Abdominal Endovascular Aneurysm Repair Simulations
Aneurysm refers to irreversible dilation of the artery, which, if left untreated, can be fatal. Although aneurysms can occur in any artery in the human cardiovascular system, most of them are found in the cerebral arteries, the thoracic and abdominal aorta. Once identified, they are typically treated through minimally invasive endovascular aneurysm repair (EVAR) surgeries, where a stent-graft is deployed at the aneurysm site using a catheter. Once deployed, this stent-graft relieves the weakened arterial wall from the hemodynamic loads and thus prevents the future of the aneurysmal artery. Some of the most common post-surgical complications associated with EVAR are stent-graft related, including thrombosis due to non-physiologic flow conditions, device migration, endoleaks and, in extreme cases, complete device failure. Computational fluid dynamics (CFD) and fluid-structure interaction (FSI) simulations offer a very lucrative pathway to quantitatively assess during the surgical planning phase if such complications can arise for a given patient or not, as elaborated below.
Abdominal Endovascular Aneurysm Repair simulations
Aneurysms found within the abdominal aorta are known as abdominal aortic aneurysms. In addition to the post-surgical complications elaborated in the section on thoracic aneurysms, another complication is clinically observed after abdominal endovascular aneurysm repair is endoleaks. Endoleaks are defined as persistent blood flow occurring between the stent-graft and the aneurysm sac after EVAR. There are four different types of endoleaks: Type I, II, III and IV. Key features of these endoleaks are summarized below:
- Type I endoleaks – These occur when blood seeps into the aneurysm sac from the attachment site, either proximally (Type Ia) or distally (Type Ib).
- Type II endoleaks involve flow reversal from the inferior mesenteric artery or lumbar artery back into the aneurysm sac.
- Type III endoleaks – These endoleaks occur due to the failure of the stent-graft itself, such as a tear due to a fractured stent or modular disconnection.
- Type IV endoleaks – Filtration of blood plasma through the porous graft wall accounts for these endoleaks.
Type I endoleaks are the most common and are usually caused by improper sealing of the device with the aortic wall or distal migration of the stent-graft because of hemodynamic displacement forces acting on the device. Contrary to type I endoleaks, type III and IV are the rarest. Endoleaks pose a threat because leakage of blood into the aneurysm sac means that the intra-saccular pressure remains elevated and thus may cause a rupture of the abdominal aortic aneurysm. These endoleaks can be modelled quite effectively using FlowVision’s proprietary Gap Model. The flow in narrow channels can be resolved using a single layer of computational cells rather than a highly refined mesh which can significantly slow the CFD/FSI simulations. Some physicians also classify endotension as Type V endoleak. Endotension is defined as elevated intra-saccular pressure without evidence of an obvious endoleak, thus causing a continued expansion of the aneurysm sac. It is believed that intra-vascular pressure is transmitted from the stent-graft to the native aneurysm wall, causing the expansion of the aneurysmal cavity. Type V endoleaks are poorly understood to date.
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