Orthodontics: The State of the Art [Reprint 2016 ed.] 9781512800289

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Orthodontics

The State of the Art

Proceedings of the 1978 International of Dental Medicine,

Centennial

Orthodontic

Program

Conference,

University

of Pennsylvania

School

Edited by Harry G. Barrer

Orthodontics The State of the Art University of

Pennsylvania

Press/ Philadelphia

Í98Í %

Copyright © 1981 Harry G. Barrer All rights reserved Printed in the United States of America Library of Congress Cataloging in Publication Data International Orthodontie Conference, University of Pennsylvania School of Dental Medicine, 1978. Orthodontics : the state of the art. Conference held as part of the Centennial Program of the University of Pennsylvania School of Dental Medicine. 1. Orthodontics—Congresses. I. Barrer, Harry G. II. Pennsylvania. University. School of Dental Medicine. III. Title. [DNLM: 1. O r t h o d o n t i c s Trends—Congresses. 2. Periodontics—Congresses. 3. Surgery, Oral—Congresses. WU400 0773 1978] RK521.I59 1978 617.6'43 79-5043 ISBN 0-8122-7767-8

In Dedication

Many of us know Dr. Wilton Krogman for his many contributions to our understanding of concepts of craniofacial growth and development and its relations to the overall growth of the child. Others of us know him because of his many accolades and awards acknowledging his work. These include honorary doctorates from the University of Michigan, the University of Kentucky, and the University of Pennsylvania, his active membership in the National Academy of Science, and his receipt of the Viking Award in Anthropology, considered the highest award in his field. He has been president of the American Association of Physical Anthropologists four times in succession, a Professor at the University of Pennsylvania, and Director of the Krogman Child Growth Center at Children's Hospital of Philadelphia. Presently, his postretirement career is Director of Research at the Lancaster Cleft Palate Clinic. Extracurricularly, he is known not only as a mystery-loving Sherlock Holmes enthusiast and Baker Street Irregular, but also in a more serious view as the father of forensic anthropology. There are those of us who know Bill as a partner in a wonderful marriage to Mary and as a father to Mark and John. And then there are those of us who were fortunate enough to have had him as a professor who, while presenting a perfectly cogent and lucid lecture on the evolution and modern adaptation of the human craniofacial complex, routinely recited poetry in both English and German, in between telling some of the best jokes ever to boom off the staid walls of a lecture hall. And for those of us who are Bill's personal friends, we know him for his humility, his humanity, and his warm friendship. Wilton Marion Krogman is a unique human being, and we take pride in dedicating this work to him with out greatest respect. H.G.B.

Foreword

The International Conference on Orthodontics brought together a group of distinguished clinicians and scientists to share information gathered from laboratories around the world. The high intellectual caliber of this meeting was appreciated by essayists and students alike, and most thought this event appropriate as part of the Centennial Celebration of the School of Dental Medicine of the University of Pennsylvania. The advances in Orthodontics through the past 80 years were in many instances tied to the development of the dental school at the University of Pennsylvania, and we were extremely pleased to sponsor and host this session. We are grateful to Professor Harry Barrer for assuming the responsibility for organizing and directing this successful symposium and seeing to the publication of its proceedings. D E A N D . WALTER C O H E N University of Pennsylvania School of Dental Medicine

Preface

T w o concepts basic to this b o o k merit careful consideration. T h e first centers on the time factor: a century o f change and progress in orthodontics generally and at the University o f Pennsylvania's School o f Dental Medicine. T h e second is expressed by the word "interdisciplinary" and its implication o f w o r k ing-togetherness. In the century celebrated in the pages that f o l l o w , O r t h o d o n t i c s has moved f r o m the physical, mechanical force o f appliance therapy to the biologic understanding o f causes, effects, and prognosis in that therapy. F r o m the relative statics o f force-system to the dynamics o f the biologic processes o f g r o w t h - t i m e , a stride has been taken to greater k n o w l e d g e and deeper comprehension in Orthodontics—a dental specialty that has achieved the status o f a dental science. W e have been wont to accept that problems o f human biology require the evaluation o f many different research and clinical disciplines. This we call multidisciplinary, a term not only conveying the idea o f " m a n y , " but also suggesting a degree o f uniqueness to the point o f relative isolationism. This severalty is overcome by the interdisciplinary approach, with its implication o f integration. B u t even more than this occurs: the men o f each discipline, each avenue o f investigation, each sphere o f clinical care, talk with one another—not at or to one another. T h e S y m p o s i u m reported in this volume was characterized—nay, illuminated—by a freely expressed give and take by essayists and discussants. Fact, evaluation, interpretation, interrelationship were the order o f the day. H e r e are a few examples: O c c l u s i o n was discussed phylogenetically, ontogenetically, clinically. T h e discussion o f occlusal function considered not only the interdigitation o f teeth alone, but also all supporting oral tissues, both in health and disease. T h e theme o f bony structures offered the interrelations o f a dento-osteo-myologic trilogy. And theme o f growth and development embraced prenatal and postnatal growth processes and included consideration o f a genetic background, both in quasi-unit traits and in syndromes. I f I may be permitted to end on a commerical note, this b o o k is what Madison Avenue would call a "package deal" o f the state o f the art appraised f r o m the perspective o f 100 years. WILTON MARION KROGMAN, PH. D .

Prologue

T h e Centennial of the University of Pennsylvania School of Dental Medicine was celebrated by a series of scholarly symposia m a r k i n g the progress of dental k n o w l e d g e over the last 100 years. At one of these symposia, the Department of Orthodontics undertook the task of presenting the intricacies of its disciplines in relation to other major disciplines involved in treating the orthodontic patient. In his text, Current Orthodontic Concepts and Techniques, Graber characterized orthodontics as "the practice of esoteric, cult-oriented systems." O r thodontists tend to be mechanists rather than applied biologists w h o utilize the entire spectrum of dental knowledge. It has n o w become obligatory for orthodontists to avoid the restrictions of a narrow "latest and best technique." T h e y m u s t apply to the p r o b l e m s presented to t h e m for treatment, their extensive discretionary knowledge and skill gained by comprehensive study, and broad-based training in many disciplines. Orthodontic treatment is not a simple mechanical procedure utilizing a single appliance that will automatically correct the aberrations of growth, development, and dental malocclusion. Orthodontics is a complex treatment method dependent on an understanding of all the biologic, pathologic, and medicomechanical procedures used in the total field of dentistry. T h e r e f o r e , it was to this end that the planning committee for the symposium developed a program of lectures on six major disciplines intimately related to orthodontics. Thirty-six researchers and clinicians of international renown participated in this interdisciplinary seminar. O n a lecture-discusser basis they exposed leading-edge knowledge concerning the basic science of their discipline, its potential application to current orthodontic procedure, and finally its actual clinical use in orthodontic treatment. T h e information thus presented expands the orthodontist's ability to treat his patients as he includes these advanced concepts in his clinical utilization of the state of the art of Orthodontics. H.G.B.

Contents

Participants

I 1

xvii

OCCLUSION AND FUNCTION The Origin of Mammalian Occlusion

3

A. W. CROMPTON

2

Emergence of Hominid Oral Mechanisms

19

E. LLOYD DU BRUL

3

The Development of Occlusion and Facial Balance

33

JAMES A. MCNAMARA, JR.

4

The Concept of a Rational Condylar Position

47

EUGENE H. WILLIAMSON

5

A Clinical Concept of Functional Occlusion in Orthodontics: The Missing Answer S3

Comprehensive

PAUL W. STÖCKLI

II THE INTERRELATIONSHIP OF ORTHODONTICS AND PERIODONTICS 6

Structural and Biochemical Features of the Attachment Apparatus

71

IAN C. MACKENZIE

7

Structural and Biochemical Features of the Attachment Apparatus

75

ANTHONY H. MELCHER

8

Tissue Reactions of the Periodontal Ligament as a Factor in Ortho-Perio Treatment MELVYN WEINSTOCK

9

Remodeling of the Periodontium during Tooth Movement A. RICHARD TEN CATE AND E. FREEMAN

97

79

10

11

III

12

Clinical Interrelation of Orthodontics BJÖRN U. ZACHRISSON

and Periodontics

Periodontal Problems Associated with Orthodontic ROBERT L. VANARSDALL

105

Treatment

115

THE INTERRELATIONSHIP OF ORTHODONTICS AND SURGERY The Organization

of Bone and the Mechanism

of Calcification

127

MELVINJ. GLIMCHER

13

The Mechanism

of Mineralization

139

IRVING M. SHAPIRO

14

Form-Function Relationships ELSDON STOREY

15

A Rationale for Surgical H. P. FREIHOFER, JR.

of Bone

Correction

145

of Jaw Deformities

159

IV ADHESIVES AND BONDING IN ORTHODONTICS 16

Adhesive

Bonding

of Various Materials

to Hard Tooth

Tissues:

Tracer Study of

Adsorption on Enamel 163 THOMAS EARL GILLS A N D RAFAEL LEE BOWEN

17

Adhesion Principles for Orthodontics B. DAVID HALPERN

18

Basic Considerations for Orthodontic

173

Bonding

177

MICHAEL G. BUONOCORE

19

The Scientific Basis for I. LEON DOGON

20

Indirect Bonding:

21

V

22

Using Direct

Simplicity

Bonding

in Action

193

Clinical Applications of Direct GEORGE V. NEWMAN

and Indirect Bonding

in Orthodontics

GROWTH AND DEVELOPMENT Mechanisms

of Craniofacial

Skeletal Growth

Mechanisms

of Craniofacial

Growth

223

DONALD H. ENLOW

24

185

ROYCE G. THOMAS

KALEVI KOSKI

23

in Orthodontics

The Biology of Tooth Eruption 227 YAEL MICHAELI A N D M A X M. WEINREB

209

201

Mordant

25

The Fibroblast Migration Hypothesis of Tooth Eruption with a Note on the Tissue-Fluid Pressure Hypothesis

239

B. K. B. B E R K O V I T Z

26

The Heritability of Malocclusion: Implications for the Orthodontic Practitioner J A M E S E. HARRIS

27

A Discussion of the Heritability of Malocclusion

269

C O E N R A A D F. A. M O O R R E E S

VI

TISSUE REACTION IN ORTHODONTICS

28

Embryogenesis of the Craniofacial Complex

275

M A L C O L M J O H N S T O N , K. K. SULIK, A N D R. M I N K O F F

29

Congenital Craniofacial Malformations: Perspectives in Susceptibility H A R O L D C. SLAVKIN

30

Tissue Reaction and Bone Turnover in Orthodontic Treatment ZE'EV DAVIDOVITCH

31

Mechanical and Electrical Effects on Bone and Cartilage Cells: Translation of the Physical Signal into a Biologic Message 315 G I D E O N A. R O D A N

32

Syndromology's Message for Craniofacial Biology M. M I C H A E L C O H E N , JR.

