Torque measurements of the ossicular chain: implication on the MRI safety of the hearing implant Vibrant Soundbridge.

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.