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Effects of the flexibility of the arterial wall on the wall shear stresses in Abdominal Aortic Aneurysms
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Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)
permeation studies to determine the Young and aggregate moduli of the solid matrix and the Darcy permeability. Biochemical properties of the ILT samples were also studied. Matrix Metallo Proteinases (MMP-8, MMP-9) and Tissue Inhibitors (TIMP-1, TIMP-2) were quantified on paired samples obtained from the ILT samples at each location of harvest. The preliminary results show that the ILT is non homogeneous and that its mechanical and biochemical properties vary widely across the thickness. This stratification of the material properties may be related to ILT formation, which occur in time by successive layers. 7689 Mo-Tu, no. 3 (P65) PET-CT imaging - A new approach predicting abdominal aortic aneurysm rupture? C. Reeps 1, E Heider 1, O. Wolf 1, M. Hanke 1, S. Seidl 2, H.H. Eckstein 1.
1Department of Vascular Surgery and 2Institute of Pathology Klinikum rechts der Isar, Technical University of Munich, Munich, Germany Despite of emergency repair rupture of abdominal aortic aneurysm (AAA) is fatal in 70-90%. At present, increasing aortic aneurysm diameter, expansion rate and calculated tensile wall stress assessed by conventional CT imaging are accepted predictors of AAA rupture. However, rupture also occurs unexpected in small AAA. Aneurysm development is accompanied by subacute inflammatory processes and consecutive enzymatic degradation of the aortic wall exposed to pulsatile stress of blood pressure. Especially at site of rupture increased proteolytic activity is observed. Distinct biological activity is associated with unspecific accelerated glucose metabolism and can be displayed by intensified radioactive lSF-fluorodeoxyglucose (FDG) uptake in conventional Whole Body Positron Emission Tomography (PET). Significant lSF-FDG accumulation is described previously only in acute symptomatic aneurysms and aortic graft infections, but not in clinically asymptomatic patients. With PET-CT the integrated dual information from PET and CT enhances the sensitivity, specificity and tissue resolution of PET imaging and provides a more graduated picture of tissue activity. With AAA patients, through PET-CT in parallel with morphological routine information subacute inflammation and vascular remodelling processes within the aneurysm wall may be detected at lower levels before acute aneurysm onset. To assess the capabilities of PET-CT we subjected selected patients with asymptomatic aortic aneurysm to PET - C T consecutively, having prophylactic conventional open aneurysm repair. Specimens of the aneurysm wall with elevated preoperative lSF-FDG uptake were retrieved and compared with the corresponding histopathologically findings with special respect to collagenous fibrous tissue components and inflammatory cell infiltrate as determinants of wall strength and metabolic activity. First results are presented and the potential role of biological imaging in anticipating aneurysm rupture will be discussed alternatively to conventional diagnostic markers. 5793 Mo-Tu, no. 4 (P65) Numerical model for predicting dilatation of an Abdominal Aortic Aneurysm (AAA) F. Helderman 1, O. Schouten 2, I. Manoch 1, H. Buffart 1, A.F.W. van der Steen 1, R. Krams 1. 1Cardiology, Erasmus MC, Rotterdam, The Netherlands,
2Anaesthesiology, Erasmus MC, Rotterdam, The Netherlands Background: An AAA is a widening of the abdominal aorta. An aneurysm enlarges until it ruptures, most likely with fatal consequences. Pre-rupture diagnosis, allows for life saving intervention. However, intervention mortality is 5%. Clinicians decide for intervention on aneurysm diameter alone. We have built a numerical model to simulate the dilatation process and estimate when the aneurysm ruptures. Methods: The aneurysm geometry is taken from CT-angiographic images. Stress is calculated by finite element methods. We used a Young's modulus of 2 106 N/m 2, a Poisson ratio of 0.45 and a pressure of 5 103 N/m 2. If stress locally exceeds 3 104 N/m 2 then the Young's modulus is lowered, mimicking collagen breakdown. As a result the aneurysm bulges locally. The dilatation simulation is a series of bulges adjusting the geometry. When the wall stress exceeds the yield stress, rupture is expected. Data of 76 patients is used to find the relation between growth factors and rates. By varying the model parameters for an aneurysm a relation between growth rate and model parameters can be made. Simulation of dilatation is performed for six aneurysms. Results: The simulated dilatation of each aneurysm is compared with echo measurements. The maximum diameter in the simulations didn't fit the measurements. In three cases the diameter already differed more than 20mm to start with. In three simulations dilatation was to fast and in one simulation to slow.
