Temporal Changes in Human Temporomandibular Joint Size and Shape

Measurements approximating the size of the temporomandibular joint were taken on a series of genetically homogeneous populations from early Nubia, which span almost 10,000 years and embody a shift from a primarily hunting and gathering adaptation to a completely agricultural lifeway. A generalized trend of reduction of temporomandibular joint size was observed. In addition, a decrease in sexual dimorphism was apparent for all measures of joint size; a change primarily mitigated by reductions in male dimensions. The observed variation in size and form of the temporomandibular joint is most likely the result of the reduction in masticatory muscle robusticity and resultant changes in craniofacial form which have been documented for the transition from a hunting and gathering to an agricultural subsistence in Nubia. The temporomandibular joint is an anatomical structure which, as the point of articulation between the cranium and the lower jaw, is of considerable importance to an understanding of human oral function. Since the only other contact between cranium and mandible occurs indirectly through the teeth implanted in both upper and lower jaws, an interdependence between stresses on the dentition and those on the joint seems likely. Histological studies of joint structure (Sicher, '52; Rees, '54; Scheman et al., '74) coupled with studies of condyle-fossa relationships during jaw movements have confirmed that joint tissues are indeed designed to be forcebearing in those regions where the condyle is seated during tooth contact. Recent theoretical biomechanical analyses (Barbenel, '69, '72; Hekneby, '74; Hylander, '75) have also indicated tha t considerable stresses may impinge on the temporomandibular joint as a consequence of occlusal loading. The probability that the temporomandibular joint may be loaded during jaw function has not been lost on researchers in clinical dentistry, who have long sought to understand the interactions between stresses on the dentition, occlusal relations and the functional integrity of the temporomandibular joint. AM. J. PHYS. ANTHROP. (1979) 50: 325-334. Much of this research has, understandably, focused on the etiology of the multifaceted symptoms of "temporomandibular joint syndrome" (e.g., Sarnat, '64) so often encountered in clinical practice, and has thus tended to couch joint alterations due to function in a pathological framework. A heightened appreciation of the reactions of the temporomandibular joint to functionally-induced stresses is possible from a specifically anthropological perspective: i.e., by examining situations in which occlusal stresses (and presumably stresses on the joint) are undeniably several times greater than those encountered in modern-day dental patients and for which a time depth of often thousands of years allows an assessment of joint response to a changing functional environment to be made. Bite force data on extant human populations suggest that, irrespective of body size, occlusal forces are appreciably greater in aboriginal peoples pursuing a hunting and gathering lifeway than in urbanized Europeans, becoming as much as three times ' This research wafi supported in part by Wenner-Gren Foundation for Anthropological Research Grant 2959 and by USPHS DE-03610 from the National lnstitutes of Health. Requests for reprints should be sent to Dr. David S. Carbon, Center for Human Growth and Development, 1111 E. Catherine, Ann Arbor, Michigan 48109.