Biomechanical Evaluation of Sacral Load Redistribution Following Unilateral and Bilateral Sacroiliac Joint Disruption: A Three-Dimensional Finite Element Comparison of Three Fixation Strategies

Sacroiliac joint (SIJ) disruption alters posterior pelvic ring stability and can produce abnormal sacral stress redistribution; the symmetry of sacral load transfer following different fixation strategies remains controversial. This study compared sacral stress patterns under unilateral and bilateral SIJ instability for three fixation constructs using a three-dimensional finite element (FE) model. A lumbosacral–pelvic FE model was reconstructed from computed tomography data of a healthy adult and validated against previously published pelvic biomechanical data. SIJ instability was simulated by reducing the friction coefficient to represent ligamentous failure. Three fixation constructs were analyzed: anterior plate combined with posterior screw fixation (Model 1), spinopelvic fixation (Model 2), and hybrid fixation (Model 3). A 750 N axial compressive load was applied to simulate static standing. Peak sacral von Mises stress, stress amplification factors (SAFs), and left–right asymmetry ratios were computed and compared with the intact reference. Model 1 produced the highest sacral stress amplification (SAF = 3.46 under unilateral instability; peak stress 265.40 MPa). Model 2 reduced peak sacral stress (125.66 MPa under bilateral instability; SAF = 1.64), but values remained above the intact-model baseline. Model 3 yielded sacral stress closest to the intact condition under bilateral instability (81.64 MPa; SAF = 1.06), with near-symmetric load distribution in the bilateral injury configuration. Fixation topology strongly influenced sacral load transfer: hybrid fixation (Model 3) produced sacral stress magnitudes closest to the intact model, particularly under bilateral instability, whereas spinopelvic fixation (Model 2) showed more consistent left–right symmetry under unilateral injury. No single construct was superior across all symmetry-related outcomes. Hybrid stabilization may provide a biomechanically balanced approach to highly unstable posterior pelvic ring injuries under the simulated static axial-loading conditions.