Mechanical characterisation of the human thoracic descending aorta: experiments and modelling.

This work presents experiments and modelling aimed at characterising the passive mechanical behaviour of the human thoracic descending aorta. To this end, uniaxial tension and pressurisation tests on healthy samples corresponding to newborn, young and adult arteries are performed. Then, the tensile measurements are used to calibrate the material parameters of the Holzapfel constitutive model. This model is found to adequately adjust the material behaviour in a wide deformation range; in particular, it captures the progressive stiffness increase and the anisotropy due to the stretching of the collagen fibres. Finally, the assessment of these material parameters in the modelling of the pressurisation test is addressed. The implication of this study is the possibility to predict the mechanical response of the human thoracic descending aorta under generalised loading states like those that can occur in physiological conditions and/or in medical device applications.

Factors influencing the mechanical behaviour of healthy human descending thoracic aorta.

In recent times, significant effort has been made to understand the mechanical behaviour of the arterial wall and how it is affected by the different vascular pathologies. However, to be able to interpret the results correctly, it is essential that the influence of other factors, such as aging or anisotropy, be understood. Knowledge of mechanical behaviour of the aorta has been customarily constrained by lack of data on fresh aortic tissue, especially from healthy young individuals. In addition, information regarding the point of rupture is also very limited. In this study, the mechanical behaviour of the descending thoracic aorta of 28 organ donors with no apparent disease, whose ages vary from 17 to 60 years, is evaluated. Tensile tests up to rupture are carried out to evaluate the influence of age and wall anisotropy. Results reveal that the tensile strength and stretch at failure of healthy descending aortas show a significant reduction with age, falling abruptly beyond the age of 30. This fact places age as a key factor when mechanical properties of descending aorta are considered.

A volumetric model for growth of arterial walls with arbitrary geometry and loads.

Stress and deformation in arterial wall tissue are factors which may influence significantly its response and evolution. In this work we develop models based on nonlinear elasticity and finite element numerical solutions for the mechanical behaviour and the remodelling of the soft tissue of arteries, including anisotropy induced by the presence of collagen fibres. Remodelling and growth in particular constitute important features in order to interpret stenosis and atherosclerosis. The main object of this work is to model accurately volumetric growth, induced by fluid shear stress in the intima and local wall stress in arteries with patient-specific geometry and loads. The model is implemented in a nonlinear finite element setting which may be applied to realistic 3D geometries obtained from in vivo measurements. The capabilities of this method are demonstrated in several examples. Firstly a stenotic process on an idealised geometry induced by a non-uniform shear stress distribution is considered. Following the growth of a right coronary artery from an in vivo reconstructed geometry is presented. Finally, experimental measurements for growth under hypertension for rat carotid arteries are modelled.

[Influence of shear stress on in-stent restenosis: in vivo study using 3D reconstruction and computational fluid dynamics]. / Influencia de la tensión de cizallamiento en la reestenosis intra-stent: estudio in vivo con reconstrucción 3D y dinámica de fluidos computacional.

INTRODUCTION AND OBJECTIVES: Local factors may influence neointimal proliferation following conventional stent implantation. In this study, the relationship between wall shear stress and luminal loss after coronary stenting was assessed using a combination of angiography, intravascular ultrasound, and computational fluid dynamics. PATIENTS AND METHOD: Seven patients with de novo right coronary lesions treated with conventional (i.e., bare metal) stents were included. Realistic three-dimensional geometric reconstructions were generated offline from angiographic and intravascular ultrasound data both immediately after stenting and at 6-month follow-up. A finite-volume model was used to calculate local wall shear stress within the stent and 4 mm proximally and distally to the stent. The mean coronary ostium entry flow velocity was assumed to be 25 cm/s in all cases. RESULTS: The mean neointimal thickness was 0.29 (0.21) mm. In five cases, weak negative correlations between wall shear stress and neointimal thickness were found: maximum r value = -0.34, minimum r value = -0.11 (P < .001). The neointimal thickness in segments in which the level of wall shear stress was in the lowest quartile was greater than that in segments in which it was in highest quartile, at 0.34 (0.21) mm and 0.27 (0.24) mm (P < .001) for quartiles 1 and 4, respectively. CONCLUSIONS: Low wall shear stress after stenting favors neointimal proliferation both within the stent and at the stent's edges.

Influencia de la tensión de cizallamiento en la reestenosis intra-stent: estudio in vivo con reconstrucción 3D y dinámica de fluidos computacional / Influence of shear stress on in-stent restenosis: in vivo study using 3D reconstruction and computational fluid dynamics

Introducción y objetivos. Los factores locales pueden influir en la proliferación neointimal tras la implantación de stents convencionales. En este estudio se evalúa la relación entre la tensión de cizallamiento y la pérdida luminal tras la colocación de stents en las arterias coronarias, utilizando la combinación de angiografía, ecografía intravascular y cálculo computacional de la dinámica de fluidos. Pacientes y método. Se incluyó a 7 pacientes con lesiones de novo en las arterias coronarias derechas tratadas con stents convencionales (no recubiertos de fármacos). Se realizó una reconstrucción tridimensional real, basada en la angiografía y en la ecografía intracoronaria, realizada offline tras la angioplastia y a los 6 meses. Mediante el modelo de volúmenes finitos se calculó la tensión de cizallamiento localmente, en el interior del segmento con stent y en los 4 mm proximales y distales, tomando como velocidad de entrada en el ostium coronario un valor de 25 cm/s. Resultados. El grosor neointimal medio fue de 0,29 ± 0,21 mm. En 5 casos se encontró una correlación negativa débil entre la tensión de cizallamiento y el grosor neointimal (valor máximo de r = -­0,34, valor mínimo de r = 0,11; p < 0,001). Los segmentos en el cuartil inferior de la tensión de cizallamiento mostraron valores más altos de grosor neointimal, comparados con los del cuartil superior (0,34 ± 0,21 frente a 0,27 ± 0,24 mm; p < 0,001, para los cuartiles 1 y 4, respectivamente). Conclusiones. La baja tensión de cizallamiento tras la implantación de stents convencionales favorece la pérdida luminal, tanto en los segmentos intra-stent como en los bordes del stent

Introduction and objectives. Local factors may influence neointimal proliferation following conventional stent implantation. In this study, the relationship between wall shear stress and luminal loss after coronary stenting was assessed using a combination of angiography, intravascular ultrasound, and computational fluid dynamics. Patients and method. Seven patients with de novo right coronary lesions treated with conventional (i.e., bare metal) stents were included. Realistic three-dimensional geometric reconstructions were generated offline from angiographic and intravascular ultrasound data both immediately after stenting and at 6-month follow-up. A finite-volume model was used to calculate local wall shear stress within the stent and 4 mm proximally and distally to the stent. The mean coronary ostium entry flow velocity was assumed to be 25 cm/s in all cases. Results. The mean neointimal thickness was 0.29 (0.21) mm. In five cases, weak negative correlations between wall shear stress and neointimal thickness were found: maximum r value =­-0.34, minimum r value = -­0.11 (P<.001). The neointimal thickness in segments in which the level of wall shear stress was in the lowest quartile was greater than that in segments in which it was in highest quartile, at 0.34 (0.21) mm and 0.27 (0.24) mm (P<.001) for quartiles 1 and 4, respectively. Conclusions. Low wall shear stress after stenting favors neointimal proliferation both within the stent and at the stent's edges