血管支架設計對易破損動脈粥狀硬化斑塊之纖維帽應力影響

Abstract

急性心肌梗塞是一種嚴重的心血管疾病，其急性症狀主要是因為大量血栓堆積於原有的硬化斑塊上，造成血管阻塞，使得人體血液無法順利流通所引起。其中與急性心肌梗塞相關的急性冠狀動脈病症多數主要是由易破損硬化斑塊的破裂所致，此種易破損硬化斑塊的組織含有肥大的脂質核心、薄纖維帽、鈣化組織以及發炎細胞浸潤；易破損硬化斑塊的破裂對於造成急性冠狀動脈病症有相當大的影響力，而易破損硬化斑塊的破裂主要受到其本身的組織形態與生物力學環境引起的應力集中現象所影響。以生物力學角度來說，在硬化斑塊上的應力，如拉伸應力，是影響硬化斑塊破裂的重要因素，因此在探討易破損硬化斑塊的易破性時，必須將生物力學環境納入考量。雖然現今醫療技術已在心血管疾病的預測與治療上有所進步，但因易破損硬化斑塊破裂而引起的急性心肌梗塞仍有相當高的發生機率。本研究目的為分析並評估自動擴張式血管支架對易破損硬化斑塊上之薄纖維帽破裂的影響性，進而提出一心血管支架的設計概念，能有效的降低薄纖維帽上的峰值應力。本研究建立含有易破損硬化斑塊的冠狀動脈之有限元素模型，此模型中包含血管內膜、中膜、外膜以及脂質核心，且其形態均依據組織學上的理想幾何形狀建構；在組織材料方面，以等向性、高彈性之材料模擬之。模型建構後，使用有限元素分析軟體模擬自動擴張式血管支架部署於含有易破損硬化斑塊的冠狀動脈內，並探討在纖維帽上的應力分布情況。研究結果顯示，以較多Crown數量的自動擴張式血管支架部署於易破損硬化斑塊時，薄纖維帽上之應力分布較為均勻；在薄纖維帽的峰值應力方面，含有較多Crown數量的血管支架所造成的應力值遠小於Crown數量較少的血管支架，而本研究中的24-Crown血管支架相對於標準血管支架，降低峰值應力之幅度高達18%。本研究提出增加自動擴張式血管支架Crown數量的血管支架設計概念，此設計方式能有效平均分散血管支架部署時對薄纖維帽產生的應力並降低其峰值應力，避免易破損硬化斑塊的破裂，提供作為新一代血管支架的設計參考。

Acute myocardial infarction (AMI) is one of the most serious condition of cardiovascular disease. Most commonly, the acute syndromes of acute myocardial infarction are caused by a local arterial occlusion with a thrombus overlying a pre-existing atherosclerotic plaque which could restrict blood supply to the downstream arteries. Further, some acute coronary syndromes defined by acute myocardial infarction result from rupture-prone or so-called vulnerable plaque consisting of an accumulation of cells, large lipid, thin-fibrous cap, calcium, and inflammatory infiltrates. Plaque rupture has represented important influences on the majority of acute coronary syndromes. Generally, rupture has mainly been associated with stress concentration, which is affected by both plaque morphology and biomechanical environment. In terms of biomechanical factors, stresses, for example plaque wall tensile stress, are regarded as important factors in plaque rupture process and have to be taken into account for plaque vulnerability assessment. Nowadays, despite major advances in prevention and treatment of cardiovascular disease, rupture of vulnerable plaque which causes acute myocardial infarction remains in high probability. The aim of this study is to evaluate the influence of endovascular self-expanding stent on the risk of the rupture of a vulnerable plaque, also on proposing a novel design concept of an intravascular stent to effectively reduce the peak stress of a fibrous cap. For this purpose, a finite element model of a coronary artery with vulnerable plaque was developed. The constructed model with idealized geometries based on histology images of human coronary arteries contained intima, media, adventitia and lipid core section. For the tissue properties, isotropic and hyperelastic material models were used. Afterward, the deployment of self-expanding stent in coronary artery with vulnerable plaque was modeled by finite element analysis and the stress distribution on fibrous cap was investigated. Results indicated the stresses induced by the increased crown of a self-expanding stent were more uniformly distributed on the fibrous cap of a vulnerable plaque. Moreover, the computed peak stress for fibrous cap was much higher with more crowns stent in comparison to that with less; also, the reduction of peak stress between 24-crown stent and standard stent was up to 18 percentage points. This study demonstrates that the new concept of stent design, that is, to increase the number of crowns of a self-expanding stent, which could be both efficient and effective, distributes stress uniformly and lowers the peak stress on the fibrous cap, avoiding the rupture of a vulnerable plaque; it also develops value-added approach to the optimization of future stent design.