U.S. patent application number 10/405902 was filed with the patent office on 2005-04-07 for flexible stent structure.
This patent application is currently assigned to AVANTEC VASCULAR CORPORATION. Invention is credited to Yan, John.
Application Number | 20050075716 10/405902 |
Document ID | / |
Family ID | 27623313 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050075716 |
Kind Code |
A1 |
Yan, John |
April 7, 2005 |
Flexible stent structure
Abstract
Luminal prostheses comprise adjacent expansible segments,
typically serpentine ring segments joined by sigmoidal links. By
properly orienting the sigmoidal links and aligning hinge regions
on adjacent serpentine rings, enhanced opening characteristics can
be obtained. Additionally, by varying the mechanical
characteristics of adjacent serpentine rings, program expansion of
the luminal prostheses over their lengths may be obtained. The
disclosed prostheses also have controllable opening characteristics
and can be crimped to small diameters.
Inventors: |
Yan, John; (Los Gatos,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
AVANTEC VASCULAR
CORPORATION
Sunnyvale
CA
|
Family ID: |
27623313 |
Appl. No.: |
10/405902 |
Filed: |
April 1, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10405902 |
Apr 1, 2003 |
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09565560 |
May 4, 2000 |
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6602282 |
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Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F 2/91 20130101; A61F
2002/91533 20130101; A61F 2/915 20130101; A61F 2230/0054 20130101;
A61F 2002/91558 20130101 |
Class at
Publication: |
623/001.15 |
International
Class: |
A61F 002/06 |
Claims
1. A radially expansible luminal prosthesis including a scaffold
comprising: a plurality of serpentine ring segments including
struts connected by circumferentially offset hinge regions, each
hinge region having an apex and side regions disposed on either
side of the of the apex; and links between at least some of the
hinge regions on adjacent serpentine rings, wherein the links are
attached on the sides of the hinge regions, and all links have
substantially similar sigmoidal shapes.
2. A radially expansible luminal prosthesis as in claim 1, wherein
each sigmoidal link comprises two connecting legs and wherein each
connecting leg is attached to a hinge region in a circumferential
direction.
3. A radially expansible luminal prosthesis as in claim 2, wherein
each sigmoidal link has a uniform width over its length.
4. A radially expansible luminal prosthesis as in claim 2, wherein
each sigmoidal link has a flared end with joins to the hinge
region.
5. A radially expansible luminal prosthesis as in claim 2, wherein
the sigmoidal link is adapted to permit the connecting legs to move
circumferentially past each other to accommodate radial crimping of
the prosthesis.
6. A radially expansible luminal prosthesis as in claim 1, wherein
the ring segments are expansible in response to a radially outward
force; wherein at least one of the ring segments opens at a
different rate or in a different amount than at least one other
ring segment when exposed to the same radially outward force.
7. A radially expansible luminal prosthesis as in claim 6, wherein
at least some struts in at least some of the serpentine rings have
different lengths to cause a different rate or amount of
expansion.
8. A radially expansible luminal prosthesis as in claim 7, wherein
at least some struts which are positioned away from the sigmoidal
links are longer.
9. A radially expansible luminal prosthesis as in claim 6, wherein
at least some of the struts have different widths than others of
the struts to cause a different rate or amount of expansion.
10. A radially expansible luminal prosthesis as in claim 6, wherein
at least some of the hinge regions have different widths than
others of the hinges regions to cause a different rate or amount of
expansion.
11. A radially expansible luminal prosthesis as in claim 1, wherein
at least some of the struts are straight over the distance between
the hinges.
12. A radially expansible luminal prosthesis as in claim 1, wherein
at least some of the struts are non-linear.
13. A radially expansible luminal prosthesis as in claim 1, wherein
the sigmoidal links are individually axially expansible and
contractable.
14. A radially expansible luminal prosthesis including a scaffold
comprising: a plurality of serpentine ring segments including
struts connected by hinge regions being circumferentially offset
with apices and side regions disposed on either side of the apices;
and links extending between and connecting at least some of the
hinge regions on adjacent serpentine rings, wherein each link is
attached on a side of each connected hinge region and wherein all
the links have substantially similar sigmoidal shapes.
15. A radially expansible luminal prosthesis as in claim 14,
wherein the connected hinge regions on adjacent serpentine rings
are axially proximate one another.
16. A radially expansible luminal prosthesis as in claim 14,
wherein less than all of the hinge regions of a serpentine ring are
connected to hinge regions of an adjacent serpentine ring.
