Biomechanical evaluation and comparison of polyetheretherketone rod system to traditional titanium rod fixation

Abstract

Background context

Polyetheretherketone (PEEK) has been increasingly used as a biomaterial for spinal implants. PEEK lumbar fusion rods have recently become available for use in posterior lumbar fusion procedures.

Purpose

To compare Polyetheretherketone Rod System to traditional titanium rod fixation in a cadaveric model and provide mechanical test data for the PEEK system.

Study design

Biomechanical testing.

Methods

Cadaveric biomechanical testing was conducted to compare Expedium 5.5 mm PEEK rods to titanium rods of equivalent diameter. Biomaterials testing was performed to determine static and dynamic performance of Expedium 5.5 mm PEEK rods with 6% BaSo4 in compressive bending and torsion.

Results

Cadaveric testing demonstrated that PEEK rods can significantly reduce the range of motion of a destabilized segment. The testing showed no significant difference in the stability provided by PEEK and titanium rods in posterolateral fusion (PLF) or posterior lumbar interbody fusion (PLIF) constructs. PEEK static compressive bending tests showed 67 degrees displacement without fracture of the rod. Torsion testing showed 30 degrees of rotation without yield or plastic deformation. Dynamic compression testing revealed two fatigue runouts at 23 degrees.

Conclusions

PEEK rods provide comparable stability to titanium rods of equivalent diameter in cadaveric testing. Mechanical testing suggests PEEK rods can withstand far beyond the angular displacements suggested by cadaveric testing and that of normal physiologic range of motion. Potential advantages to PEEK rods include better anterior column load sharing, reduced stress at bone-to-screw interface, and reduced computed tomography and magnetic resonance imaging scatter and artifact.

Introduction

The use of lumbar pedicle screw instrumentation to augment interbody and posterolateral fusion (PLF) rates has been well established [1], [2], [3], [4], [5]. Traditional instrumentation consisting of stainless steel constructs eventually evolved into titanium implants as a result of closer biomechanical properties to bone, improved biocompatibility, and reduced magnetic resonance imaging (MRI) artifact. Titanium offers sufficient strength under physiologic loads and construct stiffness closer to the modulus of elasticity of cortical bone, thus improving the stress shielding characteristics of lumbar pedicle instrumentation [3], [4], [6], [7], [8].

Recent advancements in posterior spinal fixation has centered on the concept of dynamic stabilization [9], [10], [11], [12], [13]. Often used as a nonfusion alternative, dynamic stabilization has a theoretical advantage to traditional stiff fixation of minimizing adjacent segment disc degeneration. When applied to fusion, a more dynamic stabilization may provide additional load sharing onto the anterior column and reduce stresses at the bone screw interface. In the common case of an undersized graft or graft subsidence, these advantages may be amplified as a system of lower stiffness may flex to appropriately load the interbody device. However, inherent to reduced fixation stability is the risk of pseudarthrosis. The ideal fixation system would maximize fusion rates by providing sufficient stability without excessive rigidity to allow for bone graft loading, while maintaining natural posturing and alignment to minimize adjacent level stress.

Polyetheretherketone (PEEK) has been increasingly used as a biomaterial for trauma, orthopedic, and spinal implants [7], [14]. PEEK is a thermoplastic polymer whose chemical structure maintains stability at temperatures exceeding 300°C, resists chemical and radiation damage, exhibits greater strength per mass than many metals, and offers compatibility with many reinforcing agents. PEEK as a biomaterial is fully biocompatible, with numerous studies documenting minimal systemic, intracutaneous, and intramuscular toxicity [7], [14], [15]. Furthermore, PEEK is considered to be relatively inert biologically with no evidence of inflammatory reaction to wear debris. Rivard et al. concluded after an in vivo biocompatibility study on New Zealand White rabbits that PEEK polymer is harmless to the spinal cord and it may be used safely as a component of spinal implants [16].

The purpose of this study was to compare the biomechanical performance of PEEK rods to titanium rods in a cadaveric study when used as posterior spinal instrumentation.

In addition, mechanical testing is provided that characterizes the properties of 5.5 mm PEEK lumbar rods.