Biomedical Engineering Reference
In-Depth Information
The piezoelectric transducer can provide an output close to 110 dB SPL (Kroll, Grant
et al., 2002; Shohet, 2008).
An audiologist programs the implant using a device called a
commander
. After the
device is programmed, patients are given a personal programmer that allows them to turn
the device on or off, to adjust the volume, and to remotely modify background noise filters.
The advantages of such a device are notable. Without any appliance in the external
auditory canal, the occlusion effect is eliminated. Uncoupling of the sensor and driver
eliminates most feedback.
The Envoy device faces some hurdles as development progresses. Its battery life is an
issue with an estimated life of only 3-5 years depending on use. However, it can be replaced
under local anesthetic. More importantly, removal of a portion of the incus permanently
alters the ossicular mechanism and prohibits full recovery of hearing to preimplantation
baseline levels if the device fails or is switched off. Modern ossicular reconstruction
techniques are not perfect and can restore hearing to only within 10 dB. Finally, functional
gain decreased at frequencies above 3000 Hz in the phase I study of the Envoy device.
This appears to have been substantially improved in the phase II studies.
6.7.2 Electromagnetic Hearing Devices
Electromagnetic hearing devices function by passing an electric current through a coil
to generate a magnetic flux that attracts or repels an adjacent magnet, as discussed in
Chapter 2. In a typical application the small, 50 mg magnet is attached to one of the
vibratory structures of the middle ear (e.g., tympanic membrane, ossicles) and induces the
structure to move in proportion to the strength of the applied magnetic field.. To date, all
of the research programs using this method use devices that are only partially implantable
and that still require an external hearing-aid shell to house the electromagnet. In some
cases, the external coil is sufficiently small to be housed in a CIC type of hearing-aid shell,
thus putting the electromagnet as close to the permanent magnet as possible. The major
disadvantage of this setup is that as power transmission is determined by the alignment of
the two elements so a slight shift of coil position in the outer ear can result in unpredictable
or inadequate power coupling. In addition, the anatomy of the middle ear space restricts
the size of the magnet and the coil so the performance of these devices is limited.
6.7.2.1 Vibrant Soundbridge Device
One example of an electromagnetic device is the Vibrant Soundbridge, which is shown
in Figure 6-25. Originally developed by Symphonix Devices, Inc., the Soundbridge was
the first implantable middle-ear hearing device approved by the U.S. Food and Drug
Administration (FDA) to treat sensorineural hearing loss. It was marketed and implanted
in the United States for a few years until the technology was purchased by Med-El of
Austria. It is now marketed by Vibrant Med-El and available for implantation in Europe.
Over 1400 such devices have been implanted worldwide (Traynor and Fredrickson, 2008).
The Soundbridge device is a semi-implantable device consisting of an external sound
processor and amplifier, an audio processor, and an internal vibrating ossicular prosthesis
(VORP). Sound passes into a microphone within the postauricular audio processor and
is transmitted through the skin to an implanted receiver on the VORP using amplitude
modulation. The VORP, which is implanted just behind the ear, conducts the sound to a
magnet surrounded by a coil called the floating mass transducer (FMT). The transducer is
