## 1 Introduction  
A hearing aid is a major instrument used to compensate for hearing disorders via amplifying sound. When hearing aids are used, an earplug is inserted into the ear canal to deliver sound. The coupling condition between the hearing aids and ears can influence audiological benefits, comfort, and fixation. When the earplug fits tightly to the ear, sound leak from the ear canal is reduced to suppress feedback problems. The output level from the transducer can be amplified enough to obtain sufficient benefits. The stable fixation of the earplug stabilizes the output. Unsuitable small earplug may cause feedback problems and their falling from the ear. In contrast, a tight earplug can induce sensations of fullness, pain, and skin injury. Ear shapes vary among patients. To achieve good conditions, a custom ear mold or shell is created for each patient instead of modular construction. To create the molds, the ear impression is first obtained via inserting a silicone material into the ear. After scanning the ear impressions, hearing aids are designed using specialized computer-aided design (CAD) software. This process is considered to be an established method. However, taking an ear impression involves some risks, such as the possibility of external and middle ear trauma or foreign bodies entering the canal and middle ear [1; 2; 3]. As an alternative method,three-dimensional (3D) laser scanning was developed [4], which avoids the risks related to taking the ear impression. However, this method is not popular in clinical practice. In some cases, it is difficult to achieve sufficient benefit from hearing aids. A representative example of this is aural atresia. Some patients cannot wear hearing aids because of their atretic ear. Furthermore, atretic ears often accompany microtia. A modular construction is hardly fitted to such ears, and a custom ear mold is required for the fitting. However, most cases cannot achieve sufficient benefits due to poor gains or feedback problems [5]. Instead of air conduction (AC) devices, bone conduction (BC) devices are used in these patients. Conventional BC hearing aids are the most common. The transducers of BC hearing aids are placed on the skull bone, such as the mastoid, to deliver the signals. This conduction form is effective in ears with aural atresia and microtia. Unfortunately, high contact pressure against the skull bone is required for the device to function properly. This fixation style can cause skin irritation, prolonged skin depression, and discomfort [5; 6]. An esthetic problem owing to the headband is another issue. Recently, a new type of BC device--ADHEAR (MED-EL, Innsbruck, Austria)--was used in clinical practices. This device is placed on the mastoid via attachment to the adapter stuck to the skin behind the ear and can overcome the disadvantages of conventional BC hearing aids derived from its fixation style. Unfortunately, the signals may be deteriorated because the transducer is fixed without a static force. The recommended fitting range is \(\leq\)25 dB HL [7]. Alternative options in atretic ears are implantable devices, such as bone-anchored hearing aids (BAHA; Cochlear Limited, Sydney, Australia), Bonebridge (MED-EL), and Vi-brant Soundbridge (MED-EL) [5; 7; 8; 9]. As the transducers of these devices are not fixed with high contact pressure, most of the demerits in conventional BC hearing aids are resolved. Furthermore, the signals are delivered directly to the skull bone, not percutaneousally, which can improve the audiological benefits [7]. In contrast, these devices require surgery, which is disadvantageous to conventional hearing aids. Upward of 20% of patients who undergo BC device implantation experience complications, such as tissue overgrowth, skin infection, and persistent pain at the abutment site [10]. Cartilage conduction (CC) hearing devices are a new option for atretic ears [11; 12; 13; 14; 15]. This device delivers sound to the aural cartilage using a small lightweight transducer. It can be used without static force using a headband.