Mechanical properties of open-cell, self-expandable shape memory alloy carotid stents

Mechanical properties of carotid stents are important for the selection of the implantable device. The efficacy of cell design is usually investigated on the basis of clinical follow-ups; however, for the specific carotid application the relative importance of the cell design on the stenting outcome is debated. Ideally, a carotid stent should have at the same time low bending stiffness (i.e., high flexibility) to correctly conform to the vessel curvatures in the stenotic region and high stiffness to sustain the vessel wall in the radial direction. In this study, experimental tests on six carotid open-cell design self-expandable stents were carried out to assess bending and radial stiffness. A four-point bending test was adopted to measure the bending stiffness. Radial stiffness was obtained by measuring the stent resistance to local compression in a V-shaped support. Geometrical features, such as the link positioning and their length, were examined as a possible determinant of the mechanical properties of the tested stents. The tested stents showed very different geometrical features and mechanical properties despite similar material and a common open-cell structure consisting in rings connected by roughly straight links. In particular, different strategies are evident regarding the link position. Three distinct behaviors were observed: stents with both low radial and bending stiffness, moderate bending and high radial stiffness, and high bending and moderate radial stiffness. In general, a large link length allows the stent to be more flexible under bending condition, whereas the mutual links positioning plays a key role in the radial stiffness of these devices. Furthermore, for a fixed strut layout, a correlation was found between the strut cross area, as well as metal-to-artery ratio and the resultant radial and bending stiffness.