U.S. patent application number 09/899147 was filed with the patent office on 2002-06-13 for stent with optimal strength and radiopacity characteristics.
Invention is credited to Burgermeister, Robert, Burpee, Janet.
Application Number | 20020072792 09/899147 |
Document ID | / |
Family ID | 22881625 |
Filed Date | 2002-06-13 |
United States Patent
Application |
20020072792 |
Kind Code |
A1 |
Burgermeister, Robert ; et
al. |
June 13, 2002 |
Stent with optimal strength and radiopacity characteristics
Abstract
Disclosed is a stent having improved characteristics of its
structural design and improved radiopacity characteristics.
Specifically, the present invention is a stent that has
circumferential sets of strut members at the ends of the stent and
central sets of strut members that are longitudinally placed
between the end sets of strut members. Optimal radiopacity is
achieved when the end sets of strut members are more radiopaque as
compared to the radiopacity of the central sets of strut members.
Also disclosed is the concept of adjusting the strut width of the
curved sections of the end and central sets of strut members so
that equal strain in all curved sections is achieved as the stent
is expanded even though the diagonals sections of the end sets of
strut members are shorter than the diagonal sections of the central
sets of strut members.
Inventors: |
Burgermeister, Robert;
(Bridgewater, NJ) ; Burpee, Janet; (Fair Haven,
NJ) |
Correspondence
Address: |
AUDLEY A. CIAMPORCERO JR.
JOHNSON & JOHNSON
ONE JOHNSON & JOHNSON PLAZA
NEW BRUNSWICK
NJ
08933-7003
US
|
Family ID: |
22881625 |
Appl. No.: |
09/899147 |
Filed: |
July 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60234497 |
Sep 22, 2000 |
|
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Current U.S.
Class: |
623/1.16 ;
623/1.34; 623/1.42; 623/1.46 |
Current CPC
Class: |
A61F 2002/91525
20130101; A61F 2/915 20130101; A61F 2230/0013 20130101; A61F
2250/0036 20130101; A61F 2002/91558 20130101; A61F 2250/0032
20130101; A61F 2002/91541 20130101; A61F 2002/91583 20130101; A61F
2210/0076 20130101; A61F 2250/0098 20130101 |
Class at
Publication: |
623/1.16 ;
623/1.34; 623/1.42; 623/1.46 |
International
Class: |
A61F 002/06 |
Claims
What is claimed is:
1. A stent in the form of a thin-walled, multi-cellular, tubular
structure having a longitudinal axis, the stent comprising a
multiplicity of circumferential sets of strut members, each set of
strut members being longitudinally separated each from the other,
each set of strut members being connected to adjacent sets of strut
members by longitudinal connecting links and each set of strut
members forming a closed, ring-like cylindrical portion of the
stent, each set of strut members consisting of a multiplicity of
connected curved sections and diagonal sections, each curved
section having two ends and a center situated there between, at
least one set of strut members having at least half of the curved
sections within the set of strut members having a tapered shape
wherein the width at the center of a curved section with a tapered
shape is greater than the width at the ends of a curved section
with tapered shape.
2. The stent of claim 1 wherein the curved sections of one or more
of the sets of strut members have inside and outside surfaces in
the shape of circular arcs each circular arc having a center of
curvature with the centers of curvature of the two arcs being
longitudinally displaced one from the other.
3. The stent of claim 1 wherein the width at the center of the
curved sections with a tapered shape is at least 0.001 inches
greater than the width at the ends of the curved section with
tapered shape.
4. The stent of claim 1 further comprising a multiplicity of sets
of flexible links with each set of flexible links connecting two of
the multiplicity of sets of strut members, each set of flexible
links consisting of a multiplicity of individual flexible links,
each individual flexible link being a single undulating structure
that extends generally in the longitudinal direction that is
parallel to the stent's longitudinal axis and each individual
flexible link having two ends, each one of the two ends being
fixedly attached to one curved section of the multiplicity of sets
of strut elements at an attachment point situated between the
center and the end of that curved section.
5. The stent of claim 1 wherein one or more of the curved sections
of the sets of strut members have a tapered shape with a greater
width at the center of the curved section compared to the width at
the center of at least one diagonal section.
6. The stent of claim 1 wherein all curved sections have a tapered
shape.
7. The stent of claim 1 wherein the sets of strut members include
end sets of strut members located at each end of the stent and
central sets of strut members positioned between the end sets of
strut members, the end sets of strut members having shorter
diagonal sections as compared to the length of the diagonal
sections of the central sets of strut members.
8. The stent of claim 7 wherein all curved sections of every
central set of strut members have a tapered shape.
9. The stent of claim 7 wherein the strut width at the center of
the curved sections of the end sets of strut members is less than
the strut width at the center of the curved sections of the central
sets of strut members.
10. The stent of claim 7 wherein the diagonal sections of the
central sets of strut members have a center and two ends, at least
one of the diagonal sections of the central sets of strut members
has a tapered shape wherein the width of the at least one diagonal
section is different at the center of the diagonal section as
compared to the width at either end of that diagonal section.
11. The stent of claim 10 wherein the width of the at least one
diagonal section is less at the center of that diagonal section as
compared to the width at either end of that diagonal section.
12. The stent of claim 10 wherein the width of the at least one
diagonal section is greater at the center of that diagonal section
as compared to the width at either end of that diagonal
section.
13. The stent of claim 10 wherein all the diagonal sections of all
of the central sets of strut members have a tapered shape.
14. The stent of claim 10 wherein all of the diagonal sections of
the end sets of strut members have a tapered shape.
15. The stent of claim 1 wherein the stent is coated with a plastic
material.
16. The stent of claim 15 wherein the plastic material is
parylene.
17. The stent of claim 16 wherein a drug is attached to the plastic
material.
18. The stent of claim 17 wherein the drug is from the family of
drugs that include Rapamycin.
19. The stent of claim 17 wherein the drug is Taxol.
20. The stent of claim 17 wherein the drug is heparin.
21. The stent of claim 17 wherein the drug is
phosphorylcholine.
22. The stent of claim 15 wherein the plastic material has a highly
radiopaque material mixed into the plastic material.
