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# Magnetic Guidance of Cochlear Implants: Proof-of-Concept and Initial Feasibility Study

[+] Author and Article Information
James R. Clark

Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112 james.r.clark@utah.edu

Lisandro Leon1

Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112 l.leon@utah.edu

Frank M. Warren

Department of Otolaryngology, Division of Otology and Skull Base Surgery, Oregon Health & Science University, Portland, OR 97239warrenf@ohsu.edu

Jake J. Abbott

Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112jake.abbott@utah.edu

1

Corresponding author.

J. Med. Devices 6(3), 035002 (Aug 10, 2012) (8 pages) doi:10.1115/1.4007099 History: Received July 29, 2011; Revised April 03, 2012; Published August 10, 2012; Online August 10, 2012

## Abstract

Cochlear implants have become a standard treatment for many with severe to profound sensorineural hearing loss. However, delicate cochlear structures can be damaged during surgical insertion, which can lead to loss of residual hearing and decreased implant effectiveness. We propose a magnetic guidance concept in which a magnetically tipped cochlear implant is guided as it is inserted into the cochlea. In a scaled in vitro experimental study, we record insertion forces for nonguided and magnetically guided insertion experiments and compare the results. Results indicate that magnetic guidance reduced insertion forces by approximately 50%. Using first principles, we discuss the effects of scaling down our in vitro experiments, and account for realistic clinical dimensions. We conclude that scale–down effects are negligible, but to produce the same field strength as in our experiments and provide sufficient clearance between the patient and the manipulator, the magnet dimensions should be increased by approximately four times.

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## Figures

Figure 1

Cochlear implant system with blow-up of cochlea cross-section showing the location of several cochlear structures (National Institutes of Health public domain image with added labels). Labeled items are the (1) microphone and speech processor, (2) transmitter, (3) receiver, (4) electrode array inserted into the cochlea (referred to as the “cochlear implant’’ herein), (5) auditory nerve, (6) ear drum, and (7) ossicles.

Figure 2

Some causes of intracochlear trauma during cochlear implant insertions: (a) tip scraping [4], (b) tip fold-over [1], and (c) buckling [(2),4]

Figure 3

Concept for magnetically guided cochlear implant surgery. Red wide arrows indicate the three controlled degrees of freedom.

Figure 4

Magnetic force and torque in clinical arrangement of Fig. 3. The large magnet is the manipulator magnet, with the coordinate frame origin at its dipole center (shown offset for clarity). The implant tip magnet is placed along the manipulator’s rotation axis making M and p orthogonal. Two directions of the implant tip magnet with resulting forces and torques are shown. (a) Negative z-direction (θ=0deg): force in negative y-direction. (b) Negative x-direction (θ=90deg): negative torque about y-axis.

Figure 5

Two guidance methods explored in experiments. (a) Maximum-field method: the dipoles are nearly aligned. (b) Maximum-torque method: the dipoles are nearly perpendicular. The increasing manipulator magnet size in the maximum-torque diagram indicates that the manipulator is advancing toward the cochlea.

Figure 6

Experimental setup. (top) The prototype (1) was attached to a force/torque sensor (2) with custom mounting fixtures (3) and inserted into a phantom (4) using linear stages (5). The manipulator (6) is attached to a brushed dc motor shaft and mounted to a linear stage (7), which translates the manipulator toward the phantom. The dashed line shows that the manipulator rotation axis and cochlear central spiral axis are aligned. The curved arrow above the manipulator (6) shows the manipulator rotation direction. The straight arrows on (5) and (7) show the translation direction of the linear stages. The stripe on the front of the manipulator marks the dipole direction of the enclosed axially magnetized magnet. (bottom-left) ATI Nano17 force/torque sensor with definition of positive force. (bottom-right) Insertion angle based on the phantom model [17].

Figure 7

(top) The MED-EL practice electrode shown is a straight, free-fitting electrode array with a length of 32 mm, and it tapers from a base diameter of 1.3 mm to a tip diameter of 0.5 mm. (bottom) Our 3:1 prototype has a length of 83 mm and a constant diameter of 1.6 mm.

Figure 8

Key steps for production of the wire core used in the implant prototype. (a) Wires twisted together and wrapped around a cylindrical shaft. (b) Snip off one wire at a certain length and wrap the remaining wires. Repeat this process until only one wire is left. This creates a tapered stiffness, which was observed in the MED-EL device. (c) Wrap remaining wire around tubing to create a coil that the magnet can be slid into. (d) The result of the wire wrapping prior to placing the magnet. (e) Magnet is placed at the end of the coil and encased in silicone using an acrylic mold.

Figure 9

(a–c) Force measurements for all three insertion methods. The direction of F x , F y , and F z are defined in Fig. 4 and Fig. 6. Each data point represents the average of n runs, where the collected measurement for each run is an average of 10 samples at the corresponding insertion depth. The solid vertical bar with each data point shows the two-standard-deviation interval. Vertical dashed lines mark the approximate locations where the implant first made contact with the ST outer wall. (d) Corresponding images showing implant positions at first instance of outer-wall contact.

Figure 10

The tip of the implant is directed away from the outer wall using both the maximum-torque and maximum-field methods, even at significant insertion depths

Figure 11

Images of implant through the first turn

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