EXTERNAL FIXATION DEVICES

Most orthopedic implants and materials do not pose problems for patients undergoing MR procedures. However, because of the length of the implant or the formation of a conductive loop, MR examinations may be hazardous for certain orthopedic implants, namely 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;
-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 safety 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 of the external fixation system. Importantly, the specific MRI conditions (strength of the static magnetic field, RF frequency, type of RF transmit coil, pulse sequence, body part imaged, etc.) directly impact the safety aspects of scanning patients with external fixation systems.

For example, Luechinger et al. 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 of the newly designed clamps were more than a factor 2 lower as the gravitational force on the device whereas, magnetic forces on the older devices showed over 10 times the force induced by earth acceleration of gravity. No torque effects could be found for the newly designed clamps.

Temperatures recorded at the tips of Schanz screws in the 1.5-Tesla MR system showed 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 in the 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.

In order to ensure patient safety, guidelines must be applied on a case-by-case basis. Therefore, MR users are referred to product labeling approved by the U.S. Food and Drug Administration for a given external fixation system. Notably, this information may only apply to a particular configuration for the external fixation device.

Vibration Associated With MR Procedures.
Graf et al. 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 well-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 sometimes 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.]

REFERENCES
Graf H, Lauer UA, Schick F. Eddy-current induction in extended metallic parts as a source of considerable torsional moment. Journal of Magnetic Resonance Imaging 2006;23:585-590.

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.