Modeling rod elasto-plasticity improves biomechanical simulation of adolescent idiopathic scoliosis instrumentation

In adolescent idiopathic scoliosis instrumentation surgery, spinal rods undergo substantial elasto-plastic deformation resulting in flattening, yet most biomechanical models represent rods as linear-elastic. We evaluated two approaches to account for elasto-plasticity and their improvement over conventional linear-elastic modeling in 14 patient-specific physics-based simulations with identical instrumentation constructs: a computationally efficient approximation of permanent contour loss and a material-based approach using nonlinear stress-strain behavior to estimate plastic deformation. Linear-elastic modeling overestimated postoperative rod curvature, sagittal correction and forces, whereas both elasto-plastic approaches better reproduced postoperative rod shape and global alignment. Incorporating rod elasto-plasticity improves the biomechanical fidelity of surgical simulation.