Study design:

Retrospective chart review.

Objective:

Identify demographic and sagittal alignment parameters that are independently associated with femoral nerve position at the L4-5 disc space.

Summary of background data:

Iatrogenic femoral nerve or lumbar plexus injury during lateral lumbar interbody fusion (LLIF) can result in neurologic complications. The LLIF “safe zone” is the anterior half to 2/3 of the disc space. However, femoral nerve position varies and is inconsistently identifiable on MRI. The safe zone is also narrowest at L4-5.

Methods:

An analysis of patients with symptomatic lumbar spine pathology and MRIs with a visibly identifiable femoral nerve evaluated at a single large academic spine center from 1/1/2017 to 1/8/2020 was performed. Exclusion criteria were transitional anatomy, severe hip osteoarthritis, coronal deformity with cobb >10 deg, > grade 1 spondylolisthesis at L4-5 and anterior migration of the psoas.Standing and supine lumbar lordosis (LL) and its proximal (L1-L4, PLL) and distal (L4-S1, DLL) components were measured. Femoral nerve position on sagittal imaging was then measured as a percentage of the L4 inferior endplate. A step-wise multivariate linear regression of sagittal alignment and lumbar lordosis parameters was then performed. Data are written as (estimate, 95% CI).

Results:

Mean patient age was 58.2±14.7 years, 25 (34.2%) were female and 26 (35.6%) had a grade 1 spondylolisthesis. Mean femoral nerve position was 26.6±10.3% from the posterior border of L4. Female sex (-6.6, -11.1 to -2.1) and supine PLL (0.4, 0.1 to 0.7) were independently associated with femoral nerve position.

Conclusions:

Patient sex and proximal lumbar lordosis can serve as early indicators of the size of the femoral nerve safe zone during a transpsoas LLIF approach at L4-L5.