Blog 2019-2: Percutaneous bone conductor or (active) middle ear implant to treat mixed hearing loss? Osseo presentation


To rehabilitate patients with mixed hearing loss, implantable devices like the Vibrant Soundbridge middle ear implant (VSB) or percutaneous bone conductors (BCDs) have been applied. Such implantable devices become the next option whenever conventional hearing devices (e.g. behind-the-ear device) or reconstructive surgery are not possible or probably not effective (e.g. in case of aural atresia or chronic draining ears). Using recent publications, describing clinical trails, a comparison is made between audiological outcomes obtained with either power BCDs and VSBs. Studies in patients with severe mixed hearing loss were considered only if the mean sensorineural hearing loss component was at least 35 dB HL (arbitrary choice).

Medline was used to find relevant studies, published between 2016 and early 2019. Of the 30 detected studies, 25 studies were excluded (see Materials and Methods in the appendix) and five studies remained:

Busch et al. (2016) studied 78 patients with mixed hearing loss using the VSB with its actuator coupled to either the round window (RW) or the oval window (OW); Lee et al., (2017) and Zahnert et al. (2019) also published VSB data, including 19 and 24 patients, respectively. Bosman et al. (2018) published data on the application of a recently introduced percutaneous power BCD, viz. the Baha 5SP, applied in 10 patients and  Bosman et al. (2019) published also data on another percutaneous power BCD, the Ponto 3SP device, applied in 18 patients.

Separately, for reference purposes, data obtained with the Codacs middle ear implant device were included (Lenarz et al., 2013). This device, developed for severe mixed hearing loss was on the market from 2013 until 2018. It was the most powerful middle ear implant (MEI) ever. In contrast to the other devices, the maximum power output (MPO) of the Codacs is so high that it had to be limited when applied in patients (Lenarz et al., 2013; 2014).

The Methods and Materials section is added as an appendix.


Figure 1 presents the frequency-specific gain values related to the (bone-conduction) threshold; it should be noted that a VSB with actuator coupled to the cochlea and BCDs directly stimulate the cochlea, bypassing the impaired middle ear (and thus the air-bone gap). Therefore, to assess the effectiveness of the device fitting, only the ‘effective gain’ is of importance, defined as the cochlear (bone-conduction) thresholds minus the aided thresholds (e.g. Busch et al., 2016; Snik, 2018).

The figure presents the ‘effective gain’ divided by the cochlear hearing loss, referred to as the gain-threshold (GT) ratio, plotted as a function of frequency (0.5 kHz to 4 kHz). If GT=0, it means that the aided threshold and the cochlear thresholds coincide, indicating no ‘compensation’ of the cochlear hearing loss. Regarding the VSB, the subfigure at the right shows the calculated results using Zahnert et al. (2019), Lee et al. (2017) and Busch et al. (2016), separately. As these curves show a similar trend, data were averaged after weighting for the number of patients per study. The mean VSB data in the left subfigure show the highest GT ratio at 2 kHz with roll-off in higher and lower frequencies. For the Baha 5SP, a broader frequency response is seen comparable to that of the Ponto 3SP; obviously, the latter device is less powerful (lower GT values).

The target GT ratio (0.45), according to the well-validated NAL-RP fitting procedure, is also indicated (Dillon, 2012; see also Appendix). Systematic differences are seen between the measured GT ratios and that NAL-target GT ratio, especially in the high and low frequencies.  At 2 kHz the measured values are closest to the NAL-GT target.

Dia1blog 5 pp oud

Figure 1. The gain-threshold (GT) ratio presented as a function of frequency. Results of the included studies are shown (individual data of the 3 VSB studies, presented in right subfigure, were averaged). According to the NAL-RP fitting rule, the target GT ratio is approx. 0.45 (indicated in the figure)

The NAL fitting rule is well established for sensorineural hearing loss (Dillon, 2012). However, compared to conventional behind-the-ear devices, BCDs and middle ear implants have a limited dynamic range caused by a relatively low MPO (maximal power output). Therefore, a modified, pragmatic version of the NAL fitting rule has been introduced for these implantable devices (Snik et al., 2019, see Appendix), meant primarily for validation purposes. Figure 2 shows target-aided thresholds as obtained with this pragmatic validation rule, minus the measured aided thresholds. A value of 0 means that the result is adequate according to that rule; negative values indicate insufficient gain. Mean (pooled) VSB data and, separately, the results of the Ponto 3SP study and Baha 5SP study are presented. In agreement with Figure 1, an obvious discrepancy is found at 0.5 kHz for the VSB device. The Ponto 3 SP has relatively little gain at 2 kHz.

