**2. The auditory prosthetic solutions**

The first prosthetic used for the management of deafness on record in human history was the *Ear Trumpet or Horn*. These were funnel shaped devices which collected sound waves and led them into the ear, thus strengthening the impact of sound energy on the ear drum and improving hearing. The use of these devices date back to the 17th century with the earliest description given by the French Jesuit priest and mathematician Jean Leurechon in his work "*Recreations Mathematiques"* (1634). Commercial production of these devices began much later in the 1800s and many notable personalities of that era including pianist Ludwig van Beethoven are known to have used them.

#### **2.1 Candidacy assessment**

Hearing impaired individuals usually seek help only when they have reached a stage that they can no longer ignore their hearing loss. Often, it is not the individual themselves, but the family member/caretaker who notes that the concerned individual is struggling with their hearing impairment. In either situation, the preaudiometric assessment which involves collection of relevant medical history and clinical examination of the hearing-impaired person is an important first step of the auditory rehabilitation process. It is important to identify patients in whom medical or non-prosthetic surgical management of their hearing loss should be attempted prior to dispensing of the hearing device. After the clinical assessment, the candidacy process involves a complete audiological evaluation using the following tests.

**Pure Tone Audiometry (PTA)** – This is a subjective test involving hearing threshold evaluation in both air conduction and bone conduction.

**Speech Audiometry** – This test provides information concerning hearing for speech, the type and degree of hearing impairment, and to check the reliability of the pure tone thresholds.

**Brainstem Evoked Response Audiometry (BERA) –** This test is an objective and non-invasive method of hearing assessment which detects electrical activity along the auditory pathway (from inner ear to inferior colliculus) in response to sound impulse.

*Audio-Vestibular Neurosensory Prosthetics: Origins, Expanding Indications and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.95592*

**Oto-Acoustic Emissions (OAE) –** This involves recording of a low-level sound emitted by the cochlea either spontaneously or evoked by an auditory stimulus thus indicating integrity of the outer hair cells of the cochlea.

**Cortical Auditory Evoked Potentials (CAEP) –** This test is an objective and non-invasive assessment of electrical activity from the level of the inferior colliculus to the primary auditory cortex in response to sound stimulus.

**Aided Audiometry –** This test is similar to PTA but involving the estimation of hearing thresholds during the use of a hearing prosthesis.

#### **2.2 The hearing aid (HA)**

#### *2.2.1 History*

The simplest and most widely used auditory prosthetic solution is the Hearing Aid. The first electronic hearing aids in the mid-19th century were large instruments that sat on a table. The subsequent invention of the transistor and miniature electron tubes made the hearing aid small enough to fit behind the ear. In the late 1960s the introduction of minicomputers opened the doors to real-time signal processing for people with hearing loss. Although not fast enough, these were computer-controlled analog systems which were used for amplification in hearing aid devices. In 1975, Daniel Graupe developed the first digital hearing aid in which high speed digital-array processors were used. This made it possible to process audio signals digitally in real time. Finally, in the early 1980s, the Central Institute of the Deaf (CID) developed the first practical wearable digital hearing aid which set precedent to the current type of aids in use [4].

#### *2.2.2 Design*

The modern digital hearing aid is a marvel of sophisticated engineering and miniaturization. The microphone is the first component to receive the sound signal and it converts this energy into electricity. These microphones may be unidirectional, bidirectional or omnidirectional. A more sophisticated type of an omnidirectional microphone is an "adaptive directional microphone" which works together with the hearing aid's noise reduction technology to highlight speech sounds and important environmental sounds while suppressing background noise. These small signals generated by the microphone are sent to the amplifier which makes them more powerful. Compression amplifiers are used to amplify the signal while avoiding distortion and decreasing its dynamic range and can represent sound in either an analogue (mimicking acoustic waveforms) or digital (representing signals as a string of numbers) manner. The signal then goes through a filter which is used to change the relative amplitude of the high, mid and low frequency characteristics of a signal. Thus, the sound signal can be altered by the user or clinician to suit the type of hearing loss. Finally, the receiver converts this modified and amplified electrical signal back into sound with the help of electromagnetism which is similar to how headphones work. The hearing aid performs all these functions with the help of electrical power from a detachable battery. Based on where they are worn, hearing aids are classified as body (pocket aids), spectacle, behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC) and completely-in-canal (CIC). BTE are further classified based on location of the receiver as receiver-in-the-aid (RITA) where the receiver lies within the hearing aid case or receiver-in-the-ear (RITE) where the receiver lies within the ear canal.

