Congenital Aural Atresia: Hearing Rehabilitation by Bone-Anchored Hearing Implant (BAHI)

Auris atresia (AA) is a congenital pathology characterized by aplasia or hypopla-sia of the external ear with associated middle ear malformation. The AA has a different degree of severity, and the severe form of the disorder presents no identifiable ear canal (complete atresia) and absence or significative underdevelopment of the middle ear structures. Sometimes AA is associated with a malformation of the ear called “microtia.” The alterations of the external auditory canal and of the middle ear structures are responsible for the conductive hearing loss which affects the patients. The hearing restoration procedures may recreate a normal external and middle ear anatomy to favor the recovery of the hearing function, or the surgeon may simply restore the hearing capacity through bypassing the malformed structures by bone-anchored hearing implants (BAHIs). The restoring of normal anatomy is generally associated with episode of restenosis of the external ear canal due to bony regrowth. The formulation of a therapeutic strategy may be supported by using Jahrsdoerfer classification to identify the severity of malformation. In the chapter we discuss vari-ous bone anchoring prostheses currently used (Baha, Ponto, Alpha2 by Sophono, Bonebridge) and the results that can be obtained by the use of these implants.


Introduction
Congenital aural atresia (CAA) is a congenital malformation of the ear that causes both esthetic and functional impairments. The malformation presents different severities of impairment; CAA may be the only malformation in the body or be associated with other malformation as observed in syndromic patients.
CAA is a failure in the ear development that happens in the first gestation' weeks; the failure may be complete by affecting the external and the middle ear or partial, as, for example, a stenotic external canal with normal middle ear structures.
In all cases, the hearing function is impaired, despite having different hearing threshold.
Our group showed that independently from the severity of the hearing impairment, children with hearing loss present a reduction of the memory function and scholar abilities [1], so the restoration of hearing function should be considered

Embryology and etiopathogenesis of CAA
The mandibular (I) and the hyoid (II) branchial arches contribute to the auricular development, and both may be involved in the etiopathogenesis of CAA. Auricular pinna starts to develop between the third to sixth weeks of embryonic life, when hillocks appear on the arches, and its formation is complete at the fourth month of gestation. The basis of tragus, the helical root, and the superior part of the helix comes from the anterior three hillocks, derived from the first arch. The posterior hillock that derives from second arch is responsible for the formation of the antihelix, antitragus, and lobule. The middle ear cavity derives from the first pharyngeal arch starting from 4 weeks of gestation. The pinna develops around the external meatus which becomes canalized at week 28 of embryologic life. At 8 weeks the middle ear cleft is formed, and the cavity is complete developed at 30 weeks. The first arch cartilage generates malleus and incus by 8 weeks of gestation that start to ossify at the 4 months of pregnancy. From the second arch, cartilage comes out the stapes except the medial lamina of the footplate which derives from the otic capsule [10]. At week 9, ectodermal cells proliferate, fill the meatus lumen, and form the "meatal plug" (MP); then on week 10, the MP extends in a disclike fashion by following a horizontal plane, and the internal part of MP starts to thin for generating the future tympanic membrane. At the same time, the plug in the proximal portion of the neck starts to be resorbed. At week 13 the MP is in contact with the primordial malleus, and this contact will contribute to the thinness of internal part of MP that will create the tympanic membrane at week 15. At week 16 the external ear canal is fully patent but still narrow and curved. At week 18 the meatus is fully extended and starts its opening that will be completed at 28 weeks [11].
Any type of adverse event that occurs during the 4 and 25 weeks of gestation and interrupts one of more of these developments may be responsible for one of the different types of CAA. The adverse event may be related to genetic aberrations, vascular accident (fetal hypoxia), teratogenic substances (aminoglycoside antibiotics, hydantoin, alcohol, nicotine, herbicides), maternal infection (rubella, Cytomegalovirus, measles, hepatitis, toxoplasmosis, lues), and maternal metabolic disease (deficiency of thyroid hormone or diabetes) [12].

