Body Mass Index and the Risk of Postoperative Complications After Total Knee Arthroplasty.

Introduction: Given the growing prevalence of obesity, it is crucial to understand the effect of obesity on complications after total knee arthroplasty (TKA). This study aims to assess the relationship between body mass index (BMI) and postoperative periprosthetic joint infection (PJI), medical complications, and surgical complications after TKA.

Methods: The Premier Healthcare Database was used to identify all primary elective TKAs between 2016 and 2021. The primary outcome was risk of PJI within 90 days of surgery. Using logistic regression, restricted cubic splines were generated to assess the relationship between BMI as a continuous variable and PJI risk. Bootstrap simulation was then done to identify a BMI inflection point on the final restricted cubic spline model past which the risk of PJI increased. The relationship between BMI and composite 90-day medical and surgical complications was also assessed.

Results: A direct relationship was observed between increasing BMI and increasing risk of PJI with a BMI changepoint of 31 kg/m2 identified as being associated with increased risk. Above a BMI of 31 kg/m2, there was an average relative risk increase of PJI of 13.6% for every unit BMI. This relative risk per unit BMI increased from 5.8% for BMI 31 to 39 to 11.5% between BMI 40 and 49 kg/m2, and 21.3% for BMIs ≥50 kg/m2. Similarly, a direct relationship was also found between increasing BMI and both medical and surgical complications with BMI changepoints of 34 and 32 kg/m2 identified, respectively.

Discussion: Obese patients with a BMI >31 kg/m2 were at increased risk of PJI. Although the relative risk increase was small per unit BMI above 31 kg/m2, the cumulative increase in risk may be marked for patients with higher BMIs.

Conclusion: These data should be used to inform discussions that involve shared decision making between patients and surgeons who weigh the risks and benefits of surgery.