Intraarticular injection of magnesium sulphate and/or bupivacaine for postoperative analgesia after arthroscopic knee surgery

BACKGROUND: Intraarticular bupivacaine is often used for prevention of pain after arthroscopic knee surgery. Intraarticular magnesium, a N-methyl-D-aspartate receptor blocker, would be of particular interest in either producing postoperative analgesia or enhancing the analgesic effect of intraarticular bupivacaine. We designed this study to determine whether intraarticular magnesium sulfate or bupivacaine results in a decrease in visual analog scale (VAS) score followed by a decrease in analgesic requirement and whether their combination would provide more reduction in VAS, and subsequently less analgesic requirement, than either drug alone. METHODS: One-hundred and eight patients undergoing arthroscopic meniscectomy were randomized blindly into one of four parallel groups. The saline placebo group (group S) received 20 mL of isotonic saline, and the magnesium group (group M) received 20 mL of isotonic saline containing I g magnesium sulfate. The bupivacaine group (group B) received 0.25% (20 mL) bupivacaine, whereas the magnesium with bupivacaine group (group MB) received bupivacaine 0.25% and 1 g of magnesium sulfate in 20 mL. The postoperative analgesia was assessed using VAS recorded after surgery at 1, 2, 4, 6, 8, 12, and 24 h. Patients were evaluated at rest and with movement. RESULTS: Group MB had a significantly reduced VAS both at rest and on movement, a significantly increased time to first postoperative analgesic request, as well as significantly reduced total analgesic requirement than other groups. CONCLUSION: Magnesium combined with bupivacaine produces a reduction in postoperative pain when given intraarticularly in comparison to either bupivacaine or magnesium alone, or to saline placebo.