Effect of different lumbar–iliac fixation and sacral slope for Tile C1.3 pelvic fractures: a biomechanical study

Background Lumbar–iliac fixation (LIF) is a common treatment for Tile C1.3 pelvic fractures, but different techniques, including L4–L5/L5 unilateral LIF (L4–L5/L5 ULIF), bilateral LIF (BLIF), and L4–L5/L5 triangular osteosynthesis (L4–L5/L5 TOS), still lack biomechanical evaluation. The sacral slope (SS) is key to the vertical shear of the sacrum but has not been investigated for its biomechanical role in lumbar–iliac fixation. The aim of this study is to evaluate the biomechanical effects of different LIF and SS on Tile C1.3 pelvic fracture under two-legged standing load in human cadavers. Methods Eight male fresh-frozen human lumbar–pelvic specimens were used in this study. Compressive force of 500 N was applied to the L4 vertebrae in the two-legged standing position of the pelvis. The Tile C1.3 pelvic fracture was prepared, and the posterior pelvic ring was fixed with L5 ULIF, L4–L5 ULIF, L5 TOS, L4–L5 TOS, and L4–L5 BLIF, respectively. Displacement and rotation of the anterior S1 foramen at 30° and 40° sacral slope (SS) were analyzed. Results The displacement of L4–L5/L5 TOS in the left–right and vertical direction, total displacement, and rotation in lateral bending decreased significantly, which is more pronounced at 40° SS. The difference in stability between L4–L5 and L5 ULIF was not significant. BLIF significantly limited left–right displacement. The ULIF vertical displacement at 40° SS was significantly higher than that at 30° SS. Conclusions This study developed an in vitro two-legged standing pelvic model and demonstrated that TOS enhanced pelvic stability in the coronal plane and cephalad–caudal direction, and BLIF enhanced stability in the left–right direction. L4–L5 ULIF did not further improve the immediate stability, whereas TOS is required to increase the vertical stability at greater SS.