RESUMEN
<p><b>OBJECTIVE</b>To introduce the location and course of S1, S2 sacral nerve root tunnel and to clarify the significance of the anterior aspect of sacral nerve root tunnel on placement of iliosacral screw on the standard lateral sacral view.</p><p><b>METHODS</b>Firstly the data of 2.0 mm slice pelvic axial CT images were imported into Mimics 10.0, and the sacrum, innominate bones, and sacral nerve root tunnels were reconstructed into 3D views respectively, which were rotated to the standard lateral sacral views, pelvic outlet and inlet views. Then the location and course of the S1, S2 sacral nerve root tunnel on each view were observed.</p><p><b>RESULTS</b>The sacral nerve root tunnel started from the cranial end and anterior aspect of the vertebral canal of the same segment and ended up to the anterior sacral foramen with a direction from cranial-posterior-medial to caudal-anterior-lateral. The tunnel had a lower density than the iliac cortex and greater sciatic notch on the pelvic X-rays,especially on the standard sacral lateral view, on which it showed up as a disrupted are line and required more careful recognition.</p><p><b>CONCLUSION</b>It can prevent the iliosacral screw from penetrating the sacral nerve root tunnel and vertebral canal when recognizing the anterior aspect of sacral nerve root tunnel and choosing it as the caudal-posterior boundary of the "safe zone" on the standard lateral sacral view.</p>