Abstract:
A self-expanding stent delivery apparatus having a reinforced sheath for the safe, effective and accurate deployment of self-expanding stents. The sheath is formed from an inner polymeric layer, an outer polymeric layer and a reinforcement layer sandwiched therebetween. The reinforcement layer comprises flat metallic wire to provide the requisite radial and axial strength. In addition, flat wire reduces the profile of the device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 09/631,002 filed on Aug. 2, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to stents for use within a body passageway or duct which are particularly useful for repairing blood vessels narrowed or occluded by disease, and more particularly, to systems for delivering such stents.  
         BACKGROUND OF THE INVENTION  
         [0003]    Various endoprosthesis assemblies, which include expandable stents, have been proposed or developed for use in association with angioplasty treatments and other medical procedures. The endoprosthesis assembly is percutaneously routed to a treatment site and the stent is expanded to maintain or restore the patency of a body passageway such as a blood vessel or bile duct. A stent is typically cylindrical in shape comprising an expandable open frame. The stent will typically expand either by itself (self-expanding stents) or will expand upon exertion of an outwardly directed radial force on an inner surface of the stent frame by a balloon catheter or the like.  
           [0004]    Stents for endovascular implantation into a blood vessel or the like, to maintain or restore the patency of the passageway, have been deployed percutaneously to minimize the invasiveness associated with surgical exposure of the treatment site during coronary artery bypass. Percutaneous deployment is initiated by an incision into the vascular system of the patient, typically into the femoral artery. A tubular or sheath portion of an introducer is inserted through the incision and extends into the artery. The introducer has a central lumen which provides a passageway through the patient&#39;s skin and artery wall into the interior of the artery. An outwardly tapered hub portion of the introducer remains outside the patient&#39;s body to prevent blood from leaking out of the artery along the outside of the sheath. The introducer lumen includes a valve to block blood flow out of the artery through the introducer passageway. A distal end of a guide wire is passed through the introducer passageway into the patient&#39;s vasculature. The guide wire is threaded through the vasculature until the inserted distal end extends just beyond the treatment site. The proximal end of the guide wire extends outside the introducer.  
           [0005]    For endovascular deployment, a stent, in an unexpanded or constricted configuration, is crimped onto a deflated balloon portion of a balloon catheter. The balloon portion is normally disposed near a distal end of the balloon catheter. The catheter has a central lumen extending its entire length. The distal end of the balloon catheter is threaded onto the proximal end of the guide wire. The distal end of the catheter is inserted into the introducer lumen and the catheter is pushed along the guide wire until the stent reaches the treatment site. At the treatment site, the balloon is inflated causing the stent to radially expand and assume an expanded configuration. When the stent is used to reinforce a portion of the blood vessel wall, the stent is expanded such that its outer diameter is approximately ten percent to twenty percent larger than the inner diameter of the blood vessel at the treatment site, effectively causing an interference fit between the stent and the blood vessel that inhibits migration of the stent. The balloon is deflated and the balloon catheter is withdrawn from the patient&#39;s body. The guide wire is similarly removed. Finally, the introducer is removed from the artery.  
           [0006]    An example of a commonly used stent is given in U.S. Pat. No. 4,733,665 filed by Palmaz on Nov. 7, 1985. Such stents are often referred to as balloon expandable stents. Typically the stent is made from a solid tube of stainless steel. Thereafter, a series of cuts are made in the wall of the stent. The stent has a first smaller diameter which permits the stent to be delivered through the human vasculature by being crimped onto a balloon catheter. The stent also has a second or expanded diameter. The expanded diameter is achieved through the application, by the balloon catheter positioned in the interior of the tubular shaped member, of a radially outwardly directed force.  
           [0007]    However, such “balloon expandable” stents are often impractical for use in some vessels such as superficial arteries, like the carotid artery. The carotid artery is easily accessible from the exterior of the human body. A patient having a balloon expandable stent made from stainless steel or the like, placed in their carotid artery might be susceptible to sever injury through day to day activity. A sufficient force placed on the patients neck, such as by falling, could cause the stent to collapse, resulting in injury to the patient. In order to prevent this, self-expanding stents have been proposed for use in such vessels. Self-expanding stents act similarly to springs and will recover to their expanded or implanted configuration after being crushed.  
