*2.4.1 History*

Alvar Wilska, a Finnish physiologist, is credited with the first attempt at mechanical stimulation of the auditory system with the help of an electromagnetic driver. In 1935, he sprinkled iron filings on to the tympanic membrane of a patient lying on his side and placed a ear-phone over the man's ear that produced no sound, but an electromagnetic signal, and the patient reported hearing. Subsequently, in 1959, Rutschmann devised a method of fixing a tiny permanent magnet to the tympanic membrane at the umbo with water soluble glue. By introducing an alternating current he produced pure tones in the range of 2 kHz to 10 kHz [16]. In 1973, Goode and Glattke refined this work by introducing an electromagnetic coil on the postauricular skin to drive the magnet fixed on the umbo. Heide reported an important modification to these previous studies in 1988, by replacing the postauricular transducer with an in-the-canal electromagnetic induction coil located millimeters from a magnet fixed at the umbo [17]. Prior to this in 1986, Maniglia at Case Western Reserve University had already begun investigating one of the first contactless electromagnetic based middle ear implant systems. In that same year, Kartush and Tos, at the Michigan Ear Institute, collaborated with Smith & Nephew Richards, to create an electromagnetic-based partially implantable middle ear device that used an in-the-canal electromagnetic coil with a custom ear mold housing [18]. The use of piezoelectric crystals in middle ear implants became evident in 1984, when the RION device became the first commercially approved piezoelectric-based middle ear device to be implanted. Since this time, several additional middle ear implants have used this technology, including the Impex TICA and the Envoy Esteem. This was roughly a century after Piezoelectricity was first discovered by Jacques and Pierre Curie in 1880 after observing that certain solid substrates develop an electrical charge proportional to an applied mechanical stress. In 1996, Geoffrey Ball pioneered development of the VIBRANT SOUNDBRIDGE which became the first FDA approved Active Middle Ear Implant (AMEI) system for implantation in patients with SNHL, receiving approval in August 2000 [19].

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

These can be classified as *totally implantable* if all these parts are beneath the skin or *partially implantable* if only the receptor and transducer are implanted beneath the skin. Depending on the type of energy the transducer utilizes, MEI are classified as: *Piezoelectric* – when energy is transmitted to a piezoelectric crystal, which deforms it or changes its volume generating a vibratory signal; *Electromagnetic* – when energy is transmitted to a coil that generates an electromagnetic field that causes a magnet located close to the ossicular chain or the inner ear to vibrate and thus produce a vibratory signal; or; *Electromechanical* – which are similar to the electromagnetic implants but the coil and magnet are closely related to each other and the ossicular chain [20]. The main advantages of the AMEI over hearing aids is that they induce direct mechanical vibration of the ossicular chain or *Audio-Vestibular Neurosensory Prosthetics: Origins, Expanding Indications and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.95592*

(and) the intracochlear fluids and thus generate greater functional gain, especially over the acute frequencies, with less distortion and feedback, increasing spokenword discrimination. In terms of subjective results, patients report better intelligibility and quality of sound and a more natural perception of their own voice than obtained with hearing aids [21].

#### *2.4.3 The Med-El VIBRANT SOUNDBRIDGE*

The VIBRANT SOUNDBRIDGE (VSB) is currently the most widely implanted middle ear device worldwide. This AMEI consists of an externally worn processor that contains the microphone, electronic signal processor and battery and an implantable part called the Vibrating Ossicular Prosthesis (VORP). This VORP contains a magnet (that enables the external part to be coupled), a receiver unit, a demodulator (that filters the signal received), a conductor link for the electrical signal and a floating mass transducer (FMT). Sound signal received by the external audio processor is transmitted transcutaneously to the implanted device generating vibratory movements of the FMT and conduction of sound to the inner ear. The FMT coupled to the Incus is the original indication devised for patients with a healthy middle ear and moderate–severe sensorineural hearing loss in the higher frequencies with a discrimination >50% at conversational intensity who were not satisfied with their hearing aid or who had repeated external otitis. The VSB is the only AMEI approved for use in children since 2009. It is indicated in children with external auditory canal atresia and bilateral malformations of the ossicular chain who have sensorineural conductive or mixed hearing loss and meet the same audiological criteria as for adults.

#### *2.4.4 The Cochlear CARINA*

This is a totally implantable AMEI which consists of two parts, the electronic capsule that contains the microphone, the batteries, the digital processor and the connector, and the middle ear transducer that contains the receiver unit and the electromechanical transducer. All these parts are placed beneath the skin with no need for externally worn processors. This AMEI is indicated for adults with moderate to severe SNHL with a hearing threshold between 30 and 85dBHL especially in the higher frequencies.

#### *2.4.5 CODACS (direct acoustic Cochlear implant)*

The CODACS implant is a semi-implantable device that is still in a preliminary phase and is gradually being used in some European centers. This AMEI is indicated for patients with severe to profound mixed hearing loss due to otosclerosis either as a primary indication or after stapedial surgery has failed.

#### *2.4.6 The envoy ESTEEM implant*

The Envoy ESTEEM implant is a fully implantable device which is indicated in patients with moderate to severe and severe sensorineural hearing loss. The system uses two piezoelectric transducers (PZTs). Sound is received via a PZT sensor that picks up eardrum vibrations and transforms them into an electric signal. This signal is filtered, modified, amplified and transferred to a PZT driver which mechanically drives the stapes thus conducting sound to the inner ear. The sound processor also contains a power source, which is an implantable lithium iodide battery [22]. For candidacy, hearing thresholds should be stable and between 35 and 85dBHL for

audiometric frequencies of 500-4000 Hz with a word recognition score of 40% or greater. It is currently indicated only in patients older than 18 years.

