Are today’s implantable hearing devices better than conventional devices for patients with conductive or mixed hearing loss?
In March 2014, we published a paper in ENT & Audiology news, with the same title. The present overview should be considered as an update. Since 2014, several new implantable hearing devices have been introduced, existing device types have been further developed and, unfortunately, some devices have been withdrawn from the market. Below, an overview is given of devices, which are available early 2021, with the focus on their effectiveness; only acoustical implantable devices are considered. The central question is: what are the choices for a patient with a particular hearing loss? In this overview, a division is made between patients with conductive/mixed hearing loss (part 1) and those with sensorineural hearing loss (part 2). This overview can be considered as a summary of the website snikimplants.nl. In the present text, regularly, a reference is given to a specific (sub)chapter of the website for more detailed information; these references are colored red.
Part 1; patients with conductive and mixed hearing loss
Patients with conductive or mixed hearing loss become candidates for amplification if reconstructive surgery is not an option or might lead to poor post-surgical hearing thresholds (see e.g. Nadaraja et al., 2013). The amplification options available nowadays are: the conventional acoustic behind-the-ear devices (BTE) or in-the-ear devices, conventional (non-surgical) bone-conduction devices (BCDs), (semi-)implantable BCDs and active middle ear implants with their actuator coupled to one of the cochlear windows. This overview aims primarily at comparing the audiological capacity (effectiveness) of non-surgical and implantable BCDs, mainly based on literature reviews.
1.1 The limitations of air-conduction devices in conductive and mixed hearing loss
BTEs are effective hearing devices for sensorineural hearing loss, however, less effective, if the patient has an additional attenuation (air-bone gap) on top of the sensorineural hearing loss. Obviously, if the gain of a BTE, measured on an ear simulator, is 50 dB and the maximum output is 115 dB SPL (representative values), and if the hearing impaired patient has sensorineural hearing loss with additionally an air-bone gap of 45 dB, then the gain reduces to just 5 dB (50-45) and the maximum output to just 70 dB SPL (115-45). Wardenga et al. (2020), studying patients with otosclerosis or tympanosclerosis (no contra-indication for using an air-conduction device) indeed showed that the width of the air-bone gap is an important success factor for a BTE fitting.
Fitting a conventional BCD is not necessarily more effective than a BTE, as the bone-conduction route is far less effective than the air-conduction route. Amongst others, Skoda-Türk and Welleschik (1981) showed that the bone-conduction route is even approximately 50 dB less effective. In other words, the first 50 dB produced by the amplifier of such a traditional BCD are ‘lost’, making this solution in principle as (in)effective as a BTE for the patient with a 50 dB air-bone gap. So, both options are not effective; however, if the air-bone gap is limited, below 50 dB HL, then the BTE might be the better choice.
In the case of a malformed, atretic ear or chronically inflamed middle ear, BTEs cannot or should not be used, and thus, BCDs are the next option.
1.2 Conventional, transcutaneous BCDs
Figure 1 shows a conventional BCD, which comprises a sound processor (in a BTE housing) and, separately, the actuator (bone vibrator). The actuator is pressed against the skin, behind the ear, by means of the steel spring headband. This traditional set-up has been updated during the last decades. Instead of the steel headband to keep the actuator in place, a so-called softband has been introduced, specifically developed for young children (Baha Softband; Verhagen et al., 2008). Nowadays, the BCD manufacturers have such a solution available.
Figure 1, a traditional, conventional bone-conduction device; source Internet
More recently, a BCD with a magnetic coupling (needing surgery; see section 1.2.1) and a BCD with an adhesive coupling, the Adhear device, were introduced (e.g. Favoreel et al. 2020; Osborne et al. 2020).
Below, a comparison is made of these devices, based on a systematic review of published papers (period: 2005-early 2021). Papers presenting audiological data, comparing two different types of the above-introduced BCDs, were included. The search string included device names, and ‘bone-conduction’ and ‘hearing thresholds’. The following eight papers were identified and included in this overview: Verhagen et al., (2008); Christensen et al. (2010); Denoyelle et al. (2015); Powell et al. (2015); Giannantonio et al. (2018); Skarzynski et al. (2019); Favoreel et al. (2020) and Osborne et al. (2020).
The hypothesis is that the new coupling options are approximately as effective as the traditional coupling with the steel headband.
