Numerical investigation of the intravascular coronary stent flexibility

Symmetrical structure design of PLGA Biodegradable sinus stents and structure optimization based on surrogate models

This study aims to enhance the degradation uniformity of PLGA sinus stents to minimize fracture risk caused by stress corrosion. Symmetric stent structures were introduced and compared to sinusoidal structure in terms of stress and degradation uniformity during implantation and degradation processes. Three surrogate models were employed to optimize the honeycomb-like structure. Results showed honeycomb-like structures exhibited the superior stress distribution and highest degradation uniformity. The kriging model achieved the smallest error and degradation uniformity of 83.24%. In conclusion, enhancing the symmetry of stent structures improves degradation uniformity, and the kriging model has potential for the optimization of stent structures.

A comparative study on the deformation behavior and mechanical properties of new lower extremity arterial stents

BACKGROUND AND OBJECTIVE

The lower extremity movement involves a complex and large amplitude extremity movement process, and arterial stents implanted in the lower extremity are prone to complex mechanical deformation behavior. Hence, the lower extremity arterial stent is required to have favorable comprehensive mechanical properties.

METHODS

In this study, a new lower extremity arterial stent (New) was proposed, and its deformation behavior and mechanical properties were analyzed by numerical simulations under different deformation modes, such as radial compression, axial compression/tension, bending, and torsion. Stents with different diameters were modeled to compare the effect of diameter size on their biomechanical properties. Additionally, a comparative analysis was conducted between this new stent and seven commercially available stents.

RESULTS

The results demonstrated that the stent diameter exerted a significant effect on its deformation behavior and mechanical properties. Specifically, with the increase of the stent diameter, the radial expansion rate, radial shrinkage rate, radial support stiffness, axial compression stiffness, and axial tensile stiffness tended to decrease, and the expansion inhomogeneity, stenosis rate, bending stiffness, and torsional stiffness tended to increase. In contrast, the stent diameter exerted a small effect on the stent axial shortening rate and ellipticity. The new lower extremity arterial stent was validated to outperform other stents in terms of most performance indicators. Especially, the radial expansion rate and ellipticity of the New stent were better than those of all commercially available stents. Moreover, the New stent presented favorable mechanical properties and flexibility under the premise of ensuring the support performance.

CONCLUSIONS

Based on these findings, this lower extremity arterial stent may play a better therapeutic effect in clinical application. Furthermore, these analysis results may provide reference for the clinical application and selection of the stent.

Impact of inverse unequal height strut structure on the functional performance of an additively manufactured cardiovascular stent

Recently, additive manufacturing (AM) has been investigated as an innovative method to manufacture stents due to its capability in producing complex and customized structures. In this paper, the cardiovascular stents of M-type and N-type with inverse unequal height strut structure and N-type with equal height strut structure were designed and manufactured by Selective Laser Melting (SLM). Following surface polishing, balloon expansion, plane compression and three-point bending experiments were carried out to evaluate the mechanical performance of the stent. The stents designed with inverse unequal height strut structure showed higher radial support performance and lower radial recoil when compared to the stents with uniform design. This study proved the feasibility of SLM in rapid manufacturing of cardiovascular stents that can be used for performance evaluation in design stage.

Effects of Tapered-Strut Design on Fatigue Life Enhancement of Peripheral Stents

Peripheral stent could fracture from cyclic loadings as a result of our blood pressures or daily activities. Fatigue performance has therefore become a key issue for peripheral stent design. A simple yet powerful tapered-strut design concept for fatigue life enhancement was investigated. This concept is to move the stress concentration away from the crown and re-distribute the stresses along the strut by narrowing the strut geometry. Finite element analysis was performed to evaluate the stent fatigue performance under various conditions consistent with the current clinical practice. Thirty stent prototypes were manufactured in-house by laser with a series of post-laser treatments, followed by the validation of bench fatigue tests for proof of concept. FEA simulation results show that the fatigue safety factor of the 40% tapered-strut design increased by 4.2 times that of a standard counterpart, which was validated by bench tests with 6.6-times and 5.9-times fatigue enhancement at room temperature and body temperature, respectively. Bench fatigue test results agreed very well with the increasing trend predicted by FEA simulation. The effects of the tapered-strut design were significant and could be considered as an option for fatigue optimization of future stent designs.

Mechanical and hydrodynamic effects of stent expansion in tapered coronary vessels

Percutaneous coronary intervention (PCI) has become the primary treatment for patients with coronary heart disease because of its minimally invasive nature and high efficiency. Anatomical studies have shown that most coronary vessels gradually shrink, and the vessels gradually become thinner from the proximal to the distal end. In this paper, the effects of different stent expansion methods on the mechanical and hemodynamic behaviors of coronary vessels and stents were studied. To perform a structural-mechanical analysis of stent implantation, the coronary vessels with branching vessels and the coronary vessels with large bending curvature are selected. The two characteristic structures are implanted in equal diameter expansion mode and conical expansion mode, and the stress and mechanical behaviors of the coronary vessels and stents are analyzed. The results of the structural-mechanical analysis showed that the mechanical behaviors and fatigue performance of the cobalt-chromium alloy stent were good, and the different expansion modes of the stent had little effect on the fatigue performance of the stent. However, the equal diameter expansion mode increased distal coronary artery stress and the risk of vascular injury. The computational fluid dynamics analysis results showed that different stent expansion methods had varied effects on coronary vessel hemodynamics and that the wall shear stress distribution of conical stent expansion is more uniform compared with equal diameter expansion. Additionally, the vortex phenomenon is not apparent, the blood flow velocity is slightly increased, the hydrodynamic environment is more reasonable, and the risk of coronary artery injury is reduced.

