Research Papers

Scala-Tympani Phantom With Cochleostomy and Round-Window Openings for Cochlear-Implant Insertion Experiments

[+] Author and Article Information
Lisandro Leon

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

Matt S. Cavilla

Department of Mechanical Engineering,
University of Utah,
Salt Lake City, UT 84112
e-mail: matt.cavilla@utah.edu

Michael B. Doran

Department of Mechanical Engineering,
University of Utah,
Salt Lake City, UT 84112
e-mail: mike.doran@utah.edu

Frank M. Warren

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

Jake J. Abbott

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

1Corresponding author.

Manuscript received January 7, 2014; final manuscript received May 1, 2014; published online August 19, 2014. Assoc. Editor: Rafael V. Davalos.

J. Med. Devices 8(4), 041010 (Aug 19, 2014) (10 pages) Paper No: MED-14-1003; doi: 10.1115/1.4027617 History: Received January 07, 2014; Revised May 01, 2014

Experiments with scala-tympani (ST) phantoms are used to evaluate new electrode arrays and cochlear-implant insertion techniques. To date, phantoms have not accounted for clinical orientations and geometric differences between round-window (RW) insertions and anteroinferior cochleostomy insertions. For improved assessments of insertion experiments, we present a scala-tympani phantom that offers three distinct benefits over previous phantoms: it mimics the standard otologic position, it accommodates for both round-window and anteroinferior cochleostomy insertions, and it incorporates a visual coordinate system based on industry consensus making standardized angular measurements possible.

Copyright © 2014 by ASME
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Fig. 1

Diagram of the standard otologic positioning

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Fig. 2

The method to generate the cochlear view of Fig. 3 is summarized. (a) While the X-ray and film plane are maintained orthogonal to each other, the skull is positioned against the film such that the angle between it and the midsagittal plane is approximately 50 deg. (b) Next, the skull is adjusted so that the angle between the X-ray and the infraorbitomeatal plane (IOP) is near zero. Upon completion, the modiolar axis is nearly parallel to the X-ray. Modified image from Ref. [10] reproduced with permission of Wolters Kluwer Health.

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Fig. 3

The skull positioning of Fig. 2 results in the cochlear view, which contains a 2D image of the electrode array (shown as a series of squares) as a nonoverlapping spiral in the basal and middle turns. The spiral center is determined by fitting a mathematical spiral template to the position of the electrodes. The line cv2 passes through the top of SSC and the midpoint of the vestibule (V). The line cv1 passes through the spiral center and is orthogonal to cv2. Angular insertion depth (θ) is measured from the geometric 0 deg reference, which is the line from the spiral center through the intersection of cv1 and cv2. The location of the RW entry, which is near the intersection of cv2 and the electrode array, is measured from the 0 deg reference and shown here as θ3. LSC is the lateral-semicircular canal. Modified image from Ref. [12] reproduced with permission of Wolters Kluwer Health.

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Fig. 4

The skull positioning of Fig. 2 was confirmed through more rigorous measurements using numerous temporal bones [12]. To confirm Fig. 2(a), they computed the angle A between the line passing the lower arm of the posterior semicircular canal (PSC) and the midsagittal plane, and assumed that A is nearly identical to A′ (left image). The mean value of A (for n = 102) is 52.5 deg. C is the complementary angle of A and is identical to θ1 in Table 1 and Fig. 2. Next, to confirm Fig. 2(b), they computed the angle B between the lateral-semicircular canal (LSC) and the modiolar axis (bottom-right image). The mean value of B (for n = 10) is 28.2 deg. Since the LSC forms an angle of 30 deg upward from the infraorbitomeatal plane, they concluded that the modiolar axis is nearly parallel to this plane. Viewing the film plane in the direction indicated by the arrows results in the cochlear-view radiograph shown in Fig. 3. Modified left image is from Ref. [12] reproduced with permission of Wolters Kluwer Health. The right images are generated using software available for public use [15].

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Fig. 5

Top: Orientation angles of Table 1 and cochlear-view axes (lines cv1 and cv2) are shown relative to the three orthogonal reference planes of the body: the sagittal plane (SP), coronal plane (CP), and the transverse plane (TP). θ3 is measured in the plane formed by cv1 and cv2; modified public domain image. Bottom: cochlea orientation angles (shown as a series of successive rotations of a Cartesian frame originally aligned with the reference planes).

