## 4 Discussion
Regarding aidedthresholds, despite the lack of a static force, CC hearing devices can provide the same hearing levels as those of conventional BC hearing aids [13]. The patients in the present study achieved benefits comparable to those reported in previous studies [17]. CC hearing devices have better aided hearing levels than ADHEAR [19], which may result from the difference not only in the transducer output, but also place of fixation, thickness of the fixation tape, and mass of the transducer. Regarding the fixation place, the thresholds for cartilage stimulation are better than those for mastoid stimulation at low frequencies when the simple-type transducer is used [28]. In ADHEAR and CC hearing devices with simple-type transducers, the surface of the transducer in contact with the body is regular. In contrast, ear chip-type transducers are created based on the shape of the ear. The surface of the transducer in contact with the body shows varies between patients. Different from ADHEAR and CC hearing devices with simple-type transducers, the process of designing the shape of the transducer is extremely important to obtain optimal results. The process of designing a hearing device based on the ear impression is an established method used in clinical practice. However, the process of designing a hearing device based on CT images was not optimized and must be improved before clinical use. Some current demerits of transducers designed using CT images may be improved via optimization. Compared to the conventional process, the merits of the design using CT images are as follows: no risks related to taking the ear impression, the advantage of understanding the shape of the ear in 3D, no physical transport or shipment of an ear impression is necessary, and CT images can be sent instantly via the Internet; therefore, CC hearing devices can be created without visiting the hospital. Furthermore, CT-based devices can be utilized in patients with allergy to the impression materials. In contrast, the disadvantages include the requirement of a CT scanner and reliance of the ear chip shape details on the CT image slice width. If a CT scan was not performed for diagnosis or other purposes, radiation exposure and examination costs are other demerits. The aim of this study was to investigate the possibility of creating transducers using CT images in place of ear impressions, not to compare the benefits between both devices. As a result, CT images obtained in clinical practices could be used for creating the transducer, and the performance of the CT-based transducer was not remarkably inferior to that of the impression-based transducer. As this was an exploratory study, further studies are needed to establish the design process and clarify the influence of the CT slice width on the transducer quality and audiological benefits. This study did not evaluate the coupling of the ear mold and shell of AC hearing aids designed using CT images of the ear. For CC hearing devices, the coupling condition to the ear is more important because it directly influences sound transmission. Therefore, the current results suggest that CT images can also be utilized to create ear molds and shells for AC hearing aids. Conceptualization, T.N. ; data curation, C.M., T.O. and R.S. ; writing--original draft preparation, T.N. ; writing--review and editing, T.N. ; supervision, T.K. and H.H. All authors have read and agreed to the published version of the manuscript. This research was supported by JSPS KAKENHI Grant Number 19K09874 and 21K09588. Institutional Review Board StatementThe study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of Nara Medical University (No. 1981). Informed Consent StatementWritten informed consent was obtained from the patients to publish this paper.