Self expanding bifurcated endovascular prosthesis Number:7,520,895 from the United States Patent and Trademark Office (PTO) owispatent Disclosed is a bifurcated tubular endoluminal vascular prosthesis, useful in treating, for example, an abdominal aortic aneurysm. The prosthesis comprises a self expandable wire support structure surrounded at least in part by a flexible tubular membrane. A delivery catheter and methods are also disclosed.<H endovascular, prosthesis, Self, expanding, , 7520895.html, Patent, pto, 7520895

Title: Self expanding bifurcated endovascular prosthesis

Abstract: Disclosed is a bifurcated tubular endoluminal vascular prosthesis, useful in treating, for example, an abdominal aortic aneurysm. The prosthesis comprises a self expandable wire support structure surrounded at least in part by a flexible tubular membrane. A delivery catheter and methods are also disclosed.

Patent Number: 7,520,895 Issued on 04/21/2009 to Douglas, et al.

Inventors: Douglas; Myles S. (Phoenix, AZ), Madrid; Gilbert (Laguna Niguel, CA), Shokoohi; Mehrdad M. (Rancho Palos Verdes, CA)
Assignee: Endologix, Inc. (Irvine, CA)

Appl. No.: 10/119,525

Filed: April 8, 2002

Current U.S. Class: 623/1.35

Current International Class: A61F 2/06 (20060101)

Field of Search: 606/191-198 623/1.11,1.35,1.15

References Cited [Referenced By]

