The Mechanical Effect of CP Titanium Vs. PEEK Rods On the Spinal Implants and the Operative and Adjacent Levels After TLIF

Purpose: To evaluate the effect of rod material (6.35mm PEEK vs. 5.5mm commercially pure titanium) on bone screw and interbody device strain, maximum translation, and adjacent segment disc pressure.

Methods: 12 fresh frozen lumbar cadaver motion segments were instrumented with either 5.5 mm CP Ti rods or 6.35 mm oval PEEK rods and four multi-axial bone screws and an interbody device. The inferior left bone screws were instrumented with three (axial, shear, and bending) strain gauges, and compression and shear strain gauges were affixed to the interbody devices. Compression and flexion testing was performed on each segment using custom pure moment fixtures in a servohydraulic test frame. Normalized flexion translations (presented as a % of intact values) and normalized caudal level disc pressure measurements (% of cage only state) were also recorded.

Results: Screw strain data was found to be higher in flexion (up to 4 Nm) and compression (for test duration) for Ti compared to PEEK. Conversely, interbody device strain was higher for PEEK constructs in both tests. Caudal adjacent segment translation showed a statistical trend toward decreased motion with PEEK (p=0.065). Caudal level disc pressure measurements of Ti vs. PEEK were approaching a significant decrease (p=0.055) from the cage only in compression. No significant pressure difference was observed in flexion (p=0.818).

Conclusions:  More rigid instrumentation with Ti rods resulted in increased screw strain (bone-screw interface forces) and less interbody device compression (stress shielding), as compared to PEEK rods.  Furthermore, there was a trend for increased motion with the more rigid instrumentation (Ti) at the caudal segment, and a trend toward decreased caudal intradiscal pressure, as compared the cage only state. These trends suggest that segments instrumented with PEEK more closely mimicked intact physiological loading in the subadjacent level which may reduce the likelihood of adjacent level disease.