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Research Papers

Spinal Implant Development, Modeling, and Testing to Achieve Customizable and Nonlinear Stiffness

[+] Author and Article Information
Eric Dodgen, Eric Stratton, Anton Bowden

Department of Mechanical Engineering,  Brigham Young University, Provo, UT 84602

Larry Howell1

Department of Mechanical Engineering,  Brigham Young University, Provo, UT 84602lhowell@byu.edu

1

Corresponding author.

J. Med. Devices 6(2), 021010 (May 07, 2012) (8 pages) doi:10.1115/1.4006543 History: Received August 17, 2011; Revised February 21, 2012; Published May 07, 2012; Online May 07, 2012

The spine naturally has a nonlinear force-deflection characteristic which facilitates passive stability, and thus there is a need for spinal implants that duplicate this behavior to provide stabilization when the spine loses stiffness through injury, degeneration, or surgery. Additionally, due to the complexity and variability in the mechanics of spinal dysfunction, implants could potentially benefit from incorporating a customizable stiffness into their design. This paper presents a spinal implant with contact-aided inserts that provide a customizable nonlinear stiffness. An analytical model was utilized to optimize the device design, and the model was then verified using a finite element model. Validation was performed on physical prototypes, first in isolation using a tensile tester and then using cadaveric testing on an in-house spine tester. Testing confirmed the performance of the implant and it was observed that the device increased mechanical stability to the spinal segment in flexion-extension and lateral-bending.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Prototype of the baseline configuration

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Figure 2

Deflected positions of the baseline configuration

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Figure 3

Contact-aided flexure shown deflected on a circular contact profile

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Figure 4

Contact-aided flexure and two different contact profiles

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Figure 5

Contact-aided attachment for baseline configuration

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Figure 6

Baseline configuration with contact surface dimensions defined

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Figure 7

Interaction of the flexures of the baseline configuration with the contact surface of the insert provides a tailorable nonlinear force-deflection response. The flexure was sectioned into two parts for analysis: one part in contact (dotted) and one part free of contact (solid).

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Figure 8

Test data collected for each surface compared to the analytical and FEA anticipated results. Compression and decompression are symmetrical about origin; for easier visualization, only compression results are displayed.

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Figure 9

Comparison of displacements predicted by the analytical model with those predicted by FEA using ANSYS. The three curves are, from top to bottom, ra=0.95,ra=1.00,ra=1.15.

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Figure 10

Modified ASTM F1717 test fixture

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Figure 11

Prototype to be tested

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Figure 12

Flexion-extension torque-rotation results

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Figure 13

Lateral bending torque-rotation results

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Figure 14

Axial rotation torque-rotation results

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