Note: Descriptions are shown in the official language in which they were submitted.
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AXIALLY FLEXIBLE STENT
Technical Field
The present invention relates to a stent having axial flexibility and
resilience
in its expanded form.
Background Art
A stmt is commonly used as a tubular structure left inside the lumen of a
duct to relieve an obstruction. Commonly, stents are inserted into the lumen
in a
non expanded form and are then expanded autonomously (Tihon et al. (1994) U.S.
Patent No. 5,356,423) or with the aid of a second device in situ. Although a
number
of designs have been reported, these designs have suffered from a number of
limitations. These include; restrictions on the dimension of the stmt (Cardon
et al.
1995 U.S. Patent No 5,383;892 ). Cardon et al. describes a stent that has
rigid ends
(8mm) and a flexible median part of 7-2lmm. This device is formed of multiple
parts and is not continuously flexible along the longitudinal axis. Another
stent
design that has rigid segments and flexible segments has been described by
Pinchasik et al. US Patent 5,449,373 (1995).
Other stents are described as longitudinally flexible (Lau et a1.(1995) U.S.
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Patent No. 5,421,955 also EP application 540290 A2, A3) but consist of a
plurality of
cylindrical elements connected by flexible members. This design has at least
one
important disadvantage, for example, according to this design, protruding
edges
occur when the stent is flexed around a curve raising the possibility of
inadvertent
retention of the stmt on plaque deposited on arterial walls. This may cause
the stent
to embolize or move out of position and further cause damage to the interior
lining
of healthy vessels. (see Figure 1(a) below).
Thus, stems known in the art, which may be expanded by balloon
angioplasty, generally compromise axial flexibility to permit expansion and
provide
overall structural integrity.
Summar~of The Invention
The present invention overcomes some perceived shortcomings of prior art
stents by providing a stent with axial flexibility. In a preferred embodiment,
the
stent has a first end and a second end with an intermediate section between
the two
ends. The stent further has a longitudinal axis and comprises a plurality of
longitudinally disposed bands, wherein each band defines a generally
continuous
wave along a line segment parallel to the longitudinal axis. A plurality of
links
maintains the bands in a tubular structure. In a further embodiment of the
invention, each longitudinally disposed band of the stent is connected, at a
plurality
of periodic locations, by a short circumferential link to an adjacent band.
The wave
associated with each of the bands has approximately the same fundamental
spatial
frequency in the intermediate section, and the bands are so disposed that the
waves
associated with them are spatially aligned so as to be generally in phase with
one
another. The spatially aligned bands are connected, at a plurality of periodic
locations, by a short circumferential link to an adjacent band.
In particular, at each one of a first group of common axial positions, there
is a
circumferential link between each of a first set of adjacent pairs of bands.
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At each one of a second group of common axial positions, there is a
circumferential link between each of a second set of adjacent rows of bands,
wherein, along the longitudinal axis, a common axial position occurs
alternately in
the first group and in the second group, and the first and second sets are
selected so
that a given band is linked to a neighboring band at only one of the first and
second
groups of common axial positions.
In a preferred embodiment of the invention, the spatial frequency of the wave
associated with each of the bands is decreased in a first end region lying
proximate
to the first end and in a second end region lying proximate to the second-end,
in
comparison to the spatial frequency of the wave in the intermediate section.
In a
further embodiment of the invention, the spatial frequency of the bands in the
first
and second end regions is decreased by 20% compared with the spatial frequency
of
the bands in the intermediate section. The first end region may be located
between
the first end and a set of circumferential links lying closest to the first
end and the
second end region lies between the second end and a set of circumferential
links
lying closest to the second end. The widths of corresponding sections of the
bands
in these end regions, measured in a circumferential direction, are greater in
the first
and second end regions than in the intermediate section. Each band includes a
terminus at each of the first and second ends and the adjacent pairs of bands
are
joined at their termini to form a closed loop.
In a further embodiment of the invention, a stent is provided that has first
and second ends with an intermediate section therebetween, the stent further
having
a longitudinal axis and providing axial flexibility. This stent includes a
plurality of
longitudinally disposed bands, wherein each band defines a generally
continuous
wave having a spatial frequency along a line segment parallel to the
longitudinal
axis, the spatial frequency of the wave associated with each of the bands
being
decreased in a first end region lying proximate to the first end and in a
second end
region lying proximate to the second end, in comparison to the spatial
frequency of
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the wave in the intermediate section; and a plurality of links for maintaining
the
bands in a tubular structure. The first and second regions have been further
defined
as the region that lies between the first and second ends and a set of
circumferential
links lying closest lying closest to the first end and second end.
In a further embodiment the widths of the sections of the bands, measured in
a circumferential direction, are greater in the first and second end regions
than in the
intermediate section.
Brief Description of the Drawings
The foregoing aspects of the invention will be more readily understood by
reference to the following detailed description, taken with the accompanying
drawings, in which:
Figs 1 (a) and 1 (b) are side views of a stent having circumferentially
disposed
bands wherein the stent is in axially unbent and bent positions respectively,
the
latter showing protruding edges.
Figs 1 (c) and 1 (d) are side views of an axially flexible stent in accordance
with the present invention wherein the stent is in unbent and bent positions
respectively, the latter displaying an absence of protruding edges.
Fig. 2 is a side view of a portion of the stmt of Figs. 1(c) and 1(d) showing
the
longitudinal bands, spaces, and inner radial measurements of bends in the
bands
being measured in inches.
Figs. 3 (a) and 3 (b) show a portion of the stent of Fig. 2 with two bands
between two circtunferential links (a) before expansion in the undeformed
state; and
(b) after expansion, in the deformed state.
Fig. 4 is a view along the length of a piece of cylindrical stent (ends not
shown) prior to expansion showing the exterior surface of the cylinder of the
stent
and the characteristic banding pattern.
Fig. 5 is an isometric view of a deflection plot where the stent of Fig. 2 is
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expanded to a larger diameter of 5mm.
Fig. 6 shows a two-dimensional layout of the stent of Fig. 4 to form a
cylinder
such that edge "A" meets edge "B", and illustrating the spring-like action
provided in
circumferential and longitudinal directions.
Figure 7 shows a two dimensional layout of the stent. The ends are modified
such that the length (LA) is about 20% shorter than length (LB) and the width
of the
band A is greater than the width of band B.
Detailed Description of Specific Embodiments
Improvements afforded by embodiments of the present invention include (a)
increased flexibility in two planes of the non-expanded stent while
maintaining
radial strength and a high percentage open area after expansion; (b) even
pressure
on the expanding stem that ensures the consistent and continuous contact of
expanded stent against artery wall; (c) avoidance of protruding parts during
bending; (d) removal of existing restrictions on maximum length of stent; and
reduction of any shortening effect during expansion of the stmt.
In a preferred embodiment of the invention, an expandable cylindrical stent
is provided having a fenestrated structure for placement in a blood vessel,
duct or
lumen to hold the vessel, duct or lumen open, more particularly for protecting
a
segment of artery from restenosis after angioplasty. The stent may be expanded
circumferentially and maintained in an expanded configuration, that is
circumferentially rigid. The stent is axially flexible and when flexed at a
band, the
stent avoids any externally protruding component parts. Figure 1 shows what
happens to a stent, of a similar design to a preferred embodiment herein but
utilizing instead a series of circumferentially disposed bands, when caused to
bend
in a manner that is likely encountered within a lumen of the body. A stent
with a
circumferendal arrangement of bands (1) experiences an effect analogous to a
series
of box cars on a railway track. As the row of box cars proceeds around the
bend, the
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corner of each car proceeding around the bend after the coupling is caused to
protrude from the contour of the track. Similarly, the serpentine
circumferential
bands have protrusions (2) above the surface of the stent as the stmt bends.
In
contrast, the novel design of the embodiment shown in Figs. 1 (c) and 1 (d)
and
Figure ? in which the bands (3) are axially flexible and are arranged along
the
longitudinal axis, avoids the box car effect when the stmt is bent, so the
bent bands
(4) do not protrude from the profile of the curve of the stmt. Furthermore,
any
flaring at the ends of the stmt that might occur with a stent having a uniform
structure is substantially eliminated by introducing a modification at the-
ends of the
stent. This modification comprises decreasing the spatial frequency and
increasing
the width of the corresponding bands in a circumferential direction (LA and A)
compared to that of the intermediate section. (LB and B). Other modifications
at the
ends of the stent may include increasing the thickness of the wall of the
stent and
selective electropolishing. These modifications protect the artery and any
plaque
from abrasion that may be caused by the stent ends during insertion of the
stent.
The modification also may provide increased radio-opacity at the ends of the
stent.
Hence it may be possible to more accurately locate the stent once it is in
place in the
body.
The embodiment as shown in Figs. 2 and 6 has the unique advantage of
possessing effective "springs" in both circumferential and longitudinal
directions
shown as items (5) and (6) respectively. These springs provide the stmt with
the
flexibility necessary both to navigate vessels in the body with reduced
friction and to
expand at the selected site in a manner that provides the final necessary
expanded
dimensions without undue force while retaining structural resilience of the
expanded structure.
As shown in both Figs. 2, 4 and 6, each longitudinal band undulates through
approximately two cycles before there is formed a circumferential link to an
adjacent
band. Prior to expansion, the wave associated with each of the bands may have
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approximately the same fundamental spatial frequency, and the bands are so
disposed that the waves associated with them are spatially aligned, so as to
be
generally in phase with one another as shown in Fig. 6.
The aligned bands on the longitudinal axis are connected at a plurality of
periodic locations, by a short circumferential link to an adjacent band.
Consider a
first common axial position such as shown by the line X-X in Fig. 4 and 6.
Here an
adjacent pair of bands is joined by circumferential link 7. Similarly other
pairs of
bands are also linked at this common axial position. At a second common axial
position, shown in Fig. 6 by the line Y-Y, an adjacent pair of bands is joined
by
circumferential link 8. However, any given pair of bands that is linked at X-X
is not
linked at Y-Y and vice-versa. The X-X pattern of linkages repeats at the
common
axial position Z-Z. In general, there are thus two groups of common axial
positions.
In each of the axial positions of any one group are links between the same
pairs of
adjacent bands, and the groups alternate along the longitudinal axis of the
embodiment. In this way, circumferential spring 5 and the longitudinal spring
6 are
provided.
A feature of the expansion event is that the pattern of open space in the
stent
of the embodiment of Fig. 2 before expansion is different from the pattern of
the
stent after expansion. In particular, in a preferred embodiment, the pattern
of open
space on the stmt before expansion is serpentine, whereas after expansion, the
pattern approaches a diamond shape (3a, 3b). In embodiments of the invention,
expansion may be achieved using pressure from an expanding balloon or by other
mechanical means.
In the course of expansion, as shown in Fig. 3, the wave shaped bands tend to
become straighter. When the bands become straighter, they become stiffer and
thereby withstand relatively high radial forces. Fig. 3 shows how radial
expansion
of the stmt causes the fenestra to open up into a diamond shape with maximum
stress being expended on the apices of the diamond along the longitudinal
axis.
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When finite element analyses including strain studies were performed on the
stmt,
it was found that maximum strain was experienced on the bands and links and
was
below the maximum identified as necessary to maintain structural integrity.
The optimization of strain and "pop" pressure of the stmt is achieved by
creating as large a turn radius as possible in the wave associated with each
band in
the non-expanded stent while preserving a sufficient number of bands and links
to
preserve the structural integrity of the stmt after expansion. The number of
bands
and the spatial frequency of the wave they describe on the longitudinal axis
also
affects the number of circumferential links. The circumferential links
contribute
structural integrity during application of radial force used in expansion of
the stent
and in the maintenance of the expanded form.
The stent may be fabricated from many methods. For example, the stent may
be fabricated from a hollow or formed stainless steel tube that may be cut out
using
lasers, electric discharge milling (EDM), chemical etching or other means. The
stmt
is inserted into the body and placed at the desired site in an unexpanded
form. In a
preferred embodiment, expansion of the stent is effected in a blood vessel by
means
of a balloon catheter, where the final diameter of the stent is a function of
the
diameter of the balloon catheter used.
In contrast to stents of the prior art, the stent of the invention can be made
at
any desired length, most preferably at a nominal 30mm length that can be
extended
or diminished by increments, for example l.9mm increments.
It will be appreciated that a stent in accordance with the present invention
may be embodied in a shape memory material, including, for example, an
appropriate alloy of nickel and titanium; or stainless steel. In this
embodiment after
the stmt has been formed, it may be compressed so as to occupy a space
sufficiently
small as to permit its insertion in a blood vessel or other tissue by
insertion means,
wherein the insertion means include a suitable catheter, or flexible rod. On
emerging from the catheter, the stent may be configured to expand into the
desired
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configuration where the expansion is automatic or triggered by a change in
pressure, temperature or electrical stimulation.
An embodiment of the improved stent has utility not only within blood
vessels as described above but also in any tubular system of the body such as
the
bile ducts, the urinary system, the digestive tube, and the tubes of the
reproductive
system in both men and women.
