1. 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. lost of elastin  early aortic expansion
Biological approach
Measurement of MMPs
lost of elastin  early aortic expansion
collagen breakdown  late expansion and rupture
Dobrin et al, 1984
MMPs are endogenous proteases implicated in aneurysm pathobiology
MMP-2 and MMP-9 are specific to elastase
MMP-1 and MMP-8 are specific to collagenase
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 material’s strength
stress
strength
Statistical model (thickness,curvature,family hystory, age, sex)
Numerical model
Predictive realistic model: geometry + material properties (AAA+ILT) + physiologic loads

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

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

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

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

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

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

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

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. Material properties - Experimental tests
Compression tests on ILT
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
R.Spirito, Centro Cardiologico Monzino, Milano

14. Methods: Material properties - Experimental tests
Compression tests on ILT – choice of the constitutive model
ILT  soft hydrated tissue (fibrin, cells, 90% water)  biphasic theory
Fluid phase
Inviscid
Uncompressible
Solid phase
Isotropic
Elastic
Uncompressible
K, 
Measured parameters
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)

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

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

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

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

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