Association Between Body Mass Index and Thirty-Day Complications After Total Knee Arthroplasty

Although previous studies have evaluated the effect of obesity on the outcomes of total knee arthroplasty (TKA), most considered obesity as a binary variable. It is important to compare different weight categories and consider body mass index (BMI) as a continuous variable to understand the effects of obesity across the entire range of BMI. Therefore, the objective of this study is to analyze the effect of BMI on 30-day readmissions and complications after TKA, considering BMI as both a categorical and a continuous variable.

The National Surgical Quality Improvement Project database was queried from 2011 to 2015 to identify 150,934 primary TKAs. Thirty-day rates of readmissions, reoperations, and medical/surgical complications were compared between different weight categories (overweight: BMI >25 and ≤30 kg/m²; obese: BMI >30 and ≤40 kg/m²; morbidly obese: BMI >40 kg/m²) and the normal weight category (BMI >18.5 and ≤25 kg/m²) using multivariate regression models. Spline regression models were created to study BMI as a continuous variable.

Obese patients were at increased risk of pulmonary embolism (PE) (P < .001), while morbidly obese patients were at increased risk of readmission (P < .001), reoperation (P < .001), superficial infection (P < .001), periprosthetic joint infection (P < .001), wound dehiscence (P < .001), PE (P < .001), urinary tract infection (P = .003), reintubation (P = .004), and renal insufficiency (P < .001). Transfusion was lower in overweight (P < .001), obese (P < .001), and morbidly obese (P < .001) patients. BMI had a nonlinear relationship with readmission (P< .001), reoperation (P < .001), periprosthetic joint infection (P = .041), PE (P < .001), renal insufficiency (P = .046), and transfusion (P < .001).

Obesity increased the risk of readmission and various complications after TKA, with the risk being dependent on the severity of obesity. Relationships between BMI and complications showed considerable variations with some outcomes like readmission and reoperation showing a U-shaped relationship. Based on our findings, a potential BMI goal in weight management for obese patients could be established around 29-30 kg/m², in order to decrease the risk of most TKA postoperative complications.