Abstract:
A low profile stent graft for treatment of vascular abnormalities such as aneurysms. The endoskeleton of the stent graft has first and second self-expandable stents. The stents are spaced apart and connected together by at least one, and preferably three, flexible strut members. A tubular graft is placed around the endoskeleton while the stents are in a partially compressed state, and attached to the stents. A bifurcated configuration is disclosed that can be employed where the stent graft is to be used for repair of an aneurysm of the abdominal aorta.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a low profile device for treatment of vascular abnormalities, such as the repair of aneurysms. 
     In the past, treatment of aneurysms involved an open surgical procedure which exposed the affected lumen and bypassing the aneurysm with an artificial tubular graft. 
     More recently stent grafts have been employed as a minimally invasive alternative to traditional graft bypass surgery. There are a number of stent graft designs that have been tried. Current designs are limited to use with patients having large vessels because of the large size of the delivery systems. Such large sizes require surgical exposure of the vascular system and in smaller patients, often females, such stent grafts are simply too large to use at all. In addition, such larger stent graft designs can be difficult to position and may cause damage to the blood vessels. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a stent graft design that has a low cross sectional profile that can be delivered percutaneously and used in patients with small and tortuous vessels. 
     It is a further object to provide a stent graft that is highly flexible. 
     It is a still further object to provide a stent graft that is simple and inexpensive to construct. 
     These and other objects are achieved by providing a stent graft that has a minimal metallic endoskeleton that holds the graft material in place until secondary deployment of additional stents. Such a minimal metallic endoskeleton allows the delivery system to have a lower cross-sectional profile than conventional stent grafts. 
     The endoskeleton of the stent graft of this invention has a first self expandable metallic stent located at the distal end thereof and a second self expandable metallic stent located at the proximal end thereof. At least one flexible longitudinal strut member extends between and is connected to the first and second stents. The strut member can have a fixed length or can be a compound strut comprised of two or more strut sections that are fastened together in a manner that allows a sliding motion between the strut sections so that the compound strut can be elongated by applying traction to the proximal stent. 
     The endoskeleton of the stent graft of this invention may be bifurcated for use with aneurysms of the abdominal aorta. 
     The endoskeleton of the stent graft of this invention is inserted into a tube of a graft material and the tube attached to the first and second stents. In the case of the bifurcated system, the graft material is also connected to the third contralateral stent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a straight tubular form of the endoskeleton of the stent graft of the present invention; 
     FIG. 2 is a plan view of a portion of a stent used in the invention; 
     FIG. 3 is a plan view, partially in section, of the straight tubular form of the endoskeleton attached to the inside a tubular graft; 
     FIG. 4 is a plan view of a bifurcated form of the endoskeleton of the stent graft of the present invention; 
     FIG. 5 is a plan view, partially in section, of the bifurcated form of the endoskeleton attached to the inside of a tubular graft; and 
     FIG. 6 is a partial plan view of the bifurcated stent attached to a two component strut to permit sliding elongation, shown in its collapsed condition; 
     FIG. 7 is a partial plan view of the strut of FIG. 6 shown in its extended condition; and 
     FIG. 8 is a plan view of the bifurcated stent of the FIG. 6 embodiment shown with tubular graft material attached thereto. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The endoskeleton  10  of the straight tubular form of stent graft  30  is illustrated in FIG.  1 . 
     Endoskeleton  10  is comprised of a first stent  12  and a second stent  14 . Stents  12  and  14  are identical in construction, each being a cylindrical spring assembly made from a single piece of fine gauge stainless steel spring wire or nitinol having a diameter of about 0.4 to 0.5 mm that is bent into a sinusoidal configuration comprised of a plurality of arms  16  and outer and inner elbows  18  and  19 , respectively, as best seen in FIG.  2 . Elbows  18  and  19  are illustrated as being simple arches, but other elbow configurations, such as a recurved arch or aperatured arch may be used. 
     Struts  20 ,  21 , and  22  extend between first and second stents  12  and  14 , as shown. Struts  20 - 22  are constructed of fine gauge stainless steel spring wire or nitinol having a diameter of about 0.3 to 0.6 mm. Struts  20 - 22  are attached to stents  12  and  14  in any suitable manner, such as soldering to outer elbows  18  at solder joint  24 ; they may, alternatively be attached by bending, or cut from a single piece of metal. The struts are substantially evenly spaced apart around cylindrical stents  12  and  14 . 
     Although three struts are preferred, a single strut may be used. More than three struts may also be used, but the number used should not be such that the flexibility of the stent graft is compromised. 
     A straight tubular stent graft  30  is formed by compressing stents  12  and  14  and inserting endoskeleton  10  into elongated cylindrical graft tube  31 , as best seen in FIG.  3 . The graft tube  31  is attached to stents  12  and  14  by sutures  32  passed through graft  31  and outer elbows  18 . 
     Graft tube  31  may be formed of any conventional graft material which is known to be substantially biologically inert, non-biodegradable, and durable. A suitable such graft material is polytetrafluoroethylene (“PTFE”) sold under the trademark “IMPRA” by Bard of Tempe, Arizona. The diameter of the graft tube  31  can be between about 6 and about 40 mm in diameter and between about 1.5 and about 400 mm in length. 