Verhagen et al. (2008) and Christensen et al. (2010) compared the Baha Softband with traditional BCDs with steel headband. Combining these 2 studies (in total n=22) a small difference in aided thresholds was seen of 2.3 dB in favor of the softband. Favoreel et al. (2020), Osborne et al. (2020) and Skarzynski et al. (2019) compared the Adhear with the softband. Combining the data from these 3 studies (n=37), a minor difference was found of 1.6 dB, in favor of the Adhear. So, the ‘traditional’ transcutaneous BCD has been updated and, as such, is still on the market. A major advantage of these devices is that surgery (implantation) is not involved.
1.2.1 Conventional BCDs, transcutaneous solutions that require surgery
In 2011, another type of coupling was introduced; the magnetic coupling (Sophono device; e.g. Denoyelle et al., 2015). In contrast to coupling the actuator with a headband or with an adhesive, surgery is involved. One magnet is placed subcutaneously, connected to the skull, behind the pinna, and the other magnet is attached to the actuator of the BCD. Some years later, the Baha Attract was released with the same type of coupling (e.g. Powell et al., 2015). Denoyelle et al. (2015) compared the (pre-surgery worn) Baha Softband and the Sophono device. No differences in aided thresholds were found. Aided thresholds obtained with the Baha Attract and Sophono were compared in two other studies (Powell et al. 2015; Giannantonio et al. 2018). After pooling their data (n=32) a difference of 3 dB was found in favor of the Baha Attract.
Next, the mean aided threshold (0.5, 1, 2, 4 kHz) was calculated for all subjects from any of the studies, using one of the non-implantable transcutaneous solutions (in total n=109) and, separately, those using a semi-implantable transcutaneous device with magnetic coupling (n=79). The calculated mean aided threshold was 28 dB HL (s.d. of 3 dB HL, range 20-33 dB HL) and 29 dB HL (s.d. of 2 dB HL, range 25-33 dB HL), respectively. Therefore, as hypothesized, in terms of aided thresholds, all these transcutaneous BCDs seem to be equally effective. So, the type of coupling doesn’t play a part. It should be noted that the reported aided thresholds, in the range of 25-30 dB HL, are relatively poor, especially for young children, who need to develop speech and language (see section 1.6).
Apart from effectiveness, other factors play a role in the selection of a particular BCD, like ease of device use, costs, cosmetics and whether or not the device is comfortable to use. If implanted, additional factors that should be considered as well are: the stability and safety of the implanted part of the BCD, extra costs and MRI compatibility.
The softband is easier to use and better accepted than a steel headband (Verhagen et al., 2008); in contrast to the softband, skin problems occurred more frequently with headband, owing to the rather high local pressure to keep the actuator in place and effective, with the steel headband (Snik et al., 2005). With the adhesive BCDs, problems with the skin have been reported as well. Neumann et al. (2019) reported that 2 of the 12 subjects using the Adhear device discontinued using their device owing to persistent irritation of the skin, while other studies reported no skin problems with the adhesive BCD (Favoreel et al., 2020). Concerning the BCDs with the magnetic coupling; reviewing the literature, Bezdjian et al. (2017) reported that skin problems occurred in 29% of subjects using the Sophono device; in 3.5% of the subjects these problems were severe. Dimitriadis et al. (2017) reported minor skin problems in 4 % of their Baha Attract users and serious skin problems in 3 %.
In short, skin problems are not uncommon and might occur with all these transcutaneous devices; however, the percentage of serious problems leading to non-use or even revision surgery is low.
Regarding children, it should be noted that the youngest age at which the Sophono and Baha Attract magnets can be implanted is 5 years, according to the manufacturers. So, for young children (< 5 years), only the non-surgical options remain.
1.3 BCDs with percutaneous coupling
Two more types of BCD are available that bypass the skin and subcutaneous layers. These layers attenuate the vibrations produced by any transcutaneous BCDs. These two types of BCD comprise a BCD with percutaneous coupling to the skull and a BCD with its actuator implanted, under the skin, connected by a transmission link to an externally-worn sound processor (see below, section 1.3.1).
