Assessment of Residual Pain and Dissatisfaction in Total Knee Arthroplasty: Methods Matter.

Background: Residual pain after total knee arthroplasty (TKA) refers to knee pain after 3 to 6 months postoperatively. The estimates of the proportion of patients who experience residual pain after TKA vary widely. We hypothesized that the variation may stem from the range of methods used to assess residual pain. We analyzed data from 2 prospective studies to assess the proportion of subjects with residual pain as defined by several commonly used metrics and to examine the association of residual pain defined by each metric with participant dissatisfaction.

Methods: We combined participant data from 2 prospective studies of TKA outcomes from subjects recruited between 2011 and 2014. Residual pain was defined using a range of metrics based on the WOMAC (Western Ontario and McMaster Universities Osteoarthritis Index) pain score (0 to 100, in which 100 indicates worst), including the minimal clinically important difference (MCID) and patient acceptable symptom state (PASS). We also examined combinations of MCID and PASS cutoffs. Subjects self-reported dissatisfaction following TKA, and we defined dissatisfied as somewhat or very dissatisfied at 12 months. We calculated the proportion of participants with residual pain, as defined by each metric, who reported dissatisfaction. We examined the association of each metric with dissatisfaction by calculating the sensitivity, specificity, positive predictive value, and Youden index.

Results: We analyzed data from 417 subjects with a mean age (and standard deviation) of 66.3 ± 8.3 years. Twenty-six participants (6.2%) were dissatisfied. The proportion of participants defined as having residual pain according to the various metrics ranged from 5.5% to >50%. The composite metric Improvement in WOMAC pain score ≥20 points or final WOMAC pain score ≤25 had the highest positive predictive value for identifying dissatisfied subjects (0.54 [95% confidence interval, 0.35 to 0.71]). No metric had a Youden index of ≥50%.

Conclusions: Different metrics provided a wide range of estimates of residual pain following TKA. No estimate was both sensitive and specific for dissatisfaction in patients who underwent TKA, underscoring that measures of residual pain should be defined explicitly in reports of TKA outcomes.

Level of evidence: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.