Body mass index, not local adiposity, best predicts surgical site infection following surgical fixation of distal femur fractures

Body mass index (BMI) is a recognized risk factor for infection in distal femur fracture management, but the role of local subcutaneous fat thickness at incision sites remains unclear. This study evaluated whether local adiposity independently predicts surgical site infection (SSI) after open reduction internal fixation (ORIF) and compared its predictive value to BMI. We conducted a retrospective case–control study of 302 adults who underwent ORIF for distal femur fractures at a Level 1 trauma center (2021–2024). Subcutaneous fat depth was measured at lateral, prepatellar, and, when available, medial sites on plain radiographs. The primary outcome was SSI requiring irrigation and debridement within 1 year. Logistic regression assessed the predictive value of fat thickness and BMI, with subgroup analysis of periprosthetic fractures. Nineteen patients (6.3%) developed SSI. On univariate analysis, greater lateral, prepatellar, medial fat, and BMI were significantly associated with infection. Logistic regression showed lateral and prepatellar fat predicted SSI, but these effects were lost once BMI was included, confirming BMI as the stronger independent predictor. In the periprosthetic subgroup, neither BMI nor adiposity predicted infection. In distal femur fractures, BMI outperformed local fat thickness as a predictor of SSI, supporting its use as a practical risk stratification tool. While greater local adiposity correlated with reoperation, these associations diminished when BMI was considered. In periprosthetic fractures, no predictors were identified, highlighting the need for larger studies to clarify whether implants alter infection risk.