The mechanical significance of morphological variation in the macaque mandibular symphysis during mastication.

Catarrhine symphyseal morphology displays considerable variation. Although this has been related to dentition, phylogeny, sexual dimorphism, and facial orientation, most emphasis has been given to the functional significance of the symphysis to mechanical loading during mastication. The current state of knowledge regarding the mechanical significance of the symphysis is based on a combination of in vivo experimental and comparative studies on Macaca fascicularis. These approaches have provided considerable insight into the stereotypical patterns of loading in the symphyseal region during chewing and hypotheses related to the associated symphyseal morphologies. Finite element analysis (FEA) was used to assess how in silico manipulation translates into the mechanical loading hypotheses previously proposed experimentally. In particular, this study tests the form-function relationship of the symphysis of an adult M. fascicularis mandible during lateral transverse bending and dorsoventral shear of the mandibular symphysis, and a series of modified hypothetical morphologies including absence/presence of tori and variation in the inclination and depth of the symphysis. FEA results of this study support previous findings that stresses associated with lateral transverse bending and dorsoventral shear of the mandibular symphysis can be minimized via an increased labio-lingual thickness in the superior transverse torus, an oblique symphyseal inclination, and/or an increased symphyseal depth. The finding that reduction of strains related to lateral transverse bending and dorsoventral shear can be achieved through a number of different morphologies contributes to our understanding of the influence of morphological and/or developmental constraints, such as dental development, on symphyseal form.