Impact of low body mass index on reoperation risk and complications after joint arthroplasty: a cohort study

Abstract

Purpose: The risks associated with low body mass index (BMI) in arthroplasty patients are underexplored. While outcomes of patients with elevated BMI are well-documented, low BMI patients may also face unique challenges, including malnutrition, osteopenia, and increased surgical risks and postoperative complications. To evaluate the impact of low BMI on reoperation risk and other complications compared with normal BMI among patients undergoing total hip or knee arthroplasty. Methods: This retrospective cohort study analyzed electronic health records of patients with BMI < 25 kg/m² who underwent hip or knee arthroplasty at Sunnybrook Holland Orthopaedic & Arthritic centre, Toronto, Canada between April 2, 2012, and April 6, 2023. Patients were stratified into low BMI (< 20 kg/m²) and normal BMI (20–24.9 kg/m²) groups, with their outcomes followed until November 2024. The main exposure was BMI categorized as low or normal. Other covariates controlled for were relevant demographics and comorbidities. The primary outcome was the risk of reoperation. The secondary outcome was composite complications (persistent pain, wound issues, and radiographic abnormalities). Survival analysis was performed with probabilities visualized with Kaplan-Meier curves. Multivariate Cox proportional hazards models were employed adjusting for potential confounders. Results: Among 1,162 included patients (mean [standard deviation] age, 68.8 [11.1] years; 70.1% women), 182 (15.7%) had low BMI and 980 (84.3%) had normal BMI. Kaplan-Meier curves demonstrated significantly higher risks of reoperation and composite complications in patients with low BMI compared to those with normal BMI (both p < 0.001). After adjusting for other covariates, low BMI was independently associated with increased risks of reoperation (adjusted Hazard Ratio (aHR), 5.8; 95% confidence interval (CI), 2.8–12.1; p < 0.001) and composite complications (aHR, 7.5; 95% CI, 3.9–14.5; p < 0.001). Conclusions: In this large cohort of arthroplasty patients, BMI < 20 kg/m² was associated with elevated risks of reoperation and composite complications. These findings emphasize the importance of tailored preoperative optimization and vigilant postoperative care for this high-risk population. Level of evidence: Level III.