Patent Publication Number: US-11376114-B2

Title: Introducer for deploying a stent graft in a curved lumen and stent graft therefor

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/839,115, filed Dec. 12, 2017, which is a continuation of U.S. application Ser. No. 12/609,066, filed Oct. 30, 2009 (now U.S. Pat. No. 9,855,128), which claims priority to United Kingdom Patent Application No. 0820061.0, filed Oct. 31, 2008 entitled “Introducer For Deploying a Stent Graft in a Curved Lumen and Stent Graft Therefor.” This application is related to United Kingdom Patent Application No. 0820066.9, filed Oct. 31, 2008 
    
    
     FIELD OF THE INVENTION 
     The present application relates to an introducer for deploying a stent graft within a curved lumen. It also relates to a stent graft for deployment within a curved lumen. 
     BACKGROUND 
     Stent grafts are used to replace or repair vessels of the body such as arteries. A stent graft is usually formed from a tubular body of a biocompatible graft material with one or more stents mounted into or onto the tubular body to provide support therefore. The stents may be balloon expandable stents or self-expanding stents. 
     Endovascular methods have been proposed for treatment of aneurysms of the aorta particularly where the aneurysm is adjacent the aorta bifurcation. However, when an aneurysm occurs higher up in the aorta, in the region of the descending aorta adjacent the aortic or thoracic arch or in the ascending aorta, endovascular techniques for treating these aneurysms are somewhat more difficult because of the tight curvature of the aortic or thoracic arch, the occurrence of major arteries in the region and the proximity to the heart. Placement of a substantially cylindrical prosthesis in such a curved region can cause problems. 
     Stent grafts are typically deployed using endovascular techniques on an introduction device in which the stent graft is retained in a radially contracted condition by a sheath. Upon withdrawal of the sheath and release of any retention arrangement where provided, for example in cases in which the stent graft has self-expanding stents, the stent graft can expand under the action of the self-expanding stents towards the vessel walls to redefine the blood flow path. The introduction device is withdrawn after deployment. 
     Currently, stent grafts are deployed in curved lumens by causing these to follow the curvature imparted to the introducer. However, this can result in the stent graft not sitting properly in the blood vessel and in the lumen of the prosthesis being closed off or reduced in lumen diameter. 
     Furthermore, when deploying a stent graft that is substantially straight in a curved aorta there is a danger that the proximal end of the stent graft, that is, the end nearest the heart, will not lie flat against the walls of the aorta (i.e., the end “face” is not positioned perpendicularly to the wall of the vessel) and blood can flow underneath the edge of the graft, particularly on the inner side of the curve of the aortic or thoracic arch and cause the stent graft to buckle and close off thereby causing serious problems. 
       FIGS. 1 and 2  illustrate this problem. The introducer and stent graft generally have a substantially straight configuration but as a result of their pliancy are urged into a curved orientation by the walls of the lumen. However, as the stent graft is held on the introducer, which itself tends to maintain its straightened configuration to the extent that it keeps to the outside of the curve of the lumen, the stent graft also tends to maintain a lesser curved configuration as it is being deployed. When the stent graft is released from its ties to the introducer, it expands in a manner which continues to tend to the straightened configuration and in particular to push the inner edge of the stent graft forwardly. This leads to an increased Proximal Face Angle (PFA), that is, to an increased angle between the line formed by the proximal end of the stent graft and the line perpendicular to the walls of the lumen. It also leads to an increased Proximal Normal Gap (PNG), that is, a gap between the lumen wall and the proximal end of the stent on the inner part of the bend in the lumen. These are shown in  FIGS. 1 and 2 . The Proximal Normal Gap allows blood pressure to build between the outside of the stent graft and the lumen wall, which will tend to bias this side of the stent graft inwardly into the lumen and thus towards closing of the lumen of the stent graft. Thus, the seal at the proximal end of the stent graft may not be as effective as desired. This gap can also cause the graft material itself to flap in the fluid flow, leading to unnatural fluid flow and possible premature wear and tear of the stent graft. 