     The endoskeleton  40  of a bifurcated form of stent graft  60  of this invention is illustrated in FIG.  4 . Endoskeleton  40  is comprised of a first stent  42  and a second stent  44  having struts  46 ,  47 , and  48  extending therebetween and attached thereto in the same manner as for the straight tubular form  10  discussed above. Stents  42  and  44  are cylindrical spring assemblies of the same configuration and material as discussed above, except that stent  42  has approximately twice the diameter of stent  44 . Stent  50  is attached to stent  42  by means of a single strut  52 . 
     Bifurcated stent graft  60  is formed by positioning a partially compressed bifurcated endoskeleton  40  inside a bifurcated tubular graft  61  as shown in FIG.  5 . Graft  61  is formed of the same material as graft  31 . 
     Stent  44  is located inside, and adjacent the end of, channel  62  and sewn to graft  61  by sutures  63 , only two of which are shown. Stent  44  is in a partially compressed state so that an outward force is applied to the inner side of channel  62 . 
     Stent  50  is located inside, and adjacent the end of, channel  64  and sewn to graft  61  by sutures  65 , only two of which are shown. Stent  50  is in a partially compressed state so that an outward force is applied to the inner side of channel  64 . 
     Stent  42  extends outwardly from the end of channel  66  and is sewn to graft  61  by sutures  67 , only two of which are shown. Being in a partially compressed state at its inner end, stent  42  exerts an outward force on the inner side of channel  66 . 
     Stent  42  is not covered by graft material  61  to allow placement of the stent graft  60  across critical branch vessels without causing occlusion, the blood flowing through the open structure of the uncovered stent. This is especially true in the abdominal aorta where the renal arteries are just above the level of the aneurysms. An uncovered stent  42  can be placed safely across the origins of the renal arteries and still allow a fixation site for the stent graft  60 . 
     Graft material  61  is configured to wrap around stent  50  and is sewn along stitch line  68  to form channel  64 . There is no communication between channel  62  and channel  64  along the stitch line  68 , but both channels  62  and  64  communicate with channel  66 . 
     Bifurcated stent graft  60  is used to repair aneurysms of the abdominal aorta which typically occur where the aorta branches into smaller (iliac) arteries. Bifurcated stent graft  60  can be used in conjunction with the straight tubular stent graft  30  described above by inserting straight tubular stent graft  30  into channel  64  of bifurcated stent graft  60  and sewing it into place. The end of bifurcated stent graft  60  containing channel  66  is located in the aorta with channel  50  (extended by insertion of a stent graft  30 ) and channel  62  being located in the adjacent iliac arteries. 
     A variation of the endoskeleton of bifurcated stent graft  60  is shown in FIGS. 6 and 7. In this variation, single strut  52  of FIG. 4 is replaced by a two component strut comprised of fixed strut  152  and sliding strut  252 . Fixed strut  152  is attached to stint  142  and sliding strut  252  is attached to stint  150 , the reference numbers of the components in FIGS. 6 and 7 common to those in FIG. 4 being the same but increased by 100 or, in the case of the sliding strut, by 200. 
     Sliding strut  252  has a ring  254  attached at its outer end. Ring  254  fits over fixed strut  142  in sliding relationship. In FIG. 6 sliding strut  252  is shown in its unextended (collapsed) configuration. In FIG. 7 sliding strut  252  is shown in its extended configuration. Cap  154  attached to the outer end of fixed strut  152  prevents passage of ring  254  thereover. 
     FIG. 8 illustrates the bifurcated configuration employing the sliding strut configuration illustrated in FIGS. 6 and 7. In other respects, the bifurcated configuration of FIG. 8 is the same as the bifurcated configuration illustrated in FIG. 5, and identical components of the FIG. 8 configuration have the same reference numbers as those used in FIG. 5, but increased by 100. 
     In FIG. 8, the sliding strut  152 / 252  (not shown) is in the collapsed configuration shown in FIG.  6 . That portion of graft  161  covering sliding strut  152 / 252  and stent  150  is collapsed, accordion style, since it must have a length long enough to accompany stent  150  (to which it is attached by sutures  165 ) as strut portion  252  is extended. 
     The endoskeletons of both the straight stent graft  30  and bifurcated stent graft  60  are easily compressed into a small diameter endoprosthesis package for percutaneous delivery to the required aneurysm repair site since a minimum number of stents are employed (two in straight stent graft  30  and three in stent graft  60 ). For example, straight stent graft  30  can be collapsed to a diameter of about 3.3 mm and bifurcated stent graft  60  can be collapsed to a diameter of about 4.0 mm. 
     The stent grafts  30  and  60  of the invention are introduced through a conventional sheath with a hemostatic valve. The sheath and taper dilator is initially positioned over an intravascular guide wire. The tapered dilator is removed and the stent graft is advanced through the sheath with a blunt pushing device. The distal tip of the sheath is demarcated with a radiopaque marker. 
     After stent graft  30  or stent graft  60  is secured in place in the artery of a patient, additional (secondary) stents can be inserted inside the graft tube along the length thereof.