U.S. patent application number 14/090663 was filed with the patent office on 2014-03-27 for tapered helical stent and method for manufacturing the stent.
The applicant listed for this patent is Thomas O. Bales, JR., Scott Jahrmarkt, Charles Slater. Invention is credited to Thomas O. Bales, JR., Scott Jahrmarkt, Charles Slater.
Application Number | 20140088689 14/090663 |
Document ID | / |
Family ID | 39742440 |
Filed Date | 2014-03-27 |
United States Patent
Application |
20140088689 |
Kind Code |
A1 |
Bales, JR.; Thomas O. ; et
al. |
March 27, 2014 |
Tapered Helical Stent and Method for Manufacturing the Stent
Abstract
A tapered helical implantation device for peripheral arteries
and other body lumina of locally varying diameter, including struts
connected by loops and shaped in a helical winding. The helical
winding defines turns and, when expanded, has a first end with a
first expanded circumference and a second end with a second
expanded circumference greater than the first expanded
circumference to form a stent tapering outward from the first end
towards the second end. Bridges connect adjacent ones of the turns.
The struts, the loops, and the bridges define a series of pores
aligned along the helix, the pores having a substantially uniform
pore size. The pores has a substantially similar area. The pore
size limits a size of a particle that can pass therethrough. In
particular, the pore size limits a size of a spherical particle
that can pass therethrough.
Inventors: |
Bales, JR.; Thomas O.;
(Coral Gables, FL) ; Slater; Charles; (Fort
Lauderdale, FL) ; Jahrmarkt; Scott; (Miami,
FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bales, JR.; Thomas O.
Slater; Charles
Jahrmarkt; Scott |
Coral Gables
Fort Lauderdale
Miami |
FL
FL
FL |
US
US
US |
|
|
Family ID: |
39742440 |
Appl. No.: |
14/090663 |
Filed: |
November 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11715859 |
Mar 8, 2007 |
8623070 |
|
|
14090663 |
|
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Current U.S.
Class: |
623/1.16 |
Current CPC
Class: |
A61F 2002/91558
20130101; A61F 2/91 20130101; A61F 2002/91541 20130101; A61F
2002/91508 20130101; A61F 2/90 20130101; A61F 2/915 20130101; A61F
2/88 20130101; A61F 2250/0039 20130101; A61F 2002/91516 20130101;
A61F 2002/91525 20130101; A61F 2230/0078 20130101; A61F 2/885
20130101 |
Class at
Publication: |
623/1.16 |
International
Class: |
A61F 2/88 20060101
A61F002/88 |
Claims
1. A tapered helical implantation device for peripheral arteries
and other body lumina of locally varying diameter, comprising:
struts connected by loops and shaped in a helical winding, said
helical winding defining turns and, when expanded, having a first
end with a first expanded circumference, and a second end with a
second expanded circumference greater than said first expanded
circumference to form a stent tapering outward from said first end
towards said second end; bridges connecting adjacent ones of said
turns; and said struts, said loops, and said bridges defining a
series of pores aligned along said helix, said pores having a
substantially uniform pore size from said first end to said second
end even when a diameter of the stent varies along its length from
said first end to said second end to form the outwardly tapered
stent from said first end to said second end when expanded.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/715,859, filed on Mar. 8, 2007, the entire disclosure
of which is hereby incorporated herein by reference in its
entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
FIELD OF THE INVENTION
[0003] The present invention relates to vascular stents. In
particular, the invention relates to tapered helical stents for
peripheral arteries and other body lumina of locally varying
diameter, e.g., the carotid artery.
BACKGROUND OF THE INVENTION
[0004] Stents are used in various lumina of the body, including the
biliary tree, the venous system, peripheral arteries, and coronary
arteries. Stents open or hold open a lumen that has been blocked
(occluded) or reduced in size (stenosed) by some disease process,
such as atherosclerosis or cancer. Previously developed stents for
use in the biliary, venous, and arterial systems have been of two
broad classes: balloon-expanded and self-expanding. In both of
these classes, stents have been made by different techniques,
including forming from wire and machining from a hollow tube; such
machining may be done by photo-chemical etching, laser-cutting,
stamping, piercing, or other material-removal processes. Other
manufacturing techniques have been proposed, such as vacuum or
chemical deposition of material or forming a tube of machined flat
material, but those "exotic" methods have not been widely
commercialized.
[0005] The vast majority of stents for use in the arterial and
venous systems have been made by machining (typically, laser
machining) a pattern of struts and connecting elements from a
metallic tubular pre-form. Of these machined-tube stents, there
have been two basic architectures: circumferential and helical.
Circumferential configurations are based upon a series of
cylindrical bands joined longitudinally by bridges to make a
tubular structure. Helical configurations include a continuous
helical structure (typically made of a string of cells or an
undulating pattern of struts and end-loops) with joining structures
(referred to below as "bridges") joining adjacent turns of the
helix to provide mechanical integrity to the tubular structure (to
prevent unwinding, kinking, and buckling). As used herein, "struts"
are lengthwise elements aligned either along the axis of a stent or
perpendicular to the axis of the helical winding. Struts are
connected to one another by 180-degree connector curves, referred
to herein as "loops". Between adjacent turns of a helical winding,
are the "bridges," which join loops of one turn to nearby loops of
an adjacent winding.
[0006] Prior art helical stents are discussed in various patents
and publications. An exemplary set of such helical stents include
United States Patent Publication No. 2002/0116044 to Cottone et al.
(corresponding to International Publication no. WO 01/89421), and
U.S. Pat. No. 6,423,091 to Hojeibane, U.S. Pat. No. 5,913,897 to
Corso (forms stent with helically-wound serpentine wire shown in
FIG. 1), U.S. Pat. No. 6,042,597 to Kveen (forms stent by machining
a tube shown in FIG. 2), U.S. Pat. No. 6,013,854 to Moriuchi
(helical stent formed of angle-shaped repeating wire units as set
forth in FIG. 3), U.S. Pat. No. 5,922,021 to Jang (balloon-expanded
tapered stent as shown in FIG. 4), U.S. Pat. No. 5,938,697 to
Killion (stent having different diameters as shown in FIG. 5), U.S.
Pat. No. 5,061,275 to Wallsten (self-expanding prosthesis made by
braiding individual elastic metal wires shown in FIG. 6), U.S. Pat.
No. 5,741,333 to Frid (self-expanding stent constructed by plaiting
elastic metal wires as shown in FIG. 7), U.S. Pat. No. 6,652,576 to
Stalker (variable stiffness stent from heat treating),
International Publication Nos. WO 98/53759 to Yadav (carotid stent
formed from braided individual elastic wires shown in FIGS. 8 and
9) and WO 98/34668 to Roubin et al. (corresponding to United States
Patent Publication Nos. 2004/0267350A1 and 2003/0055490A1 and to
U.S. Pat. Nos. 6,764,506 B2, 6,475,236 B1, 6,106,548 A, and
5,827,321 A; stent made from a tube by laser machining as shown in
FIG. 10), and U.S. Pat. No. 6,569,193 to Cox (tapered
self-expanding stent with varying outward pressure as shown in
FIGS. 11 and 12)
[0007] Use of Self-Expanding Stents in Carotids and Other Tapered
Locations
[0008] Certain locations in the body have non-uniform diameters in
their natural, un-diseased state. For example, the carotid artery
tapers abruptly from a typical diameter of six to eight mm to a
diameter of four to six mm over a short length where the common
carotid artery branches to the internal carotid artery, an area
which is a common site for atherosclerotic disease. See, e.g., FIG.
13. Other locations, such as the common bile duct and the location
where the iliac artery joins the superficial femoral artery, have
sharply-tapered segments. When a physician wishes to implant a
stent in such a naturally tapered location or in a location where
disease processes have caused a sharp taper that is not resolvable
by angioplasty, it may be desirable to use a tapered stent so that
the stent does not apply inappropriately high forces to the
smaller-diameter portion of the vessel, duct, or other body
lumen.
[0009] The technique of stenting a tapered location varies,
depending upon whether a self-expanding or balloon-expandable stent
will be implanted. To implant a balloon-expandable stent in a
tapered location, it is necessary to use a tapered balloon or to
progressively dilate the vessel along its length; typically, a
non-tapered stent (one having uniform size and properties along its
length) is used and it is dilated more in the larger-diameter
portion of the lumen to be stented. However, in such a situation,
it may be advantageous to use a stent having different properties
along its length, so that the desired relationship of stent to
lumen may be maintained over the length of the stented region, even
though the diameter varies. In particular, it may be advantageous
to use a stent that, when expanded to the desired tapered shape,
has uniform stiffness, flexibility, metal coverage, opening size,
or other properties; only by making a stent with varying geometry
or properties along its length may this result be achieved.
