Biomechanical Investigation about Traditional and Cortical Trajectory Screws of Static and Dynamic Lumbar Fixation

Abstract

Background To evaluate kinetic and kinematic behavior in both static and dynamic CBT fixation a finite element study was designed. Two types of screw trajectories are commonly used in lumbar surgery. Both traditional trajectory (TT) and cortical bone trajectory (CBT) provide equivalent pull-out strengths of a screw. Dynesys fixation of TT screws, but not dynamization of CBT screws, has been used extensively in lumbar surgery. This study aimed to simulate the biomechanics of CBT-based dynamic system for an evaluation of CBT dynamization.Methods A validated nonlinearly lumbosacral finite-element model was used to simulate four variations of screw fixation. Responses of both implant (screw stress) and tissues (disc motion, disc stress, and facet force) at the upper adjacent (L3-L4) and fixed (L4-L5) segments were used as the evaluation indices. Flexion, extension, bending, and rotation of both TT and CBT screws were simulated in this study for comparison.Results The results showed that the TT static was the most effective stabilizer to the L4-L5 segment, followed by CBT static, TT dynamic, and the CBT dynamic, which was the least effective. Dynamization of the TT and CBT fixators decreased stability of the fixed segment and alleviate adjacent segment stress compensation. The 3.5-mm diameter CBT screw deteriorated stress distribution and rendered it vulnerable to bone-screw loosening and fatigue cracking.Conclusions A systematic analysis of the effects of TT and CBT fixation constructs on kinematic and kinetic responses in a full lumbosacral model is currently lacking. This study examined both the static fixation effect and its dynamic counterpart and identified that dynamization of CBT have slightly inferior structural stiffness than dynamic TT and cautious preoperative evaluation is essential if 3.5-mm diameter CBT screws are used. Therefore, 4.5-mm or 5.5-mm diameter CBT screws, or as big as tolerated, are recommended to avoid loosening and cracking.