U.S. patent application number 12/721834 was filed with the patent office on 2010-07-01 for expandable body cavity liner device.
This patent application is currently assigned to BOSTON SCIENTIFIC SCIMED, INC.. Invention is credited to Alejandro Berenstein, Joseph C. Eder.
Application Number | 20100168781 12/721834 |
Document ID | / |
Family ID | 28452693 |
Filed Date | 2010-07-01 |
United States Patent
Application |
20100168781 |
Kind Code |
A1 |
Berenstein; Alejandro ; et
al. |
July 1, 2010 |
EXPANDABLE BODY CAVITY LINER DEVICE
Abstract
The present invention is an aneurysm treatment device for
treating aneurysms of various shapes and sizes.
Inventors: |
Berenstein; Alejandro; (New
York, NY) ; Eder; Joseph C.; (Los Altos Hills,
CA) |
Correspondence
Address: |
VISTA IP LAW GROUP LLP
12930 Saratoga Avenue, Suite D-2
Saratoga
CA
95070
US
|
Assignee: |
BOSTON SCIENTIFIC SCIMED,
INC.
Maple Grove
MN
|
Family ID: |
28452693 |
Appl. No.: |
12/721834 |
Filed: |
March 11, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10107689 |
Mar 27, 2002 |
7695488 |
|
|
12721834 |
|
|
|
|
Current U.S.
Class: |
606/191 |
Current CPC
Class: |
A61B 2017/1205 20130101;
A61B 17/12113 20130101; A61B 17/12136 20130101; A61B 17/12177
20130101; A61B 17/12022 20130101; A61B 17/12172 20130101 |
Class at
Publication: |
606/191 |
International
Class: |
A61M 29/00 20060101
A61M029/00 |
Claims
1. A aneurysm occlusion device, comprising: an expandable liner for
delivery within an aneurysm; and a first extender coil disposed
within said expandable liner, said first extender coil having a
restrained shape in axial alignment with a delivery device when
restrained by said delivery device, and a curved relaxed shape when
removed from the delivery device such that said first extender coil
bears against the expandable liner causing the expandable liner to
expand toward the walls of an aneurysm.
2. The aneurysm occlusion device of claim 1, further comprising a
second extender coil disposed with said expandable liner, said
second extender coil having a restrained shape in axial alignment
with an delivery device when restrained by the delivery device, and
a curved relaxed shape when removed from the delivery device such
that said second extender coil bears against the expandable liner
causing the expandable liner to expand toward the walls of an
aneurysm
3. The aneurysm occlusion device of claim 2, wherein said first
extender coil and said second extender coil are biased to extend in
opposite directions when removed from the delivery device.
4. The aneurysm occlusion device of claim 2, further comprising a
reinforcing layer disposed around a proximal portion of said
expandable liner to strengthen the proximal portion of the
expandable liner to prevent the expandable liner from expanding
through a neck of an aneurysm and into a parent vessel of the
aneurysm upon inflation of the expandable liner within the
aneurysm.
5. The aneurysm occlusion device of claim 1, wherein the expandable
liner is perforated to allow blood to escape from the expandable
liner.
6. The aneurysm occlusion device of claim 1, wherein the expandable
liner is configured to be detachably connected to a delivery
catheter.
7. An aneurysm occlusion device, comprising: an elongated delivery
catheter having a proximal end, a distal end and a lumen
therethrough; an expandable liner for delivery within an aneurysm,
said expandable liner detachably connected to the distal end of
said elongated catheter; a guidewire extending through said lumen
of said delivery catheter and into said expandable liner; a first
extender coil disposed within said expandable liner and over said
guidewire, said first extender coil having a restrained shape in
axial alignment with said guidewire when restrained by said
guidewire, and a curved relaxed shape when said guidewire is
withdrawn from said first extender coil such that said first
extender coil bears against the expandable liner causing the
expandable liner to expand toward the walls of an aneurysm.
8. The aneurysm occlusion device of claim 7, further comprising a
second extender coil disposed with said expandable liner and over
said guidewire, said second extender coil having a restrained shape
in axial alignment with said guidewire when restrained by said
guidewire, and a curved relaxed shape when said guidewire is
withdrawn from said second extender coil such that said second
extender coil bears against the expandable liner causing the
expandable liner to expand toward the walls of an aneurysm
9. The aneurysm occlusion device of claim 8, wherein said first
extender coil and said second extender coil are biased to extend in
opposite directions when removed from the delivery device.
