Measuring the population impact of knee pain and disability with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)

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Abstract
This study has used the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) in an unsolicited postal questionnaire to investigate the impact of knee pain and disability in the general older population. The study provides WOMAC population data for those aged over 50 and demographic and psychosocial associations with severity of WOMAC scores. A pilot survey (n=240) and repeatability study (n=80) were undertaken to test completion of the WOMAC in this new setting. The main questionnaire was mailed to 8,995 men and women aged over 50 registered with three general practices in North Staffordshire, UK. Completion rates for WOMAC items were high. Substantial reliability was found for pain and physical function scales (both >0.80). Fourteen percent of the over 50 population in this study had severe knee pain, 20% had severe difficulty with at least one area of physical functioning, 12% had both. The strongest link with severe difficulty with physical functioning was chronicity (odds ratio (OR)=6.49, 95% CI 4.65, 9.04). Other independent links were age over 75 years (odds ratio (OR)=4.11, 95% confidence interval (CI) 3.03, 5.58), depression (OR=2.80, 95% CI 2.22, 3.54), bilateral knee injury (OR=2.23, 95% CI 1.63, 3.06) and body mass index>30 (OR=2.00, 95% CI 1.51, 2.64). Similar associations were found for severe pain. The findings suggest that the WOMAC is a reliable measure for use in postal surveys. It has advantages over other instruments when measuring pain and physical function difficulty related to the knee. Chronicity, older age, injury, obesity and depression were all linked with higher WOMAC scores for knee pain severity and disability among knee pain sufferers in the general older population.