## 1 Introduction  
Cartilage conduction hearing aids (CC-HA) or bone conduction hearing aids are the treatments of choice to improve hearing in patients who cannot use air conduction hearing aids (AC-HA) because of aural atresia or continuous otorhea [1, 2]. In regards to bone conduction hearing aids, the conventional eyeglass and headband types are large and are problematic in terms of aesthetics, as well as comfortability due to the compression of the vibrator [3]. In recent years, however, bone conduction implants such as bone anchored hearing aids (Baha; Cochlea Corp., Sydney, NSW, Australia) [4] and bone conduction hearing aids with an adhesive adapter sticking to the skin, such as the ADHEAR system (MED-EL, Imsbruck, Austria), are available in Japan, and the range of choices for the above cases are broad.  
CC-HA is a device that utilizes cartilage conduction which has been in use in Japan since 2017 [5]. Cartilage conduction is the third type of hearing conduction pathway proposed by [PERSON] [6], followed by air and bone conduction [7]. It is preferred in patients who cannot use AC-HA and whose hearing level cannot be improved with appropriate surgery, and moreover, there are no restrictions on hearing level. Aural atresia is a good indication for CC-HAs and they are also expected to be effective and preferred in patients with persistent otorhea due to chronic otitis media [8]. We previously reported that CC-HA is effective in pediatric cases [9]. We also reported that CC-HA enabled a novel noninvasive therapeutic procedure for microtia patients using 3D printing technology without reducing hearing gain [10].  
Baha is preferred in patients with bilateral hearing loss, with bone conduction hearing levels of 55 dB HL or less in at least one ear, and with difficult or inadequate hearing improvement using either air, bone, or cartilage conduction hearing aids [11]. The qualifications for Baha are external auditory canal atresia, continuous otorrhoea from the external and middle ear, and cases in which hearing improvement cannot be expected or cannot be obtained with appropriate surgery. It is also an option for those in which the contralateral side is deaf or has severe hearing loss, so as to avoid the risk of complications of hearing loss due to surgery. Baha requires surgery for placement of the implant in the temporal bone, however, patients can try the device to know how it hears without surgery using a sound arc.  
ADHEAR is a new bone conduction hearing device consisting of a thin adhesive adapter and a sound processor. The adapter sticks to the skin behind the ear and the sound processor clicks onto it. Because ADHEAR is a non-implanted device, it does not require surgery. There is work recommending ADHEAR to patients who cannot or do not want to undergo surgery [12]. It also overcomes the problems of aesthetics and comfortability associated with conventional bone conduction hearing aids. Furthermore, ADHEAR is small and can be discreetly placed behind the ear where it does not exert pressure on the skin. Additionally, ADHEAR's volume can be changed by turning the dial. Although it is simple, there is the problem that fine adjustments of the hearing gain is not possible in Japan compared to other devices.  
There are many overlaps in the characteristics for conduction hearing aid devices, and it is difficult for otologists and audiologists to objectively distinguish the indications based on patient-side factors such as hearing levels and disease conditions. In our department, we conducted a comparative trial of CC-HA, Baha, and ADHEAR, and provided patients with an opportunity to select conduction hearing aids, based on hearing gain and subjective evaluation. In this study, we evaluated the performance of these devices, the likelihood of subjectively reported improvement and examined the necessity of a comparative trial.