Cross-cultural adaptation and validation of the Estonian version of the Oxford Knee Score

Objective: The aim of this study was to adapt the Estonian version of The Oxford Knee Score (OKS) and test its psychometric properties. Methods: The original OKS questionnaire was translated into Estonian using the forward and back-translation method recommended in the literature. The face validity of the Estonian OKS was then tested. 150 consecutive patients who had undergone total knee replacement (TKR) received the Estonian version of OKS, the Short Form 36 Health Survey (SF-36), and a visual analogue scale (VAS) for pain by mail twice with an interval of 1 month. OKS total score, its functional, and pain subscales were examined. Floor and ceiling effects were assessed. Reliability was evaluated by measuring test-retest reliability and internal consistency. Construct validity was measured by examining the correlation of OKS, its functional and pain subscales with VAS and the subscales of SF-36. Results: The Estonian version of OKS was assessed equivalent to the original and was satisfactorily accepted by patients on face validity testing. The Estonian OKS performed well on psychometric testing with good test-retest reliability (Spearman's rho 0.8887, P < 0.001, for the overall score) and internal consistency (Cronbach's alpha for the overall score 0.9353 and 0.9177 on the first and second assessment, respectively). It showed expected correlations with SF-36 subscales (strongest with the physical functioning and pain subscales and weakest with the emotional functioning subscale) and a strong correlation (Spearman's rho -0.8618 and -0.6967 on the first and second assessment respectively) with VAS for pain. A small proportion of patients demonstrated a ceiling value on both assessments, whereas a floor effect was not evident. Conclusion: The Estonian version of OKS is a reliable and valid instrument providing a standardized measure of patient-reported outcome for evaluating the results of TKR.