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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3591_Библиотеки_им_академика_М_И_Перельмана.pdf
Balloons andStents
DeepakJustineViswanathan, SrinivasMeharwade,
andS.H.Chandrashekhara
9
Key Messages
1. Balloon ination is governed by the law of Laplace and
its primary mechanism of action is controlled fracture of
the obstructing plaque.
2. Based on compliance, balloons are divided into semicompliant balloons and non-compliant balloons.
3. Majority of the balloons are composed of polyethylene
terephthalate (PET).
4. Generally, balloons are sized such that their diameter is
5–10% more than the diameter of adjacent normal
vessel.
5. Two main designs are rapid exchange and over the wire,
with preferences based on the procedure and
complexity.
6. Main complications of balloon angioplasty include dissection, vessel rupture, and balloon rupture. Flowlimiting dissection requires bail-out stenting.
7. Risk of abrupt closure, lack of durability, and high restenosis paved the way for the next level of devices called
stents.
8. They are used in both vascular and non-vascular
procedures.
9. Many types including balloon-mountable, selfexpandable, drug-eluting, and ow-diversion stents.
10. The mechanism of a stent differs from that of a balloon
in that it pushes the plaque and vessel wall aside to
enlarge the lumen, resulting in a more uniform opening
of the vessels.
9.1 Introduction
Balloons and stents are crucial tools in interventional radiology. Balloon ination is governed by the Law of Laplace [1]
and acts by controlled fracture of the obstructing plaque.
D. J. Viswanathan · S. Meharwade · S. H. Chandrashekhara (*)
Department of Radiodiagnosis and Interventional Radiology, All
India Institute of Medical Sciences, Delhi, India
Based on compliance, balloons are divided into semicompliant balloons and non-compliant balloons. Noncompliant balloons are preferred in angioplasties, whereas
semi-compliant/compliant balloons are mainly used as
occlusion balloons. Majority of the balloons are composed
of polyethylene terephthalate (PET); however, nylon balloons are preferred in highly calcied plaques due to its
superior scratch resistance [2]. Generally, balloons are sized
such that their diameter is 5–10% more than the diameter of
adjacent normal vessel [3]. Two main designs are rapid
exchange and over the wire, with preferences based on the
procedure and complexity [4]. Trackability, pushability, balloon proles, and compliance are the major factors that inuence the clinical needs of a balloon. Complications include
dissection, vessel rupture, and balloon rupture; minor dissection is common, but ow-limiting dissection may necessitate
bail-out stenting [5].
Stents are implantable devices that support a luminal passage in the body. They are used in both vascular and nonvascular procedures. The term stent was named after a dentist
named Charles T. Stent, who used gutta-percha for dental
implants, which became known as Stent’s material [6] and
gradually found its way into surgical procedures, referring to
any articially implanted structural support. In interventional radiology, stents were rst used in non-vascular procedures to keep any lumen anatomically open, especially in the
GI tract. In 1985, Julio Palmaz inserted the rst vascular
stent, a balloon-expandable stent, into a peripheral artery [7].
Following that, acceleration in technology resulted in the
introduction of balloon-mountable, self-expandable, drugeluting, and ow-diversion stents. The main complications
that paved the way for stenting in vascular diseases were
those of balloon angioplasty, which was limited by the risk
of abrupt closure (1%), a lack of durability due to early vessel recoil (5–10%), and restenosis [8]. The mechanism of a
stent differs from that of a balloon in that it pushes the plaque
and vessel wall aside to enlarge the lumen, resulting in a
more uniform opening of the vessels.
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
S. H. Chandrashekhara (ed.), Textbook of Interventional Radiology, https://doi.org/10.1007/978-981-97-9601-4_9
77
78
T=PR×
Inflation pressure
B
Balloon diameter
get lesion
D. J. Viswanathan et al.
9.2 Balloons
9.2.1 Basics
A balloon catheter, also known as an angioplasty balloon, is
a compact tube with a balloon attached at its far end. Balloon
ination is governed by the Law of Laplace, which states that
the outward force or tension (T) exerted by the balloon’s wall
on the nearby vessel wall is directly proportional to the pressure (P) within the balloon and its radius (R).
Thus, the dilating force of the balloon can be increased by
inating the balloon to a higher pressure or by choosing a
balloon of larger diameter. Moreover, larger balloons need a
lower ination pressure compared to smaller balloons to
generate the required dilating force and consequently have a
lower threshold pressure for rupture [9].
9.2.2 Mechanism ofAction ofBalloons
The primary mechanism of action is controlled fracture of
the obstructing plaque [3]. The secondary mechanism of
action is stretching of tunica media; this mechanism is predominant in venous strictures as they are mainly brotic in
nature [10]. Although plaque particles may microembolize
distally post-balloon ination, it is typically clinically insignicant [11]. Additionally, minor dissection within the lesion
is commonly observed in angiograms after balloon angioplasty, usually requiring no active intervention [12].
applied force [13]. Based on compliance, balloons can be
broadly divided into semi-compliant balloons and noncompliant balloons (Fig.9.1).
9.2.4 Semi-Compliant Balloons (Fig.9.2)
Semi-compliant balloons have a propensity to expand less in
the area of stenosis and expand maximally where there is
least resistance. When inated in resistant lesions, it causes
“dog boning” and increased vessel wall distortion on each
side of the lesion. This raises the likelihood of dissection [13,
14]. Their primary use is to temporarily occlude blood ow
(e.g., balloon retrograde transvenous obliteration), to sweep
away thrombus in balloon thrombectomy [3] and in emergency stabilization of hemorrhagic shock with bleeding
below the level of diaphragm by temporary balloon occlusion of abdominal aorta (Resuscitative Endovascular Balloon
Occlusion of the Aorta (REBOA)) [15]
A- Non-compliant balloon
B- Semi-compliant balloon
Nominal pressure
A
Nominal diameter
9.2.3 Compliance ofaBalloon
Compliance is a key characteristic of a balloon that affects its
capacity to expand the lesion. It is a measurement of the
degree of expansion of the balloon after reaching its predetermined size (nominal diameter or length) in response to an
Fig. 9.2 “Dog-boning” of a semi-compliant balloon when it is inated in a resistant stenosis. The balloon expands less at the stenotic site and
expands maximally adjacent to it leading to the conguration of a “dog-bone”
Fig. 9.1 Graph depicting the response of non-compliant and compliant
balloons during ination. With continued increase in ination pressure,
non-compliant balloon (A) does not increase in size beyond its nominal
diameter, whereas semi-compliant balloon (B) continue to increase in
size beyond its nominal diameter with application of pressure
Dog boning
Ta r
Non compliant balloon
Ta rget lesion
Working rangeNominal pressureRated burst pressure
max
0
9 Balloons andStents
Fig. 9.3 Opening up of the stenotic site by non-compliant balloon without deformation. Non-compliant balloons expand maximally at the stenotic
site without formation of a “dog-bone” and, hence, are preferred in angioplasty
79
9.2.5 Non-compliant Balloons (Fig.9.3)
Non-compliant balloons expand minimally once their predetermined nominal diameter is reached. Increasing the pressure beyond the nominal level does not result in a noticeable
change in size, but tension increases. Consequently, noncompliant balloons deliver a greater dilating force in a lesion
and cause less deformation of the vessel wall on either side
[14, 16]. These balloons are used in balloon angioplasties
and are preferred to dilate resistant or calcied lesions [17]
(Table9.1).
9.2.6 Composition ofBalloons
The composition of a balloon is the main factor inuencing
compliance, burst pressure, and scratch/puncture resistance
and thus determines the performance and suitability of a
balloon [18]. Polyvinyl chloride (PVC) has largely been
abandoned as a material for balloons due to its tendency to
burst at low ination pressures and its inconsistent diameter when inated [19]. The majority of balloons today are
composed of polyethylene terephthalate (PET) or nylon
[2].
PET is a polyester derivative that can withstand extremely
high ination pressures despite having relatively thin walls.
However, PET balloons have low scratch resistance and
therefore should not be used in highly calcied lesions [19].
Nylon balloons provide better scratch resistance than PET
balloons but at the cost of compliance [18].
