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Computer modelling of stent implantation: expansion and fluidynamics. Francesca Gervaso. Stents a Rilascio di farmaco. Aspetti clinici e tecnologici, Milano, 9 Maggio 2007. Introduction. Arterial diseases like atherosclerosis are the leading causes of death in the industrialised world.

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Computer modelling of stent implantation: expansion and fluidynamics

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Computer modelling of stent implantation expansion and fluidynamics

Computer modelling of stent implantation: expansion and fluidynamics

Francesca Gervaso

Stents a Rilascio di farmaco. Aspetti clinici e tecnologici, Milano, 9 Maggio 2007


Computer modelling of stent implantation expansion and fluidynamics

Introduction

Arterial diseases like atherosclerosis are the leading

causes of death in the industrialised world

A reduction of the blood flow occurs because of the narrowing or occlusion of the affected arteries

Stent implantationis a common procedure with a high rate of success when compared with angioplasty alone

The purpose of a stent is to maintain the arterial lumen open by a scaffolding action that provides radial support

Restenosisis a re-narrowing or blockage of an artery at the same site where treatment (such as a balloon angioplasty or stent procedure) has already taken place

However, some limitations are still present and the major ones are those associated with the ‘in-stent restenosis’ process


Computer modelling of stent implantation expansion and fluidynamics

Introduction

Clinical trials showed the reduction of restenosis using a

Drug Eluting Stent

In fact…

it is difficult to evaluate the effective release of the drug in the vascular tissue

  • stent design(i.e. strut thickness and shape)

  • drug(i.e. hydrophilic/hydrophobic)

  • coating type(i.e. pore size, continuous/reservoir)

Moreover…

  • the expansion of the struts and the interaction with the vascularwallinfluences the outcome of the stenting procedure

(Grenacher et al. 2006, Rogers and Edelman 1995)

  • fluidynamics is strongly influenced by the presence of the stent and correlations between wall shear stresses and restenosis exist

(LaDisa et al., 2004)


Computer modelling of stent implantation expansion and fluidynamics

Introduction

FEMs are a useful tool to assess the influence of the factors affecting the drug release

Finite Element (FE) models allow to

stent-arterial wall interaction

  • predict phenomena

fluidynamics

drug release

  • interpret the available clinical data


Computer modelling of stent implantation expansion and fluidynamics

Introduction

Stent-arterial wall interaction

1) Coated vs. Reservoir stent

2) Balloon expandable stent

stent expansion

drug elution

a stent is expanded inside an atherosclerotic coronary artery by means of the commercial finite element code ABAQUS (Abaqus Inc, RI, USA).

the deformed coronary artery and stent are used as input geometries on which the drug eluting model analysis is carried out


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent

To analyse the drug release of two different DES resembling commercial products by means of numerical models based on the Finite Element Method

Reservoir stent

(Medsystem – Conor)

Coated stent

(Cordis – Cypher)


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: aim of the study

  • Quantitative evaluation of the mechanical interaction between the two stents and an atherosclerotic artery in terms of:

  • stressesinduced in the vascular wall

  • tissue prolapse within the stent cells

  • radial stretch ratioof the vascular tissue

Rationale:

  • non physiological stress state field

  • tissue prolapse

Restenosis

1)

variation in tissue permeability due to the radial stretch ratio

2)

Influence on the drug release


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Materials & Methods

CAD models

  • balloon expandable

  • AISI 316L

Coated stent

Reservoir stent

Medsystem Conor

Cordis Cypher


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Materials & Methods

Geometry

Cordis Cypher

Medsystem Conor

= 1.2 mm

Thickness = 0.1 mm

3.7 mm

3.68 mm

Mesh

shell elements number: 1400 ÷ 2100

Young modulus 193 GPa

Poisson ratio 0.3

Material model: elasto-plastic


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Materials & Methods

Stent expansion: boundary conditions

experimentaltests:

At the end of the balloon inflation process the stent is

UNIFORMLY EXPANDED in the radial direction

a radial displacement up to 3 mm in diameter is imposed


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Materials & Methods

Coronary artery

Realisticgeometryobtained from medical images

Simplified symmetric geometry

influenced by simplified hypotheses

models and results near reality

reduced computational costs

high computational costs

utility for comparative analyses

difficulties for comparative analyses


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Materials & Methods

Material model:

Artery

  • homogeneous

  • isotropic

  • incompressible

(Hayashi and Imai, 1997)

  • hyperelastic

Artery and plaque

Geometry:

  • two hollow co-axial cylinders (no sliding)

  • artery internal diameter: 2.15 mm; thickness: 0.5 mm

  • plaque internal diameter: 1.25 mm; thickness: 0.45 mm

  • stenosis: 66%

Plaque

Mesh

  • 9230 Hexahedral elements (C3D8H)


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Materials & Methods

Simulations:

STEP1: PRETENSIONING of 4% (Holtzapfel, 2005)

axial stress equal to 0.006 MPa

STEP2: PRESSURIZATION

internal pressure of 100 mmHg

STEP3: STENT EXPANSION up to 3 mm


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Results

  • von Mises

  • triaxial

plaque and artery

MPa

0.35

0.29

0.23

0.17

0.11

0.06

0.00

MPa

4.52

3.67

2.62

1.81

0.97

0.53

0.00

Stresses

0.35

4.52

splaque~ 10 sartery

Cordis Cypher

Medsystem Conor

scordis~ 3 sconor


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Results

  • von Mises

  • triaxial

plaque and artery

MPa

MPa

MPa

0.20

-0.07

-0.34

-0.61

-0.87

-1.14

-1.41

2.26

1.90

1.54

1.19

0.83

0.47

0.11

0.40

0.21

0.01

-0.18

-0.38

-0.57

-0.76

Stresses

Radial stress

Medsystem Conor

Circumferential stress

Axial stress

Cordis Cypher


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Results

MPa

0.27

0.20

0.13

0.06

0.00

-0.07

-0.14

  • von Mises

  • triaxial

plaque and artery

Stresses

Circumferential stress

Axial stress

Radial stress

scordisslightly higher thensconor

Cordis Cypher

Medsystem Conor


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Results

  • tissue prolapse (tp)

  • radial stretch ratio (lr)

ln lr

-0.09

-0.19

-0.28

-0.37

-0.46

-0.55

-0.65

lr

increase of

Deformed configuration

Medsystem Conor

Cordis Cypher

3.00

2.90

2.94

3.00

tp = 0.1 mm

tp = 0.06 mm

Influence on the drug release

decrease of tissue permeability


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: Conclusions

  • It is difficult to find a strict correlation between prolapse, the in-stent restenosis and the geometrical design.

The FEM of the stent expansion developed allowed:

 to mechanically evaluate two stent designs in terms of

effects on the arterial wall

 to provide the initial geometry for the Drug Elution model

A similar study could be useful when joined with a clinical trial aimed at comparing the influence of the stent design on the degree of restenosis.

Clinical trials are present in the literature, but results are hardly comparable to those from previous trials, even for the same stent design, because of the different methods adopted (patient recruitment, primary or secondary endpoints, …).


Computer modelling of stent implantation expansion and fluidynamics

Coated vs. Reservoir stent: limitations and future work

Arterial wall

  • only one vessel layer considered

  • absence of plaque fracture

  • absence of vessel curvature

Methodology

  • absence of the inflation/deflation of the balloon

  • only one stent unit analyzed

  • New material model: 3 layers (Holtzapfel et al.,2005)

  • Stent expansion by balloon inflation


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Introduction

Stent-arterial wall interaction

1) Coated vs. Reservoir stent

2) Balloon expandable stent

Stent expansion

Fluidynamics

Two distinct computational models related to the implantation of balloon-expandable stent and the subsequent fluidynamics generated by stent presence were built