33

Syndrome Recognition R O B E R T J. G O R L I N

353

323

297

289

257

Participants

MODERATORS James L. Ackerman, Professor of Orthodontics,

Harry G. Barrer, Professor of Orthodontics University

of

D.D.S. School of Dental Medicine,

University

of

Pennsylvania

D.D.S. and Director of Graduate and Postdoctoral Orthodontics,

School of Dental

Medicine,

Pennsylvania

Irving D. Buchiti, Professor of Orthodontics,

D.D.S. School of Dental Medicine,

University

of

D. Walter Cohen,

D.D.S.

Professor of Periodontics,

and Dean of the School of Dental Medicine,

Ze'ev Davidovitch,

University

of

Pennsylvania

D.M.D.

Associate Professor of Orthodontics,

Bratnerd Swain, Professor of F. Orthodontics,

Pennsylvania

School of Dental Medicine,

D.D.S.of Dental Medicine, School

University

University

of

of

Pennsylvania

Pennsylvania

LECTURERS Rafael L. Bowen,

D.D.S.

Associate Director of the American of Standards

Dental Association

Michael G. Buonocore, M.S., Chair, Dental Materials

M. Michael Cohen, Jr., D.M.D.,

A. W. Crompton,

Ph.D.

Director of the Museum

of Comparative

Ze'ev Davidovitch,

Eastman Dental

D.M.D.

Bureau

Center

Ph.D.

of Oral and Maxillofacial

Associate Professor of Orthodontics,

Research Unit, National

D.M.D.

and Chemistry,

Professor in the Departments Washington

Health Foundation,

Zoology,

Surgery,

The Agassiz

School of Dental Medicine,

Orthodontics

Museum, University

and Pediatrics,

Harvard of

University

Pennsylvania

University

of

xviii H. P. Freihof er, Jr., M.D., Assistant

D.M.D.

Professor of Oral and Maxillofacial

Melvin J. Glimcher,

Surgery,

Kieferchirurgische

Klinik,

University

of

M.D.

Harriet M. Peabody Professor of Orthopedic Surgery, Harvard Medical School; Orthopedic Chief Children's Hospital Medical Center, Boston, Massachusetts

James E. Harris, D.D.S., Professor and Chairman

of the Department

of Orthodontics,

University

of

Kalevi Koski, L.Odont.,

Michigan

Ph.D.

Professor of Orthodontics, School of Dentistry; Professor of Anatomy, Investigator, Dental Research Center, University of North Carolina

School of Medicine;

L.D.S.,

Institute

R.D.S.,

of Dentistry,

B.D.S.,

Associate Dean for Research and Program Development,

James A. McNamara, Jr., D.D.S.,

University

F.D.S., University

of Turku,

Finland

Ph.D.

Ph.D. Craniofacial

Research Center for

D.D.S.

of Anatomy

and Embryology,

Elsdon Storey, Ph.D., Royce G. Thomas,

Hebrew

D.D.Sc.

Professor of Child Dental Health,

Department

University-Hadassah

of Conservative

Medical School,

Dentistry,

University

of

Jerusalem Melbourne

D.D.S.

Associate Clinical Professor, St. Louis

Melvyn Weinstock, D.D.S.,

University

Ph.D.

Assistant Professor, Faculty of Medicine; Assistant Medical Research Scholar, McGill University

Eugene H. Williamson, D.D.S., Chairman

Senior

of Iowa

Assistant Professor of Anatomy and Program Director of the Experimental Human Growth and Development, University of Michigan

Department

and

D.Odont.

Professor of Pedodontics and Orthodontics,

Yael Michaeli,

Surgeon-in-

M.S.

Malcolm Johnston, D.D.S.,

Ian C. Mackenzie,

Zurich

of the Department

Björn U. Zachrisson,

Faculty of Dentistry;

Canadian

M.S.

of Orthodontics,

D.D.S.,

Associate Professor of Orthodontics,

Professor of Orthodontics,

Medical College of Georgia

Ph.D. University

of Oslo,

Norway

DISCUSSANTS William H. Bell,

D.D.S.

Associate Professor, Division

B. K. B. Berkovitz, Lecturer in Anatomy,

of Oral Surgery,

B.D.S.,

University

M.Sc., Ph.D.,

Medical School,

University

I. Leon Dogon, L.D.S.R.C.S.,

Health Science Center at Dallas

L.D.S.R.C.S.

of Bristol

D.M.D.

Associate Professor and Head of the Department

E. Lloyd Du Bruì, M.S., D.D.S.,

of Texas,

of Operative

Dentistry,

Harvard School of Dental

Medicine

Ph.D.

Professor and Head of the Department of Oral Anatomy, College of Dentistry, College of Medicine, University of Illinois at the Medical Center

Professor of

Anatomy,

xix Donald H. Enlow,

Ph.D.

Chairman of the Department of Orthodontics; Hill Distinguished Professor of Oral Biology,

B. David Halpern,

Thomas

Ph.D.

President, Polysciences,

Inc.

Robert J. Gorlin, D.D.S., Professor and Chairman

Anthony

Assistant Dean for Graduate Studies and Research; Case Western Reserve University

M.S.

of the Division

H. Melcher, M.D.S.,

of Oral Pathology,

H.D.D.,

University

of

Minnesota

Ph.D.

Professor and Director of the Medical Research Council Group in Periodontal Physiology,

University

of

Toronto

Coenraad F. A. Moorrees, Professor of Orthondontics,

Harvard

George V. Newman, Diplomate,

American

D.D.S. School of Dental Medicine,

Gideon A. Rodan, M.D.,

University

Irving M. Shapiro,

B.C.S.,

Ph.D.

Professor and Chairman Pennsylvania

of the Department

Harold C. Slavkin,

Bureau of Health,

of Connecticut

Center

Newark,

New

Jersey

of Biochemistry,

Health

School of Dental Medicine,

University

of

D.D.S.

Professor in the Department California

of Biochemistry

Paul W. Stöckli, D.M.D.,

of the Orthodontic

Department,

Β.Sc.,

of the Division

Robert L. Vanarsdall,

and Nutrition,

School of Dentistry,

University

of

Southern

M.S.

A. Richard Ten Cate, B.D.S.,

Assistant

Chief Orthodontist,

Ph.D.

Associate Professor of Oral Biology,

Professor and Chairman Dentistry

Center

D.D.S.

Board of Orthodontics;

Professor and Chairman

Forsyth Dental

University

of

Zurich

Ph.D.

of Biological Sciences; Dean of the University

of Toronto School of

D.D.S.

Professor of Periodontics,

PLANNING COMMITTEE Harry G. Barrer, Chairman James Ackerman Ze'ev Davidovitch Brainard Swain Wilton M. Krogman

School of Dental Medicine,

University

of

Pennsylvania

I

Occlusion and Function

The Origin of Mammalian Occlusion

1 A. W.

Crompton

PATTERN OF JAW MOVEMENTS In most mammals jaw movements are complex. If a mammal is viewed from the front during a normal masticatory cycle, when the molars and premolars are being used during opening and the initial stages of closing, the mandible of the active side—that is, the side in which chewing takes place—moves toward the labial side (Fig. 1-1). During occlusion, the jaw ramus on the active side moves in a lingual and dorsal direction. At this time, the multiple shearing surfaces on the lower molars and premolars are drawn across matching shearing surfaces on the corresponding upper teeth. This requires precise control of the mandible. The jaw movements in herbivores and carnivores are basically similar. The main difference is that in herbivores the transverse component of jaw movement during occlusion is more extensive than in carnivores (Fig. 1-1). The exaggerated component and larger number of small shearing surfaces on the molars of herbivores are effective in breaking down plant material, whereas the more vertical movement during occlusion in carnivores and the small number of large shearing blades on the carnassial teeth are effective in slicing flesh. Each of these specialized modes of feeding arose independently from ancestral mammals that possessed a moderate transverse component of jaw movement during occlusion. 8 In some specialized mammals, such as rodents, the jaw moves more in an anterior than medial direction during occlusion, but this is clearly a refinement of the ability to move the jaw horizontally. The complex jaw movements of mammals stand in sharp contrast to those of reptiles, in which jaw movements are usually limited to the vertical plane. In some forms, anterior or posterior movement accompanies tooth contact, but transverse movement of the ramus of the mandible on the active side toward the midline during biting never occurs in reptiles.

JAW MUSCLES The precise control of the lower jaw during occlusion in mammals is possible because of the organization of the jaw muscles. The masseter muscles (deep

3

4

/ Cat

A. W.

Crompton Rabbit

Figure 1-1. Jaw movement in a carnivore (cat) and a herbivore (rabbit) when viewed from the front. Accurate control of the lower jaw is necessary for the lower teeth to occlude precisely with the upper teeth. The horizontal component of the lower jaw movement during occlusion is more marked in herbivores than in carnivores.

