Finite element simulation of slotted tube (stent) with the presence of plaque and artery by balloon expansion

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Abstract

In this paper, results are presented of nonlinear finite element simulations performed to analyse the interactions between the slotted tube (stent) and the plaque or artery using finite element method. The post-expansion response of the slotted tube in the presence of the plaque and artery was also investigated. The analysis was performed using finite element packages LS-DYNA explicit solver for solution of the problem and ANSYS (version 5.7) as pre- and post-processor. The slotted tube stent was subject to a pressure load applied on the inner surface of the balloon. Then the slotted tube expands and presses the plaque against the arterial wall. In order to reduce the solution time, only one-eighth of the model was developed by taking advantage of symmetry. A bi-linear elasto-plastic and hyperelastic material models were assumed for the slotted tubes and the balloons, respectively. The material models used for the plaque and arterial wall were defined as linear isotropic and nearly incompressible. The deformation characteristics of the slotted tube were analysed. The stresses in the plaque and artery due to the interference of slotted tube stent were discussed.

Introduction

Implantation of coronary stent in a human artery to treat coronary artery disease is a complicated process and the task is even difficult to perform. Literature studies show that the behaviour of the stent expansion by experimental tests during the stent deployment in an artery is full of complication and difficult to predict [1], [2], [3]. However, experimental evidence indicates that the interaction between the stent and the artery as a significant cause for the activation of stent-related scular injuries which contributes greatly to the recurrence of restenosis [4], [5], [6], [7], [8], [9], [10].

Computer simulations of stent deployment process were inactive and began to receive a little attention not until the last decade when a number of software tools became available and gained the recognition from engineering prospect. Many researchers deem this approach as an alternative to experimental test as it possesses a few advantages such as cost, the nature of the structure, experimental set up, etc. [11], [12], [13], [14]. To date, a lot of researchers also look to compare experimental and computer simulation results as they could compliment each other and achieve marvellous outcomes with fine conclusions. In particular, using computer simulation it allows researchers to look closely into the problem or specific element of interest in which they never thought could be possible before. A considerable amount of publications pertaining to the computational investigations within this field have been carried out to study the stent-related restenosis [15], [16], [17]. However, most of the concerned studies neglect the fact that the entire system should be taken into consideration if the problem was to be fully understood.

The main objective of the present work is to investigate the expansion characteristic of slotted tube stent (J & J Palmaz-Schatz like) with the presence of plaque and artery. The effect of the reformation of the occlusion by plaque and artery on slotted tube stent after the deployment was investigated. The correlations between stent–plaque contact area and deployment pressure was also analysed. The stresses in the plaque and artery due to the interference of slotted stent were discussed.

Section snippets

Finite element model

The ANSYS finite element package was used to develop the geometry of the balloon, slotted tube stent, plaque and artery. A three-dimensional finite element model of a stenosed artery was created and a simulation of a stent implantation procedure was performed. The slotted tube stent consists of 40 slots that are equally spaced throughout the stent model. The geometry of the slotted tube stent is shown in Fig. 1. The balloon was placed inside the slotted tube with the outside diameter of the

Loading and solution

Fig. 3 shows the load history of the simulation process. The pressure load was applied on the inner surface of the balloon. The balloon expands and causes the slotted tube stent to expand as well. As a result, the slotted tube stent will press the plaque against the arterial wall. The simulation of the expansion process was carried out with pressure load varying in three stages, i.e. pressure increasing, constant load pressure and pressure decreasing. These three loading stages were used to

Stress analysis within stent, plaque and arterial wall

Fig. 4 shows the residual stress distributed within the slotted tube stent when the balloon is deflated. The maximum stress location does not change despite the presence of plaque and arterial wall. The major von Mises stresses are localised in the corners of the slots and the minor von Mises stresses are localised in the middle of the body struts. As before, the minimum stresses were found to be located at the bridging struts near the end of the slotted tube stent. A small region of peak

Conclusions

The simulation successfully demonstrated the function and the application of slotted tube stent in which the artery is successfully opened and the stent remained in the artery as a scaffold. The results of the present study demonstrate that stents are able to withstand the remodelling of artery and plaque that otherwise may have contributed to restenosis. Finite element analyses demonstrated that maximum surface contact stress grew higher at a point where the plaque surface and strut surface

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