A computational approach for the lifetime prediction of cardiovascular balloon-expandable stents
Introduction
Nowadays, cardiovascular diseases represent a global health care problem, since are the leading cause of death and illness accounting for of the deaths worldwide annually [1]. The advantage of stenting intervention with respect to the classical bypass surgery has been largely demonstrated in the treatment of heart disease [2] and now stents are involved in more than of coronary intervention procedures [1]. Stents are small tube-like devices used to restore patency of blood vessels having the lumen area reduced due to atherosclerosis. The stent acts as a mechanical scaffold for the vessel and its implant is performed by a minimally invasive procedure inserting a catheter through a small incision in the femoral artery.
Several materials, as the 316L austenitic stainless steel, the cobalt–chromium alloy or the NiTi-based shape memory alloy, are well-known in the biomedical field [3], [4], [5], [6] and one of their biomedical applications regards coronary stents [7]. As an example, the 316L stainless steel is used for the PalmazSchatz (Cordis) or the Medinol/Boston Scientific NIR™ balloon-expandable stents, thanks to its good properties in the fully annealed condition, e.g., well adapted mechanical characteristics (i.e., great ductility, high tensile strength, and a raised elastic limit), biocompatibility, resistance to corrosion, as well as fatigue performances [5].
Although stent design is a relatively mature topic, fatigue failure has recently appeared as one of the main cause of stent drawback [1], [8], [9], [10], [11], [12]. In particular, stents should survive at least for 10 years without exhibiting failure, that correspond to systolic–diastolic pulsatile cycles [13]. Therefore, stent design lies within the high- and very high-cycle fatigue regime (denoted, respectively, as HCF and VHCF) [14]. Several parameters significantly affect the fatigue performance of stents and increase the complexity of the design process; among the others, we can cite the loading during the implant, the small-size of the device, manufacturing-induced features, complex material behavior, corrosive environment [1]. Stents have to withstand both their initial deployment within the artery, which involves large amounts of plastic strains, and the fatigue loading due to blood pressure, which can cause the failure of the stent even at stress levels lower than the tensile or yield strength of the material [15], [16]. The majority of stents are manufactured by drawing a very fine tube, followed by laser cutting, leading to a final structure composed of struts connected by hinges [17]. Current stent strut cross sections are approximately 50–150 m, which allow only for a few grains across (approximately 10–20), thus affecting fatigue resistance and making the application of standard bulk models inadequate [17], [18], [19], [20]. The manufacturing process induces also the presence of stress concentration features, in particular, of small radius connections affecting fatigue behavior [21].
Therefore, a deep understanding of the fatigue and fracture failure in microsize components is necessary, in order to increase stent durability. Despite the importance of such applications and the associated difficulty, most of the research work on stent failure is based on medical statistics and/or clinical studies [1] and the literature disposes of few experimental data sets; see, e.g., [21], [22], [23], [24], [25], [26], [27] for 316L stainless steel stents. Most of them refer to cycles, equivalent to an implant period of about –4 months due to systolic–diastolic pulsatile loading.
Several computational tools, as finite element (FE) analysis, have already proved their usefulness to bridge the gap between the existing experiments and the design of such components, e.g., [28], [29], [30], [31], [32], [33], [34]. Among the others, some works deal with the numerical fatigue-life assessment of metallic stents, e.g., [17], [35], [36], [37], [38], [39], [40], [41]; most of them predict lifetimes lower than the required cycles and do not focus on data from real precise experiments on stent components. Sweeney et al. [39] presented a refined FE stent fatigue methodology based on crystal plasticity theory for the prediction of crack initiation, using microstructural fatigue indicator parameters. The proposed simple criteria are however calibrated on experiments related to macroscopic specimens [42]. On the contrary, Sweeney et al. [43] presented the development of a micromechanical framework for stent fatigue design, based on a suitable experimental characterization of a biomedical CoCr alloy. Additionally, many computational modeling works on stents directly apply bulk material properties to the structure [30], [36], [37], [40], [44], [35], [41]. Among these works, dos Santos et al. [36] predicted lifetimes lower than cycles for balloon-expandable stainless steel stents by incorporating a two-scale plasticity-damage model [45]. Such an approach, however, requires the calibration of several parameters, which has been done by referring to experimental data on macroscopic specimens. Barrera et al. [37] applied the multiaxial HCF criterion by Dang Van [46] to cardiovascular stents. Such a criterion, based on a micro–macro approach, successfully meets the need of microstructural representation in crack initiation predictions. However, the work by Barrera et al. [37] does not investigate the experimental data used for the criterion calibration [22], [21] and constructs the Dang Van line, referring to only cycles, by assuming the same fatigue limits in bending and torsion. The work by Azaouzi et al. [40] predicts the fatigue-life of 316L stainless steel stents through the well-known Goodman diagram and the theory of critical distance based on the results presented in [22], [21]. The Goodman parameters are again calibrated on experimental data [22], [21] without a detailed investigation.
