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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3585_Библиотеки_им_академика_М_И_Перельмана
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2 Stenting ofBifurcation Lesions
P
SB
Fig. 2.6 Positioning a dedicated bifurcation stent. A dedicated stent
positioned optimally for access to the SB (a) and a stent with the carina
positioned on the opposite side (b)
Fig. 2.7 Failure to optimally position a dedicated stent. It may be
impossible to place the center of the petal of a dedicated stent exactly at
the center of the SB lumen, as shown on the left
MB
Fig. 2.9 Boston Scientic Petal stent. The stent is mounted on the
dedicated stent delivery system shown on the left. Ination of the spherical balloon at the SB ostium extends the petal obliquely into the
SB.Subsequently, KBI is done to optimally reshape and align the petal
a
b1 b2 b3 b4
carina
Fig. 2.8 Incorrect deployment of a dedicated stent proximal to the
carina. It is necessary to reinsert a guidewire/balloon into the SB
through a cell closer to the carina and perform KBI again
Fig. 2.10 Abbott Xience SBA stent (a). This dedicated stent is suitable
for provisional stenting, which is simply accomplished by simultaneous
stent implantation and KBI.The stent delivery system mounts a single
stent on two balloons and accommodates the SB guidewire near the tip
(b1). The stenting procedure includes the following steps: advancing
the stent until the SB balloon is just beyond the carina, releasing the SB
guidewire tip and withdrawing the wire, pulling the entire system back
slightly, advancing the guidewire with a reshaped tip into the SB (b2),
advancing the stent until its carina meets the vascular carina (b3), and
performing local KBI (b4)
2.1 Dedicated Bifurcation Stents
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Fig. 2.11 Advanced Bifurcation Systems stent
Fig. 2.12 Medtronic Y stent
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2 Stenting ofBifurcation Lesions
a b
Fig. 2.13 Tryton stent (a, b)
Fig. 2.14 STENTYS stent
2.2 Optimizing General-Purpose Stents forBifurcation Lesions
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Fig. 2.15 AXXESS stent
2.2 Optimizing General-Purpose Stents
forBifurcation Lesions
As shown in Fig. 2.16, the ideal bifurcation stent should
achieve excellent apposition of a single layer of struts over
the entire vessel wall at the bifurcation without causing SB
or MB jailing. A general-purpose stent that can provide the
ideal bifurcation scaffold needs to meet the following
requirements:
1. High exibility (conformability): Bifurcation lesions are
often tortuous (Fig.2.17), and many side branches have a
steep take-off angle (Fig.2.18). Hence, a stent for bifurcation lesions requires high conformability. Highly conformable stents usually have little edge effect. In general,
stents with fewer links are more conformable.
2. No risk of SB jailing after optimal KBI: The optimal stent
for a bifurcation lesion can dilate the lesion correctly
without causing SB jailing if optimal KBI is performed,
which is dened as ination of appropriately sized kissing balloons at the optimal pressure that does not injure
the proximal MB after SB (re)wiring through the most
distal cell near the carina.
3. Minimum risk of malapposition: The optimal stent for
bifurcation lesions must have high overall conformability
as well as being easily deployed by balloon ination.
Ease of stent expansion is inuenced by the design as
well as the material used to make it.
To achieve optimal apposition in a bifurcation lesion, a
stent with fewer links and with more and longer crowns is
preferred. In addition, a stent made of a material with
higher plasticity (e.g., stainless steel or platinumchromium alloy) is better able to undergo plastic deformation when implanted at a complicated bifurcation
163
lesion and thus achieve optimal apposition. A stent made
of cobalt-chromium alloy, which has lower plasticity (i.e.,
higher elasticity), may be slightly inferior with respect to
ne plastic deformation at bifurcation lesions.
However, a stent with higher elasticity can undergo
greater distortion before reaching the limit of elasticity,
which makes the occurrence of fracture (especially acute
fracture) or dynamic deformation less likely. Thus, the
balance between elasticity and plasticity of the stent material leads to signicant trade-offs for stent performance.
4. Little or no risk of fracture: Stents with no risk of fracture
have long been awaited, but no such stent has been developed so far. I consider that stent fractures can be classied
as “good” or “bad.” It would be ideal to completely prevent stent fractures, but this is impossible. Therefore,
stents should be designed so that any fracture is likely to
be a good fracture.
5. Drug-eluting: Unlike metal stents, BVS cannot be used
extensively for bifurcation lesions at present because performing KBI leads to collapse of the scaffold. DES will
remain the standard type of stent for bifurcation lesions,
at least for a while.
6. Polymer-free: A polymer coating may peel off after KBI
or culotte stenting. The peeling polymer can lead to
thrombosis, even if it is bioresorbable, and there is less
drug release from the part of the stent where the coating
has been lost. Therefore, a polymer-free DES is preferable for bifurcation stenting.
7. Proximal edge structure compatible with the aorto-ostial
anatomy: The aortic ostium, whether normal or occluded/
stenosed, can usually be dilated to its original diameter,
although its anatomy may prevent this. When treating an
LMT bifurcation lesion, it is often necessary to dilate the
bifurcation through the LMT ostium. Since the ostium can
be as large as 6mm in diameter, a stent for an LMT bifurcation lesion has to be expandable to a large diameter. On the
other hand, the part of the stent implanted outside the ostium
may be deformed by a guiding catheter or other device. To
prevent such stent deformation that could impair the system
integrity, the stent must have two links within 2–3mm of the
proximal edge. Two links are both necessary and sufcient
for preserving stent conformability and preventing deformation, while three or more links are not required.
A stent that meets all of requirements (1) through (4)
above should have no links, but it is virtually impossible
to implant a no-link stent without marked deformation.
Hence, the ideal stent for bifurcation lesions should have
links that can prevent excessive deformation during
implantation and lose these links as soon as possible after
implantation.
If all of the stent links facing the SB ostium can be
fractured during KBI, there is no risk of SB jailing. Thus,
I would like to discuss the concept of a stent with temporary (easily fractured) links later.
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Fig. 2.16 Optimal bifurcation stents. Whether
bifurcation stenting is performed by the MB crossover
technique (a) or the MB/SB full coverage technique (b),
the optimal stenting strategy must meet the following
requirements: (1) complete coverage, (2) complete
apposition, (3) complete expansion, and (4) optimal stent
design
Fig. 2.17 A tortuous
bifurcation lesion
2 Stenting ofBifurcation Lesions
Fig. 2.18 A bifurcation
lesion with extreme SB
angulation
Column 13 Good Versus Bad Fracture
[Bad fracture]
1. When one of the links is fractured in a stent with low conformability (e.g., a multi-link stent), both edges of the stent
will become more mobile, and this will cause static or dynamic edge effects (Fig.2.19).
2. When the long link in a stent is fractured, the stumps of the link may cause stimulation of the intima or malapposition can occur at the fractured link.
2.2 Optimizing General-Purpose Stents forBifurcation Lesions
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3. A stent with at least one link can be fractured at a strut rather than at a link (Fig.2.20). If strut fracture occurs, the
stumps of the strut will stimulate the intima. In addition, the fractured strut loses its radial strength due to loss of
continuity of the supporting arch. Thus, the fractured strut can easily be deformed by vascular recoil, narrowing the
lumen that was previously expanded by balloon ination of the stent. This mechanism may be involved in some
cases of in-stent occlusion (restenosis) at a fractured strut.
[Good fracture]
When can stent fracture be considered good? A good fracture can occur if a stent is designed with fracturable short
links between the strut peaks and valleys (Fig.2.21). If fracture occurs at a short link, it is unlikely that the stumps of
the fractured link will stimulate the intima or that the strut will lose radial strength. These outcomes are even more
unlikely if the stent has few links and high conformability.
I have long sought for a stent that can undergo good fracture. The Driver stent initially appeared to be suitable, but
it was found that fracture occurred at the struts and not at the links. However, I have found that the Nobori stent (particularly the 3.5-mm stent) is more likely to be fractured at a link than at a strut. This may account for the low restenosis
rate after implanting a Nobori stent, although this type of stent is liable to fracture if used at the RCA ostium. A modied Nobori stent that is much more likely to undergo fracture at a link than a strut is needed. Moreover, a stent with no
links or one designed with disappearing links could provide a good solution to the problem of stent fracture.
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Fig. 2.19 Bad fracture. The parts of the stent proximal and
distal to the fracture have shifted horizontally in opposite
directions
Fig. 2.20 Another example of bad fracture. The stent strut
has been fractured
Fig. 2.21 Good fracture. The stent has fractured at a short
link
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cell After disconnection
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2 Stenting ofBifurcation Lesions
[Stent “recoil”]
I often feel that something is wrong when I hear someone referring to stent deformation by external force as stent
recoil. This is because recoil means spontaneous movement of an object to return to its original shape. Thus, stent
recoil describes the tendency of a stent that has been expanded by a balloon to shrink slightly due to its inherent elasticity after deation of the balloon. Partial luminal loss after balloon deation when implanting a balloon-expanded stent
cannot be referred to as stent recoil because this phenomenon arises from recoil of both the vessel and the stent. A stent
will show minimal deformation if it is implanted in a soft lesion, but severe deformation will occur after implantation
in a hard lesion that has not been prepared sufciently.
