Introduction--Bone turnover and fracture risk.

Abstract

At its Consensus Development Panel on Osteoporosis in 2001, the National Institutes of Health defined osteoporosis as skeletal disorder characterized by compromised bone strength predisposing a person to an increased risk of fracture. Bone strength primarily reflects the integration of bone density and bone 1 . The issue of bone quality as a factor in osteoporotic fracture has received increasing attention in recent years because the reduction in vertebral fracture risk subsequent to treatment with anti-resorptive agents appears to have little relationship to the percentage increase in bone mineral density (BMD). Moreover, the increase in fracture risk with age in untreated patients is only partly a function of changes in BMD. The risk of fracture in a 75year-old woman is 4-7 times that of a 45-year-old woman with an identical bone mass, demonstrating a bone quality component to fragility that is independent of bone mass. This shows that there is a component to bone fragility that is independent of bone mass, and determined by bone quality. Bone quality is defined by four characteristics of the bone tissue matrix and skeletal physiology: (1) the rate of bone turnover; (2) the properties of the collagen/mineral matrix; (3) microdamage accumulation and (4) skeletal architecture and geometry. Rapid turnover accelerates osteoclastic resorption on trabecular surfaces that can reduce their resistance to buckling and make failure more likely 2 . Ultimately, increased resorp