STUDY DESIGN: A numerical study was conducted by simulating in situ contouring (ISC) surgery. OBJECTIVE: To quantify intraoperative correction during ISC surgery. SUMMARY OF BACKGROUND DATA: Surgical techniques correcting scoliosis, like the ISC one, lead to a complex 3-dimensional correction of the spine. Using motion analysis devices to analyze the effect of intraoperative surgical maneuvers was tedious and limited the study to the kinematics of exposed vertebrae. An alternative method consisted in simulating the surgical gestures. However, proposed models were based on rigid instrumentations, and focused attention on specific gestures of the rod-rotation and the distraction techniques through operator-dependent simulations. METHODS: This study included 10 patients with severe idiopathic scoliosis treated by ISC surgery. From a patient-specific finite-element model (T1-L5 and pelvis), all main steps of the ISC surgery were automatically simulated. A specific algorithm was developed to determine the sequences of bending maneuvers according to the rod shapes chosen by the surgeon. The accuracy of the automated surgery simulation was assessed regarding the virtual postoperative spinal configuration and postoperative clinical data. For each maneuver, vertebral kinematics was computed as well as the evolution of various clinical parameters. RESULTS: The bending maneuvers of both the first and the second rods provided complementary effects inside, but also outside the fused spinal area. These main maneuvers combined the intraoperative spinal corrections induced by maneuvers specific to the rod-rotation surgery. CONCLUSION: The automated patient-specific simulation of ISC surgery may improve the understanding of the main mechanisms involved in the scoliosis surgical correction.