Investigation of changing geometry parameters of nickel-titanium shape memory alloys wire stent in cardiovascular implants

The purpose of this investigation was to assess the effects of changing the Nitinol stent's geometrical characteristics on the superelastic behavior of the stent using the finite element method. Four different geometrical parameters were considered and analyzed in sixteen stents. These geometrical parameters consisted of stent length, number of stent's meanders, inside diameter of the stent and wire diameter. Results showed that decreasing either stent length or the number of stent meanders and increasing either the inside diameter of the stent or wire diameter generally result in more sensible superelastic behavior exposure. The effect of changing stent length is independent of the inside diameter and number of the stent's meanders. Effects of changing inside diameter and number of stent's meanders were exactly dependent together. In addition, increasing wire diameter has a great effect on the maximum force. In conclusion, the results of this study may serve as guidelines for future stent design.

Structural Design of Vascular Stents: A Review

Percutaneous Coronary Intervention (PCI) is currently the most conventional and effective method for clinically treating cardiovascular diseases such as atherosclerosis. Stent implantation, as one of the ways of PCI in the treatment of coronary artery diseases, has become a hot spot in scientific research with more and more patients suffering from cardiovascular diseases. However, vascular stent implanted into vessels of patients often causes complications such as In-Stent Restenosis (ISR). The vascular stent is one of the sophisticated medical devices, a reasonable structure of stent can effectively reduce the complications. In this paper, we introduce the evolution, performance evaluation standards, delivery and deployment, and manufacturing methods of vascular stents. Based on a large number of literature pieces, this paper focuses on designing structures of vascular stents in terms of “bridge (or link)” type, representative volume unit (RVE)/representative unit cell (RUC), and patient-specific stent. Finally, this paper gives an outlook on the future development of designing vascular stents.

The optimal structural analysis of cobalt-chromium alloy (L-605) coronary stents

A coronary stent is a conventional medical device used in percutaneous coronary intervention (PCI), and its mechanical properties have become one of the critical factors to determine the effect of surgical treatment. Based on the existing cobalt-chromium (CO-Cr) alloy L605 coronary stent, this paper established an integrated finite element model of the stent-balloon-compression shell to simulate the stent bundle expansion test and crush resistance test with parallel plates. We verified the accuracy of the analytical model by comparing the simulation results with experimental results. The effects of different parameters on the axial retraction performance, the expansion uniformity, and the flexibility of the stent are analyzed by using the support body dimension and connector form as parameters. The results demonstrate that increasing the number and length of the circumferential support can improve the axial retraction performance, expansion uniformity, and the form of the connector has a significant influence on the flexibility of the stent. Finally, the optimized CO-Cr alloy stent with improved comprehensive indicators is described.

Design and evaluation of a novel anti-reflux biliary stent with cone spiral valve

Endoscopic placement of biliary stent is a well-established palliative treatment for biliary obstruction. However, duodenobiliary reflux after stent placement has been a common problem which may lead to dreadful complications. This paper designed a novel anti-reflux biliary stent with a cone spiral valve. Fluid-Structure Interaction (FSI) simulations were established to evaluate the efficiency of the anti-reflux stent comparing with a clinically applied standard stent. According to the stress distribution of the valve, the fatigue performance in the stress concentration area was analyzed. The results show that when the antegrade flow through the valve, the cone spiral valve could stretch and open to realize adequate drainage under the normal physiological pressure of biliary tract; When the duodenal reflux through the valve, the valve would be compressed and close with a result of nearly zero at the outlet flow rate. Furthermore, the anti-reflux stent achieved improved radial mechanical performance with 2.7 times higher radial stiffness than standard stent. Finite element analysis (FEA) also indicates that compared with the standard stent, the addition of the anti-reflux valve had little negative effect on flexibility of the stent. Fatigue analysis results showed that the valve was reliable. This research provides the new stent with a cone spiral valve and proves that it is technically feasible and effective for preventing the duodenobiliary reflux while ensuring the antegrade bile flow without compromising the other biomechanical performances.

