Note: Descriptions are shown in the official language in which they were submitted.
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EXPANDABLE ENDOVASCULAR MEDICAL TUBULAR STENT
Technical Field
The present invention relates to expandable intravascular medical tubular
stems which are
applied within the peripheral or coronary arteries of a living animal or human
being to
maintain patency after a balloon angioplasty, and also relates more generally
to stents which
may be applied to the pathology of other anatomical canals, such as the
venous, biliary,
esophagus, urinary, and so forth.
Background Art
Stents are generally tubular-shaped devices whose function is to hold open a
segment of a
vessel in the human body. The term "vessel" is intended to include any of the
arteries and
body passageways found in the human body.
Stents are of two types. The first comprises a generally non-elastic, metallic
material which
is radially expandable (i.e. plastically deformable) from the inside towards
the outside under
the effect of an inflatable balloon. The second comprises an elastic metallic
material made of
metal mesh whose diameter constricts under tension. This stmt is introduced
under tension
into the lumen of the vessel whereupon release of the tension returns the stmt
to its relaxed,
larger diameter state.
Further details of prior art stmt structures may be found in U.S. Patent No.
5,514,154 (Lau et
al); U.S. Patent No. 5,041,126 (Gainturo); U.S. Patent No. 4,655,771
(Wallsten); U.S. Patent
No. 5,496,365 (Srgo); U.S. Patent No. 5,133,732 (Wiktor); U.S Patent No.
5,382,261
(Palmaz); U.S. PatentNo. 5,102,417 (Palmaz); U.S. PatentNo. 5,195,984
(Schatz); U.S Patent
No. 5,776,183 (Kanesaka); U.S Patent No. 5,800,509 ( Boneau ); U.S. Patent No.
5,800,526
(Anderson et al ); U.S. Patent No. 5,776,181 (Lee et al); U.S. Patent No.
5,800,508
(Goicoechea et al ); U.S. Patent No. 5,776,161 (Globerman); U.S. Patent No.
5,755,776 (Al-
Saadon); U.S. Patent No. 5,843,175 (Frantzen); and U.S. Patent No. 5,843,120
(Israel et al.).
These patents are incorporated herein by reference in their entirety.
Although there are several existing devices, each suffers from variety of
drawbacks which
make them less than ideal. For example, lack of flexibility in the majority of
previous art
surgical stems makes it very difficult to access the tortuous nature of some
arterial pathways,
which may result in damage to the arterial wall during deployment of the stmt.
SUBSTITUTE SHEET (RULE 26)
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Another problem which is present in most of the prior art stems is that the
axial length of the
stmt shortens upon radial expansion, making it difficult for the operating
physician to position
the stent precisely within the artery or other body lumen.
Additionally, prior art stems have a relatively limited range of radial
expansion due to their
structure and, accordingly, such stems have to be provided in a variety of
different non-
expanded diameters to cover different vessel sizes after expansion. However,
it is often
difficult for the operating physician to accurately determine the exact final
diameter size of the
stmt needed for proper vessel size. This may result in wall damage in the case
of over-
estimating of the final diameter size of the stmt, or a sub-optimal result in
the case of under-
estimation of the final diameter size of the stmt. In the latter case, since
it usually impossible
to remove the stmt once expanded, an over-sized balloon may have to be
employed to fracture
the structure of the stmt to the extent necessary to get good patency of the
vessel wall.
However, this may result in vessel wall damage or loss of the radial strength
of the stmt
(which may cause re-stenosis), or procedure difficulty due to rupture of the
balloon inside the
vessel wall which may put the life of the patient in danger.
Preferably, the constricted diameter of the stent should be as small as
possible in order to
facilitate introduction of the stmt into the vessel. Not only is the range of
expansion affected
by reducing the initial diameter of prior art stems, but at odds is that
reducing the initial
diameter of a stmt tends to reduce its axial flexibility for a given tubular
wall thickness.
Another problem present in some of the prior art surgical stems is the
relatively loose
geometry with large gaps or wire crossing which may result in relatively high
re-stenosis rate
of the stent either due to higher incidence of clot formation within the stmt
or due to weak
radial force of the stent.
With my previous stmt described in U.S. Patent 5,755,776, I manage to overcome
most of the
drawbacks that exist in the prior art surgical stems, except the necessity for
a variety of
different diameter size stems for different vessels sizes.
Summary of the Invention
The present invention provides a stmt which has flexibility substantially
along its longitudinal
axis when in its initial, constricted state (i.e. before balloon expansion on
a catheter) to allow
it to easily pass through and along highly curved body vessels and fluid
carrying tubes. In
accordance with one aspect of the invention, there is provided an endovascular
tubular stmt
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007 06.03.2001 16:52:00
3
expandable between a fast, constricted state and a second state of greater
expanded diameter
wherein the stmt comprises in its const<icted state:
a plurality of interconnected radialiy expandable rings each formed of a
plurality of
circumferentially extensible serpentine elements, each circumferentially
actensi'ble serpentine
element comprising:
first and stcond generally eit~ally-extending members each having
respective first and second ends, the first and second members being spaced-
apart in
the axial direction of the stem; and
a medial member connecting the second end of the first member with the first
end of the second member such that at Ieast a portion of the first member
circvmfcrcutially overlaps the second mdmbcr;
the first end of the first mcmbea being circumficre~ially separated from a
second Gad
of a second member of a circumfCrentially adj scent serpentine element of the
ring by a
connection member,
the radially expandable rings being expandable under influence of a radially
outwardly
directed force, whereby the first and second members of each serpentine
element move in
circumferentially oppose directions to increase the circuznferontial length of
each serpentine
element; and
at least one comiection interconnecting tech adjaeentpair ofradially
expandable rings,
each connection connectia$ one of the connection members of vne ring to an
adjacent one of
the connection nnembers of an adjacent ring;
the stmt being flexible substantially along its longitudinal axis when in its
constricted
state and being relatively more rigid along its longitudinal axis when
expanded.
In gareral, the flcxibhity of the stmt is inversely related to the number and
length of the
interconnecting members that jointhc ring sections. In orderto maximize the
flexibility in this
invention, the length of the interconnecting raember may be minimized
practically to zero,
' ailowin~g adjacent rings to be attached by at least two opposing peaks of
their serpentine
members to form one connection therebetween. The intercvnnocting member acts
as a hinge
between adjacent rings so the least number of hinges and the shortest Length
of the
interconnecting member will enhance the flexibility and the range of movement
of the ring
elements of the stmt during the constricted state. This f texibility of the
stcnt makes it easy to
adapt to the curved tortr~ous arterial pathway through which the stmt is being
passed.
It is a further object of the present invention to provide a stmt which does
not substantially
change in length or at least does not reduce in length as the stmt diameter
expands during
3S balloon inflation.
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The stmt may be deformed by the inflation of a balloon forming part of the
catheter delivery
system. The balloon expands the diameter by applying a radially-outwardly
directed force.
During the stmt expansion, the joints or points of inflection deform so as to
angularly separate
the various members that constitute the stmt. this will prevent any shortening
in the
longitudinal length after radial expansion of the stent.
