Biomechanics Study On Intravascular Stent Structure

Introvascular stent is the interventional treatment implant for cardiovascular stenosis diseases, and security and stability of the mechanical support structure is a fundamental issue in the stent study. To establish the stent structure's design guidelines and mechanical perfor-mance evaluation system is the important issue about the stent structure biomechanics study need to be solved, which has the great scientific significance and the wide range of clinical applications for stent implantation therapy; in addition, clinical applications and theoretical analysis have comfirmed that the collapse of stent structures often appear in the connecting beams of it's structural units,which fully shows the beams and structural units of stent have the crucial role in the security and stability of whole stent structure. The research is still rare in the literature, so this work is mainly carried out for both of these issues.According to the process of stent's whole structure collapse by reinforced beam fracture and structural unit instability, this paper presents a new method of stent structure study, namely, "Beam - structural unit" method. The method is to optimize two important features of reinforced beam and structural unit, and to propose a more security and stability of the stent structure. Thesis work includes the following three main elements:First, Thesis bracket stent beam reinforcement characteristics of trapezoidal cross-section optimization. The characterial style of stent structure design owns inverted tra-pezoidal beam cross-section with coefficient k about the proportion value between the inner and outer section edge length, through three proposed analysis on beams'bending strength, mechanical damage of vascular wall support and hemodynamic status reinforced beam sur-face.1, study on beam cross-section of stent network structure can reinforce the beam bending strength. Trapezoidal cross-section beam of stent structure can effectively avoid the stress conditions in a structural material collapse phenomenon. The larger the coefficient value k, the greater the bending strength of stent.2, finite element numerical results show that stent contact with the vessel wall model under load in the expansion, k value of the larger stress within stent and the vessel wall are less than k value of the smaller stress whith the model, which can improve stent's anti-collapse proper- ties and decrease the damage of vascular wall.3, studies showed that the corner in the rectangular cross-section beam surface of stent struc-ture made of is easy to form the corners of blood flow near the vortex. In this paper, the sur-face of the stent with the larger k value and the blood vessel wall in the dynamic model of blood flow don't have the vortex, which can effectively improve the blood cells, other bio-logical macromolecules, and emboli fluidity of movement.Second, research is proposed about the stent circumferential and axial connection struc-ture. From two aspects between the deformation energy principle and integrated biomechani-cal parameters multi-objective optimization, stent is finally composed of close-arc circumfe-rential connecting elements and the N-shaped axial connecting structural units.1, deformation energy principle is used to analyze the common close-arc, parallel-arc and open-arc beam element in stent structure. Closed-arc beam element with large strain energy, can absorb more energy from balloon expansion, resistretract the vessel pressure; it have a larger load displacement to get the larger expansion radius of stent.2, taked the five radial biomechanical properties, such as metal coverage, expanding radius, the maximum stress within the stent, the axial retraction rate and radial recoil rate to the ob-jective function, and taked the four characteristic parameters, such as the beam's arc con-necting form, the circumferential unit number, unit axial length and beam width to the design variables, stent circumferential structure is optimizatively designed. Through the weighted priority order calculation of integrated bio-mechanical properties, the result shows the closed-arc radial connecting beam unit in the smaller beam width and more to the circumfe-rential unit number, and the larger unit axial length of the result of stent design of the struc-ture can achieve a good overall biomechanical properties.3, FEM anlysis of bending experiment on the different axial connection stent structure show that the relative radial displacement of contact surface between N-shaped axial connection stent structure and the catheter is smaller, less stress within stent structure; and give that con-tributions on N-shaped axial connection stent structure for flexibility and adherent are more prominent, compared with straight-shaped, S-shaped, W-shaped and WD-shaped structure.Finally, to establish the balloon model of“Beam-structural unit”stent is provided to have the FEM analysis, and to analyze and compare with BX Velocity^TM stent. The result demonstrate the stent structure designed by“Beam-structure unit”method, which all have a more favorable biomechanical security and stability both with new biomedical materials WE43 and with the traditional 316L stainless steel.1,“Beam-structure unit”stent is taken to FEM analysis. The maximum stress within the stent under balloon loads is much smaller than its ultimate strength of materials, its axial retraction rate and radial recoil rate is smaller, and its ultimately expand radius to support the large blood vessels can be size requirements, which make it get the more excellent synthet-ic biomechanical performance.2, BX Velocity^TM stent is taken to FEM analysis. The maximum stress value appear in the re-gion of stent beam's frame corner, consistent with the results in related literature, which demonstrate the correctness and feasibility of theory and FEM simulaiton in the paper.3, two stent structure is comprartive analyze by FEM, differently taked WE43 magnesium alloy and traditional 316L stainless steel as material. Compared with BX Velocity^TM stent structure, both in the use of WE43 magnesium alloy and traditional 316L stainless steel as material,“Beam-structure unit”stent has more excellent mechanical properties such as radial recoil rate and maximum stress during expansion.Compared with the existing experience gduidance and contrast verification method of stent structure design,“Beam-structure unit”method proposed to characteristics optimize for the stent reinforced beam and structural unit. This method support system theory to guide the mechanical security design, to avoid reinforced beam fracture and stent structural collapse; reinforced beam of trapezoidal cross-section and closed-arc shape structural unit are two im-portant optimal features of stent structure proposed in this paper, and are the issues that scho-lars have not yet studied. "Beam-structural unit" method establishs a technology framework for the stent structure's design guidelines and mechanical performance evaluation system, and is of great scientific significance and clinical application value.