Numerical analysis of paclitaxel-eluting coronary stents: Mechanics and drug release properties

Since theoretical models provide data that cannot be otherwise gathered, numerical methods applied to medical devices analysis have emerged as fundamental tool in preclinical development. Large efforts were done to study mechanical and drug-eluting properties in stents but often the coating modelling is neglected. This work presents a finite element framework to calculate mechanical loads and drug distribution in three commercial drug-eluting stents (Palmaz-Schatz, Palmaz Genesis and Multi Link Vision), to check coatings strength and drug distribution maps in biological tissues. The promising copolymer poly(methylmethacrylate-co-n-butylmethacrylate), loaded with paclitaxel, is analyzed. Results demonstrated that the coating undergoes localized plastic phenomena, and calculated stresses are lower than the ultimate stress, ensuring coating integrity. Computed drug concentration depends on stent geometry and its values are in all cases lower than the toxicity level for this drug.

Flexibility of Biodegradable Polymer Stents with Different Strut Geometries

Objective: Biodegradable stents (BDSs) represent a new technological development in the field of cardiovascular angioplasty; good flexibility helps stents pass through tortuous vessels during delivery and reduces the amount of damage caused to blood vessels. This study investigates the relationship between flexibility and the geometry of BDS struts. Methods: Four stent struts with different geometry (circular, triangular, hexagonal, and spline curved) and the same links were modeled to evaluate their flexibility via a three-point bending experimental method and a numerical method. Results: The bending state of the four stents was well-balanced. The bending effect of the four stents was different. Under the same conditions, the circular and spline curved stents showed the best bending effects while the hexagonal stent was the worst. However, these differences were not significant. Conclusion: The flexibility of BDSs is related to the geometry of the struts and links; however, the geometry of the struts has less effect on flexibility than the links. The greater the area enclosed by the strut centerline, the better flexibility of the stent.

Study on the bending behavior of biodegradable metal cerebral vascular stents using finite element analysis

Excellent bending behavior is evaluated as the primary factor during the design of biodegradable metal cerebral vascular stents (BMCVSs), which enables vascular stents to be successfully delivered to the targeted location and avoids unnecessary damage to blood vessels. Unfortunately, this bending behavior has been barely investigated which limits the design of BMCVSs with optimal structures. Herein, six BMCVSs were designed and their bending process were simulated using finite element analysis (FEA). Then, the effects of the stent bridge connection type and structure on the bending behavior were systematically analyzed and an universal mathematical model was further established, in which the influence of the structure parameters of the stent bridge on the flexibility of stents was considered. After that, the bending mechanism of the high-stress zone of the bridge was investigated. Finally, the causes and effects of the self-contacting phenomenon as well as the inner-stent protrusion phenomenon in the bending state were analyzed theoretically, and corresponding solutions were proposed to optimize the design of stents. The numerical results show that the stents with the dislocation-line W-shaped unit have better flexibility than the other stents. The flexibility is positively correlated to the cube of the length of linear part and to the square of the curvature of curved part. The self-contacting phenomenon of the bridge during bending can constrain the formation of inner-stent protrusion, which can eliminate the negative effects of the implanted stents on the hemodynamics in blood vessels. This study is expected to provide practical guidance for the structural design of BMCVSs for clinical applications.

Numerical simulation of stent deployment within patient-specific artery and its validation against clinical data

BACKGROUND AND OBJECTIVE

One of the most widely adopted endovascular treatment procedures is the stent implantation. The effectiveness of the treatment depends on the appropriate stent expansion. However, it is difficult to accurately predict the outcome of such an endovascular intervention. Numerical simulations represent a useful tool to study the complex behavior of the stent during deployment. This study presents a numerical model capable of simulating this process.

METHODS

The numerical model consists of three parts: modeling of stent expansion, modeling the interaction of the stent with the arterial wall and the deformation of the arterial wall. The model is able to predict the shapes of both stent and arterial wall during the entire deployment process. Simulations are performed using patient-specific clinical data that ensures more realistic results.

RESULTS

The numerical simulations of stent deployment are performed using the extracted geometry of the coronary arteries of two patients. The obtained results are validated against clinical data from the follow up examination and both quantitative and qualitative analysis of the results is presented. The areas of several slices of the arterial wall are calculated for all the three states (before, after and follow up) and the standard error of the area when comparing simulation and follow up examination is 5.27% for patient #1 and 4.5% for patient #2.

CONCLUSIONS

The final goal of numerical simulations in stent deployment should be to provide a clinical tool that is capable of reliably predicting the treatment outcome. This study showed through the good agreement of results of the numerical simulations and clinical data that the presented numerical model represents a step towards this final goal. These simulations can also provide valuable information about distribution of forces and stress in the arterial wall that can improve pre-operative planning and treatment optimization.

Fast simulation of stent deployment with plastic beam elements

Coronary stent deployment is a reference cardiology intervention, used to treat atherosclerosis and prevent heart attacks. The outcomes of the intervention highly depend on the accuracy of the stent apposition, which could benefit from per-operative prediction tools. In this paper, we propose a fast and mechanically realistic 3D simulation of a coronary stent expansion. Our simulation relies on the finite element method and involves serially linked beam elements to model the slender geometry of a stent. The elements are implemented with a non-linear elasto-plastic behavior, describing realistically the complex deformation of a balloon-expandable stent. As a proof of concept, we simulated the free expansion of a coronary stent. The simulation output was compared with micro-CT data, acquired experimentally during the device expansion. Results show that the plastic beam model is able to reproduce successfully the final geometry of the stent. In addition, the use of 1D elements allows to achieve a significantly lower computational time than for equivalent literature simulations, based on 3D elements. This preliminary work highlights the compatibility of our method with clinical routine in terms of execution time. Further developments include the application of the method to more advanced simulation scenarios, with the addition of a personalized artery model.

