LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS FLUID MECHANICAL PERTURBATIONS INDUCED BY STENT IMPLANTATION: A NUMERICAL STUDY Rossella

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

Balossino R. MPF- 2006

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)

10


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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18

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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Newtonian fluid


artery


Single strut


WORKS IN PROGRESS

Carreau model: [Seo et al., 2005] 212

0 1

n

S

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Newtonian fluid


artery


Single strut


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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THANK YOU

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LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS FLUID MECHANICAL PERTURBATIONS INDUCED BY STENT IMPLANTATION: A NUMERICAL STUDY Rossella