Does bone conduction overclosure correlate with speech recognition in bone conduction devices?—a systematic review with narrative synthesis

Introduction

Bone conduction devices (BCD) have drastically advanced in the last 10–15 years. They are mainly indicated for rehabilitation in conductive or mixed hearing loss (MHL) or single sided deafness (SSD) in sensorineural hearing loss (SNHL) (1-4).

Sound can be conducted to the inner ear by two main mechanisms, air conduction (AC) and bone conduction (BC). AC is commonly referred to as the normal hearing mechanism where air-borne sound is conducted from the outer ear, through the bones in the middle ear and to the cochlea in the inner ear. In BC, sound is conducted through the skull bones with vibrations to the cochlea. The concept of BC is used in BCDs (5). BCDs are indicated in cases where conventional hearing aids are unsuitable, such as in patients with chronic otorrhoea, a history of ear surgery, ear canal stenosis or atresia, or congenital ear malformations (6).

There are many different ways in which the mechanical vibrations of the skull bones can be transmitted to the cochlea. Based on this, BCDs are categorised into direct drive with a solid coupling to the bone, such as Ponto (Oticon, Denmark), BAHA^® (Cochlear^TM, Sweden), Osia (Cochlear, Australia) and Bonebridge (Medel, Austria), and skin drive with a soft tissue interface between the device and bone, such as Adhear (MED-EL, Austria), Softband (Cochlear, Australia), Baha SoundArc (Cochlear, Australia), BahaAttract (Cochlear, Australia), Sophono (Sophono Inc., Austria) (7).

Direct-drive devices are further classified into two categories: percutaneous systems, which use an abutment (e.g, BAHA, Ponto), and active transcutaneous implants, where the active component is implanted under the skin and the external sound processor is magnetically coupled across the skin [e.g, Bonbridge, Osia and Sentio (Oticon, Denmark)] (7).

Osia and Bonebridge are the most common transcutaneous BCDs used currently. Both devices offer the advantage of aesthetics, fewer skin complications, a better safety profile, and also better tolerance in the pediatric population (8,9).

Recent studies have shown Osia performing better in mid and high frequencies (in terms of audiological gain) and better speech discrimination in diverse environments (10). However, the literature is inconsistent due to small sample sizes and there are still ongoing studies for these devices.

BAHA, BAHA attract and Ponto are the frequently used percutaneous BCDs. They offer higher maximal output, better magnetic resonance imaging (MRI) compatibility and reduced feedback (11,12).

The maximum power output (MPO) of a BCD is the strongest output level the device can produce (13). This is important in terms of dynamic range of sound for the patient, better speech recognition, especially in a noisy environment, providing enough head-room if the hearing worsens with time, and overall better audiological and subjective outcome for the patient (14,15).

There are different parameters that influence the MPO, like the position/placement of the device, type of device used (percutaneous versus transcutaneous) (16-19). MPO can be assessed using various techniques, including skull stimulator (20), probe microphone (13,21), surface microphone (22), and more recently, through clinical data combining direct patient measurements with audiometric BC thresholds (23). The MPO is typically determined by gradually increasing the input signal to the device until the output reaches its maximum level without distortion or clipping (13). It is a fixed device specification and is measured by the manufacturer or a researcher. A recent study has shown that improving the MPO improves speech recognition (14).

This study aims to evaluate whether a clinical parameter can effectively replace MPO in illustrating the performance of a BCD. Specifically, we investigate BC overclosure—defined as an aided threshold better than the pre-operative BC threshold—as a potential surrogate marker. By examining the correlation between overclosure and speech recognition outcomes, we explore the feasibility of using overclosure as a clinical alternative to MPO in assessing BCD efficacy.

We will review the literature from the last 15 years to analyze the audiological outcomes for different BCDs. The data obtained will be analyzed with regard to BC overclosure in the 500, 1,000, 2,000 and 4,000 Hz frequencies, and the average pure tone thresholds, and correlated with the speech recognition. We aim to analyze if better BC overclosure translates to improved speech recognition and hence will that help to choose the BCD that is appropriate for a patient.

Methods

The study adhered to the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines (available at https://www.theajo.com/article/view/10.21037/ajo-25-12/rc) (24).

The selection criteria were established using the Patient/Population, Intervention, Comparison, Outcome (PICO) framework, including:

• Participants of any age with unilateral or bilateral conductive hearing loss (CHL), MHL, or SNHL, such as SSD;
• Use of BCD including ADHEAR, Bonebridge, BAHA^®, Osia, Ponto, Softband, Sentio and Sophono;
• Evaluation of audiological outcomes before (unaided) and after (aided) surgery;
• Subjective and objective hearing assessment, including pure tone audiometry, sound field threshold (SFT), word recognition score (WRS) and patient-reported outcome measures (PROM);
• Retrospective and prospective cohort studies, randomized controlled trials (RCTs) and case series.

