Biomechanical considerations of hip and spine fractures in osteoporotic bone

Falls and fall-related hip fractures are among the most serious, common, and costly medical problems facing the elderly. Recently, we and others have shown that falls to the side, particularly those that end with impact on the hip, raise the risk of hip fracture from six- to thirty-fold, compared to about threefold increases in risk associated with one SD reduction in BMD at the hip. And yet, despite the apparently crucial importance of sideways falls, little is known about the mechanics of falling to the side. In addition, while previous studies have helped identify those factors that place the elderly at high risk for falling and provided assessment procedures that can be used to identify those at risk for falls, as far as we are aware no previous work has successfully identified subjects at increased risk of falling to the side. Moreover, while rigorous, multifactorial fall prevention efforts have demonstrated moderate reductions in fall incidence, such programs are expensive and potentially inefficient in that they have not consistently demonstrated reductions in the numbers of injurious falls. While trochanteric padding systems show considerable promise for hip fracture prevention by reducing impact force, they involve difficult acceptance and compliance issues and will require large and rigorous clinical trials to demonstrate effectiveness. Finally, while it also appears that osteodynamic agents can be used to increase BMD at the hip and spine, little is known about the local structural consequences of these changes and, in particular, if they are sufficient to prevent hip fracture from a severe fall. The findings reviewed here thus emphasize the continuing need for combined intervention strategies that focus on fall prevention, on reductions in fall severity, and on maintaining or increasing bone mass and strength, either through targeted exercise programs or the use of osteodynamic agents. By developing and refining the factor of risk phi, a property that captures both the contributions of bone density and the confounding influences of body habitus and fall severity, we believe these intervention strategies can be targeted more appropriately. Based on such approaches and an improved understanding of the complex interplay between fall biomechanics and bone fragility in the etiology of hip fracture, there is hope that the growing worldwide epidemic of hip fractures among the elderly can be substantially abated.