Safety Considerations of Cochlear Implantation

Electrochemical and mechanical performance of reduced graphene oxide, conductive hydrogel, and electrodeposited Pt-Ir coated electrodes: an active in vitro study

OBJECTIVE

To systematically compare the in vitro electrochemical and mechanical properties of several electrode coatings that have been reported to increase the efficacy of medical bionics devices by increasing the amount of charge that can be delivered safely to the target neural tissue.

APPROACH

Smooth platinum (Pt) ring and disc electrodes were coated with reduced graphene oxide, conductive hydrogel, or electrodeposited Pt-Ir. Electrodes with coatings were compared with uncoated smooth Pt electrodes before and after an in vitro accelerated aging protocol. The various coatings were compared mechanically using the adhesion-by-tape test. Electrodes were stimulated in saline for 24 hours/day 7 days/week for 21 d at 85 °C (1.6-year equivalence) at a constant charge density of 200 µC/cm²/phase. Electrodes were graded on surface corrosion and trace analysis of Pt in the electrolyte after aging. Electrochemical measurements performed before, during, and after aging included electrochemical impedance spectroscopy, cyclic voltammetry, and charge injection limit and impedance from voltage transient recordings.

MAIN RESULTS

All three coatings adhered well to smooth Pt and exhibited electrochemical advantage over smooth Pt electrodes prior to aging. After aging, graphene coated electrodes displayed a stimulation-induced increase in impedance and reduction in the charge injection limit (p   <  0.001), alongside extensive corrosion and release of Pt into the electrolyte. In contrast, both conductive hydrogel and Pt-Ir coated electrodes had smaller impedances and larger charge injection limits than smooth Pt electrodes (p   <  0.001) following aging regardless of the stimulus level and with little evidence of corrosion or Pt dissolution.

SIGNIFICANCE

This study rigorously tested the mechanical and electrochemical performance of electrode coatings in vitro and provided suitable candidates for future in vivo testing.

Cochlear implant electrode insertion: Jacobson's nerve, a useful anatomical landmark

The classical technique in approaching the scala tympani through the round window niche by way of facial recess has not always been successful. There have been recent reports on false insertion of the electrode array into the hypotympanic cells tracts or infralabrynithine cells tracts. In these cases the round window niche are situated posterior, or obliterated due to bony growth, or sclerosed at the basal turn of the cochlea. Therefore, successful intubation of the scala tympani much depends upon the position of the round window niche and patency of the basal turn of the cochlea. The present study was conducted in an attempt to elucidate factors that determine the ease of insertion of an electrode array, and to by-pass the initial turn of the basal cochlea (hook area). The jacobson's nerve (JN) landmark was used to perform cochleostomy by drilling anteriorly or through the nerve. The inferior segment position of the basal turn of the cochlea studied in relation to the JN and successful intubation of the scala tympani was achieved. Also, critical anatomical measurements were made pertaining to this surgical technique.

Microminiature molding techniques for cochlear electrode arrays

We provide a general method for producing a variety of small, complex electrode arrays based on injection molds produced using computer-aided drafting and machining (CAD-CAM) procedures and a novel method for connecting to the very fine electrical leads associated with the individual contacts of such arrays. Cat-sized cochlear electrode arrays with up to eight contacts were built according to these methods and their electrical contacts were characterized in vitro by impedance spectroscopy and in vivo by monitoring impedance for over 1 year of intermittent stimulation in chronically instrumented animals.

Computer-generated three-dimensional reconstruction of the cochlea

Computer-generated three-dimensional reconstructions of the nerve fibers from the organ of Corti to the spiral ganglion were used to determine the optimum maximal length of the cochlear implant electrode. The spiral ganglion within the modiolus is much shorter than the organ of Corti. The spiral ganglion has 1 3/4 turns and reaches no higher than the middle of the second turn of the organ of Corti, which has 2 3/4 turns. The spiral ganglion is concentric and basal with respect to the organ of Corti. The spiral ganglion dendrites within the osseous spiral lamina of the basal turn project radially, nearly perpendicular to the central axis of the modiolus. Upon entering the modiolus, they turn basally at an angle of approximately 120 degrees. The projection of dendrites within the osseous spiral lamina became increasingly oblique as the ganglion extended apically. The organization of the cochlear nerve results from the spiraling of the ganglion. These findings are in agreement with previous reports. Implications of these findings and their possible relevance to the optimum length of the cochlear implant electrode are discussed with reference to cochlear damage resulting from longer electrodes.

Medical or surgical complications related to the Nucleus multichannel cochlear implant

A questionnaire was sent to 152 surgeons to survey complications associated with the implantation of the Nucleus multichannel cochlear implant. Complications were categorized as life-threatening; major, if they necessitated revision surgery; or minor, if they resolved spontaneously or with minimal treatment. A total of 55 complications occurred in 459 reported operations for an overall complication rate of 11.8%. There were no deaths, but there was one life-threatening complication, a case of meningitis. There were 23 (4.8%) major complications, most of which involved flap design or electrode insertion (and included the case of meningitis). There were 32 (7%) minor complications. Most of the complications might have been avoided by proper training, planning of the operations, and careful attention to detail. We recommend that all prospective implant surgeons attend a device-specific training course and practice in the temporal bone laboratory.