Self-reported physical functioning was more influenced by pain than performance-based physical functioning in knee-osteoarthritis patients.
OBJECTIVE
To test the hypothesis that self-reported physical functioning is more influenced by pain than performance-based physical functioning.


METHODS
163 knee-osteoarthritis patients completed the performance-based DynaPort KneeTest (DPKT), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and SF-36 (self-report measures of pain and physical functioning) before, 3, 6, and 12 months after knee replacement.


RESULTS
Correlations between (two) self-reported measures of functioning and (two) pain measures were higher (0.57-0.74) than correlations between the performance-based measure of functioning and the two pain measures (0.20 and 0.26). In factor analysis, WOMAC and SF-36 pain and physical functioning subscores loaded on the first factor (eigenvalue 3.2), while DPKT KneeScore2 loaded on the second factor (eigenvalue 0.92). Before surgery, correlations between performance-based and self-reported physical functioning were higher in patients with less pain (0.43) compared to patients with more pain (0.17), for the WOMAC (as expected), but not for the SF-36. After surgery, when the pain had diminished, the correlations between performance-based and self-reported physical functioning were higher, especially for the WOMAC.


CONCLUSIONS
Our hypothesis was convincingly supported by the results of the WOMAC, and somewhat less by the results of the SF-36. We consider this as evidence for a lack of content validity of the WOMAC.
Terwee
Caroline B
CB
Institute for Research in Extramural Medicine, VU University Medical Center, van der Boechorststraat 7, Amsterdam 1081 BT, The Netherlands. cb.terwee@vumc.nl
van der Slikke
Rienk M A
RM
van Lummel
Rob C
RC
Benink
Rob J
RJ
Meijers
Wil G H
WG
de Vet
Henrica C W
HC
D000203:Activities of Daily Living; D000368:Aged; D019645:Arthroplasty, Replacement, Knee; D005163:Factor Analysis, Statistical; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D008297:Male; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000503:physiopathology* / Q000523:psychology* / Q000601:surgery; D010146:Pain / Q000209:etiology / Q000503:physiopathology / Q000523:psychology*; D010147:Pain Measurement; D018434:Sickness Impact Profile