Origin

of Mammalian

Occlusion

and superficial masseter and zygomaticomandibularis), inserting on the external surface, and the pterygoid muscles (lateral and medial pterygoid), inserting on the internal surface, effectively hold the lower j a w in a sling (Fig. 1-2). Differential contraction of these muscles can precisely control j a w m o v e ments. T h e temporalis muscles, inserting on the coronoid process and the inner surface of the process, are well situated to supply additional bite force, but they are poorly situated to control j a w movements in the horizontal plane. In herbivores with exaggerated transverse movement, the masseter and pterygoid muscles are the dominant adductors (Fig. 1-3), and the temporalis is usually relatively poorly developed. However, the large masseter and pterygoid muscles, because of their position relative to the j a w joint, tend to limit gape. In carnivores where a large gape is essential but where extensive transverse m o v e m e n t during occlusion is not important, these muscles tend to be relatively smaller, and the temporalis is the dominant adductor. In primitive mammals, such as the American opossum, the mandibular symphysis is extremely mobile 7 and it is doubtful whether significant muscle forces f r o m the nonactive side of the j a w are transmitted through the s y m physis to the active side during occlusion. Consequently, the control of each j a w ramus during occlusion depends almost entirely on the musculature of the active side. This is not true of forms that have a fused symphysis. 1 0 , 1 7 Here the forces of musculature contraction on the nonactive side are transmitted t h r o u g h the symphysis, and the analysis of the forces to which the active j a w is subjected is infinitely more complex than in primitive mammals w i t h a freely m o b i l e s y m p h y s i s . T h e i m p o r t a n t point is that in primitive mammals the j a w muscles are organized so that each j a w ramus is essentially controlled by the adductor muscles inserting on it. T h e lower j a w s of all tetrapods act as a third-class lever (Fig. 1-2), with muscle forces acting between the point of bite (load) and the j a w joint (fulcrum). This is true of mammals if only a single adductor muscle is considered active at any one time. However, when powerful biting takes place, all adductors on the active side contract simultaneously. 9 ' 1 2 Because of the direction of the main components of force of the major adductors, their simultaneous contraction results in large forces acting through the point of bite (Figs. 1-2 and 1-3), whereas those acting through the joint are comparatively extremely small. This is obviously more marked when the molars are being used than when the incisors and canines are being used. Primitive mammals thus are capable of generating large forces at the point of bite when using the muscles on the active side, without excessively loading the j a w joint, but major forces are generated at the j a w joint when any adductor muscle acts singly or when the front of the j a w is loaded. 1 1 T h e j a w joint must be buttressed to withstand these forces. T h e extent and position of the preglenoid and postglenoid flanges in carnivores are designed to help balance the forces imposed by the principal adductors acting alone or when the j a w is subjected to forces not caused solely by the adductor musculature (Fig. 1-3). In carnivores the postglenoid process apparently is designed to counteract the posterior pull of the temporalis, and the preglenoid process counteracts the forward pull of the superficial masseter. In typical herbivores, where the superficial masseter and medial pterygoid are more vertically oriented, the preglenoid process is usually absent or greatly reduced.

/

5

6

/

A. W.

Crompton

Opossum

pterygoid

masseter

diagastric (p) omohyoid sternohyoid

, , diagastric (a)

geniohyoid

Dinosaur (Heterodontosaurus)

Alligator

adductor pterygoid

depressor

jaw joint

adductor

Ύ

i

bite

Figure 1-2. Comparison of the jaw-closing muscles in mammals and reptiles. The muscles that open and close the j a w s of a m a m m a l are arranged differently f r o m those of a reptile. In the American opossum, the muscles are arranged so that the force of the muscles and the force generated between the teeth at the point of bite tend to meet, establishing a stable triangle of forces. (The arrows indicate the direction of the main component of force of the individual muscles.) Consequently, the j a w joint is not subjected to major vertical forces when the biting occurs between the molar teeth. In a reptile such as the late Triassic dinosaur Heterodontosaurus, the lower j a w acts as a third-class lever, and the vertical forces acting through the j a w joint, or fulcrum, can be greater than those acting through the point of bite. In reptiles such as the crocodile, the pterygoid muscles insert on the medial surfaces of the lower j a w and tend to pull the j a w ramus toward the midline. The j a w joint must be strong enough to withstand these horizontally directed forces. In mammals, the medially directed force of the pterygoid muscles is balanced by the force of the masseter muscles, which tends to pull the lower j a w laterally.

Origin of Mammalian

Occlusion

/

Lion

Figure i-3. Comparison of the skulls of a carnivore (lion) (A and B) and omnivore-herbivore (F) (pig). In the carnivore, the temporalis is the dominant muscle, whereas in herbivores the masse ter and pterygoid complex form the dominant adductors. In the carnivore, the horizontal component of the adductors is large relative to the vertical. Preglenoid and postglenoid flanges (C and D) help buttress the jaw joint against the horizontal components of these muscles. In herbivores, because the vertical component of the masseter pterygoid complex is larger than in the vertical, the preglenoid flanges (E) are absent or poorly developed. (DM = deep masseter; lat con = lateral condyle; med con = medial condyle; pogf = postglenoid flange; prgf = preglenoid flange; SM = superficial masseter; SQ = squamosal, or temporal; Τ = temporalis)

7

8

/

A.

W.

Crompton

In reptiles, the organization of the adductor musculature does not result in a reduction of vertical forces at the jaw joint but functions as a third-class lever. Reptiles also lack a masseter, or extensive adductor musculature on the external surface of the jaw. Without a musculature sling, independent movements of a single ramus of the mandible in a medial direction in addition to movement in the vertical plane are not possible. The principal adductors in reptiles always insert on the inner surface of the mandible; they therefore tend to pull the posterior part of the mandible toward the midline. The jaw joint must be strengthened to withstand these forces.

TOOTH REPLACEMENT In the mammal-like reptiles ancestral to mammals, jaw movements apparently were restricted to the vertical plane. Upper and lower postcanine teeth did not come in contact with one another when the jaw closed, and a narrow gape separated them when the jaws were fully closed.4 In these forms, new teeth erupted between old ones (Fig. 1-4), and this form of alternate replacement apparently continued throughout the life of the individual. 14 This type of tooth replacement cannot be maintained in mammals, because in the premolar and particularly in the molar series the shearing planes and cusps of one tooth accurately fit the matching shearing surfaces and basins on two teeth in the opposite jaw (Fig. 1-5). Alternate tooth replacement would disrupt this series of accurately fitting parts. Consequently, in mammals (and in the few reptiles that have matching shearing surfaces on upper and lower postcanine teeth) 2 corresponding upper and lower teeth are added at approximately the same time, and the molars are added sequentially from front to back. 13 Complex and precisely controlled jaw movements, accurate occlusion, and the ability to reduce the vertical forces acting through the jaw joint when the molars are being used are unique features of the mammalian masticatory apparatus. Once this group of features appeared in the first mammals, the stage was set for the evolution of the wide variety of dental types that characterize mammals. It is possible to obtain some idea of how and when this group of features arose by studying the fairly good fossil record that documents the gradual evolution of mammals from reptiles. Reptiles have their lower jaw made up of several bones rather than, as in mammals, a single bone (the dentary, Fig. 1-6). In reptiles, the dentary is only one of the many bones making up the lower jaw. Rather than having a jaw joint formed between the dentary and the temporal bone, the reptilian joint lies between two other bones, the terminal bone of the lower jaw (the articular) and a bone attached to the skull (the quadrate). One is an ossification of the posterior end of Meckel's cartilage, and the other is an ossification of the posterior end of the palatoquadrate cartilage. The articular is supported by a series of membrane bones. For this discussion, the important ones are the angular and the surangular. For more than 140 years it has been known that the angular, articular, and quadrate of the reptilian skull are homologous with the tympanic, malleus, and incus, respectively, of the mammalian middle ear.16 This conclusion, often called Reichert's theory, has been based on comparative embryology and could have been used to make several predictions about the evolutionary

Origin of Mammalian

Occlusion

/

9

Cat (juvenile)

Mammal-like reptile

(Thrinaxodon)

Figure 1-4. Comparison of the tooth replacement pattern of an advanced mammal-like reptile and a mammal. In the mammal-like reptile Thrinaxodon, the tooth replacement continues throughout life and in the postcanine series new teeth erupt between older teeth. In mammals (cat), replacement is limited to a single replacement of the milk teeth, and the molar has neither a predecessor nor successor. In mammals with several molar teeth, the teeth are added successively f r o m back to front.

10

/

A. W. Crompton

Μι

M1

ΜΣ

M2

Ms end

M3 hyld

Figure 1—5. The matching shearing surfaces on the molars of an American opossum (crown view). Corresponding surfaces on Upper (A) and lower (B) teeth are given the same numbers. Muscular control of the lower jaw must be precise so that the matching surfaces can be brought into contact with one another in the correct order, (abl = anterobuccal ridge, ac = anterior cingulum,c = cusp c, end = endoconid, hyd — hypoconid, hyld — hypoconulid, me = metacone, med = metaconid, mt = metastyle, pa = paracone, pad = paraconid, pr = protocone, prd = protoconid, ps = parastyle, sty = stylocone.)

changes that must have taken place in the origination of mammals f r o m reptiles: (1) In the forms ancestral to mammals, the dentary must have become progressively larger and the postdentary bones, including the j a w joint bones, must have become progressively smaller. (2) The muscular forces acting on the lower j a w must have been modified so that the reduced reptilian j a w joint would not have been damaged during powerful contraction of the adductor muscles. (3) When the enlarging dentary established contact with the skull to form a new mammalian j a w joint, the reduced old reptilian joint bones could separate f r o m the dentary and be incorporated in the middle ear. The fossil record adequately supports these predictions. 3 In a series of mammal-like reptiles covering a period of some 200 million years, it is possible to trace the

Origin of Mammalian

Occlusion

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Mammal (Opossum)

¡ncus= quadrate malleus= articular tympanic membrane

temporal

Reptile

(Prolacerta)

temporal stapes tympanic

tympanic membrane

membrane quadrate articular dentary

articular

Figure 1-6. Comparison of the skulls of a mammal (opossum) and a reptile (Prolacerta). In the mammal, the j a w joint lies between the dentary and the temporal bone. In the reptile it lies between the quadrate and the articular. These two bones are homologous with the incus and malleus of the mammalian middle ear.

E a r l y P e r m i a n (Dimetrodon)

Early Triassic

(Trirachodon)

adductor

postdentary bones L a t e P e r m i a n (Pristerognathoides)

dentary Late Triassic

(Diarhtrognathus)

Figure 1—7. Comparison of a series of mammal-like reptile skulls to illustrate the progressive increase in the size of the dentary, progressive decrease in the size and strength of the j a w joint bones and accompanied increase in the relative mass of the adductor musculature (parallel-lined area).

