Body Mass Index ≥35 kg/m² and Early Postoperative Complications After Primary Hip and Knee Arthroplasty: A Retrospective Cohort Study

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Background and objectiveHigher body mass index (BMI) has been associated with complications following total joint arthroplasty.However, the clinical value of a fixed BMI cutoff remains uncertain.This study assessed whether BMI ≥35 kg/m² was associated with early complications after primary total hip arthroplasty (THA) and total knee arthroplasty (TKA). MethodologyThis retrospective cohort study included elective primary THA and TKA operations performed at a regional orthopaedic centre in the United Kingdom between January 2017 and December 2018.Operations were grouped as BMI <35 kg/m² and BMI ≥35 kg/m².The primary outcome was any documented early postoperative complication.Modified Poisson regression with patient-clustered robust standard errors was used to adjust for age, ASA grade and procedure type.BMI threshold performance and receiver operating characteristic analysis were also assessed. ResultsA total of 466 operations were included, comprising 368 (79.0%) in the BMI <35 kg/m² group and 98 (21.0%) in the BMI ≥35 kg/m² group.Postoperative complications occurred in 39 (10.6%) operations in the BMI <35 kg/m² group and 14 (14.3%) in the BMI ≥35 kg/m² group (RR, 1.35; 95% CI, 0.76-2.38;p=0.307).After adjustment for age, American Society of Anesthesiologists (ASA) grade, and procedure type, BMI ≥35 kg/m² was not significantly associated with postoperative complications (adjusted relative risk (RR), 1.33; 95% confidence interval (CI), 0.73-2.44;p=0.352).ASA III was associated with a higher risk of complications (adjusted RR, 1.85; 95% CI, 1.05-3.27;p=0.034).BMI ≥35 kg/m² was not significantly associated with hospital stay (IRR, 1.01; 95% CI, 0.88-1.15;p=0.921), blood transfusion (RR, 0.79; 95% CI, 0.29-2.13;p=0.639), or haemoglobin reduction (p=0.848).BMI alone showed poor discriminatory ability with an area under the curve of 0.558 (95% CI: 0.472-0.645).These findings indicate limited discriminatory performance; clinical utility cannot be established from ROC analysis alone. ConclusionBMI ≥35 kg/m² was associated with a higher observed complication rate, but it was not an independent predictor.BMI alone had limited discriminatory performance for individual risk classification.Preoperative decisions should consider overall clinical status rather than a fixed BMI cutoff alone.