Biomechanical analysis of fixation for vertically unstable sacroiliac dislocations with iliosacral screws and symphyseal plating.

OBJECTIVES: To examine the effects of various iliosacral screw configurations with and without symphyseal plating on sacroiliac (SI) motion and hemipelvis stability in the vertically unstable pelvic model.
DESIGN: Biomechanical, human cadaver.
SETTING: Level 1 trauma center. INTERVENTION: Hemipelvis and SI motion were analyzed on a Materials Testing System before and after creation of a vertically unstable APC III pelvic injury. Posterior fixation constructs consisted of iliosacral screws: (1). one into S1, (2). two into S1, or (3). one into S1 and one into S2. Results were obtained for all posterior constructs with and without a two-hole symphyseal plate. MAIN OUTCOME MEASUREMENT: Hemipelvis and SI motion with axial loading.
RESULTS: There was no statistically significant difference between one or two iliosacral screws when hemipelvis rotational or linear displacement was examined at the SI joint. The two-hole symphyseal plate significantly increased the stability of the fixation construct in resisting linear displacement in all three planes. Without the symphyseal plate, an abnormal loading response was seen at the SI joint, resulting in paradoxical posterior translation and sagittal plane rotation. The addition of the plate restored the normal response, and anterior rotation and translation were observed as in the intact state.
CONCLUSIONS: Anterior symphyseal plating for the vertically unstable hemipelvis significantly increases the stability of the fixation construct and restores the normal response of the hemipelvis to axial loading. A significant benefit to supplementary iliosacral screws in addition to a properly placed S1 iliosacral screw was not shown.