Note: Descriptions are shown in the official language in which they were submitted.
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1
NON-FORESHORTENING INTRALUMINAL PROSTHESIS
BACKGROUND OF THE INVENTION
1. Field of the Invention
. 5 The present invention relates to an intraluminal
prosthesis~for implantation into a mammalian vessel,
and in particular, to an intraluminal stent that is
delivered~in a compressed state to a specific location
inside the lumen of a mammalian vessel and then
deployed to an expanded state to support the vessel.
The intraluminal stent is provided with a structural
configuration that maintains the prosthesis at
substantially the same length in both the compressed
and expanded states. The intraluminal stent may also
be provided with varying rigidity or flexibility along
its length.
2. Description of the Prior Art
Intraluminal prosthesis, such as stents, are
commonly used in the repair of aneurysms, as liners
for vessels,.or to provide mechanical support to
prevent the collapse of stenosed or occluded vessels.
These stents axe typically delivered in a compressed
state to a specific location inside the lumen of a
vessel or other tubular structures, and then deployed
at that location of the lumen to an expanded state.
The stent has a diameter in its expanded state which
is several times larger than the diameter of the scent
in its compressed state. These stems are also
frequently deployed in the treatment of
atherosclerotic stenosis in blood vessels, especially
after percutaneous transluminal coronary angioplasty
(PTCA) procedures, to improve the results of the
procedure and to reduce the likelihood of restenosis.
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The positioning of a stmt at the desired location
in the lumen of a body vessel is a critical factor
that affects the performance of the stent and the
success of the medical procedure. Since the region in
5 a lumen at which the stent is to be deployed is
usually very difficult for a physician to access, it
is essential that the stent's deployed diameter and
length be known before the physician can accurately
position a stent with the correct size at the precise
10 location. For example, since the diameter and the
length of the diseased or damaged segment or region of
the body vessel can vary for different body vessels,
disease states, and deployment purposes' it is
important that a stent having the precise diameter and
15 length be delivered to this region for deployment.
Careful sizing of this region of the lumen of the
body vessel may pose a difficult challenge for many
physicians who know the exact dimensions of the body
vessel at this region, but are not certain about the
20 stent's deployed diameter and length. This is due to
a foreshortening effect which is experienced by many
stents when they are expanded from their compressed
state to their expanded state.
This foreshortening effect is illustrated in FIGS.
25 1A, 1B, 2A and 2B, which illustrate portions 20 of a
st mt having a mesh-like pattern made up of V-shaped
struts or legs 22 and 24 connected at their apices 26.
Two pairs of these V-shaped struts 22, 24 are
illustrated in this portion 20 of the stent. Each of
30 these struts 22 and 24 has a length h. FIG. 1B
illustrates the portion 20 of the scent in a fully
compressed state, in which the length h has a
longitudinal or horizontal component 12 (see FIG. 2B!,
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and FIG. 1A illustrates the same portion 20 of the
stent in a fully expanded state, in which the length h
has a longitudinal or horizontal component 11 (see
FIG. 2A). As illustrated by the imaginary lines 28
and 30 in FIGS. 1A and 1B, and in FIGS. 2A and 2B, 11
is shorter than 12 because the angle which the strut
22 assumes with respect to the horizontal axis is
greater when in the expanded state, so the length of
the expanded portion 20 is shorter than the length of
the compressed portion 20 by a length of 2d. This
foreshortening is caused by the shortening of the
longitudinal component 1 of the struts 22 and 24 as
the stent is expanded from the compressed state to the
expanded state.
This foreshortening effect is troublesome because it
is not easy to determine the exact dimension of this
foreshortened length 2d. The physician must make this
calculation based on the material of the scent, the
body vessel being treated, and the expected diameter
of the stmt when properly deployed in the lumen of
the body vessel. For example, the foreshortened
length 2d will vary when the same stmt is deployed in
vessels having different diameters at the region of
deployment.
