The Oxford Knee Score: compared performance before and after knee replacement.

BACKGROUND: Self-administered quality-of-life questionnaires are now crucial to the evaluation of orthopaedic surgical patient-reported outcomes, as they reflect patient satisfaction. The Oxford Knee Score (OKS) is a validated instrument that is widely used to assess outcomes of knee osteoarthritis surgery. HYPOTHESIS: The relevance of the OKS (comprehension and relevance of the items and responses, and internal and external validity) and its discriminating performance measured based on the ceiling and floor threshold effects are better before than after knee replacement surgery.
MATERIALS AND METHODS: We included 200 patients (100 scheduled for knee replacement and 100 having had knee replacement more than 1 year earlier). The OKS questionnaire was handed to each patient during the first surgeon visit or during a follow-up visit. The American Knee Society (AKS) score was determined simultaneously.
RESULTS: The mean OKS was 43.7 (range, 21-56; SD, 6.9) before surgery and 20.5 (range, 12-45; SD, 5.6) after surgery. The floor effect was absent (0%) before surgery and substantial (33%) after surgery; a weak ceiling effect (7%) was noted before surgery and no ceiling effect after surgery. Internal consistency of the OKS was excellent. The OKS correlated negatively with the AKS knee and functional scores, both before and after surgery. DISCUSSION: The OKS is well-suited to the evaluation of knee function both before and after knee replacement surgery. Before surgery, the absence of substantial floor and ceiling effects lead to excellent discrimination. After surgery, the substantial floor effect limits the ability to discriminate among the best results. Efforts should be made to develop more demanding scoring systems. LEVEL OF EVIDENCE: Level 2. Exploratory cohort study with universally applied reference standards.