Equivalent threshold force levels for skin drive bone conduction hearing devices

Abstract

Introduction:

Bone conduction hearing devices (BCDs) are essential for individuals with conductive or mixed hearing loss who cannot use traditional air conduction devices. Prescriptive methods for skin drive BCDs have recently been developed, taking into account skin and soft tissue transmission loss. Equivalent Threshold Force Levels (ETFLs) are needed to define normative references and enable accurate prescription and verification yet are not well-established for skin drive BCDs.

Methods:

In this study, in-situ thresholds were measured in 20 normal-hearing young adults using three commercially available skin drive BCDs, and corresponding force level outputs were recorded with a skull simulator to derive ETFLs.

Results:

In-situ thresholds were significantly higher than audiometric bone conduction thresholds and varied by 2.7–15 dB across devices. After conversion to force levels, variability in measured thresholds between devices decreased but remained significant at select frequencies, with discrepancies of up to 17 dB.

Discussion:

These results indicate that ETFLs can be defined for skin drive BCDs, although device-specific differences are observed. Establishing ETFLs provides a foundation for standardized prescription and verification methods, but may need to incorporate device-specific properties.

1 Introduction

Bone conduction hearing devices (BCDs) offer an innovative solution aimed at improving audibility for individuals with conductive or mixed hearing loss who are unable to wear traditional air conduction (AC) devices. Several forms of BCDs have been developed and can be worn on the skin (transcutaneous or skin drive) or on an abutment through the skin (percutaneous). For further details on these devices, terminology, structure, and consensus, reviews are available (Maier et al., 2022; Reinfeldt et al., 2015). Briefly, in percutaneous devices, the surgically implanted abutment penetrates the skin and is fixed to the mastoid bone of the individual's skull. For surgically-implanted transcutaneous BCDs, a magnet is placed underneath the skin that is active or passive and the bone conduction processor is placed over intact skin on the mastoid and held in place by the magnet. Although surgical BCDs can offer advantages over non-surgical BCDs such as improved high frequency output (Ellsperman et al., 2021), they may not be suitable for some adults and older children due to surgical candidacy criteria or personal preference. Additionally, younger children (i.e., under about 5 years of age) do not yet have the necessary skull thickness for surgical placement of currently-available BCDs (Bagatto et al., 2022; Gordey and Bagatto, 2021; Ricci et al., 2011; Snik et al., 2005; Willenborg et al., 2023). In such cases, non-surgical skin drive BCDs coupled to a soft headband or adhesive and worn on the skin around the mastoid bone are an effective alternative for both adults and children. At this time, Cochlear BAHA, Oticon Ponto, and Med-EL ADHEAR systems are several commercially available devices that offer such alternatives.

Prescriptions and verification are components of best practice protocols for hearing aid fitting (Audiology Practice Standards Organization, 2021, 2022). Procedures for fitting air conduction hearing aids are well-established and include threshold-based prescription, followed by verification and fine tuning using real-ear measurements. Procedures for fitting BCDs have, in the past, not had available technology for prescription and verification, and has therefore used audiometric bone conduction thresholds to estimate candidacy, used aided threshold testing to quantify audibility, and used parental/patient feedback to crosscheck device settings (Gordey and Bagatto, 2021; Maier et al., 2022). These approaches are now starting to be replaced with objective techniques for in-situ audiometry with the BCD, and verification using hearing aid analyzers. These objective techniques require specific measures and transforms which have been incorporated into a prescriptive method for use with percutaneous BCDs (Desired Sensation Level [DSL] BCD v1.1: Hodgetts and Scollie, 2017). The skin drive, non-surgical BCD category cannot be used with the DSL BCD v1.1 prescription due to the expected signal transmission loss to the cochlea created by the skin and soft tissue layers under the skin drive BCD (Kurz et al., 2014). Several studies have documented large differences in output when BCDs are worn on the skin compared to an abutment, while others have raised this as an issue for further development in prescription and verification (El-Naji et al., 2024; Hodgetts and Scollie, 2017; Gascon et al., 2022; Kurz et al., 2014; Mattingly et al., 2015; Wróbel et al., 2023). Large-scale clinical studies, foundational studies with Bekesy audiometry, cadaveric studies using laser doppler vibrometry, and finite element models also indicate threshold differences from different coupling methods (Beros et al., 2022; Chang and Stenfelt, 2019; Farahmandi et al., 2022; Håkansson et al., 1984). These studies have established clear differences in threshold levels across different coupling to the head, such as percutaneous and skin drive, and link these differences to sound transmission through soft tissue and skull. Clinically, this is important in two ways: (1) the clinician can expect threshold differences based on device type; and (2) with recent equipment, the clinician can measure output force level differences between devices and at threshold using commercially-available verification equipment such as a skull simulator. The purpose of the current study was to measure force levels at threshold across brands of skin drive BCDs, in comparison to force levels at thresholds measured in past studies of percutaneous devices, vs. those measured with a commercially-available skull simulator.

