Biomechanical Evaluation of Full-Length S2 Alar-Iliac Screws Combined With Posterior Iliac Transfixion Screws for the Treatment of Unilateral Unstable Sacral Fractures

OBJECTIVE: This study aimed to examine the biomechanical properties of full-length S2 alar-iliac screws combined with posterior iliac transfixion screws for the treatment of unilateral unstable sacral fractures, thereby providing a theoretical foundation for clinical application. METHODS: A 3-dimensional finite element analysis was applied to simulate Tile C1-C3 pelvic injuries, characterized by a unilateral sacral fracture in Denis Zone II combined with fractures of the superior and inferior pubic rami. Three posterior pelvic ring fixation techniques were evaluated: (1) standard S1 screws with S2 alar-iliac screws, (2) full-length S2 screws, and (3) full-length S2 screws combined with posterior iliac transfixion screws. Vertical and rotational stability of the posterior pelvic ring under simulated standing conditions were quantified and compared. Peak stress, vertical displacement of the sacral surface, posterior rotation angle displacement of the sacrum, and stress distribution across the pelvis were recorded and analyzed for each fixation technique. RESULTS: The fixation stability achieved by full-length S2 alar-iliac screws combined with posterior iliac transfixion screws was comparable to that of standard S1 screws with S2 alar-iliac screws. Both techniques demonstrated significantly less vertical displacement and posterior rotational angle of the sacrum compared with single S2 alar-iliac screws (P