thesis final (3 files merged)

of 86 /86
I Modelling and Design Optimisation of Arterial Stents Carl McEncroe School of Aerospace, Mechanical and Mechatronic Engineering University of Sydney 2012 Honours Thesis

Author: carl-mcencroe

Post on 15-Jan-2017

161 views

Category:

Documents


17 download

Embed Size (px)

TRANSCRIPT

  • I

    Modelling and Design

    Optimisation of Arterial Stents

    Carl McEncroe

    School of Aerospace, Mechanical and Mechatronic Engineering

    University of Sydney

    2012 Honours Thesis

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    V

    Table of Contents

    StatementofContributionII

    Abstract.III

    Acknowledgements...IV

    TableofFiguresVII

    AbbreviationsandAcronyms.X

    1.Introduction.................................................................................................................................1

    2.LiteratureReview.......................................................................................................................3

    2.1.IndicationsforRevascularisation......................................................................................................3

    2.1.1.CoronaryArteryDisease..................................................................................................................................3

    2.1.2.PeripheralArteryDisease................................................................................................................................4

    2.1.3.RenovascularHypertension............................................................................................................................4

    2.1.4.CarotidArteryDisease......................................................................................................................................4

    2.2.BalloonAngioplasty................................................................................................................................5

    2.3.Stents............................................................................................................................................................6

    2.3.1.SurgicalProcedure..............................................................................................................................................7

    2.4.ArterialAnatomy.....................................................................................................................................9

    2.5.ExistingStentDesigns.........................................................................................................................10

    2.5.1.SlottedTube.......................................................................................................................................................11

    2.5.2.Coil..........................................................................................................................................................................12

    2.6.AcumenforanIdealStent..................................................................................................................15

    2.6.1.DifficultyofDelivery.......................................................................................................................................15

    2.6.2.Scaffolding...........................................................................................................................................................15

    2.6.3.DogBoning..........................................................................................................................................................16

    2.6.4.Foreshortening..................................................................................................................................................18

    2.6.5.AcuteStentThrombosis................................................................................................................................18

    2.6.6.Restenosis............................................................................................................................................................18

    2.7.ClinicalIndicationsforStentChoice:.............................................................................................19

    2.7.1.CoronaryArteries.............................................................................................................................................19

    2.7.2.TreatingCarotidArteries..............................................................................................................................22

    2.7.3.TreatingPeripheralArteryDisease(PAD)............................................................................................23

    2.8.TheFiniteElementMethod...............................................................................................................23

    2.9.FiniteElementAnalysisSoftware....................................................................................................24

    2.10.DesignOptimisation..........................................................................................................................24

    2.11.Summary................................................................................................................................................25

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    VI

    3.Methods........................................................................................................................................26

    3.1.MaterialPropertiesandCharacteristics.......................................................................................26

    3.1.1.Stent.......................................................................................................................................................................26

    3.1.2.CoronaryArtery................................................................................................................................................28

    3.1.3.Balloon..................................................................................................................................................................32

    3.2.SimulationofStentDeployment......................................................................................................33

    3.2.1.BoundaryandContactConditions............................................................................................................34

    3.2.2.Loading.................................................................................................................................................................35

    4.PreliminaryResults.................................................................................................................41

    4.1.ExpansionwithinRealisticCoronaryArtery...............................................................................41

    4.2.StentFreeExpansion...........................................................................................................................46

    5.ResultsandDiscussion...........................................................................................................48

    5.1.OriginalPalmazSchatzPS154.........................................................................................................48

    5.1.1.StressDistribution...........................................................................................................................................49

    5.1.2.LuminalGain.......................................................................................................................................................50

    5.1.3.VesselStraightening........................................................................................................................................51

    5.1.4.ElasticRecoil.......................................................................................................................................................51

    5.1.5.StentForeshortening......................................................................................................................................52

    5.1.6.Dogboning..........................................................................................................................................................52

    5.1.7.RatioofKineticandInternalEnergy........................................................................................................53

    5.2.ModifiedGeometryPS154.............................................................................................................54

    5.2.1.EightCircumferentialSlots..........................................................................................................................54

    5.2.2.SixteenCircumferentialSlots......................................................................................................................60

    5.2.3.CurvedEndPS154..........................................................................................................................................66

    5.2.4.AlteringthewidthofPS154distalstrut................................................................................................70

    5.3.OptimisedStentDesign.......................................................................................................................71

    6.ConclusionandRecommendations.....................................................................................73

    7.References...................................................................................................................................75

    8.Appendix......................................................................................................................................79

    8.1.ConsistentunitrequirementinAbaqus........................................................................................79

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    VII

    Table of Figures

    Figure1Differenttypesofvulnerableplaqueasunderlyingcauseofacutecoronary

    events(ACS)andsuddencardiacdeath(SCD).A,Ruptureproneplaquewithlarge

    lipidcoreandthinfibrouscapinfiltratedbymacrophages.B,Rupturedplaquewith

    subocclusivethrombusandearlyorganization.C,Erosionproneplaquewith

    proteoglycanmatrixinasmoothmusclecellrichplaque.D,Erodedplaquewith

    subocclusivethrombus.E,Intraplaquehemorrhagesecondarytoleakingvasa

    vasorum.F,Calcificnoduleprotrudingintothevessellumen.G,Chronicallystenotic

    plaquewithseverecalcification,oldthrombus,andeccentriclumen[8].............................4

    Figure2Carotidarterydiseaseandstentingofthecarotidarteries[12]....................................5

    Figure3Percutaneoustransluminalcoronaryangioplasty(PTCA)procedure

    demonstratingtheimprovedlumenareaafterplaqueiscompressedagainstthewalls

    ofthecoronaryarteries[13].....................................................................................................................6

    Figure5Coronaryangiographybeforeandafterangioplasty[18].................................................8

    Figure6Layersofthearterialwall................................................................................................................9

    Figure7Typicalstentconstructions:(A)closedcell,peakpeak,flexconnector,(B)open

    cell,nonflexconnector,peakpeak,(C)opencell,nonflexconnector,peakpeak,(D)

    opencell,flexconnector,peakpeak,(E)opencell,nonflexconnector,peakvalley,and

    (F)opencell,nonflexconnector,midstrut[20].............................................................................11

    Figure8Strutprogressiontowardcircumferentialorientationintwotubularstent

    designs[22]....................................................................................................................................................12

    Figure9Typicalstructureofcoilstents[22].........................................................................................12

    Figure10patternoftransientnonuniformballoonstentexpansionatdifferentstages

    duringexpansionprocess[24]..............................................................................................................17

    Figure11Lefevreclassificationofplaqueburdenatbifurcation[31]........................................20

    Figure13PhotographofthePalmazSchatzPS154balloonexpandablestentinits

    constrictedpredeploymentphase[46]............................................................................................26

    Figure14TheisometricviewoftheresultingSolidworksofthePS154stent.......................27

    Figure16ScreenshotsofthebranchedcoronaryarterymodelcreatedinScanIPwith

    hollowedbodytodefinethearterialwall.........................................................................................29

    Figure18ScreenshotstakenfromSimplewareScanIPofthesegmentationprocedureto

    procureastraightcoronaryarterysection......................................................................................30

    Figure19HollowcylindricalmodelofthearterymodelledinAbaqus......................................31

    Figure20SketchinAbaqusofthethreefoldballoontobeextrudedincludingparameters

    .............................................................................................................................................................................32

    Figure21Compliancedatafromthemanufacturersofduralynwiththelineofbestfit

    representingthelinearrelationshipofthematerial[52]..........................................................33

    Figure22Theresultingthinshellmembranemodelofthetrifoldballoonmodelledin

    Abaqus..............................................................................................................................................................33

    Figure23ScreenshotofthesetupwithinAbaqusoftherealisticcoronaryarterymodel

    illustratingtheapplicationofauniformpressureloadtothealreadymeshedinner

    surfaceofthestent.Thegreensectionrepresentstheartery,thewhitepartisthePS

    154stent,andthepinkarrowsrepresenttheselectedfacesofthefiniteelementson

    theinnersurfaceofthestentthataretobeassignedapressureloading..........................38

    Figure24ScreenshotofapreliminarysimulationofthePS154stentbythemethodof

    applyingauniformpressuretotheinnersurfaceofthestent.Excessivedogboning

    causedthedistalendsofthestenttoperforatethearterialwall,anexampleofdeep

    nodalpenetration.Theperforationshavebeencircledinred................................................39

