Imputation of 10-year osteoporotic fracture rates from hip fractures: a clinical validation study

The World Health Organization (WHO) fracture risk assessment system (FRAX) allows for calibration from country-specific fracture data. The objective of this study was to evaluate the method for imputation of osteoporotic fracture rates from hip fractures alone. A total of 38,784 women aged 47.5 years or older at the time of baseline femoral neck bone mineral density (BMD) measurement were identified in a database containing all clinical dual energy X-ray absorptiometry (DXA) results for the Province of Manitoba, Canada. Health service records were assessed for the presence of nontrauma osteoporotic fracture codes after BMD testing (431 hip, 787 forearm, 336 clinical vertebral, and 431 humerus fractures). Ten-year hip and osteoporotic fracture rates were estimated by the Kaplan-Meier method. The population was stratified by age (50 to 90 years, 5-year width strata) and again by femoral neck T-scores (-4.0 to 0.0, 0.5 SD width strata). Within each stratum, the ratio of hip to osteoporotic fractures was calculated and compared with the predicted ratio from FRAX. Increasing age was associated with greater predicted hip-to-osteoporotic ratios (youngest 0.07 versus oldest 0.41) and observed ratios (youngest 0.10 versus oldest 0.48). Lower T-scores were associated with greater predicted (highest 0.04 versus lowest 0.71) and observed ratios (highest 0.06 versus lowest 0.44). There was a strong positive correlation between predicted and observed ratios (Spearman r = 0.90-0.97, p < .001). For 14 of the 18 strata, the predicted ratio was within the observed 95% confidence interval (CI). Since collection of population-based hip fracture data is considerably easier than collection of non-hip fracture data, this study supports the current emphasis on using hip fractures as the preferred site for FRAX model calibration.