Biomechanical analysis of variants of spinopelvic fixation of longitudinal sacral fractures by the finite element method

Objective. To analyze the strength of three types of spinopelvic fixation system configurations in longitudinal fracture of the sacrum by the finite element method.

Material and Methods. Biomechanical analysis was carried out by the finite element method. A three-dimensional model of a segment of the spinopelvic complex (SPC), including the pelvic bones, sacrum and L4 and L5 vertebrae, was created on the basis of the CT scan results of a healthy patient. Then, a longitudinal fracture of the sacrum was simulated on the developed model of the sacrum on the left side in zone 1 according to the Denis classification. Further, a comparative assessment of three variants of spinopelvic fixation systems with the help of biomechanical computer modeling was carried out: bilateral spinopelvic system L4–S2Alar, bilateral spinopelvic system
L4–S2Alar with transverse connector installation, and bilateral spinopelvic system L4–S2Alar with L-shaped rod installation. The stability of fixation, as well as the amount of loads acting on the fixation elements and bone tissues were determined.

Results. As the rigidity of the structure increases by means of a transverse connector or an L-shaped rod, the load is redistributed between the screws located to the left and right of the fracture. The rigidity of the L4–S2Alar system with parallel, unconnected rods is much lower, which leads to a critical increase in loads on instrumentation and vertebrae.

Conclusion. Analysis of three variants of spinopelvic fixation of longitudinal fractures of the sacrum by finite element method revealed that bilateral spinopelvic system with pedicle screws installed in the L4 and L5 vertebrae and pelvic screws installed in the iliac bones through the lateral masses of S2, two on each side (L4–S2 Alar) and connected by two parallel rods (variant 1) is the least strong in comparison with the other variants. The strength of the fixation increases when the structure is supplemented with a transverse connector between the rods (variant 2). The L4–S2 Alar design with an L-shaped rod on the side of the longitudinal fracture of the sacrum (variant 3) proved to be the most strong.

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