The goal of this clinical trial is to evaluate whether adding low-dose ketamine to PCA morphine reduces opioid requirements after posterior spinal fusion surgery in adolescent idiopathic scoliosis patients. Selected patients aged 10-18 years undergoing elective AIS surgery at University Malaya Medical Centre will be randomised to ketamine-morphine or morphine-only PCA. The primary outcome is cumulative morphine consumption at 48 hours, with secondary outcomes including pain scores, opioid-related adverse effects, time to ambulation, and patient satisfaction. This study aligns with national priorities for safe opioid stewardship and enhanced peri-operative care in Malaysia.