Combined intrathecal fentanyl and morphine for postoperative analgesia after total knee arthroplasty: a randomised controlled trial

Total knee arthroplasty (TKA) is associated with significant postoperative pain. This study aimed to evaluate whether the combined intrathecal administration of morphine and fentanyl improves postoperative analgesia compared with either agent alone. In this randomised controlled trial, 111 patients undergoing TKA under spinal anaesthesia were assigned to receive intrathecal fentanyl 25 µg (F group), morphine 100 µg (M group), or a combination of fentanyl 25 µg and morphine 100 µg (MF group). All groups received 12.5 mg of 0.5% hyperbaric bupivacaine. The primary outcome was the resting pain score at 6 h postoperatively. Secondary outcomes included pain scores (at rest and during movement) at 12 and 24 h, cumulative 24-h opioid consumption, opioid-related adverse effects, and intraoperative haemodynamic variables. At 6 h, resting pain scores were significantly lower in the MF group than in the F and M groups (1.5 ± 0.9 vs. 4.2 ± 2.7 and 2.7 ± 2.0; P < 0.001). Pain scores at rest and during motion at 6, 12, and 24 h were consistently lower in the MF group (P < 0.001). Cumulative 24-h morphine consumption was reduced in the MF group (10 ± 7 mg) compared with the F (30 ± 16 mg) and M (15 ± 9 mg) groups (P < 0.001). Nausea and vomiting were more frequent in the M and MF groups than in the F group, with no significant difference between the M and MF group. Combined intrathecal administration of morphine and fentanyl provided superior postoperative analgesia and reduced opioid consumption compared with either agent alone, without increasing opioid-related adverse effects compared with morphine alone. This strategy represents a viable analgesic option, particularly when peripheral nerve blocks are not feasible. ClinicalTrials.gov (NCT03365115; Date of registration: 30/11/2017).