The relationship of intrapsoas nerves during a transpsoas approach to the lumbar spine: anatomic study.

STUDY DESIGN A cadaveric study. OBJECTIVE To define the relationship of the lumbar exiting nerve root and trunks within the psoas muscle with reference to the radiographic center of the intervertebral disc, the recommended disc access point for the minimally invasive lateral transpsoas approach. SUMMARY OF BACKGROUND DATA The transpsoas approach to the lumbar intervertebral body disc is a minimally invasive approach used for interbody fusion. This approach carries the potential risk of injury to the intrapsoas nerves. There are no published studies investigating the locations of the intrapsoas neural elements with reference to the transpsoas access corridor developed during minimally invasive lateral approaches to the disc. METHODS Ten human cadaveric specimens were analyzed. A guide wire was placed in each disc space center under lateral fluoroscopic guidance as has been recommended for disc access in the transpsoas fusion technique. Using calipers, the distances from the exiting nerve and trunk to the wire were measured. RESULTS In general, the nerve trunk was a mean of 14 mm posterior to the center of the disc and was a mean of 5 mm closer to the center of the disc than the exited nerve. The trunks were closer to the center of the disc caudally in the lumbar spine, with the distance ranging from a mean of 16.4 mm at L2-3 to 10.6 mm at the L4-5 level. The intrapsoas location of the exited nerve was less variable and was greater than 15 mm from the projected center of the disc. At L4-5, the trunk approximated the center of the disc in 15% of specimens. CONCLUSION This study suggests that the intrapsoas nerves are a safe distance from the radiographic center of the intervertebral disc in a majority of cases; however, anatomic variations in the location of these nerves place them at injury risk in a small number of cases. These results suggest that neural monitoring whereas traversing the psoas may be important to enhance the safety of the transpsoas approach. Care is particularly warranted at the L4-5 level.