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    VIII

    Figure25PressureRampingProfile.Theprofilerepresentstheamplitudeofthe

    applicationofloadingandunloadingforallsimulations.Thedottedredlinedepictsthe

    transitionpointbetweentheloadingandunloadingsteps.......................................................40

    Figure26Segmentcutviewofinitialsetupforrealisticcoronaryarterysimulation..........41

    Figure27YZplanecutviewofinitialsetupforrealisticcoronaryarterymodel.................41

    Figure28YZplanecutviewoffinalphaseofthesimulation........................................................42

    Figure29RatioofKineticEnergytoInternalEnergythroughoutthesimulationforthe

    realisticcoronaryarterymodel.............................................................................................................42

    Figure30Residualstressdistributioncontouronthearterialwall(withoutandwiththe

    stent).................................................................................................................................................................43

    Figure31RadialDisplacementofthreenodesontheoutersurfaceofthestent(thetwo

    distalendsandinthecentre)andtheircorrespondingvaluesofdogboning

    percentage......................................................................................................................................................43

    Figure32Superimposedimageoftheinitialpredeploymentphaseofthestent(dark

    green)andthefinalpostdeploymentinflatedgeometry(lightgreen)...............................44

    Figure33Foreshorteningofthestentvstheappliedpressureloadingfortherealistic

    arterymodel..................................................................................................................................................45

    Figure34DeformedendphaseoffreeexpansionofstentinAbaqusbyapplyinga

    uniformpressureloadtotheinnersurfaceofthestent.Notetheoverexpansionofthe

    distalendsofthestentbeyondthemaximuminflatedbodysectionofthestent

    (~3.5mm)........................................................................................................................................................46

    Figure35TheoriginalPS154stentwiththeinclusionofwireconnectorelements.The

    connectorelementsmustberestrainedfromexpandinglargerthanthemaximum

    inflatedsizeoftheballoon.......................................................................................................................47

    Figure36Initialsetupofmodelincludingtheartery(green),stent(red)andballoon

    (white)inAbaqus........................................................................................................................................48

    Figure37Sidetransparentviewoftheexpansionstages.Thetipimagedepictstheinitial

    setupofthemodel,andthebottomimagedepictsthefinalexpandedphase..................49

    Figure38Contourplotofthestressdistributioninthearterialwallatfirstimpactofthe

    distalendsofthestentwiththearterialwall..................................................................................49

    Figure39Contourplotoftheresidualstressdistributioninthearterialwallafterthe

    balloonhasbeendeflatedandremovedfromtheartery...........................................................50

    Figure40Superpositionoftheinitialandfinalgeometryofthestentfromthesideand

    frontviews......................................................................................................................................................50

    Figure41Graphoftheradialdisplacementofthearterialwallandthecorresponding

    luminalgaininareaofthevesselcomparedtotheexpansionpressureloading............51

    Figure42Graphofthepercentageforeshorteningofthestentvsthepressureloading...52

    Figure43Graphoftheradialdisplacementofnodesatthecentreandatthedistalendof

    thestentaswellasthecorresponding%dogboning.................................................................52

    Figure44RatioofKineticEnergytoInternalEnergythroughoutthesimulationofthe

    expansionoftheoriginalPS154stent...............................................................................................53

    Figure46Sidetransparentviewoftheexpansionprocess.Thetopimagedepictsthepoint

    offirstimpactbetweenthestentandtheartery,andthebottomimagedepictsthefinal

    stageofthesimulation..............................................................................................................................55

    Figure47Contourplotofthestressdistributionofthearterywhendistalendsofstent

    makefirstcontact........................................................................................................................................56

    Figure48Superpositionoforiginalpredeploymentphaseofstent(darkgreen)andthe

    geometryofthestentwhenfistmakingcontactwiththearterialwall(lightblue).......56

    Figure49Residualstressdistributionofthearterialwalloncetheballoonhasbeen

    deflated............................................................................................................................................................57

    Figure50Symbolplotdepictingthemagnitudeanddirectionofresidualprinciplestresses

    inthearterialwallattheendofthesimulation.............................................................................57

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    IX

    Figure51GraphoftheLuminalGainachievedvs.theappliedpressureloading..................58

    Figure52Graphofthepercentageforeshorteningofthestentvs.thepressureloading..58

    Figure53Graphoftheradialdisplacementofnodesatthecentreandatthedistalendof

    thestentaswellasthecorresponding%dogboning.................................................................59

    Figure54Graphoftheratioofkineticenergytointernalenergythroughouttheexpansion

    simulation.......................................................................................................................................................60

    Figure55IsometricviewoftheSolidworksmodelofthePS154stentmodifiedtoinclude

    16circumferentialslotsites...................................................................................................................60

    Figure56Sidetransparentviewoftheexpansionprocess.Thetopimagedepictsthe

    initialsetupofthestent,balloonandartery,thesecondimagedepictspointoffirst

    impactbetweenthestentandtheartery,andthebottomimagedepictsthefinalstage

    ofthesimulation..........................................................................................................................................61

    Figure57Contourplotofthestressdistributionofthearterywhendistalendsofstent

    makefirstcontact........................................................................................................................................62

    Figure58Superpositionoftheinitialandfinalgeometryofthestentfromthesideview.62

    Figure59Residualstressdistributionofthearterialwalloncetheballoonhasbeen

    deflated............................................................................................................................................................63

    Figure61GraphoftheLuminalGainachievedvs.theappliedpressureloading..................64

    Figure62Graphoftheratioofkineticenergytointernalenergythroughouttheexpansion

    simulation.......................................................................................................................................................64

    Figure63Graphoftheradialdisplacementofnodesatthecentreandatthedistalendof

    thestentaswellasthecorresponding%dogboning.................................................................65

    Figure65Sidetransparentviewoftheexpansionprocess.Thetopimagedepictsthe

    initialsetupofthestent,balloonandartery,thesecondimagedepictspointoffirst

    impactbetweenthestentandtheartery,andthebottomimagedepictsthefinalstage

    ofthesimulation..........................................................................................................................................66

    Figure66Contourplotofthestressdistributionofthearterywhendistalendsofstent

    makefirstcontact........................................................................................................................................67

    Figure67Residualstressdistributionofthearterialwalloncetheballoonhasbeen

    deflated............................................................................................................................................................67

    Figure68Symbolplotdepictingthemagnitudeanddirectionofresidualprinciplestresses

    inthearterialwallattheendofthesimulation.............................................................................68

    Figure69GraphoftheLuminalGainachievedvs.theappliedpressureloading..................68

    Figure70Graphofthepercentageforeshorteningofthestentvs.thepressureloading..69

    Figure71Graphoftheradialdisplacementofnodesatthecentreandatthedistalendof

    thestentaswellasthecorresponding%dogboning.................................................................69

    Figure72Screenshotsofthethreemodifiedstentsintermsofwidthofthedistalstrut.

    Fromlefttoright,thedistalstrutwidthis0.25mm,0.30mm(originalstentparameter),

    0.35mmand0.40mm.................................................................................................................................70

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    X

    Abbreviations and Acronyms

    BMS: Bare Metal Stent

    CAD: Coronary Artery Disease

    CTO: Chronic Total Occlusion

    DES: Drug Eluting Stent

    FDA: Food and Drug Administration

    FEA: Finite Element Analysis

    FEM: Finite Element Method

    ISR: In-stent restenosis

    PAD: Peripheral Artery Disease

    PCI: Percutaneous Coronary Intervention

    SVG: Saphenous Vein Graft

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    1

    1. Introduction

    The era of interventional cardiology had its grounds in 1964 when Charles Theodore

    Dotter and Melvin P. Judkins used a balloon-tipped catheter to treat a case of atherosclerotic

    disease in a femoral artery [1]. In wasnt until 1977 when Andreas Gruentzig went on to perform

    the first percutaneous transluminal coronary angioplasty (PTCA) on a human [2]. Angioplasty

    was a revolutionary procedure to improve blood flow through vessels throughout the body by

    inflating and then deflating a balloon within the diseased segment of the vessel. The procedure

    had the effect of dilating areas of blood vessels that had experienced narrowing from blockages

    to improve the blood flow through the vessel. The development of stents was prompted due to

    the two main shortcomings of angioplasty; acute occlusion and long-term restenosis [3]. The

    stent is a tubular metallic structure that is implanted and left within the vessel during an

    angioplasty procedure to give on-going support in the form of scaffolding to maintain vessel

    patency [4].