17. A radially expansible luminal prosthesis as in claim 14,
wherein all of the hinge regions of a serpentine ring are connected
to hinge regions of an adjacent serpentine ring.
18. A radially expansible luminal prosthesis as in claim 14,
wherein a number of the connected hinge regions of the serpentine
rings is constant along the length of the prosthesis.
19. A radially expansible luminal prosthesis as in claim 14,
wherein a number of the connected hinge regions of the serpentine
rings varies along the length of the prosthesis.
20. A radially expansible luminal prosthesis including a scaffold
comprising: a plurality of serpentine ring segments including
struts connected by hinge regions having apices and side regions,
wherein the side regions are disposed on either side of the of the
apices; and links between at least some of the hinge regions on
adjacent serpentine rings, wherein the links are attached on the
sides of the hinge regions and all links have substantially similar
sigmoidal shapes.
21. A radially expansible luminal prosthesis as in claim 20,
wherein opposed hinge regions on adjacent serpentine rings are
circumferentially offset.
22. A radially expansible luminal prosthesis including a scaffold
comprising: a plurality of serpentine ring segments including
struts and hinge regions disposed between the struts, each hinge
region having an apex and side regions disposed on either side of
the apex; links extending from one of the side regions of at least
some of the hinge regions and connecting at least some of the
adjacent serpentine rings, wherein all links have substantially
similar sigmoidal shapes; and cells formed by a boundary defined by
adjacently joined serpentine ring segments and a pair of radially
adjacent links connecting the adjacent serpentine rings, wherein
each cell has a shape which is a multiple of the smallest cell.
23. A radially expansible luminal prosthesis as in claim 22,
wherein each cell has a substantially similar shape.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 09/565,560 (Attorney Docket No.
020460-000100/______), filed May 4, 2000, the full disclosure of
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to medical devices
and methods. More particularly, the present invention relates to
radially expansible luminal prostheses, such as vascular stents and
grafts.
[0004] Luminal prostheses are provided for a variety of medical
purposes. For example, luminal stents can be placed in various body
lumens, such as blood vessels, the ureter, urethra, biliary tract,
and gastrointestinal tract, for maintaining patency. Luminal stents
are particularly useful for placement in atherosclerotic sites in
blood vessels or fistula or bypass grafts. Luminal grafts can be
placed in blood vessels to provide support in diseased regions,
such as aortic abdominal, and other aneurysms.
[0005] Both stent and graft prostheses must meet certain mechanical
criteria to function successfully. In particular, such prostheses
should be at least partly flexible or articulated (i.e., adjacent
expansible ring segments are connected by links that articulate
relative to one another) over their lengths so that they may be
advanced through tortuous body lumens, such as those of the
coronary vasculature. In addition, the prostheses should have
controllable length change properties, either to maintain their
original length or to have the ability to elongate or foreshorten,
as desired, when the prostheses assume an expanded configuration.
Further such prostheses must have sufficient mechanical strength,
particularly hoop strength after they are expanded, in order to
mechanically augment the luminal wall strength and thus maintain
lumen patency. The ability to meet these requirements is severely
limited in the case of stents and grafts which are delivered in a
radially constrained or collapsed configuration. Such prostheses
must radially expand at a target site within the body lumen, so any
adaptations which are intended to enhance flexibility must not
interfere with the ability to radially expand or to maintain
strength once expanded.
[0006] Prior luminal prostheses often have structures which present
a risk of injury as they are endoluminally delivered (i.e.,
tracked) to and/or released at a target site within a patient's
body lumen. In particular, many vascular stents comprise a
plurality of circumferentially connected and spaced-apart ring
segments which deform circumferentially as the stent is radially
expanded. The Palmaz stent described in U.S. Pat. Nos. 5,102,417
and 4,776,337, is typical of such stents. Such stent designs can
present challenges in both delivery and deployment. For example a
phenomenon called "flaring" occurs when the longitudinal elements
of the distal or proximal end of the prosthesis are bent outward to
assume a crown-like configuration due to bending forces placed on
these elements as the prosthesis passes through tortuous body
passageways. Flaring can create the same deleterious effects as the
previously described fish scaling phenomenon, injuring or
traumatizing the blood vessel wall as the prosthesis is delivered
or tracked within the blood vessel. In addition, flaring may
increase a tendency for stent movement relative to a delivery
balloon, thus causing an improperly deployed stent or, possibly,
dislodging the undeployed stent completely from the catheter.