[0010] Deployment Considerations
[0011] If a self-expanding stent is to be utilized in a tapered
lumen, it is advantageous for the stent to be sized appropriately
for the local diameter at each point in the lumen to be stented.
For example, as shown in FIG. 13, the common carotid artery might
be 7 mm in diameter, while the internal carotid artery might taper
to 5 mm in diameter. Accordingly, a stent introduced from the
common carotid artery (as is the common practice) should have a
proximal fully-expanded diameter of 8 mm and a distal
fully-expanded diameter of 6 mm, using the common practice of
applying a stent which has a fully-expanded diameter slightly
larger than the lumen to be treated. In addition to providing a
stent of the proper fully-expanded diameter for a given segment of
lumen, the stent could advantageously provide appropriate outward
forces and resistance to collapse for a lumen of that size.
[0012] Clinical Requirements
[0013] Constant Forces while Constrained in "Undersized"
Vessels
[0014] Because it is common practice to "oversize" a self-expanding
stent for the lumen to be treated (e.g., placing an 8 mm stent in a
7 mm artery), a self-expanding stent should generate the desired
outward pressure when the stent is compressed by an appropriate
amount, typically 1 mm to 2 mm for typical arteries.
[0015] Graduated Forces Along Stent Axis
[0016] In other clinical situations, it may be desirable to implant
a self-expanding stent that produces greater pressure against the
lumen wall at either the small or large end, according to the
decision of the physician in treating a particular diseased artery.
Thus, it may be advantageous to provide a tapered stent that exerts
different pressure along its length, not necessarily in proportion
to its local diameter. In fact, it may be advantageous to treat a
tapered segment of artery with a stent that expands freely to a
constant diameter along its length (i.e., a non-tapered stent) but
exerts graduated pressure along its length. Such a stent would not
appear tapered in a freely expanded condition but, because it would
exert a graduated amount of pressure along its length, it may still
be considered a tapered stent. For these reasons, a tapered stent
should achieve the desired graduated pressure along its length,
though it may or may not assume a tapered shape when fully and
freely expanded.
[0017] Retention Requirements for Self-Expanding Stents
[0018] In some cases, physicians prefer self-expanding stents to
have specific features on the distal end to aid in securing the
stent to the lumen wall as it is expanded into place. Such features
usually include arms, petals, or a circumferential ring of stent
material that engages the lumen wall as the stent is allowed to
expand into place.
[0019] Typically, self-expanding stents are introduced through the
body lumen to the position where the physician wishes to allow the
stent to expand to treat a local disease condition. Expansion is
carried out by most stent deployment systems progressively from the
distal end toward the proximal end by a retractable sheath covering
the stent and retaining the stent in its compressed state. Under
physician control, the covering sheath is withdrawn proximally,
exposing the distal end of the stent. As the compressed stent is
exposed, it expands. Stents are chosen to be of a size that, when
fully expanded, assumes a diameter larger than the lumen to be
treated; so, as the stent expands, it will contact the lumen wall,
creating a frictional engagement and preventing any relative
movement of the stent and lumen wall. This positive initial
engagement helps to assure that the deployed stent will remain in
the position intended by the physician.
[0020] To enhance this frictional engagement, it is common in the
art to form stents such that the most distal end of the stent
expands to a larger diameter than the middle portion of the stent
(when freely expanded). For symmetry, and perhaps to assure
frictional engagement at the proximal end of the stent as well, it
is common for such stents to have both distal and proximal ends so
formed. Thus, it is advantageous to provide a stent with an end or
ends that enhance the frictional engagement of the stent with the
lumen wall during deployment.
[0021] In non-helical designs, typically constructed with
circumferential rings joined axially to form a tubular structure,
the most distal and proximal ends are formed (in the fully-expanded
state) into a conical shape with the largest diameter at end of the
stent. Alternately, additional features may be added to the distal
and proximal ends of tubular structure to form this
outward-expanding conical shape. Such features take the form of
petals, struts, closed cells, tabs containing radiopaque markers,
and so forth.
[0022] End Treatments for Helical Stents
[0023] Helical stent configurations are different from non-helical
(hoop-plus-bridge) configurations because the ends of a helix do
not form an end that closely approximates the end of a right
cylinder (i.e., the end of a helix is not perpendicular to its
longitudinal axis). For example, Corso, (U.S. Pat. No. 5,913,897)
discloses a helical stent configuration with uneven ends.
[0024] It is known in the art to apply end treatments to helical
stents to make them approximate a right cylinder (or a right
conical cylinder, if the end is flared). One such treatment (e.g.,
Cottone) adds a transition segment and a cylindrical hoop to the
ends of the stent as shown in FIG. 14. Another method of making
even the end of a helical stent is taught by Hojeibane (U.S. Pat.
No. 6,423,091) and includes progressively lengthening the struts of
the final turn of the helix as shown in FIG. 15. A third method of
providing a perpendicular end to a helical stent is disclosed by
the inventor of the present application in U.S. Patent Publication
Nos. 2006/0060266, 2006/0064154, 2006/0064155, 2006/0064158, and
2006/0074480. In that method, a series of paddles is added to the
end of the stent; the length of the paddles is varied so that, even
though the helical array of stent struts ends in a helical plane,
the ends of the paddles are evenly aligned to create a right
cylindrical end to the stent as shown in FIG. 16.
[0025] All of these methods create a more even end for a helical
stent so that the ends of the stent engage the lumen wall in an
axially symmetrical way, without creating any local irregularities
where the helix ends. Also, by providing a substantially
perpendicular surface at the proximal end of the stent, the
engaging portions of the delivery system that exert axial pushing
forces on the proximal end of the stent during deployment are
simplified, and the pushing forces are more uniformly distributed
onto the stent, preventing buckling or distortion of the stent
during deployment. For these reasons it is advantageous to provide
a helical stent with one or both ends treated to approximate the
end of a right circular cylinder.
[0026] If the prior art stents are expanded into a non-cylindrical
shape (e.g., with a non-cylindrical balloon), then the openings
(referred to as pores herein) become asymmetrical. More
specifically, if a cylindrical stent having same size pores is
deformed to have one end be larger than the opposite end, the pores
at the smaller end will be smaller than the pores at the larger
end. The opening size of the pores defines the lower limit of
objects that can pass therethrough, e.g., emboli. If the pore
openings are constant, such as in the case of a prior art
cylindrical stent, then an upper emboli limit can be defined and
practiced. However, when such a cylindrical stent is deformed to a
taper, for example, the pores at the larger end are greater in size
than the pores at the smaller end. Such a configuration can
impermissibly permit passing of emboli greater than the desired
upper limit. It would, therefore, be beneficial to provide a
tapered stent having pores that are substantially uniform along the
length of the stent. As used herein, pores, pore size, and/or pore
openings refer to the openings that exist in an expanded form of
the stent and have four sides, two of which are defined by a first
set of struts and at least one loop and a second set of struts and
at least one loop on an adjacent turn of the helical winding. The
other two sides are defined by two adjacent bridges connecting the
turn on which the first set of struts exist to the adjacent turn on
which the second set of struts exist. Therefore, the size of the
pore is the area defined by these four sides when the stent is
expanded and also defines the upper emboli limit. The pore size can
also be defined by the size of the particle that can pass
therethrough or the area of the pore. Because helical stents are
typically cut from metal tubes, the pore boundary can also be
defined by a proportion of metal area to the total area within a
region, which proportion can be explained as a percentage of metal
within a given area of the stent.
[0027] It would be desirable, however, to provide a practical
non-cylindrical helical stent (i.e., tapered or flared shape),
whether self-expanding or balloon-expandable, having a varying
gradient of properties over the length thereof achieving a maximum
recovery performance and substantially no change in axial length
upon expansion. It would also be beneficial to provide measures for
smoothly changing the properties of a stent manufactured from a
tube from one end to the other.
[0028] Thus, a need exists to overcome the problems with the prior
art systems, designs, and processes as discussed above.
SUMMARY OF THE INVENTION
[0029] It is accordingly an object of the present invention to
provide a tapered helical stent and a method for manufacturing the
stent that overcome the hereinafore-mentioned disadvantages of the
heretofore-known devices and methods of this general type and that
provides a practical non-cylindrical helical stent having a
perpendicular end and a smoothly varying gradient of properties
over the length thereof achieving a maximum recovery performance
and substantially no change in axial length upon expansion, and
that provides measures for smoothly changing the properties of a
stent manufactured from a tube from one end to the other.
[0030] The tapered helical stent of the present invention provides
pores that are substantially uniform along the length of the stent.
Therefore, the tapered helical stent has the ability to define an
upper emboli limit, which limit can be explained by the area of the
cell, the boundary of the cell, the upper limit on size of a
particle that can pass therethrough, and/or the percentage of metal
in a given area of the stent. Accordingly, the tapered helical
stent of the present invention can be formed to have a
substantially constant pore area, a substantially constant pore
boundary, a defined upper particle size limit, and a constant
proportion of metal area to total area within a given region.
[0031] Constant Pressure at Each Point Along the Stent
[0032] In the case of self-expanding stents, the stent should
generate expansion forces (Chronic Outward Force, also referred to
as COF) and resistance to forces (Radial Resistive Force, also
referred to as RRF) appropriate for the lumen diameter at each
segment along its length.
[0033] While there is a paucity of clinical evidence relating
outward forces or pressure against the vessel wall with clinical
effects for different sizes and types of vessels, it is thought
that smaller vessels should require smaller expansion forces than
larger vessels. The invention of the instant application proposes
that the pressure against the vessel wall, in units of force per
unit area, should be approximately the same for large and small
vessels. Therefore, a tapered stent should apply an equal amount of
pressure to the lumen wall at every point along its length.
[0034] Balloon-expanded stents are not generally configured to
provide constant pressure against the vessel wall. However, the
expansion of the stent is achieved by pressurizing a balloon, so it
is advantageous if the resistance to expansion of the stent at each
point along its length is proportional to the intended vessel size;
thus, a balloon expanded by hydrostatic pressure (equal at all
points on the balloon surface) will cause the stent to expand to
the desired diameter at each point along its length. For example, a
stent configured according to such criterion and intended for a
vessel that tapers from 8 mm diameter to 6 mm diameter should
expand to these diameters when forced outward by a balloon inflated
to the appropriate pressure. Thus, according to the present
invention, similar design considerations are used to construct
tapered stents whether they are self-expanding or
balloon-expanded.
[0035] By providing a tapered helical stent that can linearly
increase features, the pores can be shaped to define openings both
at the smaller end and the larger end having virtually the same
shape and area.
[0036] With the foregoing and other objects in view, there is
provided, in accordance with the invention, a tapered helical
implantation device for peripheral arteries and other body lumina
of locally varying diameter, including struts connected by loops
and shaped in a helical winding, the helical winding defining turns
and, when expanded, having a first end with a first expanded
circumference, and a second end with a second expanded
circumference greater than the first expanded circumference to form
a stent tapering outward from the first end towards the second end,
bridges connecting adjacent ones of the turns, and the struts, the
loops, and the bridges defining a series of pores aligned along the
helix, the pores having a substantially uniform pore size.
[0037] With the objects of the invention in view, there is also
provided a tapered helical implantation device for peripheral
arteries and other body lumina of locally varying diameter,
including a helical winding of struts connected by loops, the
helical winding defining a first end, a second end, and helical
turns, the struts each having a length and a width, at least one of
the length of the struts, the width of the struts, and a frequency
of the struts per one of the turns being varied from the first end
to the second end to form a diameter-varying implantable stent
outwardly tapered from the first end to the second end when
expanded, bridges connecting adjacent ones of the turns, and the
struts, the loops, and the bridges defining a series of pores
aligned along the helix, the pores having a substantially uniform
pore size.
[0038] With the objects of the invention in view, there is also
provided an implantation device for lumina of locally varying
diameter, including a helical winding defining turns and, when
expanded, having a first end with a first expanded circumference
and a second end with a second expanded circumference greater than
the first expanded circumference to form a stent tapering outward
from the first end towards the second end, bridges connecting
adjacent ones of the turns, and the winding and the bridges
defining a series of pores aligned along the helix, the pores
having a substantially uniform pore size.
[0039] With the objects of the invention in view, there is also
provided a tapered helical implantation device for peripheral
arteries and other body lumina of locally varying diameter,
including struts connected by loops and shaped in a helical
winding, the helical winding defining turns and, when expanded,
having a first end with a first expanded circumference, and a
second end with a second expanded circumference greater than the
first expanded circumference to form a stent tapering outward from
the first end towards the second end, bridges connecting adjacent
ones of the turns, and the struts, the loops, and the bridges
defining a series of pores aligned along the helix, the pores being
shaped to permit particles no greater than a given size to pass
therethrough, the given size being substantially constant from the
first end to the second end.
[0040] With the objects of the invention in view, there is also
provided a tapered helical implantation device for peripheral
arteries and other body lumina of locally varying diameter,
including a helical winding of struts connected by loops, the
helical winding defining a first end, a second end, and helical
turns, the struts each having a length and a width, at least one of
the length of the struts, the width of the struts, and a frequency
of the struts per one of the turns being varied from the first end
to the second end to form a diameter-varying implantable stent
outwardly tapered from the first end to the second end when
expanded, bridges connecting adjacent ones of the turns, and the
struts, the loops, and the bridges defining a series of pores
aligned along the helix, the pores being shaped to permit particles
no greater than a given size to pass therethrough, the given size
being substantially constant from the first end to the second
end.
[0041] With the objects of the invention in view, there is also
provided a tapered helical implantation device for peripheral
arteries and other body lumina of locally varying diameter,
including metal struts connected by metal loops and shaped in a
helical winding, the helical winding defining turns and, when
expanded, having a first end with a first expanded circumference,
and a second end with a second expanded circumference greater than
the first expanded circumference to form a stent tapering outward
from the first end towards the second end, the first end having a
first end turn and the second end having a second end turn, metal
bridges connecting adjacent ones of the turns, and the struts, the
loops, and the bridges between the first and second end turns
having a substantially constant metal-to-total area proportion.
[0042] With the objects of the invention in view, there is also
provided a tapered helical implantation device for peripheral
arteries and other body lumina of locally varying diameter,
including a helical winding of struts connected by loops, the
helical winding defining a first end, a second end, and helical
turns, the first end having a first end turn and the second end
having a second end turn, the struts each having a length and a
width, at least one of the length of the struts, the width of the
struts, and a frequency of the struts per one of the turns being
varied from the first end to the second end to form a
diameter-varying implantable stent outwardly tapered from the first
end to the second end when expanded, bridges connecting adjacent
ones of the turns, and the struts, the loops, and the bridges
between the first and second end turns having a substantially
constant metal-to-total area proportion.
[0043] With the objects of the invention in view, there is also
provided a implantation device for lumina of locally varying
diameter, including a helical winding of struts connected by loops
and defining a first end, a second end, and helical turns, bridges
connecting adjacent ones of the turns, and the struts each having a
length and a width, at least one of the length of the struts, the
width of the struts, and a frequency of the struts per turn being
varied from the first end to the second end to form a
diameter-varying implantable stent outwardly tapered from the first
end to the second end when expanded.
[0044] With the objects of the invention in view, there is also
provided a implantation device for lumina of locally varying
diameter, including a helical winding of struts connected by loops
and defining a first end, a second end, and helical turns, bridges
connecting adjacent ones of the turns, and the struts each having a
length and a width, at least two of the length of the struts, the
width of the struts, and a frequency of the struts per turn being
varied from the first end to the second end to form a
diameter-varying implantable stent outwardly tapered from the first
end to the second end when expanded.
[0045] With the objects of the invention in view, there is also
provided a implantation device for lumina of locally varying
diameter, including a helical winding of struts connected by loops
and defining a first end, a second end, and helical turns, bridges
connecting adjacent ones of the turns, and the struts each having a
length varied from the first end to the second end to form a
diameter-varying implantable stent outwardly tapered from the first
end to the second end when expanded.
[0046] With the objects of the invention in view, there is also
provided a implantation device for lumina of locally varying
diameter, including a helical winding of struts connected by loops
and defining a first end, a second end, and helical turns, bridges
connecting adjacent ones of the turns, and the struts each having a
width varied from the first end to the second end to form a
diameter-varying implantable stent outwardly tapered from the first
end to the second end when expanded.
[0047] With the objects of the invention in view, there is also
provided a implantation device for lumina of locally varying
diameter, including a helical winding of struts connected by loops
and defining a first end, a second end, and helical turns, bridges
connecting adjacent ones of the turns, and the struts having a
frequency per turn varied from the first end to the second end to
form a diameter-varying implantable stent outwardly tapered from
the first end to the second end when expanded.
[0048] In accordance with another feature of the invention, the
winding defines a longitudinal axis of the stent and an axis of the
helical winding; and the struts extend one of longitudinally along
the longitudinal axis and perpendicular to the axis of the helical
winding.
[0049] In accordance with a further feature of the invention, the
bridges connect adjacent loops on adjacent ones of the turns.
[0050] In accordance with an added feature of the invention, the
bridges are placed at locations on the helix corresponding to at
least one of every third strut pair, every fifth strut pair, and
every seventh strut pair.
[0051] In accordance with an additional feature of the invention,
the bridges are disposed in a 5-up-and-5-down pattern or a
5-up-and-7-down pattern.
[0052] In accordance with yet another feature of the invention, the
stent has an overall length and a number of the struts per one of
the turns remains the same throughout the overall length.
[0053] In accordance with yet a further feature of the invention,
the struts increase in one of width and length from the first end
to the second end at least one of in steps, by linear proportion,
and by geometric proportion.
[0054] In accordance with yet an added feature of the invention,
the struts of each of the turns have the same size and the struts
increase in size from one of the turns to another of the turns from
the first end to the second end at least one of in steps, by linear
proportion, and by geometric proportion.
[0055] In accordance with yet an additional feature of the
invention, a density of the bridges is substantially even over the
overall length to have an equal number of the struts disposed
between adjacent bridges in all areas of the stent.
[0056] In accordance with again another feature of the invention, a
number of the struts per one of the turns is constant and a pattern
of the bridges remains constant along the overall length.
[0057] In accordance with again a further feature of the invention,
a density of the bridges is varied over the overall length to have
a different number of the struts disposed between adjacent ones of
the bridges in different areas of the stent.
[0058] In accordance with again an added feature of the invention,
a circumferential distance between ends of the struts at the second
end are substantially the same as a circumferential distance
between ends of the struts at the first end to create the
substantially uniform pore size.
[0059] In accordance with again an additional feature of the
invention, a number of the struts at the second end is
proportionally larger than a number of the struts at the first
end.
[0060] In accordance with still another feature of the invention,
the first end has a diameter between approximately 4 mm and
approximately 6 mm and the second end has a diameter between
approximately 6 mm and approximately 10 mm.
[0061] In accordance with still a further feature of the invention,
the stent has a length and, when expanded and placed in a lumen,
provides a constant hoop force over the length against the
lumen.
[0062] In accordance with still an added feature of the invention,
the stent has a constant metal ratio from the first end to the
second end.
[0063] In accordance with still an additional feature of the
invention, the lumen is one of a carotid artery, a common bile
duct, and a location where an iliac artery joins a superficial
femoral artery.
[0064] In accordance with another feature of the invention, the
stent is a balloon-expanded stent and has a resistance to expansion
at each point along a length of the stent proportional to an
intended vessel size in which the stent is to be implanted.
[0065] In accordance with a further feature of the invention, the
struts and the loops define a kerf having a width, the width of the
kerf between the struts defines the width of the struts, and the
kerf is varied along the stent to create a taper of the stent when
expanded.
[0066] In accordance with an added feature of the invention, a
width of the struts remains constant throughout the overall length,
a width of the kerf between the struts remains constant throughout
the overall length, a number of struts per one of the turns stays
the same along an entirety of the overall length, and the length of
the struts varies from the smaller end to the larger end.
[0067] In accordance with an additional feature of the invention,
both the width and the length of the struts transition in a
constantly increasing manner from the smaller end of the stent to
the larger end, the kerf transitions in a constantly decreasing
manner from the smaller end to the larger end, and a number of the
struts per one of the turns remains the same along an entirety of
the overall length.
[0068] In accordance with yet another feature of the invention,
both the width and the length of the struts transition in a
constantly increasing manner from the smaller end to the larger
end, the kerf remains constant throughout the overall length, and a
number of the struts per one of the turns decreases along the
overall length from the smaller end to the larger end.
[0069] In accordance with yet a further feature of the invention,
the length and the width of the struts and a count of the struts
varies along the overall length to have a number of the struts at
the smaller end be larger than a number of the struts at the larger
end.
[0070] In accordance with yet an added feature of the invention,
the stent is a tapered balloon-expanded stent and has a resistance
to expansion at each point along the overall length proportional to
an intended implantation vessel size.
[0071] In accordance with yet an additional feature of the
invention, the struts and the loops are shaped to expand
asymmetrically when the stent is expanded by a uniform expansion
force.
[0072] In accordance with again another feature of the invention,
the stent has a uniform resistance to expansion and has uniform
expansion properties over the overall length when expanded to an
asymmetric shape.
[0073] In accordance with again a further feature of the invention,
the stent is a self-expanding tapered helical stent and generates
expansion forces and resistance to external forces proportional to
an adjacent lumen diameter at each point along the overall length
to apply a substantially equal pressure to an implantation lumen
wall at every point along the overall length.
[0074] In accordance with a concomitant feature of the invention,
an opening angle between struts of a pair of the struts is uniform
over the overall length.
[0075] Other features that are considered as characteristic for the
invention are set forth in the appended claims.
[0076] Although the invention is illustrated and described herein
as embodied in a tapered helical stent and method for making the
stent it is, nevertheless, not intended to be limited to the
details shown and described because various modifications and
structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of
equivalents of the claims.
[0077] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof,
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
[0078] Thus, a need exists to overcome the problems with the prior
art systems, designs, and processes as discussed above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] The accompanying figures, where like reference numerals
refer to identical or functionally similar elements throughout the
separate views, which are not true to scale, and which, together
with the detailed description below, are incorporated in and form
part of the specification, serve to illustrate further various
embodiments and to explain various principles and advantages all in
accordance with the present invention. Advantages of embodiments of
the present invention will be apparent from the following detailed
description of the exemplary embodiments thereof, which description
should be considered in conjunction with the accompanying drawings
in which:
[0080] FIG. 1 is a fragmentary plan view of a portion of a first
prior art stent configuration;
[0081] FIG. 2 is a fragmentary plan view of a portion of a second
prior art stent configuration;
[0082] FIG. 3 is a fragmentary plan view of a portion of a third
prior art stent configuration;
[0083] FIG. 4 is a fragmentary plan view of a portion of a fourth
prior art stent configuration;
[0084] FIG. 5 is a fragmentary, side elevational view of a portion
of a fifth prior art stent configuration;
[0085] FIG. 6 is a fragmentary, side elevational view of a portion
of a sixth prior art stent configuration;
[0086] FIG. 7 is a fragmentary, side elevational view of a portion
of a seventh prior art stent configuration;
[0087] FIG. 8 is a fragmentary, side elevational view of an eighth
prior art stent configuration;
[0088] FIG. 9 is a fragmentary plan view of the stent configuration
of FIG. 8;
[0089] FIG. 10 is a perspective view of a ninth prior art stent
configuration;
[0090] FIG. 11 is a fragmentary plan view of a portion of a tenth
prior art stent configuration;
[0091] FIG. 12 is a fragmentary plan view of a portion of an
eleventh prior art stent configuration;
[0092] FIG. 13 is a fragmentary perspective view of the common,
internal, and external carotid arterial junction;
[0093] FIG. 14 is a fragmentary, side elevational view of a twelfth
prior art stent configuration;
[0094] FIG. 15 is a perspective view of a thirteenth prior art
stent configuration;
[0095] FIG. 16 is a fragmentary, plan view of a portion of a
fourteenth prior art stent configuration;
[0096] FIG. 17 is a fragmentary, enlarged, planar projection of a
portion of an exemplary configuration of a tapered helical stent
according to the invention;
[0097] FIG. 18 is a fragmentary, enlarged, planar projection of a
portion of another exemplary configuration of a tapered helical
stent according to the invention;
[0098] FIG. 19 is a fragmentary, enlarged, planar projection of a
portion of a further exemplary configuration of a tapered helical
stent according to the invention;
[0099] FIG. 20 is a fragmentary, enlarged, planar projection of a
portion of still another exemplary configuration of a tapered
helical stent according to the invention;
[0100] FIG. 21 is a perspective, enlarged, diagrammatic
representation of an exemplary configuration of a tapered helical
stent according to the invention in an expanded state;
[0101] FIG. 22 is an enlarged, side elevational, diagrammatic
representation of an exemplary configuration of a tapered helical
stent according to the invention in an expanded state;
[0102] FIG. 23 an enlarged, side elevational, diagrammatic
representation of the tapered helical stent of FIG. 22 with the
rear half removed;
[0103] FIG. 24 is an enlarged, perspective, diagrammatic
representation of an exemplary configuration of a tapered helical
stent according to the invention in an expanded state; and
[0104] FIG. 25 an enlarged, perspective, diagrammatic
representation of the tapered helical stent of FIG. 24 with the
rear half removed.
DETAILED DESCRIPTION OF THE INVENTION
[0105] While the specification concludes with claims defining the
features of the invention that are regarded as novel, it is
believed that the invention will be better understood from a
consideration of the following description in conjunction with the
drawing figures, in which like reference numerals are carried
forward.
[0106] Before the present invention is disclosed and described, it
is to be understood that the terminology used herein is for the
purpose of describing particular embodiments only and is not
intended to be limiting. It must be noted that, as used in the
specification and the appended claims, the singular forms "a,"
"an," and "the" include plural references unless the context
clearly dictates otherwise.
[0107] The present invention presents, for the first time, a
tapered stent having a helical configuration, referred to herein as
a tapered helical stent. Construction of a tapered helical stent
can be explained with reference to FIGS. 17, 18, and 19. In
general, helical stents include a continuous helical structure
(typically made of a string of cells or an undulating pattern of
struts and end-loops) with joining structures (bridges) connecting
adjacent turns of the helix to provide mechanical integrity to the
tubular structure and prevent unwinding, kinking, and buckling. If
the bridge patter is constant, prior art cylindrical helical stents
will define pores with a constant size--thereby defining an upper
size limit for a spherical particle that can pass through the stent
wall. This pore consistency is not present if such a stent is,
hypothetically, deformed to taper. The tapered helical stent of the
present invention, however, varies features of the stent to keep
constant the pore size and, therefore, keep constant the upper
spherical size limit for particles that can pass therethrough.
[0108] FIGS. 17 to 19 show three different embodiments of the
tapered helical stent of the present invention (bridges are omitted
for clarity). Each of the configurations show the helical structure
having struts 10 connected by loops 20. FIGS. 17 to 19 are planar
representations of the helical structure, much like a Mercator
projection of the spherical Earth. Here, the approximately tubular
form is cut longitudinally and flattened. To illustrate the single
helical winding connection of the structures of FIGS. 17 to 19,
dashed vertical lines 30 are included (four lines in each of the
figures). These lines 30 indicate the junction of two halves of a
single loop 20. In these figures, the struts 10 are aligned along
the axis of the stent 1, but they also can be aligned perpendicular
to the axis of the helical winding (as in the configuration of FIG.
2). Each of the five planar segments shown in FIGS. 17 and 19 will
be referred to as a "turn" of the helical winding. Between adjacent
turns of this winding are the non-illustrated bridges, which join a
loop of one turn to another nearby loop of an adjacent turn and
which are described in further detail below and shown in FIG.
20.
[0109] There are three characteristics of the helical winding that
permit the construction of the tapered helical stent 1 of the
present invention: strut width, strut length, and kerf (which is
the size and shape of the cut that creates the struts 10 and loops
20. As set forth above, helical stents are typically cut from a
solid tube of material (e.g., with a laser). The width of the kerf
between the struts is used by the present invention to define the
width of the struts 10 and the size of this kerf can be varied
along a stent to create the taper of the instant invention.
Heretofore, construction of a stent having varying strut width or
strut length or kerf was not possible. With the ability of the
present invention to vary these three characteristics, the tapered
helical stent becomes possible.
[0110] A first exemplary stent configuration according to the
invention is illustrated in FIG. 17. Here, the width of the struts
10 remains constant throughout the length of the stent 1. Also, the
width of the kerf between the struts 10 remains constant throughout
the length of the stent 1. This means that the number of struts 10
per turn stays the same along the entire length of the stent 1. It
is the strut length that varies from the smaller end 2 of the stent
1 to the larger end 3 in this embodiment.
[0111] A second exemplary stent configuration according to the
invention is illustrated in FIG. 18. Here, both the width and the
length of the struts 10 transition in a constantly increasing
manner from the smaller end 2 of the stent 1 to the larger end 3 of
the stent 1. The kerf, in comparison, transitions in a constantly
decreasing manner from the smaller end 2 of the stent 1 to the
larger end 3 of the stent 1. Like the first configuration, the
number of struts 10 per turn remains the same along the entire
length of the stent 1.
[0112] One illustrative exemplary configuration for this tapered
helical stent has a single turn length of 5.6549 mm--the turn
length is equal to the length of the vertical lines 30. This stent
is selected to have a starting strut length of 1 mm and a starting
strut width of 0.054 mm. For a stent having a total longitudinal
length of 40 mm, the ending strut length is 1.929 mm and the ending
strut width is 0.11 mm.
[0113] A third exemplary stent configuration according to the
invention is illustrated in FIG. 19. Here, both the width and the
length of the struts 10 transition in a constantly increasing
manner from the smaller end 2 of the stent 1 to the larger end 3 of
the stent 1. In contrast to FIG. 18, here the kerf remains constant
throughout the length of the stent 1. Therefore, the number of
struts 10 per turn decreases along the length of the stent 1 from
the smaller end 2 of the stent 1 to the larger end 3 of the stent
1.
[0114] By varying or keeping constant any of the strut length, the
strut width, and the kerf, the helical stent can be configured to
have any desired kind of taper. Variables for creating a final
tapered helical stent include: [0115] apex offset (longitudinal
distance from the end of the kerf at an apex to the end of the
apex; see reference numeral 22 in FIG. 18); [0116] desired first
strut length; [0117] desired last strut length; [0118] outer tube
diameter (O/D); [0119] start angle (of strut offset); [0120] strut
length growth percentage; and [0121] either: [0122] strut
length-to-width ratio; or [0123] desired first and last strut
widths.
[0124] With defined values for each of these variables, a
customized (or standardized) tapered helical stent can be
created.
[0125] FIG. 20 is an exemplary configuration for a tapered helical
stent 1 of the present invention. This exemplary stent 1 varies the
strut length, the strut width, and the strut count so that the
number of struts 10 at the smaller end 2 is slightly larger
(twenty-four pairs) than the number of struts 10 at the larger end
3 (twenty-two pairs). Bridges 40 are illustrated in FIG. 20 and
connect a loop 20 on one side of a turn to a loop 20 on an opposing
side of an adjacent turn. Here, the bridges 40 are in a
5-up-and-5-down pattern. It can be seen in FIG. 20 that bridges 40
connect each of the first turn struts 10 to one strut 10 in the
second turn. This configuration provides a good balance of flexural
stiffness and integrity in the main body of the stent with an
increased resistance to buckling at each end, providing greater
ability to accommodate the forces applied to deploy the stent; this
design also provides a gradual transition from a smaller diameter
of 6 mm to a larger diameter of 8 mm after expansion. By providing
a larger number of struts (and, hence, opening angles) at the small
end of the stent, the outward forces at that end would be lower
than those at the larger end of the stent.
[0126] In an exemplary embodiment where the number of struts per
turn stays the same (see FIGS. 17 and 18), a tapered helical stent
can be created with a larger end 3 opening of 8 mm and a smaller
end opening of 6 mm. For example, a 5-up-and-7-down configuration
can be used to create a uniform pattern of bridges in a 46
strut-per-turn design as shown in FIGS. 17 and 18. The number of
struts 10 for the 8 mm opening is selected. Then, a strut length
and a strut width are determined for a 6 mm opposite end having the
same number of struts 10. The selection criteria for this
embodiment include keeping the same opening angle in the expanded
condition. This means that the 6 mm end has struts 75% as long as
the 8 mm end. Because the strut width is proportional to the strut
length, the 6 mm end has struts 75% as wide as the 8 mm end.
[0127] In one variation of struts 10 in such a stent, the struts 10
change smoothly from the 8 mm end to the 6 mm end, each one being
different in length and width. Such a configuration is illustrated
in FIGS. 17 and 18, for example. Alternatively, in a
non-illustrated configuration, the struts 10 can change in jumps,
with all of the struts in one turn, for example, being the same
length and width. Accordingly, for a fifty-turn 50 mm stent, fifty
different strut geometries can exist, for example. In another
exemplary configuration, three segments can be created for the
stent (one for 6 mm, one for 7 mm, and one for 8 mm) with the strut
characteristics changing from one step to the other.
[0128] As set forth above, a further exemplary tapered helical
stent according to the present invention can be configured to have
a smooth or gradual incremental progression of either the number of
struts per unit length or the strut size from one end of the stent
to the other, or even both the number of struts and the strut size.
Such a gradual change in struts per turn and/or strut size cannot
be achieved in a stent configured according to the prior art,
either by the known cylindrical-hoop structures or the uniform
helical structures. When the number of struts per unit length and
their dimensions are varied in a progressive manner, when the stent
is expanded, the properties at each point along the length of the
stent are appropriate for a stent (and, hence, the stented lumen)
of that size.
[0129] Alternately, the struts may be varied in groups; for
example, groups of ten or twenty struts may all have the same
length. The desired result is that the incremental changes are
sufficiently small that there is no substantial difference in
properties from one group of struts to the next. There are
advantages of both approaches: by adjusting the length of each
successive pair of struts, the resulting pattern of bridges has
smaller dislocations; but, by having groups of pairs of struts of
equal length, the design process is simplified, analysis is
simplified, and manufacturing inspections are reduced because there
is a smaller number of distinctly sized features. There are other
alternative embodiments of the expanded tapered stent. For example,
groups of strut pairs can be varied in length and width over the
length of the taper. In another example, every three strut pairs
could have the same dimensions.
[0130] A configuration and geometry of bridges is chosen to provide
the desired amount of flexibility and integrity. In general, a
higher number of bridges provide a stent with greater resistance to
deformation but reduced flexibility and resistance to fatigue and a
lower number of bridges provide a stent with less resistance to
deformation but increased flexibility.
[0131] In a first exemplary embodiment of the present invention,
the density of bridges is maintained substantially even over the
length of the stent such that there is an approximately equal
number of struts disposed between adjacent bridges in all areas of
the stent. If the number of struts per turn is maintained constant,
and the properties of the stent are varied by some characteristic
other than circumferential frequency, the bridge pattern may remain
constant along the length of the stent.
[0132] In a second exemplary embodiment of the present invention,
the pattern of the bridges remains constant along the length of the
stent, but the attachment points are adapted to the configuration
of the struts, which can vary in length, width, or both.
[0133] In a third exemplary embodiment of the present invention,
the density of bridges is varied over the length of the stent so
that there is a different number of struts disposed between
adjacent bridges in different areas of the stent. When the number
of struts per turn is varied along the length of the stent, there
are discontinuities in the bridge configuration. Accordingly, there
is not a constant mapping relationship between the loops of one
turn and the loops of the adjacent turns because there would be
differing numbers of loops in the adjacent turns. Thus, the bridges
are adapted to the struts.
[0134] A conventional kerf shape between two struts 10 and a loop
20 is a rectangle with a rounded end as shown, for example, in the
left-most turn of FIG. 18 (smaller end 2). The "inside" kerf can be
kept small at this hairpin end by forming triangular windows at the
hairpin end. Alternatively, the inside hairpin radius can be opened
up, making it less likely to crack or break at this point, as
illustrated in FIG. 18.
[0135] If a stent 1 according to the invention is fully expanded,
it may look like the configuration shown in FIG. 21. It is noted
that the configuration of FIG. 21 is merely an illustrative
representation of a stent according to the invention because, in
practice, the change from 6 mm to 8 mm, for example, will be barely
perceptible to the naked human eye. Therefore, FIG. 21 has an
enhanced configuration that is exaggerated so that features of a
stent according to the invention can be explained. A side elevation
of such an expanded tapered helical stent 1 is shown in FIG. 22. As
can be appreciated, because of the number of struts, it is
difficult to differentiate the rear side of the stent 1 from the
front side. Accordingly, the rear half of the stent 1 of FIG. 22
has been removed in FIG. 23. An isometric perspective view is shown
in FIG. 24 and, like, FIG. 23, the representation in FIG. 25 has
the rear half removed for clarity. It is noted that FIGS. 21 to 25
are merely approximate representations of a stent that can be made
according to the invention. They are not mathematically accurate
and, especially, they are not drawn to scale.
[0136] A helical stent according to the present invention having a
tapered shape when expanded may be configured as a
balloon-expandable stent or as a self-expanding stent. Construction
considerations differ for these two applications.
[0137] Balloon-expanded stents are not generally configured to
provide constant pressure against the vessel wall. However, the
expansion of the stent is achieved by pressurizing a balloon, so it
is advantageous if the resistance to expansion of the stent at each
point along its length is proportional to the intended vessel size.
Thus, a balloon expanded by hydrostatic pressure (equal at all
points on the balloon surface) will cause the stent to expand to
the desired diameter at each point along its length. Even if the
stent is tapered, the balloon will still impart the same force at
each longitudinal point. For example, a stent configured according
to such criterion and intended for a vessel that tapers from 8 mm
diameter to 6 mm diameter should expand to these diameters when
forced outward by a balloon inflated to the appropriate pressure
and such a stent will have a resistance to expansion at each point
along its length proportional to the intended vessel size.
[0138] Because balloon-expanded stents must be made of malleable
material, flexibility must be imparted primarily by inelastic
deformation of the struts and the connecting bridges, which must be
made slender to minimize the force needed to bend the stent.
Flexibility of balloon-expanded stents is primarily of importance
during the process of introducing the stent into the anatomy (while
it is crimped onto a delivery balloon) and during expansion. Once
expanded into the anatomy, the amount of flexibility of a
balloon-expanded stent is minimal because of the low yield stress
of materials suitable for balloon-expanded stents (e.g., annealed
stainless steel, annealed chromium-cobalt alloys, gold, or platinum
alloys).
[0139] Accordingly, balloon-expanded stents are typically
configured with slender struts and connecting bridges. Even when
crimped to a small diameter for delivery, there is a significant
amount of void space in a balloon-expanded stent. The slender
elements, nonetheless, provide substantial rigidity when the stent
is expanded because of their high modulus of elasticity (Young's
Modulus) and some amount of cold work during expansion that raises
the local yield strength. Also, expanded stents typically have cell
structures with wider-open angles, which raise the circumferential
stress needed to collapse them (the "cosine effect").
[0140] In an exemplary embodiment of a tapered helical stent 1
according to the present invention optimized as a balloon-expanded
stent (Embodiment 1.sub.BE), the stent 1 is constructed of a
malleable alloy (e.g., platinum-5% iridium) having a thickness of
60 microns. The struts 10 have a width equal to 0.1 times the
length of the struts 10. In Embodiment 1.sub.BE, the size of the
struts 10 is kept proportional to the desired expanded diameter at
each station along the stent 1. In this example (a conical or
tapered stent), the distal end 3 expands to 4.0 mm diameter and the
struts 10 are 2.0 mm in length. At the proximal end 2, which
expands to 3.0 mm diameter, the struts are 1.5 mm in length. Along
the length of the stent (from the proximal end 2 to the distal end
3), the length of the struts 10 may be varied such that each
successive strut pair is incrementally longer. The width of the
struts 10 is also varied according to a mathematical relationship
with the strut length; in Embodiment 1.sub.BE, the width is
maintained proportional to the length of the strut 10. In
Embodiment 1.sub.BE, the stent 1 has twelve pairs of struts 10 per
helical turn throughout the length of the stent.
[0141] The bridge configuration of the tapered helical stent 1 is
chosen to provide sufficient integrity during introduction and
deployment without adversely affecting the flexibility of the stent
1 once deployed. In Embodiment 1.sub.BE, for example, an array of
four bridges 40 per helical turn at the distal (larger) end 3 would
decrease in the middle of the stent to three bridges 40 per helical
turn.
[0142] Balloon-expanded stents 1 according to the present invention
may assume a tapered expanded shape either by the application of a
varying expansion force over the length of the stent 1 (for
example, by pressurization of an asymmetrical balloon, or by
successive pressurizations of a balloon at different pressures) or
by the application of a uniform expansion force (for example,
uniform pressure in a symmetrical balloon) over the length of the
stent, or by a combination of these measures. The stent 1 may be
configured to expand asymmetrically when expanded by a uniform
expansion force (by varying the resistance to expansion of the
stent over its length), or the stent 1 may be configured with
uniform resistance to expansion, but with the proper structure to
provide uniform (or otherwise programmed) properties over the
length of the stent 1 when expanded to an asymmetric shape, such as
a taper. In one exemplary embodiment, groups of strut pairs can be
varied in length and width over the length of the balloon-expanded
stent 1. In another exemplary embodiment, groups including three
strut pairs could have the same dimensions. As a further example,
rather than keeping the same bridge pattern over the length of the
balloon-expanded stent 1, the pattern may be varied. In Embodiment
1.sub.BE, for example, rather than having a lower number of bridges
40 at the smaller end 2 of the stent 1, it is possible to have the
same bridge configuration (for example, four bridges 40 per helical
turn) over the entire length of the stent 1.
[0143] In the case of a self-expanding tapered helical stent 1, the
stent 1 can be made to generate expansion forces (Chronic Outward
Force) and resistance to external forces (Radial Resistive Force)
appropriate for the lumen diameter at each segment along its
length. Such a tapered helical stent might apply an equal amount of
pressure to the lumen wall at every point along its length.
[0144] A self-expanding tapered helical stent 1 according to the
present invention is made from superelastic material (e.g., Nitinol
alloy, gold superelastic alloys, or other nickel and titanium
containing superelastic alloys known to those skilled in the art)
by laser-machining from a tube. Because of the lower stiffness of
superelastic materials, which exert the substantially same force
over a wide range of deformations, it is customary to construct
self-expanding stents to contain the maximum possible amount of
metal, consistent with collapsing the stent for insertion into a
delivery system catheter. Accordingly, the struts 10 are made as
wide as possible. The stent designer may choose a number of struts
10 per helical turn, making the struts 10 as wide as possible by
separating them one from another by the smallest practicable
laser-cut kerf. The length of the struts 10 is, then, determined by
a mathematical relationship determined by the designer that will
assure that the stent 1 can be expanded and collapsed and will be
durable when affected by bending, torsion, extension-compression,
pinching, pulsatile radial compression-expansion, and other
performance requirements.
[0145] An exemplary embodiment for a self-expanding tapered helical
stent 1 includes a conical shape (being tapered shape when fully
expanded) tapering from 8 mm diameter to 6 mm diameter (Embodiment
2.sub.SE). This stent 1 is machined from a tube of Nitinol with an
outside diameter of 1.72 mm and a wall thickness of 300 microns.
After polishing, the thickness of the stent is approximately 275
microns. The number of struts 10 per revolution is selected to be
36 and constant over the length of the stent 1. The struts 10 vary
in length from 2 mm at the larger end 3 to 1.5 mm at the smaller
end 2. Strut width, proportional to the length, varies from 110
microns to 80 microns over the length. The pattern of bridges 40 is
constant over the length of the stent 1 at six per helical
turn.
[0146] Self-expanding stents are heat-treated while being held in a
certain configuration ("heat setting") so that they have a
memorized shape. To achieve a self-expanding stent that has a
tapered fully expanded shape according to the present invention,
the stent 1 is held in the desired shape (possibly, by placing it
onto a tapered tool) and heating it to approximately 450.degree. C.
(depending upon the properties of the superelastic material being
used) for approximately three minutes. Once cooled, the stent 1
will remain in that shape, and it will recover to that shape if
unconstrained after being expressed from its delivery system.
[0147] Although Embodiment 2.sub.SE will recover to a tapered shape
if unconstrained, it is possible to make a stent 1 that is heat-set
to a non-tapered shape, yet it might assume a tapered shape in a
tapered body lumen. For example, a stent 1 can be configured with
geometry or other properties varying over its length as would be
appropriate for use in a tapered body lumen, yet it might be
heat-set to a non-tapered shape to achieve certain desired results,
such as reduced outward pressure in the large-diameter segment or
for prevention of over-expansion in aneurismal lumina. In either
embodiment, groups of strut pairs can be varied in length and width
over the length of the tapered self-expanding stent 1. For example,
groups of three strut pairs could have the same dimensions. Also,
rather than keeping the same bridge pattern over the length of the
self-expanding stent, the bridge pattern may be varied. For
example, in Embodiment 2.sub.SE, rather than maintain a pattern of
six bridges 40 per helical turn, it is possible to configure a
lower number of bridges 40 at the smaller end 2 of the stent 1. It
should be noted that, although certain bridge patterns repeat
uniformly, other bridge patterns are possible. After placing a
bridge 40 every three pairs of struts 10 (i.e., six bridges per
helical turn in a stent with eighteen pairs of struts 10 per
helical turn) at the larger end 3 of the stent 1 (as described in
Embodiment 2.sub.SE) it is possible to place bridges closer
together at the smaller end 2, for example, every four pairs.
[0148] More complicated patterns of bridges may also be chosen; for
example, a bridge 40 may be placed four pairs after the first
bridge 40, then after five pairs, then after four pairs, then after
five pairs, and so forth. It should be appreciated that, in a
tapered helical stent configuration, it is not necessary for each
turn of the helix to have the same number of bridges 40, nor does
there need to be a repeating pattern of bridges 40 from turn to
turn.
[0149] The stent 1 according to the invention, in any of the
embodiments set forth above (whether self-expanding or balloon
expanded), is not limited to changing the strut length, the strut
width, or the kerf. The struts 10, loops 20, and bridges 40 can be
varied by other measures. First, the thickness of the struts 10,
loops 20, and bridges 40 may be varied over the length of the stent
1 to generate a tapered stent shape when expanded. Such a stent can
be produced by laser-machining the desired pattern into a tube that
has wall thickness varying from one end to the other; such a
variable wall thickness can be achieved by honing a tapered bore
into a tube or by grinding or polishing the outer surface of the
tube to remove more wall thickness at one end than the other. For
example, the wall thickness of 275 microns at one end of the stent
may be reduced to 200 microns at the opposite end. Second, the
material from which any part of the stent 1 is made (struts 10,
loops 20, bridges 40) may be varied over the length of the stent 1
to generate the tapered shape when the stent 1 is expanded, as
might be achieved by heat-treating, ion-implanting, or alloying one
end of the tube differently than the other. Third, by applying
coatings, platings, or attachments by such measures as mechanical
attachment, welding, brazing, soldering, adhesion, bonding,
polymerization, solvent casting, thermal deposition, fasteners, and
press fits, physical properties of the stent 1 may be varied over
the length of the stent 1 to provide different characteristics in
different locations. By varying the local properties of the stent 1
by any of these measures along its length, the stent 1 can be made
to have a tapered expanded shape.
[0150] The tapered helical stent 1 of the present invention may be
constructed so that there is a smooth or gradual incremental
progression of properties from one end of the stent 1 to the other,
even though there may not be a conical or tapered shape to the
stent 1, either in the compressed or expanded condition (that is,
either freely-expanded in the case of a self-expanding stent, or
after unconstrained expansion by a cylindrical balloon in the case
of a balloon-expanded stent). These gradually incremental or
smoothly changing properties may be achieved by any of the
variations (or by combinations thereof) including thickness,
metallurgical properties, and/or materials. Similarly, for such an
embodiment, groups of strut pairs can be varied in length and width
over the length of the taper. For example, every three, four, or
five strut pairs could have the same dimensions. Also, rather than
keeping the same bridge pattern over the length of the stent, the
bridge pattern may be varied.
[0151] There are a number of criteria that can influence the design
of tapered helical stents 1 according to the present invention. One
such design criterion includes maintaining a proportional strut
width. To provide similar properties along the length of a tapered
helical stent 1 (that is, a stent intended for use in a tapered
body lumen), and to maintain the durability (under cyclic flexion
and compression) of the stent over its entire length, it is
desirable to maintain a certain mathematical relationship between
the length and width of such features as the struts 10, loops 20,
and bridges 40. In exemplary embodiments, maintaining linear
proportionality for the dimensions of these features is desired,
although more complex mathematical relationships may be used, such
as proportionality to the square root, third root, or 3/2 root,
geometrically, or exponentially depending upon the primary design
constraints for stent performance and durability.
[0152] Another design criterion is maintaining a uniform opening
angle. To provide similar expansion characteristics, and to control
the opening sizes and material strains, it may be desired to
maintain uniform opening angles between struts of a pair. Because
quality inspection of stents may be expensive and time-consuming,
providing easily inspected features, such as uniform expansion
angles, may be of benefit.
[0153] Still another design criterion is the maintenance of uniform
maximum crimp strains. In the design of self-expanding stents, it
is important to make certain that the maximum strains at any point
on the stent does not exceed a specified level during compression
of the stent into its delivery system. Typical values for this
limit are 6% to 8%.
[0154] Other characteristics of stents 1 according to the present
invention relate to the longitudinal pressure profile. A first
exemplary profile is uniform pressure. It is usually considered
beneficial and preferred to apply a uniform amount of contact
pressure to the body lumen being stented. When the number of struts
per unit length and their dimensions are varied in a progressive
manner, the properties at each point along the length of the stent
may be made appropriate for the local diameter of the stent (and,
hence, the stented lumen) when the stent is expanded. Therefore, an
exemplary embodiment of the present invention has geometry and
material properties that assure even pressure along all segments of
the tapered or otherwise varying body lumen to be stented. However,
it is also possible to adopt other criteria, such as contact
pressure in proportion to diameter, higher contact pressures in the
middle segment of the stent, higher contact pressures at the distal
and/or proximal ends, and so forth. Another pressure profile
example includes gradients in the contact pressure. In some of the
embodiments presented, the properties of the stent 1 and/or its
geometry are configured to be graded linearly (that is to say, in a
proportional way) along the length of the stent. It is also
possible to provide, using the same methods, shape and/or
properties that vary in non-linear ways along the length of the
stent. For example, the properties might be constant over certain
parts of the distal and proximal ends of the stent, yet may vary
over a middle segment. Proportionalities other than linear
(first-order) are also possible, such as parabolic (second-order),
cubic (third-order), or any other arbitrary relationship.
[0155] Some of the helical stent geometries described herein have
been of the type where alternating struts 10 are slightly longer
and slightly shorter, so that the serpentine winding advances
helically while the struts 10 maintain substantially a longitudinal
alignment in the non-expanded shape. Other configurations are
possible, such as that taught by Corso, where the helical
serpentine advances with equal-length struts; in such a
configuration, the struts 10 are approximately perpendicular to the
helical axis when unexpanded.
[0156] Some of the foregoing embodiments describe alternative
locations for the bridges 40 that connect adjacent helical turns of
the stent. The shape of the bridges 40 may be varied from those
shown and may be longitudinal, circumferential, or may be oriented
in a diagonal or helical direction. In addition, bridges 40 may
cross the space between the adjacent helical turns such that they
join loops immediately next to one another, loops displaced from
one another, nested loops (hill-to-valley), and/or abutting loops
(hill-to-hill). In addition, other configurations are possible and
known to those skilled in the art, such as valley-to-valley (where
the connecting bridge 40 transits between the struts of a pair to
reach the connecting loop at the opposite end). Exemplary bridges
40 may be straight, curved, curvilinear, spiral, sinusoidal, or of
any other arbitrary shape.
[0157] At the proximal and distal ends 2, 3 of the tapered helical
stent 1 it is possible, and in some instances desired, to provide
features that extend axially beyond the helical portion of the
stent 1. In particular, tabs, extensions, lollipops, or
paddle-shaped extensions may be provided to assist in deployment
and anchoring of the stent 1 in the body lumen. These features may
be smooth or may be equipped with engaging features such as barbs,
prongs, or needles.
[0158] With respect to marking the stent 1 according to the present
invention, any number of prior art systems can be used. For
example, fiducial markers visible under X-ray, MRI, ultrasound,
and/or other imaging measures can be used. Some specific marker
configurations are particularly suitable for stents to be used in
tapered body lumina, such as a single distal marker. In the special
case of a stent for use in the carotid artery, and for other uses
as well, a single marker placed at the distal end of the stent 1 is
all that can be required for precise stent placement, and, by
eliminating redundant markers at the distal end or any markers at
the proximal end, the area of the stent is limited so that
non-diseased areas are not impinged upon by features of the
stent.
[0159] In the present invention of a tapered helical stent, a
particularly advantageous location for a single marker is at the
notch which naturally occurs at the termination of the helical
serpentine winding. A single marker placed at that location allows
precise positioning during delivery while eliminating unwanted
intrusion of portions of the stent into areas that are not desired
to be treated. Alternatively, the marker may be placed in, on, or
otherwise made a part of or attached to a paddle, tab, or
lollipop-shaped extension of the stent at this location.
[0160] As described above for distal markers, it is possible to
place a single marker in the helical notch at the proximal end of
the stent. While not all practitioners desire a proximal marker,
some do. Placing a marker in this position has an additional
advantage of evening out the proximal end 2 of the helical stent,
which facilitates application of the pushing-out force needed to
express a self-expanding stent 1 from its delivery system. Even if
a proximal marker is not desired, it may be advantageous to provide
a similar feature in the helical notch at the proximal end to
facilitate delivery of a self-expanding stent 1 or to make the
proximal end 2 of a balloon-expandable stent more even than would
be possible without the marker.
[0161] In addition to those described here, other markers, multiple
markers, or markers at other locations on the tapered stent 1 are
possible and may be advantageous in certain circumstances, as when
positioning the stent 1 relative to a bifurcation, branch lumen, or
other anatomical feature.
[0162] In addition to the overall tapered shape (either intrinsic
to the stent or caused by its placement in a tapered body lumen) of
the present invention, it may be preferable and advantageous to
flare the proximal end 2, the distal end 3, or both ends 2, 3 of
the stent 1 to aid in deployment and anchoring. The flared shape
may be created by a heat-set shape (in the case of a self-expanding
stent) or by the action of the expanding balloon upon features at
the ends of the stent (in the case of a balloon-expandable
stent).
[0163] There are different ways to manufacture the tapered stent
according to the invention. One exemplary method can include the
following steps: [0164] 1. A tube of nitinol alloy is provided with
a diameter of 1.800 millimeters, a wall thickness of 300 microns,
and a length of 60 millimeters; [0165] 2. The tube is mounted into
a laser-machining station, which has been programmed to make a
pattern of kerfs (cuts) of the intended design; [0166] 3. The tube
is rotated and translated in the laser-machining station relative
to the focal spot of the laser so that the intended pattern of
kerfs is cut through the wall of the tubing; [0167] 4. The cut tube
is removed from the laser-machining station; [0168] 5. The tube is
processed by mechanical and/or chemical measures to remove dross,
burrs, spatter, or other excess material from the inside and
outside surfaces; [0169] 6. The cut and deburred tubing is
thoroughly cleaned; [0170] 7. The cleaned part is pushed onto a
mandrel (by a tapered introducer), which has a diameter slightly
larger than the original internal diameter of the tubing (for
example, 2.500 millimeters) to expand the part (cut tubing); [0171]
8. The mandrel and expanded part are heated to a heat-setting
temperature (for example, 540.degree. Celsius) for one minute;
[0172] 9. The mandrel and expanded heated part are cooled to room
temperature; [0173] 10. The cooled part is removed from the mandrel
and pushed onto a larger mandrel; [0174] 11. The larger mandrel and
cooled part are, again, heated to a heat-setting temperature for
one minute; [0175] 12. Steps 9 through 11 are repeated until the
part is expanded to slightly less than the final desired diameter
of the small end of the finished stent, which can be referred to as
a pre-form; [0176] 13. The pre-form is pushed onto a conical
mandrel to expand one end more than the other end; [0177] 14. The
heating and cooling steps are performed (as described previously in
steps 8 and 9); [0178] 15. The conical pre-form is placed onto
succeedingly larger (or more-tapered) mandrels, and the
heat-treating and cooling steps are repeated until the final
expanded conical shape is achieved, which part can be referred to
as the unfinished stent; [0179] 16. The unfinished stent is removed
from the mandrel; [0180] 17. The unfinished stent is cleaned and
polished by chemical, electrochemical, mechanical, or a combination
of processes to produce the desired final surface finish on the
finished stent; [0181] 18. The finished stent is inspected and
cleaned as needed; [0182] 19. The finished stent is compressed (at
room temperature or at a lower temperature) to a diameter
sufficiently small that it can be placed into the delivery system
catheter; [0183] 20. The stent is placed into the delivery system
catheter; and [0184] 21. The delivery system and stent are packaged
and sterilized.
[0185] Although the examples illustrated herein are indicated as
being made by laser-machining, it is possible to make the stent of
the present invention by other measures, such as electro-discharge
machining (EDM), photo-chemical machining (PCM), stamping,
conventional machining, chemical deposition (for example, plating),
physical vapor deposition (PVD), chemical vapor deposition (CVD),
or other measures known to those skilled in the art.
[0186] In the foregoing examples, the stent 1 can be made from
uniform-diameter tubing (with possibly changing wall thickness) or
from tapered-diameter tubing. It is also possible to make the
tapered helical stent 1 from tubing with a tapered outer diameter.
It is, in fact, particularly advantageous to do so because a stent
1 constructed according to the present invention naturally may have
different geometry at opposite ends of the stent. For example, it
may be advantageous, as previously described, to have a larger
number of struts 10 per helical turn at the larger end 3 of the
stent 1.
[0187] While stents are customarily machined from
constant-outside-diameter tubing, it is possible to machine the
stent 1 of the present invention from tapered tubing, so that the
end requiring more struts 10 has more circumference out of which to
machine those struts 10. Particularly in the case of self-expanding
embodiments, where it is common to construct stents according to
the criterion of "maximum metal" (that is, removing the smallest
possible amount of metal from the stent tubing to make the stent),
machining a tapered stent 1 from tapered tubing would allow the
design to be optimized at both ends of the stent 1, allowing for
the minimum amount of material to be removed at all points along
the stent 1.
[0188] When machining a self-expanding stent 1 from tapered tubing,
it is advantageous to provide reduced wall thickness at the larger
end of the tubing to ease compression of the stent 1 into its
delivery system.
[0189] There are certain considerations that can be discussed with
respect to the process for expanding the tapered stent 1 according
to the present invention.
[0190] Self-expanding stents are expanded by many measures known to
those skilled in the art, including the use of tapered mandrels or
other internally-expanding tooling. The expansion is usually
accomplished in a step-wise fashion, expanding by small increments
and heat-treating after each step, in order to minimize the chance
of damage to the stent.
[0191] In the case of self-expanding helical stents 1 having a
tapered shape when freely expanded, in one exemplary process, at
least the final heat-setting step will utilize a tapered mandrel or
tool to hold the stent 1 in the desired shape during heat
treatment.
[0192] In the case of self-expanding helical stents 1 that do not
have a tapered shape when freely expanded, the final expansion and
heat-setting step will be done, for example, with a non-tapered
mandrel or tool. However, it may be advantageous to use tapered
tools during a portion of the expansion process so that the desired
configuration of the stent 1 will be achieved when deployed in a
tapered body lumen.
[0193] Balloon-expandable stents are typically machined in an
intermediate configuration part way between a fully expanded shape
and a fully crimped shape. Once machined, such stents must be
expanded partially to place them onto the balloon deployment
system, after which they are, typically, crimped to hold them onto
the balloon during deployment. Many different measures are known to
those skilled in the art to crimp, attach, and fix such stents onto
balloons. These measures can be used to create a balloon-expandable
tapered helical stent 1. Once completed, stents are customarily
polished by a combination of chemical and physical measures. In the
case of self-expanding stents, such steps generally include honing,
abrasive or bead blasting, acid pickling, mechanical polishing,
electropolishing, and passivation. In the case of balloon-expanded
stents, similar steps are employed, depending upon the material
from which the stent is made. These processes can also be used to
create the tapered helical stent 1 according to the present
invention.
* * * * *