     US 2004/0073289 discloses a stent graft for deployment within a curved portion of the aorta. 
     SUMMARY 
     The subject matter of the present application seeks to provide an improved introducer and method for deploying a stent graft within a curved lumen, as well as a stent graft for deployment in a curved lumen. 
     In general the teachings herein relate to the placement of prostheses in the aorta in the region known as the aortic or thoracic arch, where the aorta leaves the heart and curves in approximately a U-shape to the descending aorta, then into the abdominal aorta and then into the lower limbs via the iliac arteries. The teachings herein are, however, not so restricted and can relate to placement of prostheses within or in place of lumens in any portion of a human or animal body, though it is particularly relevant to curved lumens. 
     According to an aspect of the present invention, there is provided an introducer for deploying a stent graft in a curved lumen, the introducer including: a carrier for a stent graft, which stent graft is provided with a plurality of stents including a proximal stent at a proximal end of the stent graft; a release mechanism including a constraining mechanism operable to maintain the proximal stent of the stent graft in a constrained configuration at at least two points during deployment whilst allowing at least a portion of the stent graft distal of the proximal stent to expand; wherein the constraining mechanism is operable to constrain both the proximal end and the distal end of the proximal stent at said at least two points. 
     This arrangement ensures that the proximal-most stent of the stent graft can be kept constrained during deployment, allowing the rest of the stent graft to expand and permitting the proximal stent to expand thereafter, so as to enable the proximal end of the stent graft to be positioned substantially perpendicularly to the vessel wall prior to its expansion. In particular, this arrangement can promote overlap of graft material just distal of the distal end of the proximal stent and thus a better curvature of the stent graft. As a result, it is possible to achieve an improved seal between the proximal stent and the vessel wall. 
     Thus, in an embodiment, the release mechanism is operable to enable the constrained stent to expand so as to overlap with the interior of at least a portion of a distally adjacent stent in the expanded portion of the stent graft. This helps the stent graft conform to curved vasculature. 
     In a preferred embodiment, the release mechanism is operable to constrain the proximal stent at at least two, preferably three, points during deployment. In an embodiment, the at least two points are substantially evenly spaced radially around the proximal stent. 
     The release mechanism preferably includes at least one restraining wire able to hold loops of thread provided on an implantable medical device so as to constrain the implantable medical device to the introducer. The at least one restraining wire is preferably located within a lumen of the carrier of the introducer. 
     The introducer may include a center guide wire carrier provided within the lumen of the carrier. The carrier may be provided with a plurality of apertures therein for receiving the loops of thread of the implantable medical device. 
     The introducer preferably includes at least three restraining wires. It may include at least six restraining wires. 
     According to another aspect of the present invention, there is provided a stent graft for deployment in a curved lumen, the stent graft arranged to be deployed by an introducer as described above, wherein the stent graft is provided with a mechanism for allowing at least two points of the proximal stent to be constrained during deployment whilst a portion of the stent graft is expanded, wherein the at least two points of the proximal stent are able to be constrained at both the proximal end of the stent and at the distal end of the stent. 
     According to another aspect of the present invention, there is provided a stent graft for deployment in a curved lumen, including an element of graft material providing an inner lumen, a plurality of stents located longitudinally along the graft element, at least one of said stents being a proximal stent located at a proximal end of the stent graft, a plurality of loops of thread located at or adjacent proximal and distal ends of the proximal stent and extending into the inner lumen, wherein the plurality of loops are able to allow at least two points of the proximal stent to be constrained during deployment whilst a portion of the stent graft is expanded, wherein the loops provided at the at least two points of the proximal stent are able constrain both the proximal and the distal ends of the stent. 
     As such, the stent graft can provide an improved seal at its proximal end, because the proximal stent can be located substantially perpendicularly to the vessel wall prior to expansion. 
     In a preferred embodiment, the mechanism allows the proximal stent to be constrained at three points during deployment. 
     Preferably, the at least two points are substantially evenly spaced radially around the proximal stent. 