           [0008]    One type of self-expanding stent is disclosed in U.S. Pat. No. 4,665,771. The disclosed stent has a radially and axially flexible, elastic tubular body with a predetermined diameter that is variable under axial movement of ends of the body relative to each other and which is composed of a plurality of individually rigid but flexible and elastic thread elements defining a radially self-expanding helix. This type of stent is known in the art as a “braided stent” and is so designated herein. Placement of such stents in a body vessel can be achieved by a device which comprises an outer catheter for holding the stent at its distal end, and an inner piston which pushes the stent forward once it is in position.  
           [0009]    Other types of self-expanding stents use alloys such as Nitinol (Ni—Ti alloy) which have shape memory and/or superelastic characteristics in medical devices which are designed to be inserted into a patient&#39;s body. The shape memory characteristics allow the devices to be deformed to facilitate their insertion into a body lumen or cavity and then be heated within the body so that the device returns to its original shape. Superelastic characteristics on the other hand generally allow the metal to be deformed and restrained in the deformed condition to facilitate the insertion of the medical device containing the metal into a patient&#39;s body, with such deformation causing the phase transformation. Once within the body lumen the restraint on the superelastic member can be removed, thereby reducing the stress therein so that the superelastic member can return to its original un-deformed shape by the transformation back to the original phase.  
           [0010]    Alloys having shape memory/superelastic characteristics generally have at least two phases. These phases are a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austenite phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensite phase.  
           [0011]    When stress is applied to a specimen of a metal such as Nitinol exhibiting superelastic characteristics at a temperature above which the austenite is stable (i.e. the temperature at which the transformation of martensite phase to the austenite phase is complete), the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in strain but with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Thereafter, further increase in stress is necessary to cause further deformation. The martensitic metal first deforms elastically upon the application of additional stress and then plastically with permanent residual deformation.  
           [0012]    If the load on the specimen is removed before any permanent deformation has occurred, the martensitic specimen will elastically recover and transform back to the austenite phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensite phase transforms back into the austenite phase, the stress level in the specimen will remain essentially constant (but substantially less than the constant stress level at which the austenite transforms to the martensite) until the transformation back to the austenite phase is complete, i.e. there is significant recovery in strain with only negligible corresponding stress reduction. After the transformation back to austenite is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load and to recover from the deformation upon the removal of the load is commonly referred to as superelasticity or pseudoelasticity. It is this property of the material which makes it useful in manufacturing tube cut self-expanding stents. The prior art makes reference to the use of metal alloys having superelastic characteristics in medical devices which are intended to be inserted or otherwise used within a patient&#39;s body. See for example, U.S. Pat. No. 4,665,905 to Jervis and U.S. Pat. No. 4,925,445 to Sakamoto et al.  
           [0013]    Designing delivery systems for delivering self-expanding stents has proven difficult. One example of a prior art self-expanding stent delivery system is shown in U.S. Pat. No. 4,580,568 to Gianturco. This patent discloses a delivery apparatus which uses a hollow sheath, like a catheter. The sheath is inserted into a body vessel and navigated therethrough so that its distal end is adjacent the target site. The stent is then compressed to a smaller diameter and loaded into the sheath at the sheath&#39;s proximal end. A cylindrical flat end pusher, having a diameter almost equal to the inside diameter of the sheath is inserted into the sheath behind the stent. The pusher is then used to push the stent from the proximal end of the sheath to the distal end of the sheath. Once the stent is at the distal end of the sheath, the sheath is pulled back, while the pusher remain stationary, thereby exposing the stent and expanding it within the vessel.  
           [0014]    However, delivering the stent through the entire length of the catheter may cause many problems, including possible damage to a vessel or the stent during its travel. In addition, it is often difficult to design a pusher having enough flexibility to navigate through the catheter, but also enough stiffness to push the stent out of the catheter. Therefore, it was determined that pre-loading the stent into the distal and of the catheter, and then delivering the catheter through the vessel to the target site may be a better approach. In order to ensure proper placement of the stent within catheter, it is often preferred that the stent be pre-loaded at the manufacturing site. Except this in itself has posed some problems. Because the catheter exerts a significant force on the self-expanding stent which keeps it from expanding, the stent may tend to become imbedded within the wall of the catheter. When this happens, the catheter has difficulty sliding over the stent during delivery. This situation can result in the stent becoming stuck inside the catheter, or could damage the stent during delivery.  
