Comparison of adding magnesium sulfate versus dexmedetomidine to intrathecal bupivacaine in patients undergoing total knee arthroplasty: a randomized double-blind clinical trial

Total knee arthroplasty is frequently associated with substantial postoperative pain, and spinal anesthesia with intrathecal bupivacaine is commonly used to provide effective perioperative analgesia. Adjuvants such as magnesium sulfate and dexmedetomidine may enhance block quality and prolong analgesia through different mechanisms. This study aimed to compare the effects of adding magnesium sulfate versus dexmedetomidine to intrathecal bupivacaine in patients undergoing total knee arthroplasty. In this prospective, double-blind, randomized trial, 90 patients undergoing total knee arthroplasty were allocated to receive intrathecal bupivacaine alone or combined with either 10 µg dexmedetomidine or 100 mg magnesium sulfate. The study evaluated sensory and motor block profiles, intraoperative hemodynamics, and postoperative recovery metrics, specifically focusing on Visual Analogue Scale pain scores and total rescue analgesic requirements during the first 24 h. Block onset was fastest and analgesia duration was longest in the dexmedetomidine group, followed by the magnesium sulfate group and then the control group ( P < 0.001). Postoperative VAS scores and 24-hour diclofenac consumption were lowest in the dexmedetomidine group, intermediate in the magnesium sulfate group, and highest in the control group (72.50 vs. 110.00 vs. 145.00 mg, P < 0.001). Hemodynamic parameters remained clinically stable in all groups, and adverse event rates did not differ significantly, supporting dexmedetomidine as the most effective adjuvant among the three groups. Adding dexmedetomidine to intrathecal bupivacaine for total knee arthroplasty provides superior anesthesia and analgesia compared to magnesium sulfate or bupivacaine alone. Dexmedetomidine significantly accelerates block onset, extends pain-free intervals, and reduces postoperative analgesic consumption without increasing adverse events. This clinical trial was registered in the Iranian Registry of Clinical Trials (IRCT) with the code IRCT20190325043107N58 (Registration date 2025-08-02 -Retrospectively registered).