Abstract:
A stent graft leg extension ( 10 ) to extend from a bifurcated aortic stent graft into an iliac artery. The stent graft has a tubular body. ( 12 ) of a biocompatible graft material and a plurality of self-expanding stents ( 14 ) joined to and supporting the tubular body. An uncovered tubular self-expanding stent assembly ( 26 ) extends from a first end of the tubular body and is fastened thereto. The uncovered tubular self-expanding stent assembly ( 26 ) provides a smooth transition from the leg extension into the iliac artery to reduce the chance of kinks causing problems in the leg extension.

Description:
TECHNICAL FIELD 
     This invention relates to a medical device and more particularly to a medical device for use in relation to endovascular surgery. 
     BACKGROUND OF THE INVENTION 
     Bifurcated stent grafts are well known for treating abdominal aortic aneurysms. Such stent grafts typically include a tubular body which extends in the aorta towards the renal arteries of a patient and a bifurcation. The bifurcation usually has a shorter leg and a longer leg. Once the bifurcated graft is deployed, an extension leg is provided to extend down one iliac artery from the shorter leg, with the longer leg extending down the other iliac artery. 
     There can be problems with such stent grafts, however, for example in cases in which the iliac artery is extremely convoluted, as is often the case with older patients. The extension leg provided to extend down one iliac artery from the shorter leg of the bifurcated aortic stent graft can be kinked in the convoluted region, thereby blocking off blood flow to the iliac and femoral arteries. In some instances the iliac artery can kink immediately beyond the end of the extension leg, again causing blood flow restriction. 
     WO 03/053286 describes an endovascular prosthesis including a first end, a furcated second end, and an anchoring means. The first end has a longitudinally extending central lumen and means for laterally supporting the first end. The furcated second end includes at least two branches that extend from an intersection of the furcated second end. Each of the branches includes a longitudinal support means and a branch lumen in fluid communication with the central lumen of the first end. The anchoring means secures the first end within a vasculature. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a stent graft in which the potential for either of these kinking problems can be reduced or at least provide the physician with a useful alternative device. 
     Throughout this specification the term distal with respect to a portion of the aorta, a deployment device or a prosthesis means the end of the aorta, deployment device or prosthesis further away in the direction of blood flow from the heart and the term proximal means the portion of the aorta, deployment device or end of the prosthesis nearer to the heart. When applied to other vessels similar terms such as caudal and cranial should be understood. 
     According to an aspect of the present invention there is provided a stent graft device as specified in claim  1 . 
     The invention provides a leg extension stent graft device with a flexible uncovered self-expanding stent tail which can assist with controlling convolutions along the length of the iliac artery. The uncovered stent portion of the leg extension can assist in preventing any convolutions in the artery from forming kinks in the covered tubular body portion of the leg extension. The uncovered section can bend to take up any convolution without kinking whereas the same amount of bending in a covered tubular body could cause kinking. 
     In one embodiment, the uncovered tubular self-expanding stent assembly comprises, a plurality of zigzag self-expanding stents flexibly linked together. The flexible linking together can be by use of a suture thread tied to alternate apices of adjacent zig-zag stents to provide a degree of flexibility between adjacent stents. 
     In an embodiment, the uncovered tubular self-expanding stent assembly may comprise a shape memory metal tube integrally formed into a plurality of circumferential stent portions and longitudinal flexible links between the stent portions. The shape memory metal can be Nitinol™. Such a self-expanding stent may for instance be a Zilver™ Stent sold by Cook Incorporated, Bloomington, Ind., USA. 
     The plurality of self-expanding stents joined to the tubular body can comprise zig-zag Gianturco™ stents. 
     Preferably, a second end of the tubular body opposite to the first end is provided with an outside sealing surface and at least one self-expanding stent within the tubular body at the second end, in order to assist with sealing of the leg extension device into one of the legs of a bifurcated stent graft deployed into the aortic bifurcation. 
     In use, the distal end is preferably the first end of the device and the proximal end is the second end of the device. 
     The uncovered tubular self-expanding stent assembly can extend within the tubular body for a distance equivalent to at least the diameter of the tubular body, in order to assist with providing a stable transition from the covered body portion to the uncovered portion. 
     The uncovered tubular self-expanding stent assembly extending from a first end of the tubular body can have an exposed length equivalent to between a quarter of the length of the tubular body to equal to the length of the tubular body. 
     In one embodiment of the invention, the tubular body can comprises a side arm extending therefrom. The side arm can be used for deployment of an extension piece for connecting the internal iliac artery of a patient where the leg extension device is deployed into the common iliac artery and the uncovered portion extends down the external iliac artery towards the femoral arteries. 
