Validation of a proposed WOMAC short form for patients with hip osteoarthritis

BACKGROUND: The aims of this study were to propose a Spanish Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) short form based on previously shortened versions and to study its validity, reliability, and responsiveness for patients with hip osteoarthritis undergoing total hip replacement (THR). METHODS: Prospective observational study of two independent cohorts (788 and 445 patients, respectively). Patients completed the WOMAC and the Short Form (SF)-36 questionnaires before THR and 6 months afterward. Patients received the questionnaires by mailing, and two reminder letters were sent to patients who had not replied the questionnaire. Based on two studies from the literature, we selected the two shortened domains, the pain domain composed of three items and the function domain composed of eight items. Thus, we proposed an 11-items WOMAC short form. A complete validation process was performed, including confirmatory factor analysis (CFA) and Rasch analysis, and a study of reliability, responsiveness, and agreement measured by the Bland-Altman approach. RESULTS: The mean age was about 69 years and about 49% were women. CFA analyses confirmed the two-factor model. The pain and function domains fit the Rasch model. Stability was supported with similar results in both cohorts. Cronbach's alpha coefficients were high, 0.74 and 0.88. The highest correlations in convergent validity were found with the bodily pain and physical function SF-36 domains. Significant differences were found according to different pain and function severity scales, supporting known-groups validity. Responsiveness parameters showed large changes (effect sizes, 2.11 and 2.29). Agreement between the WOMAC long and short forms was adequate. CONCLUSIONS: Since short questionnaires result in improved patient compliance and response rates, it is very useful to have a shortened WOMAC version with the same good psychometric properties as the original version. The Spanish WOMAC short form is valid, reliable, and responsive for patients undergoing THR, and most importantly, the first WOMAC short version proposed in Spanish. Because of its simplicity and ease of application, the short form is a good alternative to the original WOMAC questionnaire and it would further enhance its acceptability and usefulness in clinical research, clinical trials, and in routine practice within the orthopaedic community.