U.S. patent application number 09/797520 was filed with the patent office on 2001-07-26 for tapered self-expanding stent.
Invention is credited to Cox, Daniel L., Stalker, Kent C.B., Ventura,, Joe II.
Application Number | 20010010013 09/797520 |
Document ID | / |
Family ID | 23414305 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010010013 |
Kind Code |
A1 |
Cox, Daniel L. ; et
al. |
July 26, 2001 |
Tapered self-expanding stent
Abstract
A self-expanding, tapered profile stent for implantation in a
body lumen, such as an artery, is disclosed. The stent is
constructed of a plurality of radially expandable cylindrical
elements generally aligned on a common longitudinal stent axis and
interconnected by one or more interconnecting members placed so
that the stent is flexible in the longitudinal direction. The
lengths of the cylindrical elements increase from one end of the
stent to the opposite end by increasing the lengths of the struts
and the lengths of the interconnecting members. Each cylindrical
element is formed from repeating patterns of upright V's and
inverted V's connected by straight strut arms with shoulders to
create an overall serpentine wave pattern around the circumference.
A step, continuous, parabolic, or curved taper in the stent can be
imparted by using an expansion mandrel and applying deforming
forces to the stent. The stent is made from pseudoelastic and shape
memory alloys.
Inventors: |
Cox, Daniel L.; (Palo Alto,
CA) ; Stalker, Kent C.B.; (San Diego, CA) ;
Ventura,, Joe II; (San Jose, CA) |
Correspondence
Address: |
FULWIDER PATTON LEE & UTECHT, LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE
TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Family ID: |
23414305 |
Appl. No.: |
09/797520 |
Filed: |
February 28, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09797520 |
Feb 28, 2001 |
|
|
|
09359550 |
Jul 22, 1999 |
|
|
|
Current U.S.
Class: |
623/1.15 ;
623/1.11 |
Current CPC
Class: |
A61F 2/91 20130101; A61F
2250/0039 20130101; A61F 2002/91575 20130101; A61F 2002/91525
20130101; A61F 2002/91533 20130101; A61F 2002/91516 20130101; A61F
2230/0013 20130101; A61F 2/915 20130101 |
Class at
Publication: |
623/1.15 ;
623/1.11 |
International
Class: |
A61F 002/06 |
Claims
What is claimed is:
1. A longitudinally flexible stent for implanting in a body lumen
and expandable from a contracted state to an expanded state,
comprising: a plurality of adjacent cylindrical elements, each
cylindrical element having a circumference extending around a
longitudinal stent axis and being substantially independently
expandable in the radial direction, wherein each cylindrical
element is formed from struts arranged in a serpentine wave
pattern; wherein the plurality of adjacent cylindrical elements are
arranged in alignment along the longitudinal stent axis, and
wherein a plurality of cylindrical elements include sequentially
increasing diameters to create a tapered profile; a plurality of
interconnecting members extending between the adjacent cylindrical
elements and connecting the adjacent cylindrical elements to one
another; and wherein at least one of the plurality of struts and
interconnecting members at the tapered profile increase in length
along the longitudinal stent axis.
2. The longitudinally flexible stent of claim 1, wherein the stent
includes a pseudoelastic alloy material.
3. The longitudinally flexible stent of claim 1, wherein the stent
includes a shape memory alloy material.
4. The longitudinally flexible stent of claim 1, wherein the
plurality of interconnecting members are aligned end to end.
5. The longitudinally flexible stent of claim 1, wherein the
serpentine wave pattern of a cylindrical element further comprises
a repeating strut pattern of upright V's and inverted V's.
6. The longitudinally flexible stent of claim 5, wherein the strut
pattern of upright V's and inverted V's of one cylindrical element
is in phase with the strut pattern of V's and inverted V's of an
adjacent cylindrical element.
7. The longitudinally flexible stent of claim 6, wherein the strut
pattern of upright V's and inverted V's of one cylindrical element
nest into the upright V's and inverted V's of the adjacent
cylindrical element.