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Mammal (opossum)

Mammal-like reptile (Thrinaxodon)

temporal

temporal

dentary

reflected lamina

Early Mammal

stapes quadrate articular tympanic membrane

(Eozostrodon)

dentary

¡ncus= malleus= quadrate articular

stapes

articular angular

tympanic membrane tympanic= reflected lamina

Figure 1-8. Comparison of the jaw articulation of an advanced mammal-like reptile (Thrinaxodon) and a primitive mammal (American opossum) to illustrate the similar structure of the auditory ossicles and tympanic bone in mammals with the bones forming thejaw joint in reptiles. In the early mammal, Eozostrodon, the mammalian jaw articulation is present alongside the reptilian jaw articulation between the quadrate and the articular. Once the mammalian joint was established, it was possible for the reptilian jaw joint bones to specialize solely for the transmission of vibrations from the tympanic membrane to the inner ear. progressive enlargement of the dentary until it establishes a n e w m a m m a l i a n j a w articulation w i t h the t e m p o r a l b o n e (Fig. 1-7). This is accompanied b y a progressive decrease and weakening of the bones f o r m i n g the j a w j o i n t . In a fairly a d v a n c e d m a m m a l - l i k e reptile such as Thrinaxodon several m a m m a l i a n features—for example, a secondary b o n y palate, differentiation of the dentition into incisors, canines and postcanines, and double occipital c o n d y l e — w e r e already present (Fig. 1-8). Also, several of the features p r e dicted by Reichert's t h e o r y are present—the dentary is enlarged and the p o s t dentary bones reduced. H o w e v e r , the j a w j o i n t still lies entirely b e t w e e n the quadrate on the articular, and the postdentary bones apparently have done m o r e t h a n s i m p l y f o r m the j a w j o i n t . T h e a n g u l a r b o n e possesses a large flange—the reflected lamina—that consists of a thin flange of b o n e extending b a c k w a r d and lateral to the m a i n b o d y of the j a w and f r o m w h i c h it is separated by a n a r r o w space (Fig. 1-8). T h e structure of the angular, the articular, and the quadrate, except for size, is r e m a r k a b l y similar to that of the middle ear bones of a p r i m i t i v e m a m m a l such as the o p o s s u m (Fig. 1-8). Allin 1 has suggested that in such f o r m s as Thrinaxodon these bones m a y h a v e h a d t h e s a m e f u n c t i o n t h e y d o in m a m m a l s — t h a t is, to c o n d u c t the

Origin of Mammalian Occlusion

Early Permian reptile

temporal

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Early Triassic cynodont (Thrinaxodon) medial pterygoid

bite force

vertical through jaw joint

Early Permian mammal-like reptile IDimetrodon)

coronoid process

postdentary bones

Late Permian mammal-like reptile (Pristerognathoides)

ang|e

Early Triassic cynodont (Trirachodon)

Late Triassic cynodont (Diarthrognathus)

dentary

Figure 1-9. T h e principal j a w - c l o s i n g muscles in m a m m a l - l i k e reptiles u n d e r w e n t progressive differentiation and c h a n g e o f direction w i t h the course o f time. In the early stem reptile, the vertical forces generated at t h e j o i n t s w e r e as large or larger than those generated at the p o i n t of bite. T h e progressive decrease in t h e size o f the p o s t d e n t a r y b o n e s w a s accompanied b y the d e v e l o p m e n t o f a c o r o n o i d e m i n e n c e and later a c o r o n o i d process. T h i s allowed a change in t h e direction of pull, b u t n o t in the leverage o f the t e m p o r a l i s m u s c u l a t u r e . T h e insertion of the p t e r y g o i d muscles shifted progressively f o r w a r d and eventually m o v e d f r o m the p o s t d e n t a r y b o n e s o n t o the angle of t h e d e n tary. A s the intersection of t h e lines of force o f these muscles m i g r a t e d f o r w a r d , the force generated at t h e j a w j o i n t d u r i n g c h e w i n g w o u l d h a v e decreased in m a g n i t u d e , allowing a reduction in size o f t h e p o s t d e n t a r y b o n e s and j a w j o i n t . W h e n t h e extensions o f the m a i n c o m p o n e n t s of force of t h e j a w - c l o s i n g muscles and t h e bite force meet, o n l y m i n i m a l vertical forces are generated at t h e j a w joint.

vibrations of the tympanic membrane to the inner ear. He has suggested that the reflected lamina supported a membrane between its free dorsal edge and the b o d y of the angular and in part may also have acted as a m e m b r a n e . Vibrations picked up by this large tympanic membrane could have been transmitted via the body of the angular to the articular, to the quadrate, to the stapes, and into the inner ear. The chain of ossicles involved would thus be the same in both mammal-like reptiles and mammals, and no major change in this function would have been involved in the transition f r o m reptiles to mammals. In reptiles such as Thrinaxodon the mass of the ossicles was many times greater than that of mammals, but the relatively much larger tympanic m e m -

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Figure 1-10. Dentition of the j a w structure of one of the earliest k n o w n mammals, Eozostrodon f r o m the late Triassic Period. In the figure below the drawing of the posterior part of the opossum's skull is an enlarged drawing of the auditory ossicles. In the lower figure the matching shearing surfaces on upper and lower second molars are shown. These surfaces indicate that this form possessed accurate control of the lower j a w . T h e tooth replacement pattern is mammalian.

brane would have helped overcome the inertia of the system. The larger mass of reptilian ossicles would have limited the range of sound frequencies that could be transmitted. The main difference between advanced mammal-like reptiles and mammals is not the mechanism for conducting vibrations of the tympanic membrane, but is that in mammal-like reptiles the auditory ossicles also formed the jaw joint. In the earliest mammals, a substantial mammalian jaw joint, lying between a well-developed condyle on the dentary and a wellbuttressed glenoid in the temporal bone, was present (Fig. 1-8). 15 This lay alongside a reduced reptilian jaw joint between the articular and the quadrate. Once the new mammalian jaw joint was established, it was possible for these postdentary bones to retain their sound-conducting function, to separate from the lower jaw, to become further reduced in size, and to be included in a middle-ear cavity quite separate f r o m the lower j a w . H o w the angular ( = tympanic bone) moved from a situation in which it was supported by the lower j a w to a situation in which it was supported by the bones of the basicranial region is not known, but there is little doubt that this transition took place shortly after the appearance of the earliest mammals. A reduction in the size of the postdentary bones resulted in a relative

Origin of Mammalian Occlusion weakening of the j a w joint, but this was accompanied by a dramatic increase in the mass of the jaw-closing musculature (Fig. 1-7). If the lower j a w of these forms had continued to function as a third-order lever, as it does in typical reptiles, this weak j a w joint w o u l d not have been able to withstand the p o w e r f u l vertically orientated forces acting through the j a w joint during biting. What w e observe, however, in a group of mammal-like reptiles leading toward mammals is the progressive and gradual development of a coronoid process and angle to the j a w (Fig. 1-9). T h e development of these features changed the direction of pull of the temporalis, the pterygoid, and superficial masseter muscles without decreasing their effective m o m e n t arms. T h e result of the change of the orientation of the principal adductors is to progressively reduce the vertical forces acting through the j a w joint, especially when the postcanine teeth were used for biting. Consequently, in the transition f r o m mammal-like reptile to m a m m a l , an enlargement in the relative size of the postcanines (premolars and molars) and decrease in the relative size of the incisors and canines w o u l d be expected. A comparison of Probainognathus, the most advanced of the mammal-like reptiles (Fig. 1-12), with Eozostrodon, one of the earliest mammals (Fig. 1-10), confirms this suggestion. H o w e v e r , vertical forces are not the only forces to which the j a w joint is subjected during mastication. In reptiles, the pterygoid and other adductor muscles all insert on the internal surface of the mandible and pull the posterior end of the j a w , not only u p w a r d but also inward. These horizontally directed forces also had to be decreased if the reptilian j a w joint bones were to continue to decrease in size. This was achieved by the zygomatic arch b o w i n g outward away f r o m the coronoid process or ascending ramus (Fig. 1-11) of the dentary to enable the migration of part of the external adductor musculature onto the external surface of the dentary. It is possible to observe a rapid spread of the insertion area of this muscle and an increase in its size in fossil forms that were characterized by a further reduction in the size of the j a w joint bones. As a result of the laterally directed component of the masseter muscle, the medially directed forces of the medial pterygoid could be balanced. T h e changes in orientation and organization of the jaw-closing muscles, to reduce both the vertical and horizontal forces acting through the j a w joint, were essential for the shift f r o m reptilian to mammalian articulation to have taken place. This transition involved three distinct phases. 4 ' 8 Phase I was simply a reduction in the size of the postdentary bones, accompanied by changes in the adductor musculature (Fig. 1-11). In Phase II, the surangular bone, lying above and intimately connected to the articular, established a j o i n t articulation with the temporal bone, as in Probainognathus (Fig. 1-12). In this stage, therefore, postdentary bones f o r m e d t w o distinct joints—one with the quadrate and the other with the temporal bone. In Phase III, which we encounter in the earliest mammals (Fig. 1-8), the postdentary bones, including the surangular, continued to decrease in size. T h e dentary continued to g r o w b a c k w a r d and replace the surangular f r o m the s u r a n g u l o t e m p o r a l j o i n t to establish a true mammalian j a w joint. O n c e this stage was reached, the reptilian j a w joint bones and the tympanic ( = angular) were no longer required to f o r m a j a w hinge, and they could lose their contact with the dentary to become specialized solely for the conduction of vibrations initiated by the tympanic membrane. Once the n e w mammalian j a w joint was established, it rapidly increased

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Late Permian mammal-like reptile (Pristerognathoides) "temporalis"

quadrate articular

Late Permian cynodont

(Protocynodon)

Early Triassic cynodont

(Thrinaxodon)

