U.S. patent application number 10/764841 was filed with the patent office on 2004-08-12 for nitinol alloy design for sheath deployable and re-sheathable vascular devices.
Invention is credited to Boylan, John F., Huter, Scott J..
Application Number | 20040158281 10/764841 |
Document ID | / |
Family ID | 24243271 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040158281 |
Kind Code |
A1 |
Boylan, John F. ; et
al. |
August 12, 2004 |
Nitinol alloy design for sheath deployable and re-sheathable
vascular devices
Abstract
An embolic protection device that employs a superelastic alloy
self-expanding strut assembly with a small profile delivery system
for use with interventional procedures is disclosed. The expandable
strut assembly is covered with a filter element and both are
compressed into a restraining sheath for delivery to a deployment
site downstream and distal to the interventional procedure. Once at
the desired site, the restraining sheath is retracted to deploy the
embolic protection device, which captures flowing emboli generated
during the interventional procedure. The expandable strut assembly
is made from a superelastic alloy such as nickel-titanium or
nitinol, and includes a ternary element in order to minimize the
stress hysteresis of the superelastic material. The stress
hysteresis is defined by the difference between the loading plateau
stress and the unloading plateau stress of the superelastic
material. The resulting delivery system including the restraining
sheath has a small profile and has a thin wall.
Inventors: |
Boylan, John F.; (Murrieta,
CA) ; Huter, Scott J.; (Temecula, CA) |
Correspondence
Address: |
FULWIDER PATTON LEE & UTECHT, LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE
TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Family ID: |
24243271 |
Appl. No.: |
10/764841 |
Filed: |
January 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10764841 |
Jan 26, 2004 |
|
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09561747 |
Apr 28, 2000 |
|
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6706053 |
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Current U.S.
Class: |
606/200 |
Current CPC
Class: |
A61F 2230/0067 20130101;
A61F 2002/018 20130101; A61F 2230/0006 20130101; A61F 2/01
20130101 |
Class at
Publication: |
606/200 |
International
Class: |
A61M 029/00 |
Claims
We claim:
1. An embolic filtering system for use in a body lumen, comprising:
a self-expanding strut assembly including a nickel-titanium alloy,
wherein the alloy includes a ternary element selected from the
group of elements consisting of platinum, palladium, or tantalum,
and wherein the alloy further includes a hysteresis curve with a
loading plateau at about 100 ksi to 110 ksi and an unloading
plateau at about 55 ksi to 100 ksi; and a filter element disposed
on the strut assembly.
2. The embolic filtering system of claim 1, wherein the system
includes an elastic sheath at least partially overlying the filter
element.
3. The embolic filtering system of claim 2, wherein the
self-expanding strut assembly is cut from a tube with truncated
diamond shape openings.
4. The embolic filtering system of claim 1, wherein the
self-expanding strut assembly when deployed has a generally conical
shape with a first base, and the filter element when deployed has a
generally conical shape with a second base, and wherein the first
and second bases are joined.
5. The embolic filtering system of claim 1, wherein the
self-expanding strut assembly includes a strut pattern that is
laser cut from a tube.
6. The embolic filtering system of claim 1, wherein the alloy
includes a transition temperature set below human body
temperature.
7. The embolic filtering system of claim 1, wherein the hysteresis
curve includes about a 2:1 ratio of loading stress to unloading
stress.
8. The embolic filtering system of claim 1, wherein the
nickel-titanium alloy exhibits superelasticity while inside the
body lumen.
9. A filtering system for use in a body lumen, comprising: a
self-expanding strut assembly including a nickel-titanium alloy,
wherein the nickel-titanium alloy includes a ternary element
selected from the group of elements consisting of platinum,
palladium, or tantalum conferring a substantially small stress
hysteresis with a ratio of loading plateau stress to unloading
plateau stress is about 2:1 to 1.1:1 and a loading plateau of about
110 ksi; and a filter element disposed on the self-expanding strut
assembly.
10. The filtering system of claim 9, wherein the unloading plateau
is about 55 ksi.
11. The filtering system of claim 9, wherein self-expanding strut
assembly has been heat treated at about 500 degrees C.
12. A filtering system for use in a body lumen, comprising: a
self-expanding strut assembly including a superelastic alloy,
wherein the superelastic alloy includes about 30 to 52 atomic
percent titanium, at least about 38 atomic percent nickel, and
about up to 15 atomic percent of a ternary element selected from
the group of elements consisting of platinum, palladium, or
tantalum, and wherein a stress hysteresis curve of the alloy
includes a loading plateau of about 110 ksi and an unloading
plateau of about 55 ksi; and a filter element disposed on the
self-expanding strut assembly.
13. The filtering system of claim 12, wherein the hysteresis curve
of the alloy includes an absolute .DELTA.y of about 55 ksi.
14. The filtering system of claim 12, wherein the hysteresis curve
of the alloy includes a ratio of loading to unloading plateaus
stresses of about 2:1.
15. The filtering system of claim 12, wherein the self-expanding
strut assembly expands inside the body lumen through shape memory
effect.
16. The filtering system of claim 12, wherein the superelastic
alloy includes a transition temperature below 45 degrees C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending parent
application having Ser. No. 09/561,747 filed on Apr. 28, 2000,
whose entire contents are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to filtering devices
and systems which can be used when an interventional procedure is
being performed in a stenosed or occluded region of a blood vessel
to capture embolic material that may be created and released into
the bloodstream during the procedure. More precisely, the present
invention is directed to filtering devices that include a
superelastic metal that is alloyed with a ternary element to obtain
a desired hysteresis curve that maximizes performance of the
filtering devices.
[0003] The embolic filtering devices and systems of the present
invention are particularly useful when performing balloon
angioplasty, stenting procedures, laser angioplasty or atherectomy
in critical vessels, particularly in vessels such as the carotid
arteries, where the release of embolic debris into the bloodstream
can occlude the flow of oxygenated blood to the brain or other
vital organs, which can cause devastating consequences to the
patient. While the embolic protection devices and systems of the
present invention are particularly useful in carotid procedures,
the inventions can be used in conjunction with any vascular
interventional procedure in which there is an embolic risk.
[0004] A variety of non-surgical interventional procedures have
been developed over the years for opening stenosed or occluded
blood vessels in a patient caused by the build up of plaque or
other substances on the wall of the blood vessel. Such procedures
usually involve the percutaneous introduction of the interventional
device into the lumen of the artery, usually through a catheter. In
typical carotid percutaneous transluminal angioplasty (PTA)
procedures, a guiding catheter or sheath is percutaneously
introduced into the cardiovascular system of a patient through the
femoral artery and advanced through the vasculature until the
distal end of the guiding catheter is in the common carotid artery.
A guide wire and a dilatation catheter having a balloon on the
distal end are introduced through the guiding catheter with the
guide wire sliding within the dilatation catheter. The guide wire
is first advanced out of the guiding catheter into the patient's
carotid vasculature and is directed across the arterial lesion. The
dilatation catheter is subsequently advanced over the previously
advanced guide wire until the dilatation balloon is properly
positioned across the arterial lesion. Once in position across the
lesion, the expandable balloon is inflated to a predetermined size
with a radiopaque liquid at relatively high pressures to radially
compress the atherosclerotic plaque of the lesion against the
inside of the artery wall, thereby dilating the lumen of the
artery. The balloon is then deflated to a small profile so that the
dilatation catheter can be withdrawn from the patient's vasculature
and the blood flow resumed through the dilated artery. As should be
appreciated by those skilled in the art, while the above-described
procedure is typical, it is not the only method used in
angioplasty.
[0005] Another procedure is laser angioplasty which uses a laser to
ablate the stenosis by super heating and vaporizing the deposited
plaque. Atherectomy is yet another method of treating a stenosed
blood vessel in which cutting blades are rotated to shave the
deposited plaque from the arterial wall. A vacuum catheter is
usually used to capture the shaved plaque or thrombus from the
blood stream during this procedure.
[0006] In the procedures of the kind discussed above, abrupt
reclosure or restenosis of the artery may occur over time, which
may then require another angioplasty procedure, a surgical bypass
operation, or some other method of repairing or strengthening the
stenosed area. To reduce the likelihood of the occurrence of abrupt
reclosure and to strengthen the area, a physician can implant an
intravascular prosthesis, commonly known as a stent, for
maintaining vascular patency inside the artery across the lesion.
The stent is crimped tightly onto the balloon portion of a catheter
and transported in its delivery diameter through the patient's
vasculature. At the deployment site, the stent is expanded to a
larger diameter, often by inflating the balloon portion of the
catheter.
[0007] Prior art stents typically fall into two general categories
of construction. The first type of stent is expandable upon
application of a controlled force, as described above, through the
inflation of the balloon portion of a dilatation catheter which,
upon inflation of the balloon or other expansion means, expands the
compressed stent to a larger diameter to be left in place within
the artery at the target site.
[0008] The second type of stent is a self-expanding stent formed
from, for example, shape memory or superelastic alloys including
nickel-titanium (NiTi) alloys, which automatically expand from a
collapsed state when the stent is advanced out of the distal end of
the delivery catheter into the body lumen. Such stents manufactured
from expandable heat sensitive materials allow for phase
transformations of the material to occur, resulting in the
expansion and contraction of the stent.
[0009] The above non-surgical, interventional procedures when
successful avoid the necessity of major surgical operations.
However, there is one common problem associated with all of these
non-surgical procedures. Namely, the potential release of embolic
debris into the bloodstream can occlude the distal vasculature and
cause significant health problems for the patient. In one example,
during deployment of a stent, it is possible that the metal struts
of the stent cut into the stenosis and shear off pieces of plaque
which become embolic debris that travel downstream and lodge
somewhere in the patient's vascular system. In another example,
pieces of plaque can sometimes dislodge from the stenosis during a
balloon angioplasty procedure and become released into the
bloodstream. In yet another example, while complete vaporization of
plaque is the intended goal during a laser angioplasty procedure,
quite often particles are not fully vaporized and thus enter the
bloodstream. Likewise, not all of the emboli created during an
atherectomy procedure are drawn into the vacuum catheter and, as a
result, enter the bloodstream as well.
[0010] When any of the above-described procedures are performed in
the carotid or like arteries, the release of emboli into the
circulatory system can be extremely dangerous and sometimes fatal
to the patient. Debris that is carried by the bloodstream to distal
vessels of the brain can cause these cerebral vessels to occlude,
resulting in a stroke, and in some cases, death. Therefore,
although cerebral percutaneous transluminal angioplasty has been
performed in the past, the number of procedures performed has been
limited due to the justifiable fear of causing an embolic
stroke.
[0011] Medical devices have been developed in an effort to resolve
the problem created when debris or fragments enter the circulatory
system following vessel treatment using any one of the
above-identified procedures. One approach which has had some
limited success is the placement of a filter or trap downstream
from the treatment site to capture embolic debris before it reaches
the smaller blood vessels downstream. Again, there have been
problems associated with such filtering systems, particularly
during the expansion and collapse of the filter within the body
vessel. If the filtering device does not have a suitable mechanism
for closing the filter, there is a possibility that trapped embolic
debris can backflow through the inlet opening of the filter and
enter the bloodstream as the filtering system is collapsed and
removed from the patient. The backflow is caused by the act of
collapsing the filter device, which then squeezes trapped embolic
material through the opening of the filter and back into the
bloodstream.
