Construct validity of change scores of the Chair Stand Test versus Timed Up and Go Test, KOOS questionnaire and the isometric muscle strength test in patients with severe knee osteoarthritis undergoing total knee replacement

Introduction: The Chair Stand Test (CST) is a frequently used performance-based test in clinical studies involving individuals with knee osteoarthritis and demonstrates good reliability. Aim: To assess the construct validity of change scores of the CST compared to three other measures in patients before and after total knee replacement surgery. Methods: The construct validity of change scores of the CST compared to the Timed Up and Go (TUG) test, the Knee Injury and Osteoarthritis Outcome Score questionnaire (KOOS, subscale ADL) and the isometric muscle strength test of the knee extensors (IMS sum) was measured 1-2 week before and 3 months after surgery. Results: Change (%) CST = -4.45, TUG = -2.08, KOOS ADL = 43.90, IMS sum = -13.24. Correlations CST -TUG = 0.56 (95% confidence interval (CI) 0.29, 0.74), CSTeKOOS = -0.31 (95% CI -0.57, 0.01), CST-IMS sum = -0.11 (95% CI -0.42, 0.22). Comparison of pairwise correlations: CSTeKOOS versus CST-TUG (p <0.0004), CST-TUG versus CST-IMS sum (p <0.0068), CST-KOOS versus CST-IMS sum (p <0.3100). Conclusion: For patients undergoing TKR, the CST might not be an ideal measure to assess change between pre-surgery and 3 months post-surgery. Construct validity of change scores was close to zero but the result might have been influenced by the relatively small homogeneous sample size and the chosen timespan of measurement. We ordered pairwise correlations based on the strength of correlation between the different instruments, which to our knowledge has never been done before.