The Western Ontario and Mcmaster Universities arthritis index pain scale demonstrates cross talk from co-occurring pain sites in persons with knee pain: a cross-sectional multicenter study

Abstract

Purpose: The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) Pain scale is both a stand-alone measure and also is imbedded within the Knee Injury and Osteoarthritis Outcome Score (KOOS). The WOMAC Pain scale requires the patient to judge the extent of knee pain during daily activities while also potentially experiencing pain in other body regions. It is possible that pain in other body regions could influence the way in which patients answer questions in self-report measures like the WOMAC. Our purpose was to determine if pain in other areas of the body influence WOMAC Pain scores, a phenomenon we refer to as cross talk. Methods: We used two public use datasets, the Ostoearthritis Initiative (OAI) and Multicenter Osteoarthritis Study (MOST) and a cross sectional design. 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 and the common factor model determined the extent of cross talk. Improvements in the coefficient of determination (R2) across the three models were used to determine the presence of cross talk. Our interest in using classical test theory and the common factor model was in causal explanation and not prediction. Our causal theoretical model (i.e. the common factor model) was used to test our two causal hypotheses. We hypothesized that; (1) pain in other lower extremity joints and the back influence index knee WOMAC Pain scores after accounting for generic index knee verbal pain ratings; and (2) pain in other lower extremity joints or the back moderate the association between WOMAC Pain scores and generic index knee pain scores. Results: Our causal models 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 < 0.0001) as additional pain areas were added to the models. We use two vignettes to illustrate application of our model estimates. For example, if we consider a multi-joint pain scenario vignette with a generic index knee pain score of 6 (on a scale from 0 to 10), but also with a score of 6 (on a scale from 0 to 10) for the contralateral knee, a score of 6 for the lower extremity pain scale (pain aching or stiffness in hips, feet and ankles) and a score of 3 for the back pain scale indicating severe back pain, the predicted WOMAC Pain score would be 9.2. If we consider a best-case scenario vignette for the OAI data, let’s say a patient who scores a 6 on the generic index knee pain scale, and a 0 on all other non-index joint scales, the predicted WOMAC Pain score would be 5.0. Conclusions: We found causal explanatory evidence for cross talk when applying the WOMAC Pain scale. Cross talk is greater when bodily pain burden is greater and affects more body regions. Clinicians should be aware of this threat and consider the extent of pain in other body regions when interpreting WOMAC Pain scores.