[0012] Many of the prior art filters that can be expanded within a
blood vessel are attached to the distal end of a guide wire or
guide wire-like tubing. The guide wire or guide wire-like tubing
allows the filtering device to be placed in the patient's
vasculature when the guide wire is manipulated in place. Once the
guide wire is in proper position in the vasculature, the embolic
filter can be deployed within the vessel to capture embolic debris.
The guide wire can then be used by the physician to deliver
interventional devices, such as a balloon angioplasty dilatation
catheter or a stent, into the area of treatment.
[0013] What has been needed is a reliable filtering device and
system for use when treating stenosis in blood vessels which helps
prevent the risk when embolic debris that can cause blockage in
vessels at downstream locations is released into the bloodstream.
The device should be capable of filtering any embolic debris which
may be released into the bloodstream during the treatment and
safely contain the debris until the filtering device is to be
collapsed and removed from the patient's vasculature. The device
should be relatively easy for a physician to use and should provide
a failsafe filtering device that captures and removes any embolic
debris from the bloodstream. Moreover, such a device should be
relatively easy to deploy and remove from the patient's
vasculature. Such important applications as mentioned above have
prompted designers of embolic filtering devices to use superelastic
or shape memory alloys in their designs to exploit the materials'
properties.
[0014] Although not directed to embolic protection devices, an
example of shape memory alloy as applied to stents is disclosed in,
for example, European Patent Application Publication No.
EP0873734A2, entitled "Shape Memory Alloy Stent." This publication
suggests a stent for use in a lumen in a human or animal body
having a generally tubular body formed from a shape memory alloy
which has been treated so that it exhibits enhanced elastic
properties. In particular, in the stress-strain curve exhibiting
loading and unloading of the shape memory alloy material, the
applicant suggests using a composition that results in a large
difference between the loading and unloading curves, otherwise
known as a wide hysteresis.
[0015] The wide hysteresis means that the inward force required to
compress the stent transversely once in place in the lumen is
relatively high, while the outward force that the stent exerts on
the lumen as it attempts to revert to its original undeformed
configuration is relatively low. This can mean that the lumen is
resistant to being crushed by externally applied forces which can
be a problem for lumens close to the surface such as arteries in
the thigh and neck. The publication further suggests use of
specified ternary elements in a nickel titanium alloy to obtain a
stent exhibiting a wider hysteresis in the stress-strain behavior
in a loading and unloading cycle.
[0016] The evolution of superelastic and shape memory alloy devices
progressed to use of ternary elements in combination with nickel
and titanium to obtain specific material properties. Use of a
ternary element in a superelastic stent, as opposed to embolic
protection devices, is shown in, for example, U.S. Pat. No.
5,907,893 to Zadno-Azizi et al. As a general proposition, there
have been attempts at adding a ternary element to nickel-titanium
alloys as disclosed in, for instance, U.S. Pat. No. 5,885,381 to
Mitose et al.
[0017] The conventional efforts of using a ternary element in a
superelastic material for a stent have focused only on a wider
hysteresis in the stress-strain behavior in a loading or unloading
cycle of the stent. Unfortunately, the greater the difference
between the loading and unloading stress plateaus, the stronger the
delivery system must be to accommodate any given level of stent
performance. Typically, a stronger delivery system must also be
larger and bulkier. This is a major drawback to conventional
superelastic stents and delivery systems when the stent must be
delivered through tortuous vessels at remote locations in the human
anatomy.
[0018] What has been needed and heretofore unavailable in the prior
art is a superelastic, removable filtering device and delivery
system that apply a ternary element to the superelastic alloy in
order to minimize the hysteresis. That hysteresis is defined by the
difference between the loading and unloading plateau stresses of
the material as plotted on a stress-strain curve.
INVENTION SUMMARY
[0019] The present invention is generally directed to a number of
filtering devices and systems for capturing embolic debris in a
blood vessel created during the performance of a therapeutic
interventional procedure, such as a balloon angioplasty or stenting
procedure, in order to prevent the embolic debris from blocking
blood vessels downstream from the interventional site. The devices
and systems of the present invention are particularly useful while
performing an interventional procedure in critical arteries, such
as the carotid arteries, in which vital downstream blood vessels
can easily become blocked with embolic debris, including the main
blood vessels leading to the brain. When used in carotid
procedures, the present invention minimizes the potential for a
stroke occurring during the procedure. As a result, the present
invention provides the physician with a higher degree of confidence
that embolic debris is being properly collected and removed from
the patient's vasculature during the interventional procedure.
[0020] An embolic protection device and system made in accordance
with the present invention includes an expandable filter assembly
which is affixed to the distal end of a cylindrical shaft, such as
a guide wire. The filter assembly includes an expandable strut
assembly preferably made from a self-expanding material, such as a
nickel-titanium (NiTi) alloy, and includes a number of outwardly
biased and extending struts that are capable of self-expansion from
a contracted or collapsed position to an expanded or deployed
position within the patient's vasculature. A filter element made
from an embolic capturing media is attached to the expandable strut
assembly. The filter element opens from a collapsed configuration
to an expanded configuration via the movement of the expandable
struts similar to that of an umbrella.
[0021] In particular, the expandable strut assembly of the filter
assembly includes a superelastic alloy, wherein the alloy
optionally includes a ternary element, and wherein the alloy
further includes a substantially small stress hysteresis; and a
delivery system including a sheath having a distal end and a
proximal end, wherein the filter assembly is disposed inside the
sheath at the distal end, and wherein the sheath has a small
profile.
[0022] In an exemplary embodiment, the superelastic alloy includes
binary nickel-titanium alloys that exhibit superelasticity and have
an unusual stress-strain relationship. More precisely, the
superelastic curve is characterized by regions of nearly constant
stress upon loading (referred to as the loading plateau stress) and
unloading (unloading plateau stress). The loading plateau stress is
always larger than the unloading plateau stress. The loading
plateau represents the period during which martensite is being
stress-induced in favor of the original austenitic structure. As
the load is removed, the stress-induced martensite transforms back
into austenite along the unloading plateau.