10. The aneurysm occlusion device of claim 8, further comprising a
reinforcing layer disposed around a proximal portion of said
expandable liner to strengthen the proximal portion of the
expandable liner to prevent the expandable liner from expanding
through a neck of an aneurysm and into a parent vessel of the
aneurysm upon inflation of the expandable liner within the
aneurysm.
11. The aneurysm occlusion device of claim 7, wherein the
expandable liner is perforated to allow blood to escape from the
expandable liner.
12. The aneurysm occlusion device of claim 7, wherein the
expandable liner is configured to be detachably connected to a
delivery
Description
RELATED APPLICATION DATA
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 10/107,689, filed Mar. 27, 2002, the priority
of which is claimed under 35 U.S.C. .sctn.120, and the contents of
which is incorporated herein by reference in its entirety, as
though set forth in full.
BACKGROUND OF THE INVENTION
[0002] The present invention deals with a system for treating a
vascular cavity. More specifically, the present invention is
directed to vascular cavity liners and vascular cavity neck
bridges.
[0003] While the present discussion proceeds with respect to
aneurysms, it will be appreciated that it can be applied to other
vascular cavities (such as vessels, lumens, etc.) as well. An
aneurysm or vascular malformation is a localized stretching or
distension of an artery due to a weakening of the vessel wall. For
example, "berry" aneurysms, i.e., small spherical distensions,
occur in the vessels of the brain. The distension--often referred
to as the aneurysm sac--is related to defects in the muscular
coating of the artery and is probably degenerative in origin.
Rupture of aneurysms account for the majority of spontaneous
hemorrhages. Approximately 25,000 intracranial aneurysms rupture
every year in North America.
[0004] Several methods of treating aneurysms have been attempted,
with varying degrees of success. At present, the treatment of
aneurysms with drugs is substantially ineffective. Also,
extra-vascular surgery, referred to as open craniotomy, for the
purpose of preserving the parent artery is replete with
disadvantages. A patient subject to open craniotomy for
intercranial aneurysms typically must undergo general anesthesia,
surgical removal of part of the skull, brain retraction, dissection
around the neck of the sac, and placement of a clip on the parent
artery to prevent bleeding or rebleeding.
[0005] Alternative treatments include endovascular occlusion where
the interior of the aneurysm is entered with a guidewire or a
microcatheter. An occlusion is formed within the sac with an
intention to preserve the parent artery. One means for forming a
mass is through the introduction of an embolic agent within the
sac. Examples of embolic agents include a detachable coil, which is
detached from the end of a guidewire, a liquid polymer which
polymerizes rapidly on contact with blood to form a firm mass, and
embolic particles.
[0006] Endovascular occlusion is not without drawbacks. For
example, there is a risk of overfilling the sac and consequent
embolic agent migration into the parent vessel. Overfilling of the
sac also generates additional pressure in the aneurysm.
[0007] Another means for forming a mass in the aneurysm sac
involves the placement of an elastic, expandable balloon or liner
in the aneurysm. Detachable occlusion balloons have been used for a
number of medical procedures. These balloons are carried at the end
of a catheter and, once inflated can be detached from the catheter.
Such a balloon may be positioned within an aneurysm, filled and
then detached from the catheter. Deploying the balloon within the
aneurysm can be rather difficult due to the high rates of blood
flow through the aneurysm. Elastic balloons have exhibited problems
with respect to performance and have not been used endovascularly
in some time.
[0008] This aneurysm filling technique also has its problems. As
the balloon is filled, the operator must be very careful not to
overfill the balloon due to possible risk of rupturing the
aneurysm. Accordingly, the balloon may be too small, potentially
resulting in the release of the balloon from the aneurysm into the
blood stream. Furthermore, the balloon often does not mold or shape
to the odd-shaped contours of the aneurysm leaving room for blood
to continue flowing through the aneurysm, or generating undesired
pressure on the aneurysm wall.
[0009] Aneurysm liners are composed of a liner sac which is placed
in the aneurysm and filled to occlude the aneurysm. A guidewire is
inserted in the liner. The guidewire carries the liner through the
vasculature to deploy the liner in the aneurysm.
