Cochlear implant electrode design for safe and effective treatment

Abstract

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.

Keywords: cochlear implant; electrical stimulation; pre-curved electrode; straight electrode; trauma.

Figure 1

Anatomical variation of the human inner ear. Anonymized CT scans of temporal bones with different inner ear anatomies were kindly provided by St. Petersburg ENT and Speech Research Institute, Russia (33). (A) Normal anatomy; (B) almost normal cochlea anatomy but with an enlarged vestibular sac; cochlear hypoplasia with (C) 2 turns of the cochlear lumen; (D) 1½ turns of the cochlear lumen; (E) approximately 1 turn of the cochlear lumen; (F) only ½ turn of the cochlear lumen; (G) incomplete partition type II with enlarged vestibular aqueduct sac; (H–M) severe form of cochlear hypoplasia; (N) cochlear hypoplasia with bud-like cochlea; (O) incomplete partition type I; (P,Q) common cavity; (R) cochlear aplasia with vestibular cavity; (S) cochlear aplasia; and (T) incomplete partition type III.

Figure 2

Cochlear size measured according to A-value in the oblique coronal view. CT scans showing the cochleae of different sizes are from anonymized subjects from St. Petersburg ENT and Speech Research Institute, Russia (33).

Figure 3

Postoperative images demonstrating STANDARD electrodes in three different sizes of cochleae (37). (A) An angular insertion depth (AID) of 720° in a regular-sized cochlea, (B) 630° of AID in a slightly bigger than regular sized cochlea, and (C) only 540° of AID in a significantly bigger sized cochlea. The white slanted line cutting through the electrode points to the cochlear entrance. Reproduced by permission of Williams and Wilkins Co.

Figure 4

Shape variations in the cochlear basal turn. (A) More circular, (B) elliptical, (C) extended elliptical-shaped, (D) triangular, and (E) a perimodiolar electrode with a fixed size and shape. 3D segmented inner ear from CT scans of cochleae with varying shapes of basal turn are from anonymized subjects from St. Petersburg ENT and Speech Research Institute, Russia (33).

Figure 5

Distribution of spiral ganglion cell bodies (SGCBs) (40). (A) Outline of Rosenthal’s canal housing SGCBs; (B) 3D segmentation of SGCBs from the synchrotron phase-contrast image demonstrating its presence up to 680°–720° (Source: Courtesy of Dr. Hao Li and Dr. Helge Rask-Andersen, University Uppsala, Sweden, and Prof. Hanif Ladak and Dr. Sumit Agrawal, Auditory Biophysics Laboratory, Western University, London, in Ontario, Canada); (C) percentage of SGCBs in different segments of the cochlea; and (D) cartoon version of a perimodiolar electrode unable to cover segment 4 with electrical stimulation.

Figure 6

Literature review of articles reporting on electrode angular insertion depth.

Figure 7

Neuronal coverage by electrode variants.

Figure 8

Literature review process of studies on electrode angular insertion depth and associated hearing outcomes.

Figure 9

Closer look at the human cochlea. (A) Endoscopic view of scala tympani (ST) entering through the round window (RW) entrance demonstrating blood vessels on the floor, basilar membrane on the top, porous bony wall on the right, and smooth lateral wall on the left (reproduced by permission of Dr. Richard Chole, Washington, United States). (B) Histology image of the mid-modiolar section of the human cochlea with a 28 mm-long electrode placed fully inside the ST (image courtesy: Prof. Thomas Lenarz and Peter Erfurt from Hannover Medical School, Hannover, Germany).

Figure 10

Illustration of electrode scalar deviation (A) and electrode tip fold-over (B) (30).

Figure 11

MED-EL fixation clip made of titanium. A surgical view of the fixation clip locking the electrode leads to the bony buttress of the middle ear.

Figure 12

Stiff probe (dummy) device with insertion depth markers.