#### **2.5 The Cochlear implant (CI)**

#### *2.5.1 History*

The history of the cochlear implant dates back to the first attempts at electrical hearing. The Italian scientist Alessandro Volta (1800) was the first to demonstrate that electric stimulation could directly evoke auditory sensations in humans when he invented the battery [23]. However electric stimulation of the auditory system was not subsequently reported for another 150 years until modern electronic technology appeared. SS Stevens and his colleagues conducted a series of studies to re-examine the electric stimulation of hearing using vacuum based tube oscillators and an amplifier, a copper wire serving as an electrode. They identified three mechanisms that were responsible - the first mechanism was called an "electromechanical effect" by Kiang & Moxon in 1972 in which electrical stimulation causes the hair cells in the cochlear to vibrate, resulting in a perceived tonal pitch at the signal frequency it was acoustically stimulated; the second mechanism occurs due to the tympanic membrane's conversion of the electric signal into an acoustic signal, resulting in a tonal pitch perception but at the doubled signal frequency; while, the third mechanism is due to direct electric activation of the auditory nerve. However, it was Andreev who first gave direct evidence of electric stimulation of the auditory nerve when hearing sensations were reported with electric stimulation in a deaf patient whose middle and inner ears were damaged [24]. The modern era of cochlear implants began when Djourno and Eyries successfully performed the electric stimulation of hearing in two deafened patients in 1957 [25]. Their success led to a frenzied increase in attempts to restore hearing to deaf people on the US west coast in the 1960 and 1970s. Although their methods were crude, these studies identified critical problems and limitations that needed to be considered and overcome for successful implementation of electric hearing. In 1984, the House 3 M single-electrode implant became the first Food and Drug Administration (FDA) approved device. This was followed by the Ineraid or Symbion device developed by the University of Utah, the Laura device developed by the University of Antwerp, and the Digisonix MX20 developed by the MXM laboratories in France. These devices were later phased out and are no longer commercially available [26]. At present, there are three major cochlear implant manufacturers including Med-El Corporation, Austria; Cochlear Corporation, Australia and Advanced Bionics, USA. In addition to its design, implantation criteria have evolved over the past decades. Niparko provides a detailed account of these evolving criteria in patient selection as well as the surgical, cost-utility, educational, pre and post-operative issues in cochlear implants [27]. Most notably, the audiological criteria for cochlear implantation has relaxed from bilateral total deafness (<110 dB HL) in the early 1980s to severe hearing loss (>70db HL) in the 1990s, and then to current suprathreshold speech based criteria (<50% open-set sentence recognition with properly fitted hearing aids).

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

The essential components of a cochlear implant are as follows – a microphone converts sound into an electrical signal for input to the speech processor. The processor transforms this electrical input into a set of stimuli for the implanted array of electrodes. These stimuli are sent to the electrodes via a transcutaneous link which

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

involves encoding of the stimulus information for efficient radiofrequency transmission from an external transmitting coil to an internal (implanted) receiving coil. The signal received by the internal coil is decoded to specify the electrical stimuli for the electrode array. These electrical stimuli that stimulate the hair cells have signal characteristics which determine the sound quality of the perceived stimulus.

## *2.5.3 Med-El SYNCHRONY CI*

The SYNCHRONY cochlear implant system consists of the SYNCHRONY 2 cochlear implant with the Sonnet 2 BTE or Rondo 2 sound processor. The implanted device has a removable magnet that may be temporarily taken out by a cochlear implant surgeon in the event that the MRI (up to 3.0 Tesla) is needed for the head. Med-El offers the largest selection of electrode options within the cochlear implant industry. Traditional electrode options include the standard array, medium array, compressed array and split electrodes array. The newest options include the FLEX 24, 28 and FLEX SOFT electrodes. The Sonnet 2 is a redesigned BTE sound processor for greater ease of use, better esthetics and improved reliability. It recognizes the ambient environment or scene and automatically adjusts settings to match it. The Rondo 2 is a unique sound processor option different from the Sonnet 2. It has innovative wireless charging, simple on/off button and automatically controls volume level for the recipient with up to 18 hours of battery life. The device is small and compact and sits on the head, just behind and above the ear and more comfortable to wear with glasses as compared to any BTE system.

#### *2.5.4 The advanced bionics NAIDA CI*

The HiRes Ultra 3D cochlear implant has a multi-magnet assembly which automatically provides alignment to the 3D MRI field, allowing adult and pediatric users to undergo 3.0 Tesla MRIs safely, without any preparation surgery or head bandaging. This unique magnet assembly is composed of four rotatable magnets encased in a revolving disc allowing alignment with the 3D MRI field. There is no need for head bandaging or possible surgical removal of the magnet. The electrode array options in Advanced Bionics include the HiFocus Mid-Scala electrode array – which is designed to lie in the center of the scala tympani in order to remain reasonable proximal to the modiolus (and cochlear neural elements) while not making physical contact with the delicate cochlear structures necessary to maintain residual hearing; and, the HiFocus Slim J electrode array – which is designed to be placed in the scala tympani toward the lateral cochlear wall away from the cochlear structures. The Naída CI Connect is a design-integrated solution which turns the Naída CI Q90 sound processor into a Bluetooth headset which allows for hands-free calling and direct audio streaming from any compatible device.
