Safe Placement of S1 and S2 Iliosacral Screws: The "Vestibule" Concept

OBJECTIVES:To determine the optimal starting points for placement of S1 and S2 iliosacral screws as well as the pertinent anatomy surrounding the S1 and S2 vertebral bodies. DESIGN:Normal subject study evaluating helical CT scans of thirty normal posterior pelvic rings. SETTING:Methodist Hospital, Indianapolis, Indiana, Level I trauma center. PARTICIPANTS:Consenting adults for limited pelvis CT. MAIN OUTCOME MEASUREMENTS:The three-dimensional anatomy of the posterior pelvic ring pertinent to S1 and S2 iliosacral screw placement. Safety of simulated S1 iliosacral screw placement using different lateral ilium starting points. RESULTS:The transversely placed (horizontal) iliosacral screw was the least safe of the screws tested. The safest lateral ilium starting point for our entire population was at the posterior sacral body sagitally and at the inferior S1 foramen coronally. S2 iliosacral screws had less cross-sectional area for placement than S1 screws. Placement of the S2 screw slightly to the S1 foraminal side of the S2 vertebral body increased the safety of placement. CONCLUSION:The iliosacral screw starting point at the posterior sacral body and inferior S1 foramen was the safest when considering the entire population. Careful attention to the size and orientation of the S2 vertebral body should be taken if S2 iliosacral screws are placed.