Multi-Objective Optimization Design of Balloon-Expandable Coronary Stent

PURPOSE

Recent studies suggested that suboptimal delivery and longitudinal stent deformation can result in in-stent restenosis. Therefore, the purpose of this paper was to study the effect of stent geometry on stent flexibility and longitudinal stiffness (LS) and optimize the two metrics simultaneously. Then, the reliable and accurate relationships between metrics and design variables were established.

METHODS

A multi-objective optimization method based on finite element analysis was proposed for the investigation and improvement of stent flexibility and LS. The relative influences of design variables on the two metrics were evaluated on the basis of the main effects. Three surrogate models, namely, the response surface model (RSM), radial basis function neural network (RBF), and Kriging were employed and compared.

RESULTS

The accuracies of the three models in fitting flexibility were nearly similar, although Kriging made more accurate prediction in LS. The link width played important roles in flexibility and LS. Although the flexibility of the optimal stent decreased by 13%, the LS increased by 48.3%.

CONCLUSIONS

The obtained results showed that the multi-objective optimization method is efficient in predicting an optimal stent design. The method presented in this paper can be useful in optimizing stent design and improving the comprehensive mechanical properties of stents.

Surrogate-based visualization and sensitivity analysis of coronary stent performance: A study on the influence of geometric design

The main goal of this numerical study is to assess the impact of geometric design perturbations on the performance of a representative coronary stent platform. In this context, first, a design parameterization model was defined for the stent under study. After, a set of metrics characterizing stent performance, namely, vessel injury, radial recoil, bending resistance, longitudinal resistance, radial strength, the risk of fracture, prolapse index, and dogboning were evaluated within the context of a finite element analysis. Afterwards, accurate surrogate models were developed, using the efficient global optimization algorithm, as predictive tools in the execution of tasks that normally require a high number of model evaluations, such as global sensitivity analysis and visualization. In the end, the dependence of the output response surfaces on the geometric parameters was mechanically interpreted, which allowed us to understand the complex interplay that exists between the considered design variables and the defined performance metrics.

Optimizing the deformation behavior of stent with nonuniform Poisson's ratio distribution for curved artery

Stent implantation at a highly curved artery has always been a challenge, considering the relatively high chance of in-stent restenosis (ISR) caused by severe straightening effect and high strain energy over the vessel wall. In this paper, a novel optimization based design method was proposed to manipulate the deformation behavior of the common ring-and-link stent. By changing the location of the connection point between rings and links, traditional ring-and-link structure was modified to achiever tunable Poisson's ratio (PR). With the nonuniform cellular structure design method proposed in a previous study, PR distribution of the stent structure was optimized to achieve the desired curvature. As a result, the obtained stent structure with nonuniform PR could perfectly fit into the curved artery after expansion, without causing any obvious vessel straightening. To validate the proposed method, two different vessel models were introduced. Firstly, a short vessel with a constant curvature was set as the design objective, and both numerical and experimental tests were conducted. Further, a patient-specific vessel was applied. Both test results showed that optimized stents would cause much smaller vessel straightening. Moreover, vessels stented by the optimized structures had much lower stress concentration and strain energy. All those properties will decrease the possibility of ISR significantly.

FLEXIBILITY BEHAVIOR OF CORONARY STENTS: THE ROLE OF LINKER INVESTIGATED WITH NUMERICAL SIMULATION

Flexibility is a vital property of stents and different stent structures lead to different flexibility behaviors. In this study, the finite element analysis was adopted and a virtual bending deformation was imposed to quantify the effects of linker pattern, linker number, bending direction and linker location on flexibility. Stent performance indicators, including stress distribution, deformation patterns and bending stiffness, were examined. Results indicate that higher levels of stresses are found on the linker struts, associated with much larger deformation. The linker number plays the most significant role in flexibility, and simply decreasing linker number could result in a sharp increase in flexibility and a decrease in stress. The linker pattern has great impact on stent flexibility, especially on the behavior of self-contact. Stents with different linker patterns could respond differently in the course of bending, and the stent with an offset peak-to-peak linker pattern is the best choice. It is also found that stent flexibility can be improved when fewer linkers lie in the compression area and the linker directions between two adjacent rows are consistent. The results obtained could provide useful information for the improvement of stent design and clinical choice.