    Angioplasty and stenting is a minimally invasive and relatively low risk procedure that

    has revolutionised the treatment of coronary artery disease (CAD). Although stenting is used

    predominantly to treat CAD within the coronary arteries, stents are also used to treat numerous

    diseases that cause narrowing or blockage of arteries throughout the body. Stent design has

    evolved to improve its performance and reduce the risks involved with their use as well as being

    considered for more complex situations.

    There are many different designs for arterial stents and each different design varies the

    mechanical characteristics of the stents and therefore varies their suitability for different types

    and anatomical sites of arterial lesions. The favourable stent characteristics include flexibility,

    trackability, low profile, radio-opacity, thromboresistancy, biocompatibility, reliable

    expandability, high radial strength, circumferential coverage and a low surface area. There does

    not yet exist a stent that has these ideal properties that would make its use optimal for all cases,

    therefore interventional cardiologists are required to understand the differences in the different

    types of stents to determine the best candidate for different lesion types and anatomical sites.

    In 2003 the Food and Drug Administration (FDA) approved the first drug-eluting stent

    (DES) in the United States. These stents slowly release medication that inhibit cell proliferation

    and has consequently caused a drastic reduction in restenosis rates compared to bare metal stents

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    2

    (BMS). The improved outcomes associated with DES has expanded stent usage to include

    diabetic patients and to treat lesions that were previously believed to be too complex [2]. DES

    are now used in 70-80% of all stent procedures worldwide [5].

    Persisting concerns of in-stent restenosis and thrombosis keep the design evolution

    process alive. As no ideal arterial stent yet exists, the on-going design optimisation of arterial

    stents continues. This thesis aims to identify potential design optimisation of a commercially

    available stent by finite element analysis focusing on residual stress distribution in the artery,

    stent dog-boning, foreshortening, elastic recoil and luminal gain.

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    3

    2. Literature Review

    2.1. Indications for Revascularisation

    Angioplasty is a procedure used to treat conditions that cause the narrowing and blockage

    of vessels throughout the body. The implanting of a stent is now involved in more than 70% of

    angioplasties performed worldwide to assist the vessel to remain open [6]. The most common

    conditions that require revascularisation procedures are Coronary Artery Disease (CAD),

    Peripheral Artery Disease (PAD), Renovascular Hypertension, and Carotid Artery Disease.

    2.1.1. Coronary Artery Disease

    CAD is the single leading cause of mortality worldwide with greater than 19 million

    deaths annually [7, 8]. The three main coronary arteries (left anterior descending, circumflex and

    the right coronary artery) and their respective branches supply different sections of the heart

    muscle with oxygen-rich blood. CAD is caused by atherosclerosis of the coronary arteries, which

    is a build-up of plaque on the walls of arteries. Plaque is composed of cells, lipids, calcium,

    collagen, and inflammatory infiltrates [9]. Atherosclerosis of arteries can vary in severity

    depending on the amount of plaque build up as well as the composition of the plaque build-up.

    The varying levels of severity are depicted in Figure 1 on the following page.

    The coronary arteries supply the hearts muscle tissue with its oxygen and nutrient

    requirements and when the coronary arteries experience such atherosclerosis they are not able to

    supply the heart with the blood that it demands. An imbalance between blood demand and

    supply to the heart muscle cause chest pain for the patient called angina pectoris. Vulnerable

    atherosclerotic plaques can rupture which trigger a repair response from the body, producing a

    blood clot at the site of the superficial crack to seal it. If the clot completely obstructs the already

    narrowed artery then the blood supply to the heart muscle is cut off, known as ischemia

    without oxygen and nutrients the patient suffers myocardial infarction and possible death if not

    immediately treated. 70-85% of all myocardial infarctions occur with

  • 4

    Figure 1 - Different types of vulnerable plaque as underlying cause of acute coronary events (ACS) and sudden

    cardiac death (SCD). A, Rupture-prone plaque with large lipid core and thin fibrous cap infiltrated by

    macrophages. B, Ruptured plaque with subocclusive thrombus and early organization. C, Erosion-prone plaque

    with proteoglycan matrix in a smooth muscle cell-rich plaque. D, Eroded plaque with subocclusive thrombus. E,

    Intraplaque hemorrhage secondary to leaking vasa vasorum. F, Calcific nodule protruding into the vessel lumen.

    G, Chronically stenotic plaque with severe calcification, old thrombus, and eccentric lumen [8]

    2.1.2. Peripheral Artery Disease

    PAD is caused by atherosclerotic plaque build-up in the peripheral arteries of the

    body. PAD usually affects the arteries supplying the legs but can also affect the arteries to the

    stomach, kidneys, arms and the head. The reduction or cessation of blood flow to these

    regions can cause pain and numbing and if severe enough it can cause tissue death such as

    gangrene. A patient that suffers from PAD also has an increased chance of coronary and

    carotid artery disease [10].

    2.1.3. Renovascular Hypertension

    Renovascular hypertension is a condition whereby narrowing of the renal arteries

    causes a decreased blood flow to the kidneys and in turn the kidneys release hormones to

    retain salts and water. This has the effect of increasing the patients blood pressure.

    Atherosclerosis of the renal arteries is the most common cause of renovascular hypertension.

    2.1.4. Carotid Artery Disease

    Carotid Artery Disease is a narrowing of the carotid arteries in the neck that supply the

    head and brain with oxygen-rich blood. The narrowing is also a result of atherosclerosis and

    the patient can suffer a stroke if the blood supply is restricted to the brain, in the same respect

    as myocardial infarction for the heart. Carotid Artery Disease is the cause of 20% of all

    ischemic strokes and transient ischemic attacks [11]. A stroke can cause lasting brain damage,

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    5

    paralysis or death if the blood supply to the brain is cut off for too long. Carotid artery disease

    and stenting of the bifurcation of the carotid arteries is depicted in Figure 2.

    Figure 2 - Carotid artery disease and stenting of the carotid arteries [12]

    2.2. Balloon Angioplasty

    Balloon angioplasty is a procedure performed to treat patients that have stenosed

    blood vessels by compressing atherosclerotic plaques against the arterial wall and dilating the

    lumen for improved blood flow. The procedure involves passing a balloon-tipped catheter to

    the diseased segment of a vessel, inflating the balloon to a set diameter for roughly a minute

    and then deflating and removing the balloon-tipped catheter from the patient.

    Balloon angioplasty performed on the coronary arteries is known as percutaneous

    transluminal coronary angioplasty (PTCA) as depicted in Figure 3.

    Angioplasty as a treatment alone had several shortcomings that prompted the

    development of the stent. The two main shortcomings were acute occlusion and long-term

    restenosis. Some of the compressed material tends to spring back, or recoil, after balloon

    angioplasty. The procedure damages the arterial wall to some degree, which causes

    physiological mechanisms to repair the damage. Further cell proliferation of the intima,

    known as neointimal hyperplasia, occurs 3-6 months after the procedure. The combined effect

    of these factors causes the luminal area to re-narrow, called restenosis. The Belgium

    Netherlands Stent Study (Benestent) and the Stent Restenosis Study (STRESS) were two

    major randomised trials that popularised stent usage by confirming that stenting caused a

    reduction in angiographic restenosis and clinical events post-procedure [3].

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    6

    Figure 3 - Percutaneous transluminal coronary angioplasty (PTCA) procedure demonstrating the improved

    lumen area after plaque is compressed against the walls of the coronary arteries [13].

    As stenting is now involved in the majority of percutaneous coronary intervention

    (PCI) procedures the role of balloon angioplasty is to initially prepare the passage-way and

    site of the stent prior to its deployment, and then to inflate the stent against the arterial wall if

    the stent is not self-expanding, and then for further expansion of the stent to ensure complete

    dilation, if necessary.

