Analysis of blood flow behaviour in custom stent grafts

Endovascular aneurysm repair (EVAR) is an attractive alternative to open surgery for treating abdominal aortic aneurysms (AAAs). However, the implantation of stent grafts into AAAs can result in post-operative complications such as stent graft migration, rupture or endoleak. EVAR has therefore been carried out only on selected patients. Stent grafts are usually standard commercial stent grafts (CSGs); however, custom made stent grafts (cmSGs) of various shapes and sizes are sometimes used to fit patients' anatomies. In the present study, the cmSGs were specially designed and fabricated by the surgeons at the Pitié-Salpétriére hospital in Paris. Two patients carrying cmSGs with unfavourable geometries showing tortuous shapes, angulation, widening, narrowing, curvature and kinking and one patient with a cmSG with a more favourable geometry resembling a straight tube were examined. These three clinical cases were investigated using three-dimensional numerical simulations, and the results showed that even when the cmSG geometries are unfavourable, the drag forces to which they are subjected are of a similar magnitude to those exerted on CSGs, or even smaller. The hemodynamic analysis carried out on the two unfavourable cmSGs showed the occurrence of low velocity values in the main trunk of the cmSGs, high velocities linked to recirculation areas downstream from kinking and strong distal narrowing. These flow patterns are liable to induce thrombus. However, since cmSG implantation can save the lives of patients for whom neither classical stent grafts nor open surgery are indicated, it can be concluded that these devices are useful in some cases.