Patient-Specific Simulation Models of the Abdominal Aorta With and Without Aneurysms

This research study presents computational simulation models for analysis of parameters which are in evidence of development and clinical management of abdominal aortic aneurysms (AAA). The research covers three main areas: interpretation of material parameters, implementation of the constitutive relations for computational analysis, and evaluation of the material model predictability.

The constitutive framework applied is the four fiber family (4FF) model. This model assumes that the wall is a constrained mixture of an amorphous isotropic elastin dominated matrix reinforced by collagen fibers. The collagen fibers are grouped in four directions of orientation. The purpose of the first study was to investigate whether significant risk factors related to AAA development can be identified from a specific pattern in the material parameters of the 4FF model.

Smoking is a leading self-inflicted risk factor for cardiovascular diseases in general, and AAA in particular. Results suggests that arterial stiffening caused by smoking is reflected by consistent increase in an elastinassociated material parameter, and moreover by marked increase in the collagen-associated material parameters.

The arterial stiffening appears to be isotropic, which may allow simpler phenomenological models to capture these effects. There is a pressing need, however, for more detailed histological information coupled with more complete experimental data for the systemic arteries. The second study was aimed at developing computational simulation models incorporating subject-specific geometry of the abdominal aorta (AA) as well as subject-specific blood flow conditions.

The geometry was acquired from magnetic resonance imaging, and the blood flow characteristics were acquired from ultrasound. The solid AA wall was modeled as a thick-walled cylinder allowing for inspection of the stress distributions inside the wall. The 4FF model characterizes the mechanical behavior.

The blood is assumed to be an incompressible Newtonian fluid. The fluid and solid models were implemented in a commercially available finite element software. The goal of third study was to evaluate the predictability of the 4FF model. This was achieved by combining subject-specific blood flow and age-matched material parameters of the 4FF model in a fluid-structure interaction (FSI) model.

The predicted wall dynamics were compared to in vivo wall dynamics obtained with ultrasound. Simulation results indicate that the 4FF model overestimates the displacement of the AA wall in a realistic simulation setup. This is believed to be the first study to evaluate the predictability of the 4FF model using a FSI model environment.