The study excluded non-English publication, review articles, case reports with fewer than 5 patients, a minimum follow-up period of less than three months, studies with insufficient data, and non-human experimental research.

A comprehensive literature search was conducted across major medical databases, including PubMed, Embase, MEDLINE, and the Cochrane Library. MESH terms including hearing loss, hearing aids, BC, hearing tests, speech perception and audiometry were used in search strategy (Appendix 1). The review period spanned 15 years, from October 6, 2009 to October 6, 2024, and utilized terms related to various BCDs and associated terminology.

The systematic review process utilized Covidence, a specialized data management software, for screening and automatically removing duplicate records. Two independent researchers (S.R., H.M.N.) reviewed the titles and abstracts, followed by a thorough full-text evaluation to ensure adherence to the predetermined inclusion and exclusion criteria. Data extraction was conducted on a pre-designed spreadsheet, and any discrepancies between the two authors were resolved through discussion or consultation with a third reviewer (J.M.G.).

The extracted data included research design, patient demographics, type of BCD, and audiological outcomes, as well as patient-reported quality assessment or questionnaire data. When precise figures were unavailable, data were derived or obtained from graphs or supplementary materials.

The statistical analysis was conducted using the Jamovi software (H.M.N., S.R.). It included descriptive statistics, independent sample t-tests, and correlation analyses using Spearman’s rank-order correlation and linear regression.

The level of evidence (LoE) was assessed using the Centre for Evidence-Based Medicine (CEBM) guidelines. For non-randomized studies, two independent reviewers (S.R., H.M.N.) conducted a risk of bias evaluation using the ROBINS-I tool. Sensitivity analysis was carried out to eliminate studies with a high-risk of bias from the data synthesis.

Results

The result of the systematic review is shown in Figure 1. The initial search resulted in 331 studies (PubMed, 113; Embase, 101; MEDLINE, 59; Cochrane, 58). A total of 89 duplicates were identified automatically via Covidence and manual screening. A total of 242 studies underwent abstract screening, and 97 studies were subsequently selected for full-text screening. After the final screening, 18 studies met eligibility criteria and were included in the systematic review. A total of 459 patients were obtained from the included studies.

Figure 1 PRISMA diagram. PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-analysis.

Study and patient characteristics

Patient demographics, type of hearing loss and type of device used are summarised in Table 1. There were 13 cohort studies and 5 case series included in the systematic review. Sprinzl [2023] included results for two patient cohorts (adult and pediatric) with CHL/MHL and third cohort with SSD, and thus were treated as three separate cohorts on the data summary. A total of 10 studies were prospective, 7 studies were retrospective and 1 study was both prospective and retrospective. There were 6 types of BCDs, including 9 studies on Bonebridge, 1 study on ADHEAR, 4 studies on Osia, 2 studies on BAHA Attract, 1 study on Ponto and 1 study on BAHA 5SP (Cochlear, Australia). The majority of patients have CHL or MHL (n=351, 76.5%), while the remainder have SSD (n=108, 23.5%). The mean age is 39.9 years [standard deviation (SD) 15.2]. Indication of BCD varied across studies, most commonly included chronic otitis media, aural atresia, cholesteatoma, sudden onset SNHL and infection as the most common causes.

A total of 8 studies focused on CHL and MHL (8/18) whilst 2 studies focus on only SSD (2/18), remaining 8 studies, including both CHL/MHL and SSD. All cohorts report on audiological outcomes and 13 studies report patient-reported outcomes using the rating system, such as Abbreviated Profile of Hearing Aid Benefit (APHAB) and Bern Benefit Single-Sided Deafness (BBS). The median follow-up period was 6.5 months [interquartile range (IQR): 3–12 months, range: 3–36 months].

Audiological outcomes

The reviewed studies reported audiological outcomes, with the pure tone average across 0.5, 1, 2, and 4 kHz (PTA4) used as the measure. Table 2 summarized the preoperative and postoperative pure tone average (PTA4—average of hearing threshold across 0.5, 1, 2 and 4 kHz) for AC, BC, and SFTs.

For Bonebridge devices, the majority of patients had conductive or MHL, with a mean pre-operative AC PTA4 of 61.6 (SD 5.3) dB hearing level (HL) (25-27,29-33). Four studies included patients with single-sided deafness, most of whom had non-measurable preoperative air and BC, while two studies reported a mean AC PTA4 of 96.7 (SD 0.8) dB HL (25,35).