It is further object of the present invention to supply the constricted stmt
with a minimum
diameter to ease its passage for placement through a minimal diameter vascular
point as well
as to enable it to enter through narrow lumen of constricted body tubes. It is
further object of
the present invention to provide a stmt geometry which allows both a greater
expansion ratio
for the stmt and smaller stmt diameter in order to fit into different diameter
sizes of the body
vessel. This present invention includes different arrangements of serpentine
elements with
their linear portions constructed to be aligned circumferentially or parallel
to the longitudinal
axis of the stmt in the constricted state. The serpentine members are
connected together by
a variety of different connecting members to form rings which circumscribe the
tubular
contour of the stmt. The stmt is constructed to take advantage of opposing
movements during
expansion to minimize or eliminate longitudinal extension of the stmt and to
provide
significant expansion in the circumference of the stent. The increase in the
circumference
results in an increase in the diameter of the stmt after radial expansion.
Depending on the
amount of radial expansion needed, a combination of different numbers of
serpentine elements
and their connecting members that form the rings of the stmt, as well as the
manner in which
the rings are interconnected, can be selected so as to provide a surgical stmt
which can be
radially expanded to over four times its constricted diameter.
The stmt is designed such that during expansion, local plastic deformation
occurs only at the
points of inflection distributed throughout the rings. Expansion is limited in
proportion to the
amount of expansion to ensure the resulting expanded stent is within material
fracture limits
and that adequate wall supporting structure is provided thereby. The stmt is
further designed
such that it becomes more rigid when expanded and, thereby, provides greater
resistance to
radial collapse after deployment.
These and other objects and advantages of the present invention are described
in the following
description and illustrated by way of drawings.
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Brief Description of the Drawings
Fig. 1 A is a perspective view of a stmt with the rear portion removed for
clarity, constructed
and operable according to the teachings of the present invention, shown in its
constricted but
highly flexible state prior to expansion; Fig. 1 B is a perspective view of
the embodiment of
S the stmt of FIG. lA in an expanded configuration;
Fig. 2A is a schematic drawing showing a flattened portion of the cylindrical
contour of the
stmt embodiment of Fig. 1 A; Fig. 2B is a schematic drawing of the flattened
portion of the
cylindrical contour of the stem as expanded;
Figs. 3A and 3B are cross-sectional views showing the stmt of FIG. lA in situ
before and after
expansion by a balloon forming part of its catheter delivery system;
Fig. 4A is a perspective view of an alternate embodiment of the stmt with the
rear portion
removed for clarity, shown in its constricted but flexible state prior to
expansion; Fig. 4B is
a perspective view of the embodiment of the stmt of Fig. 4A in an expanded
configuration;
Fig. SA is a schematic drawing showing a flattened portion of the cylindrical
contour of the
stmt embodiment of Fig. 2A; Fig. SB is a schematic drawing of the flattened
portion of the
cylindrical contour of the stmt as expanded;
Figs. 6A, 6B and 6C are schematic drawings showing a portion of an individual
ring of the
embodiment of Fig. 2A and variants thereof;
Figs. 7A, 7B and 7C are schematic drawings showing a portion of an individual
ring of the
embodiment of Fig. SA and variants thereof;
Figs. 8A, 8B, 8C and 8D are schematic drawings showing a portion of an
individual ring and
variants thereof based on the embodiment of Fig. 2A;
Figs. 9A, 9B, and 9C are schematic drawings showing a portion of an individual
ring and
variants thereof based on the embodiment of Fig. SA;
Fig. 10 is a schematic drawing showing a flattened portion of the cylindrical
contour of the
stmt constructed with rings as shown in Fig. 8A;
Fig. 11 is a schematic drawing showing a flattened portion of the cylindrical
contour of the
stmt constructed with rings as shown in Fig. 9A;
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Figs.12A-12S are schematic drawings of exemplary configurations of extension
elements and
expansion segments which are usable in conjunction with this invention;
Figs. 13A and 13B are schematic drawings showing a portion of further variants
of an
individual ring based on the embodiment of Fig. SA;
Figs. 13C and 13D are schematic drawings showing a portion of further variants
of an
individual ring based on the embodiment of Fig. 2A;
Fig. 14 is a schematic drawing showing another basic variant of an individual
ring;
Figs. 1 SA and 15B are schematic drawings showing portions of individual rings
based on the
embodiments of Figs. SA and 2A, respectively, but having alternative
connection members
between the serpentine elements;
Fig. 16 is a schematic drawing showing a portion of yet another individual
ring based on the
embodiment of Fig. SA, but having a different medial member;
Fig. 17 is a schematic drawing showing a portion of yet another individual
ring based on the
embodiment of Fig. SA, but having a different medial member;
1 S Figs. 18A-18D are schematic drawings illustrating alternate embodiments of
the
interconnecting member disposed between pairs of adjacent rings;
Fig. 19 is a schematic drawing showing a flattened portion of the cylindrical
contour of a stmt
constructed with rings as shown in Fig. 9A, but having an alternate manner for
interconnecting
the rings;
Fig. 20 is a schematic drawing showing a flattened portion of the cylindrical
contour of a
further stent constructed with rings as shown in Fig. 7A, but having an
alternate manner for
interconnecting the rings;
Fig. 21 is a schematic drawing showing a flattened portion of the cylindrical
contour of another
stmt constructed with rings as shown in Fig. 7A, but having another alternate
manner for
interconnecting the rings;
Fig. 22 is a schematic drawing showing a flattened portion of the cylindrical
contour of another
embodiment of a stmt made in accordance with the invention;
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Fig. 23 is a schematic drawing showing a flattened portion of the cylindrical
contour of a
further embodiment of a stmt made in accordance with the invention;
Fig. 24 is a schematic drawing showing a flattened portion of the cylindrical
contour of yet
another embodiment of a stmt made in accordance with the invention;
Fig. 25 is a schematic drawing showing a flattened portion of the cylindrical
contour of the
stent of Fig. 24 as expanded;
Fig. 26 is a schematic drawing showing a flattened portion of the cylindrical
contour of yet a
further embodiment of a stmt made in accordance with the invention; and
Fig. 27 is a schematic drawing showing a flattened portion of the cylindrical
contour of another
embodiment of a stmt made in accordance with the invention.
Detailed Description of the Invention:
Fig. lA illustrates a simple form of the invention in an expandable
endovascular medical
tubular stmt 10 shown in its constricted state, i.e. prior to deployment and
expansion. In
general, the stem 10 comprises a plurality of interconnected radially
expandable rings 12
arranged coaxially so as to form a generally tubular structure having a
longitudinal axis 14.
While three such ring elements 12a,12b,12c are shown in Fig. 1 A, the stmt of
the present
invention is operable with two or more such rings 12, the number of which is
generally
dependent on specific structure of the rings and how they are interconnected
(as will be
explained hereinbelow) as well as the desired length of the stent. The
circular lines 13a,13b
shown in phantom, represent in general the longitudinal boundaries of each
ring 12 and are
included to illustrate the cylindrical contour of the rings 12, particularly
in the rear which has
not been shown for purposes of clarity.
