The Mechanical Effect of CP Titanium and PEEK Rods on Spinal Implants at the Operative and Adjacent Levels

Introduction: Polyetheretherketone (PEEK) as a biomaterial has been used in spinal implants since the 1990s. Initially, it was used for plating and interbody systems, but recently PEEK has been utilized in the posterior regions of the spine as spinal rods. The concepts of “better load sharing” and “less bone-screw interface stresses” are terms that have arisen to describe lumbar PEEK spinal devices. However, very little biomechanical or clinical literature exists to substantiate these claims. The goal of this study was to mechanically evaluate the biomechanical differences in commercially pure (CP) titanium (Ti) rods and PEEK rods in conjunction with PEEK interbody devices in singlelevel cadaveric constructs. Basic biomechanical motions were performed to physiologically relevant loading levels. Strain on both the interbody device and a pedicle screw in multiple axes were recorded. Motion data were collected at the cephalad, treated, and caudal levels, and caudal level intradiscal pressure measurements were taken. Methods: Twelve human cadaveric spine segments were utilized. For each spine, two spinal segments, T12-L3 and L4-S1, were prepared for potting. The segments were potted so that the mid-plane of the middle disc space (L1-2), or middle vertebral body (L5) was coplanar to the x-y plane of the test frame. The threads near the crown of twelve Ti alloy 6.5x40 mm multi-axial screws (MAS) were removed down to the minor diameter to allow instrumentation of strain gauges. Twelve PEEK interbody (IB) devices were also selected for strain gauging. Clinically, interbody sizing is disc height dependant, so x-rays were taken of all specimens prior to implantation, and the correct sizes were chosen by the surgeon implanting the devices. Screws and IB devices were then professionally strain gauged (HITEC Corporation, Littleton, MA). Three uniaxial strain gauges were affixed to each screw. The orientation of the three gauges facilitated shear, tension-compression, and bending strain measurements (Figure 1). Two uniaxial strain gauges were also placed on each IB device. A uniaxial gauge was affixed in the superior-inferior axis to measure tensile and compressive strain. A shear strain gauge was orientated to measure anterior-posterior (A-P) shear (Figure 2).