Aortic Valve Level as a Candidate Zone 4/5 Boundary Landmark in Endovascular Aortic Repair

Background: The zone 4/5 boundary has been described as the mid-descending aorta-T6 level, but this correspondence has uncertain anatomic support. Purpose: To determine whether the aortic valve (AV) level approximates the descending aortic midpoint and to compare candidate boundaries relative to the critical segmental artery (CSA) origin. Materials and Methods: This retrospective study included 204 patients who underwent Adamkiewicz artery-specific CT angiography from January 2022 through February 2026. The CSA was defined as the aortic origin of the segmental artery directly or collaterally connected to the Adamkiewicz artery. Along the descending-aortic centerline, the descending aortic midpoint was the point halfway between a site 20 mm distal to the left subclavian artery origin and the celiac artery origin; the AV-level midpoint was the point halfway between the centerline intersections of the left coronary and noncoronary aortic sinus planes. Equivalence between midpoints was tested within a prespecified +/-20-mm margin. Distal CSA classifications were compared using McNemar tests. Results: The study included 204 patients (median age, 72 years [IQR, 59-79 years]; 142 men and 62 women). The mean difference between the AV-level midpoint and descending aortic midpoint was -1.2 mm (90% CI, -3.3 to 0.9 mm; P < .001 for equivalence). The T6 vertebral level was 59.0 mm proximal to the descending aortic midpoint. The CSA origin was distal to the AV-level midpoint and descending aortic midpoint in 96.6% (197/204) and 95.6% (195/204), respectively (P = .68), but distal to the noncoronary aortic sinus plane in 89.7% (183/204; P = .001 versus the AV-level midpoint). Conclusion: The AV-level midpoint approximated the descending aortic midpoint, whereas the T6 vertebral level was more proximal. Fewer critical segmental artery origins were classified as distal to the noncoronary aortic sinus plane than to the AV-level midpoint.