What is the effect of a relatively low MPO? Figure 3.1A (upper, right) shows the
audiogram of a patient with conductive hearing loss. Cochlear thresholds and the
loudness-discomfort-levels (LDLs), taken from Dillon and Storey (1998), are indicated in
Figure 3.1B (upper, left).

    

Figure 3.1. Figure A (upper left) shows the audiogram; figure B (upper, right) present the
cochlear thresholds and the LDLs. Figure C (lower figure) shows the MPO values
(labelled with ‘S’) of the applied transcutaneous ‘BCD’.

The patient’s dynamic range of hearing is, by definition, the difference between the
cochlear threshold and LDLs. Evidently, Figure 3.1C shows that the MPO values,
indicated by symbol S, split the patient’s dynamic range of hearing into two parts; an
audible part (45 dB wide, see vertical red arrows) and a second unused part (40 dB
wide). The speech area of the patient’s own voice is schematically indicated, with the
yellow part being ‘inaudible’. It should be noted that it is assumed that the amplification of
the device was 0 (which means that with the device, the AB-gap was ‘functionally’
‘closed’).. With that gain of 0 , the MPO values indicate that the ‘BCD’ saturates at 50-60
dB HL. Saturation causes annoying distortions. Patients can prevent distortions, by
turning down the volume (thus reducing the amplification). For instance, in this case, the
chosen reduction in amplification might be 20 dB. In that way, the louder part of the
patient’s own voice (yellow area), that range from 50 to 70 dB HL, will be perceived
undistorted, ranging from 30 to 50 dB HL. Now, the patient’s own speech fits nicely in the
patient’s ‘BCD’-aided dynamic range of hearing. However, that 20 dB attenuation
(negative amplification) negatively affects speech perception of someone else, talking at
a normal conversational level. Note that in the audiogram such a ‘negative gain’ of 20 dB
shows up as a ‘remaining AB-gap’ of 20 dB. This example underlines that it is better to
use a ‘BCD’ with a higher MPO, even in patients with a (sub)normal sensorineural
hearing loss component.

The choice for a ‘negative gain’ to deal with a relatively low MPO, proved to be a general
observation amongst patients using ‘BCDs’.

The next question is: in daily practise, what is the volume setting (chosen amplifications)
by ‘BCD’ users with varying degrees of cochlear hearing loss ? This question was
answered with the help of our Baha database by re-examined the data of 89 patients
using Baha devices (Snik et al., 2004). At that time, Baha devices with (predominantly)
linear amplification were used (Baha Compact, Classic, HC220, Cordelle) and with a
volume wheel that enabled the patients to choose their preferred amplification (Snik et
al., 2004).

Figure 3.2 The ‘Baha’-aided thresholds (at 0.5, 1 and 2 kHz averaged) as a function of
the mean SNHLc (sensorineural hearing loss component; same frequencies). For clarity,
individual data of the 89 Baha users were grouped into 12 classes of each 5 dB wide.
The diagonal line represents the (cochlear) hearing thresholds, thus the thresholds
without the Baha

Figure 3.2 shows that patients with (sub)normal cochlear function (low mean SNHLc) set
the volume of their Baha such that the gain is negative: the ‘Baha’-aided thresholds are
worse (higher) than the threshold without the ‘Baha’. This results in a remaining AB-gap
of around 10 dB, which is seen for SNHLc values up to approx. 25 dB HL.

Patients with more severe SNHLc need ‘positive’ amplification to be able to properly hear
other subjects talking, which, according to Figure 3.2, is found for SNHLc > 30 dB HL.

It should be noted that with increasing mean SNHLc, more and more patients used the
more powerful Baha sound processors (Snik, 2014). Per patient, the type of Baha device
used was documented and, consequently, its MPO was known. The MPO values were
used to obtain the ‘input level at saturation’ or the sound level at which the MPO was
reached. This ‘input level’ is simply the MPO value minus the amplification of the Baha (output = input + amplification). The amplification itself is easily obtained by subtracting
the Baha-aided thresholds from the cochlear thresholds (thresholds without the Baha).
These calculated (individual) input levels at which the Baha devices saturate, (again)
grouped in the 12 classes, are presented in Figure 3.2 as the blue line (with label
‘saturation’). The figure clearly shows that the input level at saturation restricts the
dynamic range of hearing with the Baha significantly. Before discussing the
consequences, Figure 3.2 was redrawn, see Figure 3.2^bis.

   

Figure 3.2^bis Redrawn of Figure 3.2 with more details, see text

The left graph of Figure 3.2 bis  shows the remaining capacity of ear of patients with SNHLc
between 0 and 60 dB HL. As said before, the diagonal line presents the cochlear
thresholds while the dotted line presents the patient’s LDL. The right graph shows
the aided situation after Baha fitting, taken from Figure 3.2. The difference between aided
thresholds and input level at saturation is the patient’s aided dynamic range of hearing or
his ‘auditory window’ to the world.

The Baha-aided dynamic range of hearing is restricted; as an example, at a SNHLc of 45
dB HL, a patient’s unaided dynamic range of hearing is 50 dB HL (95-45 dB, left-hand graph, red arrow) and with the Baha, the dynamic range is only 35 dB HL (40 to 75 dB
HL, right-hand graph, red arrow).

The subfigure on the right side presents schematically sounds from the real auditory
world. Let us consider normal conversational speech. That level fluctuates between 25
dB HL and 55 dB HL (indicated in the subfigure, right-hand side; according to Killion &
Christensen, 1998). Combining the right-side subfigure and the right-hand graph shows
that in the present patient group, the 30 dB speech range is only fully audible when the
mean SNHLc is 15 dB HL or less, while at a mean SNHLc of 60 dB HL, the audible
proportion of speech range has reduced to just 5 dB HL.

Furthermore, we learn from Figure 3.2 that the patients, irrespective of their mean
SNHLc, set the volume of their device such that the device saturates at input sound
levels of 70-75 dB HL. Indeed, this choice does enable undistorted perception of even
the loudest parts of normal conversational speech. So, the figure suggests that the
patients in this study don’t compromise with regard to the input level at saturation, but,
consequently, they do compromise with regard to the softest speech sounds that they
can hear.

As indicated before, the basic rule for desired amplificatiion in pure sensorineural hearing
loss equals approximately half the hearing loss(e.g., Dillon, 2012). As the figure shows,
that is not accomplished at all. E.g. at a mean SNHLc of 45 dB HL, the amplification is
only 7 dB (see Figure 3.2). The limited MPO plays an important, probably decisive role in
the low amplification setting, as chosen by the patients.s.

So far, the analysed data concerned the Baha device. Figure 3.3 presents similar data of
all implantable systems available (early 2017), including besides the Baha the
percutaneous Ponto (Oticon Medical, Askim, Sweden), the ‘Bonebridge’ (Med-El,
Innsbruck, Austria), Sophono (Medtronic, Boulder, Co, USA), the transcutaneous Baha
Attract (Cochlear BAS, Gothenburg, Sweden) and the Codacs device (Cochlear,
Mechelen, Belgium). A systematic review of the literature was carried out.

To find the most relevant studies it was decided, first of all, to use studies that had been
selected before by authors who published systematic reviews. This concerned reviews by
Colquitt et al., 2011; Verhaert et al., 2013; Nadaraja et al., 2013; Ernst et al., 2016;
Sprinzl and Wolf-Mangele, 2016 and Dimitriades et al., 2016. Only those studies were re-
considered that presented audiometric data; including air- and bone-conduction
thresholds as well as device-aided thresholds. Duplicates were removed as well as
studies with five patients or less and studies published before 2006. Unpublished
presentations were also excluded.

Using the reviews by Colquitt et al., Nadaraja et al. and Verhaert et al. Concerning the percutaneous Baha and Ponto mechanisms, nine publications met the inclusion criteria (Kompis et al., 2007; Priwin et al., 2007; Flynn et al., 2009; Fuchsman et al., 2010; Ricci et al., 2011; Marsella et al., 2011; Bouhabel et al., 2012; Concerning VSB using the reviews by Verhaert et al. and Ernst et al., 14 publications were included (Beltram et al. 2009; Streitberger et al. 2009; Colletti et al. 2011; Baumgartner et al., 2010; Frenzel et al., 2010; Beleites et al. 2011; Bernardeschi et al., 2011; ., 2013). Sprinzl andWolf-Mangele (2016) reviewed the outcomes of ‘Bonebridge’ application. Only two studies met the criteria (Sprinzl et al., 2013; Ihler et al., 2016). Dimitriades et al. (2016) reviewed the transcutaneous Baha Attract; three studies were included (Iseri et al., 2014; 2015; Briggs et al., 2015). Since the number of included publications regarding the Baha Attract and the ‘Bonebridge’ were low and the publications with the percutaneous Baha/Ponto outcomes were rather dated (<2012), we performed an additional literature search, using Pubmed. For Bonebridge and Baha Attract, search terms were the respective device names together with '2016'; concerning ‘Bonebridge’, four studies were added (Gerdes et al., 2016; Baumgartner et al., 2016; Smerber et al., 2016; Zernotti et al., 2016), for Baha Attract, no further study with relevant audiometric data was found. Concerning Baha/Ponto, the sound-processor's names were used and the search period was 2011-2016. In contrast to the other devices, number of patients studied had to be ten or more. Seven studies were identified (Pfiffner et al., 2011; Flynn et al., 2012; Bosman et al., 2013; Kurz et al., 2013; Desmet et al., 2013; Kompis et al. 2014; Gerdes et al. , 2016). Concerning the Sophono device, a systematic review has not yet been published. Therefore, we searched for studies using only 'Sophono' as search term and applying the introduced inclusion criteria. Six studies were included (Siegert and Kanderke, 2013; Sylvester et al., 2013; Hol et al., 2013; Denoyelle et al., 2015; Shin et al., 2016; Magliulo et al., 2015). Finally, data obtained with the Codacs device were included, which is the most powerful implantable device (Zwartenkot et al., 2014). Searching PubMed, three papers were identified comprising audiological data obtained in more than five patients (Busch et al., 2013; Lenarz et al., 2013; 2014).

Altogether, 48 studies were included in this analysis. Figure 3.3A  presents per study the
mean device-aided threshold at 0.5, 1, 2 and 4 kHz as a function of the mean cochlear
threshold, SNHLc, averaged over the same frequencies. Again, this figure shows that,
generally, when the (mean) SNHLc is below 25-30 dB HL, a remaining AB gap (data
points above the diagonal) is found, in agreement with Figure 3.2. Remarkably, Figure
3.3.b shows a rather large variability between studies.

Figure 3.3B presents the same data as Figure 3.3A; this time, trend-curves are
presented, drawn by eye through the data points for a each device. Considering the bleu
line, representing the trend for VSB, the VSB-aided thresholds are at a rather stable level
(25-30 dB HL) up to a SNHLc of approx. 45 dB HL. Above that value, the VSB-aided
thresholds seem to deteriorate exponentially, indicating that the device is no longer
powerful enough for such degrees of the SNHLc. For the Baha Attract and Sophono
devices, similar trends are seen, however, with lower cut-off SNHLc values, in between
15 dB HL and 20 dB HL. Obviously, these different types of hearing devices are not
equivalents, which conclusion is in line with the variance in objective MPO data, as
presented in Table 2.1.

For the Baha/Ponto devices, no clear cut-off value for SNHLc is found. This is caused by
the availability of several processors with increasing power, which have been applied in
the patients with more severe SNHLc.

Even taking this device-related limitation into account, an obvious variation between
studies is seen. This suggests that well-defined device-fitting procedures are needed.
This issue is the subject of the next chapter (chapter 4).

Figure 3.3.a.  Mean device-aided threshold (0.5, 1, 2, 4 kHz) as a function of the mean
SNHLc as obtained from 48 studies using the percutaneous Baha/Ponto devices (black
diamonds), the VSB (big blue dots), the ‘Bonebridge’ (red squares), the Baha Attract
(green triangles), the Sophono (small black dots) or the Codacs (green circles)

Figure 3.3.b. The same figure as Figure 3.3A, however, this time trend lines are indicated
for the VSBand the passive transcutaneous ‘BCDs’

2.1 Introduction

Nowadays, for patients with conductive hearing loss or mixed hearing loss, who need amplification, commonly used options are: 1. conventional acoustic behind-the-ear devices (BTE) or in-the-ear devices, 2. (semi-implantable) bone-conduction devices (‘BCD’) and 3. active middle ear implants with their actuator coupled to one of the cochlear windows; these implantable devices have been described in detail elsewhere (Snik, 2011).

The three options are neither equivalent nor interchangeable with respect to medical or anatomical restrictions and their technical capacities.

While there are several practical, medical, surgical and esthetical reasons to prefer one of these technologies over another, this chapter discusses one of the basal characteristics of these hearing devices, namely the maximum output capacity. Part of the following paragraphs has been published before (Zwartenkot et al. 2014, Snik, 2014, van Barneveld et al. 2017).

2.2. Air-conduction versus bone-conduction hearing aids; capacity of these devices

In principle, devices using either the air-conduction or bone-conduction route can be considered as amplification options for patients with conductive or mixed hearing loss. Generally, for patients with pure sensorineural hearing loss, BTEs are the first choice, fitted according to standardized procedures (Dillon, 2012).

Figure 2.1. A conventional BTE (behind-the-ear) hearing aid with earmould. Source: Internet

However, when an AB-gap is present (gap between the air- and bone conduction hearing thresholds), a significant part of the BTE output (equal to the width of the air-bone gap) is lost before the amplified signal reaches the cochlea. Consequently, the remaining output, to ‘compensate’ for any cochlear hearing loss, might become too low or even absent. For example, consider a patient with mixed hearing loss with an AB-gap of 50 dB, who uses a standard rather powerful BTE. According to the documentation, the MPO of that BTE is 116 dB SPL and the maximum gain is 52 dB. Consequently, the MPO as perceived by the patient is 116-50 thus 66 dB SPL, and the maximum gain is (52-50) just 2 dB. Note that the 66 dB SPL equals the sound pressure level of normal speech. Louder sounds cannot be amplified properly. Therefore, when applying BTEs in mixed hearing loss, only the most powerful BTEs should be considered and, if the AB-gap is large, benefit might be limited.

The other amplification option is a hearing device that uses the bone-conduction route; Figure 2.2 shows such a bone-conduction device or ‘BCD’. From an efficiency perspective, by far, the bone-conduction route for sounds is not as effective as the air-conduction route. Amongst others, Skoda-Türk and Welleschik (1981) showed that the air-conduction route is approximately 50 dB more effective than the conventional bone-conduction route (when a bone-conduction hearing aid is pressed against the skin behind the ear; the so-called transcutaneous route). In other words, the first 50 dB produced by the amplifier of such a ‘BCD’s are ‘lost’, making this solution as (in)effective as the BTE for the patient with the 50 dB AB-gap.

Figure 2.2. A conventional transcutaneous ‘BCD’. A standard powerful BTE is connected to a bone-conduction transducer, worn contralaterally. The headband has to keep the transducer in place (behind the pinna) and to push the transducer against the skin with the required static pressure to enable the best transcutaneous transmission. Source: Internet

A new, more effective ’BCD’ has been developed in the mid 1980s (Hakansson et al., 1984). This so-called ‘Baha’ device comprises an externally worn processor with electronics and the actuator that is coupled to the skull by means of a skin-penetrating implant, anchored in the skull bone (see Figure 2.3). As shown by Hakansson et al. (1984) this ‘percutaneous’ bone-conduction route is approximately 15 dB more efficient than the transcutaneous route, owing to the absence of the attenuating skin and subcutaneous layers.

Combining Hakansson et al.’s finding with that of Skoda-Türk and Welleschik suggests that the percutaneous route is only 35 dB (50-15 dB) less effective than the air-conduction route. This implies that if a hearing-impaired subject has an AB-gap of 35 dB, a BTE and a ‘Baha’ might be equally effective. Furthermore, the ‘Baha’ will be the most effective device for patients with an AB-gap exceeding 35 dB while the BTE might be more effective than ‘Baha’ if the AB-gap is below 35 dB. Indeed, de Wolf et al. (2011) showed that this theoretical “cross-over” point is realistic.

