U.S. patent application number 11/220900 was filed with the patent office on 2006-02-09 for expandable medical device with locking mechanism.
This patent application is currently assigned to Conor Medsystems, Inc.. Invention is credited to John F. Shanley.
Application Number | 20060030931 11/220900 |
Document ID | / |
Family ID | 26736476 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060030931 |
Kind Code |
A1 |
Shanley; John F. |
February 9, 2006 |
Expandable medical device with locking mechanism
Abstract
According to the present invention there is provided an
expandable medical device having, a plurality of elongated beams,
the plurality of elongated beams joined together to form a
substantially cylindrical device which is expandable from a
cylinder having a first diameter to a cylinder having a second
diameter. A plurality of hinges connecting the elongated beams have
a hinge width, wherein the hinge width is smaller than the beam
width. A pawl is disposed adjacent to and substantially parallel to
the hinge prior to expansion of the medical device and a plurality
of teeth are adapted to receive the pawl. The present invention
additionally provides the benefit of limiting the amount of recoil
of an expandable device by engaging a locking mechanism, thereby
retaining the expanded diameter of the device.
Inventors: |
Shanley; John F.; (Redwood
City, CA) |
Correspondence
Address: |
CINDY A. LYNCH;CONOR MEDSYSTEMS, INC.
1003 HAMILTON COURT
MENLO PARK
CA
94025
US
|
Assignee: |
Conor Medsystems, Inc.
Menlo Park
CA
|
Family ID: |
26736476 |
Appl. No.: |
11/220900 |
Filed: |
September 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10057414 |
Jan 25, 2002 |
6964680 |
|
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11220900 |
Sep 6, 2005 |
|
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60266805 |
Feb 5, 2001 |
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Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F 2/915 20130101;
A61F 2002/91541 20130101; A61F 2002/91591 20130101; A61F 2/91
20130101; A61F 2002/91558 20130101; A61F 2250/0004 20130101 |
Class at
Publication: |
623/001.15 |
International
Class: |
A61F 2/06 20060101
A61F002/06 |
Claims
1-36. (canceled)
37. An expandable medical device comprising: a plurality of
elongated beams, the plurality of elongated beams joined together
to form a substantially cylindrical device which is expandable to
form a cylinder having a first diameter to a cylinder having a
second diameter, the plurality of the elongated beams having a beam
width in a circumferential direction; a plurality of hinges having
a hinge width, wherein the hinge width is smaller than the beam
width; and an internal self locking mechanism.
38. The expandable medical device according to claim 37, wherein
the internal self-locking mechanism comprises a pawl and a
plurality of teeth adapted to receive the pawl.
39. A method of constructing an expandable medical device, the
method comprising: fabricating an expandable medical device from a
cylindrical member, wherein the expandable device is formed having
a first unexpanded diameter; retracting the expandable medical
device to an unexpanded second diameter, wherein deformation during
retraction is confined to a hinge portion of the expandable medical
device; and retaining the expandable medical device in said
unexpanded second diameter.
40. The method according to claim 39, wherein the cylindrical
member is formed of superelastic Nitinol alloy.
41. The method according to claim 40, wherein the step of retaining
the expandable medical device includes disposing a retaining means
about the unexpanded second diameter of the expandable medical
device.
42. The method according to claim 41, further including the steps
of: disposing the retracted expandable medical device onto a
delivery device; placing the delivery device within an artery of a
patient; and removing said retaining means wherein the expandable
medical device returns to said first unexpanded diameter.
43. The method according to claim 42, further including the step of
expanding the expandable medical device to an expanded diameter
larger than the first unexpanded diameter wherein a locking means
is engaged to retain the expandable medical device in said first
expanded diameter.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority to U.S. Provisional
Patent Application Ser. No. 60/266,805 filed Feb. 5, 2001, the
entirety of which is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to expandable medical devices, more
particularly to an expandable medical device including a locking
mechanism and low recoil after expansion from one diameter to a
greater second diameter.
