Note: Descriptions are shown in the official language in which they were submitted.
CA 02312566 2000-OS-31
IMPROVED STENT CONFIGURATION
BACKGROUND OF THE INVENTION
The present invention generally relates to intravascular stems and more
particularly, pertains to specialized stmt configurations for the treatment of
vascular
disease within non-uniform vessels such as, for example, a tapered artery, or
at the
ostium or bifurcation of an artery.
Stems or expandable grafts are implanted in a variety of body lumens in an
effort to maintain their patency. These devices typically are intraluminally
implanted by use of a catheter which is inserted at an easily accessible
location and
then advanced to the deployment site. The stmt initially is maintained in a
radially
compressed or collapsed state to enable it to be maneuvered through the lumen.
Once in position, the stmt is deployed which, depending upon its construction,
is
achieved either automatically by, for example, removing a restraint, or
actively by,
for example, the inflation of a balloon about which the stent is carried on
the
catheter.
Intravascular stems currently in use typically are designed to expand or to be
expanded within a diseased vessel to a given nominal diameter that is constant
along
the entire length of the stmt. The stent also typically is uniform along its
entire
length in terms of its radial strength, its longitudinal flexibility and its
coverage, i.e.,
the actual area of stmt material defining the surface of the deployed stmt
relative to
the area of vessel covered thereby. Most blood vessels, however, are not of
constant
diameter, exhibiting either a natural taper or a narrowing, particularly near
bifurcations. Blood vessels may be abruptly tapered over short lengths (less
than
20mm), as in the carotid arteries, or gradually tapered over long lengths
(greater
than 20mm), as in the iliac arteries. Examples of bifurcation sites in the
human
circulatory system include the vascular profile where the external and
internal
carotid arteries branch out, from the common carotid artery. The common
carotid
CA 02312566 2000-OS-31
_2_ Docket No. ACS 52779
artery is 7-9 mm in diameter while the internal carotid artery is 4-6 mm in
diameter.
In the event disease is present at such junction, a stmt deployed therein must
accommodate a 3-5 mm diameter change over a length of about 20-30 mm.
Another example involves the stenting of the renal arteries. In order to cover
the
S entire ostial area, it is necessary for the stmt to conform to the interior
of the renal
artery and to flare into the significantly larger aorta. Moreover, the lesions
present
at the ostium are typically hard and calcified requiring the stmt to possess
greater
strength in that specific region. Similar requirements arise in the treatment
of ostial
disease at the bifurcation in native coronary arteries or bypass grafts and
aorta-ostial
disease in the peripheral (e.g., carotid, renal and iliac) arteries. Non-
uniformities
also may be present by virtue of a curved or tortuous configuration of the
blood
vessel at the diseased site. .
By fitting conventionally configured stems, i.e., stems of uniform shape and
diameter, to such sites, a number of problems arise. In the event such stent
is fitted
to a tapered section of artery, either the artery is forced into an unnatural
shape, or
the stmt must somehow become distorted during its deployment. By forcing the
artery into an unnatural shape, certain sections of the tissue are caused to
be
overextended or other sections to be undersupported. Steps taken in an effort
to
non-uniformly expand a uniformly constructed stmt have the effect of imparting
undesired 'non-uniform characteristics to the device such as non-uniform
radial
strength, flexibility, and coverage. Another potential side effect associated
with the
use of a stmt of uniform construction is that its deployment in a tortuous
segment of
vasculature would cause an undesirable straightening of such segment.
A stmt therefore is needed with which a non-uniform vessel may be
uniformly supported to provide consistency in terms of coverage, radial
strength,
and longitudinal flexibility along its entire length. Alternatively, a stmt is
needed
that is capable of providing specific variations in terms of coverage, radial
strength,
CA 02312566 2000-OS-31
_3_ Docket No. ACS 52779
and longitudinal flexibility at certain locations along its length, in order
to
accommodate varying needs along the length of a particular vessel.
SUMMARY OF THE INVENTION
A stmt of the present invention may be constructed so as to provide uniform
coverage, uniform radial strength and/or uniform longitudinal flexibility to a
non-
uniformly shaped deployment site such as a vessel that is tapered or
bifurcated.
Alternatively, such stmt may be constructed to accommodate non-uniform
requirements of a particular site, wherein specific variations in flexibility,
radial
strength or coverage are required at specific locations along such stmt.
The desired functional differentiation is achieved with a stmt that is
structurally differentiated in a preselected manner. Dimensional or geometric
differentiation, or both dimensional and geometric differentiation, are relied
upon to
impart the desired variations of functional characteristics to a particular
stmt. Such
structural differentiation may be gradual or abrupt and may include several
different
1 S type of differentiations along the length of the stmt.
A stmt of the present invention constructed for deployment in a tapered
vessel has a gradually increasing expansion ratio along is length. Such
differentiation may be achieved with an assembly of axially aligned rings each
with
a serpentine structure, wherein each repeating pattern of serpentines defines
a single
unit cell. By selecting the size of the unit cells in successive rings to be
increasingly
wider, an increased amount of material becomes available for expansion. Upon
expansion, such stmt assumes the tapered shape of a truncated cone to match
the
shape of the tapered artery. Despite its tapered shape, the stmt nonetheless
provides
uniform coverage of the walls of the tapered vessel as well as exhibits
uniform
radial strength and longitudinal flexibility along its entire length. The stmt
may be
alternatively constructed so as to expand into any of a variety of shapes or
profiles
CA 02312566 2000-OS-31
_4_ Docket No. ACS 52779
to fit a particular application. Geometric or dimensional as well as both
geometric
and dimensional variations of the unit cell may be employed to achieve such
functional variation.
As a further alternative, a stmt of the present invention may be non-
uniformly constructed so as to attain a uniform diameter along its entire
length
while it exhibits preselected variations of coverage, radial strength, or
longitudinal
flexibility along its length. This is achievable by, for example, varying the
thickness
of the serpentine elements while the width of each serpentine element is held
constant, or by varying the number of unit cells in certain serpentine
elements. A
similar result can be achieved by simultaneously varying the dimensions (i.e.,
strut
width and/or thickness of the unit cell), or the geometry, or both the
dimensions and
the geometry of the individual unit cells.
These and other features and advantages of the present invention will become
apparent from the following detailed description of a preferred embodiment
which,
when taken in conjunction with the accompanying drawings, illustrates by way
of
example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a greatly enlarged plan view of a longitudinally severed and
flattened stent of the present invention, shown in the predeployed state.
FIG. 2 is a perspective view of the stmt shown in FIG. 1, also in the
predeployed state.
FIG. 3 is a greatly enlarged plan view of the longitudinally severed and
flattened stmt of FIG. 1, shown in the deployed state.
CA 02312566 2000-OS-31
_S_ Docket No. ACS 52779
FIG. 4 is a perspective view of the stmt shown in FIG. 3, also in the
deployed state.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Stems of the present invention are specifically constructed for particular
S deployment sites and thereby overcome the shortcomings inherent in
attempting to
fit a uniform stmt to a non-uniform site. The non-uniformity of the site may
include
a taper, a bifurcation, an ostium, or any other variation in terms of
dimensions or
support requirements. The stmt is maneuvered into place in the conventional
manner, such as by a catheter about which it is carned while in its collapsed
state.
Once in position, the stem is expanded such as by the inflation of one or more
balloons, or in the case of self expanding stems, a confining sheath is
removed to
allow the stmt to expand automatically. Subsequent withdrawal of the catheter
and
associated deployment devices leaves the stmt in place to maintain the patency
of
the vessel.
By providing a tapered stmt for deployment within a tapered artery, uniform
coverage, uniform radial strength and uniform stiffness nonetheless may be
achieved along the entire length of the stent. Alternatively, the versatility
of the
system of the present invention allows a non-uniform stmt to be constructed
that
imparts enhanced coverage, strength or stiffness at preselected locations so
as to, for
example, provide the needed support requirements in a diseased ostium.
FIG.1 illustrates stmt 12 incorporating features of the present invention and
more specifically, a stmt for deployment in a tapered artery. The stmt
typically is
tubular in its overall shape, however, the drawing shows the stmt in a
longitudinally
severed and flattened state in order to clearly display its structure. The
stmt
structure generally consists of a series of circumferentially extending
serpentine
CA 02312566 2000-OS-31
_6_ Docket No. ACS 52779
elements (14, 16,...30) that are interconnected by links 32 extending between
adjacent serpentine elements. Each of the serpentine elements may be
characterized
as being made up of a number of individual unit cells 34, wherein each such
cell
consists of link 32 attached to two adjacent U- or V-shaped ribs 36,38. In the
S embodiment illustrated, a total of four unit cells define each serpentine
element.
Adjacent serpentine elements are arranged such that the respective series of
apexes
are in phase, and in longitudinal alignment with one another. All links extend
from
the same side of the serpentine elements. In the embodiment illustrated, all
links
extend between the left edges of the individual serpentine element.
In the embodiment illustrated in FIG. 1, each serpentine element'is
differentiated relative to the adjacent serpentine elements in terms of the
width of
the serpentine pattern, i.e., the length of each rib element as well as of
each link
element: In the particular embodiment shown, each successive serpentine
element
is progressively wider than the previous element, consequently, the rib and
link
elements of the respective unit cells are longer. The number of unit cells in
each
serpentine element, however, remains unchanged, as does the thickness and
width of
all of the rib and spine elements.
FIG. 2 is a perspective view of stmt 12 as it actually appears in use prior to
deployment. The overall tubular structure is of uniform diameter and each of
the
serpentine elements extend about the entire circumference of the device. The
individual serpentine elements are recognizable as rings while the individual
links
32 extend only between adjacent rings. The diameter of the stent is selected
to be
sufficiently small to allow passage through a patient's vasculature to the
deployment
site.
FIG. 3 illustrates the stmt shown in FIGS. 1 and 2 in its deployed state. The
device again is shown longitudinally severed and flattened in order to more
clearly
display its structure. As is clearly visible, the expansion of the device
results in a
tapered shape wherein the wider serpentine elements shown towards the right
side
CA 02312566 2000-OS-31
Docket No. ACS 52779
of the device expand to a greater extent than the narrower serpentine elements
shown toward the left side of the device. Such expansion results in a
truncated cone
shape when shown in perspective as in FIG. 4. Each of U- or V-shaped ribs
36,38
assumes a wider, more open angle while the links 32 remain essentially
stationary
and aligned. The deformation and expansion of the ribs provides for the
increase in
the diameter of the device, yet the overall length of the stmt remains
essentially
unchanged during deployment of the stmt, since the links connect only adjacent
rings to each other. This is a highly desirable characteristic of a stmt, as
any
shortening would not only reduce the total area supported by the device, but
also
could cause trauma to the surrounding tissue during deployment. Additionally,
because more stmt material is present in the larger diameter rings by virtue
of the
presence of the same number of longer ribs, the radial strength, coverage and
stiffness of the stmt remains fairly constant along its entire length.
While the figures illustrate the structure of a single embodiment of the
1 S present invention and the distortion it undergoes during deployment, it
should be
understood that vast numbers of variations are possible to enable a stent to
be
tailored to the specific requirements of a particular application. By varying
or
differentiating the geometry of the stmt's structure or of the individual unit
cells, a
commensurate variation or differentiation in function is achieved. Functional
differentiation also is achievable by varying the thickness or width of the
individual
ribs. Desired functional differences also are achievable by varying the number
of
unit cells either gradually along the length of the stent or in isolated
locations
thereon. Both the number of unit cells, the geometry of such cells, and the
dimensions of such cells may be varied in any combination to achieve a
particular
functional effect.
The above-described variables can be selected such that the resulting stmt is
specifically tailored to a very specific anatomical requirement, be it the
taper,
bifurcation twist, or ostium of a vessel. In addition to fitting the
dimensional
CA 02312566 2000-OS-31
_g_ Docket No. ACS 52779
requirements of a particular non-uniformity in the anatomy, the same variables
can
be selected such that the resulting stmt is specifically tailored to provide a
desired
radial strength, longitudinal flexibility, or coverage differentiation.
A stmt of the present invention may be constructed using any number of
well-known techniques. Preferably, a stainless steel tube is laser cut with
the
desired stmt pattern as is known in the art. Digital angiography and advanced
computing algorithms are valuable tools that are readily useable to create a
structurally differentiated stmt that conforms to the natural profile of a
vessel upon
deployment. Chemical etching or electro-polishing techniques, which are well
known, subsequently may be used to selectively vary the wall thickness~of such
stmt.
Deployment maybe achieved by shaped balloons for balloon-expandable
stems whereby a tapered balloon is used to expand a tapered stmt within a
tapered
vessel. Alternatively, multiple balloons of varying size may be used to
achieve a
similar effect, as may the tapered section of an over-sized balloon. As a
further
alternative, the stent may be constructed to be self expanding by any of
various
techniques well known in the art. Deployment of a self expanding stmt can be
achieved by subjecting the collapsed stent which is constructed of a shape
memory
alloy, to certain temperatures which cause it to expand. A stmt constructed of
elastic material may be forcefully collapsed and constrained within a sheath.
Removal of the sheath allows the stmt to automatically expand.
Balloon expandable stents may be manufactured from any number of ductile
metals and alloys including, stainless steel, tantalum, and platinum-iridium
alloys,
either coated or uncoated. Self expanding stems are constructed of shape
memory
or superelastic materials or alloys such as NiTi alloys, including Nitinol,
and Cu-Zn
alloys.
While a particular form of the invention has been illustrated and described,
it
also will be apparent to those skilled in the art that various modifications
can be
CA 02312566 2000-OS-31
_9_ Docket No. ACS 52779
made without departing from the spirit and scope of the invention.
Accordingly, it
is not intended that the invention be limited except by the appended claims.
