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PURPOSE: To determine the importance of fall characteristics, body habitus, function, and hip bone mineral density as independent risk factors for hip fracture in frail nursing home residents. SUBJECTS AND METHODS: In this prospective, case-control study of a single, long-term care facility, we enrolled 132 ambulatory residents (95 women and 37 men) aged 65 and older, including 32 cases (fallers with hip fracture) and 100 controls (fallers with no hip fracture). Principal risk factors included fall characteristics, body habitus, measures of functional assessment, and hip bone mineral density by dual-energy X-ray absorptiometry. RESULTS: In multivariate analysis, including only those with knowledge of the fall direction (n = 100), those who fell and suffered a hip fracture were more likely to have fallen sideways (odds ratio 5.7, 95% confidence interval [CI] 1.7 to 18, P = 0.004) and have a low hip bone mineral density (odds ratio 1.9, 95% CI 0.97 to 3.7, P = 0.06) than those who fell and did not fracture. When all participants were included (n = 132) and subjects who did not know fall direction were coded as not having fallen to the side, a fall to the side (odds ratio 3.9, 95% CI 1.3 to 11, P = 0.01), low hip bone density (odds ratio 1.8, 95% CI 1.03 to 3, P = 0.04), and impaired mobility (odds ratios 6.4, 95% CI 1.9 to 21, P = 0.002) were independently associated with hip fracture. Sixty-seven percent of subjects (87% with and 62% without hip fracture) had a total hip bone mineral density greater than 2.5 SD below adult peak bone mass and were therefore classified as having osteoporosis using World Health Organization criteria. CONCLUSIONS: Among frail elderly nursing home fallers, the preponderance of whom are osteoporotic, a fall to the side, a low hip bone density, and impairment in mobility are all important and independent risk factors for hip fracture. These data suggest that, among the frailest elderly, measures to reduce the severity of a sideways fall and improve mobility touch on new domains of risk, independent of bone mineral density, that need to be targeted for hip fracture prevention in this high-risk group. Am J Med. 1998;104:539-545. [C] 1998 by Excerpta Medica, Inc.