Femoral nerve strain at L4-L5 is minimized by hip flexion and increased by table break when performing lateral interbody fusion.

STUDY
DESIGN: Anatomic studies have demonstrated that nerves and blood vessels have excursion with extremity range of motion. We have measured femoral nerve excursion with the lateral lumbar transpsoas interbody fusion (LLIF) procedure with changes in table flexion and ipsilateral hip flexion on both sides of 5 cadavers.
OBJECTIVE: To determine the effect of hip range of motion on femoral nerve strain near the L4-L5 disc space because it pertains to the LLIF procedure. SUMMARY OF BACKGROUND DATA: Postoperative thigh symptoms are common after the LLIF procedure. Although nerve strain in general has been shown to impair function, it has not been tested specifically with LLIF.
METHODS: Five cadavers were placed in the lateral position as though undergoing the L4-L5 LLIF procedure. Radiographical markers were implanted into the femoral nerve. Lateral and anteroposterior fluoroscopic images were recorded with 0° initial table flexion and the hip at 0, 20, 40, and 60° flexion. The table was flexed to 40°, and the process repeated. Examination was repeated on the contralateral side and nerve strain and excursion were calculated.
RESULTS: Table flexion results in preloading the femoral nerve when approaching L4-L5. Nerve strain was highest with the table flexed to 40° and the hip at 0° (average, 6%-7%). Strain in the femoral nerve decreased with increasing hip flexion for both table flexion angles. Anterior displacement of the nerve by approximately 1.5 mm was noted at 40° table flexion compared with 0°.
CONCLUSION: Strain values with table flexion of 40° approached those associated with reduced neural blood flow in animal studies. Table flexion should be minimized to the extent possible when performing L4-L5 LLIF. Additionally, hip flexion to 60° can neutralize the neural strain that occurs with aggressive table flexion. LEVEL OF EVIDENCE: N/A.