In addition, there are certain body vessels that
experience a change in vessel lumen diameter, anatomy
or disease state along their lengths. Stents to be
deployed at such vessels will need to be capable of
addressing or adapting to these changes.
An example of such a body vessel are the carotid
arteries. Blood is delivered from the heart to the
head via the common carotid arteries. These arteries
are approximately 8-10 mm in lumen diameter as they
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make their way along the neck up to a position just
below and behind. the ear. At this point, the common
carotid artery branches into a 6-8 mm lumen diameter
internal carotid artery, which feeds blood to the
5 brain, and a 6-8 mm lumen diameter external carotid
artery, which supplies blood to the face and scalp._
Atherosclerotic lesions of the carotid artery tend to
occur around this bifurcation of the common carotid
artery into the internal and external carotid
10 arteries, so stents often need to be deployed at this
bifurcation.
Another example are the iliac arteries, which have a
lumen diameter of about 8-10 mm at the common iliac
artery but which decrease to ~a lumen diameter of about
15 ~ 6-7 mm at the external iliac artery. The common iliac
arteries experience more localized stenosis or
occlusive lesion which are quite often calcific and
usually require a shorter stent with greater radial
strength or rigidity. More diffused atherosclerotic
20 disease of the iliac system will commonly involve both
the common and external iliac arteries, and
necessitate a longer stent having increased
flexibility that is suitable for deployment in the
tortuous angulation experienced by the iliac system.
25 The femoropopliteal system similarly experiences
localized and diffused stenotic lesions. In addition,
the flexibility of a stent is important where deployed
at locations of vessels that are affected by movements
of joints, such as the hip joint or the knee joint.
30 The renal. arteries provide yet another useful
example. The initial 1 cm or so at the orifice of a
renal artery is of ten quite firmly narrowed due to
atheroma and calcification, and is relatively
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straight, while the remainder of the length of the
renal artery is relatively curved. As a result, a
stmt intended for implantation at the renal arteries
should be-relatively rigid for its first 1.5 cm or so,
5 and then become more flexible and compliant.
Thus, there remains a need for an intraluminal
prosthesis that maintains a consistent length in both
its fully compressed and fully expanded states, and in
all states between its fully compressed and fully
expanded states. There also remains a need far a
stent which can accomodate body vessels having varying
lumen diameters, different anatomies, and different
disease states.
SUMMARY OF THE DISCLOSDRE
In order to accomplish the objects bf~zhe dent
invention, there is provided a stent having a-
plurality of annular elements. Each annulax dement
has a compressed state and an expanded stake, and has
a longitudinal dimension which is smaller in the
expanded state than in the compressed state. A
plurality of connecting members connect!; adjacent
annular elements, with the connectinc, tn~embers
operating to compensate for the small.~r longitudinal
dimension of each annular element xn:'~he expanded
state.
In one embodiment of the present invention, each
annular element includes a plur;al~ty of struts and
apices connected to form an annular configuration.
The connecting members are connected to the apices of
the adjacent annular elements 'l~he plurality of
struts of the annular elemen.'.s include left and right
struts, with each pair of left and right struts
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connected to each other at an apex. Each strut has a
longitudinal dimension which is smaller when the
annular element is in the expanded state than in the
compressed state.
5 In one embodiment of the present invention, at least
one of the annular elements may have a closed
configuration such that the plurality of alternating
struts and apices are connected to each other to form
a closed annular element. In addition, it is also
10 possible for at least one of the annular elements to
assume an open configuration such that the plurality
of alternating struts and apices are not connected at
at least one location.
In a preferred embodiment of the present invention,
15 the connecting members have a plurality of alternating
segments.
In one embodiment, the connecting members have a
plurality~of alternating curved segments defining
alternating top and bottom curved apices. In another
20 embodiments the connecting members have a plurality of
alternating curved and straight segments. In a
further embodiment, the connecting members have a
plurality of alternating and angled straight segments.