SUMMARY OF THE RELATED ART
[0003] In the past, permanent or biodegradable devices have been
developed for implantation within a body passageway to maintain
patency of the passageway. These devices are typically introduced
percutaneously, and transported transluminally until positioned at
a desired location. These devices are then expanded either
mechanically, such as by the expansion of a mandrel or balloon
positioned inside the device, or expand themselves by releasing
stored energy upon actuation within the body. Once expanded within
the lumen, these devices, called stents, become encapsulated within
the body tissue and remain a permanent implant.
[0004] Known stent designs include monofilament wire coil stents
(U.S. Pat. No. 4,969,458); welded metal cages (U.S. Pat. Nos.
4,733,665 and 4,776,337); and, most prominently, thin-walled metal
cylinders with axial slots formed around the circumference (U.S.
Pat. Nos. 4,733,665, 4,739,762, and 4,776,337). Known construction
materials for use in stents include polymers, organic fabrics and
biocompatible metals, such as, stainless steel, gold, silver,
tantalum, titanium, and shape memory alloys such as Nitinol.
[0005] U.S. Pat. Nos. 4,733,665, 4,739,762, and 4,776,337 disclose
expandable and deformable intraluminal vascular grafts in the form
of thin-walled tubular members with axial slots allowing the
members to be expanded radially outwardly into contact with a body
passageway. After insertion, the tubular members are mechanically
expanded beyond their elastic limit and thus permanently fixed
within the body.
[0006] Many of the known stents display a large elastic recovery,
known in the field as "recoil," after expansion inside a lumen.
Large recoil necessitates over-expansion of the stent during
implantation to achieve the desired final diameter. Over-expansion
is potentially destructive to the lumen tissue and is known to
cause higher rates of restenosis. Known stents of the type
described above experience recoil of up to about 9 to 21% from
maximum expansion.
[0007] Large recoil also makes it very difficult to securely crimp
most known stents onto delivery catheter balloons. As a result,
slippage of stents on balloons during intralumenal transportation,
final positioning, and implantation has been an ongoing problem.
Many ancillary stent securing devices and techniques have been
advanced to attempt to compensate for this basic design problem.
Some of the stent securing devices include collars and sleeves used
to secure the stent onto the balloon.
[0008] Some materials have intrinsic properties that are beneficial
in some aspects of stent design, and undesirable in other aspects.
U.S. Pat. No. 5,545,210, for example, discloses a thin-walled
tubular stent geometrically similar to those discussed above, but
constructed of a nickel-titanium shape memory alloy ("Nitinol").
Martensitic Nitinol has a very low Young's Modulus, and a low,
nearly horizontal "de-twinning" stress plateau that provide an
exceptionally large strain range before plastic deformation
commences. When incorporated into conventional stent designs, these
properties produce stents that have unusually good flexibility,
deliverability, conformability and radiopacity, but also
unacceptably high recoil and poor radial strength. For example,
recoil of a typical design Nitinol stent ranges from 10% to 25%,
typically about 12% to 16%.
[0009] One approach to remedying material-based recoil and radial
strength problems is to employ locking or detent features in the
stent design. A number of such locking stent designs have been
proposed for conventional materials like stainless steel, but none
has proven workable in practice. Many of the designs are simply
impossible to manufacture on a small scale. More fundamentally,
most of the designs fail to provide the basic mechanical
requirements of a ratchet or detent. mechanism: 1) two
degree-of-freedom differential motion between the engaging
elements, and 2) a restoring or spring force between the elements.
It is relatively easy to provide these two elements in larger
mechanisms comprised of discrete components, but much more
difficult to provide this functionality while cutting all features
into a very small, continuous, cylindrical surface.
