Overview of Osseointegrated Bone-Conducting Hearing Protheses
Hearing loss is a major health issue globally that tends to become more common and severe as we age. People with hearing loss often struggle with understanding speech and identifying where sounds are coming from, especially in places like the workplace or social gatherings. Older adults with hearing loss may feel socially isolated, less self-reliant, and could even be at risk for early signs of dementia. The most common treatment for hearing loss is the usage of hearing aids, which are devices that make sounds louder.
Hearing aids are often the first choice for treating hearing loss because they are non-invasive and far less expensive than other options. These devices include a microphone to capture sound, an amplifier to make the sound louder, and a speaker to broadcast the louder sound. Despite its proven positive effects on daily communication, emotional health, and brain function, many people do not consistently use their hearing aids because they may not work perfectly, producing issues like insufficient volume increase, echo-like feedback, problems with detecting sound direction, and the sensation of a blocked ear.
There are two main types of hearing aids: air conduction and bone conduction hearing aids. Air conduction hearing aids use a microphone, amplifier, and speaker placed in the ear canal to make sounds louder. Bone conduction hearing aids work similarly, but the sounds are translated into vibrations by the device, which are then sent directly to the inner ear. To make this possible, the device has to be positioned near the hard part of the skull behind the ear, usually attached to a headband or eyeglasses.
There’s another device called an osseointegrated bone conduction prosthesis, which is surgically implanted by a specialist in ear, nose, and throat conditions. This can be done in a straightforward one or two-step procedure usually with local or general anesthesia. The device is embedded directly into the skull bone behind the ear and becomes a part of it, a process known as osseointegration. This implanted device provides certain benefits over conventional hearing aids. It gets rid of any need for in-ear or in-canal devices, which can cause discomfort and lead to buildup of moisture in the ear. It also stops the device from pressing on soft tissues, which can be an issue with bone conduction hearing aids. Lastly, this solution could be a suitable choice for those who cannot use conventional hearing aids due to birth defects, changes in ear anatomy, or chronic ear infections. The effectiveness of this device can be predicted in patients who have hearing loss caused by issues with sound conduction through the ear, who have noticed better hearing with conventional bone conduction aids. People who have had better hearing with these aids are likely to be satisfied with the implanted device due to the lack of soft tissues in between the device and the skull bone affecting sound transmission.
Anatomy and Physiology of Osseointegrated Bone-Conducting Hearing Protheses
The system responsible for helping us hear, also known as the auditory system, is divided into three parts: the outer, middle, and inner ear. Each of these parts plays a critical role in how we hear sounds.
The outer ear includes the parts you can see, like the auricle or pinna, the external part of your ear canal, and the eardrum or tympanic membrane. The main job of the outer ear is to capture and recognize sounds and to create a pathway to the middle ear. The eardrum helps to separate the outer ear from the middle ear, and it also turns sound waves into vibrations.
Next is the middle ear, which is a small, hollow space in your temporal bone (the bone that surrounds your ear) that is filled with air. Inside the middle ear are three tiny bones together called the ossicles, along with their respective muscles and ligaments. These three bones are named the malleus, incus, and stapes. The malleus connects to the eardrum on one side and to the incus on the other; the incus connects to the stapes, which in turn connects to a membrane known as the oval window in the cochlea. The job of the middle ear is to amplify the vibrations created by the eardrum and deliver them to the inner ear, which contains organs for hearing and maintaining balance.
The inner ear is surrounded by an outer bony and inner membranous labyrinth, which is located at the petrous portion of the temporal bone. The outer bony labyrinth is filled with a fluid called perilymph, and the inner membranous labyrinth is filled by an endolymph fluid. The parts of your inner ear that helps with hearing are the cochlea and the vestibulocochlear nerve. The cochlea is an organ shaped like a snail that has three parts: the vestibular duct, cochlear duct, and tympanic duct. The vibration that comes from the oval window enters the vestibular duct and travels along the cochlea toward the tympanic duct. The pressure at the oval window creates a change in the pressure of the perilymphatic fluid, which moves across the outer ducts and triggers a vibration of the basilar membrane in the cochlear duct. This vibration makes the hair cells situated at the organ of Corti move. The hair cells’ job is to make neural impulses from mechanical impulses. Once the mechanical impulses are created, the vestibulocochlear nerve sends them to the central nervous system, which helps us understand and interpret the sounds.