The connecting members have a larger longitudinal
25 dimension when each annular element is in the expanded
state than in the compressed state to compensate for
the smaller longitudinal dimension of the annular
element in the expanded state.
The stent according to the present invention further
30 includes a plurality of apertures defined by adjacent
annular elements and connecting members. In one
embodiment, it is possible for the apertures of
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different segments of the stent to have different
sizes.
The stent according to the present invention further
provides a plurality of segments, at least two of
which have a different degree of flexibility. In one
embodiment, the varying flexibility is accomplished by
forming a plurality of gaps. These gaps may be formed
by omitting one or more of the connecting members, or
portions of connecting members, between adjacent
annular elements, or by omitting one or more of the
struts, or by omitting connecting members and struts.
In another embodiment, the varying flexibility is
accomplished by providing the apertures of different
stent segments with different sizes.
The st mt according to the present invention may
further provide segments that assume different
diameters when the stent is in its expanded state.
The differing diameters may be accomplished by
providing the stent in a tapered or a stepped
configuration.
In a preferred embodiment according to the present
invention, the stent is made from a shape memory
alloy, such as Nitinol, although other materials such
as stainless steel, tantalum, titanium, elgiloy, gold,
platinum, or any other metal or alloy, or polymers or
composites, having sufficient biocompatibility,
rigidity, flexibility, radial strength, radiopacity
and antithrombogenicity can be used for the scent
material.
Thus, the stent according to the present invention
maintains a consistent length in both its fully
compressed and fully expanded states, and in all
states between its fully compressed and fully expanded
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states. As a result, the stmt according to the present
invention facilitates accurate sizing and deployment,
thereby simplifying, and possibly reducing the time needed
for, the medical procedure. In addition, the stmt
according to the present invention provides varying
flexibility and rigidity along its length and/or
circumference, as well a~~ varying diameters along
different segments of the stmt, thereby facilitating the
treatment of body vessels having varying lumen diameters,
different anatomies and different disease states.
In a broad aspect, then, the present invention
relates to a stmt comprising: a plurality of annular
elements, each annular element having a compressed state
and an expanded state, wherein each annular element has a
longitudinal dimensions which is smaller in the radially
expanded state than in the compressed state; and
connecting members connecting adjacent annular elements;
wherein the annual elements and connecting members are
made of Nitinol, with each connecting member preset with
an elasticity which causes the connecting member to
elongate longitudinally when the annular elements are in
their expanded state to compensate for the smaller
longitudinal dimension of the annular elements in the
expanded state.
In another broad aspect, then, the present invention
relates to a stmt having a first segment and a second
segment, with each segment having a diameter and
comprising: a plurality of annular elements, each annular
element having a compressed state and an expanded state;
at least one connecting member connecting adjacent annular
elements; a plurality of apertures defined by adjacent
annular elements and connecting members, each aperture
having a size and a geometric configuration; with the
first and second segments having substantially the same
diameter in the compressed state; and wherein the
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apertures of the first and second segments have different
sizes but substantially the same geometric configuration
when the first and second segments are in the expanded
state.
In yet another broad aspect, then, the present
invention relates to a stmt having a plurality of
segments and comprising: a plurality of annular elements,
each annular element having a compressed state and an
expanded state; a plurality of connecting members
connecting adjacent annular elements; and a plurality of
gaps formed by omitting at least one of the connecting
members between adjacent annular elements so as to provide
two of the plurality of segments of the stmt with
different degrees of flexibility.
In a further broad aspect, then, the present
invention relates to a stmt having a plurality of
segments and comprising: a~ plurality of annular elements,
each annular element. having a compressed state and an
expanded state; a plurality of connecting members
connecting adjacent annular elements; and wherein each
annular element comprises a plurality of alternating
struts and apices connected to each other to form a
substantially annular configuration, and wherein the
connecting members are connected to the apices of the
adjacent annular elements; and wherein a plurality of gaps
are formed by omitting at least one of the struts so as to
provide two of the plurality of segments of the stmt with
different degrees of flexibility.