[0010] FIG. 1 shows a typical prior art "expanding cage" stent
design. The stent 10 includes a series of axial slots 12 formed in
a cylindrical tube 14. Each axial row of slots 12 is displaced
axially from the adjacent row by approximately half the slot length
providing a staggered slot arrangement. The material between the
slots 12 forms a network of axial struts 16 joined by short
circumferential links 18. The cross section of each strut 16
remains constant or varies gradually along the entire length of the
strut and thus the rectangular moment of inertia and the elastic
and plastic section moduli of the cross section also remain
constant or vary gradually along the length of the strut. Such a
strut 16 is commonly referred to as a prismatic beam. Struts 16 in
this type of design are typically 0.005 to 0.006 inches
(0.127-0.1524 mm) wide in the circumferential direction. Strut
thicknesses in the radial direction are typically about 0.0025
inches (0.0635 mm) or less to keep expansion forces within
acceptable levels. However, most stent materials must be
approximately 0.005 inches (0.127 mm) thick for good visibility on
conventional fluoroscopic equipment. This high ratio of strut width
to thickness, combined with the relatively high strut length and
the initial curvature of the stent tubing combine to cause the
instability and bucking often seen in this type of stent design.
When expanded, the stent structure of FIG. 1 assumes the roughly
diamond pattern commonly seen in expanded sheet metal.
[0011] Another stent described in PCT publication number WO
96/29028 uses struts with relatively weak portions of
locally-reduced cross sections that on expansion of the stent act
to concentrate deformation at these areas. However, as discussed
above non-uniform expansion is even more of a problem when smaller
feature widths and thicknesses are involved because manufacturing
variations become proportionately more significant. The
locally-reduced cross section portions described in this document
are formed by pairs of circular holes. The shape of the locally
reduced cross section portions undesirably concentrates the plastic
strain at the narrowest portion. This concentration of plastic
strain without any provision for controlling the level of plastic
strain makes the stent highly vulnerable to failure.
[0012] U.S. Pat. No. 6,241,762, entitled "EXPANDABLE MEDICAL DEVICE
WITH DUCTILE HINGES" filed Oct. 10, 1998 and assigned to Conor
Medsystems Inc., is incorporated herein by reference in its
entirety. This patent discloses a stent design that provides a
stent with large, non-deforming strut and link elements, which can
contain holes without compromising the mechanical properties of the
strut or link elements, or the device as a whole. Further, these
holes may serve as large, protected reservoirs for delivering
various beneficial agents to the device implantation site.
[0013] In view of the drawbacks of prior art stents, it would be
advantageous to have an expandable medical device with low radial
recoil and high radial force (circumferential crush strength)
independent of the intrinsic properties of the material of
fabrication.
[0014] It would further be advantageous to have a tissue-supporting
device that could be expanded to a range of final diameters
independent of the means of expansion or the force levels
applied.
[0015] It would also be desirable to control the maximum material
strain to a desired level wherein when the expandable medical
device is deployed the material remains below its elastic limit,
and may be expanded to a greater diameter if necessary without
plastically deforming the material.
SUMMARY OF THE PRESENT INVENTION
[0016] The present invention makes novel use of hinges to provide
both the spring force and two degrees-of-freedom motions required
for a true self-locking expandable medical device design. In
addition to that above the present invention provides a novel
expandable medical device that is capable of self-expansion or
expansion through the use of a balloon catheter or similar device.
Further still, the expandable medical device according to the
present invention may be utilized to deliver a beneficial agent to
the area adjacent to the expanded medical device.
[0017] In accordance with one aspect of the present invention there
is provided an expandable medical device including a plurality of
elongated beams. The plurality of elongated beams are joined
together to form a substantially cylindrical device. The
cylindrical device is expandable from a cylinder having a first
diameter to a cylinder having a second diameter, the plurality of
the elongated beams having a beam width in a circumferential
direction. A plurality of hinges connecting the elongated beams
have a hinge width, wherein the hinge width is smaller than the
beam width. A pawl is disposed adjacent to and substantially
parallel to the hinge prior to expansion of the medical device and
a plurality of teeth are adapted to receive the pawl.
[0018] In accordance with another aspect of the present invention
there is provided an expandable medical device including a
cylindrical tube and a plurality of axial slots formed in the
cylindrical tube in an arrangement to define a network of elongated
struts, wherein each of the elongated struts are radially displaced
from adjacent struts, and each elongated strut further includes at
least one tooth disposed thereupon. A pawl formed between the
elongated struts has a distal end adapted to be received by the
tooth. A plurality of hinges formed between the elongated struts
allow the cylindrical tube to be expanded from a first diameter to
a second diameter by bending of the hinges and engaging the distal
end of the pawl with the tooth.