     Preferably the constraining mechanism is operable to constrain the distal end of the proximal stent substantially entirely therearound. 
     In an embodiment, the constraining mechanism is operable to constrain at least two adjacent struts of the proximal stent to the introducer at each of the at least two points of the proximal stent. 
     The loops are preferably substantially evenly spaced around the circumference of the proximal stent. 
     The portions of the loops extending into the inner lumen may be located at or adjacent junctions of two adjoining struts forming the proximal stent. 
     There may be provided a plurality of threads of material for forming the loops. There may be provided a single length of thread providing said plurality of loops. 
     According to another aspect of the present invention, there is provided an assembly including an introducer for deploying a stent graft in a curved lumen, the introducer including a carrier for a stent graft, which stent graft is provided with a plurality of stents including a proximal stent at a proximal end of the stent graft; a release mechanism including a constraining mechanism operable to maintain the proximal stent of the stent graft in a constrained configuration at at least two points during deployment whilst allowing at least a portion of the stent graft distal of the proximal stent to expand; wherein the constraining mechanism is operable to constrain both the proximal end and the distal end of the proximal stent at said at least two points; the assembly including a stent graft, the stent graft including: an element of graft material providing an inner lumen, a plurality of stents located longitudinally along the graft element, at least one of said stents being a proximal stent located at a proximal end of the stent graft, a plurality of loops of thread located at or adjacent proximal and distal ends of the proximal stent and extending into the inner lumen, wherein the plurality of loops are able to allow at least two points of the proximal stent to be constrained during deployment whilst a portion of the stent graft is expanded, wherein the loops provided at the at least two points of the proximal stent are able constrain both the proximal and the distal ends of the stent. 
     Preferably the loops are able to constrain the distal end of the proximal stent substantially entirely therearound. 
     In an embodiment, the mechanism includes at least one wire-receiver for cooperating with a release wire of the introducer. The wire-receiver may be a loop of material, such as a suture loop. 
     The term thread as used herein is intended to include any filamentary material which can perform the stated function and could, for example, be of conventional suture material, a multi-filamentary structure formed of yarns for example and of a natural or synthetic material such as cotton, other biocompatible material or a polymer material such as polyester, or a mono-filamentary structure of a natural material, other biocompatible material, a metal such as gold or an alloy such as Nitinol. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: 
         FIGS. 1 and 2  are schematic illustrations of problems with deployment of a stent graft in a curved lumen; 
         FIGS. 3 and 4  show an example of an implant deployment device that can be used with the teachings herein; 
         FIG. 5  shows a perspective view of an embodiment of a stent graft mounted on a deployment device; 
         FIG. 6  shows a top view of the stent graft and deployment device of  FIG. 5 ; 
         FIG. 7  shows a transverse cross-section through a portion of the stent graft and deployment device of  FIG. 5 ; 
         FIG. 8  shows an enlargement of a part of  FIG. 7 ; 
         FIG. 9  shows a longitudinal cross-section through a portion of the deployment device shown in  FIG. 5 ; 
         FIGS. 10 to 12  illustrate deployment of the stent graft of  FIG. 5 ; 
         FIG. 13  illustrates another embodiment of a stent graft; 
         FIG. 14  illustrates another view of the stent graft of  FIG. 13 ; 
         FIG. 15  illustrates another view of the stent graft of  FIG. 13 ; and 
         FIG. 16  illustrates a modified arrangement of suture threads that may be used with the stent graft of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the Figures are schematic and do not show the various components to their actual scale. In many instances, the Figures show scaled up components to assist in understanding their structures and functions. 
     In this description, when referring to a deployment assembly, the term distal is used to refer to an end of a component which in use is furthest from the surgeon during the medical procedure, including within a patient. The term proximal is used to refer to an end of a component closest to the surgeon and in practice in or adjacent an external manipulation part of the deployment or treatment apparatus. 
     On the other hand, when referring to an implantable medical device such as a stent or stent graft, the term proximal refers to a location which in use is closest to the patient&#39;s heart, in the case of a vascular implant, and the term distal refers to a location furthest from the patient&#39;s heart. 
