Scaling relationships between Haversian canal‐to‐secondary osteon and midshaft femur cortical‐to‐total area in a human autopsy sample

Lifestyle variables, including physical activity and diet, are key determinants of bone remodelling. However, remodelling is also spatially limited by the anatomical form of bone. The extent to which these macroscopic, microscopic and lifestyle variables co-vary allometrically in humans is subject to ongoing investigation. We hypothesised that femur midshaft size and biomechanical properties would have a dimensional effect on the size and density of secondary osteons produced during remodelling, independently of age and sex, and that this effect would manifest under different lifestyle conditions. We examined femur midshaft microradiographs in 73 samples part of the Melbourne Femur Research Collection (Australia). We measured cortical/total area (CA/TA) and the ratio of axes of the largest and smallest femur rigidity areas (I_max/I_min); secondary osteon area (On.Ar), the ratio of Haversian canal to On.Ar (H.Ar/On.Ar), and total population density of intact and fragmentary (partially remodelled) secondary osteons (OPD) from the anterior, posterior, medial and lateral anatomical femur axes. Out of all the bone parameters, CA/TA and H.Ar/On.Ar showed low to high negative correlations (p < 0.001, r range - 0.323 to -0.752) and negative allometry relationships in the entire sample, in males and in a sub-group that consisted of individuals who were sedentary but well nourished. In these groups, higher log values of CA/TA were associated with smaller log values of H.Ar/On.Ar, which translated to femoral midshafts with thicker cortices having less porous secondary osteons (smaller Haversian canals relative to thicker lamellar bone). However, this relationship was not evident in females and other age and lifestyle groupings. We suggest that effects of allometry on bone histology, or even basic variables such as cortical thickness, are included in future remodelling and lifestyle assessments to ensure that histological interpretations are not confounded by bone size.