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Christine Scotti — 2004-05 Fellow

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Abdominal aortic aneurysms (AAAs) are an increasingly prevalent cardiovascular disease characterized by a dilation of the infrarenal segment of the abdominal aorta. Currently, AAAs are estimated to affect about 2 million Americans; within men aged 65- to 75-years old the incidence rate is about 3-5% and the likelihood is 10 times greater than similarly aged women. AAAs have been recently identified as a condition closely associated with cigarette smoking, however other factors present in a continuously aging population such as hypertension, familial history, and chronic obstructive pulmonary disease (COPD) are also linked to the presence of AAAs. Therefore, the number of aneurysms diagnosed is expected to increase as the population ages and diagnostic tools continue to improve.

While the exact mechanism for AAA expansion is unknown, it is associated with a cumulative effect initiated by a weakening of the arterial wall and the disturbed flow patterns near the bifurcation of the iliac arteries. As an aneurysm develops, there exists a changing biomechanical environment that creates localized stress gradients; these gradients are predicted to further contribute to the weakening of the arterial wall until the point where mechanical failure occurs. Failure of the aneurysm wall is characterized by its rupture, an event associated with a 90% overall mortality rate, representing the 13th leading cause of death in the U.S.

While AAAs remain largely asymptomatic, they are treatable with surgical intervention once properly diagnosed. One technique used to restore normal blood flow through the aneurysm and reduce the pressure exerted on the diseased arterial wall is referred to as endovascular aneurysm repair (EVAR), as shown in Figure 1. It is a minimally invasive procedure in which an endovascular graft (EVG) is placed inside the aneurysm by guiding a catheter through a small incision in the femoral artery and subsequently passing the graft through the catheter. The EVG is then expanded and attached by either an active or passive mechanism to the wall of the aortic neck and iliac arteries. An active mechanism requires the use of stents, barbs, or hooks at the proximal and distal ends. Alternatively, a passive system can be used in which an extension cuff acts to oversize the graft and hold it in place by a balance of frictional forces.

While EVAR is a clinically advantageous technique, it is also shows an increased rate of secondary interventions due to the development of endoleaks and graft migration. Endoleaks occur when either the attachment site or graft body are permeated, allowing perigraft flow and subsequent repressurization of the aneurysm sac, increasing its susceptibility to rupture. Of particular interest to this research is the failure mechanisms as they occur at the proximal attachment site (along the aortic neck) referred to as Type I endoleaks. The incidence rate for this type of endoleaks is 10% while for graft migration it is nearly 15%.

The objectives of this proposal are to integrate computational modeling and experimental validation tools to assess the biomechanical environment that leads to graft failure and set guidelines for an improved graft design that minimizes their occurrence.