This prospective randomized double-blind study evaluates the efficacy of erector spinae plane (ESP) blocks on postoperative pain in patients undergoing cervical or lumbar posterior spinal fusion. Patients will receive either 0.5% bupivacaine plus liposomal bupivacaine or 0.5% plain bupivacaine. Outcomes include opioid consumption over 72 hours, pain scores, length of stay, and time to ambulation.