NUMERICAL INVESTIGATIONS OF THE FLEXIBILITY OF INTRAVASCULAR BRAIDED STENT

Braided stents are commonly used to treat cerebral aneurysm, but there is little information about the bending characteristic of braided stent used for cerebral aneurysm. This paper investigates how geometrical parameters of braided stent influence its flexibility. Eight groups of braided stent models with different geometries (i.e., nominal diameter, length, braiding angle, number of wires, diameter of wire, frictional coefficient among wires and porosity) were constructed. Parametric analyses of these models were carried out by using Abaqus/Explicit. When the nominal diameter varied from 2[Formula: see text]mm to 5.5[Formula: see text]mm, the forces required for flexural deformation decrease from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; when the axial length varied from 10[Formula: see text]mm to 40[Formula: see text]mm, the forces required for flexural deformation decrease from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; when the braiding angle increases from 30[Formula: see text] to 75[Formula: see text] (the number of wires is 48 and the diameter of the wire is 0.026[Formula: see text]mm), the forces required for bending deformation decrease from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; when the diameter of wires increases from 0.026[Formula: see text]mm to 0.052[Formula: see text]mm (the number of wires is 24 and the braiding angle is 60[Formula: see text]), the forces required for flexural deformation increase from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N; and when the number of wires increases from 14 to 48 (the braiding angle is 75[Formula: see text] and the diameter of the wire is 0.026[Formula: see text]mm), the forces required for flexural deformation increase from [Formula: see text][Formula: see text]N to [Formula: see text][Formula: see text]N. From the data above it can be seen that the diameter of wires, the number of wires and braiding angle have a larger impact on bending characteristics of braided stent; and the axial length and nominal diameter have a smaller impact on bending characteristics of braided stent. Results of the present study may provide theoretical guidance for the design of self-expanding braided stent and its clinical practice.

Computational fluid dynamics study of common stent models inside idealised curved coronary arteries

The haemodynamic behaviour of blood inside a coronary artery after stenting is greatly affected by individual stent features as well as complex geometrical properties of the artery including tortuosity and curvature. Regions at higher risk of restenosis, as measured by low wall shear stress (WSS < 0.5 Pa), have not yet been studied in detail in curved stented arteries. In this study, three-dimensional computational modelling and computational fluid dynamics methodologies were used to analyse the haemodynamic characteristics in curved stented arteries using several common stent models. Results in this study showed that stent strut thickness was one major factor influencing the distribution of WSS in curved arteries. Regions of low WSS were found behind struts, particularly those oriented at a large angle relative to the streamwise flow direction. These findings were similar to those obtained in studies of straight arteries. An uneven distribution of WSS at the inner and outer bends of curved arteries was observed where the WSS was lower at the inner bend. In this study, it was also shown that stents with a helical configuration generated an extra swirling component of the flow based on the helical direction; however, this extra swirl in the flow field did not cause significant changes on the distribution of WSS under the current setup.

A Novel Approach to Design Lesion-Specific Stents for Minimum Recoil

Stent geometries are obtained by topology optimization for minimized compliance under different stenosis levels and plaque material types. Three levels of stenosis by cross-sectional area, i.e., 30%, 40%, and 50% and three different plaque material properties, i.e., calcified, cellular, and hypocellular, were studied. The raw optimization results were converted to clear design concepts and their performance was evaluated by implanting them in their respective stenosed artery types using finite element analysis. The results were compared with a generic stent in similar arteries, which showed that the new designs showed less recoil. This work provides a concept that stents could be tailored to specific lesions in order to minimize recoil and maintain a patent lumen in stenotic arteries.

Simulated Bench Testing to Evaluate the Mechanical Performance of New Carotid Stents

Our group recently developed a novel covered carotid stent that can prevent emboli while preserving the external carotid artery (ECA) branch blood flow. However, our recent in vitro side-branch ECA flow preservation tests on the covered stents revealed the need for further stent frame design improvements, including the consideration to crimp the stent to a low profile for the delivery of the stent system and having bigger cells. Hence, the current work aims to design new bare metal stents with bigger cell size to improve the crimpability and to accommodate more slits so that the side-branch flow could be further increased. Three new stent designs were analyzed using finite element analysis and benchmarked against two commercially available carotid stents in terms of their mechanical performances such as crimpability, radial strength, and flexibility. Results indicated that the new bare metal stent designs matched well against the commercial stents. Hence our new generation covered stents based on these designs can be expected to perform better in side-branch flow preservation without compromising on their mechanical performances.

Mechanical response of cardiovascular stents under vascular dynamic bending

Backround

Currently, the effect of vascular dynamic bending (VDB) has not been fully considered when studying cardiovascular stents’ long-term mechanical properties, as the previous studies about stent’s mechanical properties mostly focus on the effect of vascular pulsation (VP). More and more clinical reports suggested that the effect of VDB have a significant impact on stent.

Methods

In this paper, an explicit-implicit coupling simulation method was applied to analyze the mechanical responses of cardiovascular stents considering the effect of VDB. The effect of VP on stent mechanical properties was also studied and compared to the effect of VDB.

Results

The results showed that the dynamic bending deformation occurred in stents due to the effect of VDB. The effects of VDB and VP resulted in alternating stress states of the stent, while the VDB alternate stresses effective on the stent were almost three times larger than that of the VP. The stress concentration under VDB mainly occurred in bridge struts and the maximal stress was located in the middle loops of the stent. However, the stress distributed uniformly in the stents under the effect of VP. Stent fracture occurred more frequently as a result of VDB with the predicted fracture position located in the bridging struts of the stent. These results are consistent with the reported data in clinical literatures. The stress of the vessel under VDB was higher, than that caused by VP.