2 Threshold transforms

Prescriptive algorithms for air conduction hearing aids use a combination of Hearing level (HL)-referenced audiometric thresholds and their corresponding Reference Equivalent Threshold Sound Pressure Levels (RETSPLs). RETSPLs can be added to audiometric air conduction hearing thresholds to convert to the sound pressure level (SPL) scale, often in a coupler (see review by Bagatto et al., 2005). This supports comparing an individual's hearing threshold to the levels associated with normal hearing, and also facilitates visualization of thresholds on an SPL-referenced graph (i.e., SPLogram; Seewald et al., 2005) for use during verification with real-ear measurement.

For skin drive BCDs, the fitting may not be based on audiometric thresholds in HL, because the force tension of the skin-driven system differs from the force tension of an audiometric headband. Instead, clinical guidelines recommend in-situ threshold measurements using the individual's BCD, in follow-up to audiometric bone conduction thresholds, as part of a complete fitting protocol (Bagatto et al., 2022). The hearing aid settings are fitted using the device-specific in-situ thresholds, since they incorporate the effects of skin, soft tissue, and device characteristics (Mattingly et al., 2015; Stenfelt, 2012; Wróbel et al., 2023). This is of specific relevance to infants and young children who are more likely to be fitted with skin drive BCDs and who need early and appropriate access to sound (Bagatto et al., 2022).

With the recent availability of clinical tools and procedures, in-situ hearing thresholds can now be measured using device-specific software (Gascon et al., 2022). Also, BCD verification tools have now been developed. Specifically, two methods have been proposed to measure the skin transmission loss: (1) using a surface microphone (Hodgetts et al., 2018); or (2) using a commercially-available skull simulator (Håkansson and Carlsson, 1989). The use of either of these strategies allows quantification of specific transforms that may aid in developing prescription and verification strategies for skin drive BCDs. These include specific transforms for threshold data, discussed below.

3 Threshold force levels

Similar to RETSPLs for air conduction hearing aids, Reference Equivalent Threshold Force Levels (RETFLs) are used to quantify hearing thresholds against normative values for BCDs. RETFL values, across frequency, represent the average force level (in dB re: 1 μN) needed to reach the average hearing threshold levels of normal-hearing listeners. Standard RETFLs for audiometry support calibration of audiometric bone conduction transducers and are designed for use with audiometric bone oscillators (International Standards Organization, 2016). When RETFLS or RETSPLs are not available for a specific device category, Equivalent Threshold Levels in either force (ETFL) or sound pressure level ETSPL, (e.g., Folkeard et al., 2019) can be gathered to begin defining their characteristics. For example, ETFL values have been published for percutaneous BCDs and referenced to a TU-1000 skull simulator (e.g., Carlsson et al., 1995; Carlsson and Håkansson, 1997), which is helpful for defining in-situ thresholds and prescriptive targets for this device category.

Transform procedures for converting clinical audiometric thresholds from the dB HL scale to either dB SPL or force level have been defined for use in prescribing both air conduction and bone conduction hearing aids (Bagatto et al., 2005; Hodgetts and Scollie, 2017). For example, the air conduction procedures combine the standard RETSPL values with a 2cc-referenced real-ear-to-coupler difference (RECD), if insert earphones are used for audiometry. Similar transforms for use with bone conduction include the real-head-to-coupler difference (RHCD), which references percutaneous force levels against force levels on a skull simulator. Equations 1 and 2 contrast how conceptually similar transforms are used for both air conduction devices and percutaneous BCDs within DSL prescriptions.

Air conduction devices

Abbreviations: dB SPL, decibel sound pressure level; dB HL, decibel hearing level; RETSPL, reference equivalent threshold sound pressure level; RECD, real-ear-to-coupler difference.

Bone conduction devices (percutaneous)

Abbreviations: dB FL, decibel force level; ETFLdbc, equivalent threshold force level direct bone conduction; RHCD, real-head-to-coupler difference.

