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The factor subdimensions of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) help to specify hip and knee osteoarthritis. a prospective evaluation and validation study.

OBJECTIVE
To determine whether it is possible to specify different score patterns for hip and knee osteoarthritis (OA), and to identify the degree of responsiveness and the validity of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) factors, which are alternative health dimensions obtained by factor analysis of the WOMAC items.


METHODS
WOMAC scales and WOMAC factors were compared in a prospective setting examining patients with hip and knee OA before and after rehabilitative inpatient intervention (n = 317). In a partial sample (n = 103), the validity of the WOMAC factors was determined by a global rating of their activities.


RESULTS
The WOMAC factor "ascending/descending" was significantly different for hip and knee OA in the health state before therapy (score in hip 5.09, in knee 6.59; p < 0.001); this was also true of the effect size after therapy (hip 0.39, knee 0.65; p = 0.012). The WOMAC scales did not differ for the 2 conditions. The WOMAC factor "ascending/descending" was the most responsive dimension in knee OA (effect size 0.65), but in hip OA the WOMAC pain scale was most responsive (effect size 0.55). Most of the WOMAC factors correlated moderately (r = 0.52-0.69) with the patient's self-rating on the validation questionnaire.


CONCLUSION
The WOMAC factors are valid measures. Analyzing the WOMAC by the WOMAC factors facilitates and improves the differential relevance and accuracy of the WOMAC for specific conditions such as hip and knee OA.