[0010] All of the present systems for treating aneurysms have
disadvantages as well. For example, while the aneurysm liner
concept is intuitively attractive, it has posed a number of
technical challenges. One primary challenge involves the difficulty
in producing a material that is robust enough to contain embolic
material without inhibiting the ability of the embolics to conform
to the aneurysm geometry itself, rather than the geometry of the
liner. For example, the elastic materials discussed above generally
require to much force to deform, and inelastic materials that
deform readily do not have adequate memory to conform to the
aneurysmal wall.
[0011] Different types of aneurysms also present different
challenges. For example, aneurysms which have a particularly wide
opening between the aneurysm sac and the parent vessel ("wide neck
aneurysms") present difficulties concerning the retention of
embolic materials. Specifically, wide neck aneurysms make if very
difficult to maintain the embolics, or occlusive materials, within
the aneurysmal sac. This is especially true of liquid embolic
materials. Of course, should the embolic material enter the parent
vessel, it poses an undesirable risk of occlusion in the parent
vessel.
[0012] Some current aneurysm liner concepts are inadequate in
treating larger aneurysms.
[0013] For example, some liner concepts involve forming the
aneurysm liner of a woven or braided polymeric material such as
polypropylene, polyester, nylon, urethane, teflon, etc. However,
these mesh materials are difficult to use in treating aneurysms
larger than, for example, twelve millimeters in diameter. Such mesh
materials result in an assembly which is too bulky when collapsed
down onto the catheter for delivery. In other words, the amount of
materials required to fill a relatively large aneurysm is very
difficult to collapse down into a constrained, low profile,
delivery configuration small enough to be delivered and deployed
without excess friction on the walls of the delivery catheter or
other delivery lumen.
SUMMARY OF THE INVENTION
[0014] The present invention is a vascular cavity treatment device
for treating vascular cavities of various shapes and sizes and will
be discussed by way of example as an aneurysm treatment device.
[0015] In one embodiment, the aneurysm treatment device includes an
aneurysm liner formed of material having very low yield strength
and very low elasticity so that, with a relatively low amount of
internal pressure exerted by, for example, embolic material, the
aneurysm liner readily plastically deforms to the internal geometry
of the aneurysm sac. A second, reinforcing layer is deployed on the
first material. The reinforcing layer is more elastic than the
first material and has a much higher yield strength. The
reinforcing layer is illustratively disposed at the neck of the
aneurysm liner device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIGS. 1A-1C illustrate the deployment of an aneurysm liner
in an aneurysm.
[0017] FIGS. 2A-2C illustrate an embodiment of an aneurysm liner
being formed of materials with two different characteristics, one
of them having a very low yield strength and the other having a
high yield strength and a greater elasticity.
[0018] FIG. 2D illustrates the embodiment shown in FIGS. 2A-2C,
with perforations therein.
[0019] FIGS. 3A-3C illustrate an embodiment of an aneurysm liner
being formed of a balloon material having two different
characteristics, portions thereof being weaker than other portions
thereof.
[0020] FIG. 3D illustrates the embodiment shown in FIGS. 3A-3C with
perforations therein.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0021] FIGS. 1A-1C illustrate a portion of an aneurysm treatment
device 10 in a vessel 12 which has an aneurysm 14 therein, and thus
illustrate the general context of the present invention. Though the
embodiments discussed herein are discussed in conjunction with an
aneurysm, it will be appreciated that they can be used in
substantially any vascular cavity or other bodily cavities.
Aneurysm 14 is defined by aneurysmal sac 16 and neck 18. Device 10
includes, in the embodiment illustrated, delivery catheter 18, a
pair of extender coils 21 and 22 and an expandable liner 24
(aneurysm liner sac). Delivery catheter 20 has a proximal end that
extends proximally to a position where it is manipulable by an
operator. The distal end of catheter 20 is releaseably connected to
the liner 24 and coil 21. Coils 21 and 22 can either be attached to
the liner or catheter, or unattached. In addition, there can also
be one or more coils disposed between coils 21 and 22 and axially
aligned therewith.
