Analysing The Cross-Section of The Abdominal Aortic Aneurysm Neck and Its Effects on Stent Deployment

Abstract

Stent graft devices for the treatment of abdominal aortic aneurysms (AAAs) are being increasingly used worldwide. Yet, during modelling and optimization of these devices, as well as in clinical practice, vascular sections are idealized, possibly compromising the effectiveness of the intervention. In this study, we challenge the commonly used approximation of the circular cross-section of the aorta and identify the implications of this approximation to the mechanical assessment of stent grafts. Using computed tomography angiography (CTA) data from 258 AAA patients, the lumen of the aneurysmal neck was analysed. The cross-section of the aortic neck was found to be an independent variable, uncorrelated to other geometrical aspects of the region, and its shape was non-circular reaching elliptical ratios as low as 0.77. These results were used to design a finite element analysis (FEA) study for the assessment of a ring stent bundle deployed under a variety of aortic cross-sections. Results showed that the most common clinical approximations of the vascular cross-section can be a source of significant error when calculating the maximum stent strains (underestimated by up to 69%) and radial forces (overestimated by up to 13%). Nevertheless, a less frequently used average approximation was shown to yield satisfactory results (5% and 2% of divergence respectively).

Figure 1

For every patient, 2 CTA slices were examined, one at the proximal (left column) and one at the distal end (right column) of the aneurysmal neck. For each image (b) a border on the intima was manually drawn (c) and then turned into a binary image (d1). Figure (d2) shows the distal cross-section of a patient with the equivalent ellipse superimposed on top of it, allowing the definition of $ER$. Thrombus and calcification (transparent yellow and white regions in (a)) were included in the cross-section. Source of the image of the torso: GermanVectorPro/Shutterstock.com.

Figure 2

Dimensional and angular variables related to the AAA neck.

Figure 3

Schematic visualization of $ER$ results. ‘Average’ corresponds to the averaging of the proximal and distal site results.

Figure 4

The ring stent bundle and vessel model used for the FEA study (a) and a schematic of the vascular cross-section (b). The elliptical ratio changed from 1 (brown circle) up to 0.75 (black ellipse) at the pressurized state. For every one of the ellipses, a circle with diameter equal to its minor axis (blue dotted circle) and an average diameter circle (green dashed circle) were examined. Additionally, 3 angles of deployment were studied for each elliptical section. In (c), the logarithmic strains of the ring deployed in the most extreme elliptical vessel are visualized.

Figure 5

Maximum mean strain developed on the ring (top) and COF exerted by the ring at the end of systole (bottom) versus the elliptical ratio of the pressurized vessel. Ring deployed in all circles and the ellipses at 0° degrees. The dotted line refers to oversize greater than 33%.

Figure 6

Maximum mean strain developed on the ring (top) and COF exerted by the ring at the end of systole (bottom) versus the elliptical ratio of the pressurized vessel. Ring deployed in the ellipses at various deployment angles.

Figure 7

The effect of stenting on the ellipticity of the vessel.