U.S. patent application number 11/216293 was filed with the patent office on 2006-04-06 for stent and method for manufacturing the stent.
This patent application is currently assigned to PST, LLC. Invention is credited to Thomas O. Bales, Scott L. Jahrmarkt, Charles R. Slater.
Application Number | 20060074480 11/216293 |
Document ID | / |
Family ID | 37831748 |
Filed Date | 2006-04-06 |
United States Patent
Application |
20060074480 |
Kind Code |
A1 |
Bales; Thomas O. ; et
al. |
April 6, 2006 |
Stent and method for manufacturing the stent
Abstract
A stent includes a stent body having a circumference and struts
disposed helically about the circumference in turns. At least two
of the struts have respective strut ends. At least two
paddle-shaped markers extend away from a respective one of the
strut ends. The markers have respective marker extreme ends and
different overall longitudinal lengths substantially aligning the
marker extreme ends approximately along a single circumference of
the stent body. A method for manufacturing a helical stent includes
the steps of providing a stent body with struts disposed about the
circumference thereof in turns and with bridges connecting the
struts in adjacent turns. The stent body is expanded and,
thereafter, some of the bridges, in particular, sacrificial
bridges, are removed.
Inventors: |
Bales; Thomas O.; (Coral
Gables, FL) ; Slater; Charles R.; (Fort Lauderdale,
FL) ; Jahrmarkt; Scott L.; (Miami Beach, FL) |
Correspondence
Address: |
FELDMANGALE, P.A.
MIAMI CENTER, 19TH FLOOR
201 SOUTH BISCAYNE BOULEVARD
MIAMI
FL
33131
US
|
Assignee: |
PST, LLC
|
Family ID: |
37831748 |
Appl. No.: |
11/216293 |
Filed: |
August 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60606261 |
Sep 1, 2004 |
|
|
|
Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F 2002/91541
20130101; A61F 2002/91533 20130101; A61F 2/88 20130101; A61F 2/844
20130101; A61F 2002/0864 20130101; A61F 2/91 20130101; A61F 2/848
20130101; A61F 2002/9665 20130101; A61F 2250/0098 20130101; A61F
2/915 20130101; A61F 2002/91583 20130101; A61F 2002/91558
20130101 |
Class at
Publication: |
623/001.15 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1. A stent, comprising: a stent body having: a circumference;
struts disposed helically about said circumference in turns, at
least two of said struts having respective strut ends; and at least
two paddle-shaped markers extending away from a respective one of
said strut ends, said markers having respective marker extreme ends
and different overall longitudinal lengths substantially aligning
said marker extreme ends approximately along a single circumference
of said stent body.
2. The stent according to claim 1, wherein: said stent body has a
longitudinal axis; and said single circumference is substantially
orthogonal to said axis.
3. The stent according to claim 1, wherein: said stent body has a
longitudinal axis; and said markers extend away from a respective
one of said strut ends substantially parallel to said longitudinal
axis.
4. The stent according to claim 1, wherein: said marker has a body
with a first imaging characteristic, said body has at least one
portion with a second imaging characteristic different from said
first imaging characteristic; said first and second imaging
characteristics are selected from the group consisting of
ultrasound imaging characteristics, fluoroscopy imaging
characteristics, x-ray imaging characteristics, and magnetic
resonance imaging characteristics.
5. The stent according to claim 4, wherein said portion is a
structure selected from at least one of the group consisting of a
depression, a hole, a recess, a notch, a slot, a cylinder, a
coating, a filling, a sphere, a texture, a porosity, a second
material attached to said marker, and a particle.
6. The stent according to claim 1, wherein said marker extreme ends
are substantially flat in a circumferential direction and are
shaped to receive a deployment catheter.
7. The stent according to claim 1, wherein said struts are disposed
in a single helix having one start at a proximal end of said helix
and one end at a distal end of said helix.
8. The stent according to claim 1, wherein: said stent body has a
proximal end and a distal end; and said struts are disposed in a
multiple helix with at least two helices each having one start at
said proximal end and one end at said distal end.
9. The stent according to claim 8, wherein said multiple helix has
4 helices each having one start at said proximal end and one end at
said distal end.
10. The stent according to claim 7, wherein: said struts are
s-shaped struts; and said helix has a continuous repetition of said
s-shaped struts throughout a length of said helix.
11. The stent according to claim 8, wherein: said struts are
s-shaped struts; and each of said helices has a continuous
repetition of said s-shaped struts throughout a length of said
helices.
12. The stent according to claim 1, wherein said struts have curved
segments and said markers extend away from said curved ends.
13. The stent according to claim 1, wherein said markers are
radiopaque markers.
14. The stent according to claim 13, wherein said radiopaque
markers are of a material selected from at least one of the group
consisting of tungsten, tantalum, molybdenum, platinum, gold,
zirconium oxide, barium salt, bismuth salt, hafnium, and bismuth
subcarbonate.
15. The stent according to claim 1, wherein said markers are
ultrasound markers.
16. The stent according to claim 15, wherein said ultrasound
markers are features selected from at least one of a group
consisting of abrasions, holes, voids, porous materials, porous
coatings, hollow balloons, and layered materials having different
sonic properties.
17. The stent according to claim 15, wherein said ultrasound marker
is a hole 0.50 millimeters in diameter filled with a composite of
glass microballoons and tungsten powder suspended in an epoxy
matrix.
18. The stent according to claim 1, wherein said markers are
magnetic resonance imaging markers.
19. The stent according to claim 18, wherein said magnetic
resonance imaging markers are of materials selected from at least
one of the group consisting of paramagnetic, diamagnetic, and
ferromagnetic.
20. The stent according to claim 18, wherein said magnetic
resonance imaging markers are of at least one of the group
consisting of gadolinium, gadolinium salts, gadolinium foil,
gadolinium powder, hematite, oxides, nanocrystalline iron oxide,
and iron powder.
21. The stent according to claim 1, wherein said markers are
ultrasonic markers.
22. The stent according to claim 21, wherein said ultrasonic
markers are of glass or ceramic microballoons
23. The stent according to claim 1, wherein said markers are
combinations of radiopaque, ultrasound, and magnetic resonance
imaging markers.
24. The stent according to claim 1, wherein one of said markers has
a relatively shorter extension portion adjacent an end of said
helix turn and others of said markers have a relatively larger
extension portion increasing in size along said helix turn in a
direction away from said relatively shorter extension portion.
25. The stent according to claim 24, wherein said markers have
paddle portions with aligned ends.
26. The stent according to claim 24, wherein: said stent body has a
longitudinal axis; and said markers have paddle portions with ends
opposite said extension portions defining a circumferential plane
substantially orthogonal to said longitudinal axis.
27. The stent according to claim 24, wherein said extension
portions have a given width in a circumferential direction of said
stent body and said paddle portions have a width in said
circumferential direction greater than said given width.
28. The stent according to claim 24, wherein said extension
portions have a given width in a circumferential direction of said
stent body and said paddle portions have a width in said
circumferential direction equal to said given width.
29. The stent according to claim 25, wherein said paddle portions
are non-circular.
30. The stent according to claim 1, wherein said extreme ends of
adjacent ones of said markers are separated by a distance no
greater than 18 microns.
31. The stent according to claim 1, wherein: said struts have a
reduced state and first expanded state defining an outer
circumferential cylinder with a first circumference; at least two
of said struts have respective strut ends; at least two of said
paddle-shaped markers extend away from a respective one of said
strut ends and have respective marker extreme ends; and said
markers have a second expanded state in which a second
circumference defined by said marker extreme ends is greater than
said first circumference.
