The Functional and Adaptive Significance of the Supraorbital Torus.

Abstract

The supraorbital torus, or browridge, is a bar of bone extending continuously above the bony orbits. It is found only in some genera of the primate order. Among the hominids, the torus has taken several forms, being lightly developed in the australopithecines, heavy and bar-like in Homo erectus, and undulating in archaic Homo sapiens. In living human populations, supraorbital development is greatest among the Australian Aborigines. Because no muscles of consequence attach directly to the torus, it has been argued that the torus is not functional. However, in-vitro strain gauge experiments demonstrate that when the anterior teeth are loaded, the supraorbital region acts as a bent beam, pulled downward on each end by masticatory muscle forces and pushed upward centrally by bite force. Clinical and experimental data indicate that in response to repeated dynamic bending stress, mechanically guided cellular activity reconstructs skeletal material until bending stresses are neutralized. With these facts in mind, the hypothesis that supraorbital development is, in part, a predictable ontogenetic response to in-vivo bending stresses which concentrate over the eyes during anterior tooth loading was tested by means of a biomechanical model. The Bent Beam Model states that supraorbital bending is a function of the area moment of inertia of the forehead (relative to the direction of the bite force), of masticatory muscle force and its moment arm and of the component of vertical anterior bite force tangential to the height of the face. This model was tested using a large series of Australian Aboriginal crania. Estimates of the true values of the independent variables were obtained craniometrically. Significant relationships were found between browridge development and forehead area moment, muscle force and its moment arm and their product, bending moment. It was concluded that an important function of the torus is to resist bending stress concentrated over the eyes during anterior biting and that supraorbital development is proportional to the amount of such stress which cannot be resisted by the unadorned frontal squama.