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

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Table of Contents
Statement of Contribution………………………………………………………………………………… II
Abstract……………………………………………………………………………………………………………. III
Acknowledgements…………………………………………………………………………………………... IV
Table of Figures………………………………………………………………………………………………… VII
Abbreviations and Acronyms……………………………………………………………………………. X
1. Introduction ................................................................................................................................. 1
2. Literature Review ....................................................................................................................... 3
2.1. Indications for Revascularisation ...................................................................................................... 3
2.1.1. Coronary Artery Disease .................................................................................................................................. 3
2.1.2. Peripheral Artery Disease ................................................................................................................................ 4
2.1.3. Renovascular Hypertension ............................................................................................................................ 4
2.1.4. Carotid Artery Disease ...................................................................................................................................... 4
2.2. Balloon Angioplasty ................................................................................................................................ 5
2.3. Stents ............................................................................................................................................................ 6
2.3.1. Surgical Procedure .............................................................................................................................................. 7
2.4. Arterial Anatomy ..................................................................................................................................... 9
2.5. Existing Stent Designs ......................................................................................................................... 10
2.5.1. Slotted‐Tube ....................................................................................................................................................... 11
2.5.2. Coil .......................................................................................................................................................................... 12
2.6. Acumen for an Ideal Stent .................................................................................................................. 15
2.6.1. Difficulty of Delivery ....................................................................................................................................... 15
2.6.2. Scaffolding ........................................................................................................................................................... 15
2.6.3. Dog‐Boning .......................................................................................................................................................... 16
2.6.4. Foreshortening .................................................................................................................................................. 18
2.6.5. Acute Stent Thrombosis ................................................................................................................................ 18
2.6.6. Restenosis ............................................................................................................................................................ 18
2.7. Clinical Indications for Stent Choice: ............................................................................................. 19
2.7.1. Coronary Arteries ............................................................................................................................................. 19
2.7.2. Treating Carotid Arteries .............................................................................................................................. 22
2.7.3. Treating Peripheral Artery Disease (PAD) ............................................................................................ 23
2.8. The Finite Element Method ............................................................................................................... 23
2.9. Finite Element Analysis Software .................................................................................................... 24
2.10. Design Optimisation .......................................................................................................................... 24
2.11. Summary ................................................................................................................................................ 25

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3. Methods ........................................................................................................................................ 26
3.1. Material Properties and Characteristics ....................................................................................... 26
3.1.1. Stent ....................................................................................................................................................................... 26
3.1.2. Coronary Artery ................................................................................................................................................ 28
3.1.3. Balloon .................................................................................................................................................................. 32
3.2. Simulation of Stent Deployment ...................................................................................................... 33
3.2.1. Boundary and Contact Conditions ............................................................................................................ 34
3.2.2. Loading ................................................................................................................................................................. 35
4. Preliminary Results ................................................................................................................. 41
4.1. Expansion within Realistic Coronary Artery ............................................................................... 41
4.2. Stent Free Expansion ........................................................................................................................... 46
5. Results and Discussion ........................................................................................................... 48
5.1. Original Palmaz Schatz PS 154 ......................................................................................................... 48
5.1.1. Stress Distribution ........................................................................................................................................... 49
5.1.2. Luminal Gain ....................................................................................................................................................... 50
5.1.3. Vessel Straightening ........................................................................................................................................ 51
5.1.4. Elastic Recoil ....................................................................................................................................................... 51
5.1.5. Stent Foreshortening ...................................................................................................................................... 52
5.1.6. Dog‐boning .......................................................................................................................................................... 52
5.1.7. Ratio of Kinetic and Internal Energy ........................................................................................................ 53
5.2. Modified Geometry – PS 154 ............................................................................................................. 54
5.2.1. Eight Circumferential Slots .......................................................................................................................... 54
5.2.2. Sixteen Circumferential Slots ...................................................................................................................... 60
5.2.3. Curved End PS 154 .......................................................................................................................................... 66
5.2.4. Altering the width of PS 154 distal strut ................................................................................................ 70
5.3. Optimised Stent Design ....................................................................................................................... 71
6. Conclusion and Recommendations..................................................................................... 73
7. References ................................................................................................................................... 75
8. Appendix ...................................................................................................................................... 79
8.1. Consistent unit requirement in Abaqus ........................................................................................ 79

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Table of Figures
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] ............................. 4
Figure 2 ‐ Carotid artery disease and stenting of the carotid arteries [12] .................................... 5
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]. .................................................................................................................... 6
Figure 5 ‐ Coronary angiography before and after angioplasty [18] ................................................. 8
Figure 6 ‐ Layers of the arterial wall ................................................................................................................ 9
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] ............................................................................. 11
Figure 8 ‐ Strut progression toward circumferential orientation in two tubular stent
designs [22] .................................................................................................................................................... 12
Figure 9 ‐ Typical structure of coil stents [22]. ........................................................................................ 12
Figure 10 ‐ pattern of transient non‐uniform balloon‐stent expansion at different stages
during expansion process [24] .............................................................................................................. 17
Figure 11 ‐ Lefevre classification of plaque burden at bifurcation [31] ........................................ 20
Figure 13 ‐ Photograph of the Palmaz Schatz PS 154 balloon‐expandable stent in its
constricted pre‐deployment phase [46]. ........................................................................................... 26
Figure 14 ‐ The isometric view of the resulting Solidworks of the PS 154 stent ....................... 27
Figure 16 ‐ Screenshots of the branched coronary artery model created in ScanIP with
hollowed body to define the arterial wall ......................................................................................... 29
Figure 18 ‐ Screenshots taken from Simpleware ScanIP of the segmentation procedure to
procure a straight coronary artery section ...................................................................................... 30
Figure 19 ‐ Hollow cylindrical model of the artery modelled in Abaqus ...................................... 31
Figure 20 ‐ Sketch in Abaqus of the three‐fold balloon to be extruded including parameters
............................................................................................................................................................................. 32
Figure 21 ‐ Compliance data from the manufacturers of duralyn with the line of best fit
representing the linear relationship of the material [52] .......................................................... 33
Figure 22 ‐ The resulting thin shell membrane model of the tri‐fold balloon modelled in
Abaqus .............................................................................................................................................................. 33
Figure 23 ‐ Screenshot of the setup within Abaqus of the realistic coronary artery model
illustrating the application of a uniform pressure load to the already meshed inner
surface of the stent. The green section represents the artery, the white part is the PS
154 stent, and the pink arrows represent the selected faces of the finite elements on
the inner surface of the stent that are to be assigned a pressure loading. ......................... 38
Figure 24 ‐ Screenshot of a preliminary simulation of the PS 154 stent by the method of
applying a uniform pressure to the inner surface of the stent. Excessive dog‐boning
caused the distal ends of the stent to perforate the arterial wall, an example of deep
nodal penetration. The perforations have been circled in red. ............................................... 39

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Figure 25 ‐ Pressure Ramping Profile. The profile represents the amplitude of the
application of loading and unloading for all simulations. The dotted red line depicts the
transition point between the loading and unloading steps....................................................... 40
Figure 26 ‐ Segment cut view of initial setup for realistic coronary artery simulation. ......... 41
Figure 27 – YZ‐plane cut view of initial setup for realistic coronary artery model ................. 41
Figure 28 – YZ‐plane cut view of final phase of the simulation ........................................................ 42
Figure 29 ‐ Ratio of Kinetic Energy to Internal Energy throughout the simulation for the
realistic coronary artery model ............................................................................................................. 42
Figure 30 ‐ Residual stress distribution contour on the arterial wall (without and with the
stent). ................................................................................................................................................................ 43
Figure 31 ‐ Radial Displacement of three nodes on the outer surface of the stent (the two
distal ends and in the centre) and their corresponding values of dog‐boning
percentage. ..................................................................................................................................................... 43
Figure 32 ‐ Superimposed image of the initial pre‐deployment phase of the stent (dark
green) and the final post‐deployment inflated geometry (light green) ............................... 44
Figure 33 ‐ Foreshortening of the stent vs the applied pressure loading for the realistic
artery model. ................................................................................................................................................. 45
Figure 34 – Deformed end‐phase of free expansion of stent in Abaqus by applying a
uniform pressure load to the inner surface of the stent. Note the over‐expansion of the
distal ends of the stent beyond the maximum inflated body section of the stent
(~3.5mm). ....................................................................................................................................................... 46
Figure 35 ‐ The original PS 154 stent with the inclusion of wire connector elements. The
connector elements must be restrained from expanding larger than the maximum
inflated size of the balloon. ...................................................................................................................... 47
Figure 36 ‐ Initial setup of model including the artery (green), stent (red) and balloon
(white) in Abaqus. ....................................................................................................................................... 48
Figure 37 ‐ Side transparent view of the expansion stages. The tip image depicts the initial
setup of the model, and the bottom image depicts the final expanded phase. ................. 49
Figure 38 – Contour plot of the stress distribution in the arterial wall at first impact of the
distal ends of the stent with the arterial wall. ................................................................................. 49
Figure 39 ‐ Contour plot of the residual stress distribution in the arterial wall after the
balloon has been deflated and removed from the artery. .......................................................... 50
Figure 40 ‐ Superposition of the initial and final geometry of the stent from the side and
front views. ..................................................................................................................................................... 50
Figure 41 ‐ Graph of the radial displacement of the arterial wall and the corresponding
luminal gain in area of the vessel compared to the expansion pressure loading. ........... 51
Figure 42 ‐ Graph of the percentage foreshortening of the stent vs the pressure loading. .. 52
Figure 43 ‐ Graph of the radial displacement of nodes at the centre and at the distal end of
the stent as well as the corresponding % dog‐boning. ................................................................ 52
Figure 44 ‐ Ratio of Kinetic Energy to Internal Energy throughout the simulation of the
expansion of the original PS 154 stent ............................................................................................... 53
Figure 46 ‐ Side transparent view of the expansion process. The top image depicts the point
of first impact between the stent and the artery, and the bottom image depicts the final
stage of the simulation. ............................................................................................................................. 55
Figure 47 – Contour plot of the stress distribution of the artery when distal ends of stent
make first contact ........................................................................................................................................ 56
Figure 48 ‐ Superposition of original pre‐deployment phase of stent (dark green) and the
geometry of the stent when fist making contact with the arterial wall (light blue). ...... 56
Figure 49 ‐ Residual stress distribution of the arterial wall once the balloon has been
deflated. ........................................................................................................................................................... 57
Figure 50 ‐ Symbol plot depicting the magnitude and direction of residual principle stresses
in the arterial wall at the end of the simulation. ............................................................................ 57

