关键词: 椎弓根螺钉, 腰椎, 脊柱固定, 应力分析, 拔出强度, 改道置钉, 有限元分析, 生物力学

Abstract: BACKGROUND: Currently, pedicle screw fixation technology is recognized as the gold standard for lumbar posterior fusion surgery. However, this technique is associated with several complications such as suboptimal screw placement, loosening, and fracture. Addressing these issues, it requires a thorough investigation into the mechanical properties of screw reinsertion to optimize surgical procedures and enhance success rates and safety. 
OBJECTIVE: By combining finite element analysis with biomechanical experiments, this study aims to compare and analyze the mechanical performance of traditional trajectory pedicle screws during multiple extraction processes. The goal is to reveal patterns in screw extraction strength over repeated withdrawals, providing scientific insights into the safety and effectiveness of pedicle screw reinsertion for clinicians.
METHODS: Based on CT scan data, a three-dimensional reconstruction of the L4 vertebra model was performed. Three-dimensional printing technology was used to create biological experimental samples. A pull-out experiment was conducted according to a screw placement plan. Utilizing CT data and standard pedicle screw parameters, a finite element model of the L4 vertebra and a pedicle screw model (diameter 6.0 mm, length 45 mm) were established. The model was divided into five operating conditions based on screw placement angle and cycles. A finite element model was developed to simulate axial pull-out testing, analyzing stress distribution in the vertebral body and maximum axial pull-out strength of the screw. Mechanics experimental results of three-dimensional printing were compared and analyzed against simulation outcomes.
RESULTS AND CONCLUSION: (1) A dedicated experimental setup for pedicle screw extraction from single vertebrae was designed and constructed. (2) In the three-dimensional printing experiment, our groups of models were compared between correctly placed screws and once improperly placed screws. The correctly placed screws group exhibited a maximum pull-out force of (1 422.63±23.80) N. Furthermore, with increasing deviation angles in screw placement, the maximum pull-out forces of each group gradually decreased. (3) Comparing the condition of a single improper nail placement with repositioning the nail correctly, when the offset angle of the improper placement exceeded that of Model 3, correctly repositioning the nail helps to increase the screw’s pull-out resistance. (4) Comparing the scenario of two consecutive improper nail placements with repositioning correctly after two improper placements, correctly repositioning the nail reduced the screw’s pull-out resistance. Without replacing the screw, it was not advisable to attempt a third nail placement. (5) Experimental pull-out strength of three-dimensional printing correlates significantly with finite element simulation results, with a correlation coefficient of 0.98. There is no significant difference in the outcomes between the two methods (P > 0.05).

Key words: pedicle screw, lumbar spine, spinal fixation, stress analysis, pullout strength, redirection of screw placement, finite element analysis, biomechanics