    2.3. Stents

    A stent is a metallic tubular structure implanted and left in the diseased section of a

    vessel to restore blood flow. Stents vary greatly in their design however the main purpose is to

    give on-going assistance in holding a blood vessel open and preventing vessel recoil. Stents

    do this by providing a scaffolding feature for the arterial wall, mechanically enforcing it and

    resetting an improved luminal area, having the effect of decreasing the incidence of

    restenosis. The atherosclerotic plaques are compressed against the arterial walls, dilating the

    luminal area and maintaining vessel patency [14]. Stents can also be used after unsuccessful

    balloon angioplasty to hold back intimal flaps, close off vessel dissections and prevent plaque

    prolapse into the vascular lumen to treat threatened vessel closure [3, 4].

    In 2003 the Food and Drug Administration (FDA) approved the first drug-eluting stent

    (DES) in the United States. These stents slowly release medication that inhibit cell

    proliferation and has consequently caused a drastic reduction in restenosis rates compared to

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    7

    bare metal stents (BMS). The improved outcomes associated with DES has expanded stent

    usage to include diabetic patients and to treat specific lesion types that were previously

    believed to be too complex [2]. As of 2009, DES are used in 70-80% of all stent procedures

    worldwide [5]. Despite the dominance of DES usage, bare metal stents (BMS) still have a role

    in contemporary practice. The decision for usage of a BMS or DES comes down to both

    clinical and economic factors. DES are considerably more expensive than BMS, they require

    the patient to be on long-term dual antiplatelet therapy (DAPT) and they have been shown to

    increase the risk of late stent thrombosis. However, they significantly reduce restenosis rates

    (and therefore repeat vascularisation procedures) with no additional risk of mortality.

    Ultimately it depends on a patient-specific basis but DES are currently believed to be the

    superior choice in the majority of cases [2].

    As can be seen in Figure 4, the procedure for implanting a stent is effectively the same

    as balloon angioplasty, however, the stent remains in the diseased section of the vessel. Stents

    can be both self-expanding or balloon expandable, whereby the stent is crimped and mounted

    on a balloon-tipped catheter and expanded at the target site by balloon angioplasty. Self-

    expandable are easily deployed however they may require additional expansion by balloon

    angioplasty to ensure satisfactory dilation of the vessel [15].

    There are many different designs for arterial stents,

    the main types being mesh, slotted tube, tubular, coil, ring

    and multi-design. Each different design varies the

    mechanical characteristics of the stents and therefore varies

    their suitability for different types and anatomical sites of

    arterial lesions. The favourable stent characteristics include

    flexibility, trackability, low profile, radio-opacity,

    thromboresistancy, biocompatibility, reliable

    expandability, high radial strength, circumferential

    coverage and a low surface area [16]. There does not yet

    exist a stent that has these ideal properties that would make

    its use optimal for all cases, therefore interventional

    cardiologists are required to understand the differences in

    the different types of stents to determine the best candidate

    for different lesion types and anatomical sites [17].

    2.3.1. Surgical Procedure

    Balloon angioplasty and stenting is a minimally

    Figure 4 - Deployment of a balloon-expandable

    stent in the coronary artery [13].

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    8

    invasive procedure that takes between 30 and 60 minutes to complete. At least one week prior

    to the procedure, the patient will commence an anticoagulant drug regimen to thin their blood

    to ensure no blood clots occur during the operation. The patient is administered a local

    anaesthetic and an incision is made in an access artery, usually in the groin or the arm, as an

    entrance point to the bodys circulatory system. The patient is injected with a radio-opaque

    contrast material to circulate through the circulatory system that block x-rays, enabling the

    circulatory system to be visualised by an angiogram.

    The interventional cardiologist is able to navigate a guide wire of the diagnostic

    catheter through the circulatory system under angiogram image guidance to the target site.

    Using angiography and intravascular ultrasound (IVUS) the interventional cardiologist is then

    able to determine the length, diameter and type of balloon and stent required for each patient.

    Balloon angioplasty alone is performed before the stent is deployed to prepare the site

    of deployment and to help deliver the stent, this is known as pre-dilation. The stent is then

    advanced to the site, positioned to fully cover the lesion, and then expanded (either by balloon

    angioplasty or self-expanded). A hand-held syringe pump controls inflation of the balloon

    whereby the interventional cardiologist can monitor the inflationary pressure to ensure ideal

    dilation of the balloon and the stent. The balloon is left inflated for 30-60 seconds before

    being deflated. Once the interventional cardiologist believes the stent is satisfactorily

    expanded appositional to the arterial wall the balloon and catheters are removed from the

    body, leaving the stent behind. Figure 5 depicts coronary angiography images before and after

    an angioplasty procedure illustrating the improvement in blood flow after the deployment of a

    stent.

    Figure 5 - Coronary angiography before and after angioplasty [18]

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    9

    The operator technique for the deployment of stents is vitally important sub-optimal

    expansion is associated with increased rates of restenosis and stent thrombosis.

    Underestimating the size of stent required is the main cause of sub-optimal expansion.

    Intravascular ultrasound has been shown to improve optimal stent size selection as it is more

    accurate than angiography to determine stent length and diameter required, as well as

    identifying if stent-edge dissections occur and if the there is incomplete stent apposition [2].

    2.4. Arterial Anatomy

    The artery is a blood vessel that carries oxygen and nutrient-rich blood away from the

    heart throughout the body (with the exception of the pulmonary artery). An artery has three

    distinct layers that make up the arterial wall. The three layers of an artery are the tunica

    intima, tunica media and tunica externa (previously tunica adventitia) [19]. The hollow cavity

    in the middle of the artery through which the blood flows is known as the lumen. The three

    layers of the artery can be seen in Figure 6 and will be further discussed.

    Figure 6 - Layers of the arterial wall

    The arteries that are most commonly stented are the coronary arteries, carotid arteries,

    renal arteries, and the peripheral arteries such as the superficial femoral arteries.

    The tunica intima is the innermost layer of the artery and it is made up of connective

    tissue and endothelial cells that are in contact with the blood that flows through the artery.

    The endothelial cells make up a thin layer called the endothelium that acts as the scaffold for

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    10

    atherosclerotic plaques to build on. Several months after the deployment of a stent, the

    endothelium will grow over the top of the stent. The connective tissue consists of collagenous

    and elastic fibers that provide structural support for the artery.

    The tunica media is the middle layer of the artery and it is made up of smooth muscle

    cells and elastic tissue. The overall thickness of an artery is completely dependant on the

    thickness of the tunica media layer. Depending on the volume of blood that is flowing through

    the artery and the force that blood exerts on the artery due to the blood pressure dictates the

    amount of smooth muscle and elastic tissue required for the artery to maintain its structure

    without rupturing. The larger arteries, such as the aorta, require significantly more smooth

    muscle cells and elastic tissue than the smaller coronary or carotid arteries that accounts for

    the difference in thickness. The pressure in the artery varies between its peak (systolic

    pressure) and its minimum (diastolic pressure). The smooth muscle cells and elastic

    connective tissue allows the artery to resist and adapt to the varying pressure that it

    experiences between heart contractions. The tunica media is of utmost importance for this

    thesis as it is this layer that causes recoil of the vessel during and after stent deployment, as

    well as the source of resistance that the stent experiences over its lifetime.

    The tunica externa (formerly tunica adventitia) is the outer layer of the artery and it is

    made up of irregular connective tissue, both collagenous and elastic fibers. The function of

    the tunica externa is simply to connect the vessel to the area it is running through. The

    connective tissue of the tunica externa will connect to adjacent tissues to hold it in place and

    provide the artery with some support. Although the tunica externa provides the artery stability

    and allows it to stay in place, it does not have a significant role in maintaining vessel patency.

    The lumen of the artery is the cavity through which the blood flows. When the arteries

    experience atherosclerosis of the artery wall, the lumen size through which the blood can flow

    is reduced. The obstruction to normal blood flow that atherosclerotic plaques cause means

    that the oxygen demands of the body are not met.

    2.5. Existing Stent Designs

    There are various commercially available stents currently and they can be classified on

    the basis of their mode of expansion, their geometry design, and whether they are drug-eluting

    or bare-metal. Stents can be either self-expandable or balloon-expandable, with the latter

    being the major focus of this thesis. The materials used are plastically deformed during

    expansion by balloon angioplasty so that they retain their expanded form when the balloon is

    deflated and removed. The material has slight recoil that is related to the elastic deformation

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    11

    being reversed by relaxation. The ideal material would have a low yield so that the stent can

    be plastically deformed at manageable balloon pressures, and low intrinsic elastic recoil. 316

    L stainless steel is the material of choice in the majority of balloon-expandable stents. The

    two main types of stent geometry design for vascular means are slotted-tube and the coil

    designs and they all differ in length, percentage metal coverage, number of struts, strut

    thickness, and cross section [14].

