Abstract:Abstract: Objective To compare the analgesic effect and possible side effects of different routes of magnesium sulphate administration in cases of spinal anesthesia for knee arthroscopy. Methods 120 patients scheduled for knee arthroscopy 4 groups (30 patients in each): group A received only Bupivacaine intrathecally; group B received 50.00 mg Mg sulphate with Bupivacaine intrathecally; Group C: after 10 min intrathecal injection of Bupivacaine, received intravenous injection of 30.00 mg/kg Mg sulphate in 100.0 ml saline within 10 min then 10 mg/kg Mg sulphate intravenous drip for one hour; Group D received intra-articular injection of 800.00 mg Mg sulphate diluted in 12.0 ml normal saline (0.90% NaCl) 10 min before the end of surgery. The operating time is in 10 minutes, record active and passive VAS, the time from the end of surgery to the knee flexion, time and dosage of the first analgesic after operation, hemodynamic changes and any possible side effects. Results The time of the knee flexion after surgery, the time and dose of the first analgesic after surgery and the consumption of Pethidine at 24 h after surgery, group D is significantly better than the other groups (P < 0.05) respectively. Group C is significantly better than group B and group C as regards time taken from end of surgery until first analgesic dose (P = 0.000) and as regards amount of pethidine consumption in first postoperative 24 h (P = 0.000). VAS at rest and with passive movement in group D is significantly lower other groups (P = 0.000). Conclusion Intraarticular administration of magnesium sulphate is found to be better than intravenous magnesium and intrathecal in postoperative analgesia.