New England Society for Vascular Surgery

Beyond Fusiform and Saccular: Three Dimensional Aorta Shape and AAA Rupture

Suguna Pappu¹, Alan Dardik¹, Hemant Tagare², Richard Gusberg¹
¹Yale University, New Haven, CT;²Yale University, New Haven, CT

Introduction and Objectives:
“Saccular lesions are more prone to rupture [than fusiform]” noted Szilagyi, postulating a distinction between the natural history of the saccular abdominal aortic aneurysm (AAA), with its asymmetric bulge, and the uniform dilation of a fusiform AAA. Attempts to validate this observation have been limited by the inability to precisely define the 3-dimensional shape of an aorta. A quantitative 3-dimensional shape model may distinguish between shape classes, and identify small aneurysms at risk for rupture.
Methods:
Three-dimensional surface data of AAAs were calculated retrospectively from CT images of 15 patients with small aneurysms (< 5.5 cm maximal transverse diameter). The centerline was used to construct a shape classification based upon the orthographic projection of the centerline about its central axis. Hierarchical cluster analysis was used to identify distinct shape classes. The extent and direction of the individual deviations from a shape class were quantified as areas on the plane of projection to create a tortuosity index.
Results:
AAA shape was classified by the centerline projection as “C” class with a single direction of curvature, or an “S” class reflecting increased 3-dimensional tortuosity. Small AAAs that subsequently ruptured demonstrated significantly greater out-of-plane growth compared to the change in the maximal diameter. (Table; mean transverse diameter change 8.0% +/- 4.2%, mean tortuosity index change 84.0% +/- 15.2%; p=0.03, Kruskal-Wallis)
Conclusion:
Expanding AAAs develop specific, quantifiable shapes that are relevant to their natural history. The 3-dimensional features of this shape model provide a novel and accessible adjunct for judging rupture potential and determining intervention criteria for small aneurysms.


Patient	Maximal Transverse Diameter (cm)	% change	Centerline Length	% change	Tortuosity index	% change
1A	4.9		1.03		0.15
B	4.9	0	1.14	11	0.26	73
2A	4.3		1.22		2.39
B	4.9	14	1.61	32	3.94	65
3A	5.1		2.03		1.28
B	5.6	10	2.27	12	2.74	114

Table: 3-dimensional shape features, centerline length and tortuosity index given by centerline projection area, demonstrate significant out of plane change.