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.

Posterior spinal fusion (PSF) is the definitive surgical treatment for patients with scoliosis. However, the procedure involves extensive tissue dissection, resulting in significant postoperative pain. Although patient-controlled analgesia (PCA) with intravenous morphine remains the current standard, the large doses required are frequently associated with side effects such as nausea, vomiting, pruritus, and sedation [4-6]. These complications delay mobilisation, prolong hospital stay, increase healthcare costs, and may contribute to opioid tolerance, undermining effective pain control.

Enhanced Recovery After Surgery (ERAS) protocols strongly promote multimodal analgesia, which combines opioid and non-opioid agents to achieve synergistic pain relief while minimising opioid exposure. This strategy has been shown to reduce side effects, improve recovery, shorten hospital stay, and lower the risk of opioid-related tolerance, hyperalgesia, and potential long-term dependence. Despite these advantages, evidence for the use of ketamine-morphine PCA in scoliosis surgery remains limited, and subanaesthetic ketamine-though effective intraoperatively as an opioid-sparing agent-remains underutilised in postoperative PCA regimens. Our previous study demonstrated that co-administration of subanaesthetic ketamine (0.5 mg/kg) at induction reduced postoperative pain sensitivity and hyperalgesia typically associated with high-dose remifentanil infusion, a strong opioid analgesic [13]. This finding underscores the potential role of ketamine as an opioid-sparing adjunct.

Building on this, we propose a single-centre, double-blind, randomised controlled trial in 114 idiopathic scoliosis patients undergoing elective PSF at University Malaya Medical Centre. Participants will be randomised to receive PCA containing ketamine-morphine (1 mg/mL + 1 mg/mL) or morphine (1 mg/mL) alone, with identical syringes to ensure allocation concealment. The primary endpoint is cumulative morphine consumption at 48 hours, while secondary outcomes include pain scores, opioid-related side effects, time to ambulation, and patient satisfaction.

This study aims to provide the first Malaysian evidence on an opioid-sparing PCA regimen, addressing national ERAS priorities and contributing to global opioid stewardship.

Inclusion criteria:

1. Aged > 10 years old
2. Idiopathic scoliosis scheduled for single-stage posterior spinal fusion (PSF).
3. American Society of Anaesthesiologists (ASA) physical status I-II.

Exclusion criteria:

1. Known hypersensitivity to morphine, ketamine or formulation excipients.
2. Hepatic dysfunction (ALT or AST > 2 × upper limit of normal).
3. Renal impairment (eGFR ≤ 60 mL min-¹ 1·73 m-²).
4. Uncontrolled asthma or severe restrictive lung disease.
5. Cardiac disease or clinically significant arrhythmia.
6. Epilepsy.
7. Intellectual disability precluding PCA use.
8. Chronic opioid therapy or pre-operative pain > 3 months.
9. Concomitant monoamine-oxidase inhibitor or tricyclic antidepressant therapy.
10. History of severe postoperative delirium.

Individual participant data will not be shared due to ethical and confidentiality considerations.