In general, the stmt is manufactured as an integral structure but for the
purposes of description,
the stmt is reduced effectively to a number of different elementary
components.
As can be seen more specifically in Fig. 2A, each ring 12 of the stmt 10 in
the constricted state
comprises a series of similar serpentine elements 16 connected together in a
circular contour.
Each serpentine element 16 comprises a pair of spaced-apart first and second
members 18,20
which extend generally in the circumferential direction. A medial member 22
connects the
first and second members 18, 20 such that they overlap in the circumferential
direction. As
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will be explained, the degree of circumferential overlap effects the extent of
radial
expandability of the stmt 10.
Member 18 is considered to have first and second ends 18a, 18b and member 20
has first and
second ends 20a,20b. Thus the medial member 22 generally connects the second
end 18b of
first member 18 with the first end 20a of the second member 20. The first end
18a of the first
member 18 is connected to the second end 20b of the second member 20 of a
circumferentially
adjacent serpentine element 16 while the second end 20b of the second member
20 is
connected to the first end of the other circumferentially adjacent serpentine
element 16.
Medial member 22 is connected to the first and second members by plastically
deformable
joints 23, which may have a rounded shape as shown in Figs. lA and 2A, form a
rounded
transition between the respective members.
In this embodiment, the serpentine element 16 is connected to
circumferentially adjacent
serpentine elements 16 by way of a connection member 24. Due to the specific
configuration
of the serpentine element 16 of stent 10, the connection member 24 is
generally linear and is
disposed substantially parallel to the longitudinal axis 14 when the stmt 10
is in its constricted
state as shown in Fig. 1 A. The connection member 24 connects to the first end
18a of the first
member 18 and to the second end 20b of the second member 20 of the
circumferentially
adjacent serpentine element 16 by plastically bendable joints 26, which form
rounded corners
as shown in Figs. 1 A and 2A.
Each pair of adjacent rings 12a,12b; 12b,12c of stmt 10 is interconnected by
at least one
interconnection member 28 disposed generally parallel with the longitudinal
axis 14 of the
stmt 10 in its constricted state. Interconnecting members 28 bridge the space
30 between
adjacent rings 12 and, in this embodiment, attach a connecting member 24 of
one serpentine
element 16 to the connecting member 24 of a corresponding serpentine element
16 of an
adjacent ring 12. The interconnecting member 28 between rings 12b and 12c
exists as shown
in Figs. 2A and 2B, but is not shown in Figs. 1 A and 1 B because it is
disposed in the rear
portion which is not shown as aforesaid.
In general, the flexibility of the stmt 10 with respect to its longitudinal
axis 14 is inversely
related to the number of the interconnecting members 28. Accordingly, it is
preferred that the
3 0 number of interconnections between adj acent rings be two. Depending on
the actual structure
of the individual components and material properties, the stmt may have a
single
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interconnection between each pair of adjacent rings. More than two
interconnections between
adjacent rings starts to detract from the flexibility of the stmt in its
constricted state.
Referring now to Figs. 1B, 2B, 3A and 3B, the tubular stent 10 has an initial
constricted
diameter d (see Fig. lA) which permits intra-luminal delivery of the stmt into
the lumen 32
of the body passageway or vessel 34 (see Fig. 3A) and is controllably deformed
by application
of an radially outwardly directed force from the interior of the stmt 10, for
example, by
expandable balloon 36 of catheter 38, to an expanded diameter d'. The term
"deformed" is
used to indicate that the material from which tubular stmt 10 is manufactured
and in
particular, the rings 12, is subjected to sufficient stress which is greater
than the elastic limit
of this material that portions thereof yield plastically. The radial expansion
force results in
circumferential expansion of the rings, effectively inducing a tensile stress
in each of the
serpentine elements 16. In general, the stmt is designed through materials and
structural
considerations known to those skilled in the art such that the plastic
deformation is caused to
take place primarily in the joints or points of inflection due to bending
stresses while the
tensile forces incurred in the linear portions tend to be lower that the
plastic yield strength of
the material. Accordingly, upon radial expansion, the first and second members
18,20 of each
serpentine element 16 will start to move in generally opposed circumferential
directions as
shown in Fig. 2B and plastic deformation starts to occur in the plastically
deformable and/or
bendable joints 23,26. The first and second members 18, 20, which are
constrained by medial
member 22, move angularly from a substantially circumferential direction as
the medial
member 22 moves angularly, thereby also causing the longitudinal distance
(relative to the
stmt axis 14) between the second end 18b of the first member 18 and the first
end 20a of the
second member 20 to increase. The connection member 24 is also caused to move
angularly
as the first member 18 of one serpentine .element 16 moves circumferentially
away from the
second member 20 of the circumferentially adjacent serpentine element 16. All
of this results
in expansion of the circumference of each ring 12a,12b,12c, and hence, an
increase in the
diameter d' of expanded stmt 10' as shown in Fig. 1 B. Referring to Fig. 2B,
while the
width w of each ring 12 increases to a width w' (between phantom lines
13a',13b' shown in
Fig. 1B) the overall length 1 of the stmt either increases slightly to length
1' or remains
substantially the same due to the presence of the interconnecting members 28,
which, by
moving angularly as the connecting members 24 move angularly, operate to
decrease the
length of the spaces 30 between the rings 12, thereby offsetting the increase
in the length of
the stmt due to the increase in width of the rings 12. The widening of the
rings 12 coupled
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with the decrease in the longitudinal spacing 30 between the rings 12 results
in the stmt 10
providing more comprehensive support to the inner wall 40 of the passageway 34
over
practically the entire length of the expanded stmt 10'.
In general, the idea is to permit substantial radial expansion but not to the
extent that the
5 plurality of serpentine elements 16 and connecting members 24 become overly
straightened,
i.e. relatively circular, because such a resulting structure would generally
not provide adequate
support along the entire inner surface of the passageway 34. In addition, the
more each ring
12 is expanded, the greater the risk of failure or breakage by exceeding the
plastic limit of the
material at one or more of the joints 23,26 or even exceeding the tensile
strength of the linear
10 members, leading to plastic deformation thereof and possibly ultimate
failure. Accordingly,
it is preferred that the rings 12 are caused to expand radially to a point
whereabout the width
of the ring 12 is substantially maximized or where the medial members 22 are
disposed
generally parallel to the longitudinal axis 14 of the stmt. This point can be
ascertained in
terms of the diameter d' either empirically or through analysis of the
geometry of the ring and
provided to the user by way of an accompanying specifications sheet.
Alternatively, the joints 23,26 may engineered to take advantage of work-
hardening properties
of certain materials so as to work-harden in proportion to the amount of
plastic deformation.
