Biomechanical analysis of iliosacral and transiliac–transsacral screw combinations for fixation of undisplaced Denis II vertical shear fractures in dysmorphic sacrum

Background Percutaneous iliosacral screws (ISS) and transiliac-transsacral screws (TTS) are effective for treating posterior pelvic ring instability. However, the biomechanical stability of undisplaced sacral dysmorphism fractures remains underexplored. This study evaluated various ISS and TTS combinations to provide a clinical reference for fixing such fractures. Methods A finite element model of a complete Denis type II dysmorphic sacral fracture (extending through the sacral foramen) was developed. The stability of the posterior pelvic ring was evaluated using seven fixation techniques: S1-ISS (Group 1), S2-ISS (Group 2), S1-ISS + S2-ISS (Group 3), S2-TTS (Group 4), anterior S1-ISS + S2-TTS (Group 5), middle S1-ISS + S2-TTS (Group 6), and posterior S1-ISS + S2-TTS (Group 7). In all models, the anterior pelvic ring was fixed with pubic ramus screws. The upper sacral surface was subjected to six loading modes to simulate physiological states: standing, forward flexion, left flexion, right flexion, left rotation, and right rotation. The following parameters were recorded and analyzed: vertical displacement and sagittal angular displacement of the upper sacral surface, relative displacement of five pairs of observation points on the anterior fracture line, and maximum Von Mises stress and deformation of the S1 and S2 screws. Results The finite element analysis revealed that the maximum stress in all internal fixation groups across the six loading modes was below the yield strength of titanium alloy, indicating no risk of implant failure. Screw deformation was highest in G1 and lowest in G5 for the S1 segment, and highest in G2 and lowest in G 5 for the S2 segment. G5 also exhibited the minimum vertical and angular displacements of the sacral upper surface in all motion states. Conclusion This study demonstrates that S1-ISS combined with S2-TTS fixation provides excellent biomechanical stability for undisplaced vertical fractures of the sacral dysmorphism, particularly the combination of anterior S1-ISS and S2-TTS. This fixation method offers a promising clinical treatment option.