Basic Structure
The middle ear implant generally consists of key components such as a microphone, amplifier, speech processor, signal transmission pathway, and output transducers.
Mechanism of Action
The microphone in the middle ear implant captures external sound signals and converts them into electrical signals. After processing by the amplifier and speech processor, the signals are transmitted through the signal pathway to the middle ear's vibration system, where they are coupled (often at the ossicular chain, the round window membrane, or the mastoid bone). The electrical signals are then converted into vibrations that drive the middle ear's vibrating device, transmitting the sound signals to the inner ear via bone conduction. Currently, two widely used types of middle ear implants in clinical practice are the Vibrant Soundbridge (VSB) and the Bonebridge (BB).
VSB is a semi-implantable electromagnetic middle ear implant composed of two main parts: an external audio processor (AP) and the implanted Vibrant Ossicular Reconstructive Prosthesis (VORP). The AP includes a microphone, external chip, transmitter coil, magnet, and battery, whereas the VORP consists of a receiving coil, magnet, demodulator, wires, and a connected Floating Mass Transducer (FMT). The AP is attached to the scalp behind the ear. It collects, analyzes, and encodes the sound signals, converting them into electrical signals, which are transmitted through the skin to the receiver coil within the VORP. These electrical pulse signals drive the FMT to vibrate, which in turn either vibrates the ossicular chain or transmits vibrations directly to the inner ear through the vestibular or cochlear windows.
The BB, also a semi-implantable middle ear implant, was introduced into clinical use in 2016 in China. The Bonebridge consists of an external audio processor (AP) and a Bone-Conduction Implant (BCI). The AP includes a microphone, external coil, magnet, and battery, while the BCI comprises a receiving coil, magnet, demodulator, and a Bone Conduction Floating Mass Transducer (BC-FMT). Its working principle is similar to that of the VSB. The AP collects external sound signals, processes and encodes them, and transmits them across the intact skin flap as electromagnetic waves. The implanted coil receives these signals, which are then processed by the demodulator. The demodulator commands the BC-FMT to emit mechanical vibrations. These vibrations are transmitted through the temporal bone to the inner ear, creating the perception of bone-conducted sound.
Indications
Middle ear implants are indicated for patients with conductive or mixed hearing loss as well as those with unilateral deafness. In clinical practice, the use of BB has surpassed VSB. BB is primarily applied in cases of congenital external or middle ear malformations, residual conditions of chronic middle ear infections, and unilateral hearing loss.