Responsiveness of the WOMAC osteoarthritis index as compared with the SF-36 in patients with osteoarthritis of the legs undergoing a comprehensive rehabilitation intervention

Objective-To compare the responsiveness of the condition-specific Western O ntario and McMaster Universities osteoarthritis (OA) index (WOMAC) and the generic Short Form-36 (SF-36) in patients with OA of the legs undergoing a comprehensive inpatient rehabilitation intervention. Methods-A prospective follow up study of consecutively referred inpatients of a rehabilitation clinic was made. The patients included fulfilled the Am erican College of Rheumatology criteria for knee or hip OA and underwent bo th passive and, particularly, active physical therapy for three to four wee ks. Responsiveness assessment was performed using the standardised response mean (SRM), effect size, and Guyatt's responsiveness statistic between adm ission and discharge (end of rehabilitation) and then again between admissi on and three months later. For pain and function the SRMs were stratified b y sex and OA joint. Effects were tested by the t test and SRMs of different scales were compared by the jack knife test. Results-At the three month follow up, complete data were obtained for 223 p atients. In general, the three responsiveness statistics showed a similar o rder of responsiveness. For both instruments, the pain scales were more res ponsive than the function scales. The responsiveness of the pain scale of b oth instruments was comparable (SRM=0.723 for WOMAC and SRM=0.528 for SF-36 at the end of rehabilitation; SRM=0.377 for WOMAC and SRM=0.468 for SF-36 at the three month follow up). In the measurement of function, the WOMAC wa s significantly more responsive than the SF-36 (SRMs, end of rehabilitation : 0.628 v 0.249; three month follow up: 0.235 v -0.001). Responsiveness ten ded to be higher in women and in knee OA than in men and hip OA. Conclusions-Both. instruments, the WOMAC and the SF-36, capture improvement in pain in patients undergoing comprehensive inpatient rehabilitation inte rvention. Functional improvement can be detected better by the WOMAC than b y the SF-36. All the other scales of both instruments were more weakly resp onsive.