Trabecular Bone Variation In Mid And Upper Craniofacial Regions Of Four Anthropoids

Abstract

Some posit that robust supraorbital tori seen in fossil hominin species are an adaptation to hard object feeding, while others believe that supraorbital morphology is overdesigned for mastication in robust species. Strain gage analysis of primate craniofacial regions finds that strain is much lower in the supraorbital region relative to the zygomatic region, however, the mechanisms behind bone adaptation to loads is not fully understood. While variations in cortical bone material properties in a few primate species have been determined, no one has explored the variation in cranial trabecular bone. This study used microcomputed tomography to assess the variation in trabecular bone structure in the supraorbital and zygomatic regions of 10 humans, 4 baboons, 1 chimpanzee, and 1 gorilla. Significant differences in the degree of anisotropy, an indication of trabecular bone strength relative to orthogonal axes, were region specific, but significant differences in bone volume fraction, positively correlated with density, were species specific. Furthermore, differences in the degree of anisotropy by region were consistent with differences in cortical bone surface strain determined by other researchers using strain gage analysis. Trabecular bone structural variables also differed in distribution by species and region. The results of this study suggest that trabeculae adapt their primary orientation in response to relative strain endured during mastication, but that other variables related to density and strength may have a genetic component. Moreover, the results of this study support the hypothesis that robust craniofacial features may be genetic or epigenetic adaptations to fallback feeding strategies. There is a tremendous amount of variation in trabecular bone morphometric variables among species and between regions, suggesting that trabecular bone orientation and structure plays an essential role in craniofacial bone adaptation.