           [0015]    Another example of a prior art self-expanding stent delivery system is given in U.S. Pat. No. 4,732,152 to Wallsten et al. This patent discloses a probe or catheter having a self-expanding stent pre-loaded into its distal end. The stent is first placed within a flexible hose and compressed before it is loaded into the catheter. When the stent is at the delivery site the catheter and hose are withdrawn over the stent so that it can expand within the vessel. However, withdrawing the flexible hose over the stent during expansion could also cause damage to the stent.  
           [0016]    An example of a more preferred self-expanding stent delivery system can be found in U.S. Pat. No. 6,019,778 to Wilson et al. and issued on Feb. 1, 2000, which is hereby incorporated herein by reference. While using such a device, it is essential for the stent delivery device to be able to navigate through tortuous vessels, lesions and previously deployed devices (stents). The delivery system must follow a guide wire with out overpowering the wire in the tortuous vessels. The guidewire, when entering a new path, needs to be flexible enough to bend such that it is angled with respect to the delivery device proximal thereto. Because the guidewire extends through the distal end of the delivery device, if the distal end of the delivery device is stiff, it will not bend with the guidewire and may prolapse the wire causing the guidewire to move its position to align itself with the distal end of the delivery device. This could cause difficulty in navigating the delivery system, and may also cause any debris dislodged during the procedure to flow upstream and cause a stroke.  
           [0017]    Therefore, there has been a need for a self-expanding stent delivery system which better navigates tortuous passageways, and more easily and accurately deploys the stent within the target area. The present invention provides such a delivery device.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention overcomes the disadvantages associated with self-expanding stent deployment as briefly described above.  
           [0019]    In accordance with one aspect, the present invention is directed to a delivery apparatus for a self-expanding stent. The delivery apparatus comprises a substantially tubular shaft having a proximal end, a distal end, a guidewire lumen extending between the proximal and distal ends, and a stent bed proximate the distal end upon which the self-expanding stent is positioned. The delivery apparatus further comprises a substantially tubular sheath defining an interior volume. The sheath has a proximal end, a distal end, and an enlarged section proximate the distal end. The sheath being coaxially positioned over the shaft such that the enlarged section is aligned with the stent bed. The sheath being formed from an inner polymeric layer, an outer polymeric layer, and a flat wire reinforcement layer.  
           [0020]    In accordance with another aspect, the present invention is directed to a delivery apparatus for a self-expanding stent. The delivery apparatus comprises a shaft having a proximal end, a distal end, a guidewire lumen extending between the proximal and distal ends, and a stent bed proximate the distal end upon which the stent is mounted. The delivery apparatus also comprises a sheath defining an interior volume. The sheath having a proximal end, a distal end, and an enlarged section proximate the distal end. The sheath being coaxially positioned over the shaft such that the enlarged section is aligned with the stent bed. The sheath is formed from an inner polymeric layer, a lubricious coating on the inner polymeric layer, an outer polymeric layer, and a flat wire reinforcement layer.  
           [0021]    The delivery apparatus for a self-expanding stent of the present invention utilizes a sheath constructed in a manner that allows flexibility in navigating through tortuous vessels, provides pushability for navigating through tight passageways, and substantially prevents the stent from becoming embedded in the device. The apparatus utilizes a sheath constructed from two polymeric layers and a reinforcement layer sandwiched therebetween. The reinforcement layer is formed from flat metallic wire to ensure adequate strength with reduced profile. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]    The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.  
         [0023]    [0023]FIG. 1 is a simplified elevational view of a stent delivery apparatus made in accordance with the present invention.  
         [0024]    [0024]FIG. 2 is a view similar to that of FIG. 1 but showing an enlarged view of the distal end of the apparatus having a section cut away to show the stent loaded therein.  
         [0025]    [0025]FIG. 3 is a simplified elevational view of the distal end of the inner shaft made in accordance with the present invention.  
         [0026]    [0026]FIG. 4 is a cross-sectional view of FIG. 3 taken along lines  4 - 4 .  
         [0027]    [0027]FIGS. 5 through 9 are partial cross-sectional views of the apparatus of the present invention sequentially showing the deployment of the self-expanding stent within the vasculature.  
         [0028]    [0028]FIG. 10 is a simplified elevational view of a shaft for a stent delivery apparatus made in accordance with the present invention.  
         [0029]    [0029]FIG. 11 is a partial cross-sectional view of the shaft and sheath of the stent delivery apparatus in accordance with the present invention.  
