Magnetically Activated Implants and Devices

Various types of implants and devices incorporate magnets as a means of activating the implant. The magnet may be used to retain the implant in place (e.g., certain prosthetic devices), to guide a ferromagnetic object into a specific position, to permit the functional aspects of the implant, to change the operation of the implant (e.g., certain adjustable pressure valves), or to program the device.

Because there is a high likelihood of perturbing the function, demagnetizing, or displacing these implants, MRI procedures typically should not be performed in patients with these implants or devices. However, in some cases, patients with magnetically-activated implants and devices may undergo MRI procedures as long as certain precautions are followed (for example, see section entitled Cerebrospinal Fluid (CSF) Shunt Valves and Accessories).

Implants and devices that use magnets (e.g., certain types of dental implants, magnetic sphincters, magnetic stoma plugs, magnetic ocular implants, otologic implants, and other similar prosthetic devices) may be damaged by exposure to MR systems which, in turn, may necessitate surgery to replace or reposition them.

For example, Schneider et al. reported that an MRI examination is capable of demagnetizing the permanent magnet associated with an otologic implant (i.e., the Audiant magnet). Obviously, this has important implications for patients undergoing MRI procedures.

Whenever possible, and if this can be done without risk to the patient, a magnetically-activated implant or device (e.g., an externally applied prosthesis or magnetic stoma plug) should be removed from the patient prior to the MRI procedure. This will permit the examination to be performed safely. Knowledge of the specific aspects of the magnetically-activated implant or device is essential to recognize potential problems and to guarantee that an MRI procedure may be performed safely on the patient.

Extrusion of an eye socket magnetic implant in a patient imaged with a 0.5-Tesla MR system has been described. This type of magnetic prosthesis is used in a patient after enucleation. A removable eye prosthesis adheres with a magnet of opposite polarity to a permanent implant sutured to the rectus muscles and conjunctiva by magnetic attraction through the conjunctiva. This "magnetic linkage" is intended to permit the eye prosthesis to move in a coordinated fashion with that of eye movement.

In the reported incident, the static magnetic field of the MR system produced sufficient attraction of the ferromagnetic portion of the prosthesis to cause injury to the patient.

Certain dental prosthetic appliances utilize magnetic forces to retain the implant in place. The magnet may be contained within the prosthesis and attached to a ferromagnetic post implanted in the mandible or visa versa. An MRI procedure may be performed safely in a patient with this type of dental magnet appliance as long as it has been determined that it is properly attached to supporting tissue.

REFERENCES

Gaston A, Marsault C, Lacaze A, et al. External magnetic guidance of endovascular catheters with a superconducting magnet: preliminary trials. J Neuroradiol 1988;15:137-147.

Grady MS, Howard MA, Molloy JA, et al. Nonlinear magnetic stereotaxis: three dimensional in vivo remote magnetic manipulation of a small object in canine brain. Med Phys 1990;17:405-415.

Ranney DF, Huffaker HH. Magnetic microspheres for the targeted controlled release of drugs and diagnostic agents. Ann NY Acad Sci 1987;507:104-119.

Schneider ML, Walker GB, Dormer KJ. Effects of magnetic resonance imaging on implantable permanent magnets. Am J Otol 1995;16:687-689.

Shellock FG. Magnetic Resonance Procedures: Health Effects and Safety. CRC Press, LLC, Boca Raton, FL, 2001.

Shellock FG. Ex vivo assessment of deflection forces and artifacts associated with high-field strength MRI of "mini-magnet" dental prostheses. Magn Reson Imaging 1989;7 (Suppl 1):38.

Young DB, Pawlak AM. An electromagnetically controllable heart valve suitable for chronic implantation. ASAIO Trans 1990;36:M421-M425.

Yuh WTC, Hanigan MT, et al. Extrusion of an eye socket magnetic implant after MR imaging examination: potential hazard to a patient with eye prosthesis. J Magn Reson Imaging 1991;1:711-713.