Structural validity of the Brazilian version of the Western Ontario and McMaster Universities Osteoarthritis Index among patients with knee osteoarthritis

ABSTRACT

BACKGROUND: The original structure of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) has been contested in several languages. OBJECTIVE: To assess the structural validity of the Brazilian version of WOMAC among patients with knee osteoarthritis. DESIGN AND SETTING: Structural validity study conducted at physiotherapy clinics and primary healthcare units. METHODS: The study included males and females aged 40 to 80 years who were all native Brazilian Portuguese speakers, with knee pain in the previous six months and a diagnosis of knee osteoarthritis. We used exploratory factor analysis (EFA) followed by confirmatory factor analysis (CFA) with implementation of a polychoric matrix and the robust diagonally weighted least squares (RDWLS) extraction method. The adequacy of the model was assessed using the following fit indices root mean square error of approximation (RMSEA), comparative fit index (CFI), Tucker-Lewis index (TLI), standardized root mean square residual (SRMR) and chi-square/degree of freedom (DF). RESULTS: 203 patients with knee osteoarthritis were included. The model proposed in this study with two factors, i.e. "pain" (items 1, 2, 3 and 4) and "physical function" (items 10, 11, 16, 17, 18, 19, 21 and 22), showed adequate fit indices in CFA chi-square/DF = 1.30; CFI = 0.976; TLI = 0.970; RMSEA = 0.039; and SRMR = 0.070. The factorial loads ranged from 0.68 to 0.76 for the "pain" domain and 0.44 to 0.62 for the "physical function" domain. CONCLUSION: The Brazilian version of WOMAC with two domains, i.e. "pain" (four items) and "physical function" (eight items), presents the best structure.