Figure 2.3. The Bone-anchored hearing aid or Baha with its transducer (in the housing) connected to the skull via a titanium percutaneous implant. Source: Cochlear Company 

2.3 The maximum output-level of bone-conduction hearing implants and other implantable amplification options

BTEs cannot be used by hearing-impaired patients in case of atresia of the ear canal or stenosis or chronically infected ears (see Chapter 5). Then, ‘BCD’s are the next option. Carlsson and Hakansson (1997) studied the gain and MPO of the percutaneous Baha. The MPO, or the maximum power output, is the loudest sound that can be produced by a device. They studied the standard Baha (HC200 at that time) and showed that the MPO was 100, 112, 102, 95 dB FL (decibel-force-level) at 0.5, 1, 2 and 4 kHz, respectively, as measured on a so-called ‘skull simulator’. These data can be expressed in dB HL, using the so-called RETFLdbc (Reference Ear to Force Level for direct bone conduction; Carlsson and Hakansson, 1997), resulting in MPOs expressed in dB HL. In this case (HC200) the MPO was 53, 66, 78, 69 dB HL respectively, with a mean of 67 dB HL. As a reference, the mean MPO of a standard BTE might exceed 100-110 dB HL. Carlson and Hakansson also studied the noise-floor of that Baha, which was  inaudible. The MPO measurements have been reproduced and extended using other/newer types of Baha by Zwartenkot et al. (2014). They also studied the Ponto percutaneous ’BCD’ (Oticon Medical, Sweden), a competitor of Baha (see Table 2.1). 

Zwartenkot et al. developed an alternative technique to measure the MPO that can also be used for non-percutaneous ‘BCDs’. Knowing that the transcutaneous coupling is approximately 15 dB less effective than the percutaneous coupling (Hakansson et al., 1984), it is expected that the MPO of transcutaneous devices might be approximately 50-55 dB HL (the percutaneous MPO of 67 dB HL minus 15 dB). Indeed, measurements showed a mean MPO of 53 dB HL (Sophono transcutaneous ‘BCD’), see table. The Sophono ‘BCD’ comprises a conventional transcutaneous ‘BCD’ with magnetic coupling to the skull instead of an elastic softband or steel headband (Siegert & Kanderske, 2013; see Figure 2.4). The alternative is the Baha Attract (Cochlear BAS; Briggs et al., 2015), a standard Baha processor also with a transcutaneous magnetic coupling instead of a percutaneous one. Evidently, the Baha Attract is less powerful than the percutaneous Baha (see table).

In the late nineties, a more powerful (percutaneous) Baha was developed, the Baha Cordelle, which comprised a body-worn powerful amplifier that made this device the most powerful ’BCD’ on the market for many years. More recently, two updated super power percutaneous ‘BCDs’ became available, namely the Baha 5-super-power and the Ponto 5-super-power (head-worn devices). The Baha 5-super-power is the most powerful ‘BCD’ with a mean MPO of 88 dB HL.

Figure 2.4. The Sophono Alpha 1 device. The device is worn externally, coupled to the skull transcutaneously by coupling magnets; a (double) magnet is implanted under a closed skin. The footplate of the externally worn processor also contains such a magnet. Source: Internet  

Table 2.1. Objective measurement of the MPO of several hearing devices

Device	Mean MPO (0.5 – 4 kHz)	Reference	Manufacturer
Sophono Alpha 1	53 dB HL	Hol et al., 2013, van Barneveld et al, 2017	Sophono, Boulder, US
‘Bonebridge’	64 dB HL	Mertens et al. 2014; Ghoncheh et al., 2024	Med-El, Innsbruck, Austria
Standard Baha 5	70 dB HL	See note below this table	Cochlear BAS, Goteborg, Sweden
Standard Ponto 5	Idem	Idem	Oticon Medical, Askim, Sweden
Baha 6	77 dB HL	Idem	Cochlear BAS, Goteborg, Sweden
Baha 5-super-power=; Ponto-5-super-power	88 and 83 dB HL	Idem	Cochlear BAS, Goteborg, Sweden; Oticon Medical, Askim, Sweden
‘Vibrant Soundbridge’	81 dB HL	Zwartenkot et al. 2014, Maier, 2023	Med-El, Innsbruck, Austria
Codacs *	100 dB HL	Zwartenkot et al. 2014	Cochlear Mechelen, Belgium
‘Baha Attract’	64 dB HL	van Barneveld et al, 2017	Cochlear Mechelen, Belgium

Note. Following the procedure described by Carlsson and Hakansson (1997), the MPO can be derived from the product data sheets.
* no longer available since summer 2020

Figure 2.5. The ‘Bonebridge’ device; an active transcutaneous ‘BCD’ with its actuator implanted in the mastoid area. The actuator is driven by an externally worn audioprocessor. Source: Internet

In 2013, the first so-called active transcutaneous ‘BCD’ was released, the ‘Bonebridge’, see Figure 2.5 (e.g., Huber et al., 2013). The actuator (vibrator), which is implanted, is coupled with an externally worn speech processor (FM link). Mertens et al. (2014) measured the MPO of the ‘Bonebridge’ according to the protocol described by Zwartenkot et al. (2014) and reported a mean MPO value of 63 dB HL. Recently, Ghoncheh et al. (2024) reported a value of 65 dB HL, thus a rather similar outcome. The mean of the two studies is listed in the table.

