Computational Modelling of Fatigue Behaviour of Nitinol Peripheral Stents During in Vitro Tests

Hip and knee movements during gait produce large and cyclic deformations of the superficial femoral artery (SFA), increasing the risk of fracture of implanted Nitinol peripheral stents. Although in vitro testing offers a valid tool to comparatively assess the risk of fatigue fracture of different marketed stents, there is still a lack of validated methodologies for bench testing under the complex biomechanical environment of the SFA.
In this study, a computational approach was used to investigate the fatigue behaviour of stents when subjected to different in vitro tests. Finite element models of different commercial Nitinol stents were developed and various loading conditions were simulated using finite element analyses (Ansys Inc., Canonsburg, PA, USA). Stent geometries were reconstructed starting from images taken by optical microscopy. The same material properties were adopted for all the stent models. Stents were subjected to cyclic axial compression and bending, either in a free-expanded configuration or after deployment in a silicone tube, to reproduce two different test methods recently proposed into the literature (Nikanorov et al. 2008, Müller-Hülsbeck et al. 2010). Results were analyzed plotting the amplitudes (1a) and mean values (1m) of the cyclic first principal strain in all the points of the stent on a constant-life diagram of Nitinol.
Results indicated that: i) the two testing conditions produce quite different strain fields and fatigue fracture risk in the stent, explaining the conflicting findings reported in literature;
ii) the mechanical interaction with the arterial wall plays an important role on the stent fatigue behaviour.

The abstract is submitted to the session "SMA modeling in medical industry: from raw material to final product design"