Percutaneous endovascular stent and method for insertion thereof

An endovascular stent formed of stainless steel wire of 0.018 inches diameter and arranged in a closed zig-zag pattern. The stent is compressed into a reduced size shape of an outer diameter which is many times smaller than its expanded shape. The stent is positioned in a passageway in the vascular system by means of a sheath while the stent is retained in the compressed reduced size shape. A flat-ended catheter is used through the sheath to hold the stent in place in the passageway while the sheath is withdrawn from the passageway allowing the stent to expand in the passageway into its expanded shape to hold the passageway open and enlarged. Other possible applications of the stent are in the respiratory, biliary and urinary tracts to reinforce collapsing structures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to stents and a method for inserting a stent. 
2. Brief Description of the Prior Art 
It is desirable in various situations that means be provided for expanding 
a constricted vessel portion or for maintaining an open passageway through 
a vessel portion. Such situations arise, for example, in conjunction with 
the disease known as arteriosclerosis as well as the growth of a tumor so 
as to restrict or stop flow of blood through a blood vessel. Dr. Charles 
Dotter et al. reported in 1969 on the experimental use of coiled stainless 
steel wire stents placed in the popliteal arteries of dogs. Although the 
coils exhibited long-term patency, narrowing of the lumen occurred within 
them and only small coils could be passed percutaneously. See Dotter CT et 
al., Transluminally-Placed Coilspring Endoarterial Tube Grafts, Invest. 
Radiol., 1969; 4:329-332.sup.1. Recently, two laboratories reported on the 
use of a prosthesis constructed of a thermal shape memory alloy, nitinol, 
which is passed through a catheter. See Dotter CT et al., Transluminal 
Expandable Nitinol Coil Stent Grafting, Radiology, April, 1983; 
147:259-260.sup.2, and Cragg A. et al., Nonsurgical Placement of Arterial 
Endoprostheses, Radiology, April, 1983; 147:261-263.sup.3. Such stents can 
be complicated to use, requiring ice water or heated saline for placement. 
Also they have been found to produce luminal narrowing due to fibrin 
deposition on the stent wires. 
Other references which may have relevance to the present invention are the 
following U.S. patents: Sakura U.S. Pat. No. 4,214,587; Alfidi U.S. Pat. 
No. 3,868,956; and Simon U.S. Pat. No. 4,425,908; and the Russian Pat. No. 
978,821; also the following publications: C. Gianturco et al., A new vena 
cava filter: experimental animal evaluation, Radiology, December, 1980; 
137:835-837.sup.4 ; and M. Simon et al., A Vena Cava Filter Using Thermal 
Shape Memory Alloy, Diagnostic Radiology, 125:89-94, October 1977.sup.5. 
Still another reference publication is D. Maass et al., Radiology 
Follow-up of Transluminally Inserted Vascular Endoprostheses: An 
Experimental Study Using Expanding Sprials, Radiology, September 1984; 
152:659-663. 
Objects of the invention are to provide a stent which is easy to place and 
use that reduces flow defects, luminal narrowing and occlusion. 
SUMMARY OF THE INVENTION 
One embodiment of the stent of the present invention might include a wire 
formed into a closed zig-zag configuration including an endless series of 
straight sections joined by bends. The stent is resiliently compressible 
into a smaller first shape wherein the straight sections are arranged 
side-by-side and closely adjacent one another for insertion into a 
passageway. The stent is resiliently expandable into a larger second shape 
wherein said straight sections press against the walls of the passageway 
to maintain it open. 
One embodiment of the method of the present invention might involve 
inserting a stent by compressing a stent including a wire formed in a 
closed zig-zag configuration into a first shape wherein the zig-zag 
configuration includes side-by-side closely adjacent straight sections 
joined by bends with a stress therein. The compressed wire stent is then 
moved into a sheath. The sheath is then located with the distal end 
thereof in a passageway with the compressed wire within the distal end of 
the sheath. The sheath is then removed from the passageway while holding 
the stent in place, whereby the stress in the stent causes it to expand in 
the passageway to hold the passageway open and enlarged.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
For the purposes of promoting an understanding of the principles of the 
invention, reference will now be made to the embodiment illustrated in the 
drawings and specific language will be used to describe the same. It will 
nevertheless be understood that no limitation of the scope of the 
invention is thereby intended, such alterations and further modifications 
in the illustrated device, and such further applications of the principles 
of the invention as illustrated therein being contemplated as would 
normally occur to one skilled in the art to which the invention relates. 
Referring now more particularly to the drawings, there is illustrated in 
FIG. 1 a side elevation of a preferred embodiment of the stent 9 of the 
present invention which includes a length 10 of stainless steel wire 
formed in a closed zig-zag configuration. The wire is closed by a sleeve 
11 which is welded to or tightly squeezed against the ends of the wire to 
produce the endless configuration. Referring to FIG. 4, the stent is shown 
in a resiliently compressed first shape wherein the straight sections 12 
are arranged side-by-side and closely adjacent one another. 
