Mandibular Function and Biomechanical Stress and Scaling

Abstract

Patterns of stress were analyzed in the mandibular symphysis of Macaca fascicularis using rosette strain gages. During jaw opening, the mandibular symphysis is bent due to medial transverse bending of the mandibular corpora. Levels of stress and strain are relatively low at this time, and the source of this stress is the medially-directed component of force from the lateral pterygoid muscles. During the power stroke of mastication, the symphysis is maximally stressed. At this time the symphysis experiences dorsoventral shear and bending due to lateral transverse bending of the mandibular corpora, i.e., “wishboning.” The dorsoventral shear is due to the vertical component of the balancingside adductor muscle force; the “wishboning” is due to the laterally-directed components of the bite and jaw adductor muscle forces. Unlike dorsoventral shear, “wishboning” results in considerable levels of stress and strain, particularly along the most lingual aspect of the symphysis. The most effective way to counter this stress is to increase the thickness of the symphysis in the labio-lingual direction. The stress analysis and an allometric analysis of mandibular dimensions in female cercopithecine (Old World) monkeys indicates that allometric changes in the symphysis are readily understood if the mandible is modelled as a curved beam. With increasing body size, symphyseal thickness in cercopithecines must increase in a positively allometric fashion so as to prevent the occurrence of dangerously high levels of stress along the most lingual aspect of the symphysis. This is because increasing body size is associated with three factors thathave important consequences within the context of the biomechanics of curved beams: (1) jaw length is positively allometric to body size, (2) mandibular-arch width is negatively allometric to body size, and (3) there is a tendency to use relatively greater amounts of balancing-side jaw muscle force with increased body size because of dietary changes and allometricconstraints on total jaw muscle force.