U.S. Patent Documents


2127903	August 1938	Bowen
2845959	August 1958	Sidebotham
2990605	July 1961	Dernsyk
3029819	April 1962	Starks
3096560	July 1963	Liebig
3805301	April 1974	Liebig
4497074	February 1985	Rey et al.
4501263	February 1985	Harbuck
4503568	March 1985	Madras
4592754	June 1986	Gupte et al.
4617932	October 1986	Kornberg
4816028	March 1989	Kapadia et al.
4840940	June 1989	Sottiurai
4856516	August 1989	Hillstead
4878906	November 1989	Lindemann et al.
4907336	March 1990	Gianturco
4922905	May 1990	Strecker
4994071	February 1991	MacGregor
5019090	May 1991	Pinchuk
5035706	July 1991	Giantureo et al.
5064435	November 1991	Porter
5078726	January 1992	Kreamer
5104399	April 1992	Lazarus
5108424	April 1992	Hoffman, Jr. et al.
5133732	July 1992	Wiktor
5135536	August 1992	Hillstead
5156619	October 1992	Ehrenfeld
5178634	January 1993	Martinez
5197976	March 1993	Herweck et al.
5201757	April 1993	Heyn et al.
5256141	October 1993	Gencheff et al.
5275622	January 1994	Lazarus et al.
5282824	February 1994	Gianturco
5304200	April 1994	Spaulding
5314472	May 1994	Fontaine
5316023	May 1994	Palmaz et al.
5342387	August 1994	Summers
5360443	November 1994	Barone et al.
5366504	November 1994	Andersen et al.
5370683	December 1994	Fontaine
5383892	January 1995	Cardon et al.
5387235	February 1995	Chuter
5405377	April 1995	Cragg
5415664	May 1995	Pinchuk
5423886	June 1995	Arru et al.
5425765	June 1995	Tiefenbrun et al.
5443498	August 1995	Fontaine
5443500	August 1995	Sigwart
5456713	October 1995	Chuter
5489295	February 1996	Piplani et al.
5496365	March 1996	Sgro
5507767	April 1996	Maeda et al.
5507771	April 1996	Gianturco
5522880	June 1996	Barone et al.
5522881	June 1996	Lentz
5522883	June 1996	Slater et al.
5545211	August 1996	An et al.
5554181	September 1996	Das
5562728	October 1996	Lazarus et al.
5578072	November 1996	Barone et al.
5591229	January 1997	Parodi
5607445	March 1997	Summers
5609625	March 1997	Piplani et al.
5609627	March 1997	Goicoechea et al.
5628788	May 1997	Pinchuk
5632772	May 1997	Alcime et al.
5643339	July 1997	Kavteladze et al.
5647857	July 1997	Anderson et al.
5653727	August 1997	Wiktor
5653743	August 1997	Martin
5653746	August 1997	Schmitt
5653747	August 1997	Dereume
5662700	September 1997	Lazarus
5662702	September 1997	Keranen
5665115	September 1997	Cragg
5665117	September 1997	Rhodes
5674241	October 1997	Bley et al.
5674276	October 1997	Andersen et al.
5676696	October 1997	Marcade
5676697	October 1997	McDonald
5679400	October 1997	Tuch
5681346	October 1997	Orth et al.
5683448	November 1997	Cragg
5683449	November 1997	Marcade
5683450	November 1997	Goicoechea et al.
5683451	November 1997	Lenker et al.
5683452	November 1997	Barone et al.
5683453	November 1997	Palmaz
5690644	November 1997	Yurek et al.
5693066	December 1997	Rupp et al.
5693084	December 1997	Chuter
5693086	December 1997	Goicoechea et al.
5693087	December 1997	Parodi
5693088	December 1997	Lazarus
5695516	December 1997	Fischell et al.
5695517	December 1997	Marin et al.
5716365	February 1998	Goicoechea et al.
5716393	February 1998	Lindenberg et al.
5718973	February 1998	Lewis et al.
5749880	May 1998	Banas et al.
5755771	May 1998	Penn et al.
5800456	September 1998	Maeda et al.
5824037	October 1998	Fogarty et al.
5851228	December 1998	Pinheiro
5906640	May 1999	Penn et al.
5916263	June 1999	Goicoechea et al.
5919225	July 1999	Lau et al.
5928279	July 1999	Shannon et al.
5948018	September 1999	Dereume et al.
5961548	October 1999	Shmulewitz
6017363	January 2000	Hojeibane
6030414	February 2000	Taheri
6039749	March 2000	Marin et al.
6039755	March 2000	Edwin et al.
6039758	March 2000	Quiachon et al.
6051020	April 2000	Goicoechea et al.
6070589	June 2000	Keith et al.
6074398	June 2000	Leschinsky
6077296	June 2000	Shokoohi et al.
6077297	June 2000	Robinson et al.
6080191	June 2000	Summers
6086611	July 2000	Duffy et al.
6106548	August 2000	Roubin et al.
6117167	September 2000	Goicoechea et al.
6123722	September 2000	Fogarty et al.
6123723	September 2000	Konya et al.
6126685	October 2000	Lenker et al.
6129756	October 2000	Kugler et al.
6165195	December 2000	Wilson et al.
6168610	January 2001	Marin et al.
6187037	February 2001	Satz
6192944	February 2001	Greenhalgh
6197049	March 2001	Shaolian et al.
6283991	September 2001	Cox et al.
6331190	December 2001	Shokoohi et al.
6395019	May 2002	Chobotov
6508835	January 2003	Shaolian et al.
6520988	February 2003	Colombo et al.
6551350	April 2003	Thornton et al.
6733523	May 2004	Shaolian et al.
2006/0264801	November 2006	Bolling et al.
2006/0271163	November 2006	Shokoohi et al.
2006/0287713	December 2006	Douglas et al.
2007/0112412	May 2007	Shokoohi et al.

Foreign Patent Documents


2007648	Apr., 1991	CA
2127458	Jul., 1993	CA
2 220 141	Jul., 1997	CA
2287406	Dec., 1997	CA
0 177 330	Jun., 1991	EP
0 596 145	May., 1994	EP
0 621 015	Oct., 1994	EP
0 659 389	Jun., 1995	EP
0 740 928	Nov., 1996	EP
0 747 020	Dec., 1996	EP
0 775 470	May., 1997	EP
04-25755	Jun., 1990	JP
07-47134	Mar., 1994	JP
30-09638	Apr., 1994	JP
08-52165	Jun., 1995	JP
09-164209	Dec., 1995	JP
WO 93/13825	Jul., 1993	WO
WO 96/38101	Dec., 1996	WO
WO 96/39999	Dec., 1996	WO
WO 96/41589	Dec., 1996	WO
WO 97/10777	Mar., 1997	WO
WO 99/44536	Sep., 1999	WO
WO 99/47077	Sep., 1999	WO

Other References

US 6,413,270, 07/2002, Thornton et al. (withdrawn) cited by other.