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Fig. 6

Virtual model of the cochlea showing the basal end of the scala tympani (ST) as seen through the facial recess during surgery with an enlarged view of the RW region provided in the lower-left inset. Depictions of both insertion openings, RW opening and anteroinferior cochleostomy, are superimposed onto the virtual model to provide approximate locations with respect to the RW membrane. Basilar membrane (BM), scala vestibuli (SV), and the skull position corresponding to the cochlea orientation are provided for reference. Image is generated using software available for public use [15]. Top-right: posterior-superior lip of RW niche (black arrow) and bony projection from crista (outlined by dotted white line) restrict the angle of electrode (EL) entry so that the electrode tip (white arrow) is directed toward modiolar wall and spiral ganglion (SG) rather than the ST lumen. A well placed cochleostomy (shown as a dashed circle) can facilitate direct insertions into the ST lumen (dashed arrow). Modified image from Ref. [16] reproduced with permission of John Wiley and Sons.

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Fig. 7

Histological cross sections in the basal end of the cochlea. BM, basilar membrane; OSL, osseous spiral lamina; RWN, round-window niche; RWM, round-window membrane; SM, scala media; ST, scala tympani; SV, scala vestibuli. Modified images from Ref. [26] reproduced with permission of Lippincott Williams and Wilkins.

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Fig. 8

Steps to model the scala tympani. (1) Create sections parameterized by h and w. (2) Place the origin of each section (intersection of x′ and z′) onto the spiral at 1 mm increments from its beginning, with z′ initially aligned with z, and orient the section such that it is orthogonal to the local spiral direction. (3) Rotate section counterclockwise by φ and shift section medially by ws.

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Fig. 9

Wysocki's height data [21] is smaller than other published values [22-25]. The solid line is the average of the non-Wysocki data sets.

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Fig. 10

Our scala-tympani model (shown in gray and limited to 1.5 turns to reduce visual clutter) is compared with Cohen's silastic models [11] and Kawano's reconstructions [27]. The intersections of the outer wall of our model with the 13.5 deg and 23.8 deg lines are the respective entry points through the round-window opening and an anteroinferior cochleostomy. Modified image from Ref. [9] reproduced with permission of Wolters Kluwer Health.

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Fig. 11

Our scala-tympani wall lengths compared with published data [22,27]

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Fig. 12

Steps to design phantom in SolidWorks. (1) Create loft from imported x-y-z points that model the scala tympani. (2) Build a structure around the lofted cut to orient the phantom appropriately. (3) Create the angular grid system and an exit hole near center of the dial. For the round-window version, we trim material along the cavity boundary (red-dashed line) to reduce the tunnel effect from the phantom's wall thickness at the round-window opening. (4) Create sketch planes where the 13.5 deg and 23.8 deg lines intersect the lofted cut. (5) Create insertion openings on the defined sketch planes.

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Fig. 13

SolidWorks renderings of the cochleostomy (top-left) and round-window (top-right) versions of our scala-tympani phantom are used to manufacture the corresponding devices below. The tabletop views assume the phantom is lying on a flat surface with the observer's line-of-sight at the level of the phantom. The facial recess views approximate the surgeon's view of the insertion openings, in the spirit of Fig. 6. The top-down views are taken above the phantom with the line-of-sight along the gravity vector. The dial views assume a line-of-sight directed toward and orthogonal to the face of the dial. Standard MED-EL electrodes are inserted as far as possible before buckling (to approximately 720 deg) into both phantoms.

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Fig. 14

Insertion force measurements are compared for five different phantoms, each of which is rigidly mounted onto a force sensor with the insertion opening oriented for vertical, automated insertions. An image of phantom number 5 is not provided because its semitransparent material did not provide good visualization of the electrode.

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Fig. 15

Insertion force measurements, at clinically accurate insertion angles, are compared between the round-window phantom (left) and cadaver cochleae fixed in baskets with paraffin wax (right), mounted rigidly to the force sensor




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