Biomechanical comparison of titanium and peek rod systems in posterior fixation of subaxial cervical spine fractures

Pedicle screw-rod systems are widely used for the posterior stabilization of spinal instabilities. Semi-rigid PEEK rods, developed as an alternative to conventional titanium rods, have been used in lumbar and thoracic regions since 2007. There are no biomechanical and clinical studies in the literature regarding the use of PEEK rods in the cervical region. In this thesis study, the biomechanical performance of titanium and PEEK rod systems in the subaxial cervical spine was compared in the laboratory environment. Titanium and PEEK rods with dimensions of Ø3.0 mm🞨40 mm were subjected to static four point bending test according to ASTM F2193-20 standard. In the tests carried out with five samples; yield load, displacement at yield load, stiffness, maximum load and displacement at maximum load were calculated. Test blocks representing subaxial cervical vertebrae were manufactured according to ASTM F1717-18 standard. Two pedicle screws of Ø3.5 mm🞨14 mm were inserted into each test block. Vertebrectomy models were obtained by connecting the two test blocks to each other with titanium or PEEK rods of Ø3.0 mm🞨40 mm. The titanium and PEEK rod systems were subjected to static compression bending, static tension bending and fatigue tests according to ASTM F1717-18 standard. Six samples were used for each test. As a result of the static four-point bending test, the yield strengths of titanium and PEEK rods were calculated respectively as 572.77 N ± 9.71 N and 56.12 N ± 0.85 N, and their stiffness as 858.05 N / mm ± 12.35 N / mm and 35.66 N / mm ± 0.25 N / mm. As a result of the static compression bending test of the vertebrectomy model, the stiffness of the titanium and PEEK rod systems were calculated as 10.45 N / mm ± 0.06 N / mm and 0.34 N / mm ± 0.01 N / mm, respectively. As a result of the static tensile bending test, the stiffness of the titanium and PEEK bar systems were calculated as 0.12 N / mm ± 0.30 N / mm and 0.80 N / mm ± 0.01 N / mm, respectively. As a result of the fatigue test, the endurance limits of the titanium and PEEK rod systems were determined as 23 N and 6 N respectively. It has been determined that the strength of titanium rod systems is higher in flexion and extension directions than PEEK rod systems under both static and dynamic loading conditions. However, it is anticipated that PEEK rod systems can be a good alternative to titanium rod systems. It is expected that PEEK rod systems allow motion of the spine before fusion since the system show more flexibility without deformation. In addition, it is expected that using PEEK rod systems having closer stiffness to the bone will not cause the stress shielding as much as titanium rod systems and reduce the risk of adjacent segment disease. This thesis study shows that the pedicle screw fixation with PEEK rod is a good alternative to traditional titanium rod systems for the fixation of cervical spine instabilities in terms of biomechanical performances. However, since there are not enough studies on pedicle screw fixation with PEEK rod systems in the cervical spine, cadaver studies should be performed before clinical use of these systems.