In the late eighties a percutaneous coupling for BCDs was developed as an alternative for the transcutaneous coupling. Hakansson et al. (1984) showed that their newly developed percutaneous coupling was approx. 15 dB more effective than the transcutaneous coupling, because the vibration-attenuating skin and subcutaneous layers were bypassed. Nowadays, there is a choice between the percutaneous Cochlear Baha devices and Oticon Ponto devices. Apart from the standard sound processor, more powerful BCD processors are available, what is of importance for longevity of the treatment (Chapter 4.2). Regarding aided thresholds obtained with the first generations Baha compared to those of the conventional BCDs, a systematically difference of 6-10 dB was found in favor of the Baha (Cremers et al., 1992; Snik et al., 2004). Recently, a comparison was made between more homogenous groups, viz. children with conductive loss. Aided thresholds were more than 10 dB better with Baha (Chapter 7.2.1).
Another approach to compare BCDs, instead of studying aided thresholds, is to compare maximum output levels (MPO) of different types of BCDs (Chapter 2.3). The MPO, or the loudest sound that a BCD can produce, is a device-characteristic, objective measure. In comparison BTEs, the MPO of BCDs is rather limited; the MPO of BTEs might be 120 dB SPL (average at 0.5, 1, 2 and 4 kHz, which equals approx. 115 dB HL). Transcutaneous BCDs have a MPO, expressed in dB HL, just around 55 dB HL (e.g. Sopohono device; Chapter 2.3). The Baha Attract is an exception, whenever a power processor (BP110) is used instead of the standard processor; the measured MPO was 64 dB HL.
The MPO of standard percutaneous BCD processors is approx. 67 dB HL, while the MPO of the more powerful processors is significantly higher, up to 80-85 dB HL (Chapter 2.3). All these MPO values are limited, restricting the patient’s dynamic range of hearing with the device switched on. The patient’s dynamic range of hearing is, by definition, the difference in dB HL between the hearing (bone-conduction) thresholds and the MPO of the fitted BCD. Studying the MPO of all the different types of BCD leads to similar conclusion regarding effectiveness as that drawn from studying aided thresholds. That was expected because, to deal with that limited MPO which causes distortions of loud sounds, a lower gain setting than usual might be chosen by the subject. Doing so, on the one hand distortions will be less audible, while on the other hand aided thresholds will be worse. Indeed, Cremers et al. (1992), comparing conventional BCDs with headband, with the percutaneous Baha, showed that the aided thresholds were approx. 10 dB better with percutaneous Baha, while harmonic distortion, measured in the sound field, was significantly less with Baha, ascribed to the higher MPO.
Percutaneous BCSs are the most powerful type of BCD on the market (early 2021).
Problems with the skin around the percutaneous implant are not uncommon. A recent study showed that adverse skin reactions, needing treatment, occurred in 15% of the patients (Langerkvist et al., 2020). Complication leading to revision surgery also occur; the longer the follow-up, the higher the chance. To related revisions with follow-up, the revision ratio was introduced (Chapter 5.1.2), being the accumulated follow-up periods of all subjects in the study divided by the number of revision surgeries in that group. That ratio proved to vary between studies and surgical procedures. The traditional surgical approach includes thinning of the skin around the percutaneous implant; in this case, in adults, the ratio was 1 revision in 43/77 years of follow-up (2 studies). Recently, other approaches have been introduced, mainly to simplify the surgical procedure (no thinning of the skin layers). Then the ratio is somewhat less: 1 revision in 30/55 years (also 2 studies). These ratios seem to be acceptable. Complication rates are higher in children; see section 1.6.
1.3.1 Transcutaneous device with the actuator (vibrator) implanted, connected to the skull
In 2001, another concept was introduced; the actuator of the BCD was totally implanted, rigidly connected to the skull. That actuator is connected to the externally sound processor by a transmission link. The first device on the market was the Bonebridge device. Huber et al. (2013) studying BCD devices in cadaver heads, measuring vibration of the cochlear promontory, concluded that the Bonebridge device was as effective as the standard Baha device (BP100). In agreement with that, Gerdes et al. (2016), comparing two groups of patients, one group using the Bonebridge and the other the percutaneous Baha, reported that aided threshold and several speech recognition scores were identical. Comparing the MPOs of the two devices (the standard Baha and the Bonebridge), a difference of 4 dB was seen in favor of the Baha (Chapter 2.3).
As said before, all these MPO values are limited, restricting the patient’s dynamic range of hearing with the device. To deal with that, compression amplification is often used. In principle, compression amplification is advocated for patients with a limited dynamic hearing range caused by physiological factors. Compression can also be used to minimize the consequences of a limited dynamic range caused by a low MPO of the fitted device. However, the better option, instead of using compression, is to apply a more powerful BCD. Recently, another transcutaneous device with the actuator connected to the skull was introduced, the Osia device (Cochlear Company). This rather new device will be discussed soon on the website Snikimplants.nl.