8. The longitudinally flexible stent of claim 1, wherein the
serpentine wave pattern of a cylindrical element further comprises
a repeating strut pattern of U's connected to W's.
9. The longitudinally flexible stent of claim 8, wherein the
serpentine wave pattern of a cylindrical element is out of phase
with the serpentine wave pattern of an adjacent cylindrical
element.
10. A longitudinally flexible stent for implanting in a body lumen
and expandable from a contracted state to an expanded state,
comprising: a plurality of adjacent cylindrical elements, each
cylindrical element having a circumference extending around a
longitudinal stent axis and being substantially independently
expandable in the radial direction, wherein the plurality of
adjacent cylindrical elements are arranged in alignment along the
longitudinal stent axis and define a first end, a second and, and a
center section; wherein the center section is tapered when expanded
such that the first end includes a small diameter and the second
end includes a large diameter; each cylindrical element formed from
struts arranged in a serpentine wave pattern; a plurality of
interconnecting members extending between the adjacent cylindrical
elements and connecting the adjacent cylindrical elements to one
another; and wherein in the expanded state of the stent, a distance
between adjacent cylindrical elements increases from the first end
to the second end.
11. The longitudinally flexible stent of claim 10, wherein the
plurality of interconnecting members are aligned end to end.
12. The longitudinally flexible stent of claim 10, wherein the
serpentine wave pattern of a cylindrical element further comprises
a repeating strut pattern of upright V's and inverted V's with each
upright V and inverted V including a shoulder.
13. The longitudinally flexible stent of claim 10, wherein the
serpentine wave pattern of a cylindrical element further comprises
a repeating strut pattern of U's connected to W's.
14. The longitudinally flexible stent of claim 10, wherein the
stent includes a nickel-titanium alloy.
15. A method for providing a longitudinally flexible stent for
implanting in a body lumen and expandable from a contracted state
to an expanded state, the method comprising the steps of: providing
a plurality of adjacent cylindrical elements, wherein each
cylindrical element has a circumference extending around a
longitudinal stent axis and is substantially independently
expandable in the radial direction; arranging the plurality of
adjacent cylindrical elements in alignment along the longitudinal
stent axis to define a first end, a second end, and a center
section; providing a tapered profile in the center section when
expanded by providing small diameter cylindrical elements in the
first end and gradually increasing the diameters toward the second
end; forming a serpentine wave pattern in each cylindrical element;
providing a plurality of interconnecting members extending between
the adjacent cylindrical elements; connecting the adjacent
cylindrical elements to one another using the interconnecting
members; and wherein when the stent is in the expanded state, a
distance between adjacent cylindrical elements increases from the
first end toward the second end.
16. The method of claim 15, wherein the step of forming the
serpentine wave pattern further comprises forming a repeating strut
pattern of upright V's and inverted V's.
17. The method of claim 16, wherein the step of forming the
serpentine wave pattern further comprises nesting the upright V's
and inverted V's of one cylindrical element with an adjacent
cylindrical element by arranging the serpentine patterns in phase
and shortening the interconnecting members.
18. The method of claim 15, wherein the step of forming the
serpentine wave pattern further comprises forming a repeating strut
pattern of U's connected to W's.
19. The method of claim 15, wherein the step of providing a
plurality of adjacent cylindrical elements includes forming the
cylindrical elements from shape memory alloy material.
20. The method of claim 15, wherein the step of providing a
plurality of adjacent cylindrical elements includes forming the
cylindrical elements from a pseudoelastic alloy material.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to expandable endoprosthesis
devices, generally known as stents, which are designed for
implantation in a patient's body lumen, such as blood vessels, to
maintain the patency thereof. These devices are particularly useful
in the treatment and repair of blood vessels after a stenosis has
been compressed by percutaneous transluminal coronary angioplasty
(PTCA), percutaneous transluminal angioplasty (PTA), or removed by
atherectomy or other means.