    The two main constituents of stent design are expandable ring elements and

    connecting bridge elements. The expandable rings are generally of zig-zag patterns that make

    up the longitudinal struts of the stent. These expandable rings are positioned adjacent to each

    other and connected via the bridge elements. The bridge elements can be described as either

    flex or non-flex, which is dependant upon their shape. The designs vary where the bridges

    occur and how many there are. Stents that have all struts connected to the adjacent rings

    struts are called closed-cell, whereas when only some of the struts are the bridge connection

    points they are called open-celled. A few examples of different open and closed-cell designs

    with different combinations of connection points can be seen in Figure 7.

    Figure 7 - Typical stent constructions: (A) closed cell, peak-peak, flex connector, (B) open cell, nonflex

    connector, peak-peak, (C) open cell, nonflex connector, peak-peak, (D) open cell, flex connector, peak-peak, (E)

    open cell, nonflex connector, peak-valley, and (F) open cell, nonflex connector, midstrut [20]

    2.5.1. Slotted-Tube

    Slotted-tube design stents make up roughly 75% of all commercially available

    vascular stents [20]. The slotted-tube design is made up of a series of expandable Z shaped

    elements called struts that are connected by bridge elements as described and illustrated

    above. In general, slotted-tube stents have excellent radial strength but lack longitudinal

    flexibility compared to coil designs [21].

    As is demonstrated in Figure 8, the struts that are in the longitudinal direction rotate

    outwards to become the circumferential struts [22].

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    12

    Figure 8 - Strut progression toward circumferential orientation in two tubular stent designs [22]

    2.5.2. Coil

    Coil stents exhibit the highest longitudinal flexibility of all the stents as they have no

    longitudinal shaft [22]. However, the strength of coil stents is somewhat lacking. The mode of

    expansion for coil stents is by stretching of the circumferential struts until the desired

    diameter is achieved as the diameter of the stent is dilated the space between the struts

    increases which increases the chance of tissue prolapsing through the gaps. The most readily

    used coil stent is the Freedom stent made by Global Therapeutics Inc. [23]. A typical coil

    stent structure is illustrated in Figure 9, depicting in particular the increase in gap size

    between the struts as the stent is expanded.

    Figure 9 - Typical structure of coil stents [22].

  • Stent Manufacturer Geometry and design Drug-

    Eluting? Material Deployment

    Diameter

    (mm)

    Length

    (mm) Clinical Use Notes Picture

    Acculink Guidant

    comes in both a classical

    tube and a conical

    configuration

    - nitinol Self-

    expandable 6.0-10.0 20-40 Carotid

    The conical configuration manufactured so that

    when expanded the distal diameter is smaller than

    the proximal diameter (for being in apportioned in

    the internal carotid artery and the common carotid

    artery, respectively)

    beStent Medtronic AVE

    Slotted tube - sinusoidal

    ring modules linked via

    sigmoidal link elements.

    Radiopaque gold markers

    at both ends

    - 316 L stainless

    steel

    Balloon-

    expandable 2.5-5.5 8-25

    Regular, ostial,

    bifurcation lesions

    Large/open cell design that facilitates access to side

    branches, virtually no shortening, low elastic recoil

    beStent 2 Medtronic AVE

    Slotted tube - flexible

    radial "S" crowns and

    longitudinal "V" crowns

    crossing at a junction that

    rotates during expansion.

    Gold markers at both ends

    - 316 L stainless

    steel

    Balloon-

    expandable 2.5-4.0 9-30

    Regular coronary

    stent

    Closer strut design than beStent so not suitable for

    ostial or bifurcational lesions

    Biodivysio Biocompatibles

    Slotted tube - alternating

    sinusoidal rings with

    rectangular and rounded

    edges. Rings linked by S

    articulations

    -

    316 L stainless

    steel coated with

    phosphorylcholine

    Balloon-

    expandable 2.75-4.0 8-28

    Regular, ostial,

    bifurcational

    lesions

    Good scaffolding and open cell design that

    facilitates access to side branches. The

    phosphorylcholine coating is designed to reduce

    thrombosis

    CYPER

    system (Bx

    Velocity

    stent)

    Cordis

    Slotted tube - sinusoidal

    ring strut modules linked

    by "N" shaped flex

    segments

    Simolimus 316 L stainless

    steel

    Balloon-

    expandable 2.25-5.0 8-33

    Ostial lesions

    (aorto-ostial),

    regular lesions,

    calcified

    The CYPHER system is the popular Bx Velocity

    slotted tube stent that has been treated to be drug-

    eluting with Simolimus to decrease the incidence of

    thrombosis

    NIR (9 cell) Medinol

    Slotted tube - sinusoidal

    ring modules linked via

    curved link elements

    - 316 L stainless

    steel

    Balloon-

    expandable 2.0-5.0 9-32

    Ostial lesions

    (aorto-ostial),

    calcified lesions,

    not good for

    Designed to be a stiff stent compared to others

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    14

    configuration designed to

    improve the stent's

    scaffolding properties

    crossed vessels

    Carbostent

    Sirius

    Sorin

    Biomedica

    Cardio

    Slotted tube - with

    platinum end markers -

    316 L stainless

    steel covered with

    thin layer of

    turbostratic

    carbon

    Balloon-

    expandable 3.0-5.0 6-20

    Regular and

    difficult

    anatomies/lesions

    Turbostratic carbon with the intent to decrease its

    interaction with platelets

    Multi-Link

    RX

    ULTRA

    Guidant

    Slotted tube - corrugated

    multi-link five crown

    rounded corner zigzag ring

    module design linked with

    straight link elements

    - 316 L stainless

    steel

    Balloon-

    expandable 3.5-5.0 13-38

    Ostial lesions

    (aorto-ostial)

    Excellent longitudinal flexibility and traceability.

    Lacks in radial strength.

    PS 154 Palmaz Schatz

    Laser cute slotted tube

    microstent with 12

    circumferential slot sites

    and no bridge connectors

    - 316L Stainless

    Steel

    Balloon-

    expandable 3.5 8.06

    Regular coronary

    stent. Small

    arterial lesions

    Excellent radial strength but lacks in flexibility and

    has unfavourable levels of dog-boning

  • 15

    2.6. Acumen for an Ideal Stent

    An ideal stent can be judged on the outcomes of difficulty of delivery, scaffolding

    capability, degree of dog-boning and foreshortening, and the occurrence of acute stent

    thrombosis and restenosis.

    2.6.1. Difficulty of Delivery

    The ideal stent should be easy to deliver to the diseased site of the vessel. The two

    characteristics of stent design that are of great importance when considering the ease of

    delivery are having high longitudinal flexibility in its unexpanded state and having a low

    profile [22]. The flexibility of the stent can be described by the bending stiffness of the stent.

    Evaluating the stiffness of stents can be achieved by using the bending equation of a simple

    cantilever beam [24]:

    !" =!"

    !

    3!

    (EI = Bending Stiffness, P = Pressure, L = Length of stent, ! = Deflection)

    High longitudinal flexibility is important so that the stent can be easily advanced to the

    target through tortuous anatomical curves and bends with minimum effort, without injuring

    the intima of the arterial wall and without eliciting spasm. Smooth delivery is also termed

    high trackability.

    2.6.2. Scaffolding

    For ideal scaffolding the stent is required to have sufficient radial strength to be able to

    hold the artery open to the desired luminal area, resisting the elastic recoil of the tunica media.

    The stent must be able to cover the diseased segment of the artery in a uniform manner so that

    there is no tissue or plaque prolapse through gaps in the stent. It is also required that the stent

    have to strength to be able to tack back intimal flaps and seal off vessel dissections. Once the

    stent is deployed at the target site, the stent should be able to mould to the natural contour of

    the vessel. This is especially important if the stent is to be deployed in the curvature of a

    vessel or in a complex anatomical region such as a bifurcation. The stent must have high

    longitudinal flexibility in its expanded stage so that it doesnt tend to straighten out the vessel

    but rather have a smooth transition between the stented and adjacent arterial wall areas. The

    first Palmaz-Schatz stent had low longitudinal flexibility and it was found to straighten the

    target site if deployed at a bend this would induce stress concentrations at the ends of the

    stent and injure the arterial wall increases the incidence of restenosis and acute stent

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    16

    thrombosis. High longitudinal flexibility is found in the multi-design stents and the more

    flexible coil stents that have no longitudinal shaft or only a single longitudinal shaft such as

    the Bart XT stent [22].

