Comparison of the Effect of Intrathecal Morphine and Intrathecal Fentanyl as Adjuvants to 0.5% Levobupivacaine in Total Knee Replacement Surgery

Abstract

Background: Intrathecal opioids are commonly used as adjuvants to local anaesthetics to prolong spinal analgesia. Morphine (hydrophilic) provides prolonged analgesia; fentanyl (lipophilic) provides rapid onset but shorter duration. This study compared intrathecal morphine and fentanyl as adjuvants to 0.5% hyperbaric levobupivacaine in total knee replacement (TKR) surgery.

Methods: In this prospective, randomized comparative study, 50 patients aged 45–75 years, belonging to ASA physical status I or II and undergoing total knee replacement surgery, were included after obtaining institutional ethics committee approval and written informed consent. Patients were randomly allocated to either Group LM (3 mL of 0.5% hyperbaric levobupivacaine + 100 µg morphine) or Group LF (3 mL of 0.5% hyperbaric levobupivacaine + 25 µg fentanyl), with each solution made up to a total volume of 3.5 mL with normal saline (n = 25 per group). VAS pain scores were recorded up to 24 hours; rescue analgesia (8 mL 0.125% ropivacaine epidural) was given for VAS>4.

Results: Baseline demographics and haemodynamics were similar between groups. Fentanyl produced significantly faster onset of sensory and motor block (p<0.001). Morphine produced significantly lower postoperative VAS scores, longer time to first rescue analgesia (5.56±0.71 vs 3.88±0.88 h; p<0.001), and fewer rescue doses in 24 h (1.52±1.05 vs 5.64±1.41; p<0.001). Nausea/vomiting, pruritus and haemodynamic parameters showed no significant difference between groups.

Conclusion: Intrathecal morphine (100 µg) with 0.5% hyperbaric levobupivacaine provides significantly superior and more prolonged postoperative analgesia than intrathecal fentanyl (25 µg), with similar haemodynamic stability and side-effect profile, making it a preferable adjuvant for TKR spinal anaesthesia.