The Baha 5SP seems to be an adequate device with a rather flat curve around 0; as Figure 2 shows, the Ponto 3SP device lags behind by approx. 6 dB, what might be ascribed to the difference in MPO of the two BCDs. The MPO of the Baha 5SP is approximately 9 dB higher than that of the Ponto 3SP (Schwartz, 2018). The MPO of the VSB and the Baha 5SP are rather comparable (Chapter 2 in Snik, 2018).

As a reference, Codacs data are added in both figures, showing that the best resemblance is found with the Baha 5SP.


Figure 2. The difference between prescribed aided thresholds and measured aided thresholds as a function of frequency. The modified NAL-procedure was used. Pooled VSB data are presented, and, separately, the data of the two percutaneous BCD studies

Apart from aided thresholds, three studies presented aided speech recognition scores in quiet, however, measurement conditions and the tests used were divers. Importantly, in all studies, monosyllabic words were applied. Busch et al. (2016) and Zahnert et al. (2019) presented word scores obtained at a presentation level of 65 dB SPL, using the Freiburger monosyllable test. Bosman et al. (2018; 2019) reported only speech-in-noise data. From their databases, Dutch NVA monosyllable test scores were retrieved comprising phoneme score obtained at 60 dB SPL. These data were converted into word scores and corrected for presentation level. Lee et al. (2017) presented the words at variable levels, viz. the individual patient’s most comfortable listening level. Therefore, these data were excluded. Table 1 shows the outcomes.

Table 1. Speech recognition scores at 65 dB SPL presentation level

WRS PS score and calculated WRS Correction 60 à 65 dB SPL
VSB Zahnert* 67%
VSB Busch* 78%
Ponto 3SP*** 75%/52% 75%
Baha 5SP*** 69%/44% 70%
Codacs** 75%

Note WRS: word recognition score, * Freiburger monosyllable test; (Busch et al., 2013). ** WRS obtained with different monosyllable tests (different languages; Lenarz et al., 2013; Lenarz et al., 2014); *** NVA monosyllable test (PS at 60 dB SPL corrected to WRS at 60 dB SPL using WRS=PR2.3; correction for 65 instead of 60 dB SPL using WRS increases by 5 % per dB)

In summary, referring to Figure 2, for patients with mixed hearing loss around 35-45 dB HL, the achieved aided thresholds related to the prescribed values with either Codacs, VSB or the percutaneous Baha 5SP and Ponto 3SP are rather comparable and relatively close to prescribed values using the adaptive NAL rule (except the VSB at 0.5 kHz and the Ponto 3SP at 2 kHz). Speech recognition, a supra threshold outcome measure, is also rather comparable between systems.

Closing remarks

It is acknowledged that the VSB outcomes depend on the efficacy of the actuator coupling to the cochlea (e.g. Busch et al., 2016). Recently, a new coupling option has been developed (Lenarz et al., 2018; Mueller et al., 2018), which has a positive effect on the frequency response of the VSB, boosting gain and output with 10-15 dB around 1 kHz.

For the percutaneous BCDs, the coupling of the processor to the skull is more or less standardized and, therefore, not a major source of variance between subjects.

To choose a device, the audiological performance is highly relevant but also of importance is the invasiveness and complexity of the surgery, medical and technical complications and its risks, longevity, cost etc. Regarding the Baha 5SP, with high volume setting, it might be advised to use the behind-the-ear processor away from the ear, with a clip connected to clothing, to deal with feed-back. That might be considered as a draw back. Regarding de application of a middle ear implant, the status and anatomy of the middle ear is of importance as it might affect effective application (e.g. Manchero et al., 2017).

Note that the conclusions drawn are valid for the present devices/processors. If more powerful processors or more effective actuators are introduced or if better coupling techniques are being developed, conclusions might change.


