Numerical investigation of the intravascular coronary stent flexibility

A Scaling Parameter for Predicting Pressure Wave Reflection in Stented Arteries

A one-dimensional finite-difference model was developed to predict pressure wave reflections in stented arteries, and a parametric study of variations in stent and vessel properties was performed, including stent stiffness, length, and compliance transition region, as well as vessel radius and wall thickness. The model was solved using a combination of weighted essentially nonoscillatory and Runge–Kutta methods. Over 100 cases were tested and the magnitudes of the predicted waves were less than 0.5 mm Hg for all cases, less than 1% of the normal pulse pressure of 40 mm Hg. It was also shown that reasonable variations in these parameters could induce changes in reflection magnitude of up to ±50%. The relationship between each of these properties and the resulting wave reflection could be described in a simple manner, and the effect of all of them together could, in fact, be encompassed by a single nondimensional parameter titled “stent authority.” It is believed that stent authority is a novel way of relating the energy imposed upon the arterial wall by the stent to the fraction of the incident pressure energy that is reflected from the stented region. Based on these results, it is believed that stent design can have a significant effect on pressure wave reflections; however, it was concluded that their small magnitudes make clinical relevance of these waves unlikely, regardless of design.

Expansion and drug elution model of a coronary stent

The present study illustrates a possible methodology to investigate drug elution from an expanded coronary stent. Models based on finite element method have been built including the presence of the atherosclerotic plaque, the artery and the coronary stent. These models take into account the mechanical effects of the stent expansion as well as the effect of drug transport from the expanded stent into the arterial wall. Results allow to quantify the stress field in the vascular wall, the tissue prolapse within the stent struts, as well as the drug concentration at any location and time inside the arterial wall, together with several related quantities as the drug dose and the drug residence times.

Simulation of stent deployment in a realistic human coronary artery

Background

The process of restenosis after a stenting procedure is related to local biomechanical environment. Arterial wall stresses caused by the interaction of the stent with the vascular wall and possibly stress induced stent strut fracture are two important parameters. The knowledge of these parameters after stent deployment in a patient derived 3D reconstruction of a diseased coronary artery might give insights in the understanding of the process of restenosis.

Methods

3D reconstruction of a mildly stenosed coronary artery was carried out based on a combination of biplane angiography and intravascular ultrasound. Finite element method computations were performed to simulate the deployment of a stent inside the reconstructed coronary artery model at inflation pressure of 1.0 MPa. Strut thickness of the stent was varied to investigate stresses in the stent and the vessel wall.

Results

Deformed configurations, pressure-lumen area relationship and stress distribution in the arterial wall and stent struts were studied. The simulations show how the stent pushes the arterial wall towards the outside allowing the expansion of the occluded artery. Higher stresses in the arterial wall are present behind the stent struts and in regions where the arterial wall was thin. Values of 200 MPa for the peak stresses in the stent strut were detected near the connecting parts between the stent struts, and they were only just below the fatigue stress. Decreasing strut thickness might reduce arterial damage without increasing stresses in the struts significantly.

Conclusion

The method presented in this paper can be used to predict stresses in the stent struts and the vessel wall, and thus evaluate whether a specific stent design is optimal for a specific patient.

Topology optimization of a novel stent platform with drug reservoirs

The new generation of drug-eluting stents (DES) is required to control drug release kinetics. A novel DES (the Conor stent) with drug reservoirs on struts has been engineered. Topology optimization of one Conor stent strut was based on the commercial finite element analysis code OptiStruct, with the aim of increasing the strut stiffness while retaining its drug holding capacity. Results show that the element density distribution of the strut model was optimized with manufacturing constraints of extrusion constraint and minimum member size control. The optimal result was directly transformed to a clear, manufacturable design concept using the OptiStruct utility OSSmooth. The final manufacturing design increased the strut stiffness and yielded better stress distribution, as compared to the original strut design under the same loading. Topology optimization may help designers devise novel stent platforms for future DES with drug reservoirs and adequate scaffolding.

Modeling of stents exhibiting negative Poisson's ratio effect

Intravascular stents are metallic scaffolding structures deployed in the stenotic arteries to restore the lumen for the blood flow to the down stream tissues. Most stents are balloon expandable and are deployed from its crimped state through a balloon catheter. The efficacy of the stenting procedure mainly depends on the way the stent is deployed. Both numerical and experimental evaluations show that almost all the present day stents undergo the most undesirable effects namely: (i) longitudinal foreshortening: the axial contraction in the length, and (ii) dogboning: flaring of the distal edges, during the radial expansion of the stents. Due to the foreshortening effect, clinicians are forced to select stents longer than the plaque. Still, the final length of the stent depends on the amount of radial expansion, which is subjective during the procedure. This paper introduces a new stent model called "Murugan", which exhibits negative Poisson's ratio effect. That is, the stent may have zero axial contraction or can have extension when under radial expansion. The presence of hyperelastic balloon and the stent-balloon friction is also considered to study their effects in mechanical properties of the stents under consideration. Free expansion analysis is done using finite element method (FEM) to compare the new stent model with the present day stent geometries.

