Abstract:
The present invention is an improved endovascular device particularly useful for use in transjugular intrahepatic portosystemic shunt (TIPS) procedures. The device employs a two-part stent-graft construction that provides a low permeability membrane to line the shunt and an uncovered stent portion designed to reside in the portal vein. The device provides numerous benefits over previous stents and stent-grafts used in TIPS procedures, including being more compact to deliver, being easier to accurately deploy, a controlled compacted surface with tucked apices, an improved stent winding pattern, and being more flexible in delivery and use.

Description:
RELATED APPLICATIONS 
   The present application is a continuation of application Ser. No. 09/489,604, filed Jan. 20, 2000 and issued as U.S. Pat. No. 6,673,102, which is a continuation-in-part of application Ser. No. 09/235,219, filed Jan. 22, 1999 and now abandoned. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention The present invention relates to endoprostheses such as stents and stent-grafts, and particularly to endoprostheses that are suitable for use in transjugular intrahepatic portosystemic shunt (TIPS) applications. 
   2. Description of Related Art 
   It is known that diseased or damaged liver tissue may increase the resistance to hepatic perfusion resulting in excessive and often dangerous fluid pressure increases in the portal vascular circulation. This condition can lead to gastrointestinal variceal hemorrhage and pathological conditions such as ascites. 
   In order to decompress the portal circulation, a transjugular intrahepatic portosystemic shunt (TIPS) may be created through the liver tissue by connecting the portal vein to the inferior vena cava via the hepatic vein. This procedure first forms a fairly large puncture (for instance, using a 16 or 18 gauge needle) directly through the liver to allow direct flow between the portal vein and the hepatic vein. Next the puncture is lined with a stent (for example, an 8–12 mm stent) to form a shunt. The TIPS procedure has proven to be safe and effective at decompressing the portal system and in controlling acute variceal hemorrhage. A summary of TIPS procedures using unlined Palmaz balloon-expandable stents (available from Cordis Corp., Miami Lakes, Fla.) is provided in Zemel, et al., “Technical Advances in Transjugular Intrahepatic Portosystemic Shunts,” 12  RadioGraphics  615–622 (1992). 
   Unfortunately, for many patients conventional TIPS procedures may provide fairly short-lived improvements. Stenosis or occlusion may occur in up to half of all patients within 6 months following TIPS creation. Blockage of the shunt normally occurs due to pseudointimal formation or intimal hyperplastic response. 
   In the article by Nishimine, et al., “Improved Transjugular Intrahepatic Portosystemic Shunt Patency with PTFE Covered Stent-Grafts: Experimental Results in Swine,” 196  Radiology  341–347 (1995), the authors hypothesize that shunt patency might be improved if flowing blood could be separated from leaking bile, the exposed surface of liver parenchyma, and the injured hepatic vein. In that article the authors describe a device that comprises a thin-walled polytetrafluoroethylene (PTFE) graft (available from W. L. Gore &amp; Associates, Inc., Flagstaff, Ariz.) having single-body Gianturco-Rosch Z-stents (available from Cook Inc., Bloomington, Ind.) mounted on each end for anchorage. Once mounted in place, the authors then deployed one or two WALLSTENT stents (available from Schneider Inc., Minneapolis, Minn.) within the PTFE graft to provide mid-shunt radial support. The authors reported that a PTFE-covered stent-graft provided improved TIPS patency verses uncovered stents in a porcine model. However, the authors also indicated that accurate stent-graft placement was important in order to maintain patency, since occlusion continued to occur when a stent-graft was misplaced in the shunt or if a portion of the intrahepatic tract between the portal vein and the hepatic vein was otherwise left unlined. 
   Nishimine et al. also reported that the two-step deployment of their stent-grafts (that is, deploying a graft with anchoring stents first and then separately deploying WALLSTENT stents within the graft as a second procedure) was “cumbersome and technically challenging.” Nevertheless, as the authors explained, this two-part procedure was necessary in order to allow placement of the stent and graft combination through a small 10-French (F) (3.3 mm) sheath. 
   Accordingly, a lined or otherwise covered stent-graft device would appear to be useful in helping to maintain patency in TIPS procedures. However, a number of problems are presented to someone attempting to provide an improved TIPS stent-graft. 
   First, as Nishimine et al. reported, small profile delivery of a stent-graft device is difficult to accomplish in a single step. Generally percutaneous delivery of a stent or stent-graft requires the device to be delivered at no more than 13-F (4.3 mm). While Nishimine et al. achieved delivery of a stent and graft through a 10-F sheath, this was accomplished through a cumbersome and challenging two-step procedure in which the separate devices were combined in-situ. Other authors have reported delivering stent and graft combined structures in one-step, but have required larger introductory profiles to do so, on the order of about 14-F to 16-F (4.7–5.3 mm): Haskal, et al, “PTFE Encapsulated Endovascular Stent-Graft for Transjugular Intrahepatic Shunts: Experimental Evaluation,” Radiology: 205 (1997); Behesti, et al. “Technical Considerations in Covering and Deploying a Wallstent Endoprosthesis for the Salvage of a Failing Transjugular Intrahepatic Portosystemic Shunt,” JVIR: 9 (1998). 
   Second, also as Nishimine et al. reported, accurate device sizing and placement may be critical in order to successfully avoid a biological response resulting in stenosis or occlusion. Premature device occlusion or shunt malfunction may be caused by a variety of conditions, including: failing to maintain proper flow into and out of the shunt device; failing to position the device to completely cover the intrahepatic tract between the portal and hepatic veins; or extending the device too far in the portal circulation or inferior vena cava. 
   Third, it appears important that the graft portion of the device inhibits seepage of bile and other thrombogenic or mitogenic substances into the blood system. Maintaining bile-exclusion while also seeking to use very thin graft materials in order to achieve small device delivery profiles presents a critical design challenge. 
   Fourth, the stent-graft also needs to have sufficient structural strength to resist distortion during use. Nishimine et al. report that protrusion of tissue through an uncovered stent structure may result in up to 40% of pseudointimal thickness. The present inventors believe that this tissue encroachment may be one of the reasons that uncovered stent structures do not perform very well. While any cover should limit the extent of tissue protruding through the stent structure, it is believed desirable to provide a cover with sufficient strength that it will resist distortion from the inward force of tissue protrusion. Thus, the choice of cover material, as well as its method of attachment to the underlying stent, present design challenges. The stent component also needs sufficient radial strength to prevent the stent-graft from significantly narrowing or collapsing under pressure from scar or fibrotic proliferation. Again, with respect to both the stent and the graft components, the amount of radial strength provided must be balanced against the desire for thin graft membranes and minimal stent size needed to create an overall device with small profile. Also, a desired attribute in the TIPS procedure is that the endoprosthesis is flexible in order to accommodate the tortuous intrahepatic pathway. 
   SUMMARY OF THE INVENTION 
   The present invention is an improved endoprosthetic device specifically designed to line an intrahepatic shunt formed within liver tissue in order to maintain its effectiveness. The present invention addresses the endoprosthesis itself, as well as delivery and deployment apparatus, including associated catheters, capture and restraining means, and other related apparatus. The device of the present invention comprises an endoprosthesis designed to be deployed in the transjugular intrahepatic portosystemic shunt (TIPS) procedure. The endoprosthesis of the present invention comprises a two-part construction having a first, covered, segment that is designed to be positioned within the intrahepatic tract and hepatic vein and a second, uncovered, segment that is designed to reside in the portal vein upstream of the shunt. The endoprosthesis of the present invention provides a unique stent and graft combination that combines a self-expanding stent with high radial strength with a thin-walled graft material that is resistant to permeation by bile and other fluids. 
   The endoprosthesis of the present invention has numerous desirable features. First, the covered segment employs a thin bile exclusionary graft material that protects the shunt from bile infiltration while maintaining small wall thicknesses. Second, the device has a unique one-piece construction that allows it to be easily and accurately placed within the intrahepatic tract. Third, the present invention provides a unique multi-stage deployment procedure that again aids in the accurate placement of the endoprosthesis. Fourth, the device is highly flexible, aiding in its positioning and deployment. Fifth, the device is highly compactable, again aiding its deployment and minimizing device profile. Sixth, the device employs a unique self expanding stent pattern that is believed to be easier to manufacture, easier to deploy, and safer to use than alternative stent patterns. These and other benefits of the present invention will be appreciated from review of the following description. 

