Finite element analysis of the pedicle screw in lumbar transpedicular dynamic stabilization that penetrates the lateral cortex of the vertebral pedicle

The mechanical stability of lumbar transpedicular dynamic stabilization (LTDS) was determined by simulating the degree to which the pedicle screws penetrated the lateral pedicle cortex. A finite element model of the L4-5 segment was established. Four states of pedicle screw implantation were simulated: in Type 1, the pedicle screw was located entirely within the pedicle; in Types 2, 3, and 4, the pedicle screw broke through the lateral pedicle cortex by ≤ 2 mm, 2.1–4 mm, and > 4 mm, respectively. The L4-5 segment was fixed with LTDS with seven pedicle screw fixation models: Groups A (all screws: Type 1), B (left screws: Type 2; right screws: Type 1), C (left screws: Type 3; right screws: Type 1), D (left screws: Type 4; right screws: Type 1), E (all screws: Type 2), F (all screws: Type 3) and G (all screws: Type 4). The corresponding mechanical indicators were recorded. Among Groups A, B, C and D, Group B had the smallest overall displacement and overall strain, followed by Groups A, B, C, and D; the overall stress and pedicle screw stress of Group D were the lowest, followed by Groups C, B, and A; and there were no significant differences in overall displacement or pedicle screw stress among the groups (p > 0.05). However, there were significant differences in overall strain and overall stress among the groups (all p < 0.001). Among Groups A, E, F and G, the overall displacement and overall strain of Group E were the lowest, followed by Groups A, F, and G; the overall stress and pedicle screw stress of Group G were the lowest, followed by Groups F, A, and E; and there was no significant difference in overall displacement within the groups (p = 0.718). However, there were significant differences in overall strain, overall stress, and pedicle screw stress among the groups (all P < 0.001). Within the limits of the present model, it may be acceptable for the screw to be located within the pedicle and for the screw to penetrate past the lateral pedicle cortex by ≤ 2 mm. The mechanical strength of the pedicle screws decreases progressively when they penetrate past the lateral pedicle cortex by 2.1–4 mm and > 4 mm, so such penetration and should be avoided.