atherosclerosis

In the present study, numerical simulation of the steady blood flow through the carotid artery with a non-planar geometry model and considering mild (20%), moderate (50%), and severe (80%) occlusion was performed. In this research, the shear-thinning behavior of the blood fluid is incorporated by the Carreau–Yasuda model, and the viscoplasticity of blood was ignored. Furthermore, concentric and eccentric geometries were considered for stenosis. By comparing the non-Newtonian and Newtonian viscosity results, significant differences were found in the secondary flow lines. Shear-thinning behavior affects the secondary flow lines so that the vortices are either not formed or are smaller in size in the middle of the stenosis and subsequent sections. Moreover, axial velocity profiles in the non-planar branch decreased by increasing stenosis percentage, and in estimating the maximum wall shear stress, the Newtonian model had a significant error compared to the non-Newtonian one, and the estimated values by the Newtonian model were less than the non-Newtonian in most cases (up to 37% for an 80% stenosis). In addition, variation of velocity and shear rate caused by stenosis reveals the importance of the non-Newtonian model in calculating streamlines and velocity magnitudes. Plus, as the percentage of stenosis increased, the vessel's curvature effect, which causes the velocity field to deviate to the inner wall, decreased.

Selection the appropriate anastomosis angle for the creation of a fistula for surgery is very important. Therefore, in this study, three anastomosis angles of 45, 90 and 135 degrees, representing acute and obtuse angles, are designed and simulated in a complete pulsation cycle. Carreau non-Newtonian blood model is used and the flow is considered as incompressible flow. Finally, after modeling, important parameters such as TAWSS, OSI, RRT and maximum pressure drop are extracted and compared at different angles. After comparing the results, it is observed that the time average wall shear stress and the range of the high shear stress at anastomosis angle of 135 degree is lower than the two other angles and the probability of thrombosis disease in this angle is reduced. 80% of fistula failure is caused by thrombosis disease, therefor this angle is chosen as the most appropriate angle for fistula creation. Based on the results of the RRT in which are shows areas of sediment, it is found that at all three angles of anastomosis, the right branch of the fistula and flow separation sites have probability of sedimentation and it decreases at angle of 135 degree. This angle also has the lowest pressure drop between the main inlet and the fistula outlet.

Recently, the use of coronary stents in interventional procedures has rapidly increased. Biodegradable magnesium alloy stents gained increasing interest in the past years due to their potential prospect. However, for the magnesium alloy stents to be feasible for widespread clinical use, it is important that their performance can be compared to modern permanent stents. In this project, a finite element method is used for investigating the effect of the stent geometry and material properties on its behavior. The stent designs made with two different materials, stainless steel 304 and magnesium alloy AZ 31, and the Palmaz-Schatz geometry are modeled and their behavior during the deployment is compared in terms of stress distribution in the stent, vessel wall, plaque as well as in terms of outer diameter changes, radial recoil ratio, axial recoil ratio and Foreshortening. Moreover, the effect of stent material properties on the restenosis after coronary stent placement is investigated by comparing the stress distribution in the arteries. According to the findings, the possibility of restenosis after coronary stenting is lower for magnesium alloy stents in comparison with stainless steel 304 stent.