[0023] The superelastic, self-expanding strut assembly of the
present invention filter assembly is collapsed (that is, loaded)
and then constrained within a delivery system such as a restraining
sheath. At the point of delivery, the restraining sheath is
retracted and the filter assembly is released (that is, unloaded)
and allowed to reassume its original diameter and shape. The filter
assembly is designed to perform various mechanical functions within
the lumen, all of which are based upon the lower unloading plateau
stress.
[0024] Importantly, according to the present invention, a preferred
lower loading plateau stress relative to the unloading plateau
stress of the superelastic material in the self-expanding strut
assembly establishes the mechanical resistance the assembly exerts
against the delivery system. The superelastic material of the
self-expanding strut assembly of the present invention further
exhibits a small hysteresis and a relatively high unloading plateau
stress. The small stress hysteresis defined by the loading and
unloading stress plateaus is preferably accomplished by using a
ternary element in addition to the superelastic alloy.
[0025] As a result, the present invention filter assembly and
delivery system enjoy an overall reduced delivery system profile
for any given level of filter assembly mechanical performance.
Moreover, because of the smaller hysteresis and lower loading
plateau stress relative to the unloading plateau stress for a given
level of performance, the delivery system including the sheath can
be made of a thinner wall material, leading to better
flexibility.
[0026] In addition, the smaller hysteresis and lower loading
plateau stress ensure easy collapse and retraction of the filter
assembly into the delivery system. To be sure, as part of the
delivery system, the recovery sheath used to collapse the deployed
filter assembly can have a smaller profile with a thinner wall,
again improving overall flexibility of the system.
[0027] The present invention is therefore superior to a system that
relies on a wide hysteresis curve. The greater the difference
between the two plateau stresses is, the wider the hysteresis
curve, and the stronger the delivery system must be to accommodate
any given level of self-expanding strut performance. A stronger
delivery system entails a bulkier, larger profile device. The
device by its bulky nature is more inflexible, leading to
difficulties in accessing remote lesions.
[0028] As mentioned above, a preferred superelastic alloy is
nickel-titanium or nitinol. In the exemplary embodiment, the
ternary element may be palladium, platinum, chromium, iron, cobalt,
vanadium, manganese, boron, copper, aluminum, tungsten, tantalum,
or zirconium.
[0029] Other features and advantages of the present invention will
become more apparent from the following detailed description of the
invention when taken in conjunction with the accompanying exemplary
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is an elevational view, partially in cross-section,
of an embolic protection device embodying features of the present
invention showing the expandable filter assembly in its collapsed
position within a restraining sheath and disposed within a
vessel.
[0031] FIG. 2 is an elevational view, partially in cross-section,
similar to that shown in FIG. 1, wherein the expandable filter
assembly is in its expanded position within the vessel.
[0032] FIG. 3 is a perspective view of an expandable strut assembly
which forms part of the filter assembly of the present invention as
shown in its collapsed position.
[0033] FIG. 4 is a plan view of a flattened section of the
expandable strut assembly shown in FIG. 3 which illustrates one
particular strut pattern.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] The present invention is generally directed to a filtering
device and system for capturing embolic debris in a blood vessel
created during the performance of a therapeutic interventional
procedure, such as a balloon angioplasty or stenting procedure, in
order to prevent the embolic debris from blocking blood vessels
downstream from the interventional site. In a preferred embodiment,
the present invention filtering device incorporates superelastic
alloys conferring a small hysteresis curve with high level loading
and unloading plateau stresses.
[0035] Turning now to the drawings, in which like reference
numerals represent like or corresponding elements, FIGS. 1 and 2
illustrate a preferred embodiment embolic protection device 10
incorporating features of the present invention. In the particular
embodiment shown in FIGS. 1 and 2, the embolic protection device 10
is constructed from a filter assembly 12, which includes an
expandable strut assembly 14 and a filter element 16. The filter
assembly 12 is rotatably mounted on the distal end of an elongated
tubular shaft, such as a guide wire 18, for example.
[0036] In the side elevational and cross-sectional views of FIGS. 1
and 2, the embolic protection device 10 is shown positioned within
an artery 20 or other blood vessel of a patient. This portion of
the artery 20 has an area of treatment 22 in which atherosclerotic
plaque 24 has built up against the inside wall 26 of the artery 20.
The filter assembly 12 is placed distal to, and downstream from,
the area of treatment 22. A balloon angioplasty catheter (not
shown) can be introduced within the patient's vasculature in a
conventional Seldinger technique through a guiding catheter (not
shown). The guide wire 18 is passed through the area of treatment
22 and the dilatation catheter can be advanced over the guide wire
18 within the artery 20 until the balloon portion is appositioned
directly in the area of treatment 22. The balloon of the dilatation
catheter is inflated, thereby expanding the plaque 24 against the
inside wall 26 of the artery 20. This opens the occlusion, expands
the artery 20, and reduces the blockage in the vessel caused by the
plaque 24.
[0037] After the dilatation catheter is removed from the patient's
vasculature, a stent 25 (shown in FIG. 2) may be delivered to the
area of treatment 22 using over-the-wire techniques. The stent 25
helps to scaffold and maintain the area of treatment 22, which in
turn help to prevent restenosis from occurring in the area of
treatment 22.
[0038] Any embolic debris 27 that breaks off from the plaque 24
during the interventional procedure is released into the
bloodstream. The embolic debris 27 is carried by blood flow
(indicated by arrows) and is captured by the deployed, i.e.,
unfurled, filter element 16 of the filter assembly 12 located
downstream from the area of treatment 22. Once the interventional
procedure is completed, the filter assembly 12 is collapsed and
removed from the patient's vasculature, taking with it all embolic
debris 27 trapped within the filter element 16.
