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.

PurposeTo compare functional and clinical variables of women with fibromyalgia (American College of Rheumatology [ACR] criteria) vs women diagnosed by doctors and women with knee osteoarthritis (KOA).MethodsThis is a cross‐sectional study. We used clinical measures, namely, Widespread Pain Index (WPI), Symptom Severity Scale (SSS), Fibromyalgia Impact Questionnaire‐Revised (FIQ‐R), Numerical Pain Rating Scale (NPRS), Central Sensitization Inventory (CSI), Pain‐Related Catastrophizing Thoughts Scale (PCTS), and functional measures, such as Sit‐to‐Stand (STS) test, and Timed Up and Go (TUG) test.ResultsThe sample consisted of 91 participants divided into 3 groups: participants with KOA (n = 30), fibromyalgia diagnosed according to the ACR (FM‐ACR, n = 31), and fibromyalgia according to the medical diagnosis (FM‐Med, n = 30). In the comparisons, we observed a significant difference (P < 0.05) and a large effect size (d ≥ 0.8), between all groups, in the WPI, WPI + SSS, FIQ‐R domains, CSI, and PCTS. We did not observe significant values in the correlations between the clinical variables, SST, and TUG test.ConclusionPeople with fibromyalgia according, to the ACR, have higher levels of widespread pain, symptom severity, global impact on quality of life, central sensitization, and catastrophizing compared with people with knee OA and people with clinical fibromyalgia diagnosis not confirmed by the ACR diagnostic criteria.