Orthopedic Implants, Materials, and Devices

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.


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