Christine M. Scotti

Fluid-structure interaction in abdominal aortic aneurysms: effects of asymmetry and wall thickness

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.

Wall stress and flow dynamics in abdominal aortic aneurysms: finite element analysis vs. fluid–structure interaction

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%.

Wall apposition assessment and performance comparison of distal protection filters

Purpose: To assess the wall apposition of 3 distal protection filters used in carotid artery stenting (CAS) for cerebral protection and quantify the effect on the in vitro capture efficiency of the filters under simulated physiological flow conditions. Methods: The 3 distal protection filters (Angioguard XP, FilterWire EZ, and RX Accunet) were deployed in silicone flow models of 5.0-, 5.5-, and 6.0-mm inner diameter and were tested at a mean flow rate characteristic of the human internal carotid artery while injecting polydispersed microspheres simulating plaque emboli. The injected microspheres had a diameter larger than the pore size of the devices tested, so it was conjectured that any microspheres missed by the device traveled between the device basket and the vessel wall. Varying the diameter of the vessel phantom within the recommended vessel diameter treatment range for each device simulated the variability of vessel diameter in vivo, allowing the quantification of device wall apposition. Results: None of the devices tested completely prevented distal embolization in the flow model. The RX Accunet device has the best overall wall apposition, yielding gaps of 0.075% of the vessel cross-sectional area. The FilterWire EZ device had the best overall average filtration rate, failing to capture only 0.8% of plaque particles. There were no statistically significant differences in the wall apposition assessment or the capture efficiency of the RX Accunet and FilterWire EZ devices. Conclusion: 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.

WALL STRESSES BEFORE AND AFTER ENDOVASCULAR REPAIR OF ABDOMINAL AORTIC ANEURYSMS

Endovascular aneurysm repair (EVAR) technique is a minimally invasive procedure approach to abdominal aortic aneurysm (AAA) repair. Following EVAR, isolated aortic tissue starts remodeling after the new blood path is established. The commercially available endovascular grafts (EVG) have been found to be prone to Type I endoleak, which is re-pressurization of the degenerated AAA sac following a breach in the seal mechanism of the EVG or migration due to failure of the mechanism holding the graft in place (Chuter, 2002) These inadequacies of EVGs might be attributed to the effect of non-optimal design of graft anchoring system. In the present study, we utilized pre-operative and post-operative computer tomography (CT) data with previously derived material properties to construct three-dimensional finite element (FE) models for AAA before and after the EVAR procedure. We studied the nature of stresses acting on the aorta before and after EVAR. In particular we investigated the physical forces acting on the EVG and how they are transferred to the aortic wall at graft anchoring sites.Copyright

Computational Fluid Dynamics and Wall Mechanics of Pre- and Post-Operative Abdominal Aortic Aneurysms

In the present work we numerically evaluated the biomechanical environment of pre- and post-operative aneurysms and examined the cumulative ffect of factors such as the presence of localized thrombus, non-uniform material properties, and patient-specific geometry. Flow disturbance was intensified through the pre-operative aneurysm sac with the onset of diastolic conditions and the obstruction of the localized thrombus. This led to increased flow-induced forces, particularly in the normal direction to the wall. Similarly, the thrombus localized the Von Mises stress where changes in geometry and material property occur, while simultaneously shielding the AAA wall from elevated mechanical forces.

Fully Coupled vs. Partially Coupled Fluid-Structure Interaction Methods for Patient-Specific Abdominal Aortic Aneurysm Biomechanics

Primary among the mechanical factors linked with abdominal aortic aneurysm (AAA) rupture is peak wall stress, frequently quantified as either the maximum principal or Von Mises stress exerted along the diseased arterial wall. Intraluminal pressure, as an impinging normal force on the wall, has been hypothesized as the dominant influence on this stress and thus the majority of numerical modeling studies of AAA mechanics have focused on static computational solid stress (CSS) predictions [1,2]. Unfortunately, retrospective studies comparing the magnitude of wall stress with the failure strength of the aneurysmal wall have yet to consistently predict the outcome for patient-specific AAAs [3,4]. Previous studies have shown that hemodynamics also plays a significant role in both the biological and mechanical factors that exist within AAAs. In the present investigation, partially and fully coupled fluid-structure interaction (p-FSI and f-FSI, respectively) computations of patient-specific AAA models are presented and compared to identify the effect of fluid flow in the biomechanical environment of these aneurysms.Copyright

Computational Fluid Dynamics and Solid Mechanics Analyses of a Patient-Specific AAA Pre- and Post-EVAR

The establishment of a new pathway for blood flow immediately following endovascular aneurysm repair (EVAR) results in morphological changes and remodeling of the aneurismal sac. While EVAR is a minimally invasive surgical intervention, failure of the endovascular graft (EVG) may occur in which there is downstream migration and endoleak formation, creating a repressurization of the aneurismal sac and an increased risk of rupture. While the mechanism of aneurysm rupture and EVG failure is fundamental in nature, the factors that most significantly contribute to the end result are not yet fully understood. Mechanically, both are the consequence of an exerted force or disturbance exceeding the strength of a given material, whether it is the aneurismal arterial wall or the interaction that exists between the graft and wall. Embedded within this causal relationship are the contributions of arterial wall remodeling, intraluminal thrombus formation, and the dynamics that exists within the lumen. Several studies have been performed to examine these factors individually as they affect shear stress, the development of vortices, and the mechanical stress experienced along the arterial wall. However, a complete investigation is needed to study an anatomically realistic geometry operating under physiological conditions. The computational analyses conducted in this investigation address the confluence of these factors as they are modeled within an accurate patient-specific abdominal aortic aneurysm (AAA) reconstructed from CT scan data prior to and after EVAR. Our results verify the pressure-dominated characteristic of the flow and the negligible contribution of the dynamic and frictional force components; both are in good agreement with previously published results for analytical estimation of flow-induced forces in EVGs. [1]Copyright