## 4 Discussion  
In terms of coupling to the ear, the transducer designed via ear impression was expected to be better because of its advantages in reproducibility. For a CT-based transducer, reproducibility depends on the slice width. The surface of the transducer became smoother as the slice width decreased. The shape and mass of the CT-based transducer did not correspond to those of the impression-based transducer. The mass of the impression-based transducer was smaller in Patients 1 and 2 but larger in Patient 3. The mass of a transducer depends on its shape. During CAD, the shape is determined through considering the overall shape of the ear, and the slice width does not significantly influence the overall shape. Thus, the design based on CT images was not disadvantageous in terms of mass. The placement of the transducer unit and the arrangement of the wire influence the shape of the transducer and are determined through considering the ear structure, including the auricular and surrounding areas. However, an impression of the ear cannot inform the 3D structure of the auricular and surrounding areas. In clinical practice, information on the wire length and a photograph of the ear partially compensate for these insufficiencies. In contrast, CT images provide a 3D structure of the ear. Therefore, information concerning the wire length and photographs of the ear are not required for creating hearing aids. A CT-aided design is considered to have the advantage of completely understanding the ear structure. Wearing comfort was subjectively better for the device designed via ear impression, implying that the details of the surface may be important for comfort. No esthetic advantage was observed between the two designs. The surface of the transducer is invisible during usage, which may mask the disadvantages of the surface details. Improvement in audiometric tests is an important benefit of hearing devices. In terms of subjective evaluation, hearing aids designed using CT images were better (Patients 1 and 3). However, no significant differences were observed between the aided thresholds of the two devices. A small difference that could not be detected via the audiometric test might exist. Such an insignificant difference is considered easy to compensate for through adjustments. The dominant sound transmission pathway to the cochlea depends on the ear condition. In a normal ear, there are three candidates for theoretical transmission pathways to the cochleae: direct AC, cartilage BC, and cartilage AC [24; 25; 26]. In contrast, in atreic ears, the AC pathway does not exist because of obstruction in the ear canal. In cases with fibrotic aural atresia, fibrotic tissue may be connected to the ossicles. Sound can travel to the cochlea via these fibrotic tissues and ossicles (fibrotic tissue pathway) without skull bone vibration, which achieves excellent benefits from CC hearing devices [27; 28]. In this study, there was no fibrotic tissue pathway in four ears. Thus, the dominant sound transmission pathways to the cochlea in the four ears were considered to be cartilage BC. The coupling condition may be related to the audiometric test results. Although wearing comfort was better with the device designed based on ear impression in two patients (Patients 1 and 2), hearing was not better. This contradiction may imply that the differences were insignificant. Thus, CC hearing devices designed based on CT images are not entirely disadvantageous compared to those designed based on the ear impression and, therefore, can be considered a viable option. There are several hearing device options for aural atreic ears [7]. Among them, conventional BC hearing aids and ADHEAR are used without surgical treatment in the same manner as CC hearing devices. One of the differences among the three devices is their fixation style. Both ADEHAR and CC hearing devices do not require a static force for fixation, making them different from conventional BC hearing aids.