OCTAGON trial is a randomized, double-blinded, parallel-group non-inferiority multicenter (Helsinki and Turku University Hospitals, Finland) clinical trial according to CONSORT criteria. 194 adolescents with idiopathic (major curve \> 45 degrees) or neuromuscular scoliosis (major curve \> 50 degrees) are enrolled for the OCTAGON trial comparing pooled human plasma (Octaplas, 10mL/kg, active management) vs. Plasmalyte (10 mL/kg, placebo) before incision as part of the normal intraoperative fluid therapy. Data is collected at baseline and at each follow-up until a minimum of 2-year follow-up. Outcomes Outcomes for the OCTAGON trial include 1) intraoperative blood loss (in mL, primary outcome) and the need for allogenic red blood cell infusion (percentage of patients). Secondary outcomes include health-related quality of life (Scoliosis Research Society 24 outcome questionnaire), postoperative pain (48-hour opioid consumption), operative time (hours), drain output (mL), hidden blood loss (mL), hospital stay, and complications (skin reactions, TRALI, deep surgical site infection, neurologic deficit). Research questions and hypothesis Does prophylactic use of pooled human plasma decrease intraoperative blood loss in adolescents undergoing instrumented spinal fusion for scoliosis? We hypothesize that pooled human plasma will reduce intraoperative and total blood loss by 25% resulting in lower need for blood transfusion and fewer surgical site infections. Objectives To compare the effect of pooled human plasma vs. crystalloid fluids on intraoperative bleeding and total blood loss (drain output and hidden blood loss) in children undergoing posterior spinal fusion for AIS and NMS. Adverse events will be recorded and reported as minor (skin reaction) or major (severe allergic reaction, transfusion related acute lung injury, TRALI, deep surgical site infection, neurologic injury). Ethical aspects The PHP trial has been evaluated via European regulatory authority (EU CT: 2024-514857-31-00) and by Fimea (FIMEA/2024/006588). Informed consent is obtained from the parent(s). Results will be disseminated in high-quality peer-reviewed publications. The individual patient safety and high-quality management of fractures and scoliosis is a priority in this trial. The randomization can be opened at any stage of the treatment process. Based on the clinical decision the randomized treatment can be terminated and treatment provided accord-ing to clinical decision making even if results will be evaluated using the intention to treat princi-ple. Pain management will be prioritized in every clinical scenario and parental presence is always possible.