An In-Vitro Insertion-Force Study of Magnetically Guided Lateral-Wall Cochlear-Implant Electrode Arrays

Abstract

Hypothesis:Insertion forces can be reduced by magnetically guiding the tip of lateral-wall cochlear-implant electrode arrays during insertion via both cochleostomy and the round window.Background:Steerable electrode arrays have the potential to minimize intracochlear trauma by reducing the severity of contact between the electrode-array tip and the cochlear wall. However, steerable electrode arrays typically have increased stiffness associated with the steering mechanism. In addition, steerable electrode arrays are typically designed to curve in the direction of the basal turn, which is not ideal for round-window insertions, as the cochlear hook's curvature is in the opposite direction. Lateral-wall electrode arrays can be modified to include magnets at their tips, augmenting their superior flexibility with a steering mechanism. By applying magnetic torque to the tip, an electrode array can be navigated through the cochlear hook and the basal turn.Methods:Automated insertions of candidate electrode arrays are conducted into a scala-tympani phantom with either a cochleostomy or round-window opening. The phantom is mounted on a multi-degree-of-freedom force sensor. An external magnet applies the necessary magnetic bending torque to the magnetic tip of a modified clinical electrode array, coordinated with the insertion, with the goal of directing the tip down the lumen. Steering of the electrode array is verified through a camera.Results:Statistical t-test results indicate that magnetic guidance does reduce insertion forces by as much as 50% with certain electrode-array models. Direct tip contact with the medial wall through the cochlear hook and the lateral wall of the basal turn is completely eliminated. The magnetic field required to accomplish these insertions varied from 77 to 225 mT based on the volume of the magnet at the tip of the electrode array. Alteration of the tip to accommodate a tiny magnet is minimal and does not change the insertion characteristic of the electrode array unless the tip shape is altered.Conclusion:Magnetic guidance can eliminate direct tip contact with the medial walls through the cochlear hook and the lateral walls of the basal turn. Insertion-force reduction will vary based on the electrode-array model, but is statistically significant for all models tested. Successful steering of lateral-wall electrode arrays is accomplished while maintaining its superior flexibility.

Optimizing the Magnetic Dipole-Field Source for Magnetically Guided Cochlear-Implant Electrode-Array Insertions

“…10) will drastically impact the size of a permanent-magnet MDS, and the allowable current density will drastically impact the size of an electromagnetic MDS. Third, we have determined in our prior work [8] that the required magnetic field can vary substantially based on electrode-array models. Therefore, it may even be necessary to have electrode-array specific MDS models as well.…”

“…The necessary magnetic field to achieve successful guided electrode-array insertions has been determined by our prior work [8]. In that study, Flex-24 electrode arrays provided by MED-EL (Innsbruck, Austria) with a 4.73×10 −5 A·m 2 permanent magnet embedded in the EAT were guided successfully through a plastic scala-tympani phantom [7].…”

“…Assuming that the field magnitude necessary to achieve guided insertions of electrode arrays are known [8], then combining (2) and (4) yields the equation to compute the required minimum radius of the MDS.…”

“…As a nominal value for || m ||, we will use the magnets embedded in the EAT in our prior work [8] which has been determined to be 4.73 × 10 −5 A · m 2 . This represents the combined magnetic dipole for two 0.41-mm-long by 0.25-mm-diameter cylindrical magnets made of grade N52 NdFeB.…”

“…1) [6]. Repeatable automated insertions have recently been conducted in at-scale scala-tympani phantoms, designed with simulated cochleostomy and round-window openings [7], using clinical lateral-wall type EAs with magnets embedded at their tips [8]. Insertion forces were reduced by as much as 50% compared to nonguided insertions, and at the first turn in the basal plane, where a high percentage of basilar-membrane perforations occur, the EAT was never in contact with the lateral wall.…”