Conclusions

The results showed that the effect of VDB has a significant impact on the stent’s stress distribution, fatigue performance and overall stress on the vessel, thus it is necessary to be considered when analyzing stent’s long-term mechanical properties. Meanwhile, the results showed that the explicit-implicit coupling simulation can be applied to analyze stent mechanical properties.

An Efficient Finite Element Framework to Assess Flexibility Performances of SMA Self-Expandable Carotid Artery Stents

Computer-based simulations are nowadays widely exploited for the prediction of the mechanical behavior of different biomedical devices. In this aspect, structural finite element analyses (FEA) are currently the preferred computational tool to evaluate the stent response under bending. This work aims at developing a computational framework based on linear and higher order FEA to evaluate the flexibility of self-expandable carotid artery stents. In particular, numerical simulations involving large deformations and inelastic shape memory alloy constitutive modeling are performed, and the results suggest that the employment of higher order FEA allows accurately representing the computational domain and getting a better approximation of the solution with a widely-reduced number of degrees of freedom with respect to linear FEA. Moreover, when buckling phenomena occur, higher order FEA presents a superior capability of reproducing the nonlinear local effects related to buckling phenomena.

Computational Assessment of Stent Durability Using Fatigue to Fracture Approach

Stents are metal scaffold devices used to maintain lumen and restore blood flow of diseased artery. Despite they brought care of coronary diseases to a new level of efficacy, problem of stent fracture remains unclear even after global needs reached number of 5 × 106 devices yearly. For projected work-life of 10 years, rate of fracture occurrence in stents varies from 5% up to 25% for different designs. Analysis of such miniature devices and long-term events in realistic in vivo conditions remains impossible while experimental in vitro measurements provide limited results consuming much time and expensive equipment. The principal aim of this study was to propose procedure for numerical estimation of coronary stents durability assuming the hyperphysiological pulsatile pressure conditions. The hypothesis was whether the stent durability would be achieved safely for the projected work-life of 10 yr? The procedure was carried out within three phases: (a) initial fatigue analysis based on S-N approach; (b) fatigue lifetime assessment based on fatigue crack growth simulation using Paris power law, and (c) safe-operation, i.e., no-fatigue failure (based on Kitagawa–Takahashi diagram) as well as immediate predictions of the fracture event in the stent. For considered generic stent design, results showed that the stent durability would be achieved safely. Since special diagrams were used, the fatigue risk assessment was clearer compared to the conventional fatigue lifetimes. Moreover, it was found that crack growth was stable for both small and large scale sizes of the crack. Besides the fact that the presented procedure was shown as suitable for numerical assessment of the generic stent durability under hyperphysiological pulsatile pressure conditions, it was concluded that it might be applied for any other design as well as loading conditions. Moreover, it could be efficiently combined with experimental procedures during the process of the stent design validation to reduce manufacturing and testing costs.

Numerical models of net-structure stents inserted into arteries

INTRODUCTION

Restenosis is strongly attributed to stresses caused by stent-artery interactions generated in the artery after balloon angioplasty. Numerical methods are often used to examine the stent-artery mechanical interactions. To overcome the extensive computational requirements demanded by these simulations, simplifications are needed.

OBJECTIVE

We introduce simplified models to calculate the mechanical interactions between net-structured stents and arteries, and discuss their validity and implications.

METHODS

2D simplified numerical models are suggested, which allow cost effective assessment of arterial stresses and the potential damage factor (DF). In these models, several contact problems were solved for arteries with hyper elastic mechanical properties. Stresses were calculated for a large range of cases and for different numerical model types. The effects of model simplifications, oversizing mismatch and stenosis rate and length and symmetry on the resulting stresses were analyzed.

RESULTS & CONCLUSIONS

Results obtained from planar 2D models were found in good agreement with results obtained from complex 3D models for cases with axisymmetric constant or varying stenosis. This high correlation between the results of 3D cases with varying stenosis and the more simple 2D cases can be used as a simplified and convenient tool for calculating the arterial wall stresses in complex cases. Maximal stresses obtained by the 2D model with an asymmetric stenosis are lower than the maximal stresses obtained in the axisymmetric case with the same stenosis percentage. Therefore, axisymmetric models may provide the worst-case estimation values for a stent of interest.