[0022] When in the insertion position shown in FIG. 1A, coils 21
and 22 (and other optional coils therebetween) are axially aligned
with one another, their length is sufficient to substantially hold
liner 24 in a low profile position for insertion and manipulation
within the vasculature. In one embodiment, coils 21 and 22 are
axially aligned with one another and with catheter 20 through the
use of a guidewire 26 which is disposed within the lumen of
catheter 20, through coils 21 and 22 and liner 24, and out the
distal end of catheter 22 and liner 24. Coils 21 and 22 are held in
an axially aligned conformation by guidewire 26 such that coils 21
and 22 substantially conform to the curvature of guidewire 26.
Coils 21 and 22, rather than guidewire 26, can act to extend and
even tension liner 24.
[0023] FIG. 1B shows that treatment device 10 has been positioned
through vessel 12 and neck 18 into the sac 16 of aneurysm 14.
Similar items are similarly numbered to those shown in FIG. 1A. In
use, aneurysm treatment device 10 can be preloaded or back loaded
onto guidewire 26. Guidewire 26 is manipulated through the
vasculature from the entry site (such as the femoral artery) to the
region of vessel 12 containing the aneurysm. The distal tip of
guidewire 26 is advanced across the neck 18 of aneurysm 14 and into
the aneurysm sac 16. This can be done using any desirable
visualization technique. In one embodiment, catheter 20 is placed
over guidewire 26 prior to positioning guidewire 26 in the
vasculature, with several centimeters of guidewire 26 extending
distal of the distal tip of catheter 20. Therefore, when the distal
end of guidewire 26 has passed the aneurysm neck 18, catheter 20 is
positioned just proximal of neck 18. Treatment device 10 is then
advanced into the aneurysm sac 16.
[0024] In another embodiment, guidewire 26 is placed in the
vasculature first. Once the distal end of guidewire 26 is moved
past the aneurysm neck 18, into the aneurysm sac 16, catheter 20 is
advanced over guidewire 26 such that the extender coils 21 and 22
are pushed distally along the guidewire by the catheter 20 until
the aneurysm treatment device 10 is in place in the aneurysm sac
16.
[0025] FIG. 1C illustrates treatment device 10 deployed in aneurysm
sac 16 in accordance with one embodiment. Similar items are
similarly numbered to those shown in FIGS. 1A and 1B. Once device
10 is substantially fully within aneurysm sac 16, guidewire 26 is
retracted proximally, but liner 24 remains connected to delivery
catheter 20. The distal end of delivery catheter 20 holds
expandable liner 24 in position within the aneurysm sac 16 while
expandable liner 24 is filled with embolics. Expansion of liner 24
occurs after the distal end of guidewire 26 is retracted from the
coils 21 and 22.
[0026] As shown in FIG. 1C, once guidewire 26 has been retracted,
coils 21 and 22 recoil away from axial alignment with one another
toward the periphery of liner 24. In one illustrative embodiment,
coils 21 and 22 are biased to extend in opposite directions to
enhance deployment of, and expansion of, liner 24 within aneurysm
sac 16. If any coils are disposed between coils 21 and 22 on
guidewire 26, they simply fall away and float within liner 24.
Embolic material can now be introduced into liner 24 through
catheter 20 using substantially any desired method. Such methods
include, for example, advancing coils or particles into liner 24,
pushing the embolic material into catheter 20 with guidewire 26
completely removed, or infusing or injecting embolic material
through catheter 20 into liner 24. Liner 24 is thus filled with a
common embolic agent, such as detachable coils, particles,
acrylics, hydrogel, etc.
[0027] Once liner 24 is filled, it is unable to be removed through
aneurysm neck 18. Therefore, it is released from delivery catheter
20 and delivery catheter 20 is removed from the treatment site.
Detachment of liner 24 from catheter 20 can be accomplished using
any desired method, such as using electrolytic detachment,
traction-based detachment, or other mechanical, electrical,
heat-based, magnetic, chemical or other detachment.
[0028] FIGS. 1A-1C illustrate that device 10 is configured for
convenient treatment of aneurysm 14, and in particular, a generally
symmetrically shaped aneurysm. However, asymmetrically shaped
aneurysm sacs, or those having an otherwise irregular geometrical
shape present other problems. For example, if aneurysm sac 16 had a
cavity extending out one side thereof, it may be difficult for
liner 24 to fill that portion of the aneurysm sac.