Table 9.1 Differences between semi-compliant and non-compliant
balloons
Semi-compliant balloons Non-compliant balloons
Expand in the area of least
resistance adjacent to the lesion
Dog boning is seen Dog boning is absent
More deformation of the adjacent
vessel well, increased risk of
dissection
Table 9.2 Various balloon ratings and their denitions
Pressure rating Denition
Nominal
pressure
Rated burst
pressure (RBP)
Working range Ination range between nominal and rated burst
Fig. 9.4 Balloon ratings (working range). Working range represents
the operating range between the nominal pressure and the rated burst
pressure. Ination beyond the rated burst pressure carries a high risk of
balloon rupture
Pressure required to inate the balloon to its
predetermined diameter
The maximum pressure that a balloon is capable
of withstanding without rupturing
pressure (Fig.9.4)
Pressure scale (atm)
Provide a greater amount of
dilating force at the target
lesion
Less deformation of the
adjacent vessel wall
9.2.8 Principles ofBalloon Ination
andDeation
9.2.7 Balloon Ratings
During ination, the balloon reaches its nominal size from
its wrapped state. It should be slow and steady and can be
Every balloon catheter has three ratings as depicted in
Table9.2.
RBP denotes 95% condence that 99.9% of balloons will
not fail at or below rated burst pressure. There is an increased
risk of balloon rupture if inated above RBP [20]. The typical working range for semi-compliant balloons is 8–14atm,
while for non-compliant balloons, it is 10–20atm.
achieved by using a Luer-Lock syringe or an ination device.
Usually a 10ml Leur-Lock syringe is commonly used and
can deliver up to 10atm of pressure; smaller syringes provide more pressure [1]. On the other hand, the ination
device contains a pressure gauge to track the current pressure
and can deliver up to 30atm of pressure. A dilute contrast
mixture (1:2 or 1:3) of nonionic contrast and ush solution is
80
Entr
Crossing profile
D. J. Viswanathan et al.
used to visualize the balloon-lesion interaction [3]. While
slight pressure or discomfort is common, excessive pain during ination should be approached cautiously, as it may indicate dissection or vessel rupture.
Deation represents the transition from the wrapped state
to the normal conguration. It should always be conrmed
by the absence of contrast uoroscopically. For thorough
aspiration of the diluted contrast mixture, a 20ml syringe is
preferred over a 10ml syringe and it should be swift [1]
9.2.9 Choosing anAppropriate-Sized Balloon
foraProcedure
The appropriate balloon size for angioplasty depends on the
vessel harboring the target lesion. A common practice is to
oversize the balloon diameter by 5–10% compared to the
vessel lumen. The desired nal diameter is often determined
by referencing an adjacent normal segment of the vessel or,
when applicable, the same vessel on the contralateral side.
Alternatively, the known average size of the vessel is considered (“rule of thumb” technique) [3]. The appropriate size of
the balloon to be used according to the diseased vessel is
described in Table9.3. Dissection is more likely when a bal-
loon is too big. Conversely, undersized balloons pose the risk
of therapeutic failure [21].
If the balloon is not appropriately sized and positioned, it
has a tendency to slip within the target lesion during ination, and this is called the “watermelon seeding phenomenon.” It is commonly seen in cases of in-stent restenosis after
percutaneous coronary interventions and is associated with
poor short-term and long-term angiographic results [22].
2. Crossability: Must cross the stenotic site.
3. Dilatation: Must safely dilate the stenotic site.
The deliverability of a balloon depends on its pushability
and trackability. Crossability is primarily inuenced by the
balloon’s proles, while dilatation is mainly determined by
the compliance and composition of the balloon.
Pushability The capability of a catheter to convey the force
of a forward push from its proximal end to its distal end is
known as pushability [20, 23]. Balloons with good kink
resistance and over the wire design have better pushability
[23].
Trackability Trackability refers to the ease with which the
balloon catheter glides from point A to point B along the
guidewire in the vascular anatomy [20, 23]. Balloon catheter
designs that are exible and have a narrow prole have good
trackability. To increase trackability, they have a hydrophilic
coating of silicone that decreases friction/resistance [18].
Proles The maximum diameter of the balloon when it is
deated and wrapped around the catheter constitutes its prole. At its distal end, the balloon has two named proles:
lesion entry prole and crossing prole (Fig.9.5)
1. Lesion entry prole: First point at which the balloon cath-
eter encounters the lesion.
2. Crossing prole: This refers to a point on the distal end of
the catheter where the balloon is attached to the catheter.
It represents one of the largest diameters on the distal segment of the balloon catheter.