[0002] Stents are typically implanted within a vessel in a
contracted state and expanded when in place in the vessel in order
to maintain patency of the vessel to allow fluid flow through the
vessel. Ideally, implantation of such stents is accomplished by
mounting the stent on the balloon portion of a catheter,
positioning the stent in a body lumen at the stenosis, and
expanding the stent to an expanded state by inflation of the
balloon within the stent. The stent can then be left in place by
deflating the balloon and removing the catheter.
[0003] A bifurcated stenosis typically can occur in the carotid or
coronary arteries at the carina between adjoining arterial branches
and around the ostia of the adjoining arterial branches. Employment
of a stent for repair of vessels that are diseased at a bifurcation
requires that the stent must, without compromising blood flow,
overlay the entire circumference of the ostium to a diseased
portion and extend to a point within and beyond the diseased
portion. Particularly at a bifurcation, lesions may form along the
side walls of the blood vessel and at the carina of the
bifurcation, not only contributing to stenosis of a main branch and
side branch of the bifurcation, but also interfering with the
normal rheology of flow at the bifurcation to create eddy currents
that can contribute to formation of thrombosis.
[0004] A conventional stent might be placed so that a portion of
the stent extends into the pathway of blood flow to a side branch
of the bifurcation or extend so far as to completely cover the path
of blood flow in a side branch. The conventional stent might
alternatively be placed proximal to, but not entirely overlaying
the circumference of the ostium to the diseased portion. Such
placement of the conventional stent results in a bifurcation that
is not completely repaired. Also, where the stent does not overlay
the entire circumference of the ostium to the diseased portion, the
stent fails to completely repair the bifurcated vessel.
[0005] In a conventional method for treating bifurcated vessels,
the side-branch vessel is first stented so that the stent protrudes
into the main vessel. A dilatation is then performed in the main
vessel to open and stretch the stent struts extending across the
lumen from the side-branch vessel. Thereafter, the main-vessel
stent is implanted so that its proximal end overlaps with the
side-branch vessel. However, the structure of the deployed stent
must be recrossed with a wire by trial and error.
[0006] In another prior art procedure, known as "kissing" stents, a
stent is implanted in the main vessel with a side-branch stent
partially extending into the main vessel creating a double-barreled
lumen of the two stents in the main vessel proximal to the
bifurcation. Another prior art approach includes a so-called
"trouser legs and seat" approach, which includes implanting three
stents, one stent in the side-branch vessel, a second stent in a
distal portion of the main vessel, and a third stent, or a proximal
stent, in the main vessel just proximal to the bifurcation.
[0007] In addition to problems encountered in treating disease
involving bifurcations for vessel origins, difficulty is also
encountered in treating disease confined to a vessel segment but
extending very close to a distal branch point or bifurcation which
is not diseased and does not require treatment. In such
circumstances, very precise placement of a stent covering the
distal segment, but not extending into the ostium of the distal
sidebranch, may be difficult or impossible.
[0008] It is important for stents to be sized correctly for the
vessel into which they are implanted. In some situations, like the
carotid artery, it is desirable to place a single stent from the
common carotid artery to the internal carotid artery. The diameter
is about 2 to 3 mm smaller in the internal carotid artery, so it is
difficult to size a stent appropriately for both vessels. A stent
that is designed for a large diameter vessel is not optimal for a
small diameter vessel, and vice versa.
[0009] To address the deployment problems at a bifurcation and to
address the stent sizing problems, the present invention is
directed to a tapered stent. With such a tapered stent, the
diameter of the stent varies along the length of the stent.
[0010] Some tapered stent designs are known in the art. For
example, PCT Publication No. WO98/53759, entitled "Carotid Stent,"
by Jay S. Yadav discloses a stent for cardiovascular application
wherein a substantially cylindrical tubular member tapers from its
proximal end to its distal end. This type of tapered stent is
intended for stenting the common carotid bifurcation or the
proximal internal carotid artery.
