Emerging concepts in osteoporosis and bone strength

Objective: Osteoporosis is a systemic skeletal disorder characterized by compromised bone strength and increased fracture risk. The factors that contribute to bone strength include bone mineral density (BMD) and bone quality, which encompasses factors such as bone turnover, microarchitecture, mineralization, and geometry. The objective of this paper was to review the factors that contribute to bone strength and osteoporosis. Research design: A MEDLINE search of English language journals between 1 January 1995 and 1 March 2005 was conducted using the term 'osteoporosis' combined with 'bone strength' or 'bone quality'. Reference lists of pivotal studies and reviews were also examined. Studies were otherwise not excluded on the basis of quality or size, the aim being to present an overview of research conducted to date on osteoporosis and bone strength. Results: While there is a relationship between BMD and fracture risk, evidence suggests that BMD measurements reflect only 1 component of bone strength. For example, small changes in BMD produced by osteoporosis treatments do not fully explain the reductions in fracture risk observed after initiation of therapy, and substantial fracture risk reduction is observed before peak increases in BMD are achieved. In addition to their effects on BMD, anti-resorptive therapies for osteoporosis (i.e., bisphosphonates, selective estrogen receptor modulators, calcitonin, and estrogen) produce positive effects on bone turnover, microarchitecture, and/or mineralization, all of which can contribute to the reductions in fracture risk observed with these agents. Anabolic agents such as teriparatide also appear to have beneficial effects on bone strength independent of bone mass. New, non-invasive, high-resolution imaging methods, such as magnetic resonance imaging and computed tomography, may offer a comprehensive assessment of bone quality in the future. Conclusions: The development of clinical tools that assess bone quality independent of BMD will be essential to advance our assessment of fracture risk and response to osteoporosis treatment.