Patent Publication Number: US-6984243-B2

Title: Abrasion resistant vascular graft

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to devices and methods for repairing aneurysms and more particularly, to percutaneously and/or intraluminally delivered devices and methods for repairing aneurysms such as abdominal aortic aneurysms and thoracic aortic aneurysms. 
   2. Discussion of the Related Art 
   An aneurysm is an abnormal dilation of a layer or layers of an arterial wall, usually caused by a systemic collagen synthetic or structural defect. An abdominal aortic aneurysm is an aneurysm in the abdominal portion of the aorta, usually located in or near one or both of the two iliac arteries or near the renal arteries. The aneurysm often arises in the infrarenal portion of the diseased aorta, for example, below the kidneys. A thoracic aortic aneurysm is an aneurysm in the thoracic portion of the aorta. When left untreated, the aneurysm may rupture, usually causing rapid fatal hemorrhaging. 
   Aneurysms may be classified or typed by their position as well as by the number of aneurysms in a cluster. Typically, abdominal aortic aneurysms may be classified into five types. A Type I aneurysm is a single dilation located between the renal arteries and the iliac arteries. Typically, in a Type I aneurysm, the aorta is healthy between the renal arteries and the aneurysm and between the aneurysm and the iliac arteries. 
   A Type II A aneurysm is a single dilation located between the renal arteries and the iliac arteries. In a Type II A aneurysm, the aorta is healthy between the renal arteries and the aneurysm, but not healthy between the aneurysm and the iliac arteries. In other words, the dilation extends to the aortic bifurcation. A Type II B aneurysm comprises three dilations. One dilation is located between the renal arteries and the iliac arteries. Like a Type II A aneurysm, the aorta is healthy between the aneurysm and the renal arteries, but not healthy between the aneurysm and the iliac arteries. The other two dilations are located in the iliac arteries between the aortic bifurcation and the bifurcations between the external iliacs and the internal iliacs. The iliac arteries are healthy between the iliac bifurcation and the aneurysms. A Type II C aneurysm also comprises three dilations. However, in a Type II C aneurysm, the dilations in the iliac arteries extend to the iliac bifurcation. 
   A Type III aneurysm is a single dilation located between the renal arteries and the iliac arteries. In a Type III aneurysm, the aorta is not healthy between the renal arteries and the aneurysm. In other words, the dilation extends to the renal arteries. 
   A ruptured abdominal aortic aneurysm is presently the thirteenth leading cause of death in the United States. The routine management of abdominal aortic aneurysms has been surgical bypass, with the placement of a graft in the involved or dilated segment. Although resection with a synthetic graft via transperitoneal or retroperitoneal procedure has been the standard treatment, it is associated with significant risk. For example, complications include perioperative myocardial ischemia, renal failure, erectile impotence, intestinal ischemia, infection, lower limb ischemia, spinal cord injury with paralysis, aortaenteric fistula, and death. Surgical treatment of abdominal aortic aneurysms is associated with an overall mortality rate of five percent in asymptomatic patients, sixteen to nineteen percent in symptomatic patients, and is as high as fifty percent in patients with ruptured abdominal aortic aneurysms. 
   Disadvantages associated with conventional surgery, in addition to the high mortality rate, include an extended recovery period associated with the large surgical incision and the opening of the abdominal cavity, difficulties in suturing the graft to the aorta, the loss of the existing thrombosis to support and reinforce the graft, the unsuitability of the surgery for many patients having abdominal aortic aneurysms, and the problems associated with performing the surgery on an emergency basis after the aneurysm has ruptured. Further, the typical recovery period is from one to two weeks in the hospital and a convalescence period, at home, ranging from two to three months or more, if complications ensue. Since many patients having abdominal aortic aneurysms have other chronic illnesses, such as heart, lung, liver and/or kidney disease, coupled with the fact that many of these patients are older, they are less than ideal candidates for surgery. 