[0011] PCT Publication No. WO98/34668, entitled "Non-Foreshortening
Intraluminal Prosthesis" by Gary S. Roubin et al. discloses an
intraluminal prosthesis provided with a plurality of annular
elements. The stent may be provided with varying flexibility along
its length and/or circumference, and may include segments that have
different diameters. The differing diameters may be accomplished by
providing the stent in a tapered or a stepped configuration.
[0012] Other tapered stents include U.S. Pat. No. 5,222,964 to
Cooper, disclosing a tapered stent made of resilient material for
interconnecting portions of a Fallopian tube after a resection
procedure. U.S. Pat. No. 5,180,392 to Skeie et al. discloses a
prosthesis for use in joining hollow organ parts or systems wherein
the prosthesis may have tapered outer ends. U.S. Pat. No. 4,441,215
to Kaster discloses a vascular graft of a synthetic material
including a tubular member having a braided inner layer and a
compliant outer covering layer. This synthetic vascular graft can
have an increasing or decreasing taper.
[0013] Another tapered stent is known as the "Flamingo Wallstent."
The Flamingo Wallstent is intended for esophageal malignant
strictures. It is partially covered at the ends to protect against
tissue injury, and inside to prevent food impaction and tumor
growth. A major drawback for the Flamingo Wallstent design is its
inability to be accurately placed due to unpredictable
foreshortening after deployment. There is, however, still a need
for an improved tapered stent for deployment in, for example, the
common carotid bifurcation or the proximal internal carotid artery.
These areas are the most common sites for cerebrovascular
atherosclerotic disease.
SUMMARY OF THE INVENTION
[0014] To address the aforementioned problems, the present
invention is directed to a stent having a taper along its length
and having varying radial strength as a function of the diameter of
the stent and spacing between the struts. In a preferred
embodiment, the present invention is directed to a longitudinally
flexible stent for implanting in a body lumen and expandable from a
contracted condition to an expanded condition, comprising a
plurality of adjacent cylindrical elements, each cylindrical
element having a circumference extending around a longitudinal axis
and being substantially independently expandable in the radial
direction, wherein the plurality of adjacent cylindrical elements
are arranged in alignment along the longitudinal stent axis, and
wherein a plurality of cylindrical elements include sequentially
increasing diameters to create a tapered profile, with each
cylindrical element formed from struts arranged in a serpentine
wave pattern; and a plurality of interconnecting members extending
between the adjacent cylindrical elements and connecting the
adjacent cylindrical elements to one another; wherein the struts
and interconnecting members at the tapered profile increase in
length along the longitudinal stent axis.
[0015] Such a tapered stent with smaller diameters as well as
larger diameters has several benefits. A stent having a smaller
diameter can have greater radial strength, better coverage of the
vessel wall, and less foreshortening than is achievable with a
stent having larger diameters. Obtaining these optimized features
is especially important for the carotid application in which the
internal carotid.artery has the most significant disease, but the
common carotid artery diameter dictates many of the design
requirements of the stent.
[0016] As mentioned earlier, carotid stent procedures frequently
involve the treatment of a diseased artery where plaque extends
across the bifurcation between the common and internal carotid
arteries. Selection of an appropriate stent diameter becomes
precarious because the internal carotid artery tends to be smaller
than the parent common carotid artery. The stent selected must be
large enough to treat the common carotid artery, but using a stent
sized to the common caroAd artery can require implantation of a
stent much larger than the nominal diameter of the internal carotid
artery. This stent diameter mismatch and concomitant oversizing
could lead to vessel injury and poor clinical results.
[0017] In the present invention, each end of the stent has
preferably been designed specifically for the appropriate diameter
range. That is, when deployed, the smaller diameter end of the
stent supports the diseased portion of the internal carotid artery
while the larger diameter end of the tapered stent supports the
large diameter of the common carotid artery.
