Comparison of pedicle screw pull-out forces using two vertebral bone material mappings: a patient-specific finite element modeling study

There is no consensus regarding the effect of spine pedicle screw misplacement and its pull-out force using patient-specific finite element modeling. The aim of this study was to investigate the effect of lumbar spine pedicle screw misplacements on their pull-out forces using finite element analyses. Computed Tomography (CT) Images of a healthy 23-year-old subject were used for development of the models. L2 vertebra of the lumbar spine was segmented and meshed using medical imaging software and imported into a finite element software. Three models were created with three different pedicle screw directions: Ideal c(perfect) placement without any misalignment, medial misplacement, and lateral misplacement all according to the Abul-Kasim classification. Two different material mappings were assigned to each model. In the first material mapping strategy, the cancellous bone of the vertebra was modeled as voxel-based and the cortical bone was homogeneously modeled, while in the second material mapping strategy, the entire bone was modeled as voxel-based. Pull-out simulations were performed on each model for both material mapping strategies. Results revealed that the first material mapping strategy produced maximum pull-out forces ranging from 1535 to 1770 N, whereas the second material mapping strategy generates maximum pull-out forces ranged from 2160 to 2918 N. Notably, for the first material mapping strategy, lateral misplacement maximum forces were 6.3% higher than those of the ideal condition, while the medial misplacement maximum forces were 1.9% lower than those of the ideal condition. In contrast, for the second material mapping strategy, maximum pull-out forces in lateral misplacements were 17.4% lower than those of the ideal condition, and the medial misplacement forces were 23% smaller than the ideal condition. The agreement of the simulation results with the experimental tests in the literature shows the potential of patient-specific finite element modeling in the accurate estimation of the stability of spinal implants.