Materials and methods

Search strategy

This review was carried out in accordance with PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses; Moher et al., 2009). Medline (Pubmed) was used to find relevant studies, published between 2016 and early 2019. Search string was: ((soundbridge OR ponto OR baha) AND (mixed hearing) NOT transcutaneous). Thirty manuscripts were identified; based on the abstracts, 20 manuscripts were excluded because: the focus was on cochlear implantation (1), comprised a review, no original data (5), case reports and how-we-do-it reports (4). Manuscripts dealing with medical/surgical issues (8), technical issues (1) and application in pure sensorineural hearing loss (1) were excluded as well. The full text of the remaining 10 manuscripts was reviewed. One more manuscript was excluded because the mean cochlear hearing loss was below 35 dB HL (Iwasaki et al., 2017), three manuscripts were excluded as the bone-conduction thresholds and/or aided thresholds were not presented frequency specific (essential information for the present analyses; Brkic et al., 2019; Olszewski et al., 2017; Monini et al., 2017) and one manuscript with just 5 relevant patients (Gregoire et al., 2018). Five studies were included; regarding VSB: Busch et al. (2016); Zahnert et al. (2019) and Lee et al. (2017). Regarding BCD, two studies were identified: Bosman et al. (2018) and Bosman et al. (2019). Unfortunately, Bosman et al., didn’t present the aided thresholds; these were retrieved from their database.

Table 2 gives an overview of some number of subjects per study and mean age at intervention.  For reference purposes, data obtained with the Codacs middle ear implant were included (Lenarz et al., 2013; Lenarz et al., 2014; Busch et al., 2013). This device was the most powerful middle ear implant, on the market from 2013 to 2018. Mean age of the 43 patients (of the 3 evaluative studies) was 59 years.  On the average, the patients using BCDs are older than those using the MEIs.

Table 2. Some patient characteristics

Study; first author Lee, 2017 Busch, 2016 Zahnert, 2019 Bosman, 2018 Bosman, 2019
n 19 78 24 18 10
Device VSB VSB VSB Ponto 3SP Baha 5SP
Age, years 55±17 57±17 55 (34-75) 64 (22-80) 70 (59-77)

Note. n: number of patients; PTAbc: calculated mean bone-conduction threshold at 0.5, 1, 2, 4 kHz. Regarding the age at implantation, mean age is presented with standard deviation or the median age with range between brackets

Table 3. The mean bone-conduction thresholds (in multiples of 5 dB) per type of implant.

Thresholds (dB HL): 0.5 kHz 1kHz 2kHz 4kHz
VSB, n=121 30 dB HL 35 40 45
Baha 5SP n=10 30 40 50 55
Ponto 3SP n=18 30 40 45 50
Codacs n=43 45 45 50 50

Gain measurement

BCDs as well as the MEIs with its actuator coupled to one of the cochlear windows stimulate the cochlea directly, bypassing the impaired middle ear. Therefore, the air-bone gap doesn’t play any role when fitting these devices and, to evaluate audiological benefit, the ‘effective gain’ is of importance. The ‘effective gain’ is defined as the cochlear (bone-conduction) thresholds minus the aided thresholds (e.g. Busch et al., 2016; Snik, 2018).  The ‘effective gain’ assesses which proportion of the cochlear hearing loss is ‘compensated’ by the device. By definition, the often-used ‘functional gain’ equals the ‘effective gain’ plus the air-bone gap, and is mainly dominated by the width of the air-bone gap. In contrast, the ‘effective gain’ is a measure of the quality of the device fitting and can be compared to target gain values as prescribed by generally used, validated prescription procedures.

Given a certain hearing loss, obviously, the required ‘effective gain’ depends on the cochlear hearing thresholds (the higher the hearing loss, the higher the gain should be). Therefore, the ‘effective gain’ divided by the cochlear threshold (the gain/threshold ratio or GT ratio) is calculated; according to the widely used NAL-RP hearing aid fitting rule, this ratio should be approximately 0.45 for 1, 2 and 4 kHz, for proper amplification (Dillon, 2012).