Why do People Need Osseointegrated Bone-Conducting Hearing Protheses
To be considered for a bone conduction prosthesis, which is a type of hearing aid that uses direct bone contact to transmit sound, patients need to meet certain hearing and medical requirements.
Hearing requirements are:
1. If a person has hearing loss that’s either all or partly caused by problems with the ear’s ability to conduct sound, their bone-conducted hearing should be less than 45 decibels and they should be able to correctly identify 60% or more of words in a speech test.
2. For people with deafness in one ear, the hearing in their unaffected ear needs to be normal.
Medical conditions that might make a person suitable for the procedure include:
1. Chronic ear disease, such as persistent outer or middle ear infections that get worse when using regular hearing aids.
2. Malformations in the structure of the ear, either acquired or present from birth. This could include a closed or blocked ear canal, bony growths in the ear canal, or patients who’ve undergone ear surgeries and have issues with a hearing aid producing unwanted noises.
3. Patients who can’t tolerate the drawbacks of regular hearing aids like discomfort due to the feeling of blockage in the ear.
4. Individuals who experience hearing loss conducted through their only functioning ear.
When a Person Should Avoid Osseointegrated Bone-Conducting Hearing Protheses
There are some situations where certain people should not have a particular procedure. These are known as ‘absolute contraindications’. For example, this procedure should not be performed on:
- Children under the age of 5.
- People who have a hearing loss greater than 55 dB, which means their hearing is severely impaired.
- People who have had developmental delays, mental health conditions, or autism, particularly if they cannot keep themselves clean.
There are also ‘relative contraindications’, which are conditions that might make the procedure more risky, but it can still be done in some cases. These include:
- People who have conditions that can cause unusual bone growth, such as Paget’s disease (a condition that disrupts the normal replacement of bone tissue), osteoporosis (weakening of the bones), or those who have had radiation treatment.
- People who have a history of forming keloids, which are raised scars that go beyond the boundary of the original injury.
- People who have a disease near the ear that needs to be monitored with MRI scans.
Equipment used for Osseointegrated Bone-Conducting Hearing Protheses
Your surgeon will need specific tools to carry out the implantation of an osseointegrated bone conduction prosthesis. This is a device that helps people who can’t hear in the usual way, to hear sounds through bone conduction. The equipment is as follows:
1. A marker.
2. A 27 gauge needle, which is a small, thin needle.
3. Hemostat, used to control bleeding.
4. A ruler for precise measurements.
5. A 10 ml syringe filled with 1% Lidocaine with epinephrine 1:100,000. This is an anesthetic, which helps to numb the area to reduce pain.
6. A surgical tool with a #15 blade.
7. Bipolar cautery, used to stop blood vessels from bleeding.
8. Self Retractors, to keep the surgical area open and clear.
9. Various surgical instruments such as a dummy device, raspatorium (for scraping bone), dissector (for separating tissues), drill, drill indicator, torque wrenches (for applying a specific amount of force), screwdriver, and an abutment inserter (for placing the abutment).
10. A specific drill console provided by the manufacturer
11. Drilling disposable: guide drill and countersink drill (used to widen the drilled hole to 3-4 mm).
12. Implants: Abutment (connects the implant to the prosthesis) and Fixture (part of the implant that is screwed into the bone). Several sizes are provided by the manufacturer.
13. Punch Biopsy of 5 mm size, used to gather tissue samples.
14. 4-0 Vicryl suture, used for sewing up wounds and incisions.