[0018] The present invention can be made from a shape-memory
metallic alloy such as Nitinol or superelastic Nitinol to create a
self-expanding stent. Alternatively, the present invention stent
can be balloon expanded. With a balloon expandable stent, the shape
of the balloon can be used to control the final shape of the stent.
For example, a balloon with more than one diameter can be used to
expand a stent having two final diameters. Separate balloons can
also be used to post dilate the stent with a step in its
diameter.
[0019] The present invention tapered stent presents a logical
solution for carotid stenting across the bifurcation. The varying
stent diameter accomplishes at least two goals: it allows adequate
treatment of a lesion in both the common and internal carotid while
maintaining a suitable stent-to-artery ratio for each vessel.
[0020] Other features and advantages of the present invention will
become more apparent from the following detailed description of the
invention, when taken in conjunction with the accompanying
exemplary drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a side elevational view depicting the present
invention stent and a deployment system as the stent is carried
through a vessel.
[0022] FIG. 2 is a top plan view of a handle of the deployment
system.
[0023] FIG. 3 is a cross-sectional view of the present invention
tapered stent after deployment at the bifurcation between the
common and internal carotid arteries.
[0024] FIG. 4 is a perspective view of a preferred embodiment of
the present invention tapered stent in its expanded mode.
[0025] FIG. 5 is a plan view of a flattened strut pattern of the
present invention tapered stent in its unexpanded mode.
[0026] FIG. 6 is a plan view of a flattened strut pattern of an
alternative embodiment tapered stent in its unexpanded mode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The present invention is directed to a tapered stent with a
strut pattern that changes the spacing between struts to achieve
various design objectives. While the present invention is described
in detail as applied to the carotid artery of a patient, those
skilled in the art will appreciate that the present invention can
also be used in other body lumens as well.
[0028] FIG. 1 is a side elevational view and partial sectional view
of self-expanding stent 10 of the present invention as carried
inside stent delivery system 12. Stent delivery system 12 includes
elongated catheter body 14 for delivering and deploying stent 10
which is shown in the compressed or unexpanded state. As seen in
FIG. 1, elongated catheter body 14 is positioned within artery 16
or similar type vessel.
[0029] Stent delivery system 12 further includes housing assembly
18 attached to proximal end 20 of delivery catheter 14. Housing
assembly 18 is used to manually deploy compressed stent 10 mounted
at distal end 22 of delivery catheter 14. Delivery catheter 14
further includes inner tubular member 24 that extends within outer
tubular member 26 in a coaxial arrangement.
[0030] Outer tubular member 26 has a proximal end attached to
pull-back handle 28 that is designed to move along the longitudinal
axis of delivery catheter 14 while supported by base 30 of housing
assembly 18. When pull-back handle 28 is translated in the proximal
direction, outer tubular member 26 is likewise translated in the
proximal direction exposing compressed stent 10. Because base 30 of
housing assembly 18 remains stationary, inner tubular member 24
also remains stationary during the stent deployment.
[0031] Applying tensile force to the shaft of outer tubular member
26 during stent deployment creates an equal and opposite
compressive force on inner tubular member 24. Inner tubular member
24 possesses sufficient column strength to prevent buckling or
deformation during deployment.
[0032] Distal end 32 of inner tubular member 24 has stent holder 34
upon which compressed stent 10 is mounted. Tip assembly 36 having a
preferably tapered profile is positioned at distal end 22 of
delivery catheter 14 to help cross any areas of occlusions in the
diseased artery. Tip assembly 36 is made from a small segment of
preferably stainless steel hypotube that has internally tapered
wound coil 38 welded to the distal end of tip assembly 36. An
optional radiopaque tungsten element 40 is placed at the distal end
of tip assembly 36. An opening at the distal end of tip assembly 36
permits guidewire 42 to advance therethrough thereby allowing
delivery catheter 14 to track into the diseased artery.