[0007] In addition to challenges during delivery, prior luminal
prostheses can suffer problems during expansion, particularly
during balloon expansion of malleable stents. For example, it has
been found that balloon expansion of vascular stents often results
in the ends of the stent expanding preferentially compared to the
center of the stent. Such "dog-bone" expansion inhibits sufficient
expansion of the center or ends of the stent, thus leaving a
restricted luminal area in the fully deployed stent. Conversely,
sometimes it will be desired to flare the ends of the stent in
order to lock the stent in place and prevent the ends of the stent
from collapsing after deployment. The ability to program stent
expansion over the length of the stent has generally been lacking
in prior stent designs.
[0008] A still further problem experienced by many prior stent
designs is a lack of vessel coverage after expansion. It will be
appreciated that the ability to support luminal patency and inhibit
hyperplasia and other luminal in-growth can be enhanced if relative
coverage of the luminal wall area by the expanded stent is
increased. Thus, stent designs which afford a greater luminal wall
coverage, or which minimize the free space between stent
structures, while minimizing the amount of stent material used may
be advantageous. Such increase of luminal wall coverage, however,
should not be achieved at the expense of "crimpability."
Particularly for vascular applications, it is desirable that the
diameter of the stent be reduced as much as possible during
delivery, e.g., when crimped over a delivery balloon. By minimizing
the crimped-stent diameter, both trackability and the ability to
cross smaller lesions and access more distal lesions will be
enhanced. In addition, a larger crimped-stent diameter may increase
the risk of stent movement relative to the deployment balloon
which, in turn, could cause an improperly deployed stent or even
loss of the undeployed stent from the catheter. The ability to
reduce the stent diameter is generally limited by the amount of
material in the stent itself. Thus, designs which increase the
ability of the stent to cover the luminal wall without
significantly reducing the "crimpability" would be particularly
desirable.
[0009] For these reasons, it would be desirable to provide improved
stent, graft, and other luminal prostheses. In particular, it would
be desirable to provide improved luminal prostheses which exhibit a
high degree of flexibility with minimum losses of hoop strength and
luminal wall coverage after the prostheses are expanded. For
example, the design should be such that the expanded prostheses
will conform to both curved and straight vessels with minimal or no
straightening or other unintended deformation of the vessel wall.
Such luminal prostheses should be trackable, preferably being both
flexible and presenting minimum risk of injury to the luminal wall
as they are being delivered. In particular, the prostheses should
avoid "fish scaling" and should be highly "crimpable" so that the
prostheses diameter during delivery can be reduced. The luminal
prostheses will preferably further display superior expansion
characteristics. In particular, the prostheses designs should
permit selective programming of the expansion characteristics along
the length of the prostheses. For example, the designs should
permit preferential expansion over the central portion of the
prosthesis, or alternatively at either or both ends of the
prostheses depending on the particular application in which the
prosthesis is to be used. Still further preferably, upon expansion
the prostheses should display superior luminal wall coverage and
adequate to superior hoop strength in order to best maintain
patency of the body lumen being treated. At least some of these
objectives will be met by the luminal prostheses described and
claimed hereinafter.
[0010] 2. Description of the Background Art
[0011] Stents having expansible ring segments joined by sigmoidal
links and axial beams are described in WO 99/17680. Stents
comprising expansible rings including struts and hinges where the
hinges are configured to have different opening forces are
described in U.S. Pat. No. 5,922,020. EP 662 307 describes an
expansible stent having serpentine elements with varying degrees of
curvature to provide controlled expansion characteristics. WO
00/003,662 describes a stent delivery balloon which preferentially
opens a center region of a stent as the balloon is expanded. U.S.
Pat. No. 6,017,365, describes a stent with serpentine segments with
non-linear struts and sigmoidal links. Other patents of interest
include U.S. Pat. Nos. 4,776,337; 5,102,417; 6,017,362; 6,015,429;
and 6,013,854.
SUMMARY OF THE INVENTION
[0012] The present invention provides improved luminal prostheses
suitable for endoluminal placement within body lumens, particularly
blood vessels, and most particularly coronary and peripheral
arteries. The luminal prostheses may be in the form of stents,
intended for maintaining luminal patency, or may be in the form of
grafts, intended for protecting or enhancing the strength of a
luminal wall. Generally, the term "stent" will be used to denote a
vascular or other scaffold structure comprising expansible
components, such as ring segments, which when expanded form an open
lattice or framework which is disposed against the luminal wall. In
contrast, the term "graft" will generally denote such as luminal
scaffold which is covered by a liner, membrane, or other permeable
or impermeable layer which covers at least a portion of the
scaffold. The drawings included herein are generally directed at
stent structures, but it will be appreciated that corresponding
graft structures could be provided by incorporating a liner,
membrane, or the like, on either the outer or inner surfaces of the
stent.
