Finite element analysis of axial pull-out strength in osteoporotic lumbar pedicle screws

English Keywords:Osteoporosis  Pedicle screw  Cortical bone trajectory  Finite element analysis  Biomechanics  Tensile strength

English Abstract:

  【Abstract】 Objectives: To compare the biomechanical outcomes of six different screw reinforcement methods for posterior lumbar pedicle screw augmentation technique in osteoporotic vertebrae through finite element analysis, and provide a reference for the optimization and selection of internal fixation screws. Method: The lumbar CT scan data from seven males without lumbar pathologies or surgical history were obtained. Age: 68.0±4.9 years(60-75), height: 171.7±3.2cm (168-176), weight: 63.6±3.2kg(59-68). Continuous thin-layer scanning was performed using a 64-slice spiral CT to obtain DICOM images. The L3 vertebral 3D models were reconstructed using Mimics 21.0 software, after optimizing for surface geometry in Geomagic Wrap 2021, seven models of screw were designed and assembled in SolidWorks 2023. Screws were categorized into seven groups based on reinforcement methods: Group A(Control group, no reinforcement), Group B(increasing thread outer diameter), Group C(decreasing pitch), Group D(decreasing the part of pitch contacted cortical bone), Group E(bone cement reinforcement), Group F(penetrating the opposite cortex), and Group G(cortical bone trajectory screws). 49 finite element models(7 screw types × 7 vertebrae) were established. After material property assignments based on the data from prior research literature, the Static Structural Analysis module of Ansys Workbench 2023 was employed to simulate axial pull-out experiments on the seven established finite element models. The pull-out displacement under an axial tension of 1,200N and the maximum equivalent stress when the screw was damaged were analyzed. Intergroup comparisons employed one-way ANOVA, with independent-sample t-tests for experimental vs. control group comparisons. Results: The maximum displacements of Groups A-G were 0.142±0.029mm, 0.138±0.031mm, 0.144±0.032mm, 0.139±0.027mm, 0.119±0.024mm, 0.126±0.028mm, 0.040±0.007mm, respectively. No statistically significant differences were observed between Groups A to F(F=0.812, P=0.5489). Group G demonstrated a statistically significant difference compared to Group A(P<0.001). The maximum equivalent stress of groups A-G were 264.0±7.6MPa, 234.8±28.0MPa, 245.5±17.5MPa, 260.0±7.2MPa, 279.7±14.3MPa, 311.9±31.4MPa, 432.6±87.5MPa. Compared with group A, group B decreased by 11.1%, and group C decreased by 7%, and the differences were statistically significan(P<0.05); Group D wasn′t significantly different from group A(P>0.05); Compared with group A, groups E-G increased by 5.9%, 18.1% and 63.9%, respectively, and the differences were statistically significant(P<0.05). Conclusions: Different screw augmentation techniques demonstrated significantly different effects on axial pull-out strength. Increasing the outer diameter of the screw or decreasing the total pitch will reduce the axial pull-out strength of the screw, and reducing the pitch in the cortical bone area has no obvious positive effect on increasing the holding force of screw. Bone cement reinforcement, increasing the screw length in penetrating contralateral cortical bone, and cortical bone trajectory screw can all significantly improve the axial anti-pullout effect of the screw, among which Cortical bone trajectory screw augmentation technique has the most significant effect.