[0019] In accordance with another aspect of the present invention
there is provided an expandable medical device including, a
cylindrical expandable body of Nitinol and a locking feature for
locking the expanded body in an expanded position, wherein the
locking mechanism prevents recoil of the expanded body of greater
than 5 percent.
[0020] In accordance with another aspect of the present invention
there is provided a method of processing an expandable medical
device, the method including the steps of fabricating a cylindrical
expandable medical device from martensitic Nitinol, where the
expandable medical device is fabricated with a first unexpanded
diameter. Expanding the cylindrical expandable medical device to an
expanded second diameter, wherein deformation during expansion is
confined to a hinge and the deformation is below the elastic limit
of the material. Processing the expandable medical device, and
restoring the expandable medical device to the unexpanded first
diameter by applying heat to the expandable medical device.
[0021] In accordance with another aspect of the present invention
there is provided an expandable medical device, the expandable
medical device includes a plurality of elongated beams and a
plurality of hinges. The plurality of elongated beams joined
together to form a substantially cylindrical device that is
expandable to form a cylinder having a first diameter to a cylinder
having a second diameter. The plurality of the elongated beams have
a beam width in a circumferential direction, the beam width of the
elongated members being less than the width of the hinges. The
expandable medical device further including an internal self
locking mechanism.
[0022] In accordance with yet another aspect of the present
invention there is provided a method of constructing an expandable
medical device. The method including the steps of fabricating an
expandable medical device from a cylindrical member, the expandable
medical device is formed having a first unexpanded diameter,
retracting the expandable medical device to an unexpanded second
diameter where deformation during retraction is confined to a hinge
portion of the expandable medical device, and retaining the
expandable medical device in the unexpanded diameter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The invention will now be described in greater detail with
reference to the preferred embodiments illustrated in the
accompanying drawings, in which like elements utilize like
reference numerals, and wherein;
[0024] FIG. 1 is an isometric view of a prior art tissue-supporting
device;
[0025] FIG. 2A is a planar view of a representative portion of an
unexpanded tissue-supporting device in accordance with one
embodiment of the invention;
[0026] FIG. 2B is a sectional planar view of a single expanding
member in accordance with the present invention;
[0027] FIG. 3A is a detail view of the expandable medical device of
FIG. 2 undergoing sequential expansion;
[0028] FIG. 3B is a detail view of the expandable medical device of
FIG. 2 undergoing sequential expansion;
[0029] FIG. 3C is a detail view of the expandable medical device of
FIG. 2 undergoing sequential expansion;
[0030] FIG. 3D is a detail view of an alternative embodiment of the
expandable medical device in accordance with the present
invention;
[0031] FIG. 4 is a planar view of the expandable device of the
present invention in an expanded and locked state; and
[0032] FIG. 5 is a stress/strain curve of a martensitic Nitinol
alloy illustrating the extended sub-plastic strain range and the
horizontal "de-twinning plateau" of the material.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] Referring now to FIGS. 2A and 2B, there is shown a planar
view of a representative portion of an unexpanded tissue-supporting
medical device 200. The expandable medical device 200 includes a
series of axial slots 210 formed in a cylindrical tube (not shown).
Each axial slot 210 is displaced radially from the slots in the
adjacent rows of slots by approximately 0.010 inches. The plurality
of axial slots 210 define a plurality of elongated beams 220. The
plurality of elongated beams 220 are interconnected by a hinge 250
disposed at one end and a locking area disposed at the other end. A
U-shaped link 270 interconnects adjacent rows of beams 220.
[0034] The elongated beam 220 further includes a pawl 230 having a
distal end 235 disposed at one end of the elongated beam 220 and a
plurality of teeth 240 disposed at the other end opposite the pawl
230. The elongated beam 220 further includes a hinge 250 adjacent
the pawl 230. The hinge 250 further contains a first end 252 and a
second end 254 defining a section 265, wherein the section 265
between the first end 252 and the second end 254 is designed where
it will act as a stress/strain concentration area. Specifically,
the hinge 250 contains a first portion extending along about 1/3 of
the length of the hinge 250 and a second section gradually
tapering, extending about 2/3 of the length of the hinge 250. It is
contemplated that other ratios may be utilized as well as other
geometries in order to confine the maximum stress/strain to the
hinge section 265. Furthermore, the length and width of the hinge
can be adjusted to confine the maximum strain in the hinge 250, to
some desired value at the maximum required bend radius of the hinge
250.
[0035] For example, if the maximum desired bend angle of the hinge
250 was set at ninety (90) degrees, and the minimum hinge width was
fixed at about 0.002 inches, a hinge length could be determined
which would guarantee that the maximum strain in the hinge 250 was
well below the elastic limit of some specified material. For
example, the expandable medical device could be manufactured of
materials such as titanium, stainless steel, polymers, or
preferably a Nitinol alloy, wherein the Nitinol alloy may be either
martensitic or austenitic super-elastic.
[0036] With reference to the drawings and the discussion, the width
of any feature is defined as its dimension in the circumferential
direction of the cylinder. The length of any feature is defined as
its dimension in the axial direction of the cylinder. The thickness
of any feature is defined as the wall thickness of the
cylinder.
[0037] When manufactured as a continuous array as shown in FIG. 2,
the distal end 235 of the pawl 230 is adjacent to the teeth 240 of
the adjacent elongated member 220.
[0038] Referring now to FIGS. 3A through 3C there is shown a plan
view of a section of the expandable medical device 200 as the
device is deployed. The expandable device 200 may be expanded
radially by placing an appropriate device, such as a balloon
catheter within the inner diameter of the expandable device 200 and
expanding the balloon catheter until the expandable device has been
expanded to a desired diameter.
[0039] Additionally the expandable medical device 200 may be
constructed having a plurality of apertures for receiving a
beneficial agent, which may be released after insertion within a
patient. For a more detailed description of the apparatus and
methods for delivering a beneficial agent, See co-pending U.S.
patent application Ser. No. 09/649,217 entitled "Expandable Medical
Device With Ductile Hinges", filed Aug. 28, 2000 which is hereby
incorporated by reference in its entirety.
[0040] Furthermore, the expandable device 200 may be constructed of
materials such as polymers, biodegradable materials, or
combinations of polymers, biodegradable materials and Nitinol. The
locking mechanism of the present invention provides the ability to
decrease the recoil and increase the radial strength of expandable
devices constructed of these materials, thereby allowing an
expandable device to be constructed of these materials.
[0041] The polymer and/or the biodegradable materials may be
further adapted to receive a beneficial agent that may then be
eluted after the expandable device is deployed within a patient.
For example, the beneficial agent may be contained within the
polymer or biodegradable material, in which the characteristics of
the polymer or biodegradable material control the release rate of
the beneficial agent after the expandable device is deployed within
a patient.
[0042] As shown in FIG. 3A, a partial section of the expandable
device 200 is shown. As described in detail above, the expandable
device 200 includes a plurality of elongated members 220, spaced
apart by radial slots 210. The elongated members have a pawl 230
disposed on one end thereof and a plurality of teeth 240 disposed
on the other, and a hinge 250 adjacent to the pawl 230. The
plurality of elongated members 220 are joined together by the hinge
250 and the locking configuration. In an unexpanded state, as shown
in FIG. 3A, the pawl 230 disposed at one end of an elongated member
220 is substantially parallel to hinge 250. Further still the
distal end 235 of the pawl 230 is designed having a `chisel` shape
adapted for being received by at least one of the plurality of
teeth 240 on the adjacent substantially parallel elongated member
220.
[0043] Referring now to FIG. 3B, there is shown the partial section
of the expandable device 200 having been expanded to a second
partially expanded diameter. As indicated by arrow 300 a general
rotational motion is achieved by pawl 230 as the diameter of the
expandable device 200 is increased and hinge 250 bends. As shown,
the locus of points drawn out by distal tip 235 of the pawl 230 as
it rotates describes a non-circular arc. The hinge 250 bends
initially about a pre-determined point within the region 265 as
shown in FIG. 3B. The pre-determined bending point is determined by
narrowing the width of the hinge 250 within the section 265 as
defined by the first end 252 and the second end 254 of the hinge
250. As shown FIG. 3B, as the pawl rotates during expansion of the
device from an unexpanded state as shown in FIG. 3A to a partially
expanded state as shown in FIG. 3B the pawl 230 is no longer
substantially parallel to the hinge 250 due to the bending of the
hinge 250.