[0033] Other aspects of the delivery system are disclosed in
co-pending patent application Ser. No. 09/313,780, filed May 17,
1999, entitled "Self-Expanding Stent with Enhanced Delivery
Precision and Stent Delivery System," whose entire contents are
hereby incorporated by reference. Although this delivery system is
used in conjunction with the present invention self-expanding
stent, other types of delivery systems are contemplated. For
example, the present invention stent may be made of stainless steel
or tantalum for example, and may be deployed on a balloon catheter
delivery system and balloon expanded at the delivery site. Such
balloon delivery systems are well known in the art.
[0034] FIG. 3 provides a cross-sectional view of the present
invention as deployed in bifurcation 44 between common carotid
artery 46 and internal carotid artery 48. It is clear that
selection of an appropriate stent diameter becomes important
because internal carotid artery 48 tends to be smaller than the
parent common carotid artery 46. Furthermore, the denser strut
pattern, described below, giving greater hoop strength of stent 10
is important at this particular location to support diseased
regions 50.
[0035] As seen in FIG. 3, outer tubular member 26 has been
withdrawn in the proximal direction exposing stent 10 which
self-expands. This is accomplished by fabricating stent 10 from a
highly resilient alloy such as superelastic Nitinol or other
spring-like materials known in the art.
[0036] FIGS. 4 and 5 show expanded and contracted states of stent
10, respectively. As seen in these figures, the preferred
embodiment of the present invention stent 10 is constructed from a
plurality of nested cylindrical elements 52 arranged coaxially
along a common longitudinal stent axis to assume a tubular form.
Adjacent cylindrical elements 52 are joined at predetermined
circumferential locations by interconnecting members 54 so that
each cylindrical element 52 is independently expandable. In the
preferred embodiment shown in FIGS. 4 and 5, interconnecting
members 54 are aligned axially.
[0037] As best seen in FIG. 4, each clindrical element 52 has a
generally serpentine wave strut pattern constructed from repeating
patterns of upright and inverted V's 56.
[0038] Connecting upright and inverted V's 56 are strut arms 58.
Each strut arm 58 is generally straight but may include shoulder
60. Optional shoulder 60 is included in strut arm 58 in order to
squeeze the stent to a smaller profile in the delivery system. In
other words, inclusion of shoulders 60 in strut arms 58 permits
tighter packing of the struts of the stent.
[0039] This leads to better coverage of the vessel wall.
Conversely, if strut arms 58 were straight, the vessel coverage by
the stent struts is diminished.
[0040] Each cylindrical element 52 has a longitudinal dimension or
length as measured from peak 84 of one inverted V to valley 86 of
the next upright V. Cylindrical elements 52 are described as nested
to mean that those lengths of adjacent cylindrical elements 52
overlap each other. Thus, peaks 84 of inverted V's 56 of one
cylindrical element 52 lie within the open areas of peaks 84 of
inverted V's 56 of adjacent cylindrical elements 52.
[0041] In a similar fashion, valleys 86 of upright V's 56 of one
cylindrical element 52 lie within the open areas of valleys 86 of
upright V's of an adjacent cylindrical element 52. This preferred
strut arrangement could be described as a loose herringbone
pattern.
[0042] As best seen in FIG. 4, the lengths of interconnecting
members 54 and strut arms 58 increase from first end 64 toward
second end 62. Looking at it another way, the strut pattern under
this configuration becomes more dense toward first end 64 due to
the shorter struts. Taper 82, however, is not created by the
varying strut arm lengths 58.
[0043] Rather, the strut arm lengths 58 is enabled by the taper.
The tapering 82 is dictated by the expansion process, that is, the
shape of the expansion mandrel when the stent is fabricated. Again,
the tapered profile is best seen in the expanded mode of FIG.
4.
