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LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS www.labsmech.polimi.it Compressive properties of intraluminal thrombus from abdominal aortic aneurysms Federica.

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Presentation on theme: "LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS www.labsmech.polimi.it Compressive properties of intraluminal thrombus from abdominal aortic aneurysms Federica."— Presentation transcript:

1 LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS Compressive properties of intraluminal thrombus from abdominal aortic aneurysms Federica Boschetti

2 Introduction Abdominal aortic aneurysm (AAA) is the15th leading cause of death in the U.S. (Al-Omran et al., 2004, NCFH, 2000) Unless treated, rupture of AAA is associated with an emergency postoperative mortality rate of 47% (Resch et al., 2001) Current method for determining risk of rupture is not reliable (Darling et al., 1977, Lederle et al., 2002) The presence of an ILT is a common finding in AAAs and may potentially play an important, though as yet unknown, role in the progression of the disease (Vorp, 1997). 5.5 cm

3 Measurement of MMPs lost of elastin early aortic expansion collagen breakdown late expansion and rupture Dobrin et al, 1984 Biological approach MMP-2 and MMP-9 are specific to elastase MMP-1 and MMP-8 are specific to collagenase MMPs are endogenous proteases implicated in aneurysm pathobiology MMP-8 and MMP-9 are increased at the site of AAA rupture (Wilson et al, 2006)

4 Biomechanical approach AAA rupture is an event marked by catastrophic mechanical failure of the diseased aortic wall tissue Mechanical failure of a material occurs when the acting stress exceeds the materials strength Predictive realistic model: geometry + material properties (AAA+ILT) + physiologic loads stress strength Numerical model Statistical model (thickness,curvature,family hystory, age, sex)

5 Biomechanical approach (IDL) CT Image Segmentation 3D SmoothingIGES Splines CAD model Mesh discretisation FEM simulation

6 Biomechanical approach (IDL) CT Image Segmentation 3D SmoothingIGES Splines CAD model Mesh discretisation FEM simulation

7 Biomechanical approach (IDL) CT Image Segmentation 3D Smoothing Mesh discretisation FEM simulation IGES Splines CAD model

8 Biomechanical approach (IDL) CT Image Segmentation 3D SmoothingIGES Splines CAD model Mesh discretisation FEM simulation

9 Biomechanical approach (IDL) CT Image Segmentation 3D SmoothingIGES Splines CAD model Mesh discretisation FEM simulation Material properties ?? FEM model Structural model Fluid-dynamic model Fluid-structure interaction model

10 Material properties - Experimental tests Experimental set-up AAA specimen E.S. Di Martino J Vasc Surg Mar;43(3):570-6 Uniaxial tensile tests on AAA wall

11 Wang et al., 2002, Di Martino, 2003 Material properties - Experimental tests L Biaxial tensile tests on AAA wall

12 Material properties - Experimental tests Biaxial tensile tests on ILT Luminal layer isolated (2mm ave thickness) Tested biaxially Wang et al., 2002, Di Martino, 2003 Compressive properties are missing Aim: to evaluate the mechanical properties of ILT under compressive loads

13 ILT from O.R. cylindrical plugs liquid N2 on the day of testing: 3 slices from each plug (luminal, medial, abluminal) all mechanical tests performed in a bath of physiologic solution Material properties - Experimental tests Compression tests on ILT R.Spirito, Centro Cardiologico Monzino, Milano

14 Elastic Modulus E (from uc stress-relaxation) Aggregate Modulus Ha (from cc stress-relaxation) Permeability K (from permeation - Darcy) Porosity (from wet to dry weight) Methods: Material properties - Experimental tests Compression tests on ILT – choice of the constitutive model Measured parameters ILT soft hydrated tissue (fibrin, cells, 90% water) biphasic theory Solid phase Isotropic Elastic Uncompressible Fluid phase Inviscid Uncompressible K,

15 Compression tests on ILT – stress-strain data Methods: Material properties - Experimental tests Testing machine: Enduratec Elf 3200, 22N load cell v = 0.1 m/s

16 0 )( E step wise stress relaxation test Strain=40%-45%-50%-55% Compression tests on ILT: unconfined compression Methods: Material properties - Experimental tests radial flow

17 axial flow Compression tests on ILT: confined compression 0 )( Ha step wise stress relaxation test Strain=40%-45%-50%-55% Methods: Material properties - Experimental tests

18 Measurement of the volume flow through thrombus Application of a constant load to the fluid Measurement of the resulting pressure Darcy law Permeation tests on ILT Methods: Material properties - Experimental tests

19 Biochemical analysis Homogenization in 0.05 mol/L Tris-HCl, pH 7.6, containing proteases inhibitors. Centrifugation at rpm for 15 minutes at 4°C MMP-9 levels assessed by Elisa assay (Quantikine MMP9, R&D system) Methods: Biochemical analysis M.Camera and R.Ballerio, Centro Cardiologico Monzino, Milano

20 Results data from 3 patients

21 Conclusions ILT is a non-homogeneous material: the compression and biochemical properties vary widely across the thickness Stratification of the material properties may be related to ILT formation, which occur in time by successive layers. Work in progress and future developments: more tests comparison between ruptured and non ruptured search for correlations fluid-structure interaction remodeling law implementation

22 pressure gauge capillary capillary PE filter sample O-Ring lower cylinder upper cylinder Permeation tests on ILT Methods: Material properties - Experimental tests


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