Anatomical Safe Zone of Sacral Ala for Ventrolateral Sacral(S1) Screw Placement: Re-evaluation of Its Effectiveness

ABSTRACT

Among the various sacral fixation techniques used to enhance the strength of fixation, S1 screw placement in the sacrum is the most common method. Ventrolateral S1 screw placement through the sacral ala has been used alone or in combination with a medially-directed screw in the S1 pedicle to enhance pull-out resistance. Although the anatomical safe zone was identified, there is a risk of neurovascular injury particularly when the enhancement of fixation strength requires bicortical purchase. The purpose of this cadaver study is to re-evaluate the previous anatomical safe zone when using an S1 screw laterally directed toward the sacral ala. After dissecting the lateral safe zone of sacral ala in 12 human cadavers, K-wires were intentionally inserted deep into this zone. Each "safe" angle to the center of the safe zone was measured and the degree of risk to neurovascular structures was recorded on the basis of the distance in millimeters from the tips of the penetrating K-wires. The results are as follows the mean safe angle to the center of the anatomical safe zone was 33.5degrees+/-9.3(20-50). Between 20 and 50 degrees, the range of safe angle was too wide. The distance between the tip of the K-wire and the sacroiliac joint, lumbosacral trunk, obturator nerve was 4.8mm+/-1(4-7.5), 6.8mm+/-1(6-9.5) and 6.8mm+/-3.2(0-10) respectively, while the anterior height between sacral cortex and lumbosacral trunk, internal iliac vein was 0mm and 2.1mm+/-1.8(0-5) respectively. In 29% of cases, the iliolumbar artery, the first branch of the internal iliac artery, abnormally crossed the middle of the safe zone. The sacroiliac joint, lumbosacral trunk, internal iliac vein and iliolumbar artery were at risk from laterally-directed S1 screws. This study shows that bicortical placement of S1 screws into the sacral ala presents unnecessary risks to neurovascular structures. It is concluded that the previous anatomical safe zone for bicortical S1 screw placement into the sacral ala was not surgically safe, and when lumbosacral fixation surgery is planned, operative techniques other than bicortical screw placement should be considered.