     Referring to  FIGS. 3 and 4 , an implant deployment device  10  includes an external manipulation section  12 , a proximal attachment region  14  and a distal attachment region  16 . The proximal attachment region  14  and the distal attachment region  16  secure the two ends of the implant  18 . During the medical procedure to deploy the implant  18 , the proximal and distal attachment regions  14  and  16  will travel through the patient&#39;s vasculature, in this example, to a desired deployment site. The external manipulation section  12  at the proximal end of the implant deployment device  10 , which is operated by a surgeon to manipulate the introducer, remains outside of the patient throughout the procedure. 
     The distal attachment region  16  of the implant deployment device  10  includes a dilator tip  20 , which is typically provided with a bore  22  therein for receiving a guide wire (not shown) of conventional type. The longitudinal bore  22  also provides a channel for the introduction of medical reagents. For example, it may be desirable to supply a contrast agent to allow angiography to be performed during placement and deployment phases of the medical procedure. 
     A guide wire carrier or cannula  24 , conventionally made from a flexible thin walled metal tube, is fastened to the dilator tip  20 . The guide wire carrier  24  is flexible so that the implant deployment device  10  can be advanced along a relatively tortuous vessel, such as a femoral artery, and so that the distal end of the implant deployment device  10  can be longitudinally and rotationally manipulated. The guide wire carrier  24  carries a stent  18  or other device to be implanted in the patient. The guide wire carrier  24  extends through the implant deployment device  10  to the manipulation section  12 , terminating at a connection device  26 , in conventional manner. 
     The connection device  26  is designed to accept a syringe to facilitate the introduction of reagents into the guide wire carrier  24  and for this purpose is typically provided with a threaded luer lock connection. 
     Where provided, a pusher sheath or rod  30  (hereinafter referred to as a pusher member), typically made from a plastics material, is mounted coaxial with and radially outside of the guide wire carrier  24 . The pusher member  30  is “thick walled”, that is the thickness of its wall is preferably several times greater than that of the guide wire carrier  24 . In some instances, the pusher member  30  and the guide wire carrier  24  are the same component, possibly having different outer diameters at the location at which the stent  18  is to be carried. 
     A sheath  32  extends coaxially over and radially outside of the pusher member  30 . The pusher member  30  and the sheath  32  extend distally to the manipulation region  12 . 
     The implant  18 , which is a stent graft, is retained in a compressed condition by the sheath  32 . The sheath  32  extends proximally to a sheath manipulator and haemostatic sealing unit  34  of the external manipulation section  12 . The haemostatic sealing unit  34  includes a haemostatic seal (not shown) and a side tube  36  held to the unit  34  by a conventional luer lock  38 . 
     The sheath manipulator and haemostatic sealing unit  34  also includes a clamping collar (not shown) that clamps the sheath  32  to the haemostatic seal and a silicone seal ring (not shown) that forms a haemostatic seal around the pusher member  30 . The side tube  38  facilitates the introduction of medical fluids between the pusher member  30  and the sheath  32 . Saline solution is typically used. 
     During assembly of the implant deployment device  10 , the sheath  32  is advanced over the proximal end of the dilator tip  20  of the proximal attachment region  16  while the implant  18  is held in a compressed state by an external force. A suitable distal attachment (retention) section (not visible in this view) is coupled to the pusher member  30  and retains a distal end  40  of the prosthesis  18  during the procedure. The distal end of the prosthesis  18  may be provided with a loop of material (not shown) through which a distal restraining wire  42  extends. The distal restraining wire also extends through an aperture (not shown in  FIGS. 1 and 2 ) in the proximal attachment section  40  into an annular region  44  between the guide wire carrier  24  and the pusher member  30 . The distal restraining wire  42  extends through the annular space  44  to the manipulation region  12  and exits the annular space  44  at a distal wire release mechanism  46 . 
     A proximal portion of the external manipulation section  12  includes at least one restraining wire actuation section  50  mounted on a body  48 , in turn mounted onto the pusher member  30 . The guide wire carrier  24  passes through the body  48 . The distal wire release mechanism  46  and the proximal wire release mechanism  50  are mounted for slidable movement on the body  48 . 