[0013] The luminal prostheses of the present invention will be
radially expansible, usually by the application of a radially
outward internal force to expand a minimally resilient (usually
malleable) prosthesis structure. Such radially outward internal
force will usually be provided by an inflatable balloon, and such
balloon expansible stents are well-known in the art and described
in the background references which have been cited above and are
incorporated herein by reference. Alternatively, at least some of
the radially expansible luminal prostheses of the present invention
may be self-expanding. By fabricating the prostheses from a
resilient material, usually a metal, such as spring stainless
steel, a nickel-titanium alloy (such as Nitinol.RTM. alloy), or the
like, the prosthesis can be designed to have a large (fully
expanded) diameter in an unconstrained state. The diameter of the
prosthesis can be reduced by applying a radial constraint, e.g., by
placing the prosthesis within a sleeve, tube, or other constraining
structure. In that way, the self-expanding prosthesis can be
delivered while constrained and deployed by releasing the
constraint at the target site within the body lumen. The general
principles of constructing self-expanding stents and other luminal
prostheses are also well-known in the art and described in at least
some of the background references which have previously been
incorporated herein.
[0014] In a first aspect of the present invention, a radially
expansible luminal prostheses comprises a plurality of serpentine
ring segments including struts connected by hinge regions. The
struts may be straight or may have non-linear configurations, e.g.,
being curved, wavy, or the like. The use of non-linear struts may
be advantageous in order to increase the area of the strut which
engages the luminal wall after expansion without significantly
reducing flexibility and/or crimpability of the strut. The hinge
regions are usually formed by a short curved or C-shaped region
which permits the connected struts to reverse direction in order to
define the serpentine ring pattern. Adjacent serpentine rings are
joined by sigmoidal links, i.e., S-shaped elements which may be
malleable or elastically deformable in order to allow the adjacent
segments to flex relative to each other during prosthesis delivery
and expansion. The sigmoidal links are attached to a side of the
hinge region, typically located at the point where the hinge
attaches to or transforms into the strut. The use of such sigmoidal
links is beneficial since it permits the longitudinal expansion or
contraction of the prosthesis to accommodate length changes as the
prosthesis is expanded. Such links further permit bending of the
prosthesis since they allow differential motion of adjacent
serpentine rings. Such flexibility is particularly advantageous
since it allows improved tracking of the prosthesis as it is
delivered to an endoluminal location. The sigmoidal links also
improve the conformability of the expanded prosthesis when placed
in a native vessel, artificial graft, or other body lumen location.
Such a structure distinguishes prior art designs where a sigmoidal
link is attached at or near the apex of the link. By attaching the
sigmoidal link closer to the strut, the adjacent ring segments can
be positioned closer to each other. Moreover, because the links
attach away from the apex of the hinge region, stress at the apex
is reduced and uniform expansion of each ring segment is
enhanced.
[0015] In a second aspect of the present invention, the apexes of
opposed hinge regions on adjacent serpentine rings will be
circumferentially offset. That is, the hinge regions apices on at
least some (often all) of the serpentine rings will be aligned with
the trough regions on the adjacent serpentine ring. In this way,
the hinge regions are circumferentially offset so that the
circumferential length of the sigmoidal links connecting proximate
hinge regions can be reduced. Such a design also allows for
adjacent serpentine rings to be closer together to allow for
improved vessel coverage upon prosthesis expansion. Such a design
also permits an increase in the diameter of the curved portions of
the sigmoidal link which further improves stress distribution and
opening characteristics of the prosthesis. Preferably, the luminal
prostheses of the present invention will both have the sigmoidal
links attached to the sides of the hinge regions and have the hinge
regions circumferentially offset in order to achieve the greatest
improvement in flexibility, crimpability, and uniform expansion
characteristics.
[0016] The sigmoidal links will preferably have a S-shaped geometry
with two outer connecting legs joined to a central leg by U-shaped
joints. In some instances, it might also be possible to provide
Z-shaped sigmoidal links, but those will generally be less
preferred. In connecting the sigmoidal links to the hinge regions
of the serpentine rings, the outer connecting legs will generally
be oriented in the annular or circumferential direction and attach
to the hinge region on its side.
