bone. The width of the cartilage (L) is correctly outlined on the film (F). (b) Normal joint. The X-ray beam is not tangential, and the cartilage width is falsely depicted, being too narrow. (c) Cartilage defect on both sides of the joint. The X-ray beam is correctly tangential to the subchondral bone. Joint space narrowing is correctly visualized. (d) Cartilage defect on one side of the joint is rotated away from a tan-

gential position. Only the depth of the lower defect is depicted. (e) The upper cartilage defect has rotated away from the tangential position, and the lower joint surface has slid posteriorly. The joint space width appears normal on radiography. This picture can illustrate the situation of the knee with local cartilage degeneration posteriorly upon radiographic examination with a straight or overextended joint. (Published with permission from Nordisk Lärobok I Radiologi, Studentlitteratur, Sweden, 1993)

Table 1  Classification of knee OA.  Comparison between the Ahlbäck and the Kellgren and Lawrence grading systems Ahlbäck grade

Grade 1 Grade 2 Grade 3 Grade 4 Grade 5

Ahlbäck definition

Joint space narrowing (joint space 10 mm)

Kellgren and Lawrence grade Grade 1 “doubtful” Grade 2 “minimal” Grade 3 “moderate” Grade 4 “severe”

Kellgren and Lawrence definition Minute osteophytes, doubtful significance Definite osteophytes, unimpaired joint space Moderate diminution of joint space

Grade 4 Grade 4

Joint space greatly impaired with sclerosis subchondral bone As above As above

Grade 4

As above

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and subchondral sclerosis (Altman et  al. 1995; Altman and Gold 2007; Spector et  al. 1992). These atlases have been created for semiquantitative assessment of the degree of disease in an attempt to reduce intraobserver and interobserver variance in the evaluation of knee joints for research purposes. In patients with suspected degenerative disease, an axial view of the patella (aka a skyline or Merchant view) may also be included to evaluate the patellofemoral joint. The examination should be performed with weight bearing, knee flexion, and a vertical X-ray beam. Weight-bearing views alter the patella position and tilt and have increased sensitivity for detection of medial patellofemoral OA, with the same sensitivity for lateral OA as non-weight bearing (Skou and Egund 2017). In the measurement of the patellofemoral joint space, Buckland-Wright (1995) defined the measuring surfaces as “the articular surface of the cortex at the medial and lateral surfaces for the femur and the bright radiodense band of the subchondral cortex at the medial and lateral articular surfaces lying deep, or anterior, to the profile of the inferior articular margin for the patella.” Boegård et  al. (1998c) found the critical joint space width to be 5 mm. If the joint space is below 5  mm, there is a low sensitivity (50%) and a high specificity (94%) for MR-detected cartilage defects. Osteophytes at the patellofemoral joint are associated with MR-detected cartilage defects in the same joint. The relationship was strong for osteophytes at the lateral femoral trochlea and in joints with nar-

a

rowing (5 mm) (Boegård et al. 1998d). Extensive review articles on plain radiography in OA are available (e.g., Boegård and Jonsson 1999; Mazzuca and Brandt 1999). Stress views, previously mentioned as a tool for assessing capsuloligamentous traumatic injuries, also have a role in the assessment of OA.  There is mounting evidence that unicompartmental or partial knee replacements (UKRs or PKRs) are preferred to total knee replacements (TKRs) in patients with advanced medial tibiofemoral osteoarthritis, where there is lateral tibiofemoral joint space preservation and intact meniscus, non-severe patellofemoral OA, and intact cruciate and collateral ligaments (Griffin et al. 2007; Pandit et al. 2015; Hamilton et al. 2017; Berend et al. 2015; Knifsund et al. 2021). UKR patients have fewer complications at surgery and, in the follow-­up period, recover faster from surgery and have better reported outcomes. However, there are concerns over long-term implant survival and revision rates. To identify suitable patients, Hamilton et al. (2016) developed a decision aid, including the use of supine varus and valgus stress AP, lateral, and skyline views. The system that is used to apply stress force in a mechanically controlled and fixed method, while maintaining knee flexion and positioning, is the Oxford Stress System for Knee Arthroplasty Radiographs (OSSKAR) (Fig.  13). This uses a combination of blocks, straps, and springs to apply force with the knees in 20° of flexion. Medial and lateral joint space narrowing and col-

b

Fig. 13  The Oxford Stress System for Knee Arthroplasty Radiographs (OSSKAR) system applied to a patient, using straps and blocks with a spring force meter to obtain the stress views. (a) Valgus stress. (b) Varus stress

Radiography

a

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b

Fig. 14  AP radiographs of the same knee obtained using stress views. (a) Varus stress applied, demonstrating medial compartment joint space loss indicating OA. (b)

Valgus stress applied, demonstrating lateral compartment joint space loss, indicating OA

lateral ligament functionality can be more sensitively assessed than with weight-bearing views alone and can also be used to assess preservation or loss of lateral joint space during valgus stressing (Fig.  14). A similar system, known as the Knee Osteoarthritis Grading System (KOGS) has also been proposed by Oosthuizen et al. (2019) to grade OA and serve as a guide in choosing which patients would be suitable for UKA. In patients undergoing and following implantation of a knee prosthesis, the same technique is used as in primary OA, i.e., with weight bearing and flexion of the knee. In these patients, it is important to have the central X-ray beam parallel to the tibia plateau in the AP or PA view in order to have the same projection at follow-up examinations. If the X-ray beam is not parallel to the tibial plateau, a thin zone of lucency may be hidden. In the lateral view, the posterior elements of the femoral component of the prosthesis should be superimposed. The aim of prosthesis examination is to obtain standardized projections in order to diagnose possible complications, such as loosening, infection, or wear of the plastic coat.

4 Radiographic Measurement Techniques 4.1 Assessment of Alignment Accurate measurements of lower extremity alignment and configuration are a prerequisite for research and the clinical handling of patients with OA. Such measurements are mandatory in the preoperative planning of high tibial osteotomy and knee arthroplasty as well as for postoperative outcome assessment. The mechanical axis of the lower extremity, defined as the hipknee-­ankle (HKA) angle, can be demonstrated on long films exposing the hip, knee, and ankle (Odenbring et  al. 1993). Measurement of the HKA angle is performed with the patient bearing weight on both legs, and both legs are usually imaged. Patient positioning must be standardized for reproducibility. The patella should be central, often requiring 10° of lateral rotation. A long AP radiograph including the pelvis and hips, knee, and ankle is obtained with the film cassettes in a calibration frame. A long vertical

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cassette holder with four 14 × 17 in. graduated cassettes may be used. The printed or developed radiographs can be mounted on a similar frame on a light box. Radiographs uploaded and viewed with PACS can be digitally combined and “stitched” together into a full-length weightbearing view. In this way, the radiographs are centered correctly in relation to each other, and lines drawn on the films or images allow evaluation of the HKA angle. The lines are drawn from the center of the femoral head to the center of the knee and from the middle of the ankle joint to the middle of the knee (Fig. 15). In this way, a varus or valgus deformity of the knee can be outlined and graded. Another important angle is the HKS angle (hip-­knee-­shaft), which is the angle between the line from the center of the femoral head to the middle of the knee and the line from the middle of the femoral shaft at the level of the lesser trochanter to the center of the knee. This angle is important for correct positioning of surgical equipment at osteotomy or placement of prosthesis. There are methods described to measure the anatomical alignment on standard AP knee radiographs that estimate for the HKA, for example the femoral-­shaft-­tibial-shaft angle, but these are less accurate and reproducible and are not recommended for surgical planning (Sheehy et al. 2011). In moderate or early knee OA, patients may be operated on with high tibial osteotomy to correct the alignment of the knee and redistribute the load to the healthy compartment, i.e., to the lateral compartment in medial arthrosis (Insall et al. 1974). Traditionally, an osteotomy included removal of a bone wedge to correct the alignment, usually an open wedge osteotomy with a fixation plate, but more recently, callus distraction or hemicallotasis osteotomy has become popular in some centers (De Pablos et al. 1995). In order to perform a correct osteotomy and estimate the size of wedge or callus distraction, HKA measurement is necessary both preoperatively and postoperatively to check the result. Preoperative measurement in gonarthrosis can also be evaluated in the supine non-weight-­ bearing position with the application of stress views (Edholm et al. 1976).

Y. Berkowitz and O. Czarnecki

Fig. 15  The principle of HKA measurement. Lines are drawn from the center of the femoral head to the middle of the knee (represented by the tibial spines) and from the middle of the ankle to the middle of the joint (black lines). The femoral line is extrapolated beyond the knee joint (white line) and the angle measured between it and the tibial line to give the HKA (double-arrowhead black curved line)

HKA measurements represent the static alignment of the knee. Clément et  al. (2019) have shown, however, that the HKA measured statically may not correspond to the HKA measured dynamically during walking. They found after performing 3D kinematic analysis on 165 normal knees that there was no correlation between dynamic and static HKA in those with varus knees. This may be a focus of research and become part of the presurgical workup in the future to optimize the survival period of TKRs.

Radiography

4.2 QPR or SKI To improve the measuring routines of the knee, other methods have been described, such as Questor Precision Radiography system (QPR), which is a standardized examination method for the lower limb in the weight-bearing position. This method has now been renamed standardized knee imaging (“SKI”). The method was developed for conventional radiography by the Clinical Mechanics Group, Division of Orthopaedic Surgery and Department of Mechanical Engineering at Queen’s University, Kingston, Ontario, Canada (Siu et  al. 1991). The patient stands on a turntable inside a frame that has protractor scales for recording foot rotation (Fig.  16). The ankles are positioned against adjustable blocks to represent the location of each mid-­malleolar point, and the hips are supported by adjustable hip pads. A lateral view of the knee is obtained, and then the turntable can be rotated 90° without the patient changing position for an orthogonal AP view of the knee and hip. The QPR frame contains two Plexiglas panels spaced 10  cm apart with embedded radio-opaque markers and reference lines for correction of parallax and magnification. The panels are located in front of the patient between the X-ray source and the film. The bony landmarks, reference lines, and markers are located and digitized on a digitizing tablet. A software program on a computer processes the information and generates a display of nine angles and ten distances. Measurement of the HKA and HKS angles by more routine methods has been compared with QPR, and there is good correlation between the two methods (Sanfridsson et al. 1996). The QPR system has been adapted to computed radiography and PACS (Sanfridsson et  al. 1997), with good reproducibility. With the CR technique, it is possible to reduce the radiation dose considerably in this environment (Sanfridsson et  al. 2000). The same group has also demonstrated that with the QPR technique, it is possible to show the rotation of the tibia in relation to the femur and the migration of the patella when changing the position of the knee from slight

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flexion to fully extended position (Sanfridsson et al. 2001). It has to be pointed out, however, that this technique is not generally available because it requires special equipment and computer programs, although it is used at some centers for routine evaluation of knee alignment (Cook et  al. 1999).

Fig. 16 The examination table for QPR. (From Sanfridsson et  al. 1996, with permission from Acta Radiologica)

Y. Berkowitz and O. Czarnecki

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4.3 Roentgen Stereophotogrammetric Analysis Stereophotogrammetry (RSA) was described by Selvik (1989). The idea behind the method is that mobility between two structures can be defined and analyzed by a stereophotogrammetric technique. The structures are defined by small metallic beads of known size and density. The beads are placed in two structures that may move in relationship to each other, for instance the tibial component of a prosthesis in relation to the proximal tibia. Absolutely spherical 0.8  mm beads made of tantalum are implanted into the bone and/or the plastic coating of a prosthesis. At least three beads are required for each structure to define a “rigid body.” Two radiographs are taken simultaneously with two angled X-ray tubes in stereoscopic convergent-ray mode or at 90° to each other. Before these radiographs are taken, special calibration procedures must be performed. The films are mounted in a special analyzer that, due to the stereoscopic radiographic technique, can locate the bodies in space and define the position of each body in relation to the other by using a special software program. At

a

Fig. 17 (a, b) Total knee prosthesis examined with weight bearing. (a) Postoperative examination 8  weeks after surgery. (b) Repeat examination 1 year after surgery. The first examination (a) is not performed correctly: the X-ray beam is not tangential to the tibial plateau. By contrast, the second examination (b) is correct. Already at the first examination, there was clinical suspicion of infec-

repeat examinations, differences in position between the bodies can be revealed. The tantalum beads can be easily introduced percutaneously under local anesthesia. The knee is ideal for RSA studies. The method may be used for the analysis of knee kinematics and prosthesis fixation (Fig.  17). The accuracy of measurements from conventional radiography is in the order of 2–3 mm, but with RSA, the accuracy is 0.2–0.3  mm. Also, rotation can be estimated with the same degree of accuracy (Ryd 1992; Fridén et al. 1992). There is no other method available that can compete with RSA in terms of accuracy of measurements, and to this day, it is considered the gold standard in detecting micromotion of prostheses, but the technique requires considerable time and effort. The markers must be implanted with great care, and the radiographs must be taken with patients correctly positioned. Digitization and analysis of motion are time consuming and demand thorough knowledge of the technique. RSA is best suited for smaller well-­defined investigations that address kinematic problems and is currently mainly a research tool rather than a method in general use due to these limitations (Pijls et  al. 2018). That being said, Yuan et al.

b

tion. The zone under the tibial plateau cannot be evaluated in a, while in b, the zone is well seen. Note the tantalum beads for stereophotogrammetry in the tibia and in the plastic coating of the tibial component. In b, but not in a, both the anterior and the posterior tantalum implants are seen. This is a further check on correct X-ray beam direction

Radiography

(2018) have shown the potential viability of using conventional radiographic projections and a modified radiosteroemetric analysis procedure to produce similar accuracy as the standard system in phantom models, which could make this technique more widely available. CT has the potential to assess for prosthetic micromotion using bony and prosthetic landmarks, with or without stress or motion, but many feel that it is not yet validated for this purpose and its accuracy remains unproven in comparison with RSA (Röhrl 2020).

4.4 Femoral Condyle Configuration The configuration of the femoral condyles may influence knee stability. In a study by Fridén et al. (1993), 100 consecutive patients with anterior cruciate ligament (ACL) rupture were studied prospectively for 5  years. During this time, 16 patients developed disability, which required reconstructive surgery. The remaining 84 patients did not develop any functional limitations. The measurements showed that, compared with the non-operated patients, the 16 patients who needed surgery had femoral condyles that were more spherical. This suggests that the articular geometry of the femoral condyles is of importance for function and healing after ACL tear. Similar findings have been reproduced by others, most recently at the time of publishing by van Kuijk et  al. (2021) who examined 168 patients with ACL ruptures and 168 controls after trauma and found that those with smaller intercondylar notches and tibial eminences were more likely to develop ACL ruptures following trauma. This was established on radiographs using statistical shape modeling software to analyze the femoral and tibial configurations. Using similar software and technique, femoral condylar shape and tibial eminence size were also found to be factors associated with poor outcomes in those with ACL tears that were surgically repaired and those with tears treated conservatively, which may guide surgical decision-making (Eggerding et al. 2014).

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4.5 Patellar Position Patellar problems have previously been mentioned in trauma and degenerative disease. The position of the patella is considered to be a significant etiological factor in retropatellar chondromalacia. A distinction is drawn between high-riding patella, patella alta, and low-riding patella, patella baja. The most common and reproducible method employed to determine the height of the patella is the Insall-Salvati index, measured on a weight-bearing lateral radiograph in slight flexion (Biedert and Tscholl 2017 and Verhulst et al. 2020). In this method, the greatest diagonal length of the patella (LP) is divided by the length of the patellar tendon (LT) (Fig. 18). An Insall-Salvati index (LP/LT) of less than 0.8 indicates patella baja, which results in stresses on the central and distal portions of the retropatellar cartilage. If the Insall-­ Salvati index is greater than 1.2, it indicates patella alta, with resultant loading on the proximal portion of the retropatellar cartilage. Several other methods have been described. The reliability and interobserver variability of the Insall-­ Salvati index and several other indices have been compared (Seil et  al. 2000). There was great variability between different indices and, depending on the index used, the same knee could show patella alta, patella baja, or normality. In Seil et  al.’s study, the method described by Blackburne and Peel (1977) showed the lowest interobserver variability and discriminated best among the groups “alta,” “normal,” and “baja.” In this method, the ratio of the articular surface length of the patella to the height of the lower pole of the articular surface above a tibial plateau line is measured. A more recent systematic review undertaken by White et  al. (2021), however, suggested the Insall-Salvati index to be the most reliable in correlating clinical symptoms with radiological findings and had the least interobserver variability. The authors found insufficient data of the Blackburne and Peel (1977) method to obtain statistically significant findings. The heterogeneity of the data and studies that have been published makes determining the “gold standard” for assessment difficult,

Y. Berkowitz and O. Czarnecki

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a

b

Fig. 18  Insall-Salvati index is calculated by dividing the length of the patella (white line) by the length of the tendon (black line). (a) Patella alta. (b) Patella baja following

a quadriceps tendon avulsion. An avulsed cortical fragment is visible as well as fluid distension of the suprapatellar recess

and there is “need for greater consolidation of the most reliable radiologic measurements and their roles in the assessment of patellar instability,” as per White et al.’s review (2021). Egund et al. (1988) had described a method in which the relation of the patella and the tibia is measured in weight bearing and 30°–40° of flexion, which would appear to be the most physiological method, but it has not been subsequently reviewed or validated to our knowledge.

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25 Egund N, Lundin A, Wallengren NO (1988) The vertical position of the patella. A new radiographic method for routine use. Acta Radiol 29:555–558 Fridén T, Ryd L, Lindstrand A (1992) Laxity and graft fixation after reconstruction of the anterior cruciate ligament. A roentgen stereophotogrammetric analysis of 11 patients. Acta Orthop Scand 63:80–84 Fridén T, Jonsson A, Erlandsson T et  al (1993) Effect of femoral condyle configuration on disability after an anterior cruciate ligament rupture. 100 patients followed for 5 years. Acta Orthop Scand 64: 571–574 Griffin T, Rowden N, Morgan D et  al (2007) Unicompartmental knee arthroplasty for the treatment of unicompartmental osteoarthritis: a systematic study. ANZ J Surg 77:214–221. https://doi. org/10.1111/j.1445-­2197.2007.04021.x Gwathmey FW, Tompkins MA, Gaskin CM, Miller MD (2012) Can stress radiography of the knee help characterize posterolateral corner injury? Clin Orthop Relat Res 470:768–773. https://doi.org/10.1007/ s11999-­011-­2008-­6 Hamilton TW, Pandit HG, Lombardi AV et  al (2016) Radiological Decision Aid to determine suitability for medial unicompartmental knee arthroplasty. Bone Joint J 98-B:3–10. https://doi.org/10.1302/0301-­ 620X.98B10.BJJ-­2016-­0432.R1 Hamilton TW, Pandit HG, Jenkins C et al (2017) Evidence-­ based indications for mobile-bearing unicompartmental knee arthroplasty in a consecutive cohort of thousand knees. J Arthroplast 32:1779–1785. https:// doi.org/10.1016/j.arth.2016.12.036 Insall J, Shoji H, Mayer V (1974) High tibial osteotomy. J Bone Joint Surg 56:1397–1405 Jónsson A, Hannesson P, Herrlin K et al (1995) Computed vs film-screen magnification radiography of fingers in hyperparathyroidism. An ROC analysis. Acta Radiol 36:71–75 Jónsson A, Herrlin K, Jonsson K et  al (1996) Radiation dose reduction in computed skeletal radiography: effect on image quality. Acta Radiol 37:128–133 Keats TE (1996) Atlas of normal roentgen variants that may simulate disease, 6th edn. Mosby-Year Book, St. Louis Kellgren JH, Lawrence JS (1957) Radiologic assessment of osteoarthritis. Ann Rheum Dis 16:494–501 Kim T-H, Sobti A, Lee S-H et  al (2014) The effects of weight-bearing conditions on patellofemoral indices in individuals without and with patellofemoral pain syndrome. Skelet Radiol 43:157–164. https://doi. org/10.1007/s00256-­013-­1756-­7 Knifsund J, Niinimaki T, Nurmi H et al (2021) Functional results of total-knee arthroplasty versus medial unicompartmental arthroplasty: two-year results of a randomised, assessor-blinded multicentre trial. BMJ Open 11:e046731. https://doi.org/10.1136/ bmjopen-­2020-­046731 Körner M, Weber CH, Wirth S et al (2007) Advances in digital radiography: physical principles and system

26 overview. Radiographics 27:675–686. https://doi. org/10.1148/rg.273065075 Leach RE, Gregg T, Silber FJ (1970) Weight bearing radiography in osteoarthritis of the knee. Radiology 97:265–268 Lugo-Olivieri CH, Scott WW Jr, Zerhouni EA (1996) Fluid-fluid levels in injured knees: do they always represent lipohemarthrosis? Radiology 198:499–502 Mae T, Shino K, Hiramatsu K et al (2018) Anterior laxity of the knee assessed with gravity stress radiograph. Skelet Radiol 47:1349–1355. https://doi.org/10.1007/ s00256-­018-­2941-­5 Mazzuca ST, Brandt KD (1999) Plain radiography as an outcome measure in clinical trials involving patients with knee osteoarthritis. Rheum Dis Clin N Am 25:467–480 Merchant AC, Mercer RL, Jacobsen RH et  al (1974) Roentgenographic analysis of patellofemoral congruence. J Bone Joint Surg Am 56A:1391–1396 Odenbring S, Berggren AM, Peil L (1993) Roentgenographic assessment of the hip-knee-ankle axis in medial gonarthrosis. A study of reproducibility. Clin Orthop 289:195–196 Oosthuizen CR, Takahashi T, Rogan M et al (2019) The knee osteoarthritis grading system for arthroplasty. J Arthroplast 34:450–455. https://doi.org/10.1016/j. arth.2018.11.011 Pandit H, Hamilton TW, Jenkins C et al (2015) The clinical outcome of minimally invasive Phase 3 Oxford unicompartmental knee arthroplasty: a 15-year follow-­up of 1000 UKAs. Bone Joint J 97-B:1493–1500. https://doi.org/10.1302/0301-­620X.97B11.35634 Pavlov H, Burke M, Giesa M, Seager KR, White E (1999) Orthopaedist’s guide to plain film imaging. Thieme, New York Pijls BG, Plevier JWM, Nelissen RGHH (2018) RSA migration of total knee replacements. Acta Orthop 89:320–328. https://doi.org/10.1080/17453674.2018. 1443635 Piperno M, Conrozier T, Bochu M, Carmon Y, Fantino O, Vignon E (1998) Re-evaluation of conventional radiography in arthritis (in French). Rev Prat 48(Suppl 17):S5–S8 Ravaud P, Auleley GR, Chastang C, Rousselin B, Paolozzi L, Amor B, Dougados M (1996) Knee joint space width measurement: an experimental study of the influence of radiographic procedure and joint positioning. Br J Rheumatol 35:761–766 Resnick D, Vint V (1980) The “tunnel” view in assessment of cartilage loss in osteoarthritis of the knee. Radiology 137:547–548 Röhrl SM (2020) “Great balls on fire:” known algorithm with a new instrument? Acta Orthop 91:621–623. https://doi.org/10.1080/17453674.2020.1840029 Rosenberg TD, Paulos LE, Parker RD et  al (1988) The forty-five-degree posteroanterior flexion weight-­ bearing radiograph of the knee. J Bone Joint Surg Am 70:1479–1483 Royal College of Radiology Guidelines (2022) In: iRefer. https://www.irefer.org.uk/guidelines. Accessed 16 Jan 2022

Y. Berkowitz and O. Czarnecki Ryd L (1992) The role of roentgen stereophotogrammetric analysis (RSA) in knee surgery. Am J Knee Surg 5:44–54 Sanfridsson J, Ryd L, Eklund K et  al (1996) Angular configuration of the knee. Comparison of conventional measurements and the QUESTOR Precision Radiography system. Acta Radiol 37:633–638. https:// doi.org/10.1177/02841851960373P243 Sanfridsson J, Svahn G, Jonsson K et al (1997) Computed radiography for characterisation of the weight-­ bearing knee. Acta Radiol 38:514–519. https://doi. org/10.1080/02841859709174378 Sanfridsson J, Svahn G, Ryd L et  al (1998) Assessment of image post-processing and of measuring assistance tools in computed radiography. Acta Radiol 39: 642–648 Sanfridsson J, Holje G, Svahn G et  al (2000) Radiation dose and image information in computed radiography. A phantom study of angle measurements in the weight-bearing knee. Acta Radiol 41:310–316 Sanfridsson J, Ryd L, Svahn G, Fridén T, Jonsson K (2001) Radiographic measurement of femorotibial rotation in weight bearing. The influence of flexion and extension in the knee on the extensor mechanism and angles of the lower extremity in a healthy population. Acta Radiol 42:207–217 Scott WW Jr, Rosenbaum JE, Ackerman SJ et  al (1993) Subtle orthopaedic fractures: teleradiology workstation versus film interpretation. Radiology 187:811–815 Scott WW, Beall DP, Eng J et al (2007) Improved demonstration of cartilage narrowing in the knee joint using standing PA flexed radiographs. J Okla State Med Assoc 100:469–472 Seil R, Müller B, Georg T et  al (2000) Reliability and interobserver variability in radiological patellar height ratios. Knee Surg Sports Traumatol Arthrosc 8:231–236 Selvik GA (1989) A roentgen stereophotogrammetric system for the study of the kinematics of the skeletal systems. University of Lund, Sweden, Thesis 1974 (reprint). Acta Orthop Scand 60(Suppl 232):1–51 Sheehy L, Felson D, Zhang Y et al (2011) Does measurement of the anatomic axis consistently predict hip-­ knee-­ankle angle (HKA) for knee alignment studies in osteoarthritis? Analysis of long limb radiographs from the multicenter osteoarthritis (MOST) study. Osteoarthr Cartil 19:58–64. https://doi.org/10.1016/j. joca.2010.09.011 Siu D, Cook DV, Broekhoven LD et al (1991) A standardized technique for lower limb radiography. Investig Radiol 26:71 Skou N, Egund N (2017) Patellar position in weight-bearing radiographs compared with non-­ weight-­ bearing: significance for the detection of osteoarthritis. Acta Radiol 58:331–337. https://doi. org/10.1177/0284185116652013 Spector TD, Cooper C, Cushnaghan J et al (1992) A radiographic atlas of knee osteoarthritis. Springer, Berlin Strobel M, Stedtfeld HW (1990) Diagnostic evaluation of the knee. Springer, Berlin

Radiography van Kuijk KSR, Eggerding V, Reijman M et  al (2021) Differences in knee shape between ACL injured and non-injured: a matched case-control study of 168 patients. J Clin Med 10:968. https://doi.org/10.3390/ jcm10050968 Verhulst FV, van Sambeeck JDP, Olthuis GS et al (2020) Patellar height measurements: Insall-Salvati ratio is most reliable method. Knee Surg Sports Traumatol Arthrosc 28:869–875. https://doi.org/10.1007/ s00167-­019-­05531-­1 White AE, Otlans PT, Horan DP et al (2021) Radiologic measurements in the assessment of patellar instabil-

27 ity: a systematic review and meta-analysis. Orthop J Sports Med 9:232596712199317. https://doi. org/10.1177/2325967121993179 Whitley AS, Jefferson G, Holmes K et al (2015) Clark’s positioning in radiography, 13th edn. CRC Press, Taylor and Francis Group, Boca Raton, FL Yuan X, Broberg JS, Naudie DD et  al (2018) Radiostereometric analysis using clinical radiographic views: validation with model-based radiostereometric analysis for the knee. Proc Inst Mech Eng H 232:759– 767. https://doi.org/10.1177/0954411918785662

Computed Tomography (CT) and CT Arthrography Nuttaya Pattamapaspong and Wilfred C. G. Peh

Contents

6    CT Arthrography  6.1  Role and Indications  6.2  Technique 

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7    CT-Guided Interventions 

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8    Conclusion 

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References 

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1    Introduction 

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2    Developments in CT  2.1  Slip Rings  2.2  X-Ray Tubes  2.3  X-Ray Detectors  2.4  Helical CT  2.5  CT Fluoroscopy  2.6  Dual-Energy CT  2.7  Data Processing and Reformatted Images 

 30  30  31  31  31  32  33  33

Abstract

3    Scan Image Quality  3.1  Internal Metalwork from Fixation Devices  3.2  Image Display: CT Number, Hounsfield Unit, Window Width, and Level 

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4    CT of the Knee  4.1  Anatomy  4.2  Immobilization 

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5    Indications  5.1  Trauma  5.2  Knee Morphology and Surgery  5.3  Patellofemoral Joint  5.4  Articular Cartilage  5.5  Soft Tissues 

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The role of computed tomography (CT) has increased tremendously due to technical developments which have allowed isotropic voxel acquisition, permitting high-quality multiplanar reformation and three-­dimensional reconstruction of knee anatomy. In addition, dual-energy CT technology enables CT to perform specific detection of certain materials and improve reduction of metal artifacts. CT is widely available and fast to perform; therefore, the study is particularly useful in the evaluation of patients with acute trauma. CT remains essential when magnetic resonance imaging is contraindicated. The appropriate examination protocol will depend upon the suspected pathology and the equipment available. This chapter reviews the development in CT scanners, highlights the current relevant technologies, and presents examples of clinical applications.

N. Pattamapaspong (*) Department of Radiology, Faculty of Medicine, Chiang Mai University, Chiang Mai, Thailand e-mail: [email protected]; [email protected] W. C. G. Peh Department of Diagnostic Radiology, Khoo Teck Puat Hospital, Singapore, Republic of Singapore e-mail: [email protected]; [email protected]

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Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2023_413, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 12 April 2023

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1 Introduction Since its first development by Sir Godfrey Hounsfield (1973), computed tomography (CT) has developed rapidly over the past five decades or so. CT and magnetic resonance (MR) imaging are now established methods of investigating the knee, and both methods continue to develop. The comparative clinical values between techniques such as CT and MR imaging depend upon the condition being imaged, the model and age of scanners used, the scanning protocol, the experience and ability of the technologists operating the scanner, and the available radiologist expertise. In recent years, the role of CT has increased tremendously as technical development has allowed isotropic voxel acquisition, permitting high-quality multiplanar reformation and three-­ dimensional (3D) reconstruction of anatomical structures of the knee. In addition, dual-energy CT technology enables CT to perform specific detection of some materials and improve reduction of metal artifacts. Currently, both CT and MR imaging are complementary in providing benefits for different clinical applications. As CT is more widely available, cheaper, quicker, and easier to perform, the study is preferred for the assessment of patients with acute trauma. CT remains essential in the assessment of patients in whom MR imaging is contraindicated, e.g., patients with intracranial aneurysm clips or cardiac pacemakers or who are claustrophobic. Having decided that CT is an appropriate investigation for an individual, the precise format of the examination will depend upon the suspected pathology and the equipment available. This chapter reviews the developments in CT scanner technology, highlights the current methods to optimize image quality, and presents examples of clinical applications.

2 Developments in CT The principles of the CT image formation are based on a Cartesian coordinate map of normalized X-ray attenuation coefficients, generated by

N. Pattamapaspong and W. C. G. Peh

electronically filtered computerized back projection of X-ray transmission measurements in multiple directions through a section of the object in question. The areas where more recent developments have been made include helical scanning, multislice acquisition, and CT fluoroscopy. These developments have been made on the back of improving technology, which includes slip rings for power and data transmission to and from the gantry, higher heat-loading X-ray tubes, high-­ efficiency solid-state X-ray detectors, and faster data transmission and processing abilities of the electronics. In the development of dual-energy CT, modification of X-ray tubes or detectors adds the capability of material decomposition and virtual selection of beam energy.

2.1 Slip Rings Prior to the development of slip ring innovation, in order to acquire X-ray transmission data in all directions across a slice of the patient, the X-ray tube had to travel around the entire circumference of the CT gantry to acquire each slice. As the tube was supplied with power by cables, these had to wrap around the circumference of the gantry as the tube moved. In order to unwrap the cables, the next slice was performed by rotating the tube in the opposite direction. This design required more than 360° of tube rotation, due to the need for initial acceleration and final deceleration distances. Powerful motors and brakes were required to cope with the inertia of this system, which may include the X-ray detectors and counterweights to balance the gantry. A significant time delay was necessary between each slice acquisition to allow for these acceleration and deceleration phases. The need for these cumbersome mechanical devices changed with the development of slip ring technology. Slip rings are large-circumference electrically conducting rings which encircle the X-ray tube path and enable the transfer of power from the rings to the X-ray tube via conducting brushes. Replacing the power cables with slip rings allows the X-ray tubes within the gantry to continuously

Computed Tomography (CT) and CT Arthrography

rotate in one direction. This innovation has several advantages, including the following: (1) rapid acceleration and deceleration of the gantry are no longer required, yet a faster rotation speed can be achieved, giving rise to shorter scan acquisition times; (2) the time delay between slices needs to be no longer than that required for table movement in the conventional acquisition mode; and (3) the potential for acquiring continuously updated X-ray transmission data enables helical scanning: paving the way for volume imaging, CT fluoroscopy, and kinematic studies.

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The development of slip rings required X-ray tubes to have both a higher heat capacity and a higher maximum tube current, as the mAs required for a single slice remains much the same, but the time in which the slice is acquired is reduced. With wide-coverage helical scanning, continuous X-ray output and high heat capacity are needed to enable scanning the region of interest in the Z-direction. Current high-performance X-ray tubes allow scanning using 1300  mA at 70  kV with cone angle of 160  mm (Lell and Kachelriess 2020). Radiation dose can be reduced by applying low-kV scan protocols or automated tube current modulation (Roth et  al. 2013). Radiation dose can also be reduced by advances in detector technology, such as solid-state and photon-counting detectors.

cent channels (or arrays), have facilitated the development of multislice scanners. These scanners can acquire several sections simultaneously, which can be separately processed to give large numbers of thin sections or recombined to give fewer thicker sections with low noise. The size of each detector element determines the minimum slice thickness, which reflects spatial resolution. The current CT scanners commonly use detector elements that achieve 0.5–0.625  mm thickness, while ultrahigh-­ resolution detectors can achieve a slice thickness of 0.25  mm (Lell and Kachelriess 2020). Multidetector geometry inherently reduces radiation dose by reducing over-beaming because the wider detector array captures more of the primary beam and less of the beam fraction that projects beyond the detector array. Photon-counting detectors are an emerging technology which may replace conventional solid-state CT detectors in the near future. While solid-state detectors indirectly convert X-ray photons into visible light by scintillator-­ photodiode detectors which subsequently convert light into an electronic signal, photon-counting detectors, which use semiconductors, directly convert X-ray photons to an electrical signal (Pelc 2014; Leng et  al. 2019). Photon-counting detectors can reduce the radiation dose and noise while increasing spatial resolution. They also allow better metal artifact reduction and material decomposition (Pelc 2014; Leng et al. 2019; Lell and Kachelriess 2020).

2.3 X-Ray Detectors

2.4 Helical CT

The initial xenon gas detectors, which had a conversion efficiency (X-rays to signal strength) of around 60%, have now been replaced by solid-­ state crystal detectors, which have a conversion efficiency of nearly 100%, resulting in a 40% reduction in patient radiation dose for the equivalent scan appearances. Currently, several designs of solid-state crystal detectors are used in practice (Lell and Kachelriess 2020). Solid-state detectors, which can be stacked in parallel adja-

Helical scanning became possible only after the development of slip ring technology. Helical scanning is performed by moving the patient couch continuously through the CT gantry during the X-ray exposure, from the first slice location to the last. Thus, X-ray transmission data through a scanned patient volume is acquired in the shape of a helix, hence the term helical (or spiral or volume) scanning. To generate a CT image, the data from adjacent turns of the helical pathway are

2.2 X-Ray Tubes

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interpolated to produce transmission data, which are effectively from a single-slice location. This process can be performed at any location within the helix (except the first and last 180°s, where there is no adjacent helix of data for interpolation). This way, overlapping slices can be produced without overlapping irradiation of the patient. The relationship between the X-ray fan-beam collimation and the table movement per rotation of the gantry is called the pitch ratio. Extended or stretched pitch scans are performed with pitch ratios greater than 1. Such extended pitches can be used to trade off between greater scan volumes, shorter scan acquisition times, and lower scan radiation doses. Increasing the pitch ratio to 1.25 has little effect on the image appearances, but pitch ratios greater than 1.5 produce images with an effective slice thickness significantly greater than the nominal fan-beam collimation thickness. By increasing the number of detector arrays (i.e., in multislice scanners), several interlaced helices can be acquired simultaneously (Fig. 1), with the table increment per gantry rotation increased proportionately. Therefore, the combination of multislice and helical scanning enables faster scanning, and more extensive area of coverage and volume data acquisition.

Fig. 1  Helical CT scanning. Pictorial representation of the path followed by a single column of detectors for a four-beam multislice helical CT scan. The patient positioning illustrates a way of scanning a single knee at a time. Current CT scanners are 320-slice

N. Pattamapaspong and W. C. G. Peh

2.5 CT Fluoroscopy In conventional CT, transmission data from a 360° gantry rotation are required to generate an image. This is because there are two opposing beam paths for each ray across the imaging volume. This produces improved signal to noise and corrects for the effects of divergent X-ray beams along each ray and beam-hardening effects. Images can also be produced using 270° or even 180° gantry rotation (partial rotation) datasets. Such “partial scan” images have acquisition times proportionately shorter than full rotation scans. This can be useful for reducing movement artifacts in selected patients. For a 0.5 s per rotation scanner, the effective scan acquisition time will be one-quarter of a second (250 ms). If the gantry continues to rotate and acquire data without table movement, continuously updated transmission data will be collected from which revised images can be generated. At any one time, image data of between 0 and 250 ms will be available, i.e., data acquired taking an average time of 125  ms. With extremely rapid processors and appropriate reconstruction algorithms, further delay for image reconstruction can be minimized and a continuously updated CT image is displayed in “near real time” (Hsieh 1997). This so-­ called CT fluoroscopy is ideal for CT-guided interventional procedures. Like conventional fluoroscopic procedures, care should be taken to reduce CT fluoroscopy time to the minimum necessary and to avoid operator irradiation. Instruments designed to keep the operator’s hands out of the CT X-ray beam (Daly et  al. 1998) and use of the lowest selectable tube current sufficient for image quality (Froelich et al. 1999) are advocated. To assist in maintaining short CT fluoroscopy exposure, routine recording and auditing of fluoroscopy exposure times are advocated. An audible alarm after a preset exposure time may also assist in keeping exposures as short as possible. The use of a lead drape adjacent to the irradiated volume has been demonstrated to reduce operator exposure (Nawfel et al. 2000). Several strategies such as iterative reconstruction, automatic dose modu-

Computed Tomography (CT) and CT Arthrography

lation, high-pitch technique, and wide detector row are integrated in CT fluoroscopy to improve procedure success rate while minimizing the radiation dose (Yamamoto et al. 2021).

2.6 Dual-Energy CT Dual-energy CT scanners allow simultaneous acquisition of images at low (70–100  kVp) and high energy (140–150 kVp) levels (Walstra et al. 2019). Data from two different energy levels can be obtained by manipulation at the X-ray tube (single-source helical, single-source twin beam, single-source sequential dual scan, dual sources, or single source with rapid-kilovolt peak switching) or at the detector (dual-layered detector) (D’Angelo et  al. 2019). Standard multidetector CT scanners can perform dual-energy acquisition by sequentially scanning the patient twice using two different energy levels (typically 80 and 140 kVp), but patient motion may produce data misregistration. For dual-energy CT scanners, three major technologies are currently used, namely dual-source dual detector (Siemens Healthineers), rapid-kilovoltage switching (Canon Medical Systems, GE Healthcare), and dual-layered detector (Philips Medical Systems) (Cicero et al. 2020; Rajiah et al. 2020). In dual-source acquisition, data collection occurs simultaneously using two sets of X-ray tubes and detectors oriented at 90° to each other. Each tube emits X-rays at a different peak energy level. The rapid-kilovoltage switching technique rapidly changes tube voltages and peak energy levels (kVp) with a single X-ray tube paired with a detector. The dual-layered detector technology uses a single X-ray tube paired with a two-layer detector made of different scintillator materials. The most superficial layer selectively absorbs the low-energy photons, while the deeper layer absorbs the residual higher photons to create datasets at different energy levels. The clinical applications of dual-energy CT are based on two principles, namely material decomposition and creation of virtual monoenergetic images. Elements such as monosodium

33

urate crystals, calcium, iodine, and hydroxyproline and hydroxylysine in collagenous structures absorb more low-energy photons. This causes differences between the low- and high-energy images, which allow determination of the different chemical compositions (Mallinson et  al. 2016; Chou et al. 2017). Subsequently, the composition can be quantified, subtracted, or enhanced by post-processing software. The clinical applications of material decomposition include detection and monitoring monosodium urate crystal in gout, subtraction of calcium in bone (virtual non-calcium images) for bone marrow lesions, subtraction of iodine (virtual non-contrast images) for the detection of contrast enhancement in tumors or for arthrography, and collagen analysis for tendon and ligamentous injuries. The combination of data from polychromatic low- and high-energy photon beam enables reconstruction of virtual monoenergetic images with wide range of energies. The monoenergetic images at high-energy level are used in metal artifact reduction.

2.7 Data Processing and Reformatted Images The data at each detector element represent the sum of the X-ray attenuation of all tissues through which the beam has passed; this is also called “raw data.” The acquisition data are then processed to construct CT images. Filtered back projection (FBP) has been the standard reconstruction algorithm to transform the “raw data” to image data. In the process of FBP, image sharpening or softening filters (kernels) are applied to the image data to emphasize the details of images, depending on the organ examined and specific application. The sharpening filter enhances the image details but also increases the noise (Fig. 2). The function of FBP is efficient when dealing with noiseless data. Data with high levels of noise, e.g., data from scanning of large patients or using low-dose technique, results in noisy images. The iterative reconstruction method has been integrated in the reconstruction process. Each vendor

N. Pattamapaspong and W. C. G. Peh

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a

b

Fig. 2  Application of filters. Axial CT image of the knee reconstructed with (a) sharp and (b) soft filters. The image with sharp filter reveals the details of bone trabeculae but has more speckles due to noise

applies iterative reconstruction differently, but the remarkable advantage of this method is reduction of radiation dose and artifacts. However, images may appear “over-smooth” or “blotchy” (Geyer et al. 2015) (Fig. 3). Thin detector row and similar pixel sizes in the lateral direction help in creating a high-­ resolution isometric voxel. As the best effective Z-axis resolution for a helical scan acquisition is approximately half the X-ray beam collimation thickness, isometric voxels can be achieved by selecting an optimal combination of scan parameters (Table 1). Volume acquisitions obtained in any plane can therefore be reformatted into other planes without loss of image quality, i.e., multiplanar reformatted (MPR) images. Sagittal and coronal reformations can then be performed with a slice thickness of 1–3  mm to achieve optimal spatial resolution and good contrast-to-noise ratio (Stevens et al. 1999; Winalski and Alparslan 2008; Kalke et al. 2012).

Image processing techniques such as curved planes, 3D rendering, and minimum or maximum intensity projections can produce both visually stunning images and clinically meaningful details of knee lesions (Fig.  4). Production of 3D images has an increasing clinical role in displaying complex anatomy and 3D relationships. Reconstruction of 3D images can be performed by 3D surface rendering (e.g., shaded surface display) or volume rendering techniques. New techniques developed based on the principle of volume rendering include global illumination rendering which is also called cinematic rendering (Siemens Healthineers), global illumination rendering (Canon Medical Systems), photorealistic volume rendering (Philips Medical Systems), or volume illumination (GE Healthcare). These techniques provide a more photorealistic appearance than conventional volume rendering (Blum et al. 2020) (Fig. 5).

Computed Tomography (CT) and CT Arthrography

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a

b

c

d

Fig. 3  Use of iterative reconstruction to reduce artifacts. Axial CT images of a swollen knee with metallic surgical staples reconstructed with (a) filtered back projection (FBP) and (b–d) different levels of iterative reconstruction. Note the reduction of streak artifacts from the metal-

lic staples in images with iterative reconstruction compared to FBP (arrowheads). The appearance of the soft tissue varies among different reconstruction techniques. The outlines of soft tissue in (d) are “blotchy” (arrows) compared to other images

Table 1  Effect of field of view, collimation, and reconstruction interval on pixel and voxel sizes for CT images Field of view (cm) 12.5 25 50

Pixel diameter on 512 × 512 matrix (mm) 0.25 0.5 1.0

Collimation/reconstruction interval required for isometric voxels (spiral mode) 0.5 mm/0.25 mm 1.0 mm/0.5 mm 2.0 mm/1.0 mm

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a

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Fig. 4  Image post-processing techniques. (a) Coronal multiplanar reformatted (MPR), (b) maximum-intensity projection (MIP), and (c) 3D volume rendering CT images of the knee show a comminuted distal femoral fracture.

(d) 3D volume rendering CT image of the distal femur after electronic removal of the patella and tibia shows the fracture fragments more optimally

Computed Tomography (CT) and CT Arthrography

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Fig. 5  Comparison of 3D reconstructed CT images using (a) shade surface rendering, (b) volume rendering, and (c) cinematic volume rendering techniques

3 Scan Image Quality Optimizing image quality results from balancing between achieving high spatial and contrast resolution but minimizing noise and artifacts, while maintaining the optimal radiation exposure. The amount of noise, beam-hardening, and streak artifacts, which commonly degrade a CT image, is dependent upon the following factors: • Collimation slice thickness • Partial- or full-rotation dataset • Mass and distribution of tissue in the scan plane • Scan time/movement • High-density extraneous material (the contralateral limb, contrast medium spills, surgical metalwork) • kVp and mAs • Field of view

• Matrix size • Reconstruction algorithm • Post-processing image sharpening or softening filters • Viewing window width and level settings Most of these factors are amenable to selection or modification by the technologist operating the scanner and can markedly affect the quality of the final image. As demonstrated in Fig.  6, the relationships between image noise, mAs, and patient size are nonlinear, with a halving of patient size resulting in a quartering of image noise, while a fourfold increase in mAs is needed to halve the image noise. Figure  6 also demonstrates that for small patients, image noise is low at all mAs settings and the absolute reduction in image noise achieved by quadrupling the mAs is small. For these reasons, only one knee should be scanned wherever possible, rather than both

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shapes, and size. These artifacts may impair visualization of surrounding tissues and reduce diagnostic accuracy. Metal devices can cause beam 40 hardening, scatter, noise, and photon starvation (Wellenberg et  al. 2018). The metal edges may produce streak artifacts resulting from under-­ 30 sampling, motion, cone beam, and windmill effects (Boas and Fleischmann 2012). 20 Metal artifact reduction can be performed during and after the scan. Care in patient positioning (including using the decubitus position, where 10 necessary), combined with gantry angulation in order to align the scan plane with the long axis of any screws present, will help reduce the number 0 0 200 400 600 800 1000 of sections degraded by streak artifacts from the mAs screws to a minimum (Fig.  7). During image acquisition, increased mAs and kVp, reduced large phantom (1850sq cm) pitch, and use of thin-beam collimation help small phantom (960 sq cm) reduce artifacts. In image reconstruction, use of a soft filter combined with iterative reconstruction Fig. 6  Graph shows the influence of patient size and mAs and applying commercial metal artifact reduction on image noise for a CT scanner. Sections were performed through water density phantoms with 10  mm slice software is recommended (Wellenberg et  al. collimation 2018). In dual-energy CT, virtual monoenergetic together. This reduces image noise, and streak images allow arbitrary selection to balance and beam-hardening artifacts caused by the con- between metal artifacts and visualization of the tralateral limb. A lower mAs setting can then be surrounding structures (Fig. 8). The principle is used with consequent reduced patient radiation to reduce the effect of beam-hardening and streak dose, no X-ray tube loading limitations (cooling artifacts in the polychromatic photon beam. time, limited spiral length), and potentially However, the technique cannot reduce the effect increased tube life. of scatter (Boas and Fleischmann 2012). Virtual Streak artifacts can be generated by high-­ monoenergetic images beyond 60 keV are associdensity material within the scan plane but outside ated with reduction of beam-hardening artifacts. the field of view of the scanner. Scanner tabletops However, in practice, the optimum energies range which contain edge grooves, tracks for the fixing from 108 to 149 keV, depending on metal compoof attachments, or detachable mattresses can act sition and type of fixation (Wong et al. 2018). as traps for spilt-contrast media. Contrast droplets on the scanner gantry window may also cause image artifacts. To remove these sources of arti- 3.2 Image Display: CT Number, fact, care is needed to keep the tabletop and ganHounsfield Unit, Window try clean. Width, and Level

Image noise(SD of pixel values)

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3.1 Internal Metalwork from Fixation Devices Metal artifacts can present with different degrees of severity, depending on metal composition,

The Hounsfield unit (HU) is named after the CT pioneer Sir Godfrey Hounsfield. The HU is a quantitative scale for describing X-ray attenuation. HUs are displayed on images as CT numbers, which is the scale of numbers used to define the gray scale in CT images.

Computed Tomography (CT) and CT Arthrography

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b

Fig. 7  Effect of alignment of metallic devices on artifacts. Coronal reformatted CT images of (a) screws which align along the axis of the scan plane produce smaller artifacts (white arrows) compared to (b) the larger artifact

(black arrow) due to a screw aligned oblique to the scan plane. Note the exostoses (open arrows) in the femur and tibia

The Hounsfield unit value for any material is defined by

This formula relates the HU value to the linear attenuation coefficients of the material being measured and water. As the linear attenuation coefficients of all materials change with X-ray beam energy, there are consequently only two fixed points on the Hounsfield scale. These are −1000, which is the HU value for no X-ray attenuation (i.e., a vacuum), and zero, which corresponds to the HU value for water (at the calibration pressure and temperature for the scanner). The HU scale is open ended, with high-­



HU s = 1000 ( ms - m w / m w )



where HUs = the Hounsfield unit value for substance s. μs  =  the linear attenuation coefficient for substance s. μw = the linear attenuation coefficient for water.

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a

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Fig. 8  Utility of virtual monoenergetic images in dual-­ energy CT to reduce artifacts. Axial CT images of a knee with tumor (arrows in a) acquired using (a) polychromatic photon beam and (b–f) virtual monoenergies at (b) 90  keV, (c) 100  keV, (d) 120  keV, (e) 140  keV, and (f)

150  keV allow arbitrary selection to balance between metal streak artifacts due to the metallic implants and visualization of the tumor. The artifacts progressively decrease with increasing keV

Computed Tomography (CT) and CT Arthrography

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f

Fig. 8 (continued) Table 2  Theoretical HU values for a variety of materials at 65 keV Material Adipose tissue Water Collagen Dense cortical bone Aluminum Iron Iodine Lead

HU value −80 0 250 1600 2300 34,000 141,300 205,000

atomic-­ number, high-density materials having values way in excess of the upper end of the usual scale of CT number of the scanners (Table  2). However, some limitations such as scanner drift, calibration error, or artifacts may render the CT number inaccurate. The HU values for materials differ when examined using different beam energy. The theoretical HU value for dense cortical bone calculated at an effective beam energy of 65  keV (equivalent to a scanner operating at around

120  kVp) is in the region of 1600 (Fig.  9). At lower energies (e.g., 55 keV which is the approximate effective energy of a scanner operating at 80 kVp), the HU value for dense cortical bone is more than 2000. Other high-atomic-number materials (contrast media, aluminum and metal fixation devices) also show marked variation in HU values with differing beam energies. In contrast, the HU values of soft tissues, collagen, and fat vary very little with effective beam energy, as the linear attenuation coefficients for these materials closely follow those of water. Consequently, in scanners which allow the operating voltage to be changed, the CT number for bone can be increased by using a low kVp (i.e., around 80 kVp). This increases the dependence of the CT number on the presence of bone or calcification and is hence particularly used for quantitative measurement of mineral density. A high kVp (usually around 140  kVp) can be selected to reduce the CT number of bone and metalwork, which has some effect in reducing streak artifacts.

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42 Fig. 9  Graph shows the influence of scanner kVp on CT numbers for different tissues: bone, collagen, soft tissue, and fat

3000

CT number

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Effective kV

For materials with lower atomic numbers (e.g., soft tissue structures), the X-ray attenuation and consequent CT number are predominantly influenced by the electron density of the material; this is, in turn, closely related to the physical density of the material. Even the CT number of water is influenced by differences in temperature, as differences in density exist between water at room and at body temperatures. The presence of protein or high concentrations of salts will increase the CT number of body fluids. Measurement of the CT number of a region of interest in an image must therefore be considered only a guide to its composition. As an example which is potentially relevant to research, the CT number of ice at 0 °C (approximately −80 HU) is lower than that of fat (the CT number of which increases as it cools). Therefore, specimens scanned straight from the freezer may look quite different to that expected (Whitehouse et  al. 1993). The visual impression of the density of a region of interest is influenced by the window

and level settings of the image, the calibration of the display, and the densities in the surrounding part of the image. Within bone, the surrounding high density of bone can give a lytic lesion the visual impression of a lower density than actually exists. Consequently, actual measurement, rather than estimation, of any region of interest is essential. The window width and level are calibrated contrast and brightness settings for image display. All scanners have preset buttons allowing different window/level combinations to be instantly applied. These commonly have settings deemed appropriate for various structures, such as the bone, lung, and brain, but typically, the bone setting is aimed at an overview of trabecular bone, which may be inappropriate for subtle cortical bone lesions (Fig. 10). The most appropriate window level for cortical bone will be influenced by the bone density and the effective scan energy, while the window width may need to be quite narrow to demonstrate subtle intracortical density changes.

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Fig. 10  Selection of the most appropriate window level and width. Axial CT images show early lytic phase Paget’s disease of bone causing subtle reduction in cortical density in the distal femur. Coarsening of the trabecular pattern in the condyles is visible on a “standard” bone

window (left images), but the reduction in cortical density of the femoral shaft requires a narrower window for clear demonstration of the osteolytic areas (right images). (Courtesy of Dr. A.  Horrocks, Wythenshawe Hospital, UK)

4 CT of the Knee

knee can be demonstrated, and the clinical significance ascribed to its various structures, has improved significantly.

Application of CT to imaging of the knee started in the 1980s, as the technique became widely available and equipment improved. Descriptions of patient positioning, immobilization methods, and reformatting image datasets (including 3D reconstructions) that were then described are still adaptable to modern scanners. With isometric voxels, the detail in which the anatomy of the

4.1 Anatomy A detailed knowledge of the appearances of the knee and surrounding structures in all imaging planes is necessary for adequate interpretation.

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High-quality reformations with isometric voxels allow visualization of anatomical structures in different planes without distortion. CT is the ideal modality for the evaluation of bony structures but has limited value for soft tissue structures, as the soft tissue contrast is not as good as

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Fig. 11  Axial CT images show anatomic structures of the left knee from proximal to distal: (a–d) 1—femur, 2— patella, 3—medial femoral condyle, 4—lateral femoral condyle, 5—tibia, 6—fibula, 7—quadriceps tendon, 8— vastus medialis muscle, 9—vastus lateralis muscle, 10— iliotibial band, 11—biceps femoris muscle and tendon, 12—semitendinosus muscle and tendon, 13—semimembranosus muscle and tendon, 14—gracilis muscle and ten-

MR imaging. Hence, anatomical details and pathology of structures such as the menisci and ligaments are much better depicted by MR imaging. Keeping in mind its limitations, CT images are able to show many bone and soft tissue structures of the knee (Figs. 11, 12, and 13).

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don, 15—sartorius muscle and tendon, 16—medial head of gastrocnemius muscle, 17—lateral head of gastrocnemius muscle, 18—medial retinaculum, 19—lateral retinaculum, 20—medial collateral ligament, 21—lateral collateral ligament, 22—popliteus muscle, 23—patellar tendon, 24—popliteal vessels, 25—sciatic nerve, 26— tibial nerve, 27—common peroneal nerve, 28—great saphenous vein

Computed Tomography (CT) and CT Arthrography

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Fig. 12  Coronal reformatted CT images show anatomic structures of the left knee from anterior to posterior: (a–d) 1—femur, 2—tibia, 3—medial meniscus, 4—lateral meniscus, 5—vastus medialis muscle, 6—vastus lateralis

muscle, 7—iliotibial band, 8—medial collateral ligament, 9—lateral collateral ligament, 10—popliteus tendon, 11—biceps femoris tendon, 12—fibula, 13—popliteal vessels

4.2 Immobilization

the patient is fitted with a plaster of Paris cast or back slab. Scanning through a plaster cast does not significantly interfere with image quality, while the immobilization achieved is usually excellent, such that a temporary cast is also worth considering for occasional patients who are inadequately immobilized using other methods.

Although CT of the knee is a rapid procedure, immobilization may be necessary to prevent movement artifacts, particularly in children. Sandbags, straps, and sticking tape usually suffice. Even better immobilization can be achieved routinely in the setting of trauma if

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a

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Fig. 13  Sagittal reformatted CT images show anatomic structures of the left knee from medial to lateral: (a–d) 1—femur, 2—tibia, 3—vastus medialis muscle, 4— medial head of gastrocnemius muscle, 5—medial menis-

b

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cus, 6—patella, 7—quadriceps tendon, 8—posterior cruciate ligament, 9—popliteal vessels, 10—patellar tendon, 11—lateral meniscus, 12—lateral head of gastrocnemius muscle

Computed Tomography (CT) and CT Arthrography

5 Indications CT of the knee is particularly suited to the demonstration of bony anatomy such as the evaluation of fractures, bony morphological abnormalities, and patellofemoral tracking (Balassy and Miller 2013; Lansdown and Ma 2020). Intra-articular fractures, in particular, should be assessed by CT.  Intraosseous tumors are well demonstrated; for example, the nidus of an osteoid osteoma, which can be overlooked on MR imaging, is characteristic and clearly demonstrated on CT.  The presence of tumor matrix ossification or calcification is also clearly seen on CT. In osteomyelitis, the presence and location of sequestra are most optimally revealed. With the addition of arthrography, osteochondral lesions are well demonstrated. Soft tissue pathology is less well demonstrated compared to MR imaging, and although intravenous contrast medium injection provides less satisfactory contrast enhancement than the equivalent MR examination, valuable information such as size, extent, calcification, enhancement, and articular involvement of soft tissue lesions is still obtainable from CT.  CT can be used to guide biopsy and cyst aspiration procedures (Antonacci et al. 1998) (Fig. 14).

Fig. 14  CT-guided aspiration of an anterior cruciate ligament ganglion cyst. A posteromedial approach is used to avoid the popliteal vessels

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The accurate 3D localization of the bone anatomy from MPR images can be used to calculate the mechanical axes of long bones and the relationships of the joints. This information can be used in the preoperative planning of joint replacements. Morphological differences have also been demonstrated between patients with anterior knee pain and patients with asymptomatic knees. CT has also been used after joint replacement to identify the relationship between knee morphology, alignment of the prosthesis, and outcome. The CT scanogram (or scout view), usually used to identify the start and finish points for a CT scan, can also be used for limb length measurements (Silva et  al. 2019). CT angiography can demonstrate the relationship between bone and vascular structures (Sananpanich et al. 2020). The limitations of CT are usually described in relationship to MR imaging, with the poorer soft tissue contrast of CT being the most important one. Where MR imaging is available and not contraindicated, it is the most appropriate modality for imaging soft tissue lesions in and around the knee. The other major limitation of CT in relation to MR imaging is its ionizing radiation hazard.

5.1 Trauma In the acutely traumatized patient, speed and patient safety are important requirements for a satisfactory examination. This gives limited scope for scan technique modifications. As the primary aim of the examination is to determine the size and position of fracture fragments and joint alignment, the aim is to ensure adequate coverage of the injured region. The scanogram is routinely used to determine the appropriate start and end points for image acquisition. The smaller the collimation thickness, pitch, and reconstruction interval, the better the quality of the reformatted planar and 3D images (Fig.  15). 3D reconstructions provide an overview of fracture fragment disposition that facilitates interpretation and is particularly useful in badly comminuted and complex fractures.

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Fig. 15  Coronal reformatted CT image of a depressed tibial plateau fracture. Scan was acquired helically with 0.6  mm collimation and a pitch ratio of 0.7 and reconstructed at 2 mm increments

High-energy fractures of the distal femur and tibia plateau that are badly comminuted require CT to determine the surgical approach and fixation strategies. For instance, in fractures of the distal femur, CT helps detection of the coronal plane femoral condylar fracture, also called a Hoffa’s fracture, which is often difficult to visualize on knee radiographs (Fig. 16). The presence of Hoffa’s fragments affects the surgical strategy, as the fragment usually displaces during routine placement of angle blade plate and requires additional fixation in the anteroposterior direction (White et al. 2015; Baker et al. 2002) (Fig. 16c). For the tibial plateau fracture, the CT-based classification using the three-column system described by Luo et  al. (2010) emphasizes the need for a specific surgical incision when the posterior tibia is affected. Involvement of ligamentous or tendon attachment is well depicted by CT (Fig. 17).

N. Pattamapaspong and W. C. G. Peh

Avulsion injuries are well recognized around the knee. Sites include the tibial tuberosity and inferior pole of the patella (patellar tendon), posterior tibial plateau (posterior cruciate ligament avulsion is rare), tibial eminence (anterior cruciate ligament), lateral tibial plateau (“Segond fracture,” lateral capsular ligament), and fibular head (conjoint tendon of lateral collateral ligament and biceps femoris). Most avulsion injuries are usually evident on conventional radiographs, but the tibial eminence and posterior tibial plateau avulsion injuries, in particular, are less clearly visualized on radiographs. CT can more clearly demonstrate these fractures (Stevens et al. 1999). In dual-energy CT, virtual non-calcium images can be acquired in which bone calcium is subtracted, enabling CT to demonstrate bone marrow lesions (Fig.  18). Compared to MR imaging, dual-energy CT shows high sensitivity (84–94%) and high specificity (95–99%) in the detection of bone marrow contusion of the knee (Pache et al. 2010; Cao et al. 2015; Booz et al. 2020). As CT is readily accessible in the setting of acute trauma, this technique aids in the confirmation of radiographically occult fractures and helps in patient reassurance (Hickle et  al. 2020). Nevertheless, this technique is unable to demonstrate bone marrow lesions located near to cortical bone because of the volume-averaging effect. The high-attenuation pixels adjacent to the cortex are unintentionally subtracted as a part of threshold-­based calcium subtraction (Pache et  al. 2010; Mallinson et al. 2016). In the detection of fractures, the added value of dual-energy CT over conventional CT requires more validation. Although material decomposition of dualenergy CT allows analysis of collagenous structures, its accuracy in the detection of ligament and tendon injury and its role in clinical practice require more supportive evidence. Currently, MR imaging remains the modality of choice for imaging ligamentous and tendon injuries in and around the knee.

Computed Tomography (CT) and CT Arthrography

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Fig. 16  CT detection of a coronal plane Hoffa’s fracture. (a) Lateral radiograph shows a linear lucency, which is suggestive of a fracture (arrows). (b) Axial CT confirms

the coronal plane fracture of the femoral condyle. (c) Postoperative radiograph shows screw fixation in anteroposterior direction (open arrow)

5.2 Knee Morphology and Surgery

field variations cause image distortion) or magnification (unlike conventional radiography) of the images. This is true for CT images acquired with no gantry angulation. Scanograms suffer from magnification perpendicular to the direction of table travel but not along it. With these provisos, CT provides 3D localization by virtue of the X and Y coordinates in the CT image, and the Z coordinate by the table location.

CT is ideal for localizing anatomical reference points in 3D. The physics of the process involves clearly defined table positions and utilizes the straight-line paths of X-rays from the known tube locations to the known detector locations. Consequently, there should be no distortion (compared with MR imaging, where magnetic

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Fig. 17  Usefulness of reconstructed CT images in preoperative assessment. (a) 3D reconstructed CT image using volume rendering shows the three columns of tibial plateau. (b) 3D reconstructed CT image using cinematic vol-

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ume rendering shows fractures of posterior and medial column (arrows). (c) Sagittal reformatted CT image shows the relationship between the fracture and posterior cruciate ligament (open arrow)

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Fig. 18  Utility of dual-energy CT in demonstrating bone marrow edema. (a) Sagittal reformatted CT image shows a fracture tibial plateau (arrow). (b) Corresponding virtual

non-calcium dual-energy CT image shows the extent of bone marrow edema (arrowheads) around the fracture

The scanogram can be used to identify the structures to be localized, single transverse sections performed at each required level, and X, Y, and Z coordinates recorded. From these points, distances and angles can be calculated or ­measured directly on appropriate transverse or reformatted images. Such measurements have been used to define the “Q” angle (alignment of rectus femoris to the patellar tendon) (Ando

1999), the angle of rotation of the tibia with respect to the femur (Nagao et al. 1998), the morphology of the femoral trochlear groove (Martino et al. 1998), the location of the tibial tuberosity in flexed osteoarthritic knees (Nagamine et  al. 1997), and the knee version in anterior knee pain (Eckhoff et al. 1997). In preoperative assessment, CT can be used to develop patient-specific prosthesis for joint replacements or intraoperative

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Fig. 19  Usefulness of reconstructed CT images in postoperative assessment. (a) Sagittal reformatted CT image shows position of the tibial tunnel of the anterior cruciate

ligament graft (arrow). (b) 3D reconstructed CT image shows the relationship between screws and tunnels (arrowheads) more optimally

localization of prosthetic alignment (Sezer et al. 2021). Finally, CT has been used for postoperative assessment of anterior cruciate ligament reconstruction and planning for revision (Kim et al. 2016; Kosy and Mandalia 2018) (Fig. 19).

space narrowing, and/or tilting that is not visible on radiographic examination (Schutzer et  al. 1986). The addition of loading aimed at putting tension on the extensor mechanism further increases the sensitivity of the examination for maltracking (Shellock et  al. 1993). With the advent of helical CT scanning, yet further sophistication of the technique is possible, with continuous data acquisition during active flexion and extension of the knee to give kinematic CT imaging. This can also be performed with or without additional loading (Dupuy et  al. 1997; Demehri et  al. 2014; Drew et  al. 2016). The images obtained from static, dynamic, and loaded or unloaded studies are assessed for lateral shift or tilting of the patella, thinning of the joint cartilage, and other osteoarthritic changes.

5.3 Patellofemoral Joint Knee flexion to varying degrees has been advocated for the assessment of patellofemoral joint alignment. Radiographic assessment using skyline views in 45° of knee flexion (Merchant views) has been largely superseded by CT or MR imaging evaluation, as once flexed to this degree, the patella is forced into the intercondylar groove, even when prone to subluxation or maltracking (Walker et al. 1993). Imaging at lesser degrees of flexion is not easily or consistently achieved with radiographic techniques; hence, CT methods were developed. Single axial sections through the mid-patella with the knee fully extended and then passively placed at 10°, 20°, and 30° of flexion (Fig.  20) can demonstrate malalignment, joint

5.3.1 Static CT of the Patellofemoral Joint With the patient lying supine on the scanner table, the knee is placed flexed over a series of

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traction with the knee straight, being less marked at 30° of flexion and appearing normal at 60° flexion (Biedert and Gruhl 1997).

Fig. 20  Static unloaded patellar tracking using CT in a patient with nail-patella syndrome (hypoplastic patellae). Single 10-mm-thick low-mAs axial CT scans were performed through the patella at 10°, 20°, and 30° of knee flexion

padded wedges to produce 10°, 20°, and 30° of angulation. A single low mAs axial section through the mid-part of the patella is acquired at each angulation for an unloaded CT assessment of the patellofemoral joint. In an average-sized adult, in whom both the leg and the thigh are approximately 450 mm long, raising the back of the knee by only 40, 80, and 115  mm from its extended position achieves these respective angulations. For a static loaded examination, the knee needs to be held against resistance applied to the anterior shin, ankle, or foot to put the extensor mechanism into tension, while similar CT sections (as above) are obtained. Asking the patients to hold their foot up against a restraining bar or strap over the ankle will achieve this. Greater loading can be achieved with weighted boots worn during the same maneuver. Abnormal patellofemoral relationships have been found to be greatest during maximal quadriceps muscle con-

5.3.2 Dynamic CT of the Patellofemoral Joint To achieve a truly dynamic assessment of the patellofemoral joint, continuous imaging during movement of the knee is required. This is achievable with helical scanning performed continuously during active flexion and extension of the knee. If the rate of movement is kept relatively slow and well controlled, any movement artifacts appearing on the subsequent images are less obvious. This method has been described by Dupuy et al. (1997) and reviewed by Muhle et al. (1999). The technique can be performed with or without additional loading, with wearing of a weighted boot used to achieve greater loading. Several techniques have been described with variable degrees of knee flexion-extension range and patient position (Dupuy et  al. 1997; Muhle et al. 1999; Demehri et al. 2014). The principle is to maintain smooth and controlled motion. Patient training is important to avoid acquisition repetition, which may lead to unnecessary radiation exposure. Scanning can be performed with either intermittent acquisition with time interval less than 1  s or continuous acquisition (Demehri et al. 2014; Teixeira et al. 2015). The continuous acquisition mode is suitable in patients with a snapping joint, but the scan should be kept within 5 s, in order to maintain an acceptable radiation dose (Teixeira et al. 2015). For examination of the knee joint, wide-detector CT scanners with coverage in Z-axis at least 140 mm are recommended (Teixeira et al. 2015).

5.4 Articular Cartilage Thin-section CT combined with arthrography was previously the study of choice for measuring articular cartilage thickness and volume in the knee (Fig.  21). However, CT has now been replaced by MR imaging, which allows the evaluation of molecular properties of cartilage and underlying bone marrow lesion without radiation or need for any invasive procedure (Lansdown

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and Ma 2020). Nevertheless, CT arthrography currently remains the best modality for the detection of subtle cartilage surface lesions (Omoumi et al. 2017). This study can precisely determine the size and configuration of cartilage defects, and aids creation of a 3D printing model of knee cartilage (Michalik et al. 2017).

5.5 Soft Tissues

Fig. 21  Axial (top) and sagittal reformatted (bottom) CT arthrography images show patellar cartilage thickness. (Courtesy of Dr. S.  Bianchi, Hôpital Cantonal, Geneva, Switzerland)

Imaging of soft tissue and synovial lesions of the knee is best undertaken by MR imaging, either with or without contrast enhancement, augmented by radiographs and possibly ultrasonography. CT has a limited role where these methods are contraindicated or unavailable, but some pathologies (e.g., fatty tumors such as lipoma arborescens, calcified lesions such as synovial osteochondromatosis (Fig. 22), gouty tophi, and dense lesions such as pigmented villonodular synovitis) may have characteristic appearances on CT (Chen et al. 1999; Lin et al. 1999). CT is however unreliable for follow-up scanning of the resection site of soft tissue sarcoma (Hudson et al. 1985). Dual-energy CT helps confirm the presence of monosodium urate crystals in gouty tophi, which occasionally mimics tumors (Fig. 23). Common

Fig. 22  Synovial osteochondromatosis. Axial CT images show multiple peripherally calcified lesions present within a knee joint effusion

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Fig. 23 Utility of dual-energy CT in demonstrating gouty deposits in a 40-year-old man presenting with a knee mass. (a) Axial CT image shows a mass in the patellar tendon (arrows). (b) In the corresponding axial CT image, the green color code indicates deposition of mono-

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sodium urate crystals. (c) 3D reconstructed CT image with volume quantification shows urate volume of 8.33  cm3. (d) Repeat 3D reconstructed CT image with volume quantification obtained after 5  months of treatment shows reduction of urate volume to 3.08 cm3

Computed Tomography (CT) and CT Arthrography

locations for urate crystal accumulation in the knee are the popliteus tendon, quadriceps tendon, menisci, and cruciate ligaments, but any part of the knee can be affected (Mallinson et al. 2014). Dual-energy CT yields a high sensitivity and specificity (>80%) in the detection of gout and has high reproducibility (intra-class correlation a

Fig. 24  Combination of contrast enhancement with 3D volume rendering. (a) MIP contrast-enhanced CT image shows a vascular malformation in the left leg (arrows). (b) 3D volume-rendered reconstructed CT image from the

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coefficients >0.9) (Ramon et al. 2018). Volumetric quantification of monosodium urate crystals allows reliable monitoring of the response to treatment (Chou et al. 2017). Contrast enhancement can be used together with volume rendering to demonstrate vascular lesions in relation to the knee joint (Fig.  24). b

same data shows the malformation more optimally in relation to the knee joint. (Courtesy of Dr. T.  Srisuwan, Chiang Mai University, Chiang Mai, Thailand)

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b

Fig. 25  Utility of virtual non-contrast to confirm the contrast CT images of the knee show the bone tumor enhancing area in a patient with a tumor in the distal (black arrows) and popliteal vessels (open arrows) femur. (a) Axial contrast-enhanced and (b) virtual non-­

Similarly, soft tissue enhancement in masses or synovium can be demonstrated, but the timing is critical, with peak enhancement being later, less marked, and more variable in onset, compared to CT of the abdomen. CT scanner software which is able to pre-scan the region of interest at a low mA to detect the onset of enhancement and triggers the study at the ideal point of time may have a role. Dual-energy CT with iodine subtraction or virtual non-contrast technique can confirm the area of soft tissue enhancement without an additional precontrast scan (Fig. 25).

