BIOMECHANICAL APPROACH TO THE RISKOF BONE FRACTURE IN THE ELDERLY

Abstract

Fractures in the elderly occur mostly as a consequence of a fall, and are represented by the reduction of the bone strength. Principal sites of fracture are the proximal femur and the spine. A useful parameter in the study of the mechanism of fracture is the factor of risk, being defined as the ratio of the applied load on the skeletal element to the failure load of the bone itself. The failure load of the proximal femur is strongly correlated with bone density, and is influenced by the direction and frequency of the load and the rate of strain. High risk of bone fracture in the elderly is due to low bone density of the femur and spine. The risk of fracture of vertebrae is largely dependent on the strength of the trabecular bone. Fall severity from standing height can be reduced by soft tissue overlying the impact site as a cushion, and by protective mechanisms such as outstretching the arm, rotation of the trunk and muscle-relax. A dominant factor in susceptibility to pathological fracture is metastatic defect. Two biomechanical models are available to estimate the strength of bone exhibiting metastatic defect. First, the failure load may be estimated from the relationship between the size of defects and the reduction of bone strength. Second, the failure load may be predicted by the measurement of the structural geometry of an intact region of the bone using noninvasive imaging, even if the bone displays metastatic defects.