However, for mixed hearing loss cases, that target GT ratio of 0.45 is not easily reached, owing to, amongst others, the limited highest sound level that can be produced by BCDs or VSBs (limited output), characterised by the ‘maximum power output’ or MPO of these devices. Therefore, a pragmatically adapted NAL procedure has been introduced for these implants, to be used for validation purposes (Snik et al., 2019). This pragmatic procedure was based on the NAL rule (Dillon, 2012) and optimized using data of 31 published clinical trials, regarding all types of BCDs and active middle ear implants. Consequently, this adapted NAL rule is device independent. The assumption is that appropriate (sufficiently powerful) implantable devices have been chosen (regarding the MPO) and that amplification is (predominantly) linear. In short, following the NAL fitting procedure, aided thresholds should equal 0.55 times the cochlear (bone-conduction) threshold; a margin of 5 dB was added. However, at 1, 2 and at 4 kHz the aided threshold should be 25 dB HL or less. This latter adaptation is based on the statement that, in case of (predominant) conductive hearing loss, de first 20 dB can be missed (Dillon, 2012). Regarding 0.5 kHz, following the NAL-RP rule, calculated target aided threshold is 5 dB higher than that at 1 and 2 kHz, thus an (second) additional margin of 5 dB is set for 0.5 kHz only. Table 4 shows the thus prescribed aided thresholds as a function of the cochlear hearing loss and target aided word scores as calculated assuming the target aided thresholds are reaches, while using the SII procedure (speech-intelligibility index, Killion & Mueller, 2010).


Table 4. Target aided thresholds and word scores using the adapted NAL rule.a



Bosman AJ, Kruyt IJ, Mylanus EAM, Hol MKS, Snik AFM. On the evaluation of a superpower sound processor for bone-anchored hearing. Clin Otolaryngol. 2018;43:450-455. doi: 10.1111/coa.12989.

Bosman AJ, Kruyt IJ, Mylanus EAM, Hol MKS, Snik AFM. Evaluation of an abutment-level superpower sound processor for bone-anchored hearing. Clin Otolaryngol. 2019 Epub ahead of printing. doi: 10.1111/coa.13084.

Brkic FF, Riss D, Auinger A, Zoerner B, Arnoldner C, Baumgartner WD, Gstoettner W, Vyskocil E. Long-Term Outcome of Hearing Rehabilitation With An Active Middle Ear Implant. Laryngoscope. 2019;129:477-481. doi:10.1002/lary.27513.

Busch S, Kruck S, Spickers D, Leuwer R, Hoth S, Praetorius M, Plinkert PK, Mojallal H, Schwab B, Maier H, Lenarz T. First clinical experiences with a direct acoustic cochlear stimulator in comparison to preoperative fitted conventional hearing aids. Otol Neurotol. 2013 Dec;34(9):1711-8.

Busch S, Lenarz T, Maier H. Comparison of Alternative Coupling Methods of the Vibrant Soundbridge Floating Mass Transducer. Audiol Neurootol. 2016;21:347-355. doi: 10.1159/000453354.

Dillon H. Hearing aids, Chapter 10, 2012; Thieme Verlag, Stuttgart

Grégoire A, Van Damme JP, Gilain C, Bihin B, Garin P. Our auditory results using the Vibrant Soundbridge on the long process of the incus: 20 years of data. Auris Nasus Larynx. 2018;45:66-72. doi: 10.1016/j.anl.2017.02.007.

Killion MC, Mueller HG. A new count-the-dots method. Hear J. 2010;63(1):10-17.

Iwasaki S, Usami SI, Takahashi H, Kanda Y, Tono T, Doi K, Kumakawa K, Gyo K, Naito Y, Kanzaki S, Yamanaka N, Kaga K. Round Window Application of an Active Middle Ear Implant: A Comparison With Hearing Aid Usage in Japan. Otol Neurotol. 2017;38:e145-e151. doi: 1097/MAO.0000000000001438.

Lee JM, Jung J, Moon IS, Kim SH, Choi JY. Benefits of active middle ear implants in mixed hearing loss: Stapes versus round window. Laryngoscope. 2017;127:1435-1441. doi: 10.1002/lary.26244.

Lenarz T, Verhaert N, Desloovere C, Desmet J, D’hondt C, González JC, Kludt E, Macías AR, Skarżyński H, Van de Heyning P, Vyncke C, Wasowski A. A comparative study on speech in noise understanding with a direct acoustic cochlear implant in subjects with severe to profound mixed hearing loss. Audiol Neurootol. 2014;19(3):164-74.

Lenarz T, Zimmermann D, Maier H, Busch S. Case Report of a New Coupler for Round Window Application of an Active Middle Ear Implant. Otol Neurotol. 2018 Dec;39(10):e1060-e1063.

Lenarz T, Zwartenkot JW, Stieger C, Schwab B, Mylanus EA, Caversaccio M, Kompis M, Snik AF, D’hondt C, Mojallal H. Multicenter study with a direct acoustic cochlear implant. Otol Neurotol. 2013 Sep;34(7):1215-25.

