Technical ReviewBiomechanical evaluation and comparison of polyetheretherketone rod system to traditional titanium rod fixation
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.
Section snippets
Cadaveric testing
Four fresh human cadaveric lumbar spines from L1 to S1 were harvested and used for this experiment after screening for abnormal anatomy using anteroposterior and lateral fluoroscopy. The specimens were stored in double plastic bags at −20°C until preparation and testing. Both L1 and sacrum were rigidly fixed and potted into custom cups using wood screws and a urethane compound (SmoothCast [Smooth-On Inc., Easton, PA]) such that the specimen was in neutral posture with the L3–L4 disc oriented
Cadaveric testing
Results demonstrated that the PLIF and PLF PEEK constructs significantly reduced the range of motion (ROM) as compared with both the intact and destabilized conditions (p<.05). Results demonstrated that there was no significant difference in the stability provided by a 5.5 mm PEEK construct and a 5.5 mm Titanium construct (Table 2). After destabilization, there was no significant difference (p<.05) in the percent reduction in ROM provided by Titanium PLF, PEEK PLF, Titanium PLIF, or PEEK PLIF for
Discussion
PEEK rods have recently become available for use in posterior lumbar fusion procedures. To the best of our knowledge, there have been no published reports of the biomechanical properties of these devices. It has been suggested that stiffness of metallic spine implants/constructs may far exceed the requirements for successful fusion and predispose secondary adjacent level disc degeneration and failure [12].
The results from our cadaveric testing show that the PEEK rod construct significantly
Conclusion
PEEK rods offer an alternative to traditional Titanium rod fixation. Biocompatibility studies, biomechanical evaluation, and cadaveric testing demonstrate that PEEK rods can provide stability under normal physiologic conditions. In addition, several notable benefits of PEEK in comparison to Titanium include increased anterior column load sharing, reduced stress at bone-to-screw interface, and reduced computed tomography and MRI scatter/artifact.
Further study is needed to evaluate the clinical
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2021, Radiotherapy and OncologyCitation Excerpt :PEEK, a biocompatible polymer, has been used in spinal implants such as fusion cages since the 1990s [46]. It is bioinert and has a modulus of elasticity of 3.6 GPa which is lower than that of cortical bone, thus causing minimal stress shielding and better load sharing [47]. Additionally, PEEK is radiolucent and non-metallic, producing minimal artifacts on CT and MRI [45].
FDA device/drug status: not applicable.
The following authors acknowledge a financial relationship: ARV and TA: Royalties; HS and BZ: Other office in the company, which may indirectly relate to the subject of this research.