Studies of the association between body mass index (BMI) and the risk of revision after total hip replacement have produced mixed results. This study was designed to better understand the effect of BMI on the risk of revision surgery at 11 years following total hip arthroplasty (THA)．

Data were prospectively collected for all patients undergoing THA using data from the National Joint Registry, between April of 2003 and December of 2015. Cofounding variables included age at the time of the primary THA, gender, physical status classification, year of primary THA, type of hip replacement and fixation. Data concerning mortality at 90 days and revision were recorded at a maximum follow-up of 11.75 years.

For the 415，598 patients followed, the cumulative probability of death at 90 days, was significantly higher in the underweight than in the normal weight group (P<0.0005). In contrast, compared to the normal weight group 90-day mortality was significantly lower in the overweight (P<0.0005), Obese Class I (P<0.0005), and Obese Class II (P＝00.049). At ten year follow up, compared to the normal weight group, the cumulative risk for revision was highest in the Obese Class III (6.7%), and significantly higher than the normal group for Obese Class I, Class II and Class III (P<0.0005 for all). Statistical significance was maintained in the adjusted models.

This population based, longitudinal, cohort study demonstrated that, after total hip replacement, elevated body mass index reduces the short-term risk for mortality, but increases the long-term risk for revision surgery.