laboratory of biological structure mechanics fluid mechanical perturbations induced by stent...
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LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS
www.labsmech.polimi.it
FLUID MECHANICAL PERTURBATIONS INDUCED BY STENT IMPLANTATION: A NUMERICAL STUDY
Rossella Balossino, Francesca Gervaso, Francesco Migliavacca, Gabriele Dubini
LaBS, Department of Structural Engineering, Politecnico di Milano, ITALY
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INTRODUCTION
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A vascular stent is a small metal tube, which is inserted into an artery at the site of a narrowing to act as an internal scaffolding or a support to the blood vessel.
INTRODUCTION
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IN-STENT RISTENOSIS
Intimal thickening following a stent implantation with progressive lumen reduction
HYPOTHESIS: non physiological stress state field responsible for restenosis.
Three phases (Edelman e Rogers, 1998):
+ REMODELING
10/12 months
+ PROLIFERATION
first 3 weeks
INFLAMMATION
during implantation
MOTIVATION
[Mehran R., 2002]
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STATE OF THE ART
• Effect of wire spacing, wire diameter, vessel diameter and flow conditions [Moore et al.,2002]
• Stent design: number, thickness and width of the strut
• Deployment ratio
• Comparison of resting or maximal vasodilatation condition [LaDisa et al.,2003-2004-2005]
• Foreshortening
• Changes in vascular geometry after stent deployment
• Effect of vessel curvature [Seo et al., 2005]
• Non-Newtonian condition [Soulis et al.,2002; Seo et al.,2005; Bernard et al.,2004]QUANTITATIVELY OBSERVED PARAMETERS
• wall shear stress (WSS) distribution
• velocity vectors
• recirculation length
• velocity profiles
QUANTITIES OF INTEREST
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THE PROBLEM
Expansion under displacement control until a diameter of 3 mm was reached
The stent geometry was modelled as shell elements
FROM SIMPLIFIED MODELS …
TO PLAQUE MODEL
Healthy artery Artery with plaque
Migliavacca et al., Proceedings of 2005 Summer Bioengineernig ASME Conference
Cordis BX Velocity (Johnson & Johnson Interventional System, Warren, NJ, USA)
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This step is necessary to obtain the correct configuration for the fluid
dynamics simulations: fluid domain
METHODS
1. Preliminary step: structural analysis
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First step: creation of the fluid domain
METHODS
Point cloud
of the
deformed
configurationCreation of
the curves
and
surfaces
Creation of
each
volume
Substraction
and creation of
the final fluid
domain
1
2
3
4
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Second step: Boundary conditions
METHODS
4 cardiac cycles pulse period = 0.54 s
INLET
OUTLET
WALL
Velocity profile: parabolic and
transient
Constant fixed
pressure
No slip
condition
ASSUMPTIO
N:
- rigid vessel wall
- Newtonian fluid:Viscosity = 0.0035 kg/(m∙s)
Density = 1060 kg/m3
Fluent (Fluent Inc., Lebanon, NH, USA)
0
0.04
0.08
0.12
0.16
0.2
0 0.1 0.2 0.3 0.4 0.5 0.6
Time [s]
[m/s
]
LaDisa et al. (2005)
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0.16 s
50
0
25
dynes/cm2
OBSERVATIONS
STENTED REGION
The highest WSS magnitude can be noticed on the
stent
HEALTHY MODEL
PLAQUE MODEL
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0
9
dynes/cm2
0.16 s
OBSERVATIONS
ARTERIAL REGION INSIDE STENT STRUTS
high WSS in the regions between the stent struts
low WSS were localized around stent struts
HEALTHY MODEL PLAQUE MODEL
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AIM OF THE STUDY
Is it correct to ignore the presence of an atherosclerotic plaque ? ?
