Weight on the fixation: the influence of body mass index on lower extremity fracture fixation outcomes

BACKGROUND: Open reduction and internal fixation (ORIF) is a standard treatment for lower extremity fractures, but the influence of body mass index (BMI) on postoperative outcomes is not well established. This study aims to evaluate the impact of BMI on short term and long term postoperative outcomes in patients undergoing ORIF for lower extremity fractures. METHODS: We performed a prospective future-in-the-past cohort analysis using the TriNetX US network including adult patients undergoing lower extremity ORIF between 2003-2023 and with a minimum of 2-year follow-up. Patients were divided into obese (BMI ≥30) and nonobese (BMI <30) groups. Propensity score matching (1:1) controlled for age, race/ethnicity, and medical comorbidities. A subgroup analysis by BMI categories was also conducted. Risk ratios (RRs), 95 % confidence intervals (CIs) and p-values were calculated; categorical variables were analyzed with chi-squared tests and continuous variables with t-tests. RESULTS: At 90 days postoperatively, patients with obesity (BMI ≥30) showed significantly increased risks of pulmonary embolism (RR 1.55, P < 0.0001), deep vein thrombosis (RR 1.30, P < 0.0001), renal failure (RR 1.217, P < 0.0001), wound disruption (RR 1.198, P = 0.017), and postoperative infections (RR 1.224, P < 0.0001) compared to nonobese patient (BMI<30). At 2 years, obesity was associated with higher risks of post-traumatic osteoarthritis (RR 1.61, P < 0.0001), malunion (RR 1.241, P = 0.001), nonunion (RR 1.293, P < 0.0001), and implant removal (RR 1.05, P < 0.002). Stratified analysis showed that complication rates rose progressively with BMI, with morbidly obese patients (BMI ≥50) having the highest risk of pulmonary embolism (RR 3.16, P = 0.001) and with severely obese patients (BMI 40-49.9) having a higher risk of mechanical failures (implant removal RR 1.21, P < 0.001). Underweight patients (BMI <18.5) had higher short-term risks of admission and pneumonia but lower long-term implant removal rates (RR 0.86, P = 0.006). CONCLUSION: Higher BMI is significantly associated with increased short- and long-term postoperative complications following ORIF for lower extremity fractures. These findings emphasize the importance of preoperative risk assessment and postoperative monitoring in obese and severely obese patients to improve surgical outcomes. LEVEL OF EVIDENCE: Level III, Prospective Cohort.