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Objective: To define the association between change in body mass index (BMI) and the risk of knee and hip replacement., Methods: 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/m 2 ) 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., Results: 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 [HR adj ] 1.03 [95% confidence interval (95% CI) 1.00-1.06]) but not hip replacement (HR adj 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)., Conclusion: Public health strategies incorporating weight loss interventions could reduce the burden of knee but not hip replacement surgery., (© 2022 The Authors. Arthritis Care & Research published by Wiley Periodicals LLC on behalf of American College of Rheumatology.)

Objective: To describe the association between body weight change and the risk of knee replacement and hip replacement., Design: Time-to-event survival analysis from a population-based cohort of participants who had or were at risk of clinically significant knee osteoarthritis at baseline., Setting: Data from the Osteoarthritis Initiative (OAI), which collected data from four clinical centres in the United States., Participants: A total of 8069 knees from 4081 participants, and 8076 hips from 4064 participants (59.3% female) aged 45-79 years, with mean ± SD body mass index (BMI) of 28.7 ± 4.8 kg/m 2 , were included in the knee and hip analyses, respectively., Exposure: Body weight change from baseline as a percentage of baseline at repeated follow-up visits over 8 years., Main Outcome Measure: Incidence of primary knee or hip replacement during 8-year follow-up., Results: Body weight change had a small, positive, linear association with the risk of knee replacement (adjusted hazard ratio [HR] 1.02; 95% confidence interval [CI] 1.00-1.04). Body weight change was also positively and linearly associated with the risk of hip replacement in hips that were persistently painful at baseline (adjusted HR 1.03; 95% CI 1.01-1.05), but not in hips that were not persistently painful at baseline. There were no significant interactions between body weight change and baseline BMI in the association with knee or hip replacement., Conclusions: In people with or at risk of clinically significant knee osteoarthritis, every 1% weight loss was associated with a 2% reduced risk of knee replacement and - in those people who also had one or more persistently painful hips - a 3% reduced risk of hip replacement, regardless of baseline BMI. Public health strategies that incorporate weight loss interventions have the potential to reduce the burden of knee and hip replacement surgery., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)