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TECHNICAL NOTES

Experimental Studies in Wear: I

C. L. BRACE AND STEPHEN MOLNAR Museum of Anthropology, The University of Michigan and University of California, Santa Barbara

ABSTRACT The construction of a machine called CANIBAL which is capable of simulating the actions of the human during is reported. Powered by an electric motor, a cam activated rocker assembly transmits force by means of wires to each point on the mandible where muscles would be attached. A separately adjustable cam, rocker arm, directional pulley and wire represents each muscle. Models of dental arches cast in dental stone or other materials can be mounted and subjected to extended wear. With different cam settings, wear patterns can be produced which resemble those seen on various fossil and living human dental arches.

Extreme forms of , so fre- ture of harpoons. These and other funtions quently seen upon examination of the den- place heavy stress on the dental arches, titions of skeletal remains of "primitive" particularly the forward end ( and peoples, suggest that the teeth are fre- canines). Unfortunately, there is as yet quently used for purposes other thaneating, no report on the of these people. even though the abrasives in are prob- The various studies reporting the de- ably the major causes of wear (Dahlberg, grees and types of show that not '63). Studies on both skeletal and living all people wear their teeth in the same way, representatives of aboriginal populations and the patterns produced frequently are describe heavity worn teeth €or people sub- characteristic of certain populations (Bux- sisting on aboriginal diets (Cambell, '25; Pedersen, '38, '49; Shaw, '31; Snow, '48). ton, '20; Drennan, '29; Shaw, '31; Stead- Observations of living populations show man, '37; Campbell, '39; Goldstein, '48). that the teeth are used for a variety of pur- These studies deal mainly with the wear poses such as chewing of hides as part of planes produced on the surfaces. In the tanning process (Pedersen, '38), the addition, the edge-to-edge bite, considered chewing of coca leaves or tobacco dipped characteristic of primitive people, is men- in lime (Leigh, '28; '37), and, as can be tioned as being the result of a wearing seen in the heavy wear on the front end of down of the cusps and not as the result of the , for tools in tearing, cut- some genetic mechanism (Ritchie, '23; ting, or holding and pulling (Falero, '05; Steadman, '37; Emslie, '52; Senyiirek, '52). Noble, '26; Brace, '62).Though the litera- Leigh also ('28) states that the edge-to- ture discussing dental caries and attrition edge bite is the result of the function and is quite extensive, there are very few re- is not innate. He observes that adolescents corded observations on how the teeth are and children in an aboriginal population characteristically used. When questioned frequently show a labial overlapping of on this point, however, a person who has the superior incisors while maintaining a spent some time studying a primitive group normal position of the first molars. Adults will reply that the teeth are frequently in the same population show an edge-to- used for a variety of functions in place of edge . tools. For example, several persons who 1 The senior author would. like to express, his .grati- have visited the Seri Indians of Northern tudc to the Research Committee at the University of Calfioda, Santa Barbara, for granting support from Sonora, Mexico, over the past few years General Research Funds to defray the expenses in- mention that the Indians are "always put- curred, .startin in 1961 and continuing through 1967. In addition, tge junior author gives thanks to the ting something in their mouths." The Seri National Science Foundation for awarding an Under- graduate Participation Award in 196'2 -1963, NSF-G- have been observed using their teeth to aid 16128, and to the National Institutes of General in the making of baskets, splicing rope, un- Medical Science for awarding a Predoctoral Fellow- ship, l-FI-GM-30,609-01, without which the project tying a difficult knot, and in the manufac- could not have been completed.

AM. J. Pnw. ANTHROP.,27: 213-222. 213 214 C. L. BRACE AND STEPHEN MOLNAR

These wear patterns, whether caused by sku11 than towards demonstrating the ac- dietary abrasives or through the use of the tual functioning of the muscles involved teeth as tools, are the results of a particu- (Anthony, '03; Anthony and Pietkiewicz, lar set of movements of the mandible. '09; Baker, '22, '41; Pratt, '43; Washburn, Years of study by a great many investiga- '46, '47; Horowitz and Shapiro, '51. tors emphasize that the movements of the More recently, the development of elec- human are the results of highly com- tromyographic techniques has led to the plicated, precisely coordinated changes in discovery of the precise sequence of mus- the forces applied by the masticatory mus- cle action required to produce specific cles. In order to study the ways in which mandibular movements (Moyers, '49, '50; the various wear patterns are produced Neumann, '50; Carlsoo, '52, '56; Pruzan- and how they affect the occlusion of the dental arches, the movements of the man- sky, '52; MacDougall and Andrew, '53; dible must be thoroughly understood. Latif, '57). While great insights have been A coiicern for understanding the precise gained through the use of electromyog- movements of the human chewing appa- raphy, a number of limitations are also ratus and the problems of dental restora- present. Insertion of the electrodes causes tion and has produced an in- a certain amount of which may mod- terest in mechanical articulators and the ify normal activity, and as yet these is no problems of masticatory actions which ex- way to determine the extent of this possi- tends back into the nineteenth century - ble inhibition (Carlsoo, '52, '56; Moyers, at least on the part of the dental profession '50). Furthermore, in experiments with (Luce, 1889, 1891, '11; Walker, 1896a,b). the living, it is impossible to eliminate at The immediately practical significance of will the contributions of specific muscles such interests has meant that research in order to determine either their functions concerning the actual variations of the hu- or the functions of others which tend to man masticatory machinery had little im- contract at the same time, although some petus. As a result, theories such as the one of this has been demonstrated in the clini- suggesting that "spherical articulation" cal literature (Root, '46; Cobin, '52). was the human ideal could be offered In addition, electromyographic analysis without contradiction (Needles, '22, '23a,b). requires the cooperation of the subject, and almost no information has been sys- and it is not always possible to obtain this. tematically collected on the different ways In this latter regard, some of the dental in which the human dentition is character- conditions of greatest interest to the an- istically used. To our knowledge, only one thropologist occur among people in remote previous attempt has been made to simu- parts of the world far from the appropriate late precisely the exact forces which oper- laboratory facilities, and, finally, some of ate the human chewing mechanism, and the most interesting problems are pre- this was only a demonstration device not sented in the and teeth of skeletal re- designed for active and continued opera- mains, both recent and ancient, where no tion (Lord, '13, '37) . possibility for active cooperation by the Some statements concerning mandibular subject exists. function were simply speculations based With this as a background, it would on the assumed logic of the human chew- seem that a logical approach would be the ing apparatus (Keith, '20; Gysi, '21 ; Brace, construction of a machine which would '62) to which some insight was contri- simulate the human chewing mechanism buted from histological studies (Robinson, and which would allow the variation and '46; Rees, '54) and from clinical cases control of each significant element sepa- (Root, '46; Cobin, '52). From another di- rately. An attempt to reproduce the full rection, information has accumulated amount of capable of being gen- from the experimental modification of the erated at each point on the dental arcade chewing machinery, although this has of the living would be desirable, but so been directed more towards elucidating the far the expense of construction and instru- relationship between the growth of the mentation has proved prohibitive. While masticatory apparatus and the form of the this approach has had to be postponed, it EXPEKIMENTS IN TOOTH WEAR 215 is our hope to be able to pursue it further , eIectromyographic work has shown if funds become available. that the temporalis should not be repre- Although is has been demonstrated that sented by a single direction of pull (Latif, the force exerted by the human bite can '57). Because of the variety of directions exceed 100 kg (Black, 1895; Klaffenbach, in which the temporalis can pull, provision '36; Warner, '39; Howell and Manly, '48; was made for three components: an an- Anderson, '56), the forces norrnally gener- terior component which pulls vertically; a ated during the mastication of char- posterior component which pulls back a- acteristically encountered in the "civilized bout 15" above the horizontal; and a mid- diet are below 10 kg, averaging in the dle component which pulls back and up, neighborhood of 3 to 4 kg (Neumann, '50). halfway in between the directions of the Our intent, then, was to build a machine other two components. which was at least capable of generating All together, for the forces acting on the normal chewing and which mandible, provisions had to be made for could simulate by homologous means all of three temporal components, a masseter the actions of which the human dental component, and external pterygoid com- apparatus is capable. The mental image ponent, and a digastric component. Taking created by a machine which wears itself both sides of the mandible into account, down by chewing -in fact, eats itself - the machine had to be able to pull in 12 led us to refer to it as the CANIBAL, diff'erent directions at once. This was co- which we rationalize to Cam Actuated ordinated by deriving movement from a NIBbling AppLiance. set of cams mounted on two cam-shafts. In the course of producing a device Power was provided by a one-third - which was actually able to carry out our power A. C. motor operating through a intent, preliminary models were made worm gear transmission which achieved a which served to test the practicality of the 14:l reduction. A "V" belt and pulleys basic approach and give us some idea con- completed the transmission of power to the cerning strength of materials and magni- cam assembly. Different speeds could be tude of power source required. For in- produced by varying the pulley diameters. stance, CANIBAL 11, the immediate pred- The overhead cam-shaft and rocker- ecessor of our present model, was con- arm assembly transmitted vertical forccs structed of odd pieces of aluminum door to stainless steel cables, with changes in frame with cam and dentition mounting the directions of pull being determined by plates of three-quarter inch plywood, but variously placed guidance pulleys. In order the bend in these elements produced by to minimize the power lost to friction even a moderate application of force which had plagued CANIBAL 11, bearings (barely enough to make impressions on were used wherever possible. dental wax) was sufficient to demonstrate The framework of CANIBAL I11 was the need for much stouter construction. welded from one inch angle iron. This In anticipating the dimensions which serves as support for the and the such a machine should have, it was noted cam assembly mounting plates which were that there are five sets of muscles which cut from three-quarter inch aluniinum jig are of primary importance in producing plate. The cam assembly includes a drive mandibular movements : shaft mounted on one-half inch sealed ball 1. Temporals. bearings, one at each end, and two cam 2. Masseters. shafts mounted at both ends and in the 3. Internal pterygoids. middle by similar one-half inch bearings. 4. External pterygoids. The "V" belt and pulley turn the drive 5. Digastrics. shaft which imparts motion to the cam Actually, the masseters (both superficial shafts by means of one and one-half inch and deep portions) and the internal ptery- spur gears, three-eighths inch thick, and goids produce effects upon the mandible with a pitch of 24 teeth to the inch. There which are so nearly identical that, for pur- are eight cams per shaft, cut from one and poses of our model, one component was one-half inch bronze round stock, giving a regarded as being sufficient. On the other total of 16 possible sources of power appli- 216 C. L. BRACE AND STEPHEN MOLNAR

Fig. 1 The CANIBAL, front view. cation. Each cam has two Allen-head set pulleys made from three-quarter inch screws which enable each one to be sep- brass round stock mounted in one-quarter arately adjusted so that the timing action inch roller bearings. of the “muscle” represented could be Although many refinements can be changed. Calibrated discs fixed on the cam made, the machine is now reliable enough shafts enabled us to quantify the varia- to run untended for periods of half a day tions in cam settings from an arbitrary or more, and some tentative insights can zero point. already be reported. Among these is a The motion of the cam shafts is picked possible function for the digastric muscle up by cam follower , or rocker arms, which has been generally overlooked. It has niounted one-fourth inch roller bearings on usually been assumed that this is of im- o.ne-fourth inch rods, one parallel to each portance mainly in the opening of the cam shaft. The rocker arms were made mandible and primarily in the later phases from five-eighth inch cold rolled key stock, of opening. The CANIBAL shows, however, and in the ends which ride on the revolv- that it can also play a role in the distal ing cams were set three-eighth inch sealed shifting of the mandible. This is only a ball bearings (this had been a major area tentative suggestion, however, since we do of friction in CANIBAL 11). The motion of not have any data relating to the possible the outer ends of the rocker arms is trans- differences in function of the anterior as mitted to the mandible by 6 mm diameter contrasted with the posterior belly of the stranded stainless steel cable. Changes in digastric. As yet, electromyographic studies the direction of pull were accomplished by have been confined to the anterior belly running the wires over variously placed (Moyers, ’50). If our surmise concerning EXPERIMENTS IN TOOTH WEAR 217

Fig. 2 Side view of the CANIBAL showing how pulleys change the direction of force application from the vertical pull of the rocker arms. the possible listo-shifting function of the however, it was demonstrated that the digastric is correct, then it might be a glenoid was not a stress bearing first step to explaining why the digastric joint (Robinson, '46; Rees, '54). Robinson impressions of Palaeolithic are showed that the resultant of the directions so marked. Perhaps the digastric played a of the temporal and masseter pull would part in the fixing of the mandible against produce vertical force in the region of the the forward pull exerted when an object molars without any lever action, although held between the incisors was pulled by this does not explain how force is trans- the . With the heavy wear so char- mitted to the incisors. acteristic of the forward end of the dental In order to study the ways in which the arch during the Palaeolithic, it seems evi- forces could be transmitted through the dent that the manipulative function of the forward part of the dental arch, the man- anterior dentition was of survival value, dible was first treated as a moving body and it is possible to speculate concerning free of bony articulation. Varying the ap- this and the apparently great development plied forces one by one, it was shown that of the digastrics. biting can be produced by the Among the various other insights which masseter (and presumably the internal the CANIBAL has yielded is one which is pterygoid) and the posterior component of importance in understanding the means of the temporal alone. Basically, this can whereby a biting force is transmitted to be considered as the action of a first class the forward end of the dental arch. Keith lever, with the angle of the mandible held ('20) considered this to occur mainly as a by the masseter (and internal pterygoid) result of the action of the masseter, evi- acting as the fulcrum, while the ascend- dently envisioning the mandible as a ing ramus, powered by the posterior com- second class lever. More than 20 years ago, ponent of the temporal, acts as the power 1118 C. L. BRACE AND STEPHEN MOLNAR

Fig. 3 View of the CANIBAL from three-quarters above, showing the details of power transmis- sion and the cam and rocker arm assembly. arm. This may offer a possible explanation glenoid fossa, and, if the condyles are for the results reported by Latif ('57) who, held against it so that it can act as a ful- in an electromyographi study, found that crum, then the mandible can be regarded 22% of the subjects showed greater activ- as a second class lever powered by the ity of the posterior region of the temporal vertical pull of the major masticatory mus- than in other parts of the muscle during cles - biting force being exerted through edge-to-edge biting. the incisors. ' While the glenoid fossa Other force combinations are also cap- proper is a non-stress bearing structure, able of producing a resultant at the for- 2 In an earlier version of this paper, delivered at ward end of the dental arch. 'The anterior the annual meetings of the American Association of Physical Anthropologists at Mexico City June, 1964, and middle regions of the temporal, with wc had stressed lhe fact that removal ok the alenoid fossa assembly had no effect on the motions of the the addition of the masseters and internal mandible or on the forces generated. While strictly pterygoids, will exert considerable force speaking we were able to make this demonstration, we are indebted to Dr. C. L. DuBrul for pointing out through the incisors if the condyles are that this would require exertion on the part of the posterior component of the temporal which is out of held in a fixed position on the inclined all proportion to the size and indicated power of that plane formed by the posterior slope the muscle section in living human beings. As a result of of DuBrul's suggestions, we were stimulated to con- articular eminence. This points out the im- struct a more adequate glenoid assembly than that which was present in the earlier versions of the portance of this region as compared to the machine. EXPERIMENTS IN TOOTH WEAR 219 the articular eminence has a compact bony experiments. This will lead, with allow- surface which, with the histologic analysis ance for losses due to friction and mechan- of the overlying tissue, indicates that it is ical inefficiency, to a description of the fully capable of stress bearing functions activity of each muscle as the type of oc- (Root, '46; Rees, '54; Symons, '54). Re- clusion is altered to correspond with those cognizing the importance of the articular observed in the various fossil and recent eminence (c. f. Angel, '48), efforts were populations. made to manufacture an effective mechan- In addition to the study of the forces ical counterpart. This was accomplished of mastication, the major concern is to re- by covering a fiat strip of metal with a produce the different kinds of wear com- polyethylene pad, running it from one side monly found on fossil and recent denti- of the machine to the other, and fixing it tions. Numerous runs have been made to the frame by means of an adjustable with the machine adjusted to various cam bracket which allows alterations both in settings, and several casts were worn down the angle of inclination and the height. into patterns resembling those character- Using this, it has been possible to demon- istic of certain populations. The major strate the inadequacy of considering the difficulty at present, however, is that the mandible solely as a free moving body, material used to make the casts - den- which while it will serve to show the pro- tal stone, dental plaster, or a variety of duction of resultant forces through the resins -either chips and cracks easily or molar part of the dental arch, will not wears too slowly to be of practical value. fully account for the pressures which We entertain the hope, at least, that an ex- members of some populations regularly perimental approach such as this can cast exert on the forward part of the dentition. some light on the nature of the selective Further manipulation of the forces ap- forces which have been important in in- plied has shown that the masseter and the Auencing the course of the evolution of anterior region of the temporal provide the human dentition. To this end, work the most efficient means of applying a is continuing (a) to find a suitable cast- biting force to the incisors. While these ing material, (b) to refine various mechan- two muscles furnish the main biting power, ical features of the CANIBAL, and (c) to the external pterygoids are also brought in- design instrumentation capable of measur- to play in holding the condyle fixed in posi- ing all the forces and pressures involved. tion on the articular eminence. Others A summary of the results together with have noted that even at the rest position photographs will be presented in future the condyles are more on the slope of the papers. eminence than in the fossa (Hildebrand, '31; Angel, '48; Sicher, '52; Choukas and LITERATURE CITED Sicher, '60). In addition, the functional Anderson, D. J. 1956 Measurement of stress in significance of this structure is further in- mastication. J. Dent. Res., 35: 671-673. dicated by observations made on skeletal Angel, J. Lawrence 1948 Factors in the tem- material where it has been noted that, in poromandibular joint form. Am. J. Anat., 83: 223-246. Eskimo skulls with heavily worn denti- Anthony, M. R. 1903 Introduction a l'htude ex- tions, the eminence is greatly modificd phrimentale de la morphogenie. Bull. de la SOC. and the fossa is rather shallow (Buxton, d'Anthrop. Paris, 4-5: 119-145. '20; Richie, '23), and observations also Anthony, M. R., and W. B. Pietkiewicz 1909 Nouvelles expkriences sur le r61e du muscle made on California Indian skulls (Leigh, crotophyte (temporal) dans la constitution '28). It is hoped that future studies will morphologique du crane et de la . Comptes help to illuminate the relationship between Rendus des SBances de YAcademie des Sciences eminence form, tooth wear, and muscle de Paris, 148: 870-871. Baker, Lawrence W. 1922 The influence of the action. Since a major consideration in any forces of occlusion on the development of the vector system is magnitude as well as di- of the skull. Intern. J. Orthod., Oral rection, appropriate instrumentation is be- Surg., and Radiog., 8: 259-281. ing designed for installation on the CAN- 1941 The influence of the formative dental organs on the growth of the bones of IBAL so that exact measurements of the the face. Am. J. Orthod. and Oral Surg., 27: force magnitude can be made in future 498-506, 220 C. L. BRACE AND STEPHEN MOLNAR

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Root, Robert W. 1946 The mechanics of the Symons, N. B. B. 1954 The attachment of the temporomandibular joint: illustrated by two muscles of mastication. Brit. Dent. J., 96: 76- cases. Am. J. Orthod. Oral Surg., 32: 113-119. 81. Senyurek, Muzaffer S. 1952 A study of the Walker, W. E. 1896a Prosthetic : The dentition of the ancient inhabitants of AlaGa glenoid fossa; the movements of the mandible; the cusps of the teeth. Dent. Cosmos., 38: 34- Hoyok. Belleten, XVI: 153-224. 43. Shaw, J. C. Middleton 1931 The Teeth, The 189613 Movements of the mandibular Bony Palate and the Mandible in Bantu Races condyles and dental articulation. Dent. Cos- of South Africa. John Bale, Sons, and Daniels- mos., 38: 573-583. son, London. Warner, Howard K. 1939 Gnathodynamics: Sicher, Harry 1952 Functional anatomy of the The measurement of biting forces with a new temporomandibular articulation. Dent. J. Aust., design of gnathodynamometer. Aust. J. Dent., 24: 1-14. 43: 381-393. Snow, Charles E. 1948 Indian Knoll skeletons Washburn, S. L. 1946 The effect of removal of the zygomatic arch in the rat. J. Mammal., 27: of site Oh2, Ohio County, Kentucky. U. Kent. 169-172. Reports in Anthrop., 4: 382-554. 1947 The effect of the temporal muscle Steadman, F. St. J. 1937 Malocclusion in the on the form of the mandible. J. Dent. Res., 26: Tasmanian aborigines. Dent. Rec., 57: 213-249. 174.