Bone Conduction Devices

Overview

Bone conduction devices (BCDs) are a class of hearing devices that can be surgically implanted or worn as a peripheral device without surgery. They work by creating sound waves that propagate through the bones of the skull and directly into the cochlea—the portion of the inner ear that detects sound. 

Bone conduction devices differ in many ways from traditional hearing aids. They are most often used to treat people with hearing loss that may not be suitable for hearing aids. While traditional hearing aids amplify sound waves through the ear canal to vibrate the eardrum, BCDs bypass these mechanical structures by vibrating sound through bone. Because of this mechanism, one BCD on either side of the head directly stimulates both inner ears at once with sound. This is helpful for people who have obstructions, damage, or other disorders of their ear canal, eardrum, or middle ear. Bone conduction devices can also be used in cases of deafness in just one ear caused by nerve damage or tumors.

Bone conduction devices come in many forms, including surgical implants and nonsurgical peripherals. 

Surgically implanted BCDs are available in a few different configurations:

• Active transcutaneous BCDs: These implants employ an external processor that is the visible part of the BCD, worn outside of the body. The processor contains microphones, a computer, a transmitter, a magnet, and a battery. These processors connect magnetically to an internal, surgically placed implant that houses the sound-generating motor called a transducer. The name of this implant refers to the active and motorized nature of the internal implant. Transcutaneous refers to the nature of the radio transmitter signal between the devices, which does not require a direct connection or break in the skin around the ear. 
• Passive transcutaneous BCDs: This older model of BCD implants uses an external processor that also contains the motor and transducer. Sound generation is thus on the outside of the head. The internal component includes a magnet and a titanium connector to the bone, and has no active or moving parts (hence the term passive). There is no direct connection or break in the skin around the ear.
• Percutaneous BCDs:  Another older model of BCD implant in which an external processor and external transducer are physically attached to a metallic connection with the skull. The metal connector, also called an abutment, is surgically implanted into the bone of the skull and extends through the skin. The process clips directly to this. These are different from transcutaneous BCDs that do not require an opening in the skin behind the ear.

Using many of the same parts, there are also nonsurgicalal categories of BCDs to consider. Those devices fall into the following general categories:

• Surgically implanted BCDs: The most common BCD. See above for the previous discussions of the various configurations of implants and the naming of these devices.
• Nonsurgical soft band BCD: These BCDs secure the active hearing unit (motor and processor in one piece) to the head via a soft headband. This holds the BCD against the skull to allow sound vibrations to pass through the skin and soft tissues and into the bone. These devices are commonly used in infants and young children during amplification trials to see if they help. Some people may go on to receive a surgically implanted BCD after a successful trial of this device. They are designed to maximize device retention (keep it on the head) while maintaining good hearing performance. Still, they do have some drawbacks and limitations compared to surgically implanted BCDs.
• Nonsurgical firm band BCD: These BCDs work similarly to soft band BCDs but utilize an adjustable outer mount made of firm materials. These BCDs are designed for improved aesthetics and comfort in older children, teenagers, and adults. This type of BCD has become widely available both through medical device manufacturers and non-medical corporations alike. As with soft band BCDs, these also have some drawbacks and limitations compared to surgically implanted BCDs.

What kind of hearing loss do BCDs treat?

In general, BCDs are used for the following types of hearing loss:

• Conductive hearing loss: This can include people whose outer or middle ear doesn’t transmit sound well due to chronic ear infections or persistent ear drainage, malformed or absent ear canals, or middle ear bone or ossicle problems that limit sound conduction. In these individuals, sound cannot travel efficiently through the outer or middle ear, but the inner ear works well.
• Mixed hearing loss: If there’s both conductive and sensorineural hearing loss and inner ear function is adequate, bone-anchored devices can still help individuals, especially when traditional hearing aids aren’t sufficient.
• Single-sided deafness (SSD) or asymmetric hearing loss (AHL): For individuals who are deaf in one ear and have normal or near-normal hearing in the other, bone-anchored devices can transmit sound from the deaf ear to the hearing ear, improving sound awareness and balance. Additionally, if you have hearing loss in both ears that asymmetrically favors one ear over the other, a BCD may be indicated.
• Children who can not use traditional hearing aids: Young children with congenital ear anomalies, like atresia, often use nonsurgical soft band BCDs early in life and may transition to a surgical implant when older.
• Adults who can not tolerate traditional hearing aids:  Adults with outer ear canal issues, chronic ear infections, or discomfort with traditional hearing aids often choose BCDs as an alternative. Additionally, some individuals opt for BCDs because conventional hearing aids don’t fit well, aren’t comfortable, or cause ongoing irritation in the ear canal.

With a bone conduction device, the sound transmission process starts by picking up sounds from the external environment via a small, external sound processor with microphones. This sits behind the ear or on the side of the head. Sound is then digitally processed and transmitted to a motor called a transducer. The transducer creates gentle vibrations that travel directly through the bones of the skull to reach the cochlea, or inner ear. From this point, as is the case with normal hearing, the cochlea communicates sound signals to the brain. 

During surgery for implantable BCDs, a small incision is made behind the ear, and the soft tissues are gently cleared away from the underlying bone in that location. Once the bone is exposed, a tiny metallic implant is placed into the skull bone as an attachment point. Typically, titanium is used for this as it reacts very well with human tissue and is extremely durable and lightweight. These bone implants are very small, usually measuring three to four millimeters (about half the size of a pea). From here, depending on the configuration of the BCD implant, the surgeon will attach either an active internal transducer, a passive internal transducer, or a percutaneous metal abutment. The former two BCDs remain fully internal, while the latter extends through a hole in the skin. Percutaneous BCDs are decreasing in popularity but still may have a role for some people.

Following the procedure, you’ll wear a small pressure dressing for one to two days over the implanted area to protect it. Aside from this, there is no other special wound care or other instructions. 

After surgery, your implant needs time to heal by integrating with the surrounding bone (osseointegration). Turning the device on too early can disrupt healing and prevent the bone attachment from stabilizing. For this reason, your surgeon and medical team will not activate the new implant for hearing until one month has passed from the day of surgery.

People are usually ready for activation when the incision has healed well and if there are no signs of infection or other rare complications. Surgeons and/or audiologists will assess readiness during follow-up visits.

The activation of a BCD for hearing typically includes the following steps:

1. Attaching the external sound processor for the first time
2. Programming the device so that sounds are comfortable, clear, and accurate
3. Gradually introducing greater sound levels
4. Instructions on how to wear, adjust, and maintain the device

When a BCD is first turned on, sounds may initially seem unfamiliar, as the brain needs time to adapt to hearing through bone conduction. As the device is fine-tuned, however, hearing will improve noticeably over the course of days or weeks.