   
     DESCRIPTION OF THE DRAWINGS 
     The operation of the present invention should become apparent from the following description when considered in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a side elevation view of one embodiment of an endoprosthesis of the present invention; 
       FIG. 2  is a side elevation view of an endoprosthesis of the present invention shown mounted in its delivery apparatus; 
       FIG. 3  is a top plan view of an endoprosthesis of the present invention, the endoprosthesis being normally cylindrical but is represented in this Figure flat by making a longitudinal cut along the length of the endoprosthesis and the uncoiling the endoprosthesis along this cut into a flat sheet. This view shows in detail the winding pattern of the two segments of the stent component of the endoprosthesis of the present invention; 
       FIG. 4  is a side elevation view of an endoprosthesis of the present invention showing it in its compacted diameter (b) and in its enlarged deployed diameter (a); 
       FIG. 5  is a side elevation view of an endoprosthesis of the present invention shown bent around a rod to demonstrate flexibility via bend radius; 
       FIG. 6   a  is an end view of an endoprosthesis of the present invention at its deployed diameter; 
       FIG. 6   b  is an end view of the endoprosthesis of  FIG. 6   a , with the endoprosthesis shown compacted by folding and rolling the endoprosthesis about a central catheter shaft; 
       FIG. 6   c  is an end view of the endoprosthesis of  FIG. 6   a , with the endoprosthesis shown compacted by radially compressing the endoprosthesis about a central catheter shaft; 
       FIG. 6   d  is an end view of the endoprosthesis of  FIG. 6   a , with the endoprosthesis shown radially compacted by folding into pleats of the endoprosthesis about a central catheter shaft; 
       FIG. 7  is a side elevation view of an endoprosthesis of the present invention shown in its deployed diameter and in a compacted, pleated diameter; 
       FIG. 8   a  is a three-quarter isometric view of one embodiment of a tapered die used to establish a pleated compacted endoprosthesis of the present invention; 
       FIG. 8   b  is a side cross-section view of the tapered die of  FIG. 8   a  along line  8   b — 8   b;    
       FIG. 9  is a side elevation view of an endoprosthesis of the present invention having tether lines positioned around the circumference of one end of the endoprosthesis in specific locations such as to control the locations where the stent forms pleats as it is drawn through a tapered die such as that shown in  FIGS. 8   a  and  8   b;    
       FIG. 10  is a side elevation view of a compacted endoprosthesis of the present invention mounted on a delivery catheter and covered with a packaging sheath; 
       FIG. 11  is a side elevation view of a compacted endoprosthesis of the present invention shown partially deployed after the packaging sheath has been removed; 
       FIG. 12  is a side elevation view of the compacted endoprosthesis of  FIG. 10  with the packaging sheath and delivery catheter being inserted through a hemostatic sheath; 
       FIG. 13  is a side elevation view of the endoprosthesis of  FIG. 12  with the delivery catheter partially advanced into a transjugular sheath with the packaging sheath shown partially removed; 
       FIG. 14  is a side elevation view of a compacted endoprosthesis of the present invention shown advanced into a portal vein; 
       FIG. 15  is a side elevation view of a compacted endoprosthesis of the present invention shown confined within a hemostatic sheath and located at an intrahepatic juncture site in a portal vein; 
       FIG. 16  is a side elevation view of a TIPS endoprosthesis of the present invention being partially deployed in the portal vein through withdrawal of the hemostatic sheath; 
       FIG. 17  is a side elevation view of the TIPS endoprosthesis of the present invention being partially withdrawn to properly position the endoprosthesis within the portal vein, an unconstrained expanded portal stent segment of the endoprosthesis engaging the intrahepatic juncture to assure proper placement; 
       FIG. 18  is a side elevation view of the TIPS endoprosthesis of the present invention having been fully deployed within an intrahepatic tract and the portal vein; 
       FIG. 19  is a three-quarter isometric view of a portion of an apparatus used to test the permeability of the cover of the present invention; 
       FIG. 20  is a side elevation view of an endoprosthesis of the present invention mounted with the fixture elements illustrated in  FIG. 19 ; 
       FIG. 21  is a top plan view of a loop device used to measure radial stiffness of an endoprosthesis of the present invention; 
       FIG. 22  is a three-quarter perspective view of an endoprosthetic device mounted in the loop device of  FIG. 19 , positioned so as to connect the loop device to a tensile tester; and 
       FIG. 23  is a scanning electron micrograph (SEM), enlarged 1000×, of the surface of an expanded polytetrafluoroethylene (PTFE) cover used in the present invention, having a node and fibril structure and having bent fibrils incorporated therein. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is an improved implantable endoluminal device, and especially such a device for establishing and maintaining an intrahepatic portosystemic shunt. As is explained in greater detail below, typically this procedure is performed endoluminally through the jugular vein, connecting the portal vein to the inferior vena cava by way of hepatic vein. As a result, the procedure is commonly referred to as being a “transjugular intrahepatic portosystemic shunt” or abbreviated “TIPS” or “TIPSS.” It should be appreciated, however, that a shunt through the liver between the portal vein and the vena cava may be accomplished by other methods and that the endoprosthesis devices of the present invention are not limited in application to transjugular insertion. As such, the term “intrahepatic portosystemic shunt” as used herein is intended to include any procedure whereby pressure is relieved in the portal vein by way of a shunt from the portal to the systemic systems. 
   As has been explained, typically a shunt is established creating a large tract (for instance, by puncturing with a 16 or 18 gauge needle and then ballooning to about 10 mm) directly through the liver to allow connection between the portal vein and the hepatic vein. Following ballooning, the shunt is generally maintained by lining the puncture with an uncovered stent. The TIPS procedure has proven to be safe and effective at decompressing the portal system and in controlling acute variceal hemorrhage. Unfortunately, for many patients conventional TIPS procedures may provide fairly short-lived improvements. Stenosis or occlusion may occur in a significant number of patients. Blockage of the shunt normally occurs due to thrombosis, pseudointimal formation, or intimal hyperplastic response. While covered stents have been employed with some improvement in subsequent occlusive response, these previous covered stent devices have been difficult to properly position, have had excessively large insertion profiles, and/or are ineffective at bile exclusion—bile being linked to increased hyperplastic response. 
   The present invention addresses all of these deficiencies of previous stent devices by providing an endoprosthesis stent-graft combination that has a small introductory profile, is easily positioned and deployed within the shunt, and employs a cover that is highly resistant to bile permeation. 
   Shown in  FIG. 1  is one embodiment of an endoprosthesis  10  of the present invention. The endoprosthesis comprises a two-part stent element  12  having a first segment  14  and a second segment  16 . A cover  18  is provided along the length of the first segment  14 , while the second segment  16  is left uncovered. 
   The first segment  14  and the cover  18  are adhered together (for instance by an adhesive and/or by a wrap of an adhered film) to maintain the position of the cover  18  on the endoprosthesis  10 . The attachment of the cover to the stent element  12  also prevents the first segment  14  from excessively longitudinally elongating when longitudinal tension is applied to the endoprosthesis  10 . It is believed preferable that the cover  18  line the interior of the stent element  12 , as shown, but acceptable results may also be achieved with the cover  18  placed on the outside of the stent element  12 , with the cover being placed both inside and outside of the stent element  12 , or with the stent element  12  being embedded within the cover  18 . As such, the term “cover” as used herein is intended to include any generally continuous material that is placed inside of and/or placed outside of and/or mounted integrally with the stent element  12  of the present invention. 
   As will become clear from the following description, the second segment  16  of the stent is left uncovered to permit its permanent deployment within the portal vein and thereby allow perfusion of portal venous branches through the interstices of  16 . In order to prevent the second segment  16  from undergoing uncontrolled longitudinal elongation, a different stent pattern is employed along this uncovered segment. In this embodiment an interlocked (or “chain-linked”) stent pattern  20  is used that prevents the second segment  16  from excessively longitudinally elongating beyond a predetermined desired length. In the interlocked stent pattern  20  shown, a single wire  22  is employed that is wrapped from the cover  18  to a distal end  24  of the endoprosthesis  10  and then back to the cover  18 . This pattern allows the wire  22  to be terminated within the cover  18  and avoids having a loose end of the wire  22  exposed at distal end  24  of the endoprosthesis  10 . This wrapping pattern is explained in greater detail with regard to  FIG. 3 , below. 
   In order to facilitate placement of the endoprosthesis  10  device, a series of radiopaque markers  26 ,  28  are provided along the length of the endoprosthesis. Marker  26  identifies proximal end  30  of the endoprosthesis  10  and marker  28  identifies the junction between the first segment  14  and the second segment  16  of the endoprosthesis  10 . By employing marker  28  as a circumferential band that entirely or substantially marks the circumference of the endoprosthesis  10 , this further facilitates correct placement of the endoprosthesis  10 . 
   The stent element  12  may be formed from a variety of wire materials, including stainless steel, nickel-titanium alloy (nitinol), tantalum, elgiloy, various polymer materials, such as poly(ethylene terephthalate) (PET) or polytetrafluoroethylene (PTFE), or bioresorbable materials, such as levorotatory polylactic acid (L-PLA) or polyglycolic acid (PGA). The preferred material comprises a superelastic material, such as nitinol metal, that will withstand tight compression in a compacted configuration (diameter) and then self-expand to a deployed configuration (diameter) once released in place. Alternatively, the stent element  12  of the present invention may be constructed from a material (e.g., stainless steel) that can be mechanically enlarged in place, such as through balloon expansion. 
