Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_1348_Библиотеки_им_академика_М_И_Перельмана.pdf
Fig. 4.3 ( a ) Poorly aligned and impacted laser-cut biliary stent (Memotherm). ( b ) Attempts at dislodging the top of
the stent resulted in entanglement of the forceps
by sutures (Boubella) or additional threads of
Metal Stents
nitinol (Choo/Hanaro stent).
534 Current Designs of Self-Expanding Stents
Segmented Stents
Individual cylindrical metal baskets are connected to each other, resulting in a semirigid tube
(Fig. 4.4 ). One of the earliest designs (Gianturco-
Rösch Z-stent, Wilson Cook, Bloomington, USA)
consists of multiple stainless steel segments, connected by sutures and covered by dipping into
polyurethane. This stent has a low elasticity and
needs to be loaded into the delivery sheath prior
to use. The high straightening force prevents
alignment around the gastroesophageal junction,
at times leading to pressure necrosis and perforation of the esophageal wall [ 5 ] .
The second-generation Ella esophageal stents
(Boubella, Ella-CS, Hradec Kralove, Czech
Republic) achieved greater fl exibility with stainless steel segments; the fi rst stent based on
nitinol baskets was developed by a Korean
manufacturer (Choo/Do stent, MI-Tech, Seoul,
South Korea). Both were supplied preloaded into
their delivery system. Due to the rigidity of the
individual segments, this type of stent tends to
buckle rather than bend. Repetitive strain occasionally leads to rupture at the junction of the segments (Fig. 4.5 ). Later, models were reinforced
Laser-Cut Stents
A completely different approach to stent design
is achieved by the construction of a solid nitinol
tube, which is extensively perforated and sections
cut out of it through the use of a laser (Fig. 4.6 ).
Originally developed for the stenting of peripheral vascular stenoses, several manufacturers
have adapted these for use in the biliary tree.
These are not covered by a membrane, and their
advantage is of a high initial radial force and
the ability to fi t into a small delivery system
(6 French; French = circumference in mm). One
covered esophageal version is available; the only
enteral version was withdrawn due to complications. These stents excel by their minimal shortening (<10%) on release, which allows for more
accurate stent placement. However, there are several trade-offs for this: Due to their angular struts,
laser-cut stents cannot be repositioned or
resheathed when partly deployed. The expanding
cells of the stent skeleton act as barbs fi xing the
stent into the mucosa and engaging on the edge
of the delivery sheath. The major limitation of
54
H.-U. Laasch
Fig. 4.4 Segmented stents. ( a ) Gianturco-Z stent,
Cook, Bloomington, USA; ( b ) Boubella stent,
Ella-CS, Hradec Kralove, Czech Republic; ( c ) Hanaro
stent, MI-Tech, Seoul, Korea. The two stainless steel
stents consist of individual baskets sutured together
( arrows ), while the nitinol segments in the third stent
are only connected by the covering membrane
( arrowheads )
Fig. 4.5 Ruptured membrane: After removal of a migrated stent (Boubella) from the stomach, the lower segment with
the antire fl ux valve was found missing. It impacted in the sigmoid colon and was removed endoscopically (insert)
laser-cut stents however, is the reluctance to align
smoothly around fl exures (Fig.
4.7 ). This is due
to the high straightening force caused by the
memory effect of the tubular design. This type of
stent is more likely to stretch the stented segment
rather than conform to the natural anatomy. As a
result, reintervention can be challenging
(Fig. 4.8 ). Manufacturers have tried to address
this by reducing the size of the metal struts. The
memory effect of the metal tube also poses consistent resistance to natural fl exion. If fl exion is
frequent and repetitive, it leads to metal fatigue
554 Current Designs of Self-Expanding Stents
Fig. 4.6 Close-up of laser-cut stents. ( a ) Covered Alimaxx-E
esophageal stent; ( b ) Uncovered Luminexx biliary stent
Fig. 4.7 Better conformability of a braided esophageal
stent (Ella-HV) compared to a laser-cut stent (Alimaxx-E,
Merit Medical, South Jordan, Utah, USA)
high occurrence of stent fracture and colonic
perforation [
7– 9 ] .
Woven Stents
There are two different ways of weaving a wire
or mono fi lament into a tube and for the purpose
of clarity will be called braided and knitted
(Figs. 4.10 and 4.11 ). Braided stents consist of
crisscrossing wires which are movable against
each other. In contrast, with knitted stents, some
of the wire junctions are looped around each
other like a wire fence. This almost completely
abolishes longitudinal straightening forces,
resulting in great conformability as well as
reduced stent lengthening on compression.
