Comparison of hip fracture risk prediction by femoral aBMD to experimentally measured factor of risk.

Areal BMD (aBMD) derived from DXA is currently the gold standard for diagnosis of osteoporosis. A biomechanical approach to fracture risk assessment comparing the ratio of applied load to bone strength, termed the factor of risk (Phi), may be useful to better identify patients at risk for fracture. We obtained 73 human cadaveric femurs (48 women and 25 men, aged 74.2+/-8.7 years, range 55-98 years), measured femoral neck (FN) aBMD by DXA, and mechanically tested the femurs to failure in a sideways fall configuration. The force applied to the hip during a sideways fall was estimated from height and weight, and accounted for trochanteric soft tissue thickness. Compared to men, women had significantly lower FN aBMD and femoral strength, and tended to have higher factor of risk for hip fracture Phi. Fifty-three of 54 (98%) specimens that had a FN aBMD T-score below -2.5 also had a Phi>1. However, 10/19 (53%) specimens with FN aBMD T-score above -2.5 also had Phi>1. These data indicate that whereas an aBMD-based diagnosis of osteoporosis is highly associated with fracture risk as assessed by the factor of risk, about 50% of individuals not designated as osteoporotic by aBMD testing would be at high risk for hip fracture should they experience a sideways fall. These findings strongly support the investigation of new biomechanically-based methods of fracture risk prediction.