Anatomic mapping of lumbar nerve roots during a direct lateral transpsoas approach to the spine: a cadaveric study.

STUDY DESIGN: Cadaveric study.

OBJECTIVE: Identifying anatomic structures at risk for injury during direct lateral transpsoas approach to the spine.

SUMMARY OF BACKGROUND DATA: Direct lateral transpsoas approach is a novel technique that has been described for anterior lumbar interbody fusion. Potential risks include damage to genitofemoral nerve and lumbar plexus, which are not well visualized during small retroperitoneal exposure. Previous cadaveric studies did not evaluate the direct lateral transpsoas approach, and considering the approach being used in clinical practice, the current study was undertaken in an effort to identify the structures at risk during direct lateral transpsoas approach.

METHODS: Sixteen dissections were performed on eight fresh-frozen male cadavers. Eight were to localize proximal lumbar nerve roots, ilioinguinal nerves, and genitofemoral nerves and establish their relationship to psoas muscle and disc space. Four simulated direct lateral transpsoas approaches, with K-wire placed into mid-disc space under fluoroscopic guidance, were made. In four dissections, sequential dilators were inserted, disc space was evacuated, and interbody devices were placed. The study was conducted in three phases to identify, in a stepwise progression, which portion of the procedure placed the nerve at greatest risk. RESULTS.: With initial dissections, perforating branches of lumbar nerve roots were identified in anterior, middle, and posterior third of psoas muscle. Sympathetic chain was identified in anterior third of psoas over L1-L4. Distance from the middle of the anterior longitudinal ligament at the level of the disc to the sympathetic chain averaged 9.25 mm. The nerve roots and genitofemoral nerve were placed at risk in all dissections in which the approach was recreated. Damage secondary to K-wire placement occurred in 25% of cases at L3-L4 and L4-L5; in one case, L4 nerve root was pierced, and in another, genitofemoral nerve was pierced. K-wire was posterior to the nerve roots in 25% of cases at L3-L4 and in 50% of cases at L4-L5. The lumbar plexus was placed under tension because of sequential dilator placement.

CONCLUSION: On the basis of our results, there is no zone of absolute safety when using the direct lateral transpsoas approach. The potential for nerve injury exists when using this approach, and consequently, we recommend either direct visualization of the nerve roots intraoperatively and/or the use of neuromonitoring. In our opinion, a higher quality, larger clinical study that examines the outcomes and surgical complications of the direct lateral transpsoas approach is warranted.