Nickel-Titanium peripheral stents: can fracture mechanics shed light on their fatigue failure?

The major concern about Nickel-Titanium (Ni-Ti) stents, which are the gold standard in the treatment of occlusive peripheral disease, is fatigue and the consequent fracture in vivo. Indeed, their failure might be responsible for severe drawbacks, among which is the re-occlusion of the treated artery. Although many phenomenological approaches have been proposed to study this topic, the current literature lacks extensive knowledge on the Ni-Ti local damage mechanisms produced by the cyclic loads that promote crack nucleation and lead to the failure of thin struts, such as those of stents. Moreover, due to the super-elastic property of the alloy, the standard approach for interpreting the fracture of metals might be not accurate for this case. This work aims at increasing awareness of fatigue failure in superelastic Ni-Ti thin struts, such as those of stents. To do so, multi-wire specimens, sharing the same dimensions and thermo-mechanical treatment of the stent struts, were fatigue tested under different strain levels and the number of cycles to failure was recorded for each sample. Numerical simulations corroborated the experimental results to gain information on the local stress and strain fields during the fatigue cycles. A fracture mechanics-based fatigue model adopting the cyclic J-integral was here proposed, giving promising results for the interpretation of such failures

A Systematic Review on Cardiovascular Stent and Stenting Failure: Coherent Taxonomy, Performance Measures, Motivations, Open Challenges and Recommendations

Cardiovascular stenting is a mature topic but it is still being developed in the research community because of its importance. To provide worthy information about cardiovascular stenting environments and to give support to the researchers, attention must be given to understand the obtainable choices and gaps in this research field. This work aims to examine and examine the literature of each work related to the placement of cardiovascular stents, the failure of the stents and the models of stent designs to provide a good understanding through the investigation of articles published in various contextual aspects, such as motivations, open-challenges and recommendations to improve the field of stent placement. A systematic review is carried-out to map and examine the articles related to cardiovascular stents, the failure of the stents and the models of stent designs through a coherent-taxonomy used in three well-known scientific databases: ScienceDirect, IEEE Explore, and Web of Science. These databases involve literature that highlight arterial stenting. Based-on our inclusion and exception, a total of 90 articles composed the final set that offer various classes and sub-classes. The first class includes the development studies with (42/90) of experimental, computational and combined experimental and computational studies related to stent models performance and stent failure, the second class discussed studies that have been performed on stent design with (32/90), the third class is focused on the framework studies with (10/90), and the fourth class includes problems of stenting long-term with (6/90). The performance of stent designs, which is a research area that requires periodic controls, tools and procedures that could provide a stent design with good mechanical performance, reduce restenosis in the stent and increase fatigue resistance and durability. There have been numerous studies on stent performance that could promise good results in this field. The fields of research in stent designs vary, but all fields are fundamental equally. The expectation of this work could help to emphasize present research chances and, therefore, expand and make further research fields

Nitinol Stent Oversizing in Patients Undergoing Popliteal Artery Revascularization: A Finite Element Study

Nitinol stent oversizing is frequently performed in peripheral arteries to ensure a desirable lumen gain. However, the clinical effect of mis-sizing remains controversial. The goal of this study was to provide a better understanding of the structural and hemodynamic effects of Nitinol stent oversizing. Five patient-specific numerical models of non-calcified popliteal arteries were developed to simulate the deployment of Nitinol stents with oversizing ratios ranging from 1.1 to 1.8. In addition to arterial biomechanics, computational fluid dynamics methods were adopted to simulate the physiological blood flow inside the stented arteries. Results showed that stent oversizing led to a limited increase in the acute lumen gain, albeit at the cost of a significant increase in arterial wall stresses. Furthermore, localized areas affected by low Wall Shear Stress increased with higher oversizing ratios. Stents were also negatively impacted by the procedure as their fatigue safety factors gradually decreased with oversizing. These adverse effects to both the artery walls and stents may create circumstances for restenosis. Although the ideal oversizing ratio is stent-specific, this study showed that Nitinol stent oversizing has a very small impact on the immediate lumen gain, which contradicts the clinical motivations of the procedure.Swiss National Science FoundationResearch Council of the Kantonsspital AarauSwiss Heart FoundationGotthard Schettler Foundatio

