Lifetime Risk of Fracture

In 1992, Black et al. (11) proposed a method for calculating a woman's lifetime risk for hip fracture. The prediction was based on the woman's bone mass at menopause expressed as a percentile for her age, estimations of bone mass at subsequent ages and then estimating her risk for hip fracture at each age. The risk of hip fracture at each age was based on two factors: the risk of fracture at a particular age derived from population-based data and the risk of fracture at a particular bone mass from prospective fracture trials. Based on a review of the literature at the time, an increase in relative risk for hip fracture of 1.65 for each SD decline in bone mass at the radius was used in the calculation of risk based on the level of bone mass. Using this method, the lifetime risk of hip fracture for a 50-year-old Caucasian woman whose midradial bone mass was at the 10th percentile was 19%. If her bone mass was at the 90th percentile, her lifetime risk of hip fracture was 11%. The gradient of risk, therefore, between the 90th and 10th percentile was 1.7 (19% ■ 11% = 1.7). This model is obviously dependent on the value chosen for the increase in relative risk per SD decline in bone mass. The authors noted if the increase in relative risk was 2 instead of 1.65, the lifetime risks for the 10th and 90th percentiles would be 21 and 9%, respectively. The gradient of risk would therefore increase to 2.3.

In 1993, Suman et al. (12) developed a nomogram for predicting lifetime risk of hip fracture that was derived from the model developed by Black et al. (11) This nomogram is shown in Fig. 10-2.The z-scores utilized in the nomogram are based on the mean and SD for bone mass for 50-year-old Caucasian women. Like the concept of remaining lifetime fracture probability that is discussed next, lifetime fracture risk goes beyond the prediction of current fracture risk. A young individual with a slightly low bone mass and low current fracture risk will be identified as having a higher lifetime fracture risk. Similarly, an older individual with a low bone mass and high current fracture risk may have a lower lifetime fracture risk because of a shorter life expectancy.

Cummings et al. (13) published lifetime fracture risk data for hip fracture based on the femoral neck bone density T-score. These data are shown in Table 10-5. The risk of hip fracture used to determine lifetime hip fracture risk was derived from data from the Study of Osteoporotic Fractures in which the relative risk for hip fracture was 2.6 per SD decline in bone density at the femoral neck (9). The femoral neck T-scores are based on NHANES III reference data (14). Using Table 10-5, a 60-year-old Caucasian woman with a femoral neck T-score of -2.0 would have a lifetime hip fracture risk of 27%. An 80-year-old Caucasian woman with the same femoral neck T-score would have a lifetime hip fracture risk of 24%.

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