An argument for the use of multiple segment stents in curved arteries

Investigation of braided stents in curved vessels in terms of "Dogbone" deformation

"Dogbone" deformation that the diameters of two ends are larger than the middle diameter of the stent under the effect of the balloon expanding, is one of the important standards to evaluate the mechanical properties of vascular stents. It is a huge challenge to simulate and evaluate the "Dogbone" behaviors of braided stents in the curved vessels. In this study, the key work was to investigate the "Dogbone" deformations of braided stents in the curved vessels by designing main parameters including strut diameter, braiding angle, and the circumferential number of unit cell. Based on the "Dogbone" stents in the curved vessels, the impact of "Dogbone" on the fatigue properties of braided stents was analyzed under the pulsatile effect of vessels. The influence of "Dogbone" stents on stress distribution of vascular walls was studied. To evaluate the "Dogbone" behaviors of stents in the curved vessels, the calculation method of "Dogbone" was improved by calculating the centerline and the bus bar of the curved vessels. Braided stents with various parameters (strut diameter t = 100,125 and 152 μm, braiding angle α = 30, 40 and 50°, the circumferential number of unit cell N = 8, 10, and 12) were designed respectively. Numerical simulation method was used to mimic the "Dogbone" deformation after stent expansion. The results showed that strut diameter and braiding angle had more influence on "Dogbone" deformations than the circumferential number of unit cell. "Dogbone" deformation could adversely affect fatigue performance and vascular walls.

Determination effect of two different NiTi stents on the vessel wall and studying their flexibility using finite element method

Finite element simulation is used to analysis stent designs, extension as well as interaction between a stent and a vessel. In this paper, two different stents with different geometries have been simulated. One is Zilver stent and the other one is Navalis stent. The aim of this study is to determine the effect of stents deployment with various designs that are made of shape memory alloy (SMA) on the distribution of vessel wall stresses by using computational modeling approach. The constitutive model which described the behavior of SMA is based on Microplane model. In addition, SMA stents have been simulated under torsion loading to compare the flexibility of various designs under different conditions. The superelastic behavior and shape memory effect of SMA stents are investigated in this paper. The numerical simulation results show the different geometries of stents have significant effect on the arterial wall. The results show the Navalis stent causes less stress on the arterial wall and it is more flexible than the Zilver stent under the same torsion loading.

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.

A finite element simulation method to evaluate the crimpability of curved stents

Stenting of curved arteries is more challenging than straight vessels. There has been an increasing need for new techniques to treat lesions in highly curved locations. One generic idea is to use curved stents to treat lesions in such curved locations. Computational modeling of straight stent crimping which is being used to evaluate the structural performance of the stents has been done vastly in the past. However, there has not been much simulation work on crimping of curved stents due to the challenges associated with applying the boundary conditions. Here we propose a new method to crimp a curved stent to a smaller diameter by incorporating a simple algorithm to generate the required boundary conditions supplementing the finite element (FE) code. To achieve this, a curved crimper is modeled and used to apply crimping loading on the curved stent evaluate its crimpability. Our method provides a simple yet very useful tool which can be implemented in finite element packages to simulate crimping of curved stents using curved crimpers. This method can also be used to expand balloon expandable stents by inflating curved balloons.

Simulated biomechanical responses at a curved arterial segment after Wingspan Stent deployment in swine

OBJECTIVES

Endovascular treatment with the Wingspan Stent is frequently associated with in-stent restenosis at the curved portion, leading to late-phase stroke. To explore the cause of stroke complications after treatment with the Wingspan Stent, we simulated the biomechanical responses at a curved arterial segment using the finite element method.

METHODS

A Wingspan stent was deployed at a slightly curved ascending pharyngeal artery (APA) in swine. Several stress distributions modeling solid mechanics were analyzed with structural deformation. Histopathological analysis of the selected APA was assessed at 28 days after stenting.

RESULTS

Arterial straightening was simulated in this study. Both radial stress (RS) and circumferential stress (CS) concentrations increased at both stent ends. Marked lower axial stress (AS) concentration was observed at the outer wall of an arterial curvature. The proximal stent segment, ending in the curved portion, significantly impacted the solid mechanical environment. Eccentric neointimal hyperplasia was observed at the curved segment.

DISCUSSION

These results show that the Wingspan stent exaggerated the non-uniform stress distributions in a curved artery. The understanding of stent-arterial wall interactions is of value to identify the current limitations of intracranial stenting, and will help to improve this treatment methodology and future devices.