The fixed cortical fraction assumption in Hip Structure Analysis produces systematic, sex- and age- dependent errors in femoral neck morphology estimates

Hip Structure Analysis (HSA) estimates femoral neck morphology from dual-energy X-ray absorptiometry (DXA) images using simplifying assumptions, including that cortical bone accounts for a fixed 60% of total bone mineral content at the narrow neck (cortical fraction, f c = 0.6). The empirical basis of this assumption is poorly established, and its effect on HSA-derived structural parameters has not been systematically evaluated. PseudoDXA images were generated by direct projection of nanoCT volumes from cadaveric proximal femurs from 32 White female (ages 24–89+ years) and 30 White male (ages 18–89 years) donors. Cortical and trabecular bone were segmented separately, enabling direct measurement of f c and comparison of HSA estimates using fixed versus measured f c against nanoCT-derived reference values. Measured f c ranged from 0.23 to 0.67 and averaged 0.48 for females and 0.42 for males, both substantially below 0.6. In females, f c increased significantly with age; in males, f c decreased significantly with body weight. Using fixed f c produced large, systematic errors in HSA-derived cortical area and buckling ratio that were sex- and age-specific, reaching up to 36% for cortical area in younger individuals. Replacing fixed f c with measured f c significantly reduced errors for cortical area in both sexes and for buckling ratio in males, with significantly greater improvement in concordance for cortical area and inner radius in males. Buckling ratio errors remained large in females even with measured f c , consistent with an additional departure from the uniform cortical thickness assumption, which is hypothesized to be more pronounced in females due to the disproportionately thick inferior cortex. These findings demonstrate that the fixed f c assumption introduces systematic, sex- and age-specific errors into HSA-derived structural estimates, and that development of DXA-derived individualized f c estimation represents a meaningful step toward improving the clinical reliability of HSA for fracture risk assessment.