Finite element analysis of long SI screws in the treatment of vertically unstable bilateral sacral fractures

Objective: The finite element analysis (FEA) was used to investigate the biomechanical stability of combined long SI screws in the treatment of bilateral sacral fractures. Methods: This study was conducted between September 10, 2023, and June 13, 2024. Using the finite element (FE) method, a vertically unstable bilateral sacral fracture model and eight internal fixation groups were established: L1 (one long sacroiliac (SI) screw in S1), T1 (one transiliac-transsacral (TITS) screw in S1), 2S1 (two SI screws in S1 and S2), L1L2 (two long SI screws in S1 and S2), 2S1L2 (two SI screws in S1 and one long SI screw in S2), L1T2 (one long SI screw in S1 and one TITS screw in S2), T1T2 (two TITS screws in S1 and S2), and 2S1T2 (two SI screws in S1 and one TITS screw in S2). A pelvic standing posture was simulated to compare overall stiffness, average displacement along the fracture line, posterior rotation angle of the sacrum, and stress distribution in the SI joint among the models. In addition, stress nephograms of the eight internal fixations were analyzed. Results: Models with smaller sacral displacement generally exhibited greater stiffness. Two-segment fixation constructs (T1T2, L1T2, 2S1T2, L1L2) provided better biomechanical stability than single-segment fixations (T1, 2S1, L1). The L1T2 group demonstrated stiffness comparable to T1T2 and superior stability to 2S1T2 when two segments were fixed. Peak stress was highest in the L1 group (211.9 MPa) and lowest in the T1T2 group (107.1 MPa), with all models remaining within the safe range of titanium alloy. The stress on implants was mainly concentrated at the SI joint–screw interface. Two-segment fixations showed lower peak stress and more uniform stress distribution, suggesting better load-sharing and reduced implant fatigue risk. Sacral retroversion angles followed the same ranking as peak stress at the SI joint, with T1T2, L1T2, and 2S1T2 groups showing the smallest angles, indicating superior rotational stability. Conclusion: SI screws in dual-segment provide better biomechanical stability than those in the single-segment. Both the L1T2 and T1T2 groups demonstrate good biomechanical stability and are reliable for the fixation of vertically unstable bilateral sacral fractures. When a TITS screw cannot be inserted in the S1 segment, using a long SI screw in S1 combined with a TITS screw in S2 can achieve a comparable fixation effect.