Body mass index, weight bearing, and delayed union after femoral shaft fracture nailing: a retrospective cohort study.

PURPOSE: The influence of body mass index (BMI) on fracture healing after femoral shaft intramedullary nailing remains unclear, particularly regarding postoperative weight-bearing decisions. This study evaluated whether elevated BMI was associated with delayed union after intramedullary nailing of adult femoral shaft fractures and whether weight-bearing protocol modified this association.

METHODS

This retrospective cohort study included adults with AO/OTA 32 femoral shaft fractures treated with intramedullary nailing at a Level I trauma center between November 2021 and April 2025. Patients with more than 6 months of clinical and radiographic follow-up were included. The primary exposure was BMI ≥ 25 kg/m2. The primary outcome was delayed union beyond 24 weeks. Multivariable logistic regression adjusted for age, sex, smoking, diabetes mellitus, AO/OTA 32C fracture, and open fracture.

RESULTS

The final cohort included 174 patients: 53 with BMI < 25 kg/m2 and 121 with BMI ≥ 25 kg/m2. Delayed union occurred in 30.6% of patients with BMI ≥ 25 kg/m2 and 13.2% of patients with BMI < 25 kg/m2, p = 0.015. BMI ≥ 25 kg/m2 remained independently associated with delayed union, adjusted OR 3.14, 95% CI 1.24 to 7.91. BMI ≥ 30 kg/m2 and continuous BMI were not independently associated with delayed union. Immediate weight bearing was not independently associated with delayed union, and no significant BMI by weight-bearing interaction was observed.

CONCLUSION

BMI ≥ 25 kg/m2 was associated with delayed union, but this was not reproduced with obesity-based or continuous definitions. Elevated BMI alone should not mandate weight-bearing restriction.