[0017] In a further aspect of the present invention, a radially
expansible luminal prosthesis comprises a plurality of ring
segments which are expansible in response to a radially outward
force. The ring segments may comprise serpentine rings, as
generally described above, or may comprise zig-zag segments, box
segments, or other conventional prosthesis ring patterns. The
expansion characteristics of the luminal prosthesis may be varied
over the length of the prostheses by controlling or programming the
characteristics of each of the adjacent expansible ring segments.
For example, different ring segments can be controlled to have
different cross-sectional areas, e.g., differing widths,
thicknesses or both, so that the amount of radially outward force
needed to open the stent is lesser or greater. Alternatively, in
the case of serpentine or zig-zag ring patterns, the strut length
may be varied in order to control the force needed to open the
stent. That is, ring segments having a greater strut length will
open with a lesser force since the increased strut length will
leverage the force applied to a hinge region so that the hinge
region will open sooner. Other techniques for controlling the
expansion characteristics of an individual ring segment may also be
employed, such as those described in U.S. Pat. No. 5,922,020, the
full disclosure of which has previously been incorporated herein by
reference. Depending on the objective, the ring segments near
either or both ends of the prosthesis may be programmed to open
more or less readily so that, when applying a constant radially
outward force along the length of the prosthesis, the stent will
first open either at both ends or in the middle. It will be
appreciated that by employing balloons which also have variable
expansion characteristics, such as those described in WO 00/03662,
a wide variety of prosthesis expansion characteristics can be
provided over the length of the prosthesis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIGS. 1A and 1B are digital photographs of a coronary stent
constructed in accordance with the principles of the present
invention with FIG. 1A showing the stent in an unexpanded
configuration and FIG. 1B showing the stent in an expanded
configuration.
[0019] FIG. 2 is a "rolled out" view of a the exemplary scaffold
structure of FIGS. 1A and 1B.
[0020] FIGS. 3A through 3C are "rolled out" views of second through
fourth exemplary embodiments of scaffold structures which may be
programmed to display differential expansion characteristics over
the length of the prosthesis.
[0021] FIG. 4 is a detailed view of the scaffold structure of FIG.
2 showing preferred dimensions of the scaffold components.
[0022] FIG. 4A is a cross-sectional view of a hinge region of a
scaffold structure of the present invention.
[0023] FIG. 5 is a detailed view similar to FIG. 4, showing the
optional incorporation of non-linear struts into the scaffold
structure.
[0024] FIGS. 6A and 6B are detailed views showing the scaffold
structure of FIG. 2 in its non-expanded configuration (FIG. 6A) and
in its fully expanded configuration (FIG. 6B).
[0025] FIGS. 7A-7C show balloon expansion of a luminal prosthesis,
such as that having the scaffold structure of FIG. 3A or FIG. 3B,
which has been programmed so that it preferentially expands near
its center.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0026] The present invention provides luminal prostheses intended
for endoluminal placement in body lumens, particularly within the
vascular system for the treatment of cardiovascular disease, such
as vascular stenoses, dissections, aneurysms, and the like. The
prostheses, however, are also useful for placement in other body
lumens, such as the ureter, urethra, biliary tract,
gastrointestinal tract and the like, for the treatment of other
conditions which may benefit from the introduction of a reinforcing
or protective structure within the body lumen.
[0027] The prostheses are preferably placed endoluminally. As used
herein, "endoluminally" will mean placement through a body opening
or by percutaneous or cutdown procedures, wherein the prosthesis is
translumenally advanced through the body lumen from a remote
location to a target site in the lumen. In vascular procedures, the
prostheses will typically be introduced "endovascularly" using a
catheter over a guidewire under fluoroscopic guidance. The
catheters and guidewires may be introduced through conventional
access sites to the vascular system, such as through the femoral
artery, or brachial, subclavian or radial arteries, for access to
the coronary arteries.
[0028] A luminal prosthesis according to the present invention will
usually comprise at least two radially expansible, usually
cylindrical, ring segments. Typically, the prostheses will have at
least four, and often five, six, seven, eight, ten, or more ring
segments. At least some of the ring segments will be adjacent to
each other but others may be separated by other non-ring
structures.
[0029] By "radially expansible," it is meant that the segment can
be converted from a small diameter configuration (used for
endoluminal placement) to a radially expanded, usually cylindrical,
configuration which is achieved when the prosthesis is implanted at
the desired target site. The prosthesis may be minimally resilient,
e.g., malleable, thus requiring the application of an internal
force to expand and set it at the target site. Typically, the
expansive force can be provided by a balloon, such as the balloon
of an angioplasty catheter for vascular procedures. As will be
described below, the present invention preferably provides
sigmoidal links between successive unit segments which are
particularly useful to enhance flexibility and crimpability of the
prosthesis.
