Mechanical effects on the skeleton: Are there clinical implications?

The basic morphology of the skeleton is determined genetically, but its fin al mass and architecture are modulated by adaptive mechanisms sensitive to mechanical factors. When subjected to loading, the ability of bones to resi st fracture depends on their mass, material properties, geometry and tissue quality. The contribution of altered bone geometry to fracture risk is una ppreciated by clinical assessment using absorptiometry because it fails to distinguish geometry and density. For example, for the same bone area and d ensity, small increases in the diaphyseal radius effect a disproportionate influence on torsional strength of bone. Mechanical factors are clinically relevant because of their ability to influence growth, modeling and remodel ing activities that can maximize, or maintain, the determinants of fracture resistance. Mechanical loads, greater than those habitually encountered by the skeleton, effect adaptations in cortical and cancellous bone, reduce t he rate of bone turnover, and activate new bone formation on cortical and t rabecular surfaces. In doing so, they increase bone strength by beneficial adaptations in the geometric dimensions and material properties of the tiss ue. There is no direct evidence to demonstrate anti-fracture efficacy for m echanical loading, but the geometric alterations engendered undoubtedly inc rease the structural properties of bone as an organ, increasing the resista nce to fracture. Like all interventions, issues of safety also arise. Physi cal activities involving high strain rates, heavy lifting or impact loading may be detrimental to the joints, leading to osteoarthritis; may stimulate fatigue damage leading with some to stress fractures; or may interact phar maceutical interventions to increase the rate of microdamage within cortica l or trabecular bone.