    2.6.2.1. Radial Strength

    Radial strength is the resistance that the circumferential struts provide against the

    elastic recoil response of the vessels media [22]. The hyperelastic media induces a

    compressing pressure on the circumferential struts of the stent. The stent tends to recoil after

    the balloon is deflated and removed due to the relaxation of the elastic media but also due to

    the internal stresses within the stent [25]. In general, a stent will have higher radial strength if

    it has wide, thick longitudinal struts that during expansion rotate circumferentially [22].

    Generally, there is an inverse relationship between the radial strength of a stent and its

    longitudinal flexibility.

    2.6.2.2. Recoil

    Recoil of the stent occurs in both the longitudinal and radial directions, they represent

    the amount of contraction of the stent after the removal of the balloon catheter [24]. The

    following equations describe both longitudinal and radial recoil:

    !"#$%&'(%#)*!"#$%& =!!"#$ !!"#$%&

    !!"#$

    !"#$"%!"#$%& =!!"#$ !!"#$%&

    !!"#$

    Where L and R are the length and radius of the stent before removing the balloon

    catheter and after removing the balloon catheter. Radial recoil of the stent after the stent is

    deployed is a serious engineering concern as it decreases the luminal area of the vessel and

    affects the stability of the stent. Stents that exhibit high recoil due to their material, design and

    geometry require the operator to over-dilate the stent with respects to the desired luminal area

    so allow for the recoil this can damage the arterial wall tissue and cause intimal

    proliferation. The elastic recoil of tubular stents is greater than coil or hybrid stents [25].

    2.6.3. Dog-Boning

    Dog-boning is the phenomenon that takes place during the expansion of a stent by a

    balloon catheter whereby the distal ends of the stent are the first places to expand, resembling

    a dog-bone shape. Dog-boning causes unnecessary trauma and damage to the arterial wall

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    17

    during the expansion phase which can increase the risk of restenosis of the artery [26]. Four

    stages of the expansion of a stent can be seen below in Figure 10, with the dog-boning

    phenomenon very obvious in the second and third stages.

    Figure 10 - pattern of transient non-uniform balloon-stent expansion at different stages during expansion process

    [24]

    Dog-boning can be described by the following equation:

    !"#$"%&%# = !!"#$%&!"#$

    !!"#$%&'

    !"#$

    !!"#$%&

    !"#$

    (!!"#$%&!"#$ = the distal radius of the stent, !

    !"#$%&'

    !"#$ = the central radius of the stent [24])

    Several finite element analysis studies have been completed to determine whether

    altering the geometrical design at the distal ends of the stent would result in decreased dog-

    boning, however Wang et al demonstrated that dog-boning can be drastically reduced by

    using shorter balloons so that the overhang at either end of the stent is not as pronounced [27].

    Wang also found that increasing the width of the distal struts had beneficial results in

    decreasing dog-boning.

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    18

    2.6.4. Foreshortening

    Foreshortening is the deformation of the stent in the longitudinal direction when the

    stent is expanded [24].

    !"#$%!"#$%&%' = ! !!"#$%&

    !

    (L = the pre-expansion length of the stent, !!"#$%& = the length of the stent after expansion)

    Foreshortening is an important issue that needs to be considered when evaluating stent

    design. If the stent exhibits a large amount of foreshortening then when it is expanded there is

    a possibility that it is not covering all of the atherosclerotic plaque anymore. Simply

    overestimating the length of the stent required to avoid this problem can also damage the

    arterial wall at the ends of the stent. Foreshortening of the stent can cause damage to the

    arterial wall as the contact between the stent and the arterial wall exhibit a shearing force that

    can increase the chance of restenosis of the artery.

    2.6.5. Acute Stent Thrombosis

    The ideal stent would have a low incidence rate of acute stent thrombosis. The design

    and material of the stent are extremely important as well as correct apposition of the stents

    struts to the arterial wall. Malapposition of the struts leave gaps between the stent and the

    artery wall where thrombus is able to form. For the struts that are not well embedded into the

    vessel wall, they tend to have much friction with the wall during ventricular contraction and

    relaxation that increases the risk of acute stent thrombosis. The surface of stent should be

    smooth and burr-free by being polished or electroplated [22]. The material used by the stent

    also has a major effect on the thromboresistancy of the stent.

    2.6.6. Restenosis

    Restenosis is a condition where a vessel that has already been revascularised begins to

    narrow again. It is caused by the early recoil and late arterial remodelling already mentioned,

    but the major cause of restenosis is neointimal hyperplasia [22]. Neointimal hyperplasia is a

    result of the initial arterial tissue damage that occurs during stent implantation and the

    persistent frictional stimulation by the stent. The design of the stent must ensure full

    apposition of the struts to the arterial wall and with the least injury to the arterial wall as

    possible. As discussed earlier, drug-eluting stents have greatly reduced the incidence rate of

    in-stent restenosis compared to bare-metal stents.

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    19

    2.7. Clinical Indications for Stent Choice:

    Interventional cardiologists are faced with a very large and varied assortment of stent

    types and it is up to their decision-making process and their experience to select the most

    appropriate stent for all cases. The choice depends on the patient, the size and composition of

    the lesion, and the artery that the lesion is located in. The different characteristics of stents to

    match different clinical issues will assist in making a judicious selection of stents. The

    following discussion will outline some features that will make a stent more or less appropriate

    for certain anatomical sites and lesion types.

    2.7.1. Coronary Arteries

    2.7.1.1. Regular Coronary Lesion

    The regular coronary lesion is characterized by being a proximal and non-angulated

    lesion [28]. It is understood that procedural success in these straightforward cases can be

    achieved with almost any stent [22]. Stents recommended for use in the regular coronary

    artery are the tubular, slotted-tube and ring geometries [28]. All of the design characteristics

    such as deliverability and lesion coverage need to be determined for the specific case.

    2.7.1.2. Lesions located in curvature of vessel

    As has already been discussed, it is essential that a stent have very high longitudinal

    flexibility when treating lesions in curvature of a vessel. If the flexibility is low then the stent

    will tend to straighten the vessel and exert unwanted stresses at the ends damaging the arterial

    tissue. Stents with no longitudinal shaft have the highest flexibility such as the multi-design

    and coil stents [22].

    2.7.1.3. Ostial Lesions

    An ostial lesion is an atherosclerotic plaque that occurs at an ostium of a vessel

    where the origin of a vessel branches off from a larger parent conduit vessel. The main

    concerns when stenting ostial lesions is the difficulty involved localising the ostium

    angiographically and then optimally positioning and deploying the stent without having

    excess metal protruding in to the parent vessel [29]. It is paramount that the stent chosen for

    ostial lesions has excellent radiologic visibility several stents incorporate end markers that

    assist optimal positioning of the stent to the ostium. The elastic recoil of the parent vessel is

    generally much stronger than that of the artery being stenting, such as with the aorta and the

    coronary arteries. The chosen stent should have high radial strength and low elastic recoil to

    allow for the increased recoil response at the ostial end. Three slotted tube stents that have

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    20

    been shown to be effective for aorto-ostial lesions are the BxVelocity, the NIR, and the Ultra

    [28].

    2.7.1.4. Bifurcational Lesions

    Bifurcational lesions are atherosclerotic plaques that are found at a bifurcation of an

    artery where the artery separates in to two parts. They are a very complex subset of lesions

    and there are many different forms of plaque burden at a bifurcation that all require different

    stent choice and deployment technique. The six main types of plaque burden are described by

    the Lefevre classification in Figure 7. The main concerns with stenting a bifurcational lesion

    is the risk of excess metal protruding into the side branch and not being able to have sufficient

    coverage of the lesion in complex areas. Bifurcational lesions also have a risk of side branch

    occlusion as the plaque tends to shift and this is what prompted the development of the

    kissing balloon technique which corrects distortion of the side branch that occurs during

    deployment [30].