The studies on Osia devices included both CHL/MHL and SSD patients. The mean pre-operative AC PTA4 for the CHL/MHL patients was 56.5 (SD 7.0) dB HL (35-38). Only one study reported the pre-operative BC PTA4 for SSD patients, while the other was non-recordable (36).

Table 2 also provides the pre-operative AC and BC values for the ADHEAR, BAHA Attract, Ponto, and BAHA5SP devices. Across all devices, the pre-operative and post-operative AC and BC thresholds did not change significantly, suggesting that BCDs do not impact the patient’s residual hearing ability. A significant improvement in SFT was observed post-implantation. The functional gain, measured by comparing unaided and aided SFTs, was on average 30.6 (SD 7.3) dB for Bonebridge users with conductive MHL (CMHL), but showed substantial variation (2.4–73.3 dB HL) among SSD patients, as SSD is not a common indication for Bonebridge. The mean functional gain was 28.5 (SD 1.7) dB HL for Osia, 23.4 (SD 7.1) dB HL for BAHA Attract, 36.5 dB HL for Ponto, and 42 dB HL for BAHA5SP.

Audiological measurement at specific frequency (0.5, 1, 2, and 4 kHz) and bone overclosure

The pre-operative BC thresholds and post-operative SFT at individual frequencies were summarised in Table 3. Studies with only single-sided deafness cohorts were excluded, as there were no measurable pre-operative BC values in the affected ear. Bone overclosure was calculated by subtracting the pre-operative BC of the implanted ear from the post-operative aided SFT. This was done for the pure tone average values as well as for specific frequencies (0.5, 1, 2, 4 kHz).

The mean bone overclosure was 14.5 (SD 5.0) dB HL for Bonebridge and 10.43 (SD 5.2) dB HL for Osia. For the other devices, there was an insufficient number of studies or the available information was inadequate to calculate overclosure, and they were not included in the analysis. An independent sample t-test revealed no significant difference in the mean bone overclosure between Bonebridge and Osia studies (P=0.22, 95% CI: 2.6–11.6). Similarly, the bone overclosure across the four individual frequencies (0.5–4 kHz) showed no significant variation among the devices (P=0.65, 95% CI: −11 to 15.4)) with Bonebridge (24.5 dB at 0.5 kHz, 13 dB at 1 kHz, 5 dB at 2 kHz, 13.5 dB at 4 kHz) and Osia (22.5 dB at 0.5 kHz, 10.4 dB at 1 kHz, 3.6 dB at 2 kHz, 12.7dB at 4 kHz).

WRS

Various methods, including WRS, speech reception threshold, and speech-to-noise ratio, are commonly used to assess speech perception. This diversity can make it challenging to correlate and compare speech reception across different devices. In this study, we utilized WRS in quiet at 65 dB sound pressure level (SPL) as the standard measure, as it was the most frequently reported metric across the included studies. The change in WRS was calculated by subtracting the pre-op unaided WRS from the post op aided WRS.

The mean improvement in WRS in quiet at 65 dB SPL was 56.3% (SD 17.0) in Bonebridge (28-33) and 49.3 % (SD 8.9) in Osia (35,38). Table 3 lists the values for other devices with single-included studies.

Correlation between bone overclosure and WRS

The study aimed to investigate whether better bone overclosure correlates with improved WRS and, consequently, speech perception. Due to the limited dataset available for Osia to calculate bone overclosure, Spearman’s correlation and linear regression analysis were performed only on the available studies for Bonebridge.

The scatterplot (Figure 2) suggests a positive linear relationship between bone overclosure and WRS change, indicating that as bone overclosure increases, WRS change also increases. Spearman’s rho is 0.61, indicating a moderately strong positive relationship. However, this correlation is not perfectly linear and is not statistically significant at the 5% level (P=0.17, 95% CI: 0–0.94). The linear regression analysis also suggests limited evidence to support the hypothesis that increased bone overclosure is associated with greater improvement in WRS in Bonebridge device [t(5) =1.93, P=0.11, 95% CI: −0.2 to 0.94]. Additionally, the average bone overclosure accounts for 42.8% of the variability in the change in WRS.

Figure 2 Scatterplot of correlation between bone overclosure (dB) and WRS change (%). WRS, word recognition score.

While the correlation analysis implies a potentially meaningful trend, the findings are constrained by the small sample size and high variability in the data.

