Tooth Orientation during Occlusion and the Functional Significance of Condylar Translation in Primates and Herbivores

Introduction The temporomandibular joint (TMJ) in carnivores is a ginglymoid (hinge) joint in which the preglenoid and postglenoid processes limit anteroposterior movements of the mandibular condyle, permitting almost exclusively rotation around a transverse axis (Becht, 1953; Maynard Smith & Savage, 1959; Noble, 1973; Freeman, 1979; Crompton, 1981). In contrast, the TMJ in anthropoid primates and several orders of specialized herbivores (Perissodactyla and Artiodactyla) is a diarthroidal joint that permits the mandibular condyles not only to rotate around a transverse axis but also to slide anteroposteriorly (“translate”) relative to the glenoid fossa during jaw closure and opening. In a study on goats, De Vree and Gans (1975) pointed out that lateral shifts of the mandible in animals with a diarthroidal TMJ must occur by the translation of one condyle in its fossa coincident with rotation of the opposite condyle about a vertical axis through the second fossa. There is little precise information on how the active and balancing side condyles move relative to the glenoid fossae during a masticatory cycle. Weijs (1994) claimed that, during closing, medially directed movement of the working side mandible is essentially a pure rotation around the working side condyle, which is braced against the postglenoid flange. Extensive transverse jaw movements obviously require an anteroposteriorly long glenoid. However, it is not obvious why both condyles move anteriorly during opening and posteriorly (at different rates) during closing given that medial and lateral movement of the mandible can be accomplished by rotating the mandible about a stationary working side condyle that is buttressed against the postglenoid flange while translating the balancing side condyle. It is this aspect of condylar translation that we wish to discuss in this paper. Several hypotheses have been proposed to explain anteroposterior condylar translation in the diarthroidal TMJ. First, Moss (1960, 1975, 1983) suggested that condylar translation is an adaptation to stabilize the axis of rotation of the mandible near the location of the mandibular A. W. Crompton Daniel E. Lieberman, and Sally Aboelela