What is the Minimum Clinically Important Difference for the WOMAC Index After TKA?

Background The WOMAC score is a validated outcome measure for use in patients undergoing TKA. Defining meaningful changes in the WOMAC score is important for sample-size calculations in clinical research and for interpreting published studies. However, inconsistencies among published studies regarding key definitions for changes in the WOMAC score after TKA potentially could result in incorrectly powered studies and the misinterpretation of clinical research results.

Questions/purposes (1) To identify the minimum clinically important difference (MCID) for the total WOMAC score and its components 1 year after TKA using an anchor-based methodology. (2) To define the minimum important change (MIC) and the minimum detectable change with 95% confidence (MDC95) for the total WOMAC score and its components 1 year after TKA.

Methods Between 2003 and 2013, 3641 patients underwent primary TKA at one center. Of those, 460 patients (13%) were excluded from this retrospective study for prespecified reasons (mainly secondary OA and bilateral surgery), and 592 patients (16%) were either lost to followup or could not be included because of incomplete questionnaires. WOMAC scores were recorded preoperatively and at 1 year postoperatively. Patient demographics and preoperative Short Form-12 and WOMAC scores were no different for the 16% of patients who were lost to followup or failed to complete 1-year questionnaires and the study cohort (n = 2589). At 1 year, patients were asked “How much did the knee replacement surgery improve the quality of your life?” Their responses were recorded as: a great improvement, moderate improvement, little improvement, no improvement at all, or the quality of my life is worse. The MCID was defined as the difference in the mean change in the WOMAC score between patients with no improvement compared with those with little improvement according to the anchor question. The MIC was defined as the change in the WOMAC score relative to the baseline score for patients who reported a little improvement in their quality of life. The MDC is the smallest change for an individual who is likely to be beyond the measurement error of the scoring tool and represents true change rather than variability in the scoring measure; we report it with 95% confidence bounds defining real change rather than variability in the scoring measure (MDC95). We calculated this with distribution-based methods for the whole cohort. Patients recording a little improvement (n = 211) and no improvement (n = 115) were used as anchor responses to calculate the MCID (using regression analysis to adjust for potential confounding variables such as age, gender, BMI and preoperative Short Form-12 or WOMAC scores) and the MIC (using receiver operative characteristics curves).

Results After adjusting for confounding variables such as age, gender, BMI as well as preoperative Short Form-12 and WOMAC scores, the MCID was 11 for pain, 9 for function, 8 for stiffness and 10 for the total WOMAC score. The MIC was 21 for pain, 16 for function, 13 for stiffness and 17 for the total WOMAC score. The MDC95 was 23 for pain, 11 for function, 27 for stiffness and 12 for the total WOMAC score.

Conclusions The MCID and MIC for the WOMAC score represent the smallest meaningful effect sizes when comparing the outcome of two groups (difference in mean change between the groups) or when assessing a cohort (a change in score for the group) after TKA, respectively, helping the reader to distinguish between a clinically important effect size and a mere statistical difference. We determined that the error in measurement (based on the MDC95) for the function component and total WOMAC scores were less than the MIC, which suggests changes beyond the MIC are clinically real and not due to uncertainty in the score. These parameters are essential to interpret TKA outcomes research and to ensure clinical research studies are amply powered to detect meaningful differences. Future studies using the WOMAC score to assess TKA outcomes should report not only the statistical significance (a p value) but also the clinical importance using the reported MCID and MIC values.

Level of Evidence Level III, diagnostic study.