32. A stent, comprising: a stent body having: a circumference;
struts disposed helically about said circumference in turns, at
least two of said struts having respective strut ends; and at least
two paddle-shaped markers extending away from a respective one of
said strut ends, said markers having respective circumferentially
flat extreme ends and different overall longitudinal lengths
substantially aligning said flat extreme ends of said markers along
approximately a single circumference of said stent body.
33. The stent according to claim 32, wherein: said stent body has a
longitudinal axis; and said single circumference is substantially
orthogonal to said axis.
34. The stent according to claim 32, wherein: said stent body has a
longitudinal axis; and said single circumference is at an angle to
said axis.
35. A stent, comprising: a stent body having: a circumference;
struts disposed helically about said circumference in turns and
having a reduced state and first expanded state defining an outer
circumferential cylinder with a first circumference, at least two
of said struts having respective strut ends; at least two
paddle-shaped markers extending away from a respective one of said
strut ends and having respective marker extreme ends; and said
markers having a second expanded state in which a second
circumference defined by said marker extreme ends is greater than
said first circumference.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority, under 35 U.S.C. .sctn.
119, of U.S. Provisional Patent Application No. 60/606,261 filed
Sep. 1, 2004, the entire disclosure of which is hereby incorporated
herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] n/a
FIELD OF THE INVENTION
[0003] The invention lies in the field of vascular stents. In
particular, the invention is in the field of helical stents for
peripheral arteries, the biliary tree, and other body lumens.
[0004] Stents have been developed for use in various lumens of the
body, including the biliary tree, venous system, peripheral
arteries, and coronary arteries. Stents are used to 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 generally been made by two different techniques: either
formed from wire or machined from a hollow tube. 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 a pattern of struts and
connecting elements from a metallic tubular preform (typically, by
laser machining). 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 an undulating pattern of
struts and end-loops) with joining structures (referred to as
"bridges") joining adjacent turns of the helix to provide
mechanical integrity to the tubular structure (to prevent
unwinding, kinking, and buckling).
[0006] Fine Cell Structure of Stents
[0007] Clinicians recommend the use of stents with relatively small
openings to minimize the chances of friable material from the lumen
wall penetrating into the interior of the stent where it may result
in narrowing of the lumen by cellular proliferation or where it may
embolize downstream, causing damage or ischemia. U.S. Pat. No.
6,537,310 to Palmaz et al. teaches that it is advantageous to cover
a stent with a porous film having openings no larger than 17
microns in their smallest dimension to minimize the migration of
embolic debris and plaque into the lumen of a stent. However,
Palmaz teaches use of a stent that is very difficult to manufacture
because of the great number of very small openings in the covering
film or "web."
[0008] Clinicians have asked for stents with "thin, equi-spaced
struts for optimal wall coverage and drug elution" ("Clinical
Impact of Stent Design: Results from 10 Years Experience," C.
DiMario, TCT2003). DiMario demonstrates 15.0% restenosis versus
36.6% for stents with thin struts (50 microns, Multilink) versus
thick struts (average of all stents evaluated with struts greater
than or equal to 100 microns). DiMario also relates stent efficacy
to "integrated cell size," showing better results for the BX
VELOCITY.RTM. stent with cells of 3.3 mm.sup.2 versus stents with
larger cell sizes. DiMario reports reduced neointimal hyperplasia
for smaller struts (0.8 mm thickness for closely-spaced 125-micron
struts versus 1.54 mm thickness for wider-spaced 200-micron
struts). Because prior art stent designs have large gaps between
stent parts, drug elution about these parts does not adequately
cover all of the tissue within the bounds of the stent.
[0009] In "Clinical Impact of Stent Design: Results From Randomized
Trials" (TCT 2003), A. Kastrati reports reduced residual
percent-diameter stenosis after stenting (4.0% versus 5.7%) with
50-micron struts (Multi-link) versus 140-micron (Multi-link
Duet).
[0010] In his report "Era of Drug-Coated and Drug-Eluting Stents"
(TCT 2002), G. Grube states that the typical open-cell
configuration gives poor distribution of the drug into the arterial
wall because of the large open gaps when the stent is situated in a
bend of the artery.
[0011] Number-of-Struts to Strut-Length Ratio
[0012] U.S. Pat. No. 6,129,755 to Mathis et al. (hereinafter
"Mathis") teaches improved self-expanding stents with
circumferential hoops of struts joined by oblique longitudinal
bridges. described therein is the importance of having a large
number of struts per hoop (the number of struts counted by going
around the circumference) and minimum strut length to minimize
strains in superelastic materials and to prevent emboli from
passing through the wall of the stent. Mathis defines a figure of
merit that is the ratio of number of struts around the
circumference to the length (in inches) of a strut, measured
longitudinally. This ratio, which has the units of reciprocal
inches, will be referred to herein as the M-D Ratio because the
inventors were Mathis and Duerig. Mathis describes prior-art stents
as having a ratio of about 200 and that their improved stent has an
M-D Ratio of over 400. A representative stent produced by Cordis
Corporation according to the Mathis-Duerig invention--referred to
as the "SmartStent"--has 32 struts per circumference and strut
lengths of approximately 0.077 inch, resulting in an M-D Ratio of
approximately 416.
[0013] The M-D Ratio is determined by number of struts divided by
strut length. For a given diameter stent, assuming "maximum-metal"
configuration, which is typical for self-expanding: stents, the
number of struts around the circumference is inversely proportional
to the strut width. Thus, the M-D Ratio is inversely proportional
to the product of strut width and length.
SUMMARY OF THE INVENTION
[0014] It is accordingly an object of the invention to provide a
helical stent and a method for manufacturing the stent that
overcome the hereinafore-mentioned disadvantages of the
heretofore-known devices of this general type and that improves
helical machined-tube stents, whether balloon-expanded or
self-expanding.
[0015] The self-expended stent of the present invention is suitable
for use in peripheral arteries, the biliary tree, and other body
lumens. In particular, it will be most advantageous for use in
arteries where flexure is an important factor, such as iliac
arteries and carotid arteries. It is not traditional for
cardiologists to use self-expanding stents in coronary arteries or
coronary bypass grafts. Nonetheless, the present invention is
especially suitable for the diffuse disease often encountered in
these locations. Also, because of the high total surface area of
the present configuration, the stent is particularly suitable for
the application of drug-eluting coatings intended to reduce
restenosis or for other therapies. Specifically, the stent
according to the present invention allows virtually all tissue
within the coverage area of the stent to be in the elution areas.
In particular, the stent provides tissue coverage so that no
element of wall tissue is more than 350 microns to 400 microns away
from the nearest strut. Such a configuration assures a short
diffusion path from a strut covered with a drug-eluting agent to
any portion of the tissue.
[0016] With the foregoing and other objects in view, there is
provided, in accordance with the invention, a stent, including a
stent body having a circumference and struts disposed helically
about the circumference in turns, at least two of the struts having
respective strut ends. At least two paddle-shaped markers extend
away from a respective one of the strut ends. The markers have
respective marker extreme ends and different overall longitudinal
lengths substantially aligning the marker extreme ends
approximately along a single circumference of the stent body.
[0017] With the objects of the invention in view, there is also
provided stent, including a stent body having a circumference and
struts disposed helically about the circumference in turns. At
least two of the struts having respective strut ends. At least two
paddle-shaped markers extend away from a respective one of the
strut ends. The markers have respective circumferentially flat
extreme ends and different overall longitudinal lengths
substantially aligning the flat extreme ends of the markers along
approximately a single circumference of the stent body.