         [0030]    [0030]FIG. 12 is a partial cross-sectional view of the shaft and modified sheath of the stent delivery system in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    [0031]FIGS. 1 and 2 illustrate a self-expanding stent delivery apparatus  10  made in accordance with the present invention. Apparatus  10  comprises inner and outer coaxial tubes. The inner tube is called the shaft  12  and the outer tube is called the sheath  14 . A self-expanding stent  100  is located within the sheath  14 , wherein the stent  100  makes frictional contact with the sheath  14  and the shaft  12  is disposed coaxially within a lumen of the stent  100 .  
         [0032]    Shaft  12  has proximal and distal ends  16  and  18  respectively. The proximal end  16  of the shaft  12  has a Luer guidewire hub  20  attached thereto. As seen best from FIG. 10, the proximal end  16  of the shaft  12  is preferably a ground stainless steel hypotube. In one exemplary embodiment, the hypotube is stainless steel and has a 0.042 inch outside diameter at its proximal end and then tapers to a 0.036 inch outside diameter at its distal end. The inside diameter of the hypotube is 0.032 inch throughout its length. The tapered outside diameter is to gradually change the stiffness of the hypotube along its length. This change in the hypotube stiffness allows for a more rigid proximal end or handle end that is needed during stent deployment. If the proximal end is not stiff enough, the hypotube section extending beyond the Tuohy Borst valve described below could buckle as the deployment forces are transmitted. The distal end of the hypotube is more flexible allowing for better track-ability in tortuous vessels. The distal end of the hypotube also needs to be flexible to minimize the transition between the hypotube and the coil section described below.  
         [0033]    As will be described in greater detail below, shaft  12  has a body portion  22 , wherein at least a section thereof is made from a flexible coiled member  24 , looking very much like a compressed or closed coil spring. Shaft  12  also includes a distal portion  26 , distal to body portion  22 , which is preferably made from a coextrusion of high-density polyethylene and nylon. The two portions  22  and  26  are joined together by any number of means known to those of ordinary skill in the art including heat fusing, adhesive bonding, chemical bonding or mechanical attachment.  
         [0034]    As best seen from FIG. 3, the distal portion  26  of the shaft  12  has a distal tip  28  attached thereto. Distal tip  28  may be made from any number of suitable materials known in the art including polyamide, polyurethane, polytetrafluoroethylene, and polyethylene including multi-layer or single layer construction. The distal tip  28  has a proximal end  30  whose diameter is substantially the same as the outer diameter of the sheath  14  which is immediately adjacent thereto. The distal tip  28  tapers to a smaller diameter from its proximal end  30  to its distal end  32 , wherein the distal end  32  of the distal tip  28  has a diameter smaller than the inner diameter of the sheath  14 .  
         [0035]    The stent delivery apparatus  10  glides over a guide wire  200  (shown in FIG. 1) during navigation to the stent deployment site. As used herein, guidewire can also refer to similar guiding devices which have a distal protection apparatus incorporated herein. One preferred distal protection device is disclosed in published PCT Application 98/33443, having an international filing date of Feb. 3, 1998. As discussed above, if the distal tip  28  is too stiff it will overpower the guide wire path and push the guide wire  200  against the lumen wall and in some very tortuous settings the stent delivery apparatus  10  could prolapse the wire. Overpowering of the wire and pushing of the apparatus against the lumen wall can prevent the device from reaching the target area because the guide wire will no longer be directing the device. Also as the apparatus is advanced and pushed against the lumen wall debris from the lesion can be dislodged and travel upstream causing complications to distal vessel lumens. The distal tip  28  is designed with an extremely flexible leading edge and a gradual transition to a less flexible portion. The distal tip  28  may be hollow and may be made of any number of suitable materials, including 40D nylon. Its flexibility may be changed by gradually increasing the thickness of its cross-sectional diameter, whereby the diameter is thinnest at its distal end, and is thickest at its proximal end. That is, the cross-sectional diameter and wall thickness of the distal tip  28  increases as you move in the proximal direction. This gives the distal end  32  of the distal tip  28  the ability to be directed by the guidewire prior to the larger diameter and thicker wall thickness (less flexible portion) of the distal tip  28  over-powering the guidewire. Over-powering the wire, as stated above, is when the apparatus (due to its stiffness) dictates the direction of the device instead of following the wire.  