The aim of the present paper is the introduction of a global computational design method for the lifetime assessment of balloon-expandable stents. The proposed prediction methodology is composed of a three-dimensional mechanical analysis, followed by a fatigue analysis. The mechanical analysis calculates the elastic or plastic shakedown mechanical state of the stent under investigation, by assuming the structure, loading, and material behavior known. The fatigue analysis is then based on the investigation of the stabilized state obtained within the mechanical analysis and evaluates the number of cycles to failure. Failure is here considered as a crack initiation, since the crack propagation phase is small with respect to the global time and the involved material thicknesses are rather small.
The stent geometry discussed here is a classical coronary stent design, i.e., the Medinol/Boston Scientific NIR™ stent, made of 316L stainless steel. Due to practical difficulties in testing m-size specimens up to cycles, we consider fatigue criteria calibrated on experimental data taken from the literature and related to smooth and notched 316L steel m-size components used during industrial stent manufacturing [21], [22], [27], starting from the work by Auricchio et al. [47]. Particularly, in case of elastic shakedown, we focus on the multiaxial HCF criterion by Dang Van [46] and, in case of plastic shakedown, on the dissipated energy per cycle criterion [48], [49]. Since the Dang Van criterion in [47] refers to a fatigue life of cycles, the present work proposes also an extension of such a calibration in order to investigate the behavior of the stent at the required cycles [13].
The results from the mechanical and fatigue analysis are discussed with the purpose of providing a general discussion on the increased or decreased stent lifetime due to several conditions affecting the cardiovascular implant. Since coronary stent fatigue lifetime is affected by other cyclic movements in addition to the diastolic–systolic physiological loading (e.g., vessel deformations during the cardiac contraction [50], [51]), we also explore the effect of bending cyclic loading associated with vessel curvature. The complexity of the analysis, modeling assumptions, as well as the difficulties that can be encountered are highlighted.
The proposed approach is general and applicable to several metallic materials as 316L stainless steel or cobalt–chromium alloys, or to other micro-scale components.
The present paper is organized as follows. Section 2 presents the mechanical analysis of the stent. Then, Section 3 describes the results of the fatigue analysis. Conclusions and perspectives are finally given in Section 4.
Section snippets
Mechanical analysis
In this Section we illustrate the mechanical analysis performed with the FE method to compute the loading and the stabilized state of the stent during and after the implant.
Fatigue analysis
This Section first presents the applied fatigue criteria; then, it presents the results of the fatigue analysis.
Conclusions and perspectives
This paper has proposed a numerical design method for the lifetime prediction of stents. The methodology has been illustrated through the analysis of an idealized generic balloon-expandable stent; as such, the analysis should be understood as a general discussion and not as a specific guideline on stent geometry or clinical scenarios.
The effects of cycling loading including blood pressure and bending due to vessel curvature have been explored; the most critical elements have been identified in
Acknowledgements
This work is partially funded by: the Cariplo Foundation through the Project No. 2009.2822; ERC Starting Grant through the Project ISOBIO: Isogeometric Methods for Biomechanics (No. 259229); Ministero dell’Istruzione, dell’Università e della Ricerca through the Project No. 2010BFXRHS; the Italian-French University (UIF–UIF) through the ’Bando Galileo 2013–2014’ Grant No. 148-30174TJ.
Moreover, Regione Lombardia and CINECA Consortium through a LISA Initiative (Laboratory for Interdisciplinary
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