Surgical anatomy of the sacrum. A guide for rational screw fixation.

Anterior sacral anatomy was studied to allow safe sacral screw placement. The study included 27 bony sacra. Intrapelvic dissections were performed. Cadaveric specimens were dissected and photographed. The position and angulation of the first sacral foramen was extremely variable. After the position of the L5 root, sacral roots, and iliac vessels, was considered, safe posterior sacral screw implantation could be assured through placement above the level of the first sacral foramen directed medially toward the promontory, parallel to the superior sacral end-plate. Complications of indiscriminate screw placement included L5-root damage, iliac vein penetration, and intractable perineal pain due to sacral root injury. Preoperative computed tomography with the scanner gantry tilted to parallel the superior sacral end-plate defined the great vessels, neural foramens, inclination of the superior sacral end-plate, and prominence of the posterior ileum. Surgical accuracy was evaluated after surgery with computed tomography.