Middle Ear Implants:

Middle ear implants, as the name suggests, are designed to stimulate the structures in the middle ear, and can be used to treat conductive and sensorineural hearing losses.

Direct stimulation of the ossicles can be used to treat conductive hearing losses due to obstructions in the ear canal and defects of the tympanic membrane and proximal ossicular chain. If the ossicular chain is damaged or absent, stimulation via the round window can be effective.

Direct stimulation of the ossicles allows for increased energy input to the inner ear, so sensorineural hearing losses can also be treated. 

Middle ear implants also produce less feedback than conventional hearing aids, and avoid the occlusion effect associated with hearing aids.

For some patients the cosmetic appearance is preferable, particularly for the totally implantable devices.

How they work:

There are two ways to transmit energy to the ossicles, either using an electromagnetic transducer or a piezoelectric transducer. In an electromagnetic transducer, electric current is passed through a coil, inducing a magnetic field, which then moves a small magnet attached to the ossicular chain, transmitting vibrations to the middle ear structures and then the inner ear. This is the technology used in the MedEl Soundbridge and the Otologics Carina.
The alternative approach involves piezoelectric technology, where an electric current changes the shape of a piezoelectric crystal and hence transmits vibration energy. The energy transmitted is limited by the size of the crystal. This is the technology used in the Envoy Esteem.

A number of  other middle ear devices have been tested in the past, but currently the three mentioned above are the main devices that have received FDA/CE approval and are available for clinical use.

Indications:

Conductive hearing loss not adequately treated by middle ear surgery.

Mixed or sensorineural hearing loss where hearing aids are not effective or tolerated:

Occlusion effect or discomfort from hearing aid.

Chronic otitis externa or allergy making hearing aid use difficult.

Limited gain from hearing aid / feedback if turned up too high

Better sound quality / hearing in noise from middle ear implant

Better cosmesis with middle ear implant (small in the ear hearing aids may not be powerful enough, and more powerful hearing aids are usually bigger and more visible).

Disadvantages:

Cost: Middle ear implants are more expensive than hearing aids, and in addition there are the costs of surgery to consider.

Risks of surgery: There are always risks associated with middle ear surgery, although these risks are small in experienced hands. Some devices involve disrupting the natural hearing mechanism (Envoy Esteem).

MRI Incompatibility: MRI is contraindicated for patients who have middle ear implants based on an electromagnetic transducer.

Despite these disadvantages, the promise of better hearing quality, hearing restoration when hearing aids and middle ear surgery are not suitable, and totally implantable devices that allow for swimming and better cosmesis means that the field of middle ear implants is constantly evolving.