Responsiveness and minimal important changes for the knee injury and osteoarthritis outcome score in subjects undergoing rehabilitation after total knee arthroplasty

OBJECTIVE: The aim of this study was to evaluate the responsiveness and minimal important changes for the Knee Injury and Osteoarthritis Outcome Score (KOOS) in subjects undergoing rehabilitation after total knee arthroplasty. DESIGN: At the beginning and end of a rehabilitation program, 148 patients completed the KOOS. A global perception of change scale was also completed at the end of the program and collapsed to produce a dichotomous outcome (improved vs. stable). Responsiveness was assessed on the KOOS subscales and calculated by distribution methods (effect size; standardized response mean). The minimal important changes of the KOOS subscales were assessed using anchor-based methods (receiver operating characteristic curves) to compute the best cutoff levels between the improved and stable subjects. RESULTS: The effect sizes ranged from 0.83 to 1.35, and the standardized response means ranged from 0.76 to 1.22. The receiver operating characteristic analyses revealed an area under the curve of 0.89, 0.88, 0.94, 0.93, and 0.85 for the Pain, Symptoms, Activities of Daily Living, Sport/Recreation, and Quality of Life subscales, respectively, showing discriminative capacities; the minimal important changes were 16.7 for Pain (sensitivity: 83%; specificity: 82%), 10.7 for Symptoms (80%; 80%), 18.4 for Activities of Daily Living (82%; 82%), 12.5 for Sport/Recreation (96%; 78%), and 15.6 for Quality of Life (88%; 67%). CONCLUSIONS: The KOOS was sensitive in detecting clinical changes. The authors recommend taking the minimal important changes provided into account when assessing patient improvement or planning studies in this clinical context.