The impact of modeling choices on the assessment of Ni-Ti fatigue properties through surrogate specimens.

The implant of self-expandable Ni-Ti stents for the treatment of peripheral diseases has become an established medical practice. However, the reported failure in clinics highlights the open issue of the fatigue characterization of these devices. One of the most common approaches for calculating the Ni-Ti fatigue limit (commonly defined in terms of mean and alternate strain for a fixed number of cycles) consists of using surrogate specimens which replicate the strain distributions of the final device but in simplified geometries. The main drawback lies in the need for computational models to determine the local distribution and, hence, interpret the experimental results. This study aims at investigating the role of different choices in the model preparation, such as the mesh refinement and the element formulation, on the output of the fatigue analysis. The analyses show a strong dependency of the numerical results on modeling choices. The use of linear reduced elements enriched by a layer of membrane elements is successful to increase the accuracy of the results, especially when coarser meshes are used. Due to material nonlinearity and stent complex geometries, for the same loading conditions and element type, (i) different meshes result in different couples of mean and amplitude strains and (ii) for the same mesh, the position of the maximum mean strain is not coincident with the maximum amplitude, making difficult the selection of the limit values.

Nickel-Titanium peripheral stents: Which is the best criterion for the multi-axial fatigue strength assessment?

Ni-Ti stents fatigue strength assessment requires a multi-factorial complex integration of applied loads, material and design and is of increasing interest. In this work, a coupled experimental-numerical method for the multi-axial fatigue strength assessment is proposed and verified for two different stent geometries that resemble commercial products. Particular attention was paid to the identification of the material fatigue limit curve. The common approach for the Ni-Ti stents fatigue assessment based on the von Mises yield criterion was proven unsuitable for a realistic fatigue strength assessment. On the other hand, critical plane-based criteria were more representative of the experimental outcomes regardless of stent design.

Assessment of tissue prolapse after balloon-expandable stenting: influence of stent cell geometry.

Restenosis is a re-narrowing or blockage of an artery at the same site where treatment, such as a balloon angioplasty or stent procedure, has already taken place. Several clinical trials have shown a significant reduction in the restenosis rates with endovascular stenting. The purpose of stenting is to maintain the arterial lumen open by a scaffolding action that provides radial support. However, stenting can cause a vascular injury during the deployment. Indeed, in-stent restenosis remains a major problem in percutaneous coronary intervention, requiring patients to undergo repeated procedures and surgery. The loading imposed by the deployment of the stent on the artery is involved in the restenosis process. Furthermore, it is well known that the stent design plays a role in the outcome of the stenting interventional procedure. This study compares the mechanical effects of the expansion of five different designs of balloon-expandable stents in a coronary artery by means of numerical models based on the finite element method. An index for the evaluation of the tissue prolapse based on the expanded configuration reached by the stent cells is proposed. The effects of the balloon inflation and deflation are included in the present study. Wall stresses and tissue prolapse of the vessel wall within the stent cells are evaluated and compared among the different stent designs. Results show that the printed area does not predict prolapse, and that the proposed index (PI) does correlate with tissue prolapse.