Comparison of Fat-to-Muscle Ratio and Body Mass Index in Predicting Perioperative Complications and Functional Recovery After Total Knee Arthroplasty: A Prospective Cohort Study.

OBJECTIVES: Obesity significantly influences the future of total knee arthroplasty (TKA). However, body mass index (BMI), the prevailing proxy for obesity, has limited predictive value for TKA outcomes, necessitating a more accurate obesity indicator. This study aimed to assess the utility of the fat-to-muscle ratio (FMR) in predicting postoperative outcomes related to obesity in patients undergoing TKA and compared its predictive value with that of BMI. METHODS: After excluding patients with secondary osteoarthritis, severe joint deformity, or neuromotor deficits, prospective data from 146 unilateral primary TKA patients were analyzed, including demographics, BMI, and systemic and leg-specific FMR. Primary outcomes included complications and 12-month patient-reported function (assessed using the University of California, Los Angeles [UCLA] activity scale and the Hospital for Special Surgery [HSS] score) and secondary outcomes including hospitalization length and surgery information were analyzed. Multivariable regression models were used to identify significant obesity-related predictors of outcomes, with linear regression employed for continuous outcomes (UCLA activity score, HSS score) and logistic regression for binary outcomes (complications). RESULTS: The mean BMI was 28.6 ± 4.4 kg/m2, systemic FMR 0.70 ± 0.23, and leg-specific FMR 0.62 ± 0.19. Both systemic FMR (OR 1.094, p = 0.036) and BMI (OR 1.050, p = 0.015) predicted systemic complications, with FMR explaining more variance (partial R 2 = 0.134 vs. 0.088). Likewise, systemic FMR (RR, 0.950, p = 0.045; partial R 2 = 0.227) and leg-specific FMR (RR, 0.922, p = 0.033; partial R 2 = 0.344) showed stronger associations with HSS functional score, than BMI (RR, 0.974, p = 0.037; partial R 2 = 0.118). Only leg-specific FMR predicted wound complications (OR 1.063, p = 0.024; partial R 2 = 0.262), and HSS pain scores (RR = 0.923, p = 0.025; partial R 2 = 0.077). Neither FMR nor BMI was correlated with surgical duration, hospitalization, activity, or ULCA activity scores (p > 0.05). CONCLUSIONS: In this single-center study, FMR demonstrated statistically stronger associations than BMI with both perioperative complications and 12-month functional outcomes following TKA. FMR assessment may provide incremental value for preoperative risk stratification and functional outcome prediction in elective TKA.