Stability of the standard incus coupling of the Carina middle ear actuator after 1.5T MRI

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 Impedance curves measured when the actuator was retracted from the short process after the experiment still indicate the presence of a clear resonance peak.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 Impedance curves obtained when the actuator was retracted from the incus short process still show a clear resonance peak, indicating the device is still functional after the MRI exposures.


Revision of "Stability of the standard incus coupling of the Carina middle ear actuator after 1.5T MRI" manuscript (PONE-D-19-30029)
Dear Dr. da Costa Monsanto, dear reviewers, We thank you for your time to review our manuscript and for the constructive feedback on our work. Based on the comments you provided, we have revised the manuscript. We agree that some aspects of the work had to be clarified to provide the PLOS ONE readers with a straightforward and unambiguous overview of our experiments. On the following pages of this letter we have addressed each reviewer's comment separately. Please share these replies with them on our behalf.
As the academic editor, familiar with the topic we investigated, you requested more information on how we have reached the conclusion that "the risk on dislocating the Carina middle ear actuator when coupled to the incus short process is limited." We have reached this conclusion based on the analysis of our CT image data and actuator impedance measurements. None of the measurement modalities have indicated that the actuator had become dislocated, leading us to the stated conclusion. We understand, however, that solely mentioning this sentence is unclear and have therefore clarified the conclusion section in the manuscript.
In addition, author affiliations have been updated to fit the PLOS ONE journal guidelines. Other formatting issues were not found when checking the journal requirements as we have used the available LaTex template for PLOS ONE submissions. The funding from Cochlear Ltd. has been mentioned in the comment section as we were not able to edit the competing interest statement directly. Finally, we have created a file containing all relevant data to allow replication of our study, which is added to the submission as supporting information. In case a researcher requires additional information, he/she can contact me directly as corresponding author.
On behalf of all contributing authors Yours sincerely,

Guy Fierens
Reviewer #1: I read the manuscript entitled "Stability of the standard incus coupling of the Carina middle ear actuator after 1.5T MRI" with great interest. In this study, the authors aimed to measure the coupling stability of an actuator of a fully implantable AMEI after MRI scans. The subject of the study is clinically relevant, and the results are presented in an intelligible fashion. Although I commend the authors for tackling such an important subject, I do have a few concerns regarding the methodology that the authors could perhaps clarify.
Answer: We sincerely thank the reviewer for the positive feedback and the confidence in the clinical relevance of our work. We have updated the manuscript according to the comments below. Below we answered all questions point per point, indicating if and where we have made changes to the manuscript.
1. For stapedotomy, the angle between the prosthesis used and the incus long process is known to be ideal at 90 degrees (URL). I wonder that there must be perfect alignment between the actuator and the ossicular chain for optimal device performance. Why did the authors not perform these angle measurements?
Answer: The optimal angle for coupling between the prosthesis and the incus long process is 90 degrees as indicated by the reviewer. Measuring and reporting on the angular values would optimize coupling efficiency and possibly also coupling stability. We have intentionally not performed angular measurements in order to avoid testing a "perfect situation". We wanted to simulate a clinically relevant, yet not per se optimal solution to assess the risk of actuator dislocation in a slightly more worst-case. In addition, the coupling to the incus body, for sensorineural hearing loss indications, is different than a standard stapes surgery where the prosthesis needs to go into a stapedotomy hole. 2. The data from 3 TB CT scans was corrupted. Therefore, only 8 CT scans could be analyzed. I think this is a small sample to get further conclusions.

Reviewer #3:
The authors of the present study conducted an extremely pertinent analysis of the subject in question. They clearly introduced the importance of magnetic resonance imaging in the growing world of implantable hearing aids. The study was conducted with fidelity of technical application, respecting the necessary steps to conduct the imaging exams in order to minimize the bias of selection or variation of radiological technique. The procedure of implantation of the anatomical specimens respected the indication criteria according to the latest manufacturer's guidelines. Actuator T2 dislocations were analyzed not only according to the computed tomography exam, which could lead us to a technical bias, but also according to the average of the impedance curves before and after 10 exams. MRI scan using the manufacturer's TLA. Finally, the authors concluded that and according to the results presented, little effect of the factor (magnetic resonance) on the implant analyzed, when using the classic anvil short coupling method. Answer: We thank the reviewer for his supportive views on the presented work. We believe that MRI safety is an extremely important topic for patients with implanted devices. We hope that our work will contribute to the insights regarding the currently labelled MRI unsafe Carina. Even though we can not make statements regarding the full MRI safety of this device, the presented work already tackles a single aspect of the topic.