Variational anatomy of the human cochlea: implications for cochlear implantation.

Hypothesis: To study variations in human cochlea anatomy with potential implications for cochlear implantation surgery.

Background: A comprehension of the anatomic variations of the human cochlea is essential for understanding the degree of surgical trauma induced by inserting various electrode arrays in cochlear implantation surgery. Variations in anatomy may also limit the potential for performing hearing preservation.

Methods: We studied 73 archival, nonselected, adult, corrosion casts of human inner ears. Anatomic reference points were constructed from photographic reproductions taken at different angles, and various dimensions were assessed using planimetry. Anatomic variants with particular clinical/surgical interests were pinpointed.

Results: Results showed that the human cochlea is individually shaped, varying greatly in dimensions ("fingerprint"). The outer cochlear wall length ranged from 38.6 to 45.6 mm with a mean length of 42.0 mm. The first turn represented 53% of the total length and ranged from 20.3 to 24.3 mm. The number of quadrants varied from slightly more than 8 to 12. The facial nerve canal ran in close proximity to the upper first turn explaining facial nerve excitement during stimulation of electrodes in this region in some instances. The internal diameter (height) of the cochlear tube in the first turn varied broadly (1.6-2.6 mm), occasionally with limited space for conventional implants.

Conclusion: The human cochlea exhibits extensive anatomic variations. These variations will influence the location of cochlear implant arrays and affect the potential of hearing preservation surgery. Our results may explain the surgeon's difficulties sometimes to insert electrode arrays even in so-called "normal" cochleae.