On the Ratio between Static Pressure and Thickness of Growing AAAs

Daniele Bruschetta, Debora Di Mauro, Vincenzo Filardi, Giuseppina Rizzo and Fabio Trimarchi

Objective: Abdominal aortic aneurysms (AAAs) are characterized by structural remodelling resulting in the gradual weakening and expansion of the aortic wall. Wall stress may furnish a usable indicator to prevent the failure. In order to evaluate this risk, static pressure aging on the cap was used to perform FEA, simulating an aneurysm varying its dimensions from 10 mm to 50 mm. Analyses were carried out by imposing different thickness of the cap, and obtaining correspondent equivalent Von Mises stresses. Understanding how these stresses are distributed and what factors influence stress distributions is critical in evaluating the potential for rupture.

Methods: A representative FE model was created in order to simulate the historical evolution of the AAA. CFD analyses were performed to obtain data of the static pressure aging on the model. Patient informed consent and IRB approval were obtained. A linear law was speculated to understand thickness thinning in function of aneurysm’s growth. The obtained pressure maps were used as input to perform elastic linear analyses on the five different FE wrappings.

Results: If the bloody pressure is increased, that increases also wall shear stress, and an adaptive increase in arterial luminal size is observed. Results have evidenced peaks of stress varying from 0,004 MPa, for a diameter of 10 mm, to 0,45 MPa, for a diameter of 50 mm. Top and bottom zones of aneurysm result more solicited than the middle ones, as it can be deducted by obtained strain values, ranging from 5,84e-7, for a diameter of 10 mm, to 3,14e-4, for a diameter of 50 mm. In order to evaluate the mechanical behaviour of the cap, related to its thickness at 50 mm of diameter, different FEA were conducted varying thickness, uniformly, from 1 to 0.4 mm. As it is possible to notice, stress increases exponentially while thickness decreases.

Conclusions: Results indicate an equivalent Von Mises stress of about 0,45 MPa, close the to failure value, for a critical dimension of 50 mm. This means that failure conditions can depend at least, by two variables: thickness and pressure.