A cochlear implant is a small electronic device that electrically stimulates the cochlear nerve (nerve for hearing). The implant has external and internal parts. The external part sits behind the ear. It picks up sounds with a microphone. It then processes the sound and transmits it to the internal part of the implant. Having said that, cochlear implants are the world's most successful medical prostheses in that less than 0.2% of recipients reject it or do not use it and the failure rate needing reimplantation is around 0.5%. Having said that, cochlear implants are the world's most successful medical prostheses in that less than 0.2% of recipients reject it or do not use it and the failure rate needing reimplantation is around 0.5%.