Primary Examiner: Truong; Kevin T.
Attorney, Agent or Firm: Knobbe Martens Olson & Bear LLP

Claims

What is claimed is:

1. A self-expandable bifurcated endoluminal prosthesis, comprising: a proximal wire support section having a proximal end, a distal end and a central lumen extending therethrough; a first wire branch section at the distal end of the proximal support; and a second wire branch section at the distal end of the proximal support; wherein at least the proximal wire support section and the first wire branch section are formed from a single length of wire and the wire in the proximal wire support section comprises a series of proximal bends and a series of distal bends, creating a series of struts connecting the proximal bends and distal bends to form at least two axially adjacent tubular segments, joined by a connector therebetween and wherein the series of proximal bends, distal bends and struts form a series of triangles, each triangle being defined by one proximal bend and two distal bends or two proximal bends and one distal bend, further wherein at least one of the proximal bends is pivotably connected to at least one of the distal bends.

2. An endoluminal prosthesis as in claim 1, wherein the proximal support comprises at least three segments and two connectors.

3. An endoluminal prosthesis as in claim 1, wherein the wire in at least one of the first and second wire branch sections comprises a series of proximal bends and a series of distal bends, creating a series of struts connecting the proximal bends and distal bends to form a tubular segment wall.

4. An endoluminal prosthesis as in claim 3, wherein at least some of the struts in at least one of the proximal wire support section, the first wire branch section, and the second wire branch section are substantially linear.

5. An endoluminal prosthesis as in claim 3, wherein each segment comprises from about 4 proximal bends to about 12 proximal bends.

6. An endoluminal prosthesis as in claim 1, further comprising an eye on at least one of the proximal and distal bends in at least one of the proximal wire support section, the first wire branch section, and the second wire branch section.

7. An endoluminal prosthesis as in claim 5, wherein the at least two axially adjacent tubular segments in the proximal wire support section comprises a first tubular segment and a second tubular segment that is axially adjacent to the first tubular segment and at least one eye on a distal end of the first tubular segment that is connected to at least one corresponding eye on a proximal end of the second tubular segment.

8. An endoluminal prosthesis as in claim 6, wherein the corresponding eyes are connected with a suture.

9. An endoluminal prosthesis as in claim 6, further comprising a polymeric layer on the wire support.

10. An endoluminal prosthesis as in claim 9, wherein at least some of the coffesponding eyes are connected with a metal link.

11. An endoluminal prosthesis as in claim 9, wherein at least a first distal bend in a first of the axially adjacent tubular segments is connected to at least a first proximal bend in a second of the axially adjacent tubular segments and at least one of the first proximal bend and the first distal bend comprise an eye.

12. An endoluminal prosthesis as in claim 1, further comprising a polymeric layer on the wire support.

13. An endoluminal prosthesis as in claim 9, wherein the layer comprises a tubular PTFE sleeve surrounding at least a central portion of the prosthesis.

14. An endoluminal prosthesis as in claim 1, wherein at least one of the first wire branch section and the second wire branch section comprises a series of proximal bends and a series of distal bends creating a series of struts connecting the proximal bends and distal bends to form at least a first tubular segment and a second tubular segment that is axially adjacent to the first tubular segment, and at least one eye on a distal end of the first tubular segment that is connected to at least one corresponding eye on a proximal end of the second tubular segment.

15. An endoluminal prosthesis as in claim 1, wherein at least a first distal bend in a first of the axially adjacent tubular segments is connected to at least a first proximal bend in a second of the axially adjacent tubular segments and at least one of the first proximal bend and the first distal bend comprise an eye.

16. A one piece endoluminal prosthesis, comprising an elongate flexible wire, formed into a plurality of axially adjacent tubular segments spaced along an axis and defining a central lumen therethrough, each tubular segment comprising a zig zag section of the wire having a plurality of proximal bends and distal bends that form a series of triangles that extend circumferentially around an axis of the graft, with the wire continuing between each adjacent tubular segment, wherein the prosthesis is radially compressible into a first, reduced cross sectional configuration for implantation into a body lumen, and self expandable to a second, enlarged cross sectional configuration at a treatment site in a body lumen, said prosthesis further comprising a first branch support section and a second branch support section each in communication with the lumen and each configured to extend into the branched arteries and at least one moveable link connecting at least one proximal bend to at least one distal bend.

17. An endoluminal prosthesis as in claim 16, further comprising an outer tubular sleeve surrounding at least a portion of the prosthesis.