15. 5-0 fast absorbing plain gut suture, a string-like material used to hold tissue and skin together.
16. Thin nonadherent dressing, used to cover wounds while not sticking to the area.
17. Bacitracin antibiotic ointment, to prevent infection.
18. Healing cap, helps protect the site as it heals after the surgery.
Who is needed to perform Osseointegrated Bone-Conducting Hearing Protheses?
Ear, nose, and throat doctors (also known as otolaryngologists) and hearing specialists (audiologists) are the team who put in place a special type of hearing device through surgery, called an osseointegrated bone conduction prosthesis. Ear, nose, and throat doctors take care of treating conditions that are related to these parts of the body. Their job in putting in the hearing device is to fully examine the patient’s medical history and current health state to see if they can safely have the device implanted.
Also, these doctors decide if the procedure needs to be done all at once or in two parts. Doing the procedure in two parts is usually advised for children under 9 years or adults who have problems with a certain bone in the skull, the temporal bone. Meanwhile, the hearing specialist conducts a complete hearing test before the surgery. They also have an important role once the surgery is done.
These specialists take care of managing the sound processor (this is the part of the hearing device which amplifies sounds). They make sure the sound processor fits the patient properly and doesn’t touch the skin, as it can cause unpleasant echoing. They also make sure the surgery was done correctly. Plus, they guide the patient on how to use the processor device, like changing the battery and understanding the different control buttons.
Preparing for Osseointegrated Bone-Conducting Hearing Protheses
Before any surgery, the hospital team needs to secure proper documentation, such as consent forms, and should answer any questions or concerns you may have. Just before surgery, you will be moved to the operating room (OR), where you’ll lie flat on your back (this is called the supine position) with your head in the highest position. Before the procedure begins, the surgical team will perform a ‘time-out’; this is a final check to make sure they have the right patient, right procedure, and right side.
After this, an anesthesiologist will give you either a local anesthetic with a sedative (which numbs a particular area of your body while you remain conscious) or general anesthetic (where you are put to sleep). Once the anesthesia is in effect, the operating table will be rotated 180 degrees away from the anesthesiologist, to give the surgical team enough room to operate.
Your head will then be turned to the side opposite to where the surgery will be done to ensure a good view and easy access to the surgical area. To prepare for the surgery, the surgical team will clean the area on your body where they plan to do the surgery with a beta-dine solution, a kind of antiseptic that kills germs and prevents infection. Finally, sterile drapes will be placed around the surgical area, as if you were undergoing a mastoid procedure (this is a type of ear surgery which is similar to many other types of surgeries in how the patient is draped).
How is Osseointegrated Bone-Conducting Hearing Protheses performed
The first step in the procedure is to identify where the implant will be located. This will be in the region behind the ear, just below a natural ridge on your skull known as linea temporalis. The distance from the opening of the ear canal should be about 50-55 mm. A placeholder (dummy) of the implant can be used to ensure the right location and to prevent the sound processor from touching the ear. Next, a vertical line is marked on the skin where the cut will be made. The line will be about 4-5 cm long, and the implant site will sit at the midpoint of the line.
The cut will be about 20-30 mm from the edge of the ear and roughly 10 mm in front of where the implant is marked to be. Hair in this area will be shaved off. Then, the whole area is cleaned and covered in a sterile wrap. The thickness of the skin and underlying fatty tissues at the implant site is then measured using a thin needle. This helps to select the right implant size.
The patient is injected with a local anesthetic (1% lidocaine with epinephrine 1:100,000) at the marked sites. The amount injected will depend on whether the procedure is being done under local or general anesthesia.
A vertical cut is made in the skin with a surgical scalpel, down to the layer covering the bone. Retractors, which are medical devices that hold the skin and tissues apart, are used to expose the bone. A small area of bone is then revealed by making a cut in the periosteum (the layer covering the bone) and lifting it up.
