Stress and strain in the mandibular symphysis of primates: A test of competing hypotheses

Abstract

The primary purpose of this study was to test various hypotheses about symphyseal stress in primates. First, those patterns of symphyseal strain that would be associated with various hypothetical patterns of symphyseal stress were formulated. Then these hypothetical patterns of stress and strain were tested by comparing the formulated bone strain pattern with actual in vivo symphyseal bone strain patterns. Patterns of in vivo symphyseal bone strain were determined by bonding rosette and/or single‐element strain gages to the midline of the middle and lower third of the labial aspect of the symphysis of six adult Macaca fascicularis. Following recovery from the anesthetic, bone strain was recorded during mastication, incision, and isometric biting. Symphyseal bone strain was also recorded during yawning, licking, and threat behaviors. The data suggest that during the power stroke of mastication, the macaque symphysis is predominately sheared dorsoventrally and/or twisted about a transverse axis and bent by lateral transverse bending of the mandibular corpora. During lateral transverse bending of the mandibular corpora, the labial aspect of the macaque symphysis experiences compressive bending stress, while the lingual aspect experiences tensile bending stess. During the opening stroke of mastication and during other jaw opening behaviors, the macaque symphysis is bent by medial transverse bending of the mandibular corpora. At this time the labial aspect of the symphysis experiences tensile bending stress, while its lingual aspect experiences compressive bending stress. During both the power and opening strokes of mastication, the macaque mandible is bent in the plane of its curvature, and therefore the mandible acts as a curved beam. This is important because it results in elevated levels of stress along the lingual aspect of the macaque symphysis, particularly during the power stroke of mastication. During the power stroke of incision, the local effects of the bite force are unknown; however, at this time the lower half of the macaque symphysis is both sheared dorsoventrally and bent due to twisting of the mandibular corpora about their long axes. The results of this stress analysis have implications for understanding the mechanical attributes of symphyseal structure. In order to counter dorsoventral shear, the most important symphyseal attribute is to have adequate cross‐sectional area of bone in the plane of the applied stress. In contrast, both the cross‐sectional area of bone and symphyseal shape is important in order to counter stress effectively during symphyseal torsion and the three symphyseal bending regimes. An effective way to counter symphyseal bending due to twisting of the mandibular corpora about their long axes is to either increase the vertical height of the symphysis or to form an asymmetrical beam section by concentrating bone along the lower aspect of the symphysis, e.g., forming a simian shelf. An effective way to counter symphyseal torsion about a transverse axis and symphyseal bending due to medial or lateral transverse bending of the mandibular corpora is to increase the labio‐lingual thickness of the symphysis, e.g., forming a superior transverse torus.