#### *2.2.3 Mode of action*

The primary mode of action of a hearing aid involves amplification of the input sound signal accompanied by frequency specific sound signal modulation to produce a tailormade output sound signal aimed at supporting the targeted hearing impairment. Hearing aid technology has seen rapid growth in the past 50 years and these advances can be broadly classified into three generations, all of which are currently commercially available. The first generation were simple analog hearing aids which were adjusted with screwdriver-controlled potentiometer trimmers according to the degree of hearing loss. An analog hearing aid consists of a microphone, preamplifier, a tone controller (or automatic gain controller - AGC), an amplifier and a receiver. In such devices, an acoustic signal is converted by the microphone into an electric signal which is amplified by a preamplifier and the frequency is shaped by the tone controller. This signal is then again amplified and converted into an acoustic output signal by the receiver. These were followed by second generation analog devices which could be digitally programmed by dedicated devices or computers. This involved coding of certain parameters of the analog components such as tone control and allowing these settings to be stored in a memory that can be modified or retrieved according to user preference. Digital hearing aids, which represent the third-generation, differ from the analog devices in that the amplified electronic signals are converted into digital signals which are processed by the Digital Signal Processor (DSP) before being converted back to analog electronic signals. Thus, these devices usually have an analog-to-digital converter, a digital signal processor and a digital-to-analog converter, of which the latter function is often embedded within the receiver in the newer devices. The entire system is built on a single integrated circuit that contains all the electronic parts i.e. transistors, capacitors and resistors [5].

#### *2.2.4 Contralateral routing of signal (CROS)/binaural CROS (BiCROS)*

CROS are hearing aids worn bilaterally where the hearing aid on the affected side transmits the acoustic signal to the receiver in a hearing aid worn on the better hearing ear. In patients who have hearing loss in the better hearing ear, the hearing aid in the better ear can also be used to provide amplification to that ear in addition to the CROS input. This configuration is called BiCROS [6].

#### **2.3 The bone conduction device (BCD)**

#### *2.3.1 History*

In the 1st century, Pliny the Elder, a Roman scientist, was the first to remark about the potential of sound conduction through solid bodies. Several centuries later, Cardano demonstrated a method by which sound may be transmitted to the ear by means of the shaft of a spear held between one's teeth [7]. About a hundred years later, in the 1600s, it was Hieronymus Capivacci, an Italian physician, who realized the clinical significance of Cardano's observations. Using the same experiment he determined that if the patient heard the sound it indicated disease of the tympanic membrane, while if the patient could not hear the sound it indicated a lesion of the auditory nerve [8]. In 1757, Johann Jorissen, a German physician, published the first known dissertation dealing exclusively with hearing through teeth. But it was only in 1821, when the first primitive bone conduction device was developed by Jean-Marie Gaspard Itard, a French ear specialist who postulated that sound was conducted through the bones of the entire skull [9]. Using the concepts

*Audio-Vestibular Neurosensory Prosthetics: Origins, Expanding Indications and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.95592*

of the carbon microphone and the magnetic receiver (earphone) in 1920, Joseph Prenn of Boston, patented a mechanical bone-conductive ear [10]. However, it was Frederick Kranz of Illinois, who in 1925, patented the first real bone conduction vibrator which was handheld initially, and later attached to a headband and set the tone for bone conduction technology [11]. With the help of the Sonotone Company, Hugo Lieber developed a small wearable bone conduction receiver in 1933 [12]. Another idea was to fix the bone conduction device in eye glasses. From the 1960s to the 1990s, four different companies – Amplivox, Akumed, Otarion & Oticon made bone conduction eyeglasses which became the most widely used bone conduction hearing devices of that time. The idea to implant the vibrator into the mastoid bone originated in Sweden, after the pioneering work of the anatomist, Per-Ingvar Brånemark in bone rheology, where Anders Tjellström fitted the first 3 patients with the BAHA implant in 1977 [13].

