U.S. patent application number 10/793538 was filed with the patent office on 2004-09-02 for stent pattern with figure-eights.
Invention is credited to Ainsworth, Stephen D., Cheng, E Tina, Hong, James.
Application Number | 20040172128 10/793538 |
Document ID | / |
Family ID | 25400667 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040172128 |
Kind Code |
A1 |
Hong, James ; et
al. |
September 2, 2004 |
Stent pattern with figure-eights
Abstract
The invention is directed to an expandable stent for implanting
in a body lumen, such as a coronary artery, peripheral artery, or
other body lumen. The invention provides for an intravascular stent
having a plurality of cylindrical rings connected by links. The
rings have peaks and valleys from which extend straight and
nonlinear bar arms, forming a figure-eight The links connecting the
cylindrical rings may be straight, undulating, or include bounded
apertures for additional flexibility.
Inventors: |
Hong, James; (San Jose,
CA) ; Cheng, E Tina; (Union City, CA) ;
Ainsworth, Stephen D.; (Los Gatos, CA) |
Correspondence
Address: |
FULWIDER PATTON LEE & UTECHT, LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE
TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Family ID: |
25400667 |
Appl. No.: |
10/793538 |
Filed: |
March 3, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10793538 |
Mar 3, 2004 |
|
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09892889 |
Jun 27, 2001 |
|
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6749629 |
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Current U.S.
Class: |
623/1.16 |
Current CPC
Class: |
A61F 2/91 20130101; A61F
2002/91508 20130101; A61F 2002/9155 20130101; A61F 2250/0018
20130101; A61F 2002/91558 20130101; A61F 2/915 20130101; A61F
2002/91583 20130101; A61F 2002/91533 20130101; A61F 2230/0008
20130101 |
Class at
Publication: |
623/001.16 |
International
Class: |
A61F 002/06 |
Claims
We claim:
1. A stent including a longitudinal axis and a plurality of
connected cylindrical rings, with each ring having a plurality of
peaks and valleys, comprising: a plurality of generally straight
bar arms connected to a plurality of nonlinear bar arms having at
least one complete generally sine wave-shaped portion that includes
an open loop, the connected bar arms defining 1) pairs of
oppositely disposed open loops that form peaks and valleys, each
open loop having a first end and a second end, and each oppositely
disposed open loop having a first oppositely disposed end and a
second oppositely disposed end, 2) a diagonal bar arm connecting
the first end of each open loop to the second oppositely disposed
end of the oppositely disposed open loop, and 3) wherein the second
end of each open loop points toward but does not connect to the
first oppositely disposed end of the oppositely disposed open
loop.
2. The stent of claim 1, wherein the straight and nonlinear bar
arms have a first and second axis respectively, the axes being
parallel to each other, and wherein an adjacent peak and valley in
the same ring define a line parallel to the longitudinal axis of
the stent.
3. An intravascular stent for use in a body lumen, comprising: a
plurality of adjacent cylindrical rings aligned along a
longitudinal axis, each ring having proximal and distal ends, the
ends defining a generally cylindrical wall extending
circumferentially between the proximal and distal ring ends; a
plurality of non-linear bar arms comprising at least a portion of
each ring, the non-linear bar arms being disposed at the proximal
and distal ends of the rings and having proximal open loops with
first and second proximal open loop ends and oppositely disposed
distal open loops having first and second distal open loop ends;
wherein 1) the first proximal open loop end is diagonally connected
to the second distal open loop end, and 2) the second proximal open
loop end and the first distal open loop end generally point toward
each other, such that 3) a proximal loop diagonally connected to a
distal loop forms a figure eight; and at least one connector
joining each pair of adjacent rings.
4. The stent of claim 3, wherein the connector is a weld.
5. The stent of claim 3, wherein the figure-eight portions include
a straight bar arm and portions of two non-linear bar arms.
6. The stent of claim 3, wherein the nonlinear bar arms are of a
generally sinusoidal shape.
7. The stent of claim 3, wherein the straight bar arm also includes
a nonlinear, flexible link portion.
8. The stent of claim 7, wherein the flexible link portion includes
a bounded aperture.
9. An endovascular prosthesis, comprising: a plurality of adjacent
rings, each ring including linear bar arms connected to non-linear
bar arms and a plurality of peaks and valleys defined by the
connected linear and non-linear bar arms; and at least one link
between each adjacent pair of rings, the link being connected to
central portion of a non-linear bar arm on each ring.
10. The endovascular prosthesis of claim 9, wherein the linear bar
arm and non-linear bar arm are parallel.
11. The endovascular prosthesis of claim 10, wherein the at least
one link includes a non-linear, flexible link portion.
12. An expandable endovascular prosthesis, comprising: a pair of
expandable, undulating, end rings including figure-eight shaped
members; a plurality of interior, adjacent, expandable, undulating
rings including figure-eight shaped members and disposed between
the end rings; a plurality of links connecting adjacent rings;
wherein each figure-eight member includes an intermediate bar arm
and two adjacent portions of non-linear bar arms; and wherein the
connecting links include the non-linear bar arms and connect the
figure-eights of two adjacent rings.
13. The prosthesis of claim 12, wherein the figure-eights of each
interior ring are circumferentially unconnected.
