The Impact of Body Mass Index on the Risk of Postoperative 90-Day Infection Differs Between Primary Total Hip and...

Body mass index (BMI) is widely used to guide surgical candidacy in total joint arthroplasty, with many institutions and payors applying uniform BMI thresholds, commonly ≥ 40, to both total hip arthroplasty (THA) and total knee arthroplasty (TKA). However, several differences between these procedures may influence how BMI affects postoperative infection risk. Emerging evidence suggests that BMI-related risk may differ between THA and TKA. This study assessed whether BMI confers differential 90-day postoperative deep infection risk between THA and TKA using a large statewide arthroplasty registry.

A large United States statewide registry was queried for all primary THA and TKA procedures performed from 2019 to 2022. A total of 173,834 procedures were included (105,963 TKA; 67,871 THA). Participant BMI was analyzed categorically (less than 20, 20 to 24, 25 to 29, 30 to 34, 35 to 40, 40 to 45, and greater than 45) and continuously. The primary outcome was 90-day deep infection. Multivariable logistic regressions adjusted for demographic, clinical, and surgical factors. Nonparametric spline-smoothed logistic regressions were used to identify BMI thresholds at which infection risk significantly increased.

Overall, 90-day infection rates were 0.40% for TKA and 0.62% for THA. Infection risk increased progressively across BMI categories for both procedures. Spline analysis demonstrated distinct inflection points: for THA, risk began rising near BMI 30 and increased sharply at higher BMI values. For TKA, infection risk remained relatively stable until approximately BMI 40, after which the slope of risk steepened.

Patient BMI influences postoperative infection risk differently in THA and TKA. The THA patients experience rising infection risk beginning at a BMI greater than 30, whereas TKA risk increases modestly until a BMI greater than 40. These findings challenge uniform BMI cutoffs and support procedure-specific thresholds to improve patient selection, counseling, and perioperative optimization.