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Figure 51 ‐ Graph of the Luminal Gain achieved vs. the applied pressure loading. ................. 58
Figure 52 ‐ Graph of the percentage foreshortening of the stent vs. the pressure loading. . 58
Figure 53 ‐ Graph of the radial displacement of nodes at the centre and at the distal end of
the stent as well as the corresponding % dog‐boning. ................................................................ 59
Figure 54 ‐ Graph of the ratio of kinetic energy to internal energy throughout the expansion
simulation. ...................................................................................................................................................... 60
Figure 55 ‐ Isometric view of the Solidworks model of the PS 154 stent modified to include
16 circumferential slot sites. .................................................................................................................. 60
Figure 56 ‐ Side transparent view of the expansion process. The top image depicts the
initial setup of the stent, balloon and artery, the second image depicts point of first
impact between the stent and the artery, and the bottom image depicts the final stage
of the simulation. ......................................................................................................................................... 61
Figure 57 ‐ Contour plot of the stress distribution of the artery when distal ends of stent
make first contact. ....................................................................................................................................... 62
Figure 58 ‐ Superposition of the initial and final geometry of the stent from the side view.62
Figure 59 ‐ Residual stress distribution of the arterial wall once the balloon has been
deflated. ........................................................................................................................................................... 63
Figure 61 ‐ Graph of the Luminal Gain achieved vs. the applied pressure loading. ................. 64
Figure 62 ‐ Graph of the ratio of kinetic energy to internal energy throughout the expansion
simulation. ...................................................................................................................................................... 64
Figure 63 ‐ Graph of the radial displacement of nodes at the centre and at the distal end of
the stent as well as the corresponding % dog‐boning. ................................................................ 65
Figure 65 ‐ Side transparent view of the expansion process. The top image depicts the
initial setup of the stent, balloon and artery, the second image depicts point of first
impact between the stent and the artery, and the bottom image depicts the final stage
of the simulation. ......................................................................................................................................... 66
Figure 66 ‐ Contour plot of the stress distribution of the artery when distal ends of stent
make first contact. ....................................................................................................................................... 67
Figure 67 ‐ Residual stress distribution of the arterial wall once the balloon has been
deflated. ........................................................................................................................................................... 67
Figure 68 ‐ Symbol plot depicting the magnitude and direction of residual principle stresses
in the arterial wall at the end of the simulation. ............................................................................ 68
Figure 69 ‐ Graph of the Luminal Gain achieved vs. the applied pressure loading. ................. 68
Figure 70 ‐ Graph of the percentage foreshortening of the stent vs. the pressure loading. . 69
Figure 71 ‐ Graph of the radial displacement of nodes at the centre and at the distal end of
the stent as well as the corresponding % dog‐boning. ................................................................ 69
Figure 72 ‐ Screenshots of the three modified stents in terms of width of the distal strut.
From left to right, the distal strut width is 0.25mm, 0.30mm (original stent parameter),
0.35mm and 0.40mm. ................................................................................................................................ 70

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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

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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 wasn’t 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

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(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.

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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 heart’s 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 <30% stenosis of the
coronary arteries [7].

11. 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 patient’s 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,

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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].

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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

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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].

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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 body’s 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]

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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

17. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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

18. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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 ring’s
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].

19. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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].

20. 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

21. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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

22. 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 doesn’t 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

23. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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 vessel’s 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

24. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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.

25. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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2.6.4. Foreshortening
Foreshortening is the deformation of the stent in the longitudinal direction when the
stent is expanded [24].
!"#$%ℎ!"#$%&%' =
! − !!"#$%&
!
(L = the pre-expansion length of the stent, !!"#$%& = 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 stent’s
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.

26. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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

27. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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]

28. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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

29. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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 patient’s 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

30. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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

31. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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

32. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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 patient’s 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.

33. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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].

34. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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

35. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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, ν 0.3
Yield Stress, σY 205MPa
Ultimate Stress, σM 515MPa
Ultimate Strain, εM 0.6
Density, ρ 7850 kg/m3
= 7.85 x 10-9
tonne / mm3
Figure 15 – Realistic branched coronary
artery model created in ScanIP

36. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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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

37. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
30
perfectly straight. Analysis of the flexibility of the stent plays a major role when simulating
deployment in an artery that exhibits non-linearity.
Once the arterial wall thickness was defined within ScanCAD it was also possible to
position the stent within the artery in preparation
for expansion in FEM software. ScanCAD allows
for .sat files to be easily imported and effortlessly
positioned within ScanIP generated models. Figure
17 shows the positioning of the stent within the
branched artery model within ScanCAD.
Once the stent is positioned within the
artery the whole model can then be exported to
Abaqus for finite element analysis and simulation
of the expansion of the stent. As the entire
branched coronary artery network model is not
necessary for the finite element method expansion
simulation it is wise to cut down the size of the
model to a small segment of one of the coronary
arteries. The Saint-Venant’s Principle states that
the impact of local displacements and stresses
diminish with distance from the local area [49] –
thus it is not necessary to run a finite element
method simulation of the entire branched model when it is only the site of the stent
deployment that is of concern for finite element analysis. The chosen segment of the coronary
artery can be seen in Figure 18.
Figure 18 - Screenshots taken from Simpleware ScanIP of the segmentation procedure to procure a straight
coronary artery section
Figure 17 - Screenshots of the placement of the
PS 154 stent model within the branched coronary
artery model within Simpleware ScanCAD

38. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
31
A downside to using ScanIP to produce an artery model is that it is a requirement that
ScanIP meshes all models through its +FE module before exporting to FEM software. FEM
software such as Abaqus that are dedicated finite element software with very powerful built-in
meshing engines ideally would be used to mesh the entire model, however Simpleware does
not enable this. The meshing capability of the Simpleware +FE module is quite
computationally expensive. There are further issues that the +FE module causes regarding
selecting the inner surface of the stent that will be discussed later.
3.1.2.2. Modelling artery with Abaqus
Several papers have run stent expansion simulations modelling the artery as three
separate concentric layers representing the tunica intima, tunica media, and tunica adventitia
all with different corresponding mechanical properties [36]. It is the decision of this thesis to
model the artery as one hollow cylindrical layer, a procedure adopted by Pant et al [50]. The
single layer artery model is assumed to have the material characteristics of the tunica media
layer of the artery as this is the largest layer by volume and is the location of the smooth
muscle cells and elastic tissue. It is these tissue types that give the artery its hyperelastic
characteristics and their mechanical responses are the subject of stent expansion research. For
these reasons it is justifiable to model the artery as one hollow cylindrical layer with the
material properties of the tunica media layer.
The coronary artery was modelled as a hollow cylindrical tube of length 10.00mm,
outer diameter of 4.40mm and an internal diameter of 3.00mm. These artery parameters are
adopted from Pant et al [50] and are considered typical coronary artery proportions. The
coronary artery was made to these parameters within Abaqus CAE and the final model is
depicted in Figure 19.
Figure 19 - Hollow cylindrical model of the artery modelled in Abaqus
The method for modelling the mechanical behaviour of the arterial wall will be
isotropic hyperelastic behaviour following a reduced polynomial strain energy density

39. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
32
function of the sixth order (U) and the material density of the artery is assumed to be 1 g/cm3
[36].
! = !!" !! − 3 + !!" !! − 3 !
+ !!" !! − 3 !
+ !!" !! − 3 !
+ !!" !! − 3 !
+ !!" !! − 3 !
(where I!is the first invariant of the deviatoric Cauchy-Green tensor [50])
Experimental data from the analysis of the coronary artery to the reduced polynomial
strain energy density function model produced the following coefficients for the three layers
of the artery. This work is adopted from Gervaso et al [42].
Alternative strain energy density functions used in prior studies to model the
hyperelastic nature of the artery include the polynomial and Mooney-Rivlin [51] form.
3.1.3. Balloon
The balloon was created within Abaqus and modelled as a tri-fold balloon. The simple
tri-fold sketch was extruded to create the thin-shell membrane part. The sketch was extruded
to a length of 15mm and the membrane of the balloon was assigned a thickness of 0.02mm.
Figure 20 - Sketch in Abaqus of the three-fold balloon to be extruded including parameters
The balloon was assumed to have been made of the material, duralyn. Based on thin
shell membrane theory the balloon’s material behaviour can be derived from the compliance
Arterial Layer C10 C20 C30 C40 C50 C60
Intima 6.70E-03
0.54 -1.11 10.65 -7.27 1.63
Media 6.52E-03
4.89E-02
9.26E-03
7.60E-01
-4.30E-01
8.69E-02
Adventitia 8.27E-03
1.20E-02
0.52 -5.63 21.44 0

40. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
33
data of the true stress and strain of the material. This compliance data is provided from the
manufacturer of duralyn in Figure 21. The balloon material was derived from the line of best
fit for the compliance data for duralyn representing the elastic line corresponding to a
Young’s modulus of 920MPa and a Poisson’s ratio of 0.4. The material behaviour of the
balloon is modelled as having linear isotropic elasticity. The density of the balloon was
assumed to be 1.1 gram/cm3
. The material properties for the balloon was adopted from De
Beule et al [52].
Figure 21 - Compliance data from the manufacturers of duralyn with the line of best fit representing the linear
relationship of the material [52]
Figure 22 - The resulting thin shell membrane model of the tri-fold balloon modelled in Abaqus
3.2. Simulation of Stent Deployment
The expansion of the stents within the artery was simulated by quasi-static dynamic
analysis with the use of the commercial Abaqus code based on the finite element method. The
finite element method is a widely used and reliable engineering tool for testing of product
design and product development.

41. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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The deployment of a stent is considered a quasi-static phenomenon where inertial
forces are small [11]. A quasi-static process is a thermodynamic process that occurs infinitely
slowly and thus the way to approximate the process is to perform them infinitesimally slowly.
Abaqus approximates the quasi-static process of stent deployment by performing the
deployment at infinitesimal increments.
It should be noted that Abaqus does not use a built-in unit system and it is the user’s
responsibility to keep consistency in the use of units. As the models that have been created for
this thesis have been created in terms of millimetres it is necessary for all other unit inputs to
be scaled to be consistent with this. For instance, if entering an input for mass of the stent it
would need to be entered in tonnes (103
kg) rather than kilograms. A full conversion table is
included in the appendix of the thesis for reference.
3.2.1. Boundary and Contact Conditions
A local cylindrical coordinate system needs to be created through the medial cross-
section of the stent so that three nodes on the stent (in an equilateral triangle shape) can be
identified. These three nodes on the stent must be free to expand radially but be restricted
from rotating circumferentially or migrating longitudinally [36].
This method will ensure that the stent is ideally fixed within the artery and will ensure
that the stent does not shift out of the artery when the simulation brings the artery and stent in
contact with each other. This restriction must only be applied to nodes in the medial plane of
the stent so the effects of dog-boning and foreshortening can be observed. The distal ends of
the stent should be free from any constraints to allow for the observation of these phenomena
during the simulations. This boundary condition should be repeated for three nodes on the
artery part to keep the expansion of the artery constrained to the radial direction.
The distal ends of the artery and balloon must be pinned in all directions of the
coordinate system, as it is essential the whole artery and the balloon to be fixed at its ends.
The ends should be fixed with respect to displacement and rotation about all three axes of the
coordinated system. The fixed boundary conditions on the distal ends of the artery are
necessary to mimic the continuation of the artery longitudinally down the remainder of its
length.
Although it would have been possible to model the simulation using only one-twelfth
of the full model this thesis chose to consider the full model simulation for ease of
interpretation of the results at the sacrifice of a shorter computational processing time. This
would have been possible due to the symmetrical boundary conditions of the circular
geometry of the stent and artery assembly.

42. Modelling and Design Optimisation of Arterial Stents Carl McEncroe
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When the method of expansion for the stent was by applying a pressure to the inside
surface of the stent it was necessary to include restraints on connector elements in the model
[44]. This was necessary to ensure that the stent would not expand beyond the maximum
diameter of a balloon. If these connector element restraints were not utilised then the distal
ends of the stent will expand quicker than the body of the stent. The ends would also begin to
deform beyond the maximum diameter of a balloon as the applied pressure is uniform
throughout the step.
The Abaqus General Contact algorithm defined the contact conditions of the
simulations between the stent, balloon and the artery. The tangential contact was assumed to
be frictionless and the normal contact was set to default “hard” contact while being able to
separate after contact
3.2.2. Loading
For the stent expansion simulation in Abaqus it is necessary to first decide on the
method of expansion for Abaqus to execute. Once the method has been chosen, it is necessary
to set up time steps and to assign loads within each step period in accordance with the chosen
expansion method. Following is an in-depth overview of the steps involved in assigning loads
to expand the stent.
3.2.2.1. Method of Stent Inflation
There are three commonly used methods to simulate the expansion of the stent within
the artery and they will be discussed below.
Displacement of Stent
The simplest approach for the expansion of the stent is by applying a displacement
condition on the inner surface of the stent. The inner surface of the stent is increased until the
diameter of a fully inflated balloon is achieved. For the case of the PS 154 this is 3.5mm. This
is not an ideal method for simulating the stent deployment as a realistic expansion would
exhibit dog-boning where the distal ends of the stent expand first and at a greater rate than the
middle of the stent. This method would not be effective at simulating this expected
phenomenon.
Pressure applied to Stent
The stent expansion can be simulated by applying a uniform pressure to the inner
surface of the stent, mimicking the pressure that the balloon induces on the stent during