[0044] Of course, by imparting the length and angle of taper 82 as
well as its location on the expansion mandrel, the shape and
profiles of the individual strut arms 58 and interconnecting
members 54 are likewise formed. In the embodiment shown in FIG. 4,
taper 82 is positioned at a center portion of stent 10. Naturally,
taper 82 can be relocated as needed along the length of stent
10.
[0045] Taper 82 may be continuous or discrete, and include changes
in shape or dimension, with small flares, or tapers only at the
ends of the stent. For instance, some types of tapers contemplated
in the present invention tapered stent 10 include a step taper as
seen in the expanded state of stent 10 in FIG. 4. That is, stent 10
has first end 64 with cylindrical elements 52 at that end having a
small constant diameter; second end 62 with cylindrical elements 52
at that end having a large constant diameter; and center section 82
with sequentially changing diameters in cylindrical elements 52
along the longitudinal stent axis to achieve the step taper. Of
course, the length and location of center section 82 containing the
taper can be changed as necessary to accommodate the specific
anatomy of the patient.
[0046] The present invention in an alternative embodiment (not
shown) further contemplates straight conical tapers; that is, the
stent has an angled profile from one end to the opposite end. The
shape of the taper in the step diameter change can be varied from
straight to parabolic to other shapes as well.
[0047] Many physical parameters of the present invention stent can
be changed to achieve specific engineering objectives. For example,
the density of the strut pattern can be adjusted as needed by
varying the lengths of strut arms 58 and interconnecting members 54
to affect the amount of open areas. The included angles of the
peaks and valleys of inverted and upright V's 56 can be changed to
affect strut density. Increasing the number of inverted and upright
V's 56 in a given cylindrical element 52 can also increase strut
density. Furthermore, the degree of nesting can be adjusted by only
changing the lengths of interconnecting members 54. Shortening
interconnecting members 54, for example, would result in a more
tightly packed or nested strut pattern. Changing the phase of
inverted and upright V's 56 in one cylindrical element 52 to the
next can also affect the amount of open space in stent 10, its
flexibility, vessel coverage, etc.
[0048] Adding or decreasing the number of interconnecting members
54joining adjacent cylindrical elements 52, positioning them at
specific locations around the circumference, and aligning them in a
row such as that shown in FIGS. 4 and 5 are all different methods
of affecting the stent's hoop strength, foreshortening,
flexibility, recoil, and other engineering characteristics. The
length of stent 10 can be varied by increasing or decreasing strut
arm lengths 58 and interconnecting member lengths 54, by using more
or fewer cylindrical elements 52, and by changing the included
angles of the inverted and upright V's 56.
[0049] In general, the present invention stent is preferably
fabricated through manufacturing processes known in the art
appropriate for pseudoelastic Nitinol. Other stent materials known
in the art, such as stainless steel, are contemplated but not
explicitly described here.
[0050] First, in the preferred process, the stent strut pattern is
laser cut out of a tube stock of pseudoelastic Nitinol. Second, any
scale on the surface of the material is removed by bead blast or
acid wash.
[0051] Third, because the stent is made from Nitinol, it is
expanded on an expansion mandrel and heat set. The heat set imparts
the shape memory to the alloy, and preferably occurs at
approximately 500 to 550 degrees Celsius. After heat set, the stent
is quenched in water. Both the heat set and quenching help control
the transformation temperature between martensite and austenite of
the Nitinol material. Furthermore, the expansion and heat set cycle
is performed in stages, sometimes up to five steps, to avoid
damaging the stent. The last one or two steps are performed on
tapered mandrels to impart the tapered profile. The stent has
inherent resilience, which conforms the stent profile to the
profile of the tapered mandrels at each stage.
[0052] Fourth, the Nitinol stent is electropolished. Preferably,
the electropolish solution is a methanol based, acidic mixture.
Specifically, the mixture consists of 465 ml absolution methanol,
37.5 mil sulfuric acid (>96.5 percent), and 12.5 ml hydrocloric
acid (saturated), which combined produces approximately 500 ml of
solution.
