Evolution of the wall shear stresses during the progressive enlargement of symmetric abdominal aortic aneurysms

The changes in the evolution of the spatial and temporal distribution of the wall shear stresses (WSS) and gradients of wall shear stresses (GWSS) at different stages of the enlargement of an abdominal aortic aneurysm (AAA) are important in understanding the aetiology and progression of this vascular disease since they affect the wall structural integrity, primarily via the changes induced on the shape, functions and metabolism of the endothelial cells. Particle image velocimetry (PIV) measurements were performed in in vitro aneurysm models, while changing their geometric parameters systematically. It has been shown that, even at the very early stages of the disease, i.e. increase in the diameter <= 50%, the flow separates from the wall and a large vortex ring, usually followed by internal shear layers, is created. These lead to the generation of WSS that drastically differ in mean and fluctuating components from the healthy vessel. Inside the AAA, the mean WSS becomes negative along most of the aneurysmal wall and the magnitude of the WSS can be as low as 26 % of the value in a healthy abdominal aorta. Two regions with distinct patterns of WSS were identified inside the AAA: the proximal region of flow detachment, characterized by oscillatory WSS of very low mean, and the region of flow reattachment, located distally, where large, negative WSS and sustained GWSS are produced as a result of the impact of the vortex ring on the wall. Comparison of the measured values of WSS and GWSS to an analytical solution, calculated for slowly expanding aneurysms shows a very good agreement, thus providing a validation of the PIV measurements.