     Clamping screws  52  prevent inadvertent early release of the prosthesis  18 . A haemostatic seal (not shown) is included so that the release wires can extend out through the body  48  without unnecessary blood loss during the medical procedure. 
     A proximal portion of the external manipulation section  12  includes a pin vise  54  mounted onto the proximal end of the body  48 . The pin vise  54  has a screw cap  56 . When screwed in, vise jaws (not shown) of the pin vise  54  clamp against or engage the guide wire carrier  24 . When the vise jaws are engaged, the guide wire carrier  24  can only move with the body  48  and hence it can only move with the pusher member  30 . With the screw cap  56  tightened, the entire assembly can be moved together as one piece. 
     Once the implant deployment device  10  is in the desired deployment position, the sheath  32  is withdrawn and the proximal and distal wire release mechanisms  50 ,  46  are released to allow the prosthesis  18  to expand. 
     For some procedures, the sheath  32  may be left in place after expansion of the implant  18 . The pusher member  30  and guide wire carrier  24  may be withdrawn and replaced by a further component, using the sheath  32  as a guide. 
     It is to be understood that the guide wire carrier  24  can sometimes be described, both above and in the description which follows, as a center guide wire carrier, as a cannula or as a sheath and in all of the embodiments described herein it could take any of these forms. It is also to be understood that although some embodiments described below make use of a guide wire carrier as well as an introducer carrier, this is not an essential combination as it is envisaged that in some embodiments a guide wire may be carried within the carrier of the introducer, that is without any separate guide wire carrier or cannula. 
       FIGS. 5 to 7  illustrate an embodiment of stent graft  18 , shown mounted on an introducer  10 ′, which may be of the type illustrated in  FIGS. 3 and 4 . The stent graft  18  is formed from a tubular element of biocompatible graft material  1  provided with a plurality of stents  4 ,  4 ′ disposed along its length, the example shown including Z-stents. In this embodiment, the stent  4 ′ located at the proximal end of the tubular piece of graft material  1  is disposed on the inside of the tube  1 , whereas other stents  4  are located outside the tube  1 . In other embodiments, the various stents can be positioned differently with respect to the inside and outside of the tube and can also be all on the same side. 
     The most proximal stent  4 ′ is provided, at substantially equally spaced locations therearound, with three loops of suture material  11  at its proximal end and three loops of suture material  11  at its distal end. The loops of suture material  11  are, in this embodiment, threaded through the graft material  1  and around a strut of the stent  4 ′. Other arrangements may be envisaged in other embodiments. 
     The loops of suture material  11  are able to engage with a release wire  42  of the introducer  10 ′ to form a constraining mechanism for use during deployment of the stent graft  18 . The constraining mechanism serves to constrain the proximal stent  4 ′ on the deployment device  10 ′ at, in this embodiment, three positions around its circumference, resulting in the creation of three lobes  13  of graft material  1  at the proximal end of the stent graft. The purpose of this is described below. 
       FIGS. 8 and 9  illustrate one example of a constraining mechanism for constraining the proximal stent  4 ′ of a stent graft  18  on an introducer  10 ′. The deployment device  10 ′ in the region of the proximal retention mechanism has, in this embodiment, a guide wire carrier  24  and a restraining wire carrier  8  (typically the carrier of the introducer for carrying the medical device) located coaxially around the guide wire carrier  24 , that is such that the guide wire carrier resides in a lumen of the carrier  3  or  8 . In some embodiments the guide wire may be located directly in the restraining wire carrier  8 , thus avoiding the need for a separate guide wire carrier  24 . 
     Restraining wires  42  pass along the annular space between the guide wire carrier  24  and the restraining wire carrier  8  and exit through apertures  7  at the six retention points and then re-enter the annular space between the guide wire carrier  24  and the restraining wire carrier  8  and pass into the nose cone dilator  20  to be secured thereby. A guide wire passes through the lumen of the guide wire carrier, catheter or cannula  24  in conventional manner. 
