The Obesity Paradox in Fibular Free-Tissue Reconstruction: Does Higher BMI Improve Outcomes?

Background: Head and neck squamous cell carcinoma is widespread and projected to increase in prevalence by ~30% by 2030. This study evaluates the association between body mass index (BMI), perioperative frailty, and postoperative outcomes, including hospital length-of-stay and perioperative complications for patients undergoing fibular free-tissue reconstruction for head and neck defects. Methods: We conducted a retrospective, multi-institutional cohort study using the American College of Surgeons National Surgical Quality Improvement Program database, encompassing subjects of mandibular malignancy radical excision with free myocutaneous and fibular flap reconstruction between 2012 and 2022. Primary independent variables were BMI and modified frailty index-11 (mFI-11) scores. Descriptive statistics and logistic regression analyses were employed to explore the relationship between BMI, frailty, hospital stay, and perioperative complications. Results: 4,808 patients were included (3,082 males, 1,723 females). Within this cohort, 36.5% were normal BMI (18.5-25.0; n=1,755), 5.3% were underweight (BMI25.0; n=2,798). 2,288 (47.6%) were robust (mFI=0), 1,624 (33.8%) pre-frail (mFI=1), and 896 (18.6%) frail (mFI≥2). Perioperative complication risk was 38.4% for normal-weight patients, 55.3% for underweight patients (P<0.001), and 28.8% for overweight/obese patients (P<0.001). Multivariate analysis revealed overweight/obese patients had shorter hospital stays (β -1.0 days, 95% CI -1.5 to -0.5, P<0.001) compared to normal-weight patients. Normal-weight patient complication rates increased from 26.9% (robust) to 39.0% (frail, p=0.0003). Conversely, robust patient complication rates were lower in overweight (23.2%) and obese (23.1%) compared to underweight (42.4%). The highest-risk group consisted of underweight frail patients (56.2%). Conclusions: Underweight BMI and physiologic frailty function as independent yet synergistic risk factors for poor outcomes after free myocutaneous and fibular flap reconstruction following radical mandibular tumor excision. Notably, decreased BMI confers a greater risk than frailty. These findings challenge the obesity paradox and mandate assessing both nutritional status and physiologic reserve for optimal risk stratification.