Note: Descriptions are shown in the official language in which they were submitted.
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Radially Expandable Stent
D a s c r i p t i o n
The invention concerns a radially expandable stent for
implanting in a body vessel, in particular in the carotid, in
the form of a self-expanding hollow cylindrical element.
The purpose of stents in blood vessels is to prevent a renewed
narrowing or a renewed occlusion of the blood vessels after a
dilatation, in order to again ensure a good blood flow. In
addition to this purpose, stents in the carotids also have the
function of preventing deposits on the blood vessels from
becoming separating and being flushed into the brain vessels
so that the patient does not suffer a stroke or other
permanent damage.
Conventional stents composed of a cell or mesh structure are
only capable of holding back deposits to a limited degree. If
these deposits separate in the carotids, they are flushed into
the brain with the blood flow and can here. result in the
blockage of vessels. A blockage in the blood supply usually
induces brain damage in the form of a stroke and should,
therefore, be avoided in any case.
Stents are implanted in blood vessels to reduce or prevent the
possibility of renewed narrowing. Furthermore, so-called
stent grafts are implanted to bridge defects such as
aneurisms, ruptures, dissections, punctures, etc. or to enable
sealing of the altered or diseased material.
Stent grafts are stents which are covered with a biocompatible
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material impermeable to blood or particles that separate from
the vascular wall and can be swept into the blood stream.
In stent grafts, a difference is made between balloon
expandable and self-expanding systems. Balloon expandable
systems are distinguished therein that they can be accurately
adapted to the diameter of the vessel. With this system, it
is possible to make allowance for the narrowing of a vessel.
Self-expanding stent grafts are composed of either plaited,
woven or knitted flexible materials or a self-expandable
material such as e.g. NITINOL is used. The sheathing consists
for the most part of PETP (dacron) or PTFE (teflon).
Depending on the process, PTFE is distinguished thereby that
it can expand in a direction without this resulting in
restoring forces. In particular in balloon expandable
systems, it is possible to expand the stent to the desired
dimension and to adapt it accurately to the diameter of the
vessel.
If necessary, it is also possible to subsequently expand such
a stem, perhaps with a balloon, after it has been implanted.
Stent grafts that are covered with a PETP fabric can only be
expanded to that dimension at which the fabric is stretched.
Therefore, the dimension of the vessel to be treated must
first be accurately determined. It is only possible to adapt
the stent graft to the anatomy of the vessel to a limited
extent.
Self-expanding systems are distinguished by the special
feature that they spring back to their original position when
compressed from the outside.
i
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Self-expanding, flexible systems are preferably used,
especially in vessels which can be detected by touch from the
outside or deformed by adjacent muscles.
The object of the invention is to improve a radially
expandable stent such that it not only holds blood vessels in
the expanded state but that it also prevents deposits on blood
vessels from separating and being flushed into the brain
vessels.
The object is solved by a stent having the features of claim
1.
The invention is distinguished thereby that a self-expanding
stent is covered with a material, for example, PTFE, in such
a way that the force of the self-expansion is not sufficient
to expand the sheathing. Similar to a conventional balloon-
expandable stent, this stent must be expanded by a balloon.
The special feature of the invention is that, in contrast to
a balloon-expanding system, this system springs back into its
original position if it is deformed by an external force.
However, at the same time, it is possible to use the advantage
of balloon expanding systems and to accurately adapt the stent
graft to the anatomy of the vessel.
As already described above, one of the advantages of stent
grafts is that it is possible to fix deposits on the vascular
wall with these systems and to prevent them from reaching into
the blood stream.
A balloon expandable stent having an outer irreversible,
expandable sheathing which, in addition, has flexible
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properties, is suitable especially for use in vessels that
have different diameters over the length of the stenting. The
diameter of vessels may differ by two or more millimeters such
as, for example, the transition of the carotis communa into
the carotis interns or externs.
Current self-expanding systems are not capable of
appropriately sheathing these greatly varying diameters. In
self-expanding stents, the stent must either be selected so
large that a good fit to the wall is ensured or the stent is
suitably selected for the smaller vessel.
In the first case, the stent exerts a continuous pressure on
the vascular wall of the smaller vessel which can result
therein that this vessel expands in the course of time and
adapts to the geometry of the stent. In any event, this
effect is undesirable.
If a self-expanding stent is selected so as to be suitable for
the smaller vessel, then there is the danger that the stent
does not clearly adjoin the wall in the larger vessel or in
the area in which the vessel has a larger diameter. In
particular in stent grafts, there is the possibility that
areas are produced between the stent graft and vascular wall
in which blood clots are formed by turbulances, which are to
be avoided in any event.
The invention described offers the possibility of springing
back to the set diameter by means of the flexible properties
of the stent material when there is a possible compression.
At the same time, this system guarantees the possibility of
perfectly adapting the stent to the anatomy of the vessel by
using different balloon diameters.
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If a stent covered with an expandable material is to be
inserted in a vascular branching, then the stent can at least
have a radial opening which can be positioned in the vascular
system in such a way that a corresponding side branch is not
blocked. In the area of this opening, the sheathing has an
opening which is preferably circular after the expansion in
order not to affect the blood stream in the side branch.
Further embodiments of the stent can, if required, have
several radial openings over the length of the stmt.
The sheathing can consist of an expandable PTFE. Stents
having a sheathing of expandable PTFE are inexpensive to
manufacture. Due to the good compatibility with the patients
tissue, the sheathing can also consist of a homologous
material. Sheathings made of an animal material, in
particular of pig, beef or horse veins, are also relatively
compatible with human tissue. The self-expandable material of
the hollow cylindrical element can be a nickel/titanium alloy
(NITINOL), a spring steel or a polymer material.