     In this embodiment, three separate restraining wires  42  are provided. Each restraining wire  42  extends to, and acts to constrain, an aligned proximal and distal attachment point on the proximal stent  4 ′. 
     The apertures  7  may be equally spaced around the restraining wire carrier  8  or they may be spaced at other selected spacings. In this embodiment there are two sets of three apertures  7  spaced at approximately 120° to each other around the circumference of and in pairs along the restraining wire carrier  8 . 
     Where each restraining wire  42  exits out of an aperture  7 , a thread of suture material or other thread-like material  11  is looped around the restraining wire  42  and is fastened to a bight  114  of the graft material  1  of the stent graft  18  and tied off with a knot  112 . The restraining wires  42  extend to an external manipulation section  12  including a restraining wire actuation section  50 , such as that illustrated in  FIGS. 3 and 4 . 
     In use, in order to improve the positioning of the stent graft  18  within a curved lumen, the constraining mechanism for the proximal stent  4 ′ is only released after the remainder of the stent graft  18  has been expanded. This is described with reference to  FIGS. 10 to 12 . 
     It can be seen that the aortic or thoracic arch  130  of aorta  25  has an outer curve  33  and an inner curve  31 . A stent graft  18  is deployed into the thoracic arch  130  to span, for instance, a tear  126  in the wall of the aorta  25  which has caused an aortic dissection  27 . 
     An introducer  10 ′, having a stent graft  18  mounted thereon, is inserted into the aorta  25  and curved around the thoracic arch  130  as it passes through this, until the stent graft  18  is located at the desired site of deployment.  FIG. 10  shows the introducer  10 ′ prior to withdrawal of the sheath  32 , which covers the stent graft  18 . The introducer  10 ′ is then oriented within the vessel, such that one of the constrained portions of the proximal stent  4 ′ is facing the inner part of the curve (as can be best seen with reference to  FIG. 11 ). The orienting step is carried out in a known manner using, for example, radio-opaque markers (not shown), which may be located on the stent graft  18  and/or on the deployment device  10 ′. 
     The surgeon or clinician then withdraws the sheath  32  to expose the stent graft  18 . The stent graft  18  is allowed to expand by means of its self-expanding stents  4 , in this embodiment, or by means of a balloon expansion mechanism in the case of use of stents which are not self-expandable. 
       FIG. 11  shows the stent graft  18  after withdrawal of the sheath  32 . It can be seen that the stent graft  18  has expanded apart from at the proximal stent  4 ′, which is still constrained to the introducer  10 ′ at three points around its circumference at both its proximal end and its distal end. 
     For the final stage of deployment, the surgeon or clinician releases the restraining wires  42  of the constraining mechanism to allow the proximal stent  4 ′ to expand. The curve of the thoracic arch  130  forces the proximal stent  4 ′ and its adjacent stent  4  closer together at the inner part of the curve  31 . As the proximal stent  4 ′ expands after its adjacent stent  4 , the proximal stent  4 ′ overlaps with the interior of its adjacent stent  4  at the inner part of the curve  31 , forming a region of overlap  120 . 
     This arrangement enables the plane of the proximal stent  4 ′ to be positioned substantially perpendicularly to the vessel wall prior to release. As a result, it is possible to achieve an improved seal between the proximal stent  4 ′ and the vessel wall. In prior art systems where the proximal stent is constrained only at its proximal end, the stent forms a cone shape after partial release, and is unable to be positioned perpendicularly to the vessel wall. 
     In this embodiment of  FIGS. 5 to 12  it is important to orient the lobes of the proximal stent  4 ′ in order to ensure that no lobe  13  becomes located on the inner side of the curve of the lumen, as so doing could prevent the stent  4 ′ from overlapping into the second stent  4  as it is released and thus prevent proper positioning of the proximal stent  4 ′. 