         [0036]    The guidewire lumen  34  has a diameter that is matched to hug the recommended size guide wire so that there is a slight frictional engagement between the guidewire  200  and the guidewire lumen  34  of distal tip  28 . The distal tip  28  then has a rounded section  36  between its distal portion  32  and its proximal portion  30 . This helps prevent the sheath  14  from slipping distally over the distal tip  28 , and thereby exposing the squared edges of the sheath  14  to the vessel, which could cause damage thereto. This improves the device&#39;s “pushability.” As the distal tip  28  encounters resistance it does not allow the sheath  14  to ride over it thereby exposing the sheath&#39;s  14  square cut edge. Instead the sheath  14  contacts the rounded section  36  of the distal tip  28  and thus transmits the forces applied to the distal tip  28 . The distal tip  28  also has a proximally tapered section  38  which helps guide the distal tip  28  through the deployed stent  100  without providing a sharp edge that could grab or hang up on a stent strut end or other irregularity in the lumen inner diameter.  
         [0037]    Attached to distal portion  26  of shaft  12  is a stop  40 , which is proximal to the distal tip  28  and stent  100 . Stop  40  may be made from any number of suitable materials known in the art, including stainless steel, and is even more preferably made from a highly radio-opaque material such as platinum, gold tantalum, or radio-opaque filled polymer. The stop  40  may be attached to shaft  12  by any suitable means, including mechanical or adhesive bonding, or by any other means known to those skilled in the art. Preferably, the diameter of stop  40  is large enough to make sufficient contact with the loaded stent  100  without making frictional contact with the sheath  14 . As will be explained subsequently, stop  40  helps to “push” the stent  100  or maintain its relative position during deployment, by preventing the stent  100  from migrating proximally within the sheath  14  during retraction of the sheath  14  for stent deployment. The radio-opaque stop  40  also aides in positioning the stent  100  within the target lesion during deployment within a vessel, as is described below.  
         [0038]    A stent bed  42  is defined as being that portion of the shaft  12  between the distal tip  28  and the stop  40  (FIG. 2). The stent bed  42  and the stent  100  are coaxial so that the distal portion  26  of the shaft  12  comprising the stent bed  42  is located within the lumen of stent  100 . The stent bed  42  makes minimal contact with stent  100  because of the space which exists between the shaft  12  and the sheath  14 . As the stent  100  is subjected to temperatures at the austenite phase transformation it attempts to recover to its programmed shape by moving outwardly in a radial direction within the sheath  14 . The sheath  14  constrains the stent  100  as will be explained in detail subsequently. Distal to the distal end of the loaded stent  100  attached to the shaft  12  is a radio-opaque marker  44  which may be made of platinum, iridium coated platinum, gold tantalum, stainless steel, radio-opaque filled polymer or any other suitable material known in the art.  
         [0039]    As seen from FIGS. 2, 3 and  10 , the body portion  22  of shaft  12  is made from a flexible coiled member  24 , similar to a closed coil or compressed spring. During deployment of the stent  100 , the transmission of compressive forces from the stop  40  to the Luer guidewire hub  20  is an important factor in deployment accuracy. A more compressive shaft  12  results in a less accurate deployment because the compression of the shaft  12  is not taken into account when visualizing the stent  100  under fluoroscopic imaging. However, a less compressive shaft  12  usually means less flexibility, which would reduce the ability of the apparatus  10  to navigate through tortuous vessels. A coiled assembly allows both flexibility and resistance to compression. When the apparatus  10  is navigating through the arteries, the shaft  12  is not in compression and therefore the coiled member  24  is free to bend with the delivery path. As one deploys the stent  100 , tension is applied to the sheath  14  as the sheath  14  is retracted over the encapsulated stent  100 . Because the stent  100  is self-expanding it is in contact with the sheath  14  and the forces are transferred along the stent  100  and to the stop  40  of the shaft  12 . This results in the shaft  12  being under compressive forces. When this happens, the flexible coiled member  24  (no gaps between the coil members) transfers the compressive force from one coil to the next.  
         [0040]    The flexible coiled member  24  further includes a covering  46  that fits over the flexible coiled member  24  to help resist buckling of the coiled member  24  in both bending and compressive modes. The covering  46  is an extruded polymer tube and is preferably a soft material that can elongate slightly to accommodate bending of the flexible coiled member  24 , but does not allow the coils to ride over each other. Covering  46  may be made from any number of suitable materials including coextrusions of Nylon® and high-density polyethylene, polyurethane, polyamide, polytetrafluoroethylene, etc. The extrusion is also attached to the stop  40 . Flexible coiled member  24  may be made of any number of materials known in the art including stainless steel, Nitinol, rigid polymers. In one exemplary embodiment, flexible coiled member  24  is made from a 0.003 inch thick by 0.010 inch wide stainless steel ribbon wire. The wire may be round, or more preferably flat to reduce the profile of the flexible coiled member  24 .  
         [0041]    Sheath  14  is preferably a polymeric catheter and has a proximal end  48  terminating at a sheath hub  50  (FIG. 1). Sheath  14  also has a distal end  52  which terminates at the proximal end  30  of distal tip  28  of the shaft  12 , when the stent  100  is in an un-deployed position as shown in FIG. 2. The distal end  52  of sheath  14  includes a radio-opaque marker band  54  disposed along its outer surface (FIG. 1). As will be explained below, the stent  100  is fully deployed when the marker band  54  is proximal to radio-opaque stop  40 , thus indicating to the physician that it is now safe to remove the delivery apparatus  10  from the body.  
         [0042]    As detailed in FIG. 2, the distal end  52  of sheath  14  includes an enlarged section  56 . Enlarged section  56  has larger inside and outside diameters than the inside and outside diameters of the sheath  14  proximal to enlarged section  56 . Enlarged section  56  houses the pre-loaded stent  100 , the stop  40  and the stent bed  42 . The outer sheath  14  tapers proximally at the proximal end of enlarged section  56  to a smaller size diameter. This design is more fully set forth in co-pending U.S. application Ser. No. 09/243,750 filed on Feb. 3, 1999, which is hereby incorporated herein by reference. One particular advantage to the reduction in the size of the outer diameter of sheath  14  proximal to enlarged section  56  is in an increase in the clearance between the delivery apparatus  10  and the guiding catheter or sheath that the delivery apparatus  10  is placed through. Using fluoroscopy, the physician will view an image of the target site within the vessel, before and after deployment of the stent, by injecting a radio-opaque solution through the guiding catheter or sheath with the delivery apparatus  10  placed within the guiding catheter. Because the clearance between the sheath  14 , and the guiding catheter is increased by tapering or reducing the outer diameter of the sheath  14  proximal to enlarged section  56 , higher injection rates may be achieved, resulting in better images of the target site for the physician. The tapering of sheath  14  provides higher injection rates of radio-opaque fluid, both before and after deployment of the stent.  
         [0043]    A problem encountered with earlier self-expanding stent delivery systems is that of the stent becoming embedded within the sheath in which it is disposed. Referring to FIG. 11, there is illustrated a sheath construction which may be effectively utilized to substantially prevent the stent from becoming embedded in the sheath as well as provide other benefits as described in detail below. As illustrated, the sheath  14  comprises a composite structure of at least two layers and preferably three layers. The outer layer  60  may be formed from any suitable biocompatible material. Preferably, the outer layer  60  is formed from a lubricious material for ease of insertion and removal of the sheath  14 . In a preferred embodiment, the outer layer  60  comprises a polymeric material such as Nylon®. The inner layer  62  may also be formed from any suitable biocompatible material. For example, the inner layer  62  may be formed from any number of polymers including polyethylene, polyamide or polytetrafluroethylene. In a preferred embodiment, the inner layer  62  comprises polytetrafluroethylene. Polytetrafluroethylene is also a lubricious material which makes stent delivery easier, thereby preventing damage to the stent  100 . The inner layer  62  may also be coated with another material to increase the lubricity thereof for facilitating stent deployment. Any number of suitable biocompatible materials may be utilized. In an exemplary embodiment, silicone based coatings may be utilized. Essentially, a solution of the silicone based coating may be injected through the apparatus and allowed to cure at room temperature. The amount of silicone based coating utilized should be minimized to prevent transference of the coating to the stent  100 . Sandwiched between the outer and inner layers  60  and  62 , respectively, is a wire reinforcement layer  64 . The wire reinforcement layer  64  may take on any number of configurations. In the exemplary embodiment, the wire reinforcement layer  64  comprises a simple under and over weave or braiding pattern. The wire used to form the wire reinforcement layer  64  may comprise any suitable material and any suitable cross-sectional shape. In the illustrated exemplary embodiment, the wire forming the wire reinforcement layer  64  comprises stainless steel and has a substantially circular cross-section. In order to function for its intended purpose, as described in detail below, the wire has a diameter of 0.002 inches.  
