The success of intracochlear electrode implantation hinges on the ability of peripheral auditory neurons to survive long-term interface with an electrode array and prolonged electrical stimulation. Histopathological studies have been conducted on a series of neomycin sulphate otointoxicated cats to assess the long-term effects of electrode implantation on an impaired cochlea (analogous to potential human sensory deaf implant candidates).
The results of these studies may be summarized as follows: 1. Long-term electrode implantation can be achieved without significant loss of primary auditory neurons using an electrode array embedded in Silastic which is molded to conform to the basal scala tympani if surgical implantation has not disrupted the basilar membrane or osseous spiral lamina. 2. A fibrous tissue matrix forms around the Silastic, displacing perilymph and effectively sealing the electrode at the round window preventing perilymph fistula. 3. While osteoneogenesis will occur whenever the periosteum of the bony labyrinth is disrupted, the molded Silastic sheathing prevents electrode displacement by small areas of new bone formations. 4. Traumatic electrode insertion, with fracture of the osseous spiral lamina or rupture of the basilar membrane, produces severe loss of neural elements in the region of injury and extensive new bone formation preventing satisfactory electrical stimulation.
The findings suggest that long-term cochlear implantation is feasible in sensory deafened animals using Silastic sheathed electrode arrays which have been molded to conform to the basal scala tympani provided the integrity of the scala tympani, osseous spiral lamina and basilar membrane is maintained.