Morphology Predicts Rapid Growth of Small Abdominal Aortic Aneurysms

This study evaluated CT scans of small abdominal aortic aneurysms (AAAs) (< 5 cm) to assess anatomic features associated with rapid expansion. Serial CT scans obtained at least 10 months apart (mean 15 months) from patients with small AAAs were reviewed. Each cross-sectional image of the AAAs was analyzed using a computer-assisted design program. The circumference of the AAA in each CT image was divided into eight equal arcs, from which the apparent radius of curvature (Rc) for each segment was calculated. Flattening of the wall curvature results in an increased segmental Rc. The CT scans of nine patients with expanding AAAs (expansion ≥0.5 cm/yr) were compared to those of 10 patients with stable AAAs (expansion ≤0.2 cm/yr). To adjust for differences in AAA size, the Rc for each segment was normalized by dividing each individual Rc by the average of the eight Rcs (RcAvg) calculated for that cross-sectional CT image. Analysis of variance showed that the left posterolateral segments in expanding AAAs had larger Rc/RcAvg ratios than those segments in stable AAAs (1.14 ± 0.19 vs. 0.80 ± 0.09, p < 0.02). Laplace's law indicates that the left posterolateral segment in AAAs that grow more rapidly is subjected to greater wall tension. Flattening in the curvature of the left posterolateral wall segment was significantly associated with an increased rate of expansion in small AAAs. This finding, readily derived from standard CT scan images, may predict which small AAAs are more prone to rapid expansion.