four different stent designs previously expanded against the same stented artery
• Cordis BX Velocity stent like (Johnson & Johnson Interventional System, Warren, NJ, USA)• Jostent Flex stent like (JOMED AB, Helsingborg, Sweden)• Sorin Carbostent stent like (Sorin Biomedica S.p.A., Saluggia (VC), Italy)• Palmaz-Schatz stent like (Johnson & Johnson Interventional System, Warren, NJ, USA)
transient simulation for each model
comparison of the WSS magnitude distribution during time
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CORDIS JOSTENT
SORIN PALMAZ
STENT MODELS
RADIUS after expansion
LENGTHafter expansion
THICKNESS
CORDIS 1.5 3.53 0.1 JOSTENT 1.5 2.30 0.1 PALMAZ 1.5 2.97 0.1 SORIN 1.55 3.50 0.1
Length: 11.68 mm
Internal diameter: 2.15 mm
Thickness: 0.5 mm
Length: 3.68 mm
Internal diameter: 1.25 mm
Thickness: 0.45 mm
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8 5
9 0
9 5
1 00
0 s 0.1 6 s 0.3 2 s 0.4 s 0.4 4 s 0.5 2 s
CORDIS JOSTENT SORIN PALMAZ
WSS < 5 dynes/cm2
• correlated with sites of intima thickening and smooth muscle cells migration
• locations where stagnation of blood occurs
• prone to thrombus formation and platelet accumulation
RESULTS: WALL SHEAR STRESSES
0 s 0.16 s 0.32 s 0.4 s 0.44 s
85
90
95
100
% o
f c
ells
0 s
0.16 s
0.32 s
0.4 s
0.44 s
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CORDIS
JOSTENT
SORIN
PALMAZ5 02.5
[dynes/cm2]
RESULTS: LOW WSS
0 s
WSS < 5 dynes/cm2
0 s 0.16 s 0.32 s 0.4 s 0.44 s
85
90
95
100
% o
f c
ells
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RESULTS: LOW WSS
WSS < 5 dynes/cm2
0 s 0.16 s 0.32 s 0.4 s 0.44 s
85
90
95
100
% o
f c
ells
5 02.5
[dynes/cm2]
CORDIS
JOSTENT
SORIN
PALMAZ
0.16 s
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CORDIS
JOSTENT
SORIN
PALMAZ
0
10
20
30
40
50
0 s 0.16 s 0.32 s 0.4 s 0.44 s
[dynes/cm2]
RESULTS: MAXIMUM WSS ON STENT
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[dynes/cm2]
0
10
20
30
40
50
0.16 s
RESULTS: MAXIMUM WSS
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CORDIS
JOSTENT
SORIN
PALMAZ
0 s 0.16 s 0.32 s 0.4 s 0.44 s
[dynes/cm2]
0
5
10
15
20
RESULTS: MAXIMUM WSS ON THE ARTERIAL WALL
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LIMITATIONS AND ASSUMPTIONS
Rigid wall: valid in the stented region
Newtonian fluid
Straight vessel: neglecting the curvature of the coronary
artery
Post implant condition
Single strut
Symmetric and hyperelastic plaque
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Rigid wall: valid in the stented region
Newtonian fluid
Straight vessel: neglecting the curvature of the coronary
artery
Post implant condition
Single strut
Symmetric and hyperelastic plaque
WORKS IN PROGRESS
Carreau model: [Seo et al., 2005] 212
0 1
n
S
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Rigid wall: valid in the stented region
Newtonian fluid
Straight vessel: neglecting the curvature of the coronary
artery
Post implant condition
Single strut
Symmetric and hyperelastic plaque
WORKS IN PROGRESS
Influence of the stent length:
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WORKS IN PROGRESS
Influence of the strut thickness: comparison of different stent design with same
thickness
CORDIS JOSTENT
0.15 mm
SORIN PALMAZ
0.1 mm
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In each stent model the WSS distribution is similar:
the maximum values are located over the stent strut
the arterial wall portion delimited by the links and the stent strut showed an increasing WSS value from the zones near the stent to the centre
WSS values change during the cardiac cycle, showing an oscillatory behaviour
The comparison among the four stent models indicates that:
• Jostent shows the lowest WSS value during the whole cardiac cycle
• the best model in terms of minimal neointima thickening is the Cordis stent
• the maximum WSS on the stent and the arterial wall occurs in the Cordis stent at the systolic peak
CFD techniques have the advantages of producing accurate information on local flow variables very close to the arterial wall
CFD can thus provide a research tool by complementing experimental studies, especially
where experimental measurements are difficult to perform and affected by uncertainties.
CONCLUSIONS
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