Can central pain modulation and beta-endorphine predict acute postoperative pain in male patients undergoing knee arthroscopy?
Siv Sofie Christensen, Eva Szuchy Kristiansen, Louise Skou Nielsen, Sten Rasmussen and Parisa Gazerani 
Orthopaedic Surgery Department, Aalborg University Hospital, Denmark
Department of Biomedicine, Faculty of Medicine, Aalborg University, Denmark

Background and aims: Treating postoperative pain is still a challenging task due to high individual variability in the pain experience and analgesic response among surgical patients. Almost one third of surgical patients report moderate to severe pain in the acute postoperative period, and approximately 5% are at risk of developing severe persistent pain after surgery. The aim of this study was to investigate if conditioned pain modulation (CPM) and saliva beta-endorphine (BE) levels could identify patients at risk of developing acute postoperative pain. 
Methods: Twenty-four men scheduled for elective knee arthroscopy participated in this study. Three consecutive sessions were carried out preoperatively, 1.5 hours postoperatively, and one day postoperatively. Measurement of pressure pain threshold (PPT, test stimulus) was conducted on the operated and the control knee. CPM was induced by a cold pressor test (CPT, conditioned stimulus) for 2 minutes, while simultaneously measuring PPT. Saliva samples were collected and analyzed for BE. 
Results: CPM was present in this study population. CPM and salivary BE was affected by knee arthroscopy (<0.05), and the functioning of CPM improved after surgery. Neither CPM nor BE presented any correlation to acute postoperative pain (p>0.05). A tendency was evident that patients with a better functioning CPM preoperatively had lower pain intensity shortly after surgery. 
Conclusion: The functioning of CPM and salivary levels of BE were affected by knee arthroscopy. None of the parameters were associated to acute postoperative pain, indicating that these parameters might not be suitable for predicting acute pain after knee arthroscopy.

CAN CENTRAL PAIN MODULATION AND CIRCULATING BETA-ENDORPHINE LEVELS PREDICT ACUTE POSTOPERATIVE PAIN IN KNEE ARTHROSCOPY?