The Effect of Mechanical Loading on Bone Material Strength and Microarchitecture in Postmenopausal Women

Osteoporosis is a large public health disease, characterized by low bone mass and micro architectural deterioration of bone tissue, resulting in enhanced bone fragility and consequent increase in fracture risk. Osteoporosis is present when bone mineral density (BMD) or content (BMC), measured by dual-energy X-ray absorptiometry (DXA), is more than 2.5 SDs below the mean value of the young adult. BMD measured by DXA is a surrogate measure of bone strength and is the primary determinant of fracture risk in both men and women. However, the majority of fragility fractures occur in women and in individuals who do not have osteoporosis according to these standards, indicating that BMD is just one among several indicators of bone health and that assessment of fracture risk should also rely on other bone properties. Newer imaging methods, such as quantitative computerized tomography (QCT), can complement information from DXA-measurements due to its ability to assess volumetric BMD and bone geometry and to differentiate between cortical and trabecular bone compartments. Bones are composite materials made predominantly of living cells, extracellular matrix, water and lipids. This composite nature of the bone material enables it to absorb stresses by elastic deformation and to endure high loads before fracturing. A new in vivo measurements of bone material strength can be used to evaluate bone mechanical properties and thereby the fracture risk. It is well established that the skeleton benefits, in terms of increased density, from regular physical activity. However, changes in BMD are still the main surrogate for assessing improvements in exercise-induced bone health despite the experimental findings as well as findings in humans showing that improvements in mechanical bone properties are independent of changes in BMD. These improvements in mechanical bone properties may be due to changes in bone shape or matrix composition. It could then be argued that a decrease in BMD is on