[0039] One exemplary embodiment of the expandable strut assembly 14
is shown in FIGS. 1-2. As can be seen in these figures, the
expandable strut assembly 14 includes a plurality of radially
expandable struts 28 that can move from a compressed or collapsed
position as shown in FIG. 1 to an expanded or deployed position
shown in FIG. 2.
[0040] The expandable strut assembly 14 is preferably made from a
superelastic material so that the struts 28 have a radially outward
bias toward the expanded position. In the preferred embodiment, the
superelastic material is a nickel-titanium alloy combined with a
ternary element. The alloy is discussed in greater detail
below.
[0041] The expandable strut assembly 14 includes a proximal end 32
which is optionally rotatably attached to the guide wire 18. A
distal end 34 is free to slide longitudinally along the guide wire
18 and can rotate thereabout. The distal end 34 translates along
the guide wire 18 whenever the struts 28 move between the expanded
and contracted positions. A proximal end 32 includes a short
tubular segment or sleeve 36 which has a coil spring formed
therein, and which acts as a dampening member or element 38. The
function of the dampening element 38 is explained below. The distal
end 34 of the tubing 30 preferably includes a short segment or
sleeve 40 which is slidably and rotatably disposed on the guide
wire 18.
[0042] The filter element 16 in one preferred embodiment of the
invention includes a tapered or cone shaped section 50, as seen in
FIGS. 1 and 2. The filter element 16 optionally has a plurality of
openings 53 that allow the blood to perfuse through (indicated by
arrows), yet the openings 53 are small enough that the embolic
debris 27 is captured inside the cone shaped section 50. The filter
element 16 includes a short proximal section 52 which is integral
with the cone shaped section 50 and expands to a substantially
cylindrical shape when the struts 28 of strut assembly 14 are
deployed. An inlet opening 51 located at the short proximal section
52 of cone shaped section 50 collects embolic debris 27, directing
the debris 27 into the filter element 16.
[0043] The short proximal section 52 also functions as a
superstructure to which the filter element 16 and the struts 28 of
the strut assembly 14 can be adhesively or otherwise affixed. At
the opposite end, the filter element 16 has a short distal
cylindrical section 54 which is integral with the remaining
sections of the filter element and is attached to the distal end 34
of the expandable strut assembly 14.
[0044] As best seen in FIG. 1, the filter assembly 12 is maintained
in its collapsed or compressed position through the use of a
restraining sheath 46. The restraining sheath 46 should have
sufficient elasticity to resist the outward bias of the struts 28.
One manner of achieving the required elasticity is through
selection of the proper size and wall thickness for the sheath 46.
Another is through use of the proper elastic material that has
sufficient resilience to resist the expansive forces of the struts
28 held therein. Such sheath materials and designs are known in the
art.
[0045] Although not shown, the guide wire and the restraining
sheath 46 have proximal ends that extend outside of the patient. As
such, the struts 28 can be manipulated into the expanded position
by retracting the restraining sheath 46 via its proximal end to
expose the struts 28. Since the struts 28 are self-expanding by
nature, the withdrawal of the restraining sheath 46 allows the
struts 28 to spring open and the filter element 16 to unfurl into
their expanded positions within the artery 20. This is depicted in
FIG. 2.
[0046] The guide wire 18 optionally includes a small sphere 56
affixed thereto. The small sphere 56 is useful during the delivery
of the embolic protection device 10 into the patient's vasculature.
Specifically, the sphere 56 is approximately as large as the inner
diameter of the restraining sheath 46 and is effectively used as a
nose cone. The nose cone prevents possible "snowplowing" of the
embolic protection device 10 as it is delivered through the
patient's arteries.
[0047] When the embolic protection device 10 is to be removed from
the patient's vasculature, a recovery sheath 48 is used to collapse
and recover the filter assembly 12, as shown in FIG. 2. Generally,
this recovery sheath 48 has a slightly larger inner diameter than
the restraining sheath 46 since the struts 28 are now deployed.
Furthermore, the recovery sheath 48 must have sufficient tensile
strength and elasticity at the distal end 47 to be capable of
collapsing the expanded strut assembly 14.
[0048] The collapse of the expandable strut assembly 14 can be
accomplished by holding the guide wire 18 and moving the proximal
end (not shown) of the recovery sheath 48 forward, which moves the
distal end 47 of the sheath 48 over the struts 28. Alternatively,
the recovery sheath 48 can be held stationary while the proximal
end of the guide wire 18 is retracted back to pull the entire
filter assembly 12 into the sheath 48. Upon collapse of the filter
assembly 12, any embolic debris 27 generated and entering the
bloodstream during the interventional procedure remains trapped
inside the filter element 16 and is withdrawn from the bloodstream
when the embolic protection device 10 is removed from the patient's
vasculature.
[0049] The number of struts 28 formed on the expandable strut
assembly 14 can be any number which provides sufficient
expandability within the artery to properly deploy and maintain the
filter element 16 in place. In the embodiment shown in FIGS. 1 and
2, the expandable strut assembly has four self-expanding struts 28.
Likewise, the particular size and shape of each strut 28 can be
varied.
[0050] FIGS. 3-4 show an expandable strut assembly 14 having a
strut pattern formed from an inverted, triangular shape first
portion 60, a substantially straight center section 62, and a
second inverted triangular shaped section 64, which completes the
strut. This particular strut pattern is one preferred design that
provides greater strength in regions of the strut where there would
be a tendency for the strut to break or become weakened. These
regions include the very proximal and distal ends of each strut
which are designed with a wider base. This particular design also
allows the expandable strut assembly 14 to open and close more
uniformly. This is advantageous especially when collapsing the
struts for removal from the patient. Additionally, the center
section 62 allows the struts 28 to expand to a greater volume,
which allows a larger filter element to be placed on the strut
assembly 14, if needed.
