Relationship Between the Western Ontario and McMaster Universities Osteoarthritis Index Physical Function Subscale and Physical Performance Measures in Patients With Hip Osteoarthritis

Wright AA, Cook CE, Baxter GD, Garcia J, Abbott JH. Relationship between the Western Ontario and McMaster Universities Osteoarthritis Index Physical Function Subscale and physical performance measures in patients with hip osteoarthritis.ObjectiveTo examine the convergent and factor validity of the Western Ontario and McMaster Universities Osteoarthritis Index physical function subscale (WOMAC-PF) using a battery of physical performance measures (PPMs) in patients with non–end-stage hip osteoarthritis (OA).DesignCross-sectional data.SettingA university physiotherapy research center.ParticipantsPatients with a clinical diagnosis of hip OA (N=93) referred for participation in research.InterventionsNot applicable.Main Outcome MeasuresFactor validity of the WOMAC-PF was evaluated by using confirmatory factor analysis (CFA). Exploratory factor analysis (EFA) was run to determine whether WOMAC-PF and PPMs were capturing separate aspects of physical function. Covariates including pain, depression, Pain Beliefs Screening Instrument, body mass index, hip flexion, use of an assistive device, and lower-extremity strength were included in the EFA as independent variables.ResultsCFA of the WOMAC-PF yielded a 1-factor solution accounting for 84% of the variance in the data (eigenvalue=11). EFA yielded a 2-factor solution accounting for 68% of the variance in the data. The PPMs and the WOMAC-PF loaded on separate factors.ConclusionsWe were unable to confirm a 4-factor solution model of the WOMAC-PF as previously reported. This suggests that the WOMAC-PF shows sufficient factor validity in capturing perceived physical function in patients with non–end-stage hip OA but captures a different construct than that of PPMs, confirming the need for both when evaluating functional outcome. Furthermore, we suggest a new standardized battery of physical performance measures to serve as the criterion standard against which self-report measures could be compared.