The roles of bone mineral density, bone turnover, and other properties in reducing fracture risk during antiresorptive therapy.

Osteoporosis is a skeletal disorder characterized by compromised bone strength and increased risk of fracture. Properties related to bone strength include rate of bone turnover, bone mineral density, geometry, microarchitecture, and mean degree of mineralization. These properties (with or without bone density) are sometimes collectively referred to as bone quality. Antiresorptive agents may reduce fracture risk by several separate but interrelated effects on these individual properties. For example, antiresorptive agents have been reported to reduce bone turnover, stabilize or increase bone density, preserve or improve microarchitecture, reduce the number or size of resorption sites, and improve mineralization. Although changes in bone architecture and mineralization are not currently measurable in clinical practice, bone turnover is assessed easily in vivo and affects the other bone properties. Moreover, antiresorptive therapies that produce larger decreases in bone turnover markers together with larger increases in bone mineral density are associated with greater reductions in fracture risk, especially at sites primarily composed of cortical bone such as the hip. Reductions in fracture risk are the most convincing evidence of good bone quality. Data from well-designed randomized clinical trials with up to 10 years of continuous antiresorptive therapy have shown that certain antiresorptive agents effectively reduce fracture risk and (together with extensive preclinical data) suggest no deleterious effects on bone quality.