PROM

PROM were collected in 10 studies [5 for Bonebridge (25-27,29,30), 3 for Osia (35,36,38), 1 for Baha Attract (39), and 1 for Ponto (41)], encompassing 221 patients. The common PROM instruments utilized included Abbreviated Profile of Hearing Aid Benefit; Bern Benefit Single-Sided Deafness; Speech, Spatial and Qualities of Hearing Scale (SSQ) and Hearing Device Satisfaction Scale (HDSS), as summarised in Table 4. However, the heterogeneity in PROM reporting and measurement approaches across studies presents challenges in pooling results for analysis. The mean APHAB score improvement was 24% (SD 1.41) for Bonebridge and 26.2% (SD 19.4) for Osia users. The mean HDSS Score was 90.3% (SD 7.42) for Bonebridge users. The mean SSQ score was 6.1 (SD 9.78) for Osia and 6.2 (SD-not available) for Ponto users. Overall, the findings indicate positive outcomes across various BCD types, though the lack of standardization in PROM reporting makes it difficult to assess the relative superiority of the device.

Risk of bias assessment

The risk of bias assessment using the ROBIN-I tool (43), as summarized in Figure 3, highlighted limitations across the included studies. The analysis revealed that 4 out of 18 studies exhibited a low-risk, 10 out of 18 demonstrated a moderate risk, and 4 out of 18 had a serious risk of bias. Traffic light plots were created using the Robins tool to represent these findings visually. Overall, a moderate risk of bias was prevalent in most studies, commonly attributed to confounding factors, missing data, issues with outcome measurement, and selective reporting. It was noted that four studies (29,36,40,42) were at serious risk due to substantial missing data, which may have been related to patients’ inability to complete audiological testing during the follow-up period. The inclusion and exclusion criteria were generally well-documented across the studies, and the interventions were typically described in adequate detail. Additionally, the hearing assessments were administered to appropriate age cohorts and employed culturally validated linguistic instruments for evaluating speech performance.

Figure 3 ROBINS-I tool risk of bias assessment tool for non-randomized observational studies.

Table 1 summarizes the evidence levels based on the CEBM. Due to the ethical and surgical intervention nature of BCDs, there were no RCTs included in this review. The highest LoE was provided by CEBM level 3 prospective cohort studies, while the remaining studies were CEBM level 4 due to their retrospective nature and small sample sizes. The lack of RCTs is a common limitation in the field of surgical interventions, where ethical considerations make it challenging to conduct such trials.

Discussion

This systematic review and narrative analysis aimed to evaluate the audiological performance and the association between BC overclosure and speech recognition outcomes of BCDs. The review examined 18 studies encompassing 459 patients with conductive, mixed, and single-sided hearing loss. The findings indicate that various BCD systems, such as Bonebridge, Osia, BAHA Attract, Ponto, ADHEAR, and BAHA5SP, have demonstrated significant improvements in audiological measures and patient-reported outcomes.

Audiological performance

The Bonebridge has been extensively investigated, demonstrating marked improvements in hearing and speech discrimination (28). Audiological thresholds exhibited significant enhancements post-implantation, with mean functional gains ranging from 30.6 dB HL and WRS increases of 56.3% in quiet at 65 dB SPL, corroborating findings from the Seiwerth [2022] (33) and Sprinzl [2023] (31) studies.

Similarly, the Osia system showed substantial benefits, achieving a mean functional gain of 28.5 dB HL, consistent with the outcomes reported by Key [2024] (44). WRS improvements averaged 49.3% for Osia users.

Furthermore, the BAHA and Ponto systems provided effective audiological improvements, with the BAHA5SP demonstrating a mean functional gain of 42 dB HL and a phoneme score exceeding 70% in cases of severe-to-profound hearing loss (42). The Ponto system also exhibited significant WRS gains, particularly in SSD cohorts (41). The ADHEAR system, although non-invasive, showed benefits primarily in CHL cases but had limited impact on SSD patients due to inherent device limitations (34).

However, some variability in performance was observed, potentially related to factors such as bone condition, skin thickness, and implantation technique (1,31,45).

Patient-reported outcome

The PROM findings, including APHAB and SSQ, indicated consistent patient satisfaction with the evaluated BCDs. Bonebridge users exhibited a 24% (SD 1.41) mean improvement in APHAB scores (27,29), and Osia users reported a 26.2% enhancement (35). Similarly, studies on the BAHA Attract and Ponto systems revealed positive outcomes in speech perception and quality of life (37,41). Nonetheless, the heterogeneity in PROM approaches and scoring across the included studies impedes the ability to directly compare the relative performance of the various BCD types.

