Movement of abdominal structures on magnetic resonance imaging during positioning changes related to lateral lumbar spine surgery: a morphometric study

Object The minimally invasive lateral interbody fusion of the lumbar spine through a retroperitoneal transpsoas approach has become increasingly used. Although preoperative imaging is performed supine, the procedure is performed with the patient in the lateral decubitus position. The authors measured the changes in location of the psoas muscle, aorta, inferior vena cava (IVC), iliac vessels, and kidneys with regard to the fixed lumbar spine when moving from a supine to a lateral decubitus position. Methods Unenhanced lumbar MRI scans were performed using a 3T magnet in 10 skeletally mature volunteers in the supine, left lateral decubitus (LLD), and right lateral decubitus (RLD) positions. Positional changes in the aorta, IVC, iliac vessels, and kidneys were then analyzed at all lumbar levels when moving from supine to RLD and supine to LLD. Values are presented as group means. Results When the position was changed from supine to RLD, both the aorta and the IVC moved up to 6 mm to the right, with increased movement caudally at L3–4. The aorta was displaced 2 mm anteriorly at L1–2, and the IVC moved 3 mm anteriorly at L1–2 and L2–3 and 1 mm posteriorly at L3–4. The left kidney moved 22 mm anteriorly and 15 mm caudally, while the right kidney moved 9 mm rostrally. When the position was changed from supine to LLD, the aorta moved 1.5 mm to the left at all levels, with very minimal anterior/posterior displacement. The IVC moved up to 10 mm to the left and 12 mm anteriorly, with increased movement rostrally at L1–2. The left kidney moved 3 mm anteriorly and 1 mm rostrally, while the right kidney moved 20 mm anteriorly and 5 mm caudally. The bifurcation of the aorta was an average of 18 mm above the L4–5 disc space, while the convergence of the iliac veins to form the IVC was at the level of the disc space. The iliopsoas did not move in any quantifiable direction when the position was changed from supine to lateral; its shape, however, may change to become more flat or rounded. When the position was changed from supine to RLD, the right iliac vein moved posteriorly an average of 1.5 mm behind the anterior vertebral body (VB) line (a horizontal line drawn on an axial image at the anterior VB), while the other vessels stayed predominantly anterior to the disc space. When the position was changed from supine to LLD, the right iliac vein moved to a position 1.4 mm anterior to the anterior VB line. There was negligible movement of the other vessels in this position. Conclusions The authors showed that the aorta, IVC, and kidneys moved a significant distance away from the surgical corridor with changes in position. At the L4–5 level, a left-sided approach may be riskier because the right common iliac vein trends posteriorly and into the surgical corridor, whereas in a right-sided approach it trends anteriorly.