Mancheño M, Aristegui M, Sañudo JR.Round and Oval Window Anatomic Variability: Its Implication for the Vibroplasty Technique.Otol Neurotol. 2017 Jun;38(5):e50-e57.

Müller M, Salcher R,  Prenzler, Nhomas Lenarz, Hannes Maier. Redesign of the Hannover Coupler: Optimized Vibration Transfer from Floating Mass Transducer to Round Window.  Biomed Res Int 2018; article ID 3701954

Moher D, Liberati A, Tetzlaff J, Altman DG; PRISMA Group. Preferred reporting  items for systematic reviews and meta-analyses: the PRISMA statement. PLoS Med 2009;6:e1000097.

Monini S, Bianchi A, Talamonti R, Atturo F, Filippi C, Barbara M. Patient satisfaction after auditory implant surgery: ten-year experience from a single implanting unit center. Acta Otolaryngol. 2017;137:389-397. doi: 10.1080/00016489.2016.1258733.

Olszewski L, Jedrzejczak WW, Piotrowska A, Skarzynski H. Round window stimulation with the Vibrant Soundbridge: Comparison of direct and indirect coupling. Laryngoscope. 2017;127:2843-2849. doi: 10.1002/lary.26536.

Schwartz, J. MPO Available to the patient: A comparison of the Cochlear™ Baha® 5 Power, Cochlear Baha 5 SuperPower and the Oticon Medical Ponto 3 SuperPower.  26-4-2018.

Snik AFM. Auditory implants. Where do we stand at present? 2018; http:\\

Snik A, Maier H, Hodgetts B, Kompis M, Mertens G, van de Heyning P, Lenarz T, Bosman A. Efficacy of Auditory Implants for Patients With Conductive and Mixed Hearing Loss Depends on Implant Center. Otol Neurotol. 2019;40:430-435. doi: 10.1097/MAO.0000000000002183.

Zahnert T, Mlynski R, Löwenheim H, Beutner D, Hagen R, Ernst A, Zehlicke T, Kühne H, Friese N, Tropitzsch A, Luers JC, Todt I, Hüttenbrink KB. Long-Term Outcomes of Vibroplasty Coupler Implantations to Treat Mixed/Conductive Hearing Loss. Audiol Neurootol. 2019;23:316-325. doi: 10.1159/000495560.



Blog 2018-4: Unsubstantiated claimed ‘equivalence’ of newly introduced BCDs (bone-conduction devices) to established BCDs


Originally, (conventional) bone-conduction devices (BCDs) comprised a bone
vibrator pressed against the skin in the mastoid region. This so-called
transcutaneous coupling works, but is not effective at all. Therefore, these
conventional BCDs needed very powerful amplifiers. Nevertheless, the gain and
output was limited and these devices were only applied if there was no
acceptable alternative, thus as a ‘last resort’ solution (e.g. in case of aural atresia
or chronic draining ears).
The first implantable BCD, the Baha, introduced in the late eighties, was more
powerful, owing to its (efficient) percutaneous coupling, bypassing the vibration-
attenuating skin and subcutaneous layers (Hakannson et al., 1985).


At that time, to establish the effectiveness of the Baha device, within-subject, pre-
post intervention studies were performed comparing Baha with the patient’s
own conventional BCD, and the beneficial outcomes were accepted by the
authorities as convincing evidence. Reimbursement was admitted in many
countries, e.g. by the FDA, with the restriction that the sensorineural hearing loss
component (SNHLc) had to be less than 45 dB HL.

Equivalence claims.

Since then, several new types of implantable BCDs have been developed and
marketed on the basis of presumed equivalence to Baha (e.g. implantable
transcutaneous devices). These devices have achieved approval based on the
assumption of equivalence, rather than based on clinical trials demonstrating
their effectiveness, and the same restriction as applied for Baha (SNHLc <45 dB
HL) was claimed. However, these implantable transcutaneous BCDs cannot
compete with Baha regarding gain and output (Chapter 3 and 4.1, this website;
van Barneveld et al., 2018); this means that the <45 dB HL criterion is an
overestimation. Table 4.1, Chapter 4.1, this website, presents the more realistic
criteria, per device type, based on objective data, (partly) validated by published
clinical data.