   For application in a TIPS procedure, the endoprosthesis  10  of the present invention would typically have dimensions as follows: a length of about 5 to 12 cm, with a length of about 6 to 10 cm being preferred; a deployed diameter of about 6 to 14 mm, with a diameter of about 8 to 12 mm preferred; and a wall thickness (comprising both the cover  18  and the stent element  12  ) of about 0.1 to 1.0 mm, with about 0.1 to 0.6 mm being preferred. While the dimension “diameter” is used herein, it should be understood that this dimension is intended to define the average cross-sectional dimension of the device and is not intended to limit the present invention to devices with circular cross-sectional shapes. The device may also have multiple diameters, with the diameter of section  16 , for example, being larger (or smaller) than the diameter of section  14 . One or both of the sections  14 , 16  may also be tapered or otherwise adjusted in dimensions. 
   The endoprosthesis  10  itself will preferably have a compacted dimension of less than or equal to 16 French (5.3 mm), and more preferably a compacted dimension of less than or equal to 12 F (about 4.0 mm), and even more preferably a compacted dimension of less than or equal to 10 F (3.3 mm) or even 9 F (3.0 mm) or 8 F (2.7 mm) or less. In order to be delivered percutaneously, the endoprosthesis and its deployment apparatus should have a diameter of less than about 13 F (4.3 mm), and more preferably less than or equal to 10 F (3.3 mm). “French” measurements used herein define the size of a hole through which a device will pass. For example, a device with a measurement of “10 French” will pass through a 10 French hole (which has a diameter of 3.3 mm). Again, the device need not have a circular cross-section in order to pass through a circular 10 French hole so long as the hole is large enough to accommodate the widest cross-section dimension of the device. 
   The first segment  14  of the endoprosthesis  10  will typically comprise about 50 to 90 percent of the entire length of the endoprosthesis. Accordingly, the first segment  14  will normally be about 4 to 8 cm in length and the second segment  16  will normally be about 1 to 3 cm in length. 
   The preferred stent element  12  is constructed from a superelastic nitinol metal with the following properties: having alloy percentage of about 51% nickel, about 49% titanium (for example, SE 508 available from Nitinol Devices &amp; Components, Fremont, Calif., USA); being 40–45% cold worked; having a tensile modulus of approximately 35 to 70×10 6  kPa; being electopolished; and having a wire diameter of about 0.15 to 0.50 mm, and preferably about 0.25 to 0.30 mm. 
   The cover  18  performs a number of functions in the endoprosthesis  10  of the present invention. First, the cover  18  prevents extrusion of liver tissue through the stent element so as to help maintain the full desired diameter of the shunt. Second, as has been noted, the cover  18  helps to maintain the dimensions of the endoprosthesis and prevents uncontrolled elongation of the stent element. Third, it is believed to be highly desirable for the cover  18  to reduce or eliminate bile from permeating into the shunt. Fourth, the cover allows bending of segment  14  without kinking or compromising the luminal surface of the device. Based upon the medical literature, the inventors believe that bile permeation into the shunt may contribute to hyperplastic response and premature occlusion. It is also desirable that the cover  18  provides all of these benefits while contributing minimal additional profile to the endoprosthesis device. 
   To this end, it is desirable to provide a cover material that has the following properties: longitudinal tensile strength when mounted in the device of about 5 to 20 Kgf, and preferably about 10 to 15 Kgf; a permeability as quantified by a Gurley Number of greater than about 60 seconds per 100 cc or air per one (1) cm 2  of material; a thickness of about 0.05 to 0.25 mm, with a thickness of about 0.10 to 0.20 mm preferred; a water entry pressure of about 5 to 15 psi (34 to 102 kPa) or more, with 7 to 9 psi (48 to 62 kPa) or more preferred. Test procedures used to establish these values are described in the Examples below. 
   As the terms “bile exclusionary,” “resistant to bile permeation,” “bile resistant,” and similar terms are used herein, these are intended to encompass any cover that has a Gurley Number of greater than about 60 seconds per 100 cc air per a cm 2  material. More preferably, the Gurley Number is greater than about 70, 80, 90, 100 or more seconds per 100 cc of air per 1 cm 2  of material. 
   The preferred cover  18  material comprises a fluoropolymer and especially a fluoropolymer comprising expanded polytetrafluoroethylene (PTFE). For example, a cover may be constructed by forming an ultra-thin walled expanded PTFE base tube by methods described in British Patents 1,355,373, 1,506,432, published PCT Application WO 95/05555, and in U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, 4,187,390, 4,902,423, 5,718,973, 5,735,892, and 5,620,763, all of which are incorporated herein by reference. This process produces an expanded PTFE tube having a microstructure of polymeric nodes connected by polymeric fibrils and defining void spaces therein. 
   The extruded ultra-thin walled tube preferably has a wall thickness of about 0.05 to 0.30 mm, with a preferred wall thickness of about 0.05 to 0.2 mm. The tube preferably has a nominal fibril length of about 5 to 50 micron, with a preferred fibril length of about 15 to 35. Fibril length may be determined through conventional methods, such as Section 8.2.1.3 of ANSI/AAMI VP20-1994, incorporated by reference. 
   Over the extruded ultra-thin walled tube an expanded PTFE film is helically wrapped. The film may be manufactured in the following manner: PTFE fine powder is blended with a hydrocarbon lubricant and paste extruded through a flat die. The resulting tape is calendered to approximately 50% of is original thickness. Next, the tape is then heated to volatilize the hydrocarbons. The resulting dried tape is then heated to about 300° C., expanded at a ratio of 25:1 and then expanded again at a ratio of 8:1, and then sintered at a temperature of about 370° C. to achieve the desired properties. 
   This produces a low permeability film, with preferred properties of about 0.005 to 0.025 mm in thickness, with a more preferred thickness of about 0.010 to 0.015 mm (as measured by a Mitutoyo Snap Gauge Model Number 2804-10). The film preferably has a fibril length of approximately 50 to 100 micron, with a preferred fibril length of about 70 to 80 microns. The film further has a methanol bubble point of about 10 to 40 kPa, and more preferably a bubble point of about 20 to 30 kPa. Alternatively, a film as described in U.S. Pat. No. 5,476,589 to Bacino, incorporated by reference, may be employed. 
   Bubble point may be determined by placing the film in a one inch (2.54 cm) diameter round clamp and completely wetting the sample out with methanol from above. Pressure is then applied slowly (at the approximate rate of 40 kPa/min) to the bottom side of the wet out sample until bubbles appear on the other side of the sample. The pressure at which the first bubble appears is designated as the “bubble point.” 
   Finally, the film has a Frazier permeability of about 0.5 to 3 cm 3   air /sec/cm 2   material  @ 125 Pa back pressure, with a preferred Frazier permeability of about 0.5 to 1.5 cm 3   air /sec/cm 2   material  @ 125 Pa back pressure. 
   Frazier permeability may be determined by placing a sample of material (referred to as a “membrane”) to be tested on a 25.4 mm diameter circular clamp. One side of the clamp is sealed such that gauge pressure can be supplied to one side of the membrane and flow rates through the membrane can be measured. Ambient air (20° C.±3 C.) is slowly increased on one side of the membrane to 125 Pa. The flow of air through the membrane is measured, and expressed in units of cubic centimeters of air (cm 3   air ) flowing through a square centimeter of material (cm 2   material ) in a second (s) with a back pressure of 125 Pa. 
   To assemble the material of the cover  18 , the extruded ultra thin wall base tube is mounted over a stainless steel mandrel with approximately the same outer diameter as the inner diameter of the base tube. The film is slit to 0.75 inches (19 mm) wide and then helically wrapped in approximately 6 layers onto the ultra-thin walled base tube (with an overlap of about 16 mm for each wrap of the tube). The composite construction is cooked in an air convection oven for about 15 minutes at about 370° C. 
   After the composite tube has cooled, “stored length” may be imparted into the wrapped tube by deforming or bending the fibrils of the expanded PTFE node and fibril microstructure and setting the material in this configuration, such as in the following manner. The stored length is imparted by compressing (“scrunching”) the graft along the mandrel longitudinally up until a point where the graft material begins to buckle. After this compression, the construction is heated treated at about 320° C. for about 5 minutes. In order to achieve greater stretch, the base tube can be over-wrapped with a thin ePTFE film layer prior to scrunching. This over-wrap resists buckling of the tube, allowing more longitudinal compression to be imparted. After the construction is heat treated, the over-wrap is then unwrapped and removed, and the composite is carefully removed from the mandrel. Stored length may also be imparted to the device after the stent has been attached. This can be accomplished through a similar process except an additional 320° C. heat treatment cycle for approximately 15 minutes is performed after stent attachment and compression. 
   After “stored length” has been imparted to the tube (such as through the above described “scrunching” process), the stent can be slid coaxially over the PTFE film wrapped tube. The stent is preferably constructed of a helical pattern as described and illustrated herein with no interconnects from row-to-row, such that the stent can bend freely. The wrapped tube and stent are then mounted onto a stainless steel mandrel. An expanded PTFE tube (e.g., a GORE-TEX® Vascular Graft, available from W. L. Gore and Associates, Inc., Flagstaff, Ariz., USA) can be sandwiched between the wrapped tube and the undersized stainless steel mandrel to function as a cushioning layer. The apices of the stent are aligned as necessary, and a porous composite film of FEP and PTFE are wrapped over the construction in approximately 5 overlapping layers, with the side of the film containing FEP toward the lumen of the graft. Alignment of the stent apices can be accomplished through use of a filament threaded through apices of adjacent rows of first segment  14 . 
   The cover  18  is preferably attached to the stent element  12  by adhering the two together through use of a suitable adhesive, such as fluorinated ethylene propylene (FEP), polyurethane, cyanoacrylates, etc. Additionally, the materials may be bonded together through heat treatment (such as, sintering of the materials together) or through use of a wrap (for instance a tube, tape, or membrane) around the outside of the stent and cover (either continuous or discontinuous) that is adhered through either a thermoplastic or thermoset adhesive to the stent and cover. Alternatively, the stent may also be coated with a thermopolymer or thermoset adhesive and the cover bonded by reflowing or setting the polymer coating. 
   The FEP-coated porous expanded PTFE composite film is made by a process that comprises the steps of:
     a) Contacting a porous PTFE film with another layer which is preferably a film of FEP or alternatively of another thermoplastic polymer;   b) Heating the composition obtained in step a) to a temperature above the melting point of the thermoplastic polymer;   c) Stretching the heated composition of step b) while maintaining the temperature above the melting point of the thermoplastic polymer; and   d) Cooling the product of step c).   

