Bone-conduction devices (BCDs) convert sound energy into skull bone vibrations, thereby directly stimulating the cochlea. They have been under development since the 1970s and are widely used in patients with conductive or mixed hearing loss (HL) (Mudry and Tjellström, 2011). Because patients with chronic otitis media or aural atresia have difficulty achieving adequate results with air conduction (AC) hearing aids, BCDs can be an effective treatment option. Moreover, BCDs are a treatment option for single-sided deafness (SSD) because they provide sound input to the contralateral cochlea via bone conduction (BC; Kitterick et al., 2016; Magele et al., 2019). Patients with SSD often have worse sound localization abilities and speech recognition difficulties in noisy and other challenging listening situations due to the loss of binaural processing (Welsh et al., 2004; Yu, 2020). The use of BCDs by patients with SSD improves speech perception and provides subjective benefits (Pfiffner et al., 2009; Wazen et al., 2010; Wesarg et al., 2013; Hampton et al., 2022; Huber et al., 2022). Their effects on speech discrimination in noise have been controversial and dependent on the signal setting and noise direction; however, a systematic review that assessed the efficacy of bone-anchored hearing aids (BAHAs) concluded they have favorable effects based on speech discrimination in noisy circumstances (Kim et al., 2017).

Asymmetric hearing loss (AHL) is a condition similar to SSD. AHL is defined as an interaural asymmetry of 20 dB hearing loss at two contiguous frequencies or 15 dB HL at any two frequencies from 2 to 8 kHz (Gimsing, 2010; Durakovic et al., 2019). For patients with hearing loss who have a large left–right hearing difference, a conventional AC hearing aid is often used in the better hearing ear. For that strategy, patients give up sound source input from the worse hearing ear. Wearing a BCD in the worse ear for patients with AHL may solve this problem. To date, only a few reports on the effect of BCD on patients with AHL have been published; some reports show more favorable results for patients with AHL when compared to patients with SSD (Monini et al., 2020), while others show a deteriorative effect in noise conditions (Schwartz and Kobylk, 2016).

Although these BCD studies of patients with AHL have yielded further insights, they have not been able to fully evaluate the contribution of BAHAs in daily life. In particular, evaluations of the effects of softer sounds, such as whispering voices, are lacking. Most previous studies fixed the sound pressure level of speech signals to a level similar to that encountered in everyday life (65–75 dB) to assess the effects of BCDs on word recognition. However, BCDs can further promote the effect of softer sound stimuli in AHL. Moreover, in some scenarios, listening comprehension is worsened by BCDs when noise is presented to the BCD-wearing side. It is important to know under what circumstances and to what degree hearing performance can be compromised.

This study investigated patients with asymmetric hearing loss, marked by severe to profound hearing loss in one ear with a difference of ≥45 dB from that in the contralateral ear, who underwent implantation on the side with worse hearing. This target group was established because patients with more significant left–right differences in hearing tend to be the ones who have to give up sound input from the worse-hearing ear. A word-recognition test was performed by sweeping the sound pressure from a low speech level (40 dB). Additionally, discrimination of speech in noise was analyzed by changing the directions of the noise and the signal speaker. The findings of this study can provide insights into the use of BCDs in patients with AHL and can be applicable to patients with SSD.

The present investigation assessed the efficacy of BAHAs in individuals with hearing impairment, specifically in those with asymmetric hearing loss. In this study, asymmetric hearing loss was characterized by severe to profound hearing loss in one ear with a difference of >45 dB between the hearing abilities of the left and right ear. In such conditions, patients tend to give up sound input from the worse-hearing ear. Among the 13 subjects included in this study, 4 individuals were already using a hearing aid in their better-hearing ear (as indicated in Table 1). For the monosyllable word recognition test, the tests were carried out without the use of hearing aids. This approach was specifically chosen to examine the pure effect of BAHAs on hearing capabilities in patients with asymmetric hearing loss. By comparison, the speech discrimination in noise test was conducted with the use of hearing aids. This decision was made to simulate more realistic everyday scenarios. These audiological tests ensure a comprehensive evaluation of the BAHA's impact, both in controlled environments without hearing aids and in more practical settings with the hearing aids in use. The WRS curves were depicted by changing the sound pressure of the word signal from 40 to 80 dB SPL, depending on the hearing ability of the patient. Although the maximum WRS did not change, the sound pressure at which the maximum WRS was achieved was significantly reduced with the use of the BAHA. A WRS of 60 dB SPL was observed in approximately 65% and 85% of the patients without BAHAs and with BAHAs, respectively (Figure 2A). While previous investigations typically assessed WRS using a fixed sound pressure for the presented word, Pfiffner et al. (2009) utilized varying sound pressures (50, 65, and 80 dB SPL) to measure WRS. Despite lacking specific details regarding these values, their study demonstrated that the use of BAHAs was associated with improved WRS, at low and high sound pressure levels. These findings are consistent with the results of our investigation.

The WRS was measured by presenting the word from the BAHA-wearing side, that is, the worse-hearing ear, to analyze the effects of the BAHA in a situation where the most preferred effect was expected (Figure 3). The sound pressure of the word presentation was varied from 40 to 80 dB SPL. The results showed that the WRS significantly improved with BAHAs over a wide range of sound pressures for the presented word. This suggested that BAHAs pick up sound sources from the worse-hearing ear side and help patients recognize soft to ordinary speech sound levels. These results also indicated that measuring the effects of BCDs by varying the sound pressure of the presented word signal is useful for ensuring a more objective analysis of how BCDs affect patient hearing in various situations. Few studies have investigated the effect of BAHAs on AHL, and the left–right difference in hearing observed in participants in previous studies has been smaller than that in the participants in this study. Monini et al. (2020) used interaural threshold difference ≧15 dB HL, and Bruschini et al. (2020) used interaural asymmetry of 20 dB hearing loss at two contiguous frequencies or 15-dB HL at any two frequencies from 2 to 8 kHz as a selection criterion for eligible patients. Consequently, this study scrutinizes and compares its findings with those of previous comparable studies on SSD, which is a similar condition.

In the analysis of the SRT in noise, no significant differences were observed in the original OLSA settings (Figure 4A). Although previous studies of patients with SSD have reported that BCDs improve the SRT when speech and noise are presented from the front, a simple comparison is difficult because of the different testing procedures conducted. For example, the hearing-in-noise test (Niparko et al., 2003), monosyllabic word list (Andersen et al., 2006), and simplified Hirsch tests (Saroul et al., 2013) have been used. A recent report assessing the effects of BONEBRIDGE, a transcutaneous BCD, in patients with SSD used the OLSA to test speech discrimination in noise and found results similar to those of this study (no significant difference between patients with and without BONEBRIDGE; Huber et al., 2022). In our study, we used extreme scenarios to assess the efficacy of the BAHA. In the scenario where the BAHA was most effective, speech was presented to the side containing the BAHA implant (the worse-hearing ear), and noise was presented to the better-hearing ear. The SRT improved significantly under these conditions. Conversely, for the scenario in which the BAHA could yield negative effects, noise was presented to the BAHA-wearing side (the worse-hearing ear), and speech was presented to the better-hearing ear. The SRT decreased significantly under these conditions.

A few reports have shown negative results with BAHAs for patients with SSD. Hol et al. (2010) reported that the SNR worsened with the BAHA when the signal was presented from the front and noise was presented from the BAHA-wearing side. The results obtained in this study suggest that the current BAHA system can negatively affect listening performance in certain situations. Whether the OLSA is suitable for measuring the effects of BAHAs under noisy conditions remains an issue that should be considered. Although discontinuous noise is applied in the OLSA, a relatively steady noise is considered to be applicable in real life. In the case of steady and continuous noise, the noise reduction function of a sound processor may be effective.

