A hypothesis: the causes of hip fractures.

Neither age-related osteoporosis nor the increasing incidence of falls with age sufficiently explain the exponential increase in the incidence of hip fracture with aging. We propose that four conditions must be satisfied in order for a fall to cause a hip fracture: (a) the faller must be oriented to impact near the hip; (b) protective responses must fail; (c) local soft tissues must absorb less energy than necessary to prevent fracture, and (d) the residual energy of the fall applied to the proximal femur must exceed its strength. All of these events become more likely with aging and lead to an exponential rise in the risk of hip fracture with advancing age. This model also suggests that a combination of measurements of neuromuscular function and of bone strength may be the most accurate approach to assessing the risk of hip fracture.