In 2006, Colletti and co-workers published a paper on the coupling of the actuator of the ‘Vibrant Soundbridge’ middle ear implant (VSB; active middle ear implant) directly to one of the cochlear windows. This new option is meant for patients with conductive or mixed hearing loss (Colletti et al., 2006). Figure 2.6 shows the VSB in its classic application with its actuator (FMT; floating-mass-transducer) connected to the intact ossicular chain (to the incus) enabling the FMT to move in parallel to the stapes. The right-hand part of Figure 2.6 shows the FMT in its alternative position, coupled to the round window of the cochlea, according to Colletti et al. First report on the MPO of the adapted VSB for application in conductive and mixed hearing loss showed a MPO between 65 dB HL and 88 dB HL (Zwartenkot et al., 2014). This variability is probably owing to the variable effectiveness of the individual coupling of the actuator to the cochlea (Shimizu et al., 2011). As an estimate, ignoring Zwartenkot et al.’s two cases with, most probably, a poor coupling, a mean MPO of 82 dB HL was found. Recently, Maier (2023; their most recent cases) reported a mean MPO of 80 dB HL, thus rather similar as that reported by Zwartenkot et al. The mean of these two studies is presented in the table.

Regarding the Codacs device (e.g. Lenarz et al., 2013), developed for patients with otosclerosis, the MPO could not be determined as it exceeded the patients’ loudness-discomfort-levels (LDLs), exceeding 100 dB HL (Zwartenkot et al., 2014). Thus the Codacs is the only device that enabled full utilization of the patient’s dynamic range of hearing, from the cochlear thresholds up to the LDLs. Unfortunately, this device is no longer on the market.

   

Figure 2.6 The ‘Vibrant Soundbridge’ in classical application (for sensorineural hearing loss; left figure) with its actuator (FMT) coupled to the incus (shown enlarged) and the alternative application for conductive or mixed hearing loss with the FMT coupled to the round window membrane. Source: Internet

The VSB, developed for patients with sensorineural hearing loss, can be applied in conductive and mixed hearing loss. MPO might be equal to that of  ’BCDs’ , depending on the quality of the coupling between actuator and the cochlea

Note; nomenclature ‘BCD’s

Four ‘BCD’ subtypes are distinguished (Rheinfeldt et al, 2015);

1. the conventional transcutaneous ‘BCD’ with its actuator kept in place by a steel headband or a elastic soft band over the head (Figure 2.2),
2. the percutaneous ‘BCD’ like the Baha (Cochlear BAS, Goteborg, Sweden, see Figure 2.3) and Ponto (Oticon Medical, Askim, Sweden),
3. the passive transcutaneous ‘BCD’, e.g., Alpha 2 (Sophono Inc., Boulder, USA; Figure 2.4) and the Baha Attract (Iseri et al., 2015; Cochlear BAS, not shown)
4. the active transcutaneous ‘BCD’, a ‘BCD’ with its actuator implanted, e.g., the ‘Bonebridge’ (Med-El, Innsbruck, Austria, Figure 2.5), the Osia device (Cochlear BAS) and the Sentio device (Oticon Medical).

The first seven chapters of this website deal with amplification options for hearing impaired patients with conductive or mixed hearing loss. If reconstructive surgery is not an option, several amplification options (hearing devices) are available: besides the conventional behind-the-ear device (BTE), bone-conduction devices (‘BCDs’) and middle-ear-implants (MEIs). However, patients with conductive or mixed hearing loss cannot use a BTE if the hearing loss is caused by atresia of the ear canal or a chronic infected ear. Noteworthy, in contrast to BTEs, ‘BCDs’ and MEIs have limited capacity. Therefore, it is a challenge to find the best device for a given patient, especially as the brochures, provided by the manufacturers of these devices, are often quite optimistic. An objective comparison between the capacities and limitations of all the amplification options is needed, and that is one of the aims of this website. Second aim is to give an overview of basic knowledge in this field.

To choose a device, the output capacity and the amplification (gain) is of high importance, however, also other issues play a significant role like safety and stability, MRI compatibility, aesthetics, user’s satisfaction, costs, etc. Focus is on those parameters that can be assessed (rather) objectively and can be compared mutually, e.g., output/amplification of the devices, stability and MRI compatibility.

Chapters 2, 3 and 4 are the main chapters, addressing basal audiological/technical issues like device characteristics, categorisation of the available devices, device-fitting procedures with an overview of stability figures and complications. Special attention is paid to decreasing effectiveness of these devices in the elderly, owing to aging. Chapters 5 to 8 deal with clinical application in specific patient groups. Chapter 9 discusses borderline areas for the application of all the hearing devices, including cochlear implants (CIs).

This overview might not be fully accurate owing to on-going developments in this field, although it is updated regularly. Last partial update: early 2024. Information on relatively new active transcutaneous bone-conduction devices (the Osia device; Cochlear, Mechelen, Belgium and the de Sentio device; Oticon, Askim, Sweden) will be added later this year.

Scope of this website is to give a rather up-to-date overview of the effectiveness of
amplification options for hearing impaired subjects, written for professionals and
students. For further background information, see Appendix 1.