     The skilled person will appreciate that modifications to the above-described embodiments may be made. In particular, the suture loops  11  may be provided on the proximal stent  4 ′ or may be provided on the graft material  1 . The constraining mechanism may be provided by any suitable arrangement. The above-described arrangement is merely exemplary. In some embodiments there could be provided six restraining wires, each able to restrain a single suture loop  114 . 
     Another embodiment of a stent graft  18  for deployment within a curved lumen is described with reference to  FIGS. 13 to 15 , which shows a stent graft  18  having a proximal stent  4 ′ that is to be constrained at both its proximal and distal ends during deployment.  FIGS. 13 and 14  show the proximal stent  4 ′ in its partially deployed state, that is after release of the proximal end of the proximal stent  4 ′ but before its distal end has been fully released. 
     As can best be seen in  FIG. 13 , there is provided on the stent graft  18  a suture arrangement in which three suture loops  132  can be pulled into the center of the stent graft  18 , that is towards the guide wire carrier  24  and held thereto by a release wire within the lumen of the guide wire carrier  24  through a plurality of openings  7  therein. The arrangement of suture loops  132  differs from the arrangement of the embodiment illustrated in  FIGS. 5 to 9 , in that a plurality of pairs of struts of the proximal stent  4 ′ is constrained by each loop arrangement, thereby pulling the entirety of the distal end of the proximal stent  4 ′ towards the guide wire carrier  24 , as can be seen in  FIGS. 13 and 14 . It is preferred that the proximal stent  4 ′ is still pulled in a tri-lobed configuration, as with the embodiment of  FIGS. 5 to 9 , but in such a way that the entirety of the distal end of the proximal stent  4 ′ is pulled inwardly to some extent and thus to lie within the perimeter of the second stent  4  adjacent to it. 
     A variety of different embodiments of suture arrangements for the stent graft  18  are possible. One embodiment is shown in  FIG. 15 , in which there are provided three lengths of suture thread  132  each attached to four stent apices. Each suture thread  132  is located generally on the outside of the graft material  1  apart from a length which feeds to the inside of the graft material  1 , as can best be seen in  FIG. 15 . This length can be fed through an opening  7  in the guide wire carrier  24  and restrained by a restraining wire therein, to produce the tied arrangement shown  FIG. 13 . A similar arrangement is provided also at the proximal end of the proximal stent  4 ′ to constrain this to the guide wire carrier  24 . 
     Although not shown in the Figures, it is preferred that three restraining wires are provided, extending through the guide wire carrier  24 . Each restraining wire can then serve to engage generally aligned proximal and distal suture threads  132 . It will be appreciated that the proximal and distal constraining mechanisms may be slightly off-set with respect to one another due to the nature of the Z-stents used in this example. 
     Deployment of this embodiment is very similar to that described above with respect to  FIGS. 10 to 12 . The main difference is that orientation of the stent graft  18  with respect to the curvature of the vessel is not required. The reason for this is that since the entirety of the circumference of the distal end of the proximal stent  4 ′ is pulled inwardly towards the carrier  3  or  24 . Thus, as the stent graft is deployed, the distal end of the proximal stent  4 ′ is already in partial overlap with the second stent  4  of the stent graft and thus better able to conform to the curvature of the vessel. This therefore renders deployment more straightforward. 
     As illustrated in  FIGS. 13 and 14 , it is also preferred that withdrawal of the release wires engaged with the suture threads  132 , acts firstly to release the proximal end of the proximal stent  4 ′ and secondly to release the distal end of the proximal stent  4 ′. 
     It can be seen from the above that in this embodiment, the constraining mechanism constrains the distal end of the most proximal stent  4 ′ substantially around the entirety of its circumference yet preferably still retaining a tri-folded configuration. As is apparent from  FIG. 14 , the distal end of the proximal stent  4 ′ lies within the circumference of the second stent  4  and, when deployed in a curved lumen, will expand to overlap with the interior of the second stent  4  so as to follow more accurately the curvature of the lumen (aortic arch). This results in an improved fit within the patient&#39;s lumen. 
