The Responsiveness and Minimal Important Change of the Western Ontario Shoulder Instability Index and Oxford Shoulder Instability Score
•STUDY DESIGN: Prospective cohort study.
•BACKGROUND: Patient-reported outcome measurements (PROMs) are widely used to evaluate functional limitations. Considering PROMs for shoulder instability, information is lacking with regard to what constitutes a relevant change from baseline scores.
•OBJECTIVES: To evaluate the responsiveness of the Western Ontario Shoulder Instability Index (WOSI) and the Oxford Shoulder Instability Score (OSIS) and estimate their minimal important change (MIC).
•METHODS: One hundred five consecutive patients with shoulder instability completed 5 PROMs at baseline and at 6-month follow-up. The PROMs included the WOSI and OSIS, the Simple Shoulder Test, the Oxford Shoulder Score, and the Disabilities of the Arm, Shoulder and Hand assessment. Patients also rated their functional change on an anchor question at follow-up. Responsiveness was evaluated by testing 9 hypotheses regarding predefined correlations between the changes in PROM scores, by calculating the area under the receiver operating characteristic curve and by calculating the standardized response mean and effect size statistics. The MIC was determined by identifying the optimal cutoff on the receiver operating characteristic curve.
•RESULTS: Seven out of 9 hypotheses (78%) were confirmed; as expected, a high correlation (0.77) was found between change scores of the WOSI and OSIS, whereas the correlations of the change scores of the WOSI and OSIS with those of general shoulder PROMs were slightly lower (0.61-0.75). The area under the curve was 0.83 (95% confidence interval: 0.75, 0.91) for the OSIS and 0.82 (95% confidence interval: 0.74, 0.90) for the WOSI. The MIC was about 6 points for the OSIS and about 14 points for the WOSI.
•CONCLUSION: Both the WOSI and OSIS are able to measure change in shoulder function in patients with shoulder instability. The estimated MIC is 6 points for the OSIS (on a scale from 0 to 48) and 14 points for the WOSI (on a scale from 0 to 100).
•KEY WORDS:clinimetrics, functional outcome measures, outcome assessment, shoulder