Bio-mechanical model for brow ridge formation

Research done on this model has largely been based on earlier work of Endo (1965, 1966, 1970, and 1973). By applying pressure similar to the type associated with chewing, he carried out an analysis of the structural function of the supraorbital region on dry human and gorilla skulls. His findings indicated that the face acts as a pillar that carries and disperses tension caused by the forces produced during mastication. Russell (1982, 1985) and Oyen et al. (1979a) elaborated on this idea, suggesting that amplified facial projection necessitates the application of enhanced force to the anterior dentition in order to generate the same bite power that individuals with a dorsal deflection of the facial skull exert. In more prognathic individuals, this increased pressure triggers bone deposition to reinforce the brow ridges, until equilibrium is reached.

In their 1979(a) publication, Oyen et al. conducted a cross-section study of Papio anubis in order to ascertain the relationship between palate length, incisor load and Masseter lever efficiency, relative to torus enlargement. Indications found of osteoblastic deposition in the glabella region were used as evidence for supraorbital enlargement. Oyen et al.’s data suggested that more prognathic individuals experienced a decrease in load/lever efficiency. This transmits tension via the frontal process of the maxilla to the supraorbital region, resulting in a contemporary reinforcement of this structure. This was also correlated to periods of tooth eruption.

In a later series of papers, Russell (1985, 1986a, and 1986b) developed aspects of this mode further. Employing an adult Australian sample, she tested the association between brow ridge formation and anterior dental loading, via the craniofacial angle (prosthion-nasion-metopion), maxilla breadth, and discontinuities in food preparation such as those observed between different age groups. Finding strong support for the first two criteria, she concluded that the supraorbital complex is formed as a result of increased tension due to the widening of the maxilla, thought to be positively correlated with the size of the messeter muscle, as well as with the improper orientation of bone in the superior orbital region.

In short, the Bio-Mechanical model predicts that morphological variation in torus size is the direct product of differential tension caused by mastication, as indicated by an increase in load/lever ratio and broad craniofacial angle (Oyen and Russell, 1984, p.368-369).

References

*Endo, B (1965) Distribution of stress and strain produced in the human face by masticatory forces. Journal of the Anthropological Society of Nippon. 73:123-136.
*Endo, B (1970) Analysis of the stress around the orbit due to masseter and temporalis muscles. Journal of the Anthropological Society of Nippon. 78:251-266.
*Endo, B (1973) Stress analysis of the gotrilla face. Primates 14:37-45
*Russell, MD (1985) The supraorbital torus: “A most remarkable peculiarity.” Current Anthropology. 58:59-65.
*Oyen, OJ, Rice, RW, and Cannon, MS (1970a) Browridge structure and functionin extant primates and Neanderthals. American Journal of Physical Anthropology. 51:88-96.