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Magnetic Properties of Medtronic ENT Otology Implant Devices
November 28, 2000
RE: Magnetic Properties of Medtronic ENT Otology Implant Devices
To whom it may concern:
The effects of Magnetic Resonance Imaging (MRI) scans on Medtronic ENT otology implants are addressed below. Included in the scope of this letter are all otology implants Medtronic ENT has acquired through the purchase of Treace Medical, Inc. and Microtek Medical, Inc. Medtronic ENT otology devices, including both ossicular (middle ear) prostheses and ventilation tubes, are composed of one or more of the following available materials:
Stainless Steel (Austenitic grade, ASTM F138)
Titanium or Titanium Alloy (ASTM F67 and F136)
Platinum
Porous Polyethylene polymer or Polyethylene with Stainless Steel wire
Fluoroplastic polymer or Fluoroplastic with Stainless Steel wire
Hydroxylapatite and Hydroxylapatite composite with silicone elastomer
Generally, MRI scans are contraindicated for patients with certain metallic and metal-containing devices, such as selected aneurysm clips, select cochlear implants (i.e., Medtronic ENT Audiant, etc.), or neurostimulator TENS units. However, studies conducted on otology devices manufactured from the above materials, by Medtronic ENT as well as other otology device manufacturers, support the lack of significant hazard for patients with middle ear implants undergoing MRI scans. The following information is provided from the available published literature as cited.
| 1 |
Effects of Magnetic Resonance Imaging Fields on Stapedectomy
Prostheses
Edward L. Applebaum, M.D., Galdino E. Valvassori, M.D., Archives of Otolaryngology, Volume 111, pages 820-821, December 1985. |
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Stapes prostheses of Medtronic ENT (stainless steel Robinson), and other stapes prostheses of fluoroplastic and stainless steel wire, platinum ribbon, and stainless steel loops were evaluated by the author in a 1.5 Tesla MRI unit, operated continuously at its maximum output of 15,000 Gauss. The items were positioned on a scale in a covered glass container, moved in and out of the magnetic field, rotated 90 degrees and process repeated in positions normally occupied by a patient during clinical imaging. Items were within 1 meter of the magnet. Results observed no prosthesis displacement following any of the trials. The concluded observation was made that MRI imaging should pose no hazard to patients who have had stapedectomy surgery using the prostheses tested. The presence of metallic implants may cause artifacts that degrade the images, but these artifacts are usually proportional to the mass of the device.
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| 2 |
Interaction between Magnetic Fields and Metallic Ossicular
Prostheses
David W. White, M.D., American Journal of Otolaryngology, Volume 8, Number 2, pages 90-92, March 1987. |
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A selection of commonly used metal ossicular prostheses, including Medtronic ENT Robinson stainless steel prostheses, were tested to see if they were attracted by the 5000 Gauss magnetic field of a MRI. None of the prostheses tested were attracted by the magnetic field, but some of the metal components had an effect on the image produced by the scanner. Therefore, while metal implants may be safe in the powerful magnetic field, they can alter the field, resulting in an abnormal image.
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| 3 |
Imaging of Ossicular
Prostheses
Barry E. Hirsch M.D., et al., Journal of Otolaryngology Head and Neck Surgery, Volume 111, Number 4, pages 494-496, October, 1994. |
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A selection of eight ossicular implants composed of porous polyethylene polymer with metal stainless steel wire and dense ceramic hydroxylapatite (HA) synthetic materials were compared for computed tomography (CT) and magnetic resonance image (MRI) characteristics. Images were obtained in the same planes for each process. In CT scans the HA material was densely radiopaque, the porous polyethylene was radiolucent, except for the stainless steel wire. In MRI the HA material generated a signal void. The metal wires generated significant artifact larger and more bulbous than the wire. No movement was observed of the metal wire prostheses despite the extensive artifact generated.
