Most of the orthopedic implants, materials, and devices evaluated for MRI issues (i.e. magnetic field interactions, heating, and artifacts) are made from nonferromagnetic materials and, therefore, are safe or “MR Conditional” according to the conditions specified for patients undergoing MR procedures. However, due to the length of the implant or the formation of a conductive loop, MRI-related heating may be a problem for some orthopedic implants, especially cervical fixation devices and internal or external fixation systems (see below).
The Perfix interference screw used for reconstruction of the anterior cruciate ligament is highly ferromagnetic, as reported by Shellock, et al. (1992). Because this interference screw is firmly imbedded in bone for its specific application, it is held in place with sufficient retentive force to prevent movement or dislodgment in association with MRI. Patients with Perfix interference screws have safely undergone MR procedures using MR systems operating at 1.5-Tesla.
Patients with the “MR Safe” or “MR Conditional” orthopedic implants, materials, and devices indicated in The List have undergone MR procedures using MR systems operating at 3-Tesla or less without incident by following specific conditions defined by MRI proper testing.
External Fixation Systems. External fixation systems comprise specially designed frames, clamps, rods, rod-to-rod couplings, pins, posts, fasteners, wire fixations, fixation bolts, washers, nuts, hinges, sockets, connecting bars, screws and other components used in orthopedic and reconstructive surgery. Indications for external fixation systems are varied and include the following treatment applications:
- Open and closed fracture fixation;
- Correction of pseudoarthroses of long bones (both congenital and acquired);
- Limb lengthening by metaphyseal or epiphyseal distraction;
- Correction of bony or soft tissue defects; and
- Correction of bony or soft tissue deformities.
The assessment of MRI issues for external fixation systems is particularly challenging because of the myriad of possible components (many of which are made from conductive materials) and configurations used for these devices. The primary concern is MRI-related heating which is dependent on particular aspects (e.g., the lengths of the component parts) of the external fixation system. Importantly, the MRI conditions (strength of the static magnetic field, frequency of the RF field, type of RF transmit coil, pulse sequence, body part imaged, position of the fixation device relative to the transmit RF coil, etc.) used greatly impacts the safety aspects of scanning patients with external fixation systems.
To ensure patient safety, guidelines are typically applied on a case by case basis and, therefore, MR professionals are referred to product labeling approved by the U.S. Food and Drug Administration or other noitifed body for a given external fixation system. Notably, the acceptable MRI conditions typically apply to the specific configuration(s) used in the evaluation of a given fixation device, only. Other configurations may be unsafe for the patient in association with MRI.
Vibration Associated With MR Procedures. Graf, et al. (2006) reported that torque acting on metallic implants or instruments due to eddy-current induction in associated with MR imaging can be considerable. Larger implants (such as fixation devices) made from conducting materials are especially affected. Gradient switching has been shown to produce fast alternating torque. Significant vibrations at off-center positions of the metal parts may explain why some patients with metallic implants sometimes report feeling heating sensations during MR examinations.
MRI Simulations and Orthopedic Implants. Because orthopedic implants tend to have a variety of sizes and shapes, it is particularly challenging to assess MRI-related heating for these devices. An efficient means of addressing this issue is to conduct MRI simulations to determine the worst-case of MRI-induced heating for a given orthopedic implant of different sizes under 1.5-T/64-MHz and 3-T/128-MHz conditions and then to apply an experimental test to validate the numerical results for worst-case heating, as described by Liu, et al. (2013).
MRI at 3-Tesla and Orthopedic Implants, Materials, and Devices. A variety of orthopedic implants have been evaluated for magnetic field interactions at 3-Tesla (see The List). Most of these are considered to be acceptable for patients based on findings for deflection angles, torque, and their intended in vivo uses. For certain orthopedic implants, MRI-related heating has been evaluated. Excessive temperature rises may be a concern for some devices, especially cervical fixation devices and internal or external fixation systems.
MR healthcare professionals are advised to contact the respective manufacturer in order to obtain the latest safety information to ensure patient safety relative to the use of an MR procedure.
