Micro-structural basis of bone fragility in women and men

Introduction: The morphological basis for the higher bone fragility in women than men is incompletely understood because of the limited study of three dimensional structures. We examined sex differences in morphology of the peripheral skeleton across age using a high resolution peripheral quantitative computed tomography (HR-pQCT) to better define the sexual dimorphism in bone fragility and fracture incidence . Materials and Methods:The distal radius and tibia were scanned using Xtreme-CT scanner (Scanco Medical AG, Bassersdorf, Swiess) in 237 healthy Caucasians (178 females aged 22-98 years and 57 males aged 24-98 years). The entire volumes of interest were separated into cortical (Cort) and trabecular (Tb) regions. The Total Volumetric Bone Mineral Density (Tot. vBMD), cortical and trabecular vBMD, Cort. thickness (Th), BV/TV%, Tb.vBMD, TbTh, Tb Number (N), Tb. Separation (Sp) measured. Independent t-test was used for comparison between sex and age groups. Short term reproducibility for vBMD and trabecular structures was 0.86-1.52% and 7-8%, respectively . Results: For all subjects, at both radius and tibia, males had higher peak total vBMD than females, a difference that was sustained across life despite a decline in both sexes. Cortical thickness and cortical vBMD decreased similarly by sex while mean cortical thickness is 12.6 % higher in radius and 14.1% higher in men than women. Trabecular vBMD decrease in both sexes due to a decrease in number in woman and thickness in men. Before 50yrs, males had 17% higher total vBMD at the radius and 14% higher total vBMD at the tibia. Trabecular vBMD was 30% higher at the radius and 16% higher at the tibia . Conclusion: Sex differences at peak are important determinants of sex differences in old age. Bone fragility increases in both sexes but the structural basis differs due to high remodeling in females producing cortical thinning, loss of cortical density, probably due to increased porosity decreased trabeuclar number but not thickness while in males, the slower remodeling producing little change in cortical density, trabecular thinning not loss of connectivity. Thus the lower incidence of fractures in males is likely to be due to larger bone size, less inctracortical porosity and less loss of trabecular connectivity. We did not detect and change in total cross sectional area, an effect that may be due to secular changes in height. Cortical thinning and porosity occur in both sexes but later in males accounting for the reduction in cortical vBMD in both sites. Higher fracture rates in females are the result of more women having structural features in old age so bone strength is less than the loads imposed .