Association of exercise and race/ethnicity with atypical femur fracture.

Atypical femur fractures (AFF) resemble stress fractures. Stress fractures can result from repetitive stresses like weight-bearing exercise and femoral geometry may influence where stress fractures manifest along the bone. Atypical femur fractures risk is demonstrably greater among Asian women compared to Caucasian women, with differences in femoral geometry hypothesized to contribute to elevated risk. Using a previously established cohort of older adult women, all with bisphosphonate (BP) use sometime during the study period (2010-2017) and some with AFF, we linked self-reported information on weekly exercise to enable evaluation of whether the average weekly minutes of exercise differed by race/ethnicity and whether accounting for any differences explains the increased AFF risk seen among Asian women. Data came from the longitudinal electronic health record of Kaiser Permanente Southern California. Atypical femur fractures were identified by expert review and adjudication of femur fracture radiographs. Race/ethnicity was categorized as White, Asian, and Other. Average weekly minutes of exercise were expressed in units of 15-min blocks. Bivariable and multivariable Poisson regression models were used to estimate incidence rate ratios (IRR) and 95% CI for the associations between exercise, race/ethnicity, and AFF. Exercise levels for all three race/ethnicity categories were different from each other at  < .001. After adjusting for age, cumulative duration of BP use, and recency of BP use, the IRR for AFF risk in Asian women (compared to White women) was 4.96 (CI, 3.72-6.61). Adding exercise level to this same model did not change the estimate for Asian women meaningfully (IRR 5.03, CI, 3.77-6.71). Thus, the observed race/ethnicity variation in exercise levels does not appear to contribute to the elevated risk of AFF in Asian women.