Middle Ear Implant : Transducer, Vibroplasty

The primary reason for developing middle ear implants was to overcome the many shortcomings of a conventional hearing aid (CHA). Middle ear implants are suitable for patients who are unable to adjust to hearing aids, have issues with them, or for whom conventional hearing aids are not advisable. These are options for hearing aids implanted in the ear. Middle-ear implants are based on two basic technologies.

• Electromagnetic stimulation principle: In this system, an electromagnetic coil takes the electric impulses to generate an electromagnetic field. This electromagnetic field can then cause movement or vibrations in a magnet attached to an ossicle, and these vibrations can travel to the inner ear. 
• Piezoelectric stimulation principle: An electric input is given to a piezomagnetic active material, which will change or deform its size and shape. If this is, for example, attached to an ossicle, it will cause vibrations which are used in patients to transmit vibrations from the middle ear to the inner ear for the conduction of sound.
• There are also new technologies like infrared used in such middle ear implants. 

Non-Ideal Features of Conventional Hearing Aids Devices

Middle ear implants are considered due to these drawbacks of conventional hearing aids: 

• Insufficient amplification: If the patient has been given the best setting possible but is still not able to get the best amplification.
• Acoustic feedback: When there is a lot of acoustic feedback, like excessive noise or distortion.
• Spectral distortion occurs when there is a peak capping effect or sound waves are distorted. The ear does not sense this as the right sound. 
• Non-linear or harmonic distortion: In such cases the impulse makes the patient feel uncomfortable in using the hearing aid. 
• Occlusion of the external auditory canal: This occurs when the hearing aid device enters the external auditory canal and causes an occlusion effect. Due to this, the patient can have numerous canal problems and may develop infections. There may be multiple issues.
• Appearance/ Visibility: This is a cosmetic concern and important to the patient. Whatever the age, nobody likes to wear hearing and due to the social stigma attached to it.  Preference is for something that is invisible and can go into the ear. 
• Lack of directionality

Also Read: Acoustic Neuroma: Classification, Causes, Clinical Features, Symptoms

The Hearing Aid Problem

The problems that arise when a hearing aid is used are:

• Sound distortion and lack of clarity: The patient can hear sound, but there is a lack of clarity.  Cannot differentiate between phonetically sounding words in the speech. This will make the speech incomprehensible. This is a major problem with hearing aids. These include: 
• Feedback effects
• Effect of gain in various frequencies: The hearing aid will be amplified at a certain frequency. However, the adjacent frequencies get amplified too. This will give a sound and make the patient uncomfortable.  
• The speaker/listener is the main limiting factor in the amplification and is important in the amplification of hearing aids. This is not something that can be worked on.
• Poor cosmetic appearance
• Discomfort in using the hearing aid
• Difficulty in adjusting the settings of the hearing aid: Patients may have difficulty listening to a program with the hearing aid and may have to go to the audiologist every time to get it adjusted. This is very troublesome for the patient.

Historical Aspects Of Hearing Aid

• The first hearing aid was made in 1935 by Wilska, who kept small pieces of iron in the tympanic membrane and a magnetic coil in the canal to move the iron with the tympanic membrane. When the electromagnetic coil received an electric impulse, iron being a magnetic substance, the electromagnetic forces made it vibrate and caused movement of the tympanic membrane.
• Vernon and his colleagues studied the first piezoelectric system in 1972. A piezoelectric material changes shape and size when exposed to electrical stimulation, causing vibrations or movements. These vibrations are transmitted to the ossicles, where they help transmit sound.
• Yanagihara and Suzuki developed the first clinically used piezoelectric hearing aid.

Also Read: Otitis Media : Types, Causes, Symptoms and Treatment

Basic Principles Of Middle Ear Implant Hearing Aid

• Piezoelectric Systems: The basic principle is that when an electric current is applied to the piezoelectric system, there is a change in the volume of the crystal, and it will vibrate.  The crystal is placed inside the ear, maybe near the ossicles, and outside there is a transducer or stimulator that gives out electric signals. These signals will cause the crystal to deform in shape and size and vibrate up and down. These vibrations will be transmitted to the ossicle and cause transmission of the sound.
• Electromagnetic System: This system consists of a transmitting coil that converts electrical impulses into electromagnetic signals.  A magnet is attached to the ossicles which will move due to the electromagnetic signals and cause transmission of sound.

The Vibrant Soundbridge

This is one of the devices under middle ear implants.

• The vibrant sound bridge is a partially implantable electromagnetic system and has been approved by the US FDA. This has two components.
  • Internal component: This is called the Vibrating Ossicular Prosthesis (VORP).  It is the implanted part. It consists of
    • The internal receiver
    • The conducting link
    • Magnet
    • The Floating Mass Transducer (FMT)
  • External component: This is the external amplification system or the audio processor. This is held on to the head, hidden by the hair, with the magnet. The electronic process converts sound waves to a signal that can be received by the internal receiver. It consists of
    • The microphone
    • The battery

Surgical Implantation of The Vibrant Soundbridge

• Implantation of The Vibrant Soundbridge is a simple process in which a post-aural incision is made.
• A space is made for implanting the internal components.
• The internal receiver is placed in the skin behind the patient’s ear.
• The FMT is attached to one of the bones of the middle ear, mostly the incus.

The Middle Ear Transducer

• This implant has been made by the company Otologics LLC in Boulder, Colorado.
• It has two components.
  • External component: This is located behind the ear and consists of the microphone, speech processor, battery, and transmitter.
  • Implant:  It consists of the subcutaneous electronics package with the transcutaneous receiver.