[0051] When the precise pattern is cut into the tubing 30, a sleeve
36 which forms the proximal end 32 may optionally be formed into a
helical coil as shown in FIG. 3. The helical coil then functions as
a damping element 38 for the expandable strut assembly 14. As seen
in FIGS. 1 and 2, the sleeve 36 slides over the guide wire 18. The
proximal end 32 of the expandable strut assembly 14 is mounted
between a tapered fitting 42 and an optional radiopaque marker band
44. The tapered end fitting 42 and the marker band 44 affix the
proximal end 32 on to the guide wire 18 to prevent any longitudinal
motion, yet allow for rotation of the filter assembly 12.
[0052] FIG. 4 is a plan view of a rolled out flat sheet of the
tubing 30 used to form the struts 28. A particular design pattern
is cut into the wall of the tubing 30 in order to form each strut.
In the case of the embodiment shown in FIG. 3, that pattern
consists of a truncated diamond shape 65 which helps form the first
section 60, the center section 62' and the triangular shaped
section 64. By selectively removing portions of the tubing 30
through laser cutting, etching, stamping, or other suitable means,
each particular strut can be fashioned into a precise shape, width,
and length. This truncated diamond pattern 68 repeats, as seen in
FIG. 4, to provide uniform size to each of the struts 28 formed
therein. Narrow struts such as that shown in FIGS. 1 and 2 can, of
course, be formed as described above.
[0053] In a preferred embodiment, the expandable strut assembly 14
of the present invention is formed partially or completely of
alloys such as nitinol (NiTi) which have superelastic (SE)
characteristics. Superelastic alloys are preferably chosen for
their elastic behavior, which as explained above, is used to deploy
the filter element 16.
[0054] The exemplary expandable strut assembly 14 of the present
invention includes a superelastic material. More precisely, the
term "superelastic" refers to an isothermal transformation--that
is, stress inducing a martensitic phase from an austenitic phase.
Alloys having superelastic properties generally have at least two
phases: a martensitic phase, which has a relatively low tensile
strength and which is stable at relatively low temperatures, and an
austenitic phase, which has a relatively high tensile strength and
which is stable at temperatures higher than the martensitic
phase.
[0055] Superelastic characteristics generally allow the expandable
strut assembly 14 to be deformed by collapsing the self-expanding
struts 28, thus creating stress which causes the NiTi to change to
the martensitic phase. The expandable strut assembly 14 is
restrained in the deformed condition by the delivery or restraining
sheath 46 to facilitate the insertion into a patient's body, with
such deformation causing the phase transformation. Once within the
body lumen, the compressive forces of the restraining sheath 46 on
the expandable strut assembly 14 are removed, thereby reducing the
stress therein so that the superelastic expandable strut assembly
14 can return to its original, undeformed shape by the
transformation back to the austenitic phase.
[0056] After the filter assembly 12 has performed its function of
capturing free flowing embolic debris 27 or other friable matter,
the filter assembly 12 is withdrawn from the patient. Prior to this
withdrawal, the recovery sheath 48 is translated distally over the
filter assembly 12; or alternatively, the filter assembly 12 is
pulled proximally into the recovery sheath 48. In either case, the
deployed struts 28 of the expandable strut assembly 14 are
collapsed by the elastic forces of the recovery sheath 48. During
this collapse, the applied stress to the deployed struts 28 changes
their structure from an austenitic to a martensitic phase.
[0057] FIG. 1 also depicts a delivery system having a small
delivery profile P. This reduced profile P is an advantage of the
present invention filter assembly 14 and delivery system
(restraining sheath 46 and recovery sheath 48) as a result of the
stress-strain hysteresis curve of the superelastic material being
minimized. This novel approach is described more fully below.
[0058] The expandable strut assembly 14 is preferably formed from a
superelastic material such as NiTi and undergoes an isothermal
transformation when stressed. The expandable strut assembly 14 and
its struts 28 are first compressed to a delivery diameter, thereby
creating stress in the NiTi alloy so that the NiTi is in a
martensitic state having relatively low tensile strength. While
still in the martensitic phase, the filter assembly 12 is inserted
into the restraining sheath 46 for delivery to area of treatment
22. The NiTi expandable strut assembly 14 tends to spring back to a
larger diameter, and pushes radially outwardly against the inside
diameter of the restraining sheath 46.
[0059] In its delivery diameter P, the overall diameter of the
filter assembly 12 and its restraining sheath 46 is less than the
inside diameter of the artery 20 or the vessel in which they are
inserted. After the filter assembly 12 is delivered to the artery
20 or other vessel, the stress exerted by the struts 28 may be
released by withdrawing restraining sheath 46 in a proximal
direction, whereupon struts 28 immediately expand and return toward
their original, undeformed shape by transforming back to the more
stable austenitic phase.
[0060] When stress is applied to a specimen of a metal such as
nitinol exhibiting superelastic characteristics at a temperature at
or above that which the transformation of the martensitic phase to
the austenitic phase is complete, the specimen deforms elastically
until it reaches a particular stress level where the alloy then
undergoes a stress-induced phase transformation from the austenitic
phase to the martensitic phase. As the phase transformation
progresses, the alloy undergoes significant increases in strain
with little or no corresponding increases in stress. The strain
increases while the stress remains essentially constant until the
transformation of the austenitic phase to the martensitic phase is
complete. Thereafter, further increase in stress is necessary to
cause further deformation. The martensitic metal first yields
elastically upon the application of additional stress and then
plastically with permanent residual deformation.