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| 4 |
Magnetic Resonance Imaging of Stapes
Prostheses
Mark J. Syms, M.D., Gregory W. Petermann, M.D., American Journal of Otology, Volume 21, pages 494-498, July, 2000. |
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Medtronic ENT ossicular prostheses were evaluated by the authors in a 1.5T MRI field. The test described in the Applebaum & Valvassori paper was repeated as an in vitro screen for rotational and/or translational movement; a handheld magnet was also used as a test for attraction. The authors observed rotational movement and no translational movement for the Medtronic ENT prostheses, which led them to conclude the parts had ferromagnetic properties. The clinical significance, however, was unknown. |
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Medtronic ENT comment on the term "ferromagnetic":
Medtronic ENT feels that the use of the term "ferromagnetic" in the Syms study is inaccurate. We believe the term "paramagnetic" would be more appropriate. The stainless steel material used in implants (ASTM F138) is inherently paramagnetic (i.e., weakly magnetic) due to the Austenitic atom structure. The dipoles are generally oriented so there are no concentrations of positive and negative areas; however, slight magnetism may be evident. Conversely, ferromagnetic material is strongly magnetic; all the dipoles are aligned in the same direction when a magnetic field is present. Processing of a paramagnetic material (e.g., machining) may impart additional magnetism by creating small concentrations of dipole alignment. Medtronic ENT studies indicate, however, that these materials are still considerably less magnetic than a known ferromagnetic material by greater than two orders of magnitude. |
| 5 |
Magnetic Prosthesis Displacement in High Strength Magnetic Fields, Michelle D. Williams, M.D., Patrick Antonelli, M.D., et al., presented at the American Otological Society Annual Meeting, May 12-13, 2000 [pending publication]. |
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The authors evaluated Medtronic ENT ossicular prostheses in MRI fields ranging from 1.5T to 4.7T; the purpose of the evaluation was to determine the clinical significance of the in vitro studies completed to date. The in vitro test described in the Applebaum & Valvassori paper was repeated; in addition, in vitro angular deflection tests were performed. The results were then compared to the results of an in vivo cadaveric temporal bone test. While the author's in vitro test results supported those described in the Syms & Petermann paper, the in vivo results indicated no detectable movement in magnetic fields up to 4.7T. In addition, positive controls (i.e., sample prostheses manufactured from a known ferromagnetic material) showed no detectable movement in magnetic fields up to 1.5T; VSM testing indicated that the material used in Medtronic ENT prostheses is 1/400th as magnetic as the positive control. The concluded observation was that while stainless steel ossicular prostheses exhibit magnetic properties, the in vivo results would indicate the magnetism is relatively weak and not clinically significant.
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| Additional references that may be of interest: |
| 6 |
The Effect of Nuclear Magnetic Resonance Imaging on Metallic Middle Ear
Prostheses
Mattucci KF, Setzen M, Hyman R, Chaturvedi G., Otolaryngology Head Neck Surg, 94: 441-3, 1986.
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| 7 |
Middle Ear Prosthesis: Significance of Magnetic Resonance
Imaging
Leon JA, Gabriele OF, Magnetic Resonance Imaging, 5: 405-6, 1987.
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| 8 |
Metallic Otologic Implants: in vitro Assessment of Ferromagnetism at
1.5T
Shellock FG, Schatz CJ, American Journal of Neuroradiology, 12: 279-281, 1991.
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| 9 |
Further Studies on the Effects of Magnetic Imaging on Middle Ear
Implants
Applebaum EL, Valvassori GE, Ann Otol Rhinol Laryngol, 99: 801-4, 1990.
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| 10 |
The Interactions Between Metal Stapes Prostheses and High-Intensity Magnetic Fields During Magnetic Resonance Tomography
Garaventa G, Satrango L, Vellucci F, Pagano M, Pallestrini EA, Acta Otorhinolaryngol Ital, 11: 455-63, 1991. [Italian] |
While benchtop testing of Medtronic ENT ossicular prostheses may demonstrate low-level, paramagnetism, there are no known reports of MRI-related adverse effects to a patient due to any Medtronic ENT ossicular prostheses. Further evidence of MR safety is given in the Antonelli study above, with the addition of in vivo testing. Caution is generally advised, however, when administering MRI procedures, particularly when magnetic fields above 1.5 Tesla are used, as the effects at such levels have not been thoroughly studied. Additionally, image artifact may occur due to the material composition of the prosthesis. Because of the potential for such artifact, alternative imaging techniques, such as CT scanning, should be considered.
Sincerely,
Medtronic ENT Regulatory Affairs
Rev.11/00
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