Indications to use BAHI in patients with CAA
CAA is predominantly unilateral (ca. 70-90%) and the malformation mostly affects the right ear, perhaps because this side can suffer more frequently of hypoperfusion even rather that the left side in which the heart is located and that generally has a pressure 10 mmHg higher to the right side. The incidence of ear malformations is approximately 1 in 3800 newborns. Some children may present a bilateral CAA, when the malformation is not an isolated disease, but it is contextualized in a syndrome as, for example, CHARGE syndrome, in which children are affected by bilateral atresia up to 60% of cases [13].
Patients may be affected from different severity of external and middle ear malformation (Figure 1): due to the severity we can identify different forms of hearing loss. CAA typically results in conductive hearing loss (CHL) in 80-90% of the cases with the remaining patients demonstrating a sensorineural hearing loss (SNHL) component [10]. The CHL is typically in the moderate hearing loss range of 40-60 dB; this is the range in which BAHIs work better (Figure 2).
In the case of children suffering from unilateral CAA [14] and sensorineural hearing loss (10-20% of children with CAA), BAHI may be used for restoring the hearing function if the contralateral normal hearing function is preserved [15][16][17].
We use to utilize the Jahrsdoerfer et al. method during our decisional process for identifying the most appropriate surgery technique that has to be used. The authors proposed a CT grading system for CAA that was shown to correlate with postoperative hearing outcomes; based on the scores reached in the preliminary patient evaluation, the authors proposed different surgical options. In the case of high scores (better option), a canaloplasty with eventual ossiculoplasty may be a good option for the treatment of CAA, while, in the case of low scores, the authors' suggestion is to use a BAHI [3]. Specifically, the authors concluded that the patient with a score of 8-9 is a very good candidate (80% chance to reach postoperative PTA threshold of 30 dB or lower) for surgical reconstruction of the auditory canal and the middle ear, while a score of 5/6 or less disqualifies patients for surgery. Furthermore, they identified that syndromic patients rarely present a grade higher than 6/7 and, in general, are poor surgical candidates ( Table 1).

Stapes present 2
Oval window open 1 Based on our experience and according to the Jahrsdoerfer study, we can affirm that patients with CAA and a score of < 7 rarely benefit from an external and middle ear reconstruction with good functional results. In such cases, the implantation of a BAHI is a viable option for obtaining a stable, satisfactory, and long-term result in terms of hearing function recovery [18]. In conclusion we think that BAHI could be quite always the best method for restoring hearing function in patients with CAA, even in the case of high score in the Jahrsdoerfer grading system.

Available product
The hearing restoration by bone stimulation did not obtain an immediate success; in fact in 1920 the first electronic implantable device was the preferred method, and bone stimulation prostheses were considered as the last alternative. Furthermore, after the Second World War, with the advent of transistor, the electronic prosthesis became smaller and more manageable, similar to the actual hearing aids. Anyway, in the recent decades, thanks to the new discoveries which better explained the physiologic stimulation of the inner by bone stimulation and to the improved technologies that miniaturize the systems and make the system implantable (BAHI Figure 3), the system is widely used for the treatment of hearing impairments [21,22]. BAHI solved the old problems related to the external bone stimulation as the difficulties in maintaining constant position and correct pressure on the mastoid and, in addition, the reduced bone stimulation due to the skin impedance [23,24].
In the light of multiple observations and experiments, it was concluded that the bone pathway is a type of natural sound conduction and that the sound characteristics are normal and quite similar to that conducted by air.
The BAHI solves the problems of the traditional retro-auricular bone stimulation as poor performance due to inadequate contact between the vibrator and the skin, pain or decubitus at the site of contact, and poor esthetic acceptance of arch prostheses by combining a pin implanted in the temporal bone with an external transducer; the vibration of the transducer on the bone induces a stimulation of the inner ear cells with consequently sound perception.   The coupling between the pin and the transducer determines the type of BAHI: percutaneous or transcutaneous. Both methods of implant need a surgery.
The difference between the transcutaneous and the percutaneous system is the way in which the titanium screw is placed related to the skin plan; in the first one, the skin is surgically open, and the abutment is placed directly on the mastoid; then the surgical opening is closed by suturing the skin [25]. The percutaneous system instead consists of a titanium implant placed "through" the skin by perforating it [26].
All BAHI systems are composed of three main components: 1. An internal titanium fixture that is surgically anchored to the temporal bone behind the ear 2. An external abutment that is connected to the implant at the time of surgery 3. An external sound processor that is snapped on to the abutment All systems currently available on the market present these characteristics: high amplification power, working independently in the presence/absence of the ear canal and middle ear, a direct bone transmission giving a clear sound, may be tested preoperatively, and all systems being quite similar in terms of comfort [27][28][29] (Figures 3-5).

Temporal bone anatomy and BAHI
Surgeons have to keep in mind the normal anatomy of temporal bone because it is helpful for remembering the anatomic landmarks when severe malformations occurred in the patient that should be implanted.
When CAA is associated with microtia, the main anatomic landmarks are the zygomatic process (image 7 sagittal plane view); in fact this structure is generally quite preserved also in the case of craniofacial malformation.
In the case of the absence of zygomatic process, the squamosal suture should be identified as alternative landmarks (Figure 6).

Figure 6.
The normal anatomy of human temporal bone in sagittal and coronal views. The red circle indicates the zygomatic process, the only landmark that may be present in the case of CAA associated with craniofacial malformation (Figures 7-9).