In the POBA era, sudden luminal loss at the end of intervention was referred to as acute recoil, while late lumen loss
attributable to restenosis was called chronic recoil. Analogous with vascular luminal loss, narrowing of the stent lumen
may therefore have also been referred to as recoil. While “recoil” may reasonably be used to describe luminal loss due
to spontaneous narrowing of the target vessel after POBA, should it be used to describe narrowing of the stent lumen?
It is clear that stent luminal loss is largely due to deformation caused by external forces, rather than spontaneous
motion of the stent. In particular, chronic stent deformation (late stent luminal loss) is never due to stent recoil. If the
term “stent recoil” is used to describe stent deformation, it does not accurately express the essence of this phenomenon.
It may lead to classication of some stents as prone to “recoil” and possibly to an inappropriate focus on stent-related
factors when investigating the mechanism of stent deformation. The vascular factors potentially involved in stent
lumen loss should also be considered, in order to elucidate the mechanism applying external force to the stent and to
devise countermeasures. In this sense, I think that we should pay careful attention to use of the term “recoil.”
2.2.1 Temporary Link Stents
2.2.1.1 Concept ofthe“Temporary Link Stent”
I use the term “temporary link stent” to describe a stent with
intact link(s) until it is expanded in a vessel, after which the
link(s) are fractured or lost because of vascular movements or
with the passage of time. A link facing the SB ostium can be
easily fractured by application of horizontal force during KBI,
and the stumps of the link will not stimulate the vessel. After
loss of the links, it is desirable that the stent does not break up
into rings and that continuity of the struts is maintained.
2.2.1.2 Classication ofTemporary Link Stents
1. Bioresorbable link stent: The links of the stent are made
of a bioresorbable (drug-eluting) polymer and disappear
from several months to about 1year after implantation.
2. Separable link stent: The links are made of metal but are
designed to be easily disconnected by KBI and by vessel
movements. If the stent is fractured at a link, the stumps
of the link do not stimulate the vessel, so it can be intentionally fractured at a link to ensure high conformability
while preventing vascular injury.
A stent with disconnectable links (Fig. 2.22) has been
developed, but it has not been marketed in Japan so far.
cell
Disconnection
of link
Fig. 2.22 A stent with disconnectable links. Balloon ination in the
SB disconnects a link, producing a larger cell and resolving SB
jailing
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2.2 Optimizing General-Purpose Stents forBifurcation Lesions
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2.2.1.3 Helical Coil Stents
Loss of links in a temporary link stent means that the individual strut rings are independent of each other in the vessel
lumen and malapposition may allow movement or shift of the
stent struts. To preserve the integrity of a stent after loss of all
links, it has to have a helical coil structure. Helical coil stents
have very high conformability. Even if overlapped, such stents
rarely become rigid and are unlikely to undergo strut fracture.
However, if a helical coil stent deployed in an aorto-ostial
lesion is not optimally apposed (even temporarily), longitudinal deformation can readily occur, and the stent may sometimes be extruded into the aorta. If used for an aorto-ostial
lesion, a helical coil stent should have several proximal struts
with a durable 2-link pattern.
2.2.1.4 Optimal Design ofaStent forUse
withKBI
The optimal stent for deployment with KBI should be
designed as follows:
1. It should permit sufcient enlargement of a strut cell for
side branch access.
2. It should have few links (Figs.2.23 and 2.24) to permit
sufcient enlargement of a cell while causing less vascular
damage near the SB ostium and to minimize the risk of
SB jailing or stent malapposition.
3. It should have disconnectable links (refer to Fig.2.21).
4. It should provide an adequate scaffold for SB shoulder
support (Fig.2.24b).
5. Its struts should be uniformly expandable in every direction by KBI.
A laser-cut stent, polished and bordered, generally has
struts that are square in the cross-sectional view. A strut with
a square cross section can be bent easily by application of
force in a direction parallel to a side of the square, but not in
a direction that is at an angle to the side. Such a strut is most
difcult to bend at an angle of 45 degrees to the side of the
square.
A strut with a rectangular cross section can easily be bent
by application of force in the direction of the shorter side, but
not in other directions, especially the direction of the longer
side. A lamentous strut with a round cross section can be
bent in every direction by application of identical force
(Fig.2.25).
In principle, a stent with cylindrical struts is considered
to be best able to t into the two branches. On the other
hand, cylindrical struts have a smaller contact area with the
vessel wall and are prone to excessive deformation when
contacted by a balloon. Thus, designing a stent with cylindrical struts involves a trade-off between these two characteristics, although the actual clinical performance of stents
with different strut shapes for bifurcation stenting is
uncertain.