Widely, the equivalence rule is under debate owing to serious problems with
other implants (breast implants, pace makers etc.) that were allowed to the
market based on the equivalence principle, without appropriate effectiveness
studies (e.g.
faulty-medical-implants-harm-patients-around-worldreference). To deal with
this public debate, before the introduction of a new device, effectiveness studies

should be performed aiming at the capacity (in audiological terms) and stability
of that implantable hearing device.


Håkansson B, Tjellström A, Rosenhall U, Carlsson P. The bone-anchored hearing
aid. Principal design and a psychoacoustical evaluation. Acta Otolaryngol.
van Barneveld DCPBM, Kok HJW, Noten JFP, Bosman AJ, Snik AFM. Determining
fitting ranges of various bone conduction hearing aids. Clin Otolaryngol.

Blog 2018-2: How to quantify the gain (amplification) of a bone-conduction device; comment to the systematic review by Bezdjian et al. (2017)

During the last decades, several new types of bone-conduction devices (BCDs) have been released for patients with conductive or mixed hearing loss. One rather recent innovation is the semi-implantable (transcutaneous) Sophono device (Medtronic; Jacksonville, Fl, USA), which is based on the Otomag device (Siegert et al., 2013; chapter 2, this website). This device makes use of a transcutaneous magnetic coupling between the externally worn conventional BCD and the skull. Recently, Bedzjian et al. (2017) reviewed published clinical experience with this device. Their overview aimed at ‘functional improvement’ and peri-operative medical issues. The Bedzjian et al. paper has been discussed before (Snik, 2018); below, that discussion is elaborated.

The ’auditory’ or ‘functional’ gain

The ‘functional improvement’ or ‘auditory gain’, as introduced by the authors, was defined as the difference between aided and unaided sound-field thresholds. In sensorineural hearing loss, the ‘auditory gain’, better known as the ‘functional gain’, is a measure of the amplification provided by the (air-conduction) device. That is not the case for conductive or mixed hearing loss when using a BCD; BCDs directly stimulate the cochlea, bypassing the impaired middle ear and thus the air-bone gap. Owing to the definition of ‘auditory gain’, however, the width of that air-bone gap directly affects the ‘auditory gain’.

To illustrate this: in case of aural atresia, assuming a total hearing loss of 70 dB HL and a mean ‘auditory gain’ of 30 dB, the aided thresholds are poor, viz. 40 dB HL. In case of mild conductive hearing loss of 40 dB HL, e.g. owing to chronic otitis, an ‘auditory gain’ of 30 dB implies near-normal hearing with the device. Obviously, the latter patient has more adequate amplification, although the ‘auditory gain’ is the same for either patient. As has been suggested before, it is more useful to analyse the aided thresholds in relation to the cochlear thresholds (bone-conduction thresholds, see Appendix 2.3, this website).

Bedzjian et al. (2017) reported that the ‘auditory gain’, averaged over studies, was 31.6 dB and they concluded that this was a satisfactory result. However, it only indicates that the BCDs did work but obviously not how adequately the BCDs were fitted.

Averaging the ‘auditory gain’ over studies is also debatable because some studies comprised patients with conductive hearing loss and other studies patients with single-sided deafness. In single-sided deaf patients, the BCD is implanted at the deaf side and works as a (transcranial) CROS device, a totally different application than a BCD for conductive hearing loss. Reading the original papers, its is clear that in single-sided deaf patients the ‘auditory gain’ was defined as the thresholds of the normal ear that was blocked with an ear plug minus the aided thresholds obtained with the CROS-BCD. Thus, to determine the ‘auditory gain’ the thresholds of the blocked normal ear were considered as ‘unaided thresholds’.
Clearly, the ‘auditory gain’ of a CROS-BCD, defined as the thresholds of the blocked normal ear minus the aided thresholds obtained with the CROS-BCD, doesn’t assess the benefit of the CROS-BCD as, amongst others, it depends directly on how effectively the normal ear was blocked. Nevertheless, oftenb done (references)

In summary. To illustrate the real capacity of any BCD, aided thresholds* should be related to bone-conduction thresholds for patients with conductive or mixed hearing loss. For patients with single-sided deafness, aided thresholds should be compared to the air-conduction thresholds of the normal hearing ear to see whether or not cross stimulation with the BCD compensates effectively the acoustic head shadow (see also previous blog 2018-1).
The ‘auditory gain’ or ‘functional gain’ is not an appropriate measure to assess the effectiveness of any BCD fitting.

