Magnetic Resonance Neurography of the Lumbar Plexus at the L4-L5 Disc: Development of a Preoperative Surgical Planning Tool for Lateral Lumbar Transpsoas Interbody Fusion (LLIF).

STUDY DESIGN: Observational study.

OBJECTIVE: To demonstrate use of magnetic resonance (MR) neurography to visualize the course of the lumbar plexus at the L4-L5 disc space.

SUMMARY OF BACKGROUND DATA: Risk of injury to the lumbar plexus during lateral transpsoas approach for lumbar interbody fusion (LLIF) is significant. We describe a new technique for preoperative mapping using magnetic resonance neurography to directly visualize the course of the plexus relative to the L4-L5 disc space.

METHODS: Consecutive lumbar plexus MR neurograms (n=35 patients, 70 sides) were studied. Scans were obtained on a Siemens 3-Tesla Skyra magnetic resonance imaging scanner. T1- and T2-color-coded fusion maps were generated along with 3-dimensional models of the lumbosacral plexus with attention to the L4-L5 interspace. The position of the plexus and the shape of the psoas muscle at the L4-L5 interspace were evaluated and recorded.

RESULTS: Direct imaging of the lumbar plexus using MR neurography revealed a substantial variability in the position of the lumbar plexus relative to the L4-L5 disc space. The left-side plexus was identified in zone 2 (5.7%), zone 3 (54.3%), and zone 4 (40%) (P = 0.0014); on the right, zone 2 (8.6%), zone 3 (42.9%) or zone 4 (45.7%), and zone 5 (2.9%) (P = 0.01). Right-left symmetry 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 overlying psoas muscle identified. In patients with an elevated psoas (n = 12), the lumbar plexus was identified in zone 3 in 75% and 66% (left and right) compared with patients without psoas elevation (n = 23), 30.4% and 43.5% (left and right).

CONCLUSION: The course of the lumbosacral plexus traversing the L4-L5 disc space may be more variable than has been suggested by previous studies. Magnetic resonance neurography may provide a more reliable means of preoperatively identifying the plexus when compared with current methods.

LEVEL OF EVIDENCE: 3.