1.4 Active middle ear implants
Another option instead of a BCD is to use an active middle ear implant with its actuator coupled to one of the cochlear windows; nowadays, the only available option is the Vibrant Soundbridge device (Chapter 2.3). Regarding the audiological outcomes: the MPO is approx. 85 dB HL (Chapter 2.3) thus in the same range as the most powerful percutaneous BCD, the Baha 5 SP. Regarding aided thresholds, using data from a literature search (including a.o. 14 studies with VSB and 18 studies with percutaneous BCD, in patients with mixed hearing loss (Chapter 3, last paragraphs), a spread in outcomes is seen, however, not a structural difference between these two types of devices. This suggests that fitting either a percutaneous BCD or a VSB leads to similar aided thresholds. Both devices worked well in patients with mixed hearing loss with a sensorineural hearing loss component of up to 50-55 dB HL.
The main theoretical advantage of a VSB is that in contrast to a BCD, only the cochlea of the implanted ear is stimulated. With a BCD, owing to the low attenuation of the bone-conducted vibrations through the skull bone, the contralateral cochlea is stimulated as well, although to a lesser degree, referred to as ‘cross’ stimulation. The limited research looking into ‘cross’ stimulation shows no clear advantage of the VSB (regarding directional hearing, see Agterberg et al., 2014); however, no firm conclusion can be drawn yet.
Implantation of the VSB in an atretic ear might be challenging owing to the abnormal anatomy (Mancheno et al., 2017). In chronic inflamed middle ear, first of all, the inflammation should be dealt with, e.g. by a subtotal petrosectomy with obliteration of the middle ear with abdominal fat. Then the VSB can be applied effectively (Chapter 5.2). So, in contrast to BCD, VSB implantation in conductive/mixed hearing loss is more demanding. Regarding complications, post-VSB implantation, again, the relation between reported revision surgeries and the accumulated follow-up time was determined (5 studies). The ratio varied from one revision in 15 years up to one in 28 years (Chapter 5.1.2), thus worse than those found for percutaneous BCDs (see above section 1.3). It should be noted that most of the included studies presented combined data of the VSB applied in conductive/mixed hearing loss and in sensorineural hearing loss.
For optimal treatment for a particular patient with conductive or mixed hearing loss, choosing the best BCD, BTE or the VSB, is a challenge. Given the effectiveness and the limitations of the different options, counseling is of utmost importance. Transcutaneous BCDs are less vulnerable to skin reactions compared to percutaneous BCDs. Furthermore, the complications with percutaneous implant might require revision surgery. However, the percutaneous BCD is by far the most powerful BCD solution, thus leading to better hearing. As the processor of percutaneous devices can be changed for more powerful processors, the percutaneous BCD might be used life long, in contrast to the other types of BCDs (Chapter 4.2). For children choosing the best option is even more challenging; see next section. Compared to BCD implantation, VSB implantation is more complicated and complications requiring revision surgery are relatively high. On the other hand, as for percutaneous BCDs, longevity is not a major issue.
1.6 Application of BCDs in children
For hearing impaired children, counseling is even more important. For young children, good hearing is essential: the better the hearing the higher the chance that the child will develop normally. According to Northern and Downs (1991), indeed, children need normal hearing (15 dB HL or less) to ensure normal development of speech and language. This statement is in agreement with anecdotal data presented by Verhagen et al. (2008). Therefore, when counseling the parents of a child with a hearing loss, sufficiently powerful amplification options should be advocated.
