Osteoporosis.

Two complementary notions need to be considered in the definition of osteoporosis: a reduction in bone mineral density and the occurrence of fracture. Bone mineral density is an established determinant of the risk of future fracture, but current interventions are most usefully targeted before bone density has fallen to the levels at which fractures usually occur. Bone density may now be measured accurately and precisely, and the sensitivity and specificity of this technology for fracture risk prediction are high. It therefore seems reasonable to classify osteoporosis in terms of both a reduction in bone density and the occurrence of fracture. On this basis, a four point scale has recently been proposed by the World Health Organization: 1) normal, 2) low bone mass, 3) osteoporosis and 4) established osteoporosis. While it is likely that this diagnostic classification will change as experience of osteoporosis increases, it is important to have widely adopted guidelines of this type which act as a framework for further research.