     The tubular body can have a diameter of from 10 mm to 20 mm and a length of about 60 mm to 120 mm and the tubular side branch, if present, can have a length of about 25 mm and a diameter of 8 mm. The exposed portion of the uncovered tubular self-expanding stent assembly can have a length of from 15 mm to 120 mm and a diameter of from 10 mm to 20 mm. Hence the overall length of the stent graft leg extension device can be from 75 mm to 240 mm. 
     The biocompatible material from which the tubular body is formed is preferably non-porous so that it does not leak or sweat under physiologic forces. The graft material is preferably made of woven or knitted polyester (Vascutek Ltd., Renfrewshire, Scotland, UK). Other biocompatible fabrics, non-woven materials and porous sheets may be used as the graft material. Examples of biocompatible polymers from which porous sheets can be formed include polyesters, such as poly(ethylene terephthalate), polylactide, polyglycolide and copolymers thereof; fluorinated polymers, such as PTFE, expanded PTFE and poly(vinylidene fluoride); polysiloxanes, including polydimethyl siloxane; and polyurethanes, including polyetherurethanes, polyurethane ureas, polyetherurethane ureas, polyurethanes containing carbonate linkages and polyurethanes containing siloxane segments. In addition, materials that are not inherently biocompatible may be subjected to surface modifications in order to render the materials biocompatible. Examples of surface modifications include graft polymerization of biocompatible polymers from the material surface, coating of the surface with a crosslinked biocompatible polymer, chemical modification with biocompatible functional groups, and immobilization of a compatibilizing agent such as heparin or other substances. Thus, any polymer that may be formed into a porous sheet can be used to make a graft material, provided the final porous material is biocompatible. Polymers that can be formed into a porous sheet include polyolefins, polyacrylonitrile, nylons, polyaramids and polysulfones, in addition to polyesters, fluorinated polymers, polysiloxanes and polyurethanes as listed above. Preferably the porous sheet is made of one or more polymers that do not require treatment or modification to be biocompatible. The graft material may include a biocompatible polyurethane. Examples of biocompatible polyurethanes include THORALON® (Thoratec, Pleasanton, Calif.), BIOSPAN®, BIONATE®, ELASTHANE™, PURSIL™ and CARBOSIL™ (Polymer Technology Group, Berkeley, Calif.). As described in U.S. Patent Application Publication No. 2002/0065552 A1, incorporated herein by reference, THORALON® is a polyetherurethane urea blended with a siloxane-containing surface modifying additive. Specifically, the polymer is a mixture of base polymer BPS-215 and an additive SMA-300. 
     The graft material may also include extracellular matrix materials. The “extracellular matrix” is a collagen-rich substance that is found in between cells in animal tissue and serves as a structural element in tissues. It is typically a complex mixture of polysaccharides and proteins secreted by cells. The extracellular matrix can be isolated and treated in a variety of ways. Following isolation and treatment, it is referred to as an “extracellular matrix material,” or ECMM. ECMMs may be isolated from submucosa (including small intestine submucosa), stomach submucosa, urinary bladder submucosa, tissue mucosa, renal capsule, dura mater, liver basement membrane, pericardium or other tissues. Purified tela submucosa, a preferred type of ECMM, has been previously described in U.S. Pat. Nos. 6,206,931, 6,358,284 and 6,666,892 as a bio-compatible, non-thrombogenic material that enhances the repair of damaged or diseased host tissues. U.S. Pat. Nos. 6,206,931, 6,358,284 and 6,666,892 are incorporated herein by reference. Purified submucosa extracted from the small intestine (“small intestine submucosa” or “SIS”) is a more preferred type of ECMM for use with the described embodiments. Another type of ECMM, isolated from liver basement membrane, is described in U.S. Pat. No. 6,379,710, which is incorporated herein by reference. ECMM may also be isolated from pericardium, as described in U.S. Pat. No. 4,502,159, which is also incorporated herein by reference. Irrespective of the origin of the graft material, the graft material can be made thicker by making multi-laminate constructs, for example SIS constructs as described in U.S. Pat. Nos. 5,968,096; 5,955,110; 5,885,619; and 5,711,969. All of these references are incorporated herein by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a first embodiment of leg extension for a vascular stent grafting system according to the present invention; 
         FIG. 2  shows a cross sectional view of the leg extension shown in  FIG. 1 ; 
         FIG. 3  shows the schematic vasculature of a patient showing how the leg extension according to one embodiment of the present invention can be used within the vasculature; 
         FIG. 4  shows another embodiment of stent graft leg extension according to the present invention; 
         FIG. 5  shows a cross sectional view of an embodiment of a leg extension stent graft according to the present invention; and 
         FIG. 6  shows part of the leg extension stent graft of  FIG. 5  showing how much bending the stent graft can take. 
     
    
    
     DETAILED DESCRIPTION 
     Now, looking more closely at the drawings and in particular  FIGS. 1 and 2 , a first embodiment of stent graft according to the present invention is described. 
