Note: Descriptions are shown in the official language in which they were submitted.
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A MULTI-LAMINATE STENT HAVING
SUPERELASTIC ARTICULATED SECTIONS
Charles V. Tomonto
FIELD OF THE INVENTION
The present invention relates to an expandable intraluminal graft for use
within a
body passageway or duct and, more particularly, expandable intraluminal
vascular grafts
which are particularly useful for repairing blood vessels narrowed or occluded
by
diseased luminal grafts.
BACKGROUND OF THE INVENTION
Intraluminal endovascular grafting or stenting has been demonstrated to be an
alternative to conventional vascular surgery. Intraluminal endovascular
grafting involves
the percutaneous insertion into a blood vessel of a tubular prosthetic graft
or stent and its
delivery via a catheter to the desired location within the vascular system.
Advantages of
this method over conventional vascular surgery include obviating the need for
surgically
exposing, incising, removing, replacing, or bypassing the defective blood
vessel.
Structures which have previously been used as intraluminal vascular grafts
have
included various types of stents which are expanded within a vessel by a
balloon catheter
such as the one described in U.S. Patent 5,304,197 issued to Pinchuk et al. on
April 19,
1994. Examples of different types of stents include helical wound wires such
as those
described in U.S. Patent 5,019,090 issued to Pinchuk on May 28, 1991 and
stents formed
by cutting slots into a metal tube, such as the one described in U.S. Patent
4,733,665
issued to Palmaz on March 29, 1988.
Other types of stents include self expanding stents, typically made from a
superelastic material, such as a nickel titanium alloy (Nitinol). The prior
art makes
reference to the use of Nitinol which has shape memory andlor superelastic
characteristics in medical devices which are designed to be inserted into a
patient's body.
The shape memory characteristics allow the devices to be deformed to
facilitate their
insertion into a body lumen or cavity and then be heated within the body so
that the
device returns to its original shape. Superelastic characteristics on the
other hand
generally allow the metal to be deformed and restrained in the deformed
condition to
facilitate the insertion of the medical device containing the metal
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into a patient's body, with such deformation causing the phase transformation.
Once within the
body lumen the restraint on the superelastic member can be removed, thereby
reducing the
stress therein so that the superelastic member can return to its original un-
deformed shape by
the transformation back to the original phase.
Alloys having shape memory/superelastic characteristics generally have at
least two
phases. These phases are a martensite phase, which has a relatively low
tensile strength and
which is stable at relatively low temperatures, and an austenite phase, which
has a relatively
high tensile strength and which is stable at temperatures higher than the
martensite phase.
Shape memory characteristics are imparted to the alloy by heating the metal at
a
temperature above which the transformation from the martensite phase to the
austenite phase
is complete, i.e. a temperature above which the austenite phase is stable (the
Af temperature).
The shape of the metal during this heat treatment is the shape "remembered".
The heat treated
metal is cooled to a temperature at which the martensite phase is stable,
causing the austenite
phase to transform to the martensite phase. The metal in the martensite phase
is then
plastically deformed, e.g. to facilitate the entry thereof into a patient's
body. Subsequent
heating of the deformed martensite phase to a temperature above the martensite
to austenite
transformation temperature causes the deformed martensite phase to transform
to the austenite
phase and during this phase transformation the metal reverts back to its
original shape if
unrestrained. If restrained, the metal will remain martensitic until the
restraint is removed.
Methods of using the shape memory characteristics of these alloys in medical
devices
intended to be placed within a patient's body present operational
difficulties. For example,
with shape memory alloys having a stable martensite temperature below body
temperature, it
is frequently difficult to maintain the temperature of the medical device
containing such an
alloy sufficiently below body temperature to prevent the transformation of the
martensite
phase to the austenite phase when the device was being inserted into a
patient's body. With
intravascular devices formed of shape memory alloys having martensite-to-
austenite
transformation temperatures well above body temperature, the devices can be
introduced into
a patient's body with little or no problem, but they must be heated to the
martensite-to-austenite transformation temperature which is frequently high
enough to cause
tissue damage and very high levels of pain.
When stress is applied to a specimen of a metal such as Nitinol exhibiting
superelastic
characteristics at a temperature above which the austenite is stable (i.e. the
temperature at
which the transformation of martensite phase to the austenite phase is
complete), the specimen
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deforms elastically until it reaches a particular stress level where the alloy
then undergoes a
stress-induced phase transformation from the austenite phase to the martensite
phase. As the
phase transformation proceeds, the alloy undergoes significant increases in
strain but with little
or no corresponding increases in stress. The strain increases while the stress
remains
essentially constant until the transformation of the austenite phase to the
martensite phase is
complete. Thereafter, further increase in stress are necessary to cause
further deformation. The
martensitic metal first deforms elastically upon the application of additional
stress and then
plastically with permanent residual deformation.
If the load on the specimen is removed before any permanent deformation has
occurred, the martensitic specimen will elastically recover and transform back
to the austenite
phase. The reduction in stress first causes a decrease in strain. As stress
reduction reaches the
level at which the martensite phase transforms back into the austenite phase,
the stress level in
the specimen will
remain essentially constant (but substantially less than the constant stress
level at which the
austenite transforms to the martensite) until the transformation back to the
austenite phase is
complete, i.e. there is significant recovery in strain with only negligible
corresponding stress
reduction. After the transformation back to austenite is complete, further
stress reduction
results in elastic strain reduction. This ability to incur significant strain
at relatively constant
stress upon the application of a load and to recover from the deformation upon
the removal of
the load is commonly referred to as superelasticity or pseudoelasticity. It is
this property of
the material which makes it useful in manufacturing tube cut self-expanding
stents. The prior
art makes reference to the use of metal alloys having superelastic
characteristics in medical
devices which are intended to be inserted or otherwise used within a patient's
body. See for
example, U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445
(Sakamoto et al.).
However, in general, the foregoing structures, both balloon expandable and
self-
expanding, have one major disadvantage in common. Insofar as these structures
must be
delivered to the desired location within a given body passageway in a
collapsed state, in order
to pass through the body passageway. While it is necessary for the expanded
stent to have
enough rigidity to maintain the integrity of the vessel it is implanted into,
it also needs to have
sufficient flexibility so that it can be navigated through tortuous vessels.
For repairing blood
vessels narrowed or occluded by disease, or repairing other body passageways,
the length of
the body passageway which requires repair, as by the insertion of a stent, may
present
problems if the length of the required graft cannot negotiate the curves or
bends of the body
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passageway through which the graft is passed by the catheter. In other words,
in many
instances, it is necessary to support a length of tissue within a body
passageway by a
graft, wherein the length of the required graft exceeds the length of a graft
which can be
readily delivered via a catheter to the desired location within the vascular
system. Some
grafts do not have the requisite ability to bend so as to negotiate the curves
and bends
present within the vascular system, particularly prostheses or grafts which
are relatively
rigid and resist bending with respect to their longitudinal axes.
Accordingly, one solution to this problem has been the development of an
articulated stent. An example of an articulated stent is given in U.S. Patent
5,195,984
issued to Schatz on March 23, 1993. Such a stent is particularly useful for
critical body
passageways, such as the left main coronary artery of a patient's heart.
Schatz discloses a
stent having a plurality of expandable and deformable individual intraluminal
vascular
grafts or stents wherein and adjacent grafts are flexibly connected by a
single connector
members.
Recently, however, there has been a need to improve upon the stent disclosed
in
the Schatz reference. Specifically, it has been a desire to the technical
community to
make such a stent which is even more flexible, so that the stent can navigate
tortuous
vessels better than before. The present invention provides such a stent.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a stent for
implantation into a vessel of a patient. The stent has at least two
plastically deformable
and expandable tubular graft members for expansion within a vessel. Each of
the graft
member has a first end, a second end, a wall section disposed therebetween and
a lumen
extending therethrough. the stent further includes at least one articulation
connecting the
first end of one of the graft members with the second end of the other graft
member.
Wherein the articulation is made from a superelastic material.
BRIEF DESCRIPTION OF DRAWINGS
The foregoing and other aspects of the present invention will best be
appreciated with
reference to the detailed description of the invention in conjunction with the
accompanying drawings, wherein:
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Figure 1 is a partial perspective view of a stent made in accordance with the
present
invention.
Figure 2 is a side view of a stent made in accordance with the present
invention.
Figure 3 is a partial cross sectional view of the stent shown in Figure 2
taken along line 3-3.
Figure 4 is a view similar to that of figure 2 but showing the stent as it
would appear
when navigating a body vessel.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings in detail wherein like numerals indicate the
same
element throughout the views, there is shown in Figures 1 and 2 a stent 10
made in accordance
with the present invention. Stent 10 is shown in its collapsed condition ready
to be disposed
on a balloon catheter for delivery and subsequent deployment into a vessel of
a patient. Stent
10 includes at least two expandable grafts. Stent 10 is shown as having 3
expandable grafts
20, 30 and 40 each having a first end, 22, 32 and 42, and a second end, 24, 34
and 44, and a
wall surface, 26, 36 and 46, disposed therebetween. The wall surface has a
plurality of slots,
for example 28, 38 and 48, formed therein. The grafts are formed from a metal
tube and the
slots are cut therein, typically by laser cutting or photochemical maching.
After the slots are
cut therein, each graft comprises a plurality of loops 50, 60 and 70, wherein
adjacent loops are
connected by at least one strut 52, 62 and 72.
Grafts 20 and 30 and 40 are connected together by at least one articulation
80, and
grafts 30 and 40 are connected together by at least one articulation 90. As
seen from the
figures, the articulations connect a loop from one graft to a loop from
another graft.
However, it is preferable that the articulation does not connect one loop of
one stent to a loop
of another stent directly across it. In stead it connects the loops along a
curved path which is
angled with respect to the longitudinal axis of the stent running through the
lumen.
The particular design of the present stent and its advantages are best
understood by
describing the materials the stent is made from and how the slots and
articulations are cut
therefrom. By referring to Figure 3, stent 10 is made from a multi laminate
hollow tube, or
hypotube as it is often referred to by those skilled in the art, having at
least two layers, wherein
one layer is made from a plastically deformable material, such as stainless
steel or titanium, and
the inner layer is made from a superelastic material such as Nitinol. Stent 10
preferably has a
stainless steel outer layer 100 and a superelastic Nitinol layer 110. The
stent could have more
than two layers and still possess all the advantages of the present invention.
For example, the
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stent could be made of a superelastic material, such as Nitinol, sandwiched
between two layers
of a plastically deformable, such as stainless steel. The inner layer of
stainless is preferably
relatively thin so that the articulated sections remain elastic.
The laminate hypotube may be manufactured by any number of processes known to
those skilled in the art, as evidenced in U.S. Patent 5,858,566 issued on
January 12, 1999. In
the first process the hypotube may be produced by hot rolling a series of
layers of material into
laminate sheets. The hot rolling process may be optimized to ensure mechanical
bonding
between the layers and that the composite material has the necessary tensile
strength. The
laminate sheets may then be rolled up around a mandrel and seam welded to form
a tube. The
seam welded tube can then be drawn to final size.
The second process for making the hypotube is to place multiple layers of
tubes within
each other around a core mandrel. The composite structure may then be drawn
and heat
treated using conventional wire drawing practices until the finished tube
diameter meets the
physical properties and dimensions required. A sacrificial ductile core
mandrel is placed within
the hypotube prior to the wire drawing process. The composite system is then
drawn to finish
dimensions and the core mandrel is removed. Removal of the mandrel is achieved
by reducing
the
cross section of the mandrel. By solely pulling the mandrel, the diameter of
the mandrel can be
reduced sufficiently to be easily removed.
Once a composite hypotube is obtained, the structure may be loaded onto a
lathe type
tool and the slots and articulations may be cut from this tube using, for
example, a laser
etching cutting tool, a water saw, or an electron discharge machine. Other
ways of cutting the
slots, such as by photochemically etching, are well known by those of ordinary
skill the art.
The present invention has an advantage in that after the stent design is cut
from the
tube, the articulations can then be processed so 'that the outer layer of
stainless steel is
removes from the articulations. Therefore, the articulations are now only made
from the
superelastic Nitinol material. This additional process is done by selective
etching or laser
scribing. After all etching and selective etching is complete, the tube may
then be deburred by
a procedure such as shot peening, abrasive tumbling, honing, electropolishing
and
electroetching.
By making the stent from a multi-laminate tube, wherein one layer is
plastically
deformable and the inner layer is superelastic, a stent having relatively
rigid grafts connected
by very flexible and shape recoverable articulations can be formed. This
allows for a stent
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which is flexible enough to navigate through tortuous paths, but which is long
enough to
cover a relatively large target site within a vessel. As seen from Figure 4,
the stent 10
often has to pass through tight curves. Figure 4 shows the stent 10 loaded
onto a balloon
catheter 120 and being navigated through a vessel 130. Because articulations
80 and 90
are made from a superelastic material, they make the stent more flexible so
that it can
better pass through tight curves such as the one shown here. The stent need
not have the
particular design shown in the figures. It could simply be a stent having a
plastically
deformable layer and an elastic layer, wherein the stent has elastic zones
along its body
where the plastically deformable layer has been removed. That is the stent
could simply
be a graft member having at least two layers wherein one of the layers
comprises an
elastic layer of material extending from end to end, and said other layer
comprises a
plastically deformable material covering only a predetermined portion of the
elastic
layer.
The multilaminate stent described herein can also be used to make a crush
recoverable balloon expandable stent of the type described in U.S. Patent No.
6,086,610.
By controlling the wall thickness of the plastically deformable layer relative
to the
superelastic section, the performance of the device can be adjusted. For this
characteristic, the wall thickness of the plastically deformable layer is
targeted to
plastically deform during deployment. At the same load condition, the
superelastic layer
is targeted to accept a load greater than the lower plateau of the stress
versus strain curve
for the superelastic layer and less than the upper plateau of the stress
versus strain curve
for the superelastic layer. In this loading condition, the radial outward
force is controlled
by the plastically deformable member (yield point on the stress versus strain
curve) and
the crush resistance is controlled by the superelastic member (upper plateau
on the stress
versus strain curve). This combination will develop a stent which is balloon
catheter
deliverable to a known stent diameter, crush resistant, and exhibits no
chronic outward
force.
Although particular embodiments of the present invention have been shown and
described, modification may be made to the device and/or method without
departing
from the spirit and scope of the present invention. The terms used in
describing the
invention are used in their descriptive sense and not as terms of limitations.
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