The internal dimensions of the cochlear scalae with special reference to cochlear electrode insertion trauma

To investigate the intracochlear micromorphology with regard to frequent patterns of cochlear electrode insertion trauma.

Cochlear implantation is a widely accepted treatment for deafness and high-grade sensorineural hearing loss. Although the device and the implantation methods are continuously optimized, damage of intracochlear structures due to electrode insertion is a frequent finding in temporal bone studies. Reduction of insertional trauma is important for the preservation of residual hearing and on the background of increasing numbers of cochlear implant recipients.

This study was performed with histologic specimens from the "Wittmaack temporal bone collection" (Hamburg, Germany) to examine the diameters of intracochlear spaces and to correlate the micromorphology of cochlear ducts to frequent patterns of intracochlear insertion trauma.

The diameter of the scala tympani decreases by approximately 300 microm during the ascending part of the basal turn. In this region, the intersegmental decrease exceeds the assumed linear diameter decrease significantly (p < or = 0.001). The regression of the cross-sectional diameter is accompanied by a shift of the spiral osseous lamina toward the scala tympani and by narrowing of the bony capsule of the cochlea.

Various attempts have been made to evaluate the dimensions of the cochlea related to cochlear implantation. Little attention was paid to the distinct narrowing of the scala tympani in the region of the ascending part of the cochlear duct, although from the literature, it is known that electrode insertion trauma frequently occurs here. Individual variations of the cochlear micromorphology may additionally contribute to the failure of preformed electrode arrays, but the challenge of guiding the electrode array around the first bend of the cochlear turn, that is, the pars ascendens, is obviously impaired by the interindividually constant narrowing in this area. Therefore, this finding may have implications on the development of electrode designs and insertion methods.