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.