Younger patients and smokers report a higher level of pain after knee arthroscopy: a clinical and experimental study including synovial metabolism

PURPOSE: Factors associated with post-surgical pain are not fully explored. The aim of this study was to identify determinants of postoperative pain after arthroscopic surgery of the knee. Synovial tissue metabolism was analysed by microdialysis and the association with individual and peri-surgical factors to identify determinants important for pain management and thus patient satisfaction. METHODS: Cross-sectional study of 57 patients (22 women) with median age of 39 years. All patients were operated on with arthroscopic surgery of the knee and monitored postoperatively with synovial microdialysis. The cross-sectional cohort was investigated to determine local tissue levels of inflammatory and metabolic compounds along with postoperative pain experience. Measurements: pain was determined by visual analogue scale (VAS). Postoperative synovial tissue levels of prostaglandin E(2) (PGE(2)), glucose, and glycerol were measured by microdialysis in the knee synovium. Patients reporting VAS ≥ 4 received rescue pain medication with systemic opioids. RESULTS: Initial results indicated that patients with pain (interpreted as having VAS ≥ 4), i.e. those receiving rescue medication with systemic opioids, were of a younger age (p = 0.04), lower body weight (p = 0.02), had a lower BMI (p = 0.04) and/or were smokers (p = 0.02). A closer analysis using multinomial logistic regression showed a significantly higher amount of pain in smokers (p = 0.01) and patients of a younger age (p = 0.02). A significant correlation was also found between VAS and duration of surgery (p = 0.007). No significant correlation could be found between VAS and synovial levels of PGE(2), glycerol and glucose, but a statistically significant decline with time of PGE(2) in both groups. CONCLUSIONS: The results from this study show a significantly higher frequency of pain, post-surgery among younger patients (p = 0.02) and smokers (p = 0.01), as well as an association between pain and length of surgery (p = 0.007). These findings point out individual factors useful for the prediction of postoperative pain after arthroscopic surgery and are clinically important for personalized pain management. LEVEL OF EVIDENCE: II.