Biomechanical Comparison of Standard Iliosacral Screw Fixation to Transsacral Locked Screw Fixation in a Type C Zone II Pelvic Fracture Model

Abstract

Iliosacral screw fixation into the first sacral body is a common method for pelvic ring fixation. However, this construct has been shown to be clinically unreliable for the percutaneous fixation of unstable Type C zone II vertically oriented sacral fractures with residual fracture site separation. The objective of this study was to biomechanically compare a locked transsacral construct versus the standard iliosacral construct in a Type C zone II sacral fracture model. A Type C pelvic ring injury was created in ten embalmed cadaver pelves by performing vertical osteotomies through zone II of the sacrum and the ipsilateral pubic rami. The sacrum was then reduced maintaining a 2 mm fracture gap. Five specimens were fixed using two 7.0-mm iliosacral screws into the S1 body; the other 5 were fixed using one 7.0-mm iliosacral screw and one 7.0-mm transsacral screw exiting the contralateral ilium with a nut placed on its end, creating a locked construct. Each pelvis underwent 100,000 cycles at 250 N and was then loaded to failure using a unilateral stance testing model. Vertical displacements at 25,000; 50,000; 75,000 and 100,000 cycles and failure force were recorded for each pelvis. The locked transsacral construct performed significantly better than the iliosacral construct at all 4 measurement points (P = 0.009) and in force to failure (P value = 0.02). Fixation of unstable zone II sacral fractures using the combination of an iliosacral screw and a locked transsacral screw resists deformation and withstands a greater force to failure as compared to fixation with 2 standard iliosacral screws. This locked transsacral construct may prove advantageous, especially when a percutaneous technique is used for a Type C zone II vertically oriented sacral fracture injury pattern, which can result in residual fracture site separation.