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Background: Patient prone positioning in scoliosis surgeries modifies the spinal curves prior to instrumentation. However, the biomechanical effects of the lateral decubitus posture, used in anterior approaches and minimally invasive techniques, have not yet been investigated. The objectives were to develop and validate a finite element model simulating the spinal changes resulting from this positioning., Methods: The 3D pre-op reconstructed geometries of six adolescent patients with idiopathic scoliosis were used to develop personalized finite element models of the spine, which integrated a three-step method simulating the lateral posture. Clinical indices were measured on pre- and intra-operative radiographs to validate the finite element model., Findings: The major Cobb angle and apical vertebral translation were reduced by 44% and 37% respectively between the pre- and intra-op postures. Using appropriately oriented gravity forces and boundary conditions, the finite element model simulations represented adequately these changes, with average differences of 4 degrees for the major Cobb angle and 4mm for the apical vertebral translation with the radiographic values., Interpretation: Lateral decubitus positioning significantly reduces the spinal deformities prior to instrumentation, as demonstrated by the finite element model. This study is a first step in the development of a modeling tool for the optimal adjustments of intra-operative positioning, which remains to be further investigated with complementary clinical studies., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)