Safe placement of S1 and S2 iliosacral screws:: The "vestibule" concept

Objectives: To determine the optimal starting points for placement of S-1 and S-2 iliosacral screws as well as the pertinent anatomy surrounding the S-1 and S-2 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 S-1 and S-2 iliosacral screw placement. Safety of simulated S-1 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 S-1 foramen coronally. S-2 iliosacral screws had less cross-sectional area for placement than S-1 screws. Placement of the S-2 screw slightly to the S-1 foraminal side of the S-2 vertebral body increased the safety of placement. Conclusion: The iliosacral screw starting point at the posterior sacral body and inferior S-1 foramen was the safest when considering the entire population. Careful attention to the size and orientation of the S-2 vertebral body should be taken if S-2 iliosacral screws are placed.