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Pull-out Strength Test Simulation and Stability Study of a Patient-Specific Drill Guide Template of Thoracic Pedicle Screw Placement for Patients with Spinal Deformity Using Finite Element Analysis

Hosseini, Fahimeh | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55540 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Arjmand, Navid
  7. Abstract:
  8. We have recently designed/fabricated novel bilateral vertebra- and patient-specific drill guides for pedicle screw (PS) placements and tested their accuracy for both nondeformed and deformed thoracic spines. PS placement deviations from their preplanned positions significantly reduced when guide template were used; the success rate improved from ~72% (freehand placements) to 94% (guided placements). In the present study, we aim to use finite element (FE) analyses to evaluate the pull-out strength of these PSs inserted via either the freehand technique or our drill-guide templates. Two 3D-printed T1-T12 thoracic models of a severe scoliosis patient with a 47° thoracic dextro-scoliotic curve were analyzed (one model bilaterally screwed at all vertebrae by the freehand approach and one by the templates guides resulting in 24 PSs in each model). Total of 48 FE models were constructed using CT-scan images of the screwed vertebrae (i.e., both the vertebra and bilateral screws) and the pull-out strength forces were computed. The cancellous bone was assumed to be homogenous and the screws were simulated as isotropic titanium materials. Surface-to-surface contacts were set between the screws and vertebrae (friction coefficient of 0.6 in tangential direction and hard contact in normal direction). Failure modeling with the element deactivation feature available in ABAQUS was used to simulate failure in the bony tissue based upon a threshold of fracture strain. Results indicated that the mean pullout strength force (±SD) was 809.4±249.9 (N) and 704.2 ± 309.8 (N) for the template guided and freehand techniques, respectively (two-tailed paired t-test p-value = 0.186). As the cancellous bone was modeled as a homogenous material, the improvement in the 3D angle of the screws inserted by the drill-guide templates could be downplayed in the FE models. Nevertheless, the presented modeling approach could predict the difference in the pullout forces of the pedicles breached by screws. In this research, a qualitative criterion for reviewing the design of guide templates based on stability, with the topology optimization method, has also been proposed.Generally, existing template guides design cover a large surface area of the vertebral bone. It causes the complete mismatch of template guides with the bone surface, due to the existence of soft tissues between them. This quality criterion confirmed our design
  9. Keywords:
  10. Finite Element Analysis ; Pedicel Screw ; Patient-Specific Modeling ; Biomechanical Modeling ; Spinal Biomechanics ; Patient-Specific Guide Template ; Pull-Out Strength Test

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