Figure 1-11. T h e development of masseter muscles in mammal-like reptiles. In a primitive mammal-like reptile such as Pritherognathoides, no jaw-closing muscles inserted on the outer surface of the dentary, and the coronoid process or ascending ramus of the dentary fitted snuggly against the zygomatic arch, or temporal bar. In an early cynodont (mammal-like reptiles f r o m which m a m m a l s originated) such as Protocynodon, the j a w joint bones are further reduced in size, and part of the temporal muscle mass had migrated o n t o the outer surface of the dentary. In order for this muscle to reach the outer surface of the coronoid process, it was necessary for the zygomatic arch to b o w o u t w a r d away f r o m the dentary. In m o r e advanced cynodonts, such as Thrinaxodon, which had even smaller postdentary bones, the mass of the masseter musculature had increased its insertion area and invaded the whole back end of the dentary including the angle. Each side of the lower j a w , in this form, as in all later mammals, was thus held in a muscle sling, and horizontal forces acting through the j a w joint could be reduced and controlled.

in size and strength. Thus, the adaptions required to reduce forces at the joint apparently were no longer essential for either mastication or hearing. H o w ever, the reorganization of the muscle and the jaw, for minimizing vertical and horizontal forces at the jaw joint, also conferred a significant element of precise musculature control to each jaw ramus of the mandible. It is interesting to note that the mammalian type of dental occlusion 3 arose at the same time as the shift from one type of jaw articulation to the other was taking place—

Origin

of Mammalian

Occlusion

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17

in squamosal Figure 1—12. Skull of Probainognathus, an advanced cynodont closely related to m a m m a l s f r o m the middle Triassic of South America. In this form, in addition to the reptilian quadrate-articular j a w joint, a n e w j o i n t was established between the surangular of the postdentary bones and the temporal bone, or squamosal. In more advanced forms, in which the postdentary bones are further reduced in size, the dentary goes backward and invades this joint and ultimately replaces the surangular completely f r o m the joint to establish a mammalian dentarytemporal j a w joint.

that is, at the time when musculature control of the lower jaw must have been extremely precise so as to protect the weakened and reduced j a w joint. Also, at this time, the alternate tooth replacement pattern that is typical of dentitions that do not have precise and complex occlusion vanished in favor of the mammalian pattern.

SUMMARY The characteristic features of the mammalian masticatory apparatus—such as, complex and precisely controlled jaw movements in both the horizontal and vertical plane or a combination of these two, minimal vertical and horizontal forces acting through the joint when the molars are used, precise and complex occlusion between matching surfaces on corresponding teeth in the upper and lower jaw, and the mammalian pattern of tooth replacement—were all made possible by a series of changes that appeared initially to have been designed to improve the ability of mammalian ancestors to hear airborne sounds. The essential features of these changes are that they required the control of forces acting through the jaw joint and precise control of the jaw rami. These features were retained after the transition from reptile to mammal had taken place. They formed an essential characteristic of the mammalian masticatory apparatus and are a prerequisite for mammalian dental occlusion.

REFERENCES 1. Allin, E.F.: Evolution of the m a m m a l i a n middle ear. J. M o r p h , 147:403-38, 1975. 2. C o o p e r , J.S., and Poole, D . F . G . : T h e dentition and dental tissue of the agamid lizard, Uromastyx. J. Zool. Lond., 169:85-100, 1973. 3. C r o m p t o n , A . W . : T h e evolution of the j a w articulation in cynodonts. In Joysey, K . A . , and K e m p , T. (eds.): Studies in Vertebrate Evolution. Edinburg, Oliver and Boyd, pp. 231-54, 1971.

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Crompton 4. C r o m p t o n , A . W . : Postcanine occlusion in c y n o d o n t s and the origin of t h e t r i t y l o d o n t i d s . Bull. Brit. M u s . N a t . Hist. Geol., 21.2:29-71, 1972. 5. C r o m p t o n , A . W . : T h e dentition and relationships of s o u t h e r n African Triassic m a m m a l s Erythrotherium parringtoni and Megazostrodon rudnerae. Bull. Brit. M u s . N a t . Hist. Geol., 24.7:397-437, 1974. 6. C r o m p t o n , A . W . : B i o l o g y of t h e Earliest M a m m a l s . In S c h m i d t - N i e l s e n , K. (ed.): Comparative Physiology: Primitive Mammals. C a m b r i d g e U n i v e r s i t y Press, in press. 7. C r o m p t o n , A . W . , and Hiiemae, K . : M o l a r occlusion and m a n d i b u l a r m o v e m e n t s d u r i n g occlusion in t h e A m e r i c a n o p o s s u m , Didelphis marsupialis, J. Linn. Soc. Z o o l . , 49:21-47, 1970. 8. C r o m p t o n , A . W . , and K i e l a n - J a w o r o w s k a , Z . : M o l a r s t r u c t u r e and occlusion in C r e t a c e o u s therian m a m m a l s . In Butler, P . M . , a n d j o y s e y , K . A . (eds.): Development, Function and Evolution of Teeth. A c a d e m i c Press, pp. 249-87, 1978. 9. C r o m p t o n , A . W . , T h e x t o n , Α . , Parker, P., and Hiiemae, K . : T h e activity of t h e j a w and h y o i d m u s c u l a t u r e in the Virginian o p o s s u m , Didelphis Virginiana. In: S t o n e h o u s e , B . , and G i l m o r e , D . (eds.): The Biology of Marsupials. L o n d o n , M a c M i l l a n , p p . 287-305, 1977. 10. H y l a n d e r , W . L . : In v i v o b o n e strain in t h e m a n d i b l e of Galago crassicandatus. A m e r . J. P h y s . A n t h r o p . , 46:309-26, 1977. 11. H y l a n d e r , W . L . : Incisai bite force direction in h u m a n s and t h e f u n c t i o n a l significance o f m a m m a l i a n m a n d i b u l a r translation. A m e r . J. P h y s . A n t h r o p . , 48:1-8, 1978. 12. M c N a m a r a , J . Α . , J r . : T h e i n d e p e n d e n t f u n c t i o n s of t h e t w o heads of the lateral p t e r y g o i d muscle. A m e r . J. A n a t . , 138:197-205, 1973. 13. O s b o r n , J . W . : N e w a p p r o a c h t o zahnreihen. N a t u r e , 225:343-46, 1970. 14. O s b o r n , J . W . , and C r o m p t o n , A . W . : T h e evolution o f m a m m a l i a n f r o m reptilian dentitions. M u s . C o m p . Z o o l . Breviora, 399:1-18, 1973. 15. P a r r i n g t o n , F . R . : O n the u p p e r Triassic m a m m a l s . Phil. T r a n s . R o y . Soc. L o n d . B . , 261:231-72, 1971. 16. Reichert, C . : U b e r die Visceralboben der Wirbeltiere i m A l l g e m e i n e n u n d deren M e t a m o r p h o s e n bei d e n V ö g e l n u n d Saugetieren. Arch. A n a t . Physiol. W n s c h . M e d . , p p . 120-222, 1837. 17. Weijs, W . A . , and de J o n g h , H . J . : Strain in m a n d i b u l a r alveolar b o n e d u r i n g m a s tication in t h e rabbit. A r c h . O r a l . Biol., 22:667-75, 1977.

Emergence of Hominid Oral Mechanisms

2 E. Lloyd

Du

Bruì

Mammalian skull f o r m emerged f r o m that of its reptilian predecessors by the fortuitous concurrence of several decisive trends in cranial evolution. As a result, the mammalian skull exhibits the complete separation of feeding and breathing channels, the complete separation of feeding and hearing structures, and the establishment of dental occlusion, along w i t h the installation of a new, sturdy joint at the back of the single jawbone. Furthermore, these features have persisted despite a wide divergence in subsequent cranial adaptations. Previous studies have long focused on the elusive details of jaw-ear history resulting in the mammalian arrangement. T h e purpose of this chapter is to continue to unravel the story of feeding adaptations through the hominid line. Unmistakable influences of this long and devious history are evident in the oral functioning of modern man.

THE MAMMALIAN SKULL T h e architectural theme of the mammalian skull can be drafted as a trusswork in three parts—the neurocranium, the viscerocranium, and the mobile j a w bone. Visceral and neural segments lie in line with the horizontal vertebral column, with the long j a w b o n e seated below both. This gives a long, flat, fore-and-aft aspect to the arrangement. 6 , 8 T h e neurocranium is a recumbent cone. Its truncated apex faces forward and its base faces backward. It surrounds the brain and is cantilevered f r o m the front end of the backbone by means of the occipital condyles. Its blunted tip is fused to the snout in front. Thus its top is formed by frontal and parietal bones; its sides by parietal, sphenoid, and temporal bones; and its b o t t o m by sphenoid, occipital, and temporal bones. Its base at the back is the nuchal plane. T h e viscerocranium is an elongated pyramid also lying on its side. Its base, facing back, is fused to the blunted peak of the neural cone w h i c h projects between the orbits. It surrounds the airway so that its apex terminates in front as the nasal vent with the incisor teeth below. Thus, its top is formed

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Figure 2-1. Architectural theme of m a m malian skull (opossum). Visceral and neural components lie in a flat, horizontal, foreand-aft line spanned by zygomatic arch on the side and mandible below. T h e area for j a w lifting muscles is confined b e t w e e n dentition and joint.

b y nasal bones, its sides b y premaxillae and maxillae, and its b o t t o m by p r e maxillae, maxillae, palatine, and pterygoid bones. T h e j u n c t i o n o f n e u r a l and visceral s e g m e n t s is o b v i o u s l y w e a k e n e d w h e r e the base of the p y r a m i d meets the apex of the cone. C o n s e q u e n t l y these segments are spanned by widely flaring flying buttresses called z y g o m a t i c arches, w h i c h link the segments laterally. T h e j a w b o n e , or mandible, also spans the constriction at the neurovisceral j u n c t i o n . It is a long, l o w e r lever bar of b o n e that contacts the neural s e g m e n t in back via its j o i n t and the visceral segment in f r o n t via its teeth. A n e x p a n d e d area of the j a w w i t h a rising coronoid flange m a r k s the region of muscle force solely b e t w e e n teeth and j o i n t (Fig. 2-1). J a w s are customarily classified as third-class levers in w h i c h the effort force ( m u s c l e ) a l w a y s lies b e t w e e n t h e fulcrum ( j o i n t ) a n d t h e resistance force (teeth). 1 · 2 ' 4 " 6 · 9 · 10 T h i s is evident in lateral view, b u t it is also clear in a superior view of the j a w . In a state of equilibrium w i t h muscles of b o t h sides c o n tracting w i t h equal force, the resultant effort force is at the midline. W h e n c h e w i n g o n one side, the effort force still lies b e t w e e n the contralateral condyle and the biting teeth. In lever mechanics, a moment is " t h e tendency to p r o d u c e a m o t i o n a b o u t an a x i s , " or f u l c r u m ; a moment arm is the distance f r o m the f u l c r u m to the point of application of the t u r n i n g force, perpendicular to its direction. A moment of force = m a g n i t u d e of force X length of m o m e n t arm. T h u s , in the m a m malian j a w the effort a r m of the lever is the distance of the muscle f r o m the joint, and the resistance a r m is the distance of the biting t o o t h f r o m the j o i n t . It is therefore clear h o w effective force can be increased in j a w s : by p u t ting m o r e muscle into the system, by lengthening the effort arm, by s h o r t e n i n g t h e resistance a r m , o r b y c o m b i n a t i o n s of t h e a b o v e . B u t f o r c e a n d speed are inversely related. T h e r e f o r e , speed is increased by lengthening the resistance arm, because the tip of a long a r m m o v e s farther than the tip of a short a r m in the same length of time w h e n the rate of t u r n i n g is the same.

CONTRASTING ADAPTATIONS IN MODERN MAMMALS F r o m such a basic plan, t w o contrasting feeding behaviors have evolved repeatedly in the long course of m a m m a l i a n p h y l o g e n y . T h e y have been designated " c a r n i v o r o u s " as o p p o s e d to " h e r b i v o r o u s " adaptations, and they seem always to go h a n d in hand w i t h strikingly similar mechanical a r r a n g e ments.

Hominid Oral Mechanisms

Carnivore Model The carnivore feeds by violent pouncing prehension and kill, followed by tranquil cutting of chunks of relatively unresistant meat. This requires a wide gape and long oral slit for exposing stabbing canines and slicing back teeth. The m o m e n t arm of muscle must thus be shortened relative to the moment a r m of canine tooth. But the mass of temporalis muscle can be expanded because it anchors above and far back on the skull and j a w . O n the other hand, the masseter muscle, which covers the side of the jaw, must be shoved far back and thus be relatively constricted for access to the cutting carnassials. But because speed and m o m e n t u m are the essence of this adaptation, the puncturing canine teeth lean out f r o m the front of the snapping canine jaw, where all the force is concentrated on a minimal, pointed surface (Fig. 2-2). The wolf or dog skull exemplifies this best, and it is probably the least changed f r o m the earliest mammalian forms. The j a w joint is the classic dentary-squamosal or t e m p o r o m a n d i b u l a r articulation. Its upper moiety is a transverse groove scooped out of the under surface of the temporal squama. The groove is deepened behind by a high forwardly curved flange—the postglenoid process—and in f r o n t by a l o w backwardly curved lip. Its lower moiety is a transverse cylindroid jutting f r o m the back of the mandible and fitted to the fossa above. Both bony surfaces are carpeted with fibrocartilage

Figure 2-2. Contrasting adaptations in modern mammals. A. Carnivore model. Note the shallow projecting jaws, wide gape, pointed canine teeth at front, slicing carnassials far back, and heavily outlined attachment areas of temporalis. (Heavy line = moment arm of temporalis, dashed lines = moment arms of canine and carnassial.) B. Herbivore model. Note the deep jaws, minimal gape, lack of canine-teeth, long cheek tooth row, short zygomatic arch, heavily outlined attachment areas of the masseter far forward on maxilla. (Heavy line = moment arm of masseter, dashed line = moment arm of cheek tooth row). (From Du Bruì, E. L.: Am. J. Phys. Anthropol., 47(2):305-320, 1977.)

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and are separated by a thin fibrocartilaginous sheet, seamed all around to the capsular sleeve. The essential features of the system can be summarized thus: 1.Jaws projecting for prehension carrying long canines in strong bony buttresses and relatively short-rooted but sharply bladed back teeth. 2. Long, slim, barlike mandible with no ramus, providing a long resistance arm for speedy snapping; decrease in power compensated by greater momentum where resistance is all concentrated on one stilettolike canine point. 3. Joint firmly fitted for stability of movable, pressure-bearing parts reacting to a writhing prey and lined up for sectioning flesh. Movements are thus quite confined, enabling rotation in a vertical plane with a slight coronal shift, resulting in a screwlike action. 4. Marked development of temporalis muscle. 5. Large, high coronoid process that lengthens the effort arm of the temporalis and increases surface for its attachment. 6. Horizontally expanded temporal fossa walled with raised crests for optimal muscle attachment. 7. Masseter muscle reduced relative to temporalis. 8. Moderate origin for masseter on zygomatic arch posterior to cheek teeth. 9. Long, pointed canines at front, where jaw movement is swiftest; bladed carnassials at back to shorten resistance arm of meat-sectioning teeth.

Herbivore Model The herbivore feeds by tranquil prehension followed by prolonged forceful and laborious grinding of resistant plant fiber. This requires little gape. Therefore, there is a short oral slit that carries the cheek wall forward to keep food in the mouth on the grinding surfaces of the back teeth. With this arrangement it is possible to lengthen the moment arm of muscle and to increase the effort force by expanding the masseter anteriorly along the side of the snout. The temporalis can thus be reduced to pull mainly in the horizontal plane (Fig. 2-2). The horse skull illustrates this best and has been extensively renovated along specific mechanical lines. The contour of the classic dentary-squamosal joint has been resculptured so that the upper moiety now "unlocks" the joint. The articular groove is thus spread out and cants forward laterally from the transverse plane. In so doing, the postglenoid flange is bent backward and the anterior glenoid lip is transformed into a broad, anteroposteriorly convex, articular eminence producing a horizontally extended articulating surface. The lower moiety has also a more flattened convexity. Here the condyle sits atop a raised ramus to butt against the articular convexity above. The articulation is fitted with the same fibrocartilaginous coverings and interposed disc. The essential features of the system are: 1.Jaws lengthened anteriorly for cropping but deepened posteriorly for housing long-rooted, broadened cheek teeth. 2. Mandible deepened with high, "bent" ramus broadened below, providing huge area for masseter insertion. 3. Joint "loosely" fitted and broadened for wide, lateral-gliding movements, but stabilized in its excursions for pressure-bearing in laborious grinding exertions.

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4. Diminished development of temporalis muscle. 5. Slim, bladelike coronoid process raised to increase horizontal vector of temporalis. 6. Small temporal fossa with no sagittal crest. 7. Enormously enlarged masseter muscle. 8. Greatly lengthened origin for masseter extending far forward of the zygomaticomaxillary suture, onto the snout, almost to the midlevel of the cheek tooth row. 9. Well-developed, cropping incisors; canines de-emphasized (sometimes vestigial); corrugated, broad-surfaced, and long cheek tooth row set well back in long jaws to shorten resistance arm of grinding teeth.

PARALLEL ADAPTATIONS IN FOSSIL HOMINIDS The great multiplicity in biological forms has arisen only by the re-working of a surprisingly few fundamental architectural themes. It is now sufficiently clear that three predominant selective influences have reworked the long, flat profile of the basic mammalian model to produce the present shortened, rounded human cranial contour. These were erect bipedalism, ballooning of the brain, and modification of the oral apparatus. 4 But it has lately been possible to isolate the effects of these adaptive variables in the fossil crania of the earliest known hominids, the australopithecines. T w o distinct forms are generally recognized—a light, "gracile" form and a heavy, "robust" form (Fig. 2-3). In both it is immediately apparent that the brain had not yet begun the later tripling in size,16 and hence this

A

Β

Figure 2-3. Fossil h o m i n i d s . A. Australopithecus africanus, gracile f o r m . ( W e n n e r - G r e n casts, Sts 5 and Sts 5 2 B . ) B. Australopithecus boisei, r o b u s t f o r m . ( F r o m T o b i a s P. V . : In Olduvai Gorge, L. S. B . Leakey, ed., C a m b r i d g e U n i v e r s i t y Press, 1967.)

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E. Lloyd Du Bruì variable had no effect on the gross cranial contour of either form. This leaves the effects of the t w o remaining variables to be separated. As hominids, by definition both types had acquired erect bipedalism and its effect would induce the same cranial renovations in each form. But each f o r m had different oral modifications; therefore this variable stands clear. 4,6 T h e earliest and most extensive cranial alteration seems to have been its adjustment to upright posture. 3 ' 8 , 1 7 T h e essential change was a sharp bending between the neural and visceral components. This brought the facial unit with its sense organs and oral apparatus d o w n in front to maintain its orientation with the environmental surround, and the neural unit d o w n in back to maintain its orientation with the spinal cord and vertebral column. T h e result was the abrupt buckling up of the cranial base that forms the declivity appropriately called the clivus. Although this obviously put severe restrictions on the thus-crowded oral apparatus, the specific contrasting adaptations in feeding mechanics are still distinctly marked in the crania of the t w o fossil types. T h e y parallel remarkably the carnivore-herbivore models previously described. Australopithecus Africanus Other than the restrictions imposed by upright posture, the jaw-lever system in this f o r m apparently is not intensively reworked. Thus: 1. T h e j a w s are quite projecting, jutting forward f r o m below the orbital rims. 2. T h e mandible is relatively shallow f r o m top to b o t t o m with a long b o d y and short ramus. 3. Muscles (effort force) were moderately developed with perhaps a slight emphasis on the temporalis. Effort arms of the lever system were relatively short. 4. Temporal fossa elongated anteroposteriorly. 5. Z y g o m a t i c arch relatively slim, or gracile. 6. Teeth are interesting. T h e anterior teeth are large with well-developed canines, especially the lower, strangely pointed canines. 12 All in all, this gracile arrangement tends toward the carnivorous model. It was probably o m n i v o r o u s — w o u l d eat anything it could get—which implies some carnivory (Fig. 2-4). Australopithecus Boisei In strong contrast, this f o r m seems to be severely reworked. It had a highly differentiated jaw-lever system. Thus: 1. T h e j a w s do not project; above, they are w i t h d r a w n behind the anterior limits of the zygomatic arches to give the face its typical "dished-in" appearance. 2. T h e mandible is extremely deep f r o m top to b o t t o m , both in the b o d y of the j a w and in its huge, high ramus. 3. Muscle development was extraordinary apparently with an emphasis on the masseter-pterygoid complex, although the temporalis was also heavy. T h e effort arm of the masseter is lengthened so far anteriorly as to extend f o r w a r d of the zygomaticomaxillary suture well onto the maxilla, reminiscent of the horse. 4. T h e temporal fossa is shortened anteroposteriorly but deepened f r o m top to b o t t o m by a partial sagittal crest.