Bagheri MH, et al. Metallic artifact in MRI after removal of orthopedic implants. Eur J Radiol 2012;81:584-90.
Farrelly C, et al. Imaging of soft tissues adjacent to orthopedic hardware: Comparison of 3-T and 1.5-T MRI. Am J Roentgenol 2010;194:W60-4.
Graf H, et al. Eddy-current induction in extended metallic parts as a source of considerable torsional moment. J Magn Reson Imag 2006;23:585-590.
Knott PT, et al. A comparison of magnetic and radiographic imaging artifact after using three types of metal rods: Stainless steel, titanium, and vitallium. Spine J 2010;10:789-94.
Koch KM, et al. Magnetic resonance imaging near metal implants. J Magn Reson Imag 2010;32:773-87.
Liu Y, et al. Numerical investigations of MRI RF field induced heating for external fixation devices. Biomed Eng Online 2013;12:12.
Liu Y, et al. Effect of insulating layer material on RF-induced heating for external fixation system in 1.5T MRI system. Electromagn Biol Med 2014;33:223-7.
Liu Y, Chen J, Shellock FG, Kainz W. Computational and experimental studies of an orthopedic implant: MRI-related heating at 1.5-Tesla/64-MHz and 3-Tesla/128-MHz. J Magn Reson Imaging 2013;37:491-497.
Luechinger R, Boesiger P, Disegi JA. Safety evaluation of large external fixation clamps and frames in a magnetic resonance environment. J Biomed Mater Res B Appl Biomater 2007;82:17-22.
Lyons CJ, et al. The effect of magnetic resonance imaging on metal spine implants. Spine 1989;14:670-672.
McComb C, Allan D, Condon B. Evaluation of the translational and rotational forces acting on a highly ferromagnetic orthopedic spinal implant in magnetic resonance imaging. J Magn Reson Imag 2009;29:449-53.
Mechlin M, et al. Magnetic resonance imaging of postoperative patients with metallic implants. Am J Roentgenol 1984;143:1281-1284.
Mesgarzadeh M, et al. The effect on medical metal implants by magnetic fields of magnetic resonance imaging. Skeletal Radiol 1985;14:205-206.
Muranaka H, et al. Evaluation of RF heating on hip joint implant in phantom during MRI examinations. Nippon Hoshasen Gijutsu Gakkai Zasshi 2010;66:725-33.
Powell J, et al. Numerical simulation of SAR induced around Co-Cr-Mo hip prostheses in situ exposed to RF fields associated with 1.5 and 3 T MRI body coils. Magn Reson Med 2012;68:960-8.
Shellock FG, Crues JV, Editors. MRI Bioeffects, Safety, and Patient Management. Biomedical Research Publishing Group, Los Angeles, CA, 2014.
Shellock FG. Biomedical implants and devices: Assessment of magnetic field interactions with a 3.0-Tesla MR system. J Magn Reson Imag 2002;16:721-732.
Shellock FG, Kanal E. Magnetic Resonance: Bioeffects, Safety, and Patient Management. Second Edition, Lippincott-Raven Press, New York, 1996.
Shellock FG, Mink JH, Curtin S, et al. MRI and orthopedic implants used for anterior cruciate ligament reconstruction: Assessment of ferromagnetism and artifacts. J Magn Reson Imag 1992;2:225-228.
Shellock FG, Morisoli S, Kanal E. MR procedures and biomedical implants, materials, and devices: 1993 update. Radiology 1993;189:587-599.
Stradiotti P, et al. Metal-related artifacts in instrumented spine. Techniques for reducing artifacts in CT and MRI: State of the art. Eur Spine J 2009;18 Suppl 1:102-8.
Yang CW, et al. Magnetic resonance imaging of artificial lumbar disks: Safety and metal artifacts. Chin Med J (Engl) 2009;20;122:911-6.
Zou YF, et al. Evaluation of MR issues for the latest standard brands of orthopedic metal implants: Plates and screws. Eur J Radiol 2015;84:450-7.