Oxford Shoulder Instability Score: cross-cultural adaptation into Spanish and analysis of its methodological quality.

Background: Oxford Shoulder Instability Score (OSIS) is a patient reported outcome measure designed specifically to assess functional difficulties resulting from shoulder instability. The main aim of this study was to cross-culturally adapt the OSIS to Spanish. Secondary, it aimed to analyse its methodological quality.

Methods: A cross-cultural adaptation to Spanish has been carried out following the recommendations of COnsensus-based Standards for the selection of health Measurement Instruments (COSMIN) with a sample of 167 cases of shoulder instability. Inclusion criteria were: subjects with instability symptoms in at least one shoulder with or without clinical diagnosis; aged between 18 and 60. The following psychometric properties were evaluated: validity (construct, internal and external), reliability (internal consistency, test-retest and measurement error), discriminant ability and feasibility. The methodological quality was addressed with Quality Assessment of Diagnostic Accuracy Studies-2 (QUADAS-2) and COSMIN Risk of Bias checklist (COSMIN RoB).

Results: The construct validity obtained an OSIS correlation with the Simple Shoulder Test of r = 0.636 and with the Western Ontario Shoulder Instability of r = 0.80. The unidimensionality of the OSIS was confirmed through second-order factor analysis. Cronbach's alpha and intraclass correlation coefficient were both 0.93 (IC95%: 0.91-0.94). Standard error of measurement was 0.70, and the percentage of error and smallest detectable change were 1.46% and 1.94, respectively. No floor or ceiling effects were found. Assessing feasibility of OSIS, the participants answered all questions, had no questions and completion time was: mean 2 min 30 s; SD ± 1 min. Regarding methodological quality, the study showed low risk of bias in the areas of patient selection, index test, reference standard and flow and timing, as well as low concern regarding applicability in the domains of patient selection, index test and reference standard according to QUADAS-2. In relation to COSMIN RoB, all psychometric properties, except for content validity, obtained very good results.

Conclusions: The Spanish version of OSIS offers valid, reliable and feasible functional outcome measures for Spanish-speaking subjects with shoulder instability, based on its psychometric properties and its methodological quality.