Traditional and cortical trajectory screws of static and dynamic lumbar fixation- a finite element study

Che-Wei Liu,Jian-Cyuan Wang,Chun-Ming Chen,Shang-Chih Lin,Wei-Tsung Tsai,Yu-Kun Xu,Lu-Lin Wang

doi:10.1186/s12891-020-03437-5

Abstract

BackgroundTwo types of screw trajectories are commonly used in lumbar surgery. Both traditional trajectory (TT) and cortical bone trajectory (CBT) were shown to provide equivalent pull-out strengths of a screw. CBT utilizing a laterally-directed trajectory engaging only cortical bone in the pedicle is widely used in minimal invasive spine posterior fusion surgery. It has been demonstrated that CBT exerts a lower likelihood of violating the facet joint, and superior pull-out strength than the TT screws, especially in osteoporotic vertebral body. No design yet to apply this trajectory to dynamic fixation. To evaluate kinetic and kinematic behavior in both static and dynamic CBT fixation a finite element study was designed. This study aimed to simulate the biomechanics of CBT-based dynamic system for an evaluation of CBT dynamization.MethodsA 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.ResultsThe 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.ConclusionsModeling the effects of TT and CBT fixation in a full lumbosacral model suggest that dynamic TT provide slightly superior stability compared with dynamic CBT especially in bending and rotation. In dynamic CBT design, large diameter screws might avoid issues with loosening and cracking.

Highlights

Two types of screw trajectories are commonly used in lumbar surgery
These findings indicate the potentially insufficient stability provided by the cortical bone trajectory (CBT) dynamic to the fixed segment in bending and rotation
Disc stresses For the healthy and instrumented models, the stressdistributing contours of the adjacent and fixed discs can provide a visual comparison of the stabilizing ability and adjacent segment compensation among four fixations (Fig. 4)

Summary

Introduction

Two types of screw trajectories are commonly used in lumbar surgery. Both traditional trajectory (TT) and cortical bone trajectory (CBT) were shown to provide equivalent pull-out strengths of a screw. CBT utilizing a laterally-directed trajectory engaging only cortical bone in the pedicle is widely used in minimal invasive spine posterior fusion surgery. To evaluate kinetic and kinematic behavior in both static and dynamic CBT fixation a finite element study was designed. The most commonly adopted transpedicular fixation is the traditional trajectory (TT) of the pedicle screw, which follows the anatomic axis of the pedicle into the cancellous bone of the vertebral body. [3, 4] Convergent trajectory of TT screw increases pull-out strength by 28.6% compared with a straight-in screw [3]. The decrease in postoperative bed-time was attributed to the smaller incision size, decreased disruption of muscle attachment, and soft-tissue dissection using CBT [14]

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