6 CT Arthrography 6.1 Role and Indications Indications for CT arthrography are the demonstration of intra-articular structures in patients with contraindications to MR imaging or presence of metal hardware (Baker et  al. 2018) (Fig. 26). The added benefits of arthrography are

increased intra-articular pressure pushing the injected iodinated contrast agent into lesions such as meniscal tear or osteochondral lesions. Arthrography combined with either CT or MR imaging is recommended in the evaluation of the postoperative meniscus (Baker et  al. 2018), anterior cruciate graft tear (McCauley et  al. ­ 2003), preoperative planning, and postoperative assessment of autologous chondrocyte implantation (Kalke et al. 2012). CT arthrography is particularly useful in the evaluation of polyethylene liner-related complications in knee prosthesis (Bradshaw et al. 2014; Hsu et al. 2019).

6.2 Technique The patient lies in the supine position with the knee slightly flexed, to relax the extensor mechanism. The procedure is generally performed under fluoroscopic guidance, but ultrasound guidance or palpation can be used. Using an aseptic technique and following infiltration of

Computed Tomography (CT) and CT Arthrography

57

Fig. 26  Sagittal reformatted CT arthrography images show normal lateral and medial menisci. (Courtesy of Dr. S. Bianchi, Hôpital Cantonal, Geneva, Switzerland)

local anesthesia, a 22-gauge needle is introduced into the patellofemoral joint from either the medial or the lateral side, holding the patella and displacing it toward the side chosen for needle introduction. The patellar displacement makes the articulation wider and more prominent, assisting in needle placement. The volume capacity of the knee joint is 35–50 mL (Chung et al. 2005). Fluoroscopic screening during injection confirms the correct needle location as the contrast agent should flow rapidly away from the needle tip into the joint. A misplaced needle results in focal accumulation of the contrast agent at the needle tip. After removing the needle, gentle manipulation ensures that the contrast agent extends into all the joint capsular recesses. After

the procedure, CT should be performed with as little delay as possible, in order to avoid excessive contrast resorption. Delayed post-­ arthrography scanning may be necessary if communication between the knee joint and any nearby cyst or ganglion is suspected, as it may take 1–2  h before the contrast agent appears within a cyst (Malghem et  al. 1998). However, the scan should not be delayed for more than 3.5 h (Wagner et al. 2001). A limitation of CT arthrography is that diagnostic quality reconstructed 3D images cannot be created, owing to the inability to separate iodine from cortical bone. Occasionally, differentiation between calcium deposition in chondrocalcinosis or urate crystal from a contrast-filled meniscal tear is difficult. Dual-energy CT helps eliminate

58

these problems by distinguishing calcium and urate from iodine and using the virtual non-­ contrast technique for iodine subtraction (Sandhu et al. 2021). The optimal concentration of iodine in the mixture should be approximately 75  mg/ mL for successful subtraction. Higher iodine concentration tends to result in failed subtraction (Chai et al. 2014).

7 CT-Guided Interventions CT is being increasingly used to guide interventional procedures, particularly with the recent development of CT fluoroscopy, which enables more rapid and accurate placement of needles and interventional devices (Froelich et al. 1999; de Mey et  al. 2000). As described above (Sect. 2.5), care needs to be taken to minimize operator and patient X-ray exposure during CT-guided biopsy. CT fluoroscopy screening times of around 10  s should suffice for most biopsy procedures (Goldberg et al. 2000). Limiting the fluoroscopy to identification of the needle tip rather than the entire needle will also reduce operator and patient radiation dose (Silverman et  al. 1999). The CT section thickness should be appropriate to the size of the lesion; otherwise, partial volume averaging may include both the needle tip and the lesion in the same section, erroneously suggesting an accurate needle location. CT can be used to guide biopsy of bone and soft tissue lesions. Where primary malignancy is present, the course of the biopsy track and the compartment(s) through which it passes may need excision with the tumor at the time of definitive surgery. Biopsy of such lesions must therefore only be performed after consultation and agreement on the approach with the surgeon who will carry out the definitive treatment. The accu-

N. Pattamapaspong and W. C. G. Peh

racy of CT-guided biopsy is increased if specimens are obtained for both cytology and pathology, with an overall accuracy of around 80% being achievable (Hodge 1999). Due to their deep location, ganglion cysts of the anterior cruciate ligament may be difficult to identify on ultrasonography. CT-guided aspiration of these cysts with a wide-bore (14 G) needle followed by injection of 40 mg methylprednisolone and 1–2  mL 0.5% bupivacaine (Antonacci et al. 1998) has been successful for symptomatic relief. Using a posterior approach, care should be taken to avoid the popliteal vessels (Fig.  14). Percutaneous treatment of osteoid osteoma can also be ­performed with CT guidance. Again, a planned approach avoiding vascular structures is required, and a preliminary contrast-enhanced scan to clearly identify the vessels is valuable (Fig. 27). It is possible to treat osteoid osteomas by complete removal via CT-guided biopsy (Voto et al. 1990), but this may be difficult to achieve, unless a large-bore needle is used and several passes are made through the lesion. Imaging-guided techniques aimed at destroying the tumor include ablation techniques using radiofrequency, interstitial laser, microwave, or cryoablation (Parmeggiani et  al. 2021). The imaging features of a nidus less than 1.5  cm in size with surrounding sclerotic bone combined with typical nocturnal pain are characteristic findings of osteoid osteoma. However, when in doubt, a biopsy should be performed as histological confirmation will not be possible after ablation (Singh et  al. 2020). Sans et  al. (1999) reported osteoid osteoma mimickers to be as high as 16%. Ablation of osteoid osteoma causes severe pain, and spinal anesthesia is preferred. General anesthesia may be necessary in pediatric patients. Local anesthesia alone is generally inadequate (Pinto et al. 2002; Rosenthal et al. 2003).

Computed Tomography (CT) and CT Arthrography

a

59

b

c

Fig. 27  Axial contrast-enhanced CT images show the location of (a) arteries and (b) veins to assist in (c) CT-guided ablation of an osteoid osteoma in the proximal

tibia. Note the incidental bone island. (Images courtesy of Dr. P. Hughes, Derriford Hospital, Plymouth, UK)

60

8 Conclusion

N. Pattamapaspong and W. C. G. Peh

liner disengagement identified by arthrography. Knee 21(6):1288–1290. https://doi.org/10.1016/j. knee.2014.07.012 The continuous improvement of CT equipment Cao JX, Wang YM, Kong XQ, Yang C, Wang P (2015) and post-processing techniques, as well as the Good interrater reliability of a new grading system in detecting traumatic bone marrow lesions in the knee development of dual-energy CT, has led to an by dual energy CT virtual non-calcium images. Eur increase in the role of CT for the diagnosis and J Radiol 84(6):1109–1115. https://doi.org/10.1016/j. management of many conditions in and around ejrad.2015.03.003 the knee. Understanding its limitations and Chai JW, Choi JA, Choi JY, Kim S, Hong SH, Kang HS (2014) Visualization of joint and bone using dual-­ strengths is important toward achieving approprienergy CT arthrography with contrast subtraction: ate applications of CT in clinical practice. in vitro feasibility study using porcine joints. 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63 Winalski CS, Alparslan L (2008) Imaging of articular cartilage injuries of the lower extremity. Semin Musculoskelet Radiol 12(4):283–301. https://doi. org/10.1055/s-­0028-­1100637 Wong WD, Shah S, Murray N, Walstra F, Khosa F, Nicolaou S (2018) Advanced musculoskeletal applications of dual-energy computed tomography. Radiol Clin North Am 56(4):587–600. https://doi. org/10.1016/j.rcl.2018.03.003 Yamamoto S, Matsumoto T, Suda S et  al (2021) First experience of efficacy and radiation exposure in 320-detector row CT fluoroscopy-guided interventions. Br J Radiol 94(1120):20200754. https://doi. org/10.1259/bjr.20200754

Magnetic Resonance Imaging Manickam Subramanian , Michael S. M. Chin, and Wilfred C. G. Peh

6    MRI Protocol for Routine Examination of the Knee   88

Contents 1      Introduction 

 66

2      General Considerations 

 66

3      Basic Concepts  3.1   Signal-to-Noise Ratio  3.2   Contrast-to-Noise Ratio  3.3   Spatial Resolution  3.4   Scan Time 

 69  69  70  70  71

4      Image Contrast and Pulse Sequences  4.1   T1-Weighted Spin Echo Sequence  4.2   T2-Weighted Spin Echo Sequence  4.3   Proton Density-Weighted Spin Echo Sequence  4.4   Intermediate-Weighted Spin Echo Sequence  4.5   Fast Spin Echo Sequences  4.6   Gradient Echo Sequences  4.7   3D Sequences  4.8   Fat-Suppression Techniques  4.9   Diffusion-Weighted Imaging  4.10  Rapid Image Acquisition Techniques 

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5      Patient Positioning and Imaging Planes  5.1   Menisci  5.2   Ligaments  5.3   Bone  5.4   Hyaline Cartilage 

 83  84  85  86  87

 74  74  75  76  78  79  82  82

M. Subramanian · M. S. M. Chin · W. C. G. Peh (*) Department of Diagnostic Radiology, Khoo Teck Puat Hospital, Singapore, Singapore e-mail: [email protected]; [email protected]; [email protected]

7      M  RI Protocols for Specific Clinical Problems  7.1  Synovitis and Inflammatory Arthritis  7.2  Bone and Soft Tissue Infections  7.3  Bone and Soft Tissue Masses  7.4  MR Arthrography  7.5  MR Neurography  7.6  Cartilage Mapping Techniques  7.7  MRI of the Postoperative Knee 

 89  89  92  93  94  94  95  96

8      Recent Advances  8.1  Ultrahigh Field Strength MR Imaging  8.2  Ultrashort TE Sequences  8.3  Sodium Imaging  8.4  GAG Chemical Exchange Saturation Transfer Imaging  8.5  Synthetic MRI  8.6  MR Spectroscopy  8.7  Diffusion Tensor Imaging  8.8  MR Elastography  8.9  Radiomics in MSK Imaging 

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9      Artifacts  9.1  Motion Artifacts  9.2  Chemical Shift Artifacts  9.3  Magic Angle Phenomenon  9.4  Truncation Artifacts  9.5  Partial Volume Averaging Artifacts  9.6  Susceptibility Artifacts  9.7  Metallic Artifacts 

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10    Conclusion 

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References 

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Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_350, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 04 March 2023

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diagnostic and management road map for the referring clinician. MRI of the knee utilizes a Magnetic resonance imaging (MRI) is the combination of pulse sequences that are sensitive imaging modality of choice for comprehen- and specific for detecting pathology in the varisive evaluation of internal derangement of the ous tissues such as menisci, ligaments, tendons, knee. MRI of the knee utilizes a combination muscles, articular cartilage, synovium, and of pulse sequences that are sensitive and spe- bones. In routine practice, most institutions cific for detecting pathology in the various tis- establish a routine protocol using a combination sues such as menisci, ligaments, tendons, of proton density (PD)-, T1-, T2-, and/or muscles, articular cartilage, synovium, and intermediate-­weighted images, with or without bones in the shortest possible time. This chap- fat suppression. Images obtained in three anater aims to provide an overview of MRI sys- tomical planes provide adequate visualization of tems and basic concepts of MRI such as most of the knee structures and answers the signal-to-noise ratio and spatial resolution, as majority of clinical questions in a relatively short well as various pulse sequences, including fat-­ time. Supplementary protocols may be employed suppression techniques. Routine and supple- to address specific clinical problems. Commonly mentary protocols of knee MRI, recent used musculoskeletal pulse sequences, routine advances, and common MRI artifacts and how and supplementary protocols of the MRI knee, to correct them are also addressed. recent advances, and common MRI artifacts and how to correct them are discussed in this chapter. Abstract

Abbreviations

2D Two-dimensional 3D Three-dimensional CNR Contrast-to-noise ratio FOV Field of view FSE Fast spin echo MRI Magnetic resonance imaging NEX Number of excitations PD Proton density RF Radiofrequency SAR Specific absorption rate SE Spin echo SNR Signal-to-noise ratio T Tesla TE Time to echo TR Time to repetition

1 Introduction In most clinical radiology departments that provide a magnetic resonance imaging (MRI) service, the knee is usually the most commonly imaged joint. MRI is the investigation of choice for enabling a comprehensive examination of structures in and around the knee and provides a

2 General Considerations Diagnostic quality MRI can be performed using magnets of various field strength magnets, ranging from 0.2 to 10.5 Tesla (T), and with variety of magnet designs such as open whole-body permanent magnets, dedicated extremity magnets, and closed-bore whole-body superconducting magnets. The MR scanners are often categorized as being low field, mid field, high field, and ultrahigh field, based on the field strength of the magnet. The MR field strength is an important factor in determining the quality of the MR images produced. Although the distinctions among the different categories can vary, these classifications are generally accepted: low field (≤0.5  T), mid field (>0.5 to 100 ms will take approximately 14  min, while a T2-weighted FSE sequence with a turbo factor of 15, TR of 4770 ms, and TE of 106 ms will only

take 1–2  min. Therefore, conventional T2-weighted SE images are seldom acquired in routine clinical practice and have mostly been replaced by FSE sequences.

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a

b

Fig. 3  Usefulness of fat-suppressed T2-weighted MR images for fluid detection. (a) Axial fat-suppressed T2-weighted FSE MR image of the knee shows subcutaneous edema and fluid within the intermuscular compart-

ment of the calf indicating fasciitis (arrow). (b) The fluid is less conspicuous, being hypointense on the T1-weighted image

4.3 Proton Density-Weighted Spin Echo Sequence

SE sequence with TR of 4040 ms and TEs at 32 and 86  ms will take approximately 4–5  min. In modern practice, double echo SE sequences have been superseded by FSE sequences, in which several 180° refocusing pulses per single TR are applied with different phase-encoding gradients for each pulse, resulting in more data being acquired per TR and a significant reduction of scan time.

Proton density (PD)-weighted spin echo images have a relatively long TR (>1500 ms) and short TE (3000 ms and an effective TE of 33–60  ms and combines the contrast of PD- and T2-weighting (Crema et al. 2011). The main advantage of this sequence is that the TE is short enough to maintain sufficient signal for visualization of anatomical details (like a PD-weighted sequence), and yet long enough to be fluid sensitive (like a T2-weighted sequence). It provides a higher overall signal intensity in articular cartilage compared to standard T2-weighted sequences, so that articular cartilage can be easily differentiated from subchondral bone (Crema et  al. 2011) (Fig.  5). Furthermore, it is less prone to the magic angle

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a

Fig. 4  Usefulness of fat-suppressed PD-weighted MR images for detection of meniscal lesions. (a) Sagittal PD-weighted FSE MR image shows a longitudinal tear in

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b

the posterior horn of the medial meniscus (arrow). (b) The tear is more conspicuous with fat suppression (arrow)

artifacts that may occur in PD-weighted images with a shorter TE of 3000 ms and TE >30 ms is used to assess affecting the knee. Imaging findings should menisci and ligaments, and it is performed in all always be correlated with clinical features and three planes at our institution. Axial fat-­ serological tests, as there is considerable overlap suppressed intermediate-weighted FSE images in the imaging findings of various arthropathies. are mainly used to assess patellofemoral articular Radiographs are often the initial investigation cartilage, marrow edema, extensor mechanism, and may show features of inflammatory arthritis intercondylar notch, and joint effusion. Popliteal elsewhere such as periarticular osteopenia, bone cysts, collateral and cruciate ligaments, menisci, erosions, symmetrical joint space narrowing, and posteromedial and posterolateral corner periostitis, and enthesitis. MRI usually complistructures are also assessed on axial images. ments clinical examination and radiography and Coronal fat-suppressed intermediate-weighted has a role in early diagnosis (Jacobson et  al. FSE images are used to evaluate collateral and 2017). MRI is superior to other imaging modalicruciate ligaments, menisci, tibiofemoral articu- ties in detecting soft tissue changes and fluid lar cartilage, marrow edema, and the iliotibial within the joint and demonstrating synovitis and tract. Sagittal oblique fat-suppressed marrow edema which are the precursors to erointermediate-­weighted FSE images are useful for sive disease, as well as bone erosions, earlier than assessing menisci, cruciate ligaments, extensor conventional radiography. The MRI protocol for mechanism, femoral trochlea and tibiofemoral suspected inflammatory arthritis of the knee articular cartilage, marrow edema, joint effu- includes T1-weighted and fluid-sensitive sions, plicae, and posteromedial and posterolat- sequences such as fat-suppressed T2-weighted eral corner structures. The Hoffa fat pad can also FSE images or STIR, preferably in all three be assessed in the sagittal as well as in the axial orthogonal planes. Supplementary fat-suppressed

Plane COR COR SAG SAG AX

Sequence type T1 FSE INT FSE PD FSE INT FSE INT FSE

Fat suppression – Yes – Yes Yes

Slice thickness (mm) 3 3 3 3 3

Inter-slice gap (mm) 0.3 0.3 0.3 0.3 0.6 FOV (mm) 160 160 160 160 160

Matrix 270 × 384 270 × 384 270 × 384 270 × 384 270 × 384

TR (ms) 488 3800 2250 3800 3800

Table 1  Imaging parameters of the routine knee MRI protocol for the 1.5 T MRI system at the authors’ institution TE (ms) 12 33 33 33 33

Turbo factor 3 8 8 8 8

Flip angle 180 150 150 150 150

Bandwidth (Hz/pixel) 160 150 180 150 150

NEX 1 1 1 1 1

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Plane COR COR SAG SAG AX

Sequence type T1 FSE INT FSE PD FSE INT FSE INT FSE

Fat suppression – Yes – Yes Yes

Slice thickness (mm) 3 3 3 3 3

Inter-slice gap (mm) 0.3 0.3 0.3 0.3 0.6 FOV (mm) 160 160 160 160 160

Matrix 320 × 448 320 × 448 320 × 448 320 × 448 320 × 448

TR (ms) 692 3083 2588 3083 3083

Table 2  Imaging parameters of the routine knee MRI protocol for the 3.0 T MRI system at the authors’ institution TE (ms) Min Full 30 30 30 30

Turbo factor 3 8 8 8 8

Flip angle 142 142 142 142 142

Bandwidth (kHz) 41.67 50 31.25 31.25 62.5

NEX 2 2 2 2 2

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contrast-enhanced T1-weighted images in the same planes are recommended to delineate synovitis and pannus (Fig.  2c). Addition of gradient echo images is useful to detect chondrocalcinosis in calcium pyrophosphate dehydrate disease and to detect hemosiderin deposition in pigmented villonodular synovitis (Flemming et  al. 2014; Jacobson et al. 2017) (Fig. 7b).

7.2 Bone and Soft Tissue Infections Bone and soft tissue infections of the knee are one of the commonly encountered clinical problems. Although the diagnosis can sometimes be made clinically, it may mimic other conditions and imaging is needed to accurately detect the extent of the infection and for treatment planning. Initial investigation of suspected bone and soft tissue infections of knee should ideally start with radiographs which may show changes of osteomyelitis, but radiographs are often normal in early bone infection. For soft tissue infections, radiographs are usually nonspecific, although sometimes features such as soft tissue swelling and soft tissue gas may be seen. Although ultrasound (US) imaging and CT have roles in detecting soft tissue and bone infections, respectively, MRI is the investigation of choice to comprehensively confirm or exclude clinically suspected infection, to accurately

a

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Fig. 17  Soft tissue infections of the knee in a 50-year-old man with diabetes mellitus. Axial (a) T1-weighted, (b) fat-suppressed T2-weighted FSE and (c) fat-suppressed

define the local extent of bone and soft tissue infections, and in follow-up of patients after treatment. MRI is superior to other imaging modalities in detecting bone marrow abnormalities and to delineate the exact extent of soft tissue infections. A normal MRI virtually excludes active infection, with a 100% negative predictive value (Palestro et al. 2006). The MRI protocol for suspected bone and soft tissue infections of the knee consists of T1-weighted and fluid-sensitive sequences such as fat-suppressed T2-weighted FSE images or STIR, preferably in all three orthogonal planes. If there are findings suggestive of infection, this is followed by contrast-enhanced fat-suppressed T1-weighted images in the same planes. Although the STIR and fat-suppressed T2-weighted FSE sequences are sensitive for detecting marrow edema, they tend to overestimate the extent of osteomyelitis due to reactive marrow edema. Therefore, the true extent of marrow replacement by infection is clearly depicted in T1-weighted images as confluent areas of T1-hypointensity (Johnson et al. 2009; Nacey et al. 2017; Jo et al. 2019). Intravenous MR contrast agent administration is strongly recommended to increase the conspicuity of synovitis, to differentiate infected tissue from reactive edema, to detect abscesses, and to delineate sinus tracts (Fig.  17). Contrast administration also helps to detect nonviable tissues as nonenhancing areas which may need debridement (Hayeri et  al. 2016) and to guide

c

contrast-­enhanced T1-weighted MR images show diffuse cellulitis, myositis, and intramuscular abscesses (arrows)

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biopsy or aspiration. DWI may play a role in detecting abscess when gadolinium-based contrast agent is contraindicated (Bhojwani et  al. 2015; Hayeri et al. 2016).

7.3 Bone and Soft Tissue Masses Bone and soft tissue masses of the knee range from benign to malignant pathology. The initial evaluation of the suspected bone and soft tissue mass should ideally start with radiographs. Radiographs still play a key role in the evaluation of the morphological features and making the diagnosis, or a shortlist of differentials, of bone tumors. The role of radiographs is limited in the evaluation of soft tissue mass, but it can demonstrate calcification and subtle changes in the adjacent cortical bone, which may be challenging to see on MRI. US imaging has a role in evaluating superficial soft tissue masses, differentiating solid from cystic masses, and guiding biopsy of soft tissue masses. CT helps in depicting any mineralization, especially if subtle, and to guide biopsy. Anatomical, functional, and metabolic MRI can be performed to evaluate bone and soft tissue masses (Fayad et al. 2012). Anatomical MRI has a crucial role in the diagnosis and local staging of bone and soft tissue masses. It helps to characterize and to define the local extent of bone and soft tissue masses, invasion of adjacent structures and neurovascular bundles, in follow-up of patients to assess treatment response, and to detect recurrence. Chemical shift imaging with in phase and opposed phase is a technique with which a marrow-­replacing tumor can be recognized and differentiated from bone marrow edema or hematopoietic marrow, as the marrow-replacing tumor is devoid of microscopic fat and does not show drop in signal in the opposed-phase images (Fayad et  al. 2012). Chemical shift imaging is useful to characterize vertebral lesions and is not routinely performed in MRI of the knee. Advanced imaging techniques such as DWI, MR perfusion, and MR spectroscopy can provide additional information about bone and soft tissue

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tumors, albeit with prolonged imaging time. These techniques are not widely available and not routinely performed in routine clinical practice. The MRI protocol considerations for a suspected bone and soft tissue mass should start with an adequate FOV.  The FOV should cover the entire mass to allow local staging. When the mass is small or clinically invisible, surface markers (fish oil capsules are used in our institution) are placed on the skin to delineate the approximate extent of the lesion. Sometimes, surface markers are placed in the region of the referring physician’s or patient’s concern to confirm or to exclude a mass. In cases of primary bone malignancy, the joint above and below the lesion should be included in order to detect any skip lesion within the same bone. MRI should be performed in all three planes and the protocol consists of T1-weighted images and fat-suppressed T2-weighted FSE or STIR images to achieve accurate characterization of the lesion and to determine its full extent. Gradient echo images are not routinely performed but are useful in selected cases to detect hemosiderin deposition within the soft tissue mass (Fig. 7b) and to help distinguish hematomas from soft tissue tumors. Fat-suppressed contrast-enhanced T1-weighted imaging is routinely performed for the evaluation of bone and soft tissue masses, unless it is contraindicated or if the suspected mass is clearly due to anatomical asymmetry or a superficial lipoma (Kransdorf and Murphey 2016). Intravenous contrast administration is useful for the evaluation of cystic lesions other than a ganglion or simple Baker cyst, as T2-hyperintense components from a myxoid tumor can mimic fluid within a cyst (Nacey et  al. 2017). Contrast administration is also useful to differentiate hemorrhagic lesions from hematomas, as it allows the detection of an enhancing tumor nodule which could otherwise be obscured in unenhanced images. Subtraction imaging technique is also valuable, as it eliminates the possibility of misinterpreting T1 shortening due to hemorrhage as enhancement (Kransdorf and Murphey 2016).

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7.4 MR Arthrography MR arthrography of the knee is not routinely performed in clinical practice. This technique has a selective role in the evaluation of the postoperative meniscus, after ligament and cartilage repairs, in assessment of osteochondral injuries and to detect loose bodies (Steinbach et al. 2002; Kalke et  al. 2012). MR arthrography may be direct or indirect. In direct MR arthrography, an intra-articular injection of 20–50  mL of diluted gadolinium chelate (concentration of 2 mmol/L) is administered into the knee joint before the MRI examination, and after any aspiration of preexisting joint fluid (Chung et  al. 2005). The needle placement is usually done under fluoroscopic guidance, although alternative methods are US imaging guidance and clinical palpation. The intra-articular position of the needle tip is confirmed either by aspirating joint fluid or by injecting a small amount of iodinated contrast agent which helps to identify initial needle placement, verify a completely intra-articular injection, and assess the distribution of the fluid within the joint (Chung et  al. 2005). After confirming the intra-articular position of the needle tip, the diluted gadolinium chelate injection into the knee joint should result in adequate distention of the joint capsule and a high SNR.  Fat-suppressed T1-weighted images are acquired in all three planes along with fat-suppressed T2-weighted FSE images in at least one plane to detect marrow edema (Steinbach et al. 2002; Nacey et al. 2017). The injected intra-articular contrast material extends into a meniscal retear or unhealed repairs, and it is best demonstrated in the fat-suppressed T1-weighted images. Potential disadvantages include radiation during fluoroscopic-guided injection, discomfort to patient, and small procedural risks of infection, allergic reaction, and bleeding (Baker et al. 2018). Indirect MR knee arthrography has been used to evaluate the postoperative meniscus (Kalke et  al. 2012; Baker et  al. 2018). In indirect MR knee arthrography, a gadolinium-based contrast agent is administered intravenously approximately 90 min before the MRI examination, during which the patient exercises the knee for

10  min followed by 80  min of rest before the MRI examination to promote synovial uptake of the contrast agent. The imaging protocol is similar to direct MR arthrography. Potential advantages include no radiation exposure, less invasive method, and ability to identify sites of synovitis and periarticular inflammation. Indirect MR arthrography of the knee poses challenges such as a longer time to achieve steady-state gadolinium concentration, preexisting joint effusion, poor distention of joint when compared to direct MR arthrography, and, most importantly, fibro-­ vascular granulation tissue in the healed meniscal tears can also enhance, mimicking a tear. For all these reasons, it is not widely accepted method for MR arthrographic imaging of the postoperative meniscus (Kalke et  al. 2012; Nacey et  al. 2017).

7.5 MR Neurography MRI of nerves is known as MR neurography (MRN). MRN of knee is an uncommon examination and it may be performed to evaluate the tibial and common peroneal nerves and their branches. MRN performed using MRI systems with field strengths of less than 1.5 T results in poor conspicuity of nerves. In addition, the images do not portray optimal morphological (fascicular) or signal characteristics of the nerves, and often have inhomogeneous fat suppression (Chhabra et al. 2011). 3.0 T MR systems are preferred for various reasons such as high SNR, CNR, and capability of parallel acquisition techniques, enabling enhanced temporal resolution and reduced acquisition times. In patients with metal implants, imaging in a 1.5 T unit is preferred, due to increased susceptibility artifacts at 3.0  T (Chhabra et al. 2011). High-resolution 2D SE images, obtained with slice thickness of 3 mm and small FOV, and isotropic 3D sequences are ideal for the evaluation of nerves. The typical MRN protocol of the knee consists of axial T1-weighted FSE images for anatomical details, axial T2-weighted SPAIR images for homogeneous fat suppression and high SNR, coronal PD-weighted FSE images to

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rule out other abnormalities of the knee, and isotropic 3D SPAIR SPACE images for multiplanar reconstruction and maximum intensity projections and isotropic 3D DW-PSIF (diffusion-­ weighted reversed fast imaging with steady-state precession) to suppress blood flow in adjacent vessels and to increase the conspicuity of the nerves (Chhabra et al. 2011; Nacey et al. 2017). Contrast-enhanced MRI is not routinely performed with MRN.  Contrast-enhanced MRI is useful if there is clinical suspicion of infection or tumor and in the evaluation of postsurgical patients (Chhabra et al. 2011). MRN has several advantages over nerve conduction studies and clinical examination. These advantages include direct visualization of nerves, depiction of nerve abnormalities such as edema, enlargement or focal discontinuity and space-­ occupying lesion, secondary denervation changes in the muscles, and causes of nerve entrapment. MRN may show normal nerves and other orthopedic problems such as arthritis mimicking neurogenic pain. Moreover, it plays a vital role in the preoperative localization and characterization of nerve abnormality and in the follow-up of postoperative nerve reconstruction (Chhabra et  al. 2011; Thawait et  al. 2011). Disadvantages of MRN include technical difficulty, prolonged scan time, and inability to quantify the nerve injury and regeneration, particularly when a nerve remains in continuity (Argentieri et al. 2018a).

7.6 Cartilage Mapping Techniques Cartilage mapping techniques are a relatively new MRI strategy that is specifically designed to evaluate the composition of articular cartilage. The articular cartilage of the knee is composed of water (approximately 70–80% of cartilage volume by weight), solid extracellular matrix mainly type II collagen and proteoglycans, predominantly glycosoaminoglycans (GAG) (approximately 20–30% of cartilage volume by weight), and sparse chondrocytes (2%) (Guermazi et  al. 2015). Early chondral degeneration leads to disruption of the collagen network, reduction in proteoglycan content, and increased permeability to

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water. Cartilage mapping techniques are able to identify changes in composition of the articular cartilage due to early chondral degeneration, before structural changes become apparent on morphological cartilage imaging. Cartilage mapping techniques are not routinely performed in clinical practice. These techniques are selectively used to detect early osteoarthritis of the knee and to evaluate the status of cartilage following repair. The MRI systems that meet the current standards for cartilage mapping techniques utilize either 1.5 or 3.0  T magnets. There are several techniques that are used for compositional imaging of articular cartilage such as T2 mapping, T2* mapping, dGEMRIC, T1 rho mapping, sodium imaging, and gag CEST (GAG chemical exchange saturation transfer imaging) (Crema et al. 2011; Guermazi et al. 2015; Argentieri et al. 2018b). Sodium imaging and gag CEST are evolving cartilage mapping techniques that are discussed below under the section on Recent Advances; they are not widely available, requires specialized hardware and ultrahigh field strength (7 T) magnets, and are at present mainly used for research purposes (Argentieri et al. 2018b). T2 mapping is the most widely used cartilage mapping technique; it produces quantitative T2 measurement of the articular cartilage and is depicted as a color map. It is performed using multiecho sequences based on the FSE technique. Each echo of the echo train produces images with a different TE. Postprocessing of the images with a mono- or bi-exponential fit is done, a T2 value is calculated for each pixel, and a color map is produced (Fig. 18). T2 mapping is used to assess the water content, collagen network, and zonal variation of articular cartilage. In articular cartilage, the superficial layers have increased water content and longer T2 relaxation times, whereas deeper layers have reduced water content and shorter T2 relaxation times. Early cartilage damage shows an increase in water content, resulting in higher T2 values. T2 mapping is used to identify and quantify early changes in the water content of articular cartilage, even before detectable structural damage (Nacey et al. 2017; de Mello et al. 2019). The advantages of T2 mapping are: it is compatible with most MRI systems

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Fig. 18  T2 mapping of the patellar cartilage performed in a 20-year-old healthy volunteer. T2 mapping demonstrates normal chondral stratification with relatively shorter relaxation values in the deep chondral layer relative to superficial layers

and field strengths, there is no need for contrast agent administration, and it is a well-validated method. Disadvantages include susceptibility to magic angle effects and inability to evaluate calcified cartilage (Argentieri et al. 2018b). T2* mapping is similar to T2 mapping, in that it allows assessment of water content, collagen network, and zonal variation of articular cartilage. T2* relaxation is exclusive for gradient echo imaging because it requires a dephasing effect that is eliminated in SE sequences. A reduction in T2* value is noted in the deep layer of normal cartilage and the T2* values are increased in early cartilage damage. It results in faster acquisition and high spatial resolution and can evaluate the deep layer of cartilage but is limited by sensitivity to magnetic field inhomogeneity and magic angle artifacts (Guermazi et  al. 2015; Argentieri et al. 2018b). The dGEMRIC method depends on T1 relaxation time measurement of negatively charged gadolinium to generate a color-coded map of the GAG content in articular cartilage. Both gadolinium and GAG are negatively charged. dGEMRIC uses intravenous administration of negatively charged gadolinium chelate, followed by 10 min of exercise and delayed imaging after 80 min of rest time. Gadolinium distributes preferentially to regions with lower GAG content in the articular cartilage, due to electrostatic repulsion, and this can be quantitatively assessed using T1 mapping to identify intact and damaged cartilage, as

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damaged cartilage has low GAG and high gadolinium uptake (de Mello et al. 2019). Even though it is a well-validated method to indirectly measure GAG content in articular cartilage, it is limited by the long imaging time and need for intravenous administration of gadolinium-based contrast agent (Crema et  al. 2011; Argentieri et al. 2018b). T1 rho mapping is a technique used to measure the T1 rho (spin-lock) relaxation time and it is optimally performed at 3.0 T (Argentieri et al. 2018b). T1 rho relaxation time is a constant that differs from T1 and T2 relaxation times, and it can be measured within cartilage using a specific pulse sequence. Multiple 3D gradient echo scans with varying flip angles are used to assess GAG content. Decreased GAG content results in T1 prolongation (Nacey et al. 2017; de Mello et al. 2019). Its advantages include sensitivity to early GAG depletion. Therefore, it is useful to identify early degeneration without the need for intravenous contrast administration. However, this technique requires a special pulse sequence, time consuming multiple datasets, is not widely available, and is not a well-validated method (Crema et al. 2011; Argentieri et al. 2018b). In summary, T2 mapping is the most widely used and well-validated cartilage mapping technique that is compatible with most of the high-­ field MRI systems. Furthermore, there is no need for intravenous gadolinium contrast administration. dGEMRIC is a well-validated method but is limited by the need for intravenous gadolinium administration and long waiting time between gadolinium administration and commencement of MRI. Other methods are promising, but need validation before widespread use in routine clinical practice.

7.7 MRI of the Postoperative Knee MRI of the postoperative knee is increasingly performed in routine clinical practice, as the number of open and arthroscopic procedures performed by orthopedic surgeons has grown tremendously in recent years. Commonly performed knee surgeries are total or partial knee replace-

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ments and various arthroscopic procedures such as partial meniscectomy, meniscal repair, ligament reconstruction, and cartilage repair procedures. Postoperative MRI of the knee is frequently performed to evaluate residual or recurrent symptoms, which could be due to complications, ­failure of hardware, or reinjury. Knowledge of the commonly performed knee procedures, postoperative MRI appearance of menisci, ligaments, and cartilage, common complications, and susceptibility artifacts due to various procedures are vital to interpret the MR images accurately (Recht and Kramer 2002; Gnannt et  al. 2011). Whatever the nature of previous surgery to the knee, there is a likelihood of susceptibility artifacts appearing on the acquired MR images. Metallic implants cause susceptibility artifacts and degrade the image quality. The severity of susceptibility artifacts depends on various factors such as composition, size, and orientation of the metallic implants, field strength of magnets, imaging parameters, type of pulse sequences used, and fat-suppression methods. Implants made of titanium alloy are nonferromagnetic and produce fewer artifacts than ferromagnetic implants such as stainless steel. Imaging in lower field strength magnets is preferred when available. If there is a choice of imaging between 1.5

a

b

Fig. 19  Patient with total knee replacement presented with knee pain and swelling. (a) Sagittal fat-suppressed PD-weighted FSE image shows extensive metallic artifact. (b) Sagittal PD-weighted and (c) coronal T1-weighted

and 3.0 T magnets, 1.5 T magnets are chosen due to increased susceptibility artifacts in the latter. Non-fat-suppressed images are preferred over fat-suppressed images. FSE images are preferred to SE and gradient echo images, and when fat suppression is needed, STIR is favored over spectral fat-suppression techniques. Sequences with short TE produce fewer artifacts than sequences with long TE; therefore, PD-weighted images are preferred over T2- and intermediate-­ weighted images (Chang et al. 2014). There are several ways to reduce metallic artifacts. Metal artifact reduction sequences (MARS) are special 3D MRI sequences developed to reduce metal-induced susceptibility artifacts and are now an integral part of MRI of the postoperative knee. These include slice encoding for metal artifact correction (SEMAC) and multi-­ acquisition variable-resonance image combination (MAVRIC) (Chen et  al. 2011). These techniques permit visualization of the interface between bone and metal, apart from reducing the susceptibility artifacts, and help to identify postoperative complications (Fig.  19). Commonly used sequences such as PD-, T1-, and T2-weighted and STIR MRI can be performed using SEMAC and MAVRIC techniques (Chen et  al. 2011). SEMAC makes use of a 3D SE

c

MR images with MARS show considerably less artifact with clear delineation of the prosthesis and a large hemarthrosis (arrow)

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acquisition that resolves through-slice distortion caused by metal. The in-plane distortion is then resolved by means of a compensation gradient. MAVRIC, in contrast, uses spectrally overlapping 3D acquisitions to reduce the encoding errors (Chen et al. 2011). These strategies, to an extent, improve the soft tissue resolution around metal and mitigate the limitations of MRI in the evaluation of postoperative patients. High SAR and prolonged imaging time are important drawbacks of MARS. The knee is the second most commonly replaced joint after the hip. Advanced osteoarthritis and rheumatoid arthritis are the leading indications for knee replacement. Knee replacement can either be total or unicompartmental. Radiographs play a very important role in evaluating alignment of the prosthesis and detecting periprosthetic fracture, aseptic loosening, and failure of hardware. MRI of knee arthroplasties are not routinely performed and it may be performed to assess other associated problems such as infection, occult fractures such as fatigue fracture of the patella, and to assess the patellar and quadriceps tendons, Hoffa fat pad, ligaments, and neurovascular injuries (Gnannt et al. 2011). Evaluation of the postoperative meniscus should always be correlated with prior imaging and operative notes. The timing, location, type of meniscal surgery, and amount of meniscal tissue that is removed are all vital information that is needed to more accurately assess the postoperative meniscus. The criterion to diagnose meniscal tear in an unoperated meniscus is either finding a meniscal deformity/defect or unequivocal increased linear signal intensity in the meniscus extending up to the articular surface in two contiguous slices or in two orthogonal planes (Nguyen et  al. 2014). These criteria no longer apply to the postoperative meniscus as the meniscal deformity can result from partial meniscectomy (particularly when >25% meniscus has been removed) and linear increased signal in the meniscus could be either due to recurrent tear or granulation tissue from the healed repair. In assessment of the postoperative meniscus, T2-weighted FSE images with fat suppression

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produce adequate visualization (White et  al. 2002; Nacey et al. 2017). Demonstration of fluid signal in the meniscus extending up to the articular surface is specific for a recurrent meniscal tear or unhealed repair in the postoperative meniscus. When doubt exists, MR arthrography is useful and superior to conventional MRI to detect recurrent meniscal tear after meniscus repair or partial meniscectomy involving more than 25% of the meniscus (Baker et al. 2018). MRI of the ACL following reconstruction aims to assess the tunnel placement, integrity of the graft, whether there is impingement, and to assess the chondral and meniscal status. Additionally, it may reveal other complications such as a cyclops lesion or fibrosis. Fat-­ suppressed T1-weighted images following intravenous contrast administration are used to assess soft tissue problems such as cyclops lesions and fibrosis, and to obtain information about the degree of revascularization of the reconstructed ligament. Cartilage repair procedures are increasingly performed for cartilage injury to alleviate symptoms and to prevent early osteoarthritis. Marrow stimulation, osteochondral grafting, and autologous chondrocyte implantation are the commonly used surgical procedures to treat chondral defects. Even though arthroscopy is very useful to evaluate articular cartilage before and after repair, it is an invasive procedure and is unable to portray the deep layer of cartilage and the underlying bone. MRI is the best noninvasive method to evaluate cartilage following repair and to assess other joint tissues (Guermazi et  al. 2015). In routine clinical practice, morphological MRI still has a very important role in the assessment of cartilage before and after repair. 3D SPGR and fat-­ suppressed intermediate-weighted FSE sequences are very useful in the evaluation of the repaired cartilage to assess the status of repair and the stage of healing and to detect complications (Sanders 2011; Guermazi et  al. 2015; Argentieri et  al. 2018b). Compositional MRI such as T2 mapping can provide biochemical assessment of the composition of cartilage and cartilage repair tissue but is not routinely performed (Argentieri et al. 2018b).

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8 Recent Advances 8.1 Ultrahigh Field Strength MR Imaging In 2017, the European Union and Food and Drug Administration approved the use of whole-body 7.0 T MRI systems for clinical musculoskeletal imaging (Juras et al. 2019; Aringhieri et al. 2020). MRI of the knee using a 7.0  T ultrahigh field strength magnet provides excellent anatomical details and improved diagnostic accuracy due to higher SNR, CNR, higher spatial and temporal resolution, and multinuclear applications that are not available with high-field strength magnets. They are mainly used for the evaluation of articular cartilage. A few studies have established improved diagnostic accuracy of 7.0 T over 3.0 T MRI in detecting chondral lesions (Welsch et al. 2012; Springer et  al. 2017). Further studies are needed to consolidate the added value of 7.0  T over widely available high-field strength MRI in the routine clinical practice. Several potential disadvantages include B1 and B0 inhomogeneities, increased T1 relaxation times and decreased T2 relaxation times which need optimization, and high SAR.  The lack of widespread availability and high cost of installation will also hinder the widespread use of 7.0 T MRI in routine clinical practice, at least in the near future (Aringhieri et al. 2020).

8.2 Ultrashort TE Sequences Tendons, ligaments, menisci, the deep layer of articular cartilage, and cortical bone have very short T2 relaxation times; therefore, these tissues produce little or no signal with conventional MRI pulse sequences (Chang et al. 2015). Ultrashort TE sequences are intended to detect signal from these tissues that have very short T2 relaxation times. The mean T2 relaxation times of ligaments, menisci, and tendons range from 2 to 8 ms, while that of cortical bone and deep layer of articular cartilage range from 0.2 to 2  ms in a 1.5  T MRI system (Chang et  al.

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2015; Xiao and Yuen 2017). Ultrashort TE sequences use TEs 10–50 times shorter than routine pulse sequences, usually 0.5  ms or shorter, to detect signal from the tissues that have very short T2 relaxation times. Ultrashort TE sequences are mainly used for research purpose to differentiate the different zones of the meniscus, to visualize the layers and defects of articular cartilage, and to differentiate scar tissue from ligaments. Limitations include low SNR, additional sequences, and long scan time which may result in patient motion. Moreover, the lack of standardization and widespread unavailability limits its use in routine clinical practice (de Mello et al. 2019).

8.3 Sodium Imaging Sodium imaging is based on the fact that the sodium ions (23Na+), like hydrogen protons, possess nuclear spin momentum and have an inherent resonance frequency that can be measured with MRI (Argentieri et  al. 2018b). The use of sodium MRI has been evaluated for stroke and tumor detection, breast cancer studies, and the assessment of osteoarthritis and muscle (Madelin et al. 2014). In sodium imaging of cartilage, the positively charged sodium ions are attracted to the negatively charged GAGs ions in the intact cartilage, a method similar to dGEMRIC, but sodium imaging does not need MR contrast agent administration. Due to low concentration of sodium in the body, the signals are significantly lower when compared with hydrogen proton MRI; thus, sodium imaging of cartilage requires ultrahigh field 7.0 T magnet and specialized coils to detect signals from the sodium ions (Madelin et  al. 2018). Even though sodium imaging of articular cartilage is promising and considered as an imaging biomarker of early cartilage degeneration in osteoarthritis, sodium imaging of the articular cartilage of knee is impractical for patient care in routine clinical practice due to lack of widespread availability of 7.0  T MRI, need for specialized coils, and prolonged scan time (Nacey et al. 2017; Argentieri et al. 2018b).

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8.4 GAG Chemical Exchange Saturation Transfer Imaging Gag CEST is a recently developed compositional imaging technique used to assess the GAG content and its depletion in the articular cartilage, which is an early sign of chondral degeneration (Guermazi et al. 2015). In the extracellular matrix of articular cartilage, water is either associated with macromolecules or is in a free state. Water linked with macromolecules exhibits a wide resonance peak due to its short T2 relaxation periods. Prior to imaging, off-resonance RF pulses can be used to saturate the water associated with macromolecules. When this saturation is exchanged for free water, it results in loss of image signal intensity. Chemical exchange can be used to quantify the concentration of macromolecules in cartilage by obtaining images with and without the off-resonance pulse (Guermazi et al. 2015). In gag CEST, off-resonance RF saturation pulses are applied at frequencies specific to chemically exchangeable protons residing on the hydroxyl groups of cartilage GAGs. Gag CEST MRI is sensitive to the amount of GAG levels in cartilage, and, in addition, it allows accurate demarcation of GAG measurements from cartilage and synovial fluid (Guermazi et al. 2015). The use of CEST to measure GAG in articular cartilage holds a lot of promise for assessing cartilage degeneration and also for identifying early-stage osteoarthritis. The key drawbacks include the need for an ultrahigh field MRI system, higher energy deposition due to long saturation RF pulses, and advanced postprocessing procedures (Guermazi et  al. 2015; Argentieri et al. 2018b).

8.5 Synthetic MRI Synthetic MRI is a new emerging acceleration technique that can produce T1-, T2-, and PD-weighted and inversion recovery images based on MR quantification within a single scan.

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The TE, TR, and inversion times can be customized with a postprocessing step, resulting in desired weighting and different tissue contrast with one acquisition, thus greatly reducing scan time and improving patient comfort (Betts et al. 2016; Yi et al. 2018). Even though the synthetic MRI technique is promising in the evaluation of internal derangements of the knee, further studies are warranted for optimization and widespread use in routine clinical practice (Yi et al. 2018).

8.6 MR Spectroscopy MR spectroscopy is a noninvasive method for detecting metabolites in the region of interest, such as water, choline, creatine, and lipids. Although it is commonly used in brain, clinical uses of MR spectroscopy in the musculoskeletal system are still gaining importance and may be used to assess tumors and muscle physiology and disease (Deshmukh et al. 2014). Proton (1H) MR spectroscopy is a widely used method in the clinical practice, as there is no need for specialized hardware and can be readily incorporated with routine MRI, while phosphorus (31P) MR spectroscopy needs specialized hardware, thus limiting its widespread use in routine clinical practice (Deshmukh et  al. 2014; de Mello et  al. 2019). Proton MR spectroscopy is currently used in the evaluation of tumors, as it provides crucial information about metabolites which are produced in large quantities by malignant tumors, particularly choline-containing compounds. Other uses of MR spectroscopy that are emerging with potential application around the knee include analysis of creatine content within the muscles to evaluate muscle disorders such as chronic myopathies. The analysis of lipid content holds promise in the evaluation of muscle and bone marrow pathologies. However, further studies are needed to prove the utility of MR spectroscopy in the evaluation of muscle diseases and bone marrow pathology before its widespread use in the routine clinical practice (de Mello et al. 2019).

Magnetic Resonance Imaging

8.7 Diffusion Tensor Imaging Diffusion tensor imaging (DTI) is a new technique based on the principles of DWI. Although it has been in use to track the fibers in the central nervous system, its use in musculoskeletal and peripheral nerve disorders is still mainly in the research setting (Chianca et  al. 2017; de Mello et al. 2019). DTI is used to measure anisotropy and in the evaluation of skeletal muscle physiology, anatomy, and pathology. DTI may play a role in the evaluation of sports-related muscle injuries, which leads to disorganization of muscle fibers and altered diffusion, therefore reducing the anisotropy which can be detected in DTI, before these changes are evident on the anatomical imaging. DTI of skeletal muscles has several limitations as muscle DTI parameters depend on various factors such as age, sex, exercise status, body mass index, and temperature, and like DWI, DTI is also prone to misregistration and motion artifacts (de Mello et  al. 2019). Further studies and technical improvement in postprocessing are needed before DTI can be considered for implementation in the routine clinical practice.

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elasticity changes in the skeletal muscles of the leg and it is potentially useful to evaluate physiological process and muscle disorders (Schrank et al. 2020). Further studies, technical improvement, and advances in postprocessing may eventually provide routine clinical utility in the future.

8.9 Radiomics in MSK Imaging

Radiomics is an emerging tool in precision medicine, which makes use of quantifiable data from imaging modalities to provide an insight into tumor biology and heterogeneity. Radiomics is used to diagnose the tumor cell type and also provides an insight about intratumoral heterogeneity, which could be a constraint of limited tissue sampling such as biopsy. Radiomic MRI features may have prognostic implications for various tumors and may be useful to differentiate intermediate- and high-grade soft tissue sarcomas (Corino et al. 2018). Evaluation of tumor biology with radiomics holds promise in the near future for diagnosis, prognosis, treatment planning, and monitoring of patients with soft tissue tumors. In addition to oncology, radiomics is also anticipated to benefit imaging of osteoarthritis. New 8.8 MR Elastography atlas-based methods and deep learning approaches are aimed at automatizing cartilage MR elastography (MRE) is a noninvasive method segmentation successfully (Bach Cuadra et  al. for assessing the elasticity of tissues by phase-­ 2020). Technical improvement and postprocesscontrast MR technique, where an acoustic or ing advances may lead to future applications in mechanical driver is used to induce shear waves routine clinical practice. into the object of interest. While MRE has been widely used in clinical practice to assess liver fibrosis, its musculoskeletal applications are rap- 9 Artifacts idly emerging, and it can be used to assess the stiffness of skeletal muscles. It is well known that MRI of the knee is susceptible to several artifacts the stiffness of skeletal muscle changes signifi- that may degrade image quality. Artifacts are cantly based on its contractile state. MRE of skel- broadly divided into two categories, namely, etal muscles can be used to assess the those resulting from patient factors and those due physiological response of diseased and damaged to inherent nature of the MRI scanning techmuscles (Glaser et  al. 2012; Low et  al. 2016; niques. Certain artifacts such as chemical shift Guermazi et al. 2017). Recently, real-time MRE and susceptibility artifacts are more prominent at has been used to assess function-related visco- 3.0  T as these are directly related to the field

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strength of the magnet. Knowledge of the common artifacts and how to correct them are vital to improve the image quality and to avoid misinterpretation (Peh and Chan 2001). Detailed discussion of all possible MRI artifacts is beyond the scope of this chapter and a few of the more common artifacts are highlighted below.

9.1 Motion Artifacts Motion artifacts can be either random or periodic. These are one of the most common artifacts that degrade image quality in MRI of the knee, and they can be easily recognized and rectified to a large extent. Motion artifacts arise due to inability of the phase gradient to encode the radio waves arising from moving structures. Motion artifacts depend on various factors such as type of motion, speed and direction of moving objects, and field strength of the magnet (Tan et al. 2014). In knee MRI, motion artifacts may result from random inadvertent patient motion or repetitive periodic artifacts due to pulsations from the popliteal artery. Artifacts due to patient motion can be minimized by reassurance, positioning the patient comfortably, by applying padding within

a

Fig. 20  Pulsation artifacts from the popliteal artery on axial fat-suppressed PD-weighted MR images. (a) The pulsation artifacts cause ghost images in the anterior–posterior direction (arrows), partially obscuring distal femur

the knee coil in order that the potential for movement is reduced, by performing the study under sedation, by using sequences with short acquisition times, and by using rapid image acquisition techniques such as parallel imaging and compressed sensing (Morelli et al. 2011; Singh et al. 2014). Pulsation artifacts from the popliteal artery may result in ghost images along the phase-encoding direction. The brightness of the ghosting artifact depends on the amplitude and speed of motion. It can be corrected or reduced by flow compensation, out-of-phase saturation pulse, increased number of signals acquired, and altering the frequency- and phase-encoding directions (Peh and Chan 2001) (Fig. 20).

9.2 Chemical Shift Artifacts Chemical shift artifacts are one of the better-­ known artifacts resulting from the physical characteristics of MRI and are more conspicuous at high-field strengths (Hood et al. 1999). Chemical shift artifacts arise because hydrogen protons within fat precess at a different frequency from hydrogen protons within water, with the differences in resonance frequencies being approxi-

b

marrow along the artifacts. (b) In a different patient with synovitis, these artifacts obscuring the femur are avoided by changing the phase-encoding direction such that the artifacts are positioned in a medial–lateral direction (arrow)

Magnetic Resonance Imaging

mately 224  Hz at 1.5  T.  This difference in precessional frequency means that signals from different chemical structures (i.e., fat and water) actually located at the same spatial position may occupy different positions in the image produced. These chemical shift artifacts are seen in the frequency-­ encoding direction as areas of high signal where fat and water overlap and low signal where they separate (Peh and Chan 2001). Chemical shift artifacts can be minimized by changing the frequency- and phase-encoding gradient directions, application of fat suppression, and using a wide receiver band width (Peh and Chan 2001). Alternatively, selective water excitation techniques may be employed but they are not available on all systems.

9.3 Magic Angle Phenomenon The magic angle phenomenon is observed in short TE sequences such as T1- and PD-weighted images and is seen as spurious increased signal in tissues containing highly structured collagen

a

Fig. 21  Magic angle phenomenon in the patellar tendon. (a) Sagittal fat-suppressed PD-weighted FSE image with TE 17 ms shows spurious increased signal in the patellar tendon (arrow) due to the magic angle phenomenon. (b) Repeat sagittal fat-suppressed PD-weighted FSE image

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fibers, such as tendons, ligaments, and hyaline cartilage. These tissues behave in an anisotropic manner and normally produce little or no signal on MRI. However, when these structures are orientated at 55° to the main magnetic field, the spin–spin interactions are nullified, ensuing that the T2 decay is only controlled by the local dynamic field. As a consequence, the T2 relaxation time increases, while the T2 decay is less rapid, resulting in anisotropic structures such as ligaments and tendons appearing bright at short TE (Peh and Chan 1998). In the knee, the magic angle phenomenon affects the ligaments, tendons, menisci, and hyaline cartilage. The posterior cruciate ligament and patellar tendon are the most commonly affected structures in the knee (Fig. 21). The spurious increased signal in the ligaments, tendons, and menisci can be misinterpreted as degeneration or tears. This can be alleviated by using T2-weighted images to compare the structures, repeating the study with TE values more than 37 ms and by repositioning the patient (Peh and Chan 1998, 2001; Singh et al. 2014).

b

with TE 44 ms in the same subject, with rest of the imaging parameters unchanged, shows a normal patellar tendon. The magic angle phenomenon was identified and removed by increasing the TE

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9.4 Truncation Artifacts

ties, resulting in spatial misregistration due to magnetic field distortion. Susceptibility artifacts Truncation artifacts (as known as Gibb phenom- are usually seen around metal implants, and at ena) may arise at sites where there is an abrupt air–tissue and air–bone interfaces. Susceptibility difference in signal intensity between two adja- artifacts are directly proportional to the field cent structures. These artifacts occur due to strength of the magnets; therefore, the susceptiundersampling of phase-encoding steps, result- bility variations are twice as large at 3.0 T when ing in a series of alternating high and low signal compared to 1.5 T. It is most severe in gradient intensity lines that are seen parallel to the inter- images, resulting in a blooming artifact (Fig. 7b). face of these structures. In the knee, the trunca- Susceptibility artifacts result in geometric distortion artifact may mimic a tear in the meniscus or tion of the image, failure of fat suppression, loss produce a spurious laminar appearance of the of signal, and signal pile-up (Hargreaves et  al. cartilage (Peh and Chan 2001). Using a smaller 2011). Susceptibility artifacts also arise due to FOV, increasing the matrix size along the phase-­ intra-articular air foci which can be misinterencoding direction, swapping the phase- and preted as a lesion. Susceptibility artifacts can be frequency-­ encoding directions, and applying reduced by using low-field strength magnets, special reconstruction filters such as the Hanning FSE sequences with short echo times, increasing filter will help to resolve this artifact (Peh and the bandwidth, and using a small FOV, high-­ Chan 2001; Singh et al. 2014; Tan et al. 2014). resolution matrix, and higher gradient strength for the given FOV (Peh and Chan 2001).

9.5 Partial Volume Averaging Artifacts Partial volume averaging artifacts arise when a single voxel contains signals arising from tissues of different MR properties. The resultant signal is the average of different tissue signals produced and is marked when the voxel size is large in relation to the structures being imaged. Partial volume artifacts may be challenging during MRI of the knee in assessing small structures such as menisci, as higher signal resulting from partial volume averaging may mimic a radial tear (Peh and Chan 2001; Singh et  al. 2014). Partial volume artifacts can be reduced by using a smaller FOV, scanning with thinner slices, reducing the inter-slice gap, and applying isotropic 3D sequences (Tan et al. 2014) (Fig. 10).

9.6 Susceptibility Artifacts Susceptibility artifacts are caused by local magnetic field inhomogeneity at the interface of structures with differing magnetic susceptibili-

9.7 Metallic Artifacts Metallic artifacts are due to the fact that metal objects produce their own local magnetic fields and therefore markedly distort MR images. Even small metal fragments resulting from bone drilling can also cause significant artifacts and degrade the image quality. The degree of artifacts from metallic objects depends on many factors such as composition, size, and orientation of the metallic object, the field strength of the magnet and imaging parameters, type of pulse sequences used, and fat-suppression methods. Ferromagnetic implants such as stainless steel produce more severe artifacts than nonferromagnetic implants such as titanium alloy. The larger the implants, the more severe are the artifacts. The orientation of the metal object in relation to the main magnetic field also plays an important role; the larger the angle between the metallic object and the main magnetic field, the more severe are the artifacts. Therefore, positioning the long axis of the metallic object as closely parallel to the main magnetic field reduces the metallic artifacts.

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Metallic artifacts can also be reduced by aligning References the frequency-encoding gradient to the long axis of the implant, albeit with long imaging time. Argentieri EC, Sneag DB, Nwawka OK et  al (2018a) Updates in musculoskeletal imaging. Sports Health Imaging in a low-field strength magnet helps in 10:296–302 reducing metallic artifacts. However, if only Argentieri EC, Burge AJ, Potter HG (2018b) Magnetic resonance imaging of articular cartilage within the high-field strength magnets are available, altering knee. J Knee Surg 31:155–165 the imaging parameters such as increasing the Aringhieri G, Zampa V, Tosetti M (2020) Musculoskeletal receiver bandwidth, using high-resolution image MRI at 7 T: do we need more or is it more than matrix, using small FOV, decreasing slice thickenough? Eur Radiol Exp 4:48 ness, increasing the matrix in the frequency-­ Bach Cuadra M, Favre J, Omoumi P (2020) Quantification in musculoskeletal imaging using computational encoding direction, and using higher gradient analysis and machine learning: segmentation and strength for small voxel size will reduce metallic radiomics. Semin Musculoskelet Radiol 24:50–64 artifacts and help produce acceptable image qual- Baker JC, Friedman MV, Rubin DA (2018) Imaging the postoperative knee meniscus: an evidence-based ity (Lee et al. 2007, 2021). FSE sequences with review. AJR Am J Roentgenol 211:519–527 short echo times and long echo train length proBarth M, Breuer F, Koopmans PJ et al (2016) Simultaneous duce fewer artifacts than gradient echo sequences. multislice (SMS) imaging techniques. Magn Reson If fat suppression is needed, STIR or Dixon Med 75:63–81 methods are preferred over spectral fat-­ Bedi A, Musahl V, Cowan JB (2016) Management of posterior cruciate ligament injuries: an evidence-based suppression techniques. Vendor specific special review. J Am Acad Orthop Surg 24:e277–e289 sequences such as MARS are useful to reduce Betts AM, Leach JL, Jones BV et al (2016) Brain imaging metallic artifacts, although it cannot completely with synthetic MR in children: clinical quality assessment. Neuroradiology 58:1017–1026 eliminate these artifacts (Fig. 19). Disadvantage of MARS sequences are high SAR and prolonged Bhojwani N, Szpakowski P, Partovi S et  al (2015) Diffusion-weighted imaging in musculoskeletal scan time (Singh et al. 2014; Lee et al. 2021). radiology-clinical applications and future directions.

10 Conclusion MRI of the knee is a proven comprehensive and noninvasive modality for detecting and evaluating various disorders affecting the knee, and it also serves as an important guide to treatment planning and in follow-up of patients with various knee disorders. Advances in MRI hardware, coil technology, and introduction of new sequences continuously improve the value and application of knee MRI for the evaluation of postoperative changes, cartilage, and nerves, in addition to meniscal and ligament tears. Knowledge of the various MRI systems, imaging techniques, pulse sequences, fat-suppression techniques, routine and supplementary protocols, and common MRI artifacts and how to correct them will help improve the diagnostic accuracy and reduce the interpretation errors in MRI of the knee.

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107 Nacey NC, Geeslin MG, Miller GW et al (2017) Magnetic resonance imaging of the knee: an overview and update of conventional and state of the art imaging. J Magn Reson Imaging 45:1257–1275 Naraghi AM, White LM (2006) Magnetic resonance imaging of joint replacement. Semin Musculoskelet Radiol 10:98–106 Naraghi A, White LM (2014) MR imaging of cruciate ligaments. Magn Reson Imaging Clin N Am 22:557–580 Ng AW, Griffith JF, Hung EH et al (2013) MRI diagnosis of ACL bundle tears: value of oblique axial imaging. Skeletal Radiol 42:209–217 Nguyen JC, De Smet AA, Graf BK et  al (2014) MR imaging-­based diagnosis and classification of meniscal tears. Radiographics 34:981–999 Palestro CJ, Love C, Miller TT (2006) Infection and musculoskeletal conditions: imaging of musculoskeletal infections. Best Pract Res Clin Rheumatol 20:1197–1218 Park HJ, Kim SS, Lee SY et  al (2014) Comparison between arthroscopic findings and 1.5-T and 3-T MRI of oblique coronal and sagittal planes of the knee for evaluation of selective bundle injury of the anterior cruciate ligament. AJR Am J Roentgenol 203:199–206 Peh WCG, Chan JHM (1998) The magic angle phenomenon in tendons: effect of varying the MR echo time. Br J Radiol 71:31–36 Peh WCG, Chan JHM (2001) Artifacts in musculoskeletal magnetic resonance imaging: identification and correction. Skeletal Radiol 30:179–191 Pruessmann KP (2004) Parallel imaging at high field strength: synergies and joint potential. Top Magn Reson Imaging 15:237–244 Recht MP, Kramer J (2002) MR imaging of the postoperative knee: a pictorial essay. Radiographics 22:765–774 Ristow O, Steinbach L, Sabo G et  al (2009) Isotropic 3D fast spin-echo imaging versus standard 2D imaging at 3.0 T of the knee-image quality and diagnostic performance. Eur Radiol 19:1263–1272 Rubin DA, Paletta GA Jr (2000) Current concepts and controversies in meniscal imaging. Magn Reson Imaging Clin N Am 8:243–270 Sanders T (2011) Imaging of the postoperative knee. Semin Musculoskelet Radiol 15:383–407 Schrank F, Warmuth C, Görner S et al (2020) Real-time MR elastography for viscoelasticity quantification in skeletal muscle during dynamic exercises. Magn Reson Med 84:103–114 Servant CT, Ramos JP, Thomas NP (2004) The accuracy of magnetic resonance imaging in diagnosing chronic posterior cruciate ligament injury. Knee 11:265–270 Shapiro L, Staroswiecki E, Gold G (2010) Magnetic resonance imaging of the knee: optimizing 3 Tesla imaging. Semin Roentgenol 45:238–249

108 Singh D, Chin M, Peh WCG (2014) Artifacts in musculoskeletal MR imaging. Semin Musculoskelet Radiol 18:12–22 Springer E, Bohndorf K, Juras V et al (2017) Comparison of routine knee magnetic resonance imaging at 3 T and 7 T. Invest Radiol 52:42–54 Stabler A, Glaser C, Reiser M (2000) Musculoskeletal MR: knee. Eur Radiol 10:230–241 Steinbach LS, Palmer WE, Schweitzer ME (2002) Special focus session. MR arthrography. Radiographics 22:1223–1246 Subhas N, Kao A, Freire M et  al (2011) MRI of the knee ligaments and menisci: comparison of isotropic-­ resolution 3D and conventional 2D fast spin-echo sequences at 3 T.  AJR Am J Roentgenol 197:442–450 Sutter R, Tresch F, Buck FM et  al (2014) Is dedicated extremity 1.5-T MRI equivalent to standard large-bore 1.5-T MRI for foot and knee examinations? AJR Am J Roentgenol 203:1293–1302 Swartz PG, Roberts CC (2009) Radiological reasoning: bone marrow changes on MRI. AJR Am J Roentgenol 193:S1–S4, Quiz S5–S9 Tan HK, Bakri MM, Peh WCG (2014) Variants and pitfalls in MR imaging of knee injuries. Semin Musculoskelet Radiol 18:45–53 Thawait SK, Chaudhry V, Thawait GK et  al (2011) High-­ resolution MR neurography of diffuse peripheral nerve lesions. Am J Neuroradiol 32: 1365–1372

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Ultrasound Stefano Bianchi, Viviane Créteur, Antoine Moraux, and Giorgio Tamborrini

Contents

Abstract

1      Introduction 

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2      US Scanning Technique  2.1   Anterior Region  2.2   Medial Aspect  2.3   Lateral Aspect  2.4   Posterior Aspect 

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3      Conclusions 

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References 

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S. Bianchi (*) CIM SA, Cabinet d’imagerie médicale, Geneva, Switzerland e-mail: [email protected]; [email protected] V. Créteur Department of Radiology, Hôpital Erasme, Université Libre of Brussels, University Clinic of Brussels, Brussels, Belgium e-mail: [email protected] A. Moraux Centre d’Imagerie Jacquemars Giélée & CLIMAL, Lille, France Hôpital Privé La Louvière, Lille, France e-mail: [email protected] G. Tamborrini UZR, Ultrasound Center and Institute for Rheumatology, Basel, Switzerland Rheumatology University Hospital Basel, Basel, Switzerland e-mail: [email protected]

The hardware improvements in the last 20 years have impressively increased the capabilities of ultrasound in the assessment of the musculoskeletal system. Ultrasound is now considered in many clinical situations as the first-line imaging technique to evaluate limb disorders because of its high definition and dynamic capabilities. In addition, ultrasound is a low-cost, patient-friendly, and readily available technique. Ultrasound is less performant than magnetic resonance imaging in the evaluation of most intra-articular knee structures. However, it allows assessment of several disorders affecting tendons, vessels, and nerves, as well as other periarticular structures. This chapter describes the standard technique of ultrasound examination of knee, the normal ultrasound anatomy, as well as selected ultrasound pathologic images.

1 Introduction The refinement of broadband linear-array transducers has increased the capability of ultrasound (US) to evaluate the musculoskeletal system (Bianchi and Martinoli 2007; Grobbelaar and Bouffard 2000; De Maeseneer et al. 2014). US is less performant than magnetic resonance imaging (MRI) in the evaluation of most intra-­articular

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knee structures. However, US is highly accurate, widely available, and cost-effective in the assessment of several disorders affecting tendons, vessels, and nerves, as well as other periarticular structures (Grobbelaar and Bouffard 2000; Lee and Chow 2007; De Maeseneer et  al. 2014; Giokits-Kakavouli et al. 2016). The purposes of this chapter are to describe the US scanning technique of the knee, illustrate the normal US anatomy, and briefly present the appearance of a variety of disorders that may be assessed by US.

2 US Scanning Technique A detailed note from the referring clinician, with the indication of the presumptive clinical diagnosis, allows a US examination focused on a limited area of the knee, reduces the time of scanning, and permits a more accurate assessment. The patient history must be collected and recent radiographs, when available, must be reviewed. Correlation with radiographs is essential for correct interpre-

tation of challenging US images related to disorders that are obvious on plain radiographs. Routine examination of the knee can be achieved with broadband linear-array transducers up to 24MhZ for e.g. the assessment of superficial enthesis operating at a frequency band range of 5–15  MHz (Bianchi and Martinoli 2007; Tamborrini and Bianchi 2020). The use of an extended-field-of-view technology allows a panoramic view of the examined region which facilitates interpretation for the referring clinician (Fig.  1). Sonoelastography may be used to determine the elastic properties of soft tissues, including tendon, muscle, nerve, joint, and ligament, in healthy, pathological, and healing situations (Fig.  1) (Klauser et  al. 2014; Dirrichs et al. 2018). The two most useful techniques of sonoelastography are compression elastography and shear wave elastography (SWE), the latter being considered as more reproducible, dynamic, and quantitative than the compression elastography technique (Taljanovic et al. 2017). Color and power Doppler are useful

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Fig. 1  Technical considerations. (a) Extended-field-of-­ view technology image obtained over the anterior aspect of the knee shows in a single panoramic sonogram the quadriceps tendon (large arrowhead), the patella, the patellar tendon (small arrowhead), the tibia, and the femur. (b, c) Shear wave elastography of healthy patellar tendon. Arrows indicate the region of interest for elasto-

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gram. The qualitative color code shows intermediate velocity (green color) within the tendon and low velocity (blue color) within the Hoffa fat pad. The quantitative longitudinal Young’s modulus (E), expressed in kiloPascal (kPa), representing elastic properties of tissues, is measured at 39.9 (normal values = 30.09 ± 13.58)

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in the evaluation of the popliteal vessels, in the assessment of the vasculature of periarticular masses, and in depicting soft tissue hyperemia. For routine examination of the knee, the patient is lying down in supine, lateral, or prone position, depending on which knee face is evaluated. The knee is examined static and dynamic at different degrees of flexion and extension. As an example, grayscale assessment of the anterior tendons is better achieved in slight 20 to 30 degrees flexion of the joint, while evaluation of the trochlear cartilage is performed by anterior transverse sonograms obtained on a forcefully flexed knee when scanning the suprapatellar region, while the leg is kept extended to examine the cartilage in the infrapatellar region (Martino et al. 1998). On the other hand, when performing tendon color Doppler examination, the anterior tendons must be examined relaxed to avoid false negative examination related to intratendinous vessel stretching. Although examination of the contralateral knee is not routinely performed, it can be helpful in selected cases when US only shows subtle changes. The possibility to perform a dynamic examination is a peculiar advantage of US. Ligaments and tendons can be examined at rest, during stress maneuvers, or during active muscle contraction. The comparison between findings obtained at rest and during movements adds supplementary dynamic data to the morphologic information. Table 1 summarized the knee structures suitable to US examination.

2.1 Anterior Region The anterior aspect of the knee is evaluated examining the patient lying supine. We first realize grayscale images with the knee flexed at 20–30° to stretch of the extensor tendons (Fig. 2). Flexion allows adequate assessment of the quadriceps and patellar tendon (Bianchi et  al. 1994) and avoids artifactual hypoechogenicity, due to tendon anisotropy, which can mimic pathologic changes (Fornage 1987). Then we perform color Doppler examination with the knee extended. In this position, tendons are relaxed, avoiding intratendinous vessel collapses.

Table 1 Anterior region

Medial region

Lateral region

Posterior region

Suprapatellar region Quadriceps tendon Suprapatellar synovial recess Suprapatellar fat pad and pre-femoral fat Distal femoral metaphysis and trochlea Patellar region Patella Medial and lateral patellar retinacula Parapatellar synovial recesses Infrapatellar region Patellar tendon Prepatellar bursa, deep and superficial infrapatellar bursae Hoffa’s fat pad Anterior cruciate ligament Medial collateral ligament Medial femorotibial joint space Medial meniscus Pes anserinus tendons and bursa Lateral collateral ligament Popliteus tendon, subpopliteal joint recess Lateral femorotibial joint space Lateral meniscus Iliotibial band Medial region Tendons of pes anserinus Semimembranosus tendon Gastrocnemius–semimembranosus synovial bursa Middle region Medial head of the gastrocnemius Popliteal artery and veins Tibial nerve Intercondylar notch Posterior cruciate ligament Lateral region Biceps and gastrocnemius lateral head muscles and tendons Common peroneal nerve and branches Lateral condyle and overlying cartilage.

The anterior aspect can be divided in three regions: the suprapatellar, patellar, and infrapatellar regions.

2.1.1 Suprapatellar Region 2.1.1.1  Normal US Anatomy The quadriceps muscle lies in the anterior thigh and is formed by the vastus lateralis, the vastus medialis, the superficial rectus femoris, and the

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Fig. 2  Anterior aspect. Suprapatellar region. Quadriceps tendon. (a) Probe positioning for longitudinal examination. (b) Corresponding sonogram. Arrowheads = quadriceps tendon, PF pre-femoral fat, SR suprapatellar synovial recess, SF suprapatellar fat. Note the multilayered appear-

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ance of the quadriceps tendon which shows a regular fibrillar pattern. The suprapatellar synovial recess does not contain fluid and is located between the suprapatellar and pre-femoral fat pad

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Fig. 3  Anterior aspect. Suprapatellar region. Quadriceps tendon. (a) Probe positioning for longitudinal examination. (b, c) Corresponding radiograph and sonogram. The superficial rectus femoris tendon (white arrowheads) is separated by the deep quadriceps tendon laminae (aster-

isks) by a thin layer of fat (black arrowheads) which is evident both in the standard radiograph and US. This is a normal anatomic variant that must be differentiated from a delamination of the tendon

deep vastus intermedius. The presence of the fifth, sixth, seventh, or eighth head varies (Olewnik et al. 2021) tendon is formed by overlaying of three different tendon laminae: the superficial lamina coming from of the rectus femoris, the intermediate lamina coming from the vastus lateralis and medialis, and the deep lamina coming from the vastus intermedius. The multilayered appearance is readily evident at US

and can be well imaged particularly in longitudinal sonograms (Bianchi et  al. 1994). While the superficial tendon is always well evident, the differentiation of the intermediate and deep tendon laminae can be more difficult (Fig. 2). Different amount of fat can be seen between the superficial lamina and the deep laminae. This anatomic variation should not be confused as a horizontal delamination of the tendon (Fig. 3). The possibil-

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ity to discriminate the different components of the tendon allows differentiation between complete tears, which are treated by surgery, and partial tears involving only one or two laminae, which can be treated conservatively. Transverse sonograms can help in assessing the extension of the tear in the axial plane. Asymptomatic older subjects can present small irregularities of the distal tendon related to calcific enthesopathy. The suprapatellar synovial recess or suprapatellar bursa lies deep to the quadriceps tendon, between the suprapatellar fat pad and pre-­femoral fat (Fig. 2). In the uterine life, the synovial recess is completely separated from the articular cavity by a septum. A perforation of the septum normally occurs at the end of the fifth fetal month and allows communication between the two synovial spaces. In a small percentage of cases, an incomplete resorption of the septum leads to the presence of a suprapatellar synovial plica which, if an effusion is present, can be imaged at US.  US depicts the suprapatellar recess as a hypoechoic band result-

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ing from the overlaying of its anterior and posterior wall separated by a variable amount of fluid. US detects intra-articular fluid more accurately than clinical examination (Hauzeur et  al. 1999). Very small effusions can be imaged only on transverse sonograms of the lateral or medial pouches of the suprapatellar recess. Small effusions can be detected in suprapatellar longitudinal sonograms only at dynamic examination obtained during isometric contraction of the quadriceps muscle. It must be stressed, however, that a modest amount of intra-articular fluid is normal in asymptomatic subjects and that correlation with clinical data and with the contralateral knee is mandatory. When analysis of synovial fluid is essential, US can guide arthrocentesis US can also guide synovial biopsy: https://symbiosisonlinepublishing.com/ rheumatology-arthritic-diseases/rheumatologyarthritic-diseases19.pdf (Fig. 4). US leads to careful selection of the site of the puncture, particularly in small effusions, and proves the intra-articular positioning of the needle tip. Usually US-guided arthrocentesis is less painful than blind puncture.

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Fig. 4  Anterior aspect. Suprapatellar region. Suprapatellar synovial recess. US-guided arthrocentesis. (a) In small effusions US can exactly guide the tip (void arrowhead) of the needle (white arrowheads) at the center of the synovial effusion (asterisk). Note parietal synovial hypertrophy (small arrowheads) that can hinder blinded aspiration. In this patient we used a 27 G/4 cm needle. The

procedure was almost pain free and allowed aspiration of a small sample for fluid analysis. (b) In large effusions, when evacuation of the articular effusion is needed, or in larger knees, a 19–21 G needle/6–8 cm is placed under US guidance at the center of the recess distended by synovial fluid (asterisk)

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Different approaches can be used (Park et  al. 2011). US guidance of knee injections resulted in better accuracy than anatomical guidance (95.8% versus 77.8%, P, 0.001) (Berkoff et al. 2012). The suprapatellar fat pad is located cranial to the patellar upper pole, posterior to the quadriceps tendon, and anterior to the suprapatellar synovial recess. It is imaged on longitudinal sonograms as a triangular hyperechoic structure. The pre-femoral fat appears as hyperechoic fat located anterior to the femoral cortex. Focal nodular projections of hyperechoic femoral fat inside the suprapatellar bursa is a frequent normal finding and should not be misdiagnosed as hypertrophied synovium or a lipoma arborescens. A greater size and frond-like appearance are typical of lipoma arborescens (Learch and Braaton 2000; Patil et al. 2011).

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The femoral trochlea and the overlying cartilage are easily assessed on transverse images obtained with the knee completely flexed (Kazam et al. 2011) (Fig. 5). The hyaline cartilage appears as a hypo-anechoic band overlying the regular hyperechoic subchondral bone. In normal conditions, cartilage is thicker at its central portion and presents regular borders and homogeneous echogenicity (Fig. 5). The sulcus angle and the trochlear depth can be calculated with US and correlate well with CT (Martino et al. 1998). 2.1.1.2  Selected US Pathologic Images US examination of the suprapatellar region can assess a variety of disorders. Examples of calcific tendinopathy of the quadriceps tendon with associated hyperemia in crystal arthropathy (e.g.,

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Fig. 5  Anterior aspect. Suprapatellar region. Trochlear cartilage. (a) Probe positioning for transverse examination. (b, c) Corresponding sonogram an anatomic preparation. F femur, black arrowheads  =  trochlear cartilage, HFP Hoffa fat pad, white arrowheads = patellar tendon. US shows the cartilage as a hypo-anechoic band with

smooth surface overlying the subchondral bone. The trochlear angle can be appreciated. (Figure (c)—Courtesy by Prof. X. Demondion, Service de Radiologie et Imagerie Musculosquelettique, CCIAL, CHU de Lille, 59037 Lille, France)

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Fig. 6  Anterior aspect. Suprapatellar region. US pathologic images. (a) Longitudinal US of calcific enthesopathy of the quadriceps tendon with associated hyperemia in a patient with gout. Note calcific deposits (black arrowheads) at the insertion of the swollen hypoechoic tendon. The local hypervascular changes are related to local inflammation. (b) Impingement of the distal quadriceps tendon on a metallic wire (black arrowheads) after patel-

lar osteotomy. The wire appears as a hyperechoic structure surrounded by the hypoechoic tendon. Note local hypervascularization. (c) Partial tear of the quadriceps tendon affecting the superficial lamina. The black arrowheads point to the distal retracted stump of the torn superficial laminae. More distally an irregular area (asterisk) is due to local edema. The deep lamina (white arrowheads) of the tendon is normal

gout), impingement of the distal tendon on a metallic wire after patellar osteotomy, and partial tear of the quadriceps tendon (superficial tear) are presented in Fig. 6. Figures 7 and 8 show the US appearance of anatomic variants and several disorders of the suprapatellar recess (focal lateral nodular fat hypertrophy, lipoma arborescens, synovial hypertrophy in rheumatoid arthritis, calcified loose body inside the suprapatellar pouch, and post-traumatic lipohemarthrosis) (Bianchi et al. 1995a). Typical features of femoral trochlea dysplasia, chondrocalcinosis (Andrés et al. 2020; Aghaghazvini et  al. 2020; Möller et  al. 2008), and cartilage changes in femoro-patellar osteoarthritis are displayed in Fig. 9.

2.1.2 Patellar Region 2.1.2.1  Normal US Anatomy (Fig. 10) In the patellar region, US allows assessment of the patella, the medial and lateral patellar retinaculum, and the parapatellar synovial recess. These structures are best examined on transverse images. The anterior cortex of the patella appears as a regular hyperechoic line that can present some small local interruption due to nutrients vessels that can be well demonstrated at color Doppler. Superficial to the patella a thin hyperechoic band corresponds to the most anterior fibers of the rectus femoris tendon which runs distally to gradually blend in the patellar tendon and patellar periosteum. These two structures cannot be differentiated with US (Fig. 10).

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Fig. 7  Anterior aspect. Suprapatellar region. US pathologic images. (a, b) Axial US (a) and corresponding T2-weighted MRI (b) in focal lateral nodular fat hypertrophy (white arrowheads). The nodular fat appears as homogeneous and presents a regular hyperechoic surface. No

internal edema is depicted by MRI. (b, c) Axial US (c) and corresponding T2-weighted MRI (d) in a patient with lipoma arborescens (black arrowheads). The mass is larger and shows a frond-like appearance. MRI shows internal edema

In patella bipartita, an interruption of the anterior cortex, typically located at the superolateral quadrant, is evident (Blankstein et al. 2001). The US appearance of bipartite patella may be confused with a fracture. However, asymptomatic bipartite/tripartite patella borders are usually smooth, color Doppler does not show inflammatory changes, and local pressure with US probe remains painless. Patella position can be roughly evaluated by US in both children and adults. In children, an advantage of US is that the evaluation is independent of the degree of patellar ossification (Jozwiak and Pietrzak 1998). In adult, US determination of patellar position may be of

interest for orthopedic purposes, before and after total knee arthroplasty (Creteur et al. 2021). The prepatellar bursa, located in the subcutaneous tissues, overlays the lower pole of the patella and the proximal patellar tendon. In normal condition, because of the thin wall and virtual absence of internal fluid, the bursa cannot be demonstrated by US. As a result, any internal fluid must be regarded as a sign of prepatellar bursitis. The patellar retinacula arise from the medial and lateral side of the patella to insert on to the femur and act as stabilizing structures. US revealed the retinacula as bilaminar structures with discrete superficial and deep layers (Starok

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Fig. 8  Anterior aspect. Suprapatellar region. US pathologic images. (a) Axial US in rheumatoid arthritis shows the thickened pannus (black arrowheads) and the central joint effusion (asterisk). (b) Longitudinal sonograms in a patient with an intra-articular loose body. The calcified fragment (LB) is located in the suprapatellar recess. It appears as a hyperechoic structure (calipers) with posterior shadowing. The recess contains a small amount of fluid (asterisk). The white arrowheads point to the quadri-

ceps tendon. (c, d) Longitudinal US (c) and corresponding horizontal bean lateral radiographs (d) in lipohemarthrosis in a patient with occult intra-articular fracture of the tibial plateau. US shows a fluid effusion inside the suprapatellar recess with a layered appearance. The three layers correspond to fat, serum, and red blood cells. Radiograph confirms the presence of medullary fat at the top of the joint effusion

et  al. 1997). The medial articular facet of the patella can be roughly imaged by US if the patella is tilted and displaced internally by examiner’s hand. Unfortunately, the lateral facet, which is commonly affected by osteoarthritis and chondromalacia, cannot be imaged at US. Deep to the retinacula the medial and lateral parapatellar synovial recess can be visualized.

pressure and if local hypervascular changes are evident (Fig. 11a, b) and US-guided pressure is painful. The most frequent disorders of the patellar region affect the prepatellar bursa and include bursitis and calcific deposits (Fig. 11).

2.1.2.2  Selected US Pathologic Images Fractures of the syndesmosis in patella bipartita are rare (Bianchi 2020). The condition can be suspected if pain is reproduced by local US probe

2.1.3 Infrapatellar Region 2.1.3.1  Normal US Anatomy The patellar tendon originates from the lower pole of the patella and inserts into the anterior tibial tuberosity (Fig. 12). The tendon has a flattened appearance, regular borders, and internal

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Fig. 9  Anterior aspect. Suprapatellar region. US pathologic images. (a) Axial US in a normal subject shows the regular cartilage (black arrowheads) covering the hyperechoic subchondral bone. (b) Axial sonogram in trochlear dysplasia shows flattening of the trochlear cartilage (black arrowheads) and of the subchondral bone plate. (c) Chondrocalcinosis. Axial sonogram images several small

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Fig. 10  Anterior aspect. Patellar region. (a) Probe positioning for longitudinal examination. (b) Corresponding sonogram shows the anterior cortex of the patella and normal prepatellar tissues (PPT). In normal conditions the prepatellar synovial bursa cannot be detected by US. (c)

calcific deposits located in the middle third of the cartilage corresponding to crystal of calcium pyrophosphate. Note an ulceration of the cartilage surface (curved arrow). (d) Osteoarthritis. Axial US demonstrates complete cartilage loss of the lateral facet of the trochlea (void arrowhead). The cartilage of the medial facet (black arrowhead) is nearly normal

Probe positioning for axial examination of the medial aspect and (d) corresponding sonogram. US images the medial patellar retinaculum (white arrowheads) inserting into the medial edge of the patella and medial collateral ligament (arrow)

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Fig. 11  Anterior aspect. Patellar region. US pathologic images. (a, b) Oblique color Doppler sonograms obtained at the superolateral portion of patella bipartita in a patient with traumatic fracture of the synchondrosis. In (a) the symptomatic patella shows hypervascular changes inside the traumatized synchondrosis. Note normal appearance of the contralateral patella (b). (c, d) Prepatellar bursitis.

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Longitudinal color Doppler US (c) shows thickening and hypervascularity of the prepatellar bursa that contains a fluid effusion (asterisks). A lateral radiograph (c) shows swelling of the prepatellar tissues (diamonds). (e) Prepatellar calcific bursitis. US shows hyperechoic calcific material inside the bursa (white arrows). Note posterior shadowing (black arrows)

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Fig. 12  Anterior aspect. Infrapatellar region. Patellar tendon. (a) Probe positioning for longitudinal examination of the patellar tendon. (b, c) Corresponding sonogram realized in 30° of flexion (b) and complete extension (b). In

(b) note the wrinkled appearance of the patellar tendon (arrowheads) associated with focal hypoechogenicity. (d) Axial US images the wide patellar tendon and the deep Hoffa fat pad (HFP)

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hyperechoic fibrillar structure. A slight increased thickness of the distal third is normal and must not be interpreted as a localized tendinopathy. Internal hypoechoic areas can also be found in asymptomatic elite athletes (Cook et  al. 1998). This has implications for clinicians managing athletes with anterior knee pain. Deep to the patellar tendon, comprised between it and the anterior portion of the condyles, the intracapsular Hoffa pad appears as an adipose structure containing internal fibrous septa. Although MRI is the technique of choice in assessing the Hoffa fat pad (Jacobson et al. 1997; Draghi et al. 2016), US seems to be a promising tool in detecting infrapatellar fat pad impingement (Mikkilineni et  al. 2018). Recently, a defect in the lateral retinaculum/capsule has been diagnosed (Moraux et  al. 2019). Herniation through this defect of a significant amount of fat from the Hoffa fat pad may result in a local asymptomatic mass that can be detected by US (Moraux et al. 2018).

The pretibial deep infrapatellar bursa is a small synovial structure located between the distal portion of the patellar tendon and the anterior aspect of the tibial epiphysis (Draghi et al. 2015). In normal subjects, high-resolution transducers can demonstrate a small amount of fluid inside this bursa. 2.1.3.2  Selected US Pathologic Images US can accurately demonstrate disorders of the patellar tendon (Fig.  13). Patellar tendinopathy (Jumper’s knee) most frequently affects the proximal portion of the tendon in sportsmen. The tendon shows a local swelling and a focal hypoechoic appearance associated frequently with local hypervascularity especially also in the adjacent part of the Hoffa fat body (Bode et  al. 2017). When the distal part is affected, the pretibial bursa can show an associated effusion. Calcifications and ossification of the distal portion of the tendon can be found in asymptomatic subjects and are usually related to sequelae of Osgood–Schlatter

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Fig. 13  Anterior aspect. Infrapatellar region. US pathologic images. (a, b) Proximal patellar tendon. Jumper’s knee. Longitudinal conventional (a) and color Doppler (b) sonograms show a swollen and hypoechoic proximal patellar tendon (arrows). Note marked irregularity of the fibrillar pattern and hypervascular changes. The middle tendon is regular (white arrowhead). (c, d) Distal patellar tendon. Distal tendinopathy, Osgood–Schlatter disease. (c) Longitudinal color Doppler sonogram in a patient with

distal tendinopathy shows a swollen and hypoechoic distal patellar tendon (arrow) containing internal hypervascular changes. Note pretibial bursitis with internal effusion (asterisk). (d) Osgood–Schlatter disease. Longitudinal color Doppler sonogram shows a swollen and hypoechoic distal patellar tendon (arrowheads) and fragmentation (arrows) of the ossification center of the anterior tibial tuberosity (ATT)

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disease. Postsurgical changes (harvesting of the mid-third of the tendon for anterior cruciate ligament reconstruction) can be detected in transverse sonograms as presence of two tendon cords separated by a low signal bridge. This aspect must not be confused with a longitudinal tendon split (Adriani et al. 1995) in a knee flexion, the anterior part of the anterior cruciate ligament can be visualized well (Wu et al. 2022). A diagnosis of infrapatellar bursitis can be made when the bursa contains an effusion or when pain is elicited by applying local pressure with the probe. Color Doppler can show parietal inflammation of the bursa. Thrombosis of a vein inside the lateral retinaculum/capsule defect can result in localized pain (Moraux et al. 2020).

2.2 Medial Aspect 2.2.1 Normal US Anatomy The medial aspect is examined asking the patient’s leg externally rotated (Figs. 14 and 15). The medial collateral ligament is formed by two portions. The superficial portion originates from

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the superior aspect of the medial condyle and inserts on the medial tibial metaphysis. The deep portion links the medial meniscus to the femur (meniscofemoral ligament) and to the tibia (menisco-tibial ligament). US depicts both components as regular, hyperechoic laminae separated by a hypoechoic area related to fat and loose connective tissue. A synovial bursa can be located between the two components but cannot be demonstrated by US in normal condition. Dynamic US improves the assessment of medial collateral ligament integrity, by measuring the distance between the tibia and femur during valgus stress, as compared with the contralateral knee. Although the possibility to evaluate meniscal tears with US has been reported (Gerngross and Sohn 1992), MR imaging is the technique of choice. Nevertheless, US can readily detect and assess meniscal cysts (Rutten et al. 1998) which are always associated to meniscal tears. When suspected on standard radiographs, US can confirm the diagnosis of meniscal ossicle (Martinoli et  al. 2000) that represents a vestigial structure usually located at the posterior portion of medial meniscus (Nguyen et al. 2014).

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Fig. 14  Medial aspect. Medial collateral ligament. (a) Probe positioning for longitudinal examination. (b, c) Corresponding sonograms obtained at its cranial (b) and distal (c) part. The ligament is formed by a thick superficial component (black arrowheads) joining the tibia and

the femur and a thin deep component (white arrowheads) joining the medial meniscus (M) to the tibia and femur. A synovial bursa lies between the two components. It cannot be imaged by US in normal knees

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Fig. 15  Medial aspect. Pes anserinus tendons. (a) Probe positioning for longitudinal examination. Corresponding conventional (b) and color Doppler (b) sonograms. The tendons lie close to the cortex of the proximal metaphysis

of the tibia. Accurate scanning can differentiate the semitendinosus, gracilis, and sartorius tendons. In (c) the small medial inferior genicular artery (black arrowhead) runs between the tendons and the tibia

The pes anserinus tendons (sartorius, gracilis, and semitendinosus tendons) insert at the anteromedial portion of the tibial metaphysis (Lee et al. 2014; Curtis et  al. 2019). The different tendons may be distinguished by US at the cranial level. As the tendons approach the distal insertion, they blend together and are hardly differentiated. Dynamic US examination is invaluable in proving snapping pes anserinus syndrome (Shapiro et  al. 2017). Different synovial bursae located among the tendons and between them and the tibia lower the friction among these structures. In normal conditions these bursae cannot be demonstrated by US, while they are readily evident when distended by an effusion and in bursitis. The saphenous nerve is the terminal branch of the femoral nerve (Bianchi et al. 2006). At the thigh, the nerve runs close to the femoral artery. At the lower part of the adductor canal, it descends behind the sartorius muscle. It runs then between the tendons of the sartorius and gracilis muscles and becomes subcutaneous to follow the great saphenous vein. High-resolution US transducers allow detection and assessment of the nerve (Le Corroller et al. 2011; Riegler et al. 2018).

2.2.2 Selected US Pathologic Images Although US is not the technique of choice in assessing menisci, it can diagnose perimeniscitis (Fig.  16) and meniscal cysts (Seymour and Lloyd 1998) (Fig. 17). In addition, US allows a real-­time, fast, and accurate perimeniscal steroid injection or PRP injection after aspiration (Chen 2015) (Fig.  17). Most interestingly, US can accurately judge complications after meniscal suture and anterior cruciate reconstruction (Bianchi et  al. 2020) which can be difficult at MRI and facilitate local injections (Figs. 18 and 19). Tears of the medial collateral ligament can be isolated from accompanying anterior cruciate ligament and medial meniscus tears and can be diagnosed with US (Lee et al. 1996; Mathieu et al. 1997). US can predict patient outcome on the basis of the location of the medial collateral ligament injuries (Lee et al. 1996). Accurate scanning of the bone cortex can reveal thickening and hypervascularization of the periosteum, suggesting a fatigue fracture in the correct clinical setting (Khy et al. 2012; Bianchi 2020) (Fig. 20).

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Fig. 16 Medial aspect. US pathologic images. Perimeniscitis. (a) Longitudinal color Doppler sonogram obtained over the medial face shows a local bulging of the soft tissues superficial to the medial meniscus (M). Note local hypervascular inflammatory changes related to perimeniscitis. (b) Corresponding coronal T2-weighted fat sat

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MR image shows increased soft tissues signal (black arrowheads) between the medial meniscus (M) and the medial collateral ligament. The bone edema of the tibial plateau (white arrow) apparent at MRI was not detectable at US

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Fig. 17  Medial aspect. US pathologic images. Meniscal cysts. (a) Longitudinal sonogram obtained over the posteromedial aspect of the joint line shows two tears of the medial meniscus (M) associated with a meniscal cyst (white arrowheads) that cause bulging of the capsule. (b, c) US-guided injection of a small meniscal cyst. (a) The

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cyst (white arrowheads) is located close to the meniscus (M). (c) Under US guidance a needle (black arrowheads) has been introduced into the cyst. The curved arrow points to the needle tip. Note accurate injection of steroid (gray arrowheads) inside the cyst

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Fig. 18 Medial aspect. US pathologic images. Complications after meniscal arthroscopic repair. (a) Longitudinal sonogram obtained over the medial aspect of the joint in a patient with previous meniscal suture and persistent local pain. US shows extrusion of a small metal-

Fig. 19  Medial aspect. US pathologic images. Complications after anterior cruciate ligament reconstruction. (a) Longitudinal sonogram obtained over the medial aspect of the proximal tibia in a patient with previous anterior cruciate ligament reconstruction and persistent local pain. US shows extrusion of the interference screw (arrow). The screw can be seen projecting outside of the tibial cortex. An inflammatory area (arrowheads) due to impingement of the screw with the adjacent soft tissues appears as a hypoechoic cystic mass corresponding to the painful swelling detected at clinical examination. (b) US-guided injection in a patient presenting similar findings. Under US guidance note the needle (black arrowheads) placed at the contact of the screw (arrows) for optimal steroid injection

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lic orthopedic hardware (arrow) that causes local inflammatory changes (white arrowheads). (b) Under US guidance note the needle (black arrowheads) placed at the contact of the hardware for optimal steroid injection. (c) Corresponding T2-weighted MR coronal image

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Fig. 20  Medial aspect. US pathologic images. Stress fracture of the tibia. (a, b) Axial conventional (a) and longitudinal color Doppler (b) sonograms obtained over the medial aspect of the proximal tibia in a patient with local mechanical pain. US shows thickening of the periosteum,

local soft tissues edema, and hyperemia. The US appearance together with the clinical data suggest a stress fracture. (c) Corresponding sagittal T2-weighted fat sat MR image confirms a stress fracture (arrow) surrounded with hyperintense bone edema

2.3 Lateral Aspect

thin fibrillar structure that inserts on a tibial eminence, the tubercle of Gerdy, located at the anterolateral aspect of the proximal tibial epiphysis (Fig. 21) (Bonaldi et al. 1998; Gyaran et al. 2010). The more distal portion of the iliotibial band normally widens just before its insertion into the tibia. This normal appearance must not be confused with tendinopathy. The standard US examination of the lateral region must also always include transverse sonograms of the proximal tibiofemoral joint and of the anterolateral compartment of the leg. A recent study showed most of the ligaments of the proximal tibio-peroneal joint can be visualized by means of US (Scarciolla et al. 2021). Intramuscular ganglia can originate from the proximal tibiofemoral joint, frequently extend inside the anterolateral leg compartment, and may provoke compression of the fibular nerve (Bianchi et al. 1995b). At the proximal edge of the popliteal space, the sciatic nerve splits into the posterior tibial nerve and the common peroneal nerve. US can assess both nerves and their typical internal

2.3.1 Normal US Anatomy The lateral aspect of the joint is examined asking the patient to rotate internally the lower extremity (Fig. 21). The lateral collateral ligament appears as a cord-like hyperechoic structure, located at the posterolateral aspect of the joint, joining the peroneal head and lateral femoral condyle (Fig. 21). The ligament at its superior portion lies anterior to the tendon of the biceps femoris, while the two structures are located close one to the other at the distal part. At this level frequently, the ligament runs between a split of the tendon to reach its lower insertion (Guillin et al. 2010). The study of the lateral meniscus shares the similar limits to those of the medial meniscus. A forceful flexion of the knee allows better visualization of meniscal cysts. The distal tract of the iliotibial band is found at the anterior aspect of the lateral region of the knee (Jiménez Díaz et al. 2020). It appears as a

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Fig. 21  Lateral aspect. Lateral collateral ligament and iliotibial band. (a, b) Probe positioning and corresponding longitudinal US of the lateral collateral ligament. The ligament (white arrowheads) appears normally thick at its cranial insertion. Note the popliteus tendon (PoT) running (dotted arrow) below it and inserting in the popliteal fossa (black arrowheads) located at the lateral aspect of the

external condyle. M lateral meniscus. (c, d) Probe positioning and corresponding longitudinal US of the iliotibial band. The thin hyperechoic iliotibial band (small black arrowheads) shows physiological thickening (large black arrowhead) at its distal pre-insertional portion into the tibia. Note the relationship of the band with the lateral aspect of the external femoral condyle

fascicular echotexture due to multiple hypoechoic parallel but discontinuous linear areas separated by hyperechoic bands. The US appearance correlates well with the internal structure composed of neural fascicles (hypoechoic areas) separated by the epineurium (hyperechoic bands) (Bianchi et al. 2011). Careful US axial scanning allows an accurate evaluation of the common peroneal nerve located close to the medial edge of the biceps femoris muscle. After reaching the region of the head of the fibula, it circumvents laterally and the nerve rests on the lateral cortex of the neck of the fibula, located in an osteomuscular tunnel. The nerve then divides between the origins of the peroneal longus muscle into two terminal branches: the deep peroneal nerve and the superficial peroneal nerve (Bianchi et al. 2006).

lateral femoral condyle. It is mostly found in runners. US is helpful in the diagnosis and in guiding local treatment. Impingement of orthopedic hardware on the adjacent soft tissues results in edema that is readily demonstrated by US (Fig.  23) while MRI analysis is limited by metallic artifact. Due to its superficial anatomic location and close relations with the cortex of the fibula, the common fibular nerve is exposed to local acute and chronic trauma. High-frequency transducers allow a detailed evaluation of the nerve pathologic changes (Fig. 24).

2.3.2 Selected US Pathologic Images (Figs. 22, 23, and 24) Iliotibial band friction syndrome (Fig. 22) is due to repeated impingement of the band against the

2.4 Posterior Aspect The patient is asked to lie prone with the knee extended, to allow examination of the posterior region. The US examination is performed in the transverse and sagittal planes starting with the evaluation of the medial region followed by the median and lateral regions.

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Fig. 22  Lateral aspect. US pathologic images. Iliotibial band friction syndrome. (a) Longitudinal US of the iliotibial band obtained at the level of the lateral condyle. The iliotibial band (curved arrows) is normal. Between the band and the femur note thickening of the local soft tissues (white arrowhead) and a local effusion (asterisk) consistent with bursitis. (b) US images demonstrate intrabursal steroid injection. Arrowheads = needle

2.4.1 Posterior Aspect, Medial Region (Figs. 25, 26, and 27) The pes anserinus tendons can be imaged at this level. The sartorius muscle is mainly composed of muscle fibers and is located medially to the other tendons of the pes anserinus complex. The gracilis tendon is located posteriorly to the sartorius and, as suggested by its name, it is the thinnest of the medial tendons. The semitendinosus tendon is superficial to the semimembranosus muscle proximally and to the semimembranosus tendon distally. The semimembranosus tendon lies in a more lateral position and inserts by its direct tendon on the posteromedial aspect of the tibial epiphysis. Its indirect tendon cannot be detected at US.

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Fig. 23 Lateral aspect. US pathologic images. Complications after centro-medullary nailing. (a) A-P radiograph show a fixation screw protruding from the tibial cortex. (b) Longitudinal US obtained at the level of the lateral condyle shows the screw’s head (black arrow) which impinges on the local soft tissues that show hypoechoic swelling (white arrowheads). Note the posterior metallic artifact of the screw (white arrows)

Between the semimembranosus tendon and the medial head of the gastrocnemius is located the gastrocnemius semimembranosus synovial bursa (Fritschy et al. 2006) which, in normal condition, is not depicted by US. In young subjects the bursa does not communicate with the knee joint while in adults it is connected by a short pedicle with the synovial articular space. The bursa has a deep and a superficial component. The smaller deep component is located between the medial head of the gastrocnemius and the posterior aspect of the knee and communicates with the joint through a break in the capsule. The larger component is located in the subcutaneous soft tissues, superficial to the gastrocnemius medial head. Although not depicted by US in normal conditions, the location of the bursa must be known by sonologists since it represents the

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Fig. 24  Lateral aspect. US pathologic images. Common fibular nerve contusion in a patient crossing the legs regularly. (a) Axial US obtained proximal (a) and at the level (b) of the nerve trauma. In (a) the common fibular nerve (white arrowheads) appears formed by an anterior and posterior component which show a normal size. More distally (b) note swelling and rounded appearance of the two components (black arrowheads). The internal fascicules are more evident. Electroneurographic studies confirmed local impairment of the nerve function

Fig. 25 Posterior aspect. Internal region. Axial images. (a) Probe positioning and (b) sonogram with (c) corresponding PD FS MRI. MC medial condyle, asterisks = cartilage of the medial condyle, SM semimembranosus tendon, MHG medial head of the gastrocnemius and its tendon (arrows), ST semitendinosus tendon, Gr gracilis tendon, Sar sartorius muscle

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commonest site of Baker’s cysts (Park et  al. 2020). Flexion of the knee opens the communication with the joint space and can lead to a better distension and assessment of the cyst pedicle. Baker’s cyst appears as bursal fluid dilatation with wall presenting different thickness and vascular hyperemia. The cysts can contain septa, hypertrophied synovium, or loose bodies (Bianchi and Martinoli 1999). Rarely, extraarticular Synovial Osteochondromatosis in Baker’s Cysts may occur (Sankepally et al. 2022). US can accurately guide an aspiration of the cyst. The procedure is safe and quick to perform. It decreases the internal pressure in the cyst, thus allowing reduction of pain, examination of synovial fluid for diagnostic purposes, and/or intracystic injection of corticosteroid/anesthetic. US can only assess the posterior cartilage of the medial condyle. Unfortunately, the weight-­ bearing cartilage, located at the junction of the middle and posterior third of the condyle, cannot be assessed.

2.4.2 Posterior Aspect, Middle Region (Fig. 28) The medial head of the gastrocnemius has a triangular shape when examined on transverse sonograms. Its tendon is located at the medial aspect

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Fig. 26  Posterior aspect. Internal region. Axial images. (a) Probe positioning and (b, c) corresponding sonograms obtained with different tilting of the transducer in the axial plane. MC medial condyle, MHG medial head of the gastrocnemius. The posteromedial tendons have different orientations and show different echogenicity depending on the orientation of the US bean. In (a) the tendon of the medial head of the gastrocnemius (small black arrow) is

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hypoechoic, while the tendons of the semimembranosus (large white arrow) and semitendinosus (white arrowhead) are hyperechoic. After change in the tilting of the transducer note change in the tendon echogenicity. The medial head of the gastrocnemius is now hyperechoic while the semimembranosus and semitendinosus appear hypoechoic

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Fig. 27  Posterior aspect. Internal region. Longitudinal images. (a) Probe positioning and (b) corresponding sonogram. MC medial condyle, asterisks = cartilage of the

medial condyle, arrows = semimembranosus tendon, arrowheads = semitendinosus tendon, MM medial meniscus

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of the muscle and can be demonstrated as a comma-shaped hyperechoic structure. The popliteal artery is located laterally to the medial head. The vein is located posterior and lateral to the artery and the nerve posterior and lateral to the vein. So, in transverse sonograms, the popliteal artery, vein, and nerve are located in a line which is directed from anterior to posterior and from medial to lateral. The caliber, thickness, and appearance of the wall as well as pulsatility of the artery can be well assessed with US. Since the patient is examined prone, the vein can be collapsed and hardly detectable by US. In this case, a small elevation of the foot, obtained by flexing the knee, fills the vein and leads to its accurate assessment. Flattening of the vein obtained by local compression with the probe excludes vein thrombosis. Both artery and vein, however, are better evaluated by US color Doppler sonograms. The posterior tibial nerve is larger than the common peroneal nerve and can be followed from its origin to the ankle region. Accurate scanning in

the axial plane can allow to detection of the medial root of the sural nerve that arises from the posterior aspect of the tibial nerve. This small branch directs posteriorly to reach the lateral root of the sural nerve (branch of the common fibular nerve) to form the sural nerve (Bianchi et al. 2018). The cruciate ligaments are hard to evaluate by US. The posterior cruciate ligament can be demonstrated as a hypoechoic band on longitudinal sonograms in normal subjects. The distal portion of the ligament and its insertion into the tibia are more superficial and are more easily assessed than the proximal deep portion. Due to its internal location and oblique orientation, the anterior cruciate ligament is barely visible. US indirect evaluation of the integrity of cruciate ligaments can be obtained by measuring the tibia subluxation during stress maneuvers (Poboży et al. 2016; Gebhard et  al. 1999). US detection of a localized hematoma at the postero-cranial portion of the anterior ­cruciate ligament correlated with acute tears (Ptasznik et al. 1995).

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2.4.3 Posterior Aspect, Lateral Region (Fig. 29) The biceps muscle consists of two heads, a long head which origins from the ischial tuberosity and a short head which origins from the femoral shaft. The two heads join together at the proximal edge of the popliteal space to form a strong tendon which inserts into the fibular head. The biceps muscle and tendon are well evaluated by longitudinal and transverse sonograms. Anatomic variations of the distal tendon include the presence of an accessory tendon inserting into the tibial epiphysis (the tibial arm). This variation can cause knee snapping and is easily demonstrated by dynamic US (Guillin et  al. 2010). Proximal images must include the myotendinous junction of the long head since this is a common site of sport-related tears. Transverse sonograms show both the lateral collateral ligament and the biceps tendon and allow their differentiation. The common peroneal nerve at the level of the peroneal head is located in the subcutaneous tisFig. 29 Posterior aspect. Lateral region. Axial images. (a) Probe positioning and (b) sonogram with (c) corresponding PD FS MRI. BM biceps muscle, LHG lateral head of the gastrocnemius, LC lateral condyle, asterisks = cartilage of the lateral condyle, void arrows = common fibular nerve

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sues between the skin and the bone cortex. This area must be accurately examined since this is the typical location at which the nerve can be injured by local traumas. Accurate scanning allows identification of the lateral root of the sural nerve, located medial to the common peroneal nerve. In a deeper location, the lateral head of the gastrocnemius muscle, the posterior portion of the lateral condyle, and the hyaline cartilage can be assessed. The lateral head of the gastrocnemius muscle is smaller than the medial head. Its tendon may contain the fabella, a sesamoid bone that appears as a curvilinear hyperechoic structure showing posterior shadowing. Care must be done not to confuse the fabella with an intra-articular loose body. The intratendinous location and the fixed position during dynamic examination exclude an intra-articular fragment. Differentiation from a capsular calcification is better achieved by lateral plain films. US allows assessment of common peroneal compression by the fabella (Cesmebasi et al. 2016).

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2.4.3.1  Selected US Pathologic Images The most frequent disorders affecting the posterior region of the knee are Baker’s cyst made by fluid effusion located inside the semimembranosus-­ medial head gastrocnemius bursa (Figs. 30, 31, and 32). Most cysts contains synovial fluid and have thin walls made by synovial membrane although they can contain loose bodies formed inside the joint and migrated into the cyst (Fig.  31).US can detect rupture of the cysts mostly located at the lower pole of the superficial component (Fig. 30). When clinically indicated, US can guide aspiration/injection of the cyst (Fig. 32). Ganglia are mucoid cysts with fibrous wall that when located in the intercondy-

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lar notch are optimally assessed by MRI.  US allows only detection of the most posterior part of these ganglia but can guide percutaneous treatment (Fig. 33). Popliteal vessels thrombosis can be easily detected by US (Fig. 34).

3 Conclusions In conclusion, although US has definite limitations in the assessment of several intra-articular disorders it has also several advantages in the evaluation of the knee. General advantages of US include absence of contraindications, absence of

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Fig. 30  Posterior aspect. US pathologic images. Baker’s cyst. (a) Longitudinal and (b) axial sonogram obtained at the medial aspect of the posterior region. The Baker’s cyst appears formed by a superficial (SP) and a deep (DP) portion. The two portions communicate with a thin pedicle located between the semimembranosus (large arrow) and

the medial head (small arrow) tendons. MC medial condyle, asterisk  =  cartilage of the medial condyle, MHG medial head of the gastrocnemius. (c) In a patient with osteoarthritis of the knee note several calcified loose bodies (void arrows) located inside the cyst

Ultrasound Fig. 31  Posterior aspect. US pathologic images. Baker’s cyst rupture. (a) Longitudinal sonogram obtained at the medial aspect of the posterior region. (b) Axial sonogram and (c) corresponding PD FS MRI obtained at the proximal calf. In (a) note a tear (white arrow) of the lower part of the superficial portion (SP) of the cyst allowing fluid to leak (dotted arrow) in the subcutaneous tissues. (b, c) show distal extension of the fluid at the calf region

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Fig. 32 Posterior aspect. US pathologic images. US-guided Baker’s cyst aspiration/injection. (a–d) Sequential longitudinal sonograms obtained at the medial aspect of the posterior region. Images show complete aspiration of the cyst (b) followed by confirmation of

intracystic injection of a small amount of cortisone (c) and then complete filling during real-time US (d). SP superficial portion of the Baker’s cyst, MHG medial head of the gastrocnemius muscle, white arrows indicate the injected cortisone

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Fig. 33  Posterior aspect. US pathologic images. Cruciate ligament ganglion cyst. Longitudinal (a) sonogram and (b) corresponding PD FS MRI. The ganglion cyst (asterisks) is located in the intercondylar notch close to the

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anterior cruciate ligament (arrow). US allows a real-time guidance of a needle (arrowheads) inside the ganglion for evacuation of the mucoid content and/or intralesional steroid injection

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Fig. 34  Posterior aspect. US pathologic images. Popliteal vessels thrombosis. (a–c) Longitudinal (a) and axial (b, c) color Doppler sonograms in a patient with popliteal artery thrombosis. The axial sonograms were obtained at the level indicated by the dotted arrows in (a). Note a thrombus (black arrowhead) completely filling the distal artery. The proximal artery (void arrowheads) is patent and

shows internal flow signals. (d, e) Longitudinal conventional (d) and color Doppler sonograms (e) in a patient with popliteal vein (PV) thrombosis. In (d) an echoic thrombus fills the popliteal vein that does not have flow signals at color Doppler (e). Note the normal adjacent popliteal artery (PA)

Ultrasound Table 2 Summary of main advantages of US in examining the knee Dynamic examination can be realized during knee movements Assessment of the vessels Real-time guidance of diagnostic and therapeutic procedures Quick and accurate assessment of local peripheral nerves Possibility to assess soft tissues in operated knee also in the presence of metallic hardware.

invasivity, wide availability, low cost, and possibility to perform a dynamic examination. In addition, US is very well tolerated by patients that are examined in a comfortable position without limitations related to gantry size and necessity to avoid movements. Patients can follow the examination step by step and can communicate freely with the sonologist asking questions and eventually referring pain at pressure with the transducer. Specific advantages of US in knee examination are listed in Table 2.

References Adriani E, Mariani PP, Maresca G et  al (1995) Healing of the patellar tendon after harvesting of its mid-third for anterior cruciate ligament reconstruction and evolution of the unclosed donor site defect. Knee Surg Sports Traumatol Arthrosc 3:138–143 Aghaghazvini L, Tahmasebi MN, Gerami R et al (2020) Sonography: a sensitive and specific method for detecting trochlear cartilage pathologies. J Ultrasound 23:259–263 Andrés M, Sivera F, Pascual E (2020) Progresses in the imaging of calcium pyrophosphate crystal disease. Curr Opin Rheumatol 32:140–145 Berkoff DJ, Miller LE, Block JE (2012) Clinical utility of ultrasound guidance for intra-articular knee injections: a review. Clin Interv Aging 7:89–95 Bianchi S (2020) Ultrasound and bone: a pictorial review. J Ultrasound 23:227–257 Bianchi S, Martinoli C (1999) Detection of loose bodies in joints. Radiol Clin N Am 37:679–690 Bianchi S, Martinoli C (2007) Ultrasound of the musculoskeletal system. Springer, New York Bianchi S, Zwass A, Abdelwahab IF et al (1994) Diagnosis of tears of the quadriceps tendon of the knee: value of sonography. AJR Am J Roentgenol 162:1137–1140 Bianchi S, Zwass A, Abdelwahab IF et  al (1995a) Sonographic evaluation of intramuscular ganglia. Clin Radiol 50:235–236

135 Bianchi S, Zwass A, Abdelwahab IF et  al (1995b) Sonographic evaluation of lipohemarthrosis: clinical and in vitro study. J Ultrasound Med 14:279–282 Bianchi S, Martinoli C, Demondion X (2006) Écho-­ anatomie et pathologies des nerfs autour du genou. In: Vande Berg B, Bianchi S, Nizard R et al (eds) Le Genou. Une approche pluridisciplinaire. Sauramps Médical, Montpellier Bianchi S, Draghi F, Beggs I (2011) Ultrasound of the peripheral nerves. In: Allan P, Baxter G, Weston M (eds) Clinical ultrasound. Churchill Livingstone Elsevier, Edinburgh Bianchi S, Droz L, Deplaine CL et al (2018) Ultrasound of the sural nerve: normal and pathological appearance. A pictorial essay. J Ultrasound Med 37:1257–1265 Bianchi S, Fischer B, Prues-Latour V et  al (2020) Ultrasound of complications after arthroscopic anterior cruciate ligament reconstruction: A pictorial essay. J Ultrasound Med 39:169–179 Blankstein A, Cohen I, Salai M et  al (2001) Ultrasonography: an imaging modality enabling the diagnosis of bipartite patella. Knee Surg Sports Traumatol Arthrosc 9:221–224 Bode G, Hammer T, Karvouniaris N et al (2017) Patellar tendinopathy in young elite soccer—clinical and sonographical analysis of a German elite soccer academy. BMC Musculoskelet Disord 18:344 Bonaldi VM, Chhem RK, Drolet R et al (1998) Iliotibial band friction syndrome: sonographic findings. J Ultrasound Med 17:257–260 Cesmebasi A, Spinner RJ, Smith J et  al (2016) Role of sonography in the diagnosis and treatment of common peroneal neuropathy secondary to fabellae. J Ultrasound Med 35:441–447 Chen H (2015) Diagnosis and treatment of a lateral meniscal cyst with musculoskeletal ultrasound. Case Rep Orthop 2015:432187 Cook JL, Khan KM, Harcourt PR et al (1998) Patellar tendon ultrasonography in asymptomatic active athletes reveals hypoechoic regions: a study of 320 tendons. Victorian Institute of Sport Tendon. Clin J Sport Med 8:73–77 Creteur V, De Angelis R, Absil J et al (2021) Sonographic and radiographic evaluation of the extensor tendons in early postoperative period after total knee arthroplasty. Skelet Radiol 50:485–494 Curtis BR, Huang BK, Pathria MN et  al (2019) Pes anserinus: anatomy and pathology of native and harvested tendons. AJR Am J Roentgenol 213: 1107–1116 De Maeseneer M, Marcelis S, Boulet C et  al (2014) Ultrasound of the knee with emphasis on the detailed anatomy of anterior, medial, and lateral structures. Skelet Radiol 43:1025–1039 Dirrichs T, Quack V, Gatz M et  al (2018) Shear wave elastography (SWE) for monitoring of treatment of tendinopathies: a double-blinded, longitudinal clinical study. Acad Radiol 25:265–272 Draghi F, Corti R, Urciuoli L et al (2015) Knee bursitis: a sonographic evaluation. J Ultrasound 18:251–257

136 Draghi F, Ferrozzi G, Urciuoli L et  al (2016) Hoffa’s fat pad abnormalities, knee pain and magnetic resonance imaging in daily practice. Insights Imaging 7:373–383 Fornage BD (1987) The hypoechoic normal tendon. A pitfall. J Ultrasound Med 6:19–22 Fritschy D, Fasel J, Imbert JC et  al (2006) The popliteal cyst. Knee Surg Sports Traumatol Arthrosc 14:623–628 Gebhard F, Authenrieth M, Strecker W et  al (1999) Ultrasound evaluation of gravity induced anterior drawer following anterior cruciate ligament lesion. Knee Surg Sports Traumatol Arthrosc 7:166–172 Gerngross H, Sohn C (1992) Ultrasound scanning for the diagnosis of meniscal lesions of the knee joint. Arthroscopy 8:105–110 Giokits-Kakavouli G, Karokis D, Raftakis I et al (2016) Ultrasound of the knee in rheumatology: pitfalls, what is new? J Rheumatol 27:151–160 Grobbelaar N, Bouffard JA (2000) Sonography of the knee, a pictorial review. Semin Ultrasound CT MR 21:231–274 Guillin R, Mendoza-Ruiz JJ, Moser T et  al (2010) Snapping biceps femoris tendon: a dynamic real-time sonographic evaluation. J Clin Ultrasound 38:435–437 Gyaran IA, Spiezia F, Hudson Z et al (2010) Sonographic measurement of iliotibial band thickness: an observational study in healthy adult volunteers. Knee Surg Sports Traumatol Arthrosc 19:458–461 Hauzeur JP, Mathy L, De Maertelaer V (1999) Comparison between clinical evaluation and ultrasonography in detecting hydrarthrosis of the knee. J Rheumatol 26:2681–2683 Jacobson JA, Lenchik L, Ruhoy MK et al (1997) MR imaging of the infrapatellar fat pad of Hoffa. Radiographics 3:675–691 Jiménez Díaz F, Gitto S, Sconfienza LM et  al (2020) Ultrasound of iliotibial band syndrome. J Ultrasound 23:379–385 Jozwiak M, Pietrzak S (1998) Evaluation of patella position based on radiologic and ultrasonographic examination: comparison of the diagnostic value. J Pediatr Orthop 18:679–682 Kazam JK, Nazarian LN, Miller TT et  al (2011) Sonographic evaluation of femoral trochlear cartilage in patients with knee pain. J Ultrasound Med 30:797–802 Khy V, Wyssa B, Bianchi S (2012) Bilateral stress fracture of the tibia diagnosed by ultrasound. A case report. J Ultrasound 15:130–134 Klauser AS, Miyamoto H, Bellmann-Weiler R et al (2014) Sonoelastography: musculoskeletal applications. Radiology 272:622–633 Le Corroller T, Lagier A, Pirro N et al (2011) Anatomical study of the infrapatellar branch of the saphenous nerve using ultrasonography. Muscle Nerve 44:50–54 Learch TJ, Braaton M (2000) Lipoma arborescens: high-­ resolution ultrasonographic findings. J Ultrasound Med 19:385–389

S. Bianchi et al. Lee MJ, Chow K (2007) Ultrasound of the knee. Semin Musculoskelet Radiol 11(2):137–148 Lee JI, Song IS, Jung YB et al (1996) Medial collateral ligament injuries of the knee: ultrasonographic findings. J Ultrasound Med 15:621–625 Lee J-H, Kim K-J, Jeong Y-G et al (2014) Pes anserinus and anserine bursa: anatomical study. Anat Cell Biol 47:127–131 Martino F, De Serio A, Macarini L et  al (1998) Ultrasonography versus computed tomography in evaluation of the femoral trochlear groove morphology: a pilot study on healthy, young volunteers. Eur Radiol 8:244–247 Martinoli C, Bianchi S, Spadola L et  al (2000) Multimodality imaging assessment of meniscal ossicle. Skelet Radiol 29:481–484 Mathieu P, Wybier M, Busson J et  al (1997) The medial collateral ligament of the knee. Ann Radiol 40:176–181 Mikkilineni H, Delzell PB, Andrish J et  al (2018) Ultrasound evaluation of infrapatellar fat pad impingement: an exploratory prospective study. Knee 25:279–285 Möller I, Bong D, Naredo E et  al (2008) Ultrasound in the study and monitoring of osteoarthritis. Osteoarthr Cartil 16:S4–S7 Moraux A, Bianchi S, Le Corroller T (2018) Soft tissue masses of the knee related to a focal defect of the lateral patellar retinaculum. J Ultrasound Med 37:1821–1825 Moraux A, Bianchi S, Tassery F et al (2019) The lateral patellar retinaculum defect: anatomical study using ultrasound. Skelet Radiol 48:1753–1758 Moraux A, Bianchi S, Le Corroller T (2020) Anterolateral knee pain related to thrombosed lateral patellar retinaculum veins: unusual anterolateral pain of the knee [published online ahead of print, 2020 Apr 1]. J Clin Ultrasound 48:275–278 Nguyen JC, De Smet AA, Graf BK et  al (2014) MR imaging-­based diagnosis and classification of meniscal tears. Radiographics 34:981–999 Olewnik Ł, Tubbs RS, Ruzik K, Podgórski M, Aragonés P, Waśniewska A, Karauda P, Szewczyk B, Sanudo JR, Polguj M (2021) Quadriceps or multiceps femoris?-Cadaveric study. Clin Anat 34(1):71–81. https://doi.org/10.1002/ca.23646. Epub 2020 Sep 9. PMID: 32644202 Park Y, Lee SC, Nam HS et  al (2011) Comparison of sonographically guided intra-articular injections at 3 different sites of the knee. J Ultrasound Med 30:1669–1676 Park GY, Kwon DR, Kwon DG (2020) Clinical, radiographic, and ultrasound findings between simple and complicated baker’s cysts. Am J Phys Med Rehabil 99:7–12 Patil PB, Kamalapur MG, Joshi SK et al (2011) Lipoma arborescens of knee joint: role of imaging. J Radiol Case Rep 5:7–25 Poboży T, Kielar M (2016) A review of ultrasonographic methods for the assessment of the anterior cruciate ligament in patients with knee instability - diagnostics

Ultrasound using a posterior approach. J Ultrason 16(66):288–95. https://doi.org/10.15557/JoU.2016.0029. Epub 2016 Sep 7. PMID: 27679732; PMCID: PMC5034023. Ptasznik R, Feller J, Bartlett J et al (1995) The value of sonography in the diagnosis of traumatic rupture of the anterior cruciate ligament of the knee. AJR Am J Roentgenol 164:1461–1463 Riegler G, Jengojan S, Mayer JA et al (2018) Ultrasound anatomic demonstration of the infrapatellar nerve branches. Arthroscopy 34:2874–2883 Rutten MJ, Collins JM, van Kampen A et  al (1998) Meniscal cysts: detection with high-resolution sonography. AJR Am J Roentgenol 171:491–496 Sankepally P, Khatoon HM, Srirambhatla A, Arora AJ (2022). Extraarticular Synovial Osteochondromatosis in Baker’s Cysts Bilaterally: a Rare Presentation. Maedica (Bucur) 17(4):999–1001. https://doi.org/10.26574/ maedica.2022.17.4.999. PMID: 36818241; PMCID: PMC9923057. Scarciolla L, Herteleer M, Turquet E et  al (2021) Anatomical study of the proximal tibiofibular ligaments using ultrasound. Insights Imaging 18(12):27

137 Seymour R, Lloyd DC (1998) Sonographic appearances of meniscal cysts. J Clin Ultrasound 26(1):15–20 Shapiro SA, Hernandez LO, Montero DP (2017) Snapping pes anserinus and the diagnostic utility of dynamic ultrasound. J Clin Imaging Sci 17(7):39 Starok M, Lenchik L, Trudell D et  al (1997) Normal patellar retinaculum: MR and sonographic imaging with cadaveric correlation. AJR Am J Roentgenol 168:1493–1499 Taljanovic MS, Gimber LH, Becker GW et  al (2017) Shear-wave elastography: basic physics and musculoskeletal applications. Radiographics 37:855–870 Tamborrini G, Bianchi S (2020) Ultrasound of the knee (adapted according to SGUM guidelines). Praxis (Bern 1994) 109:991–1000 Wu WT, Lee TM, Mezian K, Naňka O, Chang KV, Özçakar L (2022) Ultrasound Imaging of the Anterior Cruciate Ligament: A Pictorial Essay and Narrative Review. Ultrasound Med Biol 48(3):377–396. https:// doi.org/10.1016/j.ultrasmedbio.2021.11.004. .Epub 2021 Dec 21. PMID: 34949491.

Part II Clinical Applications

The Pediatric Knee Timothy Shao Ern Tan and Eu-Leong Harvey James Teo

Contents

Abstract

1      Introduction 

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2      Normal Development and Variants  2.1   Normal Irregular Ossification  2.2   Fibrous Cortical Defects and Avulsive Cortical Irregularity  2.3   Fabella  2.4   Posterior Distal Femoral and Proximal Tibial Metaphyseal Stripes on MRI  2.5   Discoid Meniscus  2.6   Developmental Angulation of the Knee 

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3      P  athological Conditions Specific to Pediatrics  3.1   Angular/Alignment Deformities  3.2   Skeletal Dysplasias  3.3   Trauma  3.4   Inflammatory  3.5   Bone and Soft Tissue Tumors 

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References 

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The knee joint in children is commonly imaged to evaluate for traumatic, inflammatory, developmental, or neoplastic conditions. The imaging appearances of the developing knee and normal variants should be known to avoid diagnostic errors. Traumatic conditions occur more commonly in older children and adolescents due to increased participation in sports while conditions such as infections or inflammatory arthritis are more often observed in younger children. Benign and malignant tumors occur less commonly but are still important causes of knee pathology and will be addressed in another chapter dedicated to tumors. This review describes normal development, variants, and pathological findings related to the knee in the pediatric age group.

1 Introduction

T. S. E. Tan · E.-L. H. J. Teo (*) Department of Diagnostic Imaging and Intervention, KK Women’s and Children’s Hospital, Singapore, Singapore e-mail: [email protected]

The knee is commonly imaged in children because many pathologies occur in this region. This chapter will focus on normal development, normal variants, and pathological conditions unique to the pediatric population. Tumors and infections will be dealt with in chapters dedicated to these conditions.

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_352, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 04 March 2023

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2 Normal Development and Variants 2.1 Normal Irregular Ossification Rapid growth of the epiphysis between the ages of 2 and 6  years may result in a fragmented appearance of the femoral condyles due to nonuniform ossification of the epiphysis (Fig.  1) (Orth 2013). These ossification centers may appear separate from the main secondary ossification center and should not be misinterpreted as osteochondritis dissecans. Epiphyseal irregularity may also normally occur in the tibial tuberosity which can be difficult to distinguish from patients with Osgood–Schlatter disease (Fig. 2).

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Clinical correlation is therefore needed as normal patients are asymptomatic while patients with Osgood–Schlatter disease have point tenderness at the tibial tuberosity. The patella ossifies from several different foci between 3 and 5 years of age and is usually complete by the second decade (Fig. 3). This ossification may be irregular resulting in the development of a bipartite patella which occurs when a secondary ossification center fails to fuse to the main part of the patella. This is usually asymptomatic but may cause anterior knee pain if chronic or direct trauma involves the synchondrosis between the ossification centers (Thapa et  al. 2012). In symptomatic cases, MRI may show abnormal signal within the synchondrosis, edema in the bipartite fragments, and overlying hyaline cartilage discontinuity (Tyler et al. 2010). Irregular ossification may also result in a dorsal defect of the patella. This is an area of fibrosis situated in the superolateral aspect of the articular surface of the patella. It is present in 0.3–1% of the population. It appears as a rounded lucency with sclerotic borders on plain radiographs, measuring between 4 and 26  mm in diameter. The cartilage over the defect is usually intact. The lesion is heterogeneous in signal intensity on T1-weighted MR images and equal to or greater than that of the cartilage on gradient echo images (Fig. 4). The lesion disappears spontaneously as the child grows older (Ho et al. 1991).

2.2 Fibrous Cortical Defects and Avulsive Cortical Irregularity

Fig. 1  Plain radiograph of a 4-year 7-month-old boy shows irregularity and fragmentation of the epiphysis of the distal femur. This is a normal developmental finding in a child of this age

Fibrous cortical defects are the most frequent benign lesions of the pediatric skeleton, occurring in older children and early adolescents, and often incidentally detected on radiography. These lesions are often situated in the posteromedial part of the distal femoral metaphysis. They appear as well-defined cortical-based lucencies with sclerotic margins on frontal radiographs and as cortical irregularities on the lateral projection.

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Fig. 2 (a) Lateral plain radiograph of the right knee in a 13-year-old boy shows irregularity of the tibial tuberosity (arrow). The patient had no symptoms at this site. (b) Lateral plain radiograph of the right knee in a 14-year-old rugby-playing boy who presented with chronic pain and tenderness at the site of the tibial tuberosity. There is irregularity and fragmentation of the tibial tuberosity

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Fig. 3 (a) Plain radiograph of a 6-year-old boy shows irregularity and fragmentation of the patella which is a normal developmental finding in a child of this age. (b) Frontal radiograph in a 7-year-old boy showing a bipartite

(arrow). Due to the presence of symptoms at this site, Osgood–Schlatter disease was diagnosed. (c) T2 fat-­ saturated sagittal image of the same knee shows the presence of bone edema in the irregular and fragmented tibial tuberosity confirming the diagnosis of Osgood–Schlatter disease

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patella (arrow). (c) Coronal T2 fat-saturated MR image in a 14-year-old boy showing edema in the bipartite fragments and at the synchondrosis

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Fig. 4  Axial T1 MR image in a 15-year-old boy shows the presence of a dorsal patella defect (open arrow). The articular cartilage overlying this lesion is intact (solid arrow)

These lesions migrate away from the growth plate as the child matures (Fig.  5). The MRI appearance of the lesion varies depending on the degree of maturation, ranging from high signal on T2 in the early stages to low signal in mature lesions. If the lesion measures more than 2 cm, it is called a nonossifying fibroma, and may be complicated by pathological fractures. Avulsive cortical irregularity or cortical desmoids are seen in children between the ages of 10 and 15 years old. They may be bilateral and occur on the posteromedial aspect of the distal femoral metaphysis due to repetitive activity involving the adductor magnus or medial head of the gastrocnemius muscles. They appear as areas of cortical irregularity and sclerosis on plain radiographs. On MRI, these lesions are low in signal intensity on T1 and high on T2-weighted images with a surrounding low signal rim (Fig. 6) (Pai and Strouse 2011).

2.3 Fabella The fabella (Latin for “little bean”) is a sesamoid bone in the tendon of the lateral gastrocnemius muscle (Fig. 7). Occasionally it may occur in the medial head of the gastrocnemius muscle. It is stabilized by the fabellofibular ligament. It may be oval or multipartite in appearance. Prevalence rates range between 3% and 87% of the population, being either unilateral or bilateral. It is usually asymptomatic but may cause pain and tenderness in some teenagers (Berthaume et  al. 2019).

2.4 Posterior Distal Femoral and Proximal Tibial Metaphyseal Stripes on MRI

Fig. 5  Plain radiograph of the knee in a 15-year-old boy shows the presence of a well-defined fibrous cortical defect situated in the lateral diametaphyseal region of the femur (arrow)

The periosteum comprises an inner cambium layer and an outer fibrous layer. The cambium layer is situated just adjacent to the bony cortex and plays a role in intramembranous bone growth. This layer is highly vascularized in the young infant. On MR imaging, it is seen as a smooth,

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Fig. 6 (a) Sagittal PD MR image shows the presence of a well-defined lesion in the posteromedial aspect of the distal femur with intermediate signal intensity (long arrow). It is at the point of attachment of the medial head of the gastrocnemius muscle (short arrow). (b) Sagittal T2 fat-­

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saturated MR image shows the lesion to have an intermediate to high signal intensity (long arrow). (c) Axial T2 fat-saturated MR image shows the lesion in the posteromedial aspect of the distal femur (arrow). The patella is subluxed

cortex from the hypointense fibrous periosteum (Fig. 8). It should not be mistaken for subperiosteal disease caused by tumor, fracture, or osteomyelitis which appears as uneven and irregular stripping of the fibrous periosteum away from the cortex. Surrounding edema and bone destruction also help to distinguish these entities from the normal metaphyseal stripe (Zember et al. 2015).

2.5 Discoid Meniscus

Fig. 7  Sagittal PD MR image shows the presence of a fabella (arrow)

circumferential area with high signal intensity on T2- and T1-weighted contrast-enhanced imaging, measuring 1–2 mm in width. It separates the

The normal meniscus is semilunar in shape. A discoid meniscus is disc-like and may have a spectrum of different shapes which may be flat, wedged, or lens-shaped. The meniscus may be completely discoid, with the meniscus covering the entire tibial plateau, incomplete or semilunar in appearance, or the Wrisberg type, characterized by the absence of the coronary and posterior meniscocapsular ligament. The discoid meniscus occurs in about 0.4–17% of patients. It occurs more commonly in the lateral than in the medial meniscus. The discoid meniscus is more prone to injury than the normal meniscus and this is likely to be due to both mechanical function and histological changes within the discoid meniscus (McKay et  al. 2013). The discoid meniscus is

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146 Fig. 8 (a) Sagittal PD MR image in a 12-year-old girl shows the presence of a thin metaphyseal stripe (arrow) just adjacent to the bony cortex. (b) Sagittal T2 fat-saturated MR image shows the presence of the thin metaphyseal stripe showing high signal intensity (arrow). This represents the cambium layer of the periosteum

Fig. 9 (a) Coronal T2 fat-saturated MR image shows a discoid meniscus extending medially toward the tibial spine (arrow). (b) Sagittal Proton Density MRI shows the presence of a lateral discoid meniscus with normal upper and lower meniscocapsular fascicles (arrows). These are missing in the Wrisberg variant of the discoid meniscus

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easily identified on coronal MR images by identifying meniscal tissue extending medially toward the tibial spine (Fig.  9). On sagittal images, the continuity between the anterior and posterior horns is seen in three or more contiguous slices 4–5  mm thick. If the slices used are

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thinner than 4–5 mm, this “bow-tie” sign can be adjusted accordingly. Most meniscal injuries in children less than 10  years of age occur within discoid menisci. A high signal band may be observed in many cases of discoid menisci possibly due to early degeneration.

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2.6 Developmental Angulation of the Knee In normal development, genu varus (bowing) is seen in neonates. This usually corrects within 6 months of walking or between 18 and 24 months of age. Thereafter, genu valgus occurs up to a maximum angulation of 10–15° at about 3 years of age. There is gradual reduction of valgus to the normal adult pattern by 6–7 years of age. If exaggerated varus angulation occurs during the second year of life, this is deemed to be physiological bowing (Cheema et al. 2003). This condition occurs more frequently in Afro-Caribbean children, early walkers, and in heavier children. The bowing occurs in a posteromedial direction and radiographs show mild enlargement and depression of the proximal tibial metaphyses posteromedially without fragmentation or breaking. The medial tibial cortices are thickened and the ankle joints are tilted with the medial side higher. The metaphyseal-diaphyseal angle is the angle between a parallel line drawn along the top of the metaphysis and a line drawn perpendicular to the long axis of the tibia, tangential to the cortex (Fig. 10). This angle is 5° ± 2.8° in physiological bowing. Physiological bowing should be differentiated from Blount’s disease which is an acquired growth disorder of the posteromedial aspect of the proximal tibial physis and epiphysis resulting in growth suppression. The cause of Blount’s disease is unknown but is thought to be the effects of weight on the growth plate, resulting in dyschondrosis of the medial physis of the proximal tibia. Two forms, based on age of onset, have been described: infantile and adolescent. The infantile form develops before 3 years of age. Physiological bowing develops initially, but instead of resolving, the bowing worsens. It is usually bilateral. Irreversible asymmetric medial proximal epiphysiodesis develops between 6 and 8  years of age and conservative treatment is no longer effective. If left untreated, the deformity worsens and premature osteoarthritis develops between 30 and 50 years of age. Spontaneous regression has occasionally been reported to occur. The adoles-

Fig. 10  A 2-year-old male with physiological bowing. The metaphyseal-diaphyseal angle is the angle CED. Line CE is perpendicular to line AB which is drawn parallel to the tibial shaft. Line ED is drawn along the top of the tibial metaphysis. In this case, the metaphyseal-diaphyseal angle is 9°, which is the borderline. The bowing resolved on follow-up

cent form is less common than the infantile form and occurs after 10  years of age. It is usually unilateral. In Blount’s disease, the metaphyseal-­ diaphyseal angle is 16° ± 4.3° (Fig. 11). Children with indeterminate angles between 8° and 11° should be followed up at least 6 monthly. Other radiographic findings of Blount’s disease include depression and irregularity, fragmentation of the tibial metaphysis posteromedially, and deficiency of the epiphysis medially. Lateral subluxation of the tibia and genu recurvatum may occur in late cases. MRI is helpful in cases of neglected or delayed forms after 4 years of age but before the development of radiographic epiphysiodesis. Thin-slice 3D gradient echo T1 sequences with

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Patients with uncontrolled obesity, progressive varus malalignment, and MRI evidence of impaired growth plate vascularization invariably progress to asymmetrical epiphysiodesis. In doubtful cases, MRI scanning should be repeated after 4–6 months. Once epiphysiodesis has developed, MRI scanning is part of the preoperative structural workup. However, the exact prognostic value of MRI findings in early-stage Blount’s disease is still under evaluation (Janoyer 2019).

3 Pathological Conditions Specific to Pediatrics 3.1 Angular/Alignment Deformities

Fig. 11  A 4-year-old girl with Blount’s disease shows bowing, depression, irregularity, fragmentation of the medial aspect of the tibial metaphyses (arrows), and deficiency of the epiphyses medially

fat suppression may show bars of epiphysiodesis. Measurement of the cartilaginous tibial slope is more accurate than the bony tibial slope measured on radiographs. Dynamic MRI with gadolinium provides information of the blood supply of the epiphysis. Conservative treatment may be performed in patients where the MRI shows a homogeneous vascular supply with no signal abnormalities at the growth plate. MRI also adds further information on the menisci, cruciate ligaments, and distal femoral epiphysis.

3.1.1 Genu Recurvatum Genu recurvatum is a condition where there is marked hyperextension of the knee joint with limited flexion. The condition can be unilateral or bilateral, congenital or acquired. The congenital form may occur as an isolated phenomenon or be part of a generalized hypermobility syndrome. The acquired form may be secondary to trauma, cerebrovascular accident, poliomyelitis, physeal arrest, Osgood–Schlatter disease, or prolonged casting. It may be associated with other orthopedic abnormalities such as developmental dysplasia of the hip or clubfoot. Fibrous contracture of the quadriceps muscle and tightening of the posterior cruciate ligament may be present (Austwick and Dandy 1983). Symptoms include pain, weakness, instability, leg-length discrepancy, and decreased range of motion. Symptomatic genu recurvatum occurs most commonly in adolescent girls and is associated with popliteal pain and an increased incidence of anterior cruciate ligament injury. Obese children are also more likely to be affected and suffer lower extremity pain. Three primary types of genu recurvatum have been described: (1) pure osseous deformity, in many cases due to damage to the tibial tubercle growth plate; (2) chronic hyperextension secondary to soft tissue laxity either from trauma or from gradual tissue stretching; and (3) a mixed-­type

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deformity resulting from a combination of osseous and soft tissue abnormalities. Genu recurvatum can also be idiopathic (Dean et al. 2020). Radiologically, the angle of extension is measured at the point of intersection between lines drawn along the shafts of the diaphyses of the femur and tibia. In genu recurvatum, there is hyperextension of the knee joint greater than 0° (Fig. 12). Prominence of the posterior aspect of the femoral condyles in the popliteal fossa may be present causing the reduction in flexion. Increased transverse skin folds over the anterior surface of the knee may also be noted. Treatment includes orthotic correction of biomechanical faults, improving knee proprioception, muscle control (especially quadriceps

Fig. 12 Newborn with hyperextension of the knee. Sagittal US image performed soon after birth shows hyperextension of the tibia with respect to the femur. P patella, F femoral epiphysis, T tibial epiphysis

Fig. 13 (a) Frontal radiograph in a newborn shows knee dislocation. (b) Sagittal US image in the same patient shows the tibia (T) anteriorly situated with respect to the femur (F). P patella

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strength) and gait, and maintaining good knee alignment during functional activities.

3.1.2 Congenital Dislocation of the Knee Congenital dislocation of the knee (CDK) is a rare disorder with an incidence of 1/100,000 live births, which is about 80–100 times less than developmental dysplasia of the hip. CDK is often associated with other musculoskeletal anomalies with DDH being the most common deformity. It is more common in females and can be unilateral or bilateral. It may be idiopathic or occur as part of a syndrome, e.g., Larsen’s syndrome, arthrogryposis multiplex congenita, and myelomeningocele. The exact etiology of CDK is unknown but may be due to extrinsic factors such as abnormal intrauterine pressure leading to intrauterine malposition or intrinsic factors such as genetic abnormalities and neuromuscular imbalances. A few different classifications of CDK have been proposed with the classification by Leveuf and Pais being the most well-known (Leveuf and Pais 1946). This classification separates the deformity into three groups: Grade 1 is the most common type and not a true dislocation and accepted as congenital hyperextension. Nearly 15–20° of hyperextension can be detected and passive range of flexion is maximum 90°. In Grade 2, congenital subluxation with joint incongruency is observed (Fig. 13). Passive flexion of the knee is impossible and 25–40° of hyperextension can be

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achieved. In Grade 3, there is dislocation with no contact noted between the joint surfaces of tibia and femur. This classification is based only on radiological views without an assessment of clinical manifestations. Ultrasound may diagnose the condition antenatally. This modality has also been shown to be able to reliably differentiate patients into one of the three groups of classification of Leveuf and Pais (Rössig et al. 1998). Soft tissue structures within the knee joint such as the anterior and posterior cruciate ligaments can be demonstrated in some cases. Obliteration of the suprapatellar recess and fibrotic changes in the quadriceps muscle may be seen indicating the necessity of surgical intervention. Management starts with closed reduction and cast or splinting. Surgical reduction is indicated when conservative measures fail or when the child is referred after the age of 1 year.

