Structural and Biomechanical Basis of Sexual Dimorphism in Femoral Neck Fragility Has Its Origins in Growth and Aging

The structural basis for sex differences in femoral neck (FN) fragility was studied in 1196 subjects and 307 patients with hip fracture. The absolute and relative patterns of modeling and remodeling on the periosteal and endocortical envelopes during growth and aging produce changes in FN geometry and structure that results in FN fragility in both sexes and sexual dimorphism in hip fracture risk in old age. Introduction: Femoral neck (FN) fragility in old age is usually attributed to age-related bone loss, while the sex differences in hip fracture rate are attributed to less bone loss in men than in women. The purpose of this study was to define the structural and biomechanical basis underlying the increase in FN fragility in elderly men and women and the structural basis of sex differences in hip fracture incidence in old age. Materials and Methods: We measured FN dimensions and areal bone mineral density in 1196 healthy subjects (801 females) 18-92 years of age and 307 patients (180 females) with hip fracture using DXA. We then used the DXA-derived FN areal bone mineral density (BMD) and measured periosteal diameter to estimate endocortical diameter, cortical thickness, section modulus (a measure of bending strength), and buckling ratio (indices for structural stability). Results: Neither FN cortical thickness nor volumetric density differed in young adult women and men after height and weight adjustment. The sex differences in geometry were confined to the further displacement of the cortex from the FN neutral axis in young men, which produced 13.4% greater bending strength than in young women. Aging amplified this geometric difference; widening of the periosteal and endocortical diameters continued in both sexes but was greater in men, shifting the cortex even further from the neutral axis maintaining bending strength in men, not in women. In both sexes, less age-related periosteal than endocortical widening produced cortical thinning increasing the risk for structural failure by local buckling of the enlarged thin walled FN. Relative to age-matched controls, women and men with hip fractures had reduced cortical thickness, but FN periosteal diameter was increased in women and reduced in men, differences are likely to be originated in growth. Conclusions: The absolute and relative patterns of modeling and remodeling on the periosteal and endocortical envelopes during growth and aging produce changes in FN diameters, cortical thickness, and geometry that results in FN fragility in both sexes and sexual dimorphism in hip fracture risk in old age.