The smallest worthwhile effect is superior to the MCID for estimating acceptable benefits of knee arthroplasty

Traditionally, the minimal clinically important difference (MCID) is used to judge the meaningfulness of outcomes in total knee arthroplasty (TKA). However, MCID estimates do not consider patient costs, potential side effects, and inconveniences. MCIDs vary substantially across TKA studies and have several conceptual and psychometric problems. A more scientifically sound alternative for estimating benefits patients expect TKA is the smallest worthwhile effect (SWE), measured with the benefit-harm trade-off method.

We recruited 121 participants and followed them for 6 months after surgery. All participants completed Knee Injury and Osteoarthritis Score (KOOS) Pain and Function, and underwent an interview using the benefit-harm trade-off method.

The absolute SWE at the 50th percentile (approximating the average patient) was 31 points KOOS Pain improvement and 28 points for KOOS Function, daily living. Construct validity was supported with strong associations between meeting SWE (yes or no) and satisfaction with 6-month outcome (yes or no) using Pearson Chi Square (24.5, P < 0.001).

Current MCID-based methods for interpreting KOOS change scores have flaws while the SWE method is conceptually superior. We determined the SWE of TKA that would justify the costs, risks, and inconveniences of surgery. SWE estimates suggest that patients expect a substantially greater change in KOOS scores than would be expected using MCID estimates. Clinicians can use SWE estimates when discussing likely outcomes and potential TKA benefits and risks with their patients.