Biomechanical effects of sacroiliac screw selection on sacroiliac screws combined with a lumbar iliac fixation for vertical fractures of the sacrum: a biomechanical finite element analysis

[Objective] To examine the impact of altering the sacroiliac screw's position and length on the biomechanical properties of the triangle osteosynthesis used to treat unilateral vertical sacral fractures and serve as a clinical reference. [Methods] Unilateral sacral fractures of the Denis II type were modeled using finite elements to represent Tile C pelvic ring injuries. Six sacroiliac screws combined with lumbar-iliac fixation patterns were used to fix the sacral fracture. To compare sacral stability, maximum pressure, and stress distribution after fixation of sacral fractures with each internal fixation modality. [Results] The best vertical stability of the internal fixation model was achieved when the S1 segment was fixed with lengthened sacroiliac screws. This was followed by using normal sacroiliac screws for the S1 segment. There was no statistical difference between the vertical stability of the S1 plus S2 dual-segment fixation model and the S1-segment fixation model.The maximum pressures under a vertical force of 600 N, L5LS1 <L5NS1<L5S1S2<L5LS2<L5NS2<L5NS1S2. [Conclusions] In unilateral vertical fracture of the sacrum (AO C3.1 DENISS II) treated with triangular osteosynthesis, using triangular jointing combined with unilateral iliolumbar fixation + sacroiliac screw fixation, it is recommended that the best vertical stability of the sacrum is obtained with a single lengthened sacroiliac screw for the S1 segment, with less maximum compression on the internal fixation. If it is not possible to apply a lengthened sacroiliac screw, it is recommended to use a plain sacroiliac screw for the S1 segment. Adding an S2 segment screw does not significantly increase the vertical stability of the sacrum.