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Materials and Methods

FE models of balloon, stent and coronary artery were created using the commercial code ABAQUS/Explicit 6.5

Balloon

Material

7.8 mm

3.2 mm

linear-elastic

Young modulus 1.4 GPa

Poisson coefficient 0.3

Mesh

11762 M3D4R/M3D3R membrane elements

11872 nodes

No radial and tangential displacements

No axial and tangential displacements

P<0


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Materials and Methods

FE models of balloon, stent and coronary artery were created using the commercial code ABAQUS/Explicit v.6.5

Stent

Material

elasto-plastic

Young modulus 193 GPa

Poisson coefficient 0.3

3.68 mm

Mesh

14951 C3D8R solid elements

25724 nodes

1.2 mm

0.14 mm

No axial and tangential displacements


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Materials and Methods

FEMs of balloon, stent and coronary artery were created using the commercial code ABAQUS (Abaqus Inc, RI, USA)

Coronary artery

Mesh

Material

39420 C3D8R

solid elements

Cauchy Stresss (KPa)

Three hyperelastic and isotropic layers

Experimental data from tensile test on

intima, media and adventitia

(Holzapfel et al., 2005)

Reduced Polynomial n=5


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Materials and Methods

The whole model – Simulations

Step 2:

Balloon pressurization

(0.7 MPa)

Step 1:

Coronary pressurization

(100 mmHg)

Step 3:

Balloon deflation

(P=0 MPa)


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Results

I phase:

balloon’s inflation

II phase:

dogboning effect

IV phase:

deflation of the balloon

III phase:

fully inflation of the balloon


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Results

[MPa]

[MPa]

500

0.9

250

0.45

0

0

Von Mises stresses

II phase:dogboning effect


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Results

[MPa]

0.9

[MPa]

0.45

500

0

250

0

Von Mises stresses

Von Mises stresses

IV phase:deflation of the balloon


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Fluidynamics

Outlet

Inlet

Velocity profile:

parabolic and transient

Constant fixed pressure

4 cardiac cycles

pulse period = 0.54 s

[ La Disa et al., 2004 ]

Assumptions

  • rigid vessel wall

  • Newtonian fluid (Viscosity = 0.0035 kg/(m∙s), Density = 1060 kg/m3)


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Fluidynamics

0.65

0.52

0.39

0.26

0.13

0

2.0

1.6

1.2

0.8

0.4

0

[Pa]

1.1

0.88

0.66

0.44

0.22

0

WSS values alternate across the vessel during the cardiac cycle

[Pa]

[Pa]


Computer modelling of stent implantation expansion and fluidynamics

Balloon expandable stent: Conclusions

This study presents an approach to simulate the interaction of a coronary stent with the vascular wall and the Fluidynamics into the arterial wall

The stent expansion model allows:

  • to mechanically evaluate the stress effect generated by the stent on the arterial wall

  • to provide the initial geometry for the Fluidynamics

The fluidynamics model allows

  • to evaluate the fluidynamic changes following a stent procedure


Computer modelling of stent implantation expansion and fluidynamics

A

B

C

D

E

G

high

F

low

Balloon expandable stent: limitations and future work

Limitations

  • absence of vessel curvature

  • absence of plaque

  • only one unit analysed

In collaboration with the Erasmus Centre - Rotterdam

Future works

  • correlation between the structural and fluidynamics results, analyzing different stent designs

jostent

multilink

cordis

carbostent


Computer modelling of stent implantation expansion and fluidynamics

Acknowledgements

THANKS

Rossella Balossino Claudio Capelli Francesco MigliavaccaLorenza Petrini Gabriele Dubini

This work has been supported by the Fondazione Cariplo, Milan, Italy, under the project “Modellistica Matematica di Materiali Microstrutturati per Dispositivi a Rilascio di Farmaco”

[email protected]

LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS

www.labsmech.polimi.it


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