Radiological evaluation of S1 dorsal screw placement.

Twelve bony pelves were used in this study. S1 dorsal screws were inserted in the anteromedial, anterior, anterolateral, and anteroinferior directions. When the screws were inserted within 5-10 mm beyond the anterior sacral cortex, radiographs were obtained in the anteroposterior, modified inlet, modified outlet, and lateral projections to evaluate the position of the screws and penetration of the anterior cortex. In addition, 30 dried sacral were obtained for anatomic evaluation focused on the anterior and superior aspect of the sacral alae and its relationship to the anterior aspect of the sacrum. The results showed that a screw penetration of the anterior cortex of the S1 vertebral body or ala is best detected in the modified inlet view. A misdirected screw into the S1 anterior foramen is best seen in the modified outlet view. The lateral view also is useful in determining the extraosseous penetration. The average distance from the anteriormost limit of the ala to the anterior sacral cortex was approximately 11 mm. The average sagittal angle between the superior surface of the ala and S1 dorsal aspect was approximately 51 degrees. This study suggested that the modified inlet and lateral radiograph views are most useful for detecting screw penetration of the anterior cortex of the sacrum. The modified outlet projection is the best for determination of a screw violating the S1 anterior foramen. Also, the modified inlet projection will show the screw orientation relative to the mediolateral plane (the sacral canal and sacroiliac joint), and the lateral view will show the screw direction relative to the superoinferior plane.