title: Evaluation of the translational and rotational forces acting on a highly ferromagnetic orthopedic spinal implant in magnetic resonance imaging.
abstract: OBJECTIVE
  To assess the translational and rotational forces acting on a highly ferromagnetic orthopedic spinal implant in 1.5T and 3.0T magnetic resonance (MR) systems.
  
  
  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.
  
  
  CONCLUSIONS
  The translational and rotational forces acting on the implant in both 1.5T and 3.0T MR systems are substantial, but based on theoretical considerations are unlikely to result in implant migration or rotation.
authors.affiliation: Department of Clinical Physics, Institute of Neurological Sciences, Southern General Hospital, Glasgow, UK. christie.mccomb@nhs.net
authors.forename: Christie
authors.initials: C
authors.lastname: McComb
authors.forename: David
authors.initials: D
authors.lastname: Allan
authors.forename: Barrie
authors.initials: B
authors.lastname: Condon
mesh_terms: D006801:Humans; D008279:Magnetic Resonance Imaging / Q000009:adverse effects; D019736:Prostheses and Implants; D012399:Rotation; D013123:Spinal Fusion / Q000295:instrumentation*; D019415:Torque