Abstract
In 1990 osteoporosis was redefined, for the first time in nearly a century, as a condition of skeletal fragility due to decreased bone mass and to microarchitectural deterioration of bone tissue, with consequent increased risk of fracture. This definition was conceptually important because it both acknowledged and encouraged a shift in thinking about osteoporosis from an anatomic to a dynamic condition. Low bone mass became a risk factor for fracture, rather than, as formerly, the defining feature of the disease. This redefinition accompanied a growing recognition that osteoporosis is not a single disorder but a group of more or less discrete fracture syndromes, multifactorial both in etiology and in pathogenesis.