Computational Analyses and Simulations of Fluid-structure Interactions Applied to Stented Abdominal Aortic Aneurysms

Abstract

An abdominal aortic aneurysm (AAA) is the localized dilation, bulging, or ballooning of an abdominal aorta segment due to a degenerative arterial disease causing local wall weakness. Sudden AAA rupture could result in mortality up to 80%. Approximately 200,000 people in the United States are diagnosed to have AAAs and 15,000 Americans succumb every year. Since the introduction of endovascular techniques in the early 90s, endovascular aneurysm repair (EVAR) has stimulated considerable interest. In EVAR, starting from the femoral artery, a stent-graft is deployed into the affected segment thereby forming a new blood vessel and shielding the weakened AAA-wall from the pulsatile blood flow. There are clear benefits compared with conventional open surgery in terms of minimal incision, early-recovery, reduced mortality and morbidity. However, post-operative complications, such as stent-graft migration, endoleaks, endotension and device failure may still occur.

Because blood vessels and stent-grafts are flexible, interactions between blood flow and wall deformation can involve a wide range of fluid-mechanical phenomena. The flow will affect movement of the walls and wall movements in turn influence the flow field. Hence, simultaneous fluid-structure interactions (FSIs) should be considered when studying the hemodynamics and biomechanics of stented aneurysms. Presently, FSIs relevant to non-stented aneurysms have been only discussed by a few investigators; but, so far there are no publications of computational FSI results for stented AAAs. The complex fluid-structure interactions occur between the lumen blood, stent-graft, cavity blood with possible intraluminal thrombus, and AAA wall. Post-operative problems, such as endoleaks, stent-graft migration, endotension as well as device failure are all examples of FSI dynamics. Hence, transient 3D FSI is very important to study the hemodynamics and biomechanics of stented AAAs.

The main objectives of this research include: (i) transient 3-D fluid-structure interaction simulations to generate physical insight of blood flow and wall stress coupling; (ii) AAA rupture analysis; (iii) impact of endoleaks, factors leading to migration, mechanism of endotension, proper stent-graft placement as well as optimal surgical recommendations; and (iv) improved stent-graft design.

The AAA-rupture risk analysis shows that the most likely rupture site is located near the anterior distal side for anterior-posterior asymmetric AAAs and the right distal side in lateral asymmetric AAAs. The rupture risk of lateral asymmetric AAAs is higher than that of anterior-posterior asymmetric AAAs. The neck angle impacts flow fields and wall-stress distributions remarkably, while the iliac bifurcation angle affects blood flow patterns insignificantly but plays an important role in wall-stress contributions. The aneurysm monitoring program based on eight biomechanical risk factors can evaluate the severity of AAA-rupture risk and hence provides useful recommendations for endovascular surgeons.

The transient 3D FSI simulations demonstrate that a secure stent-graft placement can significantly reduce sac pressure, mechanical stress, pulsatile wall motion, and maximum AAA diameter, and hence may prevent AAA rupture effectively. However, in spite of the absence of endoleaks, a certain sac-pressure level may be caused by fluid-structure interactions between the luminal blood, stent-graft, cavity blood, and AAA wall. The multi-factorial, time-varying sac (or cavity) pressure can be predicted by the stent-graft/ AAA compliance ratio.

The simulation results indicate that stent-graft migration depends on multi-factors, including blood flow conditions, stent-graft and aneurysm geometries, as well as wall mechanical properties. Excessive AAA neck angle, iliac bifurcation angle, neck aorta-to-iliac diameter ratio, stent-graft size, aorto-uni-iliac stent-graft, hypertension, and blood waveform are the key factors causing stent-graft migration.

Transient FSI simulations can provide physical insight to the mechanisms of endoleaks. An endoleak is a minute net influx of blood into the cavity. It was found that Type I endoleaks can elevate the sac-pressure to a patient's systemic level with a reduced pulsatility. The sac pressure caused by Type II endoleaks depends on the inlet branch pressure. Coexistence of Type II and Type I endoleaks results in a sac pressure determined by both lumen and branch pressures. A Type III has the same risk as Type I endoleaks. The AAA-wall stress is elevated remarkably by endoleaks; but, the stent-graft wall stress is reduced in the presence of all endoleaks. The time-varying leakage rate depends on the pressure difference between AAA cavity and lumen/branches. At elevated sac-pressure due to endoleaks, they may mitigate the risk of stent-graft migration.

In the present stent-graft models, a higher von Mises stress is observed near the bending point of stent wires with large curvatures. Nitinol diamond-shaped stents can produce more than 20% of neck oversize and generate hoop forces of 10 to 15N. In contrast, stainless steel is not suited for diamond stents because it cannot provide sufficient neck-oversizing to secure stent-graft anchoring. A z-bend stent is very ductile and suitable for largely angulated necks; but, its hoop force is very low, which implies that hooks or barbs should be considered to secure the neck anchor. The ultra-thin 0.1mm ePTFE graft material is not suited for z-bend stent-grafts due to its low yield stress. Woven polyester (PET) graft is appropriate for both diamond and z-bend stent-grafts. Z-bend stent-grafts have a large wall-compliance and are good at matching irregular necks or aneurysm geometries, while PET stent-grafts are better in reducing the level of endotension. Stents with a PET graft sheath generate minimum axial shortening and are suitable for precise device-deployment in aneurysms with complex geometries.

Validated fluid-structure interaction simulations for blood flow and stent-artery structure interactions are realistic, predictive and powerful tools to generate physical insight of blood flow and wall stress coupling mechanisms, AAA rupture analysis, biomechanical factors of migration, endoleaks and endotension as well as optimal surgical recommendations for improved stent design and proper stent-graft placement.