Low bone mineral density and fracture burden in postmenopausal women.

BACKGROUND: The study objectives were to determine fracture rates in relation to bone mineral density at various central skeletal sites, using the World Health Organization definition for osteoporosis (T-score -2.5 or less), and to contrast fracture patterns among women 50 to 64 years of age with those among women 65 years of age and older.
METHODS: Historical cohort study with a mean observation period of 3.2 (standard deviation [SD] 1.5) years. The study group (16,505 women 50 years of age or older) was drawn from the Manitoba Bone Density Program database, which includes all bone mineral density results for Manitoba. Baseline density measurements for the lumbar spine and hip were performed with dual-energy x-ray absorptiometry. Outcomes included the percentage of osteoporotic fractures and the rates of fracture and excess fracture (per 1000 person-years) among postmenopausal women with osteopenia and osteoporosis relative to those with normal bone mineral density (according to the classification of the World Health Organization).
RESULTS: The mean age was 65 (SD 9) years, and the mean T-scores for all sites fell within the osteopenic category. There were 765 incident fractures (fracture rate 14.5 [95% confidence interval, CI, 13.5-15.6 [per 1000 person-years). Fracture rates were significantly higher among women 65 years of age or older than among women 50-64 years of age (21.6 [95% CI 19.7-23.4] v. 8.6 [95% CI 7.5-9.7] per 1000 person-years, p < 0.001). Although fracture rates were significantly higher among women with osteoporotic T-scores, most fractures occurred in women with nonosteoporotic values (min-max: 59.7%-67.8%).
INTERPRETATION: In this study, most of the postmenopausal women with osteoporotic fractures had nonosteoporotic bone mineral density values. This finding highlights the importance of considering key clinical risk factors that operate independently of bone mineral density (such as age) when assessing fracture risk.