In a still further broad aspect, then, the present
invention relates to a stmt comprising: a plurality
annular elements, each annular element having a compressed
state and an expanded state, and each annular element
including a plurality of alternating struts and apices
connected to each other to form a substantially annular
configuration; and at least one connecting member
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connected adjacent annular elements; wherein at least one
of the annular elements is closed such that the plurality
of alternating struts and apices are connected to each
other to form a closed annular element, and wherein at
least one of the annular elements are open such that the
plurality of alternating struts and apices are not
connected at at least one location.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a side elevational view of a portion of a
prior art stmt in its e~;panded state;
FIG. 1B is a side elevational view of the portion of_
FIG. 1A in its compressed state;
FIG. 2A illustrates the longitudinal component of a
strut of the stent of FIGS. 1A and 1B when the stmt is in
its expanded state;
FIG. 2B illustrates the longitudinal component of a
strut of the stmt of FIGS. 1A and 1B when the stmt is in
its compressed state;
FIG. 3 is a perspective view of a stmt according to
the present invention;
FIG. 4A is a side elevational view of a portion of
the stmt of FIG. 3 in its expanded state;
FIG. 4B is a side elevational view of the portion of
FIG. 4A in its compressed state:
FIG. 5A illustrates the longitudinal component of a
strut and its connecting member of the stmt of FIGS. 4A
and 4B when the stmt is in its expanded state;
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FIG. 5B illustrates the longitudinal component of a
strut and its connecting member of the stent of FIGS.
4A and 4B when the stent is in its compressed state;
FIG. 6A is a side elevational view of the scent of
5 FIG. 3 in its expanded state;
FIG. 6B is a side elevational view of the stent of
FIG. 6A in its compressed state;
FIGS. 7 and 8 illustrate alternative embodiments of
the connecting member according to the present
10 invention;
FIG. 9 is a side elevational view of a portion of
the stmt of FIG. 3 illustrating a modification
thereto;
FIG. 10 is a side elevational view of a portion of
15 the stent of FIG. 3 illustrating another modification
thereto; and
FIGS. 11A-11C illustrate modifications to the stmt
of FIG. 3.
20 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following detailed description is of the best
presently contemplated modes of carrying out the
invention. This description is not to be taken in a
limiting sense, but is made merely for the purpose of
25 illustrating general principles of embodiments of the
invention. The scope of the invention is best defined
by the appended claims.
The intraluminal prosthesis according to the present
invention is a stent, although the principles of the
30 present invention are also applicable to other
prosthesis such as liners and filters. The stent is
delivered to a desired location in the lumen of a body
vessel in a compressed state, and is then deployed by
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expanding it to its expanded state. The stent
maintains substantially the same length in both its
fully compressed and fully expanded states, and in all
states between these two states. The stent may be
5 provided with varying flexibility or rigidity along
different segments thereof to allow the stent to be
deployed in body vessels having different anatomies
and different disease states. The stent may also be
provided in a configuration in which the same stem
ZO has varying diameters along different portions of the
st mt to facilitate implantation in body vessels that
have varying diameters.
The stem according to the present invention can be
a self-expanding stent, or a stent that is radially
15 expandable by inflating a balloon or expanded by an
expansion member, or a stent that is expanded by the
use of radio frequency which provides heat to cause
the stmt to change its size. The stmt may also be
coated with coverings of PTFE, dacron, or other
20 biocompatible materials to form a combined stmt-graft
prosthesis. The vessels in which the stent of the
present invention can be deployed include but are not
limited to natural body vessels such as ducts,
arteries, trachea, veins, ureters and the esophagus,
25 and artificial vessels such as grafts.
1. A Preferred Embodiment
A stent 40 according to the present invention is
illustrated in FIGS. 3-6 in its expanded state.
