S2 Alar Iliac Screw and S1 Pedicle Screw Fixation for the Treatment of Sacral Fractures : A Finite Element Study

Abstract Background: Five different sacral fracture fixation methods were compared using finite element (FE) analysis to study their biomechanical characteristics.Methods: Denis type I sacral fractures were created by FE modeling. Five different fixation methods for the posterior pelvic ring were simulated: sacroiliac screw (SIS), lumbopelvic fixation (LPF), transiliac internal fixator (TIFI), S2-alar-iliac (S2AI) screw and S1 pedicle screw fixation (S2AI-S1), and S2AI screw and contralateral S1 pedicle screw fixation (S2AI-CS1). Four different loading methods were implemented in sequence to simulate the force in standing, flexion, right bending, and left twisting, respectively. Vertical stiffness, relative displacement, and change in relative displacement were recorded and analyzed.Results: As predicted by the FE model, the vertical stiffness of the five groups in descending order were S2AI-S1, SIS, S2AI-CS1, LPF, TIFI. In terms of relative displacement, S2AI-S1 and S2AI-CS1 groups displayed a smaller mean relative displacement, although the S2AI-CS1 group exhibited greater displacement in the upper sacrum than the S2AI-S1 group. The SIS group displayed a moderate mean relative displacement, although the displacement of the upper sacrum was smaller than that of the S2AI-CS1 group. The LPF and TIFI groups displayed larger mean relative displacements. In terms of change in relative displacement, the TIFI and LPF groups displayed the greatest fluctuations in their motion, while the SIS, S2AI-S1, and S2AI-CS1 groups displayed smaller fluctuations.Conclusion: Compared with SIS, unilateral LPF and TIFI, S2AI-S1 displayed the greatest biomechanical stability of the FE models and can be used as a new method for the treatment of sacral fractures. When the S1 pedicle screw insertion point on the affected side is damaged, S2AI-CS1 can be used as a good alternative to S2AI-S1.