Great Vessel Excursion: Prone Versus Supine Position

Background: It is theorized that pedicle screws could be placed into the anterior vertebral cortex to increase biomechanical strength by 20% to 25%. Although stereotactic navigational tools allow for accurate docking of spinal implants, no data exist regarding operative positioning as it relates to great vessel alignment. Our hypothesis is that the great vessels fall anteriorly, with prone positioning providing a safer margin for implantation of screws from a posterior approach.

Methods: Volunteers underwent magnetic resonance imaging of the spine. Twenty healthy volunteers, affiliated with the academic medical center performing the study, underwent magnetic resonance imaging in both the supine and prone positions. Measurements were taken of the distance (mm) from the projected tip of a pedicle screw to the neighboring great vessels.

Measurements were made at every other vertebral level from T3 to L5 by bisecting the pedicle in the transverse and sagittal planes and projecting the trajectory of a screw to the anterior cortex. We then evaluated great vessel position in relationship to the tip of the projected pedicle screw at the anterior cortex in the supine and prone orientations.

Results: The vessels were found to lie in a range of 2 to 10 mm from the anterior cortex. The comparison between the supine and prone positions showed that the great vessels in the lumbar region are held securely by the surrounding soft tissue. However, in the thoracic spine, anterior excursion does occur, just not as we expected. The aorta moves anteriorly while prone by 1.4 to 5 mm; however, its movement causes it to slide forward along the vertebra, shortening the distance to the anterior cortex. As a result, the screw trajectory is in a riskier geographic location. In the thoracolumbar area, the inferior vena cava translates 1.7 to 2.9 mm.

Conclusions: These data suggest that the risks of vascular injury from anterior cortical fixation of the vertebra using pedicle screws placed posteriorly in the prone position are apparent. In the lumbar region, the upper thoracic region around the aortic arch, and the thoracolumbar junction the great vessels remain close to the vertebra. While in the mid-thoracic region, the aorta moves closer to the area of screw penetration anteriorly when the subject is prone.

Clinical Relevance: Spine surgeons commonly attempt pedicle screw placement into the anterior cortex of the vertebral body. Our study helps elucidate the inherent risks of this technique due to vessel positioning when prone.