Poster Presentations Conclusion: Aneurysm geometry is important for the growth rate. Therefore the relation between growth rate and model parameters must be determined for each aneurysm separately. The uncertainty in echo-measured diameter is probably bigger than the assumed 3 mm.
5587 Mo-Tu, no. 5 (P65) Effects o f the flexibility of the arterial wall on the wall shear stresses in Abdominal Aortic Aneurysms A.V. Salsac 1, M. Fernandez 2, P. Le Tallec3, J.M. Chomaz 4. 1Mechanical
Engineering Dept., UCL, London, England, UK, 21NRIA Rocquencourt, Le Chesnay, France, 3LadHyX, Ecole Polytechnique, Palaiseau, France, 4LMS, Ecole Polytechnique, Palaiseau, France As an abdominal aortic aneurysm forms and enlarges, large changes occur in the composition and structure of the arterial wall, which result in its stiffening. So far, most studies, whether experimental or numerical, have been conducted assuming the walls to be rigid. A numerical simulation of the fluid structure interactions is performed in order to analyze the effects that the wall compliance might have on the mechanical stimuli developping inside the aneurysm. The results of the numerical simulations are compared with measurements obtained experimentally by PIV (Particle Image Velocimetry) in rigid models of aneurysms. The numerical code couples a fluid solver (resolution of the incompressible Navier Stokes equations in a ALE configuration) and a solid one (arterial wall modeled as a non-linear shell) with a message passing library. Both symmetric and non-symmetric models of aneurysms are considered, all idealistic in shape. The mechanical properties of the aneurysm wall are varied in order to simulate their progressive stiffening. The wall is fixed at its extremeties, preventing any displacement or rotation. A physiologically correct flow rate is imposed at the inlet cross-section and a relationship between pressure and flow at the outlet. The results are obtained for resistive outlet boundary conditions. In the case of compliant walls, a small decrease in the wall shear stresses is observed (<10%). This result indicates that studies in rigid models should provide a decent approximation of the spatial and temporal distributions of wall shear stresses. However, a fluid structure interaction approach is necessary to quantity the pressure field and the internal wall tension. 6842 Mo-Tu, no. 6 (P65) Endovascular wall pressure-forces in a progressively enlarged aneurysm model P. Neotytou 1, S. Tsangaris 1, M. Kyriakidis 2,3. 1School ef Mechanical
Engineering, Fluids Section, National Technical University of Athens, Greece, 2Medical School, University of Athens, Greece, 3Cardiology Clinic, Metropolitan Hospital, Faliro, Greece The flow field induced by the unsteady flow in the vicinity of an abdominal aortic aneurysm model is numerically investigated with the use of Computational Fluid Dynamics (CFD). The computational code used is based on the finite volume method with collocated arrangement of variables and it has already used in various 2D [1] and 3D [2] studies of blood-flow related applications. As inlet boundary condition, the realistic flow rate waveform in the abdominal aorta is used. For modelling the rheological behaviour of blood, the Quemada [3] model is employed, which is suitable for simulating the two-phase character of the blood namely a suspension of blood cells in plasma. In view of the haemodynamical mechanisms related to the enlargement and subsequent rupture of an aneurysm the investigation is focused on the distribution of the flow-induced pressure on the interior wall of the aneurysm. In order to study the development of the distribution with the gradual enlargement of the aneurysm, three different degrees of aneurysm-growth have been assumed. Finally and for examining the effect of the distribution to the aneurysm growth, a comparison is made between the pressure-distributions for each growth-degree and also the areas of maximum strain on the arterial wall are identified namely the areas of the highest pressure and highest pressure-variation. References [1] P. Neofytou. Comparison of blood rheological models for physiological flow simulation. Biorheology 2004; 41: 693-714. [2] P. Neofytou, S. Tsangaris. Flow effects of blood constitutive equations on 3D models of vascular anomalies. International Journal for Numerical Methods in Fluids, in press. [3] D. Quemada. Rheology of concentrated disperse systems II1. General features of the proposed non-Newtonian model. Comparison with experimental data. Rheologica Acta 1978; 17: 643-653.
Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)
permeation studies to determine the Young and aggregate moduli of the solid matrix and the Darcy permeability. Biochemical properties of the ILT samples were also studied. Matrix Metallo Proteinases (MMP-8, MMP-9) and Tissue Inhibitors (TIMP-1, TIMP-2) were quantified on paired samples obtained from the ILT samples at each location of harvest. The preliminary results show that the ILT is non homogeneous and that its mechanical and biochemical properties vary widely across the thickness. This stratification of the material properties may be related to ILT formation, which occur in time by successive layers. 7689 Mo-Tu, no. 3 (P65) PET-CT imaging - A new approach predicting abdominal aortic aneurysm rupture? C. Reeps 1, E Heider 1, O. Wolf 1, M. Hanke 1, S. Seidl 2, H.H. Eckstein 1.
1Department of Vascular Surgery and 2Institute of Pathology Klinikum rechts der Isar, Technical University of Munich, Munich, Germany Despite of emergency repair rupture of abdominal aortic aneurysm (AAA) is fatal in 70-90%. At present, increasing aortic aneurysm diameter, expansion rate and calculated tensile wall stress assessed by conventional CT imaging are accepted predictors of AAA rupture. However, rupture also occurs unexpected in small AAA. Aneurysm development is accompanied by subacute inflammatory processes and consecutive enzymatic degradation of the aortic wall exposed to pulsatile stress of blood pressure. Especially at site of rupture increased proteolytic activity is observed. Distinct biological activity is associated with unspecific accelerated glucose metabolism and can be displayed by intensified radioactive lSF-fluorodeoxyglucose (FDG) uptake in conventional Whole Body Positron Emission Tomography (PET). Significant lSF-FDG accumulation is described previously only in acute symptomatic aneurysms and aortic graft infections, but not in clinically asymptomatic patients. With PET-CT the integrated dual information from PET and CT enhances the sensitivity, specificity and tissue resolution of PET imaging and provides a more graduated picture of tissue activity. With AAA patients, through PET-CT in parallel with morphological routine information subacute inflammation and vascular remodelling processes within the aneurysm wall may be detected at lower levels before acute aneurysm onset. To assess the capabilities of PET-CT we subjected selected patients with asymptomatic aortic aneurysm to PET - C T consecutively, having prophylactic conventional open aneurysm repair. Specimens of the aneurysm wall with elevated preoperative lSF-FDG uptake were retrieved and compared with the corresponding histopathologically findings with special respect to collagenous fibrous tissue components and inflammatory cell infiltrate as determinants of wall strength and metabolic activity. First results are presented and the potential role of biological imaging in anticipating aneurysm rupture will be discussed alternatively to conventional diagnostic markers. 5793 Mo-Tu, no. 4 (P65) Numerical model for predicting dilatation of an Abdominal Aortic Aneurysm (AAA) F. Helderman 1, O. Schouten 2, I. Manoch 1, H. Buffart 1, A.F.W. van der Steen 1, R. Krams 1. 1Cardiology, Erasmus MC, Rotterdam, The Netherlands,
2Anaesthesiology, Erasmus MC, Rotterdam, The Netherlands Background: An AAA is a widening of the abdominal aorta. An aneurysm enlarges until it ruptures, most likely with fatal consequences. Pre-rupture diagnosis, allows for life saving intervention. However, intervention mortality is 5%. Clinicians decide for intervention on aneurysm diameter alone. We have built a numerical model to simulate the dilatation process and estimate when the aneurysm ruptures. Methods: The aneurysm geometry is taken from CT-angiographic images. Stress is calculated by finite element methods. We used a Young's modulus of 2 106 N/m 2, a Poisson ratio of 0.45 and a pressure of 5 103 N/m 2. If stress locally exceeds 3 104 N/m 2 then the Young's modulus is lowered, mimicking collagen breakdown. As a result the aneurysm bulges locally. The dilatation simulation is a series of bulges adjusting the geometry. When the wall stress exceeds the yield stress, rupture is expected. Data of 76 patients is used to find the relation between growth factors and rates. By varying the model parameters for an aneurysm a relation between growth rate and model parameters can be made. Simulation of dilatation is performed for six aneurysms. Results: The simulated dilatation of each aneurysm is compared with echo measurements. The maximum diameter in the simulations didn't fit the measurements. In three cases the diameter already differed more than 20mm to start with. In three simulations dilatation was to fast and in one simulation to slow.
