Magnetic properties of middle ear and stapes implants in a 9.4-T magnetic resonance field.

HYPOTHESIS: A 9.4-T magnetic resonance (MR) field may cause motion displacement of the middle ear and stapes implants not previously observed with 1.5- and 3.0-T magnets. BACKGROUND: Publications have described the safety limitations of some otologic implants in 4.7-T field and resulted in several companywide patient safety-related recalls. To date, no studies have been reported for otologic implants in a 9.4-T MR field nor have comparisons been made with 4.7-T field strengths. METHODS: Twenty-three commonly used middle ear and stapes prostheses were selected and exposed to 9.4-T MR fields in vitro within petri dishes, and eight of the 23 implants were further studied ex corpus in human temporal bones (TBs) in a 9.4-T MR field. This study has been approved by the institutional review board. RESULTS: Eight prostheses in petri dishes grossly displaced at 9.4 T, three of which had not previously moved in either the 1.5- or 3.0-T magnets. The eight TB preparations showed no avulsions or motion indicators after exposure at 9.4 T. CONCLUSION: Middle ear and stapes implants can move dramatically in petri dishes at 9.4-T MR field, more so than at 1.5 and 3.0 T. The absence of avulsions in the TB group strongly suggests that the surgical means used to fixate the middle ear implants to the middle ear structures successfully overcomes the magnetic moment produced at MR field strengths up to 9.4 T. The use of MR imaging is not contraindicated by this study's findings.

Ferromagnetic movements of middle ear implants and stapes prostheses in a 3-T magnetic resonance field.

HYPOTHESIS: A 3-T magnetic resonance field may cause motion or displacement of middle ear implants not seen in studies with 1.5-T magnets. BACKGROUND: Previous publications have described the safety limitations of some otologic implants in 1.5-T magnetic resonance fields. Several company-wide recalls of implants were issued. No studies to date have been reported for otologic implants within a 3-T magnetic resonance field, nor have there been comparisons with a 1.5-T field strength. METHODS: Eighteen commonly used middle ear implants and prostheses were selected. In Part 1, the prostheses were placed in Petri dishes and exposed to a 3-T magnetic resonance field. In Part 2, the particular prostheses that showed movement in Part 1 were placed into their intended use positions within temporal bone laboratory specimens and exposed to a 3-T field. Both parts were repeated in a 1.5-T field. RESULTS: In Part 1, three prostheses moved dramatically from their start positions when exposed to the 3-T magnetic resonance field. In Part 2, the three particular prostheses that showed movement in Part 1 showed no gross displacement or movement from their start positions within the temporal bone laboratory specimens. No implants moved in the 1.5-T field in either Part 1 or Part 2. CONCLUSION: Certain stapes prostheses move dramatically in Petri dishes in 3-T fields. When placed into temporal bone laboratory specimens, the same prostheses show no signs of movement from the surgical site in a 3-T field, and it appears that the surgical position holds the implants firmly in place. Results of published 1.5-T field studies should not be used directly for safety recommendations in a 3-T magnetic resonance. Heat, voltage induction, and vibration during exposure to the magnetic resonance fields should be considered as additional possible safety issues. Preference should be given to platinum and titanium implants in manufacturing processes and surgical selection.