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

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.

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.

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.

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.

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.