   The FEP adhesive coating on the porous expanded PTFE film may be either continuous (non-porous) or discontinuous (porous), depending primarily on the amount and rate of stretching, the temperature during stretching, and the thickness of the adhesive prior to stretching. The preferred composite film has a thickness of about 0.0004″ (0.01 mm), a methanol bubble point of about 1.5 psi (10 kPa), a Frazier number of about 21 ft 3   air /min/ft 2   material  @ 0.5″ H 2   0  back pressure (10.7 cm 3   air /sec/cm 2   material  @ 125 Pa back pressure), and an approximate weight ratio of FEP to PTFE of 18%. 
   Weight ratio is calculated by using 2.1 g/cc as the density of FEP and 1.5 g/cc as the density of dried tape. The total mass of FEP going into the expansion process is divided by the total mass of dried PTFE tape going into the expansion process. 
   After this wrap, the construction is cooked in an air convection oven at about 320° C. for 15 minutes. After cooking, the construction is cooled to ambient temperature and removed from the mandrel. 
   As has been noted, it is believed to be desirable to exclude bile and other biological liquids from permeating through the cover  18 . The above described cover  18  construction produces a material that the inventors believe is significantly resistant to transmural fluid (bile) permeation. The ability of a material to exclude bile can be approximated by measuring the Gurley Number of the material. The test for this procedure is detailed in the Examples, below. 
   The markers  26 ,  28  may comprise any suitable radiopaque material that will appear on a fluoroscope or similar device. Suitable materials may include: metal (such as gold, iridium, platinum, or stainless steel); or filled polymers (such as carbon filled expanded polytetrafluoroethylene, or barium filled FEP).The preferred markers  26 ,  28  comprise greater than 99.9% gold. 
     FIG. 2  illustrates one embodiment of the deployment apparatus  30  that may be used to install an endoprosthesis  10  of the present invention. This deployment apparatus  30  comprises: an introducing (or packaging) sleeve  32 ; a constraining sleeve  34 ; a distal shaft  36 ; a proximal shaft  38 ; a strain relief  40 ; a deployment port  42 ; a deployment knob  44  mounted within the deployment port  42  that is connected to a deployment line  46  attached to a constraining sleeve  34  surrounding the endoprosthesis  10 ; a side arm adapter  48 ; a flushing port  50 ; and a guidewire port  52 . A radiopaque marker  54  may be provided on the distal shaft  36  to aid in the remote positioning of the endoprosthesis  10 . The operation of the deployment apparatus  30  is explained in detail below with reference to  FIGS. 10 through 18 . 
   The winding pattern of the preferred stent element  12  of the present invention is illustrated in  FIG. 3 . As can been appreciated through examination of this Figure, a single wire  22  may be employed to form both the first segment  14  and the second segment  16  of the stent element  12 . Over the length of the first segment  14 , the wire  22  may be formed in an undulated or serpentine pattern  54 . Since the cover is used to maintain the relative positions of the first undulated pattern  54 , each winding  56  of the undulated pattern around the circumference of the stent element  12  does not need to be attached to adjacent windings. It should be appreciated that some link, such as a fiber, wire extension, etc., may be employed to facilitate alignment of adjacent rows of windings if desired. 
   Along the length of the uncovered second segment  16 , however, some means should be provided to prevent the second segment  16  from elongating in an uncontrolled fashion. In the preferred embodiment shown, this is accomplished by employing the interlocked (or “chain-linked”) stent pattern  20 . In this pattern the single wire  22  is wrapped from the first segment  14  to the distal end  24  and then back to the first segment  14 . Along the length of the second segment  16  the wire  22  is provided with a second undulated pattern  58  along a first pass  60  and a third undulating pattern  62 , interlocking with the second undulating pattern  58  along a second pass  64 . By interlocking the second undulating pattern  58  and the third undulating pattern  62 , the stent pattern  20  permits the second segment  16  to be longitudinally compressed, thus imparting flexibility; but the stent pattern prevents the second segment  16  from being longitudinally elongated beyond a predetermined maximum length. It should be noted that the interlocked stent pattern  20  also imparts columnar support when the device is in a radially compressed configuration and less so when it is deployed. 
   It is even more desirable for the interlocked stent pattern  20  along uncovered second segment  16  to provide some degree of resistance to longitudinal compression. This helps to prevent the second segment  16  from longitudinally collapsing upon itself once deployed in vivo. It has been determined that additional longitudinal stiffness may be provided to the second segment  16  by using a modified “cross-over” stent pattern  63  along one side of the second segment  16 . As is shown, the cross-over pattern  63  links between every third row of the interlocked stent pattern  20 . In other words, instead of the winding pattern linking the wire  22  with an adjacent row of the interlocked pattern, the wire  22  “crosses over” two wires and interlocks with a wire in a row two removed from the starting row. Any desired cross-over pattern may be incorporated without departing from the present invention, including one that skips one, two, three, four or more rows. The amount of longitudinal stiffness may be modified using this technique by skipping more rows to impart greater stiffness or skipping fewer rows to impart lesser stiffness. 
   Longitudinal stiffness may also be provided or supplemented by structures that will assist in holding the second segment  16  in an extended position. Stiffness may also be imparted by selectively binding portions of the interlocked pattern to each other using threads or wires or similar structures (for example, attaching interlocked junction  65  together with a thread that allows no, or only limited, actuation between pattern  58  and pattern  62  ). 
   There are a number of benefits achieved with the uncovered second segment  16  of the present invention. First, the stent patterns described above provide a clinically desirable amount of radial stiffness to the second segment  16 . “Radial Stiffness” or “compressive stiffness” signifies the resistance of the endoprosthesis of the present invention to undergo circumferential compression. The testing parameters for radial stiffness are set forth in the Examples, below. Briefly, this value is the amount of force necessary to compress the device radially at a rate of 12.7 mm device circumference/min to 75% of its original diameter with a 1 cm wide loop while the force is recorded. After completing the test, the slope of the load vs. displacement curve is calculated. This calculation is performed by recording the circumferential displacement and corresponding force at 5% and 20% diametric compression (i.e., the outer diameter of the stent is 95% and 80% of its original diameter loaded with a 0.05 Kgf load, respectively). Slope is then calculated by dividing the difference in these two forces by the differences in the corresponding displacements. The result, the slope from 5% to 20% diametrical compression of the force verses circumferential displacement, is then multiplied by pi (Π) to obtain the slope of the load vs. diameter curve, expressed in Kgf/mm device diameter. 
   It is preferred that the device of the present invention will have a radial stiffness of at least 0.1 Kgf/mm for the covered segment and at least 0.12 Kgf/mm for the uncovered segment. More preferably, the device should have a stiffness of at least 0.1, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or 0.2 Kgf/mm or more for the covered segment, and 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.21, 0.22, 0.23, 0.24, or 0.25 Kgf/mm or more for the uncovered segment. The radial stiffness of the uncovered second segment  16  helps assure that the inlet to the shunt remains open in use. Additionally, since formation of the shunt is sometimes difficult to establish with accuracy, the radial stiffness of the second segment can assist in widening the portal vein or any collateral vein that might have been entered by accident. 
   Second, the open structure of the second segment allows for continued flow through the portal vein while simultaneously relieving excess pressure by way of the shunt. This is believed to be important for maintaining flow to the liver while also relieving excess pressures. 
   As has been noted, it is believed desirable that the endoprosthesis  10  of the present invention is highly resistant to any longitudinal elongation beyond its normal deployed length (that is, the longitudinal length of the device that it assumes upon deployment without tension applied to it). With respect to the uncovered second segment  16 , it is desirable that the second segment  16  will resist any attempt to elongate it beyond about 25% of its normal deployed length. “Normal deployed length” comprises the length the second segment  16  naturally assumes when deployed on a table top, extended by hand to its fully extended length, and then released without being subjected to further elongation forces. More preferably, the second segment  16  resists elongation beyond about 20%, and even more preferably beyond about 15%, and still more preferably beyond 10%, of its normal deployed length. By the term “resist any attempt to elongate” beyond a given length, it is meant that the second segment  16  has a given maximum longitudinal length that will not be exceeded under normal conditions encountered once implanted. This can be tested in vitro by attaching the device to a 2 kg weight and allow the weight to hang under normal gravity for one (1) minute. The second segment  16  should not elongate more than 20% of its normal deployed length under these conditions. 