The site for the implant is then prepared by drilling a small hole in the bone. The drill is set to a high speed and the hole is drilled to a depth of 3 mm, then 4 mm, making sure to avoid damaging any underlying structures. The hole is then widened to accommodate the implant. The drill is again set to a high speed for this.
Next, the implant and its holder (the abutment) are placed into the hole at a low drill speed. The drill will stop when the implant is at the correct depth. Then the drill and holder for the implant are gently removed.
The cut in the skin is stitched up using absorbable sutures, and a small piece of skin overlaying the holder of the implant is removed with a biopsy punch (a device that removes a small circular section of skin). A healing cap is placed over the holder, which is then covered with a special dressing soaked in a non-prescription antibiotic ointment. Finally, the area is covered with a protective dressing.
Possible Complications of Osseointegrated Bone-Conducting Hearing Protheses
When a bone conduction prosthesis is inserted into the bone (a procedure called osseointegration), there can sometimes be complications. These problems can either affect the bone or the surrounding soft tissue, with the latter being more frequent. Soft tissue complications can include irritation or inflammation of the area, overgrowth of the skin around the implant or the formation of thickened scars and keloids, which are hard, raised scars.
Most of these issues can be managed with regular outpatient treatment. To help lower the risk of any complications with the surrounding tissue, it’s important for the patient to receive clear instructions on how to keep the area clean and healthy. Regular check-ups with an ear, nose, and throat specialist (otolaryngologist) and an audiologist (a healthcare professional who treats hearing and balance disorders) are also crucial. If these steps aren’t taken, the chances of preventable tissue complications might increase.
Problems with the integration of the prosthesis into the bone can also occur. This is when the implant doesn’t bond successfully with the bone and can lead to expulsion of the implant. If this happens, the implant will need to be removed and inserted again. This issue is more common in children under 5 years old because their skulls are thinner and their bone quality isn’t as robust yet.
What Else Should I Know About Osseointegrated Bone-Conducting Hearing Protheses?
An osseointegrated bone prosthesis is a reversible surgical treatment that can help people who have trouble hearing and can’t use hearing aids because of issues like deformed ears or ongoing ear infections. ‘Osseointegrated’ simply means that the prosthesis is designed to attach to the bone.
People with permanent hearing loss in one ear who have been fitted with this prosthesis say they have a better quality of sound and can hear better in both quiet and noisy places. As per the feedback in questionnaires, they feel that the device improves their quality of life and general well-being. Most patients (around 90%) are very compliant, using the device for more than eight hours a day, every day. Patients with hearing loss in both ears may experience even better results with two of these devices.
Previously, people with deafness in one ear would be given hearing aids that transferred the sound from the deaf ear to the good ear. These hearing aids were often disliked by the patients because they were uncomfortable, looked unpleasant, and didn’t work well in noisy areas. Recent studies have shown that the osseointegrated bone prosthesis, which is surgically implanted, results in better hearing, like understanding speech in noisy environments, compared to these old-style hearing aids. However, it must be mentioned that neither this device nor the old-style hearing aids helped with figuring out where the sound was coming from.
The osseointegrated bone prosthesis has been successfully implanted in children with hearing loss, often caused by birth defects or continuous ear infections. However, it must be noted that the thickness of the skull in children is not fully developed until they are about five years old, meaning that surgeons usually delay the implantation procedure until then. Conducting surgery before this age may lead to complications. Kids younger than five years old could try using traditional hearing aids attached to a soft band while they wait for their skulls to mature sufficiently for the implant.
After getting these implants, children show a big improvement in their hearing ability, from an average hearing threshold (the softest sound a person can hear) of 65 decibels (dB) before surgery to 30 dB after. These devices can help kids develop their speech at a normal pace while leaving the door open for corrective surgery in the future if needed. Studies comparing early reconstructive surgery to treat hearing loss with the osseointegrated bone prosthesis showed the latter had better results.
