Cochlear Coverage – Impact of insertion depth of flexible electrodes on hearing outcome and surgical complications

The increasing prevalence of hearing loss requires effective therapy and rehabilitation approaches. Cochlear implants (CI) are used to restore hearing in sensory hearing loss. The aim of this study was to investigate complications related to the electrode design, such as incomplete insertion and kinking of the electrode tip. In addition, the influence of cochlear size on angular insertion depth (AID) and its hearing outcomes after complete insertion was investigated. We retrospectively analyzed CT data of 42 FLEX28-CI patients with normal cochlear anatomy. The 3D slicer DICOM viewer was used to visualize and measure the cochlear size, position verification of the electrode array, electrode tip-fold over and AID. The hearing outcome with CI was measured with word recognition test (Freiburger Speech Test) at 65 dB and 80 dB and correlated with the AID of the most apical electrode. The study showed that the electrode array was fully inserted into the cochlea in all patients without tip-foldover. The cochlea size varied between 8.3 mm and 10.0 mm in the diameter of the basal turn. Only 3 of 42 ears showed scalar dislocation of the electrodes. The AID of the most apical electrode ranged from 450° to 760° and correlated negatively with cochlear size. Apart from a few special cases, it can be concluded that the AID of the most apical electrode correlates positively with the hearing outcome. A 28 mm long, flexible electrode can be fully inserted into the cochlea, with the cochlear size influencing the AID. This electrode minimizes structural damage and tip-foldover. The AID appears to have a positive impact on hearing outcomes.