# Knee extensor muscle strength, physical function and patient-reported outcomes the first year after total knee arthroplasty: a prospective longitudinal study

## TL;DR

This study tracks how knee muscle strength and physical function recover after knee replacement surgery in men and women over one year.

## Contribution

The study provides new insights into sex-specific and preoperative factors influencing muscle strength recovery after total knee arthroplasty.

## Key findings

- Knee extensor muscle strength initially decreased but improved significantly by one year after surgery.
- Preoperative physical function and lower BMI were linked to better recovery of normalised muscle strength.
- Patient-reported outcomes and gait improved faster than muscle strength and chair stand performance.

## Abstract

Knee extensor muscle strength is crucial for optimal knee function. Understanding the recovery process after total knee arthroplasty (TKA) and the impact of preoperative factors on recovery is therefore clinically significant. The primary aim was to investigate changes in knee extensor muscle strength in men and women during the first year after TKA. The secondary aim was to describe changes in physical function and patient-reported outcomes.

A prospective observational study, with data collection before and at 12, 26 and 52 weeks after TKA. Participants (n = 57) were scheduled for elective primary TKA due to knee osteoarthritis. The primary outcome was changes in absolute and normalised maximum voluntary isometric contraction (AMVIC and NMVIC) for knee extensors in the operated knee, which was analysed with a linear-mixed effect model. Changes in physical function (6MWT and 30CST) were analysed with a paired samples t-test, and KOOS-scores were analysed with the Wilcoxon signed rank test.

The change in estimated marginal means for AMVIC at 12 weeks was − 27 N (95% CI -43;-11, p = .001) and − 3.1 percentage points for NMVIC (-5.7;-0.5, p = .013). At 52 weeks, the change was 42 N (21;63, p < .001) for AMVIC and 5.4 percentage points (2.1;8.6, p < .001) for NMVIC. The change at 26 weeks for AMVIC and NMVIC was not significant. AMVIC was affected by time, sex (both p < .001), statistical interaction between time and sex (p = .043) and the baseline covariates age (p = .010), BMI (p = .011) and 30CST (p = .048). NMVIC was affected by time, sex (both p < .001) and the baseline covariates BMI (p = .004), 6MWT (p = .023) and 30 CST (p = .036). 6MWT and KOOS-scores increased at all time points. 30CST increased from 26 weeks.

Better preoperative physical function and lower BMI appear beneficial for postoperative recovery of normalised knee extensor muscle strength. Following TKA, patient-reported outcomes (symptoms, pain, ADL and knee-related QoL) and gait performance appear to improve more rapidly than knee extensor muscle strength and 30CST. Preoperative exercise and weight loss (if obese), as well as continued postoperative exercise even if pain is alleviated and walking is improved may be advisable.

Retrospectively registered 9 February 2022 (http://clinicaltrials.gov, ID: NCT05248815).