Morphometric aspects of extraforaminal lumbar nerve roots

OBJECTIVE: In the posterolateral extraforaminal and anterolateral retroperi toneal approaches to lumbar spinal lesions, the neural structures in the lu mbar extraforaminal region are unfamiliar to many spinal surgeons. The purp ose of this study was to determine the normal anatomic morphometric paramet ers for all lumbar nerve roots around their exits, from the intervertebral foramen to the surrounding bony structure. METHODS: A total of 15 adult fixed cadavers were studied. The extraforamina l course of the lumbar nerve roots and the forming plexus were measured seg mentally, using standard calipers, and we selected the shortest distance fr om the bony landmarks to the nerve roots in the horizontal plane. The bony landmarks were the most medial superior border of the transverse process (T P), the most medial inferior border of the TP, the tip of the superior arti cular process, and the most dorsolateral margin of the intervertebral disc space. In addition, the angle of each root exiting from the intervertebral foramen was measured using a goniometer. RESULTS: The mean distance from the medial superior border of the TP to the upper segment of the nerve root was 5.1 to 6.4 mm at L2-L5. The mean dista nce from the medial inferior border of the TP to the corresponding nerve ro ot was 8.5 mm at L2 and L3 and 6 mm at L4 and L5. The mean distance from th e tip of the superior articular process to the most dorsal border of the de scending nerve trunk was 19 mm at L2 and L3 and 22 mm at L4 and L5. The mai n lumbar nerve trunk was located close to the most dorsolateral surface of the vertebral body and the intervertebral disc space, and it was topographi cally arranged dorsoventrally from the L5 to L2 nerve components. The avera ge widths of the nerve trunk were 10, 14, and 25 mm at L3-L4, L4-L5, and L5 -S1, respectively. The mean angles of the exiting roots in the extraforamin al region were 16 degrees at L2 and L3 and 25 degrees at L4 and L5. CONCLUSION: The lumbar nerve component, including both the lumbar trunk and each exiting nerve root in the extraforaminal region (the so-called "dange r zone"), was located anteriorly at a distance more than 5 mm from the TP, more than 19 mm from the superior articular process, and up to 25 mm from t he intervertebral disc space. Based on our results, the danger zone occupie d up to 25 mm forward from the intervertebral foramen at the lower lumbar s egments. Therefore, during operations such as percutaneous posterolateral p rocedures and open posterolateral or anterolateral approaches, great care s hould be taken within 25 mm of the extraforaminal region, especially for th e lower lumbar spine.