# 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

## TL;DR

This study shows that the fat-to-muscle ratio is a better predictor of complications and recovery after knee replacement surgery than body mass index.

## Contribution

The study introduces the fat-to-muscle ratio as a more accurate obesity indicator for predicting TKA outcomes compared to BMI.

## Key findings

- Systemic FMR explained more variance in complications than BMI.
- Leg-specific FMR showed stronger associations with functional scores than BMI.
- Only leg-specific FMR predicted wound complications and pain scores.

## Abstract

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.

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).

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).

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.

Our findings highlight that preoperative FMR assessment could reliably benefit surgeons in patient education and surgical timing determination, as FMR demonstrates superior predictive capacity compared to BMI for both complications and 12‐month patient‐reported outcomes following knee arthroplasty.