[0018] With the objects of the invention in view, there is also
provided stent, including a stent body having a circumference and
struts disposed helically about the circumference in turns and
having a reduced state and first expanded state defining an outer
circumferential cylinder with a first circumference. At least two
of the struts have respective strut ends. At least two
paddle-shaped markers extend away from a respective one of the
strut ends and have respective marker extreme ends. The markers
have a second expanded state in which a second circumference
defined by the marker extreme ends is greater than the first
circumference.
[0019] In accordance with another feature of the invention, the
stent body has a longitudinal axis and the single circumference is
substantially orthogonal to the axis. Alternatively, the single
circumference is at an angle to the axis.
[0020] In accordance with a further feature of the invention, the
the stent body has a longitudinal axis and the markers extend away
from a respective one of the strut ends substantially parallel to
the longitudinal axis.
[0021] In accordance with an added feature of the invention, the
marker has a body with a first imaging characteristic, the body has
at least one portion with a second imaging characteristic different
from the first imaging characteristic and the first and second
imaging characteristics are elected from the group consisting of
ultrasound imaging characteristics, fluoroscopy imaging
characteristics, x-ray imaging characteristics, and magnetic
resonance imaging characteristics.
[0022] In accordance with an additional feature of the invention,
the portion is a structure selected from at least one of the group
consisting of a depression, a hole, a recess, a notch, a slot, a
cylinder, a coating, a filling, a sphere, a texture, a porosity, a
second material attached to the marker, and a particle.
[0023] In accordance with yet another feature of the invention, the
marker extreme ends are substantially flat in a circumferential
direction and are shaped to receive a deployment catheter.
[0024] In accordance with yet a further feature of the invention,
the struts are disposed in a single helix having one start at a
proximal end of the helix and one end at a distal end of the
helix.
[0025] In accordance with yet an added feature of the invention,
the stent body has a proximal end and a distal end and the struts
are disposed in a multiple helix with at least two helices each
having one start at the proximal end and one end at the distal
end.
[0026] In accordance with yet an additional feature of the
invention, the multiple helix has 4 helices each having one start
at the proximal end and one end at the distal end.
[0027] In accordance with again another feature of the invention,
the struts are s-shaped struts and the helix has a continuous
repetition of the s-shaped struts throughout a length of the
helix.
[0028] In accordance with again a further feature of the invention,
the struts are s-shaped struts and each of the helices has a
continuous repetition of the s-shaped struts throughout a length of
the helices.
[0029] In accordance with again an added feature of the invention,
the struts have curved segments and the markers extend away from
the curved ends.
[0030] In accordance with again an additional feature of the
invention, the markers are combinations of radiopaque, ultrasound,
and magnetic resonance imaging markers. The radiopaque markers can
be tungsten, tantalum, molybdenum, platinum, gold, zirconium oxide,
barium salt, bismuth salt, hafnium, and/or bismuth subcarbonate.
The ultrasound markers can be abrasions, holes, voids, porous
materials, porous coatings, hollow balloons, and/or layered
materials having different sonic properties. In particular, the
ultrasound marker can be a hole 0.50 millimeters in diameter filled
with a composite of glass microballoons and tungsten powder
suspended in an epoxy matrix. The magnetic resonance imaging
markers can be of paramagnetic, diamagnetic, and ferromagnetic
materials. The magnetic resonance imaging markers can be of
gadolinium, gadolinium salts, gadolinium foil, gadolinium powder,
hematite, oxides, nanocrystalline iron oxide, and/or iron powder.
The ultrasonic markers can be of glass or ceramic microballoons
[0031] In accordance with still another feature of the invention,
one of the markers has a relatively shorter extension portion
adjacent an end of the helix turn and others of the markers have a
relatively larger extension portion increasing in size along the
helix turn in a direction away from the relatively shorter
extension portion.
[0032] In accordance with still a further feature of the invention,
the markers have paddle portions with aligned ends.
[0033] In accordance with still an added feature of the invention,
the stent body has a longitudinal axis and the markers have paddle
portions with ends opposite the extension portions defining a
circumferential plane substantially orthogonal to the longitudinal
axis.
[0034] In accordance with still an additional feature of the
invention, the extension portions have a given width in a
circumferential direction of the stent body and the paddle portions
have a width in the circumferential direction greater than the
given width.
[0035] In accordance with another feature of the invention, the
extension portions have a given width in a circumferential
direction of the stent body and the paddle portions have a width in
the circumferential direction equal to the given width.
[0036] In accordance with a further feature of the invention, the
paddle portions are non-circular.
[0037] In accordance with an added feature of the invention, the
extreme ends of adjacent ones of the markers are separated by a
distance no greater than 18 microns.
[0038] In accordance with a concomitant feature of the invention,
the struts have a reduced state and first expanded state defining
an outer circumferential cylinder with a first circumference, at
least two of the struts have respective strut ends, at least two of
the paddle-shaped markers extend away from a respective one of the
strut ends and have respective marker extreme ends, and the markers
have a second expanded state in which a second circumference
defined by the marker extreme ends is greater than the first
circumference.
[0039] The present invention relies on a helical configuration with
much shorter struts and significantly higher number of struts
around the circumference than the prior art. Indeed, helical stent
configurations according to the present invention are not limited
to even-integral numbers of struts--as are "hoop" configurations
taught by Mathis. In fact, odd-integral numbers of struts around
the circumferential or even non-integral numbers of struts around
the circumference are possible in the helical configuration of the
present invention because there is no requirement for the struts to
rejoin themselves to make complete hoops. In other words, a helical
stent could have 31.567 struts per revolution, or any other
arbitrary number. Mathis teaches that increasing the M-D ratio
increases the rigidity of a stent, yet the rigidity of two
comparative stents bears this relationship only if the stents being
compared are expanded to comparable opening angles between the
struts. In fact, with commercially available stent product lines
produced to the M-D configuration, stents of different diameters
frequently have the same number of struts. Even so, such a
configuration family has smaller opening angles in smaller sizes
than in larger sizes; this is because similar stent preforms are
used to make a range of final stent sizes. The smaller stents in a
product family sharing the same preform configuration (including
the number of struts) have smaller opening angles, of course,
resulting in lower chronic outward force (COF) and lower radial
resistive force (RRF) to collapse, because the effective
bending-lever length is longer in struts with lower opening angles.
Mathis teaches M-D Ratios of over 400 and numbers of struts up to
32 or more but does not teach or suggest ratios of near or over
eight-hundred (800), let alone over one thousand (1000). Mathis,
specifically, does not mention what effects a much larger number of
struts would have, and does not imply implementation of
significantly shorter struts.
[0040] In the present invention, an exemplary configuration for an
8 mm diameter stent incorporates 46 struts around the helical
circumference and the struts have a length of approximately 0.99 mm
(0.039 inches). The M-D Ratio for this exemplary configuration
according to the present invention is, therefore, 1180--nearly
three times the ratios taught in the prior art. Stents according to
the present invention have new and unexpected properties, even
though they require greater attention to opening angles (and,
hence, have a more limited useful size range for a given
configuration).
[0041] In comparison with prior art stents having cells of 3.3
mm.sup.2, the present invention gives an integrated cell size of
1.6 mm.sup.2 per cell unit in an 8 mm diameter stent. In a
configuration with bridges every three cell units, the total
integrated cell size would be 4.8 mm.sup.2, which is
proportionately smaller than that of the BX VELOCITY.RTM. 3 mm
stent.