18. An endoluminal prosthesis as in claim 16, wherein the plurality of axially adjacent tubular segments comprises a first tubular segment and a second tubular segment that is axially adjacent to the first tubular segment and at least one distal bend on the first tubular segment is connected to at least one proximal bend from the second tubular segment.

19. An endoluminal prosthesis as in claim 18, wherein the connection comprises a pivotable connection.

20. An endoluminal prosthesis as in claim 19, wherein the connection comprises a metal link.

21. An endoluminal prosthesis as in claim 16, wherein at least a portion of the prosthesis has an expansion ratio of at least about 1:4.

22. An endoluminal prosthesis as in claim 21, wherein at least a portion of the prosthesis has an expansion ratio of at least about 1:5.

23. An endoluminal prosthesis as in claim 16, wherein at least a portion of the prosthesis has an expanded diameter of at least about 20 mm -30 mm in an unconstrained expansion, and the prosthesis is implantable using a catheter no greater than about 16 French.

24. A prosthesis as in claim 23, wherein at least a portion of the prosthesis has an expanded diameter of at least about 25 mm, and is implantable on a delivery device having a diameter of no more than about 16 French.

25. An endoluminal prosthesis as in claim 16, wherein the first and second branch support sections are bilaterally symmetrical.

26. An endoluminal prosthesis as in claim 16, wherein the first and second branch support sections have a length within the range of about 1 cm to about 5 cm.

27. An endoluminal prosthesis as in claim 16, wherein at least one of the first branch support section and the second branch support section comprises a series of proximal bends and a series of distal bends creating a series of struts connecting the proximal bends and distal bends to form at least a first tubular branch segment and a second tubular branch segment that is axially adjacent to the first tubular branch segment.

28. An endoluminal prosthesis as in claim 27, further comprising a polymeric layer on the axially adjacent tubular segments.

29. An endoluminal prosthesis as in claim 28, further comprising an eye on at least some of the distal and proximal bends on at least one of the first branch support section and the second branch support section.

30. An endoluminal prosthesis as in claim 29, wherein at least a first distal bend in the first tubular branch segment is connected to at least a first proximal bend in the second tubular branch segment and at least one of the first proximal bend and the first distal bend comprise an eye.

31. An endoluminal prosthesis as in claim 16, further comprising an eye on at least some of the bends.

32. An endoluminal prosthesis as in claim 16, wherein at least a first distal bend in a first of the axially adjacent tubular segments is connected to at least a first proximal bend in a second of the axially adjacent tubular segments and at least one of the first proximal bend and the first distal bend comprise an eye.

Description

BACKGROUND OF THE INVENTION

The present invention relates to endoluminal repair of a vessel, and, in particular, repair of a bifurcation aneurysm such as at the iliac bifurcation of the abdominal aorta.

Endoluminal repair or exclusion of aortic aneurysms has been performed for the past several years. The goal of endoluminal aortic aneurysm exclusion has been to correct this life threatening disease in a minimally invasive manner in order to effectuate a patient's quick and complete recovery. Various vascular grafts exist in the prior art which have been used to exclude aortic aneurysms. These prior art grafts have met varying degrees of success.

Initially, straight tube grafts were used in the infrarenal abdominal aorta to exclude the aneurysmal sac from the blood stream thereby resulting in the weakened aortic wall being protected by the graft material. These straight tube grafts were at first unsupported meaning that they employed stents at their proximal and distal ends to anchor the proximal and distal ends of the graft to the healthy portions of the aorta thereby leaving a midsection of the graft or prosthesis that did not have any internal support. Although this type of graft at first appeared to correct the aortic aneurysm, it met with many failures. The unsupported nature of its midsection allowed the graft to migrate distally as well as exhibit significant proximal leakage due to the enlargement of the aorta without adaptation of the graft, such as enlargement of the graft, to accommodate the change in diameter of the aorta.

Later, technical improvements in stent design led to "self-expanding" stents. In addition, later improvements produced "Nitinol" stents which had a "memory" that was capable of expanding to a predetermined size. Coincidentally, graft designers began to develop bifurcated grafts having limbs which extended into the iliac arteries. The development of bifurcated grafts allowed for the treatment of more complex aneurysms. With the advent of bifurcated grafts, the need for at least a one centimeter neck from the distal aspect of the aneurysmal sac to the iliac bifurcation in order to treat the aneurysm with an endoluminal graft was no longer needed. However, proximal necks of at least 0.5 to 1 centimeter distance from the renal arteries to the most proximal aspect of the aneurysm are still generally required.

