Relationship between the Western Ontario and McMaster Universities Osteoarthritis Index Physical Function Subscale and physical performance measures in patients with hip osteoarthritis

Abstract

Objective: To 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).

Design: Cross-sectional data.

Setting: A university physiotherapy research center.

Participants: Patients with a clinical diagnosis of hip OA (N=93) referred for participation in research.

Interventions: Not applicable.

Main outcome measures: Factor 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.

Results: CFA 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.

Conclusions: We 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.