A hearing aid amplifies incoming sound but there must be functioning hair cells and an intact nerve to transmit that amplified signal to the central auditory pathways. A CI, on the other hand, attempts to replace a function lost by the cochlea, usually due to an absence of stimulable hair cells. In a normal hearing ear, the hair cells within the cochlea act as transducers of mechanical and hydraulic vibration of the tympanic membrane, ossicles of the middle ear and perilymph and endolymph of the inner ear to chemo-electric energy capable of stimulating the eighth nerve. The decrease in hair cells causing a sensory hearing loss results in the cochlea losing its ability to stimulate the eighth nerve. The cochlear implant replaces the function of the lost hair cells by converting mechanical energy (sound waves) into electrical energy capable of exciting the auditory nerve.CIs are surgically placed within the inner ear, bypassing the hair cells of the cochlea and directly stimulating the endings of the auditory nerve. Although there have been many variations on the theme, the basic design of an implant system has remained relatively stable over the years. It consists of an external microphone, processor and transmitter and an internal receiver-stimulator and electrode array.