Numerical simulations of fatigue for two overlapped stents in peripheral arteries.

A typical endovascular procedure to restore blood flow perfusion in stenotic peripheral arteries consists in balloon angioplasty and deployment of self-expanding Nitinol stents. However, movements during gait produce large and cyclic deformations of the superficial femoral artery, increasing the risk of fatigue fracture of implanted stents. Fatigue failure typically occurs in cases of very extended lesions, which often require the use of two or more overlapping stents. In this study, finite element models were used to study the fatigue behavior of overlapped Nitinol stents. Two different approaches were adopted: i) simulating the deployment of a stent into another stent and ii) using a single stent with double stiffness in the overlapping region. Simulations of cyclic axial compression of stents deployed in a simplified cylindrical model of a peripheral artery were then performed (Ansys Inc., Canonsburg, PA, USA). Fatigue risk was assessed on the basis of amplitudes (εI a) and mean values (εI m) of the cyclic first principal strain through the stent. Similar results were found in both approaches: strain concentrates in the links close to the overlapping portion where the abrupt change in stiffness causes higher cyclic compression (εI a = 0.19%), while in the overlapping region the value of εI a is lower (= 0.15%), due to a higher stiffness. These findings help to explain the high incidence of stent fracture observed in various clinical trials located close to the overlapping portion. Moreover, this study demonstrates the possibility of using a simplified model with a region with double stiffness, reducing significantly the computational time.