[0029] FIGS. 2A and 2B illustrate yet another embodiment of an
aneurysm treatment device 40 in accordance with another embodiment
of the present invention. Aneurysm treatment device 40 is similar,
in many ways, to the previous embodiments, in that it can
illustratively include interior extender coils 21 and 22 (and
optional coils therebetween) and can be positioned over a guidewire
26 using a detachable delivery catheter 20. Treatment device 40
also illustratively includes a liner 24.
[0030] However, treatment device 40 also includes other or
different features. FIG. 2A shows treatment device 40 having
already been positioned within an asymmetrical aneurysm sac 16,
which has a highly irregular geometry. FIG. 2A shows that treatment
device 40 not only includes liner (first portion) 24, but also
illustratively includes a reinforcing layer (or second portion) 42.
Liner 24 and layer 42 are described in greater detail below.
[0031] FIG. 2B shows partial deployment of aneurysm liner 24 after
guidewire 26 has been removed and extender coils 21 and 22 fall
away from axial alignment with one another. Liner 24 is also at
least partially expanded to the position shown in FIG. 2B through
the introduction of embolic material therein to slightly elevate
the internal pressure in liner 24 above ambient (e.g., 0-1 ATM),
using catheter 20.
[0032] In accordance with one embodiment of the present invention,
liner 24 is illustratively formed of a polymer that has a very low
yield strength and a low elasticity so that, with a minimal amount
of additional force exerted by the embolic material (e.g., 0-5 ATM
and illustratively 0-2 ATM or 1-2 ATM), the polymer material
forming liner 24 readily plastically expands to conform to the
interior perimeter of aneurysmal sac 16. This is illustrated in
FIG. 2C. In other words, liner 24 is formed of a polymer having
characteristics such that by the continued introduction of embolic
material into liner 24, liner 24 simply permanently deforms to
assume the shape of the aneurysm sac 16. The material which forms
liner 24 also has sufficient ultimate failure strength so as not to
tear during delivery or expansion thereof
[0033] In addition, reinforcement layer 42 is more elastic and of a
much higher yield strength. Reinforcement layer 42 is
illustratively located in the region of aneurysm liner 24 close to
its attachment point to catheter 20 (neck portion). This ensures
that it will be located preferentially near aneurysm neck 18 in
order to prevent aneurysm liner 24 from expanding through neck 18,
and into parent vessel 12. Thus, the distal end of treatment device
40 can easily expand into the irregular geometrical portions of the
aneurysmal sac, while the proximal portion thereof does not deform
as easily and thus prevents deformation into parent vessel 12.
Reinforcement layer 42 can also be discontinuous or formed of a
braid or mesh or polymer material or other reinforcing material and
can be radiopaque as well.
[0034] FIG. 2D shows another embodiment of treatment device 40 with
perforations formed therein. These perforations allow blood to
escape from the aneurysmal sac, through liner 24, and reinforcing
layer 42, into the parent vessel 12, as liner 24 is expanded.
However, as with the previous embodiments, the perforations are not
necessary and the blood can simply escape around the outside of
device 40 and through neck 18. Also, the perforations are shown as
being larger distally, to allow distal permeation of embolics,
although this is optional as well.
[0035] For example, spherical PVA embolics may traditionally be 500
microns in size and may be used to fill a conventional aneurysm
liner. The distal portion of device 40 can thus be perforated with
750 micron holes whereas the proximal portion near the neck 18 of
aneurysm sac 16 can illustratively be perforated with 350 micron
sized, irregularly distributed, holes. Therefore, as the embolics
are introduced into liner portion 24, they are sized to be able to
escape the distal end thereof and or occupy the irregular spaces in
the aneurysm sac 16, without escaping back into the parent vessel
12.
[0036] FIGS. 3A-3C illustrate another embodiment of an aneurysm
treatment device 50 in accordance with one aspect of the present
invention. Similar items are similarly numbered to those shown in
previous Figures. Treatment device 50 is similar, in many ways, to
the previous embodiments in that it can illustratively include
interior extender coils 21 and 22 (and optional coils therebetween)
and can be positioned over a guidewire 26 using a detachable
delivery catheter 20. Treatment device 50 also illustratively
includes a liner 51.