[0044] If the expandable device of FIG. 3B were fabricated of an
martensitic Nitinol (shape memory) alloy, it could at this point be
fully restored to the configuration of FIG. 3A by heating the
device above its transformation temperature. This is possible
because the pawl has not yet engaged the first tooth in the
adjacent strut, and the material is never stressed beyond its
elastic limit. This feature is useful for intermediate fabrication
steps, for example electropolishing, where the device can be
expanded to an "open" configuration for more effective polishing
and then thermally restored to a smaller diameter for mounting on a
delivery system.
[0045] Referring now to FIG. 3C, there is shown the section of the
expandable medical device 200 in an expanded state during
engagement of the locking mechanism. As shown, a significant
curvature develops in the hinge 250, specifically in the section
265. As this curvature occurs, the pawl 230 and the distal end 235
of the pawl 230 become capable of both continued angular
deflection, as indicated by arrow 300, and also a linear motion
along the axis of the pawl 230, as indicated by arrow 320. The
angular motion 300 and the linear motion 320 are both possible in
this region because the axis of the pawl 230 is no longer directly
aligned with the now-curved, centroidal axis of the hinge 250.
Rather, the motion of the pawl 230 along its own axis requires only
additional bending of the hinge 250 near second end 254 of the
hinge 250. This ability of the hinge 250 to provide both the
rotational motion 300 and the axial motion 320 allows the distal
tip 235 of the pawl 230 to follow the contour of the teeth 240
without local plastic yielding of either feature. Additionally, the
elastic energy stored in the hinge 250 provides a means for
creating a spring return force (not shown) that can be resolved at
the distal tip 235 of the pawl into components parallel and
perpendicular to the axis of the pawl. When the expansion device
has positioned the distal tip 235 of the pawl 230 beyond one of the
locking teeth 240, and the expansion device is then withdrawn,
these spring return forces force the distal tip 235 to contact the
locking tooth 240, thereby locking the expandable device 200 into
an expanded state as shown in FIG. 4. The mating faces of the
distal tip 235 and the locking tooth 240 are contoured according to
well-known techniques to insure that forces that are externally
applied to the tissue-supporting device 200 act to further lock the
features in position. Furthermore, throughout the description
reference is made to the teeth 240, it shall be understood that
this reference should not be considered limiting. For example, the
teeth 240 may comprise many different geometrical shapes, which are
adapted to receive the distal end 235 of the pawl 230. For example,
the teeth 240 may be comprised of depressions adapted to receive
the distal end 235 of the pawl 230. Thus, the teeth 240 as shown
and described are not to be considered limiting and are exemplary
only; it is contemplated that the teeth 240 as well as the distal
end 235 of the pawl 230 may comprise many different shapes as shall
be apparent to one skilled in the art.
[0046] For example, when the expandable medical device 200, as
described above, is fabricated from a Nitinol alloy, an
exceptionally large sub-plastic strain range is available, and very
large deformations are possible without exceeding the material's
elastic limit. If the locking teeth of the expandable device 200
were absent, the device could be restored to its original shape by
heating, since it is never plastically deformed during the
expansion sequence of FIG. 3.
[0047] Additionally, the hinge 250 may be contoured as described
above in order to control the bending pattern of the hinge 250 and
thus the motion of the pawl during the bend sequence. For example,
when the width of the hinge is narrowest near the pawl proximal end
232 of the pawl 230, the hinge tends to bend in this area first. As
a result, the instant center of rotation of the pawl is initially
closer to the proximal end 232 of the pawl 230, and the arc traced
by the distal tip 235 of the pawl 230 quickly passes through the
region 238 as indicated in FIGS. 3A-3C. This may be visualized by
imagining the limiting case of a simple pivot point located at end
252 of hinge 250 the arc traced by distal tip 235 would in this
case be circular, and the initial motion of the tip 235 would be
orthogonal to the axis of pawl 230 (i.e., downward, or directly
toward the adjacent strut 220). The second limiting case would
correspond to a pivot point at the other end 254 of hinge 250. The
arc traced by distal tip 235 would again be circular, but in this
case, the initial motion of the tip 235 would be parallel to the
axis of pawl 230, the tip of the pawl would move away from contact
with adjacent strut 220 from the outset, and no locking would be
possible with teeth 240 of the adjacent strut 220. By contouring
the shape of hinge 250 between these extremes, the relative motion
of pawl 230 with respect to adjacent strut 220 containing teeth 240
may be optimized.
