Responsiveness and clinically important differences for the WOMAC and SF-36 after total knee replacement

To study responsiveness and establish the minimal clinically important differences (MCIDs) and minimal detectable change (MDC) in patients undergoing total knee replacement (TKR) using the Short Form 36 (SF-36) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).

Prospective observational study in three public hospitals of all consecutive patients on waiting lists to undergo TKR intervention with diagnosis of knee osteoarthritis (OA). Patients were asked to complete before the intervention and at 6 months and 2 years afterward the SF-36 and the WOMAC health-related quality of life questionnaires (HRQoL), and additional transition questions which measured the changes in their joint at 6 months. In both questionnaires the possible range of values is from 0 to 100 points.

In WOMAC improvement at 6 months after a TKR was between 27 (stiffness) and 31 points (pain). The SF-36 showed improvements between the 28.3 points of role physical and 2.79 of general health. From 6 months to 2 years, WOMAC improvements were between 2 and 6 points. The MCID ranged from 14.52 (stiffness) to 22.87 (pain) on the WOMAC and in the physical domains of SF-36 from 11.56 (physical function) to 16.86 (bodily pain). On the WOMAC, the MDC ranged from 13.11 (function) to 29.12 (stiffness), and on SF-36 from 19.50 (physical function) to 41.23 (social functioning).

The MCID for TKR is around 15 on WOMAC, while with the SF-36 of at least 10 points. These values should not be considered as absolute thresholds.