Evaluation of the Translational and Rotational Forces Acting on a Highly Ferromagnetic Orthopedic Spinal Implant in Magnetic Resonance Imaging

Purpose: To assess the translational and rotational forces acting on a highly ferromagnetic orthopedic spinal implant in 1.5T and 3.OT magnetic resonance (MR) systems. Materials and Methods: The translational forces and rotational forces, or torques, acting on the implant were measured using existing methods and assessed using the guidelines produced by the American Society for Testing and Materials (ASTM). Results: The measured translational forces were many times greater than for any other orthopedic implant previously recorded in the literature and, based on deflection angle criteria, would be considered unsafe in both MR systems. However, due to the rigid fixation of orthopedic implants in bone, implant migration is considered highly unlikely. Several constituent components of the implant were subjected to large torques, in some cases an order of magnitude greater than the corresponding torque due to gravity. However, the counterbalancing effect of the configuration of the combined implant results in a net torque that is less than the torque due to gravity. Conclusion: The translational and rotational forces acting on the implant in both 1.5T and 3.OT MR systems are substantial, but based on theoretical considerations are unlikely to result in implant migration or rotation.