Preferential sacral fracture sites in fragility fractures of the pelvis type IVb and comparison of internal fixation methods: CT-based morphological mapping and finite element analysis

PURPOSE: Fragility fractures of the pelvis (FFP) classified as Rommens-Hoffman type IVb are associated with spinopelvic dissociation and are generally considered to require surgical intervention. This study aimed to clarify the localization patterns of FFP type IVb and compare the biomechanical stability of different internal fixation techniques. METHODS: In this retrospective study, morphologic mapping of sacral fracture lines was performed in 36 patients with FFP type IVb. Based on the mapping results, a finite element (FE) model of FFP type IVb was developed to evaluate the biomechanical stability of ilio-sacral screw (ISS) fixation, trans-sacral screw (TSS) fixation, spinopelvic fixation (SPF; On each side, L5 pedicle screw was connected to two iliac screws with a rod, and the bilateral constructs were linked using a cross-connector.), and bilateral triangular fixation (one TSS at S1 combined with SPF mentioned above) using finite element analysis (FEA). RESULTS: Morphologic mapping showed that the sacrum fracture transverse line tended to pass between the S1-2 transverse lines. Although bilateral triangular fixation and SPF provided the highest stability in both U-type and H-type fractures, a TSS for U-type and two TSSs for H-type also demonstrated comparable levels of stability. ISS-based methods showed greater displacements. CONCLUSION: TSS-based fixation may provide stability comparable to bilateral triangular fixation and SPF in FFP type IVb, with less invasiveness when anatomy permits. Further studies are needed to optimize treatment strategies for this complex injury.