Finite element analysis of balloon-expandable coronary stent deployment: influence of angioplasty balloon configuration

Today, the majority of coronary stents are balloon-expandable and are deployed using a balloon-tipped catheter. To improve deliverability, the membrane of the angioplasty balloon is typically folded about the catheter in a pleated configuration. As such, the deployment of the angioplasty balloon is governed by the material properties of the balloon membrane, its folded configuration and its attachment to the catheter. Despite this observation, however, an optimum strategy for modelling the configuration of the angioplasty balloon in finite element studies of coronary stent deployment has not been identified, and idealised models of the angioplasty balloon are commonly employed in the literature. These idealised models often neglect complex geometrical features, such as the folded configuration of the balloon membrane and its attachment to the catheter, which may have a significant influence on the deployment of a stent. In this study, three increasingly sophisticated models of a typical semi-compliant angioplasty balloon were employed to determine the influence of angioplasty balloon configuration on the deployment of a stent. The results of this study indicate that angioplasty balloon configuration has a significant influence on both the transient behaviour of the stent and its impact on the mechanical environment of the coronary artery.

Patient-specific simulations of stenting procedures in coronary bifurcations: two clinical cases

Computational simulations of stenting procedures in idealized geometries can only provide general guidelines and their use in the patient-specific planning of percutaneous treatments is inadequate. Conversely, image-based patient-specific tools that are able to realistically simulate different interventional options might facilitate clinical decision-making and provide useful insights on the treatment for each individual patient. The aim of this work is the implementation of a patient-specific model that uses image-based reconstructions of coronary bifurcations and is able to replicate real stenting procedures following clinical indications. Two clinical cases are investigated focusing the attention on the open problems of coronary bifurcations and their main treatment, the provisional side branch approach. Image-based reconstructions are created combining the information from conventional coronary angiography and computed tomography angiography while structural finite element models are implemented to replicate the real procedure performed in the patients. First, numerical results show the biomechanical influence of stents deployment in the coronary bifurcations during and after the procedures. In particular, the straightening of the arterial wall and the influence of two overlapping stents on stress fields are investigated here. Results show that a sensible decrease of the vessel tortuosity occurs after stent implantation and that overlapping devices result in an increased stress state of both the artery and the stents. Lastly, the comparison between numerical and image-based post-stenting configurations proved the reliability of such models while replicating stent deployment in coronary arteries.

Modeling stented coronary arteries: where we are, where to go

In the last two decades, numerical models have become well-recognized and widely adopted tools to investigate stenting procedures. Due to limited computational resources and modeling capabilities, early numerical studies only involved simplified cases and idealized stented arteries. Nowadays, increased computational power allows for numerical models to meet clinical needs and include more complex cases such as the implantation of multiple stents in bifurcations or curved vessels. Interesting progresses have been made in the numerical modeling of stenting procedures both from a structural and a fluid dynamics points of view. Moreover, in the drug eluting stents era, new insights on drug elution capabilities are becoming essential in the stent development. Lastly, image-based methods able to reconstruct realistic geometries from medical images have been proposed in the recent literature aiming to better describe the peculiar anatomical features of coronary vessels and increase the accuracy of the numerical models. In this light, this review provides a comprehensive analysis of the current state-of-the-art in this research area, discussing the main methodological advances and remarkable results drawn from a number of significant studies.

EFFECTS OF THE WALL STRESS–STRAIN NONLINEARITY AND VISCOELASTICITY IN A STENTED VESSEL

In this work, we study the behavior of the outflow of an incompressible viscous Newtonian fluid in large vessels with the presence of a stent having a nonlinear stress–strain relation. Here, we present a mathematical approach that allows us to get analytical solutions of the bidimensional flow in elastic stented vessel proportional to the mean axial flow. Thereafter, the one-dimensional (1D) model is obtained by averaging the two-dimensional (2D) incompressible Navier–Stokes equations over the radius of the vessel. We use the perturbative approach to solve the 1D approximation finally obtained, which helps to deduce the results of the final analytical solutions. The numerical simulation then helps to evaluate the effects of the stiffness, the nonlinearity of the wall, and the viscoelasticity of the stented vessel. We show that the increase of stiffness and nonlinearity of the stented vessel cause the distortions to the level of the swelling zones of prosthesis that contributes to reduce the life span of the stent and damages of the wall.

Shape optimization of stress concentration-free lattice for self-expandable Nitinol stent-grafts

In a mechanical component, stress-concentration is one of the factors contributing to reduce fatigue life. This paper presents a design methodology based on shape optimization to improve the fatigue safety factor and increase the radial stiffness of Nitinol self-expandable stent-grafts. A planar lattice free of stress concentrators is proposed for the synthesis of a stent with smooth cell shapes. Design optimization is systematically applied to minimize the curvature and reduce the bending strain of the elements defining the lattice cells. A novel cell geometry with improved fatigue life and radial supportive force is introduced for Nitinol self-expandable stent-grafts used for treating abdominal aortic aneurism. A parametric study comparing the optimized stent-graft to recent stent designs demonstrates that the former exhibits a superior anchoring performance and a reduction of the risk of fatigue failure.

Compliance and Longitudinal Strain of Cardiovascular Stents: Influence of Cell Geometry

A systematic study on the influence of the cell geometry of a cardiovascular stent on its radial compliance and longitudinal strain is presented. Eight stent cell geometries—based on common lattice cells—are compared using finite element analysis. It is found that, for a given strut thickness, the radial compliance depends on the shape of the cell and is intimately connected with the longitudinal strain through effective Poisson’s ratio, which depends on the cell geometry. It is demonstrated experimentally that a hybrid stent containing both positive and negative Poisson’s ratio cell lattice geometries exhibited very low values of longitudinal strain. This study indicates that cell geometries may be tailored to minimize longitudinal stresses imposed by the stent onto the artery wall.

