Patent Publication Number: US-6663667-B2

Title: Towel graft means for enhancing tissue ingrowth in vascular grafts

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The instant application for U.S. Letters Patent is a Continuation-In-Part of U.S. Ser. No. 09/473,618, filed Dec. 29, 1999, now U.S. Pat. 6,344,056 which is currently pending as an allowed case before the United States Patent and Trademark Office. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to prosthetic vascular grafts and, more particularly, to a vascular graft having an exterior texture to promote tissue ingrowth into the graft to enhance fixation and, inter alia, preclude perigraft leakage. 
     2. Discussion of the Related Art 
     An aneurysm is a ballooning of the wall of an artery resulting from weakening due to disease or other condition. Left untreated, the aneurysm may rupture, resulting in severe loss of blood and potentially death. An aneurysm in the abdominal aorta is the most common form of arterial aneurysm. The abdominal aorta, which extends downward from the heart in front of and parallel to the spine, through the thorax and abdomen, and branches off in a plurality of side vessels, connects the ascending aorta at the heart and to the circulatory system of the trunk and lower body. Branched vessels of the aorta supply the two kidneys via oppositely-directed renal arteries. Below the renal arteries, the abdominal aorta continues to about the level of the fourth lumbar vertebrae and divides at a Y-junction into the left and right iliac arteries, which supply blood to the lower extremities. 
     A common location for an aortic aneurysm is in the section of aorta between the renal and iliac arteries. Without rapid surgical intervention, a rupture of the abdominal aorta is commonly fatal because of the high volume of blood flow within the aorta. Conventional surgical intervention involves penetrating the abdominal wall to the location of the aneurysm to reinforce or replace the diseased section of the aorta. Typically, a prosthetic tube graft replaces the area of, or proximal and distal zones abutting, a potential rupture portion of the aorta. Unfortunately, conventional surgical intervention has resulted in substantial morbidity rates, and at the very least a protracted recovery period. Likewise, cost and other constraints militate for a longstanding need for endovascular intervention. 
     In recent years, methods and devices have been developed to treat an aortic aneurysm without opening up the abdominal wall. These new techniques typically involve a catheter-carried tubular graft delivered upward from the femoral artery through the iliac artery and into the region of the aneurysm. The graft normally includes a tubular graft body supported by an expandable stent, either self-expanding or balloon-expanding. The balloon-expanding type of stent naturally requires an expansion balloon, while the self-expanding type is simply deployed from the end of a tubular sheath. Implacement issues impact upon both known techniques. 
     If the aneurysm affects the Y-junction between the abdominal aorta and the iliac arteries, a bifurcated graft is typically used. A trunk portion of the bifurcated graft is secured to a healthy section of the abdominal aorta just below the renal arteries, and branched legs of the graft are secured within each of the iliac arteries, sometimes via a tubular extension graft. This procedure does not involve cardiopulmonary bypass, and thus blood continues to flow downward through the abdominal aorta. Certain complications arise in anchoring the graft to the inner wall of the vessel, because of the high blood flow both during the procedure and afterward. Indeed, the risk of grafts migrating within a vessel is a problem in many locations, not just in the abdominal aorta. In addition, the abdominal aorta may be aneurysmic very close to the renal arteries, which results in a fairly poor substrate within which to secure a repair graft. In fact, surgeons require various minimum lengths of healthy aortic wall below the renal arteries before an endovascular graft repair is indicated, or else a conventional invasive technique must be used. Moreover, the same consideration of a minimum healthy portion of the host vessel applies in other areas, especially with regard to the portion of the aorta adjacent the branching subclavian or carotid arteries. Additionally, leaking is likely to occur if a vessel is deformed due to an aneurysm. 
     A number of techniques have been proposed for anchoring grafts to vessel walls, most notably the use of barbs or hooks extending outward from graft that embed themselves into the vessel wall. Although these devices secure the graft, the intrusions into the vessel wall may damage the wall and cause complications. Alternatively, some devices have portions extending beyond the upstream end of the graft body which may be bent outward into contact with the vessel wall, either from a pre- or shape memory-bias, or from expansion of a balloon in this region. However, the optimal way to attach stentgrafts is to allow them to become naturally fixed to the vessels, such as by tissue ingrowth. This need is largely unaddressed in the art. 
     For example, the ZENITH AAA brand of Endovascular Graft from Cook, Inc. of Bloomington, Ind., utilizes an undulating wire support having barbs for supra-renal fixation of the graft. However, because these wires extend across the opening of the branching renal arteries they present a certain impediment to blood flow therethrough. 
     Moreover, any structure placed in the path of blood flow may tend to initiate the blood clotting cascade, which in turn, may generate free-floating emboli that would adversely impact the kidneys, or other organ that is perfused through the affected side branch. Because the kidneys are highly susceptible to injury from incursion of such emboli, it is highly desirable to avoid even the possibility of blood clotting at the mouth of the renal arteries. 
