摘要
血管内支架是用于心血管重度狭窄疾病介入治疗的植入器械,支架结构的力学安全稳定性是血管内支架研究工作的根本性问题。建立支架结构的设计指导及力学性能评价体系是血管内支架结构生物力学研究的重要课题,亟待解决,这对于支架植入手术具有重大的指导意义;此外,临床应用和理论分析都已证实,支架结构的塌陷经常出现在其结构单元的连接筋梁处,这充分说明支架的筋梁、结构单元力学性能对其整体结构的安全稳定性有着至关重要的作用,但这方面的研究却还未见之于文献报道。本文的工作,主要是针对以上两个问题进行的。
依据支架筋梁断裂、结构单元失稳进而整体结构塌陷的过程,本文提出了一种新的支架结构研究方法,即“筋梁-结构单元”法。该方法对支架筋梁和结构单元两个重要的特征进行优化,提出一种更具生物力学安全稳定性的血管内支架结构。本文工作包括以下三个主要研究内容:
一、本文首先对支架筋梁进行倒梯形横截面的特征优化。从支架网状结构筋梁的抗弯强度、支架对血管壁的力学损伤以及筋梁表面血流动力学状况三方面进行分析,提出了一个外、内边截面比例系数k值大的倒梯形横截面支架筋梁特征。
1、对支架网状结构的筋梁横截面进行力学研究,提高筋梁在扩张弯曲变形过程中的抗弯强度。筋梁的外、内边截面比例系数k值越大,其抗弯强度越大,则倒梯形横截面的支架筋梁可有效避免支架受力时出现结构塌陷现象。
2、支架与血管壁的接触模型的有限元数值计算结果显示,在扩张载荷的作用下,k值较大的模型的支架内应力以及血管壁内应力,均小于k值较小的模型。k值较大的筋梁横截面,可提高支架的抗塌陷性能和血管壁的抗损伤程度。
3、有研究表明,支架结构筋梁的矩形横截面使得支架表面边角附近,容易形成血液流场的涡流;而支架与血管壁表面的血流动力学模型在k值较大时,支架倒梯形横截面筋梁边角区域无涡流现象,可有效改善血液中细胞、生物大分子等的运动流畅性。
二、进行支架结构单元的特征优化。从变形能原理、综合生物力学参数多目标函数优化两方面,进行支架结构单元的优化,最终设计出一种由闭口弧形周向结构单元和N形轴向连接结构单元组成的血管内支架。
1、利用变形能原理分析了支架结构中常见的闭口、平行和开口弧形结构单元,闭口弧形结构单元具有较大的变形能,可吸收较多的球囊扩张能量,可用于抵抗血管回缩压力;同时,闭口弧形结构单元可拥有较大加载位移量,以得到较大的支架扩张半径。
2、对各种支架周向结构模型进行有限元数值模拟计算,并以模型的弧连接形式、周向单元数、单元轴长和筋梁宽度作为结构特征设计变量,以金属覆盖率、扩张半径、最大应力、轴向回缩率和径向回缩率五个生物力学参数作为目标函数,进行支架周向结构综合生物力学参数的加权优序计算,结果显示在较小的梁宽度、较多周向单元数及较大单元轴长时,闭口弧形结构单元可使支架结构获得良好的综合生物力学性能。
3、进行不同轴向连接结构的支架弯曲有限元分析,结果显示N形结构单元的支架结构,与弯曲导管的径向相对位移较小、结构最大内应力较小。该结构单元对支架结构贴壁性和柔顺性的贡献,较之直杆、S形、W形和WD形结构单元更为突出。
三、建立“筋梁-结构单元”支架球囊扩张模型,进行有限元力学分析,并与BX Velocity~(TM)支架进行分析和比较,论证了“筋梁-结构单元”法设计出的支架结构具有更为良好的生物力学安全稳定性。
1、首先,进行“筋梁-结构单元”支架结构的有限元力学分析。支架结构在球囊扩张载荷作用下,其内部最大应力远小于材料极限强度,轴向、径向回缩率较小,最终扩张半径可达到疏通大血管尺寸要求,可见其综合生物力学性能较为优异。
2、对BX Velocity~(TM)支架进行有限元分析,其扩张后最大应力值出现在支架结构弯角连接区域,该结果与相关文献一致,论证了论文理论分析以及有限元数值计算方案的正确性和可行性。
3、对两种支架结构分别使用传统316L不锈钢和WE43镁合金作为材料,进行有限元比较分析。与BX Velocity~(TM)支架结构相比,无论在使用316L不锈钢还是WE43镁合金作为材料时,“筋梁-结构单元”支架的径向回缩率和扩张最大应力等力学性能均更为优异。
与现有支架结构设计的经验指导性和对比验证方法相比,“筋梁-结构单元”法提出了针对支架筋梁和结构单元的特征优化方法,系统理论地进行支架结构的安全性力学设计,避免了支架的筋梁断裂和结构塌陷;筋梁倒梯形横截面形状和闭合弧形结构单元,是本文提出的支架结构两个重要特征优化设计,也是目前尚未有学者研究过的问题。“筋梁-结构单元”法,为支架结构的设计指导及力学性能评价体系建立了技术框架,具有重大的科学意义和临床应用价值。
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.
引文
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