[0061] If the load on the specimen is removed before any permanent
deformation has occurred, the martensite specimen elastically
recovers and transforms back to the austenitic phase. The reduction
in stress first causes a decrease in strain. As stress reduction
reaches the level at which the martensitic phase transforms back
into the austenitic phase, the stress level in the specimen remains
essentially constant (but less than the constant stress level at
which the austenitic crystalline structure transforms to the
martensitic crystalline structure until the transformation back to
the austenitic phase is complete); i.e., there is significant
recovery in strain with only negligible corresponding stress
reduction. After the transformation back to austenite is complete,
further stress reduction results in elastic strain reduction. This
ability to incur significant strain at relatively constant stress
upon the application of a load and to recover from the deformation
upon the removal of the load is commonly referred to as
superelasticity.
[0062] The prior art makes reference to the use of metal alloys
having superelastic characteristics in medical devices which are
intended to be inserted or otherwise used within a patient's body.
See, for example, U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat.
No. 4,925,445 (Sakamoto et al.).
[0063] FIG. 5 illustrates an example of a preferred stress-strain
relationship of an alloy specimen, such as an expandable strut
assembly 14, having superelastic properties as would be exhibited
upon tensile testing of the specimen. The relationship is plotted
on x-y axes, with the x axis representing strain and the y axis
representing stress. For ease of illustration, the x-y axes are
labeled with typical pseudoelastic nitinol stress from 0 to 110 ksi
and strain from 0 to 9 percent, respectively.
[0064] Looking at the plot itself in FIG. 5, the line from point A
to point B represents the elastic deformation of the specimen.
After point B the strain or deformation is no longer proportional
to the applied stress and it is in the region between point B and
point C that the stress-induced transformation of the austenitic
phase to the martensitic phase begins to occur. There also can be
an intermediate phase, called the rhombohedral phase, depending
upon the composition of the alloy. At point C moving toward point
D, the material enters a region of relatively constant stress with
significant deformation or strain. This constant or plateau region
is known as the loading stress, since it represents the behavior of
the material as it encounters continuous increasing strain. It is
in this plateau region CD that the transformation from austenite to
martensite occurs.
[0065] At point D the transformation to the martensitic phase due
to the application of stress to the specimen is substantially
complete. Beyond point D the martensitic phase begins to deform,
elastically at first, but, beyond point E, the deformation is
plastic or permanent.
[0066] When the stress applied to the superelastic metal is
removed, the material behavior follows the curve from point E to
point F. Within the E to F region, the martensite recovers its
original shape, provided that there was no permanent deformation to
the martensitic structure. At point F in the recovery process, the
metal begins to transform from the stress-induced, unstable,
martensitic phase back to the more stable austenitic phase.
[0067] In the region from point G to point H, which is also an
essentially constant or plateau stress region, the phase
transformation from martensite back to austenite takes place. This
constant or plateau region GH is known as the unloading stress. The
line from point I to the starting point A represents the elastic
recovery of the metal to its original shape.
[0068] Binary nickel-titanium alloys that exhibit superelasticity
have an unusual stress-strain relationship as just described and as
plotted in the curve of FIG. 5. As emphasized above, the
superelastic curve is characterized by regions of nearly constant
stress upon loading, identified above as loading plateau stress CD
and unloading plateau stress GH. Naturally, the loading plateau
stress CD is always larger than the unloading plateau stress GH.
The loading plateau stress represents the period during which
martensite is being stress-induced in favor of the original
austenitic crystalline structure. As the load is removed, the
stress-induced martensite transforms back into austenite along the
unloading plateau stress part of the curve.
[0069] FIG. 5 is also useful for explaining the different
approaches to defining the hysteresis. In one approach, the
difference between the stress values at loading plateau stress CD
and unloading plateau stress GH defines the hysteresis of the
system. This difference is identified as .DELTA.y of the curve in
FIG. 5. If the loading plateau stress CD is at 110 ksi and the
unloading plateau stress GH is at 55 ksi, then the .DELTA.y of the
curve is the difference between 110 and 55, which is 55 ksi. Under
this approach, .DELTA.y of the curve is defined as an "absolute"
difference in stress plateau values. This absolute difference
definition of .DELTA.y is commonly used in the superelastic
materials industry.
[0070] In an alternative approach, the hysteresis of the curve may
be defined as a ratio of the unloading plateau stress GH to the
loading plateau stress CD. Many design engineers adopt this
definition of hysteresis in their work with superelastic alloys.
Referring to the FIG. 5 example, the hysteresis of the curve under
this alternative definition is expressed as the ratio of 55 ksi to
110 ksi, or 1:2. An example of a more preferable hysteresis ratio
would be an unloading plateau stress GH of 100 ksi to a loading
plateau stress CD of 110 ksi for a smaller hysteresis ratio of
1:1.1. Therefore, the hysteresis of the curve can be defined by the
"relative" magnitudes of the loading and unloading plateau
stresses.
[0071] Under either definition of hysteresis of the curve described
above, the present invention seeks to minimize the hysteresis of
the superelastic material used to fabricate the expandable strut
assembly 14. The expandable strut assembly 14 of the present
invention embolic protection device 10 is preferably constructed
with a superelastic material having a low loading plateau stress CD
relative to the unloading plateau stress GH. This is contrary to
the teachings of the prior art.
[0072] A higher loading plateau stress CD establishes the
mechanical resistance the expandable strut assembly 14 exerts
against the delivery system, and specifically the restraining
sheath 46. It represents the stress exerted by the expandable strut
assembly 14 when it is loaded into restraining sheath 46. A high
loading plateau stress relative to the unloading plateau stress is
undesirable because of the large and bulky restraining sheath 46
that is needed to deliver the embolic protection device 10.
[0073] In FIG. 5, the segment labeled "deploy" represents release
of the expandable strut assembly 14 from the delivery sheath 46.