Many bifurcated grafts are of a two-piece design. The two-piece designs require the insertion of a contralateral limb through a separate access site. These types of grafts are complex to deploy and have the potential for leakage at the connection site of the two limbs of the graft.

One piece bifurcated grafts are also well known in the art. For example, U.S. Pat. No. 2,845,959 discloses a one piece seamless woven textile bifurcated tube for use as an artificial artery. Yarns of varying materials can be used to weave the bifurcated graft including nylon and plastic yarns. U.S. Pat. Nos. 3,096,560 and 3,029,9819 issued to Liebig and Starks, respectively, disclose woven one piece bifurcated grafts which are constructed by performing specific types of winding and weaving about a smooth bifurcated mandrel.

U.S. Pat. No. 4,497,074 describes a one piece bifurcated graft which is made from a preformed support in the shape of the bifurcated graft. In a first stage, a gel enabling a surface state close to that of the liquid-air interface to be obtained at the gel-air interface is deposited by dipping or coating the preform with a sol which is allowed to cool. A hardenable flexible material such as a silicone elastomer is applied by dipping or spraying the material on the mold in a second stage. Finally, after hardening of the material, the prosthesis is removed from the mold. In U.S. Pat. No. 4,816,028 issued to Kapadia et al., there is shown a one piece woven bifurcated vascular graft having a plurality of warp threads running in the axial direction and a plurality of weft threads running in the transverse direction. Further, U.S. Pat. No. 5,108,424 issued to Hoffman, Jr. et al. discloses a one piece bifurcated collagen-impregnated dacron graft. The bifurcated graft includes a porous synthetic vascular graft substrate formed by knitting or weaving with at least three applications of dispersed collagen fibrils.

The Herweck et al. patent, U.S. Pat. No. 5,197,976, discloses a continuous one piece bifurcated graft having plural longitudinally parallel tube structures which are attached to one another over at least a portion of their longitudinal exteriors. The tube structures can be manually separated to form a branched tubular structure. The prosthesis is manufactured by paste forming and stretching and/or expanding highly crystalline unsintered polytetrafluoroethylene (PTFE). Paste forming includes mixing the PTFE resin with a lubricant, such as mineral spirits, and then forming the resin by extrusion into shaped articles.

Although all of the above-described one piece bifurcated grafts have eliminated the problems of leakage and graft failure at the suture or juncture site associated with two piece bifurcated grafts which join together two separate grafts to form the bifurcated graft, problems still exist with these one piece bifurcated grafts. For example, the previously described one piece bifurcated grafts do not include an integral support structure to prevent the deformation, twisting or collapse of the graft limbs. Further, the same problems with graft migration that existed with straight tube grafts still exist with the one piece bifurcated grafts. Accordingly, there is a need for a stable and durable bifurcated vascular graft which is structured to prevent the migration of the graft and the deformation and obstruction of the blood flow through the limbs of the bifurcated graft.

Endoluminal implantation is a common technique for implanting vascular grafts. Typically, this procedure involves percutaneously inserting a vascular graft or prosthesis by using a delivery catheter. This process eliminates the need for major surgical intervention thereby decreasing the risks associated with vascular and arterial surgery. Various catheter delivery systems for prosthetic devices are described in the prior art.

For example, bifurcated vascular grafts have been created by combining grafts with stents on delivery systems in order to secure the graft ends to the blood vessel thereby stabilizing the bifurcated graft. In U.S. Pat. No. 5,360,443 issued to Barone et al., a method for repairing an abdominal aortic aneurysm is described. The method comprises the steps of (1) connecting an expandable and deformable tubular member, such as a stent, to each of the tubular passageways of a bifurcated graft, (2) disposing the bifurcated graft and deformable tubular members within the aortic and iliac arteries, and (3) expanding and deforming each deformable tubular member with a catheter to secure each tubular passageway of the bifurcated graft within the appropriate artery. This reference only discloses a catheter delivery method for deploying the aortic portion of the bifurcated graft. The same catheter is supposedly used to also expand and secure the associated stents within the iliac arteries.

The Palmaz et al. patent, U.S. Pat. No. 5,316,023, describes a method and apparatus for repairing an abdominal aortic aneurysm in an aorta at the iliac arteries. This method includes the steps of connecting a first tubular graft to a first deformable and expandable tubular member, connecting a second tubular graft to a second deformable and expandable tubular member, disposing the first tubular graft and first tubular member upon a first catheter having an inflatable portion, disposing the second tubular graft and second tubular member upon a second catheter having an inflatable portion, intraluminally delivering the first and second tubular grafts, tubular members and catheters to the aorta and disposing at least a portion of each tubular graft within the abdominal aortic aneurysm, and expanding the tubular members with the inflatable catheters to secure them and at least a portion of their associated tubular grafts within the aorta. This patent reference employs two separate unconnected straight grafts which are employed within an aorta to form a bifurcated graft.

Further, U.S. Pat. No. 4,617,932 issued to Komberg discloses a device for inserting a graft into an artery comprising a plurality of nested tubes each having an upper and lower end. A first outer tube has a means for guiding and positioning an arm means at its upper end. The arm means is movably attached to the upper end of another tube located inside of the first tube and extending above the first outer tube. The lower ends of the tubes are adaptable for fastening means and the inside tube extends below the end of the first outer tube. Delivery and placement of a bifurcated graft is illustrated. U.S. Pat. No. 5,522,883 issued to Slater et al. describes an endoprosthesis stent/graft deployment system which includes a tubular delivery catheter, a radially expandable prosthesis positioned over the catheter, a removable endoprosthesis support assembly located adjacent the catheter opening and having an arm extending through the catheter which keeps the endoprosthesis in a compressed state, and a release mechanism insertable through the catheter for removing the support assembly.

U.S. Pat. No. 5,104,399 issued to Lazarus also describes an artificial graft and delivery method. The delivery system includes a capsule for transporting the graft through the blood vessel, a tube connected to the vessel which extends exterior to the vessel for manipulation by a user, and a balloon catheter positioned within the tube. Finally, U.S. Pat. No. 5,489,295 issued to Piplani et al. discloses a bifurcated graft and a method and apparatus for deploying the bifurcated graft. The Piplani et al. graft includes a main tubular body, first and second tubular legs joined to the main tubular body in a bifurcation, a first expandable attachment means for anchoring the main body located adjacent the opening for the first body, and a second expandably attachment means located adjacent the opening of the first tubular leg for anchoring the first tubular leg. The graft is intraluminally implanted using a catheter that is inserted into the aortic bifurcation through a first iliac artery so that the first attachment means adjacent the opening of the main body can be anchored in the aorta and the second attachment means adjacent the opening of the first tubular leg can be anchored in the first iliac artery. The second tubular leg is deployed into the second iliac artery by using a pull line attached to the second tubular leg. The Piplani et al. patent also discloses a deployment device consisting of a capsule catheter, a balloon catheter, and a separate expandable spring attachment means.

The previously described deployment methods, systems and devices do not allow for a bifurcated graft which is fully supported with self-expandable stents to be delivered and implanted within an arterial bifurcation. A use of any of the previously described deployment devices or systems to implant the structural supported bifurcated graft of the present invention would result in failure due to the inability of those devices and systems to deliver and anchor the second supported limb within the second iliac artery. The previously described methods and systems simply do not allow for the delivery and implantation of a bifurcated vascular graft whose three open ends are supported by stents. Accordingly, not only is there a need for a structurally supported stable and durable bifurcated graft which is not susceptible to migration and leaking, but there is also a need for a delivery apparatus and method for deploying and implanting such a bifurcated graft.

SUMMARY OF THE INVENTION

There is disclosed in accordance with one aspect of the present invention, a bifurcated endoluminal prosthesis. The prosthesis comprises a proximal wire support section having a proximal end, a distal end, and a central lumen extending therethrough. A first wire branch section is provided at the distal end of the proximal support, and a second wire branch section is also provided at the distal end of the proximal support. At least the proximal support section and the first branch section are formed from a single length of wire.

Preferably, the proximal support comprises at least two axially adjacent tubular segments, joined by a connector there between. The wire in each segment is formed into a series of proximal bends and a series of distal bends, creating a series of struts connecting the proximal bends and distal bends to form a tubular segment wall. Preferably, the wire decreases in cross-section from a relatively large cross-section in the proximal wire support section to a relatively small cross-section at the distal end of at least one of the first and second wire branch sections.