9.2.10 Clinical Needs ofaBalloon
A balloon has to satisfy the following clinical needs:
1. Deliverability: Must reach the stenotic site.
Table 9.3 Approximate diameter and length of the balloons to be used
in the respective vessels [3]
Vessel
Internal carotid artery
(cervical)
Subclavian artery 6–7 2–4
Subclavian vein 8–16 4–6
Abdominal aorta 10–16 2–4
Renal artery 5–6 2–4
External iliac artery 6–7 2–4
Iliac veins 8–16 4–6
SFA 5–6 2–20
Popliteal artery 4–5 2–6
Balloon diameter
(mm)
5–6 2–4
Balloon length
(cm)
9.2.11 Balloon Catheter Design
Catheter design is one of the important factors that determines the pushability of the assembly. It is of two types:
rapid exchange design and over-the-wire design.
9.2.11.1 Rapid Exchange Design (Fig.9.6)
The rapid exchange design is gaining popularity, driven by
the availability of a diverse range of guide catheters and
DISTAL END OF BALLOON CATHETER
Ballon (wrapped state)
y profile
Fig. 9.5 Balloon proles. The crossing prole is one of the largest
diameters of a balloon catheter
Guide wir
Guide wire
Guide wir
Guide wire
9 Balloons andStents
81
Proximal endDistal end
Balloon
Radio-opaque markers
e
Guide wire lumen
Balloon lumen
Fig. 9.6 Rapid exchange balloon catheter design. Note that the guidewire exits from the side of the catheter
guide wire port (skive)
Balloon lumen
Distal end
Guide wire port
Balloon
Balloon inflation port
Radio-opaque markers
e
Guide wire lumen
Balloon lumen
Guide wire lumen
Balloon lumen
Balloon inflation port
Proximal end
Fig. 9.7 Over-the-wire balloon catheter design. Guidewire lumen is present throughout the entire extent of the catheter. It offers better
pushability
sheaths. In this system, the proximal end accommodates the
balloon lumen, while the guidewire port is situated on the
side of the catheter, roughly 40cm proximal to the balloon,
referred to as the skive [19]. It offers quick catheter exchanges
and is predominantly a single operator technique.
9.2.11.2 Over-the-Wire Design (Fig.9.7)
In this type, the guidewire tracks along the entire length of
the catheter. The continuous guidewire support over the
entire catheter provides greater pushability [23]. The proximal end has two separate ports: one for the balloon lumen
and the other for the guidewire lumen.
Advantages
– Single operator dependent [24].
– No need for exchange length guidewires [19].
– Quick exchanges.
– Better operator control of the wire.
Advantages
1. Greater pushability.
2. It is possible to exchange the wire through the lumen in
difcult cases.
– Small prole [4].
Disadvantages
Disadvantages
– Reduced pushability: difcult to push through tortuous
vessels or tight stenosis.
1. Not single operator-dependent.
2. Usually requires exchange length guidewires [23].
3. Larger prole [4].
– Absence of a port to inject contrast [19].
Specic Uses
– Ideal for simple cases where no extra guidewire support is
required, e.g., coronary, carotid, and renal interventions [4].
Specic Uses
Treatment of complex lesions which requires an easy
exchange of wires, extra catheter pushability, and added
wire support.
82
D. J. Viswanathan et al.
9.2.12 List ofApplications ofaBalloon
Catheter
• Angioplasty.
• Stent delivery.
• Drug delivery.
• Occlusion balloon catheters.
9.2.13 Complications
• Dissection: The ination of a balloon exposes the adja-
cent normal vessel wall to torsional and longitudinal
stress, elevating the risk of dissection. Following balloon
angioplasty, minor dissections are frequently observed
and typically do not necessitate active treatment [12].
However, obstructive dissections may require bailout
stenting.
• Vascular rupture: It presents as persistent severe pain,
accompanied by tachycardia and hypotension after balloon deation, and is typically addressed by maintaining
guidewire access across the lesion followed by re- inating
the balloon either across or proximal to the lesion.
Additional options for management include reversing
anticoagulation, placing a stent graft, or opting for open
surgical repair. [3].
• Balloon rupture: Usually seen when the balloon is
inated more than the rated burst pressure. Although there
are different patterns of balloon rupture, longitudinal rupture along the length of the balloon is desirable as it has
minimal risk of distal embolization [19].