[0042] Specifically, configurations according to the present
invention have much smaller openings when expanded and,
particularly, when the expanded stent is flexed in bending. The
substantially smaller openings result in greatly improved
resistance to the passage of emboli through the stent wall.
[0043] Another characteristic of stents according to the present
invention is a greatly increased flexibility and resistance to
buckling in bending or torsion. Stents according to the present
invention also have improved fatigue life in real-life
applications, resulting from a large number of struts and bending
segments to absorb irregular, localized deformations caused by the
anatomy--as opposed to such local deformations being placed on a
small number of struts and bending segments, which results in
over-straining some of these elements.
[0044] Stent configurations optimized for a particular expanded
diameter will have struts as wide as possible, consistent with the
maximum allowable strain during storage and compression. The result
of such a criterion is that stent configurations according to the
present invention, with a greater number of struts of shorter
length and narrower width than prior art configurations, will allow
greater bending deflections, resulting in greater possible opening
angles. Constructing an expanded stent with greater allowed opening
angles also results in a relatively shorter projected lever-arm
length acting on the struts and bending segments when the stent is
expanded in the anatomy. These shorter lever arms result in higher
outward forces applied to the vessel walls when the stent is
expanded.
[0045] It should be noted that the present invention results in
configurations that are optimized for a small range of expanded
sizes, creating the need to have individualized configurations for
each expanded size of stent. This approach deviates from the prior
art and results in higher configuration and validation costs, but
results in stents with significantly improved flexural and fatigue
properties while, at the same time, providing optimized radial
outward forces and collapse resistance for each size.
[0046] Another characteristic of stents made according to this
invention is the increased difficulty of collapsing the stent when
preparing it for insertion into a delivery catheter. The struts of
stents made according to the present invention are proportionately
narrower and, hence, less stiff in bending (in proportion to the
cube of the width of the struts) when compared to prior art stent
designs. This decrement in stiffness may be offset by increasing
the opening angle of the stent, as described elsewhere herein, but
the reduced stiffness of the struts (and also the increased opening
angles) results in a tendency for portions of the helix to buckle
when subjected to the stresses and strains required to fully
collapse the stent prior to insertion into its delivery system. The
result of this buckling is that a series of struts and loops
forming a portion of the helical winding will resist collapsing
uniformly along the helical axis, but rather buckle away from the
helical axis (usually remaining in the plane of the cylindrical
surface of the stent). When a portion of the helix buckles, the
struts of that turn may begin to interfere or interdigitate with
the struts of an adjacent helical turn. Thus, stents made according
to the present invention are more difficult to compress into their
delivery system.
[0047] This tendency for a series of struts and loops to buckle
away from the helical axis is aggravated when the struts are very
narrow, when the opening angles are higher, and when there is a
long series of struts between the connecting bridges. The presence
of the connecting bridges that join adjacent turns of the stent
stabilizes the stent during compression; this stability is greater
when there are only a few struts between bridges, and the stability
is reduced when there is a large number of struts between bridges.
For example, stents made with series of seven or nine struts
between bridges have a high tendency toward buckling when
compressed; stents made with five struts between bridges have an
intermediate tendency toward buckling when compressed; and, stents
with only three struts between bridges have a low tendency toward
buckling when compressed. It should be noted that this tendency
toward buckling does not adversely affect the characteristics of
the stent when expanded in the body, because the compressive
strains experienced in the body are insufficient to cause the
buckling seen during compression into the delivery system. However,
it has been found that stents with very low numbers of struts
between bridges (e.g., one or three), though they are very easy to
fully compress, do not have flexibility as great as that of stents
with larger numbers of struts between bridges (e.g., seven or
nine). As a result, it has been found that there is a tradeoff
between design choices which create a stent that is easy to
compress versus choices which make the stent flexible. It has been
found that stents made according to this invention, configured with
an M-D ratio in the range of 1000, have the most favorable balance
of flexibility and buckling during compression when the number of
struts between bridges is in the range of three to five.
[0048] Other features that are considered as characteristic for the
invention are set forth in the appended claims.
[0049] Although the invention is illustrated and described herein
as embodied in a helical stent and a method for manufacturing the
stent, it is, nevertheless, not intended to be limited to the
details shown 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.
[0050] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is a fragmentary, enlarged partially cross-sectional
and partially plan view of a stent delivery system configured to
implant a stent according to the invention in a vessel;
[0052] FIG. 2 is a fragmentary, enlarged plan view of the stent of
FIG. 1 expanded and implanted in the vessel;
[0053] FIG. 3 is a fragmentary, enlarged plan view of a portion of
a first embodiment of the stent of FIG. 1;
[0054] FIG. 4 is a fragmentary, enlarged plan view of a portion of
a second embodiment of the stent of FIG. 1;
[0055] FIG. 5 is a fragmentary, enlarged plan view of a portion of
the second embodiment of the stent of FIG. 4 with circular
markers;
[0056] FIG. 6 is a fragmentary, enlarged plan view of a portion of
the first embodiment of the stent of FIG. 3 with flat-ended
markers;
[0057] FIG. 7 is a fragmentary, enlarged plan view of a further
enlarged portion of the first embodiment of the stent of FIG. 3
with some sacrificial bridges removed;
[0058] FIG. 8 is a fragmentary, side elevational view of a portion
of an expanded stent according to the invention with a protruding
bridge;
[0059] FIG. 9 is a fragmentary, enlarged plan view of a further
enlarged portion of the first embodiment of the stent of FIG. 7
with the sacrificial bridges having break points;
[0060] FIG. 10 is a plan view of a flat cut pattern representing
the laser-cutting path to be created around a circumference of
tubing from which the stent according to the invention is to be
created;
[0061] FIG. 11 is a fragmentary, enlarged, perspective view from
the side of a stent according to the invention;
[0062] FIG. 12 is a fragmentary, further enlarged, perspective view
of a portion of the stent of FIG. 11;
[0063] FIG. 13 is a fragmentary, enlarged, perspective view from an
end of the stent of FIG. 11;
[0064] FIG. 14 is a fragmentary, further enlarged, perspective view
of a portion of the stent of FIG. 13;
[0065] FIG. 15 is a fragmentary, enlarged plan view of a portion of
an expanded stent according to the invention illustrating a largest
embolism area; and
[0066] FIG. 16 is a fragmentary, enlarged plan view of a portion of
a prior art stent illustrating a largest embolism area.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0067] Referring now to the figures of the drawings in detail and
first, particularly to FIG. 1 thereof, there is shown is a helical
stent 1 according to the present invention fitted on a delivery
catheter 20 of an exemplary delivery system 10. The helical stent 1
is about to be implanted in a vessel 30. The helical stent 1 is in
its unexpanded state and loaded into/onto the delivery system 10
that has traveled to an implantation site. FIG. 2 illustrates the
helical stent 1 implanted in the vessel 30 after being expanded,
whether by a balloon of the catheter 20 or by self-expansion due to
a shape memory of the material of the stent 1.
[0068] The helical stent 1 has proximal 2 and distal 3
ends--defined by a blood flow direction A. The helix of the stent 1
can be a single coil with one start at the proximal end that winds
all the way to the distal end. Such a configuration is possible
with the present invention because the helical stent 1 has very
short struts, which will be explained in further detail below.
Another configuration alternative usable with short struts is a
multiple-helix configuration (shown in FIG. 2), where more than one
helixed start is present, for example, a double-lead, a
triple-lead, and so on. With an exemplary 8 mm size of the helical
stent according to the present invention, up to 4 leads are
practical.
[0069] FIGS. 3 and 4 show enlarged views of a portion of the body
of the helical stent 1 of the present invention. Each turn 4 of the
helix is formed, in a preferred embodiment, by a continuous
repetition of s-shaped struts 5 throughout the length of the helix.
The struts 5 have straight portions 6 and curved portions 7
connecting respectively adjacent straight portions 6. Connecting
bridges 8 have a width substantially similar to a width of the
straight and curved portions 6, 7 and connect adjacent turns 4 of
the helix. Also connecting adjacent turns of the helix are
sacrificial bridges 9, which have a width smaller than a width of
the straight and curved portions 6, 7. Both of the bridges 8, 9
will be described in greater detail below.
[0070] Stents 1 may be made according to the present invention with
struts 5 that are aligned with the longitudinal axis 10 of the
stent 1, as shown in FIG. 3, or the struts 5 may be aligned
perpendicular to the helical direction 11, as shown in FIG. 4.
There are advantages and differences to both configurations. The
longitudinally aligned straight portions 6 of the struts 5 produce
a stent 1 that requires lower force to deploy from a confining
sleeve because there are no oblique, twisting, knife-edges to cut
into or grip the sleeve. One characteristic of this embodiment, is
that the struts 6 are not of equal length (there is an equal number
of short and long struts) and, therefore, it is not possible to
fully balance the flexibility of these struts to fully utilize the
properties of the material used to build the stent 1. In
comparison, the configuration shown in FIG. 4 with helically
aligned straight portions 6 of the struts 5 has the advantage of
equal strut lengths. This configuration, in comparison, has a
higher friction when the stent 1 is engaged inside a deployment
system.
[0071] Other advantages and differences exist for these two
configurations, including ease of manufacture, ease of inspection,
and stability during expansion or deployment of the longitudinal
and helically aligned strut configurations. But, either may be used
to practice the teachings of the present invention.
[0072] Lollipop Crown and Retention Levers
[0073] It is customary to provide radiopaque markers on stents so
that they can be easily visualized by using x-rays for assisting
their placement and deployment. The present invention provides a
convenient area at which to locate these markers, specifically,
beyond the ends of the helical pattern of struts. If the markers 12
are paddle-shaped (that is, having a substantially disk-like
enlarged portion with a narrow extension that joins it to the
structure of the stent), they may be attached to the ends of the
180-degree bending segments 7 (or to other locations on the bending
segments 7 or straight portions 6). It is advantageous to dispose
the markers so that a paddle with a short extension is located near
the end of the helix (the extreme end of the helical pattern) and
paddles on longer connectors are located at other locations around
the circumference. In such a configuration, the extreme ends of the
paddles are even, providing a relatively planar end to the stent 1.
However, the marker portions 12 need not be paddle-shaped. They can
merely be rod-shaped to extend away from either or both of the
distal and proximal ends 2, 3 of the stent 1. These rods can be
expanded for better seating in the vessel and, even with a smaller
surface area as compared to the paddle-shaped markers, can still
provide sufficient area for receiving indicators that allow for
better imaging.
[0074] The flat end provided by the paddle-shaped markers 12 of
FIG. 6, for example, facilitates pushing the stent 1 out of a
deployment device (although a shaped pusher that conforms to the
helical end of the stent could be used but is harder to manufacture
and align). During deployment of a self-expanding stent, a pusher
component of a delivery catheter exerts a (distally-directed)
counter-force onto the proximal end of the stent while a covering
sleeve is retracted from its position over the stent. As the
covering sleeve is retracted relative to the stent and the pusher,
the distal end of the stent is exposed and, therefore, expands to
contact the interior of the vessel. Thus, it is important for the
pusher to be able to apply evenly the distally directed force onto
the proximal end of the stent during deployment. Also, for most
medical indications, physicians prefer stents with flat ends
substantially perpendicular to the longitudinal axis of the device
so that there is an even transformation from the end of the stent
to the unsupported (unstented) portion of the vessel wall.
[0075] The paddle shaped markers 12 described above can be spaced
from the helical end of he stent by narrow connectors as shown in
FIGS. 5 and 6, or by full-width connectors (i.e., markers that are
of uniform width from their ends to the point where they join the
struts or loops of the stent), or by directly connecting them to
the other elements of the stent. FIG. 5, for example, illustrates
three paddle-shaped markers 12 attached by narrow connectors to the
helical end of a portion of a stent 1.
[0076] While the disk-like enlarged portions of paddle-shaped
markers 12 can be rounded, it is preferable for the extreme outer
ends to be relatively straight. As such, the paddle-shaped markers
12 may be provided with non-circular ends 13 to facilitate
engagement of the pushing device of the deployment -catheter with
which the stent is implanted. For example, FIG. 6 shows flat-ended
paddle-shaped markers 12 that maximize contact between the paddles
and the pushing device.
[0077] In addition, the paddle-shaped markers 12 may be used to
help anchor the stent 1 during and after deployment. Specifically,
the paddles may be radially expanded further than the struts 5, 6,
7 so that they form a funnel-shaped end to the stent 1 once
expanded.
[0078] While the present drawings show paddle-shaped markers
without separate radiopaque inserts, it should be noted that pieces
of radiopaque materials, such as tungsten, tantalum, molybdenum,
platinum, or gold, might be inserted into the markers to enhance
their visibility under x-rays. For example, inserted cylinders of
tantalum 0.50 millimeters in diameter and having a thickness equal
to or less than that of the marker paddles, may be pressed, glued,
riveted, threaded, or otherwise attached into holes or depressions
formed in the paddles.
[0079] Circumferential Bridges and Fixation Structures
[0080] According to the present invention, there is an array of
connecting bridges 8 that connect adjacent turns or columns of
struts 4 to provide the desirable overall stent flexibility as well
as structural integrity. It is advantageous to form these bridges 8
in a substantially circumferential direction, as shown in FIG. 7.
Two advantageous characteristics emerge by so forming the
connecting bridges 8. First, the vertical (circumferential) offset
caused by the bridges 8 ensures that, after expansion, the adjacent
180-degree bending segments (the vertices of the expanded strut
pairs) are offset from one another and, thus, will interdigitate,
allowing the stent 1 to bend easily. Second, these circumferential
bridges 8 are curved sharply in the plane perpendicular to the axis
10 of the stent 1, which curvature results from the stent 1 being
formed from small-diameter tubing. By careful control of the
expansion process, it is possible to expand the stent 1 while
retaining substantially all of the curvature of these bridges 8. In
the resulting expanded stent 1, these bridges 8, then, extend
radially away from the cylindrical surface of the stent 1 and
present edges perpendicular to the axis 10 of the stent 1. Thus,
during and after implantation, these features engage the vessel or
body lumen wall 30, preventing migration of the stent 1. The
enlargement of a bridge 8 in FIG. 8 illustrates how these
structures protrude beyond the wall of a stent 1 in this
manner.
[0081] Multi-mode Markers for Ultrasound, X-ray, and MRI
[0082] Customarily, radiopaque materials such as gold, tantalum,
zirconium oxide, barium and bismuth salts, hafnium, molybdenum,
etc., are attached to stents to enable visualization by x-rays. The
present invention is suitable for incorporating such markers,
especially at the location of the paddles 12, 13, as described
above.
