Association Between Change in Body Mass Index and Knee and Hip Replacements: A Survival Analysis of Seven to Ten Years Using Multicohort Data.

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To define the association between change in body mass index (BMI) and the risk of knee and hip replacement. We used data from 3 independent cohort studies: the Osteoarthritis Initiative (OAI), the Multicenter Osteoarthritis Study (MOST), and the Cohort Hip and Cohort Knee (CHECK) study, which collected data from adults (45-79 years of age) with or at risk of clinically significant knee osteoarthritis. We conducted Cox proportional hazards regression analysis with clustering of both knees and hips per person to determine the association between change in BMI (our exposure of interest) and the incidence of primary knee and hip replacement over 7-10 years' follow-up. Change in BMI (in kg/m2 ) was calculated between baseline and the last follow-up visit before knee or hip replacement, or for knees and hips that were not replaced, the last follow-up visit. A total of 16,362 knees from 8,181 participants, and 16,406 hips from 8,203 participants, were eligible for inclusion in our knee and hip analyses, respectively. Change in BMI was positively associated with the risk of knee replacement (adjusted hazard ratio [HRadj ] 1.03 [95% confidence interval (95% CI) 1.00-1.06]) but not hip replacement (HRadj 1.00 [95% CI 0.95-1.04]). The association between change in BMI and knee replacement was independent of participants' BMI category at baseline (i.e., normal, overweight, or obese). Public health strategies incorporating weight loss interventions could reduce the burden of knee but not hip replacement surgery.