Surgical method of implant and timing
The FDA recommends to apply the device in children not younger than 5 years. Authors say that the ideal age for implanting a BAHI is between 2 and 4 years, because at this age the thickness of retromastoid bone (2.5-3 mm) allows to perform a totally safe surgery, without risk of damaging the dura. Additionally, regarding the age of implant, we have to consider that children need to possess a sufficient manual dexterity to maintain the device and a good psychological tendency to follow the suggestions of parents and caregivers. We usually correct hearing aids before 5 years by using an elastic band (Softband).
The Tjellström et al. technique, proposed in 1977, suggest the creation of a thin muscle-cutaneous flap. The main disadvantage is an altered vascularization that may lead to infection and wound healing problems, dysesthesia in the retroauricular area and alopecia. For solving these problems, a less invasive technique by using a linear retro-auricular incision has been proposed and currently is the mostly used. The main benefit of this new technique is the good preservation of the vascularization in the surgical area that allows to avoid all the problems of muscle-cutaneous flap (infection, healing problems, dysesthesia, etc.). Another suggested low invasive technique is based on the removal of a very small piece of skin in the area in which the titanium implant should be placed. The skin removed has the same dimension of a biopsy. This technique also presents the advantages of the liner incision, due to the reduced trauma on tissue and vascular structures [30].

Alpha 2 (www.sophono.com)
The Alpha 2 by Sophono is another bone-anchored prosthesis without percutaneous screw and consists of a processor that is coupled to the skin transcutaneously with a titanium component implanted subcutaneously, containing two magnets. The magnetic attraction allows to hold the processor in place and to transmit acoustic energy [31]. The limits of this kind of device are represented by energy loss through the skin layer and possible discomfort and complications due to soft tissue pressure [21] (Figures 12 and 13).

Bonebridge (https://www.medel.com/hearing-solutions/bonebridge)
The Bonebridge by MED-EL is a semi-implantable bone conduction auditory system comprising a processor that is coupled transcutaneously to a titanium component and silicone implanted subcutaneously. The transducer, lodged in the   mastoid bone, transmits mechanical vibrations directly to the inner ear jumping middle and outer ear. The conduction system is light (∼10 g), and it has an area of ∼8.7 mm (height) × 15.8 mm (diameter). Two titanium screws-responsible for the vibrations transduced to the system-are located to a distance of around 24 mm between them.
Bonebridge is the unique BAHI with active mechanism; the information analyzed by the audio-processor are sent to the antenna and transmitted through the skin to the system. It converts the received signals into mechanical vibrations, which are transmitted to the inner ear by bone conduction. The transductor is surgically positioned at the seno-dural angle. However, due to its slightly bulky internal implant, the most optimal location for placing BB implant should be carefully selected preoperatively using 3D reconstruction software [32].
As compared to percutaneous BCI, Bonebridge's transcutaneous technology enables the avoidance of several complications including skin reaction, growth of skin over the abutment, implant extrusion, and wound infection.
BB presents the same risks of the other BAHIs (skin infection and skin necrosis), and currently nobody described severe complications by using this implant. We suggest to use a double flap for minimizing the skin trauma and improving the outcomes (Figures 14 and 15).

Our results
We widely use BAHIs both in the case of aural atresia (singular or bilateral form) [1,15,21] and as treatment of single side deafness [1], and our results are supported by other authors' observations [16-19, 29, 30].
We observed that BAHI not only improved the hearing abilities of children in noise condition but also their dictation capacity during the school activity [1,15,21].
We compared children wearing BAHI immediately after the implant and then 3 months later they were implanted, and we noticed that their abilities in speech perception increased time by time by reaching the same scores observed in healthy children after 3 months [1]. In addition, we analyzed the short and working memory abilities in children with BAHI, before and after implantation, and we identified that both these memory functions improved by restoring the hearing with BAHIs. The memory abilities, as the speech perception done, increased time by time by overlapping the score obtained by healthy children in the same age range [1]. Finally, we observed that children after being implanted with BAHI improved their school scores and increased their relationship with friend and environment [21].
Reported complications are slight skin problems as localized irritation and hypertropic scare around the titanium implant. The skin inflammation may be explained as an immune answer to an external body (the screw). In the literature the rates go from 5 to 7.5% for skin regrowth and from 1.3-10% for extrusion of the abutment [15].

Conclusions
We think that BAHIs are a very good solution for restoring hearing abilities. Although we have a wide experience on children, due to the excellent results obtained on these patients, in terms of speech discrimination but much more for the impressive benefit obtained in memory function, we suggest BAHIs as treatment of hearing loss in adults' population too.
As we have shown in our chapter, BAHIs are simple to implant, with very low side effects, and their surgery is poor and time-consuming. We speculate that due to the new theories on BAHIs' stimulation of the inner ear [33][34][35][36][37], these implants could be a valid alternative to the traditional hearing aids in slight-mild form of asymmetric hearing loss; we suggest BAHIs not only in the case of conductive hearing loss (CHL) but also in the sensorineural form (SNHL).