#### *2.3.2 Design, mode of action & candidacy*

Bone conduction devices are prosthetic devices that aid hearing by converting sound energy into vibrational energy. The vibrations that are produced are transmitted to the skull bone, and then to the inner ear bypassing a hearing impairment in the external or middle ear, thus overcoming the air conduction defect. Below is a basic classification of bone conduction devices (BCD) based on their mode of action: *Direct-drive* (sound vibration is sent to bone directly in the presence of a skin defect), *Skin-drive* (where sound vibration is sent to bone via intact skin) & *In-themouth* (where sound vibration is sent to the teeth). BCDs can also be classified as *non-implantable* (conventional bone conduction devices) and *semi-implantable* devices (where some part of the device is implanted).

The **Conventional Skin-Drive Non-Implantable BCD**s usually consist of a processor attached to soft headbands (softbands), steel spring headbands or spectacles. Common drawbacks of these devices are that a high static skin pressure is required to transmit the vibrations to the cochlea, leaving the skin compressed for prolonged periods, which might lead to discomfort and skin problems. Also, skin attenuates the high-frequency sound signal and therefore sound that reaches the cochlea has a lower content of high frequencies. These devices are commonly used to confirm candidacy before formal implantation of a BCD. Alternatively, they are used when surgery is contraindicated or refused. Candidacy requirements for this type of BCD include a permanent conductive hearing loss secondary to microtia, atresia or syndromic; bilateral mixed hearing loss with bone conduction average of 35 dB or less and single sided deafness where bone conduction average is 15 dB or less in the better ear. There are no age restrictions. Below are some of the common conventional skin-drive non implantable BCDs available.

#### *2.3.3 The Cochlear BAHA SoundArc*

This is a conventional skin-drive non-implantable BCD that is designed to be worn behind the head with the BAHA sound processor attached to the connector disc just behind the ear.

#### *2.3.4 The Softband with BAHA processor attached*

This is a conventional skin-drive non-implantable BCD that consists of a bone conduction processor fitted on a soft headband and is indicated in the pediatric population with hearing loss as part of their pre-implant assessment till formal surgery.

#### *2.3.5 The Med-El ADHEAR*

This is a conventional skin-drive non-implantable BCD consisting of an adhesive adapter which is placed behind the pinna on the mastoid skin and a bone conduction processor which gets attached to that adapter.

The semi-implantable BCDs have several advantages over the conventional BCDs which include a lack of static skin compression and preservation of the high frequency signals. These BCDs are further classified as *direct-drive percutaneous* devices, *direct-drive active transcutaneous* devices (with an implanted transducer) and *skin-drive passive transcutaneous* devices (with an implanted magnet).

The **Semi-implantable Direct-drive Percutaneous BCD**s involve use of a biocompatible osseointegrated implant to which a percutaneous abutment is fixed. The sound processor is then clicked on to the abutment to activate hearing. The major benefit of these devices is an efficient percutaneous transmission of sound vibrations providing maximum amplification. Candidacy requirements for this type of BCD include a conductive hearing loss with an air bone gap of more than 30 dB; mixed hearing loss with a conductive component more than 30 dB and sensorineural component of up to 65 dB HL and single sided deafness where bone conduction average is 15 dB or less in the better ear. Suitable for ages 5 years and above.

#### *2.3.6 The Cochlear BAHA CONNECT*

The BAHA CONNECT system was the first commercially available bone conduction device. This direct-drive percutaneous system consists of a titanium implant which is osseointegrated into the skull, along with a sound processor which transmits these vibrations through a percutaneous abutment which connects the sound processor to the implant. The skin is left intact around the abutment using *'Dermalock'* technology. The Cochlear BAHA 5 sound processor features the *'BCDrive'* electromagnetic transducer. This transducer creates vibrations that are sent through to the cochlea via an abutment in the case of BAHA CONNECT, or an implanted and an external magnet in the case of BAHA ATTRACT (described later). This sound processor can also be used in the conventional way by connecting to either the Softband or the SoundArc. The BAHA 5 System consists of a range of sound processors including the BAHA 5 Sound Processor (for up to 45 dB SNHL), the BAHA 5 *Power* Sound Processor (for up to 55 dB SNHL) and the BAHA *SuperPower* Sound Processor (for up to 65 dB SNHL) which address different levels of hearing loss.