23. The stent of claim 22 wherein the radiopaque material is
tungsten.
24. The stent of claim 22 wherein the thickness of the coating is
greater at the ends of the stent as compared to the thickness of
the coating at the longitudinal center of the stent.
25. A stent in the form of a thin-walled, multi-cellular, tubular
structure having a longitudinal axis, the stent comprising a
multiplicity of circumferential sets of strut members, each set of
strut members being longitudinally separated each from the other,
at least one set of strut members having a tapered shape wherein
the width at the center of a strut portion with a tapered shape is
greater than the width at the ends of a strut portion with a
tapered shape.
Description
PRIORITY
[0001] This application bases its priority on the application
entitled, "Stent With Optimal Strength And Radio-opacity
Characteristics," Ser. No. 60/234,497, filed Sep. 22, 2000.
FIELD OF USE
[0002] This invention is in the field of stents for implantation
into a vessel of a human body.
BACKGROUND OF THE INVENTION
[0003] Stents are well known medical devices that are used for
maintaining the patency of a large variety of vessels of the human
body. A more frequent use is for implantation into the coronary
vasculature. Although stents have been used for this purpose for
more than ten years, and some current stent designs such as the
CORDIS BX Velocity.RTM. stent, Cordis Corporation, Miami Lakes,
Fla., have the required flexibility and radial rigidity to provide
an excellent clinical result, they are not always clearly seen
under standard fluoroscopy.
[0004] Many current tubular stents use a multiplicity of
circumferential sets of strut members connected by either straight
longitudinal connecting links or undulating longitudinal connecting
links. The circumferential sets of strut members are typically
formed from a series of diagonal sections connected to curved
sections forming a closed-ring, zig-zag structure. This structure
opens up as the stent expands to form the element in the stent that
provides structural support for the arterial wall. A single strut
member can be thought of as a diagonal section connected to a
curved section within one of the circumferential sets of strut
members. In current stent designs such as the BX Velocity.RTM.
stent, these sets of strut members are formed from a single piece
of metal having a uniform wall thickness and generally uniform
strut width. Although a stent with uniform width of the strut
members will function, if the width is increased to add strength or
radiopacity, the sets of strut members will experience increased
strain upon expansion. High strain can cause cracking of the metal
and potential fatigue failure of the stent under the cyclic stress
of a beating heart.
[0005] Existing highly radiopaque stents, such as the gold plated
NIROYAL stent sold by Boston Scientific, Inc., Natick Mass., can
obscure the inside of the vessel due to the high radiopacity over
the entire length of the stent. The BeStent sold by Medtronic,
Inc., Minneapolis Minn., has small gold markers at the ends of the
stent. Those markers only mark an end point without allowing
visualization of the entire end set of strut members.
[0006] Fischell et al, in U.S. Pat. No. 6,086,604, discloses a
stent with the end sets of strut members being gold plated. Such a
stent would have ideal radiopacity but may be subject to the
corrosive effects incurred through placement of dissimilar metals
in an electrolytic solution such as blood. There has also been
significant evidence that gold is a poor surface material for
stents because it may increase the risk of subacute thrombosis or
restenosis. Further, Fischell et al, in U.S. Pat. No. 5,697,971
discloses in its FIG. 7, a stainless steel stent with increased
width diagonal sections in all the circumferential sets of strut
members.
SUMMARY OF THE INVENTION
[0007] An ideally radiopaque stent would have end sets of strut
members that are highly radiopaque so that they can be readily
seen, even using low power fluoroscopy, and would further contain a
central section that is visible but not too bright so as to obscure
the lumen when high power cine film angiograms are taken. The stent
should also have only one material on its outside surface to avoid
potential corrosion; that material should not promote subacute
thrombosis or restenosis.
[0008] The present invention is a stent that is designed to have
optimal strength and radiopacity with good biocompatibility.
Unfortunately, the choices of appropriate biocompatible metals
available as thin wall tubing for stent construction are somewhat
limited. To achieve optimal radiopacity, the stent design of the
present invention is adjusted to the specific radiopacity and
strength characteristics of the metal from which the stent is
fabricated. What is more, coatings such as parylene may be needed
to avoid corrosion from stents with less biocompatible materials
and/or dissimilar metals on the stent's outer surface. Of extreme
importance to the present invention is the achievement of optimal
radiopacity in a stent that ideally is only 0.004 inches wall
thickness or less. Such a stent would have a pre-deployment outer
diameter (profile) that would be at least 0.003 inches less than
currently marketed stents. Ideally, the stent described herein
would have a wall thickness between 0.0025 inches and 0.004
inches.
[0009] Described herein are the novel design elements for stents
formed from the following materials:
[0010] 1. A highly radiopaque metal such as tantalum;
[0011] 2. Metals somewhat more radiopaque than stainless steel,
such as the cobalt based alloy L605;
[0012] 3. Stents coated or plated with highly radiopaque materials
like gold; and
[0013] 4. Layered materials such as alternative layers of tantalum
and stainless steel.
[0014] 5. The novel design elements that are described herein
include:
[0015] 1. Tapered Strut Width for Stents Formed from Highly
Radiopaque metals.
[0016] Although reducing the width of the longitudinally diagonal
section alone will reduce radiopacity without significantly
affecting radial strength, by having a taper on the curved sections
of the circumferential sets of strut members, a greatly reduced
level of strain upon stent expansion can be achieved without
sacrificing radial strength. This is extremely important, as it
allows a stent to be made much stronger than a stent with uniform
width of the strut members while staying within the same strain
limit for the material.
[0017] Tantalum is a metal that has been used in stents; which
metal is highly radiopaque. The optimal radiopacity for a stent
design using tantalum could have uniform width for the
circumferential sets of strut members and a wall thickness of about
0.0025 inches. To provide more radial strength and to reduce the
probability of the stent ends flaring out during deployment, a wall
thickness of about 0.003 inches to 0.035 inches would be highly
desirable. With uniform width sets of strut members, a 0.035 inches
wall thickness tantalum stent would be too bright under cine
angiography. To reduce the radiopacity of the design without
significantly impacting the radial strength of the deployed stent,
the present invention envisions curved sections and diagonal
sections, either or both of which could have a variable or tapered
width. The curved sections should be tapered (wider at the center
compared to the ends) to reduce strain as previously described. The
longitudinally diagonal sections can be thinner in the center than
at the ends, to reduce radiopacity for the central sets of strut
members.