Correlation between bone overclosure and WRS

The analysis examined the relationship between BC overclosure, which is the difference between pre-operative BC and post-operative aided sound-field thresholds, and speech perception outcomes. Although the findings suggested a potential positive correlation between BC overclosure and speech recognition performance, indicating that better bone overclosure was associated with improved WRSs, the correlation was not statistically significant at the 5% level. This was likely due to the limited dataset available for analysis and the high variability in the data, which constrained the statistical power to detect a meaningful relationship.

The mean bone overclosure was 14.5 (SD 5.0) dB HL for Bonebridge (27-33) and 10.4 (SD 5.2) dB HL for Osia (35,37,38). While a moderate positive correlation was observed between bone overclosure and WRS improvement, the results were not statistically significant, again reflecting the limited dataset and variability. The review focused primarily on the Bonebridge and Osia BCDs, as they had the most comprehensive data available for analysis. The lack of robust data on other BCD types limited the ability to draw comparative conclusions about their relative performance.

Existing literature also explores methods to predict suitable BCDs for patients. Hua et al.’s prospective study demonstrated that the MPO of the BC implant influenced speech recognition in quiet and in noise (46). High MPO implants showed a 10.5% mean improvement in WRSs in quiet, and 81% of subjects rated the sound quality as significantly better (46). Wimmer et al. found that both aided SFTs and word recognition correlated with the preoperative BC thresholds of the better hearing ear, but no such correlation was observed for the poorer ear (47). Additionally, Ghoncheh et al. presented a method for predicting the maximum output of a transcutaneous bone conductor using patients’ audiometric data, utilizing only the audiometric BC threshold and direct BC threshold to determine the frequency-specific and side-specific maximum output HL (23).

In our study, we hypothesized that bone overclosure may predict the maximum output of the implant by correlating it to the WRS. However, the statistical analysis revealed that the BC overclosure does not impact the WRS and the outcome of the device. Further research with larger, more robust samples is needed to clarify the relationship between bone overclosure and speech perception outcomes.

Risk of bias and limitations

The majority of included studies were cohort-based, with only five case series. While prospective cohort designs (10 studies) provided the highest LoE in this review, the absence of RCTs remains a limitation. The ROBIN-I tool identified moderate risks of bias in most studies, attributed to confounding factors, missing data, and selective reporting. Four studies exhibited serious bias due to incomplete audiological testing during follow-up periods.

Despite these limitations, the findings strongly support the use of BCDs as effective and safe options for auditory rehabilitation in CHL, MHL, and SSD. Further research with larger, more robust samples is needed to better clarify the nature and strength of the association between BC overclosure and speech perception outcomes. Standardization of outcome measures and follow-up assessments would also strengthen the comparability and quality of evidence across studies.

The data obtained from these papers were limited, as the overclosure values were only reported for combined conductive and MHL cases, and data for individual hearing loss types (e.g., pure conductive, pure sensorineural) were not available from the included studies. This restricted the ability to assess whether the relationship between BC overclosure and speech recognition outcomes differed based on the underlying hearing loss pathology. Obtaining separate data for the various hearing loss classifications would have allowed for a more nuanced analysis of the potential predictive value of bone overclosure measures. The lack of granular data stratified by hearing loss type represents a key limitation of the current evidence base, which constrained the depth of the statistical analysis and conclusions that could be drawn.

Conclusions

This is the first systematic review and narrative analysis to investigate the correlation between BC overclosure and speech recognition outcomes with BCDs. The BC overclosure parameter represents an indirect measure of the power capability or maximum output of the device, which could potentially influence the user’s speech perception performance. While previous studies have examined the relationship between preoperative BC thresholds and postoperative WRSs (30), this review represents the first attempt to directly assess the association between the degree of bone overclosure and the corresponding speech recognition results.

This review identified several key limitations that constrain the generalisability and robustness of the findings. The heterogeneity in study methodologies, outcome measures, and reporting standards hindered effective pooling of data for analysis. Additionally, the exclusion of non-English studies may have introduced language bias, potentially omitting valuable evidence from diverse populations. This is particularly relevant for BCDs, as speech understanding and perception are closely tied to the patient’s native language and cultural context. Despite these limitations, this review provides a comprehensive summary of the current evidence on the audiological and functional outcomes of BCDs, including the first examination of the relationship between BC overclosure and speech recognition performance. More data with AC-aided thresholds should be published, and we believe this will help audiologists to refine their audiological criteria/indications for BCDs.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the PRISMA reporting checklist. Available at https://www.theajo.com/article/view/10.21037/ajo-25-12/rc

Peer Review File: Available at https://www.theajo.com/article/view/10.21037/ajo-25-12/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://www.theajo.com/article/view/10.21037/ajo-25-12/coif). J.M.G. receives consultancy fees for workshops, R&D, expert opinions from Cochlear Ltd. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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