Cochlear implant electrode arrays are designed with specific characteristics that allow for the preservation of intra-cochlear structures during the insertion process, as well as during explantation. Straight lateral wall (LW) electrode arrays and pre-curved modiolar hugging (MH) electrode arrays are the two types that are commercially available. Although there is a third type of electrode array called the mid-scala (MS), which is positioned in the middle of the scala tympani (ST), and is usually considered as an MH type of electrode. Different lengths of straight LW electrode arrays are currently available which allow for insertion across a range of different sized cochleae; however, due to manufacturing limitations, pre-curved MH electrodes are generally only available to cover the basal turn of the cochlea, while the spiral ganglion cells are distributed in the Rosenthal's canal that extends into 1.75 turns of the cochlea. Both straight LW and pre-curved MH electrodes can cause a certain degree of intra-cochlear trauma, but pre-curved MH electrodes tend to deviate into the scala vestibuli from the scala tympani more often than the straight LW electrodes, resulting in damage to the osseous spiral lamina/spiral ligament which could initiate new bone formation and eventually affect the cochlear implant users' hearing performance. Structural damage to the cochlea could also affect the vestibular function. With pre-curved MH electrodes, higher degrees of trauma are related to the fixed curling geometry of the electrode in relation to the variable coiling pattern of individual cochleae, the orientation of the electrode contacts in relation to the modiolus wall, and how effectively the stylet was handled by the surgeon during the procedure. Wire management, metal density, and the shore hardness of the silicone elastomer all contribute to the stiffness/flexibility of the electrode. It is important to acknowledge the impact of bringing the stimulating contacts closer to the modiolus wall with an MH electrode type in terms of the resultant damage to intra-cochlear structures. The presence of malformed cochleae should be identified and appropriate electrodes should be chosen for each specific cochlea, irrespective of the cochlear implant brand. In order to utilize drug therapy, the cochlea should be free from any trauma.

Determine the occurrence rate of cochlear implant (CI) electrode tip fold-over and electrode scalar deviation as reported in patient cases with different commercial electrode types. PubMed search for identifying peer-reviewed articles published till 2018 on CI electrode tip fold-over and scalar deviation. Key-words for searching were "Cochlear electrode tip fold-over", "Cochlear electrode scalar position" and "Cochlear electrode scalar location". Only if electrode related issues were investigated in patient cases. 38 articles met the inclusion-criteria. 13 articles on electrode tip fold-over issue covering 3177 implanted ears, out of which 50 ears were identified with electrode tip fold-over with an occurrence rate of 1.57%. Out of 50 ears, 43 were implanted with pre-curved electrodes and the remaining 7 with lateral-wall electrodes. One article reported on both tip fold-over and scalar deviation. 26 articles reported on the electrode scalar deviation covering an overall number of 2046 ears out of which, 458 were identified with electrode scalar deviation at a rate of 22.38%. After removing the studies that did not report on the number of electrodes per electrode type, it was 1324 ears implanted with pre-curved electrode and 507 ears with lateral-wall electrode. Out of 1324 pre-curved electrode implanted ears, 424 were reported with scalar deviation making an occurrence rate of 32%. Out of 507 lateral-wall electrode implanted ears, 43 were associated with scalar deviation at an occurrence rate of 6.7%. This literature review revealing the fact of higher rate of electrode insertion trauma associated with pre-curved electrode type irrespective of CI brand is one step closer to obsolete it from the clinical practice in the interest of patient's cochlear health.

OBJECTIVES: Capture the human inner-ear malformation types in 3D by segmenting the inner-ear structures from clinical CT (computed tomography) and MR (magnetic resonance) image datasets. Volumetric analysis was done to find the variations in the volume of cochlear part alone from complete inner-ear followed by 3D printing from the 3D segmented models. MATERIALS and METHODS: Using 3D slicer freeware, the complete inner-ear structures were segmented from anonymized CT and MR image by setting a tight grey-scale threshold to avoid capturing unwanted structures followed by volumetric analysis of the cochlear part alone. 3D printing was done using Form labs desktop 3D printer. RESULTS: We identified 2x normal anatomy (NA) cochlea, 1x enlarged vestibular aqueduct syndrome (EVAS), 3x incomplete partition (IP) type-I, 4x IP type-II, 3x IP type-III, 5x common cavity (CC) and 5x cochlear hypoplasia (CH). 3D segmented models along with the 3D printed models showed huge variation in size, shape and the anatomy among the image data-sets analyzed. Volumetric analysis showed that on average, volume of CC was above 150mm3, volume of CH fell below 80mm3, Volume of NA, EVAS and IP-I were all around 85-105mm3 whereas the volume of IP-II was around 50mm3. CONCLUSION: Visualizing human inner-ear malformation types in 3D both as computer models and as 3D printed models is a whole-new experience as demonstrated in this study. The volumetric analysis showed a huge variation among the volume of cochlear part alone among the malformation types. Cite this article as: Dhanasingh A, Dietz A, Jolly C, Roland P. Human Inner-ear Malformation Types Captured in 3D. J Int Adv Otol 2019; 15(1): 77-82.