    Figure 11 - Lefevre classification of plaque burden at bifurcation [31]

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    21

    Stents for bifurcational lesions should have excellent radiologic visibility for optimal

    positioning and have large side openings between the struts for passage of further stenting of

    the side branch using the modified T-technique for lesions proximal to the bifurcation [31].

    The slotted tube stents are found to have the greatest side opening between struts and are the

    best choice for stenting bifurcational lesions [28].

    2.7.1.5. Calcified Lesions

    As discussed previously, atherosclerotic plaques can vary greatly in composition.

    Expanding a stent at a calcified lesion will result in a less than desired luminal size compared

    to non-calcified lesions [28]. Stents for calcified lesions must have high radial strength to

    resist the elastic recoil and to maintain vessel stabilisation. Preparing the site prior to stent

    deployment by using rotational atherectomy is advised with highly calcified lesions. Highly

    calcified lesions have been shown to hinder optimal expansion of stents, especially self-

    expandable stents. The NIR, BxVelocity and Ultra are considered sound choices for calcified

    lesions [28].

    2.7.1.6. Chronic Total Occlusions

    Chronic total occlusion (CTO) is defined as an older than 3 month old total

    obstruction of an artery. They are a still a very difficult condition to treat effectively and

    recanalisation is performed prior to stent deployment to allow passage for the balloon and

    stent. A stent for treating CTO should have high radial strength to hold back the remaining

    plaque, and have good coverage with a closed-cell design so that there is no plaque prolapse

    in to the lumen. More recently drug-eluting stents have also been shown to be superior to

    bare-metal stents in the treatment of CTO.

    2.7.1.7. Small Vessels

    Stents were not initially developed for very small vessels, however there is now a

    modest range of stents available to treat vessels less than three millimetres in diameter. They

    have improved flexibility, capacity to reach distal lesions, and thin strut structure which are

    all very important attributes when the navigating through and deploying in very small vessels.

    2.7.1.8. Saphenous Vein Grafts

    A saphenous vein graft (SVG) is a cardiac intervention to treat vascular occlusions in

    the coronary arteries by removing the diseased segment and grafting in a saphenous vein to

    improve the blood flow. Saphenous vein grafts are susceptible to atherosclerotic plaque build-

    up and stenosis also once they take the place of the coronary artery. Stenting of the SVG is

    different to a regular coronary artery stenting procedure because it involves a larger vessel

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    22

    diameter and there is significant risk of distal embolization. The lesions in SVG tend to be

    considerably longer than regular lesions so the stent has to be available in longer lengths.

    Self-expanding stents such as the Wallstent or the nitinol NIR are a good choice for SVG

    lesions as they minimise trauma and damage to the arterial wall lowering the risk of distal

    embolization [32].

    Despite all of the theoretic and practical considerations provided for selecting a

    particular stent to treat a specific lesion, the individual experience and confidence of the

    operator are paramount. No rationale for choosing a specific stent for a specific lesion is yet

    supported by randomized trials.

    Except for the use of a stent to prevent threatened occlusion, stents are implanted with

    the intent to prevent restenosis. The operator should strive to reach this goal while

    maximizing the patients safety. Judicious stent selection, balloon sizing, and lesion

    preparation to achieve an optimal final lumen dimension remain the most important goals in

    percutaneous coronary interventions [28].

    2.7.2. Treating Carotid Arteries

    Stents used to treat carotid artery disease improve the blood flow to the head and

    brain. If the carotid arteries become completely blocked, or if some of the atherosclerotic

    plaque becomes dislodged and carries to the brain then the patient can suffer a life-threatening

    stroke. Carotid endarterectomy had been the general procedure for the treatment of

    atherosclerotic carotid arteries however stenting of the carotid arteries is recently being

    acknowledged as a safe and cost-effective alternative to carotid endarterectomy [11]. Stenting

    is the preferred treatment compared to

    angioplasty alone as stenting largely avoids

    plaque dislodgment, intimal dissection,

    elastic recoil and late restenosis [33].

    Stenting for the treatment of carotid artery

    disease has been shown to be an effective

    and relatively safe treatment [34].

    Atherosclerosis of the bifurcation of the

    common and internal carotid arteries is

    responsible for the majority of strokes [33].

    Intrathoracic, ostial common carotid and

    intracerebral lesions are also associated with

    Figure 12 - Acculink stent: both cylinder and conical designs

    are available

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    23

    stroke events too. The differing anatomies and size of these lesions warrants different stents.

    Much of the discussion of different stent usage for coronary arteries carries over in the

    concerns for the carotid arteries. Ostial lesions require high radial strength and precise

    positioning. Carotid bifurcation lesions may require stents of varying diameters as the lumen

    area of the common carotid and the internal carotid artery vary greatly. The Guidant Acculink

    stent comes in both a classic cylinder but also a conical design as seen in Figure 12. The

    conical configuration is designed so that the smaller distal end is placed in the internal carotid

    artery whereas the larger proximal end is appositioned within the common carotid artery. The

    intracerebral arteries have a much smaller diameter and have tortuous curvature. These

    arteries require extremely small diameter stents that have high longitudinal flexibility as the

    intracranial arteries are very sensitive to spasm [33].

    2.7.3. Treating Peripheral Artery Disease (PAD)

    There are several stents that have now been designed specifically for peripheral artery

    disease to treat pain, numbness and tissue death in the extremities. The legs are the most

    commonly affected area of the body as the femoral arteries that supply the legs with blood

    become stenosed. Patients with severe cases of PAD are at the risk of limb amputation,

    however as PAD is a major risk factor for coronary artery disease, their associated risks often

    overshadow those of PAD [10]. This is due to PAD being a marker for systemic

    atherosclerosis, so the risks to limbs are low compared to the risk to life of the patient [35].

    Because of this patients are often stented in the coronary and peripheral arteries in the same

    procedure.

    The main arteries that are stented to treat PAD are the aorta, iliac, femoral, popliteal,

    tibial and peroneal arteries. Each of these arteries varies greatly in size, elastic recoil and

    blood flow conditions. Stents are chosen on a case specific basis for much of the reasons

    already discussed.

    2.8. The Finite Element Method

    The finite element method is a numerical technique that provides approximate

    solutions to a series of differential equations that describe a range of physical and non-

    physical problems. It can be applied in structural analysis to determine stresses and

    deformations of a model. The finite element method discretises a large complex model into

    many simple finite elements. Shared nodes connect each of the finite elements. The collection

    of all of the finite elements and the nodes creates the mesh of a model. The fundamental

    variable of a stress analysis is the displacements of the nodes. Once these displacements are

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    24

    calculated the stresses and strains of all finite elements can be determined. The quality of the

    mesh can be described by the average size of the finite elements that make up the model.

    Finer mesh qualities include more elements and nodes in the mesh and should converge upon

    an approximate solution closer to the realistic solution. A finer mesh quality involves

    increased degrees of freedom of the nodes in the model thus the solution more accurately

    describes the stresses and deformation of the whole model. Increasing the quality of the mesh

    involves increasing the number of differential equations that need to be solved to determine

    the approximate solution.

    One must consider the weigh-off between accuracy of the solution and the

    computational time required to come to the solution. A convergence test can run to determine

    the ideal mesh quality of the model to ensure a balance between an accurate solution and a

    reasonable simulation time.

    2.9. Finite Element Analysis Software

    After reviewing the methodology and practices of several stent optimisation papers it

    was concluded that Abaqus and ANSYS were the two main finite element analysis (FEA)

    software packages used for similar simulations [14, 23, 24, 27, 36-42]. It was decided that

    Abaqus would be the best choice for the FEA simulation in this thesis as Abaqus is a more

    robust package compared to ANSYS when considering non-linear and dynamic models. The

    numerical analysis required in this thesis involves large levels of deformation and contact

    making the Abaqus code a clear choice as it provides a stable general contact algorithm [11].

    Abaqus is a powerful engineering simulation program that can has the capability of modelling

    and running finite element analysis of very complex models. As much of the analysis that will

    be done in this thesis will involve complex geometries of artery models and of the stents

    themselves, Abaqus is the best choice for a finite element method program.

    Abaqus has the ability of studying more that simply structural problems it is also

    able to simulate problems relating to acoustics, mass diffusion, thermal management of

    electrical components, soil mechanics, piezoelectric analysis and heat transfer. This thesis

    however will only be using structural (stress/displacement) component of the software.

