Abstract
Limited data is available concerning the safety of active middle ear implants (AMEI) during Magnetic Resonance Imaging (MRI). Measurements in temporal bones are the gold standard for preclinical assessment of device safety. In this study the coupling stability of an actuator as used in a fully implantable AMEI was determined in temporal bones. Eleven temporal bones were implanted with the actuator according to the manufacturer's surgical guidelines. The actuator was coupled on the incus short process as recommended for sensorineural hearing loss. Temporal bones were exposed 10 times to the MRI magnetic field by entering the MRI suite in a clinically relevant way. Computed Tomography (CT) images were acquired before and after the experiment to investigate the risk of actuator dislocation. Based on the electrical impedance of the actuator, the loading of the actuator to the incus was confirmed. Relative actuator displacement was determined on the CT images by comparing the initial with the final actuator position in 3D space. Impedance curves were analyzed after each exposure to check the loading of the actuator to the ossicles. Analysis of CT images with a 0.30.6 mm in-plane resolution indicate no actuator displacement. The maximum detected change in impedance for all actuators was 8.43 Ω at the actuator's resonance frequency. Impedance curves measured when the actuator was retracted from the short process after the experiment still indicate the presence of a clear resonance peak. No actuator displacement or dislocation could be detected in the analysis of CT images and the measured impedance curves. Impedance curves obtained when the actuator was retracted from the incus short process still show a clear resonance peak, indicating the device is still functional after the MRI exposures.
Highlights
Over the last decades, a number of different active middle ear implants (AMEI) have shown a succesful outcome in patients with different types of pathologies and hearing losses
The day after implantation, the Temporal bones (TBs) were transported to Bicetre University Hospital (Le Kremlin-Bicetre, France) where the actuators are delicately mounted into the fixation system and loaded to the incus body (Fig 1) using the CochlearTMCarina1 interface, the intraoperative test system provided by the manufacturer
The present study has provided more insights on how severe the risk of actuator migration could be for patient’s implanted with a CochlearTMCarina1 fully-implantable AMEI
Summary
A number of different active middle ear implants (AMEI) have shown a succesful outcome in patients with different types of pathologies and hearing losses. One of the concerns are magnetically-induced forces and torques on magnetic components in the implant [8], which could lead to the displacement of the component resulting in patient harm. Similar magnetically-induced forces could lead to a displacement of the actuator which could result in dislocation of the actuator from the ossicles or even damage the ossicular chain itself. Studies by Todt et al [12, 13] indicated that the implanted actuator could move due to magnetically-induced forces, leading to a number of actuator dislocations. For the Carina implant, knowledge is lacking on the stability of the actuator when being exposed to a 1.5 T magnetic field. The present study will investigate the risk of dislocating the actuator from its standard fixation on the incus body when exposed to a 1.5T magnetic field. In this location the homogeneity of the field is maximal, and the induced torque will be maximal as well [8]
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