3.2 Skeletal Dysplasias Skeletal dysplasias are a large group of congenital disorders caused by genetic mutations resulting in abnormalities of the bone, cartilage growth, or texture. A detailed discussion of these conditions is beyond the scope of this chapter. This section will deal with the more commonly encountered skeletal dysplasias and their effects on the knee joint in children. Achondroplasia is the most commonly encountered non-lethal skeletal dysplasia. It occurs due to a mutation in the fibroblast growth factor receptor 3 gene which is the gene responsible for making the protein fibroblast growth factor receptor 3. This protein is important for the production of collagen and other components in tissue and bones. Absence of this protein results in the cartilage not being able to fully develop into bone, causing the individual to become shorter in height. Increased ligamentous abnormality and rhizomelic shortening of the bones are noted. The affected long bones are short and thick with metaphyseal cupping and flaring. Irregularity of the growth plates is also present, but the epiphyses appear normal. The epiphyses are situated closer to the metaphyses giving the

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appearance of an increase in the depth of the articular cartilage space. A ball-and-socket appearance is seen with the metaphyses having a V-shaped configuration and the epiphyses situated within the limbs of the V (Panda et al. 2014). The fibula, however, shows relative overgrowth with the fibula heads situated at the tibial plateau (Fig.  14). Genu varum occurs in 40% of all achondroplastic children and progresses rapidly between 3 and 4 years of age and again between 6 and 7 years. The final progression takes place during the growth spurt. Genu recurvatum and internal tibial torsion have also been reported. Internal derangements of the knee found in symptomatic children include discoid menisci frequently associated with tears, patella baja, increased ACL-Blumensaat line, PCL angles, and deeper A-shaped femoral notches that extended more anteriorly than in the normal population (Akyol et al. 2015). Diastrophic dysplasia (DD) results from mutations in the diastrophic dysplasia sulfate transporter (DTDST) or SLC26A2 gene which encodes a transmembrane protein that transports

Fig. 14 A 4-year-old boy with achondroplasia. Radiograph of both knees shows the epiphyses situated close to the metaphyses giving the appearance of an increase in the depth of the articular cartilage space. A ball-and-socket appearance is seen with the metaphyses having a V-shaped configuration and the epiphyses situated within the limbs of the V. The fibula heads are situated at the level of the tibial plateau

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sulfate into chondrocytes to maintain adequate sulfation of proteoglycans. Thus, it is essential for the normal development of cartilage into bone. Patients have an unusual clubfoot, scoliosis, long-bone shortening, and hitchhiker thumbs. In the knees, excessive valgus deformity, deformation of the epiphysis before 6  years of age, delayed appearance of the bony epiphysis of the patella with fragmentation patella infera, and hypoplasia of the fibula have been observed (Peltonen et al. 1999). Early degenerative changes can be seen in the articular cartilage from 6 years of age with fragmentation of the subchondral bone, cartilaginous intrusions from the growth plate into the metaphysis, abnormal menisci, and thinned or rudimentary ACL and PCL (Peltonen et al. 2003). Multiple epiphyseal dysplasia (MED) is a genetically heterogeneous entity caused by mutations in multiple genes. A feature of MED is the bilateral symmetrical involvement of the epiphyses of multiple bones in the body (Panda et  al. 2014). In the knee, irregularity, segmentation of

Fig. 15  Frontal (a) and lateral (b) radiographs in an 8-year-old boy with multiple epiphyseal dysplasia show the epiphyses of the femur, tibia, and fibula to be small, irregularly shaped with areas of fragmentation. A double patella is shown on the lateral view (arrows)

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the epiphyses, widening of the joint space, and genu valgum deformity are the dominant findings before epiphyseal closure (Fig. 15). After epiphyseal closure, the most characteristic findings are shallow femoral trochlear grooves, early onset osteoarthritic changes, and genu valgum (Miura et al. 2000). Double-layered patella is an important diagnostic clue for MED.  It may, however, also be seen in pseudoachondroplasia. This is likely due to the fact that pseudoachondroplasia and recessive-type MED are caused by mutations affecting the same COMP gene (Panda et  al. 2014).

3.3 Trauma The knee is commonly injured due to increasing participation in sports in children. The patterns of injury are similar to the adult population in older adolescents but differ in skeletally immature patients especially when the physis has yet to heal (Strouse 2010).

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3.3.1 Bony Injuries 3.3.1.1  Physeal Injuries Physeal fractures occur in children because the physis is an area of relative weakness prior to fusion (Leschied and Udager 2017). The undulating shape of the distal femoral physis often leads to complex shearing forces during the fracture, which may lead to a higher risk of bony bar formation and premature physeal fusion (Bailey et al. 2020). The distal femur also contributes up to 35% of longitudinal growth and the high rate of growth in this bone may result in greater deformity and limb-length discrepancy. The Salter– Harris classification is used to classify physeal fractures about the knee, and the Salter–Harris type 2 fracture of the distal femur involving the lateral distal femoral metaphysis with fracture extension through the medial physis is the commonest fracture noted. Salter–Harris fractures are usually diagnosed on plain radiographs but occasionally MRI or CT is used to confirm the presence of a physeal injury. Findings on MRI include physeal widening, edema, fracture lines, periosteal elevation, and cortical disruption (Fig.  16). Premature physeal fusion is a complication of physeal fractures and may cause growth deformity in the affected bone (Fig. 17). Chronic physeal stress injury may result in microvasculature blood supply disruption to the physis. Radiographs show broad physeal widening in the femur or tibia. MRI evaluation demonstrates extension of physeal signal into the metaphysis that is isointense to normal physeal cartilage (Laor et al. 2006). Failure of the patient to rest may result in growth impairment or malalignment. Focal periphyseal edema (FOPE) is a unique pattern of bone marrow edema occurring in adolescents centered around the central physis of the distal femur, proximal tibia, or fibula. This zone of periphyseal edema may represent hemorrhage or increased vascularity at a site of early normal physeal closure in athletic adolescents (Zbojniewicz & Laor 2011). This fixed point of central early physeal closure is thought to result in abnormal mechanics in the surrounding metaphysis and epiphysis, resulting in microtrauma giving rise to the edema-like signal on

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either side of the growth plate (Fig.  18). This FOPE zone should resolve with closure of the physis as the adolescent matures. 3.3.1.2  Tibial Spine Fracture Tibial spine fractures (TSF) are avulsion fractures that typically occur in children before physeal fusion. It is relatively uncommon and accounts for 2% of all knee injuries in children (Shin et  al. 2015). The mechanism of injury is forced knee flexion with simultaneous tibial external rotation or hyperextension of the knee occurring during athletic activity or trauma. The anterior cruciate ligament (ACL) attaches to the tibial spine and is relatively stronger than the tibial eminence due to incomplete ossification. The same forces that cause a rupture of the ACL in older adolescents and adults will cause a TSF

Fig. 16  Coronal T2-weighted fat-saturated MR image in a 15-year-old boy shows a fracture line involving the medial aspect of the physis extending into the epiphysis (arrow) involving the joint. This is a Salter–Harris type 3 fracture

The Pediatric Knee Fig. 17 (a) Sagittal T1 fat-saturated MR image in a 10-year-old boy with previous physeal fracture shows premature fusion and bony bridging across the physis (arrow) resulting in anterior angular deformation of the epiphysis in relation to the femoral shaft. (b) Coronal T1 fat-­saturated MR image in the same patient shows the same bone bridge on the coronal view (thin arrow). The other parts of the distal femoral physis are also irregular (open arrow) due to the injury. The tibia was also injured previously and is similarly irregular and fused in some areas (solid arrow)

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Fig. 18  Sagittal T2-weighted fat-saturated MR image in a 13-year-old girl shows high signal intensity on either side of the distal femoral physis suggestive of FOPE

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in children (Bailey et al. 2020). However, injury to the ACL can still occasionally occur in conjunction with a tibial spine fracture. Meyers and McKeever classified TSFs using the lateral view of the knee on plain radiographs. The classification system they proposed was based on the degree of displacement (Shin et al. 2015). Type 1 fractures are nondisplaced. Type 2 fractures are partially displaced but retain an intact posterior hinge. Type 3 fractures are completely displaced. Zaricznyj described comminuted TSFs and classified these as type 4 fractures (Ellis et al. 2021). This classification helps in guiding the treatment for these patients. Operative treatment is usually recommended for type 3 and type 4 fractures, which is often performed arthroscopically. Arthroscopic fixation affords significant advantages in the postoperative rehabilitation. Tibial spine fractures can be quite subtle radiographically and initial diagnosis may be on MRI (Strouse 2010). Cross-sectional imaging with CT and MRI is also useful in planning surgery and in detecting concurrent bony and soft tissue injury which is crucial in guiding treatment (Fig. 19). Meniscal tears, meniscal entrapment at the fracture site, non-ACL ligamentous injury, and bony contusions are frequently noted. The

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Fig. 19  A 7-year-old boy with a fracture of the tibial spine. Frontal (a) and lateral (b) radiographs of the knee show a fracture of the tibial spine (arrows). A fracture of the inferior pole of the patella is also noted on the lateral Fig. 20 (a) Lateral plain radiograph in a child shows the presence of a patella sleeve fracture (arrow). Patella alta is also noted. (b) Sagittal T2 fat-saturated MRI shows the fracture fragment on the inferior aspect of the patella (arrow). A small joint effusion is noted

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rate of these complications increases with the grade of fracture as per the Meyers and McKeever classification (Strouse 2010). 3.3.1.3  Patella Sleeve Fracture A patella sleeve fracture is a unique fracture occurring in children. It is an avulsion fracture occurring at the inferior patella resulting in an osteochondral fragment remaining attached to the patella tendon. This fracture results from a powerful contraction of the quadriceps muscle with the knee in a flexed position. Plain radiographs and

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view. (c) Sagittal proton density image shows the tibial spine fracture to be almost completely displaced from the main tibia (arrow). The ACL is also thickened and increased in signal intensity consistent with a sprain

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MRI demonstrate joint effusion, displaced inferior patellar pole fracture, and patella alta (Fig.  20). MRI is more sensitive and is useful in assessing the extent of soft tissue injury. Radiographically occult nondisplaced osteochondral fractures may be shown (Pai and Strouse 2011). 3.3.1.4 Sinding-Larsen–Johansson Lesion and Osgood–Schlatter Disease Chronic avulsive stress injury to the inferior pole of the patella is known as Sinding-Larsen–

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Johansson syndrome. Plain radiographs show irregularity of the inferior pole of the patella (Fig. 21). This can be difficult to distinguish from normal irregularity seen in this area. The presence or absence of tenderness in this region should point to the correct diagnosis. MRI in stress injury may show focal edema in this area (Kan et al. 2015). Osgood–Schlatter disease results from chronic stress at the attachment of the distal patella tendon on the tibial tubercle. The proximal tibial epiphysis beneath the tubercle is an area of relative weakness, and repeated stress due to patella tendon contraction may result in inflammation and avulsion. Imaging shows thickening or calcification of the distal patellar tendon, fragmentation of the tibial tubercle, and overlying soft tissue thickening. Fragmentation of the tibial tubercle in the absence of soft tissue changes may be a normal variation of apophyseal ossification (Dupuis et al. 2009).

Fig. 21  Lateral plain radiograph in a child shows the presence of a bony fragment on the inferior end of the patella (arrow). The patient had chronic pain and point tenderness in this area and a diagnosis of Sinding-Larsen– Johansson was made

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3.3.1.5  Nonaccidental Injury Nonaccidental injury (NAI) is an important cause of child mortality and morbidity, and detailed discussion of this condition is beyond the scope of this book. NAI is associated with major physical and mental health issues that can extend into adulthood. Radiologists play a key role in the recognition of NAI.  Any discrepancy between the clinical history and the injury and/or the detection of unsuspected fractures should raise the suspicion of NAI. The classic metaphyseal lesion (CML) is a metaphyseal fracture that is considered pathognomonic of NAI (Fig.  22) (Marine and Forbes-Amrhein 2021). The mechanism is that of a shearing force across the metaphysis resulting in fractures in the immature bone. “Corner” and “bucket-handle” are terms used to describe the appearance of CML in tangential views. The distal femur and proximal tibia are the most common sites of the CML and should be

Fig. 22  16-Day-old neonate who had suffered child abuse. Lateral view of the left knee as part of a skeletal survey for suspected NAI shows a corner fracture (arrow). This fracture is considered to be pathognomonic of NAI

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imaged in frontal and lateral views. The periosteum may be sheared off the bone resulting in subperiosteal hemorrhage and the subsequence development of subperiosteal new bone formation. When a fracture is identified, the radiologist should search for other occult fractures by performing a skeletal survey. The radiologist should attempt to date the fracture(s) because it offers a time frame for the likely occurrence of the event. Subperiosteal new bone formation may be seen from 4 days after the injury, peaking between 10 and 14 days. Absorption of the fracture line and soft callus formation both peak between 14 and 21  days, with hard callus formation peaking between 21 and 42 days. The final stage of bony remodeling peaks 1 year after injury. It is important to remember that before a diagnosis of NAI is made, differential diagnoses, such as, but not limited to, osteogenesis imperfecta, skeletal dysplasias, rickets, and Ehler–Danlos/hypermobility syndrome, should first be excluded as these conditions may be misinterpreted as being caused by NAI. Communication with the referring clinician is therefore of paramount importance.

3.3.2 Soft Tissue Injuries 3.3.2.1  Osteochondritis Dissecans Osteochondritis dissecans (OCD) is a common cause of knee pain in children. Juvenile OCD

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Fig. 23  Frontal and lateral radiographs (a) and (b), respectively, in an 11-year-old girl shows a focal area of irregularity and fragmentation with areas of sclerosis and lucency in the medial femoral condyle (arrow) consistent with OCD. (c) Sagittal proton density MR image shows the area of fragmentation to have variable signal intensity

occurs in children before closure of the physeal plate while adult OCD occurs after physeal fusion. It is a condition that affects subchondral bone formation, which may result in an unstable subchondral fragment, disruption of adjacent articular cartilage, and possible separation of the fragment. It is believed that an unknown insult causes a disturbance of a small area of the epiphyseal growth plate resulting in a localized delay or cessation of ossification. The rest of the epiphysis ossifies normally while the localized area remains cartilaginous. This creates the appearance of a radiolucent crater. This area may develop laminar calcifications or may ossify partially or completely. This lesion then separates from the main bone. This separation begins at the deep bony part of the fragment and extends to the overlying articular cartilage with eventual ­separation of the fragment. Osteonecrosis is absent (Gorbachova et  al. 2018). Trauma is believed to play a crucial role as OCD is often seen in active children, particularly high-level athletes. The most common location of the OCD in the knee is in the intercondylar aspect of the medial femoral condyle although it can occur in any location (Fig.  23). MRI is the study of choice in the evaluation of OCD.  MRI can determine instability of the osteochondral frag-

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(arrow). (d) T2 fat-saturated sagittal MR image shows bony edema surrounding the fragmented bone (vertical arrow) and a small cyst (horizontal arrow). The signal intensity of the edema is less than that of joint fluid and the diameter of the cyst is less than 5 mm in size and these findings do not indicate that the fragment is unstable

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ment, which is an important factor in prognosis. It is important to note that the criteria for instability between juvenile and adult OCD differ. A high T2 signal intensity rim or cysts surrounding an adult OCD lesion are unequivocal signs of instability. A high T2 signal intensity rim surrounding a juvenile OCD lesion indicates instability only if it has the same signal intensity as adjacent joint fluid, is surrounded by a second outer rim of low T2 signal intensity representing organized fibrous tissue or sclerotic bone at the interface, or is accompanied by multiple breaks in the subchondral bone plate on T2-weighted MR images. Cysts surrounding a juvenile OCD lesion indicate instability only if they are multiple greater than 5 mm in size (Kijowski et al. 2008). Surgical treatment is often the treatment of choice in symptomatic patients with unstable lesions. The choice of technique depends on whether the fragment is salvageable or not (Carey and Grimm 2015). An unstable fragment may be unsalvageable when it consists of cartilage only (no bone on the deep surface), is composed of multiple pieces, or contains damaged or absent articular cartilage (Shea et  al. 2016). MRI is a valuable diagnostic tool that provides critical information about the composition, stability, and integrity of the OCD fragment. 3.3.2.2  Impingement of Superolateral Hoffa’s Fat Pad Patellofemoral misalignment, as a result of alterations affecting the anterior extensor mechanism including patella alta, patellofemoral dysplasia, and lateralization of the tibial tuberosity (Orth 2013), may cause chronic impingement on the superolateral aspect of Hoffa’s fat pad between the patellar tendon or lateral patellar facet and the most lateral aspect of the trochlea. This condition is predominantly seen in adolescent girls aged 15–16 years who typically present with chronic anterior knee pain. Symptoms are worse on knee hyperextension, when the patella moves superolaterally. Patella alta is usually seen along with increased T2-weighted signal in Hoffa’s fat pad on MRI (Fig. 24).

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Fig. 24  12-Year-old girl with anterior knee pain. T2 fat-­ saturated MR image shows increased signal intensity in Hoffa’s fat pad. This finding is consistent with the diagnosis of impingement of superolateral Hoffa’s fat pad

3.3.2.3  Cruciate Ligament Injuries Anterior cruciate ligament (ACL) injuries are common in adolescence, in part due to greater participation in sports and athletics. As adolescents approach skeletal maturity, ACL-related injuries occur more commonly in boys and are associated with either tibial spine avulsion fractures or partial-thickness tears. This is attributed to the relatively weak tibial eminence at the osteochondral junction that is less resistant to pivot forces compared to the strong ACL.  A similar osteochondral fracture pattern may occur with the posterior cruciate ligament (PCL) complex and posterolateral corner structures including the lateral collateral ligament and popliteus tendon (Prince et al. 2005). After skeletal maturity, ACL injuries occur more commonly in females and are associated with full-thickness tears. This is thought to be contributed by the increased valgus angle of the

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female knee compared with males, size and shape of the intercondylar notch, increased sloping of the lateral tibial plateau, ligament thickness, hormonal differences, and differences in training methods (Kim et al. 2008; Leschied and Udager 2017). Treatment of ACL injuries is mainly surgical, in order to prevent recurrent knee instability and meniscal/chondral injuries. Basic knowledge of ACL reconstruction techniques is important for the radiologist evaluating pediatric knee MRI, which mainly revolves around disturbance of growth remaining for the individual, usually determined by Tanner staging. These patients would undergo transphyseal, transepiphyseal, or physeal-sparing surgery. For example, a tibial graft that crosses the physis may be utilized and the interference screw does not violate the physis, hence limiting physeal damage. The iliotibial band may also be harvested proximally and subsequently pulled “over-the-top” around the physis and through the intercondylar notch posteriorly (Leschied and Udager 2017). PCL injuries in the pediatric population are rarer than ACL injuries and are less commonly investigated with imaging. These often respond well to nonoperative management, especially with partial-thickness tears. Surgical intervention is largely reserved for patients with multi-­ ligamentous injuries. 3.3.2.4  Patella Dislocation Lateral patella instability and patella dislocation (PD) is a frequent injury in the pediatric and adolescent age groups occurring between 23.2 and 43 per 100,000 (Grimm et al. 2020). Females are at a higher risk than males. Noncontact injuries account for a majority of the cases. The risk of recurrence is high in patients aged 18 years and under at around 34–38%. Major risk factors include trochlear dysplasia (TD), patella alta, increased tuberosity to the trochlear groove (TT-TG) distance, and abnormal lateral tilt of the patella. Additional risk factors include insufficient medial patellofemoral ligament (MPFL), torsional limb malalignment, and genu valgum.

Trochlear Dysplasia

TD is a condition where the TG is shallow, flat, or concave. This predisposes to PD because after about 30° of flexion, the TG is the main factor for stability of the patella. In individuals with TD, this results in loss of lateral patella tracking ­predisposing to PD (Dejour et  al. 1994). While plain radiographs can be used to diagnose TD, studies have shown that plain radiographs may be inaccurate in the evaluation of TD due to rotation on lateral radiographs or underestimation of the sulcus angle on axial radiographs (Koëter et al. 2006). MRI has been shown to be highly accurate and reproducible for the evaluation of TD (Diederichs et al. 2010). Trochlear dysplasia can be evaluated at MR imaging by determining lateral trochlear inclination, trochlear facet asymmetry, or trochlear depth. Lateral Trochlear Inclination

This is determined by measuring the inclination angle which is the angle between a line drawn along the subchondral bone of the lateral trochlear facet and a tangential line connecting the posterior aspect of the femoral condyles. The superior-most axial image showing the trochlear cartilage is used for this measurement. An inclination angle less than 11° has a sensitivity of 93% and specificity of 87% (Fig. 25). Trochlear Facet Asymmetry

Trochlear facet asymmetry is the ratio of the length of the medial trochlear facet to the length of the lateral trochlear facet measured 3 cm above the tibiofemoral joint cleft expressed as a percentage (Fig. 26). TD is present if the trochlear facet ratio is less than 40% (sensitivity of 100%, specificity of 96%). Trochlear Depth

Trochlear depth is assessed at the same level as trochlear facet asymmetry is determined. It is the distance of the trochlear groove from the trochlear floor to a tangential line connecting the most anterior points of the medial and lateral facets of the femur (Fig. 27). Trochlear dysplasia is present if the trochlear depth is 3 mm or less (sensitivity of 100%, specificity of 96%).

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Fig. 25  Axial PD MR image illustrating the inclination angle. This is the angle between a line drawn along the subchondral bone of the lateral trochlear facet (AC) and a tangential line connecting the posterior aspect of the femoral condyles (OD). Line AB is parallel to line OD. The angle CAB is 10°, in this case indicative of trochlear dysplasia. An inclination angle less than 11° has a sensitivity of 93% and specificity of 87% for trochlear dysplasia

Fig. 26  Axial PD MR image showing the lengths of the lateral and medial trochlear facets (17.9 and 13.1  mm, respectively). Trochlear dysplasia is present if the ratio of the length of the medial trochlear facet to the length of the lateral trochlear facet is less than 40%. The ratio in this case is 73%, which is within normal limits, and trochlear dysplasia is absent

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Fig. 27  Axial PD MR image showing the trochlear depth which is the distance of the trochlear floor to a tangential line connecting the most anterior points of the medial and lateral facets of the femur. The measurement in this image is 1.9 mm, indicating the presence of trochlear dysplasia. Note that the patella is laterally subluxed

Patella Alta

Patella alta (PA) is a condition where the patella is too high above the trochlear fossa. This is due to a patella tendon that is too long. This results in patellofemoral misalignment because the degree of flexion needs to be higher than in a normal knee in order for the patella to be situated within the trochlear. This reduced contact between the patella and the trochlear leads to instability and a higher chance of dislocation. The Insall–Salvati index is the most common index used to determine patellar height. The index is obtained on either plain radiographs or MRI by calculating the ratio of the length of the patellar tendon to the longest superoinferior diameter of the patella (Fig. 28). The normal ratio reported is 1.1 (standard deviation, 0.1). Patella alta is defined as a patellar height ratio of more than 1.3, which is the normal ratio plus two standard deviations (sensitivity of 78%, specificity of 68%). Distance from Tibial Tubercle to Trochlear Groove

In a normal joint, the tibial tubercle is directly inferior to the femoral sulcus so that during knee bending, the force on the patella is directly

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Fig. 28  Sagittal PD MR image showing the length of the patellar tendon to the longest superoinferior diameter of the patella. The ratio of Insall–Salvati ratio in this case is 1.26, which is borderline for patella alta. Patella alta is present if the ratio is more than 1.3

downwards. If the tibial tubercle is laterally situated, the patella may be pulled laterally resulting in dislocation during knee bending. The normal tibial tubercle to trochlear groove distance is less than 15  mm. A value of more than 20  mm is abnormal and values between 15 and 20 mm are borderline (Fig. 29).

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substance, or femoral insertion. Full-thickness tears can be recognized by the tendon appearing discontinuous and retracted or with a wavy appearance surrounded by soft tissue edema. Partial tears are characterized by an irregular appearance and partial discontinuity of the ligament with surrounding tendinous or ­peritendinous fluid (Diederichs et al. 2010). Patella subluxation and large effusions can often be seen. Contusion of the medial aspect of the patella and the femoral condyle resulting in bone edema in these areas are typically seen. Chondral or osteochondral lesions of the medial patella and avulsion injuries in this same region can be detected. Central cartilage defects of the posterior patella may also be recognized. Defects in the trochlear cartilage and a fracture of the lateral condyle are rare but should not be overlooked. In severe cases, edema and hemorrhage into the vastus medialis adjacent to the MPFL can be seen on coronal T2-weighted images. Loose intra-­articular bodies may be present. Meniscal, collateral ligament, and occasionally cruciate injuries may occur. Management may be conservative or surgical. The most common surgical procedures are MPFL reconstruction, medial capsular plication, and lateral release. Reconstruction of a dysplastic trochlea (trochleoplasty) and tibial tuberosity transfer in patients with a TT-TG distance may also be performed.

3.4 Inflammatory

3.4.1 Juvenile Idiopathic Arthritis Juvenile idiopathic arthritis (JIA) encompasses a heterogeneous group of chronic arthritides which share the following features: (a) disease onset MRI Findings After Patella Dislocation before 16 years of age, (b) a minimum duration Deformity or edema of the inferomedial patella of 6 weeks, and (c) exclusion of other causes. JIA and the lateral femoral condyle, in conjunction is an autoimmune inflammatory disease of with medial patellar femoral ligament (MPFL) unknown etiology, characterized by synovial disruption and patellar lateralization, are typical inflammation, and is the most common rheumatic findings of recent PD.  These findings are well complaint in children. The knee is the most comseen on MRI (Fig.  30). MPFL disruption can mon site of disease, followed by the ankle, wrist, occur at the insertion on the patella, its mid-­ and hand. It often presents with local pain, heat,

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Fig. 29 (a) The transverse distance between the tibial tubercle and the trochlear groove can be obtained by first drawing a line bisecting the trochlear groove (TG) sulcus. This line is perpendicular to a line along the posterior femoral condyles. (b) Similarly, another line bisecting the

Fig. 30  A 12-year-old girl with acute patellar dislocation. Axial T2 fat-saturated MR image shows a subchondral fracture of the medial aspect of the patella (horizontal arrow). There is rupture of the MPFL (vertical arrow). The patella was manually reduced at the time of injury

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tibial tuberosity (TT) is then drawn. The distance between TT and TG is the transverse distance between these two lines. In this case, the distance is 3.7 mm, which is within normal limits (normal 4 mm in thickness and >14 mm in length (Lecouvet et al. 1998). When conservative management is appropriate, this consists of non-weight bearing or protected weight bearing with a knee brace, nonsteroidal anti-inflammatory drugs, analgesics, and bisphosphonates (Sibilska et al. 2020). Surgical treatment may be indicated due to large size of the lesion, worrying MRI features, or failure of conservative management. Surgical options include arthroscopic debridement, core decompression, osteochondral autograft, high tibial osteotomy, or eventually unicompartmental or total knee arthroplasty (Sibilska et al. 2020). An evolving treatment option for patients with SCIF is subchondroplasty, which involves the subchondral injection of a calcium phosphate bone substitute (Gorbachova et al. 2018).

References Ahlbäck S, Bauer GC, Bohne WH (1968) Spontaneous osteonecrosis of the knee. Arthritis Rheum 11(6):705–733 Akamatsu Y et  al (2012) Low bone mineral density is associated with the onset of spontaneous osteonecrosis of the knee. Acta Orthop 83(3):249–255 Anderson MW, Greenspan A (1996) Stress fractures. Radiology 199(1):1–12 Arendt E, Agel J, Heikes C, Griffiths H (2003) Stress injuries to bone in college athletes: a retrospective review of experience at a single institution. Am J Sports Med 31(6):959–968 Beck BR et al (2012) Tibial stress injury: relationship of radiographic, nuclear medicine bone scanning, MR imaging, and CT Severity grades to clinical severity and time to healing. Radiology 263(3):811–818 Berger FH, de Jonge MC, Maas M (2007) Stress fractures in the lower extremity. The importance of increasing awareness amongst radiologists. Eur J Radiol 62(1):16–26 Boden BP, Osbahr DC (2000) High-risk stress fractures: evaluation and treatment. J Am Acad Orthop Surg 8:344–353 Davies AM, Evans N, Grimer RJ (1988) Fatigue fractures of the proximal tibia simulating malignancy. Br J Radiol 61:903–908 Fayad LM et  al (2005) Distinguishing stress fractures from pathologic fractures: a multimodality approach. Skeletal Radiol 34(5):245–259 Fredericson M, Bergman AG, Hoffman KL, Dillingham MS (1995) Tibial stress reaction in runners. Correlation of clinical symptoms and scintigraphy with a new

213 magnetic resonance imaging grading system. Am J Sports Med 23(4):472–481 Gaeta M et al (2005) CT and MR imaging findings in athletes with early tibial stress injuries: comparison with bone scintigraphy findings and emphasis on cortical abnormalities. Radiology 235(2):553–561 Gmachowska AM et al (2018) Tibial stress injuries – location, severity and classification in magnetic resonance imaging examination. Pol J Radiol 83:e471–e481 Gorbachova T et al (2018) Osteochondral lesions of the knee: differentiating the most common entities at MRI. Radiographics 38(5):1478–1495 Harrast MA, Colonno D (2010) Stress fractures in runners. Clin Sports Med 29:399–416 Hussain ZB et  al (2019) The role of meniscal tears in spontaneous osteonecrosis of the knee: a systematic review of suspected etiology and a call to revisit nomenclature. Am J Sports Med 47(2):501–507 Karim AR et  al (2015) Osteonecrosis of the knee. Ann Transl Med 3(1):6 Kijowski R et al (2012) Validation of MRI classification system for tibial stress injuries. AJR Am J Roentgenol 198(4):878–884 Koshino T (1982) The treatment of spontaneous osteonecrosis of the knee by high tibial osteotomy with and without bone-grafting or drilling of the lesion. J Bone Joint Surg Am 64(1):47–58 Lassus J et  al (2002) Bone stress injuries of the lower extremity: a review. Acta Orthop Scand 73(3):359–368 Lecouvet FE et al (1998) Early irreversible osteonecrosis versus transient lesions of the femoral condyles: prognostic value of subchondral bone and marrow changes on MR imaging. AJR Am J Roentgenol 170(1):71–77 Lecouvet FE, Malghem J, Maldague BE, Berg BCV (2005) MR imaging of epiphyseal lesions of the knee: current concepts, challenges, and controversies. Radiol Clin North Am 43(4):655–672 Lotke PA, Abend JA, Ecker ML (1982) The treatment of osteonecrosis of the medial femoral condyle. Clin Orthop Relat Res 171:109–116 Manco LG, Schneider R, Pavlov H (1983) Insufficiency fractures of the tibial plateau. AJR Am J Roentgenol 140(6):1211–1215 Marshall RA et al (2018) Imaging features and management of stress, atypical, and pathologic fractures. Radiographics 38:2173–2192 Mont MA et al (2008) Bone scanning of limited value for diagnosis of symptomatic oligofocal and multifocal osteonecrosis. J Rheumatol 35(8):1629–1634 Mulligan ME (1995) The “gray cortex”: an early sign of stress fracture. Skeletal Radiol 24:201–203 Nativ A et al (2013) Correlation of MRI grading of bone stress injuries with clinical risk factors and return to play: a 5-year prospective study in collegiate track and field athletes. Am J Sports Med 41(8):1930–1941 Niva MH et  al (2006) Bone stress injuries causing exercise induced knee pain. Am J Sports Med 34(1):78–83 Pathria MN, Chung CB, Resnick DL (2016) Acute and stress related injuries of bone and cartilage: pertinent

214 anatomy, basic biomechanics, and imaging perspective. Radiology 280(1):21–38 Pentecost RL, Murray RA, Brindley HH (1964) Fatigue, insufficiency, and pathologic fractures. JAMA 187(13):1001–1004 Pollack MS et al (1987) Magnetic resonance imaging in the evaluation of suspected osteonecrosis of the knee. Skeletal Radiol 16(2):121–127 Prasad N et al (2006) Insufficiency fracture of the tibial plateau: an often missed diagnosis. Acta Orthop Belg 72(5):587–591 Reddy AS, Frederick RW (1998) Evaluation of the intraosseous and extraosseous blood supply to the distal femoral condyles. Am J Sports Med 26(3):415–419 Roemer FW et  al (2009) MRI-detected subchondral bone marrow signal alterations of the knee joint: terminology, imaging appearance, relevance and radiological differential diagnosis. Osteoarthr Cartil 17(9):1115–1131 Sibilska A, Góralczyk A, Hermanowicz K, Malinowski K (2020) Spontaneous osteonecrosis of the knee: what do we know so far? A literature review. Int Orthop 44(6):1063–1069 Symeonides PP (1980) High stress fractures of the fibula. J Bone Joint Surg Br 62-B(2):192–193 Takeda M et  al (2008) Spontaneous osteonecrosis of the knee: histopathological differences between

J. Murphy et al. early and progressive cases. J Bone Joint Surg Br 90(3):324–329 Vidoni A et al (2018) Metaphyseal burst sign: a secondary sign on MRI of subchondral insufficiency fracture of the knee. J Med Imaging Radiat Oncol 62(6):764–768 Wall J, Feller JF (2006) Imaging of stress fractures in runners. Clin Sports Med 25(4):781–802 Warden SJ, Burr DB, Brukner PD (2006) Stress fractures: pathophysiology, epidemiology, and risk factors. Curr Osteoporos Rep 4(3):103–109 Wright AA et  al (2015) Risk factors associated with lower extremity stress fractures in runners: a systematic review with meta-analysis. Br J Sports Med 49(23):1517–1523 Wright AA et  al (2016) Diagnostic accuracy of various imaging modalities for suspected lower extremity stress fractures: a systematic review with evidence-­ based recommendations for clinical practice. Am J Sports Med 44(1):255–263 Yamamoto T, Bullough PG (2000) Spontaneous osteonecrosis of the knee: the result of subchondral insufficiency fracture. J Bone Joint Surg Am 82(6):858 Yates PJ et  al (2007) Early MRI diagnosis and non-­ surgical management of spontaneous osteonecrosis of the knee. Knee 14(2):112–116 Zwas TS, Elkanovitch R, Frank G (1987) Interpretation and classification of bone scintigraphic findings in stress fractures. J Nucl Med 28(4):452

The Knee: The Menisci Hema N. Choudur and Samir M. Paruthikunnan

Contents

Abstract

1

Technical Considerations 

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2

MR Sequences 

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3

 ormal Anatomy (Renstrom and Johnson N 1990; Petersen and Tillmann 1998)   217

4

 arameniscal Structures Mimicking P Tears 

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5

Significance of Signal Alterations 

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6

 lassification and Types of Meniscal C Tears 

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7

Indirect Signs 

 231

8

 ccuracy of MRI for Meniscal Tears and A Diagnostic Errors 

 233

MR Arthrography for Meniscal Tears 

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9

10 Management of the Meniscal Tears 

 234

11 Conclusion 

 235

References 

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H. N. Choudur (*) Department of Radiology, McMaster University, Hamilton, ON, Canada Hamilton General Hospital, Hamilton Health Sciences, Hamilton, ON, Canada e-mail: [email protected] S. M. Paruthikunnan York and Scarborough Teaching Hospitals NHS Foundation Trust, York Hospital, York, UK

The knee is one of the most frequently imaged peripheral joints on MR imaging. Plain radiographs are the initial modality to evaluate the knee. MR imaging is subsequently used to assess acute and chronic internal and external derangements of the knee. Due to the excellent soft-tissue resolution and tissue characterization on MR imaging, it is the preferred modality to evaluate the menisci and aid in the management of meniscal pathology. Trauma is a very common indication for MR imaging of menisci. Timely diagnosis of meniscal tears is essential in directing appropriate management before the onset of early osteoarthritis from loss of the adjacent articular cartilage. Therefore, the radiologist must have an in-­ depth understanding of the types of meniscal injuries on MR imaging, their classification, and interpretation. Furthermore, correlation with clinical history helps identify age-related horizontal asymptomatic tears and prevent unnecessary surgeries (Boden SD, Davis DO, Dina TS, Stoller DW, Brown SD, Vailas JC et al (1992) A prospective and blinded inves­ tigation of magnetic resonance imaging of the knee: abnormal findings in asymptomatic subjects. Clin Orthop Relat Res (282):177–185). This chapter focuses on MR techniques to image the menisci, normal meniscal anatomy, and meniscal pathology.

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_368, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 21 April 2023

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1 Technical Considerations MR imaging is the current globally accepted standard for optimum noninvasive imaging of the knee joint. Several types of open and closed-bore magnets of varying strengths (commonly used are 1.5 and 3T) and different coils are available. Both 1.5 and 3T provide high-quality diagnostic images. However, 3T magnets provide superior spatial resolution and signal-to-noise ratio while reducing the scan time. On the other hand, 1.5T imaging has the advantage of reduced metallic susceptibility artifacts and is preferentially used in patients with hardware or metal in the field of view and those with MR-compatible medical devices (Koff et  al. 2017; Miller et  al. 2016). Dedicated knee coils are used to obtain superior quality. Regardless of the magnetic field strength, dedicated knee coils will provide the best quality images. MR imaging of the knee is performed with the patient in the supine position, feet first, with the lower body placed within the gantry. While the general-purpose extremity transmit-receive surface coil provided by most companies can be used, the phased array and quadrature coils are preferred by many centers, as they provide an improved signal-to-noise ratio and spatial resolution. As per the American College of Radiology guidelines, the knee MRI parameters include a maximum field of view of 16 cm, a maximum slice thickness of 4  mm with 50% maximum intergap spacing, and a minimum matrix of 192  ×  256 (phase  ×  frequency). Commonly, a field of view of 14–16 cm and a slice thickness of 3–4 mm are utilized. However, the values are different for imaging the pediatric population with a smaller field of view of 4.0  mm indicating PLC insufficiency (LaPrade et  al. 2008) (Fig. 18). Bilateral valgus stress AP radiographs are an objective measure of medial knee ligament stability with a side-to-side difference in the medial compartment width >3.2 mm considered indicative of a grade 3 MCL injury (Laprade et  al. 2010). Dependent on patients’ guarding for pain, it may be necessary to obtain these radiographs under anesthesia (Sawant et al. 2004). For assessment of ACL stability, two

Fig. 17  Radiographic demonstration of ACL instability. Lateral standing, semi-flexed knee radiograph of a 42-year-old woman following a knee injury 8  months after ACLR shows excessive anterior tibial translation (anterior drawer sign) indicating an ACL graft tear

methods of passive, i.e., gravitational stress radiography, have been recommended. Lateral radiographs of both knees can be obtained either with the patient supine and the ankle propped on a pad so the knee is off of the table or with the patient prone with the knee in 15 flexion elevated off the table with a board beneath the thigh and a pad beneath the ankle. With these views, the degree of anterior tibial translation compared for each knee (Fukuta et  al. 2000; Mae et  al. 2018). For patients with unilateral ACL injuries, side-to-side differences were found to average 4.6 mm for the prone gravity stress radiographs and 2.8 mm for supine passive terminal extension radiography (Mae et al. 2018). Radiographs are also routinely used to assess bone tunnel placement following knee ligament reconstruction and to screen for abnormal tunnel widening or other abnormalities. Several methods have been developed to describe the femoral and tibial tunnel positions following cruciate

The Postoperative Knee: Cruciate and Other Ligaments Fig. 18  Varus stress radiographs for detection lateral ligament insufficiency. Bilateral preoperative AP varus stress radiographs prior to right knee revision ACLR demonstrate lateral ligament insufficiency of the operative side (a) with greater lateral compartment widening than the uninjured left knee (b). A combined ACL and anterolateral ligament (ALL) reconstruction was performed

a

reconstruction. The clock face method is most commonly used to describe the femoral tunnel position on frontal radiographs and coronal images for ACLR and PCLR grafts (Fig. 19). The clock is positioned with its borders on the posterior walls of the intercondylar notch with 12 o’clock is defined as the top of the notch. For the right knee, the 9 o’clock position is at the medial border of the lateral femoral condyle and the 3 o’clock position is at the lateral border of the medial femoral condyle. For the left knee, the 9 o’clock is medial and 3 o’clock lateral (Jepsen et  al. 2007; Han et  al. 2014). For lateral radiographs or sagittal images, the quadrant method uses a line drawn from the anterior to the posterior edge of the femur along Blumensaat’s line (a line drawn along the roof of the intercondylar notch). The line is then divided into four equal parts/quadrants with the location of the bone tunnel center used to determine in which quadrant the tunnel has been placed (Bernard et al. 1997; Jepsen et al. 2007). A quadrant method can also be used to determine the position of the tibial tunnel on lateral radiographs or sagittal images. A line is drawn from the anterior to the posterior

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edge of the tibia and divided into four equal quadrants; the quadrant location of the tunnel center is then determined (Jepsen et al. 2007).

6.2 Computed Tomography (CT) Multi-detector CT provides isotropic images for multiplanar reformatted and three-dimensional (3D) volume rendered images which are ideal for assessment of bone tunnels following ligament reconstruction. As opposed to MRI, CT can differentiate calcified bone from other tissues and determine not only the precise location of bone tunnels, but also accurately quantify tunnel size and volume as well as bone loss related to complications and bone formation within the tunnels (Meuffels et  al. 2011; Bourke et  al. 2013). Although metal hardware may cause artifacts from beam hardening, various techniques for reducing such artifacts have been developed, and residual metal artifacts are usually less problematic in CT than for MRI (Lee et al. 2007; White and Buckwalter 2002). For research applications, linear and volumetric CT measurements have

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Fig. 19 Expected radiographic appearance following ACL reconstruction. Radiographs shortly after BTB ACLR document the femoral tunnel at the 10 o’clock position on the AP view (a) and centered within the posterior quadrant 4 of the roof of the intercondylar notch on

the lateral view (b). The tibial tunnel is centered in quadrant 2, to avoid impingement on the roof of the intercondylar notch at full knee extension. The bone harvest sites in the inferior patella and tibial tuberosity (arrowheads) and graft-bone plugs (asterisks) are visible

compared surgical methods and devices (Lamoria et  al. 2020). In clinical practice, CT is usually reserved for patients with multiple tunnels or other complications resulting in bone loss. When metallic artifacts interfere with MRI, CT-arthrography may be a helpful alternative for evaluation of menisci or cartilage.

axial plane) can be prescribed from sagittal images parallel to the ACL graft or roof of the intercondylar notch. Alternatively, an oblique sagittal can be prescribed from axial images parallel to the medial border of the lateral femoral condyle, or from coronal images parallel to the graft. Thin-slice coronal oblique scans along the course of the popliteus tendon that include the fibular head have been recommended to improve visualization of the PLC ligaments (LaPrade et al. 2000; Yu et al. 1996). Obtaining a single 3D acquisition using isotropic voxels less than 1 mm in dimension and subsequently creating multiplanar reconstructions is an alternative strategy to direct oblique 2D multislice acquisitions (Fig. 20). While a set of 3D isotropic images may take longer than standard 2D sequences, high quality reformatted images can be created in any oblique plane and this approach may ultimately save time (Gold et  al. 2007; Yang et  al. 2014). This strategy is especially useful following knee surgery, especially for ligament reconstructions since the precise orientation of the graft(s) may not be predictable. The metal screws and other hardware from the ligament reconstruction or concomitant sur-

6.3 Magnetic Resonance Imaging (MRI) Knee MRI is the best postoperative imaging tool to demonstrate graft placement, impingement, failure, and assessment of new injuries or recurrent/persistent instability. It is an excellent modality for detecting edema-like marrow signal and determining the cause of complications (Naraghi and White 2006). Most often standard sagittal, coronal, and axial imaging planes are adequate for diagnosis of ligament and ligament reconstruction grafts. However, imaging in dedicated off-axis planes oriented along or orthogonal to the grafts can increase the sensitivity and specificity of diagnosis (Araujo et al. 2013). For ACLR grafts, oblique coronal (a.k.a. oblique

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geries may produce susceptibility artifacts, particularly at higher magnetic field strengths, that may significantly interfere with evaluation of the grafts, menisci, cartilage, and bone. It can be difficult to predict which hardware will be problematic prior to MR imaging since different metals, their orientation relative to the magnetic field, and the position within the knee will affect how much and which areas of the joint are obscured. Even in the setting of relatively large screws, such as those that may be used for PCLR inlay surgery, adequate evaluation of graft integrity may still be possible. Imaging parameters and pulse sequences that reduce metal-related MRI artifacts include high bandwidth, 3D spinecho acquisitions, shorter echo times or inter-

echo spacing, lower magnetic field strength, thin slices, smaller fields of view, and high-resolution image matrices (higher gradient strength) for small voxel sizes (Lee et al. 2007) (Fig.  21). Short tau inversion recovery (STIR) and Dixon techniques for fat suppression are often more successful than frequency selective fat saturation techniques (Lee et al. 2007; Viala et  al. 2016). Many vendors now have pulse sequences designed specifically to reduce metal artifacts in the setting of hardware such as arthroplasties (Talbot and Weinberg 2016); however, in our experience, constraints on imaging contrast, spatial resolution, and acquisition times may limit their utility for knee evaluation.

a

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Fig. 20  Oblique imaging planes for ACL assessment. To optimally show the entire length of the graft and bone tunnels, an oblique sagittal plane (a) along the graft is prescribed from either a coronal (b) or axial (c) image.

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Oblique axial/coronal images (d) may be prescribed from the central sagittal image (e). This example shows all of these options prescribed from a single 3D SPACE acquisition obtained with isotropic voxels

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Fig. 21  MR artifacts from metallic hardware. MR susceptibility artifacts from a metal screw distort the surgical site of this inlay PCLR graft on standard PDW TSE images (a, 1.5  T, TR  =  2500  ms, TE  =  14  ms, receiver

bandwidth 195  Hz/pixel). An acquisition technique for metal artifact reduction (b) allows better assessment of the graft (1.5  T, SEMAC TR  =  4610  ms, TE  =  34, receiver bandwidth 520 Hz/pixel)

7 Expected Appearances

This period of healing is different for patellar and hamstring tendons with the hamstring tendon grafts returning to low signal intensity at 12 months while patellar tendon grafts complete the stages at 24 months (Zappia et al. 2017). The graft appearance should have a uniform low signal intensity similar to a normal ACL by 2 years; however, mild, non-fluid-like, increased intrasubstance graft signal intensity on intermediateand T2-weighted images commonly persists, even beyond 4 years, in patients with uncomplicated ACLRs (Saupe et  al. 2008). For children with open growth plates, the normalization of the MR signal of ACL grafts during the ligamentization process (remodeling phase) is slower than for adults with high signal within the grafts persisting even 2  years postoperatively (Pauvert et al. 2018). Since hamstring tendon grafts consist of several bundles combined into a single construct, linear intermediate or high signal intensity is commonly seen between the bundles without a graft tear and is an expected finding when the signal is oriented parallel to the graft (Sanders 2002). The postoperative MR appearances of ACLR and PCLR grafts evolve in a similar manner, although the PCLR graft is likely to have more

7.1 Grafts The MRI imaging appearance of ligament grafts depends on the graft type and varies with time following surgery. Graft healing after ACLR reconstruction is a continuous process that goes through early, remodeling, and maturation phases that are of debated length (Claes et  al. 2014). During this process, the graft undergoes “ligamentization” as it converts from a tendon to a structure that more closely resembles a ligament by 24  months (Amiel et  al. 1986; Claes et  al. 2014). During the first three postoperative months following ACLR, the graft has low signal intensity on T1- and T2-weighted images (Naraghi and White 2006) (Fig. 22). The graft then gradually increases in signal intensity. The periligamentous tissues may show intravenous contrast enhancement due to highly vascularized tissue around the graft fibers related to the synovial proliferation, while the less vascular graft itself shows no enhancement (Jansson et  al. 2001). During early remodeling and maturation, the graft may appear inhomogeneous and even be difficult to discern (Naraghi and White 2006).

The Postoperative Knee: Cruciate and Other Ligaments Fig. 22 Early postoperative MR appearance of ACLR autograft. Two months following ACLR, this hamstring autograft has low signal intensity on sagittal PDW with fat saturation (a) and T2W (b) images. There are thin streaks of brighter signal between the bundles of the graft. With time, the signal intensity is likely to increase during the “ligamentization” of the graft

a

pronounced postoperative signal abnormality that persists longer than for ACLR grafts (Sanders 2011). The MRI appearance of the PCLR depends on the type of graft, fixation technique, and time after the surgery. In the early postoperative period (first 3–4 months) the PCL graft appears hypointense and thickened on all sequences, and over time the thickness of the graft diminishes (Sanders 2002). Similar to ACLR grafts, during the initial postoperative 4–8 months, remodeling/ ligamentization transforms the graft into a tissue similar to the native PCL (Alcala-Galiano et al. 2014). Thus, high T2-weighted signal intensity in the first postoperative months should not be interpreted as a complication (Fig. 23). Repaired native ACLs have a different evolution of their postoperative MR appearance compared to ACLR grafts since the native ACL is undergoing a healing process rather than the remodeling, “ligamentization” process of the ACLR graft (De Smet et  al. 2019). Whether IBLA, DIS, or BEAR, the signal of the healing ACL repair is initially higher than a normal ACL and often heterogeneous (Murray et  al. 2016, 2019; Daniels et al. 2018; De Smet et al. 2019). In the first 3–6 postoperative months, the ACL may appear swollen with a higher-than-normal signal intensity. The signal intensity decreases gradually, although the abnormal signal may persist for over a year (Daniels et  al. 2018; De Smet et  al. 2019) (Fig.  24). During this active

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healing phase, the abnormal, heterogeneous signal within the repaired ACL may simulate a tear, and the diagnosis of a true graft tear may require identification of secondary signs of injury on imaging, such as pivot shift impaction, and clinical correlation (Daniels et al. 2018). Eventually, the repaired ACL appears normal in signal intensity and returns to normal size (Daniels et  al. 2018). There are fewer descriptions of postoperative imaging following other ligament reconstructions and repairs. Like the ACL, repaired ligaments and surrounding tissues may initially show brighter than normal signal on T2-weighted images that diminishes over time, and the ligament often appears thickened even years postoperatively (Sanders 2011). Postoperative evaluation of PMC and PLC ligament reconstructions requires an understanding of the surgery that was performed so complete evaluation of the grafts, tunnels, and fixation devices can be achieved. Routine imaging, in the absence of instability, is generally performed with radiographs (Naraghi et  al. 2018). MR of a normal postoperative graft will show intact fibers. As with other grafts, the MR signal intensity on T2-weighted MR images may be initially high, with thickening of the graft that eventually normalizes. The hardware associated with ligament reconstructions may interfere with assessment of the grafts on MRI (Fig. 25).

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Fig. 23  MR of three different patients following PCLR, all with good clinical results, demonstrating expected graft appearances at various postoperative intervals. Patient 1 (a, d): 8 months after surgery the Achilles tendon allograft (arrow) appears thick with high signal on PDW with fat saturation (a) and T2W (d) MR images. Poor

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Fig. 24  Expected postoperative MR appearance after ACL suture repair using a BEAR scaffold. Sagittal 3D SPACE MR images 6 months (a), 1 year (b), and 2 years (c) following suture repair of a torn ACL using a BEAR scaffold show initial abnormally bright ACL signal inten-

visualization of the graft during the first 4–8 month may be normal. Patient 2 (b, e): 1  year following PCLR the Achilles tendon allograft shows only mildly high signal and a near-normal thickness. Patient 3 (c, f): 12 years after double-bundle PCLR with hamstring autografts demonstrate nearly normal PCL signal intensity

c

sity that returns to a more normal appearance over time, reflecting the healing process. By comparison, ACLR grafts usually appear dark shortly after surgery (see Fig.  22). (Images courtesy of Kirsten Ecklund, MD, Boston Children’s Hospital)

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Fig. 25 Combined ACLR and posterolateral corner (PLC) reconstruction. Two week postoperative AP knee radiograph (a) of a 26-year-old woman underwent ACLR with a metallic cross-pin (asterisk) and tibial staples also had PLC reconstruction for tears of the fibular collateral ligament (FCL), iliotibial band, popliteus tendon, and biceps tendon. The FCL was repaired with a Larson technique (see Fig. 16) using tendon allograft through a fibular

bone tunnel (white arrow) and fixed to the femur over a screw-post (black arrow). At 2 years postoperatively, coronal PDW fat-saturated MR image (b) evaluates most of the graft (arrowheads) and its fibular attachment (arrow); however, metal screw artifact obscures the femoral attachment of the graft. The screw was removed at 2.5  years postop (c) allowing evaluation of the entire, intact graft

Following ALL reconstruction, the graft can be found on axial images near the lateral epicondyle, deep to the iliotibial band (ITB) and anterior to the popliteus tendon and FCL (Fig.  26). Since the course of the graft is from posterosuperior to anteroinferior, the entire length of the graft cannot be visualized on a single coronal image, and the graft is difficult to assess on sagittal images (Lôbo et al. 2020).

the middle of the patellar tendon usually persists (Recht et al. 2000; Kartus et al. 1999) (Fig. 27). Following QT autograft harvest, surgical changes will be apparent within the central quadriceps tendon. With partial-thickness har­ vest, the deep layer of the tendon and the suprapatellar pouch remain unaffected; however, following full-thickness harvest, scarring and suture material may be seen within the suprapatellar capsule and fat pad (Emerson et al. 2019). The tendon defect will initially have edema signal scar tissue within and anterior to the tendon that diminishes in signal intensity with time, and the defect size will depend on whether the peripheral quadriceps tendon bundles were surgically reapproximated (Emerson et al. 2019). There will be a bone defect in the superior pole of the patella when QT graft includes a bone plug. The time sequence for the postoperative MR appearance of hamstring tendon donor sites has been elucidated by a small, cross-sectional study showing “regeneration” of the tendons, a.k.a. “neotendons,” by 15 months (Rispoli et al. 2001; Curtis et  al. 2019). For the first 6 postoperative weeks, fluid or edema are noted within the harvest tracts; however, over the next 3  months to

7.2 Harvest Sites The harvest sites for autografts may be visible on imaging, especially when bone plugs are taken. On early postoperative radiographs after ACLR with a BTB autograft, the bone plug harvest sites are often visible as rectangular lucencies, one each in the tibial tuberosity and the inferior patella (Figs. 8 and 19). MR imaging will similarly demonstrate the bone plug harvest sites. In the early postoperative period, the patellar tendon will appear diffusely thickened where the central third of the tendon was taken (Somanathan et al. 2019; Zappia et  al. 2017). The appearance will normalize with time, although a small defect in

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Fig. 26  Expected MR appearance following ALL reconstruction. Eleven months following a combined ALL reconstruction and revision ACLR, axial T2W fat-­ suppressed MR images superior to the femoral epicondyles (a) shows the ALL gracilis autograft attachment. The femoral screw is not completely within the femur; however, there was no clinical effect from this positioning. At the popliteus tendon origin (b), the graft (ALL) is deep to the iliotibial band (ITB) and anterior to the popliteus tendon (PLT) and fibular collateral ligament (FCL).

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Inferior to the knee joint (c), the fixation screw in the tibial bone tunnel is posterior to Gerdy’s tubercle, and the ACLR graft and interference screw (ACLR) seen. Since ALL grafts are angled, each coronal image will demonstrate only a portion of the graft. Coronal PDW fat-­ suppressed image (d) shows the distal half of this intact graft (arrowheads) and its tibial insertion. The origin of the femoral ACLR tunnel (asterisk) appears on the same image since it is anterior to the ALL graft attachment site (see Fig. 8 for radiographs of this patient)

The Postoperative Knee: Cruciate and Other Ligaments

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Fig. 27  Expected MR appearance of BTB autograft harvest site. Shortly after surgery (a, b) an inferior patellar bone defect is visible (coronal T1W image a) as well as a vertical defect in the patellar tendon (axial T2W image b).

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Ten years postoperatively, PDW fat-suppressed MR images from a different patient show that both the patellar bone defect (coronal, c) and the patellar tendon defect usually heal (axial, d)

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Fig. 28  Expected MR appearance following hamstring autograft harvest. Postoperative T1W axial image of both thighs 10  years following left sided tendon harvest for ACLR demonstrate atrophy of the semitendinosus (st) and

gracilis muscles on the operated side. More inferiorly, the distal tendon attachments were intact with continuous, regenerated “neotendons” (not shown)

1  year, structures with tendon-like signal progressively evolve from proximal to distal with “complete normalization of the proximal tendon” to within 1–2  cm of the tibial insertion for the patient imaged 2 years 8 months postoperatively (Rispoli et al. 2001). More proximally, the muscle bellies may show some atrophy, even with an intact “neoteondon” (Fig. 28).

MRI artifacts; however, they are usually relatively radiolucent limiting radiographic assessment (Fay 2011). Extracortical posts and screws are usually made of metal and may cause significant artifacts on MR imaging, with less severe artifact on CT. Extracortical suspensory devices are usually metal, but the degree of MR and CT artifact, as well as their distance from the joint, is usually less problematic than screws and posts. Transtunnel fixation devices, such a cross-pins, may be metal or bioabsorbable. Interference screws are positioned within the bone tunnel between graft and tunnel wall, firmly pressing the graft against the tunnel wall to immobilize the graft during graftbone incorporation. Ideally, the interference screw should be aligned with the bone tunnel (Sharp et al. 2018). For grafts with bone plugs (e.g., BTB autografts), both bone-to-­bone healing as well as tendon–bone incorporation are required to result in an appearance similar to a native ACL attachment (Petersen and Laprell 2000; Delay et  al. 2002). All soft tissue grafts (e.g., hamstring tendons) heal with bone incorporation into the tendon with results similar to a fibrous tendon insertion (Petersen and Laprell 2000).

7.3 Tunnels and Fixation Devices Successful ligament reconstruction requires integration of the graft into the bone tunnels. Graft fixation devices function to hold the grafts in place within the bone tunnels until there is stable incorporation of the graft. There are numerous types of devices used to secure grafts including intra-tunnel interference screws, transtunnel devices, or suspensory, and screws, posts, or staples attached to the bone’s cortex (Martin et  al. 2002). The postoperative imaging appearance will depend on the material composition. Bioabsorbable, biocomposite, and plastic interference screws have the advantage of not causing

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Fig. 29  Incorporation of a BTB graft into the tibial bone tunnel. Serial CT scans of the tibia of a patient following ACLR with a BTB autograft using a rapidly resorbing polylactide carbonate (PLC) interference screw were reformatted oriented to the tibial tunnel and registered between time points. Day 1 following surgery (a), the interference screw is holding the cortical bone plug against the posterior tunnel wall (anterior is to the right).

By 6  months (b), the screw has degraded and the bone plug has become incorporated into the tibia. At 1 year (c), the bone plug has remodeled further and the proximal tunnel opening is closing. Two years after surgery (d), the bone plug is fully incorporated as trabecular bone, there is new bone formation within the tunnel, and both tunnel openings have closed

Because there is bone-to-bone graft incorporation, BTB autografts are considered to have faster healing and superior fixation to hamstring autografts, which allows for earlier rehabilitation (Beynnon et  al. 2005a). In animals, BTB autografts had bone incorporation by 3 weeks while tendon autografts showed bone incorporation at 12  weeks (Tomita et  al. 2001). As the grafts mature, their imaging appearances within the tunnel follow the incorporation process. Based on CT appearance, bone plugs may evolve over at least 2 years with conversion of the initially cortical bone to cancellous bone (Fig.  29). Bioabsorbable fixation hardware, however, will degrade at different rates according to the type of composite material, and this will influence the imaging appearances as well as the timing of these changes or the timing of any adverse tissue response (Konan and Haddad 2009a). Radiography has been used to demonstrate tunnel position and outline, while CT can evaluate the bone components within the tunnels (Hatipoglu et  al. 2021; Sundaraj et  al. 2020). MRI is superior for evaluating the soft tissue component of the graft, radiolucent fixation hardware, bone marrow surrounding the tunnels, and identification of ganglia or adverse tissue responses (Lajtai et  al. 1999; Sundaraj et  al. 2020). As an example, biocomposite interference

screws designed to degrade and be replaced by bone may show fatty bone marrow signal on MRI suggestive of bone formation while the CT demonstrates a persistent bone void (Sundaraj et al. 2020) (Fig. 30) or new bone formation (Fig. 29). Varying, though usually mild, degrees of edema-­ like bone marrow signal may be seen on MRI around the bone tunnels that may persist postoperatively, but usually resolves within the first year (Lajtai et al. 1999). Following ACLR, the femoral and tibial bone tunnel diameters following ligament reconstruction usually change over time with ~20% tibial and ~40% widening expected during the first year (Lamoria et  al. 2020). However, over 2 years, tunnels widths may ultimately narrow, a finding more pronounced with BTB grafts (Hatipoglu et al. 2021; Pinczewski et al. 2001). Hamstring tendon autograft tunnels radiographically stabilize after 2 years, but BTB graft tunnels may continue to narrow even at 5 postoperative years (Pinczewski et  al. 2001). While likely multifactorial, angular or longitudinal graft motion at the articular tunnel apertures, the “windshield wiper” or “bungee” effects, have been proposed to explain tunnel enlargement overall and measurable differences in tunnel widening between ACLR graft types and fixation methods (Iorio et al. 2007; Cheung et al. 2010;

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Fig. 30  Serial imaging of a tibial PLLA-HA bioabsorbable interference screw from 2 to 13 years. Serial sagittal PDW MR images show the slow degradation of the tibial poly-l-lactic acid with hydroxyapatite (PLLA-HA) screw. The screw is nearly intact at 2 years (a) and only partially resorbed at 5  years (b). After 13  years, the screw is no

longer visible, replaced by fat signal on MR (c). While this may appear to be bone on MR, a CT scan (d) performed at the same time shows the screw replaced by fat without visible bony trabeculae. (From Sundaraj et  al. 2020)

Sabat et  al. 2011; Lamoria et  al. 2020). In a CT-histological analysis of an ACLR animal model, a direct relationship was found between tunnel size and motion between the tunnel and graft with osteoclast formation at the intra-articular tunnel aperture suggesting that graft motion partially accounts for tunnel widening and variations of the incorporated graft histology by location within the tunnel; however, the exact cause of tunnel widening has not been proven (Rodeo et  al. 2006). Some surgeons recommend graft fixation near the articular apertures of the tunnels to minimize graft motion (Barber et  al. 2003). Tunnel widening is less for autografts than allografts, less for BTB than for tendon autografts, and less for transtunnel fixation than suspensory fixation (Giaconi et  al. 2009). Clinical outcomes appear to be unaffected by this “expected” tunnel widening, although one report found an association between greater tunnel widening and greater knee laxity (Lamoria et al. 2020; Järvelä et al. 2008). However, excessive tunnel widening may complicate revision ligament surgery (Ghazikhanian et al. 2012) and may signal a pathological process within the tunnel such as ganglion formation or an abnormal response to the graft or fixation device.

The positions of the bone tunnels are fundamental for the proper function of any graft and can be evaluated by radiographs as well as by MRI and CT.  For single-bundle ACLR, the articular aperture of the femoral tunnel is ideally at the posterior aspect (posterior 25%, quadrant) of Blumensaat’s line, and, to avoid a posterior femoral “blow-out” fracture, 1–2 mm anterior to the posterior margin of the femoral cortex (Giaconi et  al. 2009; Bencardino et  al. 2009). On AP or coronal images, the femoral tunnel aperture will be at either the 10 or 2 o’clock position (superior intercondylar notch is 12 o’clock), although some surgeons may still choose the 11 or 1 o’clock position (Giaconi et  al. 2009; Bencardino et  al. 2009; Bernard et  al. 1997). As seen on full extension lateral radiographs or sagittal MR/CT images, the anterior margin of the articular aperture of the tibial tunnel for ACLR should be at, or posterior to, the intersection of an extended Blumensaat’s line with the tibia (Giaconi et  al. 2009). The more anteriorly the tibial tunnel is positioned, the more stable the knee; however, a tunnel that is too far anterior, i.e., anterior to Blumensaat’s line, may lead to graft impingement on the intercondylar roof (Rayan et  al. 2015). With

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d­ ouble-bundle ACLR, the tunnel for the anteromedial (AM) bundle is placed at the 10–11 (or 1–2) o’clock position and the posterolateral (PL) bundle at the 9–10 (or 2–3) o’clock position (Giaconi et  al. 2009). Judgment of tunnel position on the lateral radiographs or sagittal images is based on a percentage of the AP dimension of the tibia (Amis and Jakob 1998). The tibial tunnel for the AM bundle should be centered 36% posterior from the anterior tibia and the center of the PL bundle tunnel at 52% (Giaconi et al. 2009). For single-bundle PCLR, the femoral articular opening on lateral radiographs or sagittal images is positioned in the anterior 25% (quadrant) of Blumensaat’s line, 8–10 mm posterior to the femoral articular surface, at the junction of the roof and lateral wall of the intercondylar notch, and at the 11 (or 1) o’clock position on AP or coronal images (Pinczewski et  al. 1997; Mariani et al. 1999). With double-bundle PCLR grafts, lateral or sagittal images show the femoral tunnel for the anterolateral (AL) bundle within the anterior third of intercondylar notch while the posteromedial (PM) bundle is in the posterior or middle third (Alcala-Galiano et  al. 2014). On AP or coronal images, the AL bundle tunnel will be at the 11 (or 1) o’clock position and the PM bundle at the 9 (or 3) o’clock position (Alcala-Galiano et al. 2014). For single- and double-bundle PCLR, whether a tibial tunnel or an inlay procedure, the tibial insertion on lateral or sagittal images should be at the posterior 25% of the retrospinal tibial surface at the medial margin of the midline of the tibia as viewed on axial images (Alcala-Galiano et  al. 2014; Mariani et al. 1999).

knee instability in both the pre- and postoperative periods. However, MRI is most useful for direct visualization of the grafts, tunnels, and radiolucent hardware.

8 Complications Radiography may be able to detect fractures, ­secondary signs of graft rupture, metallic hardware loosening, and some tunnel complications. As previously described, stress radiography can be used a functional imaging tool to measure

8.1 Graft Complications 8.1.1 Graft Tear Graft failure for ligament reconstruction may be caused by technical factors including poor graft revascularization, graft laxity from improper tunnel positioning or graft movement within the tunnels prior to incorporation, or from complete or partial graft tears (Vergis and Gillquist 1995). Ligament graft tears are usually from a reinjury and may result in knee laxity and instability and revision reconstruction may be indicated. Ligament grafts are felt to be most susceptible to tears during the time of revascularization and ligamentization (Vergis and Gillquist 1995). The primary MR signs of a complete graft tear are similar to a native ligament tear with complete disruption of the graft fibers, a fluid-filled defect within the graft, or abnormal course of the graft. Secondary signs include signs of ligament laxity, e.g., anterior tibial translation (by more than 5  mm), pivot-shift-type marrow edema, or Segond fracture for an ACL graft tear and abnormal orientation of the graft (Collins et al. 2008) (Figs. 17, 31, 32, 33, and 34). Partial graft tears may be more challenging to diagnose, especially since other factors, including revascularization, synovial heterogeneity within the bundles of hamstring grafts, and graft impingement may cause increased signal within the graft. However, T2-weighted images may prove useful in identification of partial graft tear because the presence of focal fluid-like signal intensity within the graft, with some fibers remaining intact, favors a partial tear (Ilaslan et  al. 2005; Bencardino et  al. 2009) (Fig.  35). Following ACLR with multistrand hamstring grafts, a partial graft tear that only involves only one or a few of the strands may occur. In such cases, the torn strand may become displaced

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Fig. 31  Reinjury following ACLR with complete graft tear. AP knee radiograph (a) of a 19-year-old man 1 week following a skiing injury and 2.5 years after ACLR shows a lateral tibial Segond avulsion fracture (arrowhead) Fig. 32  Complete tear of ACLR graft. Knee MR of an 18-year-old woman 2 weeks after falling on her knee and 3 years following ACLR. Sagittal fat-saturated PDW (a) and T2W (b) images show a proximal tear with displacement of the graft into the anterior joint. The banded signal intensity of graft is likely the result of magic angle artifacts

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anteriorly at the base of the intact portion of the ACLR graft near the tibial tunnel and simulate localized anterior arthrofibrosis creating a “pseudocyclops” appearance (Simpfendorfer et  al. 2015) (Fig. 36). An oblique sagittal plane parallel to the graft tunnels may also optimally differentiate partial and complete graft tears by demonstrating the torn strand of the graft exiting the tibial tunnel, as well as the other intact fibers (Figs. 31 and 36). Other oblique imaging planes, or reformatted images from a thin slice 3D acquisition, parallel to the ligament graft, e.g., an oblique coronal plane for the intercondylar notch

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indicative of a graft tear. The sagittal T2W MR image (b) confirms complete disruption of the graft. Oblique sagittal image (c) reformatted from a 3D SPACE acquisition more accurately demonstrates the site of the graft tear (arrow)

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for the ACL, may improve both diagnostic accuracy and confidence for grading ACL graft injury (Moon et al. 2008).

8.1.2 Impingement Graft impingement occurs when there is pressure or rubbing of the graft during joint function. Following ACLR, this frequently occurs when the graft contacts the intercondylar notch either because the tibial bone tunnel is too far anterior, the intercondylar notch is too small for the graft size, with osteophyte formation, or if there is fixed anterior tibial translation from tight

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Fig. 33  Complete mid-substance tear of PCLR graft. Fifteen years following PCLR, this 48-year-old man developed acute knee pain while long distance running. Consecutive T2W sagittal MR images show the proximal Fig. 34 Complete proximal PCLR tear. Five years following a single-bundle inlay PCLR with Achilles tendon allograft, this 23-year-old man sustained a proximal graft tear during a slip and fall. Consecutive sagittal T2W MR images show the stump of the PCL graft at the femoral tunnel opening (arrow, a). The remainder of the torn graft is lying flat against the tibia (arrowheads, b)

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p­ osterior capsular structures (Viala et  al. 2016; Bencardino et  al. 2009). MR imaging reveals abnormal bright signal in the distal two thirds of the impinged graft (Recht et al. 1996). The graft may appear to be draped over the anterior inferior edge of the intercondylar roof or be posteriorly bowed on the sagittal MR images (Ilaslan et al. 2005). Graft impingement often presents with symptoms of limited terminal knee extension and

graft (arrowheads, a) attached to the femur, but completely separated from the distal end of the graft lying on the tibial surface (arrowheads, b). The lateral meniscofemoral ligament (arrow, b) remained intact

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may result in graft fibrosis (localized anterior arthrofibrosis) (Fig. 37), partial tearing, or eventually, a complete graft tear (Naraghi and White 2006). Less commonly, the tibial tunnel is placed too far laterally, there is lateral tibial shift, or there are osteophytes so the graft impinges on the side wall of the intercondylar notch (Fig. 38). Or, if the tibial tunnel is too medial and vertical, the graft may contact the PCL limiting knee flexion

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Fig. 35  Low grade partial tear. One year following a staged lateral meniscal repair and hamstring autograft ACLR, this 18-year-old woman retore her lateral meniscus playing soccer. Sagittal T2W MR image (a) shows

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Fig. 36  Tear of single bundle of hamstring graft (“pseudocyclops” lesion). Sixteen-year-old woman sustained a noncontact injury 9  months after a 5 strand hamstring autograft ACLR. Sagittal T2W FSE image (a) shows one of the 5 hamstring strands has torn and is displaced anteriorly (arrow). An oblique sagittal reconstruction from a 3D SPACE acquisition oriented along the tibial tunnel (b)

disruption of the anterior fibers of her ACL graft (arrows), confirmed at arthroscopy (b). The majority of the graft was intact and the knee stable. The frayed fibers were debrided, but no revision ACLR was indicated

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clearly shows the torn strand of the graft (white arrow) arises from the tibial bone tunnel (arrowheads) and is separate from the remaining, intact graft (asterisk). Since the knee was stable, no revision surgery indicated. This appearance should not be confused with localized anterior arthrofibrosis (cyclops lesion, compare with Fig.  47) or complete graft tear (compare with Fig. 32)

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Fig. 37  Intercondylar roof graft impingement in two patients. Sagittal intermediate weighted (IW) MR image of patient 1 (a) the ACLR graft is displaced around an osteophyte (arrow) on the intercondylar notch. The impingement is exacerbated by anterior placement of the

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tibial tunnel and has resulted in degeneration of the graft and localized anterior arthrofibrosis (asterisk). An oblique sagittal IW MR image of patient 2 (b) shows fraying of the anterior surface of the graft where it impinges on the roof of the intercondylar notch (arrowhead)

(Zappia et  al. 2017). Treatment by resection of small amount of bone from the impinging intercondylar roof or sidewall, a “notchplasty” is recommended to avoid progression to graft failure (Sanders 2002). Following notchplasty, the graft signal intensity usually normalizes within 12 weeks (Howell et al. 1991).