[0037] However, treatment device 50 also illustratively includes
other or different features. FIG. 3A shows treatment device 50
having already been positioned within an asymmetrical aneurysm sac
52, which has a highly irregular geometry.
[0038] In the embodiment shown in FIG. 3A, liner 51 is
illustratively formed as a detachable balloon. The balloon material
illustratively has a plurality of areas 54 disposed on its surface
which are weaker (or more elastic) than the remainder of the
surface of liner 51. In one illustrative embodiment, areas 54 are
simply formed of thinner balloon material than the remainder of
liner 51. Of course, they could be formed of different, more
elastic (or weaker) material, or the remainder of liner 51 (other
than areas 54) can be enclosed in a braid, a mesh, a polymer
material or otherwise coated with a material which precludes that
portion of liner 51 from expanding beyond a predetermined geometry
and may be radiopaque as well.
[0039] FIG. 3B illustrates aneurysm treatment device 50 expanded
under a first predetermined pressure. In one illustrative
embodiment, liner 51 is inflated with a contrast medium, or saline
solution, or another fluid introduced through catheter 20. As the
pressure in liner 51 increases, liner 51 inflates to a first
predetermined dimension in which areas 54 are not expanded beyond
the remainder of liner 51.
[0040] FIG. 3C illustrates liner 51, after it has been subjected to
additional internal pressure. It can be seen that liner 51 has now
assumed an irregular shape because the weaker regions 54 have
expanded to fill void spaces of aneurysm sac 52. This allows liner
51 to substantially fill even irregularly shaped aneurysm sac 52.
FIG. 3C also illustrates that, in a region of liner 51 proximate
neck 18 of the aneurysm there are no weak zones 54. This helps to
preclude any portion of the aneurysm liner 51 from expanding into
parent vessel 12, and thereby fully or partially occluding the
vessel.
[0041] It should also be noted that, in one illustrative
embodiment, liner 51 need not even substantially fill the entire
aneurysm sac 52. Instead, liner 51 can simply be inflated to a
geometry in which enough of the weaker regions 54 have been
expanded into void spaces or lobes of aneurysm sac 52 to securely
anchor liner 51 within aneurysm sac 52 and to block the inflow zone
through neck 18. In that embodiment, even if the entire aneurysm
sac 52 is not filled, the neck 18 is blocked and device 50 is
anchored in place to inhibit further growth of the aneurysm.
[0042] In another illustrative embodiment, aneurysm liner 51 can be
filled with embolics or other polymeric materials, or coils. This
may enhance the long term stability of liner 51 within aneurysm sac
52.
[0043] FIG. 3D is another illustrative embodiment of the present
invention. FIG. 3D is similar to the embodiment illustrated in
FIGS. 3A-3C, except that it has perforations therein. Weak regions
54 are illustrated by dashed lines while the perforations are
illustrated by either points or circular or oval shaped regions.
The perforations allow introduced embolic material to seek out the
void spaces, or irregular lobes, of aneurysm sac 52. The
perforations also allow blood that is being displaced in aneurysm
sac 52 to re-enter parent vessel 12 through aneurysm liner 51.
Further, the embodiment in FIG. 3D shows that the perforations can
be formed to preferentially permeate embolics distally. In other
words, the distal perforations are larger than the proximal
perforations such that embolics can permeate the distal
perforations but not the proximal perforations. However, the
presence of the perforations are optional, as is the sizing of any
perforations which may be used.
[0044] Further, weak regions can be other shapes as well, such as
annular rings around liner 51, axial stripes or substantially any
geometric shape.
[0045] It should further be noted that all of the embodiments
discussed herein can optionally have biodegradable, cell growth
enhancing material such as polyglycolic acid (PGA) or polylactic
acid (PLA) disposed thereon in a region that will illustratively be
deployed in a neck region of the aneurysm. Of course, other
material or combinations of these materials may be used as
well.
[0046] Also, the devices described herein can be releasably
attached to guidewire 26 instead of the catheter.
[0047] It can thus be seen that the present invention provides a
number of different embodiments for treating aneurysms. These
embodiments address many of the various deficiencies and
disadvantages associated with prior aneurysm treatment devices.
[0048] Although the present invention has been described with
reference to illustrative embodiments, workers skilled in the art
will recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *