Numerical study of the uniformity of balloon-expandable stent deployment.

Stents are small tubelike structures, implanted in coronary and peripheral arteries to reopen narrowed vessel sections. This endovascular intervention remains suboptimal, as the success rate is limited by restenosis. This renarrowing of a stented vessel is related to the arterial injury caused by stent-artery and balloon-artery interactions, and a local subsequent inflammatory process. Therefore, efforts to optimize the stent deployment remain very meaningful. Several authors have studied with finite element modeling the mechanical behavior of balloon-expandable stents, but none of the proposed models incorporates the folding pattern of the balloon. We developed a numerical model in which the CYPHER stent is combined with a realistic trifolded balloon. In this paper, the impact of several parameters such as balloon length, folding pattern, and relative position of the stent with respect to the balloon catheter on the free stent expansion has been investigated. Quantitative validation of the modeling strategy shows excellent agreement with data provided by the manufacturer and, therefore, the model serves as a solid basis for further investigations. The parametric analyses showed that both the balloon length and the folding pattern have a considerable influence on the uniformity and symmetry of the transient stent expansion. Consequently, this approach can be used to select the most appropriate balloon length and folding pattern for a particular stent design in order to optimize the stent deployment. Furthermore, it was demonstrated that small positioning inaccuracies may change the expansion behavior of a stent. Therefore, the placement of the stent on the balloon catheter should be accurately carried out, again in order to decrease the endothelial damage.

Finite element analysis and stent design: Reduction of dogboning.

In Western countries, cardiovascular disease is the most common cause of death, often related to atherosclerosis. This paper offers a brief introduction into some aspects of this disease and its treatment, where the use of stents is gaining increasing importance. Stents are supporting - mostly metal - tubular mesh structures which are opened in an obstructed artery in order to reopen it, and to offer radial strength to prevent elastic recoil of the dilated vessel. In addition to a variety of experimental tests to study the behavior of (new) stent designs, advanced numerical models (e.g. Finite Element Models) may offer interesting insights in the mechanical behavior of stents and will undoubtedly influence the design of future generation stents. A brief literature review on numerical studies dealing with the mechanical behavior of stents is presented. Subsequently, the finite element method is exploited to investigate and compare different designs of a "first generation" Palmaz Schatz stent in order to reduce the dogboning (i.e. ends of stent open first during expansion) to a minimum. Our computational models (Abaqus ) are described in terms of geometry, constitutive material models, numerical aspects and output quantities. Altering the original symmetric stent design to asymmetric designs decreased the dogboning from 27.24% to less than 10% for the vast majority of the studied asymmetric designs. For one particular configuration, the dogboning effect vanished completely. For this reason, taking asymmetry into account in the design of stents seems very promising, at least from the perspective of dogboning. However, as the dogboning only takes into account the radii (R) at the central and distal part of the stent, nothing can be concluded concerning the uniformity of the complete stent expansion. The mean value (Rm) and the root mean square (R(RMS)) of radii (differences) of the stent at the end of the loading phase (P = 0.7 N/mm2) are much better parameters to give a clear indication of the uniformity of the expanded stent's shape. Although the model is suitable to study basic aspects of stent deployment, further research is necessary, especially accounting for newer generation stent geometries and more realistic balloon-stent interaction.