Comparative Computational Study of Mechanical Behavior in Self-Expanding Femoropopliteal Stents

The use of the stent to treat peripheral artery disease (PAD) is increased and the proportion of failures also increases. The femoropopliteal artery (FPA) experiences a high deformation ratio compared to the cardiovascular artery due to limp flexion and daily activities that could lead to stent failure, as well as increasing the number of observed mortality and morbidity. In the present work, two of the common PAD stent design models represented as STENT I and STENT II were analyzed by using of finite element method (FEM) to simulate the most mechanical loading modes that could occur in FPA, such as axial tension and compression, torsion, three-point bending and radial compression to give a good understanding of deformation that affected stent inside the in-vivo. The gradual force load was used to simulate all modes, the force values are 0.25 N, 0.5 N, 1.5 N, 2.5 N, 3.5 N and 5.5 N until the stent models obtain the yield-point. The comparison of stent models (STENT I, STENT II) was performed in terms of graphs of total deformation, force-stress and stress-strain for all test modes. The similarity ratio of the total deformation in axial tension and the compression mode for STENT I and STENT II was 17% and that may indicate that STENT I obtained a high deformation value instead of STENT II, while, the torsion similarity ratio was 86% which could show a good agreement in this mode, as well as the similarity ratio, was 78% of the total three-point bending deformation and the value of the similarity ratio in the radial compression mode was 23%. Still unclear what is the clinical mode of mechanical deformation that is more important than others with changing the length of the lesion and stent diameter, and the fatigue life test provides a better understanding of the mechanical tests that must be sought

A computational study of mechanical behavior of bioresorbable polymeric stents

Coronary artery disease (CAD) is a leading killer of human life worldwide. Clinically, stent implantation with percutaneous coronary intervention has become a standard and effective method to treat coronary artery disease. A large amount of research work has been carried out to investigate the mechanical, degradation and fatigue behavior for permanent metallic stents, but not for bioresorbable polymeric stents. Such research gaps are urgently required to be addressed, as bioresorbable polymeric stents are regarded as the next generation medical devices, even replacing metallic stents. In this thesis, pioneering efforts have been made to systematically study the mechanical behavior of polymeric stents using finite element method, which are novel and have not been reported in literature yet. [Continues.

Stent’s Manufacturing Field: Past, Present, and Future Prospects

From the introduction of stents, nobody was able to predict the advances that will occur in stent technology over the upcoming decades. Since their appearances, it became evident that this device had significant limitations, such as vessel occlusion and/or restenosis. Despite that, this medical device is the best clinical solution for cardiovascular vessel occlusions. Stents require a deep analysis, in terms of thrombogenicity, manufacturing process, geometrical aspects, and mechanical performance, among many other characteristics. The surface quality obtained in their manufacture process is crucial to blood compatibility, prevents the activation process of thrombosis, and improves the healing efficiency. The forecast stent market makes necessary continuous studies on this field, which help to solve the medical and engineering problems of this device, which are in constant development. Stents have been the center of many research lines over the last decades. The present chapter aims to summarize the state of the art of this medical device in the last years in the fields of design, manufacturing, and materials

A discussion about multi-axial fatigue criteria for NiTinol cardiovascular devices