9.2.14 Occlusion Balloons
These balloons, categorized as compliant/semi-compliant,
are crafted to stretch between 100% and 800% of their original size [25]. Typically made from polyurethane, silicone, or
latex, their primary applications include the balloon retrograde transvenous obliteration (BRTO) procedure to occlude
the gastrorenal/gastrocaval shunt and in stabilizing acute
hemorrhagic shock with the source below the diaphragm
through temporary occlusion of the abdominal aorta
(Resuscitative Endovascular Balloon Occlusion of Aorta
(REBOA) [15].
Balloon Positioning
• Center the balloon over the lesion.
• Radio-opaque markers assist in conrming its position.
Contrast Usage
• Utilize dilute contrast (1:2–3 contrast to ush solution)
for enhanced visualization.
Stabilization During Ination
• Stabilize the balloon at the sheath to minimize movement
during ination.
Ination Technique
• Slow and steady ination using a 10ml syringe or insufation device.
• Ination times: ~30–45 seconds for arterial lesions;
1–2minutes for venous lesions.
Deation Process
• Rapidly deate the balloon using a 20ml syringe or insufation device.
Withdrawal Technique
• Apply continuous negative pressure and a counterclockwise motion during withdrawal.
• Maintain guidewire access until the procedure is considered complete.
9.2.16 Advances inBalloon Technology
1. Scoring balloon: It has nitinol-based helical scoring ele-
ments that is wrapped over a non-compliant balloon. It is
useful in dilating resistant calcied plaques [26].
2. Cutting balloon: It has 3–4 microsurgical blades that are
bonded longitudinally over a non-compliant balloon. It is
particularly useful in balloon dilatation of in-stent restenosis and calcied plaques [27].
3. Drug-coated balloon: It has a coating of antiproliferative
drugs such as paclitaxel and sirolimus that prevents neointimal hyperplasia and reduces the risk of in-stent restenosis [28]
9.3 Stents
9.2.15 Angioplasty–Pearls
Balloon Sizing
• Inuenced by vascular bed and site; initial preference for
an undersized balloon.
• Size adjustments can be made if the effect is suboptimal.
9.3.1 Basic Design (Fig.9.8)
The basic unit of any stent is a cell. Hoops and connectors
form stents. Hoops can be in phase or out of phase with
each other, and connectors can be straight, curved, or weldlinked. Hoops provide the most important attribute of the
stent, i.e., radial strength, which is “the amount of resis-
9 Balloons andStents
Fig. 9.8 Basic design of a
stent
tance the stent can impart against an external compressive
force that is trying to collapse the stent.” Another important
design parameter that determines performance is the connectors, which hold hoops together. Reducing the number
of connectors increases exibility and conformability while
decreasing longitudinal strength and reducing fracture
potential.
83
9.3.2 Types andTheir Engineering
Balloon-expandable (BES) and self-expanding stents (SES)
form the major types of stents. The material used and the
manufacturing technique known as stent engineering determine the design of either of these two stents, which has a
signicant impact on the device’s clinical performance. The
stent’s performance and clinical course after deployment are
ultimately determined by six major pillars of stent engineering (Fig.9.9). Before we discuss stent engineering, it is critical to understand the radial/hoop strength. It is the amount of
resistance that the stent can offer to an external compressive
force attempting to collapse the stent. It enables the stent to
withstand the radially compressive forces of stenotic vessels
following dilation.
9.3.3 Stent Materials andCharacteristics
Regardless of the type, to avoid an overreactive host immune
reaction, the stent material must be extremely biocompatible
and corrosion-resistant. They should be sufciently radiopaque and produce fewer MRI artefacts. Balloonexpandable stents are made of materials that can undergo
plastic deformation upon balloon ination. Except for a
Fig. 9.9 Six pillars of stent engineering
slight recoil caused by the elastic component of the deformation, the stent keeps its expanded shape after the balloon
deates [18]. In BES, recoil is prevented by an innite elastic
modulus, while stent crimping on the delivery system is
made easier by a low yield strength, which permits stent
expansion by balloon pressures. High tensile qualities help
provide radial strength during expansion using a small
amount of foreign material inserted.
Balloon-expandable stents have a smaller diameter and
are made to be delivered over a balloon, which is then
balloon- dilated to the expanded shape at the target site inside
the vessel. In contrast, self-expanding stents are intended to
expand before being compressed and conned within a
delivery system. They self-expand after they are taken out of
the delivery system. Therefore, the function of the material is
determined by its elastic properties. For substantial elastic
strains, the material should have a high yield stress and a low
elastic modulus [29–31].
