Keywords
thoracolumbar junction
transpedicular fixation
intermediate screws
finite element analysis
flexion
How to Cite
Abstract
Objective. To evaluate the stress–strain state in an A1 fracture of the Th12 vertebra under flexion loading and to compare different configurations of short-segment transpedicular fixation.
Materials and Methods. A detailed finite element model of the Th9–L5 spinal segment with an A1 fracture of the Th12 vertebral body was developed. Four fixation configurations were analyzed: M1 with short screws without intermediate screws; M2 with long screws without intermediate screws; M3 with short main screws with additional short intermediate screws placed into the Th12 vertebral body; and M4 with long main screws with additional short intermediate screws placed into the Th12 vertebral body. Flexion was simulated by applying a 350 N load. The evaluation criterion was the equivalent von Mises stress.
Results. In the Th12 vertebra, increasing screw length in constructs without intermediate screws led to an increase in stress from 1.6 to 2.3 MPa, whereas in constructs with intermediate screws stress remained at 1.6–1.7 MPa. In the L1 vertebra, changes were minimal. At the vertebral endplates, stress at the inferior endplate of Th11 ranged from 1.9 to 2.1 MPa, while at the superior endplate of L1 stress decreased from 8.3 to 7.7 MPa. At the screw entry zones, stress in the Th11 pedicle decreased from 8.0 to 5.6–5.7 MPa; at the Th12 pedicle it remained approximately 2.1 MPa; at the L1 pedicle, stress increased with long screws from 10.3 to 11.1 MPa regardless of the presence of intermediate screws. In the rods, the use of intermediate screws increased stress by nearly 2.2 times—from 170–172 to 380–383 MPa.
Conclusions. Intermediate screws provide local stress reduction and stabilization of the injured level but increase stress in the rods. Fixation strategy should therefore be individualized: M3–M4 are preferable when prioritizing protection of the bone–screw interface and the fracture zone, whereas M1–M2 may be chosen when minimizing rod loading is critical, with careful selection of caudal screw length.
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