    2.10. Design Optimisation

    This thesis aims to optimise the design of a commercially available coronary stent in

    order to reduce the incidence of restenosis and thrombosis. Reklaitis et al [43] describe

    optimisation succinctly in the following quote: In general terms, optimisation theory is a

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    25

    body of mathematical results and numerical methods for finding and identifying the best

    candidate from a collection of alternatives without having to explicitly enumerate and

    evaluate all possible alternatives.

    The design optimisation procedure that this thesis undertakes is a qualitative approach

    whereby finite element method software is utilised to observe improvements in results from

    simulations of several stent designs to select the best candidate. FEM is a major tool of the

    design optimisation process as it is an inexpensive and fast way to evaluate changes in design

    geometry and material properties without having to manufacture and implant first.

    Experimental measurement would be very expensive and time consuming. FEM software is

    utilised to determine if stress can be minimised as well as improving on other expansion

    outcomes such as dog-boning, foreshortening and elastic recoil.

    2.11. Summary

    Stenting has become a very common and relied upon technique employed in

    interventional cardiology and increasingly in other blood vessels also. With persisting

    concerns of in-stent restenosis and thrombosis, stents are not yet a perfect treatment of

    atherosclerotic arteries. The ongoing mechanical design evolution of the stent has seen

    improvements in clinical outcomes over the past decades, in addition to the introduction of

    drug-eluting stents in recent years. However as the ideal arterial stent does not yet exist, there

    is potential for further design optimisation so that the stent can become a safe, effective, and

    reliable device to restore vessel patency for the patients lifetime.

    This thesis will focus on the design optimisation of a balloon-expandable slotted tube

    stent utilising finite element analysis software. Many studies have previously been conducted

    on this topic and this thesis will attempt to validate and build upon this work by determining

    mechanical factors of the stent that can be altered to improve the long-term performance of

    arterial stents.

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    26

    3. Methods

    The remainder of this thesis will focus on the Palmaz Schatz PS 154 balloon-

    expandable coronary artery stent (Johnson & Johnson, Warren, NJ, USA) in particular. The

    PS 154 will be the subject of finite element method (FEM) simulation and analysis to propose

    recommendations on design optimisation of the original stent design.

    One of the main challenges of this thesis is to develop a general and easily repeatable

    finite element procedure to model the mechanical characteristics of the stent and the arterial

    wall. This thesis initially considered simulation of stent deployment in a realistic coronary

    artery. Modelling and simulating the deployment of a stent in a realistic coronary artery has

    been achieved by Zahedmanesh & Gijsen et al [44, 45] but not attempted by many others.

    Modelling of a realistic coronary artery comes with the complexity of producing the model,

    the complications associated with the complex non-linearity of the geometry and the difficulty

    in producing an easily repeatable FEM procedure.

    Initially a model for the arterial wall was created using the Simpleware software,

    ScanIP. It was found that continuing to use the ScanIP generated artery would not be a wise

    choice as it was not completely compatible with the FEM software being used. The final

    decision was to create the artery natively within Abaqus CAE in the form of a perfect hollow

    cylinder, which was also the chosen finite element method software package for analysis in

    this thesis. This was deemed necessary to reduce the computational workload in completing

    the simulations, and also to ensure consistent placement and fixing of the several stent

    expansion simulations that were analysed. The assumption of the artery being a straight,

    hollow cylindrical tube is the basis of the mast majority of literature on modelling and design

    optimisation of arterial stents.

    3.1. Material Properties and Characteristics

    3.1.1. Stent

    Figure 13 - Photograph of the Palmaz Schatz PS 154 balloon-expandable stent in its constricted pre-deployment

    phase [46].

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    27

    The Palmaz Schatz PS 154 stent is a laser-cut slotted tube stent (see Figure 13). The

    PS 154 stent was created in Solidworks under parameters set out in Gu et al [46] and can be

    viewed in the following table.

    PS 154 Stent Parameters

    Outer Diameter 1.47mm

    Length 8.06mm

    Thickness 0.10mm

    Direction Number of Slots

    Longitudinal 2

    Circumferential 12

    Slot Dimensions

    Length 3.62mm

    Width 0.22mm

    Distal Strut Length 0.30mm

    Inner Strut Length 0.22mm

    Metal Strut Width 0.14mm

    Figure 14 - The isometric view of the resulting Solidworks of the PS 154 stent

    As the elastic strain of the stent is very small in comparison to the plastic strain it can

    be assumed that the stent material exhibits isotropic elasticity [47]. The plastic properties of

    the PS 154 stent were not made available by the manufacturer so the guidelines set out in

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    28

    Migliavacca et al [41] were adopted and are detailed below. The plastic behaviour of the

    model assumed a piece-wise linear isotropic hardening between the yield and ultimate stress

    to mimic the plastic region [46].

    3.1.2. Coronary Artery

    The initial intentions of the thesis were to model the expansion of a balloon-

    expandable stent within a realistic coronary artery. A realistic coronary artery was generated

    with the use of ScanIP. As complications in producing repeatable results due to the

    complexity and non-linearity of the realistic coronary artery became apparent it was deemed

    necessary for the thesis to consider the a simpler artery model. Subsequent simulations were

    performed with a hollow cylindrical artery model created natively within Abaqus CAE for its

    comparative simplicity in setting up simulations and producing repeatable results with

    multiple stent designs.

    3.1.2.1. Modelling artery with Simpleware software

    The realistic coronary artery model created for

    analysis was produced using the Simpleware ScanIP

    software. The input data used to create the model came in

    the form of a DICOM (Digital Imaging and

    Communications in Medicine) set that accompanied the

    Simpleware software. DICOM is a data set that a variety of

    medical imaging outputs (including Magnetic Resonance

    Imaging and X-Ray Computed Tomography) can be saved

    to for storing, transmitting, printing or for post-processing.

    ScanIP is able to import DICOM data sets and assist the

    user in creating 3D models from the data set.

    PS 154 Stent Material Properties

    Material Stainless Steel 316L

    Young's Modulus, E 196GPa

    Poisson Ratio, 0.3

    Yield Stress, Y 205MPa

    Ultimate Stress, M 515MPa

    Ultimate Strain, M 0.6

    Density, 7850 kg/m3 = 7.85 x 10

    -9 tonne / mm

    3

    Figure 15 Realistic branched coronary

    artery model created in ScanIP

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    29

    The DICOM set used to create the model was of the heart and the coronary arteries

    from a computed tomography angiography (CTA) data set. The blood that was present inside

    of the arteries could be selected by using the threshold floodfill tool. The section of the

    coronary artery to be modelled was then created in to a new mask and the excess data was

    cropped from the segment. The threshold floodfill feature is not perfect and it usually requires

    the user to make manual fixes to the mask. The use of a smoothing and Gaussian filter tool

    enables the user to refine the smoothness and continuity of the model surface.

    The branched coronary artery blood flow mask can be seen in Figure 15. The DICOM

    data set was not of a high enough quality to be able to distinguish the arterial wall from other

    tissues in the CT images so the arterial wall needed to be created by exporting the blood mask

    to ScanCAD and then use an offset tool to create the arterial wall at a fixed distance from the

    blood mask. The thickness assigned to the ScanIP mask was set at 0.7mm which equates to a

    typical coronary artery thickness [48]. One issue with this method of creating the arterial wall

    is that the thickness of the arterial wall varies greatly at all points along the length of arteries

    and it has been found to be directly related to the luminal area of the artery. As can be seen in

    Figure 15 the thickness of the branched arteries varies greatly and strictly would have varying

    corresponding arterial wall thicknesses. An assumption of a fixed arterial wall thickness can

    be justified for this model as the artery segment that will be eventually analysed will be

    cropped from the remainder of the branched artery model. The final model mask for the

    arterial wall branched network can be seen below in Figure 16.

    Figure 16 - Screenshots of the branched coronary artery model created in ScanIP with hollowed body to define

    the arterial wall

    A benefit of using a realistic coronary artery model is to evaluate the vessel

    straightening that occurs during the deployment of a stent as no realistic artery is ever

  • ModellingandDesignOptimisationofArterialStents CarlMcEncroe

    30

    perfectly straight. Analysis of the flexibility of the stent plays a major role when simulating