Visual prosthesis

Retinal implants are prostheses for low -vision and blind people, their receptor cells of the retina have ( retina) lost due to illness their function whose optic nerve still intact and transmits to the brain, as va occurs in advanced retinitis pigmentosa ( RP).

Principle of operation

There are different approaches to replace the function of degenerated receptor cells of the retina artificially. The operating principle is identical but essentially: Photos from the area are converted into electrical impulses and transmitted to the nerves.

In recent years, two promising systems have been developed, the subretinal implant and the epiretinal implant. The subretinal implant is inserted into the eye below the retina, while the epiretinal implant is implanted on the retina.

In contrast to the implants of the hearing organ ( cochlea implant), the subretinal retinal implants are still in an experimental stage, which allows the perception of light and shadow at best, but no recognition of objects or orientation in space allows. Initial tests in patients have been made, a routine clinical use, however, is currently not yet in sight. However, the development has progressed and now the research team (Tübingen / Reutlingen) has Eberhart Zrenner Professor the subretinal chips so far developed that form visualization is possible. Miikka T. was able to distinguish in Tübingen an apple of a banana and read his name in giant letters ( and thereby a typo in it discover ). On 18 December 2009 Professor Eberhart Zrenner received in Munich Karl -Heinz Beckurts Award for his commitment to research and development of subretinal chip.

The subretinal implant

The subretinal retinal implant consists essentially of

• Photodiode array
• Microchip
• Stimulation electrode array
• Power supply unit

The implant is located between the retina and the choroid, and is only fixed by the intraocular pressure.

In simplified converts the photodiode array, the incident on the retinal image in a 2- dimensonale distribution of electrical impulses. In this case, each diode element provides a pulse according to the intensity of the irradiating light. The spatial resolution of the image on the retina primarily depends on the spatial density of the diode array. This diode arrays come with 1500 diodes on an area of ​​approximately 3x3 mm ² used. This corresponds to a field of view of 10 ° to 12 °. So far, a visual acuity of more than 0,037 could be achieved

The electrical signals from the array are then amplified by the integrated circuit in the microchip and passed through the stimulation electrodes to the nerve cells in the intact retina. The energy required for the amplification process is input from the outside by infrared rays or induction in the system. Since the photo- diodes and the stimulation electrodes are arranged next to each other on the chip, no further processing of the signals is necessary.

In July 2013, Retina Implant AG, Reutlingen has successfully completed the conformity assessment procedure for the Retina Implant Alpha IMS and the implant since then labeled with the CE mark.

The epiretinal implant

The epiretinal implant consists of

• Video camera ( usually integrated into a pair of glasses )
• Microchip
• Transmission unit for transmitting the processed camera data to the implant
• Stimulation electrode array
• Power supply unit

The implant is located on the retina. Because the vitreous is removed routinely at implantation, the implant on the retina needs to be fixed. The surgery for epiretinal implants is simple and secure than the subretinal approach.

Epiretinal implant when the image is taken by an external video camera and converted into electric signals. These are then transmitted wirelessly to an implanted electrode array, which stimulates the nerve in accordance with the retina. The biggest drawback of the epiretinal implant is that the image is not included in the eye, but by an external camera. Thereby, not the natural movement of the eye can be used to capture the surroundings. It must be the camera to be rotated to achieve its change of the viewing direction. Moreover, in the epiretinal implant the signal processing is performed before the transmission of external signals to the electrode array, whereas in the sub-retinal implant, the " signal processing " is performed by the eye itself. This leads to an increased complexity of the system.

The first epiretinal implant, the ARGUS unit, consisting of a platinum - array with 16 electrodes. The Phase I clinical trial with ARGUS began in 2002 with the implantation of six patients with the device. The ARGUS II device contains 60 electrodes. Preliminary results in 30 patients were published in 2012.