8.2 Tunnel and Fixation Hardware Complications

Fig. 38  Impingement on ACLR graft by the lateral wall of the intercondylar notch. Coronal PDW fat-saturated MR image shows the ACLR graft displaced by an osteophyte (arrowhead) on the lateral wall of the intercondylar notch. The impingement is worsened by lateral subluxation of the tibia (demonstrated by vertical line)

While the bone tunnels for ligament reconstructions typically enlarge 20–40% during the first year (Lamoria et al. 2020), more extreme widening, or new or continued widening after the first 2 years may indicate a complication such as ganglion formation, abnormal biological response to a bioabsorbable material, or infection. Proposed mechanisms for ganglion formation and tunnel enlargement from following ACLR and PCLR include persistent communication between the bone tunnel and joint (communicating cysts) or cystic degeneration of the graft; of note, no

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r­elationship with graft failure has been demonstrated (Papakonstantinou et  al. 2003; AlcalaGaliano et  al. 2014; Ghazikhanian et  al. 2012). While reported as an unusual occurrence, a comparative study between ACLR with metal and bioabsorbable screws found high rates of small, nonsymptomatic ganglia 13  years postoperatively on MR images with ~50% incidence of cyst formation, more commonly with bioabsorbable than metal screws in femoral tunnels, but equally common in the tibial tunnels (Sundaraj et al. 2020). Patients with tunnel ganglia are often asymptomatic, but ganglia may cause postoperative pain or limit range of motion (Bencardino et al. 2009; Ghazikhanian et al. 2012). Graft ganglia appear as fluid MR signal intensity mass-like cystic structures adjacent to the graft, which may extend into the intercondylar notch, distinguishing them from graft rupture or the thin linear signal that may be seen normally between the strands of hamstring grafts (Zappia et al. 2017). Extension of the ganglion into the soft tissues may produce a palpable pretibial mass (Fig. 39). Pretibial cysts with communication through the bone tunnel with the joint space are most often treated by simple excision but bone grafting may be required (Simonian et al. 1998). Correct tunnel placement is required for proper postoperative ligament function with incorrect placement resulting in ligament laxity or graft impingement (Bencardino et  al. 2009). The most common errors in PCLR tunnel placement are a femoral tunnel that is too high and posterior and/or an excessively proximal tibial tunnel location, leading to a vertically oriented graft that is unable to adequately resist posterior tibial translation (Noyes and Barber-Westin 2005). In addition, the posterior position of the tibial tunnel increases the risk of injury to the neurovascular structures of the popliteal fossa, especially if there is an aberrant anterior tibial artery (Klecker et al. 2008; Zawodny and Miller 2010). During tunnel drilling for PLC reconstruction, the peroneal nerve may be at risk for surgical injury (MacDonald and Vo 2015). Additionally, improper fibular tunnel placement may result in blow-out fractures (MacDonald and Vo 2015).

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Close proximity of multiple bone tunnels for multiligamentous reconstruction can also compromise osseous strength and predispose to fracture. Hardware failure or complications requiring surgical intervention are uncommon following ligament reconstruction, but may result from reactions to bioabsorbable materials, device breakage or migration with loss of graft tension or intra-articular consequences. Foreign body reactions to bioabsorbable or biocomposite interference screws or fixation devices can result in abnormal tunnel widening, focal bone resorption, formation of cysts or sterile abscesses, or synovitis, should the degradation material enter the joint (Fay 2011). The reactions occur during resorption of the hardware. Many different polymers are used for these hardware, each designed with a different rate of degradation (Cox et al. 2014). Thus, the timing of such reactions may vary from months to years (Fay 2011; Konan and Haddad 2009a). When the breakdown products of the polymers are released from the hardware, a locally acidic environment may develop that can cause tissue reactions, increase resorption rates, and inhibit bone formation (Agrawal and Athanasiou 1997; Fay 2011). Some biocomposite materials have added components to buffer the acidity of the breakdown products (Agrawal and Athanasiou 1997). Rapid degradation may weaken the hardware and lead to breakage or migration of device fragments, and possible loss of graft fixation. When bioabsorbable transtunnel, a.k.a. “crosspin,” devices degrade, they may fracture resulting in postoperative ACL laxity as well as local and/or diffuse osteolysis (Bakhru et  al. 2011; Choi et al. 2010) (Fig. 40). Biological reaction to a degrading interference screw within the femoral or tibial bone tunnel may result in focal or diffuse tunnel expansion. If the reaction occurs before the bone tunnel opening has closed, or if the screw has not been fully imbedded in the bone, patients may present with soft tissue masses or cysts simulating infection that may require debridement; the extruded portions of the device that may even become visible on physical examination (Figs. 41 and 42) (Konan

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Fig. 39  Pretibial ganglion following ACLR. This ACLR patient presented with a palpable mass on the anterior tibia. Axial T2W MR image (a) at the level of the mass shows a loculated fluid-signal ganglion (asterisk) that extends through the tibial opening of the tibial tunnel

(arrow, a). Sagittal T2W images at the center of the knee (b) and more medial (c, d) show the ganglion (asterisk, d) along the entire length of the bone tunnel with intra-­ articular communication (arrows, b, c)

and Haddad 2009b; Martinek and Friederich 1999; Sassmannshausen and Carr 2003; Cox et  al. 2010). Protrusion of a tibial interference screw into the joint may cause damage to the femoral cartilage on the opposing articular sur-

face (Fay 2011). Loss of graft fixation with migration of the graft and/or fixation hardware is uncommon but may also result in ligament laxity (Ilaslan et al. 2005). Displaced bone graft and metallic hardware may be initially d­ iagnosed

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on radiographs (Fig. 43), but MRI better shows the location of the displaced device, graft integrity/position and is necessary to identify displaced radiolucent hardware (Fig. 44).

8.3 Arthrofibrosis

Fig. 40  Broken transtunnel cross-pin with adverse biological reaction. Coronal fat-suppressed PDW MR image from a patient with a broken bioabsorbable cross-pin shows the marked biological reaction with osteolysis within the femur and tibia. The entire ACLR graft shows marked mucoid degeneration-like changes

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Fig. 41  Pretibial cyst from reaction to a degrading bioabsorbable screw. This 29-year-old professional athlete with 3 months of anterior knee pain and focal tibial soft tissue swelling presented 3  years following ACLR using biocomposite polylactic acid-β-tricalcium phosphate (PLAβTCP) interference screws. AP radiograph (a) demonstrates tibial tunnel widening and mass-like focal soft tissue swelling (arrows). Coronal T1W (b) and fat-­

Arthrofibrosis is the formation of intra-articular scar tissue and/or fibrous adhesions that may occur following joint injury or ligament ­reconstruction surgery and result in joint stiffness, loss of motion with an extension or flexion deficit, pain, snapping or effusion (Viala et  al. 2016; Millett et al. 2001). Arthrofibrosis may be generalized with diffuse scarring of joint capsule within all compartments or localized to just one region (Viala et al. 2016). MR imaging of postsurgical knees with limited flexion/extension or stiffness may help localize the scar tissue and can differentiate arthrofibrosis from graft impingement or other causes of diminished range of motion. Although physical therapy may be

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suppressed PDW (c) MR images show degradation of the tibial screw with small fragments of the PLA-βTCP material within the tibial tunnel with a cyst-like mass (asterisk, c) extruding anteriorly through the cortical opening of the tunnel. There is a severe inflammatory reaction; however, there was no infection. The tibial tunnel was debrided and packed with bone graft substitute

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Fig. 42 Biocomposite interference screw with rapid resorption and pretibial soft tissue mass. CT scans of the tibia oriented to the tibial tunnel and registered between time points immediately (a) and 6 months postoperatively (b) of a patient following ACLR with a BTB autograft

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using a polylactide carbonate (PLC) interference screw. Although the screw was implanted flush with the tibial cortex, it expanded during degradation and extruded through the tibial cortical opening into the pretibial soft tissues

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Fig. 43  Displacement of femoral interference screw. Five years after BTB autograft ACLR, this 39-year-old man presents with knee pain and limitation of flexion. Initial AP (a) and lateral (b) radiographs obtained 2 months postoperatively show normal femoral interference screw position (arrowheads). Five years later (c,

d), the femoral screw has displaced with an increase in the distance between the screw head and roof of the intercondylar when compared to the initial radiographs (arrowheads). When removed at arthroscopy, the screw was imbedded in scar tissue, but the ACLR graft was intact

successful, diffuse arthrofibrosis usually requires arthroscopic lysis of adhesions; localized arthrofibrosis is usually treated arthroscopically (Viala et al. 2016). On MR imaging, diffuse arthrofibrosis appears as thickening of the joint capsule within the suprapatellar pouch, medial and lateral parapatellar gutters, posterior capsule, the articular surface

of the infrapatellar (Hoffa) fat pad, and around the cruciate ligaments (Fig. 45). The tissue usually appears intermediate signal on T1-weighted and proton density-weighted images and intermediate to low signal on T2-weighted images (Viala et al. 2016). Thick bands of fibrous tissue connecting the patella and tibia may interfere with normal patellar motion and should be

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Fig. 44 Displaced broken tibial interference screw. Forty-eight-year-old man with knee locking and effusion 18 months following ACLR. Fat-suppressed coronal PDW (a) and axial T2W (b) MR images show a large, displaced fragment of the tibial interference screw (arrow, b) in the

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medial parapatellar recess. Midline sagittal PDW fat-­ suppressed image (c) shows the other fragment remaining partially within the tunnel. At arthroscopy, the ACLR graft was intact

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Fig. 45 Generalized arthrofibrosis following cruciate ligament reconstruction in two patients. Patient 1 (a) is a 42-year-old woman with continued knee stiffness 1 year following an ACLR complicated by postoperative septic arthritis. Despite two previous debridements, sagittal T2W MR image (a) shows severe thickening of the entire joint capsule with low signal scar tissue (arrows). ACLR graft is intact. Patient 2 (b, c) is a 24-year-old man with

severe knee pain with limited flexion and extension 6  months after hamstring autograft PCLR.  The diffuse thickening of the joint capsule (arrows) from generalized arthrofibrosis is more readily visualized on sagittal T2W (b) than PDW fat-suppressed (c) MR images. The scar tissue was lysed arthroscopically, and the PCLR graft was intact

distinguished from the mild thickening of articular surface of the Hoffa fat pad that is commonly seen postoperatively. Anterior interval scarring, which refers to fibrosis localized to the articular surface of the

infrapatellar fat pad connecting the patella to the anterior tibia, may be seen with a variety of knee surgeries including cruciate ligament reconstruction (Steadman et al. 2008). When severe enough, it may cause infrapatellar contracture syndrome

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with patella infera and limited patellar motion leading to chronic anterior knee pain and an extension deficit (Dragoo et al. 2012). MRI demonstrates the thick band of fibrous tissue between the patella and tibia (Fig. 46); however, the imag-

ing appearance may not correlate with the clinical signs and symptoms. The localized form of arthrofibrosis most commonly occurs following ACLR and may be referred to as “localized anterior arthrofibrosis,” “cyclops lesion,” or “ACL nodule” (Millett et al. 2001; Recht et al. 1995). A focal mass of dense fibrous tissue develops at the anterior margin of the tibial tunnel, usually attached to the graft, causing impingement between the graft and the roof of the intercondylar notch and blocking knee extension (Millett et  al. 2001). It is the second most common cause for loss of full knee extension following ACLR, having been reported in as many as 10% of postoperative knees (Papakonstantinou et  al. 2003). The tissue nodules usually appear intermediate signal on T1-weighted and proton density-weighted images making them difficult to distinguish from fluid, but intermediate to low signal on T2-weighted images with some lesions having regions of higher signal (Fig.  47) (Recht et  al. 1995; Bradley et  al. 2000). The lesions may be difficult to differentiate from intra-articular fat on fat-suppressed images. For ACLR patients with limited extension undergoing arthroscopy, MRI had an 85% sensitivity and specificity for detection of any size localized anterior arthrofibrosis lesion with 100% specificity for cyclops lesions >1  cm in at least one dimension (Bradley et  al. 2000). The etiology of localized arthrofibrosis is unknown and is likely multifactorial with

Fig. 46  Anterior interval scarring. Forty-year-old man with anterior knee pain, but full range of motion 14 months after ACLR. Sagittal T2W image shows a thick band of fibrous tissue along the articular border of the infrapatellar fat pad (arrow) connecting the inferior patella and anterior tibia. In some patients this scar tissue can interfere with patellar motion and cause infrapatellar contracture syndrome Fig. 47 Localized anterior arthrofibrosis (cyclops lesion). Focal fibrous tissue formation anterior to the ACLR graft (arrows) appearing as intermediate signal tissue on T2W (a) and PDW fat-suppressed (b) MR images may cause limitation and/or pain with knee extension

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Fig. 48  Two patients with patellar fracture through the graft harvest site following BTB ACLR. Patient 1: AP radiograph (a) 8 weeks after BTB autograft ACLR shows a fracture line initiated at the graft harvest site and extending superiorly and exiting the lateral patellar cortex

(arrows, a). Patient 2: Postoperative coronal PDW fat-­ suppressed FSE MR image through the patella shows a similarly oriented bright fracture line extend from the bone defect of the BTB autograft harvest site (arrow, b)

p­roposed causes including graft impingement, debris from drilling the tibial tunnel, remnants of the ACL stump or torn graft fibers (Millett et al. 2001). A “pseudocyclops” lesion consisting of anteriorly displaced fibers of a partially torn ACL graft may simulate localized anterior arthrofibrosis and should be considered if the lesion appears to contain graft fibers extending into the bone tunnels (Simpfendorfer et al. 2015) (Fig. 36).

(Emerson et  al. 2019). If the QT graft harvest extends too far proximal or lateral, it may injure vessels within the quadriceps resulting in a hematoma at the myotendinous junction, a finding that should not be mistaken for quadriceps tendon tear (Emerson et al. 2019). Donor site complications following hamstring tendon grafts include fluid collections at the donor site (Somanathan et al. 2019). However, failure of regeneration of a “neotendon” to replace the harvested tendon may result in hamstring muscle atrophy (Curtis et al. 2019).

8.4 Donor Site Abnormalities Donor site abnormalities are uncommon. Following BTB ACLR, patellar fractures, partial or complete tears of patellar tendon, and patellar entrapment syndrome have been reported (Ilaslan et  al. 2005). The reported incidence of postharvest patellar fractures and patellar tendon ruptures is less than 0.2% (Fig. 48) (Lee et al. 2008). Reported complications for QT harvest sites include myotendinous junction hematoma, quadriceps tendon tear and, if a bone plug was obtained, patellar fracture (Emerson et al. 2019). Very low patellar fracture rates similar to those for BTB grafts have been reported (Slone et al. 2015), while quadriceps tendon rupture is rare

9 Conclusion In this chapter we have reviewed the surgical techniques and imaging findings for the most commonly performed knee ligament surgeries. Imaging is a critical part of postoperative patient evaluation. Early imaging with radiographs may document hardware and bone tunnel placement. MRI is the modality of choice for evaluation of symptomatic patients since it can directly image the grafts, radiolucent hardware, and assess other joint structures. CT is especially useful in determining bone tunnel size and location prior to

The Postoperative Knee: Cruciate and Other Ligaments

revision surgery. CT-arthrography may be preferred for meniscal and cartilage evaluation in the setting of metallic hardware that significantly degrades MR images. A thorough understanding of the surgical techniques, the surgical procedure performed for each patient, and time-dependent change in graft appearance during healing is critical for proper image interpretation.

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The Postoperative Knee: Arthroplasty, Arthrodesis, Osteotomy Winnie A. Mar, Joseph Albert Karam, Michael D. Miller, and Mihra S. Taljanovic

Contents 1    Introduction 

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2    Normal Arthroplasty Appearance  2.1  Arthroplasty Types 

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3    Complications  3.1  Infection  3.2  Aseptic Loosening  3.3  Osteolysis  3.4  Polyethylene Wear  3.5  Instability  3.6  Fractures  3.7  Patellar Complications 

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W. A. Mar (*) Department of Radiology, University of Illinois Hospital and Health Sciences Center, Chicago, IL, USA e-mail: [email protected] J. A. Karam Department of Orthopedic Surgery, The University of Illinois at Chicago, Chicago, IL, USA e-mail: [email protected] M. D. Miller University Orthopedic Specialists, Tucson, AZ, USA e-mail: [email protected] M. S. Taljanovic Department of Radiology, University of New Mexico, Albuquerque, NM, USA e-mail: [email protected]

4    Knee Osteotomy  4.1  Blount Disease  4.2  Tibial Tuberosity Transfer 

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5    Knee Arthrodesis 

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6    Conclusion 

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References 

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Abstract

Imaging has an important role in the assessment of the painful total knee arthroplasty (TKA), as well as after arthrodesis and osteotomy. The knee is the most commonly replaced joint. It is important for radiologists and orthopedic surgeons to be familiar with the normal appearance as well as the most common causes of complication after TKA, which are infection, instability, aseptic loosening, and articular bearing surface wear. Infection and instability can occur at any time after TKA but are most commonly early complications. Loosening and polyethylene wear are the most common late complications. Other causes for pain after TKA that will be discussed are periprosthetic fracture and patellar complications. Arthrodesis is most commonly used for limb salvage after failed TKA but can also be used as a temporary measure to provide stability in patients with an infected TKA and antibiotic spacer. Osteotomy about the knee is used to

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_358, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 23 March 2023

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correct angular deformities of the knee, to temporize medial knee osteoarthritis, and to help correct patellar malalignment.

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The femoral component should be in 4–6° of valgus alignment on the frontal view. The mechanical axis is a line drawn from the femoral head center to the talar dome center on a standing radiograph. This has historically been the target for a neutral mechanical alignment; however, 1 Introduction some now believe restoring the patient’s normal anatomy of slight varus or valgus may be more Total knee arthroplasty (TKA) is the most com- appropriate for patient satisfaction and soft tissue mon joint replacement surgery. It is important to balancing. The anterior femoral flange is the recognize the normal appearance of TKA as well anterior lip of the femoral component and should as be familiar with the most commonly associ- be directly abutting the bone (Lesh et al. 2000). If ated complications. The most common indication there is space between the bone and the anterior for TKA is primary knee osteoarthritis, with less femoral flange, there may be a higher rate of common indications being inflammatory arthritis loosening and “overstuffing” the patellofemoral or post-traumatic osteoarthritis. Primary knee joint (an increase in the thickness of the patelloosteoarthritis is extremely common, particularly femoral joint), leading to problems with flexion. with rising rates of obesity. The United States has Anterior femoral notching adjacent to the antethe highest incidence of TKA with 235/100,000 rior flange has been shown to predispose to peripersons, with a mean utilization in Organisation prosthetic femoral fracture in biomechanical for Economic Co-operation and Development studies; however, although femoral notching is (OECD) countries of 150 cases per 100,000  in best avoided, a recent study showed no increased 2011 (Pabinger et al. 2015). The projected num- risk of fracture (Puranik et al. 2019). ber of primary TKA in the United States by 2030 The tibial component should be perpendicular is around 2–3.5 million procedures (Singh et al. to the long axis of the tibia on the AP view but 2019; Kurtz et al. 2007). may be in slight flexion (tibial slope), up to 3° on the lateral view depending on the type of implant and preservation or sacrifice of the PCL 2 Normal Arthroplasty (Taljanovic et  al. 2003). The size of the tibial Appearance component should match the size of the native tibia. A tibial component that is too large, particTotal knee arthroplasty (TKA) generally consists ularly with medial overhang, can predisposed to of a femoral bicondylar component and a tibial medial ligamentous pain (Nielsen et al. 2018). A component, which can be a nonmodular all-­ tibial component that is too small may predispose polyethylene component, but in modern designs to subsidence or settling of the component into is usually composed of a metal tibial tray and a the bone as the implant fails to engage the stronpolyethylene insert. Patellar resurfacing with a ger bone at the periphery of the tibia (Manaster patellar polyethylene component can also be 1995). used, although some surgeons may leave the patella unresurfaced. Metal-backed polyethylene patellar components were found to have an 2.1 Arthroplasty Types increased incidence of loosening, osteolysis, and metallosis and are no longer used. Most knee There are different types of arthroplasty with the arthroplasties are cemented, although the use of most common being posterior cruciate ligament cementless implants has been on the rise in the (PCL)-retaining, PCL-substituting, varus–valgus United States. Stable lucencies 2  months). Early infections are thought to result from implantation due to endogenous skin flora or exogenous organisms introduced during surgery (Kapadia et  al. 2016). Late infections are most commonly a result of hematogenous seeding. The most common organisms are Staphylococcus aureus and Staphylococcus epidermidis. When there is concern for PJI, two nonspecific markers of inflammation, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), are obtained, which have a sensitivity of 75% and 88%, respectively (Berbari et al. 2010). If they are both negative, the workup for infection can usually be stopped. If one or both are positive, or if clinical suspicion is high despite normal serum markers, knee aspiration is indicated. Aspiration is usually performed after withholding antibiotics to decrease the rate of false-negative results. Diagnosing PJI is challenging, and criteria have been established by the musculoskeletal infection society (MSIS) and later modified by the international consensus meeting (ICM) in 2018 to

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enhance the diagnosis. Findings in favor of a chronic PJI include a high synovial fluid leukocyte count (>3000/mm3), a high neutrophil percentage (>80%), and positive cultures (Parvizi et al. 2018). The new ICM definition criteria also include the use of leukocyte esterase and alpha defensin. Leukocyte esterase dipstick test can be applied to the joint aspirate with a sensitivity of 90% and specificity of 97% in a recent meta-analysis (Wang et al. 2018). However, false positives may occur in other inflammatory pathologies such as inflammatory or crystalline arthropathy. Alpha defensin lateral flow assay, which tests for an antimicrobial peptide released from neutrophils, shows promise in diagnosing infection with a sensitivity and specificity of 97% and 96%, respectively (Gehrke et al. 2018). When PJI is suspected, radiographs as well as aspiration are indicated (Expert Panel on Musculoskeletal Imaging et  al. 2017). Radiographs are neither sensitive nor specific, as PJI can be present with normal radiographs. Other findings that can be seen are joint effusion, periprosthetic lucency, and periosteal reaction (Figs. 7 and 8). Soft tissue gas is highly specific for infection but uncommonly present (Fig.  9). However, a recent study showed that the presence of soft tissue gas 14 days after TKA is predictive of infection with a sensitivity of 54% and specificity of 99% (Li et  al. 2020). Importantly, changes between radiographs, such as increased periprosthetic lucency, are helpful. CT is also indicated and is more sensitive for lucencies and can also show fluid collections (Expert Panel on Musculoskeletal Imaging et  al. 2017). On CT, associated osteomyelitis in TKA infection will be seen as periosteal reaction and destructive/erosive bone changes (Taljanovic et  al. 2018) (Fig.  10). MRI currently plays a limited role in TKA evaluation secondary to cost and artifact from the metal prosthesis; however, metal artifact reduction techniques are improving (Expert Panel on Musculoskeletal Imaging et  al. 2017). MRI can be helpful to show abscess and extracapsular spread of infection (Fig. 11), as well as a hyperintense peripheral laminar appearance of the synovium which has a different appearance

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Fig. 7  TKA infection. Frontal (a) and lateral (b) radiographs of a cemented posterior stabilized (mobile-bearing left total knee arthroplasty showing knee joint effusion,

Fig. 8 Infected TKA. Frontal (a) and lateral (b) radiographs of a varus–valgus constrained right TKA show large periprosthetic lucency about the tibial component (white arrows) and periosteal reaction (black arrows)

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from the synovitis in aseptic loosening and polyethylene wear which is low to intermediate signal intensity on fluid-sensitive sequences and more dense (Fritz et al. 2015).

tibial periprosthetic lucency (white arrows), and lateral tibial periosteal reaction (black arrow))

b

Nuclear medicine studies may also be helpful. On a triple phase bone scan, there will be uptake on all three phases in an infected TKA.  Uptake only on the delayed phase is not specific for

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a

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e

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Fig. 9 (a–f—Images of the same knee). TKA infection after quadriceps tendon rupture and two repair attempts, with subsequent allograft reconstruction of the quadriceps mechanism and patella, complicated by infection with purulent drainage and thigh abscess, with subsequent arthrodesis and cured infection. Frontal (a) and lateral (b) radiographs showing cruciate sparing right total knee arthroplasty without patellar resurfacing and two tibial tuberosity screws related to quadriceps mechanism/patellar allograft reconstruction, with large knee joint effusion with soft tissue air (arrows) and intra-articular air related

c

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to infection. Frontal (c) and lateral (d) images of the right knee after arthroplasty explant, placement of antibiotic dissolvable calcium sulfate beads impregnated with antibiotics and static antibiotic cement spacer composed of methylmethacrylate impregnated with antibiotics with a retrograde intramedullary nail showing a satisfactory immediate postoperative appearance. Frontal (e) and lateral (f) radiographs of the right knee with arthrodesis using an intramedullary rod after antibiotic spacer and antibiotic bead removal following resolution of infection show a satisfactory appearance

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Fig. 10 (a–d—Images of the same knee). Soft tissue abscess and osteomyelitis. Coronal reformatted CT images (a, b) showing soft tissue abscess (arrowheads) extending to medial tibial component with adjacent periosteal reaction (arrows in b) related to osteomyelitis. Cultures showed Streptococcus agalactiae. Frontal (c)

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and lateral (d) radiographs after right total knee arthroplasty explant with placement of antibiotic beads and an articulating spacer made of a nonmodular all-­polyethylene tibial component and primary posterior stabilized metal femoral component. An intra-articular drain is present with intra-articular air

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Fig. 11 (a–d—Images of the same knee). TKA infection. Frontal (a) and lateral (b) views of a cemented cruciate-­ retaining right TKA with patellar resurfacing show a large knee joint effusion in this patient with infected TKA with Corynebacterium and Pseudomonas. The patellar component is also incidentally shifted superiorly. Axial STIR (c)

MR image shows a large knee joint effusion with lamellated and hyperintense appearance of the peripheral synovium (arrow). Sagittal T2 (d) nonfat saturated MR image shows a sinus tract at the anteromedial aspect of the knee (black arrow)

PJI. Increased radiotracer uptake is normal during the first year after TKA on the first two phases and can be physiologic for an indefinite period of time on the third phase (Love et  al. 2001). Tagged white blood cell indium 111 (In-111) scan at 20 h is very specific, with a negative scan highly spe-

cific for lack of infection. However, a positive scan is nonspecific as there is a high false-­positive rate with approximately 50% of asymptomatic implants showing increased uptake (Larikka et al. 2001). When performed in conjunction with sulfur colloid imaging, diagnostic accuracy improves.

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Most infected TKAs are treated with a two-­stage arthroplasty in contrast to other complications. Initially, the infected prosthesis is removed, and an antibiotic spacer is placed. The patient is then treated with IV antibiotics, typically for 6  weeks, although a recent multicenter randomized controlled trial showed more favorable outcomes with a longer antibiotic course of 12 weeks (Bernard et al. 2021). Subsequently after some delay, and documentation of lack of infection by joint aspiration, serum markers, and clinical evaluation, a new TKA will be implanted. Early PJI cases within 30 days may be treated with debridement and implant retention (Zahar and Sarungi 2021). Single-stage revision for knee PJI is more accepted outside the United States, but it has been witnessing increasing interest in North America in select patients, especially thanks to the decreased morbidity compared to two-stage revision (Pangaud et al. 2019).

2 mm is seen (Fig. 12). Small lucencies which are stable or smaller than 2 mm are generally not clinically significant and do not signify loosening. A continuous radiolucency around the baseplate and keel (stem) on multiple projections also indicates loosening. Definite loosening should only be called when there is new change in the positioning of the components (Fig. 13). The tibial baseplate is more commonly affected. Subsidence can also be seen with loosening, most commonly involving the tibial baseplate. Radiographs are not able to differentiate between infection and aseptic loosening which can appear similarly. Periprosthetic lucency on CT is also nonspecific; however, contrast-enhanced CT could also show periarticular fluid collections or significant proliferative synovitis which favor infection.

3.2 Aseptic Loosening

Osteolysis, or granulomatous particle disease, is a cell-mediated inflammatory response generally to polyethylene particles after arthroplasty. It occurs much less commonly after TKA than after hip arthroplasty as the polyethylene wear pattern in TKA results in larger, less biologically active

Aseptic loosening is the most common late cause of revision arthroplasty and comprised 39% of TKA revisions in a study by Sharkey et al. (2014). Generally, periprosthetic lucency wider than Fig. 12 Aseptic loosening. Frontal (a) and lateral (b) views of a cemented left TKA with patellar resurfacing show continuous radiolucency at the anterior femoral component, around the entire tibial implant (arrows), subsidence into varus of tibia, with reactive bone formation along medial tibia tray related to micromotion of the tibia, related to aseptic loosening

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3.3 Osteolysis

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The Postoperative Knee: Arthroplasty, Arthrodesis, Osteotomy Fig. 13 Aseptic loosening. Frontal (a) and lateral (b) views of a cemented left total knee arthroplasty with patellar resurfacing show periprosthetic lucency (arrows) about the entire tibial component where there is also subsidence into extension, related to aseptic loosening. The keel has migrated anteriorly, and sclerotic bone is seen deep to the posterior tibial tray with reactive bone formation posteriorly related to micromotion and subsidence

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particles. Smaller particles induce cytokine release, resulting in activation of osteoclasts and suppression of osteoblasts (Peters et  al. 1992). There is increased frequency of osteolysis in noncemented (6–30%) versus cemented (0–16%) TKA (Gupta et al. 2007). Backside wear of polyethylene adjacent to the tibial baseplate can also occur. Common locations for osteolysis include the periphery of the arthroplasty, femoral condyles at the collateral ligamentous attachment sites, and along access channels to the cancellous bone of the tibia (Figs.  14, 15, and 16) (Gupta et  al. 2007). Other locations include the medial and lateral aspects of the tibial baseplate and adjacent to the patellar component (Naudie et al. 2007). The cystic lucencies of osteolysis may predispose to pathologic fracture or loosening. CT and MRI are more sensitive for osteolysis, and CT can aid with preoperative planning (Expert Panel on Musculoskeletal Imaging et al. 2017; Li et al. 2016).

3.4 Polyethylene Wear Due to increased durability of polyethylene liners, polyethylene wear is no longer a common

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indication for a late revision surgery. However, malalignment and instability can accelerate polyethylene wear (Fraser et al. 2015). Polyethylene wear can be evaluated on radiographs by noting the shortest distance from each femoral condyle to the tibial tray; however, this distance can be affected by multiple factors including patient positioning, a nontangential radiograph, and ligamentous laxity. Additionally, some types of arthroplasties such as the Smith and Nephew JOURNEY II arthroplasty have an asymmetric-­ bearing geometry with the tibial insert having a concave medial and convex lateral shape, resulting in a normal appearance of medial tibiofemoral compartment asymmetric narrowing (Fig. 17) (Grieco et al. 2018; Iriuchishima and Ryu 2019).

3.5 Instability Instability after TKA occurs with abnormal displacement of the knee, and is the third most common complication, comprising 7.5% of revisions in one series (Sharkey et al. 2014). It is usually secondary to surgical error and poor prosthesis selection but can also be a result of ligamentous tear or attenuation. Dislocation is the worst form

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Fig. 14  Loosening, osteolysis, and instability. Frontal (a) and lateral (b) radiographs of a left cruciate-substituting noncemented (pretty sure this is cemented, just was a poor mantle of cement and the cement is dissipated because of gross movement) left TKA with patellar resurfacing show linear lucencies about the tibial component and to a lesser

Fig. 15  Osteolysis with medial femoral condyle fracture. Frontal and lateral views of the right knee show a cemented cruciate-retaining TKA with patellar resurfacing and large lucencies adjacent to the fractured medial femoral condyle (black arrow), near the attachment site of the medial collateral ligament, as well as at the lateral tibial baseplate (white arrow), around the tip of the tibial stem, and a smaller lucency deep to the medial tibial baseplate related to osteolysis

extent about the femoral component (arrows). The tibial component is also angulated, resulting in varus angulation of the knee. Cement and bone debris are seen (arrowheads) along with smaller amounts of cobalt chromium metal particles

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Fig. 16 Osteolysis. Frontal and lateral views of a noncemented left TKA with patellar resurfacing show a very large lucency about the tibial component extending into the proximal tibial diametaphysis (arrows) with abnormal angulation and subsidence of the tibial base plate. Smaller lucency is seen about the medial and anterior femoral component (arrowheads)

of instability but is less common with an incidence of 0.15–0.5% (Fig.  18) (Mulcahy and Chew 2014). Instability may occur in the coronal plane (medial or lateral) or sagittal plane. Valgus, or medial instability, is suggested when the medial joint space appears wider, most commonly secondary to a preoperative valgus knee with tight lateral structures with insufficient release. The medial collateral ligament (MCL) may also be deficient. The main types of sagittal instability are extension, flexion, and global instability (Figs.  19, 20, 21, 22, and 23). There are two types of extension instability, asymmetric and symmetric. Asymmetric instability is more common and results from incomplete correction of a varus or valgus deformity. Symmetric extension instability is a result of excessive femoral bone removal. Flexion instability can result from an undersized femoral component or steep tibial slope. Flexion instability may also result from PCL insufficiency after a cruciate-retaining arthroplasty, which is treated by revision to a posterior stabilized arthroplasty. Global instability may result from polyethylene wear or inadequate thickness of the polyethylene liner, resulting in Fig. 17  Smith and Nephew JOURNEY II asymmetric-­ laxity developing of the surrounding soft tissues. bearing geometry. Frontal view of a cemented left TKA Midflexion instability is an increasingly recogshows the normal appearance of the Smith and Nephew nized entity, part of sagittal instability, and is JOURNEY II arthroplasty which has an asymmetric medial polyethylene liner resulting in the appearance of medial compartment narrowing

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Fig. 18  TKA displacement. Frontal (a) and lateral (b) radiographs of right distal femoral replacement hinge total knee arthroplasty with patellar resurfacing show anterior and lateral displacement of the femoral component in a Zimmer Biomet Compress® Implant. The superior cap is porous coated and covered with hydroxyapatite. It gains permanent attachment through axial loading of the cap

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against bone. Because of lack of ingrowth, there was enough stress and motion to break the traction bar, resulting in displacement of the remaining distal femoral replacement as well as rotation about the rotating platform as the femur is rotated 180° on the lateral radiograph

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Fig. 19 Instability (a–d images of the same knee). Frontal (a) and lateral (b) radiographs of a cemented left TKA without patellar resurfacing show medial and posterior displacement of the tibia with respect to the femur related to posterior dislocation of the polyethylene cam post. The patient had instability in both flexion and exten-

sion, coronal and sagittal planes, but worse in flexion, resulting in the cam post dislocation during extreme flexion and rotation. Frontal (c) and lateral (d) radiographs of left TKA revision show improved alignment after placement of a varus–valgus constrained polyethylene in the primary posterior stabilized implant

thought to be mostly related to elevation of the joint line. Radiographs are the main modality for the evaluation of instability and can be performed

in weight-bearing, full extension, full flexion, or with stress maneuvers (Expert Panel on Musculoskeletal Imaging et al. 2017).

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Fig. 20 Instability (a–d images of the same knee). Frontal (a) and lateral (b) radiographs of the posterior stabilized right TKA with patellar resurfacing show severe posterior and moderate lateral tibial displacement, and angulation and lateral patellar dislocation. There was a dehiscence of the medial retinaculum closure, with lateral patellar dislocation. The cam post dislocated out of the

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Fig. 21 (a–d Images of the same knee). Instability. Frontal (a) and lateral (b) radiographs of a noncemented cruciate-retaining left TKA with patellar resurfacing show moderate medial displacement and lateral angulation of the tibia, resulting in valgus angulation. This was second-

3.6 Fractures

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box with flexion and rotation secondary to flexion laxity and no anterior constraint. Frontal (c) and lateral (d) radiographs show improved alignment after revision to a hinged right TKA, due to MCL incompetence. The femur is also externally rotated slightly more than usual to facilitate patellar tracking

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ary to global laxity in flexion and extension likely secondary to ligamentous and soft tissue stretching over time and polyethylene thickness undersizing. Frontal (c) and lateral (d) radiographs after revision to a varus–valgus constrained TKA show improved alignment

prosthesis, type 2 displaced fracture and intact prosthesis, and type 3 nondisplaced or displaced Fractures after TKA are uncommon and most fracture with nonintact prosthesis (Rorabeck and commonly involve the distal femur after a low-­ Taylor 1999). A nonintact prosthesis is defined as energy injury (Fig.  24) (Mulcahy and Chew loosening or other failure including instability 2014). The Lewis and Rorabeck classification for and polyethylene wear. Tibial periprosthetic fracsupracondylar femoral fractures after TKA is as tures after TKA are uncommon and are classified follows: type 1 nondisplaced fracture and intact according to the Felix classification (Felix et al.

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Fig. 22  Displaced polyethylene liner. Lateral radiograph (a) and sagittal reformatted CT using metal artifact reduction algorithm (b) of a cruciate-substituting cemented

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Fig. 23  Displaced TKA fragment. AP (a) and lateral (b) views of the left knee after semiconstrained TKA with patellar resurfacing show a displaced linear metallic fragment in the anterior intercondylar notch (arrows), corresponding to a broken screw used to secure the polyethylene to the baseplate, along with polyethylene loosening. There is also anterior translation of the tibia

TKA with patellar resurfacing show an anteriorly displaced polyethylene liner (arrows)

1997). Type 1 involves the tibial plateau, type 2 involves the tibial stem, type 3 is distal to the tibial stem, and type 4 involves the tibial tuberosity. Additional modifiers include a well-fixed prosthesis (a), loose prosthesis (b), or intraoperative fracture (c). Patellar fractures are relatively common and are more often vertical at the lateral facet of the patella and do not affect the extensor mechanism (Mulcahy and Chew 2014). They can occur in the patella both with and without resurfacing. Transverse patellar fractures are less common. In a nonresurfaced patella, internal fixation should be considered. In a resurfaced patella, surgical treatment is needed for patellar fractures with extensor mechanism disruption or patellar component loosening, with conservative management if neither is present as the outcome of surgical management in these cases is very guarded (Putman et al. 2019).

3.7 Patellar Complications Anterior knee pain from patellofemoral syndrome is common after TKA, seen in up to 20%

The Postoperative Knee: Arthroplasty, Arthrodesis, Osteotomy Fig. 24 TKA periprosthetic fracture. AP (a) and lateral (b) views of a cementless left cruciate sparing TKA without patellar resurfacing show a comminuted distal femoral periprosthetic fracture with moderate dorsal angulation of the principal distal fragment, and an intact prosthesis, consistent with a Lewis and Rorabeck Type II fracture. Joint bodies are also seen at the posterior aspect of the knee, likely migrated into the Baker’s cyst. There is a large knee joint effusion

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of patients (Park et  al. 2016). This most commonly results from internal rotation of the tibial and/or femoral components with resultant patellar maltracking, but can also be a result of overstuffing of the patellofemoral joint, patellar osteonecrosis, or ligamentous or tendinous dysfunction (Putman et al. 2019). Surgical treatment should only be performed in patients with a clearly identifiable source of pain. Most currently used patellar components are completely polyethylene, with multiple small pegs (Mulcahy and Chew 2014). Metal-backed components, used in the 1980s, were found to result in increased complications. Recent cementless metal-backed patellar designs have however demonstrated very promising short- and mid-term survivorship (Grau et al. 2021). Additionally, cemented patellar components with multiple pegs result in less complications than those with one larger peg. Obese patients have a 6.3-fold increased association with loosening and 1.7-fold increased association with fracture (Meding et  al. 2008). There is also increased risk of fracture and ­osteonecrosis with resurfacing of a thin patella. A remnant patellar bone thickness of at least 12 mm is recommended (Hamilton et al. 2017). Radiographs, especially the axial, patellofemoral view, can be used to evaluate for patellar

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fracture or patellar component loosening (Fig. 25). CT is more sensitive for patellar component loosening. In patellar component displacement, the component is often displaced into the infrapatellar fat pad (Mulcahy and Chew 2014). Osteonecrosis will appear as increased sclerosis and possible fragmentation or fracture of the patella. When fracture or loosening of the patellar component is present, surgical management is indicated. Otherwise, treatment can be conservative (Putman et al. 2019). Alignment of the patella on an axial view should also be evaluated on radiographs. The lateral portion of the patella can impinge on the trochlea, resulting in pain (Fig. 26) (Cercek et al. 2011). Lateral facetectomy with lateral retinaculum release may be helpful in this scenario. CT can be used to evaluate for femoral and tibial component internal rotation. Internal rotation of the femoral component is assessed by measuring the posterior condylar angle, which should be close to 0°. This is performed by drawing a line at the transepicondylar axis (connecting the lateral epicondylar prominence and medial sulcus of the medial femoral condyle) and another line parallel to the femoral condyles (Fig. 27) (Berger et al. 1998). Tibial component rotation is measured as the angle between the

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370 Fig. 25 Patellar component displaced. Lateral (a) and patellofemoral (b) radiographs after right TKA with patellar resurfacing show that the polyethylene patellar component is not seen on the lateral view (a) and on the patellofemoral view (b) is located adjacent to the medial aspect of the patella (arrow)

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Fig. 26  Lateral patellar impingement on the trochlea. Patellofemoral view in a patient with pain after right total knee arthroplasty with patellar resurfacing shows severe lateral patellar tilt with osseous erosion and sclerosis of the lateral patella (arrow). Medial femoral bone avulsion is also present

tibial component axis and the tibial tuberosity axis. The tibial component axis is measured with a line perpendicular to the transverse axis of the tibial component, paralleling the posterior tibial polyethylene. The tibial tuberosity angle is drawn from a line connecting the tibial geometric center and the tibial tuberosity. An angle between these two lines of 18° is considered neutral tibial com-

Fig. 27  Measurement of femoral condylar internal rotation. The posterior condylar angle should be close to 0°. This is the angle between the transepicondylar axis (TEA) (dotted line) connecting the lateral epicondylar prominence and medial sulcus of the medial femoral condyle, and a line parallel to the posterior condyles (solid line)

ponent rotation. Combined internal rotation of both the tibial and femoral component is thought to be proportional to the degree of patellofemoral instability. The combined internal rotation is obtained by adding the femoral component and the tibial component internal rotational angle.

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One study showed that 1–4° of combined internal rotation was associated with lateral patellar tracking and tilting, with combined internal rotation of 3–8° correlating with patellar subluxation and large combined internal rotation of 7–17° associated with early patellar dislocation or late prosthesis failure (Berger et al. 1998). Patellar and quadriceps tendon ruptures are less common than patellar fractures with an incidence of 0.17% and 0.1%, respectively ­ (Vajapey et  al. 2019). Extensor tendon rupture results in a deficit of active knee extension and severely impairs knee function (Bonnin et  al. 2016). Ultrasound can be used to evaluate the integrity and quality of the extensor mechanism. Better outcomes have been obtained with graft reconstruction of tendons than primary repair. In patellar clunk syndrome, patients present with painful catching of the patella at 30–45° of flexion, secondary to nodular scarring at the posterior aspect of the quadriceps tendon insertion that gets entrapped within the intercondylar box of a PCL-substituting arthroplasty (Putman et al. 2019). This occurs less commonly as newer femoral implant designs have a box that is more posteriorly located and rounded anterior edge to prevent the nodule from being caught in the box. Although the diagnosis is clinical, on MRI, nodular scarring can be seen at the junction of the superior patella articular surface and quadriceps tendon (Heyse et  al. 2012). Arthroscopic resection of the nodule can be performed and yields excellent outcomes (Costanzo et al. 2014).

4 Knee Osteotomy Knee osteotomy can be performed for a variety of causes. Some of the main indications for knee osteotomy will be discussed, including medial compartment knee osteoarthritis, Blount disease, and tibial tuberosity transfer for patellar tracking disorders. High tibial osteotomy (HTO) can be used as a temporizing measure to treat medial compartment knee osteoarthritis by realigning the knee to distribute forces on the less affected lateral compartment, and it is generally used in younger or

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more active patients (Wright et al. 2005). It has been shown to promote repair of the articular cartilage, by offloading the medial compartment (Jung et al. 2014). Within the last 10 years, HTO is commonly performed in conjunction with cartilage repair, meniscal transplantation, and ligament reconstruction (Gao et  al. 2019). HTO is used uncommonly to solely correct malalignment (Wright et al. 2005). Compared to TKA, it is less costly and carries less risk of infection and venous thromboembolism. The most commonly used osteotomy procedure for knee osteoarthritis is medial wedge opening osteotomy HTO (Fig. 28). Patient selection is important and the ideal patient for HTO is a nonsmoker, has a low BMI, stable knee, and focal medial joint line pain (Gao et  al. 2019). Arthroscopic indications are medial compartment grade 2 or 3 chondral loss, with a varus deformity and an intact lateral compartment. Contraindications include rheumatoid arthritis or greater than 20° varus angulation. A lateral closed wedge tibial osteotomy can be performed if ­cartilage loss is posteromedial, and there is associated ACL deficiency, as this procedure decreases the tibial slope (Noyes et  al. 2000). Distal femoral osteotomy is uncommonly performed but could be considered in a younger active patient with valgus deformity in or near extension and isolated lateral compartment degenerative changes or patellofemoral symptoms (Fig. 29) (Gao et al. 2019). Tibial condylar valgus osteotomy (TCVO) can be used in patients with more advanced knee osteoarthritis, varus– valgus instability, and more severe varus deformity, compared to high tibial osteotomy (Higuchi et al. 2019). Potential complications after HTO include nonunion, delayed union, infection, hardware failure, or loss of correction (Kobayashi et  al. 2017). Lateral hinge fractures at the lateral tibia predispose to all of the above complications apart from infection. Lateral hinge fractures are classified by their relationship to the proximal tibiofibular joint, with a type 1 fracture proximal or within the proximal tibiofibular joint, type 2 fracture at the distal portion of the proximal tibiofibular joint, and a type 3 fracture extending

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372 Fig. 28  High tibial osteotomy for medial compartment osteoarthritis. Frontal (a) and lateral (b) radiographs of the right knee show anteromedial plate and screws transfixing a high tibial osteotomy. Interference screw in the distal femur is also seen related to prior anterior cruciate ligament reconstruction

Fig. 29  Distal femoral osteotomy for genu valgus. Frontal (a) and lateral (b) radiographs of the left knee show a medial closing wedge distal femoral osteotomy transfixed by lateral plate and multiple screws. Chronic healed post-traumatic deformity of the proximal tibia is also seen

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into the lateral tibial plateau (Takeuchi et  al. 2012). Type 1 fractures generally remain stable and with a low likelihood of complication. Radiographs are generally sufficient to evaluate

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for healing or complication after HTO; however, CT is helpful to evaluate healing of the anterior flange and to better delineate the amount of healing of the posterior cortex (Kobayashi et  al.

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2017). CT may be considered for patients with prolonged knee pain to evaluate posterior cortex healing, or those that need early removal of hardware due to infection, to determine when weightbearing can occur.

4.1 Blount Disease Blount disease is a developmental abnormality of the posteromedial tibia, resulting in genu varum seen predominately in obese children and adolescents (Birch 2013). The treatment for Blount disease includes guided growth with hemiepiphysiodesis (Fig.  30), and high tibial osteotomy for acute correction (Figs.  31 and 32), versus gradual correction in conjunction with external fixation (Sabharwal 2009). The goal of high tibial osteotomy is full to overcorrection in the infantile form and full correction in the adolescent form of the tibial varus, flexion, and internal rotation deformity (Birch 2013). The use of acute correction with proxi-

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Fig. 30  A 12-year-old girl with adolescent Blount disease and lateral proximal tibial hemiepiphysiodesis. Frontal (a) and lateral (b) radiographs of the right tibia and fibula show medial proximal tibial deformity second-

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mal tibial osteotomy has been the treatment of choice; however, recently the use of osteotomy has decreased due to neurovascular injury and inability to address associated limb shortening (Sabharwal 2015). It is still sometimes performed with proximal tibial inverted U osteotomy. Medial tibial plateau elevation for epiphyseal deformity in early onset Blount disease remains controversial but can be performed with a metaphyseal osteotomy that extends into the intercondylar area, with grafting of the space beneath the articular fragment (Birch 2013; Sabharwal 2015).

4.2 Tibial Tuberosity Transfer Patellofemoral tracking disorders are a common cause of anterior knee pain, particularly in adolescents and young adult patients (Middleton et al. 2019). Although commonly managed conservatively, in some patients failing conservative management, surgical treatment is

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ary to Blount disease. There has been placement of lateral plate and two screws, one above (into the epiphysis) and one below (into the metaphysis) the lateral proximal tibial physis for hemiepiphysiodesis/guided growth

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Fig. 31  A 12-year-old boy with adolescent Blount disease and high tibial osteotomy. Frontal view of the right knee (a) shows tibia vara and deformity of the proximal medial tibial metaphysis related to Blount disease. Three months after high tibial osteotomy, frontal (b) and lateral

(c) radiographs of the right knee show improved alignment with partial healing of a proximal tibial osteotomy transfixed by anterolateral plate and screws. There is also a healed proximal fibular osteotomy

indicated. This can include soft tissue procedures such as reconstruction of the medial patellofemoral ligament or release of the lateral retinaculum, and/or bony interventions, namely tibial tuberosity osteotomy, especially if an increase in the tibial tuberosity–trochlear groove (TT-TG) distance is identified (Thompson and Metcalfe 2019). The latter is best assessed on CT or MRI by measuring the distance between two lines perpendicular to the posterior condylar axis, the first bisecting the tibial tubercle and the second bisecting the femoral trochlear groove. A value greater than 2 cm is considered abnormal and generally warrants surgical correction with a tibial tuberosity osteotomy. The Maquet technique moves the tibial tuberosity anteriorly and reduces the patellofemoral joint load. This technique is uncommonly used currently however due to higher

complication rates (Grimm et  al. 2018). The Elmslie Trillat procedure involves isolated medialization of the tibial tubercle. This procedure is indicated for patients with patellar instability or lateral maltracking but should not be performed in patients with degenerative change. The Fulkerson osteotomy combines elements from the Maquet and Elmslie Trillat osteotomies and involves an anteromedial transfer of the tibial tubercle (Fig.  33). It is indicated for patients with patellar maltracking or instability and can also be performed on patients with patellofemoral arthritis. Further modifications of these techniques have also been reported. Anteromedialization of the tibial tubercle improves patellar tracking and helps to realign the extensor mechanism. Tibial tuberosity distalization can also be performed to correct patella alta, typically in conjunction with

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Fig. 33  Tibial tuberosity transfer. Lateral radiograph of the knee immediately after Fulkerson tibial tuberosity transfer transfixed with two screws

Fig. 32  A 4-year-old girl with bilateral infantile Blount disease and proximal tibial osteotomy. Frontal view of the bilateral tibia and fibula shows healed bilateral proximal tibial osteotomies transfixed by three staples with full correction on the right and overcorrection on the left. There are also healed bilateral proximal to mid-fibular shaft osteotomies

medial patellofemoral ligament reconstruction (Thompson and Metcalfe 2019). Complications of tibial tuberosity osteotomies are uncommon, with a 1–3% rate of tibial fracture and 1% rate of nonunion (Grimm et al. 2018; Johnson et al. 2017). Screws may also result in pain and need to be removed. Instability recurs in approximately 5% of patients.

5 Knee Arthrodesis Knee arthrodesis is currently performed primarily for limb salvage after failed TKA, in severe joint destruction after trauma and in cases of

tumor resection (Somayaji et  al. 2008). It can also be performed in patients with infected TKA as a temporizing measure to provide additional stability for weight-bearing (Fig. 34) (Wood and Conway 2015). Arthrodesis can be performed with intramedullary nailing, internal fixation with compression plates, or external fixation (Figs.  9 and 35) (MacDonald et  al. 2006). Contraindications include active infection, bilateral involvement, and ipsilateral THA. It carries a risk of complications ranging from 20% to 80%, including nonunion, infection, and limb length discrepancy (Wood and Conway 2015). Nonunion can be treated by bone grafting and plating. In cases of infection, the rod should be removed, which can be technically challenging in cases of good union. Limb length discrepancy should be treated initially using a shoe lift, with limb lengthening surgery in cases of failed conservative treatment.

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Fig. 34 (a–e—Images of the same knee). Right total knee temporary arthrodesis with antibiotic spacer and fracture. Frontal (a) and lateral (b) radiographs of a right knee arthrodesis with intramedullary antibiotic-coated pin and antibiotic spacer placement with distal femoral com-

minuted fracture. Frontal (c) and lateral (d) radiographs of the right knee after distal femoral resection, and placement of static antibiotic spacer and antibiotic cement. Axial (e) CT image showing fluid related to abscess about the static antibiotic spacer

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Fig. 35  Knee arthrodesis secondary to TKA infection. Frontal radiographs of the right hip, right knee, right tibia, and lateral radiograph of the right knee show an anterograde

intramedullary nail with one proximal and one distal interlocking screw transfixing the right knee joint. Shrapnel is also seen in the soft tissue

6 Conclusion

the need for arthroplasty. Postoperative radiographs aid in monitoring for bony union and deformity correction. Proximal tibial osteotomies can also be used to address infantile or adolescent Blount’s disease, with hemiepiphysiodesis as a potential alternative for the latter. Patients with patellar instability can benefit from tibial tuberosity transfer especially if they have an underlying anatomical predisposition identified by an increased TT-TG distance on CT or MRI. Knee arthrodesis is nowadays rarely seen; it can be performed with a variety of constructs and represents the ultimate salvage treatment short of above knee amputation in recurrent knee PJI or severe lower extremity trauma.

In conclusion, TKA procedures are exponentially growing so the radiologist and orthopedic surgeon are reviewing an increasing number of TKA imaging studies. With one in five patients being dissatisfied with the procedure, a critical analysis of imaging is essential to identify radiographic evidence of failed arthroplasty. Knowledge of a normal appearance of total knee replacement in its different varieties is essential. A comprehensive review evaluating component positioning and scrutinizing for radiolucent lines around the implants, displacement on serial imaging, periosteal reaction, osteolysis, or signs of instability helps identify potential causes for arthroplasty failure and guide treatment accordingly, including revision knee replacement surgery. Osteotomy at the knee, whether at the distal femur or proximal tibia, can help address varus or valgus deformities and/or early unicompartmental osteoarthritis in the young patient and thus delay

References American Academy of Orthopaedic Surgeons (2019) Diagnosis and prevention of periprosthetic joint infections. https://www.aaos.org/quality/quality-­programs/

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The Postoperative Knee: Arthroplasty, Arthrodesis, Osteotomy Kahlenberg CA, Nwachukwu BU, McLawhorn AS et  al (2018) Patient satisfaction after total knee replacement: a systematic review. HSS J 14:192–201. https:// doi.org/10.1007/s11420-­018-­9614-­8 Kapadia BH, Berg RA, Daley JA et al (2016) Periprosthetic joint infection. Lancet 387:386–394. https://doi. org/10.1016/S0140-­6736(14)61798-­0 Kobayashi H, Akamatsu Y, Kumagai K et  al (2017) Radiographic and computed tomographic evaluation of bone union after medial opening wedge high tibial osteotomy with filling gap. Knee 24:1108–1117. https://doi.org/10.1016/j.knee.2017.06.002 Kurtz S, Ong K, Lau E et al (2007) Projections of primary and revision hip and knee arthroplasty in the United States from 2005 to 2030. J Bone Joint Surg Am 89:780–785. https://doi.org/10.2106/JBJS.F.00222 Larikka MJ, Ahonen AK, Junila JA et al (2001) Improved method for detecting knee replacement infections based on extended combined 99mTc-white blood cell/ bone imaging. Nucl Med Commun 22:1145–1150. https://doi.org/10.1097/00006231-­200110000-­00015 Lesh ML, Schneider DJ, Deol G et  al (2000) The consequences of anterior femoral notching in total knee arthroplasty. A biomechanical study. J Bone Joint Surg Am 82:1096–1101. https://doi. org/10.2106/00004623-­200008000-­00005 Li AE, Sneag DB, Greditzer HG et al (2016) Total knee arthroplasty: diagnostic accuracy of patterns of synovitis at MR imaging. Radiology 281:499–506. https:// doi.org/10.1148/radiol.2016152828 Li N, Kagan R, Hanrahan CJ, Hansford BG (2020) Radiographic evidence of soft-tissue gas 14 days after total knee arthroplasty is predictive of early prosthetic joint infection. AJR Am J Roentgenol 214:171–176. https://doi.org/10.2214/AJR.19.21702 Love C, Tomas MB, Marwin SE et  al (2001) Role of nuclear medicine in diagnosis of the infected joint replacement. Radiographics 21:1229–1238. https:// doi.org/10.1148/radiographics.21.5.g01se191229 MacDonald JH, Agarwal S, Lorei MP et al (2006) Knee arthrodesis. J Am Acad Orthop Surg 14:154–163. https://doi.org/10.5435/00124635-­200603000-­00006 Manaster BJ (1995) Total knee arthroplasty: postoperative radiologic findings. AJR Am J Roentgenol 165:899– 904. https://doi.org/10.2214/ajr.165.4.7676989 Math KR, Zaidi SF, Petchprapa C, Harwin SF (2006) Imaging of total knee arthroplasty. Semin Musculoskelet Radiol 10:47–63. https://doi. org/10.1055/s-­2006-­934216 Meding JB, Fish MD, Berend ME et al (2008) Predicting patellar failure after total knee arthroplasty. Clin Orthop Relat Res 466:2769–2774. https://doi. org/10.1007/s11999-­008-­0417-­y Middleton KK, Gruber S, Shubin Stein BE (2019) Why and where to move the tibial tubercle: indications and techniques for tibial tubercle osteotomy. Sports Med Arthrosc Rev 27:154–160. https://doi.org/10.1097/ JSA.0000000000000270 Mulcahy H, Chew FS (2013) Current concepts in knee replacement: features and imaging assessment. AJR

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Patellar and Quadriceps Mechanism: Clinical, Imaging, and Surgical Considerations Breann K. Tisano, Jay P. Shah, and Avneesh Chhabra

Contents 1    Patellar and Quadriceps Mechanism Anatomy  1.1  Embryology  1.2  Anatomy  1.3  Soft Tissue Restraints  1.4  Biomechanics 

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2    Clinical Findings 

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3    Imaging  3.1  Radiography 

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4    Pathologic Conditions, Imaging, and Treatment  4.1  Patellofemoral Pain Syndrome  4.2  Patellar Instability  4.3  Symptomatic Bipartite Patella  4.4  Osteochondritis Dissecans/Chondral Defects 

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B. K. Tisano · J. P. Shah Department of Orthopedic Surgery, UT Southwestern Medical Center, Dallas, TX, USA A. Chhabra (*) Department of Orthopedic Surgery, UT Southwestern Medical Center, Dallas, TX, USA Department of Radiology, UT Southwestern Medical Center, Dallas, TX, USA e-mail: [email protected]

4.5  Q  uadriceps and Patellar Tendinopathy and Rupture  4.6  Osgood–Schlatter Disease  4.7  Sinding-Larsen–Johannsen Disease  4.8  Post-op Failure and Iatrogenic Conditions 

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References 

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Abstract

Patellofemoral disorders and extensor mechanism dysfunction are common causes of knee pain and lifelong disability if untreated. All ages including young are frequently affected due to such developmental aberrations and acquired disorders of anterior compartment of the knee joint. This chapter reviews normal patellofemoral and extensor compartment anatomy, its pathologies and related clinical symptoms and syndromes. The work is further enriched with prudent discussion of imaging strategies, measurement criteria, and treatment approaches in this domain with illustrative case examples.

1 Patellar and Quadriceps Mechanism Anatomy Patellofemoral articulation, along with the attached quadriceps and patellar tendons, forms the extensor mechanism of the knee. Patella is the largest sesamoid bone in the body that increases biome-

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_359, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 04 March 2023

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chanical pull of the quadriceps through the aforementioned tendons to extend the knee (Tecklenburg et al. 2006; Flandry and Hommel 2011).

1.1 Embryology In embryonic development, the patella first appears around 14 weeks of gestation as a cartilaginous mass with symmetric medial and lateral facets (Fox et al. 2012; Gray and Gardner 1950). With continued development, the lateral facet becomes more prominent so that, by 23 weeks of gestation, the fetal patella displays facet asymmetry consistent with the adult morphology (Ogden 1984). The patella remains largely cartilaginous until ossification starts between 4 and 6  years old. Multifocal ossification centers appear in the central patella. Though these quickly coalesce, rapid growth may lead to an a

irregular granular appearance of the bone in childhood (Ogden 1984). Accessory ossification centers may form at the peripheral margins, with subsequent failure to fuse leading to the presence of a bipartite patella in 2–6% of population (Matic and Flanigan 2015). The Saupe classification is used to describe the location of these persistent ossification centers (Saupe 1921). Type III, located at the superolateral pole, is the most common, seen in 75% of those with bipartite morphology (Fig. 1). Type II affects the lateral margin in 20% and type I, at the inferior pole, is the least common, representing only 5% (H S 1943).

1.2 Anatomy The patella is contained within the retinacular layer of the extensor mechanism (Flandry and

b

c

Fig. 1  Saupe type III bipartite patella on X-rays (a, b) and axial PDW (proton density-weighted) MR image (c)

Patellar and Quadriceps Mechanism: Clinical, Imaging, and Surgical Considerations

Hommel 2011). Typically, patellar dimensions are larger in males than in females but average dimensions for the population at large for height, width, and thickness are 41.7 mm, 43.4 mm, and 19.2 mm, respectively (Jain et al. 2019). The anterior surface is slightly convex, with a rough surface proximally for the quadriceps tendon attachment and an apex at the distal third for the origin of the patella tendon (Fox et al. 2012). The inferior surface has a concavity defined by a midline bony ridge which fits into the trochlear groove. The distal pole, representing about 25% of the inferior surface, is nonarticular and contains vascular channels carrying blood supply from the infrapatellar fat pad (Tecklenburg et al. 2006). The articular portion of the patella consists of two major facets—a lateral and a medial, and uncommonly medial most, aka “odd facet” (Tecklenburg et al. 2006). Articular cartilage on inferior surface is thickest centrally, averaging 4–5  mm (Fox et  al. 2012). The lateral facet is usually larger than the medial, and its superior, middle, and inferior subfacets sequentially contact the lateral femoral condyle through knee flexion (Fox et al. 2012; Kwak et al. 1997). The Wiberg classification defines patellar morphology by the relative size of the facets, with a smaller medial facet being the most common (Table  1) (Wibeeg 1941). The medial facet is separated from the odd facet on its medial border by a secondary ridge. The odd facet articulates with the lateral border of the medial femoral condyle in deep knee flexion (Tecklenburg et  al. 2006; Fox et al. 2012; Kwak et al. 1997). The femoral portion of the articulation consists of the trochlear groove, a sulcus where the medial and lateral femoral condyles meet anteriorly. The trochlea is shallow proximally, only Table 1  Wiberg classification of patellar morphology Proportion of Type population (%) 1 10 2

65

3

25

Definition Equal medial and lateral facets Medial facet smaller than lateral Medial facet smaller and convex, rather than concave

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Fig. 2 Axial fsPDW (fat-suppressed proton density-­ weighted) image. Normal cartilage thickness and signal intensity of patellar and femoral trochlear facets (arrows)

about 5.2  mm in depth, and becomes deeper moving distally toward the femorotibial joint (Fox et  al. 2012). The articular cartilage of the trochlea averages 3–4  mm in thickness and is more robust laterally (Fig.  2) (Fox et  al. 2012). The lateral side is also larger, extending more proximally and projecting further anteriorly (Jain et  al. 2019). This more prominent lateral trochlear ridge offers a bony restraint to the patella translation when it articulates around 45° of flexion (Flandry and Hommel 2011). Vascular supply to the patella branches from a rich anastomosis originating from the genicular arteries. Extraosseous supply comes from a prepatellar vascular network with almost no direct supply from the adjacent tendons or retinaculum (Wang et  al. 2019). A proportion of the blood supply originates from the infrapatellar fat pad. Intraosseous vascularity is derived from nutrient arteries that penetrate through the mid-anterior cortex and through the distal inferior pole (Crock 1962).

1.3 Soft Tissue Restraints The quadriceps is constituted by four muscles, rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The vastus intermedius inserts directly onto the superior pole of the

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patella while the remaining three muscles coalesce forming an aponeurosis as the quadriceps tendon that becomes confluent with the retinacular layer (Flandry and Hommel 2011). Vastus medialis inserts at a 50° angle off the axis of the femur, and the vastus medialis obliquus (VMO) originates off the adductor tubercle and inserts into the retinacular layer at a 65° angle (Flandry and Hommel 2011). The retinacular layer is the intermediate of the three soft tissue layers of the extensor mechanism. The superficial or arciform layer is a thin peritendinous sheath extending from the sartorial and biceps fascia (Flandry and Hommel 2011). The retinaculum comprises the anterior third of

the knee capsule and also contains the patellofemoral ligaments (Fig. 3). This layer continues over the patella and continues into the superficial patellar tendon and tibial periosteum (Flandry and Hommel 2011). The patellar tendon originates off the inferior pole of the patella and inserts onto the tibial tubercle. Of note, in the growing child, the tibial tubercle appears as a secondary ossification by 11–12  years of age. The apophysis then expands proximally and finally fuses distally (Ogden 1984). The soft tissue stabilizers of the patella are found as distinct thickenings of the retinacular layer. The most clinically discussed of these is the medial patellofemoral ligament (MPFL),

a

b

Fig. 3 (a) Axial PDW image. Normal medial and lateral retinacula (arrows). MPFL is the thickest portion on the medial side. (b, c) Axial PDW images. (b) Normal medial

c

patellomeniscal ligament (arrow). (c) Normal medial patellotibial ligament (arrow)

Patellar and Quadriceps Mechanism: Clinical, Imaging, and Surgical Considerations

which is the main structure that provides medial static stability. Biomechanical cadaver studies quantified the MPFL contribution to represent 53–60% of the force to resist lateral patellar displacement at 20° of knee flexion (Conlan et  al. 1993; Desio et  al. 1998). Anatomic dissections have described the origin of the MPFL in a groove of the distal medial femur between the adductor tubercle and medial epicondyle, with an average width of 10.6  mm in adult specimens (Baldwin 2009; LaPrade et al. 2007). This point is described as average 1.9  mm anterior and 3.8 mm distal to the adductor tubercle and 9.5– 10.6 mm proximal and 5–8.8 mm posterior to the medial epicondyle (LaPrade et al. 2007; Nomura et al. 2005). From its origin, the MFPL courses obliquely toward the patella at an angle of about 15.9° (Nomura et al. 2005). Where it crosses over the leading edge of the superficial MCL, the width averages 37.5 mm and tapers to about 12 mm at its midpoint (Baldwin 2009; Nomura et al. 2005). During its course, the anterior portion becomes confluent with the border of the VMO tendon for approximately 35% of its length (Baldwin 2009; LaPrade et al. 2007; Nomura et al. 2005). Average length in adult cadaveric specimens is reported to be 58.8–65.2  mm to its insertion point on the superior two-thirds of the patella (Baldwin 2009; LaPrade et  al. 2007; Nomura et  al. 2005). The ligamentous insertion footprint spans 28.2  mm and is centered 27–41% of patellar length from the proximal pole (Baldwin 2009; LaPrade et al. 2007). The medial side of the patella also is the origin of the medial patellomeniscal ligament. Traveling from a small origin on the deep inferomedial patella (3–5 mm) and inserting broadly onto the anterior horn of the medial meniscus, it ­contributes approximately 13% of resistance to lateral translation (Desio et al. 1998; Tuxøe et al. 2002). The medial patellotibial ligament, also originating on the inferomedial patella, inserts onto the anteromedial tibia near the proximal physeal scar but does not significantly add to medial soft tissue restraints (Flandry and Hommel 2011; Desio et al. 1998).

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Similar soft tissue structures exist on the lateral side of the knee. The lateral patellofemoral ligament travels from the lateral epicondyle to the proximal lateral patella. Iatrogenic release can cause medial patellar subluxation (Flandry and Hommel 2011; Merican et  al. 2009). Inferiorly, the lateral patellotibial ligament runs from the distal lateral patella to the lateral border of the tibial tubercle (Flandry and Hommel 2011).

1.4 Biomechanics The predominant function of the patella is to increase the biomechanical advantage of the extensor mechanism. The patella serves as a fulcrum to increase the lever arm of the quadriceps tendon as it transmits force to the tibia, through the patellar tendon attachment, in order to extend the knee (Fox et  al. 2012; Kwak et  al. 1997). In the final 15° of extension, which require twice as much torque, the patella engages against femur and displaces the extensor mechanism from the axis of rotation of the knee, increasing efficacy of the moment arm (Lieb and Perry 1968; Wendt and Johnson 1985). The patellofemoral articulation, composed of hyaline cartilage on both sides, reduces the friction coefficient from the alternative of tendon interacting directly against the distal femur (Kaufer 1979). The efficacy is emphasized by the increase in quadriceps force required for knee extension after patellectomy, which is reported to be 130% of that required in the native knee (Kaufer 1971). Patellar tracking is least stable in the first 30° of flexion as the trochlear groove is more shallow proximally (Feller et al. 2007). As such, stability is more reliant on soft tissue restraints in early flexion. At 20° of flexion, the MPFL is the primary restraint to lateral subluxation, contributing approximately 60%. Thirteen percent is derived from the medial patellomeniscal and 10% from the lateral retinaculum. The medial patellotibial ligament and superficial medial retinaculum do not contribute significantly (Desio et  al. 1998). The patella lies more laterally with the knee

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extended and then moves medially with knee flexion into the groove, where it engages around 30–45° of flexion (Flandry and Hommel 2011; Feller et  al. 2007). In deeper flexion, therefore, patellar stability is conferred by the bony anatomy of the sulcus distally. Coronal plane forces on the patella are represented by the Q angle. Described by Insall in 1976, it is an angle formed between the quadriceps and patellar tendons, as represented by vectors from the ASIS to the center of the patella and from the center of the patella to the center of the tibial tubercle (Insall et  al. 1976). A normal Q angle is 12–14°, higher in women, and greater than 20° is considered abnormal. A high Q angle equates to a larger lateral force on the patella. Elevated values have been associated with femoral anteversion and external tibial torsion, together creating the so-called miserable malalignment (James 1979). However, if the patella is already in laterally subluxed position, the Q angle can be falsely low, and trochlear groove:tibial tuberosity (TT-TG) distance on axial CT represents a better quantitative marker (Fig. 4) (Goutallier et al. 1978). Through 0–60° of flexion, the lateral facet has a greater contact area than the medial facet at each

a

point (Salsich et  al. 2003). Mapping of contact forces on the patella correspond to orientation and stiffness of the underlying cancellous bone (Townsend et al. 1977). Contact stress at all points on the patellar increase with knee flexion (Wibeeg 1941). Quadriceps contraction does not increase force or compression across the patella (Salsich et  al. 2003). Joint reactive forces of the patellofemoral joint are reported to be 385 N with walking and 2400–2500 with stair climbing (Heino Brechter and Powers 2002; Brechter and Powers 2002). Stair climbing also produces a large shearing force across the central medial patella, particularly between 50° and 60° of flexion (Townsend et al. 1977). An increase in Q angle of 10° results in a 45% increase in contact pressures (Huberti and Hayes 1984). Though patella alta causes similar joint reactive forces to the normal knee, there is less area of contact, resulting in greater calculated stress (Ward and Powers 2004).

2 Clinical Findings Clinical findings vary with specific pathology, which will be discussed in more detail later, but patellofemoral disease typically presents as a

b

Fig. 4  TT-TG distance as measured on CT (a) and MRI (b) from the deepest groove (usually at the thickest posterior condylar femoral cartilage) to the center of tibial tubercle (patellar tendon)

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vague retro-patellar or anterior/anterolateral knee pain. The majority are activity related, exacerbated by loading of the patellofemoral joint through knee flexion by climbing, squatting, or jumping. Mechanical symptoms, such as locking, popping, and catching, may be present. Disruption of the extensor mechanism will present with an inability to straight leg raise. Age and developmental status should be taken into consideration as well, as some conditions such as Osgood– Schlatter and Singing-Larsen–Johannsen are inherent in a skeletally immature state.

3 Imaging Imaging supplements and complements clinical suspicion of patellofemoral disease and pain conditions. Radiography is initial screening modality. Advanced imaging includes 3D CT, MRI, and functional CT and MR imaging.

