Simulazione FEM delle operazioni di folding e gonfiaggio di palloncini per angioplastica per il trattamento di occlusioni croniche totali in arterie periferiche

Il presente lavoro si è sviluppato nell’ambito della progettazione e ottimizzazione di un catetere endovascolare, utile al trattamento di occlusioni croniche totali (CTO) nell'arteria superficiale femorale tramite angioplastica subintimale. Per questo si è proceduto allo sviluppo di modelli agli elementi finiti per simulare la fase di gonfiaggio del palloncino, tenendo conto della complessa configurazione iniziale del pallone ripiegato all’interno del catetere sia del posizionamento in sede anatomica. In una prima fase è stato quindi modellato il processo di folding, attraverso il quale un pallone da angioplastica viene ripiegato cosicché possa essere contenuto in un diametro inscritto molto minore rispetto alla sua dimensione originale. In una seconda fase i modelli di palloni così ripiegati sono stati utilizzati come configurazione iniziale per simulazioni di gonfiaggio sia libero sia all'interno di un modello anatomico in presenza o meno di un’occlusione cronica totale (CTO)

Modelling of Atherosclerotic Plaque for Use in a Computational Test-Bed for Stent Angioplasty

A thorough understanding of the diseased tissue state is necessary for the successful treatment of a blocked arterial vessel using stent angioplasty. The constitutive representation of atherosclerotic tissue is of great interest to researchers and engineers using computational models to analyse stents, as it is this in silico environment that allows extensive exploration of tissue response to device implantation. This paper presents an in silico evaluation of the effects of variation of atherosclerotic tissue constitutive representation on tissue mechanical response during stent implantation. The motivation behind this work is to investigate the level of detail that is required when modelling atherosclerotic tissue in a stenting simulation, and to give recommendations to the FDA for their guideline document on coronary stent evaluation, and specifically the current requirements for computational stress analyses. This paper explores the effects of variation of the material model for the atherosclerotic tissue matrix, the effects of inclusion of calcifications and a lipid pool, and finally the effects of inclusion of the Mullins effect in the atherosclerotic tissue matrix, on tissue response in stenting simulations. Results indicate that the inclusion of the Mullins effect in a direct stenting simulation does not have a significant effect on the deformed shape of the tissue or the stress state of the tissue. The inclusion of a lipid pool induces a local redistribution of lesion deformation for a soft surrounding matrix and the inclusion of a small volume of calcifications dramatically alters the local results for a soft surrounding matrix. One of the key findings from this work is that the underlying constitutive model (elasticity model) used for the atherosclerotic tissue is the dominant feature of the tissue representation in predicting tissue response in a stenting simulation

An investigation into stent expansion using numerical and experimental techniques

Extensive finite element analyses have been carried out by researchers to investigate the difference in the mechanical loading induced in vessels stented with various different stent designs and the influence of this loading on restenosis outcome. This study investigates the experimental validation of these numerical stent expansions using compliant mock arteries. The development of this in-vitro validation test has the prospect of providing a fully validated preclinical testing tool which can be used to optimise stent designs. Mock arteries were developed as straight cylindrical vessels using a specially designed rig such that they had an inner lumen diameter of 3 mm and a thickness of 0.5 mm, thus representing a typical porcine coronary artery geometry. These mock arteries were manufactured from compliant Sylgard elastomer 184 (Dow Corning). This material was chosen mainly due to its inherent variable elastic properties which are determined by its curing process and ratio of elastomer to curing agent. Extensive testing was carried out on samples of porcine coronary arteries and differing ratios of Sylgard to identify a close match in mechanical properties to those of porcine coronary arteries. Driver stents (Medtronic AVE) were expanded both freely and inside these mock arteries and the subsequent deformation recorded using a video extensometer. The Driver stent was numerically modelled with a strut thickness of 0.09 mm and an overall length of 9 mm such that each modular element had a length of 1 mm. The material for the stent was described using an elasto-plastic material model whereby the linear elasticity was defined using values for MP35N cobalt chromium alloy: Young's Modulus of 232 GPa, Poisson's Ratio of 0.26. A piecewise linear function was used to represent the non-linear plasticity of the material through a von Mises plasticity model with isotropic hardening. Due to symmetry, only one-quarter of the geometry was modelled in the circumferential direction. The mock artery was represented as a hyperelastic material, the constitutive equation determined by fitting to the uniaxial tension tests of Sylgard elastomeric material. A uniform pressure was applied to the internal surface of each stent to represent a balloon expansion. This study identified a suitable material for use as a blood vessel substitute such that experimental stent expansions could be carried out within the mock artery and the results used to evaluate the accuracy of the numerical methods. Finite element analyses were carried out to examine two separate methods for stent expansion such that the most accurate and effective method could be determined. Results show that the numerical methods used in simulating the free expansion, and expansion inside a mock artery of the Driver stent, can accurately describe the in-vitro stent expansion. Both experimental and numerical models were found to achieve similar amounts of foreshortening, longitudinal recoil and radial recoil