[0083] In addition to the prior-art use of radiopaque markers, it
is possible to use other types of fiducial markers to enable
placement, deployment, and subsequent location and diagnosis of the
stent 1. Specifically, other non-illustrated markers can be made
that are easily imaged by ultrasound, such as abraded surfaces,
holes, voids, porous materials and coatings, hollow balloons, and
layered materials of different sonic properties, to name a few. For
example, a hole 0.50 millimeters in diameter may be filled with a
composite consisting of glass microballoons and tungsten powder
suspended in an epoxy matrix. Such a composite marker would be
highly visible under ultrasound imaging as well as x-ray imaging.
Additionally, markers having varying textures have improved
anchoring characteristics.
[0084] Magnetic resonance imaging may be enhanced by inclusion of
paramagnetic, diamagnetic, and ferromagnetic materials that locally
change the magnetic-field-producing spin-energy transitions in
odd-number nuclei such as hydrogen, carbon-13, fluorine-19, and
other nuclides known to those skilled in the art of magnetic
resonance imaging. Specifically, small pieces of gadolinium or
gadolinium salts (paramagnetic) provide visible changes to the
image formed by hydrogen nuclei in their vicinity, thus, such
materials can be incorporated into fiducial markers. Nano-scale
ferromagnetic materials, such as hematite or other oxides, can also
provide useful MRI artifacts without troublesome image
distortion.
[0085] Magnetically active elements, salts, and compounds can be
incorporated individually or n combination with other marker
materials, such as radiopaque materials or ultrasound-visible
structures or materials, to make multi-mode markers. Composite
markers may contain materials with magnetic properties suitable to
present fiducial marks on images made by magnetic resonance imaging
(MRI) as well as other imaging modalities. Examples include
combinations of radiopaque materials (such as, tungsten powder,
zirconium oxide, bismuth subcarbonate, and gold powder),
magnetically active materials such as diamagnetic or ferromagnetic
materials (including gadolinium foil and powder, gadolinium salts,
nanocrystalline iron oxide, and iron powder, for example), and
ultrasonically visible material such as glass or ceramic
microballoons.
[0086] Manufacturing
[0087] The standard method for manufacturing machined tubular metal
stents is to begin with a small-diameter metallic tube, typically,
of stainless steel, platinum alloy, or chromium-cobalt alloy for
balloon-expanded stents and of a nickel-titanium alloy for
self-expanding stents. This tubing is mounted in a laser machining
system that rotates the part around a stationary axis so that the
focal point of a laser beam impinges upon the surface of the tube.
When laser power is applied along with a coaxial jet of gas (either
air, oxygen, or an inert gas such as argon), the material is
perforated by the laser energy (and possibly assisted by chemical
reaction with air or oxygen). The tubing is moved under the laser
beam in at least two axes, rotational and longitudinal, so that a
continuous cut (or kerf) is made while the laser energy is applied.
The laser beam is switched on and off under computer control in
coordination with the longitudinal and rotational motions so that a
discontinuous pattern of cuts is applied to the tubing.
[0088] Following the laser-cutting operation, excessive material is
removed from the interior and exterior surfaces of the tubing, and
the tubing is further processed to produce either a
balloon-expandable or a self-expanding stent. In the case of a
balloon-expandable stent, the laser-cut tubing preform is polished
and cleaned using a combination of chemical, mechanical, and
electrochemical measures to produce a finished stent that is, then,
for example, crimped onto a balloon catheter. In the case of a
self-expanding stent, the laser-cut tubing is expanded by forcing
it onto a succession of larger and larger mandrels. At each step of
expansion, the tubing is subjected to an appropriate heat-treating
step to thermally set the expanded step. For example,
nickel-titanium tubing may be heat treated at 480 degrees Celsius
(480.degree. C./896.degree. F.) for thirty seconds while expanded
on a mandrel to set that stage of expansion. Typically, two to six
expansion stages are necessary to fully expand a nickel-titanium
self-expanding stent. After expansion, the stent is finished by a
combination of chemical, mechanical, and electrochemical polishing
to produce a smooth, biocompatible surface suitable for
implantation. The finished stent is, then, chilled (to transform it
to the soft and deformable martensitic condition) and compressed
radially to a size small enough to be placed into catheter of the
stent delivery system.
[0089] The Importance of Uniform Expansion During Manufacturing
[0090] One manufacturing problem that must be overcome with
self-expanding stents having the fine structures as described in
the present invention is uneven opening occurring during
thermo-mechanical expansion of the as-cut tubing to the final,
expanded stent. The standard manufacturing process involves
stretching the laser-cut stent over progressively larger
tapered-end cylindrical mandrels and heat-treating the material at
several stages while supported by these mandrels. The stent can be
expanded by stretching it onto the successive expansion mandrels
either at a low temperature (in the soft, martensitic condition) or
at ambient temperature (in the springy, austenitic condition). Once
expanded onto a mandrel, the stent is exposed for a short period
(several seconds to a few minutes) of high temperature, typically
in the 450 to 500 Celsius range, to "shape-set" or anneal the stent
at that level of expansion.
[0091] While the expansion process has been well understood by
stent manufacturers in the past, it is problematic because great
care must be exercised to make sure that no portion of the stent is
over-strained (over-stretching or over-bending) during the stages
of expansion. Over-straining can damage permanently the
superelastic material of which the stent is formed (typically a
nickel-titanium superelastic alloy), resulting in hidden defects
within the material that might cause immediate fracture or, worse,
fatigue failure after the stent has been implanted. Therefore,
manufacturers typically expand stents in several fractional steps,
and may employ elaborate measures, either by human skill or
tooling, to prevent any portion of the stent from being
over-strained. Over-straining is most commonly seen as a pair of
struts having an unusually large opening angle at their vertex
relative to the angle of other strut pairs in the vicinity. This
condition must be controlled and identified by in-process
inspection because it may be hidden by later expansion steps and
because it is an inherently unstable condition. That is, during a
given expansion step, once a pair of struts begins to open
excessively, that vertex becomes weakened, and the opening strains
tend to be further concentrated on that particular pair of struts,
so that it becomes progressively more over-strained.
[0092] Sacrificial Bridges
[0093] The present invention provides a process for preventing this
local over-straining. In the present invention, as compared to the
original number of bridges 8, 9 originally existing between
adjacent columns (or helical turns) of strut pairs in the
unfinished stent, only a few bridges 8 exist in the finished stent,
which remaining bridges 8 provide the desired flexibility and
resistance to fatigue. In the as-cut condition and during the steps
of expansion, additional sacrificial bridges 9 connect the bending
segments joining strut pairs in adjacent turns or columns. Thus,
when the stent 1 of the present invention is being expanded, it has
greatly improved robustness, and each pair of struts is connected
at the maximum number of points to adjacent parts of the expanding
stent. What is referred to herein as sacrificial bridges 9 provides
these additional connections and causes the expansion strains to be
much more evenly shared by all the elements of the stent, which
sharing results in a significant increase in the evenness of
strains during expansion. The result is an expanded stent with
vertex opening angles that have much less variation.
[0094] It is true that the sacrificial bridges 9 substantially
reduce the flexural (bending) flexibility of the stent 1. Thus,
they must be removed prior to finishing the stent 1. These
sacrificial bridges 9 may be removed at any stage after expansion,
but, preferably, they are removed immediately after the final
expansion heat-treating step, prior to any material-removal or
polishing steps, so that any burrs left by removal will be reduced
or eliminated during the polishing steps. Alternatively, the
sacrificial bridges 9 may be removed after some of the expansion
stages, but prior to one or more final expansion stage because it
has been found that, once the stent 1 has been partially expanded
in a very even manner, subsequent expansion steps do not generally
introduce unevenness among the opening angles. In any case, it is
only necessary to remove the extra, sacrificial bridges 9 at some
point prior to implantation so that the finished stent 1 has the
desired flexibility in its final, implanted form.
[0095] Bridge Removal Processes
[0096] To facilitate removal of the sacrificial bridges 9, special
features can be engineered into the as-cut structure to provide
prescribed locations for cutting or breaking the sacrificial
bridges 9. These features are illustrated in FIG. 9 as, for
example, notches 14 formed at one or both of the ends of the
sacrificial bridges 9 connected to the struts of adjacent turns 4.
While providing notches 14 is only one example to form the
cutting/breaking location, alternative exemplary methods of
removing sacrificial bridges include chemical etching, abrasive
blasting, grinding, electrochemical etching or polishing, shearing,
or laser cutting.
[0097] Final Burr Removal Processes
[0098] Customarily, stents are finished by a combination of
abrasive blasting, glass-bead honing, chemical etching, mechanical
polishing, and electrochemical polishing. All of these processes
assist removal of any remaining burr left by the removal of the
sacrificial bridges 9. In addition, other measures, such as
grinding, shearing, mechanical polishing, and cutting may be used
to locally smooth and remove burrs left by the sacrificial bridges
9.
[0099] FIG. 10 illustrates a flat cut pattern representing the
laser-cutting path that will be created around the circumference of
tubing from which the stent 1 is to be made. For clarity, the
pattern in FIG. 10 is broken along a longitudinal line to represent
it as a flat, two-dimensional pattern. In practice, however, this
two-dimensional flat pattern (representing width and length) is
transformed into a two-dimensional cylindrical pattern
(representing rotation and length) by the programming of the
computer-controlled laser-cutting machine so that the cut pattern
is arrayed continuously around the cylindrical surface of the tube.
The resulting cut pattern produces a cylindrical or helical array
of struts 5 to form the stent 1.
[0100] FIGS. 11 to 14 illustrate a portion of a stent 1 according
to the invention with the s-shaped struts 5 oriented in the
configuration shown in FIG. 3, i.e., the straight portions are
substantially aligned with the longitudinal axis of the stent 1
before expansion. In FIGS. 11 to 14, the right end of the stent is
not depicted and the left end is shown with flat-ended markers 13
extending from respective curved portions 7. The narrow portions of
the markers 13 have the same length and, therefore, the extreme
left flat ends of the markers 13 do not align along a single planar
surface orthogonal to the longitudinal axis of the stent 1 once the
stent is expanded. The embodiment of FIGS. 11 to 14 shows the stent
1 in an expanded state after the sacrificial bridges 9 have been
removed. As can be seen in each of FIGS. 11 to 14, the bridges 8
align along a circumference of the interior cylinder defined by the
stent 1. The interior cylinder depicted in FIGS. 11 to 14 is only
presented for illustrative purposes.
[0101] Using Very Narrow Kerfs in Stents with High Strut Count
[0102] It has been discovered that the manufacture of the stent 1
according to the present invention, in particular, the laser
cutting and expansion steps, are made substantially more difficult
when the size of struts 5 is reduced and the number of struts 5 is
increased. For example, it has been found that normal laser cutting
processes yield a finished kerf width (after material removal
processes needed to provide a stent with the desired polished
finish) of approximately 25 to 40 microns. If, for example, a total
of 46 struts were disposed around a circumference, then the total
circumferential width of kerfs would be at least 46.times.25
microns, or 1150 microns (1.15 millimeters). Of this kerf space,
half is not collapsible during compression of the stent, because
half of the kerfs are at the inside of the 180-degree bends that
join the ends of the struts. Hence, a stent of the current
configuration made by conventional manufacturing processes has at
least 0.57 millimeter of incompressible circumference resulting
from the kerfs at the 180-degree bends (corresponding to 0.18
millimeter of diameter reduction). However, by reducing the total
kerf from 25 microns to 18 microns according to the present
invention, the diameter after compression is reduced by 0.05
millimeters--a significant difference in fully collapsed diameter.
Moreover, by reducing the kerf from the conventional 25 microns to
18 microns, a further advantage is obtained--the remaining strut
widths are increased due to the fact that less metal is removed. In
the present example, reducing total kerf loss from 25 microns to 18
microns, assuming a pre-cut tubing diameter of 2 millimeters and 46
struts, the resulting strut width increases from 112 microns to 119
microns, resulting in a relative stiffness of (119/112).sup.3, or
120%, because stiffness is proportional to the cube of width.
[0103] The use of these very narrow kerfs is particularly
advantageous to the present invention because of the large number
of struts 5 in the configuration--strut counts from 36 to 50, as
compared with traditional stents customary strut count, typically
in the range of 24 to 32.
[0104] Cell Opening Size
[0105] The maximum embolus size that can pass through the wall of
an expanded stent is determined by the size of the openings between
the straight portions 6 and bending segments 7. More precisely, the
maximum embolus size is described by the largest circle that can be
inscribed within the openings of a particular stent in its open
configuration. It is, therefore, desirable to minimize the maximum
embolus size to prevent adverse results of embolization in
patients.
[0106] Referring to FIG. 6 of U.S. Pat. No. 6,129,755 to Mathis et
al. (which is hereby incorporated by reference in its entirety), it
can be seen that the maximum size embolus that can be passed
through the openings between struts has a diameter described by the
largest circle that can be inscribed within the space between two
adjacent struts and the vertex of a strut pair on the adjacent
column of struts. The volume of such an embolus is proportional to
the cube of the diameter. So, it can be seen that the volumetric
size of the largest embolus that can pass through the stent wall
becomes smaller by the third power as the strut geometry is
proportionally reduced in size (assuming otherwise similar geometry
of the strut openings). From this analysis, it can be appreciated
that the clinical effect of emboli can be substantially reduced by
using a greater number of shorter struts; hence, clinical safety
increases sharply with increases in the M-D Ratio, particularly in
regions of the vasculature, such as the carotid arteries, where
emboli are poorly tolerated and can have significant deleterious
effects upon the patient.
[0107] An expanded helical stent 1 according to the present
invention has openings sized to prevent a body (for example an
embolus or a substantially spherical body) of greater than
approximately 800 microns in diameter from passing therethrough. In
a preferred configuration, the expanded helical stent 1 according
to the present invention contains 46 struts of 120-micron width and
1000-micron length, for example. Such a configuration results in
openings that would allow an inscribed circle 15 of 610 microns.
This feature is illustrated in FIG. 15. By comparison, the Cordis 8
mm.times.50 mm SmartStent allows a much larger inscribed circle.
FIG. 16 shows the best-case alignment of the alternate rows of the
struts in the SmartStent, allowing an inscribed circle 16 of 1080
microns. The volume of an embolus of 1080 microns versus that of a
610-microns embolus is 5.5 times larger. Thus, it can be seen that
the present invention allows a much-increased ability to prevent
the passage of clinically significant emboli through its pores.
[0108] Another advantage of the present invention in prevention of
embolization is realized in the case where the stent 1 is implanted
in a bent, or non-straight, configuration. In prior-art stents,
bending causes opening of the space or gap between adjacent turns 7
or straight portions 6 of struts on the outside of the bend.
Because the present invention teaches the use of very short struts
(on the order of between approximately 600 and 1200 microns in
length) and, hence, a shorter helical pitch or column-to-column
distance, a bending deformation to a stent results in opening of
the gaps between several adjacent turns or columns of struts 4.
Thus, the distance by which any given gap is widened is reduced in
proportion to the number of gaps involved. For example, a stent 1
with struts 5 that are half as long will have twice as many gaps
affected by a bend, and the widening of each of these gaps will be
reduced by a factor of two.
[0109] Smooth Stiffness Gradient from High Bridge Frequency
[0110] Because stents 1 made according to the present invention
have a relatively high number of features compared with stents made
according to the prior art, and because there is a larger umber of
these features, including the straight portions 6 and the
180-degree loops 5 that provide local flexibility as well as the
bridges 8 joining adjacent turns or columns of struts 4 that
provide structural integrity to the overall structure, it is
possible to fine-tune the flexibility and compression/expansion
properties to a much finer extent than in prior-art stents with a
substantially smaller number of features. A typical prior-art stent
of the same size, for example, the Cordis 8 mm.times.50 mm
SmartStent, has approximately 700 struts. In comparison, for
example, an 8-millimeter diameter, 50-millimeter long stent 1
according to the present invention has approximately 1500
struts--more than a 100% increase.
[0111] It is possible to adjust the size and width of struts 5
along the length of the stent 1. However, the present invention
allows for much more precise use of this conventional construction
technique--because the features of the stent 1 are smaller, there
are more of them and, thus, the designer has a greater number of
features over which to create a gradient of properties such as
stiffness, radial outward force, flexural stiffness, surface area
(for drug-coating application), and diameter.
[0112] In a similar manner, because of the large number of
connecting bridges 8, 9 in the configurations taught by the present
invention, it is possible to introduce other property gradients
along the length of the stent 1. Among the properties affected by
bridge frequency and location are flexural stiffness and torsional
stiffness. Therefore, it is possible to construct a stent with
greater torsional rigidity in the central portion than in the ends,
or vice-versa. Similarly, it is possible to provide the stent 1
with more bending flexibility at its ends (and, hence, lower
stresses applied to the vessel walls) than in the central segment
by placing fewer connecting bridges 8, 9 at the ends of the stent 1
than in the middle. (Of course, the opposite possibility also
exists, providing a stent 1 with stiff ends and a more flexible
central segment, suitable for use in an area of the body where
flexion takes place.)
[0113] Short-Pitch Helix
[0114] Also, it can be seen that the short length of struts 5
results in a greater helix angle (or, a helical axis more closely
approaching perpendicular to the longitudinal axis) for a given
circumference of stent because the shorter struts 5 result in a
reduced helical pitch. There are several advantages to such an
increase in helix angle. First, the unevenness of the distal and
proximal ends of the stent is reduced because the step where the
end of the helix joins the previous turn is smaller (approximately
equal to the strut length). Such a reduced step provides for a
stent 1 with a substantially square-cut end (as is typically
desired by physicians) in an easier manner.
[0115] Second, the increased helix angle results in a stent 1 that
has a reduced tendency to twist as it is expanded. It can be easily
imagined that a helical stent with a very low helix angle, similar
to a corkscrew, would tend to wobble and twist when released from a
confining sheath. As the helix angle is increased toward
perpendicular (by reducing the strut length or helical pitch), a
helical stent behaves more and more like a non-helical stent
constructed of joined cylindrical hoops, resulting in even,
non-twisting behavior as it expands when released. Even though some
of the resulting properties of a stent with a high helix angle
approach those that are advantageous in a non-helical stent (such
as a nearly square end and resistance to twisting during
expansion), the advantageous properties intrinsic to a helical
stent are maintained, such as greater design freedom, lack of
distinct rigid and flexible zones along the length of the stent,
and more-uniform distribution of applied stresses and strains.
[0116] As set forth above, another configuration alternative that
becomes practicable with the very short struts 5 of the present
invention is the employment of a multiple-helix configuration. As
the number of starts is increased in the helix, the ends of the
stent 1 begin to become more square-cut in appearance; for example,
a triple-helix configuration would have three "notches" at the end
where the three loose ends are joined to the adjacent turn. Because
it is common to provide radiopaque markers at the ends of stents,
these three notches are advantageous locations for three markers,
resulting in a symmetrical, even end to the stent 1.
[0117] Torsional Compliance and Torsional Fatigue Resistance
[0118] The greater number of struts 5 and bridges 8, 9 of the
present invention result in the spreading of local forces and
deflections brought about in use to a larger number of features, so
that these local deformations are spread over a larger number of
deforming elements. As a result, each element is proportionately
less deformed. It is understandable that a stent with 1500 struts
will more readily absorb deformation and in flexion and torsion
than a stent with half as many struts, with an attendant reduction
in localized loads and deformations to the vessel or other body
lumen in which it is placed.
[0119] Torsional compliance in a helical stent is determined by the
ability of the helical strand of struts 5 to lengthen and shorten.
Hence, a longer strand of more numerous struts 5 and their joining
bending segments 7 will be better able to absorb lengthening and
shortening. The result is, for stents of a given radial compressive
strength and outward force, a configuration with a greater number
of short struts 5 that will be more easily torsioned than one with
a smaller number of longer struts 5. A related result is that,
because torsionally induced strains are reduced, any tendency
toward fatigue failure caused by torsional motions in-vivo is also
reduced.
[0120] Flexibility and Bending Fatigue Resistance
[0121] In the same way as torsional flexibility and fatigue
resistance is improved by increasing the number of flexing
elements, the flexural (or bending) flexibility and fatigue
resistance are also improved. Bending of a stent 1 causes adjacent
turns or columns of struts 4 to be forced either toward each other
(on the inside of a bend) or spread apart (on the outside of the
bend). Because connecting bridges 8 join adjacent turns or columns
4, the local deformations caused by stent bending are spread over
the struts 5 and bending segments 7 (the 180-degree loops that join
the ends of struts) between the connecting bridges 8. Thus, the
more elements (struts 5 and bending segments 7) that exist between
the connecting bridges 8, the greater number of elements there are
to absorb the deformations caused by stent bending. Also, in a
configuration with shorter struts 5, there is a greater number of
turns or columns 4 acted upon by bending the stent 1, so the total
number of elements deformed by bending the stent 1 is further
increased, resulting in much smaller deformations to each of the
elements. As deformations are reduced and strut widths are reduced,
the effective strains in the stent material are significantly
reduced, resulting in much improved fatigue resistance.
[0122] Enhanced Surface Area for Drug Elution
[0123] The large number of struts 5 of shorter length in a stent 1
made according to the teachings of the present invention has
greater surface area. For example, a stent 1 according to the
present teachings will have over twice as much kerf length than an
otherwise similar prior art stent with half as many struts around
the circumference. In self-expanding stents, kerf area (the area of
the cut radial faces of the stent's elements) is the major
contributor to total surface area because the area of the inner and
outer surfaces is relatively smaller, due to the high aspect ratio
(thickness to width) of the struts. Thus, the total surface area of
a stent 1 made according to the present teachings is substantially
larger than that of a stent made according to prior-art
configurations and, thus, it provides a larger surface area on
which to apply medicated coatings. This larger surface area allows
virtually all tissue within the coverage area of the stent to be in
the drug elution areas. In particular, the stent provides tissue
coverage so that no element of wall tissue is more than 350 microns
to 400 microns away from the nearest strut. Such a configuration
assures a short diffusion path from a strut covered with a
drug-eluting agent to any portion of the tissue.
* * * * *