In case of a congenital conductive hearing loss, whenever the tympanic membrane is at least partly visible (typically the air-bone gap will be near 40 dB HL), BTEs might be an effective solution. A transcutaneous BCD with headband or softband should be used in all the other cases. Nowadays, the adhesive coupling is also an option. As the aided sound field thresholds (mean data of 28-29 dB HL; see above) are significantly above the target of 15 dB HL, replacement of the transcutaneous BCD with the more powerful percutaneous BCD or Bonebridge, should remain on the agenda and, meanwhile, speech and language development should be monitored (Verhagen et al., 2008). The use of percutaneous BCDs involves implantation of a skin penetrating titanium coupling. Concerning the youngest age at which such implantation is feasible, thickness of the skull plays an important role: 3 mm or more is preferred (Snik et al., 2005). This implies that the child must be approximately 4 years old. Even then, loss of implants is a problem. Recently, a literature review was published aiming at complications and implant loss in children (comprising 952 percutaneous implants; Kruyt et al., 2020). Implant loss occurred in 14% and revision surgery was needed in 17% of the implants. However, implant loss with more recently introduced new percutaneous implants with a wider diameter, seemed to be better than that of older types, viz. 8,2% versus 25,5%. Although these figures are significantly worse than the reported complication rates with transcutaneous devices, the higher effectiveness of percutaneous devices should be taken into account while counseling. However, once more, it should be noted that, on the average, aided thresholds in children with conductive hearing loss, using percutaneous BCDs, are more than 10 dB HL better than those in children using conventional transcutaneous BCDs (see review, Chapter 7.2.1).
The VSB with the transducer coupled to one of the cochlear windows has been applied in children as young as 2 months (Mandala et al., 2011). Since then, little has been published. Only one paper was found (with n>5); Leinung et al. (2017) presented results obtained in 13 young children with aural atresia with an average age of 2.5 years. While the mean bone conduction threshold (at 0.5, 1, 2, 4 kHz) of the whole group was 8 dB HL, the mean aided threshold was just 40 dB HL, which is rather poor. In order to advocate VSB implantation in toddlers and young children, more evidence is needed. Studying elderly children, in total 60 children from 4 studies, a better mean aided threshold was found, viz. 31 dB HL, which still is approx. 10 dB worse than that of percutaneous BCDs (Chapter 7.2.1). That figure might improve with growing surgical experience.
In summary, regarding children, choices have to be made, based on the child’s development, audiological aspects (effectiveness) and the drawbacks of the devices. Good hearing is of utmost importance and, as long as the rehabilitation is not optimal in audiological terms, monitoring the child’s progress remains of prime importance.
It should be noted that application of bilateral BCDs in children with bilateral conductive hearing loss, congenital or acquired, is of utmost importance (Chapter 6.2); the vast majority of the children accepts two devices. Papers on bilateral application of the VSB in children are scares. Application of a BCD in unilateral congenital conductive hearing loss, however, is still debatable; profit is limited and non-use is unacceptably high, even with a percutaneous BCD (Chapter 6.2). Within four years after implantation about 40% of the children stopped using the BCD and 25% used it occasionally. Similar long-term data on the use of VSB in such children is lacking.
However, binaural hearing scores in patients with unilateral acquired conductive hearing loss using a percutaneous BCD were much better than those found in congenital cases (adult data; Agterberg et al., 2011). This suggests that previous experience with bilateral input is an important success factor.
Part 2; Acoustic implants for patients with sensorineural hearing loss
The BTE is the standard treatment for patients with mild to severe sensorineural hearing loss; whenever the hearing loss is severe/profound, cochlear implantation is the treatment of choice.
An alternative amplification option for patients with moderate/severe sensorineural hearing loss is an active middle ear implant. Nowadays, two semi-implantable devices, the Vibrant Soundbridge (VSB; Chapter 2) and the Maxum device (Ototronix, USA), and one fully implantable device, the Esteem device (Envoy Medical, St. Paul, MN, USA) are on the market. In 2014, two reviews of the literature were published comparing these devices amongst others, with BTEs; it was reported that no structural audiological benefit was found compared to BTEs (Chapter 8.1). As the implantable devices are expensive and involve surgery, the cost-utility ratio of these devices is unfavorable compared to BTEs. Since the two reviews of the literature were published, in 2014, to our knowledge, just two more clinical trials with audiological data were published; one regarding the Maxum device and one with the Esteem device, as published by company-independent research groups. In contrast, since 2014, 11 papers were identified with audiological data obtained in patients with sensorineural hearing loss using the VSB.
Although these middle ear implants have, on the average, no convincing audiological advantages compared to BTEs, they might be of major benefit for patients who cannot use a BTE (e.g. patients that don’t tolerate ear molds owing to chronic external otitis; see e.g. VSB fact sheet, Med-El, 2015). Edfelt et al. (2014) even showed that for such patients the application of the VSB device is cost effective.
Nijmegen, March, 10th 2021, Ad Snik, Martijn Agterberg
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