However, ETFLs for skin drive BCDs are less well-defined and may be device-specific. Development of ETFLs can follow a standard procedure (International Standards Organization, 2009) in which in-situ detection thresholds are measured on a group of normal hearing adults using a commercial skin drive BCD placed on the mastoid. Output force levels of the bone conduction transducer are then measured using a skull simulator designed for verification of percutaneous BCDs. The differences between the in-situ thresholds and dial levels are the RETFLs, across frequency, referenced to force levels on an abutment. This procedure allows direct comparison between skin drive BCDs as well as to previously-published ETFLs for percutaneous BCDs (Carlsson et al., 1995; Carlsson and Håkansson, 1997).

In summary, the ETFL values represent normal threshold levels for in-situ audiometry for skin drive BCDs across several brands. Two objectives of this study were to: (1) quantify ETFLs across frequencies on modern skin drive BCDs worn on a soft headband; and (2) assess whether different BCDs produce different ETFLs. This work aimed to refine verification methods by developing ETFLs for use in prescription of skin drive BCDs.

4 Materials and methods

4.1 Participants

Twenty-one normal hearing young adults with an average age of 23 years (range: 18–24 years; female = 16; male = 5) were recruited from within Western University in London, Ontario, Canada. Recruitment occurred through an approved database, as well as through posters at Western University's Elborn college. Participants were eligible for the study if they met the following inclusion criteria: age 18–25 years, a passing score on the “Questionnaire for Hearing Test” (International Standards Organization, 2009), hearing within normal limits (thresholds < 20 dB HL) bilaterally as determined by audiometric threshold testing using insert earphones coupled to foam tips from 250 to 8,000 Hz. One participant was deemed ineligible for the study, and so a total of 20 participants were enrolled. The study was approved by Western University's Research and Ethics Board prior to recruitment (ID: 104924).

4.2 Bone conduction threshold measurement

Otoscopic examination was initially completed in each ear to confirm no ear canal obstructions or outer ear pathology. Immittance measures and bone conduction audiometry were also completed to evaluate middle and inner ear function. For each participant, bone conduction thresholds were obtained using a B-71 audiometric bone conduction transducer coupled to a metal headband. In-situ bone conduction thresholds were obtained from each participant using three different commercially available functional demo BCDs: Ponto 3 SP, Ponto 5, and BAHA 6 Max, coupled to each device's corresponding soft headband. The maximum force level output of these three devices were within 8 dB, averaged for 500, 1,000, 2,000, and 4,000 Hz (Figure 1). All bone conduction threshold measurements were obtained with ears unoccluded. The B-71 was placed on the same side as each BCD and each participant was tested monaurally, with the side randomized. The B-71 and BCDs were coupled monaurally to the skin posterior to the pinna on the superior mastoid bone corresponding to the test ear. Each BCD soft headband was adjusted on each participant's head to allow the experimenter's first and second fingers to slide comfortably under the headband, placing the transducer on the prominence of the mastoid bone. Previous evaluations of this type of snug but comfortable fitting, vs. headband tensions deemed either too tight or too loose, indicated that contact force levels are consistent across a wide range of headband tension (Hodgetts et al., 2006).

Figure 1

Pure tone audiometric thresholds with the B-71 were tested first in all cases in order to ensure normal hearing status prior to in-situ threshold determination. In-situ BCD thresholds were obtained with BCD brand order randomized across participants. Audiometric thresholds with the B-71 were measured according to a local clinical protocol, at octave and interoctave frequencies from 250 to 4,000 Hz, using a 5 dB step size. In-situ thresholds were measured with the same step sizes and at the same frequencies, with the addition of 6,000 and 8,000 Hz. The in-situ thresholds were measured using commercially-available software tools that provide linear audiometric attenuation steps through the device, essentially using the device as a non-standard audiometric transducer rather than as a hearing aid with its associated signal processing: this mode disables noise reduction and directionality as well as mutes the device microphone. Both audiometric and in-situ testing were performed in a sound treated booth. Each device was connected to the in-situ software on the tester side through the patch panel of the booth for delivery of test signals to the BCD. This in-situ instrumentation allowed for the same testing environment for both in-situ and audiometric testing. All procedures for this study were conducted in one session with breaks taken as needed. Participants were reminded they could opt out at any time.

