The functional morphology of subchondral and trabecular bone in the hominoid tibiotalar joint

Abstract

This dissertation investigated whether the internal bone morphology of the distal tibia and talus may help to further clarify the controversial mosaics of external morphological traits previously found in fossil hominin ankle bones that have suggested varying levels of obligate bipedal and arboreal abilities. The morphology of both subchondral bone and trabecular bone has independently been hypothesized to reflect habitual loads incurred during life. This dissertation examined these properties concurrently in a single joint to test if these properties were consistent with each other and consistent in reflecting known and assumed habitual joint loads. Using micro-CT scans from humans, chimpanzees, gorillas, orangutans, and baboons, this study quantified and compared the radiodensity and thickness of subchondral bone and the architecture and orientation of trabecular bone in nine regions across each joint to determine which morphological variables can discriminate among these taxa and may be indicators of different modes of locomotion. The distributions of subchondral bone properties and of the underlying trabecular bone were only weakly correlated in both the distal tibia and talus. Based on morphological, kinetic, and kinematic evidence in the human tibiotalar joint, the distribution of subchondral bone properties is suggested to reflect areas of habitual joint loads common to the opposing joint surfaces while the distribution and orientation of trabecular morphology reflects attenuation of loads through the bone. The distribution of subchondral bone properties, but not trabecular bone properties, was mirrored in the opposing surfaces of the distal tibia and talus. Species were found to have distinct patterns of subchondral bone and trabecular bone morphology across the tibiotalar joint that can be understood in light of differences in observed habitual locomotor behavior, which lends support for the use of these properties in the analysis and interpretation of fossil bones. Application of this analysis to a 1.6 Ma fossil hominid talus reveals an internal morphology that is largely consistent with that of modern humans but also has features shared with other hominoids, in particular those with relatively greater locomotor variability.