Strategies to reduce fracture risk must be based on the understanding of the mechanisms that underline the increased incidence of fractures with age and with bone diseases that reduce bone stock. There is evidence that in addition to bone minerals density, other factors influence bone strength. This study reviews the biomechanical aspects of age-related fractures, including the interacting roles of traumatic loading and bone strength, and the factors that determine the resistances of bones to fracture. Although low bone mineral density (BMD) is among the strongest risk factors for fracture, a number of clinical studies have demonstrated the limitations of bone mineral density measurements in assessing fracture risk and monitoring the response to therapy. These observations have brought renewed attention to the broader array of factors that influence skeletal fragility, including bone size, shape, micro-architecture and bone quality. Bone fragility can be defined by biomechanical parameters, including ultimate force, ultimate displacement and energy absorption. Many osteoporosis treatments build bone mass but also change tissue quality. Antiresorptive therapies, such as bisphosphonates, substantially reduce bone turnover, impairing micro-damage repair and causing increased bone mineralization, which can increase the brittleness of bone. Anabolic therapies, such as teriparatide, increase bone turnover and porosity, which offset some of the positive effects on bone strength. Osteoporosis therapies may also affect bone architecture by causing the redistribution of bone structure. Restructuring of bone during treatment may change bone fragility, even in the absence of drug effects on BMD.
Keywords: bone; osteoporosis fracture.