The optimal placement of a cochlear implant (CI) electrode inside the scala tympani compartment to create an effective electrode–neural interface is the base for a successful CI treatment. The characteristics of an effective electrode design include (a) electrode matching every possible variation in the inner ear size, shape, and anatomy, (b) electrically covering most of the neuronal elements, and (c) preserving intra-cochlear structures, even in non-hearing preservation surgeries. Flexible electrode arrays of various lengths are required to reach an angular insertion depth of 680° to which neuronal cell bodies are angularly distributed and to minimize the rate of electrode scalar deviation. At the time of writing this article, the current scientific evidence indicates that straight lateral wall electrode outperforms perimodiolar electrode by preventing electrode tip fold-over and scalar deviation. Most of the available literature on electrode insertion depth and hearing outcomes supports the practice of physically placing an electrode to cover both the basal and middle turns of the cochlea. This is only achievable with longer straight lateral wall electrodes as single-sized and pre-shaped perimodiolar electrodes have limitations in reaching beyond the basal turn of the cochlea and in offering consistent modiolar hugging placement in every cochlea. For malformed inner ear anatomies that lack a central modiolar trunk, the perimodiolar electrode is not an effective electrode choice. Most of the literature has failed to demonstrate superiority in hearing outcomes when comparing perimodiolar electrodes with straight lateral wall electrodes from single CI manufacturers. In summary, flexible and straight lateral wall electrode type is reported to be gentle to intra-cochlear structures and has the potential to electrically stimulate most of the neuronal elements, which are necessary in bringing full benefit of the CI device to recipients.

Signal processing algorithms are the hidden components in the audio processor that converts the received acoustic signal into electrical impulses while maintaining as much relevant information as possible. Signal processing algorithms should be smart enough to mimic the functionality of external, middle and the inner-ear to provide the cochlear implant (CI) user with a hearing experience as natural as possible. Modern sound processing strategies are based on the continuous interleaved sampling (CIS) strategy proposed by B. Wilson in 1991, which provided envelope information over several intracochlear electrodes. The CIS strategy brought significant gains in speech perception. Translational research activities of MED-EL resulted in further improvements in speech understanding in noisy environments as well as enjoyment of music by not only coding CIS-based envelope information, but by also representing temporal fine structure information in the stimulation patterns of the apical channels. Further developments include “complete cochlear coverage” made possible by deep insertion of the intracochlear electrode, elaborate front end processing, anatomy based fitting (ABF), triphasic pulse stimulation instrumental in the suppression of facial nerve stimulation, and bimodal delay compensation allowing unilateral CI users to experience hearing with hearing aids on the contralateral ear. The large number of hardware developments might be exemplified by the RONDO, the world's first single unit audio processor in 2013. This article covers the milestones of translational research around the signal processing and audio processor topic that took place in association with MED-EL. Graphical Abstract

With cochlear implantation (CI) being the standard of care for profoundly deaf cases, more and more patients with low frequency residual hearing are currently being treated with CI. In view of preserving the residual hearing, the ultimate aim of both the surgeons and the CI companies is to achieve zero‐degree of electrode insertion trauma. Variations in the size and shape of cochlea, cross‐sectional dimensions of ST, electrode insertion techniques with and without metal stylet rod and the experience level of the operating surgeons, all play a role in the electrode array related insertion trauma. An effective electrode design must include flexible array to accommodate the cochlear shape variation, electrode with variety of array lengths to support the concept of cochlear size specific electrode array and finally smaller cross‐sectional dimensions of electrode array in matching the cross‐sectional dimensions of ST. As per published reports, FLEX electrode array design offers minimal degree of electrode insertion trauma along with the possibility of patient specific electrode array length matching their cochlear size. Looking at the cross‐sectional dimensions of FLEX electrode array along with its volume, it appear to be highly safe to the cochlea by not taking too much volume inside the ST. To offer additional support, otological pre‐planning software tool like OTOPLAN is now clinically available in measuring the cochlear size in finding the best electrode array match along with the possibilities of anatomy based post‐operative speech processor fitting.