[0018] It is envisioned that the novel stent described herein might
have wider diagonal sections for the end sets of strut members as
compared to the central sets of strut members. This feature would
enhance the radiopacity of the end sets of strut members while
retaining a moderate level of radiopacity for the central sets of
strut members. It is also envisioned to have both reduced width
diagonals and/or reduced wall thickness for the central sets of
strut members. It should be remembered that it is fluoroscopic
visualization of the end sets of strut members that is most
important for visualizing stents placed inside a coronary
artery.
[0019] 1. Thicker Diagonal Sections for Metals with Radiopacity
Slightly Better Than Stainless Steel.
[0020] The cobalt/tungsten alloy L605 is a stronger and more
radiopaque metal compared to stainless steel. To achieve optimal
radiopacity using L605 with uniform width sets of strut members,
the wall thickness is optimally equal to or greater than 0.0045
inches. To provide optimal radiopacity with such a metal in stents
of wall thickness 0.004 inches or less, the present invention
envisions wider diagonal sections in the sets of strut members.
Thus, the tapered diagonal sections would be wider than the curved
sections. The tapered curved section design for reduced strain may
also be highly desirable for stents made from the L605 alloy.
[0021] 2. End Sets of Strut Members with Thinner Curved
Sections.
[0022] Stent deliverability into curved coronary arteries is
improved when the diagonal sections of the end sets of strut
members have a decreased length as compared to the length of the
diagonal sections of the central sets of strut members. A shorter
length of the diagonal sections will also reduce outward flaring
upon expansion of the stent. Decreasing end flaring of the deployed
stent is of particular importance for stents having very thin
walls.
[0023] Previous designs that describe a stent with shorter diagonal
sections in the end sets of strut members are limited by the strain
limit allowed for the end sets of strut members. As a result, if
the end sets of strut members are made as strong as possible while
being limited by the maximum allowable strain for that metal, the
central sets of strut members will not have optimized radial
strength. The present invention envisions optimizing the radial
strength for all sets of strut members, i.e., the metal in all sets
of strut members just reach the maximum allowable strain at the
limiting diameter for the stent's expansion. To achieve this
desired attribute, the stent described herein has the curved
sections of the end sets of strut members being less wide than the
curved sections of the central sets of strut members.
[0024] 3. Good Side Branch Arterial Access While Maintaining Small
Cell Size.
[0025] The stents described herein are typically closed cell
stents, having a curved section of a central set of strut members
connected to an adjacent set of strut members by a longitudinally
extending link. In one embodiment of the present invention, the
circumferential sets of strut members are joined by undulating
longitudinal connecting links with each link having a multiplicity
of curved segments so as to increase the perimeter of the stent's
closed cells. One aspect of the present invention is that the
perimeter of each of the stent's closed cells should be at least 9
mm long. This design parameter allows each cell of the stent to be
expanded to a circular diameter of approximately 3 mm (i.e.,
9/mm.about.3 mm). This feature allows the "unjailing" of side
branches of the artery into which the stent is placed. The ideal
design to be radially strong, prevent plaque prolapse and still
allow sidebranch access will have a maximum deployed cell area of
less than 0.005 in..sup.2 while having a cell perimeter that is at
least 9 mm in length, so as to allow unjailing of side branches. A
good cell for side branch access should have a perimeter length
between 9 mm and 11 mm. (i.e. an expandable circular diameter
between 2.86 mm and 3.5 mm). Cell perimeters between 9.5 and 10 mm
are optimal.
[0026] 4. Flexible Undulating Longitudinal Links with Good Support
between Adjacent Sets of Strut Members.
[0027] To provide a strong bridge connection between adjacent
circumferential sets of strut members, the flexible undulating
longitudinal connecting links should have nearly equal extension in
the circumferential direction on each side of a line drawn between
the attachment points of the flexible undulating longitudinal
connecting link to the curved sections of adjacent sets of strut
members. "N" and inverted "N" shapes for the connecting links
inherently have equal circumferential displacement on each side of
the line connecting their attachment points. The specially designed
"M" or "W" shapes of the present invention also provide this
desirable attribute. Nearly equal circumferential lengths on either
side of a line drawn between the attachment points of the flexible
undulating longitudinal connecting links help in preventing plaque
from pushing the "M" or "W" shaped link inward into the lumen of
the stent when the stent is deployed into an artery.
[0028] The "M" and "W" shapes are of particular advantage in
obtaining the desired attribute of small area cells that have good
side branch access capability because of an increased perimeter
length. It should also be understood that the "M" and "W" shapes
each add an additional half cycle of undulating link length to the
cell perimeter as compared to an "N" shaped link design, thus
improving the stent's longitudinal flexibility. It should also be
noted that a "W" link is simply an inverted "M" link.
[0029] 5. Variable Thickness Radiopague Coatings. The NIROYAL.TM.
stent has a uniform thickness of gold plating, which makes the
center too radiopaque as compared to the radiopacity of the end
sets of strut members. Fischell et al., U.S. Pat. No. 6,086,604,
teaches stents having gold placed at the end sets of strut members.
This creates a potential for corrosion from dissimilar metals,
namely, gold and stainless steel. The present invention envisions a
gold coating that is sufficiently thick on the end sets of strut
members to provide optimal radiopacity with a thin coating of gold
on the rest of the stent. This design prevents obscuring of the
arterial lumen while providing an exterior surface for the stent
that is a single metal, thus avoiding electrolytic corrosion.
[0030] 6. Polymer Coatings for Stents Coated with Gold or Having
Dissimilar Metal Surfaces.
[0031] For stents with non-biocompatible or dissimilar metals, the
present invention envisions the use of a polymer such as parylene
to coat the entire outer surface of the stent. This would improve
biocompatibility and also allow attachment of organic compounds
such as heparin or phosphorylcholine for reduced thrombogenicity or
drugs, such as taxol or rapamycin, for reduced cell proliferation
and a decreased rate of restenosis. It is also known that highly
radiopaque materials like tungsten can be mixed into polymers. A
stent coating including a plastic with mixed in radiopaque metal
could be used to enhance both radiopacity and biocompatibility.
Such a polymer coating could also be advantageous with a
gold-coated stent.
[0032] 7. Providing a Variable Wall Thickness.
[0033] The present invention also envisions next generation
manufacturing techniques using photo-etching, whereby a stent
pattern is etched into a thin-walled metal tube. These techniques
already can produce variations in wall thickness as well as strut
width for any stent pattern. The present invention envisions use of
these techniques to create stents with optimal radiopacity. In
particular for a stent formed from a single metal or alloy, thicker
metal at each end of the stent could increase radiopacity there as
compared to the central section of the stent. Perhaps more
important is the use of multi-thickness etching techniques with a
two- or three-layered tube where one of the layers is a highly
radiopaque material such as tantalum. For example, a two-layer tube
having one layer of stainless steel and a second layer of tantalum
could be etched to provide the end sets of strut members with 0.001
inches of tantalum and 0.0025 inches of stainless steel while the
remainder of the stent would have less than 0.0005 inches of
tantalum with a stainless steel layer of 0.003 inches. It is also
envisioned that there could be tantalum only on the end sets of
strut members. Thus, one could produce a stent with enhanced
radiopacity at the ends with the stent having a uniform wall
thickness.
[0034] One could even have a stent with increased wall thickness of
a metal at the central region of the stent but still having a
decreased radiopacity at that central region if, for example, the
stent had tantalum end struts with stainless steel center struts.
Such a stent would be strongest in the center where the thickest
plaque must be restrained.
[0035] It is also envisioned that any of the above optimal
radiopacity stent designs may be used with plastic coatings such as
parylene, antithrombogenic coatings such as heparin or
phosphorylcholine, or anti-proliferative coatings such as taxol or
rapamycin.
[0036] Thus it is an object of the present invention to have a
stent with tapered curved sections, the center of the curved
sections being wider than ends of the curved sections so as to
reduce plastic strain as the stent is expanded as compared to a
curved section with uniform width.
[0037] Another object of the present invention is to have a stent
with tapered diagonal sections in the sets of strut members where
the center of the diagonal section is narrower than the ends to
reduce the radiopacity of central sets of strut members of the
stent as compared to a stent with diagonal sections having a
uniform width.
[0038] Still another object of the invention is to have a stent
with decreased wall thickness at the central struts compared to the
end struts so as to have a comparatively higher radiopacity for the
end sets of strut members.
[0039] Still another object of the present invention is to have a
stent with tapered diagonal sections for one or more of the sets of
strut members where the center of the diagonal section is wider
than the ends to increase the radiopacity of the end sets of strut
members as compared to a stent with uniform width of the diagonal
sections.
[0040] Still another object of the present invention is to have end
sets of strut members having both shorter diagonal sections and
thinner width curved sections as compared to those sections in the
central sets of strut members.
[0041] Still another object of the present invention is to have a
tantalum stent with wall thickness less than 0.035 inches having
tapered sets of strut members whereby the diagonal sections are
less wide than the width at the center of the curved sections.
[0042] Still another object of the present invention is to have a
closed cell stent design with maximum post-deployment cell area
less than 0.005 square inches and a cell perimeter length that is
equal to or greater than 9 mm.
[0043] Still another object of the present invention is to have a
stent with a radiopaque metal coating where the radiopaque metal
coating has greater wall thickness on the end sets of strut members
as compared to thickness on the sets of strut members at the center
of the stent.
[0044] Still another object of the present invention is to have a
stent etched from a multi-layer metal tube having one layer
significantly more radiopaque than at least one other layer; the
etched stent being formed with increased wall thickness of the more
radiopaque layer on the end sets of strut members as compared with
the sets of strut members at the center of the stent.
[0045] Still another object of the present invention is to have a
closed cell stent design with "M" or "W" shaped flexible undulating
longitudinal connecting links wherein the circumferential extent of
the flexible undulating longitudinal connecting links is
approximately equal on each side of a line drawn between the
proximal and distal attachment points of the flexible undulating
longitudinal connecting link.
[0046] Still another object of the present invention is to have the
stent with optimized radiopacity formed with an outer surface that
is plastic coated to improve biocompatibility.
[0047] Still another object of the present invention is to have the
stent with optimized radiopacity that is coated with a plastic
material and an additional organic compound to prevent thrombus
formation and/or restenosis.
[0048] Still another object of the present invention is to have a
stent coated with a plastic material that includes a radiopaque
filler material.
[0049] These and other objects and advantages of this invention
will become apparent to the person of ordinary skill in this art
field upon reading of the detailed description of this invention
including the associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 is a flat layout of a prior art stent having uniform
strut width for the circumferential sets of strut members.
[0051] FIG. 2 is a flat layout of a prior art stent design having
"M" and "W" flexible connecting links.
[0052] FIG. 3 is an enlargement of the "M" link of the stent design
of FIG. 2.
[0053] FIG. 4 is an enlargement of the improved "M" link design of
the present invention.
[0054] FIG. 5 is a flat layout of the present invention stent
design for a highly radiopaque metal.
[0055] FIG. 6 is a flat layout of part of the present invention
stent design of FIG. 5.
[0056] FIG. 7 is a flat layout of an alternate embodiment of part
of the present invention stent design of FIG. 5.
[0057] FIG. 8 is a flat layout of the present invention stent
design for a somewhat radiopaque metal.
[0058] FIG. 9 is a flat layout of the present invention stent
design for a stent coated with a radiopaque metal.
[0059] FIG. 10 is a flat layout of an alternate embodiment of the
present invention stent including an "N" shaped flexible connecting
link.
[0060] FIG. 11 is a flat layout of the present invention stent
design as photo-etched from a tube.
[0061] FIG. 12A is an enlargement of a section of the photo-etched
stent of FIG. 11.
