#167 Femoral Nerve Strain at L4-5 is Minimized by Hip Flexion and Increased by Table Break when Performing Lateral Interbody Fusion

General Session: Lateral Interbody Fusion

Presented by: J. O'Brien

Author(s):

J.R. O'Brien (1)
Z.A. Dooley (2)
A.W.L. Turner (2)

(1) George Washington University Hospital, Washington, DC, USA
(2) NuVasive, Inc., San Diego, CA, USA

Abstract

Introduction: Anatomic studies have demonstrated that nerves and blood vessels have excursion with extremity range of motion. Nerve tension has been documented in the clinical setting as a pain generator. For instance, the femoral nerve stretch test, straight leg raise, and Lasegue test all can cause pain in patients that have lumbar disc herniation. Extra-foraminal neural motion within the psoas has not been studied. Animal models have shown that relatively low strains can decrease or even block neural blood flow. The purpose of this study was to determine the effect of hip range of motion on femoral nerve strain near the L4-5 disc space as it pertains to the XLIF procedure.

Methods: Five cadavers with intact proximal femurs were placed in the lateral position as though undergoing the L4-5 XLIF procedure. The lumbar plexus was dissected to identify the femoral nerve. Radiographic markers were implanted into the nerve. A spherical marker was implanted in L5 to scale dimensions on the radiographs. With the operating table initially flat (0° table break), the pelvis was stabilized to the table to minimize movement during hip flexion. The table was oriented to provide true A-P and lateral fluoroscopic C-arm images aligned with the L5 superior endplate.

Lateral and A-P fluoroscopic images were digitally recorded with 0° initial table break and the hip at 0, 20, 40 and 60° flexion. The table was then broken to 40°, , and images were recorded at the same hip flexion angles. The contralateral side of the specimens was evaluated using the same methods. Images were analyzed to determine the position of the markers with respect to coordinate systems created at the L5 superior endplate. Nerve strain for each condition was calculated from the change in distance between the markers with respect to baseline, divided by the baseline distance (DL/L). Baseline was the lowest nerve strain condition, which was with the table at 0° and the hip flexed to 60°. The position where the nerve crossed the L5 superior endplate was also determined.

Results: Table break results in preloading the femoral nerve when approaching L4-5. Nerve strain was highest with the table broken to 40° and the hip at 0° (average 6-7 %). Strain in the femoral nerve decreased with increasing hip flexion for both table break angles. Hip flexion can reduce the preload on the femoral nerve in cases where the table is broken to 40 degrees for access to L4-5, however residual strain may exist unless table break is reduced. Anterior displacement of the nerve by approximately 1.5 mm was noted at 40° table break compared with 0°.

Conclusion: Strain values with table break of 40° approached those associated with reduced neural blood flow in animal studies. Table break should be minimized beyond that needed to access L4-5. These data may be considered when attempting to minimize post-operative dysesthetic leg pain after extreme lateral interbody fusion at L4-5.