The Relation Between the Arterial Stress and Restenosis Rate After Coronary Stenting

Two commercially available stents (the Palmaz–Schatz (PS) and S670 stents) with reported high and low restenosis rates, respectively, have been investigated in this paper. Finite element models simulating the stent, plaque, and artery interactions in 3 mm stenosed right coronary arteries were developed. These models were used to determine the stress field in artery walls after stent implantation. The material properties of porcine arteries were measured and implemented in the numerical models. The stress concentration induced in the artery by the PS stent was found to be more than double that of the S670 stent. It demonstrated a good correlation with the reported restenosis rate. The effects of stent structures, compliance mismatch, plaque geometry, and level of stenosis were studied. Results suggested that stent designs and tissue properties cause alterations in vascular anatomy that adversely affect arterial stress distributions within the wall, which impact vessel responses such as restenosis. Appropriate modeling of stent, plaque, and artery interactions provided insights for evaluating alterations to the arterial mechanical environment, as well as biomechanical factors leading to restenosis.

Fast virtual deployment of self-expandable stents: method and in vitro evaluation for intracranial aneurysmal stenting

INTRODUCTION

Minimally invasive treatment approaches, like the implantation of percutaneous stents, are becoming more popular every day for the treatment of intracranial aneurysms. The outcome of such treatments is related to factors like vessel and aneurysm geometry, hemodynamic conditions and device design. For this reason, having a tool for assessing stenting alternatives beforehand is crucial.

METHODOLOGY

The Fast Virtual Stenting (FVS) method, which provides an estimation of the configuration of intracranial stents when released in realistic geometries, is proposed in this paper. This method is based on constrained simplex deformable models. The constraints are used to account for the stent design. An algorithm for its computational implementation is also proposed. The performance of the proposed methodology was contrasted with real stents released in a silicone phantom.

RESULTS

In vitro experiments were performed on the phantom where a contrast injection was performed. Subsequently, corresponding Computational Fluid Dynamics (CFD) analyzes were carried out on a digital replica of the phantom with the virtually released stent. Virtual angiographies are used to compare in vitro experiments and CFD analysis. Contrast time-density curves for in vitro and CFD data were generated and used to compare them.

CONCLUSIONS

Results of both experiments resemble very well, especially when comparing the contrast density curves. The use of FVS methodology in the clinical environment could provide additional information to clinicians before the treatment to choose the therapy that best fits the patient.

Finite element shape optimization for biodegradable magnesium alloy stents

Biodegradable magnesium alloy stents (MAS) are a promising solution for long-term adverse events caused by interactions between vessels and permanent stent platforms of drug eluting stents. However, the existing MAS showed severe lumen loss after a few months: too short degradation time may be the main reason for this drawback. In this study, a new design concept of MAS was proposed and a shape optimization method with finite element analysis was applied on two-dimensional (2D) stent models considering four different magnesium alloys: AZ80, AZ31, ZM21, and WE43. A morphing procedure was utilized to facilitate the optimization. Two experiments were carried out for a preliminary validation of the 2D models with good results. The optimized designs were compared to an existing MAS by means of three-dimensional finite element analysis. The results showed that the final optimized design with alloy WE43, compared to the existing MAS, has an increased strut width by approximately 48%, improved safety properties (decreased the maximum principal stress after recoil with tissue by 29%, and decreased the maximum principal strain during expansion by 14%) and improved scaffolding ability (increased by 24%). Accordingly, the degradation time can be expected to extend. The used methodology provides a convenient and practical way to develop novel MAS designs.

Tensile and compressive properties of fresh human carotid atherosclerotic plaques

Accurate characterisation of the mechanical properties of human atherosclerotic plaque is important for our understanding of the role of vascular mechanics in the development and treatment of atherosclerosis. The majority of previous studies investigating the mechanical properties of human plaque are based on tests of plaque tissue removed following autopsy. This study aims to characterise the mechanical behaviour of fresh human carotid plaques removed during endarterectomy and tested within 2h. A total of 50 radial compressive and 17 circumferential tensile uniaxial tests were performed on samples taken from 14 carotid plaques. The clinical classification of each plaque, as determined by duplex ultrasound is also reported. Plaques were classified as calcified, mixed or echolucent. Experimental data indicated that plaques were highly inhomogeneous; with variations seen in the mechanical properties of plaque obtained from individual donors and between donors. The mean behaviour of samples for each classification indicated that calcified plaques had the stiffest response, while echolucent plaques were the least stiff. Results also indicated that there may be a difference in behaviour of samples taken from different anatomical locations (common, internal and external carotid), however the large variability indicates that more testing is needed to reach significant conclusions. This work represents a step towards a better understanding of the in vivo mechanical behaviour of human atherosclerotic plaque.