Morphometric Measurements of the Incomplete Partition Type II (IP-II) Cochlea and Implications on Cochlear Implantation.

Hypothesis: The objective of this study is to obtain comprehensive morphometric measurements of the incomplete partition type II (IP-II) cochlea to provide a better understanding of intracochlear anatomy and important considerations for electrode selection and insertion.

Background: IP-II is the most common bony inner ear malformation that often requires cochlear implantation. Currently, there is significant controversy on electrode selection due to a lack of research that can provide reliable, high-resolution measurements.

Methods: Three-dimensional reconstructions of the cochlea were made from hematoxylin and eosin-stained slides from 11 archival human temporal bones from 8 adult IP-II patients (one paired) and 2 fetuses. Detailed measurements of the angular and linear length of the spiral ganglion neurons and cochlear duct at the modiolar and lateral wall of the scala tympani as well as cross-sectional areas and vertical height measurements of the scala tympani at 90-degree intervals were measured.

Results: The spiral ganglia neurons terminated at 540.5 ± 45.4 degrees, which corresponded to the beginning of the interscalar septal defect. The corresponding Rosenthal's canal length was 12.75 ± 0.82 mm, and the lateral wall length was 23.95 ± 1.04. The average cochlear duct length was 32.44 mm ± 1.58 mm, corresponding to an average angular distance of 951.6 ± 80 degrees. The modiolar height demonstrated less variation within the scala tympani but was significantly smaller at 0 and 90 degrees compared with the normal cochlea. The lateral wall height was also significantly smaller at 0, 180, and 540 degrees. There was a drastic decrease in lateral wall height at 540 degrees to 0.4 mm, which is smaller than the apical dimension of many electrodes.

Conclusion: This is the first study to provide detailed morphometric measurements of the IP-II cochlea including spiral ganglion neuron length and scala tympani height. These measurements directly relate to electrode selection for cochlear implantation.