14. The prosthesis of claim 12, wherein the figure-eights of the
end rings are circumferentially connected.
15. The prosthesis of claim 14, wherein each figure-eight of the
end rings has a crest that is connected to the non-linear bar arm
of an adjacent figure-eight.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of co-pending application
Ser. No. 09/892,889 filed on Jun. 27, 2001, which is herein
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to endoluminal prostheses such as
vascular repair devices, and in particular intravascular stents,
which are adapted to be implanted into a patient's body lumen, such
as a blood vessel or coronary artery, to maintain the lumen's
patency. Stents are particularly useful in the treatment of a
therosclerotic stenosis and are most frequently used in connection
with coronary angioplasty.
[0003] Stents are tubular, usually cylindrical devices which hold
open a segment of blood vessel or other body lumen. They also are
suitable to support and hold back a dissected arterial lining that
can occlude the lumen. At present, numerous models of stents are
marketed throughout the world. While some of these stents are
flexible and have the appropriate strength and rigidity needed to
hold open a lumen such as a coronary artery, each stent design
typically represents a compromise between the stent's flexibility
and its radial strength. What has been needed, and heretofore
unavailable, is a stent which has a high degree of flexibility so
that it can be advanced through tortuous lumen and readily
expanded, and yet have the mechanical strength to hold open the
lumen or artery into which it is implanted and provide adequate
vessel wall coverage.
[0004] At least some in the stent industry also perceive a problem
with "fishscaling." Fishscaling, describes the twisting or bending
of stent struts, which results in the struts not conforming to a
generally cylindrical plane around the circumference of the stent.
Fishscaling can result from the manufacturing process, as in the
case of the Medinol, Ltd. NIR.RTM. stent. Fishscaling also can
occur during the stent placement process, such as when portions of
the stent surface are forced outward as the stent bends while
advancing through tortuous lumen. Some in the stent art believe
that fishscaling can damage the blood vessel through which the
stent is being advanced. Therefore, there is a perceived need for a
stent that reduces or eliminates fishscaling.
INVENTION SUMMARY
[0005] The present invention is directed to an endoluminal
prosthesis, such as an intravascular stent, which is highly
flexible along its longitudinal axis to facilitate delivery through
tortuous body lumens, but which is strong and stable enough
radially in its expanded condition to maintain the patency of a
body lumen when the stent is implanted therein. The stent also
reduces fishscaling.
[0006] The stent of the present invention includes a plurality of
generally cylindrical elements, also known as rings, that are
interconnected to form the stent. The stent typically is mounted on
a balloon catheter if it is balloon expandable, or else it can be
mounted on a catheter without a balloon if it is
self-expanding.
[0007] Each of the cylindrical rings or elements has a proximal end
and a distal end and a cylindrical plane defined by a cylindrical
outer wall surface that extends circumferentially between the
proximal end and the distal end of the cylindrical ring. In the
preferred embodiment, cylindrical rings are interconnected by three
links which attach one cylindrical ring to an adjacent cylindrical
ring. The links are positioned substantially within the cylindrical
plane of the outer wall surface of the cylindrical rings. The
design of these highly flexible, interconnected members provides
for uniform scaffolding and a high percentage of vessel wall
coverage.
[0008] The cylindrical rings typically are formed of a plurality of
peaks and valleys. A straight strut, also called a bar arm, and a
curved, or nonlinear bar arm extend from each peak or valley. In
several preferred embodiments, this arrangement gives each ring the
appearance of a series of figure-eights. In this configuration, at
least one link attaches each cylindrical ring to an adjacent
cylindrical ring. In the preferred embodiments, each cylindrical
ring has six peaks and valleys and three to six connecting links.
The cylindrical rings and flexible links are preferably not
separate structures, although they have been conveniently referred
to separately for ease of identification.
[0009] Typically, a balloon expandable stent is made from a
stainless steel alloy or similar material. The cylindrical rings of
the stent are plastically deformed when expanded by the
balloon.
[0010] The links may take various configurations. One such
configuration is a straight link. Another is an undulating or
serpentine shape, which makes the links more flexible. The
undulating links can include bends connected by straight portions,
wherein the substantially straight portions are perpendicular to
the stent's longitudinal axis. Another configuration places one or
more apertures, such as an oval, rectangle, or dog bone shape, in a
straight link. The apertures are typically longer in one direction
than another, with the longer direction oriented preferably
perpendicular to the longitudinal axis of the stent. These links
are described in two U.S. patent applications by Ainsworth and
Cheng, Ser. No. 09/746,746, filed Dec. 22, 2000, and assigned to
Advanced Cardiovascular Systems, Inc., Santa Clara, Calif. (ACS)
the assignee of the present application, and of Ser. No.
09/564,151, filed May 3, 2000, also assigned to ACS. The entire
earlier applications are incorporated herein by reference.
[0011] In the case of the undulating links that interconnect the
cylindrical rings, the positioning of the unexpanded links also
enhances the flexibility by allowing uniform flexibility when the
stent is bent in any direction along its longitudinal axis. The
cylindrical rings of the stent can expand radially outwardly
without a balloon when the stent is formed from a superelastic
alloy, such as nickel titanium (NiTi) alloys. These so-called
"self-expanding" stents expand upon application of a temperature
change or when a stress is relieved, as in the case of a
pseudo-elastic phase change.
