Local variation in age-related cortical expansion within and between cross-sections of distal human long bones

Cortical bone expansion is an age-related process where, through endosteal resorption and periosteal apposition, cross-sectional diameter increases while cortical thickness decreases in long bone diaphyses. Local variation has been observed in this process between cross-sectional locations, but few studies have examined directional variation within cross-sections. In this study, we assess variation in age-related cortical expansion both within and between cross-sections using peripheral quantitative computed tomography (pQCT) images of the tibia, radius, and ulna from a living sample of adults ages 40-80 years (n = 126; 78 females, 48 males). Cross-sectional slices were generated at four locations along tibial and forearm length (20%, 35%, 50%, and 65%), and intra-slice variation was assessed through inter-landmark distances of endosteal and periosteal landmarks along eight directional radii. Results demonstrate local variation in age-related differences of these geometric measurements both within and between cross-sections, but that variation by direction within cross-sections is not adequately explained by age-related differences alone. Patterns of intra-slice variation of endosteal and periosteal distances to the centroid exhibit similarity, suggesting that while variable at different directions of the cross-section, age-related endosteal resorption and periosteal apposition are coupled. Finally, we observe shifting degrees and directional patterns of cortical expansion along the length of each bone, particularly in the radius. These results indicate that age-related cortical expansion occurs non-uniformly throughout distal elements, which suggests that systemic factors of aging (e.g., hormonal decline, decreased muscle strength, oxidative stress) are mediated locally by factors such as mechanical loading.