84
D. J. Viswanathan et al.
Initial stents were made of 316L stainless steel, a metal
alloy that contains iron, nickel, chromium, and molybdenum.
It was used to make BES because of its high elastic modulus
and tensile strength. It was also corrosion-resistant and strong
enough to scaffold the vessel and prevent recoil. However,
because it contained iron (Fe), it was less radio- opaque and
MR-incompatible, so the thickness of the strut was increased
to maintain radio-opacity and radial strength, thereby reducing deliverability and increasing restenosis [30].
High elastic strains are necessary for materials used in selfexpanding stents. The most often employed material in SES
manufacturing is nitinol, a nickel-titanium alloy that can recover
elastic deformations of up to 10%. Superelasticity is the term
used to describe this unusually wide elastic range. SES also has
“shape memory,” which means that after deployment, the stent
expands to its pre-set diameter at body temperature without the
use of a balloon and, more importantly, returns to this shape
after being deformed during exion, extension, or external compression. Furthermore, due to the addition of specic markers, it
has less foreshortening, is more biocompatible, is less expensive
than conventional stents, and is radiopaque [31].
Plastically deformable stainless-steel stents could become
crushed and impair blood ow in the carotid arteries of the
neck and the supercial femoral arteries of the legs. Because
of these indications, self-expanding, crush-recoverable stents
were required, as well as a different material. It is also critical to remember that all stents are MR conditional, which
means they are deemed safe only in a specied MRI environment with a specic device and MRI scanner conditions.
9.3.4 Raw Material Form
9.3.5 Fabrication
The raw material form used primarily determines the method
of fabrication chosen. Standard wire-forming techniques,
such as coiling, braiding, or knitting, can be used to form
wires into stents. A wire stent’s most basic shape is a coil.
Most stents treating coronary artery disease, as well as the
majority used in peripheral vascular disease, are manufactured by laser cutting tubing.
The use of exible nitinol wire in wire stents is a recent
advancement (compared to other rigid ones). One nitinol
wire is looped six times on one end of the stent before being
braided together. It supports and mimics the natural structure and movement of the vessels, which twists, bends,
shortens, and compresses, resulting in more vasculomimetic
and better clinical outcomes. The rough surface of the stents
is acid- pickled, followed by electrochemical polishing, to
remove depositions and burrs formed on the surface during
the laser cutting production process, which, if not treated,
causes thrombus adhesion and neointimal growth [33, 34].
9.3.6 Geometry
Sequential rings are the most common on the market and are
classied into two types (Fig.9.10):
– Closed cells, a stent with bridging elements linking all
internal inexion points of the structural framework.
– Open cells, a stent with some or all internal inexion
points of the structural framework not connected by
bridging elements.
Based on their raw material form, stents are classied into
three types: wire, slotted tube, and modular. Standard wireforming techniques, such as coiling, braiding, or knitting,
can be used to form wires into stents. The latter is used to
make the vast majority of self-expanding stents.
– Slotted Tube Design
Stents with slotted tube designs are made of metallic
tubes. The laser-cut design provides greater radial force but
less exibility and deliverability. These are used to make
biliary stents and stent grafts [32].
– Modular Tube Design
It is constructed from multiple repeat modules fused to
form a stent tube, providing increased exibility and side
branch access. It is used in venous stenting, where more exibility and high radial force are required to overcome compression from adjacent structures [32].
This also has an impact on drug delivery because the gap
region contains fewer drugs than the pinch region, whereas
the closed-cell design has a more uniform drug concentration (Fig.9.11).
9.3.7 Additions
The addition of specialized materials, such as radiopaque
markers for improved visibility, coatings such as drugs to
reduce complications, or the addition of a covering to act as
a conduit, known as a stent graft, is the nal step in stent
engineering.
– Radiopacity enhancements:
Stent materials, such as stainless steel or nitinol, are difcult to see uoroscopically. As a result, gold, platinum, or
tantalum (most commonly used) markers are frequently
attached to stents to improve X-ray visibility (Fig.9.12).
