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OBJECT  The purpose of this study was to analyze MR images of the lumbar spine and document: 1) the oblique corridor at each lumbar disc level between the psoas muscle and the great vessels, and 2) oblique access to the L5–S1 disc space. Access to the lumbar spine without disruption of the psoas muscle could translate into decreased frequency of postoperative neurological complications observed after a transpsoas approach. The authors investigated the retroperitoneal oblique corridor of L2–S1 as a means of surgical access to the intervertebral discs. This oblique approach avoids the psoas muscle and is a safe and potentially superior alternative to the lateral transpsoas approach used by many surgeons.    METHODS  One hundred thirty-three MRI studies performed between May 4, 2012, and February 27, 2013, were randomly selected from the authors’ database. Thirty-three MR images were excluded due to technical issues or altered lumbar anatomy due to previous spine surgery. The oblique corridor was defined as the distance between the left lateral border of the aorta (or iliac artery) and the anterior medial border of the psoas. The L5–S1 oblique corridor was defined transversely from the midsagittal line of the inferior endplate of L-5 to the medial border of the left common iliac vessel (axial view) and vertically to the first vascular structure that crossed midline (sagittal view).    RESULTS  The oblique corridor measurements to the L2–5 discs have the following mean distances: L2–3 = 16.04 mm, L3–4 = 14.21 mm, and L4–5 = 10.28 mm. The L5–S1 corridor mean distance was 10 mm between midline and left common iliac vessel, and 10.13 mm from the first midline vessel to the inferior endplate of L-5. The bifurcation of the aorta and confluence of the vena cava were also analyzed in this study. The aortic bifurcation was found at the L-3 vertebral body in 2% of the MR images, at the L3–4 disc in 5%, at the L-4 vertebral body in 43%, at the L4–5 disc in 11%, and at the L-5 vertebral body in 9%. The confluence of the iliac veins was found at lower levels: 45% at the L-4 level, 19.39% at the L4–5 intervertebral disc, and 34% at the L-5 vertebral body.    CONCLUSIONS  An oblique corridor of access to the L2–5 discs was found in 90% of the MR images (99% access to L2–3, 100% access to L3–4, and 91% access to L4–5). Access to the L5–S1 disc was also established in 69% of the MR images analyzed. The lower the confluence of iliac veins, the less probable it was that access to the L5–S1 intervertebral disc space was observed. These findings support the use of lumbar MRI as a tool to predetermine the presence of an oblique corridor for access to the L2–S1 intervertebral disc spaces prior to lumbar spine surgery.

Retroperitoneal oblique corridor to the L2–S1 intervertebral discs: an MRI study