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

Abstract

The hominoid wrist joint has changed substantially during the course of primate evolution. One of these changes is the retraction of the ulna from direct contact with the carpals and the insertion of a triangular fibrocartilaginous disc. This change permits a greater degree of mobility at the wrist in apes and is functionally linked to suspensory behaviors. However, the implications of this more mobile wrist joint on how loads are distributed from the hand to the forearm across the wrist joint remains unclear. The goal of this study is to investigate forearm loading by estimating 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). Cortical area was used as a proxy to estimate stress in several points throughout the length of the radius and ulna in both humans and non-human primates. Results indicate that although significant differences are found in the cortical area distribution of gibbons and humans, great apes and monkeys do not differ in initial values or trends. This suggests that the more mobile wrist joint evolved by great apes as a consequence of fully retracting the ulna from the carpals may not have significant effects on how these groups are loading their forearms.