Referring to FIG. 3, the stmt 40 has a tubular
30 configuration and is made up of a plurality of pairs
of substantially V-shaped struts connected at their
apices, and by connecting a plurality of connecting
members to the apices of each pair of V-shaped struts.
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FIGS. 4A and 4B illustrate a portion of the stent 40
in greater detail. The scent 40 has a plurality of
pairs of alternating left struts 42 and right struts
44. Each pair of left and right struts 42, 44 is
5 connected at an apex 46 to form a substantially V~-
shape for the pair. The left strut 42 is defined as
being to the left of each apex 46, and the right strut
44 is defined ae being to the right of each apex 46.
The left struts 42 and right struts 44 are alternating
10 since the left strut 42 of one pair of V-shaped struts
is also the left strut of the adjacent pair of V-
shaped struts, and the right strut 44 of one pair of
V-shaped struts is also the right strut of the
adjacent pair of V-shaped struts. In this manner, the
15 alternating left and right struts 42 and 44 extend in
an annular manner around the tubular stent 40 to form
an annular element. Each apex 46 is connected to
another apex 46 by a connecting member 48. Therefore,
the stent 40 resembles a tubular lattice formed by
20 pairs of V-shaped struts 42, 44 connected to
themselves and having their apices 46 connected by the
connecting members 48. As shown in FIG. 3, both ends
of the stmt 40 are defined by a plurality of
alternating left and right struts 42. 44, with the
25 extremity of.both ends defined by the apices 46 of
these alternating left and right struts 42, 44.
The connecting members 48 have a configuration that
includes a plurality or pattern of alternating
segments. A non-limiting first preferred embodiment
30 of the connecting member 48 is illustrated in FIGS. 4A
and 4B. Each connecting member 48 extends
longitudinally along a longitudinal extension 52 from
an apex 46, then slopes upwardly along a curved
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segment 54 to a top curved apex 56, at which point the
connecting member 48 slopes downwardly along a curved
segment 58 to a bottom curved apex 60. The connecting
member 48 then slopes upwardly along a curved segment
5 62 to another top curved apex 64. From the top curved
apex 64, the connecting member 48 slopes downwardly_
along a curved segment 66 to a longitudinal extension
68 of the ppposing apex 46. Thus, the connecting
member 48 has a plurality of alternating curved
10 segments that are deffined by the alternating top and
bottom apices 56, 60 and 64.
The connecting members 48 are provided to perform
two functions. First, the connecting members 48
connect pairs of apices 46. Second, the connecting
15 members 48 function to compensate for the
foreshortening experienced by the longitudinal
component of each strut 42 and 44, thereby maintaining
the stmt 40 at substantially the same length at all
times. This is accomplished by providing the
20 connecting member 48 with a natural bias and a springy
nature, which together with its alternating segments,
combine to shorten its length when compressed. When
allowed to expand, the connecting member 48 is biased
to return to its natural or original position, which
25 lengthens the connecting member 48 to compensate for
the foreshortening experienced by the longitudinal
component of each strut 42 and 44.
This effect is illustrated in FIGS. 4A, 4B, 5A and
5B. When the stent 40 is in its compressed state, the
30 connecting member 48 has a length of L2, which is less
than the length L1 when the connecting member 48 is in
its expanded state. When the connecting member 48 is
1n the COfltpreSSed state, its alternating curves have a
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higher amplitude and a smaller wavelength than when it
is in the expanded state (compare FIGS. 4A and 4B).
Thus, the difference between L2 and L1 compensates for
the difference between 11 and 12 of the struts 42, 44
at both ends of the connecting member 48. The lines
70 and 72 in FIGS. 4A and 4B also show that the
relevant. portion of the stent 40 does not experience
any foreshortening, and the lines 74 and 76 in FIGS.
6A and 6B show that the entire stent 40 maintains a
consistent length through all its states.
Although the connecting members 48 have been
described in FIGS. 4A, 4B, 5A and 5B as assuming a
particular configuration, it will be appreciated by
those skilled in the art that the connecting members
48 can assume other configurations without departing
from the spirit and scope of the present invention.
For example, the connecting members 48 can be provided
in any curved, partially curved, or other
configuration as long as they function to compensate
for the foreshortening experienced by the longitudinal
component of each strut 42 and 44.
FIG. 7 illustrates a non-limiting second preferred
embodiment of the connecting member, in Which the
connecting member 48a has alternating curved segments
80 and straight segments 82. When the connecting
member 48a is compressed, its curved segments 80 also
have a higher amplitude and a smaller wavelength than
when it is in its expanded state. FIG. 8 illustrates
a non-limiting second preferred embodiment, in which
the connecting member 48b has alternating straight
segments 84 and 86 that are angled with respect to
each other.
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When the scent 40 is in its fully expanded state, it.
preferably has an outer diameter that is slightly
larger than the inner diameter of the region of the
body vessel at which it is to be deployed. This
5 allows the stent 40 to be securely anchored at the
desired location and prevents the stent 40 from
migrating away from the deployed location.
The stent 40 can be provided with varying
flexibility or rigidity at different portions or
10 segments along its length to facilitate deployment in
body vessels that require such varying flexibility or
rigidity. The varying flexibility or rigidity can be
accomplished by omitting connecting members 48 and
struts 42, 44, or by not connecting one or more struts
15 32, 44 and/or connecting members 48, thereby creating
"gaps" at one or more locations along the stent 40.
These locations can be anywhere along the length
and/or the circumference of the stem 40. In
addition, varying degrees of flexibility in the stmt
20 40 can be accomplished by varying the patterns of
these gaps. A non--limiting example would be to
provide a substantially spiral pattern of omitted
struts 42, 44 and/or connecting members 48, such as
illustrated in FIG. 9. The omitted struts 42, 44 and
25 connecting members 48 are illustrated in FIG. 9 in
phantom (i.e., the dotted lines) by the numerals 47 .
(for the struts 42, 44) and 49 (for the connecting
members). For example, the omitted struts 47 assume a
relatively spiral pattern along the length of the
30 stent 40 from the top left corner of FIG. 9 to the
bottom right corner of FIG. 9, and can extend around
the circumference of the stent 40. Similarly, the
omitted connecting members 49 assume a relatively
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spiral pattern along the length of the scent 40 from
the bottom left corner of FIG. 9 to the top right
corner of FIG. 9, and can extend around the
circumference of the stent 40.
5 Other non-limiting alternatives include providing
such gaps 49 at one or both ends of the stent 40 only,
or at a central portion of the stent 40. Further non-
limiting alternatives would be to increase the number
of these gaps 47, 49 from one or both ends of the
10 stent 4o towards the center of the stmt 40, or to
increase the number of these gaps 47, 49 from the
center of the stent 40 towards one.or both ends of the
stent 40. It is also possible to omit only a portion
of certain connecting members 48 and not the entire
15 ones of these connecting members 48. A portion of the
stmt 40 having a larger number of gaps 47, 49 would
have greater flexibility or reduced rigidity.
As a result of the omitted struts 47, it is possible
that some of the annular elements that are made up of
the alternating struts 42, 44 may be closed or
constitute completely connected annular elements,
while some of these annular elements will be open
annular elements.
The varying flexibility or rigidity can also be
25 accomplished by providing a structural configuration
where the size of the open areas or apertures 78 (for
example, see FIGS. 4A and 10) defined between the
struts 42, 44 and the connecting members 48 is varied
at different portions or segments of the stent 40,
30 along the length and/or circumference of the stent 40.
In a non-limiting embodiment, all the apertures 78 in
one segment of the stmt 40 have substantially the
same first size, and all the apertures 78 in another
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segment of the stem 40 have substantially the same
second size, the first and second sizes being
different. Additional segments, each having apertures
78 with substantially the same size as the other
5 apertures 78 in that segment but having a different
size as the apertures 78 in other segments, can also
be provided.
Varying the size of apertures 78 can be accomplished
by varying the lengths of the struts 42, 44 and the
10 connecting members 48. For example, a smaller
aperture 78 can be provided by shortening the lengths
of the struts 42, 44 and the connecting members 48
that define the particular open area 78. Portions of
the stent 40 with smaller apertures 78 are more rigid
15 and less flexible than portions of the stent 40 with
larger apertures 78. This allows the stent 40 to be
deployed in body vessels that require a stent to be
more rigid at one end, and to be increasingly flexible
from the rigid end. Examples of such body vessels
20 include the renal and iliac arteries discussed above.
Varying the sizes of the apertures 78 also serves
other important purposes. For example, providing
smaller apertures 78 at the opposing ends of the stent
40 provides increased or closer coverage of the vessel
25 wall, thereby improving support of the diseased vessel
wall and preventing fragments of the plaque from being
dislodged as embolic debris. The dislodgement of
debris can be dangerous in certain vessels, such as
the carotid arteries, where dislodged debris can be
30 carried to the brain, possibly resulting in a stroke.
As another example, providing larger apertures 78 at
central portions of the stent 40 provides wider open
areas that may be important in preventing the
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obstruction of side branches of other body vessels.
These wider open areas also allow the passage of
guidewires, catheters, scents, grafts, and other
deployment devices through the body of the stent 40
into these side branches.
The stent 40 can also be provided in a manner in .
which it assumes a constant diameter in its compressed
state, but in which different portions of the stmt 40
can assume different diameters when in their fully
10 expanded states. Providing an expandable stent 40
with the capability of assuming different diameters at
different portions is important where the stent 40 is
used in certain body vessels or branches of body
vessels where the lumen diameters may vary. Examples
of such body vessels and branches include the carotid
and iliac arteries discussed above, and the esophagus.
The varying stem diameter can be provided in a
number of ways. A first non-limiting alternative is
to provide a gradually tapered configuration of the
stent 40a, as shown in FIG. 11A. A tapered
configuration is best-suited for use in body vessels
which experience a gradual narrowing. A second non-
limiting alternative is to provide an abrupt
transition, such as a stepped configuration, between
two scent segments each having a relatively
consistent, but different, diameter. The step can be
for a step-up 40c, as shown in FIG. 11C, or for a
step-down 40b, as shown in FIG. 11B. In addition, a
stent 40 can be provided with several changes in
30 diameter along its length to match specific anatomical
requirements.
The tapering or transitioning of the stent
configuration can be accomplished by pre-shaping, and
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18
can be enhanced by variations in (1) the thickness of
the stent material, (2) the size of apertures 78, and
(3) the gaps 47, 49.
In addition to the above, it will be appreciated by
those skilled in the art that varying flexibility and
rigidity can also be accomplished by varying the width
or thickness of the scent material at certain
locations along the length and/or circumference of the
stmt 40.
10 A number of materials can be used for both the stmt
40 and its struts 42, 44 and connecting members 48,
depending on its method of deployment. If used as a
self-expanding scent, the stent 40 (including its
struts 42, 44 and connecting members 48) is preferably
15 made of a shape memory superelastic metal alloy such
as Nitinol, which has the unusual property of
"mechanical" memory and trainability. This alloy can
be formed into a first predetermined shape above a
transition temperature range. The alloy may be
20 plastically deformed into a second shape below the
transition temperature range, but the alloy will
completely recover to its original (first
predetermined) shape when raised back above the
transition temperature range. The Nitinol preferably
25 has a composition of about 50% nickel and about 50%
titanium. The properties of shape memory alloys such
as Nitinol and their use in stents have been well-
documented in the literature, and reference can be
made to the article by T.W. Duerig, A.R. Pelton and D.
30 Stockel entitled "The Use of Superelasticity in
Medicine", a copy of which is attached hereto and
specifically incorporated into this specification by
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19
specific reference thereto as though fully set forth
herein.
