The lumbosacral nerves in relation to dorsal S1 screw placement and their locations on plain radiographs

Seven adult cadaver lumbopelvises were harvested to study the anatomic rela tionship of the L4 and L5 nerves to Sl dorsal screw placement and the locat ion of the 14, L5, and S1 nerves on plain radiographs. The mean lateral ang le of S1 screw trajectory toward the L4 nerve was 31+/-8 degrees, and the m ean screw trajectory length was 53+/-8 mm. The mean lateral angle of the sc rew trajectory toward the L5 nerve was 21+/-8 degrees, and the mean screw t rajectory length was 38+/-4 mm. On both inlet and outlet radiographs, the l ateral angle of the nerves increased from 14 to S1. The L4 nerve coursed ov er the middle third of the superior ala in the inlet view and the middle th ird of the lateral mass in the outlet view. The L5 nerve coursed over the i nner third of the superior ala and inner third of the lateral mass. On the lateral view, the mean distances from the sacral promontory to the 14, L5, and S1 nerves along the anterior border of the sacrum were 4+/-7 mm, 12+/-5 mm, and 28+/-8 mm, respectively. This study suggests that S1 sacral screws be directed between 30 degrees and 40 degrees lateral to avoid compromisin g the lumbosacral trunk and sacroiliac joint.