     Pulling the distal end of the proximal stent  4 ′ to the carrier  24  has another important advantage in that the stent graft  18  is then held to the guide wire carrier  24  during deployment. When in the aortic arch, the introducer  10 , and thus the guide wire carrier  24 , will tend to follow the outside of the curve of the aortic arch, that is, the region of greater length. Thus, the stent graft  18  will be urged against the lumen wall at the outside of the curve with those parts at the inside of the curve held away from the vessel wall by being held close to the guide wire carrier  24 . Thus, the portions of the stent graft  18  which will be positioned on the shorter part of the inside of the curve will only come into contact with this once they are released from the guide wire carrier  24  and thus already in a curved configuration. This assists in ensuring that the distal end of the proximal stent  4 ′ on the inside of the curve overlaps with the proximal end of the second stent thereby to provide good conformity with the vessel wall. 
     The skilled person will appreciate that there are of course many modifications that could be made to this embodiment. 
     In a modification, one or more suture threads  132  may be located preferably substantially entirely on the outside of the graft layer  1  in order to minimize the amount of suture material  132  on the inside thereof which could interfere with medical devices subsequently introduced through the stent graft, such as during a subsequent medical procedure.  FIG. 16  illustrates such a modification, in which portions of suture thread located on the outside of the graft material are shown as a dashed line, while portions of suture thread  130  on the inside of the graft material  1  are shown as a thicker solid line. It can be seen that suture material  132  is located on the inside of the graft material  1  only between the struts of the proximal stent  4 ′, and towards the apices thereof. In this way, the struts act to hide or shield the loops of suture material  132  from any later used medical device. The suture threads  132  are thus somewhat protected by the struts of the proximal stent  4 ′ from snagging on a later inserted medical device. 
       FIG. 16  shows the use of a single length of suture material  130  circumscribing the entirety of the circumference of the stent graft  1 . This facilitates the manufacture of the stent graft  1  as well as facilitating its mounting onto an introducer, in particular in cases where there are provided many portions  132  of suture material extending into the inside of the graft tube and thus each being selectively able to be held by the constraining wires. On the other hand, there could be provided separate lengths of suture material, as in the embodiment of  FIG. 15 , but in which the portion  132  extending into the graft tube is located between the apex (trough when viewed as in  FIG. 15 ) of two stent struts in order to provide the shielding described above. 
     In the preferred embodiment (as described above), three restraining wires are used, one for each suture thread  132 , but each engaging with both a proximal and a distal suture thread  132 . However, it is possible to use only a single restraining wire, which engages with all three suture threads  132 , though this arrangement may interfere with the position of the guide wire. In another modification, six restraining wires may be used, each engaging with a single proximal or a single distal suture thread  132 . An advantage of this embodiment is that it allows improved control of release of the proximal stent  4 ′. 
     It is to be appreciated that the proximal end of the proximal stent  4 ′ is preferably retained by a similar arrangement of suture threads  130  such that when the stent graft is first deployed the proximal stent  4 ′ is held both at its proximal and at its distal ends by the constraining mechanisms in, preferably, a tri-lobed manner but in which the lobes do not extend beyond the diameter of the second stent  4 . During deployment, once the remainder of the stent graft  1  has been deployed, the proximal end of the stent  4 ′ is released, allowing the proximal end to open up in a manner as shown in  FIGS. 13 and 14  in particular. This allows the proximal end of the stent graft  1  to engage the walls of the patient&#39;s vessel and to be held thereto, particularly in the case where fixing barbs are provided. Then, the distal end of the stent  4 ′ can be released, allowing this to expand and in particular allowing the distal end of the stent  4 ′ at the inside of the curve to expand in overlapping manner into the second stent  4 . This can ensure good curvature of the stent graft  1  particularly at its proximal end and thus better conformity with the vessel wall and better sealing. 
     What has been described and illustrated herein is a preferred embodiment of the invention along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims, and their equivalents, in which all terms are meant in their broadest reasonable sense unless otherwise indicated. 
     The disclosures in United Kingdom patent application 0820061.0, from which the present application claims priority, and in the abstract accompanying this application are incorporated herein by reference.