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Technical Briefs

Shape Memory Alloy Expandable Pedicle Screw to Enhance Fixation in Osteoporotic Bone: Primary Design and Finite Element Simulation

[+] Author and Article Information
Majid Tabesh, Vijay Goel

 Dynamic and Smart Systems Laboratory, Engineering Center for Orthopedic Research Excellence, University of Toledo, 2801 West Bancroft, Toledo, Ohio, 43606

Mohammad H. Elahinia1

 Dynamic and Smart Systems Laboratory, Engineering Center for Orthopedic Research Excellence, University of Toledo, 2801 West Bancroft, Toledo, Ohio, 43606

1

Corresponding author.

J. Med. Devices 6(3), 034501 (Aug 20, 2012) (8 pages) doi:10.1115/1.4007179 History: Received May 15, 2010; Revised July 05, 2012; Published August 20, 2012; Online August 20, 2012

The properties of shape memory alloys, specifically the equiatomic intermetallic NiTi, are unique and significant in that they offer simple and effective solutions for some of the biomechanical issues encountered in orthopedics. Pedicle screws, used as an anchoring point for the implantation of spinal instrumentations in the spinal fracture and deformity treatments, entail the major drawback of loosening and backing out in osteoporotic bone. The strength of the screw contact with the surrounding bone diminishes as the bone degrades due to osteoporosis. The SMArtTM pedicle screw design is developed to address the existing issue in degraded bone. It is based on the interaction of bi-stable shape memory-superelastic elements. The bi-stable assembly acts antagonistically and consists of an external superelastic tube that expands the design protrusions when body temperature is attained; also an internal shape memory wire, inserted into the tube, retracts the assembly while locally heated to above the body temperature. This innovative bi-stable solution augments the pull-out resistance while still allowing for screw removal. The antagonistic wire-tube assembly was evaluated and parametrically analyzed as for the interaction of the superelastic tube and shape memory wire using a finite element model developed in COMSOL Multiphysics® . The outcomes of the simulation suggest that shape memory NiTi inserts on the SMArtTM pedicle screw can achieve the desired antagonistic functionality of expansion and retraction. Consequently, a parametric analysis was conducted over the effect of different sizes of wires and tubes. The dimensions for the first sample of this innovative pedicle screw were determined based on the results of this analysis.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Pedicle screw implantation in a spinal vertebra

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Figure 2

NiTi insert used in the SMArt™ pedicle screw. (a) Initial low-temperature form; (b) final form attained at body temperature. A portion of the insert from both ends expands and the remaining contracts to grip the screw.

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Figure 3

The screw assembly before placement

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Figure 4

The screw assembly after placement (when reached to body temperature)

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Figure 5

(a) SMArt screw concept evaluation: lag screw enhanced with Nitinol wires. (b) Control screw. (c) Experimental setup showing the screw inserted in the foam block and mounted on a tensile testing machine.

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Figure 6

Results of the axial tensile test: force versus displacement. The tensile strength is selected to be the force required to displace the screw in the block as much as the screw thread pitch.

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Figure 7

Schematic procedure representing the functionality of the antagonistic bi-stable tube-wire assembly in engaging with and disengaging from the bone. (a) and (b) Initial memorized shapes of the assembly components attained by proper shape setting. Assembly at (c) body temperature (d) elevated temperature.

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Figure 8

Discretization of the wire-tube assembly with quadratic Lagrange brick elements. (Inset) Temperature profile at the three stages of the simulation.

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Figure 9

The deflection of the wire under a tip load at the three stages of the operation

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Figure 10

The deflection of the tube under a tip load at the three stages of the operation

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Figure 11

Load-displacement plots for the wire-tube assembly at the second stage. The equilibrium is reached at a common level of force where the sum of displacements is equal to the initial gap; δA2=5.6andδB2=0.1.

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Figure 12

Load-displacement plots for the wire-tube assembly at the third stage. The equilibrium is reached at a common level of force where the sum of displacements is equal to the initial gap; δA3=5.5andδB3=0.2.

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Figure 13

First principle strain (plotted on the undeformed configuration for more clarity)

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