Zimmer: MRI Compatibility & Metal Sensitivity

Click for a PDF file with: Information for Physicians who Believe They May Have a Patient With a Sensitivity to Metal

Magnetic Resonance Imaging of Implants

Magnetic Resonance Imaging (MRI) is a commonly accepted and widely used medical procedure.  It is often safe to perform MRI on an individual that has an orthopaedic implant device. However, it may be contraindicated in certain situations because the magnetic field present in the MRI environment may, under certain circumstances, result in movement or heating of a metallic orthopaedic implant device.  Metals that exhibit magnetic attraction in the MRI setting may be subject to movement (deflection) during the procedure.  Both magnetic and non-magnetic metallic devices of certain geometries may also be subjected to heating caused by interactions with the magnetic field.  Of secondary concern, is the possibility of image artifacts that can compromise the procedure and image quality.

Implant Materials

Orthopaedic devices are manufactured from metals, polymers and ceramic materials.  Zimmer metallic implants are produced from cobalt, titanium and tantalum in either the alloy or pure form.  Stainless steels are also sometimes used.  With the exception of stainless steels, these materials have no magnetic attraction in the MRI environment and implant movement / deflection is not expected.  Stainless steels used in some Zimmer devices may exhibit small magnetic interaction in the MRI environment.

Implant Movement

There are currently several researchers as well as an ASTM committee exploring methods for accurately assessing the MRI compatibility of implant devices.  The primary focus of the research has been the measurement of implant movement in response to a magnetic field.  Shellock and co-workers 1-3 conducted several studies in which the movement/deflection of various orthopaedic implants was measured in the high magnetic field (0.3-1.5 Tesla) region of MRI units.  The results of these studies show no measurable movement of implants fabricated from cobalt, titanium and stainless steel alloys.

In an effort to improve imaging capabilities, MRI units employing larger magnetic fields (3.0 Tesla) are also being used.  It is possible that implants exhibiting weak interactions in a 1.5 Tesla magnetic field may show a stronger interaction with the 3.0 Tesla field.  Dr. Shellock4 has also examined the movement/deflection of selected orthopaedic implants in a 3.0 Tesla MRI unit and found that devices fabricated from cobalt, titanium and stainless steel exhibited little or no movement/deflection.

Implant Heating

Whereas several studies examining the movement/deflection of metallic implants have been performed, comparatively little information has been developed for the potential heating of implants caused by MRI induced electrical currents. MRI induced heating of an implant is important in that it has the potential to damage both the implant and the tissues surrounding the implant.

MRI induced heating of implanted devices is caused by the absorption of magnetic energy during the procedure. The rate of energy absorption is referred to as the specific absorption rate (SAR).  Implant heating in both 1.5 and 3.0 Tesla units is likely to be similar because the Food and Drug Administration has established the same specific absorption rate guidelines for each MRI unit. 5-6

In general, only minor heating of passive (non electrified) metallic implants has been found in the MRI setting.5-6 However, significant heating is possible for implants that have an elongated shape (e.g. pacemaker lead) or those that provide a conductive loop (large all metallic external fixators).5-7 External fixators such as the Zimmer WristoreTM or Trans FXTM are combinations of structural polymers, composites, and metallic materials.  Polymers and composites do not conduct electric current and this minimizes the chance of significant heating of these devices.

Recommendations

Although studies indicate that the MRI procedure has minimal effects on most Zimmer orthopaedic implant devices, there are certain devices, such as TransFX and other external fixation devices that may have magnetic components and should not be used in an MRI environment. Patients should note that there are several different manufacturers and generations of MRI equipment available, and Zimmer cannot make claims regarding the compatibility of Zimmer implants with any specific MRI unit.  It is recommended that the patient contact the surgeon or the manufacturer of the MRI equipment to discuss the compatibility of Zimmer implants with the MRI equipment before undergoing any test.

If more information is needed, please call Cheryl R. Blanchard, Ph.D., 800-613-6131, extension 4467 or Michael E. Hawkins, Ph.D., 800-613-6131, extension 4624.

References

  1. “High Field Strength MR Imaging and Metallic Biomedical Implants: An Ex Vivo Evaluation of Deflection Forces”, Frank G. Shellock, John V. Crues, AJR: 151, August 1988, pp. 389-392

  2. “MR Procedures and Biomedical Implants, Materials, and Devices: 1993 Update”, F.G Shellock, S. Morisoli, E. Kanal, Radiology, 1993, 189, pp. 587-599

  3. “MR Imaging and Metallic Implants for Anterior Cruciate Ligament Reconstruction: assessment of ferromagnetism and Artifact”, F. G. Shellock, J.H. Mink, S. Curtin, M. J. Friedman,  Journal of Magnetic Resonance Imaging 1992, 2: pp. 225-228

  4. “Biomedical Implants and Devices: Assessment of Magnetic Field Interactions with a 3.0 Tesla MR system”, F. G. Shellock, Journal of Magnetic Resonance Imaging 2002, 16:721-732

  5. "High Field Strength MR Imaging and metallic Biomedical Implants: an in Vitro Evaluation of Deflection Forces and Temperature Changes Induced in Large Prostheses”, F. Shellock, J. Crues, Radiology 1987, 165 (P): 150

  6. “3.0-Tesla MR Safety Information for Implants and Devices,” available at
    http:www.mrisafety.com, under the “Safety Information” tab.

  7. “Cardiac Pacemaker: In Vitro Assessment at 1.5T,” F. G. Shellock et al., American Heart Journal 2006, 151: 436-443

  


(updated 02/15/07)