[0030] Alternatively, the prosthesis can be self-expanding. Such
self-expanding structures are provided by utilizing a resilient
material, such as a tempered stainless steel or a superelastic
alloy such as a Nitinol.RTM. alloy, and forming the body segment so
that it possesses its desired, radially-expanded diameter when it
is unconstrained, i.e. released from the radially constraining
forces of a sheath. In order to remain anchored in the body lumen,
the prosthesis will remain partially constrained by the lumen. The
self-expanding prosthesis can be tracked and delivered in its
radially constrained configuration, e.g., by placing the prosthesis
within a delivery sheath or tube and removing the sheath at the
target site.
[0031] The dimensions of the luminal prosthesis will depend on its
intended use. Typically, the prosthesis will have a length in the
range from about 5 mm to 100 mm, usually being from about 8 mm to
50 mm, for vascular applications. The small (radially collapsed)
diameter of cylindrical prostheses will usually be in the range
from about 0.5 mm to 10 mm, more usually being in the range from
0.8 mm to 1.25 mm for vascular applications. The expanded diameter
will usually be in the range from about 1.5 mm to 50 mm, preferably
being in the range from about 2.5 mm to 30 mm for vascular
applications.
[0032] The ring segments may be formed from conventional materials
used for body lumen stents and grafts, typically being formed from
malleable metals, such as 300 series stainless steel, or from
resilient metals, such as superelastic and shape memory alloys,
e.g., Nitinol.RTM. alloys, spring stainless steels, and the like.
It is possible that the body segments could be formed from
combinations of these metals, or combinations of these types of
metals and other non-metallic materials. Additional structures for
the body or unit segments of the present invention are illustrated
in U.S. Pat. Nos. 5,195,417; 5,102,417; and 4,776,337, the full
disclosures of which are incorporated herein by reference.
[0033] Referring now to FIGS. 1A and 1B, an exemplary luminal
prosthesis 10 particularly intended for implantation in the
coronary vasculature comprises from 4 to 50 ring segments 12 (with
7 being illustrated). Each ring segment 12 is joined to the
adjacent ring segment by at least one of sigmoidal links 14 (with
three being illustrated). Each ring segment 12 includes a
plurality, e.g., six, strut/hinge units (described in more detail
in connection with FIGS. 2-5 below), and two out of each six
hinge/strut structures on each ring segment 12 will be joined by
the sigmoidal links 14 to the adjacent ring segment. FIG. 1A shows
the prosthesis 10 in a collapsed or narrow diameter configuration
while FIG. 1B shows the prosthesis in its expanded
configuration.
[0034] Referring now to FIGS. 2 and 4, a first embodiment of a
luminal prosthesis 20 constructed in accordance with the principles
of the present invention will be described in detail. The
prosthesis 20 comprises serpentine ring segments 22, where each
ring segment has essentially identical characteristics. The ring
segments 22 comprise a plurality of linear struts 24 joined by
curved hinge regions 26 and 26a. As illustrated also in FIG. 4, the
hinge regions 26 are free from other structure, i.e., they are not
linked to adjacent hinge regions or other prosthesis structure. The
hinge regions 26a, in contrast, are connected or joined to
sigmoidal links 28 which secure the adjacent ring segments 22. In
the embodiment of FIG. 2, each adjacent serpentine ring segment 22
is joined by three sigmoidal links 28. The number of lengths,
however, could vary from one, two, up to the total number of hinge
regions, i.e., six in the illustrated embodiment of FIG. 2.
[0035] The sigmoidal links 28 are adjoined to the hinge regions 26a
so that a first outer leg segment 30 connects to the hinge region
at its base i.e., where the hinge opens into the strut 24.
Similarly, a second outer leg segment 32 is joined to hinge region
26a on the adjacent serpentine ring 22 at the base of that hinge
region. The legs 30 and 32 are generally oriented in a
circumferential or annular direction at the point where they attach
to the hinge regions 26a. The legs are joined by a pair of U-shaped
regions which join a central leg 34 to complete the sigmoidal link.
This design of the sigmoidal link has a number of advantages. For
example, by orienting the leg segments 30 and 32 circumferentially,
the legs can move circumferentially past each other to accommodate
radial crimping of the prosthesis as well as facilitate radial
opening of the stent. Additionally, the structure permits axially
shortening and elongation to permit bending of the prosthesis as it
is being introduced through tortuous regions of a blood vessel or
other body lumen.