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(Heavy lines = moment arm o f temporalis above, moment arm o f masseter below; dashed line = m o m e n t arm o f cheek tooth r o w . ) B. A. boisei. N o t e the extremely deep retruded j a w s . Skulls brought to same size show little difference in temporalis moment arm, marked increase in masseter arm, beyond zygoma onto maxilla. (From D u Bruì, E. L.: A m . J . Phys. Anthropol., 47(2):305-320, 1977.)

5. T h e zygomatic arch is sturdy. T h e zygomatic bone at its anterior limit is twisted laterally and forward to bring the attachment o f the masseter distinctly forward o f the front o f the face. 6. Here the teeth are astounding. T h e anterior teeth, including canines, are diminutive in this massive face; their dimensions fall within the range o f the teeth o f modern man. T h e cheek teeth, however, are enormous; even the premolars have about three times the occlusal breadths o f modern man (Fig. 2-5). All in all this " r o b u s t " arrangement falls clearly into the specializations reflecting heavy herbivory (Fig. 2-4). But an examination o f the contrast in j a w joints o f these animals is even more convincing. In A. africanus the articular fossa is shallow, and the low articular eminence flattens further anteriorly onto a preglenoid plane. Laterally the eminence is limited by an articular tubercle and medially by a vertical ridge. This boundary separates the articular surface from the foramen ovale which lies behind the limit o f the lateral pterygoid plate as in modern man. Thus, in chewing on the cheek teeth, the lower j a w had to be swung to the chewing side by rotating the contralateral condyle forward and medially onto the preglenoid plane, around an axis somewhere close to the condyle o f the chewing side. But no directly transverse glide on the eminence was possible because it is walled on its medial side (Fig. 2-6). A. boisei presents completely contrasting articular contours. T h e articular fossa is deep and the eminence extremely steep. T h e eminence is terminated abruptly by a sharp rim bordering the temporal fossa above. There is absolutely no preglenoid plane. Laterally the eminence is limited by a sharp crestlike articular tubercle, but medially the eminence is entirely different from the gracile form. There is no medial wall; instead there is a broad medial glenoid

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Figure 2—5. Teeth of A. boisei. Note the unusually well-preserved dentition of robust form. The anterior teeth are the same size as in modern man, and the cheek teeth are enormously broadened, including the premolars. (Wenner-Gren casts, OH5; Peninj mandible.)

plane that extends to the rim of the foramen ovale. This orients the articular surface of the temporal bone entirely in the transverse plane. Furthermore, the lateral pterygoid plate f r o m which the lateral pterygoid muscle arises extends backward along the medial wall of the foramen ovale well behind its posterior margin (Fig. 2-6). This means that the muscle must pull directly along the transverse plane in complete conformity with the articular surface. This arrangement has been found nowhere else in hominids. Therefore, in chewing on the cheek teeth the jaw was shifted completely transversely to the chewing side by sliding the condyle down the medial aspect of the eminence onto the medial glenoid plane. Everything in the construction of the skull seems to be in harmony with this re-engineering of the jaw joint. The zygomatic arches bow widely outward to accommodate extensive lateral shifts of the contained coronoid processes of the mandible, and the extraordinarily broadened occlusal table of the back-tooth row provides an extensive transverse milling platform (Fig. 2-5). The joint of modern man in comparison to the fossil forms shows some features of each. This includes an extended preglenoid plane and a small medial glenoid plane (Fig. 2-6). The existence of this medial plane explains the presence of the so-called "Bennett shift" in the present hominid population.

ADAPTATIONS IN MODERN MAN The lower jaw remains a third-class lever system; its musculature is the effort force, its biting tooth the resistance force, and its joint is the fulcrum. Furthermore, the stubborn persistence of the basic architectural theme of j a w joint construction throughout mammals establishes, incontestably, that this construct is a major determinant in jaw mechanics. As a fulcrum, then, the joint must be adapted to maintain its stability throughout complicated move-

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Figure 2-6. Jaw joints of hominids. A. Left. A. ajricanus. Note the low slope of the articular eminence (a); preglenoid plane (b); posterior edge of the lateral pterygoid plate anterior to foramen ovale, as in modern man (c); foramen ovale (d); raised medial ridge (e). B. Right. A. boisei. Note the deep, steep slope of articular eminence (a); lack of preglenoid plane (b); lack of medial ridge, with a broad medial glenoid plane present instead (c); foramen ovale (d); and posterior edge of the lateral pterygoid plate far posterior to the foramen (e). C. Below. Modern Man. Note: the intermediate slope of articular eminence (a); preglenoid plane (b); narrow medial glenoid plane (c); foramen ovale (d); and the posterior edge of the lateral pterygoid plate anterior to the foramen (e). (From Du Bruì, E. L.: Am. J. Phys. Anthropol. 47(2):305-320, 1977.)

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E. Lloyd Du Bruì ments and to withstand pressure, because this is the reaction force explicit in Newton's third law of motion. 5 Joint Adaptations for Pressure Bearing Bones of the various limbs of the body are designed specifically to transmit and absorb forces acting on their articulations. The sharply defined internal trabecular patterning of the femur has been intensively studied, and it has been demonstrated mathematically to represent force trajectories coursing through it 11 —"a diagram of the lines of stress . . . in the loaded structure." 15 The head of the femur rests on a neck bent inward to meet its articulation with the hip bone. This puts an eccentric load on the femoral head. Seen in frontal section, the bone's internal trabeculae curve in elegant arches that cross each other at right angles and meet the articular surface at right angles.11 This arrangement coincides with directions of stress from the vertical compressive forces of the weight of the body and horizontal compressive forces imposed by the massive hip musculature, which pulls the head into its socket. Careful dissection of the forwardly bent head of the mandible in lateral view reveals an identical pattern, which disposes similar compressive forces of jaw musculature acting on it (Fig. 2-7). A section of the more symmetrically positioned femoral condyles shows parallel vertical trabeculae also meeting their articular surfaces at right angles, and these are braced at right angles by crossing studs. Again, this pattern is exactly duplicated in a transverse section of the mandibular condyle, where the condyle is seen more erectly poised on the mandibular neck (Fig. 2-7). Since this arrangement has been demonstrated with such nicety to be a pressure-bearing adaptation in all other joints, its similar function in the jaw joint is unmistakable. Osteons in the cortical bone seem to follow a parallel pattern. 13,14 Joint Adaptations for Dynamic Stability The craniomandibular joint is a compound joint. There is an articulation on each end of a single bone, the mandible. But the articulation of each side is actually two joints, one between temporal bone and articular disc and the other between disc and mandible. The jaw joint is therefore a "double-double" joint that must be considered a single functional unit because one side cannot move without movement of the opposite side. This functional unit works as a roving fulcrum around which turning of the jaw-lever occurs at all points along the temporal articular surface. As a consequence of these complexities, special engineering features must be provided to the system to limit the movements of the fulcrum and to stabilize it continuously along its functional excursions. Ligaments. Ligaments limit movement; they do not guide movement. The temporomandibular ligament is adapted in two distinct components. An outer, oblique, fan-shaped band arises from a roughened region on the outer side of the articular tubercle. It runs down and back to insert on the neck of the mandible, below and behind the lateral pole of the condyle (Fig. 2-8). An inner, horizontal, straplike band arises from the tubercle medial to and fused with the oblique band. It diverges from the oblique band to run straight back and anchor partly on the lateral condylar pole and partly on the posterolateral aspect of the articular disc. The ligamentous arrangement is not so distinct on

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fMttímí

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D

Figure 2-7. J o i n t adaptations for pressure bearing. A. Frontal section o f head of f e m u r . N o t e the G o t h i c - a r c h - l i k e crossing of trabecular struts f r o m cortical b o n e b e l o w to articular surface. B . Sagittal section of head of mandible. N o t e the similar archlike crossing of trabecular struts f r o m cortical b o n e b e l o w to articular surface. C. Frontal section of f e m o r a l condyles. N o t e the vertical trabecular struts to t h e articular surface braced by transverse struts. D. Frontal section o f m a n d i b u l a r condyle. N o t e the similar vertical struts to the articular surface braced b y transverse struts. ( F r o m D u Bruì, E. L.: Sicher's Oral Anatomy, 7th ed. M o s b y , St. Louis, 1980.)

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Figure 2-8. Joint adaptations for dynamic stability. Ligaments. A. Lateral view. Note the outer oblique band running from articular tubercle to neck of condyle below the lateral pole, and the inner horizontal band running from tubercle to lateral pole and disc. B. Superior view. The roof of the joint is removed, the roof of the external auditory meatus is at the left behind the condyle, anterior toward the right, lateral at top. (Note that the inner oblique band runs from the cut stump of the origin back alongside of the disc to insert on the posterolateral corner of the disc at left.) Muscles. C. Temporalis muscle. Note the special segment of horizontal fibers above the external auditory meatus, running forward to turn sharply down over the root of the zygomatic arch as a tendon that inserts on the depth of the mandibular notch. D. Lateral pterygoid. Note that the upper head arises from the greater wing of the sphenoid above the level of the joint; the fibers run down and back to turn horizontally and insert high on the mandibular neck and disc. (From Du Bruì, E. L.: Sicher's Oral Anatomy. Mosby, St. Louis, 1980.) the medial side of each condyle; however, a horizontal band is present at a lower level. But because both condyles must always move together, only the outer ligaments of each side seem essential. This arrangement provides a system of checkreins adapted to limit the compass of condylar excursion. The oblique band acts as a swinging mooring of constant radius that prevents the condyle f r o m drifting f r o m a stable contact with eminence and disc as the condyle glides d o w n and around onto the

