The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Estimating load distribution across the radius and ulna in humans and non-human primates


1Department of Anthroplogy, Stony Brook University, 2Department of Anatomical Sciences, Stony Brook University

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The morphology of the wrist joint in extant hominoids is fundamentally different from other living anthropoid primates. One distinguishing feature is the retraction of the ulna from direct contact with the carpals. This feature is often linked to increased forearm pronation/supination motion and may allow for increased suspension/climbing behaviors in this group. Morphological changes associated with an increase in wrist joint mobility may also bring with it a unique pattern of load distribution between carpals and forearm bones (i.e., radius and ulna) during weight support. The goal of this study was estimate how loads may be distributed into and between the radius and ulna in primates that have different wrist joints morphologies (i.e., with and without ulnar-carpal contact; monkeys vs. hominoids, respectively). Cortical area (CA), our proxy for compressive strength and derived from CT scans, was calculated from cross-sections in several locations throughout the length of the radius and ulna in sample of humans and non-human primates (n=75). As predicted, CA in the ulna (relative to CA in the radius at the same level) increases from distal to proximal in all taxa; this transition increases at a significantly higher rate in monkeys. Contrary to expectations, however, relative CA in the distal end of the ulna is not different between hominoids and monkeys. Our results suggests that the more mobile wrist joint evolved by hominoids as a consequence of fully retracting the ulna from the carpals may not have significant effects on how compressive forces are transmitted into their forearms.

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