#### *2.3.7 The Oticon PONTO 4*

The Oticon PONTO 4 system is a direct-drive percutaneous device that consists of three parts – a 4 mm titanium implant that is surgically implanted into the skull, an abutment that is seamlessly placed percutaneously through the skin and a sound processor that clicks easily to the abutment and sits discreetly behind the ear. In addition to its remarkably small size, the PONTO 4 system also connects wirelessly to the internet using an IFTTT network.

The **Semi-implantable Skin-drive Passive Transcutaneous BCD**s have a major advantage in complete elimination of the soft tissue & skin complications of a percutaneous abutment but their disadvantages include the large artifact area on post-implant MRI scans and the possible dampening of high-frequency sound signals through the skin. Candidacy requirements for this type of BCD are similar to the direct drive percutaneous BCDs.

*Audio-Vestibular Neurosensory Prosthetics: Origins, Expanding Indications and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.95592*

#### *2.3.8 The BAHA ATTRACT*

The BAHA ATTRACT system is a skin-drive passive transcutaneous device that transmits sound vibrations to the inner ear through a magnetic connection between the sound processor and the implant. The magnet which lies on the inside to the skin is attached to the underlying skull bone with a screw, and the BAHA sound processor is attached to a magnet plate on the skin via a soft pad to equalize the force distribution over the attachment surface. This BCD offers the benefit that there is no skin penetrating abutment, thus providing a good esthetic outcome with no need for daily care.

#### *2.3.9 Medtronic ALPHA 2 MPO*

The Medtronic ALPHA 2 MPO system is a skin-drive passive transcutaneous device that consists of a surgically implanted internal plate containing two airtight sealed magnets and the external digital sound processor coupled to the base plate containing twin magnets corresponding to the internal ones. In order to overcome the skin problems related to high static skin pressure, it uses a larger surface area so that the static force is widely distributed alleviating dermal compression [14].

In **Semi-implantable Direct-drive Active Transcutaneous BCD**s, the transducer is implanted in bone under intact skin. Hence, vibrations are transmitted directly from the transducer to the skull bone with the elimination of dermal impedance. These BCDs are labeled transcutaneous since it is an electromagnetic signal from the sound processor that is transmitted through the skin to the implanted transducer and not sound vibrations. Candidacy requirements for this type of BCD are similar to the direct drive percutaneous devices (except for age 12 years and above for Cochlear OSIA).

#### *2.3.10 The Med-El BONEBRIDGE*

The BONEBRIDGE is a direct-drive active transcutaneous system that consists of an external audio processor and an internal bone conduction implant. The internal component has a receiver coil, a magnet, a demodulator, and a cylindrically shaped bone-conduction floating mass transducer (BC-FMT) secured to the bone by two titanium screws. The power to drive the FMT is transmitted transcutaneously to the internal coil, processed by the demodulator and then relayed to the BC-FMT, which then transduces the signals into mechanical energy. Osseointegration of the titanium screws, however, is not thought to be crucial.

#### *2.3.11 The Cochlear OSIA*

The OSIA System is a direct-drive active transcutaneous system that uses a Piezo-Power transducer which sits within the OSI200 implant, and is positioned under the skin to send sound to the cochlea. The OSI200 implant is positioned on top of the bone and connected to the osseointegrated BI300 Implant which gives an important single-point transmission for sound to the skull. The system has a fitting range of 55 dB SNHL. The transducer functions on the principle of the "piezoelectric effect" which is the ability of certain materials to generate vibrations when provided with an electrical charge.

#### *2.3.12 In-the-mouth BCD: SoundBite*

This in-the-mouth BCD is neither direct-drive nor skin-drive. The vibrations are generated by a piezoelectric transducer and are transmitted through the teeth to the skull. SoundBite by Sonitus was mainly developed for single sided deafness patients. A microphone is placed behind the deaf ear and sound is sent wirelessly to an in-the-mouth transducer transmitting vibrations to the upper molar teeth. These vibrations are transmitted to the skull bone and received by the healthy cochlea. This device is currently only available for investigational use for the management of single sided deafness and is not available commercially [15].