Hominid

Oral Mechanisms

flattening preglenoid plane. Because the flattening lengthens the radius of curvature of the eminence, the ligament pulls taut and halts anterior movement. When the condyle swings anteromedially for chewing on the opposite side, the mooring limits the medial course. The horizontal band tenses in posterior migration of condyle and disc. It checks backward thrust to prevent the condyle and disc f r o m impinging on the fragile neurovascular tissue trapped between bony condyle and bony ear canal. Muscles. Muscles guide, as well as limit, movement. Special arrangements of short muscles hugging joints are adapted to maintain joint integrity at all times in all highly movable articulations. These are demonstrated most clearly in joints like the shoulder, or the atlanto-occipital joint. They can be seen distinctly in the j a w joint, where they act as adjustable ligaments. A special posterior part of the temporalis muscle is made up of its shortest fibers. They run horizontally forward to the root of the zygomatic arch. Here they are continuous with a tendon that turns sharply down at a right angle, around the front edge of the zygomatic root to insert on the rim of the mandibular notch at its lowest part. Contraction of this segment of muscle when the mandible is in the rest position can only pull the condyle up more tightly to stabilize it against the posterior slope of the articular eminence (Fig. 2-8). When the j a w opens and the condyle slides forward, it changes the angle of the tendon to bring it parallel to the slope of the articular eminence. In this position the stabilizing function is lost, and contraction will pull the condyle up and back along the slope. But at this instant another specialized muscular unit takes over the stabilizing function. The upper head of the lateral pterygoid muscle arises as a distinct anatomical entity from the horizontal infratemporal surface of the sphenoid bone, well above the level of the articular eminence. Its fibers run down and back, also to the anterior root of the zygomatic arch, where they swing horizontally and slightly upward to insert high on the neck of the mandible and into the articular disc. Contraction of this rather bulky mass of fibers when the mandible is in the rest position can therefore only pull the condyle forward to stabilize it more firmly against the posterior slope of the articular eminence. N o w when the jaw opens and the condyle slides forward, the muscle pull becomes more and more vertical to stabilize the condyle and disc firmly, up against preglenoid plane (Fig. 2-8).

CONCLUSIONS Unraveling the history of vertebrate jaw adaptations has explained many of the intricate peculiarities of the oral apparatus in modern man, which could have been got at in no other way. The fact that the tiny tensor tympani muscle, hidden in an obscure recess in the middle ear, is innervated by the masticator (fifth) nerve reveals the past function of the ear ossicles—they were parts of the jaw system in reptiles. The fact that present jaw joint problems often elicit ear symptoms, then, is no longer strange. The often violent forces resisted by the joint when jaws are clamped on a frantically hauling prey, or the burdensome forces sustained by the joint during forceful and prolonged grinding, gives some evidence of the range of potential in jaw adaptations. Hence the extraordinary, directly transverse, masticatory translation of the jaw seen in certain early hominid forms gives us a crucial clue to its remnant,

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which may be what is meant by the Bennett shift and which can be exaggerated in certain troublesome cases in modern man.

REFERENCES 1. C r o m p t o n , A . W . , and Hiiemäe, K . M . : H o w mammalian molar teeth w o r k . Discovery, 5:23-34, 1969. 2. Davis, D . D . : The Giant Panda. A Morphological Study of Evolutionary Mechanisms, Fieldiana: Z o o l o g y M e m o i r s . Chicago Natural History M u s e u m , Chicago, Vol. Ill, 1964. 3. D u Bruì, E.L.: Posture, locomotion and the skull in Lagomorpha. A m e r . J. Anat., 87:277-314, 1950. 4. D u Bruì, E.L.: D e v e l o p m e n t of the hominid oral apparatus. In Schumacher, G. H . (ed.): Morphology of the Maxillo-Mandibular Apparatus, Leipzig, V E B Georg Verlag, 1972. 5. D u Bruì, E.L.: F o r m and function, biological. Encyclopaedia Britannica 3, Macropaedia 7, 15th E d . , 1974. 6. D u Bruì, E.L.: Early h o m i n i d feeding mechanisms. A m e r . J. Phys. A n t h r o p o l . , 47(2):305-20, 1977. 7. D u Bruì, E.L.: O r i g i n and adaptations of the hominid oral apparatus. In Sarnat, B . G . and Laskin, D . M . (eds.): The Temporomandibular Joint, 3rd Ed., Springfield, 111., Charles C . T h o m a s , 1979. 8. D u Bruì, E.L., and Sicher, H.: The Adaptive Chin. American Lecture Series, Pub. N o . 180, Springfield, 111., Charles C. T h o m a s , 1954. 9. Hylander, W . L . : T h e h u m a n mandible: Lever or link? A m e r . J. Phys. A n t h r o p . , 43:227-42, 1975. 10. Hylander, W . L . : Incisai bite force direction in h u m a n s and the functional significance of m a m m a l i a n mandibular translation. A m e r . J. Phys. A n t h r o p . , 48:1-8, 1978. 11. Koch, J . C . : T h e laws of bone architecture. A m e r . J. Anat., 21:177, 1917. 12. Robinson, J . T . : T h e genera and species of the Australopithecinae. A m e r . J. Phys. A n t h r o p . , 12:181-200, 1954. 13. Seipel, C . M . : Trajectories of the j a w s . Acta O d o n t o l . Scand., 8:81-191, 1948. 14. Tappen, N . C . : A functional analysis of the facial skeleton with the split line technique, A m e r . J. Phys. A n t h r o p . , 11:503-532, 1953. 15. T h o m p s o n , D ' A r c y W . : Growth and Form. N e w Y o r k , M a c m i l l a n C o m p a n y , 1945. 16. Tobias, P . V . : T h e cranium and maxillary dentition of Australopithecus (Zinjanthropus) boisei. In Leakey, L.S.B, (ed.): Olduvai Gorge. C a m b r i d g e University Press, Vol. II, 1967. 17. Weidenreich, F.: Die S o n d e r f o r m des Menschenschädels als Anpassung an den aufrechten Gang. Ztschr. M o r p h o l . Anthropol., 24:157-89, 1924.

The Development of Occlusion and Facial Balance

3 James A. McNamara,

Jr.

A clear understanding of the relationship between form and function in the growing face is of great clinical importance. Over the last 11 years, our laboratory has been engaged in a multifaceted approach to experimental studies of this relationship, studies dealing with questions about the interaction of growth and remodeling of the facial skeleton as a function of the orofacial muscles. Using both clinical and experimental animal models, we have looked specifically at early orthodontic and orthopedic intervention in growing individuals, 13,14 ' 19 ' 21 ' 22 and musculoskeletal adaptations after orthognathic surgery in adults. 22 The questions considered in forming the basis of our experimental studies were derived directly from clinical problems and experience—for example, the inability of the clinician to significantly influence the growth of the mandible in retrognathic patients and the problem of relapse after orthognathic surgery in adults due to alterations in the activity and direction of the orofacial muscles and other soft tissues. To provide information on the mechanisms regulating craniofacial growth, this chapter focuses on one of the major problem areas—the alteration of mandibular growth—by comparing and contrasting clinical studies of 8- to 10-year-old children with experimental studies of young rhesus monkeys.

FACIAL BALANCE Most contemporary orthodontic treatment is aimed at correcting skeletal and dentoalveolar malrelationships, and orthodontists have generally become quite adept at achieving functionally balanced occlusions. However, one concept is often overlooked when considering the nature of malocclusion: the craniofacial complex maintains a state of homeostasis regardless of its structural configuration or whether or not it is skeletally balanced. Abnormal skeletal or dentoalveolar configurations are counterbalanced both by atypical or This work was supported in part by USPHS grants HD-02272 and DE-03610. Illustrations were done by William L. Brudon.

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James A. McNamara, Jr. abnormal patterns of perioral and masticatory muscle function and by passive pressures of the other associated soft tissues (Fig. 3-1). Therefore, the overall form-function relationship is stable, even though each of the individual components m a y have an a b n o r m a l configuration or pattern of activity. This stability is demonstrated by the relative consistency evident in the overall skeletal and dentoalveolar relationship during the growth period—for example, though maturational changes occur, an individual at age 18 years generally resembles himself as he appeared at age 6 years. M o s t current o r t h o d o n t i c therapy, particularly that practiced in the United States, is aimed at the correction of skeletal and dentoalveolar malrelationships with little or no attention paid to the accompanying abnormal functional patterns. It is often assumed by the clinician that these abnormal functional patterns will be corrected automatically if structural balance is attained. However, in patients in w h o m such unilateral treatment is undertaken (Fig. 3-2), the hard and soft tissues of the face often do not achieve a state of balance, the result of which is a relapse of the skeletal and dentoalveolar configurations toward their original relationships. In planning the ideal therapeutic regimen, the goals of treatment should include the achievement of long-term stability, which can be obtained only if the balanced skeletal and dentoalveolar configuration exists in harmony with the associated musculature and other soft tissues after treatment (Fig. 3-3). If this goal is achieved, relapse, as used in orthodontics, can be limited primarily to alterations in tooth position. Theoretically there should be no need to mechanically retain a structural relationship that has been achieved concomitant with the elimination of compensatory muscle function. Another goal of the ideal therapeutic regimen is one that should be obvious—that is, matching the therapy to the abnormality. However, owing to the limited armamentarium available to the clinical orthodontist in the past, treatment results were often compromised. For example, extraoral traction is often used in the correction of Class II malocclusion. However, not all such patients are candidates for this treatment—for example, the patient with a retrognathic mandible (Fig. 3-4A). This type of patient should not be treated with extraoral force because this approach would result in the retraction of a normal maxilla to accommodate an abnormal mandible. Although good occlusion could be obtained with this therapy, facial balance would not be achieved. However, if during the diagnostic examination the patient is asked to posture her j a w in a forward position (Fig. 3-4B), a balanced facial profile is seen. If a permanent increase in the size of the mandible could be attained in this patient through therapeutic intervention, not only could a good occlusion be achieved, but also a mandible of adequate size could then be related in a balanced fashion with the normal-sized structures of the middle and upper face.

Figure 3-1. Characteristics of existing malocclusion. A b n o r m a l Skeletal Configuration

A b n o r m a l Muscle Function Balance

A b n o r m a l Dentoalveolar Relationships

Passive Soft Tissue Pressures

Occlusion

Treated Skeletal and Dentoalveolar Relationships Original Skeletal and Dentoalveolar Relationships Figure

3-2.

Balance