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Abbreviations

‘AB-gap’: gap between the air- and bone-conduction thresholds

AI: Audibility Index

‘BCD’: Bone-conduction device (general term)

‘iBCD’: implantable ‘BCD’

‘pBCD’: percutaneous ‘BCD’

‘tBCD’: transcutaneous ‘BCD’

BTE: Behind-The-Ear device (or air-conduction device)

CHL: conductive hearing loss

CI: Cochlear Implant

CSOM: Chronic Supprative Otitis Media 

DR: dynamic range of hearing

DSL rule: hearing-aid fitting-rule developed by Scollie et al (2005)

FMT: ‘Floating-Mass’ Transducer

ILD: Interaural Loudness Difference

ITD: Interaural Time Difference

LDL: Loudness-Discomfort-Level

MCL: Most Comfortable Listening level

MPO: Maximum (Power) Output

MRI: Magnetic Resonance Imaging

NAL rule: hearing-aid fitting-rule developed by Dillon and co-workers (Dillon 2012)

OME: Otitis Media with Effusion

PTA: Pure Tone Average

PTAbc Pure Tone Average for bone-conduction

s.d.: standard deviation

SII: speech intelligibility index

SNHLc: Sensorineural Hearing Loss component

SRT: speech reception threshold

VSB: VibrantSoundbridge

WDRC: wide-band dynamic-range compression

WRS: word recognition score

References of chapters 1 to 7

Chapters 8,9 have their references listed at the end

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Appendix 1: History of the website and
acknowledgements

In September 2014, Franco Trabalzini organised the EAONO meeting in Siena and asked me, Ad Snik, to organize a panel on Consensus on Auditory Implants. Preparing that panel, I wrote a document what can be considered as the bleu print of the present website. Later that year, I sent an updated version of that document to my colleagues from Auronet (Auronet is a private initiative to develop a ’core-set’ of patient-centred outcome measures which enable the search for the best treatment options for a patient with conductive or mixed hearing loss; Tysome et al., 2015). Comments by my Auronet colleagues, especially Bill Hodgetts and Penny Hill, encouraged me to go on and to publish the document in the form of a website. 

As a start, on April 22, 2014, chapters 1 to 3 were published. Chapters 4 and 5 were added on May 6th and June 16th. During the Osseo meeting in Lake Louise, Alberta (May 21-23, 2015), the website was introduced to the audience for the first time.

On July 28th 2015, chapter 5.3 and 6 were uploaded. Martijn Agterberg critically read this part of the website before it was published. Chapter 7 was added in August. Owing to comments by Arjan Bosman, Martijn Toll, Tove Rosenbom and Marc Flynn, the text of chapters 1 to 4 was revised (September 7th and October 8th). During the latter update, the option for the readers to respond online, was skipped. From the start until early October, more then 90 responses were collected and all of them were spam. 

On March 29th 2016, chapters 5 and 6 were revised and Appendix 2 was added and on August 12th 2016, chapters 5 to 7 and Appendix 2 were updated. The remarks made by Hannes Maier are acknowledged. Chapter 8, on the use of auditory implants in patients with pure sensorineural hearing loss, was added on August 20th 2016.

January 20th 2017, data regarding relatively new devices was added (Chapter 3). This concerned the Sophono Alpha 2 processor, the Cochlear Baha Attract and the Med-El ‘Bonebridge’. On May 2nd 2017, chapter 1 was revised and chapters 2, 3 and 5 were updated. Amongst others, new data was added concerning new super-power percutaneous ‘BCDs’. Appendix 2 was rewritten. In the month of November 2017, this website was fully re-designed.

August 2018, Chapters 3, 4 were updated and in December 2018, Chapters 6 and 8. Blog 2018-4 added and in April 2019, Blog 2019-1 added. In October 2019, Tables 2.1, 4.1 were updated as well as Figure 4.2. In December 2019, Blog 2019-2 was added.

October 2020: Chapter 7 was updated. Blog 2019-2 was removed (concerned a power-point presentation at the Osseo conference). November 2020: Chapter 8 was updated and revised as two of the discussed auditory implants were taken off the market end 2019. On February 5th, 2021, Chapters 2, 4 and paragraph 5.3 were revised. Chapter 6 was updated and Chapter 8 was extended.

5 May 2024: Chapters 2.3, 4.1, 4.2 and 5.1.2 were updated and Chapter 9 was added. In July 2024, Martijn Agterberg took over the responsibility for the website as Ad Snik retired. An option was added to choose for the original English text or a translated Dutch version. For a proper automatic translation, several parts of the English texts have been reformulated.   

About the authors: 

Ad Snik studied physics at the Eindhoven University of Technology and got the master’s degree in 1976 and in 1982, he acquired the doctor’s degree from the same university. Afterwards he specialized in medical physics and was registered in 1987 as a medical physicist/ audiologist. In 1988 the author was employed as a clinical audiologist and researcher at the ENT department of the Radboud University Medical Centre Nijmegen and appointed as a professor in 2006 until his retirement in 2017. Since 2010, he worked one day a week as a researcher at the department of Biophysics, Radboud University Nijmegen until mid-2022. Ad Snik participated as an author in more than 250 peer-reviewed papers in the field of Audiology. His last paper concerning auditory implants was a consensus paper that he wrote together with Hannes Maier. The set-up was unique, based on a multi-stakeholder approach. Not only clinicians and audiologists participated in the consensus meetings but also representatives of all the major companies, and they were involved in the final report as (one of the 90) authors (Maier et al., Consensus Statement on Bone Conduction Devices and Active Middle Ear Implants in Conductive and Mixed Hearing Loss. Otol Neurotol. 2022;43(5):513-529).This paper covers most of the subjects addressed on this website. 

