Perspectives: How many women have osteoporosis now?

T IS GENERALLY agreed that osteoporosis is a disease charI acterized by abnormalities in the amount and structural arrangement of bone tissue which leads to impaired skeletal strength and an undue susceptibility to fractures. ") These qualities are difficult to measure directly, however, and they bear a continuous relationship with fracture risk in any event. Consequently, the prevalence of osteoporosis depends on some arbitrarily chosen cut-off level of a surrogate measure, such as bone mineral density (BMD). Based on the recommendation that the cut-off level be set at 2 SD below the mean of normal young women,") it was estimated that 45% of postmenopausal white women in the United States were at increased risk of fracture as a result of low BMD in the hip, spine, or forearm. (3) Recently, however, an expert panel convened by the World Hcalth Organization devised an operational definition of osteoporosis that is more ~omprehensive. '~) As reviewed elsewhere,(') low bone mass (or osteopenia) is now defined as BMD more than 1 SD below the young normal mean but less than 2.5 SD below. Women with bone density levels more than 2.5 SD below the young normal mean are presumed to have osteoporosis at the tissue level, even in the absence of a confirmatory biopsy. To provide some comparability with older definitions, the subset of women with presumptive osteoporosis who have a history of one or more fragility fractures are deemed to have "established" osteoporosis. What influence do these revised diagnostic criteria have on the estimated number of affected women in this country? Again, data from an age-stratified random sample of Rochester, Minnesota, women were extrapolated nationalI Y . ' ~ ) Bonc density was assessed in the neck and intertrochanteric regions of the proximal femur and iri the lumbar spine by dual photon absorptiometry and in the midradius by single photon absorptiometry. The proportion of women in each age group with bone density below specified levels at any of these skeletal sites was projected to the population structure of United States white women in 1990. The proportion of postmenopausal women ( 2 5 0 years of age) in each risk category defined by the WHO panel was then summed across age groups. As can be seen in Fig. 1 , most women under age 50 have normal bone density at all four skeletal sites, and osteoporosis is rare. With advancing age, however, the population becomes smaller and a greater proportion has osteopenia or osteoporosis. Among women age 80 years and over, for example, only 3% have normal BMD at all four sites, 27% have osteopenia at one skeletal site or another, and 70% have osteoporosis. Sixty percent of the latter group (42% altogether) have experienced one or more fractures of the proximal femur, vertebra, distal forearm, proximal humerus, or pelvis as judged from the Rochester data. Overall, an estimated 16.8 million (54%) postmenopausal white women in the United States have osteopenia and another 9.4 million (30%) have osteoporosis. About 4.8 million women (51% of the osteoporotic women and 16% of all white women 2.50 years of age) are estimated to have established osteoporosis. The latter figure is probably an underestimate since most fractures in elderly women are related at least in part to low bone mass,'") but only selected fracture sites were considered here. Comparable figures involving bone density measurements at diverse skeletal sites are not yet available from other population-based studies, but data have been published with respect to bone density of the hip. Assessing BMD at the hip alone underestimates the total number of affected individuals"*4) but provides a superior determination of the risk of hip fracture,'" the predominant social problem. ") Thus, Kanis and colleagues estimated the agespecific prevalence of hip osteoporosis using normal value data for dual energy X-ray absorptiometry. ") Applying these proportions to 1990 United States whites, 7 million women aged 50 years and over (23%) would appear to have proximal femur BMD more than 2.5 SD helow the young normal mean. Using Rochester data for the hip alone, the comparable figure is just over 5 million women since about 16%) of postmeno-