Mechanical behaviour modelling of balloon-expandable stents

Endoprostheses are small struts placed by intravascular way to restore the vascular lumen and flow conditions. The purpose of this work is to provide models for evaluation and characterisation of some mechanical properties of a balloon-expandable stent by using the finite element method. Here we present the results for a metallic tubular peripheral prosthesis: the P308 Palmaz stent. We focus on the mechanisms linked to the structure expansion and its long-term behaviour. Several models are constructed in order to determine the stent shape after dilation and to assess the stress and strain fields in its wall due to this transformation. They inform us about the shortening percentage on expansion, degrees of radial and longitudinal recoil, and weaknesses of the structure. Various methods, differing in their levels of complexity, are then attempted to exhibit the predominant factors responsible for the crushing of a stent under external pressure. Moreover, the sensitivity of this critical pressure to geometric imperfections is studied. Lastly, since this kind of material is implanted for a lifetime, we test the stent with regard to fatigue life. Beyond safety considerations, this type of characterisation provides mechanical properties that are often difficult to obtain by experiments. If it was available for various stents, such information could be used to choose the appropriate prosthesis for specific applications. Moreover, confronted with observations from practitioners, they might lead to a better understanding of the failure or success of a particular design and to work on the product optimisation.