Introduction: Risk of injury to the lumbar plexus during LLIF is significant. Preforming an LLIF at L4-L5 may have greatest risk of neurological complications. We describe the technique for preoperative lumbar plexus mapping using MRN in order to visualize the course of the lumbar plexus at the L4-L5 disc space and evaluate its potential utility as a pre-operative planning tool for patients undergoing LLIF.

Methods: Consecutive lumbar plexus MR neurograms (n=35 patients, 70 sides) were studied. T1- and T2- color-coded fusion maps were generated along with 3D models. The position of the plexus and the shape of the psoas muscle at the L4-L5 interspace was evaluated and recorded.

Results: MR neurography revealed a substantial variability in the position of the lumbar plexus relative to the L4-L5 disc space. On the left, the plexus was identified in zone 2 in (5.7%), zone 3 (54.3%) and zone 4 (40%) (p= 0.0014). On the right statistically significantly greater variability was found. The plexus was identified in zone 2 (8.6%) zones 3 (42.9%) or zone 4 (45.7%) and Zone 5 in (2.9%) (p=0.001). Right-left symmetry relative to the disc space was found in 18 of 35 subjects 51.4% (p=0.865). There was no association between the position of the plexus and the shape of the psoas muscle identified on the left side (p=0.0877) or the right side (0.1811). In patients with an elevated psoas (n=12) the plexus was identified in zone 3 in 75% and 66% (left and right) compared to patients without psoas elevation (n=23) 30.4 % and 43.5% (left and right).

Conclusions: The course of the lumbosacral plexus traversing the L4-5 disc space may be more variable than has been suggested by previous studies. MRN may provide a more reliable means of preoperatively identifying the plexus when compared with current methods.

Patient Care: The purpose of this study was to describe a novel application of MR neurography to directly visualize the lumbar plexus within the LLIF surgical corridor. Application of this technology will allow spine surgeons the ability to identify the course of the lumbar plexus in patients prior to LLIF procedure

Learning Objectives: By the conclusion of this session, participants should be able to have a better understanding of 1) anatomic variability of the lumbar plexus 2) limitations of the current indirect methods for lumbar plexus identification

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