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Title: Stent, bypass graft, and stent-graft in endovascular system

Stent and graft implantations are a type of treatment for vessels with abnormal sizes in the endovascular system (also known as the circulatory system, cardiovascular system, or vascular system). If the vessel in the system has an abnormal size (much larger or smaller than the vicinity), diseases can be induced such as stenosis and aneurysm.

Stenosis and aneurysm can change the blood flow and cause serious problem. Stenosis induces a portion of the vessel to become much smaller than its vicinity and consequently obstructs the normal blood flow. An aneurysm enlarges the diameter of the vessel and this portion of vessel becomes a big sac, influencing the normal blood flow. Stenosis and aneurysm can appear anywhere in the upper body, such as cerebral arteries, carotid arteries, thoracic arteries, and abdominal aorta.

Nine percent of people over the age of 65 suffer from an abdominal aortic aneurysm and as the aneurysm becomes larger, the possibility of rupture is higher. Statistics from the American Heart Association identify coronary heart diseases as the principal cause of morbidity and mortality in the western world. It is also reported that 17% of overall deaths in the USA come from cardiovascular disease. Therefore, the treatment of vascular diseases become highly concerned during these two decades.

Types of stents and grafts
There are three types of stents and grafts commonly used in operations: stent, bypass graft, and stent-graft, which, from their names, are really confused.

Stent
The stent is a cylindrical wire net, used to treat stenosis. The stent is placed in the vessel, attaching on the inner surface of the vessel to enlarge the vessel and help blood flow fluently, the implantation can be at cerebral, carotid, thoracic, and coronary artery etc.

Bypass graft
In contrast, the bypass graft uses another way to treat the stenosis, which implants a new pathway to avoid the stenosis instead of directly enlarging that location. The bypass graft is an artificial vessel, one end connects the vessel from the upstream of the obstruction location while the other end links to the downstream of the obstruction. The implantation of bypass graft is mainly at the coronary artery, called coronary artery bypass surgery.

Stent-graft
The last one is stent-graft, this type combines the other two, using an artificial vessel with a cylindrical wire net attaching on the inner surface. Instead of enlarging the vessel, the stent-graft treats the vessel with an abnormally large size. The blood flow is guided flowing through the stent-graft instead of generating vortices when flowing into a big sac. Typically, the stent-graft is implanted at the abdominal aorta where aneurysm most likely appears, an example of this can be the endovascular aneurysm repair.

Blood flow after the stent and graft implantation
Although stents and grafts are treatments for abnormal size vessels, some serious negative reactions may still occur after the implantations: for the stent implantation, the restenosis, which means the blood flow is obstructed again by the reasons of thrombus, neo-intimal growth and etc., similar situations also happen in the implantations of bypass graft and stent-graft. Despite the factors causing these may be very complicated, many researchers   still found that two major factors contribute to the appearing of these negative reactions: the shear stress changes on the inner vessel wall and the blood flow changes made by the stent implantation. However, these two results are not able to be obtained experimentally. Therefore, researchers begin to use computational fluid dynamics to simulate the fluid field and obtain the results.

Stent
Generally, two negative reactions may happen after the stent implantation: the neo-intimal growth around the stent wire and the thrombus formation in the stent. Clinical studies found that low or oscillating wall shear stress has been at the location where the thickness of the neo-intimal is greatest. Also, it was found that there is an inverse and non-linear relationship between the neo-intimal thickness and the wall shear stress : as the shear stress decreases, the neo-intimal obtains more thickness. For the distribution of wall shear stress, the low wall shear stress can be seen obliviously around the stent wires and the junctions of the wires have a more severe extent. Low shear stress induces the development of the neo-intimal and the combination of neo-intimal and stent wires will form a larger obstruction, then, consequently induces more area with low wall shear stress.

On the other hand, the formation of the thrombus can be induced by the vortices. This kind of vortices are generated by the following process: the material brought by the stent implantation in the blood vessel was obstructed the blood flow, and the blood needed to bypass the obstructions then generated the vortices at the downstream sides of the material (as figure: Blood Flows around Stent Wires). Typically, the vortices will cause the low wall shear stress, so the formation of the thrombus induced by the vortices appears coincidently with the development of the neo-intimal induced by the low wall shear stress, which may fasten the failure of stent implantation as well as the restenosis.

Bypass graft
There are many researchers studying on this topic, looking for the relationship between bypass graft implantation and negative reactions from the perspectives of the geometry, the deployment location of bypass graft, and the degree of stenosis growth in the vessel. Freshwater et al investigated the impact of the anastomosis angles on the wall shear stress and flow profiles of the left internal mammary artery bypass graft, Frauenfelder et al examined the flow pattern and the wall shear stress for the end-to-side and side-to-side anastomosis of the coronary artery bypass graft. And recently, Arefin M S demonstrated the impact of the degree of stenosis growth on the flow field and the shear stress on the wall of both of the graft and original coronary artery.

A brief example of blood flow in the coronary artery bypass graft, as shown in the figures, when there was no stenosis in the left anterior descending coronary artery, the blood flowed with no obstruction from the inlet to the outlet. However, because of the appearance of the bypass graft connecting to the internal thoracic artery, part of the blood in the left anterior descending coronary artery was sucked to the graft at the beginning of the simulation because of the negative flow rate in the bypass graft. Then the flow rate in the graft became positive, consequently made the blood flow into the left anterior descending coronary artery, and the flow reversed again because of the negative flow rate in the bypass graft.

In the 30% stenosis case, there was no reverse flow in the internal thoracic artery because the stenosis slowed down the flow rate in the left anterior descending coronary artery and consequently increased the flow rate in the bypass graft. In the 50% and 75% stenosis cases, as the percent occupying the vessel increased, the blood flowing through the bypass graft gradually contributed the mainstream in the downstream of the left anterior descending coronary artery. Furthermore, stagnations, which may cause thrombus, began to appear at the vicinity of the outlet of the bypass graft since there was a certain angle when the blood in the bypass graft jetted into the left anterior descending coronary artery, and the area of stagnation became larger as the stenosis in the left anterior descending coronary artery grew up. In terms of the wall shear stress, as the stenosis increased, the wall shear stress on the bypass graft gradually became larger. Because there was a jetting angle for the blood jetting from the bypass graft, the bottom side of the vessel wall where the jet of blood directly shot on had the maximum wall shear stress, while the top side of the vessel wall which was the vicinity of the junction of the bypass graft and the left anterior descending coronary artery had a relatively low wall shear stress, and the stagnation also located here.

Stent-graft
Before the stent-graft implantion operation, vortices are visible in the aneurysm because of the sac shape, which cannot be seen in normal abdominal aorta. The flow pattern showed areas of high velocity in the branches of the abdominal aorta and low velocity in the region of the greatest diameter of the abdominal aortic aneurysm. After stent graft implantation, the average velocity becomes higher due to the smaller diameter, especially inside the stented aneurysm. Furthermore, the shear stress and pressure on the blood vessel wall distribute more evenly than before.

Although the stent graft implantation can relieve the aneurysm, thrombus may still be induced by this implantation. Due to the prevailing of this method, researchers studied the formation of the thrombus. Polanczyk A and Podyma M demonstrated the formation of the thrombus in the stent graft and two types of formations: partial-developed and fully-developed thrombus. The partial-developed thrombus originally formed from a cross-section, then grown and occupied the whole vessel, while the fully-developed thrombus generated a line of thrombus along the direction of blood flow then grown up and obstructed all the space.