Plasticity as a Lifesaver in the Design of Cardiovascular Stents

A common treatment to restore normal blood flow in an obstructed artery is the deployment of a stent (i.e. small tube-like structure). The vast majority of stents are crimped on a folded balloon and laser cut from 316L stainless steel tubes. Although, several numerical studies (exploiting the Finite Element Method) are dedicated to the mechanical behaviour of balloon expandable stents, there seems to be no consensus regarding the mechanical properties to describe the inelastic material behaviour of SS316L. Moreover, as the typical dimensions of stent struts (e.g. 100 μm for coronary stents) are of a similar order of magnitude as the average grain size in stainless steel (i.e. 25 μm), continuum approaches relying on macroscopic material properties may be questionable. In addition, an experimental study on stainless steel stent strut specimens showed a size-dependency of the failure strain. In this study the impact of the magnitude of the yield stress on the stent expansion behavior is examined. An increase in the yield stress (from 205 N/mm² to 375 N/mm²) results in an increase of the pressure (from about 0.3 N/mm² to approximately 0.4 N/mm²) which the clinician needs to exert for the balloon to unfold and to reach its cylindrical expanded shape. Furthermore, the effect of the size dependency behavior of the material is studied by monitoring the nominal strain during stent expansion. The maximum value of the nominal strain in the expanded stent (e.g. εn = 23 %) does not exceed the critical value of the failure strain, (i.e. εn = 33 %), moreover the critical values are nowhere exceeded in the whole stent during the expansion. Our numerical results - accounting for the presence of the balloon in its actual folded shape - correspond very well with pressure/diameter data supplied by the manufacturer. Consequently, this study shows that the free expansion of new generation balloon-expandable stents can be studied accurately with computational analysis based on the Finite Element Method (FEM) and relying on macroscopic material properties. In this context, there is no need to implement a size-based constitutive material model, but before accepting the results of the study, one should check in any case the maximum strain against the limit as shown above.

Modeling of Size Dependent Failure in Cardiovascular Stent Struts under Tension and Bending

Cardiovascular stents are cylindrical mesh-like metallic structures that are used to treat atherosclerosis. The thickness of stent struts are typically in the range of 50-150 microm. At this microscopic size scale, the tensile failure strain has been shown to be size dependent. Micromechanically representative computational models have captured this size effect in tension. In this paper polycrystalline models incorporating material fracture are used to investigate size effects for realistic stent strut geometries and loading modes. The specific loading a stent undergoes during deployment is uniquely captured and the implications for stent design are considered. Fracture analysis is also performed, identifying trends in terms of strut thickness and loading type. The results show, in addition to the size effect in tension, further size effects in different loading conditions. The results of the loading analyses are combined to produce a tension and bending failure graph. This design safety diagram is presented as a tool to predict failure of stent struts. This study is particularly significant given the current interest in producing smaller stents.

Experimental Research on Balloon-expandable Endovascular Stent Expansion

The application background and experimental research overview of medical endovascular stent are presented. Based on the analytical comparison of the current research achievements, the life cycle of medical vascular stent, which is composed of three phases of mounting, deployment and long-term in vivo service, is pointed out and the characteristics of stent expansion process in the life cycle are emphasized on. The experimental scheme of in vitro stent expansion based on the machine vision technology in LabVIEW is presented. The selected component devices and measurement program for experiment are expatiated. A special drug-loading stent was expanded on the assembled platform of selected equipments and experimental results are analyzed. The experimental scheme presented in the paper provides powerful experimental support for the computer simulation of stent expansion process by the finite element analysis.

Stent expansion in curved vessel and their interactions: A finite element analysis

Coronary restenosis after angioplasty has been reduced by stenting procedure, but in-stent restenosis (ISR) has not been eliminated yet, especially in tortuous vessels. In this paper, we proposed a finite element method (FEM) to study the expansion of a stent in a curved vessel (the CV model) and their interactions. A model of the same stent in a straight vessel (the SV model) was also studied and mechanical parameters of both models were researched and compared, including final lumen area, tissue prolapse between stent struts and stress distribution. Results show that in the CV model, the vessel was straightened by stenting and a hinge effect can be observed at extremes of the stent. The maximum tissue prolapse of the CV model was more severe (0.079 mm) than the SV model (0.048 mm); and the minimum lumen area of the CV was decreased (6.10 mm(2)), compared to that of the SV model (6.28 mm(2)). Tissue stresses of the highest level were concentrated in the inner curvature of the CV model. The simulations offered some explanations for the clinical results of ISR in curved vessels and gave design suggestions of the stent and balloon for tortuous vessels. This FEM provides a tool to study mechanisms of stents in curved vessels and can improve new stent designs especially for tortuous vessels.

