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Research Papers

Biomechanical Assessment of a PEEK Rod System for Semi-Rigid Fixation of Lumbar Fusion Constructs

[+] Author and Article Information
Matthew F. Gornet1

 Spine Research Center, The Orthopedic Center of St. Louis, 14825 North Outer Forty Road, Suite 200, St. Louis, MO 63017mfgspine@gmail.com

Frank W. Chan, John C. Coleman, Brian Murrell

Medtronic Spinal & Biologics, 2600 Sofamor Danek Dr., Memphis, TN 38132

Russ P. Nockels

Department of Neurological Surgery,  Loyola University Medical Center, 2160 S. 1st Ave., Chicago, IL 60153

Brett A. Taylor

Spine Research Center, The Orthopedic Center of St. Louis, 14825 North Outer Forty Road, Suite 200, St. Louis, MO 63017

Todd H. Lanman

 450 North Roxbury Drive, Los Angeles, CA 90210

Jorge A. Ochoa

 Exponent, Inc., 15375 SE 30th Place, Suite 250, Bellevue, WA 98007

1

Corresponding author.

J Biomech Eng 133(8), 081009 (Sep 20, 2011) (12 pages) doi:10.1115/1.4004862 History: Received December 10, 2010; Revised August 13, 2011; Published September 20, 2011; Online September 20, 2011

The concept of semi-rigid fixation (SRF) has driven the development of spinal implants that utilize nonmetallic materials and novel rod geometries in an effort to promote fusion via a balance of stability, intra- and inter-level load sharing, and durability. The purpose of this study was to characterize the mechanical and biomechanical properties of a pedicle screw-based polyetheretherketone (PEEK) SRF system for the lumbar spine to compare its kinematic, structural, and durability performance profile against that of traditional lumbar fusion systems. Performance of the SRF system was characterized using a validated spectrum of experimental, computational, and in vitro testing. Finite element models were first used to optimize the size and shape of the polymeric rods and bound their performance parameters. Subsequently, benchtop tests determined the static and dynamic performance threshold of PEEK rods in relevant loading modes (flexion-extension (F/E), axial rotation (AR), and lateral bending (LB)). Numerical analyses evaluated the amount of anteroposterior column load sharing provided by both metallic and PEEK rods. Finally, a cadaveric spine simulator was used to determine the level of stability that PEEK rods provide. Under physiological loading conditions, a 6.35 mm nominal diameter oval PEEK rod construct unloads the bone-screw interface and increases anterior column load (approx. 75% anterior, 25% posterior) when compared to titanium (Ti) rod constructs. The PEEK construct’s stiffness demonstrated a value lower than that of all the metallic rod systems, regardless of diameter or metallic composition (78% < 5.5 mm Ti; 66% < 4.5 mm Ti; 38% < 3.6 mm Ti). The endurance limit of the PEEK construct was comparable to that of clinically successful metallic rod systems (135N at 5 × 106 cycles). Compared to the intact state, cadaveric spines implanted with PEEK constructs demonstrated a significant reduction of range of motion in all three loading directions (> 80% reduction in F/E, p < 0.001; > 70% reduction in LB, p < 0.001; > 54% reduction in AR, p < 0.001). There was no statistically significant difference in the stability provided by the PEEK rods and titanium rods in any mode (p = 0.769 for F/E; p = 0.085 for LB; p = 0.633 for AR). The CD HORIZON® LEGACY™ PEEK Rod System provided intervertebral stability comparable to currently marketed titanium lumbar fusion constructs. PEEK rods also more closely approximated the physiologic anteroposterior column load sharing compared to results with titanium rods. The durability, stability, strength, and biomechanical profile of PEEK rods were demonstrated and the potential advantages of SRF were highlighted.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Instrumentation constructs for (a) the modified ASTM F1717 compression bending test, (b) the standard ASTM F1717 compression bending test, and (c) the custom A/P shear test

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Figure 2

Cadaveric biomechanical testing of a lumbar spine with L4-L5 instrumentation

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Figure 4

Summary of finite element results for PEEK rods with various cross-sectional profiles: (a) displacement and (b) stress

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Figure 5

Von Mises equivalent stress contours for (a) bending and (b) shear of PEEK rods with 6.35 x 7.2 mm oval (left), 6.35 mm circular (middle), and 5.5 mm circular (right) cross-sectional profiles

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Figure 6

Static compression bending data for various single-level constructs

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Figure 7

Compression bending fatigue curve for LPR constructs, showing two run-outs at 135 N for 5 × 106 cycles

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Figure 8

Range of motion of cadaveric L4-L5 motion segments in intact, destabilized, and instrumented conditions

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Figure 9

Range of motion at L4-L5 calculated for the lumbar spine finite element model in the intact and two instrumented conditions

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Figure 10

Intra-level distribution of the axial load calculated in the instrumented L4-L5 motion segment of the spinal finite element model

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Figure 11

Axial load carried by posterior fusion rods in the instrumented L4-L5 motion segment of the spinal finite element model

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Figure 3

Finite element model of the posteriorly instrumented lumbar spine used to determine intra-level load sharing with either PEEK or titanium rods

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