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Research Papers

Finite-Element Study of the Performance Characteristics of an Intradural Spinal Cord Stimulator

[+] Author and Article Information
Nicole M. Grosland

Department of Biomedical Engineering,
The University of Iowa,
1420 Seamans Center for the Engineering Arts and Sciences,
Iowa City, IA 52242
e-mail: nicole-grosland@uiowa.edu

George T. Gillies

Department of Mechanical and Aerospace Engineering,
University of Virginia,
122 Engineer's Way,
Charlottesville, VA 22904
e-mail: gtg@virginia.edu

Robert Shurig

Evergreen Medical Technologies, Inc.,
1350 Energy Lane, Suite 100,
St. Paul, MN 55108
e-mail: rshurig@evergreenmedtech.com

Kirsten Stoner

Department of Biomedical Engineering,
1402 Seamans Center for the Engineering Arts and Sciences,
The University of Iowa,
Iowa City, IA 52242
e-mail: kirsten-stoner@uiowa.edu

Stephanus Viljoen

Department of Neurosurgery,
University of Iowa Hospitals and Clinics,
1849 John Papajohn Pavilion,
Iowa City, IA 52242
e-mail: stephanus-viljoen@uiowa.edu

Brian D. Dalm

Department of Neurosurgery,
University of Iowa Hospitals and Clinics,
1849 John Papajohn Pavilion,
Iowa City, IA 52242
e-mail: brian-dalm@uiowa.edu

Hiroyuki Oya

Department of Neurosurgery,
University of Iowa Hospitals and Clinics,
1849 John Papajohn Pavilion,
Iowa City, IA 52242
e-mail: hiroyuki-oya@uiowa.edu

Douglas C. Fredericks

Department of Orthopaedics and Rehabilitation,
University of Iowa Hospitals and Clinics,
200 Hawkins Dr,
Iowa City, IA 52242
e-mail: douglas-fredericks@uiowa.edu

Katherine Gibson-Corley

Department of Pathology,
University of Iowa Hospitals and Clinics,
1167 Medical Laboratories,
Iowa City, IA 52242
e-mail: katherine-gibson-corley@uiowa.edu

Chandan Reddy

Department of Neurosurgery,
University of Iowa Hospitals and Clinics,
1849 John Papajohn Pavilion,
Iowa City, IA 52242
e-mail: chandan-reddy@uiowa.edu

Saul Wilson

Department of Neurosurgery,
University of Iowa Hospitals and Clinics,
1849 John Papajohn Pavilion,
Iowa City, IA 52242
e-mail: saul-wilson@uiowa.edu

Matthew A. Howard, III

Department of Neurosurgery,
University of Iowa Hospitals and Clinics,
1849 John Papajohn Pavilion,
Iowa City, IA 52242
e-mail: matthew-howard@uiowa.edu

1Corresponding author.

Manuscript received March 9, 2014; final manuscript received August 12, 2014; published online October 14, 2014. Assoc. Editor: Rita M. Patterson.

J. Med. Devices 8(4), 041012 (Oct 14, 2014) (11 pages) Paper No: MED-14-1143; doi: 10.1115/1.4028421 History: Received March 09, 2014; Revised August 12, 2014

We have used finite-element (FE) modeling to investigate the mechanical compliance, positional stability and contact pressures associated with a novel type of spinal cord stimulator that is placed directly on the pial surface of the spinal cord in order to more selectively activate neural structures for relief of intractable pain. The properties used in the model are those of the actual prototype devices employed in recent in vitro and chronic in vivo tests. The agreement between predictions and experimental observations serves to validate our FE approach, which can now be used to further optimize the device's design and performance.

Copyright © 2014 by ASME
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References

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Figures

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Fig. 1

(a) Photo of a prototype I-Patch device showing the electrode-bearing surface, lead loops, and dural cuff. This version of the device is shown without the lateral support struts, to provide a better view of the entry angles of the wires into the electrode-bearing surface. (b) I-Patch component surfaces extracted from the CAD model for purposes of meshing. Note that the surface representing the dural cuff has been eliminated so that the wires are clearly visible.

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Fig. 2

Finite-element model of the assembled I-Patch: (a) lateral view, (b) longitudinal view, (c) underside of the patch, and (d) patch positioned on the surrogate cord

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Fig. 3

Resulting stress–strain curves from a series of experimental tensile tests performed on three silicone samples. The Mooney-Rivlin hyperelastic material coefficients for the FE model were derived from these curves.

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Fig. 4

Mesh convergence plot for the monitored nodes. Based on the convergence study, each wire was comprised of ∼752 elements.

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Fig. 5

Wire loop compression validation; experimental versus computational compressive force in response to 5 mm of loop compression

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Fig. 6

Reaction forces in response to the compressive displacement required to achieve gap distances of 7, 5.5, and 7 mm. The displacements required to achieve each of these gaps are included in the accompanying ().

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Fig. 7

A cross-sectional view of the 7 mm gap model reveals liftoff of the patch from the cord at a displacement of 10 mm. An additional longitudinal displacement of the wires relative to the cord led to a dramatic liftoff beyond the 10 mm displacement.

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Fig. 8

Patch lift-off versus longitudinal displacement for the 7 mm gap model. The initial negative displacements indicate compression of the cord following the compressive displacement to achieve the 7 mm gap. As the longitudinal displacement neared 10 mm, a rapid increase in the rate of lift-off was observed.

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Fig. 9

Experimental patch lift-off measurements for various gap heights coupled with longitudinal cord displacements

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Fig. 10

Wire displacements to achieve the 7 mm (upper) and 4 mm (middle) gap heights coupled with the von Mises stress contours. Photo (lower) taken during experimental testing confirms the nature of the wire displacements predicted by the computational model.

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Fig. 11

Wire displacements to achieve the 7 mm (left) and 4 mm (right) gap distances coupled with a 10 mm longitudinal displacement, accompanied by the von Mises stress contours

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Fig. 12

The maximum von Mises stress observed in the wires for the 7 mm gap model displaced longitudinally was 96.9 MPa. As illustrated this value was near the tied wire-patch attachment site.

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Fig. 13

Stresses experienced by the surrogate cord in response to displacing the wires to achieve a 7 mm gap (upper) and with the 7 mm gap coupled with a 10 mm longitudinal displacement (lower). Note that the contour limits have been adjusted to highlight the stress distributions; stresses >1.2 × 103MPa share the same contour level. The location of the maximum stress adjacent to the patch is denoted by a solid white circle.

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Fig. 14

Stresses experienced by the surrogate cord in response to displacing the wires to achieve a 4 mm gap (upper), with the 4 mm gap coupled with a 10 mm longitudinal displacement (middle), and following an additional 5 mm of longitudinal displacement to 15 mm (lower). Note that the contour limits have been adjusted to highlight the stress distributions; stresses >1.2 × 103MPa share the same contour level. The location of the maximum stress adjacent to the patch is denoted by a solid white circle.

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Fig. 15

The contact pressure illustrated on the underside of the patch for the 4 mm gap model at maximum compression (left) and longitudinally displaced (right). Note that the two central contours align with the electrodes and the overall pattern closely mimics the observed stresses depicted on the cord (Figs. 13 and 14).

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