Humans experience age-related changes to bone geometry and material properties that impact bone strength and increase the risk of fracture. At a macrostructural level, resorption along the endosteal surface and deposition on the periosteal surface of cortical cross-sections result in expanded cross-sectional diameters and decreased cortical thickness. Additionally, intracortical porosity increases with age through a net-resorptive dysregulation of the remodeling process. The processes implicated in age-related bone loss are considered to be largely systemic, such as hormonal decline, accumulation of oxidative stress, and cellular senescence. However, previous research has demonstrated that age-related changes to bone geometry and bone mineral density (BMD) occur non-uniformly at different sites throughout the skeleton, suggesting that local factors (e.g. mechanical loading, soft tissue interactions) may mediate otherwise systemic aging processes.

This dissertation adopts a modified “whole bone” approach to assess local variation in age-related differences in cortical bone both within and between cross-sectional slices of the tibia, radius, and ulna within a living sample of adults between 40-80 years of age. Cross-sections at 20%, 35%, 50%, and 65% element length were examined. Within each cross-section, eight endosteal and periosteal semi-landmarks are established along anatomical directions to assess local variation in age-related cortical bone expansion. Corresponding sectors of the cross-section were then sampled for average cortical BMD to measure microstructural differences with age. Finally, the relationship between age-related differences in cortical bone macrostructure and microstructure were assessed using correlation analysis.

Results indicate that age-related differences in both cortical expansion and cortical BMD decline exhibit local variation both within and between cross-sections. Furthermore, the patterns of local variation are not easily explained by biomechanical expectations, suggesting that age-related processes influencing cortical bone macro- and microstructure are multifactorial and likely overlapping. Additionally, the correlations between age-related differences in cortical geometry and those of cortical BMD are moderate at best, suggesting that these processes are not strongly coupled with age. Importantly, these results illustrate that aging does not affect bone uniformly, which has implications for both clinical approaches to bone loss and anthropological interpretations of the aging experience of past groups.