     In the context of preventing leaking around the graft, some manufacturers have provided grafts having texturized surfaces. For example, one such texturized graft is knitted, and includes a plurality of loops extending from the exterior surface of the graft. One known drawback associated with this configuration is that the knitted graft is, which is composed of loosely looped fabrics or threads, is very easily expansible. Such a graft may be subject to undesirable stretching and/or deformation within the body. 
     Another texturized graft is equipped with a plurality of loosely associated fibers, which may be affixed to or sewn to the exterior surface of the graft. Used appropriately, these configurations tend to reduce the incidence of leaking from around grafts in deformed vessels. However, neither of these texturized grafts sufficiently encourages tissue ingrowth into the graft. Such ingrowth is a natural bodily process that may be used to anchor the graft, and prevent undesirable migration. 
     Despite much work in this highly competitive field, there is still a need for a more secure means of anchoring a bifurcated graft in the abdominal aorta. More generally, there is a need for a more secure means of anchoring a tubular graft in a primary vessel, be it a bifurcated endoluminal prosthesis, an aorto-uniliac or any of the above. 
     SUMMARY OF THE INVENTION 
     The present invention uses ‘floaters,’ or specifically tailored threads integrated with the graft, which extend outside of the weave of the graft material. Such threads may be viewed as forming loops. These floaters may be formed from material similar to that used to form the graft, or may be formed from a material otherwise specifically tailored to promote tissue ingrowth. The floaters promote tissue ingrowth where they are placed along the exterior of the graft body. As such, they may be used to promote tissue ingrowth along the entire length, or at specific locations of grafts including, but not limited to, bifurcated grafts, aorto-uniliac grafts or thoracic LIFEPATH® AAA stent-grafts (Edwards Lifesciences, LLC, Irvine, Calif.). 
     According to a feature of the present invention there is provided an endoluminal graft having vertical, horizontal or angled filaments forming the floaters, the filaments ranging from at least about 1 cm to about 20 cm in length. Each filament may be woven into the graft material to form one or more floaters. 
     According to another feature of the present invention there is provided an endoluminal graft having vertical, horizontal and/or angled strands to promote tissue ingrowth along the length of an outer face of the graft member. 
     According to still another feature of the present invention there is provided an endoluminal graft having vertical, horizontal and/or angled strands to promote tissue ingrowth, and to resist migration, kinking, separation and perigraft blushing, or endoleak. 
     According to yet a still further feature of the present invention there is provided an endoluminal graft having vertical, horizontal and/or angled strands to promote tissue ingrowth along the length of an outer face of the graft member while providing a smooth inner surface for the reconstructed vessel lumen. 
     In a preferred embodiment, the present invention comprises a woven vascular graft adapted for placement in a primary blood vessel that is suited to bridge a vessel side branch, the graft having a texturized exterior surface designed to increase tissue in growth. The graft comprises a tubular structure defining an outer surface, a first portion of the outer surface being sized to contact and support the blood vessel on one side of the side branch, and a second portion of the outer surface being sized to contact and support the blood vessel on the other side of the side branch. The tubular structure defines an aperture for alignment with the side branch so as to permit blood flow between the blood vessel and the side branch. 
     In one embodiment of the present invention, the texturized exterior surface takes the form of plurality of horizontal, vertical and/or otherwise angled threads extending from the surface of the graft. Such exterior surface will be referred to as having “towel” type threads. An alternate texturized exterior surface takes the form of a plurality of fibrous strands extending from the surface of the graft in a substantially angled, vertical or horizontal form with spaces for rows of threads skipped. Such exterior surface will be referred to as having a “fibrous” texture. 
     A still further alternate texturized exterior surface may have loose, vertical strands extending along the length of the graft surface. The integration of the texturized fibers with the woven graft in this manner serves to encourage the ingrowth of tissue from the wall of the blood vessel into the graft. Such ingrowth anchors the graft in position in the vessel and ensure compliance with morphology changes in the vessel over time, in addition to preventing kinking, migration or narrowing of the graft. It is known to those skilled in the art that empirical generation of a cellular matrix is promoted by having aspects or elements of a precursor to the cellular infrastructure which is to be created present at the situs of neointimal regeneration. With the present invention, the ability of the graft elements to seed and become incorporated into the cellular matrix ab inito has been show to promote a rapid and pervasive ingrowth. 
     The exterior surface of the graft may be designed to include a plurality of these surfaces. By way of example, the exterior surface may have a cuff, or extension portion, of towel type threads extending around the circumference of the ends of the graft. Such threads may have an angled orientation relative to the graft surface. The remainder of the exterior surface may be smooth. Alternatively, it may be covered with fibrous strands or loose vertical strands. 
     A preferred embodiment of the present invention is a bifurcated endoluminal prosthesis having a plurality of floater threaded portions separated by gaps or spaces along at least a substantial portion of the graft body. 
     An alternate preferred embodiment includes chevron-shaped, angled portions spanning predetermined aspects of an endovascular graft which may be an aorto-uniliac graft, a bifurcated graft or a thoracic graft. 
     In a further aspect, the invention provides a fibrously outer covered vascular graft system adapted for placement in a primary blood vessel and adjacent a vessel side branch. The system includes a tubular support graft including a first tubular structure sized to contact and support the blood vessel on one side of the side branch, and a second tubular structure spaced from and connected to the first tubular structure and sized to contact and support the blood vessel on the other side of the side branch. The system further includes a tubular primary graft sized to co-axially couple with the first tubular structure. 
     A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view through an abdominal aorta showing the branching renal and iliac arteries, and illustrating one embodiment of a graft of the present invention for supporting a trunk portion of a bifurcated graft, shown in phantom; 
     FIG. 2 is a perspective view of the graft of FIG. 1; 
     FIG. 3 is a perspective view of an alternative graft in accordance with the present invention having two planar bridging members; 
     FIG. 4 is a perspective view of a further graft of the present invention having two wire-like bridging members; 
     FIG. 5 is a perspective view of a further graft of the present invention having four bridging members; 
     FIG. 6 is a sectional view of the abdominal aorta in the region of the renal arteries illustrating a still further embodiment of a graft of present invention used to support the trunk portion of a bifurcated graft, shown in phantom; 
     FIG. 7 is a perspective view of the graft of FIG. 6; 
     FIG. 8 is an elevational view of the graft of FIG. 6 showing certain axial dimensions; 
     FIG. 9 is an axial sectional view of an abdominal aorta in the region of the renal arteries showing certain anatomical dimensions; 
     FIG. 10 is a perspective view of the exterior surface of the graft according to one embodiment of the present invention; 
     FIG. 11 is a perspective view of the exterior surface of the graft according to one embodiment of the present invention; and 
     FIG. 12 is a perspective view of the exterior surface of the graft according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a woven, tubular graft having a selected, texturized exterior surface. The exterior surface of the graft is ‘texturized’ by the presence of floaters. The floaters are formed during the weaving process by adding fibers lying along the warp of the weave. These additional fibers are partially integrated into the weave of the graft material. That is, each additional fiber skips one or more of the perpendicular fibers by not alternatively extending over, and under, adjacent perpendicular fibers. In this manner, sections of the floater fibers are woven into, and integrated with, the graft material, and sections of the floater fibers lie along the exterior of the graft surface. The floater fibers may be positioned over the entire exterior surface of the graft or at selected locations on the graft. For example, the floaters may form ‘cuffs’ around the proximal and distal ends of the graft. Alternatively, the floaters may be placed in a geometric pattern. By way of example, and not of limitation, the floaters may be arranged in straight lines, angled lines, spaced-apart groups or patterns. The tension of each floater fiber may be selected by the manufacturer of the graft material during the weaving process. In this manner, the floaters may be loose or relatively taut against the graft surface. That is, the floaters may either loosely extend from the graft surface, or be tightly laid along the surface. 
     These floaters may be provided in parallel strips, parallel bands, or in groups forming a geometric pattern. Alternatively the floaters, or strands, may be placed in a geometric pattern. Such geometric pattern may be formed by individual strands, or by groups of strands. Still further alternatively, the strands may intersect and/or overlap one another. The floaters may range in length from at least about 0.5 mm to about 10 mm, but are preferably from at least about 4 mm to about 7 mm. 
     FIG. 1 illustrates a graft  20  of the present invention deployed within a primary vessel, in this case the abdominal aorta  22 . A pair of side branches  24  is shown intersecting the primary vessel  22  at approximately the same axial location across the vessel. In the context of an abdominal aorta  22 , two important side branches are the renal arteries  24 , as shown. The abdominal aorta  22  continues downward from the renal arteries  24  and bifurcates at a Y-junction  26  into the left and right iliac arteries  28 . 
     Bands of floaters  35  may be seen extending around the proximal and distal ends of the graft  20 . The floaters  35  are extending in a direction that is substantially parallel to the length of the graft  20 . The strands which form the floaters  35  may range from at least about 1 cm in length to about 20 cm in length. The term “length” used here refers to the length of the section of the strand that extends from one end of the graft to the other end of the graft  20 . The distance between the location on the graft where a floater originates and the location on the graft where a floater terminates may be same as, or less than, the length of the floater. 
     The tubular graft of the present invention may support another tubular graft in the primary vessel in proximity to a side vessel. It should therefore be understood that although the drawings and description involve a graft in the abdominal aorta for supporting another graft in the region of the renal arteries, the same principles apply whichever primary vessel or side vessel is involved. For example, as illustrated in FIG. 1, the graft  20  could be used in the vicinity of a side branch  30  in the iliac arteries  28 . Representative conditions suitable for repair with the grafts of the present invention include the abdominal aortic aneurysm (AAA) described herein, a thoracic aortic aneurysm (TAA), and an aortic uni-iliac (AUI) aneurysm. For purpose of explanation, however, the term “side branch” will be used interchangeably herein with “renal artery,” and the term “primary vessel” will be used interchangeably with “abdominal aorta.” 
     As illustrated in FIG. 1, the graft  20  helps anchor a trunk portion  34  of a bifurcated graft  36 , shown in phantom. The bifurcated graft  36  typically comprises the trunk portion  34  that diverges at a septum  38  into a pair of legs  40 . One or both of the legs  40  may extend a sufficient distance to form a seal within the iliac arteries  28 , or tubular extensions  42  may be provided for this purpose. The end result is that the bifurcated graft  36  (and optional tubular extensions  42 ) extends from a healthy portion  44  of the abdominal aorta  22  to both of the iliac arteries  28 , spanning an aneurysmic region  46 . Once the bifurcated graft  36  is in place, blood flows therethrough and blood pressure is reduced between the aneurysm  46  and the exterior of the graft. Ultimately, the aneurysm  46  collapses inward around the graft, which remains in place. 
     With reference to FIGS. 1 and 2, the graft  20  of the present invention comprises a first tubular section  50  and a second tubular section  52  connected via at least one bridging member  54 . The first tubular section  50  is spaced from the second tubular section  52  across a gap that, in conjunction with the bridging member  54 , defines an aperture  56  for blood flow. If the first and second tubular sections  50 ,  52  are co-linear, then the bridging member  54  is generally axially disposed. Alternatively, if the graft  20  is intended for implantation in a curvilinear vessel, the first and second tubular sections  50 ,  52  may be aligned along a curvilinear axis, in which case the bridging member  54  will also be generally disposed along the same curve. Still further, the graft  20  may be multi-curvate, for example S-shaped, in which case the first and second tubular sections  50 ,  52  and bridging member  54  will follow the multiple curves. 
     As illustrated in FIG. 1, the aperture  56  is aligned with at least one of the side branches  24 . In a preferred application, the graft  20  is used to support a bifurcated graft  36  in proximity with the renal arteries  24 , and thus defines two apertures  56 , each aligned with one of the renal arteries. In this context, the first tubular section  50  is secured in contact with a supra-renal portion of the abdominal aorta  22 , while the second tubular section  52  is secured in contact with an infra-renal portion. The apertures  56  are sized large enough so that no portion of the graft  20  resides in the blood flow path of the renal arteries  24 , and also so that renal arteries that are slightly axially offset from one another can be accommodated. 
     With specific reference to FIG. 2, the graft  20  comprises a tubular graft body  60  internally supported by a stent  62 . The tubular graft body  60  may be formed of one or more pieces, typically of a biocompatible fabric such as polyester (e.g., polyterepthalate). Alternatively, the graft body  60  may be an extruded PTFE tube. In a particular preferred embodiment, the graft body  60  is one piece, with the apertures  56  formed by diametrically-opposed, generally oval-shaped windows  64  cut in the body and extending circumferentially around the body into proximity with one another. Two bridge segments  66  of the graft body  60  extend between the first and second tubular sections  50 ,  52  of the graft and separate the windows  64 . Preferably, the bridge segments  66  extend circumferentially around the graft body  60  a small arc in relation to the adjacent windows  64  so as to maximize the size of the blood flow apertures  56 . In one embodiment, the bridge segments  66  each circumferentially extends between about 1-90° around the graft body  60 , and more preferably each extends about 5-10°. 
     The blood flow apertures  56  are sized to enable alignment with side branches of varying sizes. Of course, the particular size is defined by the axial dimension and the circumferential arc of the windows  64 , which depends on the overall graft diameter and length. For instance, a graft that is designed for small arteries and small side branches will have a reduced diameter and reduced window size. Additionally, if the graft is intended to bridge only one side branch then only one window is required. In a preferred embodiment, for use in the abdominal aorta  22  to bridge the renal arteries  24 , the graft  20  has a diameter of between about 19 and 30 mm, and a length of between about 22 and 46 mm. The opposed windows  64  have an axial length of between about 6 and 20 mm, and extend circumferentially around the graft body  60  between about 90° and 189°. The renal arteries  24  typically have a diameter of between about 8-10 mm, and thus the windows  64  are desirably oversized to ensure open blood flow through the renals, and to accommodate offset or otherwise misaligned pairs of renals. 
     The stent  62  actually comprises a first stent portion within the first tubular section  50 , and a second stent portion within the second tubular section  52 . The first and second stent portions may be substantially similar in construction, or may be configured differently, as desired. Those of skill in the art will understand that a variety of different types of stents may be used to internally support a tubular graft body. 
     In a preferred embodiment, the stent  62  comprises a plurality of separate, spaced-apart wireforms  70 , each formed in an undulating, or sinusoidal pattern. Each of the wireforms  70  includes alternating peaks and valleys, with either the peaks or valleys being woven through the graft body. More specifically, as seen in FIG. 2, there are three axially-spaced rows of wireforms  70  in the first tubular section  50 , and four axially-spaced rows of wireforms in the second tubular section  52 . Either the peaks or valleys of these rows of undulating wireforms are woven through slits  72  formed in the graft body  60 . In this manner, the wireforms  70  are prevented from migrating axially within the graft body  60  with respect to one another, and thus provide a fairly uniform inner support structure for the flexible graft body. As mentioned, each wireform is either radially self-expandable to the configuration shown, or is capable of plastic deformation when balloon-expanded. In either case, the stent  62  (comprising the array of wireforms  70 ) compresses the graft body  62  against the inner wall of a tubular blood vessel to form a fluid seal therebetween. Moreover, certain materials and/or sleeve-like structures are available to enhance the seal between the exterior of the graft  20  and the vessel wall, and may be combined with the present invention. 
     A plurality of crimps  74  is visible on the exterior of the graft body  62 . The crimps  74  join free ends of each wireform  70 , which comprise one or more wire segments bent into the undulating pattern, and into the annular shape required. Though the crimps  74  are not sharp, they provide an irregular surface structure on the exterior of the graft  20 , and thus help secure the graft in position within the vessel. A plurality of floaters  35  which are designed to increase tissue ingrowth may also be seen extending around one end of the second tubular section  52 . 
     The bridging member  54  seen in FIGS. 1 and 2 comprises a reinforcing strut  80  and the aforementioned bridge segments  66  of the graft body  60 . The reinforcing strut  80  is a relatively rigid elongate member extending between the first and second tubular sections  50 ,  52  of the graft  20 . In a preferred embodiment, the reinforcing strut  80  is a biocompatible metal (e.g., stainless-steel) strip or rod secured at each end to either the graft body  60  or the stent  62 . If the ends of the reinforcing strut  80  are secured to the graft body  60  as shown, sutures are typically used to sew an eyelet, hook or other such feature (not shown) provided on each end of the reinforcing strut. If the ends of the reinforcing strut  80  are secured to the stent  62 , crimps are preferably used between juxtaposed ends of the closest wireforms and the reinforcing strut. As shown, the reinforcing struts  80  are desirably located to the outside of the bridge segments  66 , although the reverse configuration is contemplated as well. 
     The bridging members  54  serve to anchor one of the first and second tubular sections  50 ,  52  of the graft  20  with respect to the other, and desirably maintain the spacing between the tubular sections, while at the same time present very little in the way of structure that might occlude or otherwise interfere with the blood flow between the primary vessel  22  and the affected side branch  24 . The bridging members  54  must have tensile strength to withstand migratory forces that may tend to separate the first and second tubular sections  50 ,  52 . In an exemplary configuration, the upstream section  50  or  52  serves to anchor the downstream section by virtue of their connection with the bridging members  54 . In addition, the bridging members  54  may be relatively rigid in the sense that they have column strength sufficient to prevent the tubular sections  50 ,  52  from migrating toward each other after implantation. 
     The bridging members  54  have a radial dimension that is approximately the same as the rest of the graft  20 ; that is, they do not project radially into or out from the side wall of the graft. The circumferential width of each bridging member  54  depends on the intended use for the graft  20 . That is, if the graft  20  is to be used in the abdominal aorta  22  to bridge the renal arteries  24  as shown in the drawings, then there are two bridging members  54  diametrically spaced apart of relatively narrow circumferential width. In this way, the bridging members  54  each axially extend along the wall of the abdominal aorta  22  at 90° orientations from the openings to the renal arteries  24 , and there is no chance of occluding blood flow between the abdominal aorta  22  and renal arteries  24 . Alternatively, if there is only one side branch then there need only be one bridging member of relatively greater circumferential width than as shown. That is, the bridging member might extend 180° or more around the graft, with the corresponding window opening up the remaining portion. In general, as long as care is taken to orient the window(s) in registration with the side branch or branches, then the bridging member(s) will not occlude blood flow. 
     The embodiment of FIGS. 1 and 2 shows relatively rigid bridging members  54  that are constructed of, for example, wires. Alternatively, the bridging members  54  may be strips of biocompatible fabric or even sutures that provide tensile strength to prevent the downstream tubular section  52  from migrating with respect to the upstream section  50 . In the illustrated example, the upstream section  50  anchors the graft  20 , and in particular the downstream tubular section  52 , with respect to the renal arteries  24 . In this context, one or the other of the tubular sections  50 ,  52  may be designed to better anchor the graft  20  in the primary artery  22 , and the other may perform another function, such as supplementing a damaged section of the artery so that another graft may be secured adjacent the side branch  24 . Of course, however, both tubular sections  50 ,  52  can be constructed to have identical anchoring and vessel supporting characteristics if the graft  20  is used to repair a damaged length of the vessel that extends upstream and downstream of the side branch. 
     FIG. 3 illustrates an alternative graft  90  of the present invention having a first tubular section  92  separated from a second tubular section  94  across a gap  96  and connected across the gap by two bridging members  98 . Again, the graft  90  comprises a graft body  100  and an internal stent  102 . The graft body  100  may be a tubular biocompatible fabric, and in the illustrated embodiment is separated across the gap  96  into two tubular portions in the respective first and second tubular sections  92 ,  94 . Because the facing edges of the two tubular portions of the graft body  100  are circular, the gap  96  is tubular. The stent  102  again comprises a plurality of spaced-apart annular wireforms, although it should be noted that the first tubular section  92  only has a single wireform  104 . A plurality of floaters  35  which are designed to increase tissue ingrowth may also be seen extending around the first tubular section  92 . 
     The bridging members  98  are elongated planar bars or strips of relatively rigid material, such as stainless-steel or a suitable polymer connected directly to the stent  102  or to the graft body  100  in the first and second tubular sections  92 ,  94 . Again, the bridging members  98  must have tensile strength to withstand migratory forces that may tend to separate the first and second tubular sections  92 ,  94  after implantation, while at the same time must not occlude or otherwise interfere with the blood flow between the primary vessel and the affected side branch or branches. Therefore, instead of being relatively rigid, the bridging members  98  may be strips of fabric, such as polyester, or sutures for that matter. 
     FIG. 4 illustrates an alternative graft  110  of the present invention having a first tubular section  112  separated from a second tubular section  114  across a gap  116  and connected across the gap by two bridging members  118 . Again, the graft  110  comprises a graft body  120  and an internal stent  122 . The graft body  120  may be a tubular biocompatible fabric, and in the illustrated embodiment is separated across the gap  116  into two tubular portions in the respective first and second tubular sections  112 ,  114 . In this case the facing edges of the two tubular portions of the graft body  120  are uneven by virtue of a plurality of notches  123 , and thus the gap  116  is uneven as well. The stent  122  again comprises a plurality of spaced-apart annular wireforms, with the first tubular section  112  having two wireforms and the second tubular section  114  having three. 
     A plurality of floaters  35  may be seen extending around both the first tubular section  112  and the second tubular section  114 . The floaters on both the first tubular section  112  and the second tubular section  114  extend along the exterior surface of the graft and are each individually generally parallel to the length of the graft  110 . The line extending around the circumference of the graft  110  that is formed by the floaters  35  is at an angle to the length of the graft  110 . 
     The bridging members  118  each comprises lengths of wire either separate from the stent  122  or defined by extensions of one or the wireforms. If the bridging members  118  are separate from the stent  122 , they are connected directly to the stent using a crimp  124 , for example, or are connected indirectly via stitching  126  to the graft body  120 . In an exemplary embodiment as illustrated, the bridging members  118  are connected via crimps  124  to free ends of the lowest wireform in the first tubular section  112  and sewn to the graft body  120  in the second tubular section  114 . 
     FIG. 5 illustrates a still further exemplary graft  130  of the present invention having a first tubular section  132  separated from a second tubular section  134  across a gap  136  and connected across the gap by four (4) bridging members  138 . Again, the graft  130  comprises a graft body  140  and a stent  142 . The graft body  140  is desirably a tubular biocompatible fabric. The stent  142  again comprises a plurality of spaced-apart annular wireforms, with the first tubular section  132  having a single wireform  144  disposed on the exterior of the graft body  140 . The external wireform  144  can either be woven through slits in the graft body  140  as described above, or may be secured thereto with the use of suture thread. 
     A plurality of floaters  35  which are designed to increase tissue ingrowth may also be seen extending around the second tubular section  134 , and generally forming two cuffs  101 . The floaters extend along the exterior surface of the graft and are each individually generally parallel to the length of the graft  110 . 
     The four bridging members  138  are distributed generally equidistantly around the circumference of the graft  130  and each comprises a narrow strip of fabric  146  and a reinforcement strut  148 . Again, the reinforcement struts  148  may be connected directly to the stent  142  using a crimp, for example, or are connected indirectly via stitching  149  to the graft body  140 . The use of four bridging members  138  may be desirable for stability when smaller branching vessels are involved so that the windows defined between the bridging members need not be as large as the previous embodiments. 
     FIGS. 6 and 7 illustrate a still further embodiment of a graft  150  of the present invention that defines a tubular structure having a first portion  152  and a second portion  154  separated from the first portion across a gap  156 . Two bridging members  158  extend generally axially between and couple the first and second portions  152 ,  154  to prevent their relative movement before during and after implantation. In this embodiment, the first portion  152  of the tubular structure is defined solely by a stent  160 , while the second portion  154  is defined by a stent  162  internally supporting a tubular graft body  164 . As shown in FIG. 7, the second portion  154  of graft  150  is additionally equipped with a plurality of floaters  35  which are designed to increase tissue ingrowth may also be seen extending around the second portion  154 . The floaters extend along the exterior surface of the graft and are each individually generally parallel to the length of the graft  110 . 
     The upper stent  160  comprises an annular wireform  166  having alternating peaks  168  and valleys  170  and contoured curvilinear segments  172  extending therebetween. The curvilinear segments  172  are shaped so as to nest together when the graft  150  is in a radially constricted state, so as to enable smaller compaction of the graft. The wireform  166  includes one or more segments connected into the annular shape by one or more crimps  174 . The lower stent  162  includes a plurality of axially-spaced undulating wireforms woven through the graft body  164 , as previously described. 
     The bridging members  158  each comprise lengths of wire either separate from the stents  160 ,  162  or defined by extensions of one or the wireforms. If the bridging members  158  are separate from the stents  160 ,  162 , they are connected directly to the upper stent  160  using a crimp  176 , and are connected directly to the lower stent  162  using a crimp or indirectly via stitching  178  to the graft body  164 . In an exemplary embodiment as illustrated, the bridging members  158  are connected via crimps  176  to free ends of the wireform  166  in the first portion  152  and sewn to the graft body  164  in the second portion  154 . 
     FIG. 6 shows the graft  150  in place within a primary vessel  180  (e.g., the abdominal aorta) and bridging two oppositely-directed vessel side branches  182  (e.g., the renal arteries). The first portion  152  is located to contact and support the primary vessel  180  on one side of the side branches  182 , while the second portion  154  is located to contact and support the primary vessel on the other side of the side branches. The gap  156  is positioned to permit blood flow between the primary vessel  180  and side branches  182 , as indicated by the flow arrows  184 . The bridging members  158  extend axially across the gap  156  against the wall of the primary vessel  180  at approximately 90° orientations from the side branches  182 . Another graft  186  (e.g., the trunk of a bifurcated graft) is seen positioned within the second portion  154 . In this way, the graft  186  is secured within the uniform and tubular second portion  154 , which in turn is anchored within the primary vessel  180  from its own contact with the vessel wall, and by virtue of its connection to the first portion  152  via the bridging members  158 . This system of supporting one graft with another permits graft positioning very close to the vessel side branches  182 , and is especially effective when the primary vessel is distended even very close to the side branches. 
     The axial dimensions of the various grafts disclosed herein may be selected to match the particular anatomical dimensions surrounding the affected side branch. That is, the grafts, including two tubular sections with an aperture or gap therebetween and bridging members connecting the sections, are sized so as to permit blood flow through the affected side branch and any adjacent side branches. For example, the graft  150  seen in FIGS. 6 and 7 is positioned so that the first portion  152  is above the renal arteries  182  and the second portion  154  is below the renals. 
     A more detailed depiction of the relative axial dimensions for the graft  150  and region of the abdominal artery  180  near the renals  182  is seen in FIGS. 8 and 9. In addition to the renal arteries  182 , the openings for the superior mezzanteric artery  190  and the ciliac artery  192  are shown in FIG.  9 . These arteries typically project in the posterior direction, in contrast to the laterally-directed renals  182 , and are located close to but upstream of the renals. The distance from the lowest of the arteries  190  or  192  and the highest of the renals  182  is given as A, the distance from the upstream side of the highest of the renals  182  to the downstream side of the lowest of the renals is given as B, and the distance between the downstream side of the lowest of the renals to the end of the perceived healthy portion of the abdominal aorta  180  is given as C. In addition, the diameter of one of the renal arteries  182  is given as D. The axial dimensions of the graft  150  are given in FIG. 8 as: L for the overall for the tubular structure, L 1  for the first portion  152 , L 2  for the gap  156 , and L 3  for the second portion  154 . 
     In a preferred embodiment, L 2 &gt;D, and if the renal arteries  182  are offset, L 2 &gt;B. In addition, L 1  is preferably smaller than or equal to A, so that the first portion  152  does not occlude either of the arteries  190  or  192 . Finally, the length L 3  of the second portion  154  is desirably less than the length C of the healthy portion of the abdominal aorta  180 , but may be greater than C. 
     In a specific embodiment, for use in the abdominal aorta  180  to bridge the renal arteries  182 , the graft  150  has a diameter of between about 19 and 30 mm, and a length L of between about 22 and 46 mm. The renal arteries  182  typically have a diameter of between about 5-10 mm, and may be offset center-to-center up to 10 cm. Thus the gap  156  has an axial length L 2  of between about 6 and 20 mm, and is desirably oversized to ensure open blood flow through the renals and to accommodate offset or otherwise misaligned pairs of renals. The length L 1  for the first portion  152  is desirably about 6 mm, but may vary depending on need. The length C of the healthy portion of the abdominal aorta  180  should be at least 5 mm to enable the proper seal of the second portion  154  with the aorta, which is smaller than an endovascular repair would currently be indicated. The length L 3  of the second portion  154  is preferably at least 6 mm, more preferably about 10-20 mm. Of course, if the graft  20  is used to repair a longer section of vessel as a primary graft, the length L 3  of the second portion  154  can be longer than 20 mm, up to the currently accepted maximum length of straight tube vascular graft. 
     To ensure the proper size/configuration of graft, the surgeon first determines the anatomical landscape through the use of angioscopy; that is, by injecting a contrast media and visualizing flow through the affected vessels with an X-ray device. The dimensions noted in FIG. 9 can thus be obtained. A range of different sized grafts are preferably available, and the surgeon then selects the graft to match the anatomy in conformance with the above preferred guidelines. 
     During implantation, the surgeon can ensure proper placement and orientation of the grafts of the present invention with the use of radiopaque markers on the graft. For example, the stent structure, or portions thereof, could be radiopaque, or markers can be attached to the stent or graft body. In FIG. 7, for instance, the wireform  160  and the upper wireform in the stent  162  are desirably radiopaque so as to enable the surgeon to monitor the approximate axial borders of the gap  156 . Furthermore, the bridging members  158  or crimps  176  may be radiopaque to enable rotational orientation with respect to the respective side branch or branches. 
     A method of supporting a tubular primary graft in a primary blood vessel adjacent a vessel side branch, in accordance with the present invention can be illustrated with reference to the embodiment of FIG.  6 . First, the tubular graft  150  is implanted in the primary vessel  180  such that the first portion  160  contacts and supports the primary vessel on one side of a side branch  182 , in this case the two renal arteries, and the second portion  154  contacts and supports the primary vessel on the other side of the side branch. Implantation of the tubular graft  150  can be accomplished by releasing a self-expandable version of the graft from within a catheter sheath in the proper location, or positioning a balloon-expandable version of the graft and inflating a balloon within the interior of the graft. A primary graft  186  is then delivered in a radially constricted state to a position overlapping the end of the second portion  154  and radially expanded into contact therewith. Again, the primary graft  186  may be either self-expanding or balloon-expanding. 
     An alternative method comprises implanting the tubular graft  150  after the implantation of the primary graft  186 . That is, the second portion  154  of the tubular graft  150  is self- or balloon-expanded outward into contact with the primary graft  186 . Indeed, the primary graft  186  may be implanted for a significant period of time before the need for the supporting function of the tubular graft  150  is recognized. 
     As mentioned above, one tubular portion of the graft may perform an anchoring function to maintain the position of the other portion that may or may not have the same anchoring characteristics. For instance, the graft portion upstream of the side branch may anchor the downstream portion, which in turn reinforces, supplements or seals with the primary vessel so as to enable placement of another graft in that location. The present invention has been described so far in terms of self- or balloon-expandable stents for anchoring, but those of skill in the art will recognize that there are other ways to anchor. For instance, staples, bent or corkscrew, are becoming more sophisticated and effective, and may be used for anchoring. For that matter, any means for anchoring one portion of the graft can be used. 
     As shown in FIGS. 10,  11  and  12 , the graft of the present invention may have an exterior surface that is designed to promote tissue ingrowth and prevent leaking. Such surface will also assist in anchoring the graft in the vessel. The graft as shown in FIG. 10 has an exterior surface that is covered in short, looped, “towel-like” floaters  37  extending from the surface of the graft  20 . These towel-like floaters  37 , which do not significantly impact the porosity of the graft material, serve the dual purpose of preventing endoleak and encouraging tissue ingrowth into the woven graft from which they extend. As discussed in greater detail above, the towel-like floaters are incorporated with the woven graft during the weaving process. The lengths of the loop may be varied, for example by increasing the length of the floater without increasing the distance between its end points (the points at which the fiber forming the floater extend into the woven graft material). 
     The exterior surface of the graft  20  as shown in FIG. 11 has two textures. First, there is a cuff of towel-like floaters  37  extending around the circumference of the graft  20  towards its ends, forming a cuff. In one embodiment of the present invention, this cuff extends between 1 and 3 centimeters from each end along the length of the graft. In the preferred embodiment, the cuff extends approximately 2 cm from each end along the length of the graft. However, one skilled in the art would realize that this length would vary depending on the dimensions of the graft. Secondly, the graft  20  has a plurality of strands  39  extending along the length of the graft parallel to the exterior surface. These strands  39 , which are typically longer than the towel-like floaters  37 , are particularly suited for encouraging ingrowth along aneurysmic areas of the vessel. The two textures of the exterior of the graft serve the dual purpose of preventing endoleak and encouraging tissue ingrowth into the woven graft from which they extend. 
     The towel-like floaters  37  are typically present on the exterior surface of the graft in amounts ranging from at least about 138 ‘picks per inch’ (ppi) to about 222 ppi across the graft, while the vertical measure of ppi remains variable. Those skilled in the art will readily understand that variation of the density and concentration of floaters creates higher degrees of cellular matrix seeding, and therefore concomitant tissue ingrowth. Similarly, the ability to combine more than one texture in a ‘tapestry-like’ fashion is understood by artisans as permitting stent-graft users to create product with varying degrees of attachment. 
     Like the graft  20  shown in FIG. 11, the exterior surface of the graft as shown in FIG. 12 has two textures. First, there are cuffs of towel-like threads  37  extending around the circumference of the graft  20  towards its ends. In one embodiment of the present invention, this cuff extends between 1 and 3 centimeters from each end along the length of the graft. In the preferred embodiment, the cuff extends approximately 2 cm from each end along the length of the graft. However, one skilled in the art would realize that this length would vary depending on the dimensions of the graft. Secondly, the graft  20  is equipped with a plurality of fibrous, angled floaters  41  extending from the exterior surface of the graft  20 . As shown in FIG. 12, each angled floater  41  intersects at least one other angled floater. The angled floaters  41  are typically present on the exterior surface of the graft in amounts ranging from at least about 138 to about 222 ppi, preferably from about 150 to about 210 PPI. As discussed above, variation in the number, concentration and relative densities are within the ambit of the instant teachings, and would become clear to those having a modicum of skill upon a review of the claims appended hereto. 
     One skilled in the art would realize that such angled floaters  41  may alternatively be aligned so that they lie parallel to each other, or in sections of floaters  41  lying parallel to one another, so that they do not intersect another floater. These angled floaters  41 , which are typically longer than the towel-like floaters  37 , are particularly suited for encouraging ingrowth along aneurysmic areas of the vessel. 
     While the foregoing is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Moreover, it will be obvious that certain other modifications may be practiced within the scope of the appended claims. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.