Minimal clinically important differences and substantial clinical benefits for Knee Society Scores.

PURPOSE: There is a paucity of literature defining the minimal clinically important difference (MCID) for the Knee Society Scores (KSS) after total knee arthroplasty (TKA), and no data on the substantial clinical benefit (SCB) for KSS have been reported. The purpose of this study was to determine MCID and SCB for the KSS in patients with primary TKA.
METHODS: The median age of patients was 71.6 (range 50-88) years, and 60.3% were females 507 patients with TKA were prospectively enrolled. Patients completed the KSS before surgery and at second postoperative year. The MCID values of the KSS were estimated using anchor-based method, distribution-based method and receiver operating characteristic (ROC) curve analysis with calculation of the area under curve (AUC). SCB was estimated using ROC.
RESULTS: The MCID for KSS-knee score was 7.2 points by the anchor-based method, 7.2 by the distribution-based method, and using a ROC analysis the cutoff point was 8.9 points with an AUC of 0.75. For KSS-function score, the MCID values were 9.7, 6.3, and 10.3 (AUC 0.71), respectively. SCB values were 39.7 points (AUC 0.74) for the KSS-knee score, and 38.6 (AUC 0.76) for the KSS-function score. Logistic regression showed age and Charlson index to negatively affect the changes in KSS.
CONCLUSION: Different methods for MCID calculation lead to different results. With the use of ROC curve analysis, patients with an improvement of at least 9 points for KSS-knee and 10 points for KSS-function scores experience a clinically important change, whereas those who have at least an improvement of 40 points for KSS-knee and 39 points for KSS-function scores experience a substantial clinical benefit. These findings can ensure clinical improvement from the patient's perspective and also aid in interpreting results from clinical studies. LEVEL OF EVIDENCE: III.