Orthopedic Implants, Materials, and Devices

Most of the orthopedic implants, materials, and devices evaluated in the MR environment are made from nonferromagnetic materials and, therefore, are safe for patients undergoing MR procedures. However, in certain instances, due to the length or formation of a conductive loop, MRI-related heating may be a problem for some orthopedic implants, especially external fixation systems, (see below).

To date, only the Perfix interference screw used for reconstruction of the anterior cruciate ligament has been found to be highly ferromagnetic. Because this interference screw is firmly imbedded in bone for its specific application, it is held in place with sufficient retentive forces to prevent movement or dislodgment. Patients with Perfix interference screws have safely undergone MR procedures using MR systems operating at 1.5-Tesla.

The presence of the Perfix interference screw causes extensive image distortion during MR imaging of the knee. Therefore, interference screws made from materials with low magnetic susceptibility should be used for reconstruction of the anterior cruciate ligament if MR imaging is to be utilized for subsequent evaluation of the knee.

Patients with the orthopedic implants, materials, and devices indicated in The List have undergone MR procedures using MR systems operating with static magnetic fields up to 1.5-Tesla without incident.

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 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 of the external fixation system. Importantly, the MRI conditions (field strength, RF field, RF transmit coil, pulse sequence, body part imaged, etc.) used greatly impacts the safety aspects of scanning patients with external fixation systems.

In order to ensure patient safety, guidelines are typically applied on a case by case basis and, therefore, MR users are referred to product labeling approved by the U.S. Food and Drug Administration for a given external fixation system. Notably, the safe MRI conditions typically apply to the specific configuration(s) used in the evaluation of a given fixation device, ONLY. Other configurations may be unsafe.

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 during MRI examinations.

MRI Information 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). All of these are considered to be safe based on findings for deflection angles, torque, and the intended in vivo uses of these devices. For some orthopedic implants, MRI-related heating was evaluated, as needed, and is a concern for certain devices, especially external fixation systems or those implants with a relatively long length.

[MRI 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 MRI procedure.]

REFERENCES

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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 Imaging 1992;2:225-228.

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