Comparison of different in vitro testing conditions for peripheral Nitinol stents: a computational study

Fatigue resistance of Nitinol peripheral stents implanted into femoropopliteal arteries is a critical issue due to the unique biomechanical environment of this district. Potentially in vitro testing offers a valid method to compare the fatigue behavior in terms of fracture risk of different marketed stents. However, there is still a lack of validated methodologies for bench testing that mimic in a satisfactory manner the complex biomechanical environment present in the SFA. In the present study, the finite element method (FEM) was used to investigate the fatigue behavior of stents when subjected to different in vitro tests. For this purpose, two models of commercial Nitinol stents were developed and different loading conditions were simulated using finite element analyses. In order to reproduce two different test methods recently proposed in the literature, stents were subjected to cyclic axial compression, either alone in the free-expanded configuration or after deployment in a silicone tube. Results were analyzed in terms of amplitudes (ε1a) and mean values (ε1m) of the first principal strain through the stent either on a constant-life diagram or in the form of strain field. Results indicated that: i) the two testing conditions lead to quite different fatigue fracture risk and strain fields in the stent, explaining the conflicting findings reported in the literature; ii) different stent designs exhibit a variable ability to withstand in vitro loading; iii) the mechanical interaction with the arterial wall cannot be disregarded as it significantly influences the stent fatigue behavior.