To assess age-related changes in femoral strength as a component in the risk of fracture of the hip, we compared the loads at fracture of the proximal aspects of femora from the cadavera of older and younger individuals, as tested in a fall-loading configuration. To provide a basis for non-invasive in vivo estimates of femoral strength, we also determined the correlations between variables measured with dual-energy x-ray absorptiometry and these loads. Femora from the cadavera of eight older individuals (mean age, seventy-four years) and nine younger individuals (mean age, thirty-three years) were scanned with a Hologic QDR-2000 densitometer to obtain densitometric and geometric information. The femora were then tested mechanically in a loading configuration that simulated a fall on the greater trochanter. The femora from the older group were half as strong as those from the younger group (p < 0.001), and they absorbed one-third as much energy (p < 0.001). The areal bone-mineral density of the femoral neck correlated strongly with the load at fracture (R² = 0.92).
Clinical Relevance: The prevalence of fracture of the hip increases exponentially with age. Ninety per cent of fractures of the hip occur in individuals who are more than seventy years old, and more than 90 per cent of these fractures are the result of a simple fall from a standing height. Comparison of our data on the load at fracture with estimates in the literature of impact forces on the hip during falls from a standing height indicates that, on the average, impact forces exceed the strength of the femora in older individuals by approximately 50 per cent but are approximately 20 per cent less than the strength of the femora in younger individuals. These findings help to explain the lower prevalence of fracture of the hip among younger adults. However, since fewer than 5 per cent of all falls result in a fracture of the hip, our data also emphasize that other factors, such as the direction of the fall, the site of impact, and the use of passive and active energy-absorbing mechanisms, must be important determinants of the risk of such a fracture.