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Torque Measurements of the Ossicular Chain: Implication on the MRI Safety of the Hearing Implant Vibrant Soundbridge

Jesacher, Marco Otto*; Kiefer, Jan; Zierhofer, Clemens; Fauser, Claudius§

doi: 10.1097/MAO.0b013e3181d2d3f3
Basic Science
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Hypothesis: Is the human ossicular chain stabile enough to withstand the torque of a Vibrant Soundbridge middle ear hearing implant in the magnetic field of a 1.5 T magnetic resonance imaging (MRI) system?

Background: The Vibrant Soundbridge is a semi-implantable hearing device in which a tiny electromechanical transducer, called floating mass transducer (FMT), is clipped to the ossicular chain within the middle ear. The FMT contains a permanent magnet, which can generate a torque when exposed to a static magnetic field of MRI systems. Since the transducer is routinely attached to the long process of the incus, this torque could affect or even disrupt the ossicular chain. This study investigates the likelihood of a middle ear injury by an FMT in an MRI system.

Methods: Torque measurements were performed on 10 unpreserved human temporal bones. A brass fork was attached to the long process of the incus via a posterior tympanotomy, and a defined torque was applied by a calibrated torque meter. The torque was increased stepwise until an injury of the middle ear was observed.

Results: The mean torque at which the middle ear was injured was 4.3 mN·m ± 1.7 mN·m. The lowest value measured was 1.5 mN·m, and the highest was 6.5 mN·m.

Conclusion: Even the lowest torque measured is more than 1.5 times higher than the "worst-case" torque affecting the FMT during a 1.5 T MRI examination. The torque on an FMT crimped to the long process of the incus should therefore not harm the human middle ear.

*Department of Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria; †HNO-Zentrum Regensburg, Regensburg, Germany; ‡Christian Doppler Laboratory for Active Implantable Systems, Institute of Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria; and §ENT-Department, University Hospital Rechts der Isar, Technical University Munich, Munich, Germany

Address correspondence and reprint requests to Marco Otto Jesacher, Ph.D., Institute of Ion Physics and Applied Physics, University of Innsbruck, Technikerstraße 25/3, A-6020 Innsbruck; E-mail: marco.jesacher@uibk.ac.at

Sources that require acknowledgment: University Hospital Rechts der Isar, Technical University Munich, Germany.

Vibrant Med-El Hearing Technology GmbH, Innsbruck, Austria.

© 2010 Otology & Neurotology, Inc.