[0048] Referring now to FIG. 3D there is shown an exemplary
alternative embodiment of the expandable device according to the
present invention. Referring to FIG. 3D there is shown a planar
view of a representative portion of an unexpanded expandable
medical device 200' in accordance with an alternative embodiment of
the present invention. The expandable medical device 200' includes
a series of spaced apart areas 211' formed within a cylindrical
tube (not shown). The cylindrical tube being constructed of
superelastic Nitinol alloy, wherein the cylindrical tube defines a
first diameter of the expandable medical device 200'. By utilizing
a superelastic Nitinol alloy for the construction of the expandable
medical device 200', the expandable medical device 200' can be
formed in a semi-expanded state as shown in FIG. 3D. Each spaced
apart area 211' defines a plurality of elongated beams 220'. The
plurality of elongated beams are interconnected by a hinge 250'
disposed at one end and a locking area disposed at the other end. A
U-shaped link 270' interconnects adjacent rows of beams 220'. As
shown, the elongated beam 220' further includes a pawl 230' having
a distal end 235' disposed at one end of the elongated beam 220'
and a plurality of teeth 240' disposed at the other end opposite
the pawl 230'. The hinge 250' further includes a first end 252' and
a second end 254' defining a section 265', wherein the section 265'
between the first end 252' and the second end 254' is designed
where it will act as a stress/strain concentration area.
Specifically, the hinge 250' contains a first portion extending
along about 1/3 of the length of the hinge 250' and a second
section gradually tapering, extending about 2/3 of the length of
the hinge 250'. It is contemplated that other ratios may be
utilized as well as other geometries in order to confine the
maximum stress/strain to the hinge section 265'. Furthermore, the
length and width of the hinge can be adjusted to confine the
maximum strain in the hinge 250' to some desired value at the
maximum required bend radius of the hinge 250'.
[0049] As shown in FIG. 3D, it can be seen that expandable medical
device 200' is constructed in a semi-expanded state wherein the
distal end 235' of the pawl 230' has not been engaged with the
teeth 240'. By constructing the expandable medical device 200' in
this manner enables the expandable medical device 200' to be
reduced to a diameter less than the initial fabrication diameter of
the cylindrical tube. By reducing the diameter of the expandable
medical device 200' it can be loaded onto a delivery device such as
any of the delivery catheters available from many different
manufacturers. The expandable medical device 200' may be retained
in a compressed state through the use a removable sleeve or similar
device such as those known to one skilled in the art.
[0050] In use, the expandable medical device 200' would be placed
within a patient's artery in a desired location, the sleeve or
retaining device would be removed and/or activated wherein the
expandable medical device 200' would then expand to the first
diameter of the cylindrical tube which is the origional fabrication
diameter of the expandable medical device 200'. The expandable
medical device 200' will automatically expand to this first
diameter without the need to utilize any expanding means such as an
inflatable balloon disposed within the inner diameter of the
expandable medical device. The expandable medical device 200' can
then be further expanded to engage the pawl and locking teeth,
thereby locking the expandable medical device in an expanded
position at a third diameter.
[0051] Throughout this description reference has been made to
materials that may be utilized in the construction of the
expandable device 200. In a preferred embodiment the expandable
device 200 is constructed of Nitinol alloy. Referring now to FIG.
