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
2
INTRODUCTION
Balossino R.
MPF- 2006
3
INTRODUCTION
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.
Balossino R.
MPF- 2006
4
MOTIVATION
IN-STENT RISTENOSIS
Intimal thickening following a stent implantation with progressive lumen reduction
[Mehran R., 2002]
Three phases (Edelman e Rogers, 1998):
INFLAMMATION
during implantation
+ PROLIFERATION
first 3 weeks
+ REMODELING
10/12 months
HYPOTHESIS: non physiological stress state field responsible for restenosis.
Balossino R.
MPF- 2006
5
STATE OF THE ART
QUANTITIES OF INTEREST
• 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
Balossino R.
MPF- 2006
6
THE PROBLEM
FROM SIMPLIFIED
MODELS …
TO PLAQUE MODEL
Migliavacca et al., Proceedings of 2005 Summer Bioengineernig ASME Conference
Healthy artery
Artery with plaque
Expansion under displacement control until a diameter of 3 mm was reached
The stent geometry was modelled as shell elements
Cordis BX Velocity (Johnson & Johnson Interventional System, Warren, NJ, USA)
Balossino R.
MPF- 2006
7
METHODS
1.
Preliminary step: structural analysis
This step is necessary to obtain the correct configuration for the fluid dynamics
simulations: fluid domain
Balossino R.
MPF- 2006
8
METHODS
First step: creation of the fluid domain
Point cloud of
1
the deformed
configuration
Creation of
2
the curves
and surfaces
3
Creation of
each volume
Substraction and
4
creation of the
final fluid domain
Balossino R.
MPF- 2006
9
METHODS
Second step: Boundary conditions
OUTLET
Constant fixed pressure
ASSUMPTION: - rigid vessel wall
- Newtonian fluid: Viscosity = 0.0035 kg/(m∙s)
Density = 1060 kg/m3
0.2
0.16
[m/s]
WALL
LaDisa et al. (2005)
0.12
0.08
No slip condition
0.04
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time [s]
4 cardiac cycles
pulse period = 0.54 s
INLET
Velocity profile: parabolic and transient
Fluent (Fluent Inc., Lebanon, NH, USA)
Balossino R.
MPF- 2006
10
OBSERVATIONS
STENTED REGION
0.16 s
HEALTHY MODEL
dynes/cm2
50
PLAQUE MODEL
25
0
 The highest WSS magnitude can be noticed on the stent
Balossino R.
MPF- 2006
11
OBSERVATIONS
ARTERIAL REGION INSIDE STENT STRUTS
0.16 s
dynes/cm2
18
9
0
HEALTHY MODEL
 high WSS in the regions between the stent struts
 low WSS were localized around stent struts
Balossino R.
MPF- 2006
PLAQUE MODEL
12
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
Balossino R.
MPF- 2006
13
STENT MODELS
CORDIS
JOSTENT
RADIUS
LENGTH
after expansio n
after expansio n
CORDIS
JOSTENT
PALMAZ
SORIN
SORIN
1.5
1.5
1.5
1.55
THICKNESS
3.53
2.30
2.97
3.50
0.1
0.1
0.1
0.1
Length: 11.68 mm
PALMAZ
Internal diameter: 2.15 mm
Thickness: 0.5 mm
Length: 3.68 mm
Internal diameter: 1.25 mm
Thickness: 0.45 mm
Balossino R.
MPF- 2006
14
RESULTS: WALL SHEAR STRESSES
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
100
% of cells
0.16 s
95
0.32 s
0.44 s
0s
90
85
0s
Balossino R.
0.16 s
0.32 s
MPF- 2006
0.4 s
0.44 s
0.4 s
15
RESULTS: LOW WSS
WSS < 5 dynes/cm2
CORDIS
SORIN
0s
% of cells
100
95
90
85
0s
0.16 s
0.32 s
0.4 s
0.44 s
[dynes/cm2]
JOSTENT
Balossino R.
5
2.5
MPF- 2006
0
PALMAZ
16
RESULTS: LOW WSS
WSS < 5 dynes/cm2
CORDIS
SORIN
0.16 s
% of cells
100
95
90
85
0s
0.16 s
0.32 s
0.4 s
0.44 s
[dynes/cm2]
JOSTENT
Balossino R.
5
2.5
MPF- 2006
0
PALMAZ
17
RESULTS: MAXIMUM WSS ON STENT
[dynes/cm2]
50
40
30
20
CORDIS
10
PALMAZ
JOSTENT
0
0s
Balossino R.
0.16 s
0.32 s
0.4 s
MPF- 2006
0.44 s
SORIN
RESULTS: MAXIMUM WSS
[dynes/cm2]
50
40
30
20
10
0
0.16 s
Balossino R.
MPF- 2006
18
RESULTS: MAXIMUM WSS ON THE ARTERIAL WALL
19
[dynes/cm2]
20
15
10
5
CORDIS
PALMAZ
JOSTENT
0
0s
Balossino R.
0.16 s
0.32 s
0.4 s
MPF- 2006
0.44 s
SORIN
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
Balossino R.
MPF- 2006
20
21
WORKS IN PROGRESS
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
Carreau model:
Balossino R.
    0   
MPF- 2006

 
1   S
2  n 1 2
[Seo et al., 2005]
22
WORKS IN PROGRESS
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
Influence of the stent length:
Balossino R.
MPF- 2006
23
WORKS IN PROGRESS
Influence of the strut thickness: comparison of different stent design with same
thickness
0.15 mm
CORDIS
Balossino R.
0.1 mm
JOSTENT
MPF- 2006
SORIN
PALMAZ
24
CONCLUSIONS
 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.
Balossino R.
MPF- 2006
25
THANK YOU
Balossino R.
MPF- 2006