Inadvertent intraforaminal iliosacral screw placement despite apparent appropriate positioning on intraoperative...

We present the case of an intraforaminal iliosacral screw placed percutaneously with aid of C-arm using inlet, outlet, and lateral views of the pelvis. The iliosacral screw was placed above the S1 foramen on the outlet view, into the middle of S1 via the ala on the inlet view, and below the cortical shadow of the ala on the lateral view. The patient was neurologically intact postoperatively, but began to complain of severe radicular pain in the S1 distribution down to the foot within 1 week postsurgery. There was mild weakness of plantar flexion. Postoperative computed tomography scan showed that the iliosacral screw was within the S1 foramen. Because of the tangential nature of the S1 foramen, slight posterior placement of the screw into the S1 body and not into the promontory resulted in violation of the foramen despite it being above the cortical shadow on the outlet view.