         [0044]    The three layers  60 ,  62 , and  64  comprising the sheath  14  collectively enhance stent deployment. The outer layer  60  facilitates insertion and removal of the entire apparatus  10 . The inner layer  62  and the wire reinforcement layer  64  function to prevent the stent  100  from becoming embedded in the sheath  14 . Self-expanding stents such as the stent  100  of the present invention tend to expand to their programmed diameter at a given temperature. As the stent attempts to undergo expansion, it exerts radially outward directed forces and may become embedded in the sheath  14  restraining it from expanding. Accordingly, the wire reinforcing layer  64  provides radial or hoop strength to the inner layer  62  thereby creating sufficient resistance to the outwardly directed radial force of the stent  100  within the sheath  14 . The inner layer  62 , also as discussed above, provides a lower coefficient of friction surface to reduce the forces required to deploy the stent  100  (typically in the range from about five to eight pounds). The wire reinforcement layer  64  also provides tensile strength to the sheath  14 . In other words, the wire reinforcement layer  64  provides the sheath  14  with better pushability, i.e., the ability to transmit a force applied by the physician at a proximal location on the sheath  14  to the distal tip  28 , which aids in navigation across tight stenotic lesions within the vasculature. Wire reinforcement layer  64  also provides the sheath  14  with better resistance to elongation and necking as a result of tensile loading during sheath retraction for stent deployment.  
         [0045]    The sheath  14  may comprise all three layers along its entire length or only in certain sections, for example, along the length of the stent  100 . In a preferred embodiment, the sheath  14  comprises all three layers along its entire length.  
         [0046]    Prior art self-expanding stent delivery systems did not utilize wire reinforcement layers. Because the size of typical self-expanding stents is large, as compared to balloon expandable coronary stents, the diameter or profile of the delivery devices therefor had to be large as well. However, it is always advantageous to have delivery systems which are as small as possible. This is desirable so that the devices can reach into smaller vessels and so that less trauma is caused to the patient. However, as stated above, the advantages of a thin reinforcing layer in a stent delivery apparatus outweighs the disadvantages of slightly increased profile.  
         [0047]    In order to minimize the impact of the wire reinforcement layer on the profile of the apparatus  10 , the configuration of the wire reinforcement layer  64  may be modified. For example, this may be accomplished in a number of ways, including changing the pitch of the braid, changing the shape of the wire, changing the wire diameter and/or changing the number of wires utilized. In a preferred embodiment, the wire utilized to form the wire reinforcement layer comprises a substantially rectangular cross-section as illustrated in FIG. 12. In utilizing a substantially rectangular cross-section wire, the strength features of the reinforcement layer  64  may be maintained with a significant reduction in the profile of the delivery apparatus. In this preferred embodiment, the rectangular cross-section wire has a width of 0.003 inches and a height of 0.001 inches. Accordingly, braiding the wire in a similar manner to FIG. 11, results in a fifty percent decrease in the thickness of the wire reinforcement layer  64  while maintaining the same beneficial characteristics as the 0.002 round wire. The flat VVire may comprise any suitable material, and preferably comprises stainless steel.  
         [0048]    [0048]FIGS. 1 and 2 show the stent  100  as being in its fully un-deployed position. This is the position the stent is in when the apparatus  10  is inserted into the vasculature and its distal end is navigated to a target site. Stent  100  is disposed around the stent bed  42  and at the distal end  52  of sheath  14 . The distal tip  28  of the shaft  12  is distal to the distal end  52  of the sheath  14 . The stent  100  is in a compressed state and makes frictional contact with the inner surface of the sheath  14 .  
         [0049]    When being inserted into a patient, sheath  14  and shaft  12  are locked together at their proximal ends by a Tuohy Borst valve  58 . This prevents any sliding movement between the shaft  12  and sheath  14 , which could result in a premature deployment or partial deployment of the stent  100 . When the stent  100  reaches its target site and is ready for deployment, the Tuohy Borst valve  58  is opened so that the sheath  14  and shaft  12  are no longer locked together.  