   The interlocked stent pattern  20 , whereby the wire is doubled back on itself to form an interlocking pattern, provides a number of benefits. First, this winding pattern allows a single wire to be used in the stent element  12 , avoiding possible manufacturing problems inherent in joining multiple wire components together. Second, this winding pattern allows the wire  22  to be terminated within the cover  18  and avoids having a loose end  67  of the Wire  22  exposed at distal end  24  of the endoprosthesis  10 . It should be appreciated that this minimizes the risk that the loose wire end  67  might snag or cause damage to the vascular system and eliminates the need for a separate manufacturing step to terminate the wire at the distal end  24  of the stent element  12 . Third, the second segment  16  of the stent is very flexible, allowing it to be easily bent over a 90° angle with a 2 cm outer circumference. Fourth, the second segment  16  of the endoprosthesis tends to rebound when it is compressed under a force and then the force is removed. 
   As has been noted, it is believed important to provide an endoprosthesis for TIPS applications that includes both a stent element and a graft element in a single integral package that has a small compacted configuration (diameter) and fully operational deployed configuration (diameter). To this end, the present inventors have sought to create an endoprosthesis device that undergoes substantial growth between the compacted and deployed states.  FIG. 4  illustrates one embodiment of a TIPS endoprosthesis  10  of the present invention showing it enlarging from a compacted diameter (b) to in an enlarged deployed diameter (a). It is desirable for the ratio of a/b to comprise at least about 2, and more desirable for the ratio to comprise about 3 to 5 or more. The present invention employs a variety of unique materials and structures to produce highly compacted devices that can achieve these kinds of ratios. 
   Another useful attribute of the endoprosthesis device of the present invention is that it is constructed from materials that render the device highly flexible, both in its compacted and in its deployed configurations. In this regard, constructing the cover  18  from a stretchable material, such as the expanded PTFE material previously described, incorporates “stored length” of material into the device that allows the device to be readily bent without kinking.  FIG. 5  shows a TIPS endoprosthesis  10  of the present invention bent around a rod  66  having a diameter “d.” It is desirable that the first covered segment  14  of endoprosthesis  10  of the present invention be capable of being bent around a 25 mm diameter rod without kinking, and more preferably around a 20 mm diameter rod without kinking. 
   As the term “stored length” is used to describe the cover  18 , it is intended to encompass the material comprising the cover either being elastic in nature and/or having sufficient excess material incorporated within the cover to allow an outer circumference  68  of the cover  18  to “stretch” without excessively buckling an inner circumference  70  of the cover  18 . With the preferred cover material of the present invention, expanded PTFE, stored length may be achieved by compressing the node and fibril structure of the expanded PTFE in the normal deployed configuration of the device—allowing the outer circumference  68  to separate the node and fibril structure upon actuation around a rod while the inner circumference  70  remains compressed or undergoes further compression prior to buckling. Stored length can also be achieved by employing elastic materials such as polyurethane, silicone, or other biocompatable elastomers or porous materials, such as porous PET. Another suitable material is to construct the cover  18  from a stretchable material, such as an appropriately proportioned stetchable expanded PTFE disclosed in U.S. Pat. No. 4,877,661 to House, et al., incorporated by reference. 
   To determine the presence of “stored length” in a cover material of the present invention, a number of tests can be performed. First, the cover and device can be tested together to determine if the cover material possesses a recovery response whereby the cover will longitudinally elongate with the device when tensioned and then recover to, or close to, its original length upon release of tension. This can be tested by determining the “normal deployed length” of covered first segment  14  and then applying moderate longitudinal tension to the device to determine the maximum length the first segment will assume while recovering to near its original normal deployed length upon release of tension. To have “stored length” within the meaning of the present invention, the maximum length of the covered segment should be at least 5% more than the normal deployed length of the covered segment. More preferably, the maximum length of the covered segment should be 10, 15, or 20% or more of the normal deployed length of the covered segment. 
   With respect to a cover constructed from expanded PTFE material, stored length can also be determined by microscopic analysis of the node and fibril structure of the material. As has been noted, an expanded PTFE material can have stored length imparted to it by “scrunching” the material and then heat treating it to heat set the fibrils into a bent configuration. An expanded PTFE material having bent fibrils is shown in the SEM of  FIG. 23 . When formed in this manner, the fibrils will straighten (thus lengthening the cover) when longitudinally tensioned and then returned to their bent configuration when released from tension (although the fibrils will “creep” into a straight configuration if tension is maintained over an extended period of time or the cover is exposed to high heat in its tensioned configuration). For an expanded PTFE material, the presence of “stored length” can be determined by examining the material untensioned to determined if the fibril structure is similar to that shown in  FIG. 23 . A test for this property is described in U.S. Pat. No. 4,877,661 to House, et al., incorporated by reference. Again, the maximum length of the covered segment with an expanded PTFE cover material should be at least 5% more than the normal deployed length of the covered segment, and more preferably 10, 15, or 20% or more of the normal deployed length of the covered segment. 
   The term “bent around a rod without kinking” is intended to encompass the endoprosthesis 10 being actuated to place its ends at a 90° angle with respect to each other, as is shown in  FIG. 5 , with no significant folding or buckling forming in the cover  18  at the point of bending around the rod  66 . “Significant folding or buckling” is present when if there is a circumferentially directed crease along the inner circumference  70  that separates the creased portion of the endoprosthesis more than 1 mm from the rod. A similar test is described in detail in Section 8.9 of ANSI/AAMI VP 20-1994, incorporated by reference. 
   The compacting of the endoprosthesis  10  of the present invention may be accomplished through a variety of methods, as is illustrated in  FIGS. 6   a  through  6   d .  FIG. 6   a  is an end view of a TIPS endoprosthesis of the present invention at its deployed diameter  72 . Each of  FIGS. 6   b ,  6   c , and  6   d  illustrates how the endoprosthesis  10  may be compacted around a catheter  74  to achieve a compacted diameter  76 .  FIG. 6   b  is an end view of the TIPS endoprosthesis of  FIG. 6   a , with the endoprosthesis  10  shown compacted by folding and rolling the endoprosthesis  10 .  FIG. 6   c  is an end view of the TIPS endoprosthesis of  FIG. 6   a , with the endoprosthesis  10  shown compacted by radially compressing the endoprosthesis  10 .  FIG. 6   d  is an end view of the TIPS endoprosthesis of  FIG. 6   a , with the endoprosthesis  10  shown compacted by folding into pleats  78 . 
   While each of these compacting techniques works well for many applications, it has been found that the covered first segment  14  of the stent element  12  of the present invention can catch on itself or the constraining sleeve  34  during deployment if the covered stent segment is not carefully handled. In this regard, as is explained in greater detail below, it is believed that compacting the TIPS endoprosthesis  10  of the present invention with multiple folds in the form of pleats  78 , as shown in  FIG. 6   d , provides the best results. 
     FIG. 7  is a side elevation view of an undulated stent  80 , shown in a deployed diameter  82  and in a compacted, pleated diameter  84 . As can be seen, in the deployed diameter  80 , the undulated stent pattern has exposed rearward facing apices  86  and forward facing apices  88 . With apices facing in both forward and rearward directions, the inventors have found that there is a likelihood that the apices will catch and tangle a fibrous constraining sleeve. However, if the stent is compacted by pleating, one direction of apices can be formed within pleat  78  so that it is protected within the folded device below the level of the outer circumferential surface of the constrained device. This construction vastly reduces the risk of entanglement. In the compacted stent  84 , it can be seen that only the forward facing apices  88  remain exposed on the outer circumference of the compacted device, while all of the rearward facing apices  86  are tucked within pleats  78  of the compacted device. 
   It should be noted that the pleated folding methods described herein can be used to direct apices into a wide variety of folded patterns. As such, the terms “forward” and “rearward” facing apices are used only for convenience to describe sets of apices that face in one direction or an opposite direction, without regard to the actual direction the device may ultimately be deployed. 
   It has been found that the endoprosthesis of the present invention can be compacted to its maximum degree, with a high compaction efficiency, and with minimal risk of apex entanglement if a pleated folding method is employed. One method of pleating an endoprosthesis is shown in  FIGS. 8   a ,  8   b , and  9 . 
     FIGS. 8   a  and  8   b  illustrated a fluted tapered die  92  suitable for use in pleating a stent or stent-graft device. The tapered die  92  has a large opening  94  at one end and a small opening  96  at its opposite end. A number of raised flutes  98  are provided within the tapered die separated by grooves  100 , each raised flute  98  corresponding to one desired pleat to be formed in the endoprosthesis. The raised flutes  98  and/or the grooves  100  may be formed by molding or machining the shapes into the die  92 . Alternatively, as is illustrated, the flutes  98  may be formed by attaching one or more evenly spaced bands  105  per flute around the tapered die  92 , such as by using polyester or nylon thread. 
   Compacting and pleating may be accomplished by attaching a series of tether lines  104  to an endoprosthesis  10 , as shown in  FIG. 9 . The tether lines  104  may be formed from thin wires, polymer fibers, or other suitable materials. As is shown, the tether lines  104  are attached along the apices that are intended to remain exposed (in this case, forward facing apices  88 ). The tether lines  104  are centrally joined together at point  108 . 
   To compact the device, the joined tether lines  104  are then drawn through the tapered die  92 , exiting at small opening  96 . Each of the tether lines  104  is drawn through a groove  100 . In this manner, the tethered apices  106  will be folded outwardly and each of the untethered apices will be folded inwardly by the raised flutes  98  as the device is radially crushed. Once exiting the small opening  96 , the compacted device can then be captured within a constraining sleeve or capture tube (not shown) for subsequent packaging. The constraining sleeve or capture tube may also contain means, such as longitudinal slots or grooves, to assist in maintaining the pleated compacted configuration of the endoprosthesis. Alternatively, it is also possible to pleat the device using a smooth tapered die with folding ridges (for example, longitudinally applied polyester lines) attached to the exterior of the endoprosthesis prior to compression. The folding ridges may then be removed following compression. 
   Packaging of the device of the present invention can be accomplished through a variety of methods. As should be appreciated from the following description of the preferred deployment method of the device of the present invention, it is preferred that the endoprosthesis  10  of the present invention be packaged to permit multi-stage deployment. One such method to accomplish this packaging is illustrated in  FIGS. 10 and 11 .  FIG. 10  shows a device of the present invention as it is delivered for use. The endoprosthesis (not shown) is mounted on a delivery catheter  110  between two “olives”  112 ,  114 . Olives are solid bumps of increased diameter on the catheter shaft (for instance, formed from a polymer, glass, or metal material) that inhibit the compacted endoprosthesis from sliding on the catheter shaft. Olive  114  at the distal shaft  36  may contain a radiopaque marker  54  to aid in positioning of the device. An introducing sleeve  32  is attached over the endoprosthesis. The introducing sleeve  32  is preferably made of polyethylene or similar material with its inner diameter being approximately equal to the outer dimensions of the compacted endoprosthesis. One or more markers or mechanical stops may be provided on the introducing sleeve  32  to aid in initial positioning and deploying. A deployment line  46  can be seen exiting the introducing sleeve  32  and passing into the delivery catheter  110  (as was previously explained in reference to  FIG. 2 , the deployment line  46  exits the delivery catheter at the deployment port  42 ). 
   When the introducing sleeve  32  is removed, as is shown in  FIG. 11 , the second segment  16  of the endoprosthesis  10  will expand to close to its fully deployed diameter. The remainder of the endoprosthesis  10 , however, is contained within a constraining sleeve  34 . In operation, the constrained device will pass from the introducing sleeve  32  into a catheter tube (of approximately equal inner diameter) extending past the ultimate deployment site. Deployment of the second segment  16  will occur when the device is separated from the catheter tube at the deployment site. 
   The preferred constraining sleeve comprises a plurality of interwoven threads, such as that disclosed in U.S. patent application Ser. No. 09/098,103, filed Jun. 15, 1998, to Armstrong et al., incorporated herein by reference. The constraining sleeve  34  can be separately deployed by actuating deployment line  46 . 
   While the multi-stage deployment apparatus illustrated in  FIGS. 10 and 11  comprises the preferred embodiment of deployment of the present invention, it should be appreciated that different embodiments may achieve similar results. For instance, the second segment  16  may be contained in its own constraining apparatus, such as the fibers used in constraining sleeve  34  or other constraining devices or other materials. Conversely, the constraining sleeve  34  may be formed from a tubular material (like the introducing sleeve  32 ). Additionally, it may be beneficial under certain circumstances to provide more than a two-stage deployment, such as be providing multiple constraining sleeves that permit multiple stage deployment of the first segment  14  of the endoprosthesis  10 . 
   The operation of the endoprosthesis and deployment apparatus of the present invention can be appreciated through the following description illustrated by  FIGS. 12 through 18 . 
     FIG. 12  illustrates a compacted endoprosthesis of the present invention, mounted within introducing sleeve  32 , being inserted through a hemostatic valve  116  into catheter tube  117 . The catheter tube  117  has previously been advanced into the portal vein. A guide wire  118  may be provided to assist in device placement. 
   As is shown in  FIG. 13 , the endoprosthesis  10  is advanced out of the introducing sleeve  32  and into catheter tube  117 . The delivery catheter is then advanced to cause the second segment  16  to become constrained within catheter tube  117 . 
   As is illustrated in  FIGS. 14 and 15 , the endoprosthesis  10  is then advanced through the catheter tube  117  through the inferior vena cava  119   a , the hepatic vein  119   b , the intrahepatic tract (shunt)  120  formed in the liver  122 , and well into the portal vein  124 . Radiopaque tip  54  can be aligned with the end of the catheter tube  117 . The radiopaque marker  28  can be positioned distal to the intrahepatic juncture site  126 . 
   In  FIG. 16  the catheter tube  117  is withdrawn proximally, which permits the second segment  16  to fully expand within the portal vein  124 . As is shown in  FIG. 17 , the delivery catheter  110  is then withdrawn through the catheter tube  117  until the unconstrained second segment  16  engages the intrahepatic juncture  126 . Alignment can be confirmed fluoroscopically by correct orientation of marker band  28  with or without confirmatory injections of radiopaque contrast media. 
   As is illustrated in  FIG. 18 , once the device is properly aligned, the constraining sleeve  34  is removed by actuating deployment line  46 , allowing the first segment  14  of the endoprosthesis  10  to fully enlarge in place in a tip-to-hub direction. If desired, further touch-up of the endoprosthesis  10  can be performed by subsequent balloon dilation of the endoprosthesis  10 . 
   It should be appreciated that the above deployment procedure aligns the covered portion of the endoprosthesis within the intrahepatic tract (shunt)  120 . Further, the uncovered second segment  16  permits blood flow both to enter the shunt  120  and to continue through the portal vein  124 . The result is that excess pressure can be relieved from the portal system (through the shunt) without completely eliminating normal blood flow through portal vein  124 . 
   Without intending to limit the present invention to the specifics described hereinafter, the following examples illustrate how the present invention may be made. 
   EXAMPLE 1 
   A pin jig is manufactured by turning 304 stainless steel rod stock to an 8 mm diameter. Holes are then drilled in the mandrel (#67 bit) to create the pattern to wind the stent depicted in  FIG. 3 , with 5 forward facing apices (10 holes) per revolution. Beyond each end of the hole pattern on the mandrel, holes are drilled and tapped to fit 10–32 screws. 1/32 inch (0.79 mm) diameter×4 mm length steel pins are then place into these holes, such that 2 mm of the pins are imbedded in the mandrel. 
   Nitinol wire of a 0.010″ (0.25 mm) nominal diameter (acquired from Nitinol Devices and Components, Fremont, Calif.) is then wound by hand around this pin jig in the pattern shown in  FIG. 3  and the ends of the wire affixed in a tight configuration to the mandrel by 10–32 screws. The wire and the mandrel is then set in a convection oven set a 450° C. for 15 minutes, followed by a rapid quench in ambient temperature water. After the mandrel is allowed to cool, the wire stent is removed from the mandrel by removing the pins and sliding the stent from the mandrel. 
   Next, as a manufacturing aid to assist in alignment of apices relative to each other, a small diameter expanded PTFE filament (for example, CV-8.0 suture available from W. L. Gore &amp; Associates, Inc., Flagstaff, Ariz., USA) is tied to the end of the wire on the proximal end of the device  30 . The filament is spirally wound toward the distal end of the device  24 . The filament is wound through apices of adjacent rows of the stent structure. A similar winding alignment procedure is described in PCT Application PCT/US96/19669 to Martin et al., incorporated herein by reference. The filament is wound along the length of the first segment  14 , where it may then be fixed to the terminal end of the stent wire  67 . This winding procedure may be started at either end  24  or  30  of the stent with similar results. 
   Next, a thin-walled expanded PTFE tubular extrusion (wall thickness 0.004 inch (0.1 mm)), as previously described is then circumferentially wrapped with five to seven layers of an expanded PTFE film. The expanded PTFE film has a thickness of 0.0007″ (0.02 mm), a density of 0.7 g/cc, and a methanol bubble point of 3 psi (21 KPa). 
   This construction is heat treated for 15 minutes at 370° C. and cooled. The wrapped tube is then removed from the mandrel. 
   Another thin-walled expanded PTFE tube is placed into the lumen of the wrapped tube. These two tubes are then fitted over an 8 mm mandrel. The wrapped expanded PTFE tube is then over-wrapped with another layer of the expanded PTFE film. 
   The construction is then compressed longitudinally by hand to approximately a third of its original length. The construction and mandrel are then heat treated at 320° C. for 10 minutes, removed from the oven, and allowed to cool. The outer over-wrap is removed. The construction is then removed from the mandrel and the graft placed under moderate tension to restore the graft to approximately ⅔rds of its original length. 
   After trimming one end of the construction squarely, the stent is slid on the outside of the PTFE construction and the end of the interlocked section of the stent (see  FIG. 1 ) aligned with the squared end such that one row of interlocked apices is covered. The 8 mm mandrel is then inserted into the PTFE/nitinol construction. The stent apices are then manually adjusted to achieve even spacing of the apices, with the forward facing apices from each row aligned longitudinally. A PTFE/FEP film, as previously described, is then over-wrapped with the FEP side down on the exterior surface of the section of the stent with non-interlocked windings. This PTFE/FEP film comprises an expanded layer of PTFE film with a discontinuous layer of FEP. The film has a thickness of 0.0005 inch (0.01 mm) and a methanol bubble point of 1.2 psi (8 KPa). This wrap is a continuous helical circumferential wrap with overlapping edges to produce approximately 3–5 layers. 
   After completing the wrap, the wrap is spot attached using a soldering iron with a tip temperature of sufficient heat to melt the FEP. A sacrificial compression wrap is then applied by helically wrapping with moderate tension an expanded PTFE film circumferentially around the exterior of the construction. The construction is then heat treated at 320° C. for five minutes and allowed to air cool. After the construction has cooled, the over-wrap is removed. The construction is again longitudinally compressed approximately 50% of its original length, making sure that the apices remained aligned. The construction is over-wrapped with a thin expanded PTFE film that serves as a compression layer and heat treated at 320° C. for 10 minutes. After a short air cool, while the tube is still warm, moderate longitudinal tension is applied to the device by hand to pull the entire device out to approximately its original length. The excess expanded PTFE tube is then trimmed flush with the stent end. 
   Threads are attached to the covered end of the device by piercing through the cover material at each end apex of the device with a needle. Thread is fed through these holes to form loops, and about a 30 cm from the device they are tied together with even slack. The knotted end of the threads is fed sequentially though a PTFE tapered die into a PTFE capture tube. The tapered die is constructed with an approximate 15° included angle taper and had five nylon lines (outer diameter=0.018″ (0.46 mm)) positioned approximately equal distance (i.e., 72°) around the inner circumference of the taper, as is shown in  FIG. 8   a . The devices and tapered die are then sprayed with a refrigerant (MicroFreeze, MicroCare Corporation, Bristol Conn.) to reduce the temperature of the device below the nitinol&#39;s martinsitic temperature. The device is immediately drawn through the tapered die into a PTFE capture tube. As the endoprosthesis is drawn through the die, a 0.055 inch (1.4 mm) outer diameter polyethylene shaft is placed within its lumen. The resulting device can then be removed from the capture tube. 
   The deployed device is flexible in both the covered and uncovered portions. The uncovered portion can be compressed longitudinally, and the stent elastically recovers close to its original length when the digital pressure is removed. The permeability of the covered portion of the construction is low and the hoop strength of the device is substantial. The device is compressed such that the apices pointing away from the attached threads are tucked under the apices pointed toward the thread. This compression technique results in a small collapsed profile. 
   Tests performed on this example include: 
   Kink Diameter: 
   This device is bent at room temperature around progressively smaller diametrical mandrels until a kink formed in the covered graft section. The smallest diameter at which that device did not kink was recorded. A “kink” is present when if there is a crease along the inner circumference  70  that separates the creased portion of the endoprosthesis more than 1 mm from the rod. A similar test is described in detail in Section 8.9 of ANSI/AAMI VP 20-1994, incorporated by reference, although the current test is performed without internal pressure. 
   Transmural Permeability: 
   To quantify a level of “bile resistance,” the inventors used a measure of air resistance described by Tappi Standard T 460 om-88 in order to establish a “Gurley Number.” The measurement was performed using a GURLEY™ Densometer Model 4110 permeability tester available from Teledyne (Troy, N.Y., USA). Briefly, the tester measures the time that is takes to pass 100 cubic centimeters of ambient temperature air through an orifice with a pressure of 4.9 inches of H 2 O (1.2 kPa). 
   The fixture apparatus  130  used to test for the “Gurley Number” is shown in  FIG. 19 . A first end  132  of a flexible tube  134  is attached to an adapter  136 . The adapter  136  is attached to the Densometer via a 25.4 mm diameter circular plate (not shown) that attaches to the adapter  136  via orifice  138 . Sealing material  140  (e.g., a ⅛ inch (0.32 mm) thick silicone sheet) is mounted around the orifice  138  to effectuate a seal with the circular plate. This assures that the air measured by the Densometer passes through orifice  138 . A second end  142  of the flexible tube  134 , which is in direct communication with orifice  138 , is attached to a circular fitting  144 . The circular fitting  144  has an outer diameter  146  approximately the same as the inner diameter of the device  10  to be tested. The circular fitting  144  is attached to the device  10 , as is shown in  FIG. 20 , using PTFE pipe thread tape to create an air tight fitting around the circumference. A sealing plug fitting  148  is provided to attached to the opposite end of the device  10  to completely seal the second segment  16  and seal a portion of the first segment  14 , as these elements are illustrated in  FIG. 1 . 
   The resistance to flow through each of the fittings with no sample attached is measured to 1.5 seconds or less per 100 cc of air, which is significantly less time than all of the samples measured. Prior to taking a measurement, the test area is calculated by multiplying the separation of the fittings to which the sample was attached to the inner circumference of the sample. After measuring the time for 100 cubic centimeters of air to pass through the sample, this value is normalized by multiplying by the total area tested. The test areas generally range from 4 to 16 cm 2 , and multiple test areas are tested for each sample. 
   Radial Compression Testing 
   Compression testing is performed using an INSTRON tensile testing machine equipped with a heated test chamber set to 37° C. As is shown in  FIGS. 21 and 22 , a loop compression fixture  150  is fabricated using 0.1 mm thick polyester film. The film is cut into a 20 mm×160 mm rectangle. On one end of the rectangle, 5 mm is trimmed from each side from the end for 100 mm, creating an elongated tab  152 . Approximately 5 mm past the trimmings in the wide section, a slit  154  (approximately 11 mm×1 mm) is cut in the film to fit the tab  152 . The 1-cm wide tab  152  is then looped through the slit  154  to form a variable diameter loop  156 . After forming the loop  156 , ends  158 , 160  of the film are placed in opposing tensile testing grips of the tensile tester. 
   Devices to be tested are heated to 37° C. for greater than 30 minutes immediately prior to the testing. Inside the loop  156  is placed a device  162  to be tested and the opposing jaws separated until a pre-load of 0.05 kg on the variable diameter loop is detected. The device is then compressed at a rate of 12.7 mm device circumference/min to 75% of its original diameter while the force is recorded. After completing the test, the slope of the load vs. displacement curve is calculated. This calculation is performed by recording the circumferential displacement and corresponding force at 5% and 20% diametric compression (i.e., the outer diameter of the stent is 95% and 80% of its original diameter loaded with a 0.05 load, respectively). Slope is then calculated by dividing the difference in these two forces by the differences in the corresponding displacements. The result, the slope from 5% to 20% diametrical compression of the force verses circumferential displacement, is then multiplied by pi (Π) to obtain the slope of the load vs. diameter curve, expressed in kg/mm diameter. 
   Profile: 
   Profile is equal to the inner diameter of the PTFE capture tube into which the device is drawn. Within the lumen of the device constrained in the PTFE capture tube is a 0.055 inch (1.4 mm) outer diameter polyethylene tube to represent the catheter that the device is loaded. 
   Scanning Electron Microscope Photomicrographs/Bent Fibrils: 
   Scanning electron microscope photomicrographs (SEMs) are taken of the luminal surface. Bent fibrils are measured in a manner consistent with that method taught in U.S. Pat. No. 5,308,664 to House et al. Briefly, horizontal lines are drawn on the SEM photomicrographs (3 lines per photo, 3 photos per sample) (total of 9 measurements taken). Horizontal distances (H) are measured between nodes along this line. Vertical distances (V) of the bent fibrils are measured from the horizontal line. A ratio (V/H) is calculated and recorded. An SEM of a cover of the present invention showing bent fibrils therein is attached as  FIG. 23 . 
   Internal Corrugations: 
   The internal corrugations of the graft are measured using gauge pins. Gauge pin testing is accomplished by first testing the internal diameter with pins until the fit between the pin and internal diameter allows the pin to be lifted when suspending only the device. Next, the device is longitudinally compressed approximately 10% (to simulate bending) and the pin test is repeated. The percent of the compressed inner diameter to the non-compressed inner diameter is then calculated. 
   8 mm Device 
   Testing is perform as described above, with the following results: 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               Kink Diameter 
               24 mm 
             
             
                 
               Radial Stiffness 
               Covered section-0.137 kg/mm diameter   
             
             
                 
                 
               Uncovered section-0.193 kg/mm diameter   
             
             
                 
               Permeability 
               213 sec per 100 cc air per 1 cm 2   material   
             
             
                 
               Profile 
               3.2 mm 
             
             
                 
               Bent Fibrils 
               Average ratio V/H = 0.131 
             
             
                 
               Internal Corrugation 
               96% of original 
             
             
                 
                 
             
           
        
       
     
   
   EXAMPLE 2 
   A device similar to the one in described in Example 1 is made, however, the inner diameter is 10 mm, the stent pattern has 6 apices per revolution, the wire diameter is 0.011 inches (0.28 mm), and the tapered die has 6 nylon lines (spacing=60°). Testing is perform as described above, with the following results: 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               10 mm Device 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Kink Diameter 
               20 mm 
             
             
                 
               Radial Stiffness 
               Covered section-0.173 kg/mm diameter   
             
             
                 
                 
               Uncovered section-0.258 kg/mm diameter   
             
             
                 
               Permeability 
               143 sec per 100 cc air per 1 cm 2   material   
             
             
                 
               Profile 
               3.6 mm 
             
             
                 
               Bent Fibrils 
               Average ratio V/H = 0.176 
             
             
                 
               Internal Corrugation 
               99% of original 
             
             
                 
                 
             
           
        
       
     
   
   EXAMPLE 3 
   A device similar to the one in Example 1 is made; however, the inner diameter is 12 mm, the stent pattern has 6 apices per revolution, the wire diameter is 0.010 inches (0.25 mm), and the tapered die has 6 nylon lines (spacing=60°). Testing is performed as described above, with the following results: 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               12 mm Device 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Kink Diameter 
               24 mm 
             
             
                 
               Radial Stiffness 
               Covered section-0.103 kg/mm diameter   
             
             
                 
                 
               Uncovered section-0.157 kg/mm diameter   
             
             
                 
               Permeability 
               111 sec per 100 cc air per 1 cm 2   material   
             
             
                 
               Profile 
               3.6 mm 
             
             
                 
               Bent Fibrils 
               Average ratio V/H = 0.234 
             
             
                 
               Internal Corrugation 
               98% of original 
             
             
                 
                 
             
           
        
       
     
   
   While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions. It should be apparent that changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims.