Experimental Studies in Human Tooth Wear: I
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TECHNICAL NOTES Experimental Studies in Human Tooth 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 mandible during chewing is reported. Powered by an electric motor, a cam activated rocker arm 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 tooth wear, 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 (incisors and peoples, suggest that the teeth are fre- canines). Unfortunately, there is as yet quently used for purposes other thaneating, no report on the dentition of these people. even though the abrasives in food are prob- The various studies reporting the de- ably the major causes of wear (Dahlberg, grees and types of attrition 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 molar 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 dental arch, 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 occlusion. 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 jaw 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 prosthesis has produced an in- a certain amount of pain 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 jaws 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 pressure 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 hand, 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 foods 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 pressures 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.