Determinants of WOMAC function, pain and stiffness scores: evidence for the role of low back pain, symptom counts, fatigue and depression in osteoarthritis, rheumatoid arthritis and fibromyalgia

Objectives. The Western Ontario MacMaster (WOMAC) is a validated instrument designed specifically for the assessment of lower extremity pain and function in osteoarthritis (OA) of the knee or hip. In the clinic, however, we have noted that OA patients frequently have other musculoskeletal and non-musculoskeletal problems that might contribute to the total level of pain and functional abnormality that is measured by the WOMAC. In this report, we investigated back pain and non-articular factors that might explain WOMAC scores in patients with OA, rheumatoid arthritis (RA) and fibromyalgia (FM) in order to understand the specificity of this instrument. Methods. RA, OA and FM patients participating in long-term outcomes studies completed the WOMAC and were assessed for low back pain, fatigue, depression and rheumatic disease symptoms by mailed questionnaires. Results. Regardless of diagnosis, WOMAC functional and pain scores were very much higher (abnormal) among those complaining of back pain. On average, WOMAC scores for back pain (+) patients exceeded those of back pain (-) patients by similar to 65%, and 52% of OA patients reported back pain. In regression analyses, study symptom variables explained 42, 44 and 38% of the variance in WOMAC function, pain and stiffness scores, respectively. In the subset of OA patients, radiographic scores added little to the explained variance. The strongest predictor of WOMAC abnormality in bivariate and multivariate analyses was the fatigue score, with correlations of 0.58, 0.60 and 0.53 with WOMAC function, pain and stiffness, respectively. The WOMAC performed well in RA and FM, and correlated strongly with the Health Assessment Questionnaire (HAQ) disability scale and a visual analogue scale (VAS) pain scale. Conclusion. The WOMAC captures more than just knee or hip pain and dysfunction, and is clearly influenced by the presence of fatigue, symptom counts, depression and low back pain. WOMAC scores also appear to reflect psychological and constitutional status. These observations suggest the need for care in interpreting WOMAC scores as just a measure of function, pain or stiffness, and indicate the considerable importance of psychological factors in rheumatic disease and rheumatic disease assessments.