The objective was to determine the clinically important difference (CID) from primary total hip replacement (THR) and total knee replacement (TKR) surgeries using the Western Ontario McMaster University (WOMAC) osteoarthritis index.
WOMAC scores were collected at decision for and 1 year after surgery (n=2,709). Transition ratings (15-point scale) were obtained at 1 year for pain and function, as well as a global assessment of willingness to go through surgery again. A "good deal better" defined the positive CID. WOMAC change scores for transition ratings and willingness to go through surgery again were evaluated using receiver operating characteristic curves. Patient characteristics within transition rating categories were examined.
For THR, the positive CIDs were 41 of 100 for pain and 34 of 100 for function. The negative CIDs were 35 and 33, respectively. For TKR, the positive CIDs were 36 for pain and 33 for function. The negative CIDs were 30 and 25, respectively. Change scores for willingness to go through surgery again validated CID values. Postoperative complications decreased the likelihood of a positive CID.
Improvement that is "a good deal better" is an appropriate threshold for the THR/TKR positive CID. Attaining a positive CID is negatively related to postoperative complications.