This property can be employed to limit the amount of plastic deformation and,
hence, the
extent to which the rings 12 will expand so as to ensure adequate wall support
all along the
section of the passageway in which the stmt is deployed. In general, the
inflection points
(joints) are designed to "open", i.e. to increase the angle between the joined
members/elements, to a limited extent. Those joints that have an initial
included angle a of
approximately 0°, i.e. double-back joints such as deformable joint 23
shown in Fig. 2A, are
not intended to expand angularly to 180 ° (straighten), but rather are
preferably limited to a
range of less than about 150°, and more preferably, to a range of about
90°or less. Those
joints having an initial included angle (3 of approximately 90°, such
as bendable joint 26 in
Fig. 2A, while angularly expandable to about 180 °, are preferably
limited to a range of about
90 ° to 150 ° . Such limitations will ensure a significant
normal component is maintained
between the elements that the joint separates for providing a more
comprehensive supporting
structure to the passageway wall when the stmt is expanded. In addition, by
attaining a
significant normal component between the elements that the joint separates,
the stmt 10' when
expanded will have less flexibility than while it was in its constricted state
10, thereby
rendering the stent 10' less prone to collapse after deployment. If advantage
is taken of the
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work hardening property to limit expansion, then the resulting expanded stmt
10' will be even
less flexible and less subject to plastic deformation and, thus, is more
capable of resisting the
radially inwardly directed force applied by the vessel wall 40 on the stmt 10'
after deployment.
Preferably, the distance between adjacent rings, and hence the length of the
interconnecting
members 28, is kept to a minimum to ensure a maximum amount of support along
the inner
surface of the passageway yet sufficiently spaced-apart to enable the rings to
expand
longitudinally without interference from the expansion of an adjacent ring.
The tubular stmt 10 is preferably fabricated from biocompatible, low memory,
more plastic
than elastic material to permit the stent 10 to be expanded and deformed from
the
configuration shown in Figs. lA and 2A to the configuration shown in Figs. 2A
and 2B, yet
sufficiently rigid to permit the tubular stmt 10 to retain its expanded and
deformed
configuration with enlarged diameter d' and also to resist radial collapse.
Typically, stems in accordance with the teachings herein may be expanded up to
about four
times their original constricted diameters yet still have desirable properties
of good axial
flexibility in the constricted state and resistance to radial collapse and
comprehensive wall
support in the expanded state. Accordingly, stents may be provided for example
in nominal
diameters d of about 1 mm,1.5mm, and 2mm which, depending on the specific
structure, may
be expanded to 4mm, 6mm or 8mm, respectively, which should enable a minimum
number
of stems to be employed in most situations. It should be borne in mind that
the stems of the
present invention are operable over their entire range because they deform
substantially
continuously under application of an radially outwardly directed force. Upon
removal of the
force, deformation halts and the stmt remains sufficiently rigid to withstand
the radial force
of the wall which it supports.
Suitable materials for the fabrication of the tubular stmt 10 would include
silver, tantalum,
stainless steel (316 L), gold, titanium, NiTi alloy or any suitable plastic
materials such as
thermoplastic polymers. Any medically-suitable metal having work hardening
properties
which is capable of yielding plastically under the typical forces of a balloon
catheter could also
be employed. Alternatively, the stmt 10 may be made of a radioactive material
or irradiated
with a radioactive isotope. The radioactive isotope may be a beta particle
emitting
radioisotope. By using a stmt made of the radioactive material, cancer cells
in and around the
stmt can be deactivated or killed. Alternatively, the stmt can be coated with
materials that
prevent cell overgrowth. The stmt may be coated with an anticoagulating
medication
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12
substance, such as heparin, and/or a bioabsorbable material. Accordingly, when
the stmt is
used in a blood vessel, blood clotting can be prevented. Also, the stmt may
have pores,
indentations or a roughened surface capable of absorbing or retaining a drug
therein/thereon
for slowly releasing the same over time. Thus, when the stent with a drug is
implanted in the
body lumen, the drug can slowly released in the body lumen. To enhance
visibility of the stmt
when viewed by various different medical imaging devices, the end rings
12a,12c can be
formed from a radio-opaque material, such as gold, silver or platinum, which
allows both ends
of the stmt to be clearly visible through a medical imaging device during or
after implantation
of the stmt within a body lumen of the patient.
The stmt 10 is preferably formed by laser cutting technology wherein the
pattern is cut into
a cylindrical section of the appropriate material. Other suitable methods may
be used, for
example, the stent can be formed by an etching technique. Namely, a pattern of
the rings
12a,12b,12c and the interconnecting members 28 are coated on a cylindrical
metal member,
which is etched in an acid solution. Then, un-coated portions are removed.
Figs. 4A and SA illustrate an alternative tubular stmt 50 which is similar to
the stmt 10, in that
it comprises a plurality of interconnected rings 52, each of which consists of
a plurality of
similar serpentine elements 16,16a joined together in a circular band. The
circular lines
53a,53b shown in phantom in Fig. 4A, represent in general the longitudinal
boundaries of each
ring 52 and are included to illustrate the cylindrical contour of the rings
52, particularly in the
rear which has not been shown for purposes of clarity.
Each of the serpentine elements 16,16a comprises a pair of spaced-apart first
and second
members 18,20 which extend generally in the circumferential direction. A
medial member 22
connects the first member 18 to the second member 20 through plastically
deformable joints
23 such that the first and second members 18,20 overlap in the circumferential
direction. In
this embodiment, however, each serpentine element 16 is the reverse with
respect to its
circumferentially adj acent serpentine elements 16a, necessitating an even
number of serpentine
elements 16,16a for constituting each ring 52. Accordingly, the first member
18 of one
serpentine element 16 is connected to the first member 18 of a
circumferentially adjacent
serpentine element 16a while the second member 20 is connected to the second
member of the
other circumferentially adjacent serpentine element 16a. For the purposes of
description, a
connection member 24 can be considered to be provided between adj acent first
members 18,18
and adjacent second members 20,20. At least one, but preferably a pair of
diametrically
CA 02358453 2001-07-16
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13
opposed, interconnecting members 28 are provided between each pair of adjacent
rings
52a,52b; 52b,52c.
Upon application of a radially outwardly directed force from inside the
constricted stmt 50,
each serpentine element 16,16a is essentially placed under tension, causing
the first and second
members 18,20 to move in circumferentially opposite directions, as seen in
Figs. 4B and SB.
Deformation takes place in the plastically deformable joints 23 as the medial
member 22
moves angularly, thereby, increasing the width of the rings 52 from width w to
width w'.
If the interconnecting members 28 connect the first members 18 of a pair of
circumferentially
adjacent serpentine elements 16,16a to the first members 18 of a pair of
circumferentially
adjacent serpentine elements 16,16a of an adjacent ring 52 or, as shown in
Fig. SA, if the
interconnecting elements 28 connect the second members 20 of a pair of
circumferentially
adjacent serpentine elements 16,16a to the second members 20 of a pair of
circumferentially
adjacent serpentine elements 16,16a of an adjacent ring 52, then the amount of
longitudinal
extension of the stmt 50' from length 1 to 1' will be limited to the amount of
longitudinal
extension from width w to w' of a single ring 52.
Again, the objective is not to expand the stmt 50 to its fullest extent
whereat the serpentine
elements are substantially straightened, but rather to such an extent whereby
the desired
diameter d' of the stmt can be achieved while permitting significant support
of the vessel
wall 40 all over its cylindrical inner surface by having a substantive
framework of the elements
which constitute the expanded stmt in the circumferential and longitudinal
directions or as
components of those directions. In general, the inflection points (joints) are
designed to
"open" to a limited extent. Those joints that have an initial included angle a
of approximately
0°, i.e. double-back joints such as deformable joint 23 shown in Fig.
2A, are not intended to
expand radially to 180 ° (straighten), but rather are preferably
limited to a range of less than
about 150 °, and more preferably, to a range of about 90 °or
less. Such limitations will ensure
a significant normal component is maintained between the elements that the
joint separates for
providing a more comprehensive supporting structure to the passageway wall
when the stmt
is expanded.
As previously mentioned, the stmt is designed with shape, material property
and force
considerations in mind such that the majority, if not the entirety, of the
plastic deformation
takes place in the plastically deformable joints 23 and in the plastically
bendable joints 26, if
provided. Preferably, the joints 23,26 may be engineered to take advantage of
work-hardening
CA 02358453 2001-07-16
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14
properties of certain materials so as to work-harden in proportion to the
amount of plastic
deformation. This property can be employed to limit the amount of plastic
deformation and,
hence, the extent to which the rings 12 will expand so as to ensure adequate
wall support all
along the section of the passageway in which the stmt is deployed.
Accordingly, the joints
23,26 may be shaped in a variety of ways to achieve these engineering
objectives and/or for
manufacturability. Exemplary configurations of joints are shown in Figs. 6A-6C
and 7A-7C
which illustrate flattened proj ections of individual ring segments in their
constricted state.
Figs. 6A-6C show various exemplary rings 12,112,212 of type shown in Fig. 2A
wherein each
serpentine element 16,116,216 is connected to an adjacent serpentine element
16,116,216 by
connecting members 24,124,224, respectively. More specifically, the ring 12 of
Fig. 6A is of
the type as illustrated in Fig. 2A having semi-circular or rounded plastically
deformable joints
23 connecting the medial member 22 with each of the first and second
circumferentially-
extending members 18,20 and rounded plastically bendable joints 26 by way of
which the
connecting member 24 is connected to the respective first and second members
18,20 of the
1 S adjacent serpentine elements 16.
Fig. 6B shows a ring 112 comprising a series of serpentine elements 116
including first and
second members 118,120 connected in overlapping circumferential fashion by way
of medial
member 122. In this arrangement, the joints 123,126 are angular with
plastically deformable
join 123 being generally straight so as to form a double right-angled
connection between the
medial member 122 and each first and second member 118,120. Similarly, the
plastically
bendable joints 126 form generally right-angled connections between the
connection member
126 and its respective first and second members 118,120.
Fig. 6C illustrates a ring 212 similar to the ring 12 of Fig. 6A except that
the plastically
deformable joints 223 of the serpentine element 216 disposed between the
medial member 222
and each of the first and second members 218,220 form a bulbous connection
which is greater
than 180 °. This form of connection induces more consistent plastic
deformation throughout
the entirety of the joint.
Figs. 7A-7C show various exemplary rings 52,152,252 of type shown in Fig. SA
wherein each
serpentine element 16,116,216 is reversed with respect to its adjacent
serpentine element
3 0 16a,116a,216a. In Fig. 7A, the plastically deformable joints 23 connecting
the medial member
22 with the first and second members 18,20 are rounded or semi-circular. In
Fig. 7B, the
plastically deformable joints 123 connecting the medial member 122 with the
first and second
CA 02358453 2001-07-16
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members 118,120 are generally straight so as to form a double right-angled
connection. In Fig.
7C, the plastically deformable joints 223 connecting the medial member 222
with the first and
second members 218,220 form a bulbous connection similar to that shown in Fig.
6C.
In order to increase the stmt's capability to expand radially, it would be
desirable to provide
5 the first and second members with the ability to extend generally lengthwise
when placed
under tension. This may be accomplished by means of an extension elements
such, as for
example, as are illustrated in Figs. 8A-8D and 9A-9C. Fig. 8A shows a ring 60,
similar to the
ring 12 of Fig. 6A, wherein rounded U-shaped extension elements 61 are
provided
intermediate each first and second member 62,63. Extension element 61 is
designed to deform
10 plastically at its joint 64a and at the member interfaces 64b. Fig. 8B
shows a ring 65, similar
to the ring 112 of Fig. 6B, wherein rectangularly U-shaped extension elements
66 are provided
intermediate each first and second member 67,68. Extension element 66 is
designed to deform
plastically at its double right-angled joint 69a and at the member interfaces
69b. Fig. 8C
shows a ring 70, similar to the ring 212 of Fig. 6C, wherein bulbous U-shaped
extension
15 elements 71 are provided intermediate each first and second member 72,73.
Extension
element 71 is designed to deform plastically at its bulbous joint 74a and at
the member
interfaces 74b. Fig. 8D shows a ring 75, similar to the ring 112 of Fig. 6B,
wherein V-shaped
extension elements 76 are provided intermediate each first and second member
77,78.
Extension element 76 is designed to deform plastically at its angled joint 79a
and at the
member interfaces 79b. The addition of an extension element not only enhances
the stmt's
capability to expand radially, but also serves to increase the amount of
material in the stmt
which, when expanded, will provide more comprehensive support to the wall 40
of the body
passageway 34 and will serve to better resist radial collapse after expansion.
Alternately, the capability to radially expand without detracting overly from
the capability to
support the wall 40 relatively comprehensively may also be provided through
the use of an
expansion segment within the connecting member between circumferentially
adjacent
serpentine elements. As shown in Fig. 8D, an expansion segment 80 is provided
intermediate
the connecting member 82 connecting the first member 77 of one serpentine
element with the
second member 78 of its circumferentially adjacent serpentine element. As
shown, expansion
segment 80 is V-shaped, but could be any of a variety of shapes (as will be
detailed
hereinbelow) which would enable the connecting element to expand lengthwise
when placed
under tension. In this regard, expansion element 80 is designed to deform
plastically at its
angled joint 84a and at the connecting member interfaces 84b. Like the
extension elements
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16
which may be provided in the first and second elements, the addition of an
expansion segment
not only enhances the stmt's capability to expand radially, but also serves to
increase the
amount of material in the stent which, when expanded, will provide more
comprehensive
support to the wall 40 of the body passageway 34 and will serve to better
resist radial collapse
after expansion.
As described in connection with Figs. SA and SB, a connection member 24 can be
considered
to be provided between adjacent first members 18,18 and adjacent second
members 20,20 of
the ring arrangement where each serpentine element 16 is the reverse of its
circumferentially
adjacent serpentine elements 16a. In general, when the connection element 24
is entirely in-
line between the first members or second members, there is no plastic
deformation as the
segments 18,24,18 or 20,24,20 are designed to withstand the tensile stress
induced therein
during expansion of the stent without plastic deformation. However, as was the
case with the
Fig. 8D embodiment, it is possible to increase the stmt's capability to expand
by including an
expansion segment in the connecting member as is exemplified in Figs. 9A-9C.