3.1 Radiography Ideal radiographic imaging includes a combination of frontal, lateral, and axial views.

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3.1.1 AP View A standard anterior–posterior (AP) radiograph is included in the basic evaluation of the patellofemoral joint. In slight knee flexion, the patella should be centered within the trochlear groove between the medial and lateral femoral condyles. This view may demonstrate lateral subluxation or dislocation, patella alta, fracture, or bipartite patella. This view also allows assessment of medial and lateral compartments of the knee and overall knee health. 3.1.2 Lateral View Lateral radiograph of the knee should be taken in with the femorotibial joint in 30° of flexion. Blumensaat’s line, first described in 1938, is an easily identified radiographic landmark, as a linear sclerosis along the roof of the intercondylar notch of the distal femur (Blumensaat 1938). The inferior pole of the patella is expected to be at approximately the same level as Blumensaat’s line. Measuring a vertical distance from the inferior pole of the patella to a linear continuation of Blumensaat’s line, a proximal translation of greater than 10  mm defines patella alta. Limitations of this method arise from the variability of the relationship between Blumensaat’s and the femoral axis among individuals and the

Table 2  Radiographic measurements of patella Measurement Insall–Salvati ratio

X-Ray view Lateral

Blackburne–Peel ratio

Lateral

Patellar subluxation

Axial

Lateral patellar angle

Axial

Trochlear depth

Axial

Trochlear angle

Axial

Definition Ratio of the patellar tendon height to the maximal diagonal length of the patella. The patellar tendon is measured from the posterior surface of the patella to its insertion on the tibial tubercle as identified by the center of the notch on the tibia. The ratio of the distance measured from the inferior patellar articular surface to the horizontal line along the tibial plateau to the height of patellar articular surface. Lateral or medial displacement of the patella with respect to the trochlear groove, >2 mm distance between the margins of the median ridge of the patella with respect to apex of the trochlear sulcus. The angle formed between a line drawn parallel to the lateral patellar facet and a line drawn connecting the most anterior points of the femoral condyles. The inset depth of trochlear groove from the line joining the most anterior aspects of the femoral condyles. The angle defining the depth of the trochlear sulcus relative to the lateral and medial trochlea of the femur.

Normal value 0.8–1.2

0.8–1 >1 is patellar alta, 145° and congruence angle >+16° (Merchant et  al. 1974; Kazley and Banerjee 2019). Initially defined on axial CT cuts, trochlear morphology can be described within the Dejour classification (Table  3) (Kazley and Banerjee 2019; Dejour et al. 1998). Given the utility of MRI to assess the status of the MPFL after dislocation, the DeJour classification has been applied to MRIs as well (Fig.  10) (Thawait et  al. 2012; Lee et  al. 2012; Gulati et al. 2018; Chhabra et al. 2011). The tibial Table 3  DeJour classification of trochlear dysplasia Type A B C D

Definition Shallow trochlea Flat trochlear groove Hypoplastic medial facet Vertical “cliff” medial facet

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b

c

d

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Fig. 9  Grade III cartilage defects. Sagittal fsPDW (a) and fsPDW (b) images show mild superolateral Hoffa’s fat pad edema and patella alta. Axial 2D PDFS (c) and corresponding 3D PDFS (d) images show grade III cartilage

defects over medial facet and median ridge of patella. Notice more conspicuous defects on 3D imaging with sharper borders

tubercle–trochlear groove (TT-TG) distance, also measured on axial cross-sectional imaging, represents extensor mechanism alignment, with values >20  mm associated with patellar instability (Dejour et al. 1994). Normal TT-TG distance is

less than 1 cm. If the patient has TT-TG >1.5 cm with significant patellar chondrosis, more aggressive treatments may be pursued. While 4D-CT and kinematic has been found to be useful for patellar maltracking, it is not routinely used due

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a

b

c

d

Fig. 10  A 47-year-old man with anterior compartment pain. Sagittal PDW (a) and fsPDW (b) images show patella alta and full-thickness cartilage loss of the lateral femoral trochlea (arrows). Axial PDFS (c) and PDW (d)

images demonstrate grade IV defects of lateral patellar facet with subchondral cysts (small arrows) and underlying Dejour type B dysplastic trochlea (long arrow)

to technical demands and variability with clinical correlations. Routine high-resolution MRI nicely depicts the classic contusions of transient patellar dislocation, i.e., inferior medial aspect of the patella and anterolateral femoral condyle. The

reader can identify partial- or full-thickness tear of medial patellar retinaculum, determine the location of injury, e.g., at patella, mid-portion, or at femoral attachment, detect trochlear dysplasia, and find intra-articular osteochondral fragments

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a

b

d

c

e

Fig. 11  Recent patellar dislocation. Axial radiograph (a) shows avulsed bone fragment at medial retinacular attachment (arrow). Axial fsPDW images (b, c) show dysplastic trochlea, laterally subluxed patella with tilt and torn medial retinaculum and MPFL from the patellar attach-

ments (arrows). Notice typical bone contusions of patella and femur. Arthroscopy images (d, e) show denuded patellar articular surface from acute osteochondral injury and medial retinacular disruption including MPFL

from injury (Fig.  11). Significant, moderate to large joint effusions are commonly identified, along with hemarthrosis and ill-defined Hoffa’s fat pad contours consistent with synovitis.

The majority of first-time dislocations can be treated nonoperatively, with brief immobilization and gradual return to activity with stabilizing patellar sleeve. Risk factors, such as

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b

a

d

c

e

Fig. 12  Recent patellar dislocation and reconstruction. AP and sagittal radiographs (a, b) show patella alta, lateral subluxation, and large effusion. Axial fsPDW images (c, d) show dysplastic trochlea, laterally subluxed patella

f

with tilt, torn medial retinaculum and MPFL from patellar attachments (arrows). Arthroscopy images before and after patellar cartilage repair (e, f) and before and after medial retinacular reconstruction (g, h)

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g

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h

Fig. 12 (continued)

skeletal immaturity, prior instability, and radiographic trochlear dysplasia, may raise risk of recurrence as high as 88% (Jaquith and Parikh 2017). In this instance of high predicted risk or with demonstrated recurrent instability, surgical intervention is recommended. Reconstruction of the MPFL is performed with allograft and, in the setting of malalignment (TT-TG >20) (Figs.  11 and 12), combined with a Fulkerson osteotomy to move the tibial tubercle anterior and medial (Fulkerson 1983; Liu et  al. 2018; Steiner et al. 2006).

4.3 Symptomatic Bipartite Patella A bipartite or multipartite patella is present in 2–6% of the population and only about 2% of these are symptomatic (Matic and Flanigan 2015; Weaver 1977). Symptoms are thought to arise after a local trauma or repetitive overuse leads to a fracture of the synchondrosis (Ireland and Chang 1995). With a peak incidence in early adolescence associated with athletic participation, patients present with anterior knee pain and tenderness to palpation over the accessory fragment worse with activity (Oohashi et al. 2010). An AP radiograph will demonstrate the presence of bipartite patella, with the accessory fragment most commonly located superolaterally (H S

1943). This classic location can help to distinguish bipartite morphology from acute fracture. While not commonly required for diagnosis, MRI studies can be utilized to identify edema across the bipartite synchondrosis, which correlates with symptoms (Kavanagh et  al. 2007; O’Brien et al. 2011). One can also detect disruption of synchondrosis with widening of pseudojoint and excessive fluid. In those who fail conservative measures such as rest, anti-­ inflammatory medications, and steroid injections, surgery may be indicated. The most common operative intervention is arthroscopic resection of the bipartite fragment, allowing 91% symptom free return to sport (Matic and Flanigan 2015).

4.4 Osteochondritis Dissecans/ Chondral Defects Osteochondritis dissecans (OCD) is a focal cartilage lesion that results from an idiopathic avascular event of secondary physis during the development of epiphyseal cartilage and bone. Lesions in the knee are common, with a peak incidence between the age of 10 and 20 years old (Lindén 1976). Patella and trochlear lesions occur, though less commonly, representing 7% and 1% of all knee lesions, respectively (Peters and McLean 2000; Hefti et al. 1999). The classic

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a

b

Fig. 13  OCD—typical location in the lateral aspect of the medial femoral condyle. Notice signs of instability on radiograph (a) sagittal PDW MR image (b) with lucency and fluid cleft around the proud osteochondral fragment (arrows)

lesion location is lateral on the medial femoral condyle, or within the intercondylar notch (Obedian and Grelsamer 1997). Acute chondral defects may occur in the traumatic setting, often on the medial patellar facet after a lateral dislocation event. Symptoms are generally vague, with anterior knee pain worsened by activity. Mechanical symptoms, such as locking and catching, may be present with an unstable cartilage flap or loose body. Plain radiographs may identify the lesion, demonstrating either disruption of the articular cartilage surface contour or subchondral lucency beneath an intact cartilage cap. X-ray allows for determination of lesion size and location with good reliability (Wall et  al. 2015). MRI is a routinely utilized adjunct, as it helps to delineate the condition of the involved cartilage. Acute fractures are best seen as a linear hypointensity on T1-weighted imaging on opposed phase of T2 Dixon imaging, with associated hyperintense marrow edema (Gorbachova et al. 2018). For OCD lesions, MRI is utilized to determine instability through the presence of the following—hyperintense fluid-like rim surrounding the progeny fragment or extending through

the articular cartilage cap on T2, underlying extensive fluid-filled cysts, in situ body, extension of gadolinium under the fragment, or complete detachment of the fragment (De Smet et al. 1990, 1996). Generally, a fragment of size greater than 1 cm and with overlying cartilage fissuring/ flaps and proud cartilage are at risk of developing such signs of instability (Fig.  13). Stability is essential in determining treatment options, with stable lesions responding well to rest and offloading or arthroscopic drilling. Unstable lesions are fixed in place with screws to stabilize and compress the lesion. Those that are fragmented or unsalvageable present a challenging problem and various surgical methods have been utilized including microfracture, autologous chondrocyte implantation, osteochondral autologous ­transplantation, or fresh osteochondral allograft (Fig. 14).

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Fig. 14  Young boy with osteochondral lesion with failed conservative treatment. AP (a) and lateral (b) views of the knee show articular surface irregularity of the medial aspect of lateral femoral condyle with a lucency consistent with an OCD (arrows). Coronal (c) and sagittal (d, e)

e

g

MR images confirm the lesion with chondral/central osteophytes and subchondral cysts (arrows). Arthroscopy images (f, g) before and after osteochondral allograft plug repair

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4.5 Quadriceps and Patellar Tendinopathy and Rupture Jumper’s knee or chronic extensor tendinopathy results from repetitive microtrauma at tendinous insertion at the superior or inferior pole of the patella. This is common in jumping athletes, such as basketball and volleyball players, with prevalence reported as high as 32–45% in the professional setting (Figs. 15, 16, and 17) (Lian et  al. 2005). Less frequently, chronic attrition may lead to quadriceps or patellar tendon rupture. Quadriceps tendon ruptures are more common in an older population, with average age of 61 years at the time of injury compared to 39.5 for patellar tendon ruptures (O’Shea et al. 2002; Garner et al. 2015). Affected patients will complain of anterior knee pain localized to the superior or inferior pole of the patella, sometimes specifically in the jumping or landing phase of activity. In the setting of rupture, a palpable defect may be present on exam and the extensor mechanism will be nonfunctional, as demonstrated through an absent straight leg raise. On X-ray, there may be evidence of periosteal reaction at the tendinous insertion site on the patella or calcification of the tendon (Blazina et  al. 1973). Quadriceps tendon rupture will lead to

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patella baja and patellar tendon rupture to patella alta, both of which can be measured on the lateral radiograph using the Insall–Salvati, Blackburne–Peel, or Caton-­Deschamps methods (Insall and Salvati 1971; Blackburne and Peel 1977; Caton 1989). Ultrasound can demonstrate thickening of the tendon and hypoechoic intrasubstance changes of tendinopathy, as well as complete disruption if present. MRI sequences show increased signal at the patellar insertion as well as in the tendon substance or at site of tear (Fig.  15). Underlying findings of tendinopathy, enthesopathy (bony hypertrophy and/or edema/ cystic changes) at the patellar and/or tibial tuberosity attachment sites are commonly evident. In acute-subacute injuries, knee joint effusion and shearing injury of Hoffa’s fad are commonly seen with partial separation from tibia, fat pad edema, and knee synovitis. MRI allows detection of site of injury and extent of tendon tear in both anteroposterior/transverse dimensions (Figs. 15 and 16). Early activity-related stages of tendinopathy are treated with NSAIDs, ice massage, and eccentric loading exercises (Blazina et  al. 1973). Persistent symptoms may require cessation of athletic activity or, in refractory cases, surgery, with open or arthroscopic resection of degenerative tendon and the affected pole. Tendon ruptures should be addressed surgically promptly after acute injury with direct repair or intraosseous patellar tunnels. Missed or chronic injuries may require extensor mechanism reconstruction with allograft.

4.6 Osgood–Schlatter Disease

Fig. 15  High-grade partial quadriceps tendon tear from recent basketball injury (arrow)

Osgood–Schlatter disease is a traction apophysitis of the tibial tubercle at the patellar tendon insertion (Osgood 1903). As the pathology is inherent to an open physis, average age of symptom onset is 8–12 years in girls and 12–15 years in boys (Blankstein et  al. 2001). Incidence in athletes of this age group is reported 21–33% (Kujala et al. 1985; Watanabe et al. 2018). Pain is present at the tibial tubercle, sometimes associated with local swelling and increasing ­prominence, and is exacerbated by jumping or

Patellar and Quadriceps Mechanism: Clinical, Imaging, and Surgical Considerations

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c

Fig. 16  Patellar tendon tear aka jumper’s knee. Sagittal fsPDW and PDW (a, b) images show proximal patellar tendinopathy with high-grade partial tear (long arrows). Notice pre-femoral fat pad edema and remodeled anterior femoral supratrochlear cortex from long-standing patellar

Fig. 17  Young man with recent injury. Notice avulsed bone fragment (arrow) of tibial tubercle with complete patellar tendon rupture and patella alta

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b

d

malalignment anatomy (short arrows). Axial sequential fsPDW images (c, d) show lateral patellar tilt, lateral aspect pre-femoral edema, and typical location of the jumper’s knee-related patellar tendon tear at the central and inner one-third of the tendon

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a

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Fig. 18  Two different young patients with long-standing anterior knee pain in pretibial area. Lateral radiographs (a, b) show enthesopathy and detached bone fragment from tibial tubercle

running. Lateral radiographs may demonstrate fragmentation at the apophysis or a persistent ossicle after fusion of the adjacent physis in a more mature individual (Fig.  18). Ultrasound has been described as an accurate diagnostic modality, allowing for visualization of cartilaginous apophysis and surrounding soft tissue inflammation (Delle Monache et  al. 1989; Bergami et  al. 1994). While not required for diagnosis in most cases, MRI can identify the edema of early cartilaginous injury at the tibial tubercle prior to radiographic evidence as well as characterize stages of disease progression (Hirano et al. 2002). Patellar tendon thickening, tears, Hoffa’s fat pad edema, patellofemoral cartilage lesions, and pre-/deep-infrapatellar bursitis are commonly associated. The majority of patients (88–91%) obtain relief with nonoperative measures such as activity modification, ice, NSAIDs, physical therapy, and, in a few severe cases, brief immobilization in a knee immobilizer or cylinder cast (Mital et al. 1980; Hussain and Hagroo 1996). Symptoms are inherently self-­limited and resolve with fusion at skeletal

maturity. In those remaining refractory cases, surgical treatment consists of ossicle excision or drilling of the apophysis to promote closure, both of which have been reported with good outcomes (Mital et al. 1980; Orava et al. 2000).

4.7 Sinding-Larsen–Johannsen Disease Similar to Osgood–Schlatter, Sinding-Larsen– Johansson is a traction apophysitis of the patellar tendon but rather at the inferior patellar insertion. Incidence is reported 2–5% again in skeletally immature children, most commonly in association with jumping activities (Malherbe 2019). Lateral X-ray will demonstrate fragmentation of the inferior pole of the patella. Ultrasound can also be used reliably to see ­swelling and fragmentation of the cartilaginous inferior pole, thickening of the patellar tendon, and reactive infrapatellar bursitis (De Flaviis et  al. 1989). MRI findings are similar to as described above for jumper’s knee but rather at the patellar attach-

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Fig. 19  Prior lateral retinacular release and medial retinaculum imbrication with persistent anterolateral knee pain and instability in a young female. Axial PDW (a) and fsPDW (b) images show focal defect from prior lateral retinacular release (arrows). Also note superolateral

Hoffa’s fat pad edema from chronic friction/impingement. 3D fsPDW images (c, d) show the clear lateral defect (arrow), subchondral cysts of the lateral facet of patella (small arrows), and susceptibility artifact at the medial retinacular repair site (large arrow)

ment. It is imperative to detect associated patellofemoral cartilage loss for proper management. Symptoms typically resolve within 10–12 months of onset through activity modification and NSAIDs if there is no significant cartilage injury.

4.8 Post-op Failure and Iatrogenic Conditions Medial patellar instability occurs almost exclusively as an iatrogenic result of excessive arthroscopic lateral retinacular release (Hughston and Deese 1988). This was traditionally utilized as a treatment for lateral patellar instability but the debilitating nature of said complication has

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caused it to fall out of favor. Patients present with anterior knee pain and instability, often more severe in nature than the initial lateral subluxation that prompted surgical intervention. Axial stress radiographs or CT scan will demonstrate the dynamic medial patellar subluxation (Sanchis-Alfonso and Merchant 2015). MRI is used to detect failure of medial retinacular plication/imbrication and underlying cartilage loss (Fig. 19). Treatment consists of reconstruction of the deep transverse layer of the lateral retinaculum with iliotibial band autograft (Sanchis-­ Alfonso et al. 2015). To conclude, patellofemoral joint and extensor mechanism are an important compartment of the knee, and its disorders are varied and can lead to significant disability and pain affecting quality of life. Proper clinical evaluation, imaging assessment, and timely treatment are essential for improving patient outcomes and prognosis. Disclosures  AC serves as a consultant for Treace Medical Concepts Inc. and ICON Medical. AC receives royalties from Jaypee and Wolters and is a speaker for Siemens Innovations Conference. AC serves as the medical advisor and receives a research grant from Image Biopsy Lab Inc. Conflicts of Interests  None.

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406 Merican AM, Kondo E, Amis AA (2009) The effect on patellofemoral joint stability of selective cutting of lateral retinacular and capsular structures. J Biomech 42(3):291–296 Mital MA, Matza RA, Cohen J (1980) The so-called unresolved Osgood-Schlatter lesion: a concept based on fifteen surgically treated lesions. J Bone Joint Surg Am 62(5):732–739 Nietosvaara Y, Aalto K, Kallio PE (1994) Acute patellar dislocation in children: incidence and associated osteochondral fractures. J Pediatr Orthop 14(4): 513–515 Nomura E, Inoue M, Osada N (2005) Anatomical analysis of the medial patellofemoral ligament of the knee, especially the femoral attachment. Knee Surg Sports Traumatol Arthrosc 13(7):510–515 O’Brien J et al (2011) Magnetic resonance imaging features of asymptomatic bipartite patella. Eur J Radiol 78(3):425–429 O’Shea K et  al (2002) Outcomes following quadriceps tendon ruptures. Injury 33(3):257–260 Obedian RS, Grelsamer RP (1997) Osteochondritis dissecans of the distal femur and patella. Clin Sports Med 16(1):157–174 Ogden JA (1984) Radiology of postnatal skeletal development. X. Patella and tibial tuberosity. Skeletal Radiol 11(4):246–257 Oohashi Y, Koshino T, Oohashi Y (2010) Clinical features and classification of bipartite or tripartite patella. Knee Surg Sports Traumatol Arthrosc 18(11): 1465–1469 Orava S et al (2000) Results of surgical treatment of unresolved Osgood-Schlatter lesion. Ann Chir Gynaecol 89(4):298–302 Osgood RB (1903) Lesions of the tibial tubercle occurring during adolescence. Clin Orthop Relat Res 1993(286):4–9 Peters TA, McLean ID (2000) Osteochondritis dissecans of the patellofemoral joint. Am J Sports Med 28(1):63–67 Salsich GB et al (2003) In vivo assessment of patellofemoral joint contact area in individuals who are pain free. Clin Orthop Relat Res 417:277–284 Sanchis-Alfonso V, Merchant AC (2015) Iatrogenic medial patellar instability: an avoidable injury. Arthroscopy 31(8):1628–1632 Sanchis-Alfonso V et al (2015) Results of isolated lateral retinacular reconstruction for iatrogenic medial patellar instability. Arthroscopy 31(3):422–427 Saupe E (1921) Beitrag zur patella bipartita. Fortschr Rontgenstr 28:37–41 Saupe H (1943) Primare Kochenmarkseiterung der Kniescheibe. Langenbecks Arch Surg 258:386–392 Steiner TM, Torga-Spak R, Teitge RA (2006) Medial patellofemoral ligament reconstruction in patients

B. K. Tisano et al. with lateral patellar instability and trochlear dysplasia. Am J Sports Med 34(8):1254–1261 Subhawong TK et  al (2010) Superolateral Hoffa’s fat pad edema: association with patellofemoral maltracking and impingement. AJR Am J Roentgenol 195(6):1367–1373 Subhawong TK et al (2014) Patellofemoral friction syndrome: magnetic resonance imaging correlation of morphologic and T2 cartilage imaging. J Comput Assist Tomogr 38(2):308–312 Taunton JE et  al (2002) A retrospective case-control analysis of 2002 running injuries. Br J Sports Med 36(2):95–101 Tecklenburg K et al (2006) Bony and cartilaginous anatomy of the patellofemoral joint. Knee Surg Sports Traumatol Arthrosc 14(3):235–240 Thakkar RS et al (2016) Patellar instability: CT and MRI measurements and their correlation with internal derangement findings. Knee Surg Sports Traumatol Arthrosc 24(9):3021–3028 Thawait SK et al (2012) High resolution magnetic resonance imaging of the patellar retinaculum: normal anatomy, common injury patterns, and pathologies. Skeletal Radiol 41(2):137–148 Townsend PR et al (1977) The biomechanics of the human patella and its implications for chondromalacia. J Biomech 10(7):403–407 Tuxøe JI et  al (2002) The medial patellofemoral ligament: a dissection study. Knee Surg Sports Traumatol Arthrosc 10(3):138–140 Wall EJ et al (2015) Novel radiographic feature classification of knee osteochondritis dissecans: a multicenter reliability study. Am J Sports Med 43(2):303–309 Wang D et al (2019) Qualitative and quantitative analysis of patellar vascular anatomy by novel three-­dimensional micro-computed-tomography: implications for total knee arthroplasty. Knee 26(6):1330–1337 Ward SR, Powers CM (2004) The influence of patella alta on patellofemoral joint stress during normal and fast walking. Clin Biomech (Bristol, Avon) 19(10):1040–1047 Watanabe H et  al (2018) Pathogenic factors associated with Osgood-Schlatter disease in adolescent male soccer players: a prospective cohort study. Orthop J Sports Med 6(8):2325967118792192 Weaver JK (1977) Bipartite patellae as a cause of disability in the athlete. Am J Sports Med 5(4):137–143 Wendt PP, Johnson RP (1985) A study of quadriceps excursion, torque, and the effect of patellectomy on cadaver knees. J Bone Joint Surg Am 67(5):726–732 Wibeeg G (1941) Roentgenographic and anatomic studies on the femoro-patellar joint. Acta Orthop Scand 12:319–410

Infection James Francis Griffith and Margaret Ip

1 Introduction

Contents 1    Introduction 

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2    Pediatric Native Knee Infection 

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3    Adult Native Knee Infection 

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4    Prosthetic Knee Infection 

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Abstract

Knee joint infection, either within or around the joint, requires an early and specific diagnosis. Reaching this diagnosis is heavily dependent on the correct use and interpretation of imaging studies. This chapter outlines the pathophysiology and evolving imaging strategy for investigating pediatric and adult native knee joint infection as well as prosthetic joint infection.

J. F. Griffith (*) Department of Imaging and Interventional Radiology, The Chinese University of Hong Kong, Shatin, Hong Kong, China e-mail: [email protected] M. Ip Department of Microbiology, The Chinese University of Hong Kong, Shatin, Hong Kong, China e-mail: [email protected]

The presentation and investigation of knee joint infection in children and adults and those with knee joint prostheses differ in many respects. This chapter therefore addresses, in turn, the pathophysiology and clinical presentation of (a) pediatric knee joint infection, (b) adult native knee joint infection, and (c) prosthetic knee joint infection.

2 Pediatric Native Knee Infection Bone and joint infection in children are due to septic arthritis in 20%, septic arthritis with osteomyelitis in 40%, and osteomyelitis alone in 40% (Monsalve et  al. 2015). Septic arthritis is more common in children less than 2 years of age (Monsalve et  al. 2015). The knee is the second most common site of septic arthritis in children (after the hip), and the distal femur/proximal tibia is the most common site of osteomyelitis (Gibian et al. 2020; Musso et al. 2021). Both septic arthritis and osteomyelitis are mainly hematogenous in origin (Ogden 1979; Goergens et al. 2005). The changing vasculature of the metaphyseal—epiphyseal region during skeletal maturation affects the likely site of osteomyelitis (Fig.  1). The typical presenting symptoms of either septic arthritis or osteomyelitis are rapid onset of pain, restricted movement, and

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_360, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 04 March 2023

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Fig. 1 (a) In the first 18 months of life, transphyseal vascular continuity exists between the metaphysis and the epiphysis. (b) As the physis matures, transphyseal communication stops with separate metaphyseal and epiphy-

seal blood supplies. (c) After physeal close, continuity between the metaphysis and the subarticular bone is restored

inability to weight-bear. In very young children, manifestations are a refusal to bear weight, distress on movement of the affected limb, general irritability with fever, sleep disturbance, and reduced appetite (Gibian et  al. 2020). Septic arthritis of the knee is iatrogenic in 10% of children. Trauma also increases the risk of knee joint infection (Funk and Copley 2017). Blood cultures are positive in ≤40% of children with osteomyelitis (Peltola et  al. 2010; Yeo and Ramachandran 2014). The most common organisms to cause pediatric metaphyseal osteomyelitis and septic arthritis are Staphylococcus aureus (40%), followed by Kingella kingae (15%), Streptococcus pyogenes (10%), Streptococcus pneumoniae (10%), and Streptococcus agalactiae (Group B Streptococcus) (Musso et al. 2021; Peltola and Pääkkönen 2014). Two subtypes of pediatric infection around the knee deserve special mention, namely Kingella kingae infection and primary epiphyseal osteomyelitis (PEO). Kingella kingae is now the most common cause of osteomyelitis or septic arthritis in young European and Middle Eastern children (Peltola and Pääkkönen 2014; Wong et  al. 2020). Kingella kingae is a gram-negative bacillus that colonizes the pharynx of one in ten healthy children younger than 2  years. Beyond 4  years of age, oropharyngeal colonization decreases signifi-

cantly. Most Kingella infections present with mild to moderate symptoms and signs and normal or almost normal inflammatory markers (Yagupsky 2015). Typical imaging appearances are prominent synovial proliferation and extensive regional myositis (Nguyen et  al. 2018). The organism is difficult to culture. Polymerase chain reaction (PCR) techniques greatly enhance the detection and should be performed in children with culture-­negative osteoarticular infection (Jaramillo et al. 2017). The organism is sensitive to most broad-­spectrum antibiotics (Shahrestani et al. 2019). Chronic osteomyelitis and long-term sequela are rare (Yagupsky 2019). Primary epiphyseal osteomyelitis (PEO), i.e., epiphyseal infection without metaphyseal infection, can occur (particularly in the distal femur) though is much less common than metaphyseal osteomyelitis (Shah et al. 2020) (Fig. 2). As primary epiphyseal osteomyelitis (PEO) is not dependent on transphyseal spread, it can occur at any age (average 5  years, range 1–14  years) (Shah et  al. 2020). Organisms include Staphylococcus aureus and Mycobacterium tuberculosis. Swelling and limping and joint penetration are more common in tuberculous PEO while pain is the dominant feature in nontuberculous PEO. Patients typically make a good recovery (Shah et al. 2020).

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Fig. 2 (a–c) A 2-year-old girl with knee pain and fever for 3 days. (a) Frontal radiograph is normal. (b) T1-weighted coronal and (c) T1-weighted fat-suppressed coronal postcontrast MR images show inflammation alongside the medial aspect of the nonossified portion of the epiphysis (arrows) and within the ossified portion of

the epiphysis (open arrows) consistent with primary epiphyseal osteomyelitis. The metaphysis is normal. Blood culture grew Staphylococcus aureus. She responded well to intravenous antibiotics with no long-term sequalae. Note normal physeal and cambium enhancement

MRI  MRI is indicated in most children with suspected knee infection given the prevalence (40%) of coexisting septic arthritis and osteomyelitis in children, either before or after ultrasound examination (Monsalve et al. 2015; Manz et al. 2018). MRI is very accurate at detecting osteomyelitis and revealing extraosseous abnormalities (Fig. 3). The presence of a joint effusion in patients with metaphyseal osteomyelitis is ­particularly noteworthy. About 60% of children with distal femoral metaphyseal osteomyelitis on MRI will have a joint effusion and 80% of these effusions will be septic (Schallert et  al. 2015). Similarly, 45% of children with proximal tibial metaphyseal osteomyelitis on MRI will have a joint effusion and 60% of these effusions will be septic (Schallert et  al. 2015). The absence of a knee effusion completely excludes septic arthritis (Schallert et al. 2015).

children to improve visibility of inflammation, delineate abscesses, or areas of bone ischemia/ infarction (Jaramillo et  al. 2017). Whole-body T2-weighted fat-saturated coronal imaging is helpful in neonates and young children to locate additional foci of infection. Multifocal infection is much less common in older children (Jaramillo et al. 2017).

Ultrasound  Ultrasound should be performed before MRI if septic arthritis is suspected rather than osteomyelitis. Septic arthritis manifests as joint effusion ± synovial proliferation. Joint fluid should be aspirated for analysis (see the following section). Visible synovial proliferation is always abnormal and, with respect to infection, is a feature of chronic infection from bacteria such as Mycobacterium or Kingella. If moderate or severe synovial proliferation is present with relatively little joint fluid, synovial biopsy should be Detection of osteomyelitis in infants and performed testing especially for Mycobacterium young infants can be problematic due to the rela- tuberculosis infection (multinucleated giant cells, tive absence of marrow fat. As such, fat-­ caseating granulomas, positive Ziehl-Nielsen suppression sequences are relatively unhelpful stain) or PCR testing (Kingella infection). The though contrast enhancement is helpful. absence of joint effusion or synovial proliferation Epiphyseal cartilage infection (“infective chon- excludes septic arthritis. Ultrasound is helpful at dritis”) may go undetected without gadolinium detecting osteomyelitis in neonates and young enhancement (Jaramillo et  al. 2017) (Fig.  2). children though not quite as sensitive as MRI in Contrast enhancement is also helpful in older this regard (Fig. 4).

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Fig. 3  A 12-year-old boy with severe knee pain, fever, weighted and (c) T2 fat-suppressed sagittal MR images inability to weight-bear for 1 week and no trauma. (a) show infiltration and edema (open arrow) of the proximal Lateral radiograph shows a transverse fracture of the tibial metaphysis. CT-guided biopsy revealed proximal tibia and fibula (arrows) with mild proximal Staphylococcus aureus osteomyelitis tibial metaphyseal osteolysis. (b) Proton-density-­

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Fig. 4 (a) Longitudinal ultrasound of neonate with acute osteomyelitis of distal femoral metaphysis showing juxtacortical soft tissue swelling (arrowheads), focal periosteal elevation with mild early new bone formation, focal corti-

3 Adult Native Knee Infection The knee is the most frequently infected native joint (Roerdink et  al. 2019) and adults are six times more likely to get knee joint infection than children (Gunnlaugsdóttir et al. 2021). One in ten patients may have polyarticular infection (Roerdink et al. 2019). One in two cases of knee

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cal and subcortical osteolysis. The epiphysis (asterisk) is normal. (b) Color Doppler ultrasound shows moderate surrounding hyperemia

septic arthritis follow an interventional procedure such as arthroscopy (80%), arthrocentesis (10%), or open knee surgery (10%) (Gunnlaugsdóttir et  al. 2021) (Fig.  5). Also, most adult patients with knee septic arthritis have underlying knee pathology such as moderate to severe osteoarthritis or an inflammatory or crystal arthropathy (Gunnlaugsdóttir et  al. 2021).

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Fig. 5  A 48-year-old female with infected ACL graft tunnel. (a) Oblique sagittal CT image showing severe osteolysis (arrows) around the tibial screw. (b) Proton-density oblique sagittal MR image confirms this osteolysis with severe widening of the tibial tunnel (arrows) though no evidence of surrounding osteomyelitis. (c) T2 fat-­saturated

In the native joint, bacteria typically first infect the synovium leading to an infective synovitis which manifests as synovial effusion and synovial proliferation. Highly virulent infections produce rapidly progressive symptoms with a dominance of synovial effusion over highly inflamed synovial proliferation. Less virulent infections cause more insidious symptoms, with a dominance of mildly inflamed synovial proliferation over synovial fluid. Causative organisms for adult native joint infection are methicillin-­ sensitive Staphylococcus aureus (40%); coagulase-­negative Staphylococci (40%); Streptococci (S. viridans, S. mitis, S. pneumoniae) (10%); and others (Enterobacter cloacae, gram positive rods, Klebsiella oxytoca, Lactococcus cremoris, Micrococcus species) (10%). Gram-­ negative bacterial infection is rare (Gunnlaugsdóttir et al. 2021; Balato et al. 2017). Blood cultures are recommended as hematogenous spread is a common avenue of joint infection though are only positive in about 15% of patients (Weston et al. 1999). Radiographs  Radiographs should be performed to help exclude other less likely causes of acute knee pain (such as insufficiency fracture with articular surface collapse, or tumor). Radiographs

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coronal MR image showing hyperintense infected inflammatory tissue (open arrow) extending from the tibial tunnel opening toward the skin. There is essentially no metallic artifact as the fixation screws used were of low ferromagnetic material. Reoperation confirmed infection

will be normal in most cases of septic arthritis. Marginal erosions, osteolysis, and joint space narrowing are indicative of established chronic infection (Fig. 6).

Ultrasound  Septic arthritis is an emergency which requires early ultrasound examination. The differential diagnosis of an acutely inflamed joint includes crystal and inflammatory arthritis, as well as para-articular bone and soft tissue infection. Ultrasound is performed to (a) establish that infection is within the joint and not in the periarticular tissues, (b) confirm the presence of a joint effusion, and (c) assess the degree of synovial proliferation. In determining the likelihood of septic arthritis, judge whether any joint effusion or synovial proliferation is commensurate with the degree of underlying knee pathology. For example, a small to moderate effusion with mild synovial proliferation is expected in uncomplicated moderate to severe knee osteoarthritis and hence is not indicative of septic arthritis. It is often helpful to use the contralateral knee as an internal reference standard. The absence of a joint effusion or synovial proliferation or a joint effusion/synovial proliferation which is entirely commensurate with the underling knee pathology effectively excludes septic arthritis.

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412 Fig. 6 (a–c) A 78-year-old male with known multifocal methicillin-resistant Staphylococcus aureus (MRSA) infection and knee pain. (a) Frontal radiograph knee at presentation, (b) 5 months and (c) 14 months post-­ presentation show progressive joint space narrowing and increasing marginal erosions (arrows). Ultrasound at 14 months revealed severe chronic joint synovitis of the medial parapatellar recess with severe synovial thickening and relatively very little joint effusion (open arrow). No detectable hyperemia was present. Synovial biopsy (not shown) grew MRSA. F femoral shaft

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Joint Aspiration  If there is a joint effusion present which is not commensurate with the underlying knee pathology, this effusion should be aspirated under full aseptic technique (Fig.  7). There is no need to discontinue anticoagulation or check coagulation times before joint aspiration (Garcia et  al. 2008; Narouze et  al. 2018; Patel et  al. 2019). Ideally aspiration should be performed before antibiotic therapy is started, though it is not necessary to defer joint aspiration to an antibiotic-free period. Select a needle track that avoids any overlying cellulitis though overlying cellulitis is not an absolute contraindication to joint aspiration (Abdel Karim et  al. 2019). Deeper soft tissue infection, such as abscess, infected bursitis, or pyomyositis, poses a greater challenge. Such infections are readily visible on ultrasound examination prior to aspiration and should be avoided. A 21  G hypodermic needle is usually sufficient to reach the knee cavity. Target the knee recess that contains the most fluid. Local anes-

thetic should be applied to the skin. A separate needle with a thin extension tubing system should be used for aspiration as local anesthetic is potentially bactericidal or bacteriostatic (Xu et  al. 2003). Intra-articular needle tip position is determined real-time with ultrasound. For fluoroscopy, a small amount of contrast can be instilled into the joint. Low-molecular or isosmolar contrast agents are not bacteriocidal (Bruins et  al. 2011). Air-contrast injection is an alternative approach. • Begin aspiration with the needle tip within the center of the knee cavity. Avoid contacting the inflamed synovial membrane to minimize bleeding, which can impede cell count. Use your free hand to massage fluid from the other knee recesses toward the needle tip rather than moving the needle tip around the joint. Gentle suction and good needle tip positioning will minimize the risk of synovial fronds occluding the needle tip during aspiration. If this happens, stop aspiration, and move the needle

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Fig. 7 (a–d) A 70-year-old man with Streptococcal septic arthritis. Serial radiographs at (a) baseline and (b) 3 months later show moderate progressive cartilage loss with joint space narrowing at the medial femoral compartment with an enlarging proximal tibia marginal erosion

•

•

•

•

tip to another location. Synovial fluid should be sent for the following tests. Place in a sterile universal container for culture, to include AFB and fungal cultures. Ideally small-volume CSF tubes should be used rather than routine culture bottles. Send fresh specimens for cell count and microscopy. This specimen can also be used for DNA/RNA detection though need to ensure bottles are sterile. If the specimen can be sent quickly to the microbiology laboratory inoculation, fluid may be injected into aerobic and an anaerobic commercial blood culture bottles (e.g., BacTec system BD). Specific culture media for Mycobacterium and fungal inoculation are also available. Small volume sample for leucocyte esterase or alpha-defensin testing (optional).

Synovial fluid aspirate should be transferred directly to the Microbiology Laboratory from the Radiology Dept within 6  h (Fuchs et  al. 2021). Gram staining is usually performed but has low

(arrow). (c) Ultrasound shows mild fluid distension of the medial parapatellar recess with moderate synovial proliferation (open arrow) (F femoral shaft). (d) Both fluid aspirate and synovial biopsy (arrowheads) grew Group B Streptococcal infection

diagnostic performance with a false-negative rate of 78% (Stirling et  al. 2014). Crystal analysis should also be performed. Specimens for crystal analysis should be sent fresh. A synovial fluid white cell count of >50,000 per μL is indicative of native joint infection with a leukocyte count of >75–80%. Additional biomarkers may be beneficial in equivocal cases. Synovial fluid alpha-defensin-1 level is an antimicrobial peptide released by activated neutrophils in response to bacterial infection. A positive test makes joint infection almost certain while a negative test does not exclude infection. Alpha-­ defensin testing is much more expensive than leucocyte esterase testing (Wyatt et  al. 2016). Leucocyte esterase testing provides a rapid estimate of synovial white blood cell count (Parvizi et al. 2018). A 2+ is considered highly suggestive of joint infection with a sensitivity of 93% and specificity of 97% (Wyatt et  al. 2016; Parvizi et al. 2011). If there is little or no discernible joint fluid and moderate to severe synovial proliferation on ultrasound examination, then image-guided

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synovial biopsy should be performed as outlined in the next section. Synovial biopsy leads to a positive culture in one-quarter of patients with false-negative aspiration (Coiffier et al. 2018) and is a better alternative than diagnostic lavage when no fluid is available for aspiration. For diagnostic lavage, 10  mL of normal saline is injected into the joint with usually 2–3  mL aspirated. Prior consultation with the referring clinical team is recommended before undertaking lavage (Abdel Karim et al. 2019). MRI  MRI has limited role in the detection of septic arthritis. Although pericapsular edema and enhancement help distinguish aseptic arthritis from a chronic inflammatory arthropathy (Karchevsky et  al. 2004), bacterial culture is needed to unequivocally make this diagnosis. MRI though may reveal radiographically occult insufficiency fractures or periarticular osteomyelitis though the latter is much less common in

a

Fig. 8  A 55-year-old man with chronic knee pain. (a) Proton-density sagittal and (b) T2-weighted fat-­ suppressed coronal MR imaging showing severe chronic-­ type synovial thickening (arrows), a large synovial-type

adults than children (Fig.  8). MRI has a very high accuracy for the detection of osteomyelitis with a pooled sensitivity of 90% and specificity of over 80% (Dinh et  al. 2008). T1 marrow replacement is crucial for a high sensitivity in diagnosing osteomyelitis (Collins et  al. 2005). Marrow T2 signal intensity similar to fluid is quite specific for osteomyelitis in the right clinical context with lower T2-hyperintesities being less specific (Collins et  al. 2005). Periarticular marrow changes in the setting of severe joint degeneration or an inflammatory arthropathy may be confused with osteomyelitis as may insufficiency fracture or avascular necrosis. Intravenous contrast can accentuate synovitis, area of osteonecrosis or ischemia, sequestrum or intraosseous abscess (Alaia et al. 2021), That said, intravenous contrast is not necessary in most instances (Safdar et  al. 2018). CT or nuclear medicine studies are helpful in selected patients with suspected native knee joint septic arthritis (Fig. 5).

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inflammatory mass posteriorly (open arrow) arising within the popliteal tendon recess and large marginal erosions (arrowheads). Ultrasound-guided synovial biopsy revealed Mycobacterium tuberculum

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Klebsiella pneumoniae). Two-thirds of TKA infections are delayed or late onset infection and Following total knee arthroplasty (TKA), a “peri- present more insidiously with less severe pain prosthetic membrane,” also known as a “synovial-­ and restricted motion. Rest or nocturnal pain is like interface membrane” (SLIM), forms at the particularly suggestive with pain on weight-­ prosthesis–bone interface while a “neosynovium” bearing favoring mechanical loosening or instaforms beneath the joint capsule to replace the bility (Taljanovic et  al. 2018). Delayed or late excised synovium. About 2% of primary TKAs onset infections are usually caused by less viruand 5% of revision TKAs become infected (Del lent organisms (coagulase-negative Pozo and Patel 2009; Kurtz et  al. 2007; Staphylococci, including Staphylococcus epiderIzakovicova et al. 2019). As such, the prevalence midis, Staphylococcus lugdunensis, of prosthetic knee joint infection is increasing as Cutibacterium species, Mycobacterium spp.). As the total number of TKAs increases. TKA infec- expected, lower organism virulence and biofilm tion results from either intraoperative inoculation maturity make late onset infection more probor later hematogenous inoculation (Izakovicova lematic to diagnose and treat than early infection et al. 2019; Zimmerli et al. 2004). The presence in a prosthetic joint or any infection is a native of a concurrent infection (including infection in knee joint (Parvizi et al. 2018). One-tenth to one-­ another prosthetic joint), rheumatoid arthritis, quarter of TKA infections may be “culture-­ diabetes, and other comorbidities compromising negative,” which is defined by purulence around the immune system all increase the risk of TKA the prosthesis, acute inflammation of periprosinfection (Abdel Karim et  al. 2019; Siu et  al. thetic tissue histologically, a sinus tract emerging 2018; Lee et al. 2015). from the knee, and no growth on aerobic and A biofilm is secreted by bacteria within sec- anaerobic cultures (Berbari et al. 2007). onds of contacting the prosthesis. It is a thin For infection, revision is usually a two-stage (10  mg/L or the ESR • Late onset: >12 months after index surgery >30 mm/h. That said, one-fifth of patients with TKA infection may have a normal C-reactive One-third of TKA infections present early protein or ESR (Berbari et al. 2007; Suren et al. with typical symptoms of pain, swelling, redness, 2021). Peripheral white cell count, for example, warmth, and impaired range of motion. Early is usually not elevated in TKA infection. The infections are normally caused by more virulent performance of newer serum inflammatory organisms (methicillin-sensitive or -resistant markers (fibrinogen, D-dimer, interleukin-6, Staphylococcus aureus, Streptococcus dysgalac- procalcitonin) is not adequately evaluated to rectiae, Streptococci agalactiae, and other ommend for their routine usage (Sigmund et al. Streptococci, Escherichia coli and Enterococci, 2021).

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Radiographs  Radiographs should be the first-­ line imaging investigation (Figs.  9 and 10). Radiographs are expected to be normal in most cases of early onset knee joint infection. The presence of infection-related radiographic changes (marginal erosion, subarticular erosion, periarticular lucency) invariably indicates a well-­ established infection. These radiological appearances can also be seen with aseptic loosening and mechanical instability (Taljanovic et al. 2018).

Ultrasound  Ultrasound is singularly the best examination for suspected TKA infection and should be performed early. The para-articular soft tissues should be examined for evidence of cellulitis, focal inflammation, abscess/collection, or deep sinus tract. The vascularity of any synovial proliferation should be assessed with more severe hyperemia increasing the likelihood of an actively infected joint. If ultrasound reveals a joint effusion, as is expected in early infection, this effusion should be aspirated as outlined in the previous section (Sect. 3). A synovial fluid leucocyte count of > 10,0000 cells/mL with >90% polymorphonuclear leucocytes (PMNs) in acute infection or >3000 cells/mL with >70% polymorphs in chronic infection is indicative of infection (Romanò et  al. 2020). More attention should be paid to PMN% than the leucocyte count for diagnosing TKA infection (Fuchs et al. 2021). In late TKA infection, there is often an abundance of synovial proliferation with relatively little visible synovial fluid. This scenario, which will result in a “dry tap” on attempted aspiration, is seen in about one-third of patients with chronic TKA infection (Partridge et  al. 2018; Yee et  al. 2013). In this situation, image-guided (ultrasound, fluoroscopy, or CT), synovial biopsy for histology and culture should be performed (Figs.  11 and 12). Synovial fluid culture in chronic periprosthetic joint infection may be particularly susceptible to negative culture due to the low bacterial load and the presence of a biofilm.

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Synovial Biopsy  Synovial biopsy in the absence of any synovial fluid has a sensitivity of 45–70%, specificity of 98–100%, and diagnostic accuracy of 78–93% for infection (Rajakulasingam et  al. 2021; Sitt et  al. 2017). Synovial biopsy is best performed using a coaxial system, typically a 16 G coaxial needle and a 15 G side-cutting needle (Sitt et al. 2016). Local anesthetic (1% lignocaine) is injected to the skin and then, under ultrasound guidance, to just outside the joint capsule. Lignocaine should not be injected deep to the joint capsule in view its potential bactericidal effect. Generally, biopsy is undertaken where the synovium is thickest, positioning the coaxial needle tip just deep to the capsule and angling it so that the biopsy path is along the line of the synovium about midway between the joint capsule and the joint cavity (Sitt et  al. 2016). As a bacterial load is often small, at least six synovial core samples should be obtained for culture and sensitivity. For fluoroscopic (±-ultrasound-­ aided), an 18-G spinal needle is placed into the joint following a small skin incision (Rajakulasingam et al. 2021). Satisfactory intra-­ articular needle placement is confirmed with iodinated contrast injection (Rajakulasingam et al. 2021). A 22-G Wescott needle is then passed through the lumen of this spinal needle for synovial biopsy (Rajakulasingam et al. 2021). While some studies have shown reduced culture positivity (Tande and Patel 2014), other studies showed culture positivity rate was not related to concurrent antibiotic therapy (Sitt et al. 2017). In a study of over 100 patients, synovial biopsy alone had a sensitivity of 70%, specificity of 98%, and diagnostic accuracy of 93% for TKA infection (Rajakulasingam et al. 2021). Multiplanar polymerase chain reaction (PCR) analysis of synovial tissue can be available within 6  h though does suffer currently from low sensitivity (30%) as it is prone to false-negative results compared to routine microbiological culture. Although no studies have been performed to date, it is our general impression that synovial tissue culture is more sensitive than synovial fluid aspiration in confirming or excluding persistent infection in patients prior to prosthetic reimplantation.

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Fig. 9 (a) Clinical photograph of a 72-year-old female with knee pain and swelling suspicious of periprosthetic joint infection, (b) and (c) frontal radiographs 1 year apart

showing progressive tilting (open arrows) and medial migration of the tibial prosthesis (arrow) indicative of aseptic loosening

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Fig. 10 (a) Longitudinal ultrasound in the same patient as Fig. 9 shows moderately thickened knee joint synovium with severe hyperemia. A small amount of joint fluid is present (asterisk). (b) 5  mL of straw-colored fluid was

aspirated (arrows) and (c) the synovium was biopsied (arrows). F femur, T tibial prosthesis. Both the aspirate and the synovial tissue were sterile

CT and MRI Examination  Compared to ultrasound examination, CT and MRI have a lesser role in the initial diagnosis of TKA infection. Joint inflammation, joint effusion, and synovial proliferation can be readily seen with both CT and MRI but are limited in helping to determine the likely cause of this inflammation. CT, with appropriate bone windowing and metallic artifact reduction techniques, may be used to reveal bony reabsorption more effectively around the implant than radiography (Romanò et al. 2020). Low ferromagnetic prostheses and metal artifact reduction sequences have improved the benefit of MRI in assessing TKA infection. For TKA infection, MRI has a sensitivity of 70% and specificity (98%) with this diagnostic accuracy being higher in index TKA cases rather than revision TKA cases (Li et  al. 2016). Although subjective, a laminated-­type hyperintense synovial proliferation may be more indicative of TKA infection

while frond-like synovial hypertrophy seems more indicative of particle-induced synovitis (Li et al. 2016). A lamellated synovitis, defined as a hyperintense, thickened synovium consisting of multiple layers, had a sensitivity of up to 92% for septic arthritis with a specificity of 87% (Li et al. 2016). MRI examination is particularly effective in determining the presence and severity of adjacent osteomyelitis (Taljanovic et al. 2018). Edematous inflamed periarticular marrow tissue in the early infected period can be taken as indicative of osteomyelitis provided there are no compounding factor such as subarticular insufficiency ­fracture, postoperative changes, or severe osteoarthritis. Percutaneous biopsy is not necessary to confirm this osteomyelitis. In the more chronic setting, ongoing periarticular marrow signal changes do not necessarily imply ongoing infec-

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Fig. 11  A 66-year male with knee pain and swelling for 6 months. There was no fever. (a) Frontal radiograph shows periprosthetic osteolysis of the medial femoral and tibial condyles (arrowheads). (b) Transverse ultrasound of

tion. One can differentiate this reparative tissue from active chronic osteomyelitis based on the degree of inflammation present and on serial MRI examinations over a 3–6  month period showing resolution rather than progression of changes. Nuclear Medicine Examination  In Europe, bone scintigraphy (40%) and anti-granulocyte antibody scintigraphy (30%) were the most requested imaging studies, in addition to radiographs, to exclude TKA infection (Ahmad 2016). In Hong Kong, bone or bone/gallium scintigraphy is more commonly performed while white blood cell (WBC) or anti-granulocyte antibody

the lateral parapatellar recess shows a large amount of synovial proliferation (open arrows) which was moderately hyperemic (not shown) with (c) a moderate amount of fluid in the medial parapatellar recess (arrow)

scintigraphy is less available (as specific laboratory facilities or licensed pharmaceutical is needed). Positron emission tomography/computed tomography (PET/CT) has become an alternative, increasingly available imaging tool. What nuclear medicine study to request depends on local availability and the level of clinical suspicion. If the pretest probability of TKA infection is low, then bone scintigraphy or PET/CT (using fluorine-18 fluorodeoxyglucose [FDG]) can be requested as both will effectively rule out TKA infection if negative. If the pretest probability of TKA infection is high or if one wants to distinguish between aseptic loosening and infection, then a WBC/marrow or anti-granulocyte anti-

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Fig. 12  Same patient as Fig. 11. (a) Ultrasound-guided aspiration was performed with a 21 G hypodermic needle yielding 8  mL of slightly blood-stained fluid. (b) Ultrasound-guided 15 G side-cutting synovial biopsy with coaxial system. Routine culture of aspirate and synovial tissue yielded no growth though fungal culture grew the

fungus, Phaeoacremonium parasiticum. (c) Frontal radiograph 12 months later following removal of the prosthesis and replacement with temporary spacers (arrowheads) though with severe collapse of the medial tibial condyle (open arrow) and lateral translation of the proximal tibia

body scintigraphy should be requested subject to availability. These examinations acquire both planar and also often tomographic images with single-photon emission computed tomography/ computed tomography (SPECT/CT) to increase diagnostic accuracy as well as more clearly delineate the site and extent of infection.

Bone Scintigraphy  Three-phase technetium-­ 99m (99mTc) diphosphonate bone scintigraphy evaluates regional perfusion, diffusion, and late bone uptake reflecting osteoblastic activity (Figs. 13 and 14). Negative bone scintigraphy can exclude TKA infection with a high level of certainty (Niccoli et  al. 2017). However, normal

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Fig. 13 (a) Clinical photograph of knee of an 82-year-old female with knee pain, especially on weight-bearing, 5 years after arthroplasty. (b) Frontal and (c) lateral radiographs of the knee show no abnormality

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Fig. 14 (a) T99m bone scintigraphy in the same patient as Fig.  13 shows increased osteoblastic activity around the left knee prosthesis (open arrows) when compared to the normal right side (arrowhead). (b) White cell scintigraphy, reflecting leucocytic accumulation, shows increased activity mainly around the femoral prosthesis (arrow). (c)

Marrow scintigraphy, reflecting functioning marrow activity, shows much less pronounced activity around the femoral prosthesis (arrow). This is all suggestive of infection around the femoral prosthesis which was confirmed at surgery

noninfective knee prostheses can show periprosthetic activity, especially around the tibial stem, and around cementless (porous) rather than cemented arthroplasties, within 1 year (and sometimes up to 5 years) following implantation (Palestro 2014). Although activity diminishes with time, only a small number of bone scintigrams will be completely negative (Romanò et al. 2020). The overall sensitivity of bone scintigraphy for periprosthetic knee joint infection is 93% (95% CI 0.85–0.98) while specificity is 56% (95% CI 0.47–0.64) (Verberne et al. 2017).

for spatial distribution and intensity. Combined bone/gallium scintigraphy has higher accuracy (65–80%) for TKA infection than bone scintigraphy alone though increasingly WMC/marrow scintigraphy is replacing bone/gallium scintigraphy for accessing suspected TKA infection (Palestro 2014).

Gallium Scintigraphy  Bone scintigraphy may be combined with gallium scintigraphy to improve specificity. Gallium scintigraphy is performed using gallium-67 (67Ga) citrate which accumulates at infective or inflammatory areas. 67 Ga activity should be compared with 99mTc activity for spatial configuration and intensity (Love 2009). The study is negative for infection when 67Ga is spatially congruent with or smaller than 99mTc and the relative intensity of 67Ga is lower. It is positive for infection (or active inflammation) when 67Ga is spatially incongruent with at least one area exceeding 99mTc activity or the relative intensity of 67Ga is higher. It is equivocal for infection if 67Ga and 99mTc are congruent both

White Blood Cell Scintigraphy  Autologous white blood cells can be radiolabeled with technetium-­99m (99mTc-­ hexamethylpropyleneamine oxime) or indium-111 (111In-oxine). After injection, early (30–60 min), delayed (2–4 h), and late (20–24 h) images are acquired (Romanò et  al. 2020) (Figs.  13 and 14). Delayed imaging in chronic infection is mandatory due to relatively slower leucocyte biokinetics. Since WBCs accumulate not only in infected tissue but also in normal bone marrow, the main diagnostic criterion for infection is accumulated uptake between the delayed and late images (Romanò et  al. 2020). Overall sensitivity of WBC scintigraphy for periprosthetic knee joint infection is 88% (95% CI 0.81– 0.93) while specificity is 77% (95% CI 0.69–0.85) (Verberne et  al. 2017). Sensitivity for chronic infection is similar at 84% though the specificity is lower at 52% (Blanc et al. 2019).

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Bone Marrow Scintigraphy  Bone marrow scintigraphy is usually performed together with WBC scintigraphy in equivocal cases. 99mTc-colloids (size >500 nm) are injected intravenously. These colloids are phagocytized by reticuloendothelial cells in the bone marrow. Images are acquired between 30  min and 6  h post injection. White blood cells are present in areas of functioning marrow. Functioning marrow, while generally less prevalent in elderly patients, tends to accumulate nevertheless around prostheses. Marrow scintigraphy helps identify rests of functioning marrow. If WBC scintigraphy is negative, one does not need to perform marrow scintigraphy. Concordance between the biodistribution of WBC and colloid excludes infection, while discordance, with positive WBC activity but negative corresponding 99mTc colloid activity, is highly suggestive of infection (Romanò et  al. 2020) (Figs. 13 and 14). With high specificity for distinguishing periprosthetic joint infection from prosthetic loosening, combined WBC/marrow scintigraphy is the scintigraphic “gold standard” for imaging prosthetic joint infection (Teiler et al. 2020), especially for early infection. For low-­ grade chronic TKA infection, combined WBC/ marrow scintigraphy can diagnose infection with a sensitivity of 71% and specificity of 95% (Niccoli et al. 2017). Focal periprosthetic uptake may be due to a localized infection, which can be targeted for surgical intervention. The overall sensitivity of combined WBC/marrow scintigraphy for periprosthetic knee joint infection is 80% (95% CI 0.66–0.91) while specificity is 93% (95% CI 0.86–0.97) (Verberne et al. 2017).

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(HAMA), thus limiting its use to a single lifetime dose only. Sulesomab does not induce a HAMA response though is no longer commercially available in Europe. Image acquisition for full-length antibodies is similar to WBC scintigraphy with the optimal acquisition being 16–24 h post injection. The acquisition for fragmented antibodies is 1  h and 4–6  h post injection due to the faster clearance of the fragmented antibody (Romanò et al. 2020). The overall sensitivity of anti-granulocyte antibody scintigraphy for periprosthetic knee joint infection is 90% (95% CI 0.78–0.96) while specificity is 95% (95% CI 0.88–0.98) (Verberne et al. 2017).

PET Imaging  It is not yet clear whether FDG PET/CT offers a significant advantage over WBC or anti-granulocyte antibody scintigraphy for evaluating TKA infection (Jin et al. 2014). PET/ CT is very sensitive to TKA infection with a negative study effectively excluding TKA infection (Van Acker et  al. 2001). Whole-body PET/CT can detect additional infective foci in patients with known TKA infection. Remote foci of infection may be evident in about one-half of patients with an infected TKA, and in about two-thirds of these, the remote infection may not be clinically known prior to PET/CT (Roschke et  al. 2022). Overall sensitivity of FDG PET for periprosthetic knee joint infection is 70% (95% CI 0.56–0.81) while specificity is 84% (95% CI 0.76–0.90) (Verberne et  al. 2017). Gallium-68 (68Ga) is a positron emitting radionuclide which can be used for PET imaging. Early data suggests that 68 Ga-citrate PET/CT may increase specificity for TKA infection and may thus have a complimenAnti-granulocyte Antibody Scintigraphy  Anti-­ tary role to FDG PET/CT imaging (Tseng 2019). granulocyte antibody scintigraphy is very similar to WBC scintigraphy though is not universally In conclusion, the pathophysiology and imagavailable as it requires a special commercial ing strategy for investigating knee infection difpreparation kit. 99mTc-labeled monoclonal anti- fers depending on whether one is investigating bodies attach to antigens on the surface of pediatric native joint, adult native joint, or pros­granulocytes. These antibodies can be either full thetic joint infection. New developments that length (IgG anti-NCA-95 antibody, besilesomab) help with early diagnosis of knee joint infection or fragmented (Fab′ fragment anti-NCA-90, are the increasing shift toward the early use of sulesomab). As besilesomab is obtained from MRI in pediatric infection, the shift toward early mice, it can induce human anti-mice antibodies ultrasound-guided knee joint aspiration  ±  syno-

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425 periprocedural management of thrombotic and bleeding risk in patients undergoing percutaneous image-­ guided interventions. Part 2. Recommendations: endorsed by the Canadian Association for Interventional Radiology and the Cardiovascular and Interventional Radiological Society of Europe. J Vasc Interv Radiol 30:1168–1184 Peltola H, Pääkkönen M (2014) Acute osteomyelitis in children. N Engl J Med 370(4):352–360 Peltola H, Pääkkönen M, Kallio P, Kallio MJ, Osteomyelitis-Septic Arthritis Study Group (2010) Short- versus long-term antimicrobial treatment for acute hematogenous osteomyelitis of childhood: prospective, randomized trial on 131 culture-positive cases. Pediatr Infect Dis J 29(12): 1123–1128 Rajakulasingam R, Cleaver L, Khoo M, Pressney I, Upadhyay B, Palanivel S, Hargunani R (2021) Introducing image-guided synovial aspiration and biopsy in assessing peri-prosthetic joint infection: an early single-centre experience. Skeletal Radiol 50(10):2031–2040 Roerdink RL, Huijbregts HJTAM, van Lieshout AWT, Dietvorst M, van der Zwaard BC (2019) The difference between native septic arthritis and prosthetic joint infections: a review of literature. J Orthop Surg (Hong Kong) 27(2):2309499019860468 Romanò CL, Petrosillo N, Argento G, Sconfienza LM, Treglia G, Alavi A, Glaudemans AWJM, Gheysens O, Maes A, Lauri C, Palestro CJ, Signore A (2020) The role of imaging techniques to define a peri-prosthetic hip and knee joint infection: multidisciplinary consensus statements. J Clin Med 9(8):2548 Roschke E, Kluge T, Stallkamp F, Roth A, Zajonz D, Hoffmann KT, Sabri O, Kluge R, Ghanem M (2022) Use of PET-CT in diagnostic workup of periprosthetic infection of hip and knee joints: significance in detecting additional infectious focus. Int Orthop 46(3):523–529 Safdar NM, Rigsby CK, Iyer RS et  al. Expert Panel on Pediatric Imaging (2018) ACR Appropriateness Criteria® acutely limping child up to age 5. J Am Coll Radiol 15(11 suppl):S252–S262 Schallert EK, Kan JH, Monsalve J, Zhang W, Bisset GS III, Rosenfeld S (2015) Metaphyseal osteomyelitis in children: how often does MRI-documented joint effusion or epiphyseal extension of edema indicate coexisting septic arthritis? Pediatr Radiol 45(8):1174–1181 Shah MM, Gupta G, Makadia AS, Rabbi Q (2020) Primary epiphyseal osteomyelitis (PEO) in 18 children: a rare entity with atypical features. J Pediatr Orthop 40(7):361–366 Shahrestani S, Evans A, Tekippe EM, Copley LAB (2019) Kingella kingae septic arthritis in an older-than-­ expected child. J Pediatric Infect Dis Soc 8(1):83–86 Sigmund IK, Puchner SE, Windhager R (2021) Serum inflammatory biomarkers in the diagnosis of periprosthetic joint infections. Biomedicine 9(9):1128 Sitt JC, Griffith JF, Wong P (2016) Ultrasound-guided synovial biopsy. Br J Radiol 89(1057):20150363

426 Sitt JC, Griffith JF, Lai FM, Hui M, Chiu KH, Lee RK, Ng AW, Leung J (2017) Ultrasound-guided synovial Tru-­ cut biopsy: indications, technique, and outcome in 111 cases. Eur Radiol 27(5):2002–2010 Siu KT, Ng FY, Chan PK, Fu HC, Yan CH, Chiu KY (2018) Bacteriology and risk factors associated with periprosthetic joint infection after primary total knee arthroplasty: retrospective study of 2543 cases. Hong Kong Med J 24(2):152–157 Stirling P, Faroug R, Amanat S et al (2014) False-negative rate of Gram-stain microscopy for diagnosis of septic arthritis: suggestions for improvement. Int J Microbiol 2014:830857 Suren C, Lazic I, Stephan M, Lenze FW, Pohlig F, von Eisenhart-Rothe R (2021) Diagnostic algorithm in septic total knee arthroplasty failure  - what is evidence-­based? J Orthop 23:208–215 Taljanovic MS, Gimber LH, Omar IM, Klauser AS, Miller MD, Wild JR, Chadaz TS (2018) Imaging of postoperative infection at the knee joint. Semin Musculoskelet Radiol 22(4):464–480 Tande AJ, Patel R (2014) Prosthetic joint infection. Clin Microbiol Rev 27(2):302–345 Teiler J, Ahl M, Åkerlund B, Wird S, Brismar H, Bjäreback A, Hedlund H, Holstensson M, Axelsson R (2020) Is 99mTc-HMPAO-leukocyte imaging an accurate method in evaluating therapy result in prosthetic joint infection and diagnosing suspected chronic prosthetic joint infection? Q J Nucl Med Mol Imaging 64(1):85–95 Tseng JR, Chang YH, Yang LY, Wu CT, Chen SY, Wan CH, Hsiao IT, Yen TC (2019) Potential usefulness of 68 Ga-citrate PET/CT in detecting infected lower limb prostheses. EJNMMI Res 9(1):2–10 Van Acker F, Nuyts J, Maes A, Vanquickenborne B, Stuyck J, Bellemans J, Vleugels S, Bormans G,

J. F. Griffith and M. Ip Mortelmans L (2001) FDG-PET, 99mtc-HMPAO white blood cell SPET and bone scintigraphy in the evaluation of painful total knee arthroplasties. Eur J Nucl Med 28(10):1496–1504 Verberne SJ, Sonnega RJ, Temmerman OP, Raijmakers PG (2017) What is the accuracy of nuclear imaging in the assessment of periprosthetic knee infection? A meta-­ analysis. Clin Orthop Relat Res 475(5):1395–1410 Weston VC, Jones AC, Bradbury N, Fawthrop F, Doherty M (1999) Clinical features and outcome of septic arthritis in a single UK Health District 1982–1991. Ann Rheum Dis 58:214–219 Wong M, Williams N, Cooper C (2020) Systematic review of Kingella kingae musculoskeletal infection in children: epidemiology, impact and management strategies. Pediatric Health Med Ther 24(11):73–84 Wyatt MC, Beswick AD, Kunutsor SK, Wilson MJ, Whitehouse MR, Blom AW (2016) The alpha-­defensin immunoassay and leukocyte esterase colorimetric strip test for the diagnosis of periprosthetic infection: a systematic review and meta-analysis. J Bone Joint Surg Am 98(12):992–1000 Xu H, Zhang L, Arita H, Hanaoka K (2003) Antimicrobial activity of local anesthetics. Pain Res 18:19–24 Yagupsky P (2015) Kingella kingae: carriage, transmission, and disease. Clin Microbiol Rev 28(1):54–79 Yagupsky P (2019) Kingella kingae and the empiric antibiotic therapy for skeletal system infections. J Pediatric Infect Dis Soc 8(3):284 Yee DK, Chiu KY, Yan CH, Ng FY (2013) Review article: Joint aspiration for diagnosis of periprosthetic infection. J Orthop Surg (Hong Kong) 21(2):236–240 Yeo A, Ramachandran M (2014) Acute haematogenous osteomyelitis in children. BMJ 348(g66):1326 Zimmerli W, Trampuz A, Ochsner PE (2004) Prosthetic-­ joint infections. N Engl J Med 351(16):1645–1654

Arthritis Holly W. Christopher, Emma Rowbotham, and Andrew J. Grainger

Contents 1    Introduction 

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2    Imaging Overview 

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3    Osteoarthritis  3.1  Overview  3.2  Conventional Radiographs  3.3  Advanced Imaging 

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4    Calcium Pyrophosphate Dihydrate (CPPD) Arthritis  4.1  Overview  4.2  Imaging Features 

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5    Gout as It Affects the Knee  5.1  Overview  5.2  Imaging 

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6    Rheumatoid Arthritis as It Affects the Knee  6.1  Overview  6.2  Imaging  6.3  Imaging with Advanced Modalities 

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H. W. Christopher Cambridge University Hospitals, Cambridge, UK e-mail: [email protected] E. Rowbotham Leeds Teaching Hospitals, NHS Trust, Leeds, UK e-mail: [email protected] A. J. Grainger (*) Cambridge University Hospitals, Cambridge, UK Department of Radiology, Addenbrookes Hospital, Cambridge, UK e-mail: [email protected]

7    Spondyloarthritides as They Affect the Knee  7.1  Overview  7.2  Imaging 

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8    Synovial Proliferation Associated with Arthritis  8.1  Synovial Osteochondromatosis  8.2  Pigmented Villonodular Synovitis  8.3  Lipoma Arborescens 

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9    Hemophilic Arthropathy  9.1  Overview  9.2  Imaging 

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10    Conclusion 

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References 

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Abstract

In this chapter, after an initial introduction and overview we will review the appearances of common imaging features seen in different arthritides affecting the knee. The chapter will discuss both radiographic and, where pertinent, advanced imaging appearances. Subsequently, the imaging features of the common arthritides affecting the knee, including mechanical or degenerative, inflammatory, and crystal arthritis, will be discussed. In each case the chapter sets out to highlight features which will help the radiologist to make a specific diagnosis, or at least a sensible differential as to the cause of the arthropathy. It is all too easy to dismiss the

Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2022_361, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 18 March 2023

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appearances of knee arthritis as osteoarthritis, given the ubiquitous nature of this condition. It is hoped that having read the chapter, the reader will be able to appreciate imaging features that should make the radiologist identify other conditions which may have different implications to the patient and clinician.

1 Introduction The expansion of modern imaging has paralleled an unprecedented global increase in joint disease, accelerated by increased life expectancy and a global obesity epidemic; since the first human MRI images were published in 1977 worldwide obesity has almost trebled (Edelman 2014; World Health Organisation 2018). Alongside this there has been a marked increase in our understanding of the multiple forms of arthritis, driven in part by our ability to characterize joint pathology on imaging. Recent years have also seen a rapid development in the management of rheumatological disease, which makes the role of imaging more important than ever, as early detection and diagnosis of joint pathology gives access to therapeutic options that can dramatically affect outcomes. Imaging in arthritis can be condensed into understanding which modalities are of use to characterize different joint diseases, what imaging features enable us to differentiate them, and what severity of disease is present. Imaging also plays a role in assessing the response of arthritis to treatment. The knee joint is commonly affected by many forms of arthritis and involvement has a significant impact on activities of daily living. Given the overlap in imaging features that many of the arthritides show, a basic overview of how certain features are demonstrated on differing imaging modalities is given in the first section, before the chapter will go on to review the imaging of specific forms of arthritis as they affect the knee.

2 Imaging Overview While relatively nonspecific features of joint disease, such as an effusion, are recognizable on all imaging modalities, some distinguishing

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features are only demonstrated on certain modalities, or may be detected earlier on one modality than another; for example, erosions can be seen on MRI before they are evident radiographically. Despite the more advanced imaging modalities now available, for the most part conventional radiographs remain the primary imaging modality for initial evaluation of the arthritic knee and should not be overlooked, particularly as an adjunct to interpretation of the advanced imaging modalities, for example, for the demonstration of calcification in the cartilage or periarticular soft tissues. In contrast, computed tomography (CT) has limited value in the evaluation of the painful knee outside the sphere of bone trauma. Ultrasound of the knee enables high-­ resolution scanning of superficial structures and affords dynamic evaluation of certain tendons and ligaments, although its scope is limited compared to use in other joints as articular surfaces and internal structures, including cruciate ligaments and the menisci, are difficult to visualize. In the context of the arthritides, ultrasound has particular value in its ability to demonstrate synovitis, effusion, and tenosynovitis. It is also able to show erosions in bone which are accessible to ultrasound, and shows some features of enthesitis, but cannot demonstrate osteitis and intrinsic bone changes. Standard ultrasound definitions of relevant entities exist (Table  1). One further role for ultrasound is the demonstration of soft tissue changes and masses associated with joint disease, such as rheumatoid nodules and gout tophi. As is the case in other circumstances, knee ultrasound is an inexpensive modality but can be limited by its operatordependent nature. Magnetic resonance imaging (MRI) is the modality of choice in definitive investigation of the painful knee, affording the benefits of multiplanar imaging and excellent soft tissue contrast. It allows for complete assessment of the menisci and cruciate ligaments, but also demonstrates the articular cartilage. Overall, the advantage of MRI lies in its greater sensitivity for early bone, cartilage, and soft tissue changes which can expedite diagnoses and facilitate early intervention, for exam-

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Arthritis Table 1  Ultrasound definitions (Wakefield et al. 2005) Synovial fluid

Synovial hypertrophy

Tenosynovitis

Erosion

Enthesopathy

Abnormal hypoechoic or anechoic (relative to subdermal fat, but sometimes may be isoechoic or hyperechoic) intra-articular material that is displaceable and compressible but does not exhibit Doppler signal. Abnormal hypoechoic (relative to subdermal fat, but sometimes may be isoechoic or hyperechoic) intra-­ articular tissue that is nondisplaceable and poorly compressible and which may exhibit Doppler signal. Hypoechoic or anechoic thickened tissue with or without fluid within the tendon sheath, which is seen in two perpendicular planes, and which may exhibit Doppler signal. An intra-articular discontinuity of the bone surface that is visible in two perpendicular planes. Abnormally hypoechoic (loss of normal fibrillar architecture) and/or thickened tendon or ligament at its bony attachment (may occasionally contain hyperechoic foci consistent with calcification), seen in two perpendicular planes that may exhibit Doppler signal and/or bony changes including enthesophytes, erosions, or irregularity.

ple, detection of subtle high T2 signal to indicate periarticular edema and subtle erosion in early rheumatoid arthritis. It is also sensitive to the demonstration of synovitis, enthesitis, and tendon disease/tenosynovitis. A typical protocol for the atraumatic painful knee would include sagittal, coronal, and axial proton density fat-saturated imaging along with T1-weighted imaging for the assessment of the bone marrow. However, contrast-enhanced sagittal T1 imaging may also be undertaken if assessment of synovitis is required, as it can be difficult to delineate the high T2 signal from synovitis from that of a joint effusion. In certain circumstances, employing gradient echo sequences to take advantage of blooming artifact is advantageous, as lesions appear more conspicuous due to the paramagnetic effect of substances such as hemosiderin in recurrent hemorrhage.

3 Osteoarthritis 3.1 Overview Musculoskeletal disorders are a leading cause for years lived with disability and the global burden of disease is increasing (James et  al. 2018). Osteoarthritis (OA) is the most common form of arthritis and, as a major weight-bearing joint, the knee is a common site of presentation of OA. The pathogenesis of OA, which is outwith the normal aging process, occurs through hyaline cartilage damage with associated subchondral changes and deformity. This results in loss of function on both a personal and societal level. The progressive nature of OA and limited focus on early intervention leads to an increasing strain on healthcare systems, given the aging population. A prudent imaging approach not only benefits the patient’s healthcare interaction but also optimizes healthcare economics; with over 100,000 knee replacements performed in the UK per year (National Joint Registry for England Wales Northern Ireland and the Isle of Man 2019), the direct costs of knee OA are high, and the indirect costs far exceed this. Imaging is a useful adjunct to clinical diagnosis and an important aid to the orthopedic decision-making process; it may also guide future use of advancing technologies within the arena of joint preservation. OA is understood to be a multifactorial heterogenous disease representing a complex interplay between genetics, environmental factors, and biochemical and biomechanical stressors. Its increasing incidence with age reflects the cumulative exposure to such risk factors in the context of inherent changes within the aging joint. Risk factors found to be strongly associated with the development of knee osteoarthritis include female sex, obesity, and previous joint injury; these factors likely represent many different overlapping mechanistic pathways (Hunter and Bierma-Zeinstra 2019). The appearance of severe joint deformity is demonstrably also a final common pathway seen in a multitude of disease processes, and, due to its prevalence, OA changes are commonly seen as a backdrop to other pathologies. As such, it is important to be aware of the characteristic features of OA across the gamut of imaging modalities.

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The diagnosis of knee OA may be made clinically, although imaging is typically involved and has value in investigating rapid deterioration in symptoms or eliminating alternative diagnoses. Typically, after clinical history and examination, suspected OA is investigated by obtaining radiographs to assess the patellofemoral and tibiofemoral joints, but the diagnostic pathway may also incorporate US and MRI.  Each modality has its unique advantages and disadvantages for different aspects of the diagnostic and therapeutic pathway. Due to the heterogeneity of the disease and its causal factors, management in OA should be tailored to the individual. Primarily, focus should be on lifestyle intervention, with education about exercise and weight loss, facilitated by adequate nonopioid analgesia. Disease-modifying therapeutics have generated significant research interest but are not yet applicable to the general population. If conservative measures are inadequate, surgical management is the next step (Hunter and Bierma-Zeinstra 2019). Although debate continues, clinical guidelines discourage arthroscopy for those with degenerative knee disease (Siemieniuk et  al. 2017). Consequently, preoperative imaging is an important adjunct to the surgical decision-making progress; the questions being asked of imaging will vary depending on the individual patient, but can range from confirming the need for surgery, to identifying factors that may pose difficulties intraoperatively or that determine implant selection, such as flexion contracture, tibiofemoral malalignment, and large bony defects (Beaufils 2012). Imaging has a particular role in determining whether a unicompartmental knee arthroplasty (UKA) or total knee arthroplasty (TKA) is more appropriate. Central to this is evaluating the disease distribution within the joint, assessing the relative involvement of the two tibiofemoral and the patellofemoral joints. For instance, UKA is contraindicated in cases demonstrating severe wear of the lateral facet of the patellofemoral joint with bone loss and grooving (Rodríguez-­ Merchán and Gómez-Cardero 2018). The relative merits and disadvantages of UKA have been contentious over the years and have ultimately led to stringent patient selection.

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Another facet in addressing this increasing demand is to improve implant longevity, which highlights further a role for imaging as optimal component positioning in multiple planes is central to achieving this. As well as aiding planning of a surgical approach in the traditional sense, radiologists should also be aware of the potential for the application of preoperative imaging if computer-assisted components navigation is being used intraoperatively (O’Connor and Kransdorf 2013).

3.2 Conventional Radiographs Radiographs remain the initial investigation for the evaluation of the painful knee for osteoarthritis. They are also the mainstay of imaging followup for monitoring disease progression or ­ assessing the knee post-arthroplasty. As such, evaluation of sequential radiographs gives a better insight into the disease process than a single radiograph showing appearances at one point in time. In the research setting, trials of disease-­ modifying OA drugs require reliable outcome measures for evaluating disease progression. Joint space width (JSW) measured on the standing knee radiograph has been considered the gold standard for the assessment of disease progression in the knee, being used as a surrogate marker for hyaline cartilage loss. However, this remains a controversial area, partly as a result of a lack of consensus on positioning and set up (Mazzuca and Brandt 2003). As a result, a standardized positioning of imaging setup with knee in flexion has been pursued; one approach is to image in knee flexion (20–35°) with fluoroscopic guidance of knee positioning (schuss views). Since apparent joint space width is also determined by the incident angle of the X-ray beam, fluoroscopic adjustment to achieve radio-anatomic parallel alignment with the medial tibial plateau controls for this, thereby enhancing the sensitivity of radiographic joint space narrowing as a marker of OA progression. In settings where fluoroscopic guidance is either impractical or unavailable, standardizing the setup with respect to knee positioning is desirable. However, other studies have

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also highlighted the relative insensitivity of JSW measurements (Raynauld et  al. 2004) and the important observation that the meniscus accounts for a significant proportion of the variance seen in JSW through degeneration and extrusion from the joint line (Hunter et al. 2006). With the goal of accurate representation of disease severity and sensitive detection of disease progression in mind, a complete radiographic study would usually use a posteroanterior weight-­bearing radiograph to evaluate the tibiofemoral joint in flexion (taking into account the aforementioned points for standardized set up for reproducibility in serial imaging) and a lateral radiograph to assess patellar position and detect a joint effusion. Axial/ skyline views would also be undertaken if the patellofemoral joint required further evaluation. Interpretation of these radiographs benefits from referring to the hallmark radiographic changes of knee OA as described by Kellgren and Lawrence in 1957 (see Table  2) (Kellgren and Lawrence 1957; Kohn et al. 2016). Their attempts to establish a radiographic classification scheme for OA utilized a grading system from 0 (no osteoarthritis) to 4 (severe osteoarthritis). Although this was proposed in the context of knee OA, features are the same as those seen at most other joints (Fig. 1): 1. Joint space narrowing—Typically focal and asymmetric, affecting one or two compartments; in contrast, uniform narrowing may be part of the spectrum of normal aging or it may point toward inflammatory arthritides. Table 2  Kellgren and Lawrence radiographic classification of osteoarthritis Grade 0 (none): definite absence of X-ray changes of osteoarthritis Grade 1 (doubtful): doubtful joint space narrowing and possible osteophytic lipping Grade 2 (minimal): definite osteophytes and possible joint space narrowing Grade 3 (moderate): moderate multiple osteophytes, definite narrowing of joint space, some sclerosis, and possible deformity of bone ends Grade 4 (severe): large osteophytes, marked narrowing of joint space, severe sclerosis, and definite deformity of bone ends

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Fig. 1  Osteoarthritis. AP radiograph shows severe OA of the knee with predominant medial compartment disease. Note the osteophyte in the medial joint line and complete loss of medial compartment joint space. An osteochondral body is projected in the intercondylar notch (arrow)

2. Subchondral bone changes—Subchondral sclerosis arises subadjacent to the site of diseased cartilage, and subchondral cysts may be seen as juxta-articular lucencies. The pathophysiology that brings about these changes remains controversial. 3. Osteophytes—These occur at the periphery of the joint surface due to enchondral ossification and may represent attempts to preserve joint stability though increasing surface area. 4. Deformity—Characteristic of end-stage OA, due to a combination of progressive joint space loss and bony attrition/collapse. Although this grading system has had both research and clinical applications, it has attracted critical commentary over the years, and scrutiny is required to assess its ongoing

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validity. Kellgren and Lawrence’s original paper highlighted the inherent inter- and intraobserver error in grading, and it has been demonstrated that clinical symptoms and radiographic severity grading do not correlate well (Hayes et  al. 2005). In the clinical setting it is important to have bidirectional communication between the referring clinician and reading clinician; a clear clinical history is imperative, as this can provide the reading clinician with useful information. For example, a history of trauma would raise the suspicion of internal derangement which predisposes to osteoarthritis and may also provide rationale for an atypical clinical or radiographic presentation. The pursuit of standardized patient positioning and the availability of an accepted grading system in the context of a common disease entity makes the knee joint in osteoarthritis an attractive focus for the application of artificial intelligence in imaging. However, whether (or when) this replaces or augments a human reader for radiographs, the need to evaluate the knee joint with further imaging modalities remains. Osteoarthritis now has to be thought of as a disease of the whole joint and conventional radiographs do not detect changes in many of the structures present, for example, ligaments and menisci; they also remain insensitive to early cartilage damage. Since early diagnosis and intervention with joint p­ reservation techniques may allow us to delay (or potentially avoid) joint arthroplasty, employing advanced imaging early in the patient’s diagnostic pathway affords both clinical and economic benefit.

3.3 Advanced Imaging While features of advanced osteoarthritis are easily recognizable on radiographs, we know that the bony changes described previously do not give the full picture. MRI and US can show how osteoarthritis involves cartilage, synovium, ligaments, and menisci, all of which are changing our understanding of how the disease process manifests globally within the knee.

3.3.1 MRI MRI is the modality of choice in the investigation of the painful knee, affording the benefits of multiplanar imaging, soft tissue contrast, and nonionizing radiation. Although there is some radiological overlap in the appearance of OA changes on MRI and radiographic imaging, the advantage of MRI lies in its greater sensitivity for early bone and soft tissue changes (Muñoz-­ García et al. 2020) (Figs. 2, 3, and 4). As a result, there is an increasing utilization of MRI earlier in the diagnostic pathway, to enable detection of, and potential intervention for, early disease, something facilitated by expansion in MRI capacity. Increasing emphasis on MRI as a roadmap to orthopedic intervention further promotes this approach. Early osteoarthritis has been defined as meeting three main criteria (Nagai et al. 2018): • Knee pain • ≤Kellgren–Lawrence grade 2 on radiographs • Arthroscopic cartilage lesion and/or OA-­ related MRI findings such as subchondral bone marrow lesions and/or cartilage and meniscal degeneration The detection of early soft tissue changes within the knee joint on MRI is underlined by the ability to evaluate structures not demonstrated on radiographs, comprising articular cartilage, synovium, menisci, cruciate ligaments, and extra-­articular soft tissues. However, clarity on what constitutes MRI findings of OA was lacking until the Osteoarthritis Research Society International group published defining criteria (Hunter et al. 2011a), which were based on the presence of: • • • •

Osteophytes Articular cartilage loss Subchondral bone marrow lesions and cysts Meniscal damage

These features largely reiterate the characteristics seen on radiographs but focus on earlier changes rather than the description of

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a

Fig. 2  Osteoarthritis. (a) PD(fs) sagittal imaging demonstrates cartilage damage in the trochlea associated with subchondral osteophyte (arrowhead). (b) PD(fs) coronal

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b

imaging shows medial meniscal extrusion (arrow) with medial compartment cartilage damage

b

Fig. 3  Osteoarthritis. (a) Axial T1 shows there is loss of articular cartilage at the patellofemoral joint with osteophyte (broken arrow) and reactive subchondral signal

change (arrowhead). (b) Axial T1 imaging following intravenous gadolinium contrast demonstrates enhancement (arrows) in synovitis

end-stage disease such as bony attrition and deformity as discussed previously in grading severity radiographically. The cross-sectional

nature of the imaging also highlights features of the disease not seen on the conventional radiograph, such as subchondral osteophyte (Fig. 2a).

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a

b

Fig. 4  Osteoarthritis. (a) PD(fs) coronal and (b) PD(fs) sagittal MR imaging shows severe osteoarthritis, with full thickness cartilage loss in the medial joint compartment

and subchondral edema signal. The patient has an anterior cruciate ligament tear (arrow) and degenerative tearing of the medial meniscus

Of note, it incorporates evaluation of the meniscus. Damage may take the form of subluxation, maceration, or degenerative tears (Figs. 2b and 4). MRI is particularly relevant in evaluating the relationship between the integrity of the articular cartilage and meniscal damage. However, this relationship is not straightforward, and it is recognized that meniscal injury and degeneration can be both a cause of cartilage degeneration and a result of osteoarthritis (Rowbotham and Grainger 2017).

correlate with the severity of knee pain (Hill et al. 2001). However, synovitis is a nonspecific ­finding and could also point toward inflammatory arthritides or connective tissue diseases.

3.3.1.1  Joint Effusion These occur frequently in OA and the presence of a large volume of fluid-equivalent signal within the joint on MRI may correlate with degree of knee symptoms (Hill et al. 2001). 3.3.1.2 Synovitis Administration of contrast may be necessary to reliably distinguish synovitis from fluid signal of joint effusion; synovitis is represented by prominent post-gadolinium enhancement in an area of synovial thickening (Fig.  3). This is relatively common in OA, can be severe, and is thought to

3.3.1.3  Periarticular Cysts and Bursitis Periarticular tissues should be evaluated for the presence of high signal, as this may denote bursitis, and/or evidence of fluid communicating between the joint space and the gastrocnemius– semimembranosus bursa as seen in popliteal cysts. 3.3.1.4  Intra-articular Osteochondral (Loose) Bodies Although identified in a wide variety of disease processes, their presence may add support to a diagnosis of OA. 3.3.1.5  Ligaments and Tendons Evaluation of the tendons and ligaments in the case of a chronically painful knee is crucial. There is strong evidence for the interrelationship between a functioning anterior cruciate ligament (ACL) and joint surface preservation; a high

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p­ roportion of those with previous ACL tear have radiographic evidence of accelerated OA (Neuman et al. 2008) attributed to altered biomechanical loading on the joint. However, ACL tears are also seen on MRI in those with knee OA in the absence of a history of trauma. Peri-­ ligamentous edema around the MCL is a well-­ recognized feature of knee OA on MRI and, again, the clinical history will avoid this being wrongly attributed to acute trauma (Bergin et al. 2002). Semiquantitative scoring systems have been developed and refined which incorporate these features of osteoarthritis, providing a “whole-­ organ” approach to evaluating the extent of disease and its progression. The most widely recognized of these are the Whole-Organ Magnetic Resonance Imaging Score (WORMS) of the knee in osteoarthritis (Peterfy et al. 2004), MOAKS (MRI Osteoarthritis Knee Score) (Hunter et  al. 2011b), and KOSS (Knee Osteoarthritis Scoring System) (Kornaat et  al. 2005). These scoring systems are aimed at ­accurately assessing disease burden on MRI but generally are only utilized in research at this time. Although we have an incomplete understanding of the complex underlying pathophysiology in OA, the emerging discipline of identifying imaging biomarkers through quantitative image analysis, such as that seen in cartilage specific

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Fig. 5  Osteoarthritis. (a) Longitudinal imaging of the suprapatellar pouch demonstrates synovitis (arrow) and effusion (asterisk). (b) Longitudinal imaging in the medial

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imaging, may provide targets for the disease-­ modifying drug development pathway (Eckstein et al. 2006) and MRI is currently at the center of this; when considering also the ongoing interest in novel arthroscopic and pharmacological approaches to treating cartilage damage, the demand for MRI knee is likely to continue to accelerate.

3.3.2 Ultrasound Ultrasound of the painful knee allows for inexpensive, high-resolution scanning of superficial structures but provides an incomplete assessment as the cruciate ligaments and the menisci are not fully evaluated (Alves et al. 2016). Much of the hyaline cartilage also lies outside the sonographic window. Although synovitis is typically seen in inflammatory arthritides, it is also a common finding on the assessment of the osteoarthritic joint, and US can detect other common features such as osteophytes which would lend support to making a diagnosis of OA (Fig. 5). The findings of synovitis and its vascularity seem to relate more to the degree of structural change in the joint, than to pain (Hall et al. 2014). Through the ability to easily compare to the contralateral knee, ultrasound may also be used in tandem with MRI for problem-solving or refuting alternative diagnoses. It is also used in the setting of diagnostic and therapeutic inject-

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joint line shows meniscal degeneration and extrusion (asterisk) along with osteophyte

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ables employed in the management of the painful knee, which may be indicated while awaiting MRI or orthopedic intervention.

4 Calcium Pyrophosphate Dihydrate (CPPD) Arthritis 4.1 Overview CPPD arthritis has had multiple labels—CPPD disease, pyrophosphate arthropathy, and pseudogout—and terms such as chondrocalcinosis have often been used interchangeably. This confusing abundance of terminology does little to communicate the pathophysiology of CPPD arthritis, and as such, efforts have been made to standardize terms to provide clarity (Zhang et  al. 2011) (Table 3). Chondrocalcinosis is a term referring to deposition of calcium within the articular cartilage. This is most commonly calcium pyrophosphate dihydrate (CPPD) crystals, but dicalcium phosphate and calcium hydroxyapatite may also be present. This can be detected radiologically; however, imaging findings do not equate to a disease state. Chondrocalcinosis can be an incidental finding, with crystals isolated from joints in the absence of arthritis, but, conversely, radiographic detection of chondrocalcinosis also probably underdiagnoses symptomatic CPPD disease (Rosenthal and Ryan 2016). To be more accurate, Table 3  Defining CPPD arthritis Term CPPD

Asymptomatic CPPD

Acute CPPD crystal arthritis Osteoarthritis with CPPD Chronic CPPD crystal inflammatory arthritis

Clinical presentation Occurrence of calcium pyrophosphate crystals, with or without symptoms Chondrocalcinosis with or without changes of osteoarthritis, but asymptomatic Self-limiting synovitis in the setting of CPPD Typical changes of osteoarthritis in the setting of CPPD Chronic inflammatory arthritis associated with CPPD

the term CPPD arthritis requires the presence of arthritis in conjunction with the evidence of CPPD deposition within the articular cartilage. Clinically, the disease can present along a spectrum (Rosenthal and Ryan 2016). Acute monoarticular CPPD crystal arthritis presents acutely with pain, swelling, and erythema. It is often referred to as pseudogout as the clinical presentation is similar to a gout attack, although unlike gout it has a predilection for the knee joint, instead of the first metatarsophalangeal joint. In contrast, chronic polyarticular CPPD crystal inflammatory arthritis presents over a longer timescale with intermittent pain and swelling, typically in the peripheral joints of the upper and lower limb.

4.2 Imaging Features For many years, diagnosis of CPPD arthritis relied on the McCarty criteria (Ryan and McCarty 1997) which required either a sample of synovial fluid from the affected joint with confirmed chemical composition of CPPD crystals, or a combination of radiological evidence of typical crystal deposits and synovial fluid crystal analysis using polarized light microscopy. In this way, suspected CPPD arthritis is confirmed upon (1) visualization of weakly birefringent rhomboid CPPD crystals and (2) evidence of chondrocalcinosis on a radiograph, in the context of arthritis. However, accurate diagnosis can be hampered by the low sensitivity and specificity of these tests. Imaging findings remain important in the assessment of CPPD arthritis, and conventional radiographs, CT, MRI, and ultrasound all have a role in the investigation of the disease. Fundamental to the radiology is the demonstration of pyrophosphate crystal deposition which is seen in cartilage (chondrocalcinosis) and synovium (Fig. 6). Crystal deposition may also be seen in tendons and ligaments about the knee.

4.2.1 Conventional Radiographs The clinical history is important to inform image interpretation in suspected CPPD arthritis as there are several radiographic features in common with osteoarthritis, including joint space

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Fig. 6  Calcium pyrophosphate arthropathy. (a) AP radiograph demonstrates meniscal chondrocalcinosis. The lateral radiograph (b) demonstrates severe patellofemoral

joint space narrowing with pyrophosphate deposition in the synovium of the suprapatellar pouch (white arrow) and scalloped erosion of anterior femur (black arrows)

narrowing and subchondral cyst formation. Indeed, it is difficult to differentiate chronic CPPD arthritis from OA on the basis of conventional radiography alone, as chondrocalcinosis can be seen in both disease processes. Differentiating features that may be useful include CPPD tending to be more symmetrical, often causing severe articular destruction, and not predominantly affecting weight-bearing joints (glenohumeral and wrist joints commonly affected), and hook-like osteophytes predominantly at the second and third metacarpals can be seen. Characteristically in the knee there is a predilection of CPPD to affect the patellofemoral joint, with joint space narrowing. The severe joint space narrowing at the patellofemoral joint may be associated with anterior femoral erosion (Lagier 1974) (Fig. 6b). Overall, the sensitivity of radiographs for the detection of chondrocalcinosis is low, but it remains the most commonly used imaging modality for the investigation of suspected CPPD arthritis.

4.2.2 CT CT scanning is highly specific and sensitive for the detection of calcification in cartilage and soft tissues, but it is rarely employed in the evaluation of the painful knee in the absence of trauma. Nevertheless, with the ever-increasing utilization of CT scanning, it is worth remembering that those in the demographic most likely to present with a painful knee due to CPPD arthritis may have historical CT imaging. This is particularly useful to check for articular calcification at axial sites. The development of dual energy CT, using dual source imaging to scan at two different energy spectrums, has proved particularly useful for imaging calcified tissues. Proponents of DECT suggest it could be useful in distinguishing CPPD from urate crystal deposition, especially if synovial fluid analysis is not possible (Tanikawa et al. 2018). Further work is required before this is translated to clinical benefit though.

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4.2.3 MRI Although MRI is often performed to investigate a painful knee, there are factors which limit its utility in the diagnosis of CPPD arthritis (Miksanek and Rosenthal 2015). On MR imaging, calcification is represented by areas of punctate or linear decreased signal intensity; this can be detected in hyaline cartilage and may be emphasized on gradient echo imaging (Fig. 7). In contrast, meniscal chondrocalcinosis can easily be missed on MRI because it usually has similar low signal intensity to fibrocartilage. However, occasionally larger aggregates of crystals in menisci can give rise to intermediate signal within a meniscus and mimic a meniscal tear (Burke et  al. 1998) (Fig.  7b). Again, the clinical history is vital in establishing the presence of CPPD arthritis, as there is a differential diagnosis for areas of focal hypointense signal within a joint including both artifactual (magnetic susceptibility artifact particularly in the postoperative setting) and features which are more indicative of OA, such as loose bodies and marginal osteophytes, and other disease processes, for example, pigmented villonodular synovitis and hemosiderin deposition in hemarthrosis. a

In the appropriate clinical context, using MRI in addition to conventional radiographs can give confidence that the detection of discrete areas of low signal intensity on MRI within the articular cartilage represents chondrocalcinosis (especially when the areas are more apparent on gradient echo pulse sequences). It will also demonstrate active inflammation in the joint in the form of synovitis and effusion, but MRI may be more valuable in refuting alternative causes of knee pain.

4.2.4 Ultrasound With the increasing use of ultrasound, with its strengths of low cost and lack of radiation, it has been suggested as a useful adjunct in the diagnosis of CPPD in peripheral joints. A systematic review of the literature to assess its clinical utility (Filippou et  al. 2016) found that US had the potential to be a useful diagnostic tool, but that further work was needed to understand inter- and intraobserver error, and to address the fact that an accepted gold standard for reference was lacking. They did summarize sonographic findings described in the literature; hyperechoic areas (of echogenicity similar to bone) with either punc-

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Fig. 7  Calcium pyrophosphate deposition. (a) AP radiograph shows chondrocalcinosis in the menisci. (b) Sagittal PD(fs) MRI shows punctate low signal foci in the hyaline cartilage (arrowhead) representing chondrocalcinosis.

The heterogenous speckled intermediate signal in the anterior horn of the meniscus represents crystal deposition (arrow). Note also thinning and irregularity of the cartilage in the patellofemoral joint

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tate or linear morphology within hyaline cartilage, punctate within fibrocartilage and linear in tendons. Sensitivity was low at the latter site, presumably due to manifesting later in disease progression. These features seem to be well accepted by those experienced in using ultrasound to examine joints with reported high specificity of diagnosis on US. Calcium pyrophosphate crystal deposition is said to typically appear on ultrasound as bright foci within the normally anechoic hyaline cartilage, in contrast to urate crystal deposition in gout which is said to more typically form on the surface of the cartilage (“double contour sign”). However, concerns over use of ultrasound in practice to distinguish gout and CPPD have been raised (Löffler et al. 2015), with the “double contour sign” seeming to be a poor discriminator between these causes of crystal arthritis. Another study reinforces the utility of ultrasound (Filippou et al. 2013), and it was shown to be adept at characterizing polyarticular involvement; they found an average of 4.7 joints affected, with frequent involvement of tendons and fascia; the knee was most commonly affected and had the highest burden of calcification followed by the wrist, Achilles tendon, plantar fascia, and metacarpophalangeal joints. As well as distinctive sonographic features that make highly specific diagnosis possible by US, it may also be more sensitive for the diagnosis of CPPD than radiography (Miksanek and Rosenthal 2015), according to several studies which use the knee as their site of assessment, particularly in meniscal tissues and hyaline cartilage.

5 Gout as It Affects the Knee 5.1 Overview The musculoskeletal manifestations of gout are caused by deposition of monosodium urate (MSU) crystals in and around the joints secondary to hyperuricemia. It is characterized by acute episodes of severely painful joint synovitis which are typically self-limiting, with quiescent intercritical periods between; but over a longer times-

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cale, joint damage, deformity, and tophus formation can develop. The tophus is the hallmark of chronic gout; this is the soft tissue nodule formed in the granulomatous reaction to MSU crystals and is the causative agent for the characteristic erosions seen adjacent to these sites. With a male predilection and a prevalence exceeding 1% in most developed countries, it is the most common inflammatory arthritis, and the global burden of disease is increasing (Kuo et al. 2015). Significant geographic variation in this data exists as it has both environmental and genetic drivers. Despite being commonly encountered, gout is often misdiagnosed, so expert consensus recommendations for diagnosis have been proposed (Richette et al. 2020). They emphasize that the gold standard for diagnosis still relies on the demonstration of negatively birefringent MSU crystals on polarized light microscopy, and as such, synovial fluid or tophus aspirates should be obtained in all cases of suspected gout. However, there is recognition that imaging has a role to play when clinical diagnosis of gout is uncertain and crystal identification is not possible. It may also be useful for evaluating the extent of crystal deposition, predicting functional outcome by assessing structural joint damage and monitoring response to urate-lowering treatment (Chou et al. 2017).

5.2 Imaging Radiographic features of chronic gout are typically described as well-defined “punched-out” marginal erosions with a sclerotic rim in a juxta-­ articular distribution, with overhanging edges, as well as soft tissue masses corresponding to gouty tophi. Although gout classically affects the first metatarsophalangeal joint, these tophi can occur in both intra- and extra-articular soft tissues of the knee, accompanied by erosion (Figs. 8 and 9). Common intra-articular locations are the popliteus tendon and fossa and the intercondylar notch, with involvement of the cruciate ligaments (Ko et  al. 2010). However, these features often take years to develop, and since untreated gout is

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Fig. 8  Gout. Lateral radiograph showing punched-out erosions of the patella (arrow)

associated with renal and cardiovascular morbidity, as well as a chronic deforming polyarthropathy, a diagnosis should be sought earlier (Chowalloor et al. 2014). The earliest, albeit nonspecific, sign on a radiograph may be a joint effusion. To differentiate gout from other inflammatory arthritides, there is typically preservation of the joint space until late stages of the disease, and a lack of periarticular osteopenia. However, the overall value of the radiograph is limited in a patient presenting with an acute gout flare, and the most likely appearance is of a normal knee radiograph, even if erosions are identified on subsequent advanced imaging modalities (Carter et al. 2009). Ultrasound has better utility for the diagnosis of acute gout (Richette et al. 2020), in particular in demonstrating MSU crystal deposition along the surface of the articular cartilage as a hyperechoic band, “double contour” sign. While this is felt to be highly specific for crystal disease, it is not specific for gout (Löffler et al. 2015), as previously discussed. US can also show gouty tophi as hyperechoic aggregates with a hypoechoic halo in soft tissues in and around the joint. It can

Fig. 9  Gout. AP radiograph, showing partially calcified gout tophus on the lateral aspect of the knee associated with the popliteus tendon (arrow)

also detect the “snowstorm appearance” of hyperechoic foci floating within the joint space, representing MSU “microtophi” within the synovial fluid. Tendon involvement with gout is a feature of the disease and both the patellar and quadriceps tendons represent common sites of involvement (Fig. 10). Care must be taken when bright echogenic foci are seen in the joint fluid. While these may represent the microtophi of gout, other possibilities include fibrinous foci seen in rheumatoid arthritis. It is felt that the presence of bright echogenic foci, representing microtophi, in synovium is a more specific finding (Davies et al. 2019). Although not specific, synovial proliferation is another pathological finding in gout, and US can demonstrate a thickened synovium with increased vascularity on doppler imaging.

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Fig. 10  Gout. Longitudinal ultrasound showing gout involvement of the patellar tendon (arrowheads). The tendon is thickened with loss of the normal architecture and contains bright echogenic foci

Conventional CT is very sensitive in identifying the characteristic features of erosions and tophi, and can be useful in atypical presentations such as mechanical “locking” of the knee, as tophaceous deposits, when calcified, can be clearly delineated (Chatterjee and Ilaslan 2008) (Fig.  11). However, it is the advent of imaging with dual energy CT (DECT) that has garnered attention in the imaging of gout. With different atomic weights, MSU and calcium have differing attenuation characteristics when imaged at different energy levels, and therefore gouty tophi can be differentiated from calcification using DECT (Fig.  12). DECT has been demonstrated to be highly specific and sensitive in the diagnosis of gout and can be particularly useful in atypical presentations or when synovial aspirate is difficult to obtain. Moreover, it can be a useful adjunct for evaluating disease severity and monitoring response to treatment, as it allows for accurate quantification of the volume of crystal burden (Mallinson et al. 2016). MRI readily demonstrates bone erosions, typically with minimal adjacent marrow edema (Fig. 13). Tophi have a spectrum of MR appearances reflecting their variable composition, but most are isointense to skeletal muscle on T1-weighted sequences, low to intermediate on fluid-sensitive sequences, and have variable contrast enhancement following gadolinium. Depending on the activity of the disease, synovi-

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tis and effusion will also be seen. While MRI is sensitive to the soft tissue and osseous changes of gout, the findings are nonspecific and features such as distribution and site affected are often important in making a diagnosis. Given the availability and expense of MRI, ultrasound is often considered more appropriate for follow-up as it can evaluate resolution of synovial hypertrophy and joint effusion as well as showing decreasing size of tophi (Girish et al. 2013). But MRI has a role to play in cases of diagnostic confusion and the monitoring of response to urate-lowering therapies.

6 Rheumatoid Arthritis as It Affects the Knee 6.1 Overview Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease which has a prevalence of up to 1% in most populations studied, with a 2–3 times higher incidence in women than men (Gabriel 2001). Importantly, it does not just affect the joints; it is a multisystem disease with considerable morbidity beyond the musculoskeletal manifestations, which we will focus on here. The rapid advancements in the management of this condition mean that its effects on joints are no longer resulting in the severe deformities seen in previous times, and often perpetuated in textbooks. The majority of rheumatoid arthritis is now diagnosed in the fourth and fifth decades and, with the advance in therapeutic management largely of the latter half of the twentieth century, we should be reframing what we “expect” to see when imaging this disease. However, RA is a polyarticular symmetrical arthritis which is classically thought of as affecting peripheral joints. This is underlined by its diagnostic criteria—patients score higher if more small joints are involved, and the diagnosis is unlikely if it only affects a single large joint. However, since it targets the synovium, all synovial joints can be involved, and indeed large appendicular joints are frequently affected.

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Fig. 11  Gout. Coronal MPR of a conventional CT demonstrates calcified tophi seen (a) medial and (b) lateral (arrows) to the knee joint. The lateral tophus is associated

6.2 Imaging

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with the popliteus tendon sheath, a common location for tophus in the knee (see also Fig. 9)

subchondral bone changes and osteophyte can be particularly helpful in distinguishing the appearKey questions to address are how we recognize ance from OA. However, the fact that the knee is RA in the knee, and how we differentiate it from a weight-bearing joint complicates this, as RA other disease processes such as osteoarthritis. often results in altered biomechanics, which will This is important as RA of the knee can be seen accelerate secondary OA.  Therefore, appearearly in the disease process, so the patient may ances in advanced disease may reflect a mixed not have a formal diagnosis, and the knee is one picture of the RA features with secondary osteoof the commonest large joints to be affected. arthritic changes evident. Ankylosis (fibro-­ osseous fusion of joints) can occasionally be seen 6.2.1 Conventional Radiograph at the knee as a late complication of complete In all large joints, RA is characterized by uniform cartilage loss but is much more common in the cartilage loss, and at the knee the pancompart- wrist and midfoot and more typical of juvenile mental distribution differentiates it from the RA. In small joints, erosions are a characteristic selective compartmental changes described pre- radiographic feature of RA. In the knee these are viously in osteoarthritis and CPPD arthritis. less commonly seen, and when present on radioEarly features of RA on knee radiographs are graphs reflect the late stages of joint typically nonspecific soft tissue changes such as involvement. joint effusion, which on the radiograph is indisWhile the rate of arthroplasty for RA has sigtinguishable from pannus (synovial hypertro- nificantly decreased from a peak in the 1990s, phy), and prepatellar bursitis, followed by the largely thanks to the advances in therapeutic characteristic tricompartmental or non-weight-­ options, RA remains the third most common bearing cartilage loss (Fig.  14). Periarticular indication for arthroplasty (Clement et al. 2012) osteopenia may be evident, and the absence of with good outcome measures for pain relief.

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In terms of adding value when reporting a knee radiograph, preoperatively it is important to assess tibiofemoral alignment and comment on valgus or flexion deformity as this will alter choice of hardware, and it is also worthwhile specifically commenting on the patellofemoral joint, as patellar resurfacing is associated with good operative outcome. Postoperatively, knee arthroplasties in RA are subject to higher rates of complications, so it is important to exclude periprosthetic fracture and assess for any changes concerning for infection (Clement et al. 2012).

6.3 Imaging with Advanced Modalities

Fig. 12  Gout. Reformatted dual energy CT (DECT) image demonstrates extensive urate crystal deposition (coded purple) about the knee

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Both ultrasound and MRI are ideal imaging modalities for the demonstration of early inflammatory changes at the knee in the form of synovitis and effusion (Fig.  15). Associated popliteal cyst formation may also be evident. Both modalities can be used for monitoring treatment response to medical therapies with the assessment of synovitis and effusion. While ultrasound can detect erosive change in the knee, unlike MRI it is not possible to assess cartilage loss reliably or bone edema representing osteitis.

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Fig. 13  Gout. (a) Sagittal and (b) axial PD(fs) imaging shows gout tophus (arrowheads) associated with the quadriceps and its insertion onto the patella. There is associated punched-out erosion of the patella (arrow)

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Fig. 14  Rheumatoid arthritis. (a) AP and (b) lateral radiographs show uniform tricompartmental joint space loss in the absence of OA features. The lateral radiograph

Fig. 15  Rheumatoid arthritis. Ultrasound through the lateral parapatellar recess of the knee shows diffuse thickening of the synovium (asterisks) with anechoic joint effusion (F) seen centrally

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also shows a joint effusion demonstrated in the suprapatellar pouch (asterisk) and Hoffa’s fat pad

MRI is more sensitive than conventional radiographs for the nonosseous and subtle changes that occur early in the disease process, for example, demonstrating synovial hyperemia and hyperplasia (synovitis) and differentiating it from joint effusion (Fig. 16). It is also more sensitive than radiographs for bone erosion, frequently detecting erosions when they are not evident on the radiograph. Juxta-articular bone marrow edema will also be seen, often in association with erosions (Fig. 16). MRI is also able to directly image the articular cartilage and demonstrate tricompartmental loss of hyaline cartilage. Although the emphasis is on early diagnosis of RA and aggressive medical therapies, operative interventions have a role to play, and MRI is useful for preoperative assessment, in particular to establish whether synovectomy or total knee arthroplasty is indicated. For selected patients with early-stage RA, where the knee is identified as a joint site that has proven resistant to medical therapies, synovectomy (removal or destruction of the

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Fig. 16  Rheumatoid arthritis. (a) Axial PD(fs) showing synovitis (arrow) and effusion. (b) Coronal PD(FS) shows extensive posterior synovitis. Bone erosion is seen on the coronal imaging along the sides of the femoral condyles.

Note also the loss of cartilage over the femoral condyles. The axial imaging shows erosions in the posterior femoral condyles are seen associated with adjacent bone edema (arrowheads)

synovium) has been demonstrated to be effective in pain reduction (Ishikawa et al. 1986) by removing the inflamed synovium. However, it does not improve range of motion at the joint and does not delay progression of degenerative changes which often ultimately require TKA (Danoff et al. 2013). The definitive treatment for advanced joint destruction is TKA, and although the revolution in biological therapies has reduced the likelihood of developing late-stage disease, the overall increase in population size and active life expectancy, in a disease process that typically requires arthroplasty earlier than in OA, means decision-making around timing of operative intervention remains crucial, taking into account the lifetime of the prosthetic and the expected revision burden.

most express the protein product of the HLAB27 gene. Five disease subtypes are recognized:

7 Spondyloarthritides as They Affect the Knee 7.1 Overview Spondyloarthritides are a group of arthritides linked by their clinical presentation and immunopathological basis. Affected individuals are typically negative for serum rheumatoid factor and

• • • • •

Ankylosing spondylitis Psoriatic arthritis Reactive arthritis Enteropathic arthritis Undifferentiated spondyloarthritis

They predominantly affect the axial skeleton, with extra-axial manifestations such as uveitis, enthesitis, and peripheral arthritis occurring in all subtypes to varying degrees. However, in recognition of the importance of early disease detection, a classification system was put forward by the Assessment of Spondyloarthritis International Society (ASAS) which makes the distinction based on whether the affected individual has predominantly axial manifestations (axial spondyloarthritis) or only peripheral manifestations (peripheral spondyloarthritis) (Rudwaleit et  al. 2011). This classification also acknowledges the value of imaging in early disease detection, as it differentiates between those with radiographic sacroiliac (SI) joint changes in axial spondyloarthritis and those without (but may have changes on MRI).

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Extra-axial inflammatory features of axial spondyloarthritis, such as synovitis, enthesitis, and capsulitis, are believed to be rare in the absence of bone marrow edema at the SI joints, and in isolation are not sufficient for diagnosis. However, first presentations of SpA in the knee do occur. Furthermore, not all patients express HLA B27, while some will be positive for RhF so the diagnosis needs to be considered when a patient presents with new knee symptoms, and imaging can be helpful. Enthesitis has been proposed as the focus of inflammation in spondyloarthritides. This has been reinforced by the finding on MRI that the frequency of inflamed ligamentous structures in the knees of patients with SpA is higher than in patients with rheumatoid arthritis (McGonagle et al. 1998).

shows characteristic features (Table  1). For the detection of enthesitis in patients with SpA, US has been shown to have higher sensitivity than MRI, and this holds true for the patellar tendon (Kamel et  al. 2004). An expert group (Balint et al. 2018) has developed an US definition and score for enthesitis based on: • • • •

Hypoechogenicity Increased thickness at the enthesis Presence of erosions and enthesophytes Doppler signal at insertion

The other key advantage of US is in that it enables direct feedback between the patient and the radiologist—the patient points to where it hurts, and the radiologist can put a probe at this site and look for evidence of enthesitis. In addition to synovitis and effusion, MRI will 7.2 Imaging also demonstrate features of enthesitis. As well as showing soft tissue changes and erosion, MRI Although the ASAS approach compartmentalizes will show bone edema signal at the enthesis site axial and peripheral SpA, there is much cross- in active disease, representing osteitis (Figs.  19 over between these entities. Many patients with and 20). While bone edema itself is a nonspecific axial SpA have significant issues with dactylitis, finding and other causes need to be considered arthritis, and enthesitis, and the diagnostic (such as trauma), the characteristic entheseal disworkup for peripheral SpA is a lot less clear and tribution of bone edema should lead to SpA being often neglected overall as a result (Carron et al. considered (Fig. 20). 2020), as it is not afforded the binary distinction Since MRI can only usually image a single based on radiographic evidence or absence of body area and the identification of enthesitis can sacroiliitis. As such identifying a site of pain and be so useful to aid diagnosis in peripheral SpA, correctly attributing it to enthesitis can be of and because these patients often present in myrvalue in these patients, as it can lend weight to the iad ways and can provide diagnostic challenges diagnosis of SpA and, even when a patient is to the rheumatologist, problem-solving someclassified as having axial SpA, identification of times involves radionuclide imaging. Although these peripheral changes can guide management. studies have shown that specificity and sensitivIt is reported that 28% of patients with long-­ ity for identification of sacroiliitis on Tc-99m standing ankylosing spondylitis will have con- labeled bone scintigraphy is suboptimal, the ventional radiographic abnormalities in the ability to detect increased radionuclide uptake in knees, the majority bilaterally (Resnick 1974). areas of accelerated bone turnover secondary to Mostly these comprise nonspecific findings such inflammation, among other causes, means it can as joint effusion/synovitis and tricompartmental be a useful way to screen multiple joints in a joint space narrowing. However, features of patient who presents atypically (Khmelinskii enthesitis and periostitis may be recognized on et al. 2018), particularly by including spot views radiographs in the form of bone proliferation and of the peripheries. Looking to the future, wholeerosion (Figs. 17 and 18). body MRI will likely come to the fore to aid disUltrasound has been shown useful for the tinction between polyenthesitis and detection of enthesitis in patients with SpA which fibromyalgia.

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Fig. 17  Ankylosing spondylitis. (a) AP radiograph shows hyperostosis associated with the popliteus insertion (arrow) and proximal tibiofibular joint (arrowhead). (b) Lateral radiograph shows a joint effusion (asterisk). (c)

AP radiograph of the contralateral knee in the same patient. There is an erosion at the popliteus insertion (arrow)

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Fig. 18  Psoriatic arthritis. Lateral radiograph shows typical hyperostosis on the dorsal aspect of the tibia (arrow)

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Fig. 20 Ankylosing spondylitis. Coronal STIR MRI shows multiple sites of bone edema at enthesis sites; for instance, the proximal tibiofibular joint (arrow), ACL attachment (asterisk), and MCL attachment (arrowhead)

8 Synovial Proliferation Associated with Arthritis

Fig. 19  Psoriatic arthritis. Coronal PD(fs) MRI of the knee in a patient with psoriatic arthritis who had atraumatic localized tenderness at the medial collateral ligament (MCL) origin. The MCL is thickened, with high intrinsic signal, surrounding soft tissue edema, and associated bone edema in keeping with enthesitis of the MCL origin (arrow)

The synovium is a specialized membrane lining the joint surfaces, bursae, and tendons and secretes fluid for lubrication and nutrition. It can become diseased through a range of processes, including inflammation, infection, and trauma; it also can undergo neoplastic change in which case getting tissue for a histological diagnosis can be paramount. There are also a range of tumor-like conditions involving synovial proliferation which can cause diagnostic confusion and present as a monoarthritis. The knee is the most common joint to be involved, as it has a large synovial surface area and synovial proliferation at the knee. Since reaching a diagnosis in a timely manner is difficult clinically and histologically, imaging plays an important role. Radiographs might not prove that helpful as they can be nonspecific, typically demonstrating joint effusions in all of these enti-

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Arthritis Table 4  Key differences between types of synovial-based proliferative disorders

Age of presentation

Differentiating feature on radiograph/CT

Differentiating feature on MRI

Synovial osteochondromatosis Primary = 4th–5th decades Secondary = older (degenerative) •  Calcified loose bodies with “ring-and-­ arc” chondroid mineralization •  Extrinsic erosions in primary form • Degenerative changes in secondary form Variable, but will not follow fat signal and no blooming artifact

Pigmented villonodular synovitis 2nd–5th decades

•  Presence of calcification effectively excludes PVNS (unlike synovial sarcoma) •  Minimal degenerative changes and do not see joint deformity (unlike hemophilic arthropathy) •  No rice bodies (unlike osteochondromatosis) Key sequence gradient echo, blooming artifact (which would not be seen in amyloid arthropathy)

ties; ultimately MRI is the modality of choice for definitive diagnosis (see Table 4).

8.1 Synovial Osteochondromatosis Synovial osteochondromatosis is characterized by the presence of multiple intra-articular cartilaginous loose bodies, and is divided into primary and secondary forms which are distinct entities. Primary synovial osteochondromatosis is a benign, typically self-limited, neoplastic process, although rarely it can undergo malignant transformation to chondrosarcoma. However, the importance in recognizing this benign synovial proliferation on imaging is to avoid diagnostic confusion as there is histological overlap in appearance with aggressive chondroid neoplasms. In the context of knee arthritis, these appearances are more likely due to secondary synovial osteochondromatosis, with the formation of multiple intra-articular loose bodies as a result of trauma and osteoarthrosis. However, the primary form of the disease is seen within the knee (Table 5). The condition might be better described as synovial chondromatosis since the ossification of the chondroid bodies is a later feature of the dis-

Lipoma arborescens 5th–7th decades

•  Erosions and rice bodies are uncommon (unlike osteochondromatosis) •  Degenerative changes are common

Follows fat signal on all sequences and does not demonstrate blooming artifact, although chemical shift artifact can be seen at the fat–fluid interface on gradient echo

Table 5  Differences between primary and secondary synovial osteochondromatosis Feature Age of onset Background joint disease

Primary Younger Less associated degenerative change

Appearance of loose bodies

More, smaller intra-articular bodies with less variation in size and single ring of calcification

Secondary Older Prominent background degenerative change Fewer, larger intra-articular bodies with more variation in size and several rings of calcification

ease process. Three phases of the primary form of the disease are recognized. Initially metaplasia of the synovium occurs to produce chondroid nodules. In the second phase the nodules start to detach, and in the third, inactive, phase the synovial proliferation resolves leaving the cartilaginous bodies in the joint which, with time, start to ossify (Greenspan and Grainger 2018). Once the chondral bodies have started to ossify they will be apparent on radiographs and in the primary form of the condition generally have a uniform appearance. Early in the disease process there may be no radiographic features until the chondral bodies start to ossify. In the advanced stages secondary osteoarthritis may

H. W. Christopher et al.

450

a

b

Fig. 21 Primary synovial osteochondromatosis. (a) Lateral radiograph and (b) sagittal PD(fs) MRI show multiple fine chondral bodies within the joint; the radiograph

shows some of these have started to ossify in the region of Hoffa’s fat pad

develop and erosion of bone may occur. CT delineates the calcified intra-articular fragments and also identifies extrinsic bone erosion with greater sensitivity. MR imaging findings are more variable, but MRI has the advantage of being able to identify the chondral bodies before ossification has occurred (Fig. 21).

ease process can affect synovium within joints, bursae, and tendons, it is classified according to its site of origin (intra-articular or extra-articular) and pattern of growth (localized or diffuse). The diffuse form of the disease is now known as diffuse-­type giant cell tumor under the World Health Organization (WHO) classification system, but PVNS remains a common synonym. The intra-articular form of this diffuse disease most commonly occurs in the knee. Localized PVNS is most commonly extra-articular and involves tendon sheaths (localized giant cell tumor of tendon sheath) but can occur in intraarticular form. The intra-articular localized form is almost exclusive to the knee where it most commonly occurs in Hoffa’s fat pad but can be seen in the suprapatellar pouch and intercondylar notch. A feature of PVNS is its ability to

8.2 Pigmented Villonodular Synovitis PVNS is a fibrohistiocytic proliferation of the synovium which affects the knee in 80% of cases and is typically monoarticular. It comprises villous and nodular proliferations which have a characteristic yellow-brown pigmentation due to deposits of lipid and hemosiderin. Since the dis-

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invade bone producing cysts and erosions in the joint. Radiographs can be nonspecific a soft tissue density which may represent the synovial mass but is often interpreted as a joint effusion. Marginal erosions, typically well-defined with sclerotic margins may also be present, but are less commonly seen on radiographs in the knee than in other joints so differentiation from synovial chondromatosis and lipoma aborescens can be challenging. Calcifications are very rare (Lin et al. 1999) and their presence should raise concern for other disease entities, including synovial sarcoma. CT can demonstrate a hypertrophic synovial soft tissue mass, but it is MRI that is the key modality for diagnosis; the mass-like synovial proliferation typically demonstrates low T1 and T2 signal intensity (Fig. 22) and “blooming” artifact on gradient echo due to hemosiderin causing magnetic susceptibility artifact (Fig.  23). On cross-sectional imaging erosion and bone invasion may also be more evident.

451

seen at the fat–fluid interface on gradient echo (Fig. 24).

9 Hemophilic Arthropathy 9.1 Overview Hemophilic arthropathy is a joint disease that occurs due to recurrent spontaneous or minimally traumatic intra-articular hemorrhage in those affected by hemophilia. It typically affects large joints in a mono- or oligoarticular pattern, with the knee being the most commonly affected. Up to 90% of severe hemophiliacs develop ­arthropathy, and it is a significant cause of morbidity and loss of function (Doria 2010).

9.2 Imaging

Hemorrhage within the joint leads to synovial hypertrophy and hemosiderin deposition, which stimulates synovitis. Left unchecked, these episodes of synovitis lead to progressive cartilage 8.3 Lipoma Arborescens and subchondral bone damage, with resultant joint space narrowing and, eventually, end-stage This is a rare condition characterized by “frond-­ joint destruction and possibly ankylosis (Ng et al. like” deposition of mature lipocytes within the 2005). The following features can be seen at mulsynovial lining of the joints and bursae, although tiple large joints on radiography secondary to most commonly found at the suprapatellar bursa. both hyperemia and hemarthrosis, with specific It is typically sporadic in nature, although it has findings at the knee affected by hemophilic been observed to occur in the context of trauma, arthropathy demonstrated in Fig. 25. and it is common to see associated meniscal damage. • Joint effusion—hemarthrosis may have fluid-­ On radiographs it again typically presents as a fluid levels relating to blood products joint effusion. It can be polyarticular and while • Periarticular osteoporosis its etiology is unknown associations with OA, • Epiphyseal enlargement with associated gracRA and diabetes are proposed and there are often ile diaphysis features of osteoarthritis present on the radio- • Secondary degenerative process—symmetrigraph (Greenspan and Grainger 2018). CT typical pan-compartment loss of cartilage, with cally shows a fat density minimally enhancing periarticular erosions and subchondral cysts, intra-articular mass, and in comparison to synoosteophytes, and sclerosis vial osteochondromatosis, erosions are uncommon. Again, MRI is the modality of choice for Radiographs have long been used in the diagdiagnosis, as the lesion follows fat signal on all nosis of hemophilic arthropathy, and grading syssequences and does not demonstrate blooming tems such as the Arnold–Hilgartner classification artifact, although chemical shift artifact can be (see Table  6) formalizes the largely sequential

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a

b

c

d

Fig. 22  PVNS. (a, b) Sagittal T2(fs), (c) axial PD(fs), and (d) T1-weighted MRI demonstrates a synovial mass in the medial suprapatellar pouch. Note the low signal

seen on the T2 and PD sequences and intermediate T1 signal with low signal foci

changes seen at different joints. Radiography remains useful for the assessment before arthroplasty or arthrodesis in end-stage hemophilic arthropathy, but the goal of modern therapy involves initiating clotting factor replacement before changes to the joint are detectable on radiography (Ng et  al. 2005). Studies have shown

that the later prophylaxis is started after the first incidence of hemarthrosis, the higher the future risk of arthropathy (Fischer et al. 2002). With this shift in treatment paradigm toward joint preservation, the value of modern imaging is in the detection of early changes to the joint, such as synovial hypertrophy. This allows for

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a

Fig. 23  Diffuse PVNS. (a) Sagittal PD(fs) and (b) sagittal T2* GRE MRI.  There is nodular thickening of the synovium in the suprapatellar pouch, Hoffa’s fat pad, and

a

453

b

the intercondylar notch. Note the characteristic blooming artifact seen on T2* imaging as a result of hemosiderin deposition

b

Fig. 24  Lipoma arborescens. (a) Sagittal T1 and (b) coronal T1 MR imaging shows villous proliferation and thickening of the synovium with fat signal (arrowheads). Note also an osteophyte on the coronal imaging (arrow)

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a

b

Fig. 25 Hemophiliac arthropathy. (a) AP radiograph demonstrates features of chronic hemophiliac arthropathy. Note the widened intercondylar notch, flattened condyles,

and subchondral cysts. (b) The lateral radiograph shows characteristic squaring of the patella

Table 6  Arnold–Hilgartner classification—a plain radiograph grading system for hemophilic arthropathy of the knee (Arnold and Hilgartner 1977)

the removal of the hypertrophied synovium to halt the cycle of destruction and bleeding; this is initially attempted by synoviorthesis (also known as radionuclide synovium ablation), but open or arthroscopic synovectomy are also possible (Teyssler et al. 2013; Nacca et al. 2017). The use of ultrasound and MRI as modalities for early imaging assessment in this context has therefore attracted attention. MRI has the advantage that it can demonstrate all of the articular (synovium, cartilage, and bone) and periarticular structures and is sensitive for the detection of the preclinical changes undetectable on conventional radiography. Typically, synovium will be thickened with low signal on all sequences due to hemosiderin deposition

Stage 0 Normal joint Stage 1 No skeletal abnormalities, soft tissue swelling is present Stage 2 Osteoporosis and overgrowth of the epiphysis, no cysts, no narrowing of the cartilage space Stage 3 Early subchondral bone cysts, squaring of the patella, widened notch of the distal femur or humerus, preservation of the cartilage space Stage 4 Findings of stage III, but more advanced; narrowed cartilage space Stage 5 Fibrous joint contractures, loss of the joint cartilage space, extensive enlargement of the epiphyses with substantial disorganization of the joint

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a

455

b

Fig. 26  Hemophiliac arthropathy. (a) Coronal T1 and (b) coronal PD(fs) MRI shows hemosiderin laden synovium which demonstrates low signal on both T1 and the fluid-­

sensitive imaging. Note also the flattened articular surfaces and subchondral cysts (arrowheads)

(Fig.  26). Pigmented villonodular synovitis also commonly affecting the knee will show hypointense synovial lesions (Narváez et  al. 2003); if the initial diagnosis of siderotic synovitis is in doubt, then consideration of the appearances of these disease entities is important (see Sect. 8). However, the application of Doppler in ultrasound allows for discrimination between an effusion and increased vascularity of synovitis (the latter will demonstrate increased doppler flow). Although user-dependent, with its lower cost, greater accessibility, and the fact that it does not require sedation in young patients, there is a strong argument for disease monitoring in hemophiliac arthropathy using ultrasound.

Sect. 2 and allows the recognition of these disease features on radiographs and the advanced imaging modalities. Subsequently the authors have provided an overview of the common arthritides seen in the knee and reviewed the general and specific features seen on imaging as the disease process affects the knee. An emphasis has been placed on distinguishing the features of the different arthritides from each other. We have also tried to highlight the features of imaging studies that should make the radiologist think that the case they are looking at is not simply another example of osteoarthritis.

References 10 Conclusion As this chapter has shown there are common features to many arthritides affecting the knee, including synovitis, effusion, and cartilage damage. These have common features discussed in

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Tumors and Tumorlike Lesions Anish Patel, A. Mark Davies, and Daniel Vanel

Contents 1    Introduction 

 459

2    Detection 

 460

3    Diagnosis  3.1  Diagnosis of Bone Tumors  3.2  Diagnosis of Soft Tissue Tumors  3.3  CT and MR Imaging in Diagnosis 

 461  461  469  470

4    Surgical Staging 

 471

5    Imaging Follow-Up 

 476

6    Bone Tumors  6.1  Benign Bone Tumors  6.2  Malignant Bone Tumors  6.3  Patellar Tumors 

 478  478  489  496

7    Soft Tissue Tumors  7.1  Benign Soft Tissue Tumors  7.2  Malignant Soft Tissue Tumors 

 498  498  498

8    Joint Tumors  8.1  Benign Joint Tumors  8.2  Malignant Joint Tumors 

 502  502  508

9    Tumorlike Lesions of Bone  9.1  Periosteal Desmoid  9.2  Stress Fractures  9.3  Inflammatory Conditions  9.4  Brown Tumors 

 510  510  510  512  514

References 

 514

1 Introduction Bone sarcomas are uncommon when compared with other malignancies, accounting for only 0.2% of all tumors (Dorfman and Czerniak 1995). Their incidence is approximately one-tenth that of soft tissue sarcomas (Mack 1995) and one-­ sixtieth that of either lung or breast carcinoma. The annual incidence for bone sarcomas is approximately 0.8/100,000 (Dorfman and Czerniak 1995). The subject is particularly pertinent when dealing with the knee, as the distal femur and proximal tibia are the sites of predilection for many benign and malignant bone tumors (Table 1). The purpose of this chapter is to detail the role of imaging in the detection and diagnosis of bone and soft tissue tumors in and around the knee joint, as well as in the surgical staging and follow-up of patients after initial treatment. We shall also highlight the numerous tumorlike lesions, which may confuse the unwary observer. Unless otherwise stated, incidence data quoted have been calculated by combining results from several authoritative texts on the subject (Mulder et al. 1993; Campanacci 1999; Unni 2010).

A. Patel (*) · A. M. Davies MRI Centre, Royal Orthopaedic Hospital, Birmingham, UK e-mail: [email protected] D. Vanel Department of Radiology, Institut Gustav Roussy, Villejuif, France Med Radiol Diagn Imaging (2023) https://doi.org/10.1007/174_2023_414, © The Author(s), under exclusive license to Springer Nature Switzerland AG Published Online: 29 April 2023

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460 Table 1  Relative overall incidence of the three commonest sarcomas of bone (column A), incidence of each type around the knee (column B), and risk of pathological fracture (column C) Osteosarcoma Chondrosarcoma Ewing’s sarcoma

A 35% 26% 16%

B 66% 17% 22%

C 9% 12% 6%

From the WHO classification of Tumours 5th Edition (2019)

2 Detection The majority of patients with a bone or soft tissue tumor will present with pain and/or swelling. Alternatively, a pathological fracture may be the precipitous presenting feature. Occasionally, a bone tumor, typically a benign lesion, can be an incidental radiographic finding. Despite newer imaging techniques, the radiograph is the preliminary and single most important imaging investigation. Frequently, the diagnosis may be obvious to the trained eye, and further imaging, if required, is then directed toward staging the lesion. Alternatively, if an abnormality is present on the film and the precise nature is not immediately apparent, certain features will indicate a differential diagnosis and other forms of imaging can then be employed to assist in establishing a more definitive radiological diagnosis. If the initial radiograph is normal, however, with persisting and increasing symptoms, a repeat radiograph may be indicated in due course. Early signs of a bone tumor, or for that matter infection, include subtle areas of ill-defined lysis or sclerosis, cortical destruction, periosteal new bone formation, and soft tissue swelling (Fig. 1) (Rosenberg et  al. 1995). Not surprisingly, bone lesions are frequently missed or overlooked on the initial radiograph. In an audit from the 1990s, in approximately 20% of cases, neither the clinician nor the radiologist at the referring center detected the bone tumor on initial radiographs, although evidence was present on retrospective review of the films (Grimer and Sneath 1990). This is a bigger problem with tumors of flat bones, such as the pelvis, than around the knee

Fig. 1  AP radiograph of the knee in a 15-year-old boy showing an early lytic osteosarcoma of the proximal tibial metaphysis

joint. The pathological process may be well established even in the presence of a normal radiograph. At least 40–50% of trabecular bone must be destroyed before a discrete area of lucency can be discerned on the radiographs (Ardran 1951; Edelstyn et al. 1967). Erosion or destruction of the cortex is more readily apparent. It is self-evident that the smaller the bone involved, with a greater proportion of cortical to medullary bone, the easier it will be to detect an abnormality on radiographs. Therefore, were three tumors of similar size to arise around the knee joint, it is fair to assume that the lesion affecting the smallest bone, the proximal fibula, would be radiographically more conspicuous than those in the femur or tibia. Beware, however, the patella. Although it is a rare site, tumors of the patella are easily overlooked as the bone is projected over the distal femur on the anteroposterior projection. In the presence of a normal radiograph, referred pain needs to be considered. Hip joint pathology presenting with referred pain to the knee is a well-recognized entity in the child. If

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Fig. 2  Enchondroma of the proximal tibia that was initially an incidental finding on MR imaging, (a) AP radiograph; (b) sagittal and axial PDFS image shows typical lobulated high T2 signal appearances of an enchondroma

referred pain is suspected, then radiographs of the pelvis and proximal femur are indicated. On occasion, radiographically occult lesions may be detected by bone scintigraphy and/or magnetic resonance (MR) imaging. It is important to stress that, owing to its high sensitivity, MR imaging all too frequently reveals abnormalities of little or no clinical significance. An increasing number of knee MR scans are performed each year for a variety of conditions. Incidental medullary abnormalities will be revealed in the distal femoral meta-diaphysis in a small percentage of cases, which almost invariably prove to be innocuous enchondromas (Fig. 2).

3 Diagnosis 3.1 Diagnosis of Bone Tumors Once a skeletal abnormality has been detected around the knee, the next objective of imaging is to attempt to characterize the lesion and, in doing so, to indicate an appropriate differential diagnosis to the referring clinician. At this stage, important maxims that should be appreciated include not overtreating a benign lesion, not undertreating a malignant lesion, and not misdirecting the approach to biopsy, which might prejudice subsequent surgical management (Moser and Madewell 1987). Before assessing the imaging, the prudent

radiologist should establish some basic facts regarding the patient. By recognizing the relevance of certain clinical details, an extensive differential diagnosis may be significantly reduced even before the imaging is considered. Important factors to be considered include the following: 1. Age. The age of the patient is arguably the single most useful piece of information as it frequently influences the differential diagnosis. Many musculoskeletal neoplasms exhibit a peak incidence at different ages. For osteosarcoma, this is in the second and third decades (Fig.  1). Metastases and myeloma should always be considered in a patient over 40 years of age (Fig. 3). Similarly, metastatic neuroblastoma should be in the differential diagnosis at 2  years of age or under. Conversely, a tumor arising in adolescence or early adult life is unlikely to be a metastasis. 2. Gender and ethnic origin. Many bone tumors occur more commonly in boys, but this fact does not play a significant role when formulating the differential diagnosis. Ewing’s sarcoma is unusual in that it is prevalent in Caucasians but is rarely seen in Afro-­ Caribbeans. A number of nonneoplastic lesions that may on occasion simulate neoplasia also show a racial disposition, e.g., sickle cell, Gaucher’s disease, and Paget’s disease. The geographic origin of the patient may also

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Fig. 3 (a) AP and (b) lateral radiograph in a 60-year-old female with a solitary endometrial metastasis in the proximal tibial metaphysis extending into the subarticular

bone. The radiographic differential includes GCT as well as myeloma and lymphoma

be significant in that the incidence of bone and joint infection is much more common in the underdeveloped countries. 3. Family history. There is little evidence of a familial predisposition to the formation of musculoskeletal neoplasms in most instances. The exception is certain congenital bone conditions which may undergo malignant transformation, e.g., diaphyseal aclasis (Fig.  4), Ollier’s disease, and Maffucci syndrome. 4. Multiplicity. It is critical early in the management of a patient to establish whether a lesion is solitary or multiple as this will influence the differential diagnosis. Frequently, this question will not be definitively answered ­ until the staging imaging is performed.

accurate of all the imaging techniques currently available in determining the differential diagnosis of a bone lesion (Kricun 1983). Although many lesions will be instantly recognizable, it is prudent to analyze the radiographic features present. The analysis can be performed by answering the following questions: Which bone and what part of the bone are involved? What is the tumor doing to the bone (pattern of destruction)? What form of periosteal reaction, if any, is present? What type of matrix mineralization, if any, is present?

It is at this stage that attention should turn to the imaging. The radiograph remains the most

Site in Skeleton  Most bone tumors and infections occur around the knee, and as such, little diagnostic information can be deduced from noting the affected bone in most cases. There are exceptions. Most tumors arising in the patella are benign. In the proximal fibula, the commonest

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Fig. 4 (a) AP radiograph showing a peripheral chondrosarcoma in a patient with diaphyseal aclasis. (b) Axial PDFS MRI showing the peripheral chondrosarcoma (white arrow)

benign tumors were osteochondroma, enchondroma, and ABCs and the commonest malignant tumors were osteosarcoma, Ewing’s sarcoma, and chondrosarcoma (Arikan et al. 2018). Location in Bone  The site of origin of a bone tumor is an important parameter of diagnosis (Fig. 5) (Madewell et al. 1981). It reflects the site of greatest cellular activity. During the adolescent growth spurt, the most active areas are the metaphyses around the knee and in the proximal humerus. Tumors originating from marrow cells may occur anywhere along the bone. Conventional osteosarcoma will tend to originate in the metaphysis or meta-diaphysis, while Ewing’s sarcoma tends to arise in the metaphysis or, more distinctively, in the diaphysis (Fig.  6). In the child, the differential diagnosis of a lesion arising within an epiphysis can be realistically limited to chondroblastoma (Fig.  7), epiphyseal abscess (pyogenic or tuberculous), and, rarely, eosino-

philic granuloma. Following skeletal fusion, ­subarticular lesions, analogous in the adult to the epiphyseal lesions, include GCT (Fig.  8), clear cell chondrosarcoma (rare), and intra-osseous ganglion. Most cases of osteomyelitis will arise within the metaphysis of a long bone, typically the tibia and femur. It can also be helpful to identify the origin of the tumor with respect to the transverse plane of the bone. Is the tumor central, eccentric, or cortically based? For example, a simple bone cyst, fibrous dysplasia, and Ewing’s sarcoma will tend to be centrally located. Chondromyxoid fibroma and fibrous cortical defect/nonossifying fibroma (Fig. 9) are typically eccentric. Lesions that usually arise in an eccentric position may appear central if the tumor is particularly large or the involved bone is of a small caliber. Therefore, most tumors arising in the proximal fibula will appear “central.” There are numerous surface

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Fig. 6 (a) AP radiograph and (b) magnified view of an Ewing’s sarcoma of the distal femoral diaphysis. Typical features include the ill-defined bone destruction, lamellar periosteal reaction (white arrow), and Codman angle (black arrow)

Fig. 5  A composite diagram of the sites of origin of primary bone tumors and the location in the bone in which they most commonly occur. (CB chondroblastoma, GCT giant cell tumor of bone, ABC aneurysmal bone cyst, OS osteosarcoma, EC enchondroma, SBC simple bone cyst, CMF chondromyxoid fibroma, FCD fibrous cortical defect, FD fibrous dysplasia, O osteoid osteoma, AM adamantinoma, RCT round cell tumor)

lesions of bone which are related to the outer cortex (Kenan et al. 1993a; Seeger et al. 1998). The majority of the malignant surface lesions of bone are the rare forms of osteosarcoma, e.g., periosteal, high-grade surface, and parosteal osteosarcoma. Most of the cases of parosteal osteosarcoma arise from the posterior metaphysis of the distal femur. Pattern of Bone Destruction  Analysis of the interface between tumor and host bone is a good indicator of the rate of growth of the lesion. A sharply marginated lesion usually denotes slower growth than a non-marginated lesion (Fig.  10). The faster the growth, the more aggressive the pattern of destruction and the wider the zone of

transition between tumor and normal bone (Fig.  1). Aggressivity per se does not conclusively indicate malignancy, but the malignant tumors tend to be faster growing than their benign counterparts. Geographic bone destruction is the term applied to bone lesions that appear well marginated with a thin zone of transition. The thicker the sclerotic border, the longer the host bone has had to respond to the lesion and, therefore, by implication, the slower the rate of growth of the lesion. The vast majority of bone tumors in children showing a geographic pattern of destruction are benign, such as simple bone cyst (SBC), ABCs, fibrous dysplasia, and enchondroma. Moth-eaten and permeative bone destruction are terms used to describe bone destruction in which there are multiple tiny cortical lucencies with an ill-defined zone of transition. These patterns indicate the aggressive nature of these lesions in contrast to those with a geographic pattern. The rapid growth of these lesions does not allow the host bone sufficient time to react and produce a response. Typically, malignancies,

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Fig. 7 (a) AP radiograph in a 14-year-old boy with a chondroblastoma arising in the proximal tibial epiphysis (white arrow). The matrix mineralization is easily seen on

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Fig. 8 (a) AP radiograph in a 30-year-old female showing subarticular lesion in the proximal tibia. There is no matrix mineralization. (b) Sagittal T1 and (c) sagittal

the radiograph. (b) Sagittal STIR MRI showing intense bone marrow edema surrounding the lesion

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STIR sequences show typical appearances of a GCT with low signal intensity on the T1 and intermediate to low signal intensity on the STIR sequence

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the more rapid the appearance of radiographic change and vice versa. Periosteal reaction, otherwise known as periosteal new bone formation, may occur in any condition which elevates the periosteum, whether it be blood, pus, or tumor. The appearance and nature of a periosteal reaction are frequently valuable in narrowing down the differential diagnosis of a bone tumor.

Fig. 9 AP radiograph showing eccentrically located lesions in the distal femur (white arrow) and the proximal tibia (black arrow) with well-defined sclerotic margins in keeping with multiple nonossifying fibromas/fibrous cortical defects

including osteosarcoma, Ewing’s sarcoma, and neuroblastoma metastasis, exhibit a moth-eaten or permeative pattern of bone destruction (Fig. 1). Acute osteomyelitis is the “benign” condition which may also give a moth-eaten appearance. Periosteal Reaction  The periosteum is normally radiolucent but will mineralize when stimulated by an adjacent osseous or para-osseous process. The rate of mineralization is partly dependent on the age of the patient. The younger the patient,

A “shell” is used to describe a lytic lesion with bone expansion. The shell is the periosteal new bone laid in response to the growing tumor. The thicker the shell, the slower growing the lesion and vice versa. Shells are typically found in benign lesions such as SBC, ABC, fibrous dysplasia, and chondromyxoid fibroma (Fig.  11). They may also be seen with a telangiectatic osteosarcoma, which frequently mimics an ABC. In the older age group, shells are found in expansile metastases from renal and thyroid primaries and plasmacytoma. A lamellar periosteal reaction is seen in many traumatic and inflammatory conditions. The lamellated periosteal reaction, otherwise known as onionskin, is seen in Ewing’s sarcoma, osteosarcoma, eosinophilic granuloma, and acute osteomyelitis (Fig.  12). A spiculated periosteal reaction occurs when the mineralization is oriented perpendicular to the cortex and denotes a more rapidly evolving process. It is typical of malignant tumors such as osteosarcoma and Ewing’s sarcoma (Fig.  13a). It may be seen in benign tumors such as hemangioma of bone and nonneoplastic conditions such as thalassemia and thyroid acropachy, but not in relation to the knee. A Codman angle is also a type of periosteal reaction seen with a more rapidly evolving process. The periosteum does not have the time to form a complete shell around the lesion, and only the raised edge of the periosteum has time to ossify. It forms an “angle/triangle” of new bone at the edge of the lesion (Fig. 13). Matrix  A number of tumors produce a matrix, the intercellular substance, that can calcify or

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Fig. 10 (a) AP radiograph of the knee showing well-­ marginated lesion in the proximal fibula with a narrow zone of transition between the normal bone and the abnormal bone. This was an ABC. (b) AP radiograph of the

knee showing a non-marginated lesion with a wide zone of transition in the distal femoral metaphysis; note its ill-­ defined edge and wide zone of transition. This turned out to be an osteosarcoma

ossify. The radiodense foci should be differentiated from other causes of calcifications such as fracture callus, sclerotic response adjacent to a tumor, necrotic debris, and dystrophic calcification. Radiodense tumor matrix is either osteoid, chondroid, or fibrous. Tumor osteoid is typified by solid (sharp-edged) or cloud to ivory-like (ill-­ defined edge) patterns (Fig.  14a). Tumor cartilage is variously described as stippled, flocculent, ring and arc, and popcorn in appearance (Fig. 14b). Fibrous tumor matrix is seen in fibrous dysplasia, where the collagenous matrix may be

sufficiently dense to give a “ground-glass” appearance (Fig. 14c). Identifying the pattern of matrix calcification will significantly reduce the differential diagnosis, but matrix per se has no influence as to whether the lesion is benign or malignant. The distribution can be helpful. For example, both enchondroma and medullary infarction, which frequently arise in the distal femur, may show calcification of a similar nature. However, the distribution is typically central in enchondroma, while it is peripheral in medullary infarction.

