Abdominal aortic aneurysm (AAA) rupture is the clinical manifestation of an induced force exceeding the resistance provided by the strength of the arterial wall. This force is most frequently assumed to be the product of a uniform luminal pressure acting along the diseased wall. However fluid dynamics is a known contributor to the pathogenesis of AAAs, and the dynamic interaction of blood flow and the arterial wall represents the in vivo environment at the macro-scale. The primary objective of this investigation is to assess the significance of assuming an arbitrary estimated peak fluid pressure inside the aneurysm sac for the evaluation of AAA wall mechanics, as compared with the non-uniform pressure resulting from a coupled fluid-structure interaction (FSI) analysis. In addition, a finite element approach is utilised to estimate the effects of asymmetry and wall thickness on the wall stress and fluid dynamics of ten idealised AAA models and one non-aneurysmal control. Five degrees of asymmetry with uniform and variable wall thickness are used. Each was modelled under a static pressure-deformation analysis, as well as a transient FSI. The results show that the inclusion of fluid flow yields a maximum AAA wall stress up to 20% higher compared to that obtained with a static wall stress analysis with an assumed peak luminal pressure of 117 mmHg. The variable wall models have a maximum wall stress nearly four times that of a uniform wall thickness, and also increasing with asymmetry in both instances. The inclusion of an axial stretch and external pressure to the computational domain decreases the wall stress by 17%.

BackgroundAbdominal aortic aneurysm (AAA) is a prevalent disease which is of significant concern because of the morbidity associated with the continuing expansion of the abdominal aorta and its ultimate rupture. The transient interaction between blood flow and the wall contributes to wall stress which, if it exceeds the failure strength of the dilated arterial wall, will lead to aneurysm rupture. Utilizing a computational approach, the biomechanical environment of virtual AAAs can be evaluated to study the affects of asymmetry and wall thickness on this stress, two parameters that contribute to increased risk of aneurysm rupture.MethodsTen virtual aneurysm models were created with five different asymmetry parameters ranging from β = 0.2 to 1.0 and either a uniform or variable wall thickness to study the flow and wall dynamics by means of fully coupled fluid-structure interaction (FSI) analyses. The AAA wall was designed to have a (i) uniform 1.5 mm thickness or (ii) variable thickness ranging from 0.5 – 1.5 mm extruded normally from the boundary surface of the lumen. These models were meshed with linear hexahedral elements, imported into a commercial finite element code and analyzed under transient flow conditions. The method proposed was then compared with traditional computational solid stress techniques on the basis of peak wall stress predictions and cost of computational effort.ResultsThe results provide quantitative predictions of flow patterns and wall mechanics as well as the effects of aneurysm asymmetry and wall thickness heterogeneity on the estimation of peak wall stress. These parameters affect the magnitude and distribution of Von Mises stresses; varying wall thickness increases the maximum Von Mises stress by 4 times its uniform thickness counterpart. A pre-peak systole retrograde flow was observed in the AAA sac for all models, which is due to the elastic energy stored in the compliant arterial wall and the expansion force of the artery during systole.ConclusionBoth wall thickness and geometry asymmetry affect the stress exhibited by a virtual AAA. Our results suggest that an asymmetric AAA with regional variations in wall thickness would be exposed to higher mechanical stresses and an increased risk of rupture than a more fusiform AAA with uniform wall thickness. Therefore, it is important to accurately reproduce vessel geometry and wall thickness in computational predictions of AAA biomechanics.

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Several complications related to apposition of the filter basket on the vessel wall and device retrieval were detected in all the devices. It is inferred that the adaptability of the filter basket to conform to the vessel cross section at the site of deployment is the primary design variable responsible for distal embolization during CAS with cerebral protection.

Stroke is the third leading cause of death in the United States, accounting for 1.5 deaths reported per 1000 people. Carotid artery stenting (CAS) with cerebral protection is slowly becoming the gold standard for treatment of carotid artery occlusive disease in high risk patients. CAS is based on the selective cannulation of the common carotid artery by means of an introducer sheath or guiding catheter and the deployment of a wire mesh (stent) to treat the occluded artery segment. The goal for CAS is the prevention of stroke and its efficacy depends greatly on the periprocedural complications. The major concern with CAS is its potential to produce emboli that may translate into a severe neurological disorder. In this regard, several cerebral protection devices (CPDs) have been developed recently as an adjunct to CAS, with the primary function of capturing the plaque particles released from the site of vessel injury to prevent neurological events. A category of CPD that has received recent attention due to its ability to allow continued distal perfusion is the embolic protection filter. We tested in vitro one FDA approved (RX Accunet Embolic Protection System, Guidant Corporation, Indianapolis, IN) and two investigational (FilterWire EZ, Boston Scientific, Natick, MA and Angioguard XP, Cordis Corp., Coral Gables, FL) devices of this kind. The objective of this study was to assess the effectiveness of emboli capture of the devices, investigate potential intangible failure modes and complications, and set a baseline of desirable design parameters for future generations of embolic protection filters. None of the devices tested completely prevented embolization into the artery model. Overall, the RX Accunet device had the best filtration performance, failing to capture 0.16% of plaque particles when deployed in an artery model of 5.5 mm in diameter. Several complications related to device retrieval were detected in all devices on any given set of testing scenarios. Crossing profile, opening/closing mechanics and pore size were among the key design variables required for improved device designs.