   The occurrence of aneurysms is not confined to the abdominal region. While abdominal aortic aneurysms are generally the most common, aneurysms in other regions of the aorta or one of its branches are possible. For example, aneurysms may occur in the thoracic aorta. As is the case with abdominal aortic aneurysms, the widely accepted approach to treating an aneurysm in the thoracic aorta is surgical repair, involving replacing the aneurysmal segment with a prosthetic device. This surgery, as described above, is a major undertaking, with associated high risks and with significant mortality and morbidity. 
   Over the past five years, there has been a great deal of research directed at developing less invasive, endovascular, i.e., catheter directed, techniques for the treatment of aneurysms, specifically abdominal aortic aneurysms. This has been facilitated by the development of vascular stents, which can and have been used in conjunction with standard or thin-wall graft material in order to create a stent-graft or endograft. The potential advantages of less invasive treatments have included reduced surgical morbidity and mortality along with shorter hospital and intensive care unit stays. 
   Stent-grafts or endoprostheses are now Food and Drug Administration (FDA) approved and commercially available. Their delivery procedure typically involves advanced angiographic techniques performed through vascular accesses gained via surgical cutdown of a remote artery, which may include the common femoral or brachial arteries. Over a guidewire, the appropriate size introducer will be placed. The catheter and guidewire are passed through the aneurysm. Through the introducer, the stent-graft will be advanced to the appropriate position. Typical deployment of the stent-graft device requires withdrawal of an outer sheath while maintaining the position of the stent-graft with an inner-stabilizing device. Most stent-grafts are self-expanding; however, an additional angioplasty procedure, e.g., balloon angioplasty, may be required to secure the position of the stent-graft. Following the placement of the stent-graft, standard angiographic views may be obtained. 
   Due to the large diameter of the above-described devices, typically greater than twenty French (3F=1 mm), arteriotomy closure typically requires open surgical repair. Some procedures may require additional surgical techniques, such as hypogastric artery embolization, vessel ligation, or surgical bypass in order to adequately treat the aneurysm or to maintain blood flow to both lower extremities. Likewise, some procedures will require additional advanced catheter directed techniques, such as angioplasty, stent placement and embolization, in order to successfully exclude the aneurysm and efficiently manage leaks. 
   While the above-described endoprostheses represent a significant improvement over conventional surgical techniques, there is a need to improve the endoprostheses, their method of use and their applicability to varied biological conditions. Accordingly, in order to provide a safe and effective alternate means for treating aneurysms, including abdominal aortic aneurysms and thoracic aortic aneurysms, a number of difficulties associated with currently known endoprostheses and their delivery systems must be overcome. One concern with the use of endoprostheses is the prevention of endo-leaks and the disruption of the normal fluid dynamics of the vasculature. Devices using any technology should preferably be simple to position and reposition as necessary, should preferably provide an acute, fluid tight seal, and should preferably be anchored to prevent migration without interfering with normal blood flow in both the aneurysmal vessel as well as branching vessels. In addition, devices using the technology should preferably be able to be anchored, sealed, and maintained in bifurcated vessels, tortuous vessels, highly angulated vessels, partially diseased vessels, calcified vessels, odd shaped vessels, short vessels, and long vessels. In order to accomplish this, the endoprostheses should preferably be highly durable, extendable and re-configurable while maintaining acute and long-term fluid tight seals and anchoring positions. 
   The endoprostheses should also preferably be able to be delivered percutaneously utilizing catheters, guidewires and other devices which substantially eliminate the need for open surgical intervention. Accordingly, the diameter of the endoprostheses in the catheter is an important factor. This is especially true for aneurysms in the larger vessels, such as the thoracic aorta. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the potential disadvantages associated with percutaneously delivered endoprostheses as briefly described above. 
   In accordance with one aspect, the present invention is directed to an endovascular graft. The endovascular graft comprises one or more scaffold structures, a biocompatible, high tensile strength, abrasion resistant, highly durable thin-walled graft material affixed to the one or more scaffold structures, and at least one connector for connecting the graft material to the one or more scaffold structures. 
   In accordance with another aspect, the present invention is directed to an endovascular graft. The endovascular graft comprises a plurality of individual stent structures and a graft material formed from an ultra high molecular weight polyethylene yarn affixed to an outside portion of the plurality of individual stents. 
   In accordance with another aspect, the present invention is directed to an endovascular graft comprising a substantially tubular structure formed from ultra high molecular weight polyethylene. 
   The abrasion resistant stent-graft of the present invention comprises at least one stent segment and a highly durable, abrasion-resistant graft material attached thereto. The graft material may be attached to the at least one stent segment in any number of ways. The stent-graft may be utilized as a component of a larger system, for example, in a system for repairing abdominal aortic aneurysms, or as a stand-alone device. In either embodiment, the stent-graft is utilized as a fluid carrying conduit that is preferably percutaneously delivered, but may also be utilized surgically. The at least one stent segment may comprise any suitable scaffold structure and may be fabricated from any number of biocompatible materials. The at least one stent segment may be self-expanding or balloon expandable. 
   The abrasion resistant stent-graft of the present invention is preferably percutaneously delivered, and as such it is preferably designed with the smallest diameter possible. In order to achieve the smallest diameter possible, thinner graft materials are needed. However, stent-grafts are typically positioned within the body in vessels that have relatively high hydrodynamic forces, thus requiring graft materials which are able to withstand these forces. Essentially, these forces tend to wear the graft material at the points where it is connected to the at least one stent segment. Over time, the graft material may develop microleaks which obviously defeat the purpose of the stent-graft, namely, as a by-pass conduit. Accordingly, the abrasion resistant stent-graft of the present invention utilizes a biocompatible, high tensile strength, abrasion resistant, highly durable yarn which may be woven, knitted or braided into a graft material without sacrificing diameter. 
   The yarn or thread may comprise a single component or it may be blended with one or more other suitable materials to achieve various desirable characteristics, including abrasion resistance, flexibility and thinness. One such yarn comprises ultra high molecular weight polyethylene, which is commercially available. Accordingly, the abrasion resistant stent-graft of the present invention is a highly durable stent-graft which, because of its thin graft material, may be percutaneously delivered more easily than present stent-grafts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
       FIG. 1  is an elevational view of an endovascular graft in accordance with the present invention. 
       FIG. 2  is a perspective view of an expanded stent segment of the endovascular graft in accordance with the present invention. 
       FIG. 2A  is a fragmentary perspective view of a portion of the stent segment of  FIG. 2 . 
       FIG. 2B  is a fragmentary perspective view of a portion of the stent segment of  FIG. 2 . 
       FIG. 2C  is an enlarged plan view of a section of the stent segment of  FIG. 2 . 
       FIG. 2D  is an enlarged plan view of a section of the stent segment of  FIG. 2 . 
       FIG. 3  is a perspective view of another expanded stent segment of the endovascular graft in accordance with the present invention. 
       FIG. 4  is an elevational view of an endovascular graft in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to an endovascular graft which may be utilized as a component in a system for use in treating or repairing aneurysms. Systems for treating or repairing aneurysms such as abdominal aortic aneurysms and thoracic aortic aneurysms come in many forms. A typical system includes an anchoring and/or sealing component which is positioned in healthy tissue above the aneurysm and one or more grafts which are in fluid communication with the anchoring and/or sealing component and extend through the aneurysm and anchor in healthy tissue below the aneurysm. Essentially, the grafts are the components of the system that are utilized to establish a fluid flow path from one section of an artery to another section of the same or different artery, thereby bypassing the diseased portion of the artery. Current systems are preferably percutaneously delivered and deployed. 
   As stated above, the present invention is directed to one component of an aneurysm repair system; namely, the endovascular graft of stent-graft. Accordingly, the following detailed description is directed to the endovascular graft. The endovascular graft comprises at least one stent segment and a highly durable, abrasion-resistant graft material attached thereto. In other words, the endovascular graft of the present invention is supported internally by one or more individual stents, which are themselves connected to the graft in a manner which secures their position, for example, by sutures. It is important to note that while one particular stent design is discussed in detail below, the graft of the present invention may incorporate any number of suitable stent designs, including self-expanding stents and balloon expandable stents. In addition, the endovascular graft may comprise a device formed solely from the graft material. 
     FIG. 1  illustrates an exemplary embodiment of an endovascular graft  10  in accordance with the present invention. The exemplary endovascular graft  10  comprises one or more first stent segments  100 , one second stent segment  200  and a third stent segment  300 . In order to illustrate the relationship of the various components comprising the endovascular graft  10 , the endovascular graft is illustrated in the figure as though the graft material were transparent. In a typical use scenario, the third stent segment  300  would be anchored in healthy tissue below the aneurysm and the uppermost first stent segment  100  would be in fluid communication with an anchoring and/or sealing component as briefly described above. It is important to note, however, that depending on the design of the system, an anchoring and/or sealing component may not be necessary. The second stent segment  200  comprises a tapered profile, having a diameter at one end equal to that of the first stent segments  100  and a diameter at the other end equal to that of the third stent segment  300 . The length of the endovascular graft may be varied by the number of first stent segments  100  utilized. 
     FIG. 2  is a detailed perspective view of an exemplary embodiment of the third stent segment  300 . The third stent segment  300  comprises a plurality of struts  302  connected in a substantially zigzag pattern. As illustrated, the exemplary third stent segment  300  comprises three sets of zigzag-connected stents  302 , thereby forming substantially diamond-shaped cells. The non-connected apex  304  of each diamond shaped cell, illustrated in greater detail in  FIG. 2A , comprises a smooth, uniform width curved region formed at the intersection of two stents  302  of each diamond-shaped cell. This shape is cut directly into the stent segment  300  during the initial machining steps, typically laser cutting, as is explained in detail subsequently, and is maintained during all subsequent finishing processing. The junctions  306  between the zigzag-connected stents  302 , illustrated in greater detail in  FIG. 2B  occurs at the intersection of four struts  302 . Preferably, each junction  306  of four struts  302  comprises two indentations  308  and  310  as illustrated in  FIG. 2B . 
   The regions proximate the non-connected apexes  304  and the junctions  306  are generally the highest stress regions in the third stent segment  300 . To minimize the stresses in these regions, these regions are designed to maintain uniform beam widths proximate where the struts  302  interconnect. Beam width refers to the width of a strut  306 . Indentations  308  and  310  are cut or machined into the junctions  306  to maintain a uniform beam width in this area, which is generally subject to the highest stress. Essentially, by designing the junctions  306  to maintain uniform beam widths, the stress and strain that would normally build up in a concentrated area, proximate the junction  306 , is allowed to spread out into the connecting regions, thereby lowering the peak values of the stress and strain in the stent structure. 
   To further minimize the maximum stresses in the struts  302  of the third stent segment  300 , the struts  302  may have a tapering width. For example, in one exemplary embodiment, the struts  302  may be designed to become wider as it approaches a junction  306 .  FIG. 2C  is an enlarged partial view of the third sent segment  300  in its expanded conditions which illustrates the tapering width of the struts  302 . In this exemplary embodiment, the strut  302  proximate the junction  306  (width a) is about 0.025 cm and gradually tapers to a dimension of about 0.0178 cm in the mid-region of the strut  302  (width b). By tapering the struts&#39; widths, the stresses in the struts  302  adjacent the junction  306  is spread out away from the junction  306 . The tapering of the struts  302  is accomplished during the machining of the tube of material from which the stent  300  is cut, as described in detail subsequently. However, by tapering the struts  302  in this manner, there is a tradeoff. The stent segment  300  becomes somewhat less resistant to localized deformations, caused for example, by a protrusion within the vessel lumen. This localized deformation may lead to a local torsional loading on some of the struts  302 , and, therefore, since the struts  302  in this exemplary embodiment have a relatively significant portion of their length with a reduced width, their torsional rigidity is reduced. 
   If maximizing the resistance to localized deformation is preferred, the struts  302  may be maintained at a uniform width, or more preferably have a reverse taper, as illustrated in  FIG. 2D , wherein the width at point a is less than the width at point b. In this exemplary embodiment, the reverse taper struts  302  are about 0.025 cm proximate the junction  306  and about 0.028 cm in the central region of the struts. While this reverse taper tends to increase the stresses somewhat proximate the junctions  306 , this increase is very small relative to the decrease in stresses gained by having the side indentations  308 ,  310  illustrated in  FIG. 2B , as well as the uniform width connections illustrated in  FIG. 2A . In addition, since the reverse taper serves to increase the torsional rigidity of the strut  302 , the stent structure resists local deformation and tends to maintain a substantially circular cross-sectional geometry, even if the lumen into which the stent is positioned in non-circular in cross-section. 
   In a preferred exemplary embodiment, the third stent segment  300  is fabricated from a laser cut tube, as described in detail subsequently, of initial dimensions 0.229 cm inside diameter by 0.318 cm outside diameter. The struts  302  are preferably 0.0229 cm wide adjacent the four strut junctions  306  and six mm long, with a reverse taper strut width. Also, to minimize the number of different diameter combination of grafts systems, it is preferred that the third stent segment  300  have an expanded diameter of sixteen mm. Similarly, the proximal portion of the graft material forming the legs is flared, having a diameter of sixteen mm. This single diameter for the third stent segment of the graft system would enable its use in arteries having a non-aneurysmal region of a diameter from between eight and fourteen mm in diameter. It is also contemplated that multiple diameter combinations of third stent segment  300  and graft flare would be desirable. 
   Referring back to  FIG. 1 , the one or more first stent segments  100  are also formed from a shape set laser cut tube, similar to the third stent segment  300  described above. The one or more first stent segments  100  comprise a single circumferential row of zigzag or sinusoidally arranged elements. In the exemplary embodiment illustrated in  FIG. 1 , and in greater detail in  FIG. 3 , the first stent segment  100  comprises ten zigzag or sinusoidal undulations. The one or more first stent segments  100  are formed with uniform width connections at the intersections  104  of the struts  102  forming the zigzag or sinusoidal pattern. The one or more first stent segments  100  are preferably cut from tubing having an inside diameter of 0.251 cm and an outside diameter of 0.317 cm. The strut widths are preferably about 0.33 cm wide adjacent strut intersections  104  and the struts  102  are preferably seven mm long and the one or more first stent segments  100  are preferably eleven mm in diameter when expanded. 
   Referring back to  FIG. 1 , the second stent segment  200  comprises a tapered profile, having a diameter at one end which is the same as the one or more first stent segments  100 , and a diameter at the other end matching the diameter of the third stent segment  300 . The second stent segment  200  is identical to the one or more first stent segments  100  except for the taper. 
   As is explained in detail subsequently, the stent segments  100 ,  200  and  300  are secured in position by the graft material. 
   The first, second and third stent segments  100 ,  200 ,  300  are preferably self-expandable and formed from a shape memory alloy. Such an alloy may be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The application of a desired temperature causes the alloy to revert to an original heat-stable configuration. A particularly preferred shape memory alloy for this application is binary nickel titanium alloy comprising about 55.8 percent Ni by weight, commercially available under the trade designation NITINOL. This NiTi alloy undergoes a phase transformation at physiological temperatures. A stent made of this material is deformable when chilled. Thus, at low temperatures, for example, below twenty degrees centigrade, the stent is compressed so that it can be delivered to the desired location. The stent may be kept at low temperatures by circulating chilled saline solutions. The stent expands when the chilled saline is removed and it is exposed to higher temperatures within the patient&#39;s body, generally around thirty-seven degrees centigrade. 
   In preferred embodiments, each stent is fabricated from a single piece of alloy tubing. The tubing is laser cut, shape-set by placing the tubing on a mandrel, and heat-set to its desired expanded shape and size. 
   In preferred embodiments, the shape setting is performed in stages at five hundred degrees centigrade. That is, the stents are placed on sequentially larger mandrels and briefly heated to five hundred degrees centigrade. To minimize grain growth, the total time of exposure to a temperature of five hundred degrees centigrade is limited to five minutes. The stents are given their final shape set for four minutes at five hundred fifty degrees centigrade, and then aged to a temperature of four hundred seventy degrees centigrade to import the proper martensite to austenite transformation temperature, then blasted, as described in detail subsequently, before electropolishing. This heat treatment process provides for a stent that has a martensite to austenite transformation which occurs over a relatively narrow temperature range; for example, around fifteen degrees centigrade. 
   To improve the mechanical integrity of the stent, the rough edges left by the laser cutting are removed by combination of mechanical grit blasting and electropolishing. The grit blasting is performed to remove the brittle recast layer left by the laser cutting process. This layer is not readily removable by the electropolishing process, and if left intact, could lead to a brittle fracture of the stent struts. A solution of seventy percent methanol and thirty percent nitric acid at a temperature of minus forty degrees centgrade or less has been shown to work effectively as an electropolishing solution. Electrical parameters of the electropolishing are selected to remove approximately 0.00127 cm of material from the surfaces of the struts. The clean, electropolished surface is the final desired surface for attachment to the graft materials. This surface has been found to import good corrosion resistance, fatigue resistance, and wear resistance. 
   The graft material or component  400 , as illustrated in  FIG. 4 , may be made from any number of suitable biocompatible materials, including woven, knitted, sutured, extruded, or cast materials comprising polyester, polytetrafluoroethylene, silicones, urethanes, and ultralight weight polyethylene, such as that commercially available under the trade designation SPECTRA™. The materials may be porous or nonporous. Exemplary materials include a woven polyester fabric made from DACRON™ or other suitable PET-type polymers. 
   In one exemplary embodiment, the fabric for the graft material is a forty denier (denier is defined in grams of nine thousand meters of a filament or yarn), twenty-seven filament polyester yarn, having about seventy to one-hundred end yarns per cm per face and thirty-two to forty-six pick yarns per cm face. At this weave density, the graft material is relatively impermeable to blood flow through the wall, but is relatively thin, ranging between 0.08 and 0.12 mm in wall thickness. 
   The graft component  400  is a single lumen tube and preferably has a taper and flared portion woven directly from the loom, as illustrated for the endovascular graft  10  shown in  FIG. 1 . 
   Prior to attachment of the graft component  400  to the stents  100 ,  200 ,  300 , crimps are formed between the stent positions by placing the graft material on a shaped mandrel and thermally forming indentations in the surface. In the exemplary embodiment illustrated in  FIGS. 1 and 4 , the crimps  402  in the graft  400  are about two mm long and 0.5 mm deep. With these dimensions, the endovascular graft  10  can bend and flex while maintaining an open lumen. Also, prior to attachment of the graft component  400  to the stents  100 ,  200   300 , the graft material is cut in a shape to mate with the end of each end stent. 
   As stated above, each of the stent segments  100 ,  200  and  300  is attached to the graft material  400 . The graft material  400  may be attached to the stent segments  100 ,  200 ,  300  in any number of suitable ways. In one exemplary embodiment, the graft material  400  may be attached to the stent segments  100 ,  200 ,  300  by sutures. 
   The method of suturing stents in place is important for minimizing the relative motion or rubbing between the stent struts and the graft material. Because of the pulsatile motion of the vasculature and therefore the graft system, it is possible for relative motion to occur, particularly in areas where the graft system is in a bend, or if there are residual folds in the graft material, due to being constrained by the aorta or iliac arteries. 
   Ideally, each strut of each stent segment is secured to the graft material by sutures. In an exemplary embodiment, the suture material is blanket stitched to the stent segments at numerous points to securely fasten the graft material to the stent segments. As stated above, a secure hold is desirable in preventing relative motion in an environment in which the graft system experiences dynamic motion arising from pulsatile blood pressure, in addition to pulsation of the arteries that are in direct mechanical contact with the graft system. The stents nearest the aortic and iliac ends of the graft system (the uppermost first stent segment  100  and the third stent segment  300  respectively) are subject to the pulsatile motion arising from direct internal contact. These struts in particular should be well secured to the graft material. As illustrated in  FIG. 4 , the stitches  404  on the upper most first stent segment  100  are positioned along the entire zigzag arrangement of struts. The upper and lower apexes of the third stent segment may be stitched utilizing a similar configuration. It is difficult to manipulate the suture thread precisely around the struts that are located some distance away from an open end, accordingly, various other simpler stitches may be utilized on these struts, or no stitches may be utilized in these areas. 
   As illustrated in  FIG. 4 , each of the struts in the first stent segment  100  is secured to the graft material  400  which has been cut to match the shape of the stent segment  100 . The blanket stitching  404  completely encircles the strut and bites into the graft material  400 . Preferably, the stitch  404  encircles the strut at approximately five equally spaced locations. Each of the struts on each end of the third stent segment  300  is attached to the graft material, which has been cut to make the shape of the stent segment  300 , in the same manner as the first stent segment  100 . 
   A significant portion of the graft will not rest directly against vascular tissue. This portion of the graft will be within the dilated aneurysm itself. Therefore, this portion of the graft will not experience any significant pulsatile motion. For this reason, it is not necessary to secure the stent segments to the graft material as aggressively as the stent structure described above. Therefore, only point stitches  406  are necessary for securing these stents. 
   It is important to note that a wide variety of sutures are available. It is equally important to note that there are a number of alternative means for attaching the graft material to the stent, including welding, gluing and chemical bonding. 
   As stated above, In percutaneous procedures, size is a critical factor. One of the more significant determinants of the final diameter of the catheter system is the bulkiness of the graft material comprising the stent-graft. Accordingly, it is generally accepted that the highest impact on delivery catheter diameter may be achieved by fabricating stent-grafts having thinner walls. 
   Typical stent-grafts are fabricated from a woven polyester and are approximately 0.005 inches thick. For example, a stent-graft fabricated from a woven polyester low twist, forty denier, twenty-seven filament yarn having two-hundred thirty yarn ends per inch and one hundred yarn picks per inch, results in a graft material having a wall thickness of approximately 0.005 inches. The graft material is then attached to the inside or outside of a stent or multiple stent segments as described above. Appreciable gains may be achieved in having a graft material thickness in the range from about 0.002 inches to about 0.003 inches. 
   For a woven graft, as described above, the wall thickness is determined primarily by weave density and yarn thickness or bulkiness. It is desirable to have a graft which is packed tight enough to prevent significant blood seepage, but not so tight that the yarn bundles pile up on each other. The weaving parameters described above result in just such a graft for the particular yarn described. At this density, the graft material is about as thin walled as it can be without significant permeability. Also, the yarn described above is only lightly twisted, so as the yarn bundles cross over one another, they tend to flatten out. Higher twisting would both make the graft more permeable and thicker, and the yarn bundle would tend to remain cylindrical at the crossover points. The only remaining parameter that can be utilized to thin the graft is smaller yarn bundles. 
   There are two variables which influence yarn bundle size; namely, the number of filaments per bundle, and the size or weight of each individual filament. The forty denier, twenty-seven filament polyester yarn described above has a relatively small filament size and a relatively low number of filaments. However, in theory, a much smaller yarn bundle could be contemplated with either few filaments, smaller filaments, or both. For example, a twenty denier yarn bundle could be made from fourteen filaments of the same diameter as described above. If this yarn were woven into a graft material with an appropriately dense weave, one would expect a graft material having a thickness of approximately 0.0025 inches. While this may work as an acceptable graft, it is possible that the long-term integrity of such a graft may not be acceptable due to the forces described above. 
   The graft material may be formed utilizing any number of techniques, including weaving, knitting and braiding. Weaving involves the interlacing, at right angles, of two systems of threads known as warp and filling. Warp threads run lengthwise in a woven fabric and filling threads run cross-wise. Knitting is the process of making fabric by interlocking a series of loops of one or more threads. Braiding involves crossing diagonally and lengthwise several threads of any of the major textile fibers to obtain a certain width effect, pattern or style. 
   A growing concern with a number of endovascular graft systems has been that over time, holes may develop in the stent-graft wall, which can lead to blood leakage and possible aneurysm rupture. There is only a limited understanding of the mechanism of hole formation; however, it is generally believed to be related to what has been termed chronic micro-motion between the metallic stent support structures and the graft material. Eventually, this micro-motion may cause the graft material to wear away, thereby creating holes. 
   One potential way in which to overcome this problem is by more tightly binding the graft material to the stent in areas exhibiting the highest possibility of micro-motion. There are numerous ways by which the graft material may be attached to the stent, for example, polymeric sutures. Accordingly, it may be possible to simply create a thinner polyester graft material as described above, more tightly secure it to the stent in areas which exhibit the greatest potential for micro-motion, and have a lower profile, longer wear resistant stent-graft. However, it would also be beneficial to consider alternate materials for fabricating a significantly thinner graft material with high wear resistance. Higher strength and/or tougher materials may yield a much thinner stent-graft conduit without sacrificing long-term integrity. In fact, some of the materials that may be utilized are so much stronger and tougher than Dacron® polyester, that a significantly thinner stent-graft constructed of these materials may be substantially stronger and more wear resistant than currently available stent-grafts. 
   There are a number of new, higher performance fibers that are significantly stronger and tougher than polyester, and which are also biocompatible. Whereas, Dacron® polyester has a tenacity of approximately nine grams per denier, many high performance fibers have tenacities in the range from about thirty-five to about forty-five grams per denier. The more preferred fibers from a strength standpoint for consideration for use in an ultra thin walled stent-graft material, approximately, 0.002 to 0.003 inches include polyaramid, polyphenylenebenzobisoxazole, liquid crystal polymer and ultra high molecular weight polyethylene. From a purely strength standpoint, all of these materials are suitable for ultra-thin walled stent-graft applications. However, from a biostability standpoint, ultra high molecular weight polyethylene fibers may offer a slight advantage in the fact that their basic chemistry is polyethylene, which is known to be relatively inert in biological applications. 
   Another important consideration for the above-described fibers is their availability in fine denier yarns. With current stent-grafts fabricated from a forty denier polymer yarn, it would be difficult to fabricate a stent-graft having thinner walls unless the yarn is of a finer denier. A liquid crystal polymer sold under the tradename Vectran is available as a twenty-five denier yarn. A ultra high molecular weight polyethylene sold under the tradename Spectra is available as a thirty-denier yarn. Another ultra high molecular weight polyethylene sold under the tradename Dyneema is available as a twenty to twenty-five denier yarn. It is also important to consider that ultra high molecular weight polyethylene fibers only have a density of 0.97 versus 1.38, so that the same denier yarn would be bulkier in ultra high molecular weight polyethylene, however, due to the substantial improvement in tensile and abrasive properties, much less ultra high molecular weight polyethylene would be necessary to obtain equivalent material properties. 
   Polyethylene is a long chain organic polymer formed by the polymerization of ethylene. When formed under low pressure, it will form long polymer chains which increases its resistance to fracture. Ultra high molecular weight polyethylene typically has between six and twelve million ethylene units per molecule. Ultra high molecular weight polyethylene has a low coefficient of friction, a high molecular weight and a high density. Accordingly, a fabric made from ultra high molecular weight polyethylene is highly abrasion resistant, highly impact resistant, and highly resistant to damage by water, salt or fresh. Ultra high molecular weight polyethylene monofilaments have a high tensile strength with the associated advantage of stretch resistance and elasticity. These properties make it especially suitable for tortuous body passageways. 
   As stated above, polyethylene has a long documented history of biocompatability. Given this level of biocompatability, coupled with its physical attributes, ultra high molecular weight polyethylene is the preferred yarn for use as a graft material. The ultra high molecular weight polyethylene yarn may be woven, knitted or braided to form the graft material and attached to the one or more stent segments as described above. The graft material may also be used as a strand alone device for surgical applications or combined with the one or more stents for endovascular delivery. 
   In alternate exemplary embodiments, the ultra high molecular weight polyethylene yarn may be blended with a dissimilar material, for example, Dacron® polyester, to manufacture a graft material with altered bulk properties; e.g., stretch potential, while retaining strength and abrasion resistance. In yet other alternate exemplary embodiment, the monofilament of ultra high molecular weight polyethylene may be blended together with another material to attain a true blended yarn such that a fiber or monofilament of one material can be placed next to a monofilament of a second material (third, fourth . . . ) to create a resultant yarn which possesses properties that differ from each of its monofilaments. 
   Although shown and described is what is believed to be the most practical and preferred embodiments, it is apparent that departures from specific designs and methods described and shown will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention. The present invention is not restricted to the particular constructions described and illustrated, but should be constructed to cohere with all modifications that may fall within the scope for the appended claims.