At least, it can be said that a stent with rectangular struts
is not suitable for bifurcation stenting. A stent with cylindrical struts is the best option, while a stent with rectangular
struts (chamfered squares in cross section) is second best.
Fig. 2.23 Design of 6-, 3-,
2-, and 1-link stents. A stent
with fewer links is less likely
to cause link-induced SB
jailing
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2 Stenting ofBifurcation Lesions
Fig. 2.24 Stents expanded by KBI exvivo. (a)
Expanding a 3-link stent results in enlargement of a
strut cell at the SB ostium to an elliptical hole about
3mm in diameter with no zigzag strut. Hence, the
lateral border of the hole (★) is proximal to the stent
carina, with the average distance being about 2mm or
slightly more. (b) Expanding a 2-link stent results in
enlargement of a strut cell at the SB ostium to about
3mm in diameter while retaining the zigzag structure
of the struts. This allows the stent to provide a more
adequate scaffold for the SB shoulder (arrow)
compared with a 3-link stent
a
★★
Ca 2mm
b
Fig. 2.25 Cross-sectional views of stent struts. A strut with a rectangu-
lar cross section can easily be bent if force is applied in the direction of
the shorter side, but not in the direction of the longer side. A strut with
2.3 Optimal Stenting Techniques
forBifurcation Lesions
2.3.1 Need forKBI Pre-dilatation
andPractical Approach
Although many interventionalists question the use of KBI
for pre-dilatation, I often perform this technique for predilatation of a bifurcation lesion with plaques around the
a square cross section can be bent more easily if force is applied in a
direction parallel with its sides, but not in a direction at 45 degrees to a
side. A cylindrical strut can be bent in any direction by the same force
carina (including the SB ostium), provided the diameter of
the proximal MB is signicantly larger than that of the distal MB.The objectives of performing KBI in this situation
are (1) to assess the responsiveness of the proximal MB to
balloon angioplasty (Figs.2.26 and 2.27) to reduce the risk
of plaque shift after stent implantation and the risk of
occlusion/severe stenosis of the SB ostium by carina shift
during alternating balloon ination (Fig. 2.27). For predilatation, I rst perform KBI at a pressure of only 6–8atm.
a
b
c
a
b
c
g
2.3 Optimal Stenting Techniques forBifurcation Lesions
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If such low- pressure KBI cannot dilate the lesion sufciently, I perform alternating balloon ination at escalating
pressures until sufcient dilatation of the SB ostium and
the distal MB is achieved. I then repeat low-pressure
(about 10 atm) KBI and check if there is any residual
indentation.
Even if alternating balloon ination seems to have
removed all indentations in the MB and SB balloons and to
have sufciently dilated both branches, one or more indentations may reappear in the body of either or both balloons
during KBI. Such pre-dilatation failure typically occurs in
patients who have an LAD-diagonal branch (Dg) ostial
lesion with hard plaque at the shoulder of the branch. In such
lesions, alternating balloon ination often causes carina shift
(Fig. 2.28) that obscures balloon indentations, so that it
appears that the SB ostium has been dilated sufciently. For
optimal preparation of these lesions, I have often used a scoring balloon in recent years. The Lacrosse NSE is my favorite
scoring balloon. In general, I combine scoring balloon angioplasty and POBA as required to prepare bifurcation lesions
according to the size of the MB/SB and the pattern of stenosis (Fig.
2.29). IVUS guidance is absolutely required for
optimizing the combination of these angioplasty
procedures.
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Fig. 2.26 Need for KBI in pre-dilatation. At a bifurcation stenosis (a),
alternating balloon ination appears to have sufciently dilated the MB
and the SB since there are no indentations when the balloon is inated
in either branch (b). However, alternating ination has actually caused
carina shift and deformation of the SB shoulder, resulting in transient
elimination of indentations in the body of the SB balloon so that it
appears as if the SB ostium has been sufciently dilated (b). During
KBI, one or more indentations may reappear in the SB balloon, but such
indentations can be eliminated by raising the KBI pressure to 8atm. As
a result, both the MB and SB can be dilated sufciently as optimal
preparation for stenting with KBI (c)
d
e
h
f
Fig. 2.27 Need for KBI in pre-dilatation of true bifurcation lesions. If
pre-dilatation of a bifurcation lesion (a) is performed by alternating balloon ination, the carina shifts toward the SB after ination of the MB
balloon (b) and then shifts back toward the MB after ination of the SB
balloon (c). After balloon deation, it is found that only minimal dilata-
tion of the lesion has been achieved (d). Following minimal predilatation of a bifurcation lesion, placing a stent in the MB can cause
plaque shift that leads to severe stenosis or occlusion of the SB (e). With
pre-dilatation by KBI (f), the lesion will be optimally dilated (g), minimizing the risk of SB occlusion (h)
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