Stent Manufacture
The wire is looped by hand around pins inserted
into a metal mandrel. In case of nitinol, the completed stent skeleton undergoes heat treatment,
imprinting the three-dimensional shape
(Fig. 4.12 ). Following this, the metal is polished
to reduce surface debris and microcracks. For
covered versions, the stent may be dipped in liquid plastic, such as polyurethane or silicone, or
covered with a polyethylene or PTFE membrane.
Of all covering materials, PTFE is the most inert
and resistant to degradation from gastric acid and
bile, but it is also the most expensive and needs to
be sutured by hand onto the stent skeleton. The
fi nished stent is loaded manually into the delivery
system, packed, and gas-sterilized.
Braided Stents
and fracture (Fig. 4.9 ), reported as high as 6%
in vivo [ 6 ] and as high as 100% in a vascular
simulation model [
3 ] . In the gut, this occurs from
peristalsis, in the bile duct by excursion of the
liver from breathing, coughing, laughing, and
hiccups. The only laser-cut enteral stent
(Memotherm, Bard) was discontinued due to a
A landmark stent design was achieved with the
development of the biliary and enteral Wallstent
(Boston Scienti fi c, Natick, MA, USA). A continuous tube braided from cobalt alloy wires
was cut into short sections, representing the
individual stents. These stents were the fi rst to
be available for through-the-scope (TTS) place-
ment for biliary and enteral use and are still a
56
H.-U. Laasch
Fig. 4.8 Poor alignment. ( a ) Blocked Luminexx stent.
The upper end is impacted in the bile duct wall ( arrow )
making cannulation dif fi cult. ( b ) Note the sharp angula-
tion of the distorted bile duct at the lower stent end
( arrow ). The pancreatic duct is also demonstrated ( arrow-
head ). ( c ) Insertion of a knitted biliary stent (Egis double
bare) shows smoother alignment along the bile duct
Fig. 4.9 Fracture from metal fatigue. ( a ) CT reconstruction of a fractured laser-cut stent (Zilver); ( b ) After restenting
with a braided stent (Ella-SX)
preferred stents for many endoscopists due to
the high visibility on fl uoroscopy. The drawback
of this construction is the crown of sharp wire
points, which forms the stent ends. If placed
within a fl exure, these may perforate the bowel
wall. The wire ends are also at risk of intertwining when the delivery system is fed through
the working channel of the endoscope. Once
entangled, the stent may fail to open properly
(Fig. 4.13 ). The follow-on Wall fl ex stent [ 10 ]
now represents the commonest construction
used in the GI tract. A single nitinol wire is
woven continuously around a metal mandrel,
and the edges of the stent consist of loops of
wire rather than sharp points.
Braided stents are very fl exible but retain a
signi fi cant straightening force. Esophageal stents
tend to have greater radial force than enteral
stents due to thicker wire gauge being used. This
is possible, as the esophageal anatomy is essentially straight except for the gastroesophageal
junction, and larger delivery systems (18–28 Fr)
can be used. Softer stents for enteral use can be
mounted in 10–10.5 Fr delivery systems, which
574 Current Designs of Self-Expanding Stents
Fig. 4.10 Woven enteral stents; upper row braided, lower
row knitted stents ( a ) Wallstent; ( b ) Wall fl ex; ( c ) Ella-SX
enteral; ( d ) Niti-S D-stent; ( e ) ComVi; ( f ) Egis single
bare; ( g ) Egis double covered. Note the ability of the
knitted stents to align in a curve
Fig. 4.11 Close-up image showing the simple crossover of a braided construction Wallstent ( left ), and the interlocking
wires of a single Niti-S D-stent ( center ), and a double knitted Egis stent ( right )
58
H.-U. Laasch
Fig. 4.12 Manufacture of a nitinol colonic stent (Egis). ( a ) Weaving of the stent skeleton on a mandrel; ( b ) Heat treat-
ment for imprinting shape memory; ( c ) Manual loading prior to sterilization (Courtesy of S&G Biotech, Seoul, Korea)
allows placement through large working channels
( ³ 3.7 mm) of therapeutic endoscopes ( through-
the-scope , TTS). All stents lengthen when compressed into the delivery system and shorten
again on deployment. This is most marked with
braided stents (Fig.
