Many approaches have been used to determine the minimal clinically important difference (MCID) in patients undergoing total knee arthroplasty, but the MCID for outcome measures after medial opening wedge high tibial osteotomy (MOWHTO) for the treatment of medial compartment knee osteoarthritis (OA) has not been reported.
To define the MCID for the Western Ontario and McMaster Universities Arthritis Index (WOMAC) after MOWHTO and to identify risk factors for not achieving the MCID.
Case-control study; Level of evidence, 3.
Among patients with medial compartment knee OA who underwent MOWHTO, 174 patients who were followed for 2 years were included in the study. The MCID and substantial clinical benefit (SCB) for the WOMAC were determined using the anchor-based method with a 15-item questionnaire. Preoperative OA severity was measured by the Kellgren-Lawrence (K-L) grading system, and the acceptable range of the postoperative weightbearing line ratio was 50% to 70%. Patients were divided into 2 groups based on whether the MCID and SCB were achieved, and then factors related to failure to achieve the MCID and SCB were analyzed using multivariate logistic regression analysis.
The MCID for the WOMAC was 4.2 points for the pain subscale, 1.9 points for the stiffness subscale, 10.1 points for the function subscale, and 16.1 points for the total. Additionally, the SCB for the WOMAC was 6.4 for pain, 2.6 for stiffness, 16.4 for function, and 25.3 for the total. Overall, 116 (66.7%), 99 (56.9%), 127 (73.0%), and 128 (73.6%) patients achieved the MCID for the WOMAC pain, stiffness, function, and total, respectively, after MOWHTO. The odds of not achieving the MCID for the WOMAC total were 1.09 times greater (95% CI, 1.05-1.13; 
Patients treated with a MOWHTO require a 16.1-point improvement in the WOMAC total score to achieve a MCID from the procedure. Low preoperative WOMAC scores, severe OA, and undercorrection or overcorrection were related to failure to achieve the MCID.