In vivo vs. in situ measures in cochlear implantation. Invited paper.

Most cochlear prostheses in use today are multi-channel implants that are inserted into the lower turns of the cochlear canal to exploit the tonotopic distribution of remaining eighth nerve fibers. The majority of implant candidates have some abnormal finding on pre-operative CT (computerized tomography) exams, and post-mortem studies of long-term implants show extensive intracochlear bone growth is common. Important anatomical issues in implantation therefore include 1) how do normal variations and pre-existing pathologies in each patient ear affect the success of an implant and 2) what is the time course and variability of pathologic responses to intracochlear electrodes. Although CT is the principal pre and post-operative imaging technique for assessing inner ear status of implant patients, there are few quantitative assessments of the capacity for CT to resolve fine details of inner ear anatomy or metallic intracochlear prostheses. In this study, we compared three-dimensional morphometric analyses of ultra-high resolution CT (UHRCT) images of long-term implantations and normal temporal bones implanted post-mortem with light microscope data from these ears sectioned at 20 m with the electrodes in place. Sectioning the electrodes in situ prevented the distortions of tissue-electrode relationships inherent in removal of the electrode arrays and allowed comparison of CT vs. histologic findings on electrode position. "Digital dissections" based on fluid and electrode attenuation characteristics were used to form 3-D reconstructions of the inner ear labyrinth and electrode arrays. Comparisons of 3-D measures from CT and light microscopy show CT-based estimates of cochlear dimensions and electrode position vary less than 5% from histologic measurements. UHRCT techniques also reliably detected bony and fibrous cochlear canal occlusions in long-term implants. Because these techniques are equally applicable to patients and animal models, we conclude they have considerable potential for in vivo longitudinal studies of implant effects and provide a rapid, reliable survey method for post-mortem temporal bone studies.