An aneurysm is an abnormal widening or ballooning of a part of an artery due to weakness in the wall of the blood vessel. There aren’t any symptoms usually, but a ruptured aneurysm can lead to fatal complications. The risk of rupture depends on the size of the bulge. The normal diameter of the aorta is between 2-3 cm but it can bulge to beyond 5 cm with an aneurysm. As an aneurysm increases in size, the risk of rupture increases, which leads to uncontrolled internal bleeding. Without surgery the annual survival rate for patients having aneurysms of over 6 cm is 20%. CDC estimates over 55,000 deaths in the US each year due to aneurysm rupture. It can occur at any location in the circulatory system. However, the two most common types of aneurysms are brain aneurysms and aortic aneurysms. Moreover, the bulging can also be classified into two main shapes: Fusiform – where the aneurysm bulges all sides of a blood vessel, or Saccular – when the bulge is only on one side.

Unlike saccular aneurysms, fusiform aneurysms have uniform bulging of the blood vessel along all directions. Saccular aneurysms occur due to sharp turns or bifurcation in the blood flow where there is usually high Wall Shear Stress (WSS). This cannot be said for a fusiform type aneurysm as in most of the cases the flow is relatively uniform with low WSS. There has been a lot of interest in CFD based estimation and evaluation of different types of aneurysms in order to characterize the pathobiological characteristics that contribute towards aneurysm growth and rupture. Previous studies relying on Newtonian models and steady blood flow have lead to an unrealistic and hypothetical representation of fusiform aneurysm. This has also led to many contradictory theories about aneurysm rupture criterion and the famous high Vs low Wall Shear Stress debate. With this study we hope to bridge this gap. In our study, we analyze a simple fusiform type aneurysm for its various aberrant flow physics based pathobiological characteristics that can tip the balance that maintains vascular homeostasis and can cause destructive remodelling to cause aneurysm growth and its eventual rupture. The most intriguing aspect of a fusiform type aneurysm is that the flow is mostly uniform and the initiation of aneurysm can be purely due to pathobiological factors. However, once the aneurysm is initiated, a combination of both biological and hemodynamic factors contribute to its growth. Through this study we evaluate how the flow physics varies through the various stages of an aneurysm.

Three dimensional axisymmetric model is constructed with a pulsatic velocity profile for blood based on actual cardiogram measurements and a non-newtonian Carreau-Yasuda model is used for blood’s viscosity. The WSS, WSS Spatial Gradient (WSSSG) and Oscillating Shear Index (OSI) are computed along the blood vessel walls for the pulsating flow during the various stages of the aneurysm life-cycle. Four different stages of aneurysm growth are considered from inception to its rupture. The results indicate a sharp variation in the hemodynamic parameters during a pulse cycle. And as the aneurysm grows over time, a decrease in WSS and a rise in OSI and WSSSG is observed. It can be concluded that hemodynamically driven biologic pathways could be responsible for the growth of fusiform aneurysms. An inflammatory-cell-mediated pathway induced via low WSS and a high oscillatory shear index can be associated with the growth of such aneurysms.