[0062] FIG. 12B is a longitudinal cross section at 12-12 of the
enlarged section of FIG. 11 shown in FIG. 12A, the stent having a
radiopaque coating that is thickest on the end sets of strut
members.
[0063] FIG. 12C is a longitudinal cross section at 12-12 of the
enlarged section of FIG. 11 shown in FIG. 12A, as etched from a
two-layer tube where one of the tube layers is a moderately
radiopaque metal and the other layer is a highly radiopaque
metal.
DETAILED DESCRIPTION OF THE DRAWINGS
[0064] FIG. 1 shows a flat layout of an embodiment of a prior art
stent described by Fischell et al in U.S. Pat. No. 6,190,403. The
stent 5 of FIG. 1 is shown in its crimped, pre-deployed state, as
it would appear if it were cut longitudinally and then laid out
into a flat, 2-dimensional configuration. The stent 5 comprises end
sets of strut members 2 located at each end of the stent 5 and
three central sets of strut members 6 connected each to the other
by sets of longitudinally extending undulating "N" links 4. The end
sets of strut members 2 consist of alternating curved sections 7
and diagonal sections 9. The central sets of strut members 6
located longitudinally between the end sets of strut members 2
consist of alternating curved sections 3 and diagonal sections 8.
In the prior art stent 5, the longitudinally diagonal sections 9 of
the end sets of strut members 2 are shorter in length than the
longitudinally diagonal sections 8 of the central sets of strut
members 6. The shorter diagonal sections 9 will reduce the stiff
longitudinal length of metal at the ends of the stent 5 to improve
deliverability (by reducing "fish-scaling") and will also increase
the post-expansion strength of the end sets of strut members 2 as
compared with the central sets of strut members 6. In this prior
art stent, the width of the curved sections 3 and 7 and the
diagonal sections 8 and 9 are all the same. There is no variation
in width within any set of strut members or between the end sets of
strut members 2 and the central sets of strut members 6. The stent
5 is a design well suited to stainless steel having a wall
thickness of 0.0045" or greater, such as found in the CORDIS BX
Velocity.RTM. stent.
[0065] If the stent 5 were formed from a highly radiopaque metal
such as tantalum with wall thickness of 0.0030 to 0.0035 inches and
with sets of strut members 6 having widths of greater than the
0.005 inches that is necessary for good radial strength, then the
stent would be too radiopaque. In addition, with a wall thickness
of 0.003 inches or less, the end sets of strut members 2 might have
a tendency to flare outwardly into the vessel wall upon expansion.
If the end sets of strut members 2 are designed to be as strong as
possible while not exceeding metal strain limits at the largest
usable diameter of the stent 5, then the central sets of strut
members 6 with longer diagonal sections 8 will not have maximized
radial strength assuming the same strut width for both central sets
of strut members 6 and end sets of strut members 2. Optimized
strength at the longitudinal center of a stent is important, as it
is that region that must typically hold back a larger amount of
plaque than at the ends of the stent.
[0066] One embodiment of the present invention provides that each
set of strut members should have maximized radial strength rather
than having the central sets of strut members 6 being less strong
than the end sets of strut members as previously described. This
design would be similar to the stent 5 of FIG. 1 with the novel
improvement being that the width of the curved sections 3 of the
central sets of strut members 6 would be greater than the width of
the curved sections 7 of the end sets of strut members 2. The
greater width of the curved sections 3 will increase the strength
of the central sets of strut members 6 compensating for loss of
radial strength because of the longer diagonal sections 8.
[0067] The stent 60 shown in FIG. 2 is a flat layout of a prior art
stent design having "N", "M" and "W" flexible connecting links. The
stent 60 is shown in its crimped pre-deployed state, as it would
appear if it were cut longitudinally and then laid out into a flat,
2-dimensional configuration. It should be clearly understood that
the stent 60 is in fact cylindrical in shape, which cylindrical
shape would be obtained by rolling the flat configuration of FIG. 2
into a cylinder with the top points "G" joined to the bottom points
"H". The stent 60 is typically fabricated by laser machining of a
cylindrical, stainless steel tube.
[0068] A central set of strut members 62 is a cylindrical, closed,
ring-like section of the stent 60 consisting of a multiplicity of
curved sections 63 connected to diagonal sections 68. Every curved
section 63 of each central set of strut members 62 is attached to a
connecting link which is either a flexible "N" link 44, "M" link 64
or a "W" link 84. The stent 60 also has two end sets of strut
members 72 consisting of a multiplicity of curved sections 73
connected to diagonal sections 78. In this embodiment, half of the
curved sections 73 of the end set of strut members 72 are attached
to "N" links 44 with the other half of the curved sections 73
situated at the extreme ends of the stent 60. The diagonal sections
78 of the end sets of strut members 72 are shorter than the
diagonal sections 68 of the central sets of strut members 62.
Shorter diagonal sections enhance the post-expansion radial
strength of the end sets of strut members 72 as compared to the
central sets of strut members 62.
[0069] FIG. 3 is an enlargement of the "M" link 64 of the prior art
stent of FIG. 2. One disadvantage of this design relates to the
circumferential extent of the "M" link 64 with respect to a line 65
that could be drawn between the two attachment points 68 where the
"M" link 64 attaches to the curved sections 63. Because almost all
of the "M" link 64 lies above the line 65, pressure on the top of
the "M" link 64 from plaque in an artery could bend the top of the
"M" link 64 inward into the arterial lumen. This would be highly
undesirable. Ideally, an "M" or "W" link should have an equal
circumferential extent on either side of a line drawn between the
attachment points to adjacent sets of strut members as shown in
FIG. 4.
[0070] One aspect of the present invention is an improved "M" link
14 as shown in FIG. 4. The "M" link 14 has a circumferential extent
(i.e., length) L' above and L" below the line 15. The line 15 is
drawn between the attachment points 18 where the "M" link 14
attaches to adjacent curved sections 13. Such a balanced design
would diminish any likelihood of the flexible connecting link 14
from expanding into the arterial lumen.
