An investigation into stent expansion using numerical and experimental techniques
Toner, Deborah (2009) An investigation into stent expansion using numerical and experimental techniques. Master of Engineering thesis, Dublin City University.
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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.
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