P0858 - The Screw-Aorta Dilemma: Changing Patient Position in CT Scan is Critical in Documenting Aortic Mobility

PS misplacement rate is reported between 6-15%. Studies looking at misplacements on a per patient basis show up to 14% of patients have screws at risk (impinging vital structures). A screw abutting the aorta is a management challenge and often requires vascular surgery intervention. However, CT scans routinely done in supine position may overestimate screw-aorta relationship. Change in patient position may allow the aorta to roll away and, in most cases, reveal an uncompromised aorta. This will allow safe removal of pedicle screws without any vascular intervention.
111 patients with post-op CT, who underwent PSF for spinal deformity, from 2004-2009, were evaluated. Patients with concerning screw-aorta relationship underwent a prone CT scan. Mobility of the aorta was determined as described in Figure 1. This was to document general mobility of the aorta. Distance (D) was compared using prone and supine CT scans. Pair t-test and signed rank tests were utilized.
2,295 screws were reviewed, 45 screws in 27 patients were in proximity to the aorta. 36 of these were in close proximity, but not impinging (>1cm aorta-screw distance). 14 screws (7 patients) were impinging (<1cm). On prone CT, 13 out of the 14 instances the aorta moved away from the screw (median 2.6mm). The mean distance above the level of the misplaced screw was 2.97mm (p=0.17), and 3.8mm (p=0.001) below. In one instance the relationship was unchanged on prone CT. No screw was noted to violate the lumen or distort the aorta.
Supine CT-scan alone is not entirely accurate in determining screw-aorta relationship. Prone-CT scan provides additional information for better delineation. This additional diagnostic step can change the treatment option by limiting the need for vascular intervention. When in doubt, the additional use of an arteriogram can allow for improved visualization.