[0071] FIG. 5 is a flat layout view of a stent 20 that includes
some embodiments of the present invention. The design of FIG. 5 is
particularly applicable to stents made from a highly radiopaque
metal such as tantalum. The stent 20 of FIG. 5 is shown in flat,
layout view based on its pre-deployed state, as it would appear
before it is crimped onto a balloon catheter. The stent 20
comprises end sets of strut members 22 located at each end of the
stent 20 and central sets of strut members 26 connected each to the
other by sets of individual flexible "M" links 24. The "M" links 24
are similar to the "M" linkl4 of FIG. 4. The end sets of strut
members 22 consist of a multiplicity of curved sections 27
connected to diagonal sections 29. The central sets of strut
members 26 located longitudinally between the end sets of strut
members 22 consist of a multiplicity of curved sections 23
connected to diagonal sections 28.
[0072] One can also define a strut element 25 as being composed of
one adjacent curved section 23 joined to a diagonal section 28. As
seen in FIG. 5, it is clear that one can describe a central set of
strut members 26 as being a closed, circumferential, ring-like
structure comprising a multiplicity of connected strut elements 25.
An end set of strut members could be likewise defined as being a
multiplicity of connected strut elements 17.
[0073] The stent 20 is a closed cell stent having cells 19 formed
from portions of adjacent sets of strut members connected by "M"
links 24. For coronary arteries, prolapse of plaque into the
arterial lumen will be minimized if the area within the cell 19
does not exceed 0.005 square inches at all diameters up to the
maximum deployment diameter of the stent 20. An important aspect of
stent design is to be able to place a guidewire through the
expanded cell 19, into a side branch vessel. A balloon angioplasty
catheter can then be advanced over the guidewire and inflated to
enlarge and circularize the opening of the cell 19 to "unjail" the
side branch vessel. By "unjailing" is meant removing metal from the
ostium of the side branch vessel, thus improving blood flow to that
side branch. One concept of the present invention is that the cell
19 has an interior length of the perimeter that is at least 9 mm.
Since balloon dilatation of the cell 19 would cause it to be near
circular, an inside perimeter length around inside of the cell 19
would provide an inside diameter of 9/, which is approximately 3
mm. A good cell design for side branch access should have an inside
perimeter length between 9 mm and 11 mm. (i.e., an expanded inside
circular diameter between 2.86 and 3.5 mm) where cell perimeters
between 9.5 and 10 mm are optimal and would be suitable for
essentially any side branch of a coronary artery.
[0074] In the stent 20, the diagonal sections 29 of the end sets of
strut members 22 are shorter in length than the diagonal sections
28 of the central sets of strut members 26. The shorter diagonal
sections 29 will reduce the longitudinal extent of the metal strut
at the end of the stent to improve deliverability into a vessel of
the human body by decreasing fish-scaling. In the stent 20, the
width of the curved sections 23 and 27 and the diagonal sections 28
and 29 are different as compared to the prior art stents 5 and 6 of
FIGS. 1 and 2.
[0075] The exact design of the stent 20 is most clearly seen in the
expanded view of the stent section 21 of FIG. 5 as shown enlarged
in FIG. 6. FIG. 6 shows that the curved sections 23 (of the central
sets of strut members 26 of FIG. 5) have a width at the center of
the curve W.sub.c. The width of the curved sections 23 taper down
as one moves away from the center of the curve until a minimum
width Wd is reached at the center of the section 28. To achieve
this taper, the inside arc of the curved section 23 has a center
that is longitudinally displaced from the center of the outside
arc. This tapered shape for the curved section 23 provides a
significant reduction in metal strain with little effect on the
radial strength of the expanded stent as compared to a stent having
sets of strut members with a uniform strut width.
[0076] This reduced strain design has several advantages. First, it
can allow the present invention design to have a much greater
usable range of radial expansion as compared to a stent with a
uniform strut width. Second, it can allow the width at the center
of the curve to be increased which increases radial strength
without greatly increasing the metal strain (i.e. one can make a
stronger stent). Finally, the taper reduces the amount of metal in
the stent and that should improve the stent thrombogenicity.
[0077] FIG. 6 also shows a unique design for the end sets of strut
members 22. The diagonal sections of the end sets of strut members
22 have a length Lend that is shorter than the length L of the
diagonal sections 28 of the central sets of strut members 26. To
maximize the radial strength of a stent along its entire length,
each set of strut members should just reach the maximum allowable
plastic strain for the metal being used at the largest allowable
expanded diameter of the stent. In the stent of FIG. 1, the curved
sections 7 of the end sets of strut members 2 and the curved
sections 3 of the central sets of strut members 6 have the same
widths. As a result, the end sets of strut members 2 (which have
shorter diagonal sections 9) will reach the maximum allowable
diameter at a level of strain that is greater than the level of
strain experienced by the central sets of strut members 6.
[0078] An optimum strength stent design would have the same strain
at the maximum stent diameter for both the end sets of strut
members 2 and the central sets of strut members 6. For the stent
design of FIGS. 5 and 6, one desires to have the end sets of strut
members 22 reach the maximum strain limit at the same stent
diameter as the central sets of strut members 26. The present
invention teaches a design with the width at the center of the
curve W.sub.c.sub..sub.end of the curved section 27 being less than
the width W.sub.c of the curved sections 23 of the central sets of
strut members 26. This reduced width for the curved sections 23
compensates for the shorter length L.sub.end of the end diagonal
sections 29 so that there is the same strain in both the central
and end sets of strut members 22 and 26 respectively as the stent
20 is expanded to its maximum allowable diameter.
[0079] The end sets of strut members 22 can also be tapered like
the central sets of strut members 26 where the width of the strut
tapers down as one moves away from the center of the curve of the
curved sections 27 until a minimum width W.sub.d.sub..sub.end is
reached at the diagonal section 29. The curved sections 23, 27 each
have an inside (concave) arc and an outside (convex) arc. Each arc
has a center that is longitudinally displaced from the other
center.
[0080] The tapered strut design shown in FIGS. 5 and 6 also has an
advantage for stents made from highly radiopaque metals such as
tantalum. If one uses uniform strut width as seen with the stent 5
of FIG. 1, then a properly designed thin-walled (0.0025 inches to
0.035 inches) wall tantalum stent may be too radiopaque. The
reduced metal from the thinner diagonal sections 28 and 29 will
decrease the radiopacity without affecting radial strength. Nominal
dimensions and dimension ranges (all in inches) for a tantalum
stent produced using the design of FIG. 5 are as follows:
1 Element Nominal Range W.sub.c 0.006 0.0045 to 0.007 W.sub.d
0.0045 0.035 to 0.005 W.sub.c.sub..sub.--.sub.end 0.0045 0.004 to
0.005 W.sub.d.sub..sub.--.sub.end 0.0045 0.035 to 0.005 L 0.028
0.020 to 0.030 L.sub.end 0.025 0.015 to 0.026 Wall Thickness 0.003
0.0025 to 0.035
[0081] Although the present invention shows the "M" shaped flexible
link 24 being used, the present invention strut designs will
function with any link shape including "N", "W", "S" "U", "V" and
inverted "N", "U" and "V" designs. It should also be noted that the
"M" link 24 shown in FIG. 6 has exactly five longitudinally
extending curved segments 24A, 24B, 24C, 24D and 24E.
[0082] FIG. 7 is an alternative embodiment 21' of section 21 shown
in FIG. 6 of the present invention stent 20 of FIG. 5. In this
embodiment, the only difference is the shape of the diagonal
sections 28'. The diagonal sections 28 of FIG. 6 have uniform
thickness. The diagonal sections 28' of FIG. 7 are tapered from a
width W.sub.d" at the end of the diagonal section 28' where it
connects to the curved sections 23' to a width W.sub.d' at the
center of the diagonal section 28'. The advantage of the inward
taper of the diagonal sections 28' is that removal of more metal
will reduce the radiopacity of the longitudinal center region of
the stent 20 as compared to a stent with uniform width diagonal
sections 28 as seen in FIG. 6. The additional taper may also
further reduce the metal strain as the stent is expanded. Although
one could taper the diagonal sections 29 of the end sets of strut
members 22 of FIG. 5, there is an advantage in having the end sets
of strut members 22 being more radiopaque than the central sets of
strut members 26. This is because visualization of the stent ends
is the most important aspect of radiopacity for a stent. Therefore,
a preferred embodiment of the present invention is as seen in FIG.
7 to have tapered diagonal sections 28' in the central sets of
strut members 26 and uniform thickness diagonal sections 29 (having
a greater average width) for the end sets of strut members 22.
[0083] Instead of connecting every curved section with a flexible
link, an alternate embodiment may use straight links connecting
only half of the curved sections of the sets of strut members. Such
a stent could also have the advantage of a reduced strain strut
design as shown in FIGS. 5, 6 and 7.
[0084] For the stent of FIG. 5, it should also be understood that
the wall thickness of the central set of strut members 26 could be
thinner that the wall thickness of the end set of strut members 22.
Also it should be noted that the "M" links 24 also have a much
narrower width as compared to the width of any strut member of the
end set of strut members. Both these attributes of the stent 20
create the following desirable radiopacity characteristics: highly
radiopaque end sets of strut members and decreased radiopacity at
the central region of the stent 20.
[0085] FIG. 8 is a flat layout view of another embodiment of the
present invention showing a stent 30 made from a moderately
radiopaque metal such as the cobalt-tungsten alloy L605. The alloy
L605 has great radial strength and is approximately 20% to 30% more
radiopaque than stainless steel. Therefore, with L605, the same
level of radiopacity is achieved with a stent wall thickness that
is 20% to 30% less than a stent made from stainless steel. One goal
in the use of L605 would be to reduce the wall thickness by 30% but
end up with a stent that is still more radiopaque than an
equivalent stainless steel stent such as the stent 5 shown in FIG.
1.
[0086] The stent 30 of FIG. 8 is shown in a layout view based on
its pre-deployed state, as it would appear before it is crimped
onto a balloon catheter. The stent 30 comprises end sets of strut
members 32 located at each end of the stent 30 and central sets of
strut members 36 connected each to the other by sets of flexible
"M" links 34. The "M" links 34 are similar to the "M" links 14 of
FIG. 4. Each end set of strut members 32 comprises alternating
curved sections 37 and diagonal sections 39 connected together to
form a closed circumferential structure. The central sets of strut
members 36 located longitudinally between the end sets of strut
members 32 comprises curved sections 33 and diagonal sections 38
connected together to form a closed circumferential ring-like
structure.
[0087] In the stent 30, the diagonal sections 39 of the end sets of
strut members 32 are shorter in length than the diagonal sections
38 of the central sets of strut members 36. The shorter diagonal
sections 39 will reduce the longitudinal length of metal at the end
of the stent to improve deliverability into a vessel of the human
body. In the stent 30, the widths of the diagonal sections 38 and
39 are different as compared to the prior art stents 5 and 60 of
FIGS. 1 and 2.
[0088] The novel concepts of the stent of FIG. 8 are shown most
clearly in the expanded view of the stent section 31 shown in FIG.
9. In FIG. 9 it can be seen that the diagonal sections 38 of the
central sets of strut members 36 have a width at the center T.sub.c
and a width at the end T.sub.e where the width in the center
T.sub.c is larger than the width at the end T.sub.e. This allows
for increased radiopacity without affecting the design of curved
sections 33 that are the primary stent elements involved for stent
expansion. The curved sections 33 and 37 shown in FIG. 9 are
tapered similar to the curved sections 23 and 27 of FIG. 6. It is
also envisioned that the curved sections 33 and 37 could have
uniform width similar to the curved sections 3 and 7 of FIG. 1. The
diagonal sections 39 of the end sets of strut members 32 also have
a tapered shape. The diagonal sections 37 have a width in the
center T.sub.c.sub..sub.--end and a width at the end
T.sub.e.sub..sub.--end where the width in the center
T.sub.c.sub..sub.--end is larger than the width at the end
T.sub.e.sub..sub.--end. Because of the desire for the end sets of
strut members 32 to be the most radiopaque part of the stent 30,
the diagonal section 39 center width T.sub.c.sub..sub.--end of the
end sets of strut members 32 is shown in FIG. 9 to be wider than
the width T.sub.c of the diagonal section 38. A wider piece of
metal will be more radiopaque. Thus, the stent has curved sections
with a single bend connecting the diagonal sections of its sets of
strut members, and flexible connecting links connecting the curved
sections of its circumferential sets of strut members.
[0089] The stent of FIG. 10 is an alternate embodiment of the
present invention showing central sets of strut members 46 having
curved sections 43 and diagonal sections 48 with tapered shapes
similar in design to the curved sections 23' and diagonal sections
28' of the stent section 21' shown in FIG. 7. The stent 40 of FIG.
10 is shown in a layout view in its pre-deployed state, as it would
appear before it is crimped onto a balloon catheter. The stent 40
comprises end sets of strut members 42 located at each end of the
stent 40 and central sets of strut members 46. The sets of strut
members 42 and 46 are connected each to the other by sets of
individual flexible "N" links 44. The "N" links 44 are similar in
shape but slightly longer than the "N" links 4 of FIG. 1. The end
sets of strut members 42 consist of curved sections 47 and diagonal
sections 49. The central sets of strut members 46 located
longitudinally between the end sets of strut members 42 consist of
curved sections 43 and diagonal sections 48.
[0090] The stent 40 is a closed cell stent having cells 45 formed
from portions of adjacent sets of strut members connected by "N"
links 44. Prolapse of plaque through the closed cells 45 is
minimized if the expanded area of the cell 45 is less than about
0.005 in..sup.2 at any diameter up to the maximum deployment
diameter of the stent 40. It is also important for an optimum stent
design that a guidewire can be placed through the expanded cell 45
into a side branch vessel. A balloon angioplasty catheter would
then be advanced over the guidewire, through the cell 45 and
inflated to "unjail" the side branch, i.e. remove any stent strut
that is blocking blood flow into that side branch. The present
invention design should have an interior perimeter of the cell 45
that is at least 9 mm, thus allowing a nearly 3 mm diameter
circular opening to be achieved for unjailing.
[0091] FIG. 11 is a flat layout view of another embodiment of the
present invention in the form of a stent 50 that is photo-etched
from a metal tube. The stent 50 is shown in its pre-deployed state,
as it would appear before it is crimped onto a balloon catheter.
The stent 50 comprises end sets of strut members 52P and 52D
located respectively at the proximal and distal ends of the stent
50. The stent 50 also has central sets of strut members 56
connected each to the other by sets of flexible "M" links 54. The
"M" links 54 are similar to the "M" links 14 of FIG. 4. The end
sets of strut members 52P and 52D each consists of curved sections
57 and diagonal sections 59. The central sets of strut members 56
located longitudinally between the end sets of strut members 52
consist of curved sections 53 and diagonal sections 58.
[0092] The section 55 of the photo-etched stent 50 is shown
enlarged in FIG. 12A. The FIGS. 12B and 12C show two embodiments of
the present invention that can provide a stent with enhanced
radiopacity at the stent ends.
[0093] FIG. 12A shows diagonal sections 58 and 59 and an "M" link
54 connecting the curved sections 53 and 57.
[0094] FIG. 12B is a longitudinal cross section at 12-12 of the
stent section 55 shown in FIG. 12A. The stent design shown in FIG.
12B has a highly radiopaque coating that is thicker on the end sets
of strut members 52 as compared to the thickness on either the flex
links 54 or the central sets of strut members 56. FIG. 12B shows
the coating 57C on the curved section 57 of the end set of strut
members 52 being thicker than the coating 54C on the flex link 54
and also thicker than the coating 53C on the curved section 53. The
most likely coating for the stent 50 would be gold plating although
platinum, tantalum or any other highly radiopaque metal could be
used.
[0095] The present invention has the entire stent coated to provide
an exterior surface for the stent 50 that is formed from a single
metal. This reduces the potential for corrosion that can occur with
dissimilar metals on the stent's exterior surface when the stent is
placed in a saline solution such as blood.
[0096] It is also envisioned that even with the entire stent coated
with a highly radiopaque metal, an additional coating of a flexible
plastic such as parylene may be desirable. Such an organic coating
has the additional advantage of allowing the attachment of drugs
such as taxol or rapamycin to reduce restenosis. Techniques for
gold plating metals such as stainless steel and controlling the
thickness of the plating are well known in the art of metal
plating.
[0097] FIG. 12C is the longitudinal cross section at 12-12 of yet
another alternate embodiment of the enlarged section 55 of FIG. 11
shown in FIG. 12A. The stent design shown in FIG. 12C is etched
from a two-layer tube where one of the tube layers is a metal of
conventional radiopacity such as stainless steel and the other
layer is a highly radiopaque metal such as tantalum. Although the
total wall thickness of the stent of this embodiment remains nearly
constant, the end sets of strut members 52' have a thicker layer of
the radiopaque metal than the flex links 54' or the central sets of
strut members 56'. The curved section 57' of the end set of strut
members 52' has conventional metal layer 57N' and radiopaque metal
layer 57R'. The flex link 54' has a standard metal layer 54N' and a
radiopaque metal layer 54R'. The central sets of strut members 56'
have curved sections 53' with conventional metal layers 53N' and
radiopaque metal layers 53R'.
[0098] It can be seen from FIG. 12C that the radiopaque metal layer
57R' of the end sets of strut members 52' is thicker than the
radiopaque metal layers 54R' and 53R'. In recent years, multi-layer
photo-etching processes for metals that can control the thickness
of individual layers have been developed so that the embodiment of
FIG. 12C can be produced within the current state of the art of
photo-etching. Using this approach, two and three layer tubing is
now available from several manufacturers and can be photo-etched to
make a stent with an optimal design which is high radiopacity for
the end set of strut members and reduced radiopacity for the
central sets of strut members. Specifically, a stent with the
characteristics as seen in FIG. 12B or FIG. 12C would have the
desirable attribute of end sets of strut members with greater
radiopacity than the remainder of the stent.
[0099] Various other modifications, adaptations, and alternative
designs are of course possible in light of the above teachings.
Therefore, it should be understood at this time that within the
scope of the appended claims the invention may be practiced
otherwise than as specifically described herein.
* * * * *