Chapter 2. Basic considerations; Amplification options for conductive and mixed hearing loss

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

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 ABgap 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

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

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

2.3 The maximum output 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

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 5 super powerPonto 5 super power 88 dB HL83 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

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 2.6.1    figure 2.6.2

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).