4.14 ). Stent shortening can be
shortened, the whole system is withdrawn
carefully into the fi nal position and deployment
completed. This cannot be performed with lasercut stents, as the barbs fi x the stent into the
mucosa.
as much as 50% in large-caliber stents. Note that
this implies an initial doubling in length by com-
Knitted Stents
pression (e.g., 10 cm 20 cm); a shortening by
one-third implies an original increase by 50%
(e.g., 10 cm 15 cm). A braided stent has a relatively smooth outer skeleton, which allows
resheathing of a partially deployed stent. Equally
stents may be gently repositioned by traction
when already partly deployed. Indeed, this allows
for a simple strategy to correct for stent shortening; the delivery system is intentionally inserted
several centimeters too far and the stent partially
deployed. Once the distal end has expanded and
This construction results in very low straightening forces and excellent conformability to the
host tube as well as reduced stent shortening
(25–30%) on deployment compared to braided
stents. Knitted esophageal stents have been in use
for over 20 years; the commercially most successful version was the Ultra fl ex stent (Boston
Scienti fi c). Unfortunately, it was never possible
to produce this within a pullback delivery system
due to the high radial force. Instead, this stent
594 Current Designs of Self-Expanding Stents
Fig. 4.13 Enteral Wallstent placed endoscopically into a
gastrojejunostomy immediately after deployment. The
wires at the lower stent end have become entangled as the
system was advanced through the working channel. This
could be recti fi ed by forceful passage of the endoscope
was tied down to the outside of the delivery catheter by silk thread. This made the system bulky
and very rough, resulting in the highest need of
all stents for predilatation to allow passage of the
system through the stricture [ 11 ] . Being only par-
tially covered, the Ultra fl ex was also not designed
to be removable. The fi rst removable knitted
nitinol esophageal stent was introduced in 2011
(Egis, S&G Biotech, Seoul, Korea) (Fig.
4.15 ),
available with optional antire fl ux valve.
Several enteral versions of a knitted stent were
developed in South Korea. One system consists of
two separate stents, one covered one uncovered
[
12, 13 ] , but commercially more successful designs
consist of a covering membrane sandwiched
between two layers of nitinol (ComVi stent,
Taewoong and Egis double covered stent, S&G
Biotech). Intuitively, these constructions should
combine the advantages of reduced migration due
to the outer layer of metal with the reduction of
ingrowth from the covering membrane, but clinical results are ambiguous [ 14– 16 ] . The drawback
of this construction is the smaller wire gauge used,
reducing radial force and visibility of the metal
skeleton, although this is compensated for by the
use of gold markers. These stents may be repositioned by traction if part-deployed but cannot be
resheathed.
Knitted stents excel through their effortless
conformability to fl exures while retaining the
ability to remodel and expand into the original
con fi guration. This may however, take several
days, and the urge to perform balloon dilatation
immediately after insertion should be resisted.
Biodegradable Stents
Early applications of biodegradable stents in the
GI tract go back as far as 1997 [ 17, 18 ] , but only
one is commercially available at present
(Ella-BD, Ella-CS, Hradec Kralove, Czech
Republic). The stent is braided from a fi lament
of a complex polymer (polydioxanone, PDX),
which disintegrates through hydrolysis, over 3–4
months (Fig. 4.16 ). This is accelerated within an
acidic environment [ 19 ] . The PDX skeleton is
radiolucent, and only the gold markers are visible on fl uoroscopy. Combined with the signi fi cant
stent shortening, these stents require a conscientious and measured approach when used for the
fi rst time.
Due to the reduced elasticity biodegradable
stents are currently supplied outside the delivery
system and need to be loaded through a funnel
prior to use. At present only licensed for the treatment of benign esophageal strictures [
tom-made devices have been successfully used
for fi brotic strictures of the bile duct and anastomotic strictures after gastrectomy and colectomy
[
21, 22 ] .
They may have a role as an adjuvant for radical chemoradiotherapy for esophageal tumors
(Fig. 4.17 ), but there are perceived dif fi culties
with accurate radiotherapy planning and delivery
of the target dose. Furthermore, successful stenting does not guarantee adequate oral intake [ 23 ] .
20 ] , cus-
60
H.-U. Laasch
Fig. 4.14 ( a ) and ( b ) Stent lengthening of a knitted ( Upper : Niti-S D-stent) and a braided enteral stent ( Lower :
Wall fl ex)
An expansion of the licensing for Ella-BD stents
is expected over the next 2 years, and there are
fi rst case reports of alternate polymers being
applied in the GI tract [ 24 ] .
Self-Expanding Plastic Stents
One self-expanding plastic stent (Poly fl ex,
Boston Scienti fi c) is currently available. It has
Fig. 4.15 Knitted esophageal stents. Note the threedimensional conformability. E: Fully covered removable
Egis stent; U/F: partially covered Ultra fl ex stent. Purse
strings for extraction ( arrows ) or repositioning ( arrow-
head ) with forceps.
similar characteristics to a metal stent but is
cheaper. Not to be confused with the rigid plastic
Atkinson tubes used in the beginnings of esophageal stenting, this stent has a braided polyester
614 Current Designs of Self-Expanding Stents
Fig. 4.16 Biodegradable esophageal stent (Ella-BD) and
spontaneous disintegration after 18-month exposure to
room air
skeleton covered with silicone on the inside.
It requires loading prior to delivery by withdrawing it into the delivery sheath with a fl exible basket (Fig. 4.18 ). The disadvantage is the size of the
delivery system, measuring 42 French for the
biggest stents. The Poly fl ex is also radiolucent
except for three bands of radiopaque markers. It
has a rough outer texture and high radial force
and has been used in a wide variety of benign and
malignant indications [
complication rate has been reported [
migration as high as 63% [
25– 27 ] . A relatively high
28, 29 ] with
30 ] but can be removed
endoscopically by extraction with forceps. Care
must be taken that these do not cut through the
stent wall.
Although the requirements for a stent within
the esophagus and the bowel are very similar,
designs for esophageal and enteral stents have
pursued different directions.
Migration of Esophageal Stents
Covered stents are used routinely in the esophagus, thus preventing stent occlusion by tumor
ingrowth through the interstices of the stent. This
does, however, result in an increased migration
rate which is approximately 5% in the esophagus
proper, but even with dedicated antimigration
designs, at least 15% for stents placed across the
gastroesophageal junction [ 11 ] . Most esophageal
stents are of a dog-bone or a double- fl ared design
with the stent ends being of a wider diameter than
the stent trunk. In addition, some manufacturers
have tried to speci fi cally address the problem of
migration. The Ella-HV stent (Ella-CS) has forward facing wings around the circumference of
the proximal stent end, resulting in a collar, which
anchors it against the stenosis (Fig. 4.19 ). The
Niti-S double stent (Taewoong) is a dog-boneshaped stent, which has an uncovered stent segment on the outside, designed to be gripped by
the mucosa (Fig. 4.20 ). However, both stents
migrate. The outer sleeve of the Niti-S double
also stiffens the stent resulting in increased resistance to peristalsis and possibly a higher risk of
migration. Whether softer knitted esophageal
stents have a lower migration rate due to their
better conformability remains to be seen.
Several stents are available with one or both of
the ends uncovered. This allows better grip by the
mucosa but makes future removal dif fi cult, as
there is invariably a degree of mucosal ingrowth
into the uncovered mesh. This may be addressed
by inserting a second covered stent resulting in
pressure necrosis of the hyperplastic mucosa and
subsequent removal of both stents together.
Older designs aimed at reducing migration
included a conical shape (Flamingo Wallstent,
Boston Scienti fi c), but the large inlet funnel occasionally resulted in pressure necrosis of the
esophageal wall.
The Gianturco-Z stent (Cook) had small barbs
that protruded from the side of the stent to embed
in the mucosa. However, this was associated with
a high incidence of chest pain and prevented stent
removal.
The majority of stents that migrate into the
stomach do not cause signi fi cant problems. Stents
that pass into the duodenum usually proceed to
pass through the whole intestine and are often
evacuated per ano without the patient noticing.
However, impaction within the small bowel can
62
H.-U. Laasch
Fig. 4.17 Biodegradable stent supporting radical radiotherapy. ( a ) Esophageal stent insertion. Injection of contrast
through a biliary manipulation catheter ( arrow ) outlines a
long distal stricture demarcated by paper clips on the
patient’s skin. ( b ) Stent deployment: Except for the gold
markers ( arrowheads ), the stent is radiolucent. The three
occur (Fig. 4.21 ), and this may necessitate a
laparotomy. With a few exceptions, all modern
esophageal stents are removable, usually by a
plastic or wire purse string around the stent end;
lower stent markers have begun to fl are out as the delivery
sheath is pulled back ( arrow = distal metal marker on deliv-
ery sheath). ( c ) Deployed stent: Injection of contrast shows
good position but limited initial expansion. ( d ) Radiotherapy
planning CT: The stent ( arrow ) is just visible within the
circumferential esophageal tumor ( arrowheads )
this allows capture with endoscopic forceps and
fairly atraumatic extraction. This offers the option
of temporary stenting, for example, for the treatment of benign esophageal strictures.