* Note that using aided thresholds to assess the gain of a device is not straightforward if non-linear amplification (compression) is applied, see Appendix 2, this website. Using Bezdjiran et al.’s data, it seems to be justified to use the aided threshold to assess gain, as amplification seemed to be linear. That is concluded from the fact that the reported mean unaided minus aided free-field tone thresholds (assessing gain for low-level sounds) was comparable to the mean unaided minus aided speech reception thresholds (assessing gain for mid-level sounds), namely 31.6 dB versus 33.6 dB.


1. Bezdjian A, Bruijnzeel H, Daniel SJ, Grolman W, Thomeer HGXM, Preliminary audiologic and peri-operative outcomes of the Sophono™ transcutaneous bone conduction device: A systematic review, Int. J. Pediatr. Otorhinolaryngol. 101 (2017) 196-203.

2. Siegert R, Kanderske J, A new semi-implantable transcutaneous bone conduction device: clinical, surgical, and audiologic outcomes in patients with congenital ear canal atresia, Otol. Neurotol. 34 (2013) 927-934.

3. Snik A. How to quantify the ‘auditory gain’ of a bone conduction device; comment to the systematic review by Bezdjian et al. Int. J. Pediatr. Otorhinolaryngol. (2018). E-pub ahead of publication

Blog 2018-1: Implantable transcutaneous bone conductors or percutaneous bone conductors; a free choice? Review of the paper by Cedars et al., 2016

The Baha bone-conduction device (BCD) was developed in the eighties for patients with conductive and mixed hearing loss. When this device is percutaneously coupled, the BCD is a powerful hearing solution (see Chapters 2 and 3, this website) and is generally considered as the gold standard. However, in a number of patients, problems occurred with the skin around the percutaneous implant. Although well manageable in adults (Chapter 5, this website) this has lead to revision surgeries, most frequently in children.

Therefore, new transcutaneously coupled BCDs have been developed (Sophono, Attract). These devices are attached with a magnetic coupling instead of a percutaneous one, with one magnet implanted, connected to the skull. The advantage is that the skin remains intact, however, the price that has to be paid is decreased efficiency of approx. 15 dB, owing to damping of the BCD’s vibrations by skin and subcutaneous layers. The first device on the market was the Otomag device, developed by Siegert et al. Since 2009 this device is known as the Sophono device (Medtronic, Boulder, US). The Baha Attract, introduced 4 years later, uses the same type of coupling (Cochlear BAS, Gothenburg, Sweden; see Chapters 2,3).

Cedars et al. (2016) published a well-documented paper entitled ‘Conversion of traditional osseointegrated bone-anchored hearing aids to the Baha Attract in four paediatric patients’ (2016). That paper was discussed before (Snik, 2017). The reason for the conversion was medical problems ascribed to the percutaneous coupling. Their Figure 5 showed that owing to the conversion, the aided thresholds worsened by 10 to 20 dB. That is quite a high price. Firstly, let us consider the aided thresholds of the child (number 4) with bilateral conductive hearing loss. Using the aided thresholds (data from Figure 5, Cedars et al.), the audibility of speech can be assessed as well as the expected word recognition score, see Figure 1, below. The count-the-dots approach was used (Mueller & Killion, 1991) resulting in an audibility index of 0.78 and 0.35, for the original percutaneous application and the new transcutaneous application, respectively. The related word recognition score can be determined using that audibility index, and is 95% when fitted with a BCD and 60% when fitted with the Attract. Adults with good linguistic skills might be able to deal with deterioration from 95% to 60%, but for children, who need to develop speech and language, it is problematic.


Figure 1. The aided thresholds obtained with, in red, the percutaneous BCD and in black, the transcutaneous BCD. The grey area presents the speech area, taken from Mueller & Killion (1990). The audibility index can be determined by counting the dots in the grey area (max. is 100) and divide it by 100