Drug-eluting stents: a multidisciplinary success story

Coronary stenting is the most common form of interventional treatment for symptomatic coronary artery disease. In-stent restenosis following bare metal stent (BMS) placement is the most common cause of procedural failure and occurs as a result of vessel wall trauma secondary to balloon angioplasty and stent deployment that results in an overly aggressive healing response (neointimal hyperplasia) that overgrows the stent lumen and causes vascular narrowing. Drug-eluting stents (DES) are specialized vascular stents capable of delivering drugs to the arterial wall in a controlled manner such that neointimal hyperplasia is reduced or prevented, luminal patency is preserved, coronary blood flow is maintained and the patient is spared a repeat procedure to re-open the vessel. The objectives of the review are to provide an overview of the major contributions that a broad range of disciplines have made to the design and development of drug-eluting stents and to summarize future directions of these fields of research. Engineers and biomaterials scientists have explored relationships between stent design and stent performance and work continues to optimize stent design and biocompatibility of stent biomaterials. Pharmaceutical scientists are continually expanding the range of candidate drugs for pharmacological intervention, and improving the technology using novel coatings to modulate drug release. Clinical scientists are investigating issues such as long-term safety and efficacy, new applications of drug-eluting stents and optimal deployment techniques.

Comparison of Element Technologies for Modeling Stent Expansion

Numerical analysis has become a standard, and in fact required, component of the process in which new biomedical stent devices are designed and approved. Despite this, very little work has been performed comparing the efficiency of various numerical techniques currently available to the analyst. The present paper begins this study by comparing the computational efficiency and results achieved using various element technologies in the context of stent deployment simulations. The findings indicate that the methodologies most commonly adopted in the present literature/industrial practice are in many cases less than optimal with regard to computational efficiency.

Finite element modeling of a progressively expanding shape memory stent

Cardiovascular stents are small cylindrical devices introduced in stenosed arteries to reopen the lumen and restore blood flow. However, this treatment presents complications, including restenosis, which is the reclosing of the artery's diameter after the insertion of a stent. The structure of the prosthesis penetrates into and injures the walls of the patient's artery. There then follows a proliferation of cells and the formation of scar tissue around the injury, similar to the scarring of other organic tissues. This reaction to the trauma subjects the artery to close. The proposed solution is to develop a Nitinol stent with a progressive expansion device made of polyethylene, allowing smooth and gradual contact between the stent and the artery's wall by creep effect. The purpose of this paper is to describe the technology and methodology for the numerical study of this kind of stent through the finite element method. ANSYS 8.0 software is used to perform the analysis. The Nitinol is modeled with a superelastic law and the polyethylene with a yield hardening law. A first simulation determines the final geometry of the stent laser cut from a small tube. A second simulation examines the behavior of the prosthesis during surgery and over the 4 weeks following the operation. The results demonstrate that a compromise can be reached between a limited expansion prior the inflation of the expandable balloon and a significant expansion by creep of the polymer rings.

Effects of stent structure on stent flexibility measurements

Adequate delivery and placement of intracoronary artery stents is dependent on various properties of the stent itself, including flexibility and high-radial strength. Although these properties can be assessed via determination of the bending stiffness and radial stiffness of a stent, the mesh structure of the stent does not lend easily to such measurements. The goal of the present study was to determine an optimal method for determining stent bending stiffness. The four-points bending test was used to evaluate stent flexibility, and the finite element method (FEM) was employed to assess the effect of stent structure on flexibility in stents with differing link structures. The four-points bending test yielded the following bending stiffness values: 85.28 N mm2 for the stent with an S-shaped link; 41.67 N mm2 for the stent with an N-shaped link; 78.79 N mm2 for the stent with a modified W-shaped link; and 188.67 N mm2 for the stent with a W-shaped link. The stent with the point symmetric link configuration (S-, N- and modified W-shaped link) had high flexibility. FEM analysis revealed that low flexibility resulted from interference between the struts at the compressive side. Further, the flexibility predicted from FEM analysis correlated with bending stiffness of the stents. We conclude that use of the four-points bending test yields information that is critical for the design of flexible stents.

Cardiovascular stent design and vessel stresses: a finite element analysis

Intravascular stents of various designs are currently in use to restore patency in atherosclerotic coronary arteries and it has been found that different stents have different in-stent restenosis rates. It has been hypothesized that the level of vascular injury caused to a vessel by a stent determines the level of restenosis. Computational studies may be used to investigate the mechanical behaviour of stents and to determine the biomechanical interaction between the stent and the artery in a stenting procedure. In this paper, we test the hypothesis that two different stent designs will provoke different levels of stress within an atherosclerotic artery and hence cause different levels of vascular injury. The stents analysed using the finite-element method were the S7 (Medtronic AVE) and the NIR (Boston Scientific) stent designs. An analysis of the arterial wall stresses in the stented arteries indicates that the modular S7 stent design causes lower stress to an atherosclerotic vessel with a localized stenotic lesion compared to the slotted tube NIR design. These results correlate with observed clinical restenosis rates, which have found higher restenosis rates in the NIR compared with the S7 stent design. Therefore, the testing methodology outlined here is proposed as a pre-clinical testing tool, which could be used to compare and contrast existing stent designs and to develop novel stent designs.

Finite element modelling and simulations in cardiovascular mechanics and cardiology: A bibliography 1993–2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.