Stent design and arterial mechanics: parameterization tools using the finite element method

Vascular stents are medical devices used to treat stenoses blockages in arteries that restrict blood flow. Most commonly, stents are made out of stainless steel or nitinol, and are delivered to the afflicted sites via catheter-based delivery systems. Usually, stents are balloon-expandable or self-expanding. In order for the treated vessel to remain patent, it is necessary that the stents be oversized to prevent flow-induced or pressureinduced stent migration. Furthermore, stents must be rigid enough to prevent the collapse of the vessel, allowing the free passage of blood. However, it has been observed that the presence of the stent in the artery triggers adverse biological responses such as neointinal hyperplasia, often times culminating in restenosis. Extensive research external to this investigation has elucidated evidence to suggest that the abnormally high stresses imparted to the arterial wall as a result of stenting are an important factor in the treatment and development of cardiovascular diseases. Furthermore, normal physiologic diameter flcutuations between systole and diastole produce beneficial biological responses in the artery wall. The primary purpose of this study was to investigate specific stent design criteria that minimize the stress field in the arterial wall to mitigate the impact of restenosis. Commerically available finite element software was used to design the stents parametrically, and perform the stress analysis on a hyperelastic arterial model, including the effects of contact between the artery and stent. Seven stent geometries were uniquely defined by varying strut-spacing, ring amplitude, and crown radii of curvature. Stent designs with large strut spacing, large ring amplitude and a greater than zero radius of curvature imparted the less severe stress field in the arterial wall as well as maximizing vessel deflection between systole and diastole. In contrast, stents with small strut spacing, small amplitudes and zero radius of curvature at the crowns imparted significantly higher stresses. The small strut spacing and small amplitude created stiffer stents, prventing the artery from experiencing physiologic diameter fluctuations between systole and diastole. Evidence presented herein suggests that strut spacing should be as wide as possible without causing pillowing of the arterial wall into the stent.