Most orthopedic implants and materials do not pose substantial problems for patients undergoing MR procedures. However, because of the length of the implant and/or the formation of a conductive loop, MR examinations may be hazardous for certain orthopedic implants, including 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 or closed fractures 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 especially 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 the particular aspects (e.g., the lengths of the component parts) of the external fixation system. Importantly, the MRI conditions (e.g., 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.) directly impact the safety aspects of scanning patients with external fixation systems.
For example, Luechinger, et al. (2007) used MRI to study “large orthopedic external fixation clamps and related components”. Forces induced by a 3-Tesla MR scanner were compiled for newly designed nonmagnetic clamps and older clamps that contained ferromagnetic components. Heating trials were performed in 1.5- and 3-Tesla MR systems for two assembled external fixation frames. Forces acting on the newly designed clamps were more than a factor of two lower as the gravitational force on the devices whereas, magnetic forces on the older devices showed over 10 times the force induced by earth’s acceleration of gravity. No torque effects could be found for the newly designed clamps.
Furthermore, the investigators recorded temperatures at the tips of Schanz screws in the 1.5-Tesla MR system and reported a rise of 0.7 degrees C for a pelvic frame and of 2.1 degrees C for a diamond knee bridge frame when normalized to a specific absorption rate (SAR) of 2-W/kg. The normalized temperature increases at 3-Tesla MR system were 0.9 degrees C for the pelvic frame and 1.1 degrees C for the knee bridge frame. Large external fixation frames assembled with the newly designed clamps (390 Series Clamps), carbon fiber reinforced rods, and implant quality 316L stainless steel Schanz screws met acceptable safety guidelines when tested at 3-Tesla. Notably, this information pertains to the specific configuration of the fixation devices that underwent testing relative to the MRI conditions that were used.
To ensure patient safety, guidelines for external fixation devices must be applied on a case-by-case and configuration-by-configuration basis. Therefore, MR healthcare professionals are referred to product labeling approved by the U.S. Food and Drug Administration or other notified body for a given external fixation system. Importantly, this information may only apply to a particular configuration for the external fixation device.
MRI Simulations and External Fixation Systems. Because external fixation systems have a variety of sizes, shapes, and component parts 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 predict the worst-case of MRI-induced heating for a particular external fixation system type under 1.5-T/64-MHz and 3-T/128-MHz conditions and then to apply the experimental test to validate the numerical results for worst-case heating, as described by Liu, et al. (2013).
Vibration Associated With MR Procedures. Graf, et al. (2006) reported that considerable torque may act on metallic implants or instruments due to eddy-current induction in associated with MR imaging. Larger implants (such as fixation devices) made from conducting materials are especially affected. Gradient switching was shown to produce fast alternating torque. Significant vibrations at off-center positions of the metal parts may explain why some patients with metallic implants may report feeling sensations of “heating” during MR examinations.
[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.]
Graf H, Lauer UA, Schick F. Eddy-current induction in extended metallic parts as a source of considerable torsional moment. J Magn Reson Imag 2006;23:585-590.
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.
Shellock FG. External Fixation Devices and MRI Safety. Signals, No. 56, Issue 1, pp. 15, 2006.