4.3 Force level output measurements

Each BCD was coupled to the Audioscan Verifit Skull Simulator “(Dorchester, Ontario Canada)” within the test box of the Audioscan Verifit2 hearing aid test system. The Verifit Skull Simulator was developed to emulate the TU-1000 skull simulator (Håkansson and Carlsson, 1989), and allows coupling of BCDs to a simulated abutment. This commercially-available accessory supports verification of BCDs with a clinical hearing aid analyzer. In “Manual Control,” the test signal from the Verifit2 was disabled and the in-situ signal from each BCD's manufacturer software was presented to the Skull Simulator. Presentation levels were 40 dB dial level at 500, 750, 1,000, 1,500, 2,000, 3,000, 4,000, 6,000, and 8,000 Hz, and 25 dB dial level at 250 Hz. The level of these in-situ signals were measured on the Verifit2 in decibels force level. The force level at each frequency for each BCD was recorded for further analysis to determine ETFL, as described below.

Force level output dial levels from the Verifit2 were used to calculate device-specific ETFL values across frequencies, referenced to a skull simulator. First, the force level at fixed dial levels were measured on the skull simulator, and the differences between dial and force level were computed, using Equation 3. The resulting dial to force level (DFL) values were used to convert the individual participants' in-situ thresholds to force level thresholds on the skull simulator using Equation 4:

Dial to force level

Example: 18 dB DFL = 58 dB FL – 40 dB dial level

Individual ETFL

Example: 28 dB FL = 10 dB DL + 18 dB DFL

5 Results

All 20 participants passed the otoscopy and immittance measures as well as the ISO standardized questionnaire. Participants' air conduction hearing thresholds ranged from −10 to 20 dB HL (Average = 5.80 dB HL; SD = 5.34 dB HL).

Audiometric thresholds gathered using the B-71 transducer ranged from −10 to 15 dB HL (Average = 0.44 dB HL; SD = 5.44 dB HL) while in-situ thresholds ranged from −10 to 45 dB dial level across BCDs (Average = 14.11 dB dial level; SD = 6.43 dB dial level; Figure 2). A repeated-measures Analysis of Variance (RM-ANOVA, SPSS v30) was completed on the BCD in situ thresholds with device type (3 levels) and test frequency (10 levels) as within-subjects factors. Results were corrected using the Greenhouse-Geisser epsilon for lack of sphericity, and with the Bonferonni correction for multiple contrasts. Results indicated that there was a significant interaction between device type and frequency on measured in-situ threshold values in dB Dial Level [F_{(8.5, 161.9)} = 25.163, p < 0.001, η² = 0.57]. Post-hoc contrasts indicated that the BCD in-situ threshold varied significantly from one another for most devices at most frequencies (Table 1). The differences between in-situ thresholds per frequency were within two audiometric step sizes between at least one device pair at six audiometric frequencies, and within one audiometric step size at two audiometric frequencies. The differences ranged from 0.5 to 17.25 dB, based on estimated marginal means. The relation between devices, plotted against the audiometric thresholds in dB HL is shown in Figure 2.

Figure 2

In order to determine if the device type was a factor after correction to force level, the force level thresholds for all participants were submitted to a RM-ANOVA, with device type (3 levels) and test frequency (10 levels), as within-subjects factors (SPSS v30). There was a significant effect of device type averaged across frequencies, with a smaller effect size than was observed with the in-situ thresholds [F_{(8.5, 161.9)} = 14.88, p < 0.001, η² = 0.44]. Pairwise comparisons revealed significant differences between devices at most frequencies, with non-significant differences at 2000 Hz (Table 2, Figure 3). Significant differences were within either or one or two audiometric step sizes in four and eight out of 10 tested frequencies, respectively. Differences between devices ranged from 0.5 to 17 dB.

Figure 3

6 Discussion

One of the aims of this study was to assess the in-situ threshold measures and force level output of three commercially available BCD, specifically the Oticon Ponto 3 SP, Oticon Ponto 5, and Cochlear BAHA 6 Max.