Working Of the Middle ear transducer

• The middle ear transducer receives the electronic probe coupled to the body of the incus.
• The transducer sends impulses for movement of the probe with the electromagnetic waves.
• As a result, the ossicles will vibrate and sound will get transmitted.  
• The tip of the probe is made of aluminum oxide, which forms a fibrous connection with the incus.
• The ossicular chain is not touched.

Soundtec Direct Drive Hearing System

• This is another middle ear implant system. 
• It is a partially implanted middle ear device.
• It has two components:
  • External component: This is located behind the ear. It consists of:
    • A microphone, amplifier, and a power amplifier      
    • An electromagnetic coil in deep ear mold.
• Internally implanted component: Nd-Fe-Bo magnet with a total weight of 27mg. 

Procedure for implantation

• The magnet is sealed to the ear joint.
• There is an assembly in the ear canal from which there is an electromagnetic coil.
• This coil will give vibrations to the implant, which is attached to the incus. 

Also Read: Malignant Otitis Externa: Symptoms, Diagnosis and Treatment

Direct Acoustic Cochlear Implant (DACI) 

• This is not a cochlear implant but is just named so. 
• It was developed in Switzerland. 
• It consists of an
  • Electromagnetic transducer
  • External audio processor
• This type of middle ear implant is indicated when there is profound hearing loss.

The Procedure for Implantation

• The stapes is totally removed
• A second stapes prosthesis is placed parallel to the oval and is attached to the incus of the patient's own incus.
• There is an external processor that gives impulses to the implant body with the receiver coil.
• From there arises an implant actuator that connects the incus and stapes.
• From here the impulses will go to the canal.

Fully Implantable Envoy Esteem Device

• This type of implant is based on the piezoelectric sensor. It is a fully implantable device.
• The piezosensor is placed on the incus body
• This acts as an internal microphone, which is different from the previous implants discussed.
• A driver cemented to the head of the stapes. This is the piezoelectric (PE) crystal.
• When there is sensor stimulation, the PE crystal will change shape and size and will cause vibrations.
• These vibrations are then carried to the canal.

• Sound is picked up by the internal microphone and goes to the sound processor, which is recessed in the temporal lobe.
• The sound then travels to the driver which is placed in the stapes region. 

Also Read: Furunculosis: Unraveling Causes, Symptoms, and Management

Semi-Implantable Middle Ear Electromagnetic Hearing Device

• This is also called as SIMEHD
• A magnet is cemented to the body of the incus.
• An implantable electromagnetic coil is supported by a titanium frame fixed to the temporal bone.
• The SIMEHD has been implanted so far in cats.

Piezoelectric Round Window Implant with Infrared Optical Signal

• This is a combination of infra-red technology with the piezoelectric systems
• A microtransducer is placed on the round window, this can receive power and signal transmission through an infrared optical transmitter located in an external unit.
• It also has a microphone, battery, and sound processor and is placed in the ear canal.
• These will develop infrared optical signals which will go to the transducer which is kept on the round window.
• From here the vibrations will go to the inner canal. 
• This system can be implanted naturally without mastoidectomy.
• This system has not yet been tested in vivo.

Earlens Tympanic Contact Transducer

• In this system, a magnet is placed in a silicone lens that sticks to the tympanic membrane due to oil-induced surface tension.
• A small induction coil is placed in the ear canal.

Patient Selection- Ideal Candidate

• Patients in which there is high frequency sensorineural or mixed hearing loss.
• The first is to see if the patients benefit from a conventional hearing aid. If not, then middle ear implants can be considered as the hearing aids have failed to provide amplification.
• The patient should not have any skin condition that will prevent the attachment of the external component of the implant.
• The patient should be medically fit for surgery and anesthesia.

Patient Selection-Audiological Considerations

• Current devices are mostly suitable for mild to severe SNHL.
• Hearing loss should be stable or gradually progressive. 
• The ear which is severely affected is first selected for the implant. This is mainly because if the implant is not successful, the patient will have a good ear that is functioning.

Patient Selection-Otological Considerations

• There should be the absence of retrocochlear or central involvement in hearing loss. 
• No middle ear inflammation.
• The external auditory canal (EAC) must be assessed if suitable for an external processor or is fit to hold the processor.

Types of Vibroplasty

• TypeA of vibroplasty
  • When an AMEI is coupled on an intact ossicular chain and is indicated in patients with mild to moderate SNHL (Sensorineural Hearing loss).  There are different points of attachment to the ossicular chain.  The footplate is found to be most favorable. 
• Type B vibroplasty
  • In this, some parts of the ossicular chain may not be available. So, the Vibrant Sound Bridge (VBS) is attached to a remnant of the ossicular chain, mostly the stapes or its footplate.  Research is being done to determine if the power of amplification can be increased by using additional couplers or passive prostheses. 
• Type C vibroplasty
  • The actuator here is coupled to one of the middle ear window membranes. The ossicles are not used. 
• Type D vibroplasty
  • It involves direct coupling of the AMEI to the inner ear fluid using the oval window.
•  The DACI system was designed for this application. The VSB combined with conventional stapes pistons can also be used in this type.

Complications Of Vibroplasty

• Damage to the chorda tympani
• Dislocation of the FMT, particularly in Type C vibroplasty
• Extrusion 
• Tinnitus
• Deterioration of bone conduction
• Vertigo with the CODACS system 
• Device malfunction
• Facial weakness

Benefits Of Vibroplasty

• Improved quality of sound
• Elimination of occlusive effect
• Better sound quality than CHA
• Direct ossicular stimulation maximizes the ability to hear high-fidelity, naturally produced sound.
• The proximity of the implant transducer to the cochlea reduces distortion.

Hope you found this blog helpful for your ENT Residency Otology Preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs. 

Propel your ENT Residency Preparation! Access conceptual video lectures, QBank, Mock Tests, and premium study resources on the PrepLadder App. Download it today!