Both the total and subscale SSQ12 scores improved significantly with the BAHA. The mean differences in speech hearing, spatial hearing, and speech quality were 1.873, 0.7273, and 0.9394, respectively. Greater improvement in the speech-hearing subscale reflected WRS improvement. A meta-analysis of the effects of BCDs on the quality of life of patients with SSD concluded that BCDs improved the Abbreviated Profile of Hearing Aid Benefits and the SSQ but not the Health Utilities Index-3 (Hampton et al., 2022). In the same study, the mean difference was 2.03 for speech and 1.51 for spatial hearing, indicating a greater improvement than that obtained in the present study. Although objective assessments indicated that BCDs do not enhance sound localization in individuals with SSD (Kim et al., 2017), it is plausible that BCDs may affect spatial hearing ability, which cannot be measured using existing audiological evaluations.

This study had several limitations. First, BAHAs are not approved for use by individuals with SSD, which is the most common cause of severe asymmetric hearing loss, in Japan. Therefore, our findings cannot be directly compared with those of other SSD investigations. Second, the sample size was small; therefore, we were unable to perform subgroup analyses based on patients' hearing abilities. Moreover, not all tests could be conducted on every patient, further limiting the comprehensiveness of our study results. Finally, the subjective evaluation was insufficient. Only a simplified version of the questionnaire, the SSQ12, was used, which was inconsistent with an objective evaluation.

However, this study revealed the advantages of BAHA implantation in the worse-hearing ear while also highlighting the possibility of worsened hearing in noisy environments. These findings can assist clinicians in deciding whether surgical intervention is feasible, considering a patient's typical environment. It can also enable patient education regarding the potential negative consequences of BAHAs and permit them to adjust the device according to the situation. Nevertheless, additional research with a larger sample size is required to understand the effects of BAHA implantation in worse-hearing ears fully. This study examined the effects of a BCD in patients with asymmetric hearing loss who wore a BCD in the worse-hearing ear and the situations in which the device seemed beneficial and non-beneficial. We found that the BAHA device significantly improved patients' WRS under a sound pressure level of 40–70 dB SPL. Conversely, it may worsen the hearing performance in situations where noise is present on the BCD-wearing side. The findings of this study yield insights into the use of BCDs in patients with asymmetric hearing loss, including SSD. Moreover, this information could help patients comprehend the potential benefits and limitations of BCDs for their hearing.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The studies involving humans were approved by the Ethics Review Board of the University of Miyazaki (protocol code O-0702). The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants' legal guardians/next of kin because Informed consent was obtained in the following structured manner. Upon receiving approval from the Ethics Committee of the Faculty of Medicine at the University of Miyazaki, the research team disseminated detailed information about this clinical study via the university's website. This included: (i) the purpose and significance of the study, the subjects involved, and the methods employed, including the type of information utilized; (ii) the name of the implementing organization, the individual responsible for the study, and contact information for queries or complaints; (iii) the approach for managing personal information, encompassing aspects of information disclosure. This study information remained publicly accessible from the time of the Ethics Committee's approval until the conclusion of the study period. The lack of objections from study participants or their representatives was interpreted as implied consent. Opt-out Procedure: Participation in the study was entirely voluntary. In the event of a participant's decision to withdraw, all information gathered about that participant up to that point would be, as a general rule, destroyed. However, the data could be retained and used if consent was expressly given by the participant. After public dissemination of the data, such as through academic presentations or publications, destruction of the data is not feasible.

SK: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing—original draft, Writing—review & editing. AG: Conceptualization, Formal analysis, Investigation, Writing—original draft, Writing—review & editing. TN: Formal analysis, Investigation, Writing—review & editing. KK: Investigation, Writing—review & editing. ES: Investigation, Writing—review & editing. KM: Investigation, Writing—review & editing. KT: Investigation, Writing—review & editing. TT: Conceptualization, Writing—review & editing.