The risk of hip fracture rises exponentially with age, and may result from fragility of the femoral neck. Dr Paul M Mayhew (University of Cambridge, UK) and colleagues used computed tomography to scan sections of femurs from 81 adults who died suddenly. They created three-dimensional reconstructions of the bone, then obtained a cross-sectional image of the mid-femoral neck. From these cross sections, they estimated the amount of stress that each region could tolerate before becoming unstable.

The thin cortical zone of the upper femoral neck became substantially thinner with age. In women, cortical thickness fell by 64% per decade, and critical stress by 132% per decade in the region most compressed by a sideways fall. Similar, but significantly smaller results were evident for men. This thinning reduced the amount of energy that the femur could absorb before buckling.

The authors suggest that the reason for this thinning is that bipedal walking does not transmit much load to this region. By contrast, the cortex of arboreal apes is almost symmetrically thick, since tree-climbing loads this region more than walking. As people age, their amount and variety of exercise is reduced, and walking becomes the main source of load on the femur.

The authors state: "Several popular forms of exercise also involve extension of the flexed femur under load (cycling, sculling, gymnastics, weights). Some should be investigated for their potential to protect, as part of our society's drive to increase physical activity for health."

In an accompanying Comment, Charles H Turner questions whether this new theory about buckling failure is too simplistic, but welcomes the potential for more detailed assessment of the regional pattern of bone loss and targeted interventions to strengthen underused
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Contact: Joe Santangelo
j.santangelo@elsevier.com
1-212-633-3810
Lancet
7-Jul-2005