BACKGROUND:
This study correlates anatomic landmarks with radiographic densities and radiolucencies of the posterior pelvis, which assists in percutaneous iliosacral screw placement.

METHODS:
Radio-opaque markers were placed on selected surfaces of cadaveric specimens and rotated under a fluoroscopic beam. Identified surfaces were described with angular measurements and anatomic characteristics.

RESULTS:
The anterior border of the S1 body is not parallel to the anterior border of the S1 ala. The anterior surface of the S1 ala is found on the inlet view ( approximately 49 degrees ) by the superimposition of the S1 and S2 alar ridges. Superior surface of the S1 ala is difficult to visualize fluoroscopically because of the highly oblique outlet angle required by fluoroscopy ( approximately 80 degrees ). The superior ridge of the S1 foramen can best be seen during an outlet view when the projection of the superior pubic symphysis is superimposed on the S2 foraminal image. Posterior bony surfaces of the ilium, foraminal region of sacrum, and sacral body are difficult to identify, as are the bony surfaces of the sacral foramina. During screw placement, the narrowest area of bone occurs in the sacral foraminal region.

CONCLUSIONS:
Use of predetermined angles for inlet and outlet views may not provide optimal visualization of relevant bony landmarks during iliosacral screw insertion. Because of variable sacral morphologies and lumbosacral lordosis, we propose using inherent landmarks to help visualize important landmarks during screw placement. Altering the fluoroscopic angle to superimpose the anterior S1 and S2 alar opacities allows the best visualization of the anterior boundary of the S1 ala. The superior bony surface of the S1 foramen is best visualized on the outlet view.



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