Biomechanical comparison of posterior internal fixation techniques for unstable pelvic fractures

Early reduction and rigid fixation of unstable vertical shear pelvic fractures has been shown to decrease the incidence of late sequelae and facilitate early mobilization. The results of fixation of the posterior pelvic ring without anterior fixation are unknown. The purpose of this study was to perform a biomechanical comparison of the most frequently used techniques of posterior fixation for unstable pelvic sacroiliac dislocations in conjunction with ipsilateral rami fractures, i.e., an unstable vertical shear injury. The four methods of posterior fixation tested included sacroiliac (SI) screws, anterior SI plates, transiliac bars, and a combination of SI screws and transiliac bars. Six cadaveric pelvises were tested in axial compression and torsion on a biaxial servohydraulic testing machine. Compared to the intact pelvis, single posterior methods of fixation provided approximately 70-85% resistance to axial and torsional loading. By combining SI screws with transiliac bars, approximately 90% of intact pelvic stability was achieved. Our results suggest that rigid posterior fixation of sacroiliac dislocations alone may obviate the need for additional complex anterior surgical procedures to fix rami fractures.