[0053] The stent is placed in an electropolish fixture preferably
constructed from four round, Nitinol wires acting as anodes to hold
the stent. The four anode wires are placed around the circumference
of the stent, parallel to the stent's longitudinal axis. There is a
center cathode made of a platinum rod. The cathode is located at
the center of the four anode wires and extends through the center
of the stent, parallel and coextensive with its longitudinal axis.
The negatively charged center cathode is used to complete the
circuit in the solution to polish the inside diameter of the
stent.
[0054] A curved sheath cathode made of platinum mesh is located
parallel to the longitudinal stent axis and partially surrounding
the Nitinol wires. The curved sheath cathode is used to complete
the circuit in the solution to polish the outside diameter of the
stent. The curved sheath is placed just below the four holding
anodes, wherein the distance between the sheath and the stent is
determined by the size of the part needed to be polished. The
fixture and stent positioned thereon are immersed in the solution
described above and an electrical current is applied to the
circuit.
[0055] Fifth, after electropolish, the stent diameter is reduced
for fitment with a delivery system. Sixth, the stent is loaded in a
delivery system. The end result is a stent with varying diameter
along its length as illustrated in FIGS. 4 and 5.
[0056] A tapered stent such as in the present invention presents a
logical solution for carotid stenting across the bifurcation. The
varying stent diameter allows adequate treatment of a lesion in
both the common and internal carotid arteries, while maintaining a
suitable stent-to-artery ratio for each vessel.
[0057] A tapered stent can be applied to other parts of the
vascular system where a bifurcation is present such as the coronary
arteries and relevant areas where peripheral vascular disease may
exist. Tapered stent diameters, lengths, flexibility, radiopacity,
and radial hoop strength are all features that would be optimized
depending on the expected application of the present invention
stent.
[0058] FIG. 6 provides a flattened, plan view strut pattern of an
alternative embodiment stent 66. Stent 66 is preferably constructed
from a plurality of cylindrical elements 68 arranged along a
common, longitudinal stent axis to assume a tubular form. Each
cylindrical element has decreasing lengths from first end 70 to
second end 72 of stent 66, similar to the embodiment depicted in
FIGS. 4 and 5.
[0059] In FIG. 6, each cylindrical element 68 is formed from struts
arranged in a repeating serpentine wave patterns. Interconnecting
members 74join adjacent cylindrical elements 68. In this exemplary
embodiment, the serpentine wave pattern is made from alternating
U's and W's joined by straight strut arms 80. Each interconnecting
member 74 joins W's 78 to U's 77.
[0060] Formation of the taper at any portion along the length of
stent 66 can be achieved through processing steps described above.
FIG. 6 does not show the taper because stent 66 is in the
unexpanded state.
[0061] The shorter cylindrical elements 68 near second end 72
improve the radial strength and also increase vessel coverage.
Opposite second end 72 has longer straight strut arms 80 to allow
expansion to larger diameters although the radial force and vessel
coverage are reduced.
[0062] Clearly, there are other parameters that could also be
varied to optimize performance at each end. For example, the stent
struts could be varied in width or thickness. The material
processing conditions could be varied to impart different
engineering characteristics. The number of repeating patterns that
form the serpentine wave pattern around the circumference of
cylindrical element 68 could be changed. The number of cylindrical
elements 68 and the number of interconnecting elements 74 may be
varied to change flexibility and other typical design
parameters.
[0063] While the present invention has been illustrated and
described in terms of its use as carotid stents, it will be
apparent to those skilled in the art that the present invention
stent can be used in other instances in all lumens in the body.
Since the present invention stent has the novel feature of
self-expansion to a large diameter while retaining its structural
integrity, it is particularly well suited for implantation in
almost any vessel where such devices are used. This feature,
coupled with limited longitudinal foreshortening of the stent when
it is radially expanded, provide a highly desirable support member
for all vessels in the body. Other modifications and improvements
may be made without departing from the scope of the present
invention.
* * * * *