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Bio Mechanical Investigation of Instent

Restenosis Using FEM

ByAswini Kumar Muttyam

Major Professor

Dr. Linxia Gu

Committee Members

Dr. Michael A Langerman

Dr. Dana J Medlin

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Outline

Introduction Motivation

Objective and Scope of Work

Literature Review Finite Element Models & Results

Bare Stent

Stent, Plaque & Vessel

Stent 1(Uniform thickness)

Stent 2(Non-Uniform thickness)

Conclusions

Future Work

Acknowledgements

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Introduction

Coronary heart disease is the single leading cause of death in

America.

Every year around 1.2 Million Americans suffer from new or

recurrent coronary attack.

Percentage of deaths in a given year due to coronary attack isabout 38%*.

Coronary heart disease is caused by atherosclerosis.

*American Heart Association, Heart Diseases and Stroke Statistics: 2007 Update At-a-Glance. 2007, p12.

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Atherosclerosis

What

Narrowing and hardening of the artery causing blockage of

blood flow through them. It is also known as stenosis.

Why

Build up of fatty acids around the inner wall of artery.

Results Heart Stroke

Chest pain

Both

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Atherosclerosis

Retrieved from http://content.revolutionhealth.com/contentimages/images-image_popup-hb7_atherosclerosis.jpgon 02/23/2008

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Treatment

Cholesterol Medication

Medication Anti-Platelet Medication

Anticoagulants

Surgery

Angioplasty

Endarterectomy

Thrombolytic therapy

Bypass Surgery

Anticoagulants

Balloon AngioplastyBalloon Angioplasty

AngioplastyStentingStenting

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Stenting

In 1964, Dotter and Judkins.

Stenting is a technique used

to open the clogged arteriesand prevent them from re-

narrowing.

Stents are implantable,hallow cylindrical tubes

introduced in stenosedarteries by a balloon catherer.

Retrieved from http://images.google.com/images?q=stenting&ndsp=18&um=1&hl=en&start=36&sa=N on 02/23/2008

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Stenting Technique

Retrieved from http://rufusrajadurai.wetpaint.com/ on 02/23/2008

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Advantages

It is non-invasive.

It is less expensive comparedto other techniques.

Long term effectivenesscompared to PTCA andCABG.

Disadvantages

In-Stent Restenosis

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Importance of Numerical Simulations

Helps in better understanding the actual process.

Reduces number of experiments.

Saves Money.

It is important to validate the numerical models with carefully

designed experimental studies.

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Literature Review Lally, C., Dolan, F., and Prendergast, P.J., Cardiovascular Stent Design and

Vessel Stresses: A Finite Element Analysis. Journal of Biomechanics, 2005, 38(8):15741581.

Mackerle, J., Finite Element Modeling and Simulations in CardiovascularMechanics And Cardiology: A Bibliography 1993-2004. Computer Methods in

Biomechanics and Biomedical Engineering, 2005, 8(2), 59-81.

Campbell, R., Tseng, D.Y., Squire, J.C., Edelman, E.R., 1999, Balloon-arteryinteractions during stent placement a finite element analysis approach to pressure,compliance and stent design as contributor to vascular injury. Circulation, 84,

pp.378-83.

Liang, D.K., Yang, D.Z., Qi, M., Wang, W.Q., 2005, Finite element analysis ofthe implantation of a balloon-expandable stent in a stenosed artery.

International Journal of Cardiology 104, pp.314-318.

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Difference

Finite Element Analysis regarding stress correlation with in-stent

resteonsis are limited.

Different stent types and different plaque shapes are used.

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Objective

Long Term Objective

To Correlate the stresses with instent restenosis.

To Explore the key factors causing restenosis.

Scope of the Work

To develop a 3D model consists of stent, plaque and arteryin order to predict the stresses and strains on the vessel.

Comparison of stresses developed in the vessel for different

stent designs and plaque geometries.

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Description of FEM Model

Expansion of stent in stenosed

vessel.

FEM Model Consists:

Stent

Plaque

Artery

Symmetrical Model was

developed.

Software used :

ABAQUS/Standard

Stent

Plaque

Artery

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Description of FEM Model

Bare Stent Model

Model Geometry

Material Properties

Mechanical Boundary conditions

Mesh

Stent, Plaque and Artery Model Model Geometry

Material Properties

Mechanical Boundary conditions Mesh

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Model Geometry

Symmetrical model was

developed for the analysis.

1/2th in the longitudinaldirection and 1/4th in

Circumferential direction.

Length = 16 mmRadius of the stent = 0.6 mm

Thickness of stent = 0.1 mm

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Material Properties of the Stent

316-L Stainless Steel.

Linearly elastic-Perfectly

plastic model is considered.

Mechanical Properties Youngs Modulus = 300GPa

Poissons Ratio = 0.3

Yield Stress = 207GPa

Linearly Elastic-Perfectly Plastic

Constitutive Model

Yield Stress

Stress

Strain

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Mechanical Boundary Conditions

Stent

Symmetrical boundaryconditions were applied tothe nodes lying on the

symmetry planes.

