Idiopathic osteoporosis: an evolutionary dys-adaptation?

Osteoporosis is characterised by simultaneous net bone growth and net resorption on different surfaces, suggesting that systemic factors are not the sole explanation for the findings. The main clinical consequence is fracturing in the largely trabecular bones of the spine, hip, and radius, and the key problem in these areas is finding an explanation for the preferential loss of transverse trabeculae. In normal bone, local maintenance depends on a negative feedback response to intermittent compression strain, and it is concluded, from biomechanical analysis of the response required to achieve negative feedback, that a preferential loss of transverse trabeculae is indicative of a selective deficiency of radial compression loading. The only significant source of radial compression in humans is the induced strain produced by axial tension. This is a necessary component of the lifestyles of quadrupeds and arboreal primates, but in humans occurs only on the convex side when the bone is offset loaded. The resulting strain is a function of the range of movement. It is suggested that the asymmetrical pattern of bone loss in cortical and trabecular osteoporosis reflects chronic underuse of movement range, resulting from the adoption of a bipedal lifestyle. Exercise regimens based on using the whole of the available movement range should better prepare the skeleton to adjust to other factors hostile to bone maintenance.