Pain Pressure Thresholds Predict TKA Outcome at 6 Weeks and 6 Months

INTRODUCTION: 8 – 19% of patients report dissatisfaction post Total Knee Arthroplasty (TKA). The reasons for this are complex and multiple factors have been reported to influence these dissatisfaction rates. A recent meta-analysis has confirmed evidence for central sensitisation to pain in patients with knee osteoarthritis (OA). It also called for future research to identify people with knee OA in whom sensitisation is a dominant feature and for longitudinal research to predict sensitisation post TKA. The clinical response to TKA in the presence of sensitisation has not been studied in detail. The purpose of this study was to evaluate the whether pressure pain thresholds would predict the early outcome of TKA and whether these pain thresholds were related to preoperative pain catastrophizing scales. Our hypothesis was that early post op satisfaction outcomes post TKA would relate to pressure pain thresholds (PPT). METHODS: 78 knees from 76 subjects were included in the study. Pre-operatively pressure pain threshold measurements were taken at the operative site, contralateral knee and at the deltoid. Multiple psychological and social predictors, including the pain catastrophising scale and the American Society Knee Score (2011), were measured preoperatively. The American Knee Society Score (2011) which includes a validated satisfaction component was used to measure TKA outcomes at 6 weeks, 6 months and 12 months post-operatively. RESULTS: 19% of the patients had a poor PPT(<175 kPa); 62% (175-430 kPa) moderate and 19% good (>430 kPa) preoperatively. These pre-operative PPT levels corresponded with satisfaction scores at 6 weeks and 6 months. The mean satisfaction score at 6 months in the poor group was 30.7 and 36.4 in the good group (p<0.0005), a significant difference was also demonstrated at 6 weeks. A significant difference in satisfaction scores was also seen between the moderate (32.9) and good groups at 6 weeks (p<0.02) and 6 months (p<0.003). There was no significant difference in satisfaction scores between the poor and moderate groups at 6 weeks or at 6 months. Patients in the poor PPT group had a mean pain catastrophising scale score of 21, this was significantly different to the good PPT group mean score 11.7 (p= 0.02). No patient in the poor PPT group recorded high satisfaction (36 or higher) at 6 weeks, but 5/15 (33%) recorded satisfaction of 36 or higher at 6 months. In the good PPT 11/15 (73.3%) had a satisfaction score of 36 or higher at 6 months. CONCLUSIONS: We have demonstrated that PPT’s correlate to significant differences in satisfaction at both 6 weeks and 6 months post TKA thus confirming our hypothesis. Those patients with poor PPT also scored more poorly on the pain catastrophising scale preoperatively. This patient group failed to attain the high levels of satisfaction seen in the good PPT group at 6 months. Care should be exercised in selecting patients with poor PPT’s and high pain catastrophising scale scores for TKA. This patient group may benefit from altered preoperative workup and postop pain treatment.