The importance of human cochlear anatomy for the results of modiolus-hugging multichannel cochlear implants

Hypothesis: The fact that the anatomy of the basal turn of the human cochlea, especially, is essentially different from that of other species is likely to influence the outcome of cochlear implantation. Background: Multichannel cochlear implants give better speech understanding than single-channel devices. They are intended to make use of the tonotopic organization of the cochlea by selectively stimulating subpopulations of the auditory nerve. At higher stimulus levels and with monopolar stimulation, excitation of nerve fibers from other turns may interfere with this concept, especially with modiolus-hugging electrodes. Methods: A three-dimensional spiraling computer model of the human cochlea, based on histologic data, was used to test the spatial selectivity and the dynamic range before cross-turn stimulation takes place for the Clarion HiFocus implant with and without a positioner. The results were compared with a similar model of the guinea pig cochlea. Results: In humans (in contrast to the guinea pig), a well-designed modiolus-hugging electrode yielded reduced current thresholds and high spatial selectivity without reduction of the useful dynamic range. The apical turn of the human cochlea, however, is largely comparable in this respect with the guinea pig cochlea, where cross-turn stimulation reduces the dynamic range substantially. Conclusion: The clinical success of cochlear implantation in humans and the favorable results with modiolus-hugging devices depend on the anatomy of the human cochlea.