Biomechanical modeling of elastoplastic behavior of spinal rod in spinal instrumentation.

Multibody modeling was combined with elastoplastic constitutive models for the rod material to simulate spinal instrumentation for enhanced understanding of its biomechanics. We simulated spinal instrumentations with rods of commonly used cobalt-chromium-molybdenum alloy and a possible alternative alloy with higher stiffness, respectively. With low fixation implant density, yielding of the rod material occurred when average corrective forces exceeded 187-318 N as elastoplastic deformation increased significantly affecting both deformity correction and bone-implant forces. These findings emphasize the need to incorporate nonlinear elastoplastic rod behavior into biomechanical modeling and better represent surgical reality. The proposed model provides a useful tool for improved preoperative planning.