Radiologic and anatomic evaluation of the anterior sacral foramens and nerve grooves.

Study Design: The present study evaluated the anterior sacral foramen using plain radiographs and projected the positions of S1-S3 anterior sacral foramen and corresponding nerve root groove on the posterior aspect of the sacrum.

Objectives: To evaluate the plain radiographs of anteroposterior, inlet, and outlet views regarding the sacral foramen, and to determine quantitatively the location of the anterior sacral foramens on the posterior aspect of the sacrum.

Summary Of Background Data: Injury to the sacral nerve roots associated with posterior sacral screw placement remains a potential hazard. Few studies regarding the evaluation of the anterior sacral foramen and its projection on the posterior sacral surface are available.

Methods: Six bony pelves were harvested from preserved cadavers. The superior aspects of the sacral alae, the openings of the anterior and posterior foramens of S1-S2, were marked by outlining them with K-wires. Anteroposterior, inlet, and outlet plain radiographs were taken. The bony sacra were further disarticulated from the above six pelvic specimens. K-wires were drilled through the sacra to project the dimensions of the anterior foramens and nerve grooves of S1-S3 onto the posterior sacral surface. The dimensions between the perimeter of the projection and the corresponding posterior foramen were measured.

Results: The plain radiographs show that the shape and relative position between the anterior and posterior foramens vary with different projections. It was believed that outlet projection is the best view of plain and radiographs in the evaluation of the sacral foramens and corresponding pedicles. The approximate boundaries of the anterior sacral foramens' projections were 6 mm superior, 10 mm lateral, 3 mm inferior, and 3 mm medial to the corresponding margins of the posterior foramens.

Conclusions: The outlet projection is the most useful view in plain radiographs for the evaluation of sacral foramens and pedicles. Quantitative data of the anterior sacral foramen's anatomic position on the dorsal aspect of the sacrum may be helpful in the sacral pedicle screw placement.