The Oxford Shoulder Instability Score; validation in Dutch and first-time assessment of its smallest detectable change

BACKGROUND The Oxford Shoulder Instability Score (OSIS) is a short, self-reported outcome measurement for patients with shoulder instability. In this study, the OSIS was validated in Dutch by testing the internal consistency, reliability, measurement error, validity and the floor and ceiling effects, and its smallest detectable change (SDC) was calculated. METHODS A total of 138 patients were included. Internal consistency was calculated with Cronbach's α. Reliability (test-retest) was calculated with the intraclass correlation coefficient (ICC). The measurement error was calculated (SEM), and the SDC was estimated in a subgroup of 99 patients that completed the re-test after a mean of 13 days (5-30 days). Construct validity was evaluated by comparing the OSIS with the Western Ontario Shoulder Instability index (WOSI), the Simple Shoulder Test (SST), the Oxford Shoulder Score (OSS), the Disability of the Arm, Shoulder, and Hand assessment (DASH), and the Short Form-36 (SF-36). RESULTS Internal consistency was good, with a Cronbach's α of 0.88. The reliability was excellent, with an ICC of 0.87. The SEM was 3.3 and the SDC was 9 points (on a scale of 0-48). Regarding the construct validity, 80% of the results were in accordance with the hypotheses, including a high correlation (0.82) with the WOSI. No floor or ceiling effects were found. CONCLUSIONS The Dutch version of the OSIS showed good reliability and validity in a cohort of patients with shoulder instability.