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Fig. 11 (a) AP radiograph and (b) axial T2 FS MRI of an ABC of the proximal fibula. There is a typical expanded shell with multiple fluid-fluid levels on the MRI

Fig. 12 (a) AP radiograph showing a lamellar periosteal reaction (white arrow). (b) Magnified image clearly shows multiple lamellae giving the “onionskin” appearance

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and bone involvement. The absence of any bony abnormality in the presence of a clinically palpable mass immediately indicates that the pathology is of soft tissue origin, albeit with a large differential diagnosis. The radiodensity of most soft tissue masses approximates to that of water and is similar to that of muscle; such masses are, therefore, only revealed by virtue of their mass effect. In a minority of cases, part or all of the tumor may exhibit a radiodensity sufficiently different from that of water for it to be visualized directly on radiographs. Lipomas, the commonest of all soft tissue tumors, produce a low radiodensity between that of muscle and air. For this reason, lipomas are typically well demarcated from the surrounding soft tissues and, if of sufficient size, can be diagnosed on radiographs with moderate confidence (Fig. 15). It should be noted that low-grade liposarcomas may contain variable amounts of fat that will also appear relatively radiolucent on radiography. A low-kilovoltage technique will accentuate the differences between fat and muscle. Increased radiodensity may be seen in the tissues due to hemosiderin, calcification, or ossification. Hemosiderin deposition typically occurs in synovial tissues exposed to repeated hemorFig. 13  AP radiograph showing a distal femoral mixed rhage such as is seen in pigmented villonodular sclerotic and lytic lesion consistent with an osteosarcoma. synovitis. Calcification or ossification in the soft See the spiculated periosteal reaction where the periosteal tissues is a feature of a large spectrum of patholonew bone is laid down at right angles to the bone (white arrow). Codman angle is formed at the edge of the lesion gies, including congenital, metabolic, endocrine, due to new bone formation at the edge of the lesion form- traumatic, and parasitic infections. Primary soft ing an angle with the native bone (black arrow) tissue tumors are one of the less common causes of calcification that the general radiologist can expect to come across in his or her routine 3.2 Diagnosis of Soft Tissue practice. Tumors Ultrasound is an important technique in the initial assessment of a suspected soft tissue mass. The lack of contrast resolution is a well-­ First, it can confidently confirm or exclude the recognized limitation of radiography, but the presence of a mass. Second, it can to a degree value of examination should not be underesti- characterize the lesion by distinguishing purely mated in the evaluation of soft tissue masses. It cystic lesions, such as ganglia, from solid tumors. will not identify the precise diagnosis in the Doppler ultrasound can be employed to assess majority of cases, but it can still provide valuable the vascularity of a lesion, and ultrasound is ideinformation, e.g., on the presence of calcification ally suited for image-guided biopsy.

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Fig. 14  Tumor matrix production. (a) AP radiograph of the knee showing dense osteoid matrix in a patient with a proximal tibial osteosarcoma. (b) AP radiograph of the distal femur showing the typical appearance of chondroid matrix often described as flocculent, ring and arc, or pop-

corn (white arrow) in a patient with dedifferentiated chondrosarcoma. (c) Lateral tibial radiograph showing typical appearances of fibrous matrix. It has been described as being ground glass (appears like a thin veil when placed over the bone)

3.3 CT and MR Imaging in Diagnosis

physical basis of MR imaging is very different, similar morphological information can be easily identified. The exception is the signal voids of fine mineralization, which can be easily missed on MR imaging. Potentially misleading MR features that might suggest a bone sarcoma are prominent marrow edema and soft tissue edema (Hayes et al. 1992). These are, however, common with osteoid osteoma, osteoblastoma, chondroblastoma, stress fracture, and infection. Many soft tissue sarcomas will appear well defined on MR imaging owing to the presence of a pseudo-capsule, whereas inflammatory processes, such as abscesses, will appear poorly defined owing to the surrounding inflammatory exudate. The majority of tumors will have prolonged T1 and T2 relaxation times, thereby showing low to intermediate signal on T1-weighted and high sig-

The principal role of computed tomography (CT) and MR imaging in the management of the patient with a suspected musculoskeletal tumor is in staging (see Sect. 4). In selected cases, both techniques can be useful in establishing a differential diagnosis. The CT features that should be assessed are similar to those described above when evaluating the radiographs. This reflects the fact that both are radiographic techniques relying on the attenuation of an X-ray source. Cortical breaching, soft tissue extension, and faint mineralization are all more readily appreciated on CT scans than on radiographs. Assessment of CT attenuation values will allow distinction between fat-containing and fluid-containing masses. Although the

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Dynamic contrast-enhanced MR imaging has been used to differentiate benign from malignant bone lesions using the slope of the derived time-­intensity curves (Verstraete et  al. 1994). Benign bone lesions tend to show a low slope as compared with the high or steep slope of malignant lesions; however, there is considerable overlap such that this technique is of limited value in routine practice. For example, highly vascularized or perfused lesions such as ABC, eosinophilic granuloma, osteoid osteoma, and acute osteomyelitis may all show slope values in the malignant tumor range. Similarly, in the soft tissues, early myositis ossificans will show a steep slope mimicking malignancy.

4 Surgical Staging Accurate surgical staging is a fundamental requisite of all oncological imaging. The staging system regularly used for bone and soft tissue sarcomas is that adopted by the Musculoskeletal Tumor Society (Enneking et  al. 1980). This assigns one of the three grades according to the local extent of the tumor, the presence or absence of metastases, and the histological Fig. 15  AP radiograph of the mid- and distal thigh show- grade (Table  2). Clarification of the first two ing a large lesion with a lower density mass than the sur- features of the staging system relies entirely on rounding muscle consistent with a lipoma (white arrows) imaging. The value of a straightforward staging system, such as this, is that it is easily applied, nal on T2-weighted sequences. T1 shortening, correlates well with prognosis, and allows valid with a high signal intensity, will be seen in fat-­ comparison of studies of differing treatments containing tumors, subacute hemorrhage, mela- and treatment centers. An alternative staging noma metastasis, and gadolinium chelate system for bone sarcomas is the American Joint enhancement. A low signal intensity on Committee on Cancer, which identifies tumor T2-weighted images is seen with dense mineral- extension, i.e., whether it is confined to bone ization, hypocellular/fibrous tumors, signal voids (T1) or extends beyond bone (T2), grade (G1– from flowing blood, hemosiderin deposition, sur- 4), nodal involvement (N), and distant tumor gical implants, and bone cement. Fluid-fluid lev- spread (M). els are well demonstrated on both CT and MR Determination of local tumor extent around the imaging in a large number of different musculo- knee usually relies on MR imaging. One study has skeletal conditions. In the immature skeleton with shown CT to be as good as MR imaging in staging the appropriate radiographic appearances, fluid- (Panicek et  al. 1997a), although there has been fluid levels are most commonly seen in ABCs some doubt expressed as to whether the technique (Fig. 11b) (Davies and Cassar-Pullicino 1992). and quality of technology used in that multicenter

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472 Table 2  Enneking classification for the staging of musculoskeletal neoplasms Stage IA IB IIA IIB III

Site Intracompartmental Extracompartmental Intracompartmental Extracompartmental Any

study were strictly comparable (Steinbach 1998). However, where access to MR imaging remains limited, CT is an adequate alternative, albeit with a significant radiation burden. The MR scan should preferably be performed before the biopsy as the trauma of the procedure may result in hemorrhage and edema, which can exaggerate the true extent of the tumor. The tumor characteristics that should be assessed on MR imaging for the purposes of staging a suspected bone sarcoma are as follows: Extent in Bone?  To assess the extent of bone involvement by tumor, a T1-weighted sequence should be performed oriented along the long axis of the bone involved. This sequence is particularly sensitive to marrow changes. It is necessary to measure the tumor extent from a recognized anatomical reference point, which, for the purposes of a bone sarcoma arising around the knee, can be the articular cortices of the femur or tibia. A gadolinium chelate should not be used at this stage as the uptake of the contrast medium may well render the tumor iso-intense with marrow fat. This problem can be overcome by utilizing a contrast-enhanced fat-suppressed T1-weighted sequence, but this is an expensive way of achieving the same result. Many benign as well as malignant bone tumors show a variable degree of peritumoral edema, e.g., osteoid osteoma, chondroblastoma, GCT, and osteosarcoma (Fig.  16). The edema appears as a zone of intermediate signal intensity merging imperceptibly with the main tumor. With sarcomas, it can be difficult to distinguish where tumor ends and edema commences. Arguably, it is prudent to include all reduced marrow signal within the measurements of the tumor extent as malignant cells may contaminate the edematous area beyond the immediate confines of the main tumor. Some researchers have suggested that it is possible on MR imaging to distinguish between tumor tis-

Grade Low Low High High Any

Metastasis No No No No Regional or distant

sue and peritumoral edema using a dynamic contrast-enhanced sequence. This utilizes the principle that viable tumor enhances faster than the surrounding edema. Chemical shift MR imaging has been shown to be fairly reliable as a way of differentiating tumor infiltration (marrow replacing) from non-marrow replacing, e.g., tumor edema and hemopoietic red marrow, and has high sensitivities and specificities in this regard (Disler et al. 1997; Zampa et  al. 2002; Shiga et  al. 2013). Diffusion-weighted MR imaging (DWI) can also be of some help in distinguishing tumor from perilesional edema. Tumor cells restrict diffusion and high signal on DWI and low signal on the ADC map, and edema will not restrict diffusion as much and will be low signal on the DWI and high signal on the ADC map. The use of dynamic contrast MRI, chemical shift, and DWI can be helpful adjuncts to the conventional MRI sequences; however, it is difficult to believe that these techniques would be able to pick up small isolated nests of malignant cells. Fortunately, this is not a significant management problem in the majority of patients with a sarcoma arising in the distal femur or proximal tibia as increasing the length of a custom-made prosthesis by several centimeters to accommodate the edematous zone is unlikely to affect the functional outcome. Extent in Soft Tissue?  If the tumor is confined to bone, the cortex will remain intact. Cortical bone appears black on all MR sequences as it does not produce a signal. Cortical destruction with loss of the black line is a frequent and characteristic finding of bone malignancy (Fig.  17). Not infrequently, however, highly malignant sarcomas such as osteosarcoma and Ewing’s sarcoma can penetrate the cortex without frank destruction. In this situation, best demonstrated on axial images, the dark contour of the cortex will persist with a

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Fig. 16 Peritumoral edema. (a) Coronal STIR MRI image showing a cortical lesion (white arrow) with marked surrounding edema in keeping with an osteoid osteoma. (b) Axial PDFS image of a cartilage lesion in the subarticular region with marked surrounding edema in

keeping with a chondroblastoma. (c) Coronal STIR image showing an osteosarcoma (black arrow) with the surrounding perilesional edema (curved white arrow). It can be difficult to know exactly where the tumor ends and the edema starts

permeative appearance analogous to the permeative or moth-eaten pattern on radiographs. It is convention to describe any tumor tissue identified outside the cortex as extra-­osseous or soft tissue extension. Strictly speaking, this is often incorrect as the tumor can remain confined by a largely intact periosteum. Nevertheless, this convention persists and is usually only a source of problems

when resolving the findings of MR imaging versus the examination of the pathological specimen. The relatively high water content of most tumors, both bone and soft tissue sarcomas, renders them iso-intense and therefore indistinguishable from surrounding muscles on T1-weighted images. It is for this reason that to assess soft tissue extension, a T2-weighted sequence, with good contrast

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the joint will usually be excised at the time of definitive surgery, prior knowledge will prevent the surgeons from opening the joint and thereby potentially contaminating the surgical field with tumor cells. MR imaging is highly sensitive for detecting joint invasion, but false positives due to subsynovial rather than true intra-articular spread can lead to overstaging (Schima et al. 1994). This is problematic in the knee, where anterior extra-osseous spread of a distal femoral sarcoma will appear to invade the suprapatellar pouch while in reality it is frequently displacing it. Of significance is the fact that the absence of a joint effusion has a high negative predictive value for joint invasion (Schima et al. 1994). The articular cartilage is a relative barrier to tumor growth and is usually only involved in very large or late-presenting tumors. Therefore, typical sites of joint invasion in the knee are the meniscocapsular reflections and the intercondylar notch along the cruciate ligaments (Fig.  18). Transarticular spread is rare. Identification of tumor on both sides of the knee should suggest that it arose de novo in the joint rather than the bone. Fig. 17  Coronal STIR MRI of the knee shows a chondrosarcoma of the distal femur, which has breached the cortex which is seen as loss of the black line (black arrow). The extra-osseous mass can be seen extending into the pre-femoral fat pad

between tumor and muscle, is required. A disadvantage of the widely used fast spin-echo (turbo) T2-weighted sequence is slightly reduced spatial resolution and relatively high signal of fat, which may limit the contrast with tumor. This problem can be overcome with the use of fat suppression. A STIR sequence is an alternative, albeit with a poorer signal-to-noise ratio. The STIR sequence will also tend to overstage the extent of the tumor owing to its increased sensitivity to raised water content in a tissue. As in bone, the distinction of soft tissue tumor from paraneoplastic edema can be problematic (Shuman et al. 1991). Knowledge of the compartmental anatomy around the knee is essential when determining the stage of the tumor (Anderson et al. 1999). Joint Involvement?  It is important to identify knee joint invasion by a sarcoma because, although

Fig. 18  MR imaging of an osteosarcoma of the proximal tibia. The tumor extends proximally into the knee joint via the ACL (black arrow). The presence of a large joint effusion is further evidence of intra-articular extension

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Neurovascular Involvement?  Once a sarcoma has extended beyond the confines of the bone, it will tend to grow in the line of least resistance. Around the knee, this is typically into the popliteal fossa. MR imaging can demonstrate whether a tumor is close to or in contact with a neurovascular structure in the popliteal fossa, but usually cannot distinguish mere contact, adherence, or early invasion (Panicek et  al. 1997b). The axial T1-weighted sequence is the most useful when assessing the relationship to the neurovascular bundle as fat surrounds it and hence can be easily seen. Fortunately, the prevalence of neurovascular involvement in bone sarcomas is less than 4%, such that, although the positive predictive value of MR imaging for involvement is poor, the negative predictive value is over 90% (Panicek et al. 1997b). MR angiography can be used to delineate the relationship of the tumor to vessels (Lang et al. 1995; Swan et al. 1995). Skip Metastases and Lymph Node Involvement?  Small synchronous foci of tumor, usually osteosarcoma, that are present within the same bone as the primary tumor, or within a bone on the other side of an unaffected joint, are called skip metastases. Skip metastases in osteosarcoma have been reported in up to 25% of cases (Enneking and Kagan 1975), although in the author’s experience, the true incidence is less than 5%. Many will be detected by bone scintigraphy, but scintigraphically negative skip metastases have been reported (Bhagia et al. 1997). When staging a sarcoma around the knee, the best resolution images will undoubtedly be obtained utilizing the knee coil; however, it is prudent to include a single large field-of-view T1-weighted sequence along the line of the femur or tibia to exclude a skip metastasis (Fig. 19). In the author’s experience, for some unknown reason, most transarticular skip metastases are identified in the proximal tibia in patients presenting with a distal femoral osteosarcoma. Lymph node spread in bone sarcomas is uncommon and usually a late manifestation of extensive disease. As at other sites, imaging has difficulty distinguishing metastatic infiltration from reactive hyperplasia (Bearcroft and Davies 1999). MRI features which may sug-

Fig. 19  Coronal T1-weighted MRI of the tibia shows a primary proximal tibial osteosarcoma. A further lesion is noted distal to the main tumor in keeping with a skip metastasis

gest popliteal lymph nodes include size >1  cm and loss of the fatty hilum (Cleary et al. 2020). The exception is those cases of osteosarcoma with mineralization, indicating metastatic involvement, which can be easily detected on radiographs or CT and will show increased activity on bone scintigraphy. A false diagnosis of a skip metastasis may occur on scintigraphy when increased activity within an involved lymph node is projected over the distal femur when only anterior or posterior projections are obtained (Bearcroft and Davies 1999). Rarely, in patients with long-standing prostheses, regional lymphadenopathy may occur owing to a foreign body reaction in response to the lymphatic uptake of metal

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debris (Davies et al. 2001). In patients treated by amputation, post-traumatic neuromas may mimic lymphadenopathy. Gd-DTPA has little value in the initial staging of bone sarcomas (Seeger et  al. 1991). It may help distinguish subsynovial spread from true joint invasion (Schima et al. 1994), and a dynamic contrast-enhanced scan can be obtained at this stage as a baseline study for the subsequent assessment of tumor response to chemotherapy (see Sect. 5). The principles of staging soft tissue sarcomas arising around the knee are very similar to those detailed above for bone sarcomas. For both bone and soft tissue sarcomas, the exclusion or confirmation of pulmonary metastases requires chest CT.  The incidence of pulmonary metastases at presentation for soft tissue sarcomas is between 14% and 21% in a recent study (Saifuddin et al. 2021) and for osteosarcomas is approximately 16.7% (Huang et al. 2019) and chondrosarcoma is approximately 8.1% (McLoughlin et al. 2020). Due to the improved resolution of CT, small indeterminate nodules can lead to overstaging of the tumor. Overstaging with CT is a potential hazard as up to 70% of solitary nodules less than 5 mm in diameter at initial presentation in children with solid extrathoracic tumors may be benign (Grampp et al. 2000). Follow-up of indeterminate nodules will require CT follow-up. Experience from our institution has shown that for chondrosarcoma, the significance of such nodules is dependent on the histological grade of the tumor and the size and margins of the nodules (McLoughlin et al. 2020). Bone scintigraphy is used to exclude skeletal metastases. However, over 95% of scintigraphic abnormalities occurring at the time of presentation of osteosarcoma at locations distant from the primary tumor do not represent metastatic disease (Keller et al. 1984). It is, therefore, important to correlate scintigraphic abnormalities with radiographs of the relevant area. Whole-body MRI is now the preferred method for staging Ewing’s sarcoma due to it being more sensitive than bone scintigraphy in detecting

skeletal metastasis with the exception of lesions in the skull vault (Kalus and Saifuddin 2019). At the same time as the staging imaging studies are performed, it is the usual practice in the author’s unit to obtain measurement radiographs of both the affected and the contralateral lower limbs to aid the manufacture of a custommade prosthesis. Also, the bone age of the skeletally immature patients is estimated as certain designs of prosthesis allow for growth, i.e., are extendable.

5 Imaging Follow-Up The imaging follow-up for a patient with a proven sarcoma arising around the knee can be divided into short term (i.e., pre-definitive surgery) and long term (i.e., post-definitive surgery). In the short term, many patients with a sarcoma will be entered into one of the international adjuvant chemotherapy trials. After a predetermined ­number of cycles of chemotherapy and immediately before surgery, the patient is restaged with an MR scan of the primary tumor and a CT scan of the chest. This is to ensure that the stage of the tumor has not altered and that the planned surgery is still appropriate. Also, this is an opportunity to use imaging to assess the response of the tumor to chemotherapy. Histological response to chemotherapy expressed as percentage necrosis is one of the most important prognostic indicators in both osteosarcoma and Ewing’s sarcoma. Over the years, all types of imaging have been used to estimate the response to chemotherapy. Post-chemotherapeutic radiographic and CT findings do not consistently differentiate the good from the poor responder (Shapeero and Vanel 2000). For example, an increase in tumor volume may suggest a poor response but may also represent hemorrhage secondary to necrosis in a responsive tumor (van der Woude et  al. 1998). Conventional angiography is considered too invasive a procedure for monitoring tumor response to chemotherapy. Although it can identify over 90% of responders, it will miss 50% of the poor responders (Carrasco et  al. 1989). It

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remains to be seen whether MR angiography can fulfil a useful role in this respect (Lang et  al. 1995). If there is a significant extra-osseous component to the tumor, Doppler ultrasound can be used to monitor response (van der Woude et al. 1995). The technique is operator dependent, which may affect reproducibility of results on sequential scanning, and is not a routine practice in our institution. Scintigraphy using technetium-­ 99m methylene diphosphonate, thallium-201, and gallium-67 and fluorine-18 fluorodeoxyglucose positron emission tomography (PET) scanning have been advocated for the estimation of tumor response (Shapeero and Vanel 2000). Inherent to all of these methods is the limited anatomical resolution and, with PET scanning, limited availability. To date, these techniques are largely reserved for research purposes. Unenhanced MR imaging has a limited role. Increased or unchanged tumor volume and increased peritumoral edema after chemotherapy suggest a poor histological response in osteosarcoma and Ewing’s sarcoma. Virtual obliteration of the extra-osseous component combined with a hypointense rim in Ewing’s sarcoma usually indicates a good response. It is, however, impossible to exclude small foci of viable tumor without contrast medium. Standard contrast-enhanced MR imaging is also of limited value as viable tumor, revascularized necrotic tissue, reactive hyperemia, etc. may all enhance. It is for this reason that much of the work on imaging assessment of the response of sarcoma to chemotherapy has concentrated on dynamic contrast-enhanced MR imaging. A number of different techniques have been described, but all rely on the underlying principle that viable tumor enhances rapidly (i.e., within seconds of the contrast medium arriving in the adjacent artery) whereas all other enhancing tissues take much longer. It is possible on the console of most modern scanners to plot a time/ intensity curve showing the uptake of the contrast medium. By comparing the curve obtained before commencement of chemotherapy with that obtained afterwards, the tumor response can be estimated. It should be noted that this is a time-­ consuming and costly exercise, with numerous

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variables that directly influence patient management in very few cases. In any case, a more definitive assessment of the percentage of necrosis will be available on the resection specimen of the tumor in a short time following the surgery. In the long term, patients are closely monitored for evidence of local recurrence (Davies and Vanel 1998), metastatic disease (Bearcroft and Davies 1999), and complications of treatment. Local recurrence of a sarcoma is almost inevitable if the original resection margin was not wide. Recurrence may be detected on radiographs as a soft tissue mass with or without bone destruction. Locally recurrent bone sarcoma will usually occur within the soft tissues at the site of the initial surgery as the host bone will have been excised and replaced with a prosthesis. Detection on radiographs is easier if there is evidence of matrix mineralization. Recurrent tumors with the propensity to mineralize (i.e., osteosarcoma) will usually exhibit focal increased activity on scintigraphy, but it is rarely used for this purpose. MR imaging is the technique of choice in the detection of early recurrence when local control may still be surgically achievable. While ultrasound does have some attractions (Choi et  al. 1991), MR imaging will still be required for preoperative evaluation if a recurrence is identified. Depending on the presence or absence of mineralization, most recurrences will show a high signal intensity mass on T2-weighted or STIR images (Vanel et  al. 1994). Diffuse high signal intensity is frequently seen shortly after surgery or can be prolonged following radiation therapy (Richardson et al. 1996). Contrast medium may be required to distinguish enhancing recurrent tumor from seromas, hematomas, etc. Dynamic contrast-enhanced MR imaging can be helpful in differentiating small recurrences from other postoperative changes. It is generally accepted that it is usually the metastatic disease that will eventually kill the patient, and not the primary tumor itself. It is for this reason that follow-up imaging is concentrated on the site where metastases are likely to occur, namely the lungs. Chest radiographs are usually considered adequate. Serial chest CT scans are of doubtful value in view of the considerable radia-

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tion dose involved. The natural history of osteosarcoma has been modified by chemotherapy in that up to 20% of those who develop metastases will first do so in bone prior to there being any evidence of pulmonary metastases. The prognosis for a patient with osseous metastases is so poor that serial follow-up scintigraphy is unlikely to modify the outcome. Scintigraphy is indicated should a patient on follow-­up develop bone pain. Fluorine-18 fluorodeoxyglucose positron emission tomography computed tomography (PET/CT) can assess both the local surgical site and distant metastatic disease. It is superior at detecting bone metastasis than the bone scan and for the detection of soft tissue or lymph node metastasis than MRI or CT (Quartuccio et  al. 2013; Treglia et al. 2012). The detection of lung metastasis, however, is more accurate with CT chest than PET/CT. It should be recognized that the prolonged medical and surgical management of a patient with a sarcoma is not without risk of complications. Prostheses may become loose or infected or require replacement if a child has outgrown the extended length of a growing prosthesis (Kaste et al. 2001). Allografts may also become infected and are prone to fracture. In the long-­term followup of patients who have received radiotherapy, pain or functional impairment within the radiation field should lead to consideration or bone necrosis or radiation-induced sarcoma. The distal femur and proximal tibia are the commonest sites of occurrence for most bone tumors. Tumors can be broadly classified according to their tissue of origin or the tissue they most closely resemble, e.g., osseous, cartilaginous, fibrous, lipomatous, and an unknown or miscellaneous category.

6 Bone Tumors 6.1 Benign Bone Tumors 6.1.1 Osseous Tumors 6.1.1.1 Osteoma Osteoma is a benign, slow-growing focus of mature dense bone that commonly involves the

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frontal and ethmoid sinuses. It rarely (Unni 2010) arises on the outer surface of the long bones. On the distal femur, if large, it can resemble a parosteal osteosarcoma. If extensive and multifocal, the sclerosing dysplasia melorheostosis (Fig. 20) should be considered. 6.1.1.2  Bone Islands Foci of dense compact bone within the medulla are known as bone islands or enostoses (Fig. 21). Multiple bone islands clustered toward the bone ends are diagnostic of osteopoikilosis. Occasionally, bone islands may grow and show increased activity on bone scintigraphy. CT may show a brush border, which is thin sclerotic projections from the lesion. CT is useful for distinguishing bone islands from other sclerotic lesions such as osteoblastic metastasis. A mean attenuation measurement of 885 HU or above is a ­reliable threshold for a bone island with a sensitivity of 95% and a specificity of 96% (Ulano et al. 2016). 6.1.1.3  Osteoid Osteoma Osteoid osteoma is a small bone-forming tumor which characteristically produces a disproportionate amount of pain. Osteoid osteoma has a predilection for the lower limb with approximately 9% arising around the knee joint (Unni 2010). They are usually seen in children and young adults but can be seen in the older patient albeit infrequently. The radiographic feature of an osteoid osteoma is the nidus. This may be lytic, sclerotic, or mixed. The nidus is commonly surrounded by reactive sclerosis. The nidus can measure up to 1.5 cm. CT is excellent in demonstrating the nidus, which may have a variable amount of mineralization and the associated reactive sclerosis (Fig. 22). The mineralization is usually central, and there may be small feeding vessels into the nidus—the vascular groove sign (Yaniv et al. 2011; Liu et al. 2011). On MR, the nidus is usually low signal on T1-weighted imaging and usually intermediate to high signal on fluid-sensitive sequences. If the nidus is densely mineralized, it can appear low signal on all sequences. The nidus is usually surrounded by a variable amount of edema. If a large field of view and thick slices are employed, this edema can

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Fig. 20 (a) Lateral radiographs of the knee show mature dense bone on the outer cortex (white arrow). (b) T1-weighted MRI clearly shows the flowing cortical thickening (white arrow) in keeping with melorheostosis

obscure the nidus. The vast majority of cases can be diagnosed using CT and MRI, but SPECT-CT or dynamic contrast MRI can be used in equivocal cases (Pottecher et al. 2017). The larger variant of osteoid osteoma, the osteoblastoma, which is histologically similar but can have more aggressive features is uncommon in and around the knee.

6.1.2 Cartilaginous Tumors Benign cartilage tumors of bone can be divided into those that arise within the medulla and those that arise from the surface of bone. Medullary cartilage tumors are enchondromas, chondromyxoid fibroma (CMF), and chondroblastoma.

Surface cartilage tumors are osteochondroma and periosteal chondroma. It is important to stress that the presence of cartilaginous calcification is helpful in indicating the tissue of origin but does not distinguish benign from malignant cartilage tumors. 6.1.2.1 Enchondroma Enchondroma is a benign tumor of mature hyaline cartilage accounting for 4% of all primary bone tumors. Approximately 15% occur around the knee, 2–3 times more commonly in the distal femur than in the proximal tibia. The typical radiographic features are a well-defined oval or rounded lytic defect, usually central, containing a

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Fig. 21 (a) AP of the knee showing a dense focus of calcification in the distal femur in a 70-year-old male (white arrow). The differential would also include a sclerotic

metastasis at this age. (b) T1-weighted MRI in the same patient as (a) shows the brush border

variable amount of cartilage calcification within the metaphysis or diaphysis (Fig. 2a). The calcifications tend to be scattered throughout the tumor, as opposed to the peripheral linear distribution that is found in medullary infarcts, which also tend to occur in the distal femoral diaphysis. Bone expansion is not common in the femur or tibia but may be observed in thinner long bones, such as the proximal fibula. On MRI, enchondromas show a lobulated mass which is low to intermediate signal on T1-weighted images and markedly hyperintense on fluid-sensitive sequences (Fig.  2b). On the fluid-sensitive sequences, the individual lobules may be separated by thin septa. Matrix mineralization will show up as low signal foci on both T1 and fluid-­sensitive sequences.

Enchondromatosis (Ollier’s disease) is a condition marked by multiple enchondromas involving the metaphysis and meta-diaphysis of the long bones. There is no hereditary or familial tendency, and it is usually classified as a bone dysplasia. A monomelic or hemimelic distribution is common. The spectrum of skeletal change around the knee can vary enormously from tiny foci of cartilage to linear columns of dysplastic unmineralized cartilage and to major modeling deformities resulting in marked deformity. If there is any doubt as to the condition, radiographs of the hands and feet will usually clinch the diagnosis. Malignant transformation and distinction of enchondroma from low-grade chondrosarcoma are discussed in Sect. 6.2.2.

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Fig. 22 (a and b) Axial CT showing osteoid osteoma around the knee in two patients. Both cases show a mineralized nidus surrounded by a variable amount of surrounding sclerosis. In (a), note the thin vascular channel

feeding into the nidus—the vascular groove sign (white arrow). (c) Axial STIR MRI shows marked surrounding osseous edema around the nidus

6.1.2.2  Chondromyxoid Fibroma Chondromyxoid fibroma is a rare (five times less common than enchondroma) benign but locally aggressive tumor composed of varying amounts of hyaline cartilage, fibrous, and myxomatous tissue. Approximately 43% of CMFs arise around the knee (Unni 2010), with the proximal tibial meta-diaphysis a more common site than the distal femur. It is usually seen between the ages of

10 and 30 years. CMF typically has an eccentric location in the medulla with a lobulated sclerotic margin (Fig. 23). The overlying cortex is thinned and expanded and may even be absent. Most lesions occur in the metaphysis. The long bones account for 60% of cases. Matrix mineralization is uncommon. It is low or iso-intense to muscle on T1-weighted images and hyperintense on fluid-sensitive sequences.

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Fig. 23 (a) Lateral radiograph of a chondromyxoid fibroma of the proximal tibia. Typical radiographic features include the eccentric location, well-defined lobulated endosteal margin, and peripheral expansion. (b) T1-weighted MRI shows intermediate to low signal lesion

eccentrically located with thinning of the cortex but no extra-osseous mass. The differential includes OFD and adamantinoma in this case. A biopsy confirmed a chondromyxoid fibroma

6.1.2.3 Chondroblastoma Chondroblastoma is a benign cartilage tumor accounting for approximately 1% of all bone tumors. Most occur below the age of 25. Most lesions occur in the epiphyses or apophyses. If there is closure of the growth plates, lesions can extend into the metaphysis. Approximately 39% of chondroblastomas occur around the knee, with an equal incidence in the distal femoral and proximal tibial epiphyses. Radiographically, chondroblastoma appears as a well-defined lytic lesion within the medulla of the epiphysis. Matrix mineralization is seen in approximately one-quarter of cases (Fig. 24a). Breaching of the growth plate with metaphyseal involvement occurs in larger lesions with late presentation. Lesions are intermediate signal on T1-weighted images and usually hyperintense on fluid-sensitive sequences. Secondary aneurysmal bone cyst formation with

fluid-fluid levels is seen in approximately 15% of cases. MRI will demonstrate the florid inflammatory response, which is typified by surrounding marrow edema with or without an associated joint effusion (Fig. 24b). Lung metastases, albeit rarely, can be seen as can secondary sarcomatous transformation (Narhari et al. 2018). The differential diagnosis includes a Brodie’s abscess when the growth plates are open. In the adult, one must also consider a clear cell chondrosarcoma, giant cell tumor of bone, or a geode. 6.1.2.4 Osteochondroma Osteochondroma is the commonest benign bone tumor. It represents a bony protuberance (exostosis) covered by a hyaline cartilage cap. Approximately 38% (Unni 2010) arise around the knee, with the distal femur being involved twice as often as the proximal tibia. The majority are

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Fig. 24 (a) AP radiograph of the knee shows a lesion in the proximal tibial epiphysis with chondroid matrix mineralization. (b) MRI in the same patient shows the epiphy-

seal lesion but also shows florid osseous edema (black arrow), which helps make the diagnosis of chondroblastoma

single lesions but multiple, but approximately 15% occur in the setting of the hereditary form— hereditary multiple exostoses. Lesions can be broad based (sessile) or narrow based (pedunculated). Osteochondromas commonly arise at the sites of tendon insertions and growth occurs in the direction of the pull of the tendon, and they are commonly angulated away from the adjacent joint. An important diagnostic feature is the continuity of the host bone marrow with the marrow of the osteochondroma. The cartilage cap is not visible on radiographs unless it calcifies. Ultrasound is useful in differentiating the ­cartilage cap from an overlying bursa; the bursa is hypoechoic and may be compressible, whereas the cartilage cap will not be. CT is useful in looking for corticomedullary continuity with the parent bone; however, visualization of the cartilage cap in the absence of

mineralization cannot be easily differentiated from overlying muscle or an overlying adventitial bursa. MRI is the gold standard for visualizing the cartilage cap, which appears as a lobulated high T2-weighted mass which overlies the periphery of the osteochondroma. Any mineralization in the cartilage cap will appear as focal areas of low signal on all MRI sequences. The cartilage cap thickness is usually in the order of a few millimeters; however, if there is a cartilage cap thickness greater than 15  mm in a skeletally mature individual, it should be considered suspicious for secondary chondrosarcomatous transformation (Murphey et  al. 2000). Complications of osteochondromas include fracture, pressure on adjacent tendons, nerves and vessels, overlying bursitis, malignant degeneration, and impingement (Murphey et al. 2000) (Fig. 25).

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Fig. 25 (a) Lateral radiograph of the knee showing pedunculated osteochondroma from the proximal tibia (white arrow), which extends away from the joint. (b) Axial T2 FS image showing corticomedullary continuity

and a thin cartilage cap (white arrow). (c) AP radiograph shows fracture of the base of a pedunculated osteochondroma (white arrow)

6.1.3 Fibrogenic Tumors of Bone

formation by the tumor and has very similar ­signal characteristics to its soft tissue fibromatosis equivalent (Fig. 26).

6.1.3.1  Desmoplastic Fibroma Desmoplastic fibroma is a very rare benign but locally aggressive bone tumor first described by Jaffe in 1958. It is the bony equivalent of the much more commonly seen soft tissue counterpart fibromatosis. It represents 0.3% of all benign bone tumors. There is no sex predilection, and it is most commonly seen in the second and third decades. It can affect any bone but is most commonly located in the mandible, the long bones, and the ilium. When it involves long bones, it is usually metadiaphyseal. Desmoplastic fibroma does not metastasize, but there are rare reports of secondary malignant transformation to osteosarcoma. Radiographically, desmoplastic fibroma is a well-defined lucent lesion with a sclerotic outline. It has numerous internal septations giving a “soap bubble” type of appearance. There may be cortical disruption associated with larger lesions. On MRI, lesions are commonly hypointense to skeletal muscle on both the T1- and T2-weighted images, which is due to the degree of collagen

6.1.4 Osteoclastic Giant Cell-Rich Tumors 6.1.4.1 Nonossifying Fibromas/Fibrous Cortical Defects Nonossifying fibroma (NOF) and fibrous cortical defects (FCD) are benign spindle cell tumors of bone which contain osteoclast-like giant cells. NOF and fibrous cortical defect have similar radiologic and histological features, and distinction between the two entities has been arbitrarily based on size with FCD being reserved for lesions less than 2 cm and NOF for those lesions greater than 2 cm. They occur in up to 30% of the normal population in the first and second decades. Sixty percent of lesions occur around the knee. Multiple lesions are present in less than 10% of cases. They are usually metaphyseal when located in the long tubular bones. While the vast majority are asymptomatic, if they exceed 50% of the

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Fig. 26 (a) Radiographs showing an extensive well-­ sequences show an expansile lesion in the distal femur, defined lesion in the distal femur giving a “soap bubble” which has low signal on the T1 and the fluid-sensitive type of appearance. (b) Sagittal T1 and (c) sagittal STIR sequence in keeping with a fibrous lesion

transverse diameter of the bone, they can become symptomatic and are at an increased risk of a pathological fracture. Solitary lesions are most common, but multiple lesions can be seen. There is an association of multiple NOFs and neurofibromatosis. Jaffe-Campanacci syndrome is characterized by multiple NOFs and café au lait spots. Radiographs are usually pathognomonic, showing a well-defined elliptical, radiolucent defect with a sclerotic border confined to the cortex of the long bone near the growth plate (Fig. 9). CT and MRI are not usually required but can be helpful if the diagnosis is in doubt. CT will demonstrate similar findings to the radiographs but is useful to assess fracture risk. The MRI findings are variable. On T1-weighted images, the lesion is usually hypointense/iso-intense to muscle with a peripheral low signal rim. On T2-weighted images, lesion can be hyperintense or hypointense depending on the degree of fibrous tissue in the lesion. If a lesion is in the sclerotic phase, it will be low signal on both T1- and T2-weighted images. Fat can also be seen in the lesion. As the

imaging findings are so characteristic, they are considered one of the “don’t-touch” lesions and they rarely require biopsy or surgical treatment. 6.1.4.2  Benign Fibrous Histiocytoma Benign fibrous histiocytoma (BFH) is a rare bone tumor that is histologically indistinguishable from NOF/FCD but has different clinical and radiological features. BFH is usually seen in those aged over 20 years of age. There is no sex predilection. Lesions can occur in any bone but most occur in the long bones with the femur and tibia being the most common location. Lesions are diaphyseal or epiphyseal, which differentiates it from NOF. Pain is usually a feature, and pathological fracture rates can be seen in up to 60% of cases. Radiographs show a well-defined lucent lesion, which is diaphyseal or epiphyseal. When epiphyseal, it can cause confusion with giant cell tumor of bone. Expansion and cortical thinning are commonly seen. MRI findings are nonspecific as with NOF but have similar appearances on T1- and T2-weighted images.

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6.1.4.3  Giant Cell Tumor of Bone Giant cell tumor of bone (osteoclastoma) is a locally aggressive tumor consisting of mononuclear cells and osteoclast-like giant cells. Most occur between the ages of 20 and 45 years, and there is a slight female preponderance. The majority of lesions are benign, but they can rarely metastasize. They account for 5% of all primary bone tumors. These tumors are typically located at the ends of the long bones in the mature skeleton and in the metaphysis in the growing skeleton. The commonest location is around the knee, which is involved in approximately 49% of cases (Unni 2010). Radiographs demonstrate an eccentric subarticular lytic lesion with or without internal trabeculae. There is no matrix mineralization. Lesions can have a narrow or wide zone of transition, and there can be cortical destruction and extension into the adjacent soft tissue or into the knee joint. MRI demonstrates a low to intermediate signal lesion on T1-weighted sequences and can be low or high signal on fluid-sensitive sequences depending on the amount of hemosiderin deposition (Fig.  8). Secondary aneurysmal bone cyst formation is seen in up to 14% of cases (Fig. 27)

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Fig. 27 (a) AP radiograph shows lytic subarticular lesion in the proximal tibia with no matrix mineralization. (b) Coronal STIR MRI shows intermediate to high signal lesion with cyst formation. (c) Sagittal STIR sequence of

(Kaplan et al. 1987; Hudson et al. 1984; Anchan 2008). Numerous classification systems have been proposed; in our institution, we use the Campanacci classification, which grades lesions into one of the three types: Grade 1 lesions have an intact cortex and a well-defined margin. Grade 2 lesions demonstrate moderate expansion with a thinned cortex. Grade 3 lesions have indistinct margins and disruption of the cortex (Campanacci et al. 1987). 6.1.4.4  Aneurysmal Bone Cyst (ABC) Primary ABC is a benign bone tumor characterized by multiple blood-filled cystic spaces. They account for 1–2% of all bone tumors. They can occur in any bone but around the knee is the commonest site seen in approximately 22% of cases with most seen in the proximal tibia than in the other bones. Most present in the first two decades of life. Secondary ABC formation is seen in a number of benign and malignant tumors including telangiectatic osteosarcoma, chondroblastoma, giant cell tumor of bone, and osteoblastoma. Radiographs classically demonstrate an intramedullary, lytic, expansile lesion usually in the metaphysis. Most lesions occur eccentrically in

c

the knee in another patient shows subarticular lesion in the distal femur with multiple fluid-fluid levels (secondary ABC formation)

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the medulla but can also be centrally located. Periosteal location is also well described but is less common. When in the periosteal location, the lesions are usually in the diaphysis of the bone. The cortex can be markedly thinned or even destroyed. MRI is the imaging modality of choice. Classical ABC demonstrates multiloculated cystic spaces with fluid-fluid levels caused by multiple bleeds within the individual cystic spaces. The signal intensities can vary due to the chronicity of the hemorrhage, but T2-weighted images are the most sensitive for the demonstration of fluid-fluid levels (Fig. 11). The use of contrast is not routinely helpful but may demonstrate septal enhancement. The presence of nodular enhancement of any solid areas within an ABC does raise the possibility of a telangiectatic osteosarcoma. Misdiagnosis can have potentially disastrous consequences as the management of an ABC is curettage while that of a telangiectatic osteosarcoma is chemotherapy and wide surgical excision. The presence of ubiquitin specific protease 6 (USP6) rearrangement in the histopathological sample can help in making this distinction as it is highly sensitive and specific for primary ABC a

and is not seen in secondary ABC formation (Li et al. 2019). There is a solid subtype of ABC in which the predominant histology is the “solid” component of a conventional ABC. In solid ABC, MRI will show a partly solid, partly cystic lesion which may or may not contain fluid-fluid levels. Lesions are often diaphyseal and centrally located in the bone in distinction to conventional ABCs. Lesions can have surrounding perilesional edema (Ilaslan et al. 2003).

6.1.5 Other Mesenchymal Tumors of Bone 6.1.5.1  Lipomatous Tumors Intra-osseous lipoma classically arises in the anterior calcaneus. It may occur in the long bones around the knee, most frequently the proximal tibia. Radiographs show a well-defined lucency within the medulla, frequently containing dystrophic calcification (Fig.  28). In this situation, it may mimic a central chondrosarcoma. The diagnosis can be made with confidence on CT or MR imaging by identifying the predominant fat component of the lesion.

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Fig. 28 (a) Lateral radiograph shows lucent lesion in the distal femur (black arrow). (b) Axial CT scan shows that lesion has same attenuation as subcutaneous fat and has a focal area of dystrophic calcification

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6.1.5.2  Simple Bone Cyst (SBC) The simple bone cyst (SBC), also known as a unicameral bone cyst, is a nonneoplastic fluid-­ filled cavity. It is more common in males and is usually detected in the first two decades of life. Only 6% of cases occur around the knee; SBCs are slightly more common in the distal femur than the other bones. The radiographic appearances are a well-defined lytic lesion, centrally located within the metaphysis and migrating with time into the diaphysis. A typical, but not pathognomonic, sign is the so-called fallen fragment. This represents a fragment of fractured cortex that descends to the dependent portion of the cyst. The differential diagnosis around the knee includes aneurysmal bone cyst (ABC), fibrous dysplasia, and nonossifying fibroma; the latter is usually eccentrically located. 6.1.5.3  Fibrous Dysplasia Fibrous dysplasia is a developmental abnormality of bone in which normal bone is replaced with poorly structured fibrous tissue, which can cause pain, deformity, and pathological fracture. It is relatively common; however, the true incidence is unknown due to many patients being asymptomatic. Most of the cases are seen in the second and third decades. It can involve any bone, but the most common site of fibrous dysplasia is the jaw, skull, proximal femur, and ribs. It involves the knee in about 4% of cases. It is usually monostotic but can be polyostotic in up to 15%. McCune-Albright syndrome is the association of polyostotic fibrous dysplasia, café au lait spots, and multiple endocrine abnormalities. An association of polyostotic fibrous dysplasia and multiple myxomas is known as Mazabraud’s syndrome. Many fibrous dysplasia lesions can be asymptomatic and will be picked up incidentally. It can present with a limb deformity or pathological fracture. Sarcomatous transformation is mainly into osteosarcoma but less commonly into fibrosarcoma, and chondrosarcoma is seen in less than 1%. Fibrous dysplasia can have a varied radiographic appearance. Lesions can be cystic, scle-

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rotic, or a mixed pattern. Lesions are intramedullary and commonly located in the meta-diaphyseal region of the long bones. They can vary in size from small to involving the entirety of a long bone. They can be central or eccentrically located in the medulla. They can be expansile and can cause endosteal scalloping with thinning of the cortex and remodeling of bone due to the fibro-osseous mass (Fig.  14c). Extreme cases in the femur can give the classical “shepherd’s crook” deformity. The “rind” sign is thickened peripheral rim of sclerosis around a lesion. The fibro-osseous matrix gives a characteristic “ground-glass” appearance to the bone— as if a veil has been placed over it. A periosteal reaction is not seen unless there is a fracture or malignant transformation. The radiographic appearances in many cases can be very specific, and the diagnosis can be made without the need for further cross-sectional imaging or biopsy. Bone scintigraphy can be useful to look at polyostotic involvement. Lesions demonstrate increased tracer uptake on delayed imaging and increased but variable uptake in the flow and blood pool phases. MRI appearances of fibrous dysplasia can be variable and depend on the amount of internal collagen, cystic, and hemorrhagic areas. Lesions are well-defined lesions which are intermediate to low signal on T1-weighted images and are usually intermediate to high signal intensity on T2-weighted images. The sclerotic rind will be of low signal on both T1- and T2-weighted imaging. Cystic and secondary aneurysmal bone cystic areas are best seen on the T2-weighted images. We do not find the administration of contrast to be particularly useful, but there can be enhancement depending on the biological activity of the lesion. Whole-­ body MRI is a useful technique in which the extent of polyostotic disease can be determined without the need for any radiation exposure. Malignant change should be considered if there is a rapid change in the appearance of a previously stable lesion, cortical destruction, an aggressive periosteal reaction, or presence of a soft tissue mass.

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6.1.5.4  Osteofibrous Dysplasia Osteofibrous dysplasia (OFD) is a rare benign bone tumor first described in 1966 (Kempson 1966). It is peculiar in that it almost always involves the diaphysis of the anterior tibial cortex, although it is rarely seen in the fibula, radius, and ulna. Histologically, OFD is characterized by trabeculae of woven bone on a background of bland spindle cell proliferation. Radiographs typically show a lobulated well-­ defined lucent lesion in the diaphysis of the anterior tibial cortex, a “soap bubble” type of appearance (Fig.  29). When lesions are large, they can extend toward the knee. There is no periosteal reaction, and it has a narrow zone of transition. Bowing can be seen in large lesions. MRI is useful to demonstrate the precise extent of the tumor. Signal characteristics are fairly nonspecific with most lesions being hypointense/iso-­ intense to skeletal muscle on T1-weighted images

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and variably hyperintense on T2-weighted images. MRI will show if there is extension of the tumor into the medullary cavity or if there is the presence of a moth-eaten margin, which if present raises the suspicion that this could be an adamantinoma, which is a related malignant tumor. Other features favoring adamantinoma over OFD include longer cranio-caudal length of the tumor and an older patient age (Khanna et al. 2008). It is important to make a correct diagnosis as the treatments of OFD and adamantinoma are considerably different. A percutaneous CT-guided biopsy should be performed. The central portion of the lesion should be targeted as clusters of benign cells can be seen peripherally in an adamantinoma.

6.2 Malignant Bone Tumors 6.2.1 Osseous Tumors 6.2.1.1 Osteosarcoma Osteosarcoma is the commonest primary malignancy of bone after myeloma. It has an annual incidence of four per million population. There is a bimodal age distribution with a peak in childhood and a second peak in older adults, and up to 305 occur in those aged >40  years. There is a slight male preponderance. There is an increased incidence in those with Li-Fraumeni, Rothmund Thomson syndrome, and hereditary retinoblastoma. The WHO Committee for the classification of bone tumors has divided these tumors into seven distinct categories (Fletcher et al. 2019): Conventional osteosarcoma Parosteal osteosarcoma Periosteal sarcoma High-grade surface osteosarcoma Telangiectatic osteosarcoma Low-grade central osteosarcoma Small cell sarcoma

Fig. 29  Lateral radiograph of the tibia in a 5-year-old female showing anterior tibial cortex lesion (black arrow) in keeping with OFD. The differential includes adamantinoma, but this would be very unusual at this age

Three-quarters of cases are conventional osteosarcoma, also known as high-grade intramedullary osteosarcoma. Over half arise in the bones around the knee. The radiographic appear-

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Fig. 30 (a) AP radiograph showing destructive lesion in the proximal tibial metaphysis with a wide zone of transition with mixed sclerotic and lytic areas (black arrow) and subtle Codman angle (white arrow); this has the classic appearances of a conventional osteoblastic osteosarcoma.

(b) T1-weighted MRI shows the lesion with some surrounding edema and cortical destruction seen as a loss of the black cortical line (white arrow). The soft tissue component is underappreciated on the T1 sequence due to the tumor being iso-intense with muscle

ances can vary from purely lytic to purely sclerotic, but most will show a mixed appearance with permeative margins, cortical destruction, soft tissue extension, and periosteal new bone formation. The latter may appear lamellated or spiculated and, if interrupted, will have Codman angles. The diagnosis is usually fairly straightforward on the radiographs (Fig. 30a). MRI contributes little in the diagnosis but is the modality of choice for local staging (Fig.  30b). The entire bone including the joint above and below the lesion will need to be imaged to exclude a skip metastasis, which can be seen in about 5% of cases. Parosteal osteosarcoma is a low-grade malignancy and is the commonest of the surface osteosarcomas, accounting for approximately 5% of all osteosarcomas. Sixty percent arise on the pos-

terior metaphysis of the distal femur. It occurs in the slightly older individual than the conventional osteosarcoma. It has an excellent prognosis with a 90% 5-year survival unless there has been dedifferentiation. Radiographically, it appears as a dense lobulated mass attached to the outer cortex with a thin radiolucent cleft between part of the mass and the cortex as it wraps around the bone due to periphery of the tumor being less mineralized. Intramedullary extension is best visualized on CT or MR imaging and will be present in approximately one-third of cases. MRI is required for local staging and shows its relationship to the neurovascular bundle. The main part of the lesion will be low signal on T1- and T2-weighted images due to the degree of sclerosis. Dedifferentiation to high-grade osteosarcoma occurs in about 20% of cases and should be sus-

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Fig. 31 (a) Lateral radiograph showing classical parosteal osteosarcoma in the posterior femoral metaphysis with a dense calcified mass. (b) CT shows the paucity of mineralization in the periphery of the lesion (white arrow).

(c) Sagittal STIR MRI shows the tumor to be purely surface lesion, which is seen in about two-thirds of cases; the signal intensity can vary but the mineralized component will be low signal on all sequences

pected if there is a large soft tissue component to the tumor (Fig. 31). Periosteal osteosarcoma is an intermediate-­ grade surface osteosarcoma. It accounts for approximately 2% of all osteosarcomas. It has a peak incidence in the second and third decades. It most commonly arises in the distal femur and proximal tibia. It arises from the inner layer of periosteum. Radiographs can demonstrate a cortically based lesion with a perpendicular (hair on end) periosteal reaction. Chondroid calcification can be seen as these tumors produce chondroid matrix. There may be a visible soft tissue mass and saucerization of the outer cortex of the involved bone. MRI can demonstrate high T2 areas within the tumor which reflects the chondroblastic nature of the tumor. The periosteal reaction and chondroid calcification will be low signal on all sequences. The differential includes other forms of surface osteochondroma and surface chondroid tumors. High-grade surface osteosarcoma is the rarest form of surface osteosarcoma. It accounts for 0.4% of all osteosarcomas. It is most commonly seen in patients in the second and third decades of life. It mostly occurs in the metaphysis or diaphysis of the

long bones with the femur being the most commonly involved bone. Radiographic appearances can be similar to a parosteal osteosarcoma with dense ossification, cortical erosion, and an associated periosteal reaction, but it is generally larger and often there is circumferential involvement. MRI will show any intramedullary involvement, which is more commonly seen in tumors than in parosteal osteosarcoma. The differential includes the other surface tumours and conventional osteosarcoma, especially if there is extensive intramedullary involvement (Fig. 32). As with other malignant primary bone tumors, the entire bone needs to be visualized for adequate local staging. Approximately 10% of osteosarcomas are the telangiectatic variety, which occurs most ­commonly in the distal femur followed by the proximal tibia. This is a very-high-grade tumor categorized by multiple blood-filled cavities with or without fluid-fluid levels on MRI.  It is the presence of fluid-fluid levels that may cause diagnostic problems with ABC; however, the presence of nodular or solid areas and irregular walls which enhance following contrast can help to differentiate it (Fig.  33). Telangiectatic ABC will not show USP6 gene rearrangement.

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Fig. 32 (a) Lateral radiograph of a high-grade surface osteosarcoma of the distal femur. (b) Sagittal T1 and STIR images conform an aggressive surface lesion with

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intramedullary extension. (c) The axial T2 image shows that the near-circumferential extent of the tumor could be underappreciated in the sagittal plane

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Fig. 33 (a) AP radiograph shows an aggressive lesion in the distal femur (white arrow). (b) MRI shows a cystic lesion with numerous cavities containing fluid-fluid levels (white arrows). More solid-looking areas (curved

arrows) are also noted. There is a large soft tissue component. Biopsy confirmed a telangiectatic variant of osteosarcoma

6.2.2 Cartilaginous Tumors Malignant cartilage tumors, chondrosarcomas, are the third most common primary malignant tumor of bone, after multiple myeloma and osteosarcoma. They can be distinguished from many

other primary sarcomas of bone in that they occur in late adulthood rather than childhood or adolescence. Like their benign counterparts, they arise in a central or peripheral location. Approximately 25% of central chondrosarcomas arise in the

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Fig. 34 (a) Radiographs of the distal femur show typical appearances of a central chondrosarcoma, and radiographic features include deep endosteal scalloping (white arrow), chondroid matrix (black arrows), and soft tissue mass (curved arrow). (b and c) show STIR MRI images

showing the typical high T2-lobulated signal of cartilage with aggressive features such as breach of the cortex and soft tissue extension (black arrows). (d) Coronal MRI shows a dedifferentiated chondrosarcoma with the dedifferentiated portion more distally (black arrows)

femur, more commonly proximally than distally. The bones around the knee are involved in approximately 13% of all cases. Patients with Ollier’s disease are at risk of sarcomatous transformation in 5–30% of cases (Liu et  al. 1987). The radiographic appearance of central chondrosarcoma is variable. Most lesions tend to be large. One study concluded that a size greater than 5 cm was the most reliable predictor of chondrosarcoma and that all other morphological features, such as endosteal scalloping, were of little value (Geirnaerdt et  al. 1997a, b). High-grade lesions will appear permeative with cartilage mineralization, cortical destruction, and soft tissue extension (Fig. 34). In particularly aggressive tumors, dedifferentiation to a high-grade sarcoma (e.g., osteosarcoma or undifferentiated pleomorphic sarcoma) should be considered. Diagnostic problems are usually encountered with low-grade central cartilage tumors. Differentiating high chondrosarcoma from the benign enchondroma is fairly straightforward on imaging. However, differentiation between low-grade chondrosarcoma and enchondroma is challenging, not only on imaging but also on histopathology (SLICED 2007; Eefting et  al. 2009). This challenging crossover group has been classified as atypical cartilage tumor (ACT)/grade 1 chondrosarcoma

by the World Health Organization (WHO) since 2013. ACT is the term used for appendicular lesions, and grade 1 chondrosarcoma is used for axial skeletal lesions. Radiographs show a lytic lesion with chondroid calcification (often described as popcorn, dot and comma, ring and arc). Endosteal scalloping without cortical breach is common. CT is useful to look at subtle endosteal scalloping and to look for occult chondroid calcification. MRI will show the typical high T2-weighted signal of the water containing hyaline cartilage, which has a lobular outline. It can also show endosteal scalloping, cortical breach, periostitis, and presence of a soft tissue mass. The presence of cortical breach and a soft tissue mass is suggestive of a higher grade chondrosarcoma or a dedifferentiated chondrosarcoma. In terms of measurement, chondrosarcomas tend to be larger, >8 cm, compared with enchondromas which had a mean length of 5  cm (Murphey et  al. 1998; Geirnaerdt et al. 1997a, b). The presence of endosteal scalloping >two-thirds of the thickness of the cortex is seen 67% of chondrosarcomas but in only 11% of enchondromas (Murphey et al. 1998). This is in contradiction to another study which suggested that endosteal scalloping was of little value (Geirnaerdt et  al. 1997a, b). Whole-body bone scintigraphy has been shown to be of little value in

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the differentiation of enchondroma and low-grade chondrosarcoma (Ferrer-Santacreu et  al. 2016). Although dynamic contrast-enhanced MR imaging has been claimed to be of value in predicting malignancy (Geirnaerdt et al. 2000), this is not a universally held view however (Flemming and Murphey 2000; Douis et al. 2018). The use of positron emission tomography/computed tomography in the differentiation between enchondroma and ACT is also not conclusive (Subhawong et al. 2017). While high-grade chondrosarcomas need prompt biopsy and treatment, this is not the case with this contentious group. Malignant transformation in this group can happen, but the risk is small, and it may take decades to occur. Serial follow-up of such lesions seems reasonable, but there is no consensus as for how often to and how long to follow up. A proposed imaging follow-up regime for such lesions— the Birmingham Atypical Cartilage Tumour Imaging Protocol (BACTIP)—allows a stepwise imaging follow-up plan and the indications for discharge and for specialist referral for cartilage tumors of the distal femur and proximal tibia (Patel et al. 2019; Davies et al. 2019) and the proximal fibula (Davies et al. 2020). Less than 1% of solitary osteochondromas undergo malignant transformation to a peripheral chondrosarcoma. It is generally accepted that the rate of malignant change in diaphyseal aclasis is higher, but probably no more than 1% if one includes all asymptomatic cases of diaphyseal aclasis that do not present for medical treatment (Voutsinas and Wynne-Davies 1983). Clinical features that suggest malignant change include pain and increasing size following skeletal fusion. Measurement of the thickness of the cartilage cap using ultrasound, CT, or MR imaging can be helpful. A cartilage cap of less than 2 cm is likely to be benign, whereas as the cap exceeds 2 cm in thickness, the likelihood of chondrosarcoma increases. Complications of osteochondromas such as overlying bursitis and pseudoaneurysm formation can mimic malignant change.

6.2.3 Fibrous Tumors Fibrosarcoma of bone is a rare malignant bone tumor accounting for less than 5% of all malignant bone tumors. Histologically, fibrosarcoma consists of spindle cells arranged in herringbone

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fashion. The degree of collagen production depends on the degree of cellular differentiation. Lower grade tumors have higher collagen production with less cellularity and nuclear atypia compared to higher grade tumors. There is no cartilage or bone production in fibrosarcoma. Approximately 50% occur in the bones around the knee. It can occur at all ages but is commonest between the third and sixth decades. There is no sex predilection. Twenty percent of cases arise in preexisting bone lesions, including Paget’s disease, bone infarction, irradiated bone, fibrous dysplasia, and nonossifying fibroma. Typical radiographic appearances are geographic bone destruction with a wide zone of transition. Most are meta-diaphyseal, but extension into the epiphysis can occur. Lesions are intramedullary but can be cortically based. Cortical destruction with soft tissue extension is common, but periosteal new bone formation is unusual and there is no matrix mineralization. MRI is used for local staging and surgical planning. It clearly demonstrates the extent of bony involvement, the presence of a soft tissue mass, and its relationship to blood vessels and nerves. MRI signal characteristics are nonspecific with most lesions being iso-­intense/hypointense signal (relative to skeletal muscle) on T1-weighted images and ­hyperintense on T2-weighted images. Hemorrhage/ central necrosis have been described, which may show up as fluid signal on T2-weighted images and may display peripheral contrast enhancement on postcontrast images. In the older patient, the appearances can be indistinguishable from metastasis or lymphoma of bone.

6.2.4 Undifferentiated Small Round Cell Tumors of Bone 6.2.4.1  Ewing’s Sarcoma Ewing’s sarcoma is a small round cell tumor which is characterized histologically by small round cells with round nuclei and inconspicuous nucleoli. The undifferentiated nature of the cells means that they can be difficult to diagnose without immunohistochemical techniques. Strong CD99 expression is seen in 95% of cases. It is the second commonest malignancy of bone in children and adolescents, after osteosarcoma. There is a slight male preponderance (Qureshi et  al.

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2007). The classic site in long bones, as illustrated in many texts, is the diaphysis, but this is the site in only one-third of cases. Approximately 60% arise in the meta-diaphysis, with only 13% of all cases around the knee. The radiographic features are of an extensive aggressive-looking lesion with a permeative or moth-eaten pattern, cortical destruction, and periosteal new bone formation. Lesions can be lytic, sclerotic, and mixed lytic and sclerotic. Around the knee, Ewing’s sarcomas appear more lytic and can be confused with the more lytic form of osteosarcoma. Areas of sclerosis are due to the host bone stimulating new bone formation rather than osteoid production by the tumor. As the tumor extends through the cortex, it elevates the periosteum giving the classical onionskin periosteal reaction (Fig. 6). A Codman angle is seen in 27% of cases, and a a

spiculated periosteal reaction is seen in 50% of cases. A pathological fracture is seen in approximately 14% (Reinus et al. 1992). Saucerization is the radiographic appearance seen in periosteal Ewing’s sarcoma, which causes scalloping of the outer cortex due to the presence of a soft tissue mass. A soft tissue component is seen in 80% of cases. MRI is the preferred modality for the local staging of the disease and to determine the presence and size of the soft tissue component and its relationship to the critical structures. The soft tissue component of the disease is often larger than the osseous component of the disease. The cortex is often preserved with a large soft tissue mass as the tumor penetrates through the cortex via the Haversian canals (Fig.  35). Lesions are usually hypointense to iso-intense to skeletal muscle on T1-weighted imaging. On T2-weighted

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Fig. 35 (a) Bone scintigraphy showing extensive Ewing’s sarcoma of the distal femur. (b) Axial T2 MRI shows relatively intact cortex (white arrow) with tumor

infiltration of the medulla and a large soft tissue component which is classical for small round cell tumors including Ewing’s sarcoma

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sequences, tumors are usually hyperintense. CT of the thorax and whole-body MRI are used for systemic staging of disease and follow-up. 6.2.4.2  Primary Lymphoma of Bone Primary lymphoma of bone is a malignant lymphoid neoplasm. It may present as a single skeletal site with or without regional lymph node involvement or may present as polyostotic skeletal disease with no lymph node or extranodal lesions. Primary lymphoma of bone is relatively rare accounting for 3.9% of all primary malignant bone lesions (Unni 2010). Approximately 10% involve the bones around the knee (Unni 2010). Ten to forty percent are multifocal. It can occur at any age, and there is a slight male predominance. The radiographic features are that of an aggressive lesion often with extensive involvement of the bone. There is no matrix mineralization, but there can be sclerosis as a host bone response. A periosteal reaction is seen in about 60% and is usually lamellated. Cortical destruction is usually subtle, and there is often a disproportionately large soft tissue component which will often encircle the neurovascular bundle rather than displacing it. MRI will best demona

Fig. 36 (a) Sagittal STIR MRI shows primary lymphoma of bone showing a destructive bony lesion with a disproportionally large soft tissue mass (black arrow) with an

strate the medullary involvement and is the best modality to show the extent of the soft tissue mass. Like most malignant lesions, it is iso-­intense to skeletal muscle on T1 weighting and intermediate to hyperintense on fluid-sensitive sequences. As with other small round cell tumors, the cortex of the bone may appear relatively intact despite the extensive medullary and soft tissue tumor involvement (Fig. 36).

6.3 Patellar Tumors The patella is the largest sesamoid bone in the human. Unlike the distal femur and proximal tibia, it is a rare site for tumors (Kransdorf et al. 1989). Despite this, it deserves separate mention in a treatise on the knee. A multicenter case series with data from four European registries (Singh et al. 2009) identified a total of 59 patellar tumors. Of these, 39% were benign neoplasms, 15% were malignant neoplasms, and the remaining 46% were benign non-neoplastic conditions. The most common benign tumors were GCT and chondroblastoma, with brown tumors b

intact cortex. (b) Axial STIR sequence shows the large soft tissue mass encircling the neurovascular bundle

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being the most common nonneoplastic condition. Although rare, osteosarcoma was the most common malignant neoplasm. The presence of a soft tissue mass is commonly associated with malignant lesions and gout. Because of the size of the patella, the typical features usually seen in a

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the long bones are missing and therefore it can be difficult to differentiate benign from malignant disease and in many cases a biopsy is required. Imaging of patellar tumors with radiographs and MR imaging will usually suffice (Figs. 37 and 38). b

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Fig. 37 (a) Radiograph showing lytic lesion in the inferior pole of the patella. (b and c) T1 and T2 axial images showing lesion in patella, fairly nonspecific on imaging. Biopsy-confirmed CGT of bone

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Fig. 38 (a) Radiograph shows lytic lesion in patella. (b) STIR sequence shows lesions in the patella with fluid-fluid levels and surrounding edema. Biopsy confirmed a chondroblastoma with secondary ABC change

7 Soft Tissue Tumors 7.1 Benign Soft Tissue Tumors The majority of benign soft tissue masses arising in relation to the knee are nonneoplastic cystic lesions. Post-traumatic conditions around the knee which may mimic a soft tissue tumor include occult rectus femoris muscle tears, myositis ossificans, and popliteal artery pseudoaneurysms secondary to mechanical irritation from an adjacent osteochondroma. The commonest benign soft tissue neoplasms are myxomas, lipomas, vascular tumors, neurogenic tumors, and aggressive fibromatosis. The relative incidence of each group of these benign conditions in the lower extremity, excluding the hip, foot, and ankle, is myxomas, 55%; lipomatous tumors, 14%; vascular tumors, 15%; neurogenic tumors, 17%; and aggressive fibromatosis, 10% (Kransdorf and Murphey 1997). The imaging features of these conditions are well described and do not vary depending on their anatomical location. A couple of entities are briefly mentioned because of their predilection for the knee. Juxta-articular myxoma, also known as peri-­ articular myxoma (Fig.  39), is a rare variant of myxoma that occurs near joints most commonly in or around the knee. Due to the myxoid nature of the lesion, it is low signal on T1-weighted

MRI and high signal on fluid-sensitive sequences, and hence it frequently mimics a ganglion on imaging. Biopsy is required to exclude a myxoid liposarcoma. Neurogenic tumors around the knee are usually schwannomas of the popliteal or peroneal nerves. On MRI, there is the presence of a fusiform mass which may show entering and exiting nerves. The target sign is a feature on the fluid-sensitive sequence where the center of the lesion has a lower signal intensity than the periphery due to the more centrally located cellular component to the tumor. The fat split sign seen on the T1-weighted sequence is due to the preservation of the fatty tissue around the slow-growing tumor. Schwannomas tend to be eccentrically located with respect to the main mass, which helps to distinguish it from a neurofibroma (Fig. 40).

7.2 Malignant Soft Tissue Tumors Approximately 45% of soft tissue sarcomas in adults occur in the lower extremity (Varma 1999), with no particular predilection for the knee. Radiographs will reveal calcifications in extraskeletal osteosarcoma, extraskeletal chondrosarcoma, and approximately one-third of synovial sarcomas. Peripheral mineralization/ ossification suggests myositis ossificans rather

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Fig. 39 (a) T1 axial MRI shows low signal lesion around the knee. (b) T2 axial MRI shows myxoid lesion. A biopsy was required to exclude a myxoid soft tissue sarcoma and conformed the diagnosis of a juxta-articular myxoma

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Fig. 40 (a) Coronal T1 shows small neurogenic tumor, and the fat split sign is evident (white arrow). Entering and exiting tails are noted (black arrows). (b) Coronal STIR sequence showing high T2 signal intensity in the lesion

than a sarcoma. On MRI, most soft tissue sarcomas, irrespective of their tissue origin, will be iso-intense to muscle on T1-weighted images and heterogeneous but predominantly hyperintense on T2-weighted images. Cystic and hemorrhagic areas are common in high-grade sarcomas, which will show as hyperintense areas on T1-weighted images. It is prudent to consider all deep-seated solid or semisolid masses, without any specific diagnostic features, as potentially malignant until proved otherwise by biopsy. If there is marked

necrosis or cystic change, imaging may help indicate the most appropriate solid part of the sarcoma to biopsy.

7.2.1 Synovial Sarcoma The term synovial sarcoma is somewhat a misnomer as it is not derived from synovial cells. It gets its name because histologically the cells can resemble developing synovium. Synovial sarcoma is the commonest lower extremity malignancy in the child and young adult and frequently arises around the knee. Less than 10% are located

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in an intra-articular location. There are several subtypes of synovial sarcoma: monophasic, biphasic, (fibrous and epithelial), and undifferentiated. Lesions can behave in an indolent fashion and therefore can be mistaken for a benign lesion. Some lesions, however, can behave more aggressively with rapid progression. Radiographs can show calcifications which are seen in up to 30% of lesions (Murphey et  al. 2006). MRI is the imaging modality of choice for local staging. It will demonstrate a soft tissue mass and its relation to the muscles, tendons, joints, bone, and adjacent neurovascular structures. On T1-weighted images, synovial sarcomas are usually heterogenous with variable signal intensity though usually iso-intense/hyperintense relative to skeletal muscle. Areas of tumoral hemorrhage will be hyperintense on T1-weighted imaging. On T2-weighted images, there can be marked heterogeneity in the tumor signal characteristics. The “triple sign” is due to the solid, fibrotic, and cystic/hemorrhagic components of the tumor and is seen in up to 60% of synovial sarcoma tumors (Jones et al. 1993). The triple sign is however not specific for synovial sarcoma and can be seen in other heterogenous soft tissue sarcomas (Fig. 41).

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choice. The fatty component will be clearly seen as hyperintense on T1- and T2-weighted sequences. The high-grade sarcomatous component will be low to intermediate on T1-weighted sequences and will be high to intermediate signal on T2 fat-saturated or short tau inversion recovery (STIR) sequences. Ultrasound-guided biopsy of the dedifferentiated (nonfatty) component is required to confirm the diagnosis.

7.2.3 Myxoid Liposarcoma Myxoid liposarcoma is a malignant tumor which is made up of varying numbers of lipoblasts within myxoid stoma. It most commonly occurs in the fourth and fifth decades. Myxoid liposarcoma accounts for 10% of all adult soft tissue sarcomas. It usually presents as a large painless soft tissue mass. Most occur in the lower limb, but are more commonly seen in the thigh than around the knee. There is no sex predilection. Radiographs may show a soft tissue mass if the lesion is large. Ultrasound will show a heterogenous mass, which may or may not contain some fatty elements within it. They will usually be demonstrable blood flow on Doppler. MRI will demonstrate a deep-seated soft tissue mass. In approximately 25% of cases, there may be 7.2.2 Dedifferentiated Liposarcoma some fatty tissue within; however, many do not Dedifferentiated liposarcoma is an atypical lipo- have any visible fatty tissue on MRI and can be matous tumor that has progressed to a non-­ mistaken for a cyst if not careful. Lesions will lipogenic sarcoma of varying histological but enhance with contrast which will allow it to be usually high grade. The risk of dedifferentiation differentiated from a cyst, but we find that in the from an atypical lipomatous tumor (ALT) to vast majority of cases, it is not usually required. dedifferentiated liposarcoma is