The straight sections 12 of the stent are joined by bends 13 which are 
relatively sharp. Thus, in one specific embodiment of the invention, the 
bends 13 are at a radius of no more than 0.2 cm. This specific embodiment 
of the invention includes wire 10 which is stainless steel of 0.018 inch 
O.D. The stent is resiliently expandable from the compressed first shape 
of FIG. 4 into a second shape illustrated in FIGS. 1, 2 and 6, wherein the 
straight sections 12 press against the walls of passageway to maintain the 
passageway open. FIG. 2 shows the end view of the stent in its expanded 
second shape. As illustrated in FIG. 2, the stent has generally a circular 
configuration or a cylindrical configuration when it is in its second 
expanded shape. 
In order to practice the method of this invention, the stent is compressed 
into the first shape illustrated in FIG. 10 and is placed within a tubular 
cartridge 15 (FIG. 10). The cartridge 15 is inserted into the recess 16 in 
the adapter 17 of the sheath 20. The stent is then advanced through the 
sheath 20 by means of a flat-ended pusher 21. Thus in one specific 
embodiment of the invention, the flat-ended pusher was made of 8 French 
polyethylene tubing, although a flat-ended flexible metal rod is 
preferred. When the stent 10 reaches the end of the sheath as shown in 
FIG. 4, the flat-ended pusher is held while the sheath is withdrawn as 
shown in FIG. 5. This frees the stent, allowing it to expand and hug the 
vessel wall as shown in FIG. 6. If desired and if necessary for the 
particular situation, further stents can be added and can be placed in the 
blood vessel in the same fashion as above described. Thus in FIG. 7, an 
additional two stents are located one longitudinally of the first stent in 
the blood vessel and the other overlapping the first stent while in FIG. 
8, four overlapping stents are used. 
In tests of the invention, endovascular stents were designed and built in 
two sizes (5.5 cm long.times.4 cm diameter fully expanded; 3.0 cm 
long.times.2.5 cm diameter fully expanded) from stainless steel wire 
(0.018 in.) formed in a zig-zag pattern. They were placed for varying 
periods of time in the jugular vein, inferior vena cava and abdominal 
aorta of five dogs (See Table I below) and evaluated with regard to ease 
of use, dilating force, migration, patency, thrombogenicity, and local 
vascular changes. 
Five adult, mongrel dogs (18-27 kg) were used in the study. They were 
anesthetized with i.v. sodium pentobarbital (Nembutal; 30 mg/kg) and the 
jugular vein, femoral vein, and femoral artery were surgically isolated. 
An incision was made in the vessels and a 8 French Teflon sheath 
containing an 8 French Teflon catheter with a taper-tip was inserted and 
under fluoroscopic monitoring advanced just beyond the area of interest. 
The stent was compressed and placed within a Teflon cartridge which fits 
inside the adaptor of the 8 French sheath. The 8 French catheter was 
removed, the cartridge was placed in the sheath adaptor, and the stent was 
advanced through the sheath with flat-ended 8 French polyethylene tubing. 
When the stent reached the end of the sheath, the polyethylene tubing was 
held while the sheath was withdrawn. This freed the stent allowing it to 
expand and hug the vessel wall. In certain cases, stents were placed one 
inside another and/or one after another (Table I). Following placement, 
angiograms were made immediately, after one week, and then at monthly 
intervals to document stent position and vascular anatomy. The dogs were 
euthanized at the end of the study by exsanguination under deep Nembutal 
anesthesia, and a complete necropsy was performed. 
TABLE I 
______________________________________ 
Summary of vascular stent placement in five dogs. 
Stent 
Dog Size 
# (Number Dura- 
(Wt) Used) Vascular Placement tion 
______________________________________ 
416 5.5 cm Two placed one inside the other in 
1 
(5) abdominal aorta (AA) bridging the 
month 
celiac, cranial mesenteric, and right 
renal arteries 
Two placed one inside the other in 
superior vena cava (SVC) at level 
of right atrium 
One placed in the inferior vena 
cava (IVC) bridging both renal veins 
3.0 One placed in right jugular 8 cm 
above SVC, and 
(3) two placed one inside the other in left 
jugular 8 cm above SVC 
355 5.5 One placed in AA bridging the 
3 
celiac, cranial mesenteric, and 
months 
right renal arteries 
Two placed one inside the other 
in IVC bridging both 
renal veins 
3.0 Two placed one inside the other 
(3) in SVC at level of right 
atrium, and one placed 2.3 cm above 
the right atrium 
354 5.5 One placed in AA bridging the 
4 
(2) cranial mesenteric and both 
months 
renal arteries 
One placed in IVC bridging both renal veins 
505 5.5 Four placed one after another in AA 
5 
(5) beginning at diaphragm (T11) and 
months 
ending at L5 
One placed in IVC at level of 
diaphragm 
3.0 One placed inside last long stent 
(3) in AA at level of L4-L5 
Two placed one after another in 
IVC between the hepatic and renal 
veins 
______________________________________ 
No difficulties were encountered in the placement of the endovascular 
stents. They were easy to use and could be placed one inside another 
and/or one after another. The expansile strength of the stents was found 
to be dependent on stent length, diameter of stent wires, the number of 
folds in the wire of each stent, and the number of stents placed one 
inside another. Specifically, expansile force increased with decreased 
length, increased stent wire diameter, increased number of wire folds, and 
increased number of stents used. 
Angiograms made of the stented vessels showed no flow defects, luminal 
narrowing, or occlusion. Blood vessels bridged by the stents remained 
patent and showed no indication of narrowing even after six months. No 
migration was noted with 29 of the 30 stents placed. One long stent (5.5 
cm) placed alone in the inferior vena cava migrated approximately 2 cm 
cranially during the first week following placement, but no further 
movement occurred and no complications were encountered because of this 
migration. 
Postmortem examinations showed the endothelial proliferation occurred 
around the stents where the wires contacted the vessel wall. By four weeks 
following placement, venous stents were almost completely (80%) covered by 
cell growth while aortic stents were just beginning (30%) to be 
incorporated. By 12 weeks, all stents were covered witth endothelium where 
the wires contacted the vessel wall. No growth was noted on wire segments 
that bridged side branches even after 6 months. In addition, no erosion of 
the vascular walls was noted, and no clot formation was seen on any of the 
stents. 
Percutaneous expandable endovascular stents can be made of various 
diameters and lengths from stainless steel wire formed in a zig-zag 
pattern. They are easy to place percutaneously in veins and arteries and 
do not require the use of ice water or hot saline as do nitinol coils (2, 
3). The dilating force of the stent can be controlled by manipulation of 
wire size, number of wire folds, and stent length. Expansion force 
increases with larger wire, but so does the size of the collapsed stent 
which necessitates use of a larger sheath for placement. Increasing the 
number of wire folds and decreasing the stent length also increase the 
dilating force. Therefore, stainless steel vascular stents can be 
tailor-made with regard to length, diameter, and expansion force. 
Multiple stents can be employed depending on the circumstance. If the 
vessel of interest is longer than one stent, several stents can be placed 
one after another with slight overlap at the ends. In addition, if the 
expansion strength of one stent is not sufficient, several stents can be 
placed one inside another to increase the dilating force at a specific 
point. 
Following placement in a blood vessel, the stent gradually becomes 
incorporated into the vascular wall by endothelial proliferation around 
the wires where they contacted the wall. This is similar to what has been 
noted in other studies where metal wire has been placed in the vascular 
system (2, 3, 4). Radiographic studies indicated that by one week after 
placement of the stent, sufficient endothelial proliferation had occurred 
to prevent migration, but during this first week, displacement was 
possible although not probable. After being in place for one month, the 
venous stents were approximately 80% encased by endothelium while the 
aortic stents were only about 30% encased. This difference is probably due 
to the greater flow and pressure in the aorta. By three months, all stent 
wires contacting the vessel wall were completely encased in endothelium. 
This incorporation into the vascular wall reduces thrombogenicity (3), but 
no clot was found even on the bare wires after 6 months. No cell growth 
was noted on any of the wire segments not in contact with the vascular 
wall, e.g., where stents bridged side branches. This observation 
corresponds with previous reports on the use of endovascular stainless 
steel wires (4). Therefore, the stents can bridge other vessels without 
occluding them or producing luminal narrowing at the branch points. This 
has not been reported for other types of endovascular stents (2, 3). Thus 
it appears that the stainless steel stents can be placed anywhere in the 
vascular system that will accomodate them. No luminal narrowing was noted 
in the stented vessels even after six months. This differs from the 
nitinol endovascular stents which have been shown to product luminal 
narrowing within 4 weeks due to fibrin deposition on the stent wires (1, 
2, 3). 
No clot formation was found on any of the stents at the time they were 
removed. This is similar to previously reported results (2, 3). No 
vascular erosion was seen probably because the vessels were normal and 
able to expand thus reducing the force of the stent wires against the 
vacular wall. 
Results from this evaluation indicate that these stents should find various 
clinical applications. These may include re-establishment of flow in veins 
compressed by neighboring tumor (superior vena cava syndrome), maintenance 
of vascular patency after percutaneous balloon dilations, and correction 
of incomplete, long, irregular vascular stenosis. In addition, it may be 
possible to use these stents in other systems such as the respiratory, 
biliary, and urinary tracts to reinforce collapsing structures from 
extrinsic compression from neoplasma, maintain the dilatation of a balloon 
dilated segment of ureter, urethra, or bowel, aortic dissection, aortic 
aneurysm, and localization of a chronic puncture site. 
While the invention has been illustrated and described in detail in the 
drawings and foregoing description, the same is to be considered as 
illustrative and not restrictive in character, it being understood that 
only the preferred embodiment has been shown and described and that all 
changes and modifications that come within the spirit of the invention are 
desired to be protected.