[0036] As also best seen in FIG. 4, the serpentine ring segments 22
are rotationally oriented relative to each other so that the apices
on the hinge regions 26 and 26a on the first ring segment are
aligned with a trough region 34 on the adjacent ring segment. Such
relative rotational alignment of the ring segments 22 minimizes the
circumferential length of the sigmoidal links 28 need to connect
the opposed hinge regions. It will be appreciated that if the
apices of opposed hinge regions 26 and 26a were rotationally
aligned, the length of the connecting leg 34 would have to be
significantly longer. Minimizing the length permits optimum
configuration of the sigmoidal link. By minimizing the length of
the connecting legs of the sigmoidal link, the crimped diameter can
be minimized as the sigmoidal links will not be interfering with
the crimped configuration. In addition, by having shorter
connecting legs on the sigmoidal link, the gaps between adjacent
ring segments on the expanded prosthesis will be minimized.
Preferably, leg segments 30 and 32 each have flared ends 31 and 33,
respectively, which connect the sigmoidal link 28 to the adjoining
hinge region 26a. The flared ends provide stress relief as the ring
segments 22 and links 28 are expanded.
[0037] The dimensions of the hinges, struts, sigmoidal links, and
the like, of the luminal prostheses of the present invention may
vary considerably depending on the intended use. Exemplary
dimensions intended for a coronary stent (FIG. 4) are set forth in
Table I below.
1TABLE I Exemplary Dimensions (mm) A B C Broad Range 0.025 to 1.25
0 to 6.5 0.075 to 1.25 Preferred Range 0.075 to 0.15 0.15 to 0.25
0.2 to 0.4 D E F Broad Range 0.1 to 6.5 0 to 6.5 0.025 to 0.65
Preferred Range 0.25 to 0.5 0.15 to 0.3 0.035 to 0.075
[0038] Referring now to FIGS. 3A through 3C, embodiments of the
luminal prostheses of the present invention where adjacent
serpentine ring segments have different expansion characteristics
will be described. In FIG. 3A, a luminal prosthesis 30 has a
plurality of adjacent serpentine ring segments 32a-32m. Individual
serpentine ring segments 32a-32m will have different expansibility
characteristics so that the prosthesis 30 will differentially open
along its length in response to uniform opening forces. The
expansibility characteristics of the individual ring segments may
be modified in a number of the different ways. In a first general
approach, the yield profile(s) of some or all of the hinges in an
individual ring segment can be modified relative to such profiles
for others of the ring segments. This can be done by increasing or
decreasing the width, thickness, or other cross-sectional dimension
of any one or more of the hinge regions. Alternatively, the lengths
of the struts connecting the hinge regions can be increased or
decreased to change the leveraged force applied to a hinge region
when the prosthesis is being expanded by an internal radially
outward force. Other approaches, such as adjusting the radius of an
arcuate hinge region are also known.
[0039] FIG. 4A is a cross-sectional view of the hinge region 26 or
26a having a width W and a thickness T. A hinge region 26 or 26a
having a smaller width W, and/or a smaller thickness T, will have
less resistance to opening than a hinge region which has a larger
thickness T.sub.2 and/or a small width W.sub.2. Thus, as described
above, the opening characteristics of the prosthesis can be
programmed by adjusting the widths and/or thickness of the hinge
regions in particular ring segments 22.
[0040] In the particular embodiment of FIG. 3A, the serpentine ring
segments 32a-32c and 32k-32m have struts and hinge regions with a
larger width than those of the middle ring segments of 32d-32j. The
innermost ring segments 32f-32h have the smallest widths for the
struts and hinge regions. Thus, assuming the prosthesis 30 is
formed from the same material (or different materials having the
same mechanical properties) over its entire length, the hinges of
the ring segments 32 having greater widths will be stiffer and
provide more resistant to expansion. In contrast, the ring segments
having the narrowest widths (32f-32h) will be the least stiff and
have the least resistance to expansion in response to an internally
applied force. Thus, when the prosthesis 30 is expanded over a
balloon 42 on a balloon catheter 40, the prosthesis will
preferentially expand over the central region, as illustrated in
FIG. 7A-7C. In particular, the prosthesis 30 is shown in FIG. 7A
prior to expansion. As the balloon 42 is partially expanded (FIG.
7B), the prosthesis 30 begins opening in its middle sections prior
to its end sections. Finally, after the balloon is fully expanded
(FIG. 7C), uniform expansion of the prosthesis 30 along its length
can be achieved. Of course, when expanded in a body lumen, full
expansion of the prosthesis 30 may be constrained so that the
center sections will first engage the wall with the end sections
engaging the wall at a later time. Such deployment may be
advantageous since it assures that the central regions of the
prosthesis are fully engaged against the luminal wall prior to
opening of the end portions.
