Changes in geometric configuration and biomechanical parameters of a rapidly growing abdominal aortic aneurysm may provide insight in aneurysms natural history and rupture risk

Abstract

Background: Abdominal aortic aneurysms (AAA) are currently being treated based on the maximum diameter criterion which has often been proven insufficient to determine rupture risk in case of every AAA. We analyzed a rare case of an AAA which presented an extremely fast growth focusing on biomechanical determinants that may indicate a high risk profile. The examination of such a case is expected to motivate future research towards patient-specific rupture risk estimations.

Methods: An initially small AAA (maximum diameter: 4.5 cm) was followed-up and presented a growth of 1 cm in only 6-months of surveillance becoming suitable for surgical repair. Changes of morphometric characteristics regarding AAA, thrombus and lumen volumes, cross-sectional areas, thrombus maximum thickness and eccentricity, and maximum centerline curvature were recorded. Moreover biomechanical variables concerning Peak Wall Stress, AAA surface area exposed to high stress and redistribution of stress during follow-up were also assessed.

Results: Total aneurysm volume increased from 85 to 120 ml which regarded thrombus deposition since lumen volume remained stable. Thrombus deposition was eccentric regarding anterior AAA segment while its thickness increased from 0.3 cm to 1.6 cm. Moreover there was an anterior bulging over time as depicted by an increase in maximum centerline curvature from 0.4 cm-1 to 0.5 cm-1. Peak Wall Stress (PWS) exerted on aneurysm wall did not change significantly over time, slightly decreasing from 22 N/cm2 to 21 N/cm2. At the same time the area under high wall stress remained practically constant (9.9 cm2 at initial vs 9.7 cm2 at final examination) but there was a marked redistribution of wall stress against the posterior aneurysmal wall over time.

Conclusion: Aneurysm area under high stress and redistribution of stress against the posterior wall due to changes in geometric configuration and thrombus deposition over time may have implications to aneurysms natural history and rupture risk.

Figure 1

Cross-sections perpendicular to the centerline with regard to cross-sectional areas for 1st AAA-model (initial examination) and 2nd AAA-model (follow-up). Maximum sectional areas take values of 22.5 cm² and 15 cm² respectively. Color scale on the centerlines depicts distance along the centerline with 8 cm representing aortic bifurcation.

Figure 2

Surface areas of sections vertical to the centerline for total aneurysm wall and lumen for 1st (initial examination) and 2nd AAA-model (follow-up) in relation to distance along the centerline (aortic bifurcation at 8 cm).

Figure 3

Thrombus thickness as it is represented for 1st (initial examination) and 2nd AAA-model (follow-up). The reference values and color scale have been taken with respect to the 2nd model. The corresponding maximum values of ILT thickness are 0.3 cm and 1.6 cm. It is observed that ILT locates anteriorly.

Figure 4

Increase of maximum centerline curvature during follow-up suggests an anterior bulging of AAA over time. Centerline at A. 1st CT, B. 2nd CT, and C. Overlapping of the two centerlines.

Figure 5

Anterior and posterior views of wall stress distribution throughout the AAA surface for the 1st (upper row) and 2nd AAA-model (lower row). A redistribution of wall stress against the posterior aortic wall is noticed.