Finite Element Analysis Of The Mechanical Behavior Of Coronary Stent And Its Structure Optimization

The Percutaneous Transluminal Coronary Angioplasty (PTCA) has become a primary therapy of coronary heart disease in our country. But in view of the internal application and research on the coronary stent, which, as an important implanted device, is often used in the PTCA, the present situation is that limited attentions have been paid to the investigations of stent design, manufacturing and testing. Most of the central techniques about stent is still dependent on the external. And meanwhile, the in-stent restenosis has become the main bottle-neck to the development of stent technique. The ideal stent design will remain the focus of attention in the future. The main purpose of this paper is to research and develop new types of coronary stent which would possess independent intellectual property and optimize the coronary stent structure by means of the finite element method (FEM). In the paper, the technique characters, clinical application, developmental trends of coronary stent and the fundamental of elastoplastic FEM are firstly introduced. Secondly, aiming at the 316LSS coronary stent, some studies on its FEM structure designing, manufacturing and testing are carried out. The contents include designing the new types of coronary stent, exploring and practicing the processing techniques of coronary stent, simulating the expanding and compressing process of coronary stent by FEM, designing and optimizing the no-dogboning and no-shortening coronary stents, testing the stem's mechanical behavior when it is expanding and anti-compressing in vitro, and doing the stent-implanted experiments in animal. Moreover, included in the research is the developing of two instruments, that is, the stent-premounted machine and the stent strength-testing instrument. Among these, analyzing the mechanical behavior of coronary stent and optimizing its structure by FEM are the focus in our work.Through the researches mentioned above, some main results or conclusions are as follows:(1) Based on the summarizing and analyzing of the current coronary stents, the general principia to the stent design was brought forward, that is, stent is generally composed of ring-shaped supporters and individual connecters. The radial strength of stent mainly depends on the structure of its ring-shaped supporters, yet its flexibility lies on the structure of the connectors. The length of stent can be adjusted by the way of appending or cutting the supporters, and the stent expanding range can be controlled by increasing ordecreasing the number of the wave-shaped structure units that compose the supporter. According to the rules and the numerical simulation results conducted in this paper, four new coronary stent designs with different structures were presented.(2) By exploring and practicing the processing techniques of coronary stent, it can be concluded that the most suitable processing method is the three-dimensional laser-cutting, and the best processing techniques are as follows respectively: (Dlaser cutting;(2) decontaminating and degreasing through ultrasonic clearing;?removing slag and coat of oxide by acid pickling;?electrochemical polishing;?passivating.(3) From the finite element simulation and the experimental validations, all of which were undertaken around the proprietary stent designs, it can be found that:? Using a specified nonzero displacement as the load to expand stent can not reflect its actual expanding form. It is the pressure that should be used as the surface load to dilate the stent.(2) Changing the stent strut's dimensions can affect the stent expanding pressure, which is needed for its dilation, and the stent's anti-compressing behavior performed after it being dilated, but not so much influence on its radial and axial recoil. Changing the stent expansion scale also has a little influence on its radial and axial recoil, but has a visible influence on its anti-compressing behavior. Yet changing the stent's material and its local structure have a great influence on both the stent's expanding and anti-compressing performances.(3) The shortening of stent occurring in the course of expansion is not associated with the change of dimension of its strut nor the material it uses, but closely related to the stent expansion scale, the structure of its connectors and the way they connect. Through the proper adjusting, a no-shortening stent design would be obtained. Moreover, to get the best result, too much unsymmetrical factors in designing should be avoided.@ The experiment result of coronary stent's anti-compressing performance is well consisted with that of finite element simulation, indicating that the anti-compressing finite element model of coronary stent is able to be used as a standard to test their radial strength and can substitute for the stent prototype testing to some extent.(5) As the assembling of stent and balloon, the dogboning, which always occurrs in the transient expansion process of stent, can be weakened to a certain extent whether by decreasing the over length of the balloon or by increasing the distal stent strength, and only these two methods coacting, would that produce a more apparent result.(4) Considering the coronary stent of type DT-2 developed by us, the results of in vitro utmost expansion testing and the stent-implanted experiments in dogs indicated that: this kind ofcoronary stent would show a good shape with remarkable uniformity and consistency during its expanding process. It was better concerning its operating performance during implantation and the biocompatibility after being implanted, and was a stent that could achieve a good treatment for patients. Is would provide a powerful basis for the tryout of this kind of stent in clinic.(5) By operating the stent-premounted machine designed and manufactured by ourselves, we think that this machine can substitute for the handwork to crimp the stent onto the balloon uniformly. After being crimped the stent/balloon system can combine tightly with its thin profile. It has been proved that this device is capable of satisfying the needs of experiment and manufacture.(6) By the actual anti-compressing testing of coronary stents, it shows that the self-designed stent strength-testing instrument, which uses spring ring as the sensor, can well deal with such technical difficulties as the micro force, small displacement, and following the process of structure instability that may happen in some stents during the testing. So this instrument is capable of being used as an effective tool to test coronary stent's radial strength.