Alternatively, the stent 40 (including its struts
42, 44 and connecting members 48) can be made of
5 stainless steel, tantalum, titanium, elgiloy, gold,
platinum, or any other metal or alloy, or polymers or
composites, having sufficient biocompatibility,
rigidity, flexibility, radial strength, radiopacity
and antithrombogenicity.
10 Although the connecting members 48 have been
described above as having the same material as the
struts 42, 44, it is possible to provide the
connecting members 48 with a different material
without departing from the spirit and scope of the
15 present invention. Such a material should be springy
in nature and should allow the connecting members 48
to be compressed and expanded in the longitudinal
direction to compensate for the foreshortening
experienced by the struts 42 and 44. Non-limiting
20 examples of such materials can include any of the
materials described above for the stent 40.
2. Methods of Manufacture
The stent 40 can be made from one of a number of
methods, depending on the material of the stent 40 and
25 the desired nature of deployment.
In a non-limiting first preferred method, the stent
4o is fabricated from a solid Nitinol tube with
dimensions that are identical to the stent 40 when it
is in the fully compressed state. The pattern of the
30 stent 40 (i.e., its struts 42, 44 and connecting
members 48) is programmed into a computer-guided laser
cutter or lathe which cuts out the segments between
the struts 42, 44 and the connecting members 48 in a
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20
manner which closely controls the outside diameter and
wall thickness of the stent 40.
After the cutting step, the stent 40 is
progressively expanded until it reaches its fully
5 expanded state. The expansion can be performed by an
internal expansion fixture, although other expansion
apparatus and methods can be used without departing
from the spirit and scope of the present invention.
The overall length of the stent 40 must be
10 consistently maintained throughout the expansion of
the stent 40 from its fully compressed to its fully
expanded states:
Once the stent 40 has been expanded to its fully
expanded state, it is heat-treated to "set" the shape
15 memory of the Nitinol material to the fully expanded
dimensions. The stmt 40 is then cleaned and electro-
polished.
The next step is to compress the stent 40 again into
a dimension which allows for delivery into a vessel,
20 either through percutaneous delivery or through
minimally invasive surgical procedures. Specifically,
the stent 40 must be compressed into a smaller state
so that it can be delivered by a delivery device to
the desired location of the vessel. Any conventional
25 delivery device could be used, such as but not limited
to a tube, catheter, or sheath. This compression is
accomplished by cooling the stent 40 to a low
temperature, for example, zero degrees Celcius, and
while maintaining this temperature, compressing the
30 stent 40 to allow the stent 40 to be inserted inside
the delivery device. Once inserted inside the
delivery device, the stmt 40 is held by the delivery
device in the compressed state at room temperature.
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In a non-limiting second preferred method, a
balloon-expandable stem 40 can be fabricated by
connecting a plurality of wires that have been bent or
formed into the desired shapes for the struts 42, 44
and connecting members 48. The connection can be
accomplished by welding, tying, bonding, or any other
conventional method. Alternatively, wire electro-
discharge machining can be used. The wires are
capable of experiencing plastic deformation when the
scent 40 is compressed, and when the stent 40 is
expanded. Upon glastic deformation of the stent 40 to
either the~compressed or the expanded state, the stent
40 remains in this state until another force is
applied to plastically deform the stent 40 again.
While certain methods of manufacture have been
described above, it will be appreciated by those
skilled in the art that other methods of manufacture
can be utilized without departing from the spirit and
scope of the present invention.
3. Deployment Methods
The stent 40 can be deployed by a number of delivery
systems and delivery methods. These delivery systems
and methods will vary depending on whether the stmt
40 is expanded by self-expansion, radial expansion
forces, or radio frequency.
While the description above refers to particular
embodiments of the present invention, it will be
understood that many modifications may be made without
departing from the spirit thereof. The accompanying
claims are intended to cover such modifications as
would fall within the true scope and spirit of the
present invention.