     In this embodiment, the stent graft leg extension  10  comprises a tubular body  12  of a biocompatible graft material with a number of zig-zag Gianturco stents  14  on its outer surface between its ends and an inner zig-zag Gianturco stent  16  at the proximal end  18 . On the outside of the proximal end, the biocompatible graft material provides a sealing surface  20 . At the distal end  22  of the tubular body is another sealing surface  24  and extending from the end  22  is a tubular self-expanding stent  26 . The tubular zig-zag stent  26  comprises circumferential zig-zag portions  26   a  with flexible longitudinal links  26   b  between the circumferential portions. 
     As can be seen particularly in  FIG. 2 , which shows a longitudinal cross-section of the stent graft shown in  FIG. 1 , the tubular stent  26  extends within the tubular body  12  for at least a length equivalent to one diameter of the tubular body  12  and is suitably fastened, such as by stitching (not shown), inside the tubular body. The length of the tubular stent  26  beyond the distal end  22  of the tubular body  12  is equal to between a quarter to one times the length of the tubular body  12 . It will be noted that the tubular stent  26  can take a considerable degree of bending, as shown in  FIG. 1 , without kinking, whereas an extended tubular body  12  itself may well kink in the region  28  of bent to the same degree. 
     It will be seen that by this arrangement a flexible transition is provided between the leg extension and the iliac artery. 
       FIG. 3  shows the use of the leg extension stent graft of  FIG. 1  in a vasculature of a patient.  FIG. 3  shows a schematic view of the vasculature of a patient, particularly showing the aorta and aortic bifurcation extending down towards the iliac arteries. The vasculature comprises aorta  30  in the region between the renal arteries  34  and the aortic bifurcation  36 . Common iliac arteries  38   a  and  38   b  extend from the aortic bifurcation  36 . The common iliac artery  38   a  has a considerable kink in it. The aorta  30  has an aneurysm  32  which extends between the renal arteries and the iliac bifurcation. 
     To traverse the aneurysm, a bifurcated aortic stent graft  40  has been deployed into the aorta  30 . The proximal end  41  of the bifurcated stent graft  40  is engaged onto a non-aneurysed portion  43  of the aorta just distal of the renal arteries  34 . To ensure good fixation, the stent graft  40  includes a supra renal exposed stent  45  with barbs  47  engaging the wall of the aorta proximal the renal arteries  34 . 
     The stent graft  40  has a short leg  42  and a long leg  44  extending from a bifurcation  49  at its distal end  51 . The long leg  44  has a sealing surface  46  at its distal end and this engages in a sealing manner into an non-aneurysed portion of the common iliac artery  38   b.    
     A leg extension  10  of the type shown in  FIGS. 1 and 2  has been deployed into the iliac artery  38   a  with the sealing surface  20  within the shorter leg  42  of the bifurcated stent graft  40  and the leg extension extending down into the iliac artery  38   a . The tubular stent  26  allows for a degree of convolution in the iliac artery  38   a  while still allowing the sealing surface  24  at the distal end  22  of the tubular body  12  of the leg extension  10  to seal conveniently within the iliac artery  38   a.    
       FIG. 4  shows another embodiment of leg extension. In this embodiment the leg extension  60  comprises a tubular body  62  of a graft material with a side arm  64  extending from the tubular body. A tubular self-expanding stent  66  extends from the distal end  68  of the tubular body  62 . This leg extension can be used where the internal iliac artery is very close to the aortic bifurcation and the placement of a leg extension without a side arm may cause occlusion of the internal iliac artery. A covered balloon expandable or self-expanding stent (not shown) can be used to extend between the side arm  64  and the internal iliac artery. 
       FIGS. 5 and 6  shows another embodiment of leg extension in longitudinal cross section with a flexible transition according to the present invention. In this embodiment the leg extension  70  comprises a tubular body  72  of a graft material with a plurality of self-expanding stents  74  on the outside except at the proximal end  76  where the self-expanding stent  78  is on the inside to provide a sealing surface  80  on the outside of the tubular body. At the distal end  82  is a stent assembly  84  which comprises a plurality of self-expanding stents  86  connected together by flexible links  88 . In this embodiment the flexible links are formed from a suture tied to alternate apices  85  of adjacent zig-zag stents  86  to provide a degree of flexibility between adjacent stents. The proximal-most uncovered stent  90  of the stent assembly  84  is linked to the distal-most covered stent  92  by a flexible suture thread  94 . 
       FIG. 6  shows that a considerable degree of flexibility is provided by the stent assembly  84  so that a smooth transition is provided for the connection between the vasculature and the leg extension  70 . 
     Throughout this specification various indications have been given as to the scope of this invention but the invention is not limited to any one of these but may reside in two or more of these combined together. The examples are given for illustration only and not for limitation. 
     Throughout this specification and the claims that follow unless the context requires otherwise, the words ‘comprise’ and ‘include’ and variations such as ‘comprising’ and ‘including’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.