Purpose: The Knee Society Clinical Rating System (KSS) is a questionnaire evaluating knee function itself and the patient's ability to walk and climb stairs. The aim of our study is to present the validated translation of KSS into Spanish.
Methods: The validated method of translation-retrotranslation was used to translate KSS into Spanish. Three hundred and sixteen patients undergoing primary knee arthroplasty, before surgery and 6 months postoperative, completed the questionnaire (with an orthopedist's help). Psychometric properties of feasibility (percentage of no answers), validity and sensitivity to change (test's ability to detect change in patients' functional status over time) were assessed. In the second part of the study, the translated version of KSS was compared with two already validated questionnaires (SF-36 and WOMAC).
Results: During the translation process, item 3 (ROM) presented low appropriateness and null comprehensibility. The alternative writing proposed was 'for every 5°, we sum up 1 point, as if 8° were 1 point, to obtain the maximum scoring of 25 points you should bend more than 200°'. Feasibility: the item 'malalignment' obtained 15 % of missing item at visit 1. The percentage of invalid items was high in both visits (60 and 47 %).
Validity: the coefficients of convergent correlation with WOMAC and SF-36 scales confirm the questionnaire's validity. Sensitivity to change: significant differences were found in all cases between the mean scores comparing both visits.
Conclusion: The translated version 1.1 of KSS (final version) has shown to be feasible, valid and sensible to changes within the clinical practice of patients undergoing primary knees arthroplasty.