Computational Modelling of In Vitro Set-Ups for Peripheral Self-Expanding Nitinol Stents: The Importance of Stent–Wall Interaction in the Assessment of the Fatigue Resistance

Although the number of fractured stents in the femoro-popliteal district was largely reduced using second-generation devices, remarkable differences still exist among various products. In vitro testing offers a valid tool to comparatively assess the risk of fatigue fracture of different devices. However, there are no standardized methodologies for bench testing under the complex biomechanical environment of the femoro-popliteal district. A computational approach was used in the present study to investigate the fatigue testing conditions adopted in the in vitro studies to increase the understanding of the Nitinol stents fatigue behaviour, by focusing on the role of the stent–wall interaction. The finite element method was used coupled to fatigue analysis to investigate the behaviour of commercial devices during in vitro tests. Two peripheral stents were chosen for the analyses for which conflicting results are found in literature. Stent models were subjected to axial compression and bending either alone in the fully-expanded configuration or after their deployment in a silicone tube resembling the presence of the artery. Results indicate that the two testing conditions investigated produce quite a different fatigue behaviour both in terms of constant-life diagram and strain distribution in the stents. In particular, the results highlight that oversizing influence the fatigue behaviour of the devices with an effect on both the mean and amplitude values of the strain induced in the stents. Comparison with a fatigue limit typical for Nitinol indicates good agreement with the experimental results and confirms the validity of the adopted methodology. Stent oversizing plays an important role in determining the fatigue response of the devices and cannot be disregarded to assess the fatigue performance of Nitinol stents during in vitro tests. Moreover, the different behaviour found for the two stent models suggests that a careful stent design can improve the fatigue performance of these devices.