Martijn Agterberg studied Medicine and Psychology at the University of Utrecht and developed an interest in learning, memory and plasticity. In 2003 Martijn received a research grant to study plasticity of the auditory system in the laboratory of Prof. Merzenich at the University of California San Francisco (UCSF). In 2009 the PHD project ‘Neurotrophic treatment of the degenerating auditory nerve’ was finished  under supervision of Prof. Wiegant. In the same year he was appointed as Post-doc in the group of Prof. Van Opstal at the department Biophysics (Radboud University), and as researcher in the group of Prof. Snik at the department Otolaryngology (Radboudumc). In Nijmegen, he ‘bridged the gap’ between fundamental research and applied research and demonstrated that hearing impaired patients, adults and children, could be measured in advanced research facility. The work was presented on Dutch television (Pavlov, NTR NPO3). In 2010 a study demonstrating the advantage of having large ears was published. The Dutch newspaper NRC wrote about the interesting result that older adults with larger ears demonstrated improved sound localization abilities for stimuli with a limited bandwidth compared to children and young adults. So, not everything becomes worse when getting older! In 2014 Martijn received funding for the idea to build a sound localization setup in a mobile trailer (Wasmann et al., 2020). He started his own company and is now providing this sophisticated setup to other clinics in order to obtain international standardization on how to measure sound localization (Roosam B.V.). In 2018 Martijn was appointed associate professor and audiologist at the ENT department at the Radboudumc. In 2019 Martijn received the Dutch NWO Open Mind grant for the idea to study the dynamic ears of bees. The research was described in the Dutch daily newspaper ‘Trouw’, in the glossy ‘QUEST’ nr. 11, and presented on the Dutch radio (FOCUS, NPO1), and on the Dutch television (Atlas, NPO2).

• Article in the daily newspaper Trouw
• The glossy Quest published an article (Quest nr. 11)
• nl wrote an article about our work.
• Interview NPO1.

In 2021 Martijn received a NOW grant to perform a technical and commercial feasibility study for a new implant with a better noise reduction based on superior directional microphones. He founded the company BeephoniX in 2023.

In 2022 Martijn started an assistant professorship in Utrecht at the University Medical Centre Utrecht. The focus of his research is on the optimal treatment of conductive hearing loss.

‘Disclosure’:

The authors have and had no financial interests in any of the devices described in this webside. Martijn Agterberg is as founder of BeephoniX interested in the development of new superior hearing solutions. This webside is partly based on research carried out at the Radboud University, which was supported by Nobelpharma, Entific, Cochlear BAS, Symphonix, Med-El, Otologics, KNO fonds Nijmegen, Heinsius Houbolt fonds, the William Demant Foundation and European Union Grants.

Appendix 2: Quantifying the gain (amplification) of auditory implants

A2.1 Introduction

Outcome measures used to validate the fitting of a hearing device can be divided into subjective measures and objective measures. In general, subjective refers to the assessment of patients’ opinions while objective refers to data that are measured with equipment, like hearing thresholds and speech recognition (Humes, 1999).  To evaluate a hearing aid fitting, the most popular ‘objective’ outcome measure is the gain (amplification) provided by the device and the improvement in speech recognition (Gatehouse, 1998; Gurgel et al., 2012). Often, the ‘functional gain’ is reported which is the difference between the hearing thresholds without and with the fitted hearing device. However, although this approach is widely used, it has significant limitations.

Note: references are listed at the end of this Appendix 2

A2.2 Quantifying the amplification of a MEI in patients with sensorineural hearing loss

The use of the  ‘functional gain’ implicitly assumes linear amplification. Nowadays, most auditory implants don’t use linear amplification but non-linear amplification or compression amplification. The most frequently used type of compression amplification (wide dynamic range compression; WDRC) implies that for soft sounds the gain provided by the device is relatively high, while for louder sounds, the gain is gradually reduced to prevent uncomfortable loud sound levels (Dillon 2012). A direct consequence is that the hearing thresholds with the device are lower (better) than those when using linear amplification; therefore, the ‘functional gain’ is also higher. It should be noted that here, the ‘functional gain’ assesses just the amplification for soft sounds (e.g. Stelmachowicz et al., 2002).

Snik and Cremers (2001) discussed two options to estimate the amplification of an auditory implant (MEI, VSB) at more realistic listening levels (thus at supra-tone- threshold levels). They suggested determining the ‘gain at the patient’s most-comfortable-listening level (MCL)’. In short, the patient’s MCL is determined (at octave frequencies) twice, once without and once with the auditory implant switched on, and the results are subtracted. In this way, the ‘gain-at-MCL’ is determined, frequency specific. Snik and Cremers compared amplification measures in 14 VSB users with 304 processor using WDRC. Averaged at 0.5, 1, 2 and 4 kHz, the ‘gain-at-MCL’ was indeed approx. 10 dB lower than the ‘functional gain’. From the data published by Rameh et al. (2010) the ‘functional gain’ was calculated. They also provided the speech-recognition-thresholds (SRT) with and without the VSB (an alternative option for the ‘gain-at-MCL’ (Snik and Cremers, 2001), however, not frequency specific). They included 112 VSB users with sensorineural hearing loss. Using their data, a similar difference (10 dB) was found between ‘functional gain’ and the ‘gain-in-SRT’. So, the ‘functional gain’ should not be used or in combination with a supra-threshold measurement of amplification whenever compression amplification is used.

Furthermore, it should be noted that hearing thresholds with a hearing device might be further affected by advanced signal processing: viz. by expansion amplification used to prevent the patient from hearing the microphone noise of the device, or by noise reduction algorithms.

To evaluate MEI fittings in sensorineural hearing loss, the ‘functional gain’ is often used. However, when evaluating devices with non-linear amplification ‘functional gain’ doesn’t have the usual significance. Additionally, the supra-threshold  ‘gain-at-MCL’ or ‘gain-at-SRT’ should be reported.

A2.3 Qualifying amplification in patients with conductive or mixed hearing loss using MEIs or ‘BCDs’

To assess benefit of auditory implants for patients with mixed or conductive hearing loss, the ‘functional-gain’ is not an appropriate measure at all, for a second reason. It should be realized that MEIs with their actuator coupled to one of the cochlear windows as well as ‘BCDs’ directly stimulate the cochlea; consequently, the mal-functioning outer/middle ear plays no role. However, the status of the outer/middle ear, as quantified by the width of the AB-gap (the difference between the air- and bone-conduction thresholds) significantly affects the  ‘functional gain’. To illustrate this, let us assume a patient with pure conductive hearing loss of 65 dB HL (full bony atresia of the ear canal) using a powerful ‘pBCD’ that stimulates the cochlea rather perfectly (thresholds with the ‘pBCD’ at 15 dB HL). Then, following the definition, the ‘functional-gain’ is 65-15=50 dB. However, if the AB gap is 30 dB instead of 65 dB (partial atresia of the ear canal) while using the same device, then the ‘functional-gain’ is (only) 30-15=15 dB. This example shows that owing to its definition, the ‘functional gain’ depends strongly on the width of the AB-gap, and, therefore, it is an inadequate amplification measure.

How to proceed? If a patient with pure conductive hearing loss is fitted with a powerful ‘pBCD’ and if some every-day sound of 40 dB is indeed perceived as 40 dB by the patient, evidently, the gain is 0. In other words, the ‘pBCD’ has effectively compensated the conductive hearing loss. In the audiogram, this shows up as device-aided thresholds coinciding with the cochlear (thus bone-conduction) thresholds. This gain, the difference between the ‘BCD’-aided thresholds and the bone-conduction thresholds is referred to as the ‘bone-conduction gain’ (Carlsson and Hakansson, 1997) or the ‘effective gain’ (e.g., Bush et al., 2017). Note that by definition, the ‘functional gain’ equals the ‘effective gain’ plus the AB-gap. Once more; the bigger the AB-gap, the higher the ‘functional gain’. Therefore, in contrast to the ‘effective gain’, the ‘functional gain’ is not a measure of the capacity of the particular device used.

The ‘effective-gain’ might be negative, indicating that the device-aided thresholds are worse than the bone-conduction thresholds; see Chapter 3 and 4. That means that every-day sounds are (still) perceived attenuated and, in the audiogram, a persistent, although reduced AB-gap is seen. When the patient has mixed hearing loss with an obvious sensorineural hearing loss component (SNHLc) of e.g., 50 dB HL, obviously an‘effective gain’ of 0 is not enough; part of the SNHLc should be ‘compensated’ as well, to make speech sufficiently audible. The ‘effective gain’ should be positive, e.g., 22 dB (calculated target gain using the adapted NAL-rule; see Chapter 4); then, a 28 dBHL sound is just audible for his patient, using that amplification.

As said before, device-aided thresholds are affected by the kind of amplification that is used; linear versus expansion- and/or compression-amplification. Compression amplification, to deal with a relatively low maximum output of MEIs and ‘BDCs’ (Chapter 3) results in low (thus favourable) device-aided thresholds and expansion (to deal with device noise) results in higher (worse) device-aided thresholds. Consequently, the ‘effective gain’ respectively over- and under-estimates the amplification of sounds at e.g., normal conversational levels. 

In the previous section (A2.2) we discussed the use of alternative, supra-threshold amplification measures (viz. ‘gain-in-SRT’ and ‘gain-at-MCL-level’). However, these measures cannot be applied easily in conductive or mixed hearing loss. This is caused by limitations of the bone-conduction transducer (part of the audiometer) used to determine the bone-conduction stimulation. The standard B71 bone-conduction transducer has a relatively low maximum output (might be at or below the patient’s MCL levels) affecting the MCL measurements, especially in high and low frequencies (Alomari, 2014). Additionally, perception of speech produced by the B71 might be affected by distortions, especially of low-frequency sounds and the poor frequency response of the B71 (limited bandwidth). 

Evidently, to avoid or minimalize problems with quantifying the gain caused by non-linear amplification, always use auditory implants with a high MPO so that compression amplification is not needed or with reduced compression settings.

If MEI/’BCD’ are used in patients with conductive or mixed hearing loss, the ‘functional gain’ doesn’t provide any useful information on the quality MEI/’BCD’-fitting. Instead, the ‘effective gain’ should be used, as it is a fitting-quality measure. The ‘effective gain’ is by definition the difference between bone-conduction thresholds and device-aided thresholds and, therefore, might be negative (what is acceptable according to the new practise-based device-fitting procedure introduced in Chapter 4).