The WOMAC Pain Scale and Cross Talk From Co-occurring Pain Sites in People With Knee Pain: A Causal Modeling Study

Objective: The Western Ontario and McMaster Universities Osteoarthritis (WOMAC) Pain Scale quantifies knee pain severity with activities of daily living, but the potential impact of pain in other body regions on WOMAC pain scores has not been explored using a causal modeling approach. The purpose of this study was to determine if pain in other areas of the body impact WOMAC pain scores, a phenomenon referred to as “cross talk.”

Methods: Cross-sectional datasets were built from public use data available from the Osteoarthritis Initiative (OAI) and the Multicenter Osteoarthritis Study (MOST).The WOMAC Pain Scale and generic hip, knee, ankle, foot and back pain measures were included. Three nested regression models grounded in causally based classical test theory determined the extent of cross talk. Improvements in the coefficient of determination (R2) across the 3 models were used to determine the presence of cross talk.

Results: Causal modeling provided evidence of cross talk in both OAI and MOST datasets. For example, in OAI, multiple statistical models demonstrated significant increases in R2 values (P < .0001) as additional pain areas were added to the models.

Conclusions: Cross talk appears to be a clinically important source of error in the WOMAC Pain Scale, particularly for patients with a larger number of painful body regions and when contralateral knee joint pain is more severe.

Impact: This study has important implications for arthritis research. It also should raise clinician awareness of the threat to score interpretation and the need to consider the extent of pain in other body regions when interpreting WOMAC pain scores.

Keywords: Knee; Knee Osteoarthritis; Measurement: Applied; Osteoarthritis; Pain.