Reliability and validity of commonly used patient-reported outcome measures (PROMs) after medial unicompartmental knee arthroplasty.

Introduction: Many patient-reported outcome measures (PROMs) have been utilized to assess outcomes after unicompartmental knee arthroplasty (UKA). However, most are not specifically designed for UKA and the measurement properties of these PROMs have never been elucidated in the setting of UKA. This study aimed to evaluate the reliability and validity of commonly used PROMs after UKA, which includes the Oxford knee score (OKS), Knee Society Score (KSS)-function score, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and Knee Injury and Osteoarthritis Outcome Score (KOOS).

Hypothesis: The four commonly used PROMs after UKA are of good reliability and validity, but with different floor/ceiling effect.

Material And Methods: Prospectively collected postoperative follow-up PROMs scores of patients after medial UKA cases for osteoarthritis between May 2015 and June 2018 were retrospectively analyzed. All of the PROMs were finished on the same electronic questionnaires. Reliability (internal consistency, test-retest reliability, measurement error), construct validity and floor/ceiling effects were assessed.

Results: The whole cohort was composed of 207 cases, with a median age of 62.0 years and a male ratio of 59/207 (28.50%). Internal consistency was high in the OKS, weak in the KSS-function score and with redundancy in the WOMAC and KOOS scores (Cronbach alpha=0.915, 0.610, 0.953, 0.961, respectively). Each of the four PROMs had a high test-retest reliability (all intraclass correlation coefficient (ICC) >0.97). Convergent validity of the four PROMs with the physical component score of the 12-Item Short Form Health Survey (SF-12 PCS) were proven (all r>0.5; p<0.001). While no ceiling effect occurred in the OKS, one was detected in the KSS-function score with 19.81% of patients achieving the best possible score, as well as in the WOMAC sub-score for pain (54.11%) and stiffness (50.72%), in addition to the KOOS sub-score for symptoms (27.54%) and pain (38.16%).

Discussion: The four commonly used PROMs after UKA showed good test-retest reliability and construct validity. The OKS is more recommended for its better performance in internal consistency and ceiling effect than the KSS-function score, the WOMAC and KOOS scores.

Level Of Evidence: III; Diagnostic study.