Biomechanical Study Of Aortic Dissection Based On Computational Fluid Dynamics And Mechanical Experiments

Background: Aortic dissection is a high-risk cardiovascular disease which is characterized by the partial rupture of the intima of the aorta,and the blood perfused into the media through the tear,forming true and false lumen in the aorta.The wall tissue which consist of the intima and partial media between the lumens was called intimal flap.Mechanical mechanism is the key factor related to the occurrence and development of dissection.The biomechanical analysis of aortic dissection is of great significance for studying the pathogenesis and predicting the development of the disease.Here,we analyzed aortic dissection from the hemodynamics and intimal flap mechanics.In the aspect of hemodynamics,the influence of different velocity boundary conditions on the simulation results was determined by comparing computational fluid dynamics with the medical velocimetry.In the aspect of tissue mechanics,we obtained the mechanical properties and basic parameters of constitutive equation of the flap through the uniaxial tensile test and the pathological staining analysis.Both researches are closely related to morphological changes of the dissection and the safety of the stent implantation,providing theoretical basis for the study of clinical conditions and surgery planning.Methods: For the hemodynamics analysis,this paper built a patient-specific model based on computed tomography angiography(CTA)images of patient with type B aortic dissection through image segmentation and three-dimensional reconstruction and obtained the distributions of the three-dimensional blood flow velocity in the aorta and the flow rate of the key anatomical sites by 4D PCMRI and Doppler ultrasound.Using computational fluid dynamics(CFD)simulation method and the patient-specific model as the basic model,medical velocity measurements as boundary conditions,we compared the influence of different boundary conditions on the simulation results to determine the accuracy of calculation,so as to effectively explore the aortic dissection flow characteristics.In the aspect of flap mechanics,we conducted uniaxial tensile tests on the intimal flap(dissection:329 strips;normal 9 strips)which excised from 44 patients with aortic dissection undergoing open surgery and 7 cases of normal aorta undergoing aortic valve replacement and established the relationship between the stretch ratio and the Cauchy stress data,and then calculate the representative data points through the strain energy average to compare the mechanical properties of the dissection tissue and the normal tissue.Combining the HE staining and EVG staining which showed the elastic fibers of vascular tissue,we evaluated the remodeling status of dissected vessels and investigate the mechanical properties,constitutive equation of the intimal flap.Results: In the aspect of flow simulation,the results based on the boundary conditions of medical velocimetry and statistical average model are consistent qualitatively in terms of flow distribution.However,the blood flow simulation results overestimate the flow volume compared with measurements quantitatively,and the flow difference is within 25.22%.Lateral contrast among various velocity boundary conditions,the results of simulations which adopted the patient-specific velocity profiles measured by 4D PC-MRI or the statistical average model boundary conditions at the inlet are in better agreement with the measurements.In the aspect of tissue mechanics,the results showed that the intimal flap had similar mechanical properties in the longitudinal and circumferential directions with no significant anisotropy;the flap of aortic dissection with hypertension and cardio-cerebrovascular disease were stiffer than that of aortic dissection only with hypertension and the former has a higher elastic fiber content,suggesting the importance of effective control of blood pressure for patients with dissection and other vascular diseases;we also obtained the basic parameters of Mooney-Rivlin constitutive equation for the intimal flap.Conclusion: The occurrence,development and stent intervention of type B aortic dissection are closely related to its biomechanical properties.Computational fluid dynamics(CFD)is the main method to explore the clinical problems and provide guidance for the treatment.However,the results are affected by the boundary conditions;on the other hand,there is still no detailed report on the mechanical properties of the intimal flap.This study determined the feasibility of the analysis of blood flow distribution through flow simulation and numerical differences compared with the clinical measurements,and put forward a more accurate method of setting the boundary conditions combining medical velocimetry.We also determined the mechanical properties of intimal flap,mechanical characteristics of flap tissue with different complications,and constitutive equation parameters of intimal flap based on Mooney-Rivlin strain energy density function.This study can provide an effective hemodynamic simulation process and material parameters for the aortic dissection and the stent intervention,providing the necessary parameters for subsequent fluid-structure interaction analysis and theoretical basis for the clinical treatment planning based on computational mechanics.