[0012] The number of peaks, valleys, links, and cylindrical rings
can be varied as the application requires. When using flexible
links, the link typically does not expand when the cylindrical
rings of the stent expand radially outwardly, but the links do
continue to provide flexibility and to also provide a scaffolding
function to assist in holding open the artery. Each flexible link
is configured so that it promotes flexibility.
[0013] The configuration of the rings provides the stent with a
high degree of flexibility along the stent axis, and reduces the
tendency of stent fishscaling. Further, because the links do not
expand or stretch when the stent is radially expanded, the overall
length of the stent is substantially the same in the unexpanded and
expanded configurations. In other words, the stent will not
appreciably shorten upon expansion.
[0014] Other embodiments of the invention also include figure-eight
type rings with a linear and non-linear bar arm. Ring orientation
and link shape vary the features of these other embodiments.
[0015] The stent is formed from a tube by laser cutting the pattern
of cylindrical rings and flexible links in the tube. The stent also
may be formed by laser cutting a flat metal sheet in the pattern of
the cylindrical rings and links, and then rolling the pattern into
the shape of the tubular stent and providing a longitudinal weld to
form the stent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is an elevation view, partially in section, of a
stent which is mounted on a rapid-exchange delivery catheter and
positioned within an artery.
[0017] FIG. 2 is an elevation view, partially in section, similar
to that shown in FIG. 1, wherein the stent is expanded within the
artery so that the stent embeds within the arterial wall.
[0018] FIG. 3 is an elevation view, partially in section, showing
the expanded stent implanted within the artery after withdrawal of
the rapid-exchange delivery catheter.
[0019] FIG. 4 is a flattened plan view of a stent pattern which
illustrates a preferred configuration of the present invention.
[0020] FIG. 5 is a plan view of a variation of the stent pattern
shown in FIG. 4.
[0021] FIG. 6 is a plan view of another variation of the stent
pattern shown in FIG. 4.
[0022] FIG. 7 is a plan view of a variation of the stent pattern in
FIG. 4, with an undulating type of flexible connecting link.
[0023] FIG. 8 is a plan view of another embodiment of the stent
pattern in FIG. 4, with a rectangular aperture as part of the
connecting link.
[0024] FIG. 9 is a plan view of a single, straight, connecting link
with a generally rectangular aperture.
[0025] FIG. 10 is a plan view of a single, straight, connecting
link with a generally oval aperture.
[0026] FIG. 11 is a detailed plan view of a single straight
connecting link with a generally dog bone-shaped aperture.
[0027] FIG. 12 is a plan view of the stent pattern with the
adjacent rings out-of-phase.
[0028] FIG. 13 is a plan view of another stent pattern with a body
cell like a figure-eight comprised of one straight and two
nonlinear bar arms.
[0029] FIG. 14 is a plan view of another embodiment with one
straight and two nonlinear bar arms.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] The present invention improves on existing endovascular
prostheses, such as stents, by providing a more flexible device
with a uniquely designed ring pattern and novel interconnecting
members or links. A significant aspect of the invention is the
arrangement of linear and non-linear bar arms, or struts, which in
several preferred embodiments form a figure-eight. In addition to
providing better longitudinal flexibility, the stent of the present
invention also provides good radial strength and a high degree of
scaffolding of a vessel wall, such as a coronary artery. The design
of the stent struts and the interconnecting members (also called
links or connectors, and, by some in the art, struts) and their
placements provide for uniform scaffolding and a high degree of
vessel wall coverage.
[0031] The present invention is the configuration of the material
used to make an endoluminal prosthesis such as a stent. In other
words, it is the stent pattern. Various embodiments, including the
figure-eight patterns 120, 520, 620, 720, are depicted in FIGS. 4
and 12-14, and will be discussed in detail below. Other
embodiments, such as those in FIGS. 5-8, will also be explained.
First, the general use of stents will be discussed.
[0032] FIGS. 1-3 can represent any balloon expandable stent 10 with
which the various configurations of the present invention can be
used. FIG. 1 depicts a stent 10 with interconnected cylindrical
rings 40 mounted on a catheter assembly 12 which is used to deliver
the stent 10 and implant it in a body lumen, such as a coronary
artery, peripheral artery, or other vessel or lumen within the
body. The catheter assembly includes a catheter shaft 13 which has
a proximal end 14 and a distal end 16. The catheter assembly is
configured to advance through the patient's vascular system by
advancing over a guide wire by any of the well-known methods of an
over-the-wire system (not shown) or a well-known rapid exchange
catheter system, such as the one shown in FIG. 1. The stent 10 in
FIGS. 1-3 conceptually represents any type of stent well-known in
the art--one comprising a plurality of undulating cylindrical rings
40. An example of such a stent is the Tetra.RTM. stent, made by
ACS.
[0033] Catheter assembly 12 as depicted in FIG. 1 includes an RX
port 20 where the guide wire 18 exits the catheter. The distal end
of the guide wire 18 exits the catheter distal end 16 so that the
catheter advances along the guide wire on a section of the catheter
between the RX (rapid exchange) port 20 and the catheter distal end
16. As is known in the art, the guide wire lumen which receives the
guide wire is sized for receiving various diameter guide wires to
suit a particular application. The stent is mounted on expandable
member 22 (e.g., an angioplasty balloon) and is crimped tightly
thereon, so that the stent and expandable member present a low
profile diameter for delivery through the arteries.
