Matthew F. Gornet 1 Spine Research Center, The Orthopedic Center of St. Louis, 14825 North Outer Forty Road, Suite 200, St. Louis, MO 63017 e-mail: mfgspine@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 Biomechanical Assessment of a PEEK Rod System for Semi-Rigid Fixation of Lumbar Fusion Constructs 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 pedi- cle screw-based polyetheretherketone (PEEK) SRF system for the lumbar spine to com- pare 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 eval- uated 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 sta- bility that PEEK rods provide. Under physiological loading conditions, a 6.35 mm nomi- nal 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  10 6 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 dif- ference in the stability provided by the PEEK rods and titanium rods in any mode (p ¼ 0.769 for F/E; p ¼ 0.085 for LB; p ¼ 0.633 for AR). The CD HORIZON V R LEGACY TM PEEK Rod System provided intervertebral stability comparable to currently marketed ti- tanium lumbar fusion constructs. PEEK rods also more closely approximated the physio- logic anteroposterior column load sharing compared to results with titanium rods. The durability, stability, strength, and biomechanical profile of PEEK rods were demon- strated and the potential advantages of SRF were highlighted. [DOI: 10.1115/1.4004862] Keywords: PEEK, polyetheretherketone, lumbar, fusion, semi-rigid, dynamic stabilization Introduction Spinal arthrodesis is intended to provide stability by limiting long term motion in one or more vertebral segments compromised by deformity, trauma, or disease. Painful degenerative disease and instability in the lumbosacral spine, when unresponsive to con- servative measures, is often treated surgically with fusion to alle- viate painful symptoms resulting from nonphysiologic motion and compression of neural structures. While there is still much debate about treatment of low back pain with spinal arthrodesis, advances in materials and in surgical techniques have led to significant improvements in both clinical outcomes and fusion success rates [1–6]. Supplemental posterior instrumentation with a stainless steel or titanium pedicle screw/rod or plate system has been shown to improve lumbar fusion success rates by providing addi- tional stability [7–12]. While the procedure is generally successful in stabilizing the segments and diminishing pain, some patients experience recurrent symptoms. Revision spinal fusion may be required to address these issues, an outcome that adds significant cost to the healthcare system and risk to the patient. One recent study indicates that despite advances in technology and surgical procedures, the incidence of re-operation has actually increased [13]. Instrumented Spinal Fusion. Although spinal fusions have been performed for nearly a century, and metallic constructs for deformity and trauma have been in use for decades, posterior 1 Corresponding author. Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received December 10, 2010; final manuscript received August 13, 2011; published online September 20, 2011. Assoc. Editor: Beth Winkelstein. Journal of Biomechanical Engineering AUGUST 2011, Vol. 133 / 081009-1 Copyright V C 2011 by ASME Downloaded 08 Nov 2011 to 128.95.104.66. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm