Management of post operative pain is challenging. Posterior spine fusion surgeries require high quality analgesia to maximize functional recovery . Adequate post-operative analgesia helps with early mobilisation, brief hospital stay, reduced hospital costs and increased patient comfort. Magnesium sulphate through NMDA receptor blockade in the central nervous system and calcium channel blockade has been reported to be an effective anesthetic adjunct for pain relief and also in blunting somatic , autonomic and endocrine reflexes induced by noxious stimuli. Magnesium sulphate has been used in the form of bolus and /or continuous intravenous infusions intra-operatively and has been found to attenuate acute post-operative pain in total knee arthroplasty. In this randomized control trial , we aim to assess the efficacy of magnesium sulphate as an additive in intravenous patient controlled analgesia (PCA) for post operative pain management ,in patients undergoing multilevel posterior spinal surgeries between the age of 18-65 years. The participants are randomized into two study groups - group 1 receiving PCA fentanyl and ketamine and group 2 receiving PCA fentanyl ,ketamine and magnesium .Postoperatively the pain scores will be assessed at specified time intervals as specified in the attached sheet. Vitals , sedation score ,nausea ,vomiting and quality of sleep will be monitored regularly in the ward. Number of boluses delivered to each participant will be noted 24 hours after surgery .Serum magnesium levels will be checked pre operatively and at 24 ,48 hours after starting the PCA infusion . Magnesium toxicity will be monitored by regular assessment of knee jerk, urine output and respiratory rate. Participants will be followed up at 3 months of surgery for the assessment of chronic pain relief.