Nickel-Titanium (NiTinol) alloys exploit a typical super-elastic behavior which makes them suitable for many biomedical applications, among which peripheral stenting, requiring the device being subjected to the high mobility of the lower limbs. Unfortunately, this complex environment can lead to the device fatigue fracture with likely other more severe complications, e.g. restenosis. Standards require to experimentally verify stent fatigue life behavior, without giving indications on how to select the loads to be applied for resembling most critical in-vivo conditions. Moreover, different multi-axial fatigue criteria have been originally developed for standard metals to predict the behavior under cyclic loads, but none of them is specifically formulated for NiTinol. This paper presents a numerical study having two aims: i) understanding how non-proportional loading conditions due to combination of axial compression, bending and torsion induced at each patient gait on the femoro-popliteal artery affects the implanted stent stress/strain distribution; ii) understanding how stent fatigue life prediction may be affected by the choice of the fatigue criteria. Accordingly, two different peripheral stent geometries, resembling commercial ones, were analysed under different sets of loading conditions. The cyclic deformations induced over the device structure by macroscopic loads are interpreted through four different fatigue approaches recently used in Nitinol fatigue analyses: Von Mises, Fatemi-Socie, Brown-Miller and Smith-Watson-Topper. The comparison between the outputs highlights that they are strongly influenced by the loading path, recognizing the major role in fatigue due to the combined torsional and bending actions. On the other hand, the choice of the fatigue criterion impacts on the fatigue life prediction

Computational methods in cardiovascular mechanics

The introduction of computational models in cardiovascular sciences has been progressively bringing new and unique tools for the investigation of the physiopathology. Together with the dramatic improvement of imaging and measuring devices on one side, and of computational architectures on the other one, mathematical and numerical models have provided a new, clearly noninvasive, approach for understanding not only basic mechanisms but also patient-specific conditions, and for supporting the design and the development of new therapeutic options. The terminology in silico is, nowadays, commonly accepted for indicating this new source of knowledge added to traditional in vitro and in vivo investigations. The advantages of in silico methodologies are basically the low cost in terms of infrastructures and facilities, the reduced invasiveness and, in general, the intrinsic predictive capabilities based on the use of mathematical models. The disadvantages are generally identified in the distance between the real cases and their virtual counterpart required by the conceptual modeling that can be detrimental for the reliability of numerical simulations.Comment: 54 pages, Book Chapte

Design and Fatigue Study of Intravascular Coronary Stent using Finite Element Analysis

A thesis presented to the faculty of the College of Science & Technology at Morehead State University in partial fulfillment of the requirements for the Degree of Master of Science by Jared May on March 15, 2012

On Modeling Assumptions in Finite Element Analysis of Stents

Finite Element Analysis (FEA) of Nitinol medical devices has become prevalent in the industry. The analysis methods have evolved in time with the knowledge about the material, the manufacturing processes, the testing or in vivo loading conditions, and the FEA technologies and computing power themselves. As a result, some common practices have developed. This paper presents a study in which some commonly made assumptions in FEA of Nitinol devices were challenged and their effect was ascertained. The base model pertains to the simulation of the fabrication of a diamond shape stent specimen, followed by cyclic loading. This specimen is being used by a consortium of several stent manufacturers dedicated to the development of fatigue laws suitable for life prediction of Nitinol devices. The FEA models represent the geometry of the specimens built, for which geometrical tolerances were measured. These models use converged meshes, and all simulations were run in the FEA code Abaqus making use of its Nitinol material models. Uniaxial material properties were measured in dogbone specimens subjected to the same fabrication process as the diamond specimens. By convention, the study looked at computed geometry versus measured geometry and at the maximum principal strain amplitudes during cyclic loading. The first aspect studied was the effect of simulating a single expansion to the final diameter compared to a sequence of three partial expansions each followed by shape setting. The second aspect was to ascertain whether it was feasible to conduct the full analysis with a model based on the electropolished dimensions or should an electropolish layer be removed only at the end of fabrication, similar to the manufacturing process. Finally, the effect of dimensional tolerances was studied. For this particular geometry and loading, modeling of a single expansion made no discernable difference. The fabrication tolerances were so tight that the effect on the computed fatigue drivers was also very small. The timing of the removal of the electropolished layer showed an effect on the results. This may have been so, because the specimen studied is not completely periodic in the circumferential direction