Three different screw trajectories in single segment fixation: a finite element analysis and biomechanical study

Abstract

Backgroud context: Conventional pedicle screw (CPS) fixation in osteoporotic spines presents significant challenges. Cortical bone trajectory (CBT) screws can enhance screw holding power by increasing contact with cortical bone. However, the standard CBT (S-CBT) screws may encounter a series of problems such as stress concentration and diminished fatigue resistance.

Purpose: The S-CBT screw technique has been modified to accommodate longer screws, and the biomechanical behaviors of this modified CBT (M-CBT) screw technique were investigated.

Study design: A finite element analysis and biomechanical cadaveric study.

Methods: A validated nonlinearly finite element model spanning L1-S1 was employed in this study. Three L4-5 fusion models, namely CPS, M-CBT, and S-CBT, were generated using interbody fusion cages and different screw fixations. Next, the models were subjected to loading protocols to simulate flexion, extension, lateral bending, and rotation motion. The range of motion (ROM) and peak von Mises stress of the Cage, rods, screws, and intervertebral discs were analyzed. Besides, 3 types of cadaveric lumbar fusion modes were constructed using diverse screw trajectories. These models were cycled 10,000 times to measure the vertebral body displacement. Afterward, the individual screws were subjected to axial pull-out tests, and the maximum pulling-out force was documented. Finally, the data from the 3 fusion models were compared.

Results: Regarding 6 degrees of freedom movements, the 3 fixation models significantly increased the ROM of the adjacent segments (L3-4 and L5-S1) (p<.01). However, the differences in ROM increments among the 3 models were not statistically significant (p=.815). The peak von Mises stress of the cage for the M-CBT model was lower by -1.06%, 37.75%, 10.28%, and 17.55% compared with the S-CBT model during flexion, extension, right bending, and left rotation directions, respectively. Similarly, the peak von Mises stress of L5 screws for the M-CBT model was lower by 50.57%, 59.98%, 47.29%, 64.07%, 63.24%, and 50.45% compared with S-CBT during flexion, extension, left bending, right bending, left rotation, and right rotation, respectively. In the biomechanical test, the fatigue displacement results revealed that the displacement of M-CBT model was intermediate between the S-CBT and CPS models under both maximum and minimum forces, with statistically significant differences (p<.05). Additionally, the results of the antipullout test following fatigue loads demonstrated that the M-CBT group exhibited the highest maximum pull-out force (Fmax) (381.80 [119.00, 852.20]), followed by the CPS group (329.10 [117.00, 507.80]) and the S-CBT group (321.50 [196.60, 887.20]), but the differences were not statistically significant (p=.665) in the upper vertebral subgroup. Conversely, the Fmax of M-CBT group (384.20 [314.00, 851.20]) was significantly higher than that of S-CBT group (264.70 [118.80, 477.40]) and CPS group (282.20 [50.80, 595.20]) in the lower vertebral subgroup, with a significant difference between M-CBT and S-CBT (p=.037).

Conclusion: M-CBT could enhance the control force of the anterior column of the vertebral body by increasing the inserted screw length, minimizing the stress on the cages and screws, and optimizing the antifatigue performance of the internal fixation system compared to S-CBT.

Clinical significances: M-CBT screw technique shows better biomechanical properties compared to both S-CBT and CPS techniques, providing a more stable and effective internal fixation option for internal fixation in osteoporotic vertebrae.

Keywords: Biomechanical test; Cortical bone trajectory; Finite element analysis; Internal fixation system; Osteoporosis.