Following the arrows, the unloading plateau stress GH represents
the stress exerted by the deployed expandable strut assembly 14
against the vessel wall 26. It represents the expanding force
available to unfurl the filter element 16, which is one measure of
expandable strut performance. When the recovery sheath 48 is moved
over the expandable strut assembly 14 to recover the device, the
stress follows the segment labeled "recover" in FIG. 5 back to
loading plateau stress CD.
[0074] Accordingly, the greater the difference (i.e., relative or
absolute hysteresis) between the two plateaus CD and GH is, the
stronger the delivery system must be to accommodate any given level
of expandable strut assembly performance. A stronger delivery
system must necessarily be larger and bulkier, with a thicker, more
rigid restraining sheath 46. Conversely, reducing the (relative or
absolute hysteresis) difference between the two plateaus CD and GH
results in smaller hysteresis. The smaller the hysteresis is, the
smaller and lower profile the delivery system has to be to
accommodate any given level of expandable strut assembly
performance.
[0075] In accordance with the present invention, the filter
assembly 12 requires only a delivery system having a small delivery
profile P as illustrated in the cross-sectional view of FIG. 1.
Furthermore, the wall thickness of restraining sheath 46 can be
reduced as compared to a comparable performance expandable strut
assembly 14 not employing the present invention. Such a compact
delivery system permits the physician better access and flexibility
to reach tortuous arteries and vessels.
[0076] In sum, the present invention offers the potential to reduce
overall delivery profile defined by loading stress CD for any given
level of embolic protection device mechanical performance defined
by unloading stress GH. In the present invention, this is
accomplished by realizing the properties of superelastic nitinol,
preferably in addition with a ternary element, as described in
greater detail below.
[0077] The superelastic alloy of the present invention is
preferably formed from a composition consisting essentially of
about 30 to about 52 percent titanium and the balance nickel and up
to 15 percent of one or more additional ternary alloying elements.
Such ternary alloying elements may be selected from the group
consisting of palladium, platinum, chromium, iron, cobalt,
vanadium, manganese, boron, copper, aluminum, tungsten, tantalum,
or zirconium. In particular, the ternary element may optionally be
up to 3 percent each of iron, cobalt, platinum, palladium, and
chromium, and up to about 15 percent copper and vanadium. As used
herein, all references to percent composition are atomic percent
unless otherwise noted.
[0078] In another preferred embodiment, a NiTi expandable strut
assembly of the embolic protection device with SME (shape memory
effect) is heat-treated at approximately 500 degrees C. The
expandable strut assembly is mechanically deformed into a first,
larger diameter and form for deployment of the filter element.
After the filter assembly is exposed within the body lumen after
retraction of the restraining sheath, heat at 45 degrees C. is
applied causing the filter assembly to return to its fully expanded
larger diameter and be in contact with the arterial wall of the
artery. The application of 45 degrees C. of heat is compatible with
most applications in the human body, but it is not to be limited to
this temperature as higher or lower temperatures are contemplated
without departing from the invention. The 45 degrees C. temperature
can be achieved in a conventional manner well known in the art such
as by warm saline injected into the delivery system.
[0079] The shape memory characteristics allow the devices to be
deformed to facilitate their insertion into a body lumen or cavity
and then to be heated within the body so that the device returns to
its original shape. Again, alloys having shape memory
characteristics generally have at least two phases: a martensitic
phase, which has a relatively low tensile strength and which is
stable at relatively low temperatures, and an austenitic phase,
which has a relatively high tensile strength and which is stable at
temperatures higher than the martensitic phase.
[0080] Shape memory characteristics are imparted to the alloy by
heating the metal to a temperature above which the transformation
from the martensitic phase to the austenitic phase is complete;
i.e., a temperature above which the austenitic phase is stable. The
shape of the metal during this heat treatment is the shape
"remembered." The heat-treated metal is cooled to a temperature at
which the martensitic phase is stable, causing the austenitic phase
to transform to the martensitic phase. The metal in the martensitic
phase is then plastically deformed, e.g., to facilitate the entry
thereof into a patient's body. Subsequent heating of the deformed
martensitic phase to a temperature above the martensite to
austenite transformation temperature causes the deformed
martensitic phase to transform to the austenitic phase. During this
phase transformation the metal reverts back to its original
shape.
[0081] The recovery or transition temperature may be altered by
making minor variations in the composition of the metal and in
processing the material. In developing the correct composition,
biological temperature compatibility must be determined in order to
select the correct transition temperature. In other words, when the
stent is heated, it must not be so hot that it is incompatible with
the surrounding body tissue. Other shape memory materials may also
be utilized, such as, but not limited to, irradiated memory
polymers such as autocrosslinkable high density polyethylene
(HDPEX). Shape memory alloys are known in the art and are discussed
in, for example, "Shape Memory Alloys," Scientific American, Vol.
281, pp. 74-82 (November 1979).
[0082] Shape memory alloys undergo a transition between an
austenitic state and a martensitic state at certain temperatures.
When they are deformed while in the martensitic state they retain
this deformation as long as they are retained in this state, but
revert to their original configuration when they are heated to a
transition temperature, at which time they transform to their
austenitic state. The temperatures at which these transitions occur
are affected by the nature of the alloy and the condition of the
material. Nickel-titanium-based alloys (NiTi), wherein the
transition temperature is slightly lower than body temperature, are
preferred for the present invention. It is thus desirable to have
the transition temperature set at just below body temperature to
insure a rapid transition from the martensitic state to the
austenitic state when the embolic protection device of the present
invention is deployed in a body lumen.
[0083] While the present invention has been illustrated and
described herein in terms of superelastic alloy components of a
filter assembly of an embolic protection device and its delivery
system wherein the superelastic alloy contains a ternary element
conferring a small hysteresis, it is apparent to those skilled in
the art that the present invention can be used in other instances.
Other modifications and improvements may be made without departing
from the scope of the present invention.
* * * * *