Pressure 0.4MPa applied onthe inner surface of the

stent.

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FEM Mesh

Linear order Quadrilateralelements were used to mesh

stent.

A total of 6359 shell

elements were generated.

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Results of Bare stent Model

Maximum diameter of the

stent was 3.132 mm.

Maximum Von-Mises in the

stent was 207MPa.

Foreshortening of the stent

was 7.54% of total length.

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Comparison of Bare Stent Model Results

Comparison between the analysis results and published data*

1atm = 0.101325MPa

*Gu, L.X., Kumar, A.V., Santra, S., and Mericle, R.A., 2005, Finite Element Analysis of Covered MicrostentJournal of Biomechanics, 38(2005) 1221-1227.

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Convergence Study for Bare Stent Model

Mesh # of

Elements

Stent

Diameter

%

Error

1 2355 2.9566 --

2 6359 3.0338 2.54

3 22932 3.132 3.13

4 36302 3.1412 0.31

5 46686 3.1512 0.31

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Description of FEM Model

Stent

Plaque

Artery

0.751.526Artery

0.70.813Plaque

Thickness (mm)Radius (mm)Length (mm)

Symmetrical model was

developed for the analysis.

1/2th in the longitudinaldirection and 1/12th in

Circumferential direction.

Dimensions of plaque andartery

0.751.526Artery

0.70.813Plaque

Thickness (mm)Radius (mm)Length (mm)

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Description of FEM Model

L1

L2

Stent

1/2th in Longitudinal

direction and 1/12th

inCircumferential direction.

Two Stents

Stent 1 Stent 2

ThicknessStents

0.2mm0.1mmS2

0.1mm0.1mmS1

L2L1

ThicknessStents

0.2mm0.1mm

0.1mm0.1mm

L2L1

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Material Properties of the Stent

316-L Stainless Steel.

Linearly elastic-Perfectly

plastic model is considered.

Mechanical Properties Youngs Modulus = 300GPa

Poissons Ratio = 0.3

Yield Stress = 207GPa

Linearly Elastic-Perfectly Plastic

Constitutive Model

Yield Stress

S

tress

Strain

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Mechanical Boundary Conditions

Symmetrical boundaryconditions were applied to

the nodes lying on thesymmetry planes.

Pressure was applied on the

inner surface of the stent. Itis varied linearly from 0 to

1.5MPa.

FEM M h

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FEM Mesh

Linear order Quadrilateralelements are used to mesh

stent.

Linear order Hexahedronelements are used to mesh

plaque and artery.

C3D8H1267215560Artery

C3D8H34384700Plaque

S4109207Stent

Element TypeNo. of ElementsNo. of Nodes

C3D8H1267215560Artery

C3D8H34384700Plaque

S4109207Stent

Element TypeNo. of ElementsNo. of Nodes

S E i i h V l

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Stent Expansion in the Vessel

S D l i h A

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Stress Development in the Artery

Results for Stent Plaque and Artery

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Results for Stent Plaque and Artery

model for stent 1

Maximum Stress developed

in the plaque is 3.109MPa.

Maximum Stress developed

in the artery is 1.626Mpa.

Poking of the stent in to the

vessel wall.

Results for Stent Plaque and Artery

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Results for Stent Plaque and Artery

model for stent 2

Maximum Stress developed

in the plaque is 2.167MPa.

Maximum Stress developed

in the artery is 0.3759Mpa.

Poking of the stent in to the

vessel wall.

C p ri n f R lt b t n 2 t nt

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Comparison of Results between 2 stents

0.3759Mpa1.626MpaArtery

2.167Mpa3.109MpaPlaqueStent 2Stent 1

0.3759Mpa1.626MpaArtery

2.167Mpa3.109MpaPlaqueStent 2Stent 1

ArteryPlaque

Comparison of Results between 2 stents

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Comparison of Results between 2 stents

Poking of Stent 1 in to artery > Poking of stent 2 in to artery

Conclusions

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Conclusions

Bare Stent Model In the final expanded state, diameter of the stent at the free ends is higher

than at the center.

Maximum stresses developed in the stent agrees with constitutive modeladopted for the stent.

Stent, Plaque and Artery Model

Non-uniform thickness stent caused less injury to the artery. Maximum Stresses are observed at the area of contact between stent and

plaque.

Stresses in the plaque and artery are less in stent 2 than in stent 1.

Poking of stent into the artery was less in stent 2.

Future Work

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Future Work

Performing analysis for different stent type and plaque

geometries.

Comparing the stress mapping and strain mapping for the

different models.

Try to correlate the stress developed in the vessel wall with

restenosis rate.

Acknowledgements

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Acknowledgements

Dr. Linxia Gu

Dr. Michael A Langerman

Dr. Dana J Medlin

Department of Mechanical Engineering

Biomedical Engineering Programme

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