ABSTRACT
The currently accepted conceptual definition of osteoporosis is of a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture risk1. This definition of osteoporosis captures the notion that BMD is an important component of fracture risk, but that other abnormalities, both skeletal and extra-skeletal, also contribute. However, BMD can be measured with a precision and accuracy greater than other skeletal and extra-skeletal factors, and its measurement forms the basis for the diagnosis of osteoporosis. The relationship between BMD and fracture risk is continuous. An estimate of bone mineral provides an effective estimate of fracture risk in the same way that blood pressure predicts the risk of stroke. It is possible, therefore, to choose an arbitrary value for BMD which constitutes a threshold below which fracture risk is unacceptably high. Fracture thresholds may be derived from the range of density measurements made in a population with osteoporotic fractures, or from their distribution in the young healthy adult population.
