A finite element strategy to investigate the free expansion behaviour of a biodegradable polymeric stent
Introduction
Percutaneous coronary intervention (PCI) with stenting has since its introduction in 1987 grown to become the golden standard to treat coronary artery stenosis, a disease caused by atherosclerosis, which is the local disposition of greasy plaques on the inner side of the arterial wall. PCI is a minimal invasive procedure and replaces the open-chest bypass surgeries, which has led to shorter revalidation times and decreased mortality rates. Conventional durable stents, including both bare metal stents and drug eluting stents (DES), are made of metal alloys such as stainless steel, cobalt–chromium or platinum–chromium and remain permanently inside the human body after placement. However, the need for mechanical support for the healing artery is temporary and beyond the first few months there are potential disadvantages of a permanent metallic prosthesis: excessive neo-intimal tissue growth causes in-stent restenosis and prolonged exposure of the metallic stent surface to the blood stream increases the risk for late stent thrombosis (Garg and Serruys, 2010). Despite the fact that repeated percutaneous and surgical revascularization procedures due to restenosis could be reduced to 50–70% by the use of DES (Moses et al., 2003, Stone et al., 2004), biodegradable stents, which temporarily support the blood vessel and afterwards fully disintegrate, are gaining much interest. Possible advantages of biodegradable stents include abolished late stent thrombosis, facilitation of repeat treatments to the same site (surgical or percutaneous), restoration of vasomotion and prevention from restenosis (Onuma and Serruys, 2011). Bioabsorbable stents also have a potentially paediatric role because they allow vessel growth and do not need surgical removal (Ormiston and Serruys, 2009). However, developing biodegradable stents comprises challenges in making a stent that has sufficient radial strength for an appropriate duration, that does not have unduly thick struts, that can be a drug delivery vehicle and where degradation does not generate an unacceptable inflammatory response (Ielasi et al., 2011).
Two classes of biomaterials are currently being used in biodegradable stent technology: biodegradable polymers and biocorrodible metal alloys. Polymers can be tailored to have a well-defined degradation pattern but have relatively poor mechanical properties. In contrast, biocorrodible metals such as magnesium alloys have excellent mechanical characteristics but display more complex and less predictive degradation behaviour.
Several biodegradable stents are under development and are subject of clinical trials. The Bioresorbable Vascular Scaffold (BVS) of Abbott Vascular is a polymeric bioresorbable stent made of poly-l-lactic acid (PLLA) and is coated with an everolimus-eluting poly-d,l-lactic acid (PDLLA) layer (Serruys et al.,, Dudek et al., 2012). The stent has extensively been tested in the ABSORB clinical trials and has since 2012 been approved with the European CE marking (Abbott, 2011). The main stent representing the class of biocorrodible metallic stents is the AMS-stent of Biotronik that is followed up in the PROGRESS-trials (Erbel et al., 2007, Haude et al., 2012).
Despite the promising results of these clinical trials, the design of novel biodegradable stents remains a considerable challenge because of the lack of precise engineering modelling tools. Finite element analysis (FEA) has proven to be a valid and efficient method to virtually investigate and optimize the mechanical behaviour of minimal invasive devices such as stents (Mortier et al., 2011, Migliavacca et al., 2002, Azaouzi et al., 2013, Hsiao et al., 2012). It can also be combined with patient specific images to plan a surgical procedure or to classify specific devices for different kinds of atherosclerotic lesions (Auricchio et al., 2013, Iannaccone et al., 2014, Morlacchi et al., 2013). In combination with computational fluid mechanics, the finite element method is able to reveal insight into the aspects of drug elution and coating biodegradation (D׳Angelo et al., 2011, Bozsak et al., 2014, Lee et al., 2014).
Analysing the mechanical behaviour of biodegradable stents via finite element simulations poses new challenges due to the complexity of the stent materials which display transitional mechanics both in the long term range (polymer degradation and metal corrosion) and the short term range (viscoplasticity in case of polymeric stents). A limited number of FEA-studies on biodegradable stents has been performed. Soares et al. (2008) proposed a continuum degradation model which includes the effect of deformation upon the degradation rate of polylactic acid and applied it to biodegradable stents. Bokov (2011) investigated the interaction of a biodegradable stent with the vascular wall using FEA. Wu et al. (2011) developed a continuum-mechanics based corrosion model for magnesium stents and optimized stent geometry for a better corrosion resistance. A similar mechanical model has been proposed by Grogan et al. for corrosion of magnesium stents. Their work includes an arbitrary Lagrangian–Eulerian approach to model stent erosion (Grogan et al., 2011, Grogan et al., 2013).
In this paper, we will consider the commercially available Absorb BVS (Abbott Vascular, Santa Clara, CA, USA) and as a case study assess its mechanical performance via FEA. In particular, we want to investigate the influence of the deployment rate during stent balloon expansion. Because of the high degree of viscoplasticity of the polymeric stent material, the expansion rate might have an important influence on the mechanical performance and integrity of the deployed stent. To be able to do so, a sufficiently representative material model was selected and its parameters were fitted to the available experimental data. The commercial finite element solver Abaqus/standard (Dassault Systems Simulia, Providence, RI, USA) was used to solve the quasi-static stent deployment simulations. To overcome part of the contact problems which are inherent to this type of simulation, a simplified balloon model was used. The level of internal stresses and the degree of post-dilational recoil will be compared for a direct and a stepwise deployment procedure.
Section snippets
Methods
This section subsequently describes the use of a cylindrical anisotropic balloon model, the generation of a finite element mesh for the stent and the implementation of a hyperelastic-viscoplastic material for PLLA to be combined within an implicit finite element framework to model the balloon deployment of the bioresorbable BVS stent.
Results and discussion
This section discusses the fitting of the material models for the balloon and PLLA to experimental data and the application of the developed finite element framework to the balloon inflation to the Absorb BVS.
Conclusions
This study shows that is possible to use an implicit finite-element solver to model stent balloon expansion. To our knowledge, until now, only explicit solvers have been for this purpose. The use of an implicit solver benefits the accuracy and the reliability of the outputted stresses. It avoids dynamic inertia effects and therefore simplifies the interpretation of the simulation outcomes. We were able to use an implicit solver through reduction of the contact problem, by using a cylindrical
Conflicts of interest statement
Matthieu De Beule and Benedict Verhegghe are shareholders of FEops, an engineering consultancy spin-off from Ghent University, and have served as consultants for several medical device companies.
Acknowledgements
We would like to thank P. Smits (Maasstad Hospital, Rotterdam, Netherlands) and FEops (Ghent, Belgium) for the supply of the BVS samples, the Centre for X-ray Tomography of the Ghent University (UGCT) for the micro-CT scanning of the BVS stents and the company Admedes (Pforzheim, Germany) for the tensile test experiments.
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