Anatomy of L4 to S3 Nerve Roots

PURPOSE: To study the course of L4 to S3 nerve roots and their distance with the ala of the sacrum and the sacroiliac joint. METHODS: The embalmed left half pelvis of 25 male and 27 female cadavers aged 30 to 91 (mean, 68) years were studied. The ventral rami of the L4, L5, S1, S2, and S3 nerve roots were dissected along their courses from the corresponding intervertebral foramina to the lesser pelvis. The distances between each nerve root and the most anterior and inferior parts of the sacroiliac joint were measured in both coronal and sagittal planes. RESULTS: There were 3 locations of fusion of L4 and L5 nerve roots: above, on, or below the level of the most anterior part of the sacroiliac joint. Fusion of S1 and S2 nerve roots at the most inferior part of the sacroiliac joint was encountered in 10% of pelvises. Most S1 and S2 nerve roots lay medial to the sacroiliac joint and closed to the anterior cortex of the ala of the sacrum. CONCLUSION: Sacral nerve roots may fuse at different levels. Most L4 to S3 nerve roots lie close to the anterior surface of the sacroiliac joint and the ala of the sacrum. To prevent nerve root injury, dissection with a sharp instrument should be avoided at such area and 5 to 7 mm medial to the sacroiliac joint.