Cochlear implant electrode insertion: The round window revisited

OBJECTIVE: To examine aspects of round window (RW) anatomy that are relevant to its use as a portal for atraumatic insertion of cochlear implant electrodes. STUDY DESIGN: Anatomic study using human cadaveric temporal bones. METHODS: A series of 30 temporal bones was dissected to permit microscopic study of the RW region. RESULTS: The bony overhangs of the RW niche limit visibility of the RW membrane during surgery. Measurements of RW membrane area visible through a facial recess opening before and after drilling the overhangs in 15 temporal bones showed that RW membrane visibility is typically increased by a factor of 1.5 to 3 times after drilling and by as much as 13 times when the opening of the RW niche is relatively small. Observations from within the scala tympani in 15 cochlear dissections showed substantial variability in size of the RW opening available for electrode insertion. Area measurements of the portion of the RW covered by the vertical segment of the RW membrane ranged from 0.8 to 1.75 mm in these specimens. In addition, irregularities in contour of the RW margin may make insertion challenging, which may necessitate drilling the anterior-inferior margin of the RW. Drilling in this region should be approached with care because of the close proximity of the cochlear aqueduct opening. CONCLUSION: RW insertion can be performed in a manner that is potentially less traumatic than the standard cochleostomy insertion. It may therefore be advantageous in cases in which hearing preservation is the goal.