Poster Presentations Conclusion: Aneurysm geometry is important for the growth rate. Therefore the relation between growth rate and model parameters must be determined for each aneurysm separately. The uncertainty in echo-measured diameter is probably bigger than the assumed 3 mm.
5587 Mo-Tu, no. 5 (P65) Effects o f the flexibility of the arterial wall on the wall shear stresses in Abdominal Aortic Aneurysms A.V. Salsac 1, M. Fernandez 2, P. Le Tallec3, J.M. Chomaz 4. 1Mechanical
Engineering Dept., UCL, London, England, UK, 21NRIA Rocquencourt, Le Chesnay, France, 3LadHyX, Ecole Polytechnique, Palaiseau, France, 4LMS, Ecole Polytechnique, Palaiseau, France As an abdominal aortic aneurysm forms and enlarges, large changes occur in the composition and structure of the arterial wall, which result in its stiffening. So far, most studies, whether experimental or numerical, have been conducted assuming the walls to be rigid. A numerical simulation of the fluid structure interactions is performed in order to analyze the effects that the wall compliance might have on the mechanical stimuli developping inside the aneurysm. The results of the numerical simulations are compared with measurements obtained experimentally by PIV (Particle Image Velocimetry) in rigid models of aneurysms. The numerical code couples a fluid solver (resolution of the incompressible Navier Stokes equations in a ALE configuration) and a solid one (arterial wall modeled as a non-linear shell) with a message passing library. Both symmetric and non-symmetric models of aneurysms are considered, all idealistic in shape. The mechanical properties of the aneurysm wall are varied in order to simulate their progressive stiffening. The wall is fixed at its extremeties, preventing any displacement or rotation. A physiologically correct flow rate is imposed at the inlet cross-section and a relationship between pressure and flow at the outlet. The results are obtained for resistive outlet boundary conditions. In the case of compliant walls, a small decrease in the wall shear stresses is observed (<10%). This result indicates that studies in rigid models should provide a decent approximation of the spatial and temporal distributions of wall shear stresses. However, a fluid structure interaction approach is necessary to quantity the pressure field and the internal wall tension. 6842 Mo-Tu, no. 6 (P65) Endovascular wall pressure-forces in a progressively enlarged aneurysm model P. Neotytou 1, S. Tsangaris 1, M. Kyriakidis 2,3. 1School ef Mechanical
Engineering, Fluids Section, National Technical University of Athens, Greece, 2Medical School, University of Athens, Greece, 3Cardiology Clinic, Metropolitan Hospital, Faliro, Greece The flow field induced by the unsteady flow in the vicinity of an abdominal aortic aneurysm model is numerically investigated with the use of Computational Fluid Dynamics (CFD). The computational code used is based on the finite volume method with collocated arrangement of variables and it has already used in various 2D [1] and 3D [2] studies of blood-flow related applications. As inlet boundary condition, the realistic flow rate waveform in the abdominal aorta is used. For modelling the rheological behaviour of blood, the Quemada [3] model is employed, which is suitable for simulating the two-phase character of the blood namely a suspension of blood cells in plasma. In view of the haemodynamical mechanisms related to the enlargement and subsequent rupture of an aneurysm the investigation is focused on the distribution of the flow-induced pressure on the interior wall of the aneurysm. In order to study the development of the distribution with the gradual enlargement of the aneurysm, three different degrees of aneurysm-growth have been assumed. Finally and for examining the effect of the distribution to the aneurysm growth, a comparison is made between the pressure-distributions for each growth-degree and also the areas of maximum strain on the arterial wall are identified namely the areas of the highest pressure and highest pressure-variation. References [1] P. Neofytou. Comparison of blood rheological models for physiological flow simulation. Biorheology 2004; 41: 693-714. [2] P. Neofytou, S. Tsangaris. Flow effects of blood constitutive equations on 3D models of vascular anomalies. International Journal for Numerical Methods in Fluids, in press. [3] D. Quemada. Rheology of concentrated disperse systems II1. General features of the proposed non-Newtonian model. Comparison with experimental data. Rheologica Acta 1978; 17: 643-653.