[0041] Under other circumstances, however, it may be desired to
preferentially open the end portions of the luminal prosthesis
first. In such instances, the luminal prosthesis 30 could be
modified so that the end segments 32a-32c and 32k-32m open
preferentially with respect to the central ring segments 32d-32j. A
variety of other opening characteristics, such as tapered would
also be possible. For example, a tapered opening could be achieved
by providing a stiffness gradient where segments at one end, such
as 32a, are the least stiff with ring segments becoming
progressively stiffer in the direction of ring segment 32m.
[0042] Differential expansion of different ring segments can be
achieved in a variety of ways. For example, as shown in FIG. 3B,
instead of selecting different widths or other cross-sectional
dimensions for the hinge regions of the ring segment, the strut
length could be varied. As shown in FIG. 3B, the struts in end
segments 50a and 50f are the longest, with the strut lengths in the
inner ring segments 50b-50e becoming progressively shorter. Ring
segments having longer strut lengths will apply a greater force to
the hinge regions of those struts in response to an equal radially
outward expansion force. Thus, as shown in FIG. 3B, the end ring
segments 50a and 50f will preferentially open with respect to the
inner ring segments 50b-50e. Still further ways for controlling the
expansion characteristics of an individual ring segment could also
be utilized. For example, the hinge regions could be weakened
and/or strengthened, as described in detail in U.S. Pat. No.
5,922,020. Alternatively, the diameters of the hinge regions could
be varied in successive ring segments, as described generally in
European patent application 662 307.
[0043] Referring now to FIG. 3C, serpentine ring segments 60 can be
programmed to have different expansion rates by adjusting the
lengths of some, but not all, of the struts 64 in any ring. The
serpentine rings 60 will be joined by sigmoidal links 62, generally
as described above. Rather than having struts 64 with identical
lengths, those struts which terminate in hinges 66a which are not
adjacent to the sigmoidal links 62 can be made longer than those
struts 66b which are adjacent the sigmoidal links. This ability
offers an additional degree of freedom in programming the expansion
rates of the individual serpentine rings as well as the overall
prosthesis made from such rings.
[0044] In an optional aspect of the present invention, at least
some of the serpentine ring segments may employ non-linear struts.
As shown in FIG. 5, ring segments 12 may comprise non-linear struts
50 which are joined by sigmoidal links 52 in a manner described
above in connection with other embodiments of the present
invention. The use of non-linear struts is advantageous in that it
increases the length and therefore the amount of strut available
for engagement against a luminal wall without increasing the length
of the prosthesis. The ability to increase the coverage of the
stent against the luminal wall is well recognized in the art. In
addition, the use of non-linear struts can reduce the crimped-stent
diameter since the wall thickness of the stent itself can be
decreased without loss of expanded hoop strength. Another advantage
of a non-linear strut design is that it increases the amount of
material within the strut thus improving upon the fluoroscopy
characteristics of the stent.
[0045] Referring now to FIGS. 6A and 6B, the improved expansion
characteristics of the luminal prosthesis 20 of FIG. 2 will be
described. It is assumed that the prosthesis 20 has been placed
over a delivery balloon and that a constant expansion force over
the length of the prosthesis is being applied. FIGS. 6A and 6B show
a detailed section of two adjacent serpentine rings 22, with the
prosthesis shown in its fully collapsed or crimped condition in
FIG. 6A and its fully expanded condition in FIG. 6B. As pressure is
applied to the delivery balloon, an outward radial force is applied
to the prosthesis. This radial force causes the hinge regions of
the serpentine rings to flex open as the prosthesis is expanded.
For a material such as stainless steel, the stresses within the
hinge region become higher than the yield strength and enter the
plastic region of the material. This allows the prosthesis to
remain open following removal of the delivery balloon. For a
shape-memory or other resilient alloy, such as Nitinol.RTM., the
natural state of the prosthesis is in the expanded configuration.
For such "self-expanding" designs, a sheath is holding the
prosthesis in a crimped configuration for delivery to the lesion
site. Once the desired deployment location has been reached, the
sheath is retracted and the prosthesis is expanded to its natural
position. The hinges act as springs in causing such expansion.
[0046] While the above is a complete description of the preferred
embodiments of the invention, various alternatives, modifications,
and equivalents may be used. Therefore, the above description
should not be taken as limiting the scope of the invention which is
defined by the appended claims.
* * * * *