Initial findings from the study indicate that in-situ thresholds for all BCDs were significantly higher than audiometric bone conduction thresholds, as expected because the force tension of audiometric headbands exceeds that of soft headbands used for skin drive BCDs (Hodgetts et al., 2006). This finding is consistent with several previous studies. Specifically, Wróbel et al. (2023) found that both indirect and direct bone conduction device thresholds differed from audiometric bone thresholds on average, although frequency-specific values were not reported. Our results showed somewhat closer agreement between audiometric and BCD thresholds compared to those of Wrobel, although device type and other methodological differences may account for this pattern. Another comparison study by Gascon et al. (2022) examined the same devices as were used in this study, on a sample of BCD users as well as users with normal hearing. The normal hearing datasets in the Gascon study and the present study are very similar. Both data sets reveal larger threshold discrepancy in the high frequencies, with a peak/notch configuration occurring between 750 and 1,500 Hz. At 2,000 Hz, no significant differences in ETFL were observed between the three tested devices; this is similar to previously-reported results from Gascon et al., who discussed underlying factors of skin and skull resonances, skin properties, contact force, and contact area as possible contributors to increased agreement of thresholds at this frequency. The Gascon et al. listeners demonstrated a larger separation of thresholds across devices in the high frequencies compared to those in the current study, with differences that increased between 4,000 and 8,000 Hz. Due to a protocol limitation, we are not able to compare the results from the present study to those of Gascon et al. in this frequency range. Taken together, these two studies highlight the impact of device-specific characteristics on the efficacy output of BCDs (Gascon et al., 2022). They also agree with the reason for the recommendation that in-situ thresholds form the basis of BCD fitting whenever possible (Bagatto et al., 2022, 2023). The dial levels of the in-situ thresholds also varied significantly across devices and with large differences in threshold levels. This is perhaps not surprising, as the in-situ procedures are not yet standardized and have been developed to work on proprietary hardware and software (Gascon et al., 2022). It does mean that exchanging threshold values between devices or brands is likely not possible at this time.

One of the key findings from this study is the significant variability in force level output across the different BCDs. Comparing Figures 1–3, we observe that one device (BAHA 6Max) had the highest-level maximum output and the flattest frequency response (Figure 1). This device also had the lowest detection thresholds and the least deviation in threshold levels across frequencies (Figure 2). We also observed that the deviations between devices was somewhat smaller when expressed in dB force level on the skull simulator (Figure 3 vs. Figure 2). While these differences in force level output between the devices were statistically significant, the between-device agreement was closer in shape, and agreed at a greater number of audiometric frequencies, when thresholds were defined in decibels force level on the skull simulator. Gascon et al. (2022) suggest that differences in test results across devices and couplings may affect the prescribed device targets, if procedures do not account for these differences, and suggest that headworn microphones may be used to quantify key variables. Alternatively, although differences were observed, it may be possible to define ETFLs on a per-device basis, and/or to use a skull simulator to quantify device properties during verification and fine tuning. It may also be possible to further improve between-device agreement if any of the differences were affected by factors such as device noise floor, frequency specificity, or other factors not assessed in this study.

RETFL values for percutaneous BCDs have been measured and reported previously (Carlsson et al., 1995; Carlsson and Håkansson, 1997), with values ranging between nearly 60 dB force level in the low frequencies, to the mid-20s in the high frequencies. These values are markedly different than those shown for the ETFLs measured in the present study (Figure 3), with the highest discrepancy observed at 250 Hz of approximately 30 dB between the percutaneous values and the current skin drive values. This difference is not observed when comparing, in Dial Level units, to more recent studies that have used similar devices (e.g., Gascon et al., 2022). In the broader literature, differences across studies can occur. They can arise from differences to the coupling to the head and the type of transducer used within each BCD, as different transducer types are known to provide different response properties (Håkansson, 2003). Different measurement modalities and patient types can also contribute, including the small differences between on-head and on-skull simulator force levels. The difference between skin drive and percutaneous is, however, consistent with past studies of this phenomenon (Gascon et al., 2022; Mattingly et al., 2015; Stenfelt, 2012; Wróbel et al., 2023). The present study referenced ETFL values to a commercially-available skull simulator, so all values presented are specific to measurement with this device, and therefore are not appropriate for use with other forms of verification or coupling to the head or skull.

7 Conclusion

This study examined the ETFLs for three skin drive BCD. In-situ thresholds were consistently higher relative to audiometric bone conduction thresholds, likely due to skin and soft tissue transmission loss. Converting these thresholds to force levels reduced but did not eliminate variability between the ETFLs of each device. These results indicate that ETFLs can be established for skin drive BCDs but are likely device-specific. Further study would be required to determine whether it is feasible to recommend a RETFL for the device category of skin drive devices, taking into consideration a wider range of BCD instruments and the type of transducer used to drive them.

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