5, there is shown a stress/strain diagram of a martensitic Nitinol
alloy that may be utilized in the construction of the expandable
device 200. As shown in FIG. 5, the alloy 500 has a very low
Young's Modulus, a low, nearly horizontal "de-twinning" stress
plateau, and a 0.2% offset yield stress (elastic limit) of
approximately 108 ksi. These characteristics provide an
exceptionally large strain range before plastic deformation
commences.
[0052] This exceptionally large strain range, when incorporated in
the present invention, provides the benefit of a device that may be
expanded from an initial diameter to a range of pre-determined
diameters, the range of pre-determined diameters depend only on the
geometry of the locking elements and not on the forces applied by
an expansion means. By utilizing the elastic and "de-twinning"
strain ranges of Nitinol, the expandable device 200 of the present
invention remains below its elastic limit before, during, and after
the deployment of the device within an appropriate location.
[0053] Furthermore, the locking mechanism 280, which includes the
pawl 230, and the teeth 240, provides the ability to decrease the
`recoil` of the expandable device 200. As described in detail
above, stents that have been manufactured of Nitinol exhibit recoil
percentages greater than many other suitable materials, thus
Nitinol has not been commonly utilized for the construction of
expandable stents. The locking mechanism 280 of the present
invention reduces the amount of recoil associated with expandable
Nitinol devices. For example, the expandable device of the present
invention exhibits recoil between about 2% and about 8%, and
preferably between about 4% and about 6%. Additionally, due to the
locking mechanism 280 of the present invention, other materials may
be utilized in the construction of expandable medical devices that
could not have been utilized before due to large recoil percentages
of the material. For example, an expandable device may be
constructed of a polymer, a biodegradable material, or a
combination of a polymer and Nitinol.
[0054] A further feature of the expandable devices in accordance
with the present invention is the reduced necessity to over-expand
the device in service. Prior art stents, and in particular nitinol
stents often required over-expansion during deployment to achieve
an acceptable "minimum lumen diameter," or MLD, post procedure.
This is undesirable for several reasons, including the risk of
collateral damage to the lumen, including perforation, and
increased risk of long-term restenosis of the artery. Once the
desired MLD has been achieved in the present invention, however,
that diameter can be sustained without additional expansion due to
the locking mechanism. At the same time, the present invention
easily permits additional expansion, if required: due to the
plurality of teeth 240 provided on one end of the elongated member
220, the distal end 235 of the pawl 230 may be advanced to engage
another one of the teeth 240, thereby increasing the overall
expanded diameter of the expandable device 200.
[0055] In addition to the embodiments above, it is contemplated
that the expandable device 200/200' may further include a means for
retaining and delivering a beneficial agent. For example, the beams
220 may include a plurality of apertures (not shown) disposed there
through, in which a beneficial agent may be disposed. After the
expandable device 200 has been placed within a vessel/artery, the
beneficial agent is released from the apertures in a controlled
fashion. Furthermore, it is contemplated that the expandable device
200 may be constructed from many different types of materials, such
as, polymers, biodegradable materials, biocompatible materials,
super-elastic alloys or a composite of any of the materials.
[0056] Still further, constructing the expandable medical device
200' of superelastic Nitinol alloy further increases the safety of
the expandable medical device. That is, the locking mechanism of
the expandable medical devices in accordance with the present
invention provides increased resistance to circumferential
compression forces. Examples of such circumferential forces are
artery spasms and elasticity of the artery wall or recoil as
described above. Additionally, constructing the expandable medical
device of superelastic Nitinol alloy allows the device to recover
from deformations caused by forces applied perpendicular to the
longitudinal axis of the device, such as flat plate crushing
forces. For example, the expandable medical device 200' may be
utilized in a peripheral location such as the carotid artery, where
the medical device is more vulnerable to being crushed by an
external force or blow to the patient's neck. A conventional
stainless steel stent cannot be utilized in this location because
once deformed the stent will not return to its expanded state, thus
potentially blocking the artery of which it was intended to
support. By contrast, the expandable medical device 200' in
accordance with the present invention will return to its expanded
and locked state if it is crushed due to an applied force because
of the material properties of superelastic nitinol and the geometry
of the expandable medical device 200'.
[0057] While the invention has been described in detail with
reference to the preferred embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made and equivalents employed, without departing from the
present invention.
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