Friday, April 8, 2011

Hearing System is Totally Implantable

Apr 6, 2011 12:56 PM
By Rong Z. Gan, Professor of Biomedical and Mechanical Engineering - University of Oklahoma rgan@ou.edu


Biomedical researcher and University of Oklahoma Professor of Biomedical and Mechanical Engineering Rong Gan (right) listens as a colleague discusses auditory research.


Biomedical researcher and University of Oklahoma Professor of Biomedical and Mechanical Engineering Rong Gan (right) listens as a colleague discusses auditory research.

Editor’s note: The author led the University of Oklahoma research team responsible for the technology described in this article.

Challenges associated with developing the ideal implantable hearing device (fully implanted and acceptable by patients who are uncomfortable with conventional hearing aids) are three-fold: 1) minimize risks to patient’s hearing and nerves within the ear so that the driving system of the device fits the restrictions of the middle ear size with life-time function stability; 2) lower the costs associated with the development of the devices as well as surgical implantation so that implantable hearing devices can be compatible with conventional digital hearing aids in cost/benefit ratio; and 3) enhance efficacy of the device so that enough gain can be delivered to aid severe hearing loss within limitations of the capacity and recharging cycles of available batteries.

These challenges, however, appear to have been met with a newly developed totally implantable hearing system (TIHS) developed by a University of Oklahoma research team. Feasibility studies show the system capable of delivering acoustic vibrations to the middle ear ossicular chain or cochlea with minimal energy loss and distortion.

Designed for simpler surgery

While several implantable hearing devices equipped with electromagnetic or piezoelectric transducers have been investigated or developed in the US and Europe since 1990, these devices are often associated with surgical difficulty.

To overcome such difficulties, the University of Oklahoma team set out to design a system that achieves the following: minimal surgical impact on contacting nerves; no significant effect on the patient’s residual hearing; no sensation of the implant movement; and tolerance of variations in the patient’s anatomy and exact position achieved by the surgeon.

The preliminary design and function evaluation on the TIHS was completed using a 3D FE (finite element) model of the human ear and the temporal bones with laser Doppler vibrometry. The human ear model consists of accurate anatomic structures of the external ear canal, eardrum, middle ear ossicular chain, middle ear cavity, and the uncoiled cochlea. The location, orientation, and dimensions of the implant transducers including the implant magnet, ossicular attachment, and implantable coil, are determined in the model within the constraints of the middle ear and external ear canal anatomy. This model is used to conduct acoustic-structure-fluid coupled analysis as well as electromagnetic coupling.
Otologic and neuro-otologic surgeon Dr. Mark Wood examines Tony Howard, a patient with bi-lateral cochlear implants, who looks forward to an opportunity to benefit from a restorative totally implantable hearing systems under development at the University of Oklahoma.

Otologic and neuro-otologic surgeon Dr. Mark Wood examines Tony Howard, a patient with bi-lateral cochlear implants, who looks forward to an opportunity to benefit from a restorative totally implantable hearing systems under development at the University of Oklahoma.
FE model tests characterize the performance of an electromagnetic hearing device across the auditory frequency range including the following aspects: mass loading effect on residual hearing with the passive implant; efficiency of electromagnetic coupling between implanted coil and magnet; efficiency of the forward mechanical driving (the actuator implanted in ossicles) and reverse driving (the actuator placed on round window membrane); and function characterization of whole unit in response to acoustic input across the skin (implantable microphone).

The mechanism of acoustic-electrical-mechanical transmission in the TIHS is typical of electromagnetic-type transducers used in middle ear implantable hearing devices. The assembly of the implantable (or transcutaneous) microphone, DSP/audio signal processor and rechargeable battery, and the RF controlled system of the TIHS utilize the technologies similar to other such devices. However, the coil and implant transducer design and the transcanal surgical approach for implantation are different from other implantable middle ear hearing devices.

The TIHS is much simpler than other middle ear implantable hearing devices in design, manufacturing, and surgical implantation. Thus, this technology may reduce both the surgical cost of middle ear implantable device and the cost of manufacturing them.

Structure

The TIHS consists of an implant transducer (magnet) placed on the middle ear ossicles, an implantable coil placed under the ear canal bony wall, an assembly of implantable microphone, DSP-audio signal processor (sound amplifier) and rechargeable battery placed in the sub-postcranial area under the skin, and a remote control unit with a battery charger as the external components.


Figure 1. Schematic of the TIHS in the right ear.








Figure 2. Block diagram of acoustic-electric-mechanical signal transmission of the TIHS.

Figure 1 is a schematic of the TIHS in the ear with implant transducer attached to the ossicles and the coil implanted under the canal wall (the external parts are not shown). The block diagram of Figure 2 displays the acoustic-electrical-mechanical signal transmission of the TIHS. Sound signals are received by an implantable microphone and converted as electrical analog signals. The analog signals are then converted as digital signals and amplified through the DSP/audio signal processor, and finally input to the coil. The interaction between electromagnetic fields of the coil and implant magnet induces the vibration of the ossicles. Therefore, the performance of TIHS is described by the movement of the stapes, which can be derived from the FE model of the ear and measured in human cadaver ears or temporal bones.


Figure 3. Implant transducer Model I with coil placed under the ear canal.

Two implant transducers: Model I and Model II are designed to meet different patient hearing situations. Implant Model I is designed for the ears with normal ossicular chain (Figure 3). The implant is attached to the long process of the incus and the head of stapes with the ossicular attachment made from Nitinol, a shape-memory biocompatible alloy material; the implantable coil is placed in the ear canal wall. A good alignment between the implant magnet and coil within ± 5 degrees was achieved through the design with the model. The permanent magnet (e.g., Neodymium-iron boron Nd2Fe14B) is hermetically sealed inside a titanium canister.


Figure 4. Implant transducer Model II with coil placed under the ear canal.

Implant Model II is specified for the ears with disrupted ossicular chain. For the case of missing of the incus, Figure 4 shows the implant Model II as an assembly of the implant magnet and ossicular attachment placed or fixed between the malleus and stapes from the posterior-medial view. The design completed in FE model ensures the alignment of the coil and implant magnet. The implant Model II functions as active incus replacement or active partial ossicular replacement prosthesis (PORP), driven by electromagnetic coupling between the implant magnet and coil. The coil is hermetically sealed inside a titanium canister and implanted in the posterior side of the ear canal bony wall for both Models I and II.

Advantages

The advantages of the TIHS are many. With it, there is no mastoidectomy, facial recess, and manipulation of ossicular chain during the implantation of TIHS. The ossicular attachment made of the shape-memory material eliminates the destruction of the ossicles. The implantation of the coil under the ear canal wall as the trans-canal surgical approach is commonly accepted by otologic surgeons. The assembly of the implantable microphone, audio/DSP signal processor with RF telemetry assembly, and rechargeable battery is a sub-postcranial pocket and will be implanted in the post-cranial area under the skin.

These advantages make it possible for a forward-thinking company to apply this well-tested technology with a good benefit-to-cost ratio and move it from prototype to finished product. Then, 38 million Americans who have moderate-to-severe sensorineural hearing loss will have an opportunity to have restored what most of us take for granted.

3 comments:

  1. The Sensor senses vibrations from the ear drum (tympanic membrane) and middle ear bones (ossicles) and converts these mechanical vibrations into electrical signals that are sent to the Sound Processor (see picture above). The Sound Processor receives the electrical signal from the Sensor, and amplifies and filters the signal to compensate for the individual patient’s hearing loss profile. The Driver converts the enhanced electrical signal received from the Sound Processor back to mechanical energy (vibrations). The vibrations are transmitted to the inner ear where they are perceived as sound. To implant the device, the Esteem® requires disruption of the normal middle ear structure (ossicular chain) by partially removing one of the bones (incus).automateandvalidate

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  2. These challenges, however, appear to have been met with gladys

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