[0034] As shown in FIG. 1, a partial cross-section of an artery 24
has a small amount of plaque that has been previously treated by
angioplasty or other repair procedure. Stent 10 is used to repair a
diseased or damaged arterial wall as shown in FIG. 1, or a
dissection, or a flap, all of which are commonly found in the
coronary arteries and other blood. The stent 10, and the stent of
the present invention, also can be placed and implanted without any
prior angioplasty.
[0035] In a typical procedure to implant stent 10, the guide wire
18 is advanced through the patient's vascular system by well-known
methods, so that the distal end of the guide wire is advanced past
the plaque or diseased area 26. Prior to implanting the stent, the
cardiologist may wish to perform an angioplasty or other procedure
(i.e., atherectomy) in order to open and remodel the vessel and the
diseased area. Thereafter, the stent delivery catheter assembly 12
is advanced over the guide wire so that the stent is positioned in
the target area. The expandable member or balloon 22 is inflated by
well-known means so that it expands radially outwardly and in turn
expands the stent radially outwardly until the stent is apposed to
the vessel wall. The expandable member is then deflated and the
catheter withdrawn from the patient's vascular system. The guide
wire typically is left in the lumen for post-dilatation procedures,
if any, and subsequently is withdrawn from the patient's vascular
system. As depicted in FIGS. 2 and 3, the balloon is fully inflated
with the stent expanded and pressed against the vessel wall, and in
FIG. 3, the implanted stent remains in the vessel after the balloon
has been deflated and the catheter assembly and guide wire have
been withdrawn from the patient.
[0036] The stent 10 holds open the artery after the catheter is
withdrawn, as illustrated by FIG. 3. In the preferred embodiment,
the stent is formed from a cylindrical tube with a constant wall
thickness, so that the straight and undulating components of the
stent are relatively flat in transverse cross-section, so that when
the stent is expanded, its flat surface is pressed into the wall of
the artery, and as a result does not interfere with the blood flow
through the artery. After the stent is pressed into the wall of the
artery, it eventually becomes covered with endothelial cell growth
which further minimizes blood flow interference. The undulating
portion of the stent provides good tacking characteristics to
prevent stent movement within the artery. Because the cylindrical
rings 40 are closely spaced at regular intervals, they provide
uniform support for the wall of the artery, and consequently are
well adapted to tack up and hold in place small flaps or
dissections in the wall of the artery, as illustrated in FIGS. 2
and 3.
[0037] The stent 10 in FIG. 3 has fourteen cylindrical rings 40.
The rings are connected by links 50. For the purpose of the present
invention, the cylindrical rings 40 could also be connected by
welds in a manner similar to the "S" series of stents presently
sold by Medtronic Ave, Inc., or in some other manner.
[0038] FIGS. 4-14 depict various configurations and features of the
present invention. Turning to FIG. 4, a portion of a stent 110 is
shown in a flattened condition so that the pattern can be clearly
viewed, even though the preferred embodiment is not made this way.
The stent is typically formed from a tubular member, but it can be
formed from a flat sheet such as the portion shown in FIG. 4 and
rolled into a cylindrical configuration. Although welding flat
sheets or rings is not a preferred method of manufacture, it can be
used.
[0039] FIG. 4 represents three cylindrical rings 140 of stent 110.
The stent can have any number of rings, and, depending upon the
size of the rings, it is preferred that a stent of the present
invention have more than the three rings 140 shown in FIG. 4. For
reference, line A-A represents the longitudinal axis of a stent
using the pattern depicted in FIG. 4. Each cylindrical ring 140 has
a cylindrical ring proximal end 146 and a cylindrical ring distal
end 148. The proximal ring ends 146 and distal ring ends 148 can
also be considered to form a pattern of peaks and valleys, i.e., an
undulating shape. In the embodiment in FIG. 4, the stent axis A-A
passes through adjacent peaks and valleys. The ring ends, or peaks
and valleys, 146 and 148 are curved. They are connected by a linear
bar arm 150 and a nonlinear bar arm 152. Each cylindrical ring 140
is connected by one or more connecting links 154. For reference,
line B-B represents the longitudinal axis of linear bar arm 150,
while C-C represents the axis of nonlinear bar arm 152. The
preferred embodiment of the invention depicted in FIG. 4 shows B-B
and C-C parallel to each other. While this is a preferred
embodiment, the invention is not so limited. Nonlinear bar arm 152
preferably comprises at least one full, three hundred sixty degree
sine wave element 160. The sinusoidal portion 160 can possess more
pointed or more rounded peaks and valleys with more linear
connections, like a zigzag shape, and the same is true for the
peaks and valleys 146 and 148 of the cylindrical rings.
[0040] The present invention frequently results in a repeating
pattern of figure-eights. In FIG. 4, the figure-eight is made up of
a straight bar arm 150 and portions of non-linear bar arms 152. The
tops and bottoms of the figure-eight 120 are the peaks 146 and
valleys 148 of rings 140. The peaks and valleys are sometimes
generically referred to as crests. The selection of which crest is
a peak and which crest is a valley is arbitrary and done for ease
of reference. Those in the art will understand that, depending upon
one's reference, a peak can be a valley, and vice versa. Moreover,
those in the art will understand from context the meanings of peak,
valley and crest.
[0041] FIG. 5 represents a modified embodiment of the present
invention. Much of the pattern is the same. The principal
difference is that in FIG. 5 the axis C'-C' of nonlinear bar arm
152 forms an acute angle with axis B'-B+ of linear bar arm 150a.
The present invention is intended to cover all configurations in
which the angle between the axes of the bar arms 150 and 152 ranges
from zero degrees (i.e., the axes are parallel) to more than
90.degree.. This can be understood more fully in the context of
FIG. 4. Axes B-B and C-C intersect the longitudinal axis A-A of the
stent, forming angles X and Y. Because B-B and C-C are parallel,
acute angles X and Y are equal. The relationship between axes B-B
and C-C and their corresponding bar arms 150 and 152 can be altered
by changing angles X and Y. The change in the angles can be of
equal or different magnitude. As those in the art will appreciate,
such changes can affect the size and shape of each ring 140 and the
flexibility of the stent.
[0042] FIG. 6 is a partial stent pattern that represents another
embodiment of the present invention, in which the linear bar arm
150 has been replaced by an additional nonlinear bar arm 152. In
other words, all of the bar arms, also known as struts, of the
cylindrical rings of the stent comprise sinusoidally shaped or
generally undulating structures.
[0043] FIGS. 7-11 depict more variations of the present invention.
In particular they address the flexibility of the stent by
increasing the flexibility of the connecting link. This can be
accomplished in a number of ways. FIG. 7, for example, depicts a
link 170 comprised of a straight portion 166 and an undulating
portion 164. The undulating or corrugated links 164 permit the
stent to flex more than a straight link along the longitudinal
axis, which substantially enhances delivery of the stent to the
target site. The size and shape of the undulations can be varied to
achieve different degrees of flexibility. This is discussed more in
the above-referenced application, Ser. No. 09/746,746, assigned to
ACS by Ainsworth and Cheng.
[0044] FIGS. 8 and 9 depict another type of link that provides
improved stent flexibility in tortuous vessels. A portion of a
stent 210 has cylindrical rings 240 connected by links 254. Link
254 has a straight portion 260 that is preferably parallel to the
stent's longitudinal axis and a generally rectangular aperture 280
bounded on all sides. Link portions 282 define two sides of
rectangular aperture 280 and are generally perpendicular to the
stent's longitudinal axis. Link portions 284 connect the link
portions 282. Tapered link portions 285 are connected to
perpendicular link portions 282 at radii 287.
[0045] The increase in stent flexibility created by aperture 280
and the surrounding structure is easily understood. As a stent
passes through a curved vessel, half the stent is in tension at the
widest part of the curve, and the other half of the stent is in
compression, at the narrowest part of the curve. Thus, one half of
the stent wants to expand, and the other half wants to contract and
the link aperture permits either. As those in the art will
appreciate, less force is required to deform the structure bounding
aperture 280 than would be necessary to elongate or compress a link
if were a straight structure like link portion 260.
[0046] FIG. 10 is conceptually similar to FIG. 9. Aperture 380 is
shaped like an oval or ellipse, with the major axis of the ellipse
running perpendicular to the stent's longitudinal axis. In other
words, the long part of the ellipse 382 is perpendicular to the
stent's longitudinal axis, the short elliptical part 384 is
parallel to the longitudinal axis. The link 354 also includes
tapered portion 385 and radius portion 387. The structural portion
surrounding the elliptical aperture 380 responds to stress in much
the same way as the rectangular structure in FIG. 9. As the ellipse
is stretched in tension it becomes more circular and less
elliptical. As the ellipse is placed in compression, it becomes
more elliptical, approaching the shape of a thin rectangle, a slit,
or even two separate rounded apertures separated by a contact
point.
[0047] In the one embodiment of the flexible link with an oval or
elliptical aperture, the major and minor axis of an ellipse are
parallel and perpendicular to the longitudinal axis of the stent.
Thus, in a general sense, the ellipse can be thought of as similar
to a rectangle to the extent it has two long sides and two short
sides. Preferably, the long sides, i.e., those associated with the
major axis of the ellipse, are transverse to the longitudinal axis
of the stent.
[0048] FIG. 11 depicts a link 454 with a dumbbell or dog bone
shaped aperture 480. Straight portion 460 of link 454 intersects
transverse portion 482. The aperture 480 is bounded by transverse
and parallel portions 482 and 484 and four curved portions 486. As
tension is applied to link 454, radius portions 487 and curved
portions 486 tend to straighten as distance between transverse
portions 482 increases. When the link 454 is compressed, transverse
portions 482 will approach each other to the point of touching.
[0049] As one of ordinary skill in the art will appreciate, the
flexibility of links 254, 354, and 454 in FIGS. 9-11 can be
controlled by the dimensions of various portions of the links, such
as tapering straight portions 460 where it meets transverse portion
482. One could also, separately or in combination with the taper,
vary radius portion 487 or modify the widths of various structural
elements, such as portions 460, 482, 484 or 486. Although perhaps
obvious, it should be emphasized that the term straight is used in
a relative sense. In other words, straight portion 460 should be
considered straight despite the taper depicted in FIG. 11.
Similarly, linear bar arm 150 could have a varying width or
thickness at different points while still defining a straight line
following axis B-B. Common sense, however, means that nonlinear bar
arm 152 is not straight, despite having a straight axis.
[0050] FIG. 12 depicts another embodiment of the present invention.
This is a 6-crown, 3-link stent pattern 510 with one straight bar
arm 550 and one non-linear bar arm 552 that ultimately form a
series of figure-eights 520. Each ring 540 is positioned
"out-of-phase" relative to its adjacent ring, potentially
increasing flexibility relative to an "in-phase" configuration. The
preferred stent will comprise more than the rings shown in FIG. 12.
Starting from the proximal end 512 of the stent 510, straight links
554 are used, with the proximal end extending from approximately
the middle 553 of the non-linear bar arms 552 to the crests 546 in
the adjacent ring 540b. In the second ring 540b, there are no
linkage connections at the mid-section 553 of the non-linear bar
arms 552; instead the links are joined at the peak of the distal
crests 546 in the second ring 540b to the mid-section 553 of the
bar arms 552 in the third ring 540c. The linkage connection pattern
of the first two rings is therefore repeated. Although a straight
link is used in this pattern, a non-linear link can also be used in
this pattern without compromising the stent's profile.
[0051] FIG. 13 depicts yet another embodiment, stent pattern 610,
with figure-eights 620. This is a 6-crown, 6-link design. FIG. 13
depicts an exemplary five-ring stent section. The figure-eight
section of ring 640 uses one straight bar arm 650 and two
non-linear bar arms 652, extending out from upper crests 646 and
lower crests 648. The non-linear bar arms extend from the crests,
and follow through with a single turn. These non-linear bar arms
extend from the upper crest 646 of one ring 640 to the more distal
lower crest 648 of the adjacent ring. Hence, the non-linear bar
arms also act as connecting links 654 between adjacent rings. Due
to this unique connection, the crests of the interior rings 640 are
not connected circumferentially, but are connected longitudinally.
As for the end rings 641, a similar non-linear bar arm 652 extends
from the end crests 646, 648 and connects the body rings 640. In
addition, a secondary non-linear bar arm 653 connects the crests
646, 648 nearest the body and follows through in a U-loop to form a
connection with the head of the adjacent figure-eight element 620
in the same ring 641. The U-loop feature adds a greater expansion
range and serves to connect the figure-eights to form a cylindrical
structure. Hence, the crests 646,648 of the end rings 640 are
connected both circumferentially to one adjacent figure-eight
element 620 in the same ring and longitudinally to another
figure-eight element 620 in an adjacent ring. Although a straight
link is used in this pattern, a non-linear link can also be used
without compromising crimp profile.
[0052] Another embodiment, FIG. 14, is a 6-crown, 6-link partial
stent pattern 710 with figure-eights 720. Like pattern 610, it is
composed of one straight bar arm 750 and two non-linear bar arms
752 extending out from the upper and lower crests 646, 648 to form
the figure-eight. Stent pattern 710 combines features of stent
patterns 510 and 610. Pattern 710 uses the non-linear bar arms 752
to act as the links 754 between adjacent interior rings 740. Like
stent pattern 510 however, the rings 741 at the end of stent 710
have their links 754 connected to an interior portion of the
non-linear bar arm 752. Due to this unique configuration, the
crests 746,748 of the rings 741 in the interior of stent 710 are
not connected circumferentially, but are connected longitudinally.
Hence, the crests of the end rings 740 are connected both
circumferentially to each other and longitudinally to the adjacent
ring 740 of figure-eight elements. Although a straight link is used
in this pattern, a non-linear link can also be used without
compromising crimp profile.
[0053] Other embodiments of the invention, although not shown, are
easily developed and fall within the scope of the present
invention. One link may include more than one non-linear portion.
For example, one could create a straight link with two differently
shaped apertures. Alternatively, one could combine an undulating
link with an aperture. Links are bar arms could have varying
thickness.
[0054] The stent of the present invention can be made in many ways.
One method of making the stent is to cut a thin-walled tubular
member, such as stainless steel tubing, to remove portions of the
tubing in the desired pattern for the stent, leaving relatively
untouched the portions of the metallic tubing which are to form the
stent. In accordance with the invention, it is preferred to cut the
tubing in the desired pattern by means of a computer controlled
laser equipment, as is well known in the art Such methods are
described in U.S. Pat. Nos. 5,759,192 and 5,780,807 to Saunders,
which are incorporated herein by reference in their entirety.
[0055] The tubing may be made of suitable biocompatible material
such as stainless steel or another metal alloy. The stainless steel
tube may be alloy type: 316L SS, special chemistry per ASTM F138-92
or ASTM F139-92 grade 2. Special chemistry of type 316L per ASTM
F138-92 or ASTM F139-92 stainless steel for surgical implants in
weight percent.
1 Carbon (C) 0.03% max. Manganese (Mn) 2.00% max. Phosphorous (P)
0.025% max. Sulphur (S) 0.010% max. Silicon (Si) 0.75% max.
Chromium (Cr) 17.00-19.00% Nickel (Ni) 13.00-15.50% Molybdenum (Mo)
2.00-3.00% Nitrogen (N) 0.10% max. Copper (Cu) 0.50% max. Iron (Fe)
Balance
[0056] The tubing is mounted in a rotatable collet fixture of a
machine-controlled apparatus for positioning the tubing relative to
a laser. According to machine-encoded instructions, the tubing is
rotated and moved longitudinally relative to the laser, which is
also machine controlled. The laser selectively removes the material
from the tubing by ablation, thereby cutting a pattern into the
tube.
[0057] The process of cutting a stent pattern into the tubing is
automated, except for loading and unloading the length of tubing.
In one example, a CNC opposing collet fixture for axial rotation of
the length of tubing is used in conjunction with a CNC X/Y table to
move the length of tubing axially relatively to a
machine-controlled laser. The entire space between collets can be
patterned using the CO2 laser set-up of the foregoing example. The
program for control of the apparatus is dependent on the particular
configuration used and the pattern to be a blated in the
coating.
[0058] Cutting a fine structure (e.g., a 0.0035 inch web width)
requires minimal heat input and the ability to manipulate the tube
with precision. It is also necessary to support the tube yet not
allow the stent structure to distort during the cutting operation.
In order to successfully achieve the desired end results, the
entire system must be configured very carefully. The tubes for
coronary stents are made typically of stainless steel with an
outside diameter of 0.060 inch to 0.066 inch and a wall thickness
of 0.002 inch to 0.004 inch. Dimensions for peripheral stents and
other endoluminal prostheses may be different. These tubes are
fixtured under a laser and positioned utilizing CNC equipment to
generate a very intricate and precise pattern. Due to the thin wall
and the small geometry of the stent pattern, it is necessary to
have very precise control of the laser, its power level, the
focused spot size, and the precise positioning of the laser cutting
path.
[0059] Minimizing the heat input into the stent structure prevents
thermal distortion, uncontrolled burn out of the metal, and
metallurgical damage due to excessive heat, and thereby produces a
smooth debris free cut. A Q-switched Nd--YAG, typically available
from Quantronix of Hauppauge, N.Y., is utilized. The frequency is
doubled to produce a green beam at 532 nanometers. Q-switching
produces very short pulses (<100 nS) of high peak powers
(kilowatts), low energy per pulse (<3 mJ), at high pulse rates
(up to 40 kHz). The frequency doubling of the beam from 1.06
microns to 0.532 microns allows the beam to be focused to a spot
size that is 2 times smaller, therefore increasing the power
density by a factor of 4 times. With all of these parameters, it is
possible to make smooth, narrow cuts in the stainless tubes in very
fine geometries without damaging the narrow struts that make up the
stent structure. The system makes it possible to adjust the laser
parameters to cut a narrow kerf width, which minimizes the heat
input into the material.
[0060] The positioning of the tubular structure requires the use of
precision CNC equipment, such as that manufactured and sold by
Aerotech Corporation. In addition, a unique rotary mechanism has
been provided that allows the computer program to be written as if
the pattern were being cut from a flat sheet. This allows both
circular and linear interpolation to be utilized in
programming.
[0061] The optical system, which expands the original laser beam,
delivers the beam through a viewing head and focuses the beam onto
the surface of the tube. It incorporates a coaxial gas jet and
nozzle that help to remove debris from the kerf and cool the region
where the beam cuts and vaporizes the metal. It is also necessary
to block the beam as it cuts through the top surface of the tube
and prevent the beam, along with the molten metal and debris from
the cut, from impinging on the opposite, inner surface of the
tube.
[0062] In addition to the laser and the CNC positioning equipment,
the optical delivery system includes: a beam expander to increase
the laser beam diameter; a circular polarizer, typically in the
form of a quarter wave plate, to eliminate polarization effects in
metal cutting; provisions for a spatial filter; a binocular viewing
head and focusing lens; and, a coaxial gas jet that provides for
the introduction of a gas stream that surrounds the focused beam
and is directed along the beam axis. The coaxial gas jet nozzle
(0.018 inch I.D.) is centered around the focused beam with
approximately 0.010 inch between the tip of the nozzle and the
tubing. The jet is pressurized with oxygen at 20 psi and is
directed at the tube with the focused laser beam exiting the tip of
the nozzle (0.018 inch dia.). The oxygen reacts with the metal to
assist in the cutting process, similar to oxyacetylene cutting. The
focused laser beam acts as an ignition source and controls the
reaction of the oxygen with the metal. In this manner, it is
possible to cut the material with a very fine, precise kerf. In
order to prevent burning by the beam and/or molten slag on the far
wall of the tube I.D., a stainless steel mandrel (approx. 0.034
inch dia.) is placed inside the tube and is allowed to roll on the
bottom of the tube as the pattern is cut. This acts as a
beam/debris block protecting the far wall I.D.
[0063] Alternatively, burning may be prevented by inserting a
second tube inside the stent tube. The second tube has an opening
to trap the excess energy in the beam, which is transmitted through
the kerf and which collects the debris that is ejected from the
laser cut kerf. A vacuum or positive pressure can be placed in this
shielding tube to remove the collection of debris.
[0064] Another technique that could be utilized to remove the
debris from the kerf and cool the surrounding material would be to
use the inner beam blocking tube as an internal gas jet. By sealing
one end of the tube and making a small hole in the side and placing
it directly under the focused laser beam, gas pressure could be
applied, creating a small jet that would force the debris out of
the laser cut kerf from the inside out. This would eliminate any
debris from forming or collecting on the inside of the stent
structure. It would place all the debris on the outside. With the
use of special protective coatings, the resultant debris could be
easily removed.
[0065] In most cases, the gas utilized in the jets may be reactive
or non-reactive (inert). In the case of reactive gas, oxygen or
compressed air is used. Compressed air is used in this application
since it offers more control of the material removed and reduces
the thermal effects of the material itself. Inert gas such as
argon, helium, or nitrogen can be used to eliminate any oxidation
of the cut material. The result is a cut edge with no oxidation,
but there is usually a tail of molten material that collects along
the exit side of the gas jet that must be mechanically or
chemically removed after the cutting operation.
[0066] The cutting process utilizing oxygen with the finely focused
green beam results in a very narrow kerf (approx. 0.0005 inch) with
the molten slag re-solidifying along the cut. This traps some
scrap, thus requiring further processing. In order to remove the
slag debris from the cut, it is necessary to soak the cut tube in a
solution of HCL for approximately eight minutes at a temperature of
approximately 55.degree. C. Before it is soaked, the tube is placed
in an alcohol and water bath and ultrasonically cleaned for
approximately one minute. This removes the loose debris left from
the cutting operation. After soaking, the tube is then
ultrasonically cleaned in the heated HCL for one to four minutes,
depending upon the wall thickness. To prevent cracking or breaking
of the struts attached to the material left at the two ends of the
stent pattern due to harmonic oscillations induced by the
ultrasonic cleaner, a mandrel is placed down the center of the tube
during the cleaning and scrap removal process. At the completion of
this process, the stent structure is rinsed in water and is now
ready for electropolishing.
[0067] The stents are preferably electrochemically polished in an
acidic aqueous solution such as a solution of ELECTRO-GLO#300, sold
by ELECTRO-GLO Co., Inc,. Chicago, Ill., which is a mixture of
sulfuric acid, carboxylic acid, phosphates, corrosion inhibitors
and a biodegradable surface active agent. The bath temperature is
maintained at about 110.degree.-135.degree. F. and the current
density is about 0.4 to about 1.5 amps per in. Cathode to anode
area should be at least about two to one. The stents may be further
treated if desired, for example by applying a biocompatible
coating.
[0068] It will be apparent that both focused laser spot size and
depth of focus can be controlled by selecting beam diameter and
focal length for the focusing lens. It will be apparent that
increasing laser beam diameter, or reducing lens focal length,
reduces spot size at the cost of depth of field.
[0069] Direct laser cutting produces edges which are essentially
perpendicular to the axis of the laser cutting beam, in contrast
with chemical etching and the like which produce pattern edges
which are angled. Hence, the laser cutting process essentially
provides strut cross-sections, from cut-to-cut, which are square or
rectangular, rather than trapezoidal. The struts have generally
perpendicular edges formed by the laser cut. The resulting stent
structure provides superior performance.
[0070] Other methods of forming the stent of the present invention
can be used, such as chemical etching; electric discharge
machining; laser cutting a flat sheet and rolling it into a
cylinder; and the like, all of which are well known in the art at
this time.
[0071] The stent of the present invention also can be made from
metal alloys other than stainless steel, such as shape memory
alloys. Shape memory alloys are well known and include, but are not
limited to titanium, tantalum, nickel titanium and
nickel/titanium/vanadium. Any of the superelastic or shape memory
alloys can be formed into a tube and laser cut in order to form the
pattern of the stent of the present invention. As is well known,
the superelastic or shape memory alloys of the stent of the present
invention can include the type known as thermoelastic martensitic
transformation, or display stress-induced martensite. These types
of alloys are well known in the art and need not be further
described here.
[0072] Importantly, a stent formed of shape memory or superelastic
alloys, whether the thermoelastic or the stress-induced
martensite-type, can be delivered using a balloon catheter of the
type described herein, or in the case of stress induced martensite,
be delivered via a sheath catheter or a catheter without a
balloon.
[0073] While the invention has been illustrated and described
herein in terms of its use as an intravascular stent, it will be
apparent to those skilled in the art that the stent can be used in
other body lumens. Further, particular sizes and dimensions, the
configuration of undulations, number of crowns per ring, materials
used, and other features have been described herein and are
provided as examples only. Other modifications and improvements may
be made without departing from the scope of the invention. For
example, the cylindrical rings can be octagonal, hexagonal, or some
other polygon, thus possessing corners. Each ring is essentially a
short tube, (or hoop or ring) whose length is preferably shorter
than its diameter and which has a significant percentage of the
tube surface removed. Other modifications could include the use of
polymers in portions of the links and/or bar arms so that the stent
would be more radiopaque. Alternatively, one could place electrical
discontinuities in the stent to minimize the Faraday Cage effect
and make the stent more visible under Magnetic Resonance
Imaging.
[0074] While the present invention has been illustrated and
described herein in terms of a radiopaque nitinol stent, it is
apparent to those skilled in the art that the present invention can
be used in other instances. Other modifications and improvements
may be made without departing from the scope of the present
invention.
* * * * *