Gap - Less drug
distribution
9 Balloons andStents
85
Fig. 9.10 Geometry in stent
manufacturing
Fig. 9.11 Impact of
geometry in drug delivery
Open cell design Closed cell design
More flexible
Conforms to vessel wall better
Less metal : artery ratio
Increased side branch access
Increased plaque prolapse
Less radial force
Decreased side branch access
Less flexible
Less conformability
Increased metal : artery ratio
Greater plaque coverage
More radial force
Pinch - More drug Closed cells - More uniform drug
A supera stent, or vasculomimetic system, is a wire mesh
stent made by interlacing the nitinol wire into a tube form
commonly used in peripheral angioplasties.
9.3.8 Drug-Eluting Stents
In-stent restenosis (ISR) has been the “Achilles heel” of
bare-metal stent technology since its debut. It often requires
repeat revascularization using percutaneous or surgical
methods. Neointimal hyperplasia, an excessive arteryhealing reaction to vascular trauma brought on by angioplasty and stent insertion, is the root cause of ISR.The poor
“efcacy” of BMS was the primary factor driving the development of drug-eluting stents (DES) [35, 36]. As a result,
inhibiting neointimal hyperplasia with anti-proliferative
drugs resulted in a signicant reduction in restenosis rates.
The therapeutic agents in DES primarily included sirolimus (Rapamycin) derivatives, which inhibit the cell cycle’s
transition from G1 to S phase, or paclitaxel, which stabilizes
the microtubule polymer and prevents it from disassembling.
All of them resulted in cell division inhibition.
Fig. 9.12 A self-expanding stent with tantalum markers at edges—
providing enhanced visibility and accuracy during placement
86
Balloon- expandable stent
a
D. J. Viswanathan et al.
9.3.9 Mechanism ofExpansion
A BES is pre-mounted on a balloon, and the stent expands
plastically in accordance with the balloon diameter as the
balloon is inated. It has the same size as angioplasty balloons, i.e., 5–10% more than the measured normal lumen,
and is deployed from both ends toward the middle. If overdilated, a slight shortening during expansion is observed.
SES material, on the other hand, automatically expands to
a predetermined size when the outer covering sheath is
removed. Distal to proximal stent deployment is necessary
for xation since it depends on the stent’s exact apposition to
the artery wall. The unconstrained diameter ought to be
10–20% greater than the target vessel’s typical diameter.
(Fig.9.13).
9.3.10 Applications ofStents
9.3.10.1 Arterial Indications
• Residual or recurrent stenosis post-angioplasty: A 30%
post-angioplasty restenosis is used as a general threshold
for continued intervention.
• Pressure gradient: A pressure gradient of >10 mm Hg
(systolic) after angioplasty usually indicates residual ste-
nosis or dissection that requires treatment.
• Post-angioplasty dissection: Stent placement should be
considered for any signicant dissection after angioplasty,
which is called bailout stenting. Post-angioplasty dissec-
tions are graded by the NHLBI classication.
• Occlusion: Balloon angioplasty followed by Stenting has
shown improved results, as the latter helps in stabilizing
residual thrombus that could embolize from the lesion
site, especially if covered stents were used.
• Placement of stent-assisted coils.
Which Stent Is for Which Lesion?
Balloon expandable stents recoil less than self-expandable
stents when placed in calcied lesions, so they are still preferred in renal and coronary stenting. In this conguration,
the balance between hoop strength and placement precision
is better.
Hence, a BES is preferred in aortic branch orice lesions,
e.g., proximal innominate, common carotid, subclavian, visceral, or renal arteries, while in exible arteries such as the
SFA, popliteal, and distal subclavian arteries, an SES is preferred (Fig.9.14).
General Principles of Stent Placement
• Delivery over guidewire: to preserve access through the
lesion following deployment.
• Predilatation of extremely constricted lesions with an
angioplasty balloon guarantees the lesion’s pliability and
facilitates stent positioning.
• Lesion length and diameter determination: length suf-
cient to encompass the lesion plus minimal extension into
normal areas.
• Post-dilatation: ensure complete expansion and apposi-
tion in the vessel/lumen.
9.3.10.2 Venous Indications
• Chronic venous occlusion recanalization.
• Extrinsic compression caused by malignancy.
• Recurrent or brotic stenosis.
Venous Stenting
The venous system has a thin muscle layer and a low- pressure
system compared to the arterial system. Hence, venous stents
are designed to have higher radial force and higher
exibility.
b
Fig. 9.13 Types of stents based on mechanism of expansion
Self- expandable stent