Lumbar Plexus Anatomy within the Psoas Muscle: Implications for the Transpsoas Lateral Approach to the L4-L5 Disc

• Introduction: – Many advocate access to the lumbar intervertebral disk space with use of minimally invasive transpsosas lateral retroperitoneal approach. It is reportedly suitable for anterior spinal arthrodesis of disk spaces proximal to L5/S1. Some advantages of this approach include less post op pain and decreased manipulation of the aorta and IVC

– Various retractors have been designed to enhance access to disc space

– Nonetheless, neurologic complications have been reported to be anywhere from 8% to 30%, ranging from paresthesias to femoral nerve palsy.

– Despite introduction of retractors, what’s key is the understanding of the relevant anatomy. Previous studies were based on cadavers dissected in the supine position, and concentrated on where nerve exited the psoas mm rather than within a psoas mm.

– Purpose of this article: describe lumbar plexus anatomy, relevant to this approach, with emphasis on the L4/5 disk space

• Materials and Methods: – 18 cadavers: dissected in lateral decub position, relevant to the recommended surgical technique

– 18-ga spinal needles placed under fluoro in mid-coronal planes of disk spaces of L2-L3, L3-L4 and L4-5, because this is where most retractors are placed, and they remained in place until dissection over.

– Proceeded with a meticulous dissection to expose the femoral nerve, distance from distance at L4/5 disk space exposed, width measured at the confluence of L2,3 & 4 nerve roots, and then traced back to the spinal foramen.

• Results: – Course of L2 and L3 nerve roots after exiting the neural foramina was immediately adjacent to the lateral aspect of pedicle of the vertebral level below and dorsolateral to the vertebral body of the level below

– Lateral femoral cutaneous nerve: branched off the lumbar plexus at the level of the L3-L4 foramen. Identified anterior to the transverse process of L4 and anterior and lateral to the lateral tip of the L5 transverse process

– Femoral nerve was largest at the level of the L4-L5 disc space. The mean diameter was 13 mm (range, 8 to 17 mm), most often being measured at the L4-L5 disc level, but occasionally more distal.

– If you look at figure 2: femoral nerve was displaced by the needle placed in the mid-coronal plane of the L4-L5 disc – this was seen in 3 specimens

– And in 10 specimens: the femoral nerve was dorsal to the needle and ventral to the posterior aspect of the L4-L5 disc. Therefore, in thirteen of the eighteen specimens, the femoral nerve was located ventral to the posterior border of the disc space. In the remaining five specimens, the femoral nerve was dorsal to the posterior aspect of the L4-L5 disc (zone P).

• Discussion: – In 13/18 of the specimen, the femoral nerve was formed from the L2 to L4 nerve roots at the level of the L4-L5 disc space and lay ventral to its posterior aspect. So proper technique, using larger dilators, mobilization of neural structures without splitting them, and avoidance of cutting instruments until disk space is fully seen. And most importantly, traction should be gentle

– Figures 4 (neurogram) and 5 (cadaveric photo) show the theoretical placement of the dilator, the potential compression of the nerve at that level

– How much compression is too much? Stretch injuries in animal models have shown complete intraneural ischemia with a 15% increase of in vivo length. Because nerve roots lack an epineurium and perineurium, even small forces can potentially damage to nerve roots and nerve root attachments

– Safety zone: to avoid branches of the lumbar plexus by Moro et al. However. a recent anatomic study based on MRI concluded that the transpsoas lateral safe corridor, which theoretically avoids the lumbar plexus, narrows considerably as one moves distally from L1-L2 to L4-L. And there was no mention of the location, diameter, and effect on the lumbar plexus of a dilated retractor

– Due to anatomy at the L4-L5 disc space, the risk of traction or compression on the femoral nerve is high. A thorough understanding of the anatomy and the effect of dilators and retractor blades on the nerves is crucial. Positioning of retractors and monitoring how long they’ve been retracted are crucial. Serial postop exams important