Epidural morphine improves postoperative analgesia in patients after total knee arthroplasty: A randomized controlled trial

Background Patients after total knee arthroplasty (TKA) often develop moderate to severe pain. This study compared the analgesic effect of low-dose epidural morphine vs. a comparable saline injection in patients following TKA surgery. Methods This randomized, double-blinded, and placebo-controlled trial was conducted in a tertiary hospital in Beijing between July 1, 2017 and May 30, 2018. One hundred and ten patients following TKA under combined spinal-epidural anesthesia were randomized to receive either epidural morphine (2 mg diluted to 5 ml normal saline, the epidural morphine group) or placebo (5 ml normal saline, the placebo group). For all patients, single-injection femoral nerve block was performed, and a supplementary patient-controlled intravenous analgesia pump was provided. The severity of pain was assessed with the numerical rating scale (NRS, an 11-point scale where 0 = no pain and 10 = the worst pain) at 6, 12, 24, 36, and 48 hours after surgery. The primary endpoint was moderate to severe pain (NRS pain score >= 4) within 48 hours after surgery. Results The percentage with moderate to severe pain within 48 hours was lower in the epidural morphine group than in the placebo group (58.2% [32/55] with epidural morphine vs. 76.4% [42/55] with placebo; OR 0.43, 95% CI 0.19-0.98; p = 0.042). Furthermore, the cumulative morphine consumption within 48 hours was lower (18.4 +/- 6.1 mg vs. 22.4 +/- 7.3 mg; p = 0.002) whereas the mental component summary score of 30-day quality of life was higher (63.8 +/- 2.9 vs. 61.9 +/- 4.2; p = 0.008) in the epidural morphine group than in the placebo group. Conclusions For patients following TKA, the addition of epidural morphine to single-injection femoral nerve block improves the quality of analgesia within 48 hours, without increasing adverse events.