BCDs are also applied in patients with single-sided deafness (SSD). If someone is talking at the deaf side, it is poorly understood owing to acoustic head shadow. This is a high frequency phenomenon; attenuation is approx. 5 dB in the low frequencies and around 10-15 dB, up to 3-4 kHz. Above that frequency, increased attenuation is found, exceeding 25 dB HL at 4-6 kHz (see the classical data of Shaw, 1974). To deal with this, amongst others, a CROS-BCD can be applied. The BCD is placed at the deaf side and works as a transcranial CROS device (e.g. Hougaard et al., 2017).
However, with a transcutaneous CROS-BCD, owing to dampening of the vibrations by the skin and subcutaneous layers, especially in the high frequencies, effective CROS stimulation might be low. Cedars et al. presented data of 3 children with SSD (patients 1 to 3; owing to incomplete data only the results of patients 1 and 2 are discussed) who were using a percutaneous CROS-BCD that was replaced by a transcutaneous one. To obtain CROS-aided thresholds, the normal hearing ear was blocked during the measurements.
Figure 2 shows the original CROS-aided thresholds minus the air-conduction thresholds (normal hearing ear) for the transcutaneous (full lines) and the percutaneous CROS device (broken lines), presented as a function of frequency. Within these two patients an obvious difference is seen between the devices. Attenuation is seen especially with the transcutaneous CROS. That attenuation is worse than the attenuation caused by head shadow, using Shaw’s data (the blue line); in other words, when stimuli are presented at the impaired side, unaided thresholds are better than transcutaneous CROS aided thresholds. The Figure indicates that for these two patients, the application of the transcutaneous CROS-BCD is not an effective solution, in contrast to the percutaneous application (although, unfortunately, information above 4 kHz is missing).


Figure 2. Attenuation caused by head shadow (in blue, according to Shaw, 1974), the attenuation of the percutaneous CROS-BCD (broken lines) and transcutaneous CROS-BCD (full lines) of the two patients (P1 in red and P3 in black) calculated from Figure 5, Cedars et al. (2016)

Hougaard et al. (2017) reported non-use and relatively low daily use of the Baha Attract in their 13 adult patients with SSD, and suggested that it probably could be ascribed to poor compensation of head shadow. Furthermore, Hougaard et al. also reported aided thresholds of seven more patients with conductive hearing loss using the transcutaneous Baha. They concluded that the aided thresholds were worse compared to published aided thresholds with the percutaneous Baha. This is in agreement with the data of Cedars et al.’s patient 4.

In summary, indeed problems with the skin around a percutaneous implant are recurrent and annoying. The 12 reported clinical visits for patient 4 in Cedar et al. (2016) might obviously be worrying and upsetting for both the child and the parents. However, according to the review above, in audiological terms, a transcutaneous solution is a major step backwards. Furthermore, transcutaneous BCDs are not free of medical complications either (e.g. Cedars et al., 2016).

The search for the best bone-conduction device, especially for children, should be continued; the options are 1) usage of percutaneous BCDs with more skin-friendly implants, better implantation procedures and improved after care, 2) transcutaneous BCDs with significantly more powerful (and feedback-free) processors.

E. Cedars, D. Chan, A. Lau, et al., Conversion of traditional osseointegrated bone-anchored hearing aids to the Baha Attract in four pediatric patients, Int. J. Pediatr. Otrhinolaryngol. 91 (2016) 37-42.
G. Mueller, M. Killion, An easy method for calculating the Articulation Index, Hear. J. 43 (9) (1990) 14-17.
E.A.G. Shaw, The external ear, in: W.D. Keidel, W.D. Neff (Eds.), Handbook of Sensory Physiology, vol. V/1, Springer Verlag, New York, 1974, pp. 455-490.
Snik A. Letter to the editor. Int J. Pediatr. Otorhinolaryngol. 2017;93:172.
Hougaard DD, Boldsen SK, Jensen AM, Hansen S, Thomassen PC. A multicentre study on objective and subjective benefits with a transcutaneous bone-anchored hearing aid device: first Nordic results. Eur. Arch. Otorhinolaryngol. 2017;274:3011-3019.
Dimitriadis PA, Carrick S, Ray J. Intermediate outcomes of a transcutaneous bone conduction hearing device in a paediatric population. Int J Pediatr Otorhinolaryngol. 2017;94:59-63.

It should be noted that using aided thresholds to assess the performance (gain) of a device is not straightforward if non-linear amplification is applied (see appendix 2, this website). Whether or not non-linear amplification was used in the reviewed studies was not reported. With regard to the Hougaard et al. data, it seems that amplification was linear, which is concluded from the fact that the reported ‘gain’ derived from free-field tone thresholds (19.8 dB), was comparable to the gain derived from the ‘supra-threshold’ speech reception thresholds (estimated from their Figure 4b), being 20 dB. That is indicative for linear amplification. Such an analysis was not possible with the data of Cedars et al., as supra threshold measures were lacking.