Anatomy of the L5 Nerve Root

Abstract

S1 pedicle screw and sacral alar screw are used as the standard sacral anchor for fixation to treat various spinal disorders and pelvic trauma. Bicortical fixation, wherein the screws penetrate the anterior sacral cortex, is often used to achieve greater stability than that with monocortical fixation. However, screws penetrating the anterior sacral cortex have a risk of neurovascular injury, and we have experienced two cases of postoperative L5 nerve root injury caused by anterolateral malpositioning of the S1 pedicle screws. Therefore, we investigated the anatomy of the L5 nerve root in the pelvis to clarify a safety zone for sacral screw placement. All of the L5 nerve roots coursed outward after exiting the intervertebral foramina. L5 nerve roots tended to take an outward course after changing their angle at the most anterior surface of the ala of the sacrum. L5 nerve roots were positioned close to the ala of the sacrum and had poor mobility. These findings suggest that surgeons should insert the S1 pedicle screw medially toward the inside of the S1 anterior foramina, and the sacral alar screw laterally with an angle >30° to prevent iatrogenic L5 nerve root injury.