The Finnish Paediatric Orthopaedic Study Group will perform a prospective randomized, multicenter, clinical trial comparing Gelatin matrix with human derived thrombin (Floseal, Baxter) and without use of Floseal in adolescents undergoing posterior spinal deformity surgery for AIS (adolescent idiopathic scoliosis between 45 and 90 degrees) in Turku, Tampare and Helsinki Children's Hospitals. Sixty patients will be randomized into the intervention group receiving Floseal (n=30) and control group not receiving Floseal (n=30). Each University Hospital will treat 10 Floseal and 10 conventional treated scoliosis patients (block randomization). Both treatment groups will be allowed to use conventional blood saving methods.