Safe placement of S-1 and S-2 iliosacral screws: The "vestibule" concept

Objectives: To determine the optimal starting points for placement of S-1 a nd 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 p osterior 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 p elvic ring pertinent to S-1 and S-2 iliosacral screw placement. Safety of s imulated S-1 iliosacral screw placement using different lateral ilium start ing points. Results: The transversely placed (horizontal) iliosacral screw was the leas t safe of the screws tested. The safest lateral ilium starting point for ou r entire population was at the posterior sacral body sagitally and at the i nferior S-1 foramen coronally. S-2 iliosacral screws had less cross-section al 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 bod y and inferior S-1 foramen was the safest when considering the entire popul ation. Careful attention to the size and orientation of the S-2 vertebral b ody should be taken if S-2 iliosacral screws are placed.