Architecture of cortical bone determines in part its remodelling and structural decay

Bone remodelling accelerates and becomes unbalanced after menopause; less bone is deposited than resorbed from the surface of canals traversing the cortex. The canals enlarge so the intracortical surface area enlarges. We hypothesized that cortical bone with a larger internal surface area, due to more or larger canals, is more liable to being remodelled, further enlarging the internal surface area and facilitating more remodelling and structural deterioration. For 95 monozygotic twin pairs aged 40-61 years, we measured internal cortical surface areas and structure of the distal tibia using high resolution peripheral computed tomography, and three circulating bone remodelling markers. Using principal component (PC) analyses, we identified one summary measure of intracortical and endocortical bone surface areas, cortical porosity and volumetric bone mineral density (structure PC), and one summary measure of bone remodelling markers (remodelling PC). We applied a twin regression analysis (Inference on Causation by Examination of Familial Confounding; ICE FALCON) to assess consistency with a causal component in the association between a predictor (X) and an outcome (Y) by testing if the regression coefficient for the X value of the co-twin decreases after adjusting for the X value of the twin herself. With Y = remodelling PC, the regression coefficient for structure PC in the co-twin was 0.29 (p < 0.001) before, and 0.18 (p = 0.03) after, adjusting for her own structure PC (40% lower; p = 0.06). With Y = structure PC, the regression coefficient for remodelling PC in the co-twin was 0.17 (p = 0.01) before, and 0.20 (p < 0.001) after, adjusting for her own remodelling PC (22% higher; p = 0.7). The structure of bone, its surface area to bone matrix volume configuration, might contribute in part to its own remodelling and deterioration, but not vice versa.