Fig. 9A
1 S illustrates a ring 90, similar to the ring shown in Fig. 7A, including a
rounded, U-shaped
expansion segment 91 disposed intermediate each connecting member 24 and hence
between
adjacent first members 18,18 of circumferentially adjacent serpentine elements
16,16a and
between adjacent second members 20,20 of circumferentially adjacent serpentine
elements
16a,16. As is the case with all of the remainder of the stmt structure, the
expansion segment
is designed to deform or bend plastically at its points of inflection, i.e. at
its joints. Fig. 9B
shows an alternate ring 93 arrangement, similar to the ring 152 of Fig. 7B,
wherein
rectangularly U-shaped expansion segments are disposed intermediate the
connecting member
124. Fig. 9C shows yet another ring 96 arrangement, similar to the ring 252 of
Fig. 7C,
wherein bulbous U-shaped expansion segments 97 are provided intermediate the
connecting
member 224. In this case, the expansion segment 97 includes over-rounded
corners 98.
As with the rings 12 of Fig. 2A and the rings 52 of Fig. SA, a plurality of
rings 60,65,70,75 of
Figs. 8A-8D or the rings 90,93,96 of Figs. 9A-9C are interconnected to form a
generally
tubular stmt. By way of example, a plurality of rings 60 of Fig. 8A are
arranged coaxially to
form a stmt shown in its constricted state 100 in Fig. 10. One or more
interconnecting
members 28 serve to interconnect each pair of adjacent rings 60a,60b; 60b,60c
and, more
specifically, to attach the connecting member 24 between a pair of
circumferentially adjacent
serpentine elements 16 to a connecting member 24 between a longitudinally
adjacent pair of
circumferentially adjacent serpentine members 16. Similarly, Fig. 11 shows a
stmt 102 in its
CA 02358453 2001-07-16
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17
constricted state comprised of a plurality of rings 90a,90b,90c of the type of
ring 90 shown in
Fig. 9A. Each pair of adjacent rings 90a,90b; 90b,90c are interconnected by at
least one
interconnecting member 28 disposed intermediate expansion segment 91 of one
connecting
member 24 and the expansion segment 91 of the connecting member 24 between an
longitudinally adjacent pair of circumferentially adjacent serpentine elements
16,16a.
As mentioned above, the extension elements and expansion segments can take the
form of a
variety of configurations. Figs. 12A-12S illustrate exemplary configurations
99a-99s,
respectively, which may be utilized to achieve the desired lengthwise
extension or expansion
of the element in which they are implemented. The shapes of the elements or
segments of
Figs. 12A to 12D are, respectively: rectangular U-shaped; V-shaped; rounded U-
shaped; and
bulbous U-shaped, each of which extends transversely relative to the end-to-
end relationship
element or segment. In Fig. 12E, two transversely-extending but alternating,
rectangular U-
shapes are shown. Fig. 12F shows two transversely-extending but alternating V-
shapes. Fig.
12G also shows two transversely-extending but alternating V-shapes which could
equally be
characterized as a longitudinally-extending Z-shape. Fig. 12H shows two
transversely-
alternating rounded U-shapes which could equally be characterized as a
longitudinally-
extending S-shape while Fig. 12I shows two transversely-alternating bulbous U-
shapes. Fig.
12J shows a plurality of transversely-extending, rectangular U-shapes, Fig.
12K shows a
plurality of transversely-extending V-shapes, and Fig. 12L shows a plurality
of transversely-
extending rounded U-shapes which could equally be characterized as a
longitudinally-
extending double S-shape. In this regard, any number of singular elements may
be combined
to form a flexuous element. Fig. 12M shows a plurality of transversely-
extending alternating
bulbous U-shapes. The expansion element 99n of Fig. 12N is S-shaped, but
unlike the S-
shaped expansion elements 99h or 991 whose respective ends 99h',991' are in
line, the ends
99n' of expansion element 99n are offset with respect to one another.
Similarly in Fig. 120,
the ends 990' of transversely alternating, rectangular U-shaped expansion
element 99o are
offset as compared with the ends 99j' of element 99j shown in Fig. 12J. Fig.
12P shows a
plurality of rectangular U-shapes which alternate transversely. Fig. 12Q shows
a plurality of
V-shapes which alternate transversely. Fig.12R shows a plurality of rounded U-
shapes which
alternate transversely and Fig. 12S shows a plurality of bulbous U-shapes
which alternate
transversely. The elements/segments 99a-99s shown in Figs. 12A-125, many of
which have
sinusoidal derivations, are intended to exemplify the variety of shapes and
configurations and
by no means are meant to be limiting. In common with all of the
elements/segments 99a-99s
CA 02358453 2001-07-16
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18
is that plastic deformation due to bending occurs at their points of
inflection so that included
angles open under tension applied between the ends. Included angles which are
approximately
0 ° are limited to expand to less than 180 °, preferably less
than 150 ° and even more preferably
to about 90 ° or less. Included angles which are less than 90 °,
i.e. the acute-angled V-shapes,
are also limited to expand to less than 180 °, preferably less than 150
° and even more
preferably to about 90 ° or less.
The elements 99a-99s shown in Figs. 12A-12S are not limited for use in
association with the
first and second members 18,20 and connecting members 24, but may also be used
in other
elements where expansion or extension is desired, such as in medial member 22
and/or in
interconnecting member 28.
To illustrate further the manner in which the extension elements and expansion
segments 99a-
99r could be implemented in a stent according to the invention, reference is
made to Figs.
13A-13D. Fig. 13A shows a ring 130, similar to ring 90 of Fig. 9A, in which an
expansion
segment 132 is included as part of the connecting member 24 disposed between
adjacent first
members 18,18 of circumferentially adjacent serpentine elements 16a,16 and as
part of the
connecting member 24 disposed between adjacent second members 20,20 of
circumferentially
adjacent serpentine members 16,16a. In this case, the expansion segment is of
the form of
segment 991 shown in Fig.12L. Fig.13B shows a ring 140 similar to the ring 130
of Fig. 13A,
with the exception that expansion segment 142 is of the form of segment 99r as
shown in Fig.
12R. As can be seen in both Figs,13A and 13B, the medial member 22 connects
first member
18 to second member 20 in at least partially circumferentially overlapping
fashion.
Fig. 13C shows a ring 155, similar to ring 60 of Fig. 8A, in which an
extension element 157
is disposed generally centrally in each first and second member 18,20.
Extension element 157
is of the form of segment 991 as shown in Fig. 12L. Fig, 13D shows a ring 160,
similar to the
ring 150 of Fig. 13C, with the exception that the extension element 162
disposed generally
centrally in each first and second member 18,20 is provided with an additional
undulation (as
compared with the extension element 157) which not only provides enhanced
radial
expandability of the ring 160, but also adds to the amount of material
available for wall
support.
It will be appreciated that with the variety of components shown and described
that any
combination thereof could be used to construct a stmt in accordance with the
invention. For
example, it is not necessary that the series of serpentine elements be the
same or alternating
CA 02358453 2001-07-16
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19
as shown in Figs. 6A or 7A. The ring could be comprised of a plurality of
serpentine elements
arranged generally randomly or in another sequence, such as in ring 170 shown
in Fig. 14.
Ring 170 comprises a pair of similarly oriented serpentine elements 171
between which is
interposed a reversely oriented serpentine element 172. It will also be
appreciated that the
extension element used for example in the first member could be different from
that used in
the second member.
Figs. l SA and 1 SB show alternate arrangements of the connecting members
disposed between
adjacent serpentine elements which generally expand as opposed to extend. In
Fig. 15A,
alternating connecting members 177,178 having U-shaped expansion segments are
disposed
between circumferentially alternating serpentine members 176,177 of ring 175.
In Fig. 1 SB,
connecting members 182 having S-shaped expansion segments are disposed between
circumferentially similar serpentine members 181 of ring 180.
It will also be appreciated that the expansion/extension principles described
and illustrated
herein can be applied to both the medial members and the interconnecting
members and that
for this purpose, the shapes shown in Figs. 12A-12S as well as others, could
be utilized. For
example, Fig. 16 illustrates a ring 185 which is similar in form to ring shown
in Fig. 7A,
having circumferentially alternating serpentine elements 186,186a comprised of
circumferentially extending first and second members 187,188. Medial member
189 is
connected between the first and second members 187,188 such that they at least
partially
circumferentially overlap. In this case, the medial member 189 consists of an
N-shaped
(alternating U-shaped) arrangement in which the linear portions thereof extend
generally in
the circumferential direction. In Fig. 17, ring 190 is comprised of a series
of alternately-
oriented serpentine elements 191,191 a, each consisting of circumferentially
extending first
and second members 192,193. Medial member 194 is connected between the first
and second
members 192,193 such that they at least partially circumferentially overlap.
In this case, the
medial member 194 consists of an S-shaped (or alternating U-shaped)
arrangement, in which
the linear portions are disposed generally parallel to the longitudinal axis
14 of the stmt in its
constricted state.
It will also be appreciated the manner in which the rings are interconnected
can also be
effected in a number of ways. In general, the interconnection can take the
form of an
interconnecting member as described above, with or without the capability for
extension/expansion, or it can be as simple as integrally forming one or more
portions of one
CA 02358453 2001-07-16
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ring with that of an adjacent ring. Figs. 18A-18D show schematically the use
of extendable
sections in the interconnecting member disposed between adjacent pairs of
rings 195. In Fig.
18A, a U-shaped extendable section 196, similar to the element 99c shown in
Fig. 12C, is
disposed transversely of the interconnecting member. A second U-shaped
extendable section
5 196a is disposed in the opposite interconnecting member in such a manner
that if the stmt as
shown in Fig. 18A were rotated 180° about its longitudinal axis, the U-
shaped extendable
section 196a would appear in the same orientation as the U-shaped extendable
section 196 as
shown in Fig. 18A. In Fig. 18B, alternating V-shaped extendable section 197,
similar to the
element 99f shown in Fig. 12F, is included in one interconnecting member. An
alternating V-
10 shaped extendable section 197a is disposed in the opposite interconnecting
member in such
a manner that if the stmt as shown in Fig. 18B were rotated 180 ° about
its longitudinal axis,
the alternating V-shaped extendable section 197a would appear in an upside-
down orientation
as compared with the alternating V-shaped extendable section 197 as shown in
Fig.18A. Both
of the interconnecting members 196,197 illustrated in Figs. 18A and 18B are of
such a
15 configuration that their ends are disposed along a line which is generally
coaxial with respect
to the longitudinal axis of the stmt. However, depending on the actual shape
of the extendable
section employed in the interconnecting member, the ends thereof could be
circumferentially
offset, such as is shown in Fig. 18C. In this variant, a longitudinally
extending S-shaped
extendable section 198, similar to element 99n of Fig. 12N, is disposed
between the ends of
20 each interconnecting member. Due to the configuration of S-shaped member
198, the ends
of the interconnecting member are not disposed in a line which is generally
coaxial with
respect to the longitudinal axis of the stem, but rather, they are
circumferentially offset. In Fig.
18A, it can be seen that the interconnecting member 196 interconnects proximal
portions
195b,195c of adjacent rings 195. It will be appreciated that the
interconnecting member may
be used to interconnect proximal (195b,195c) or distal (195a,195d) portions of
adjacent rings
195 or, as shown in Fig. 18D, to connect a proximal portion 195a of one ring
at a first end
199a with a distal portion 195d of the other ring 195 at a second end 199b.
This manner of
interconnection is also illustrated in Fig. 11. Transversely extending,
rectangularly U-shaped
extendable section 199, similar to element 99a of Fig. 12A, is disposed in the
interconnecting
member of Fig. 18D.
As indicated above, the interconnecting member can be as simple as integrally
forming one
or more portions of one ring with that of an adjacent ring. For example, Fig.
19 shows a
section of stent 200 comprised of a plurality of alternating rings 201,202 of
the type shown in
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21
Fig. 9A. At selective locations 203, the U-shaped connecting member 204a of
one ring 201
is integrally formed, fused or otherwise attached to the connecting member
204b of an adj acent
ring 202. Fig. 20 illustrates a portion of a stmt 205 comprised of alternating
rings 206,207 of
the type shown in Fig. 7A. In this arrangement, relatively short
interconnecting members 208
interconnect adjacent rings 206,207 at selected locations. Preferably, there
are not more than
two interconnecting members provided per pair of adjacent rings 206,207. The
short length
of the interconnecting members 208 enhance the flexibility of the stmt while
in constricted
state to facilitate easy maneuver through tortuous arterial pathway.
Fig. 21 illustrates a section of a stmt 230 comprised of alternating rings
232,234 of the type
shown in Fig. 7A. In this case, the interconnection is effected by lengthwise
integrally forming
selected adjacent circumferentially extending first/second members 236. In
Fig. 22, a section
of a stmt 240 is shown comprising a plurality of alternating adjacent rings
241,242, each
consisting of a plurality of serpentine elements 243 each having
circumferentially extending
first and second members 244,245 connected by an undulating or flexuous medial
member
246. While the undulating characteristic of the medial member 246 can enhance
expansion in
the tensile direction, even where the stmt is not expanded to such an extent
so as to take
advantage of its expansion capability, the undulating characteristic provides
substantial surface
area coverage which results in more comprehensive wall support as compared
with a linear
counterpart.
An expandable, U-shaped connection member 247 connects some of the serpentine
elements
243 circumferentially while a linear connecting member 248 connects others. At
selected
locations 249, the connecting member 247a of one ring 271 is integrally
formed, fused or
otherwise attached, to a connecting member 247b of adjacent ring 272. 'This
arrangement has
been found to provide excellent axial flexibility of the stmt. Because of the
substantive usage
of expansion/extension elements and the compactness thereof, this stmt has a
relatively great
range of radial expansion and substantial wall support.
Fig. 23 shows a section of a stent 255 comprising a plurality of alternating
rings 256,257, each
consisting of a series of similar serpentine elements 258 connected
circumferentially by linear
connecting member 259. Each serpentine element 258 comprises circumferentially
extending
first and second members 260 connected by way of undulating medial member 262.
Adjacent
rings 256,257 are interconnected at selected locations by interconnecting
members 263. As
can be seen, the section of the stmt 255 has a relatively high, initial
surface density (ratio of
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22
material area to area of gaps/spaces) which will result in reasonably good
surface density after
expansion since the extent to which the rings will expand will be limited as
mentioned above
in conjunction with the description of Figs. 2B and SB.
A variation of the stent 240 of Fig. 22 is illustrated at 270 in Fig. 24,
comprising a plurality
of alternating rings 271,272. Each ring 271,272 consists of a plurality of
alternating serpentine
elements 278,278a connected by alternatingly-oriented U-shaped connecting
members 276.
The rings 271,272 are positioned such that adj acent U-shaped connecting
members either have
adjacent apexes or adjacent openings (i.e. opposed apexes). Relatively short
interconnecting
members 279 are provided at selected locations between the apex of connecting
member 276a
of one ring 271 and the adjacent apex of a connecting member 276b of an
adjacent ring 272.
In accordance with the teachings of the invention, interconnecting member 279
could comprise
a linear element as shown or could comprise an expansion element of the type
shown in Figs.
12A-125. The configuration of stmt 270, which represents the best mode, has
been found to
provide an exceptional working range respecting expansion (on the order of
four times the
original diameter), without significantly compromising the strength of the
stmt when
expanded or detracting overly from the extent to which the stmt can provide
coverage and
support throughout the entire inner surface of the passageway wall. Just as
important,
however, is the fact that the configuration of stmt 270 results in a high
axial flexibility when
in its constricted state, which does not only prove advantageous when
navigating the tortuous
paths of some arterial pathways, but also permits the stmt to be provided in
relatively smaller
initial diameters thereby permitting easier access and reducing the risk of
any damage. A
further advantage of this particular configuration is that radial expansion
results in
substantially little if any longitudinal extension or contraction which
facilitates positioning of
the stent in the passageway. Fig. 25 shows schematically the initial expansion
of the stmt 270
of Fig. 24. As can be seen, a relatively small opening up (approximately 30
°) of the angle a'
from a, which was substantially 0 ° as shown in Fig. 24, results in an
almost two-fold increase
in the circumference of the stent 270'. It will be appreciated that while only
connecting
members 276 open up and serpentine elements 278,278a move angularly upon
initial
expansion, as the generally elongate serpentine elements 278,278a become
aligned more
circumferentially (i.e. the first and second members 280,281 move angularly
towards a parallel
orientation with respect to the axis of the stmt), the tensile stress on the
serpentine elements
278,278a will induce plastic deformation at the rounded joints 283 and, thus,
expansion of the
serpentine elements 278,278a in a generally circumferential direction and,
hence, even greater
CA 02358453 2001-07-16
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23
expandability. The stmt of Fig. 25 can easily expand to four times its initial
diameter while
still maintaining comprehensive wall support and its resistance to radial
collapse.
Fig. 26 shows a stent 310 which is similar to the stmt 270 of Fig. 24,
comprising a plurality
of alternating rings 311,312. Each ring 311,312 consists of a plurality of
alternating serpentine
S elements 318 connected by alternatingly-oriented U-shaped connecting members
316. Whereas
the stmt 270 of Fig. 24 utilizes a relatively short interconnecting member 279
between
selected portions of adjacent rings, the stmt 310 of Fig. 26 includes in its
interconnecting
member 319 an extendable section 399. The extendable section 399 provides
additional
flexibility to the stmt along its longitudinal axis when in its constricted
shape. While
extendable section 399 is shown as being U-shaped, it will be appreciated that
any shape, such
as those illustrated in Figs. 12A-12S could be employed.
Fig. 27 shows yet another embodiment of a stmt 320 comprising a plurality of
alternating rings
321,322 of a form similar to the rings 271,272 of the stmt 270 of Fig. 24,
except that the
medial member 323 of stmt 320 includes one less undulation (i.e. the medial
member 323 is
single S-shaped) and that the space between the legs of the U-shaped
connecting member 326
is greater. The rings 321,322 are positioned such that adjacent U-shaped
connecting members
either have opposed or adjacent apexes. Whereas in the stmt 270 of Fig. 24 the
interconnecting member 279 bridges adjacent apexes, the at least one
interconnecting member
329 between each pair of adjacent rings 321,322 of stent 320 connects opposed
apexes. Such
an arrangement ensures no longitudinal contraction of the stmt will occur upon
radial
expansion.
For exemplary purposes, the stems illustrated herein have been shown as
comprising three or
four interconnected rings. However, due to the minute sizes of the elements
involved, typical
stems would usually comprise several more rings. To give an idea of the
dimensions involved,
a typical stmt for use in coronary arteries such as is shown in Fig 24, might
have an
unconstricted diameter on the order of 1.2mm. The width of each ring would be
on the order
of about lmm. Accordingly, there would be 15 or so rings on a stmt which is
initially 15mm
in length. The circumferentially adjoining combination of a connecting member
276, a
serpentine element 278a, another connecting member 276 and a serpentine
element 278 repeats
about every 1.25mm circumferentially. Accordingly, for a l.2mm diameter stmt,
three
serpentine elements 278, three serpentine elements 378a and six connecting
members 276
would form each ring 271,272. The gaps between parallel portions of linear
members, such
CA 02358453 2001-07-16
WO 00/42945 PCT/CA99/00632
24
as the struts forming the legs of U-shaped connecting members 276 are on the
order of about
O.OSmm. The width of the material which forms the various elements, members,
joints, etc.,
while variable to achieve the plastic deformation at the points of inflection,
are generally also
on the order of about O.OSmm. The thickness of the material, i.e the stent's
tubular wall
thickness is on the order of about 0.05-0.2mm. The cross-sectional
configuration of the
material can be varied, although it will likely depend upon the manner in
which the stmt is
manufactured. For, example, using laser cutting on a piece of tubular
material, the resulting
members which are disposed in the circumferential direction will have roughly
rectangular
cross-sections while the members generally parallel to the longitudinal axis
will likely have
a slightly trapezoidal cross-section if the axis of the laser intersects the
axis of the tubular
material. A more rectangular cross-section would be obtainable with an
appropriate offset of
the laser's axis.
Having described this invention with regard to specific embodiments, it is to
be understood
that the invention has been described with respect to a limited number of
embodiments. It will
be appreciated that many variations, modifications and other applications of
the invention may
be made. Accordingly, the invention is therefore to be limited only by the
scope of the
appended claims.
