Active Middle Ear Implants

K. Böheim

• Editore: S. Karger
• Anno: 2010
• ISBN:
  9783805594714
• Argomento: Medicine
• Categoria: Surgery & Surgical Medicine

In recent years, methods for coupling active implants to the middle ear, round window or combinations of passive middle ear prostheses have progressed considerably. Patient selection criteria have expanded from purely sensorineural hearing losses to conductive and mixed hearing losses in difficult-to-treat ears. This book takes into consideration recently developed methods as well as devices in current use. It begins with a fascinating and authentic history of active middle ear implants, written by one of the main pioneers in the field. In the following chapters, leading scientists and clinicians discuss the relevant topics in otology and audiology. Treatments for sensorineural hearing loss, conductive and mixed hearing losses, and results on alternative coupling sites such as the stapes footplate and the oval window are also covered, as well as articles on candidacy and cost-effectiveness.This publication is a must for ENT professionals and surgeons seeking out the latest knowledge on current research and clinical applications of active middle ear implants for all types of hearing loss.

Böheim K (ed): Active Middle Ear Implants.
Adv Otorhinolaryngol. Basel, Karger, 2010, vol 69, pp 72–84

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Totally Implantable Active Middle Ear Implants: Ten Years’ Experience at the University of Tübingen

H.P. Zenner · J. Rodriguez Jorge

Department of Otolaryngology, Head and Neck Surgery, University of Tubingen, Tubingen, Germany

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Abstract

Active middle ear implants do not produce acoustic sounds but, rather, micromechanical vibrations. The stimulating signal does not leave the transducer as sound, but as a mechanical vibration, directly coupled to the auditory system and bypassing the normal route via air. In this paper, we review our experience with the TICA® and the Carina™ middle ear implants. Both are totally implantable and are coupled to the ossicular chain or to perilymph. The design requirements for electronic hearing implants for patients with conductive hearing loss differ from those for sensorineural hearing loss. Conductive hearing loss requires an implant that replaces impedance transformation and acts as an impedance transforming implant (ITI). In many respects, there are fewer demands on an ITI than on an electronic hearing aid for patients with sensorineural hearing loss.

Copyright © 2010 S. Karger AG, Basel

International research efforts have yielded a number of options to improve the hearing of individuals with middle ear problems. These options supplement tympanoplasties and stapes surgery and include external bone anchoring hearing aids, developed by Tjellström and Bränemark [1-5] and the Japanese group of Suzuki and Yanagihara [6-14]. A subcutaneous, bone-anchoring, partially-implantable hearing system was also available for implantation (Audiant®) [15, 16]. The pathophysiological demands on such implants used for middle ear hearing loss can easily be overseen: only the lacking ability of the middle ear to adapt for impedance needs to be compensated [17, 18]. This places no particularly great demands on the amplification and electronic processing of the sound signal. For this reason, partially implanted devices (BAHA®, the Japanese middle ear implant (MEI) and Audiant) with their uncomplicated signal processing have been clinically available for several years. In addition, research has led to the development of the TICA® Total Implant for middle ear diseases, which is no more available clinically [19-21].

The situation is different for individuals with hearing loss due to inner ear problems. The demands placed on these implants are much greater. They range from individual frequency-specific amplification to improvement in speech comprehension and discrimination (also in the presence of background noise). Completely different therapeutic approaches may be used depending on whether (1) the outer hair cells are affected with inner hair cells being mostly intact, or (2) the inner hair cells are also affected.

For the first and more common situation, the partially implantable electromagnetic floating mass transducer (FMT) system was developed by Ball [6] and is clinically available (MED-EL Vibrant® Soundbridge). The first totally implantable piezoelectric system was developed by Zenner and Leysieffer (TICA®LZ) for sensorineural hearing loss (SNHL) [6, 22-25], and was clinically available until 2000. At present, the Otologics Carina and the Envoy Esteem are totally implantable active middle ear implants (AMEIs) and are available for clinical use. To treat the second situation (inner hair cells also affected), cochlear implants and electric-acoustic implants are used more and more often [26].

Transducers

Leysieffer [27] gave an important overview of fundamental demands on an electromechanical transducer for implantable hearing aids compensating a sensorineural hearing loss. Middle ear implants (MEI) transform sound into amplified electrical signals. Unlike hearing aids, these signals are not reconverted into sound but are transformed into micromechanical vibrations. Instead of using a loudspeaker as conventional hearing aids do, the MEIs use an electromechanical transducer. The audio signal leaves the transducer not as sound (i.e. not as a compression air wave), but as a mechanical vibration (mass movement), which, by bypassing the air, is micromechanically coupled to the auditory system. The transducer is a vibrator rather than a loudspeaker.

