Impact of Competing Risk of Mortality on Association of Cognitive Impairment With Risk of Hip Fracture in Older Women

Previous studies examining the association of cognitive impairment and dementia with fracture outcomes in older adults have usually used standard approaches that did not take into account the competing risk of mortality. However, ignoring mortality may not provide accurate estimates of risk of fracture because dementia in older adults strongly predicts death, making mortality a competing risk. A total of 1491 women (mean age 87.6 years) participating in the prospective Study of Osteoporotic Fractures (SOF) Year 20 exam were cognitively assessed and followed to ascertain vital status (deaths verified by death certificates) and hip fractures (confirmed by radiographic reports). Cognitive status was categorized as normal, mild cognitive impairment (MCI), or dementia, based on a standardized evaluation. Absolute probability of hip fracture by category of cognitive function was estimated using traditional Kaplan-Meier method and cumulative incidence function accounting for competing mortality risk. Risk of hip fracture by cognitive function category was determined using conventional Cox proportional hazards regression and subdistribution hazards models with death as a competing risk. During an average follow-up of 5.6 years, 139 (9.3%) women experienced a hip fracture and 990 (66.4%) died before experiencing this outcome. Among women with dementia, the risk of hip fracture was 11.7% (95% confidence interval [CI] 7.3-17.2) at 5 years and 18.6% (95% CI 9.1-30.9) at 10 years using traditional survival analysis versus 7.9% (95% CI 5.1-11.6) at 5 years and 8.8% (95% CI 5.8-12.8) at 9.8 years using a competing risk approach. Results were similar for women with MCI. Women with MCI and dementia have a higher risk of hip fractures than women with normal cognition. However, not taking into account the competing risk of mortality significantly overestimates the risk of hip fracture in women in the ninth and tenth decades of life with cognitive impairment.