Adjacent relationship and clinical significance between anterior branches of lumbar nerves 4,5 and lumbosacral trunk with sacroiliac joint

Objective:To provide anatomical basis for the exploration and operation in fracture of posterior pelvic ring with or without the injuries of nerves nearby. Methods: 20 adult cadavers fixed with formalin were dissected and horizontal distances between anterior branches of lumbar nerves 4, 5, lumbosacral trunk(LST) and sacroiliac joint were measured with a caliper and a Kirschner wire. Results: The distance between the anterior branches of lumbar nerve 5 exiting from the intervertebral foramen and the sacroiliac joint, the distance between anterior branches of lumbar nerve 4 branch and the sacroiliac joint at the level where the lumbar nerve 5 exiting the intervertebral foramen, the distance between the beginning of the LST and the sacroiliac joint , the proximity of the LST to the sacroiliac joint at the promontorium and the proximity of the LST to the sacroiliac joint at the pelvic brim were as follows: (23.4±4.0)mm,(17.8±4.8)mm,(13.3±2.8)mm,(11.8±3.2)mm and (9.6±3.6)mm . Among these distances, the shortest distance between the nerve and the joint was the one between the LST and the sacroiliac joint at the pelvic brim. During dissection of the nerves, anterior branches of the lumbar nerves 4, 5 and the LST were found to be fixed relatively well to the sacral bone with fibrous connective tissue. Anterior branches of lumbar nerves 4, 5 did not merge into the LST before entering the lesser pelvis separately in 3 cadavers. Conclusions: During the exploration and operation for fracture of posterior pelvic ring with or without injury of nerves nearby, great care should be taken to identify the LST because the distance between the LST and the joint is the shortest.