Benefits of Currently Available, Fully-Implantable MEI Systems

• Good sound quality

• Open external auditory canal

• Low distortion (<0.5%, with hearing aids up to 5%)

• Improved speech comprehension

• Broad transmission bandwidth from 100 Hz to approximately 10 kHz for better speech comprehension and music appreciation

• Risk of feedback is reduced

• Unrestricted usage while working (e. g. in a dusty or hot environment), telephoning, showering, sleeping, swimming, and exercising

• Auditory rehabilitation is possible without any demand on the patient's manual dexterity; an advantage for persons compromised by motor and degenerative diseases of the hand

• Hidden from view, thus preventing stigmatization

Piezoelectric transducers (Envoy Esteem, TICA) consist of piezo-sensitive crystals which change their form in response to electrical current and produce vibrations. Electromagnetic devices use a magnet and a coil (Otologics Carina). Two anatomic localizations are used to implant present transducers: the tympanic cavity and the mastoid.

The transducer should be placed as close as possible to the calvarium, ossicles, or oval window to ensure that the vibrations are as free as possible from any nearby elements [28, 29]. In the middle ear, about 2-3 mm of space is available for fitting the transducer, as is done with the Envoy® system. In the mastoid cavity, a space of 1 cm³ is usually available with a maximum diameter of 1 cm (Otologics Carina, TICA) [30, 31]. The advantage of a larger space is weighed against the possible disadvantage that a connector is also required.

Sensors

Depending on technical circumstances, a sound transducer, a microphone, may be implanted in one of several possible locations. These are listed below:

(1) In the middle ear

(2) At the tympanic membrane and/or the ossicles

(3) In the wall of the auditory canal

(4) In the mastoid

(5) Under the skin of the calvaria

The space available in the middle ear and in the mastoid should not be overestimated. Maassen et al. [30, 31] have illustrated quite well how many steps are required to adapt a sensor to the anatomy of the ear, using as an example a membrane sensor (TICA) implanted in the posterior auditory canal wall. For a piezoelectric sensor coupled to an ossicle (Envoy Esteem®) and intended for middle ear implantation, Weber et al. [32] discussed surgical problems related to minimally available space.

Acoustic Aspects

Oscillation

Vibrational transducers produce no sound: their oscillations must be transformed into physiological oscillations in the middle and inner ear. Low excitation amplitudes suffice for middle ear hearing loss in which outer hair cells are intact because the signal can be amplified 100- to 1,000-fold by outer hair cells once the signal arrives in the cochlea [17, 18]. The situation is different with inner ear hearing loss and outer hair cell loss. In that case, the implant must amplify the travelling wave by a substantially greater amount so that the inner hair cells can be stimulated. According to physiological data on implants coupled to the ossicles, in the frequency range up to 1 kHz oscillations between 100 and 1,000 nm correspond to a subjective perception equivalent to 100 to 125 dB SPL in healthy ears. Above 1 kHz, in the octave range from 5 to 10 kHz, 5 to 50 nm suffice for the same auditory perception. At 10 kHz, an oscillation of 50 nm corresponds to a sound level of 140 dB SPL [27].

Dynamic Behavior

If an implant is to faithfully reproduce a complex sound signal, it must work rapidly and should not lag in its response times [40]. Otherwise, an acoustic distortion can arise. Attack and decay times are important terms here. Upon biological burdening of the ears the attack and decay times for an excitation of <0.1 ms at 1 kHz should not exceed 1-3 ms if artifacts within the acoustic envelope are to be minimized. At higher frequencies, the starting amplitudes require that, as frequency increases, the speed should also increase rather than the amplitude decrease. At 10 kHz, this means that the maximum attack time should be 100 (μs [27].

Distortion

Distortion disrupts sound transmission and quality. Linear and nonlinear distortion are two different types. Linear distortion refers to the dependency of amplitude and phase-frequency responses on frequency. Because hearing is relatively insensitive to phase shifts, we do not need to consider phase-frequency responses further. However, this is not true for amplitude. A change in amplitude is perceived immediately as a change in volume. Resonances within perceptible ranges lead to amplitude distortions. Amplitude reductions with increasing frequency may be perceived as quiet signals. An optimal transducer should, therefore, ensure a flat profile over the entire frequency range, also when the ear is biologically stressed (e.g. in the case of an acute otitis media) and should show a minimal ripple effect of at most ± 3 dB. Resonances (endogenous frequencies) of more than 3 dB within the perceptible range should be...