         [0050]    The method under which delivery apparatus  10  deploys stent  100  may best be described by referring to FIGS.  5 - 9 . In FIG. 5, the delivery apparatus  10  has been inserted into a vessel  300  so that the stent bed  42  is at a target diseased site. Once the physician determines that the radio-opaque marker band  54  and stop  40  on shaft  12  indicating the ends of stent  100  are sufficiently placed about the target disease site, the physician would open Tuohy Borst valve  58 . The physician would then grasp the Luer guidewire hub  20  of shaft  12  so as to hold shaft  12  in a fixed position. Thereafter, the physician would grasp the Tuohy Borst valve  58 , attached proximally to sheath  14 , and slide it proximal, relative to the shaft  12  as shown in FIGS. 6 and 7. Stop  40  prevents the stent  100  from sliding back with sheath  14 , so that as the sheath  14  is moved back, the stent  100  is effectively “pushed” out of the distal end  52  of the sheath  14 , or held in position relative to the target site. Stent  100  should be deployed in a distal to proximal direction to minimize the potential for creating emboli with the diseased vessel  300 . Stent deployment is complete when the radio-opaque band  54  on the sheath  14  is proximal to radio-opaque stop  40 , as shown in FIG. 8. The apparatus  10  can now be withdrawn through stent  100  and removed from the patient.  
         [0051]    [0051]FIGS. 2 and 9 show a preferred embodiment of a stent  100 , which may be used in conjunction with the present invention. Stent  100  is shown in its unexpanded compressed state, before it is deployed, in FIG. 2. Stent  100  is preferably made from a superelastic alloy such as Nitinol. Most preferably, the stent  100  is made from an alloy comprising from about 50.5 percent (as used herein these percentages refer to atomic percentages) Ni to about 60 percent Ni, and most preferably about 55 percent Ni, with the remainder of the alloy Ti. Preferably, the stent  100  is such that it is superelastic at body temperature, and preferably has an Af in the range from about twenty-one degrees C to about thirty-seven degrees C. The superelastic design of the stent makes it crush recoverable which, as discussed above, can be used as a stent or frame for any number of vascular devices for different applications.  
         [0052]    Stent  100  is a tubular member having front and back open ends a longitudinal axis extending there between. The tubular member has a first smaller diameter, FIG. 2, for insertion into a patient and navigation through the vessels, and a second larger diameter for deployment into the target area of a vessel. The tubular member is made from a plurality of adjacent hoops  102  extending between the front and back ends. The hoops  102  include a plurality of longitudinal struts  104  and a plurality of loops  106  connecting adjacent struts, wherein adjacent struts are connected at opposite ends so as to form a substantially S or Z shape pattern. Stent  100  further includes a plurality of curved bridges  108 , which connect adjacent hoops  102 . Bridges  108  connect adjacent struts together at bridge to loop connection points which are offset from the center of a loop.  
         [0053]    The above described geometry helps to better distribute strain throughout the stent, prevents metal to metal contact when the stent is bent, and minimizes the opening size between the features, struts, loops and bridges. The number of and nature of the design of the struts, loops and bridges are important factors when determining the working properties and fatigue life properties of the stent. Preferably, each hoop has between twenty-four to thirty-six or more struts. Preferably the stent has a ratio of number of struts per hoop to strut length (in inches) which is greater than two hundred. The length of a strut is measured in its compressed state parallel to the longitudinal axis of the stent.  
         [0054]    In trying to minimize the maximum strain experienced by features, the stent utilizes structural geometries which distribute strain to areas of the stent which are less susceptible to failure than others. For example, one vulnerable area of the stent is the inside radius of the connecting loops. The connecting loops undergo the most deformation of all the stent features. The inside radius of the loop would normally be the area with the highest level of strain on the stent. This area is also critical in that it is usually the smallest radius on the stent. Stress concentrations are generally controlled or minimized by maintaining the largest radii possible. Similarly, we want to minimize local strain concentrations on the bridge and bridge to loop connection points. One way to accomplish this is to utilize the largest possible radii while maintaining feature widths which are consistent with applied forces. Another consideration is to minimize the maximum open area of the stent. Efficient utilization of the original tube from which the stent is cut increases stent strength and it&#39;s ability to trap embolic material.  
         [0055]    Although shown and described is what is believed to be the most practical and preferred embodiments, it is apparent that departures from specific designs and methods described and shown will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention. The present invention is not restricted to the particular constructions described and illustrated, but should be constructed to cohere with all modifications that may fall within the scope of the appended claims.