FIELD OF THE INVENTION 
     The present invention relates to intravascular stent implants for maintaining vascular patency in humans and animals and more particularly to a stent in the form of a braided stent. 
     BACKGROUND OF THE INVENTION 
     Percutaneous transluminal coronary angioplasty (PTCA) is used to increase the lumen diameter of a coronary artery partially or totally obstructed by a build-up of cholesterol fats or atherosclerotic plaque. Typically a first guidewire of about 0.038 inches in diameter is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the first guidewire to a point just proximal of the stenosis. The first guidewire is then removed. A balloon catheter on a smaller 0.014 inch diameter second guidewire is advanced within the guiding catheter to a point just proximal of the stenosis. The second guidewire is advanced into the stenosis, followed by the balloon on the distal end of the catheter. The balloon is inflated causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten reclosure of the dilated vessel or even perforations in the vessel wall. Implantation of a metal stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. It has also been shown that the use of intravascular stents can measurably decrease the incidence of restenosis after angioplasty thereby reducing the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary. 
     An implanted prosthesis such as a stent can preclude additional procedures and maintain vascular patency by mechanically supporting dilated vessels to prevent vessel reclosure. Stents can also be used to repair aneurysms, to support artificial vessels as liners of vessels or to repair dissections. Stents are suited to the treatment of any body lumen, including the vas deferens, ducts of the gallbladder, prostate gland, trachea, bronchus and liver. The body lumens range in diameter from small coronary vessels of 3 mm or less to 28 mm in the aortic vessel. The invention applies to acute and chronic closure or reclosure of body lumens. 
     A typical stent is a cylindrically shaped wire formed device intended to act as a permanent prosthesis. A typical stent ranges from 5 mm to 50 mm in length. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be radially self-expanding or expandable by the use of an expansion device. The self expanding stent is made from a resilient springy material while the device expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during a single angioplasty procedure by using a catheter bearing a stent which has been secured to the catheter such as in U.S. Pat. No. 5,372,600 to Beyar et al. which is incorporated herein by reference in its entirety. 
     The stent must be reduced in size to facilitate its delivery to the intended implantation site. A coil stent is delivered by winding it into a smaller diameter and fixing it onto a delivery catheter. When the device is positioned at the desired site, the coil is released from the catheter and it either self-expands by its spring force or it is otherwise mechanically expanded to the specified dimension. 
     As with many stents, the deformation of the stent when it is assembled on the delivery catheter causes a strain in the stent material. If the strain is too large the material will experience plastic deformation to such an extent that the stent will not recover to the intended dimensions following deployment. This is true of superelastic or pseudoplastic alloys such as disclosed in U.S. Pat. No. 5,597,378 issued to Jervis, which is incorporated herein by reference in its entirety. Thus a maximum allowable strain based on material is a limiting parameter in stent design. 
     Two parameters influence the amount of strain a stent will experience during the deformation described above. The first is the degree of deformation applied to the stent and the second is the thickness of the stent material. For a given deformation, the strain experienced by a material is proportional to the thickness of the material. Since it is desirable to deliver a stent on the smallest delivery system possible it follows that the thickness of the stent material should be reduced to keep the strain within acceptable parameters. When forming a stent with a single solid strand (such a length of solid wire), a limit will be reached where the thickness of material becomes so small that the stent will meet the maximum allowable strain but will no longer have the hoop strength to provide adequate scaffolding. 
     Current helical coil stents are delivered on the smallest profile catheter that the stent will allow. Strain on the stent during assembly on the catheter is the limiting factor with stents made from solid round or flat wire helical coil stents. 
     U.S. Pat. No. 5,342,348 to Kaplan for “Method and Device for Treating and Enlarging Body Lumens” discloses a single helically wound strand and two counterwound delivery matrix filaments. A two stranded stent is shown in U.S. Pat. No. 5,618,298 to Simon for “Vascular Prosthesis Made of Reasorbable Material”. 
     Mesh stents are disclosed in U.S. Pat. No. 5,061,275 to Wallsten et al. for “Self-Expanding Prosthesis”, U.S. Pat. No. 5,064,435 to Porter for “Self-Expanding Prosthesis Having Stable Axial Length”, U.S. Pat. No. 5,449,372 to Schmaltz et al. for “Temporary Stent and Methods for Use and Manufacture”, U.S. Pat. No. 5,591,222 to Susawa et al. for “Method of Manufacturing a Device to Dilate Ducts in Vivo”, U.S. Pat. No. 5,645,559 to Hachtmann et al. for “Multiple Layer Stent”, U.S. Pat. No. 5,718,169 to Thompson for “Process for Manufacturing Three-Dimensional Braided Covered Stent”. 
     Woven mesh stents typically have warp and weft members as disclosed in U.S. Pat. No. 4,517,687 to Liebig et al. for “Synthetic Woven Double-Velour Graft”, U.S. Pat. No. 4,530,113 to Matterson for “Vascular Grafts with Cross-Weave Patterns”, U.S. Pat. No. 5,057,092 to Webster for “Braided Catheter with Low Modulus Warp” and EP 122,744 to Silvestrini for “Triaxially-braided Fabric Prosthesis”. The warp strands are typically the strands in the longitudinal direction on a prosthesis. The weft strands are typically the strands which are shuttled through warp strands to form a two dimensional array. 
     WO 95/29646 to Sandock for a “Medical Prosthetic Stent and Method of Manufacture” discloses a geometric pattern of cells defined by a series of elongate strands extending to regions of intersection and interlocking joints at regions of intersections formed by a portion of at least one strand being helically wrapped about a portion of another. 
     Various helical stents are known in the art. U.S. Pat. No. 4,649,922 to Wiktor for “Catheter Arrangement Having A Variable Diameter Tip and Spring Prosthesis” discloses a linearly expandable spring-like stent. U.S. Pat. No. 4,886,062 to Wiktor for “Intravascular Radially Expandable Stent and Method of Implant” discloses a two-dimensional zig-zag form, typically a sinusoidal form. U.S. Pat. No. 4,969,458 to Wiktor for “Intracoronary Stent and Method of Simultaneous Angioplasty and Stent Implant” discloses a stent wire coiled into a limited number of turns wound in one direction then reversed and wound in the opposite direction with the same number of turns, then reversed again and so on until a desired length is obtained. 
     Braiding is a well known craft. See Braidmaking by Barbara Pegg, published by A &amp; C Black Ltd, 35 Bedford Row, London WC1R 4JH, pp. 9-16 which is hereby incorporated by reference. 
     It is an object of the invention to produce a stent which has the ability to tolerate greater deformations, yet has a smaller profile to permit the use of a smaller delivery system thereby reducing the amount of trauma experienced by the patient. It is a further object of the invention to produce a stent which would recover to specified dimensions with maximized radial hoop strength and resistance to lateral force. 
     SUMMARY OF THE INVENTION 
     The present invention is accomplished by providing an apparatus for a radially expandable stent for implantation within a body vessel, comprising one or more continuous, discrete, metal strands. At least three strands repeatedly cross over each other to form a bundle. The strands are joined at the proximal and distal end such that the strands are free to adjust their position relative to each other in response to compression forces. One or more bundles are wound together to form an elongate hollow tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a wound down, helical coil stent under strain; 
     FIG. 2 is a strand with an unstrained radius curvature; 
     FIG. 3 is a strand with a strained radius curvature; 
     FIG. 4 is a bundle of strands; 
     FIG. 5 is a cross-section of a four stranded bundle with worst case stacking; 
     FIG. 6 is an helical coil stent; 
     FIG. 7 is a detail of the helical coil stent of FIG. 6 using a four stranded cross-over braid; 
     FIG. 8 is a cross-section of the detail of the bundle of FIG. 7; 
     FIG. 9 is a three stranded braid; 
     FIG. 10 is a four stranded cross-over braid; 
     FIG. 11 is a five stranded braid; 
     FIG. 12 is a six stranded round braid; 
     FIG. 13 is an alternate six stranded flat braid; 
     FIG. 14 is an eight stranded alternating braid; 
     FIG. 15 is an eight stranded braid; 
     FIG. 16 is an eight stranded twisted braid; 
     FIG. 17 is a nine stranded double braid; 
     FIG. 18 is an eleven stranded braid; 
     FIG. 19 is an eleven stranded alternating braid; and 
     FIG. 20 is a twelve stranded cross-over braid. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     During assembly onto the delivery system (catheter), a helical coil stent  10  is deformed into a reduced diameter  30 . This deformation imposes a strain in the stent material. If the strain is too great, the stent  10  will experience plastic deformation to such an extent that the stent will not recover dimensionally to the specified size during deployment. When a stent  10  is reduced to a given catheter diameter  30  the strain experienced by the stent  10  material is proportional to the thickness  35  of the stent material. 
     The present invention applies to any helical coil stent  10  where deformation is limited by the applied strain. The stent  10  is formed of multiple strands  15 . Each strand is continuous and discrete. Multiple strands  15  of material are formed into a bundle  40 , each strand  15  having a fine thickness. The resulting hoop strength of the stent  10  formed of one or more bundles  40  will be the cumulative strength of all of the strands  15  in the bundle(s)  40 . The strain on the other hand, will be limited to that of a single strand  15 . By using multiple fine strands  15  which are formed into a bundle  40 , the required strength of the stent  10  can be maintained, while allowing the increased stent  10  to be deformed (wound down) onto a smaller diameter delivery catheter than would otherwise be possible with a single solid strand  15  stent material. Bundles  40  can be formed by braiding or by other means to enable the strands  15  to slide relative to one another when compressed or released; this is necessary to reduce friction. One or more bundles  40  are then formed into the elongate hollow tubular stent  10 . 
     The increased deformation capacity of multiple strands  15  which are formed into a bundle  40  is possible because strain is proportional to a single strand  15  thickness, not the thickness of the bundle  40  of strands  15 . The width of the braided bundle  40  of strands is significantly greater than that of a round wire. Multiple strands  15  braided together into a bundle  40  provide support to one another, providing resistance to lateral forces as well as to crushing forces. By increasing the number of strands  15  in the braid, the width can be increased resulting in greater lateral strength. The increase in the number of strands  15  also provides increased radial or “hoop” strength. The braided wire coil stent  10  provides a means to deliver a decreased profile stent while still providing accurate deployment thereby resulting in a less traumatic stent  10  delivery. 
     When a smaller delivery catheter is needed and the strain on a strand  15  increases, stent  10  deformation will increase when assembling the stent  10  onto a smaller delivery catheter. With a single strand  15 , such as a length of wire, a limit will be reached where the following parameters can be optimized no further and the strand  15  thickness can no longer realistically be reduced. These parameters include the delivery catheter size, the hoop strength, the lateral strength. 
     The preferred number of strands  15  would be unique from one stent application to another. Any number of three or more strands would be possible. A larger diameter  20  stent  10  would generally require more strands  15  than a smaller diameter  20  stent  10  to provide adequate radial and hoop strength. Depending on the anatomy being targeted, a stent  10  might require more strands  15  to increase the resistance to compression, as in a stent  10  intended for implantation in the popliteal artery. Some stents  10  might require fewer strands  15  to minimize the amount of blood contact with metal. Others, such as a biliary stent would require more strands  15  or a flatter braid pattern to provide total coverage of the orifice being stented to prevent tissue in-growth. 
     The balloon expandable stent  10  can be made of a round wire or of a flat wire using a springy, inert, biocompatible material with high corrosion resistance that can be plastically deformed at low-moderate stress levels. Acceptable materials include tantalum, stainless steel or elgiloy. The preferred embodiment for a self-expanding stent  10  includes superelastic (nickel titanium) NiTi such as Nitinol manufactured by Raychem or Forukawa. Any of the braided patterns could be made from a round wire or from a flat wire. 
     FIGS. 4-5 and FIGS. 7-20 depict braided stents of  3 - 6  strands,  8 - 9  strands, and  11 - 12  strands with alternative  6  (FIGS.  12  and  13 ), alternative  8  (FIGS. 14 and 16) and alternative  11  (FIGS. 18 and 19) stranded embodiments. Those skilled in the art would recognize that these examples are not the only braided patterns that could be used for the bundle of strands stent concept. Potentially any braid pattern could be used, as for example, a seven or a ten stranded braid. Preferably, the braid is a flattened braid which is formed into a stent  10  with a flat side of the braid forming the stent cylinder so as to minimize the delivered profile of the stent and to maximize the luminal diameter of the stent. 
     To braid multiple strands  15 , conventional ribbon braiding equipment can be used. After braiding, the helical coil stent  10  could be formed by affixing the ends of the desired length of strands  15  to each other and wrapping the braided bundle  40  around a conventional mandrel to form the desired diameter  20 . The ends can be affixed with any welding technique such as, plasma welding, laser welding, RF welding or TIG welding. In addition, brazing, soldering or crimping could be employed to affix the stent ends to each other. By heat treating the assembly the helical coil shape can be “memory set” into the braided bundle  40 . 
     The following applies whenever devices are deformed and is not limited to stents  10 . Stents  10  are placed in a strained state (see FIGS. 1 and 3) during the assembly process where the stents  10  are taken from a free unstrained state (see FIGS. 6 and 2) and are wound onto a delivery catheter  105  at a much smaller diameter. As a braided bundle  40  is formed into a helical coil, the strands  15  may shift with respect to each other. Induced strain is higher when strands  15  stack exactly on top of each other as in FIG.  5  and less if the strands are offset as in FIG.  4 . 
     Strain is highest at the inner edge of the stent coil while in the assembled state (see FIG. 1) and can be represented by the following equation: 
     Strain=(d(R 1÷ R 2 −1))÷(2R 1 −d) where: 
     R 1  is the unstrained radius of curvature  25   
     R 2  is the strained radius of curvature  30   
     d is the wire strand  35  thickness (wire diameter depending on whether the strand is round or flat) as opposed to the overall stent  10  diameter. 
     Three stent designs will be mathematically approximated to, for the smallest diameter stent  10  that can be wound down on a delivery catheter without exceeding the 8% strain permitted with Nitinol as the metal. These examples show that the smallest delivery profile achievable is that of a braided multi strand  15  stent  10 . All three stents have a nominal outer diameter of 9 mm (0.354 inches) and it is assumed will provide adequate hoop and lateral strength. The material in each example is Nitinol which has a maximum 8% allowable strain. 
     EXAMPLE I 
     The first example is a helical coil stent  10  formed from a single member round 0.013 inch wire. A 9 mm outer diameter  20  stent  10  requires a round wire with a minimum diameter of 0.013 inches to provide the necessary hoop strength and lateral stiffness. The applied strain is 8%. For this stent  10  design, the unstrained radius of ad curvature  25  is 0.1705 inches and the outer diameter of the strand  15  is 0.013 inches. Solving the equation for R 2 ,=R 1 ÷[[ε÷d] (2R 1 −d)+1]the strained radius of curvature  30  is therefor 0.0565 inches. Solving the equation for D=2R 2 +d, where D is the outer diameter  20  of the helical coil stent  10  and d is the wire strand thickness or diameter  35 , yields a stent outer diameter  20  of 0.126 inches. With the maximum stent  10  outer diameter  20  profile of 0.126 inches, the required introducer size is at least 9.6 French. The delivery of the device would require an introducer sheath or a guide catheter large enough to accommodate the maximum stent  10  outer diameter  20  profile of 0.126 inches or 9.6 French. The stent  10  would therefor pass through a delivery catheter  105  with a  10  French inner diameter of 0.131 inches. 
     EXAMPLE II 
     The second example is a 9 mm outer diameter  20  helical coil stent  10  formed from a single strand  10  of 0.008 inch×0.025 inch flat wire. This size wire is wide enough to provide lateral stability which is lost when the thickness of the wire is reduced to 0.008 inches. Using the same method as for the Example 1 round wire above, the unstrained radius of curvature  25  is 0.173 inches and the outer diameter  20  is 0.008 inches. Solving the equation for R 2 =R 1 ÷[[ε÷d](2R 1 −d)+1], the strained radius of curvature  30  is therefor 0.087 inches. Solving the equation for D=2R 2 +d, where D is the outer diameter  20  of the helical coil stent  10  and d is the wire strand thickness or diameter  35 , yields a stent outer diameter  20  of 0.087 inches. With the maximum stent  10  outer diameter  20  profile of 0.087 inches, the required introducer size is at least 6.6 French. Due to differences in the wire forming process, the flat wire can only withstand a 7% strain. With a 7% applied strain the maximum device profile is 0.095 inches with a required 7.3 French introducer size. The applicant has been unable to achieve acceptable shape memory results with a strain greater than 7% for flat wire stents. The stents did not return to the nominal diameters following deployment as they were undersized, a function of the flattening process during the raw wire manufacture. With an 8% applied strain, the maximum stent device outer diameter  20  profile is 0.067 inches, with at least a 5.1 French introducer size. 
     EXAMPLE III 
     The third example is a helical coil stent  10  formed from multiple braided 0.005 inch strands  15 , as for example five strands  15  seen in FIG. 4 or four strands  15  seen in FIG.  5 . Then, R 1=( 0.354/2)−3r=(0.354/2 — −3(0.0025)=1.1695 inches. R 2 =0.267 the outer diameter, D=2(R 2 =3r)=0.0684 inches. This corresponds to approximately a 5.2 French introducer. 
     Braided bundles  40  can be of any number of strands. FIG. 9 is a three stranded braid. Each strand  15  could be a bundle  40  with one to four or more strands. FIG. 10 is a four stranded cross-over braid. Each strand  15  could be a bundle  40  with one to four or more strands. FIG. 11 is a five stranded braid. FIG. 12 is a six stranded round braid. FIG. 13 is a six stranded flat braid. FIG. 14 is an eight stranded alternating braid. FIG. 15 is an eight stranded braid. FIG. 16 is an eight stranded twisted braid. FIG. 17 is a nine stranded double braid. FIG. 18 is an eleven stranded braid. The eleven stranded FIG. 19 is an eleven stranded alternating braid which is braided in the same pattern as the eight stranded FIG. 14 but using three additional strands. Any number of strands, however, could be used in this alternating pattern. FIG. 20 is a twelve stranded cross-over braid made with four bundles  40  with three strands  15  each and braided in the pattern of FIG.  10 . Any number of strands could be used in the bundle(s). 
     The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, however, that other expedients known to those skilled in the art or disclosed herein, may be employed without departing from the scope of the appended claims. 
     
       
         
               
               
             
           
               
                   
               
               
                 No. 
                 Component 
               
               
                   
               
             
             
               
                 10 
                 Stent 
               
               
                 15 
                 Strand 
               
               
                 20 
                 D - Outer Diameter of Stent 
               
               
                 25 
                 R 1  - Unstrained Radius of Curvature 
               
               
                 30 
                 R 2  - Strained Radius of Curvature 
               
               
                 35 
                 d - Wire Strand Thickness 
               
               
                 40 
                 Bundle 
               
               
                 45 
                 Strand 1 
               
               
                 50 
                 Strand 2 
               
               
                 55 
                 Strand 3 
               
               
                 60 
                 Strand 4 
               
               
                 65 
                 Strand 5 
               
               
                 70 
                 Strand 6 
               
               
                 75 
                 Strand 7 
               
               
                 80 
                 Strand 8 
               
               
                 85 
                 Strand 9 
               
               
                 90 
                 Strand 10 
               
               
                 95 
                 Strand 11 
               
               
                 100  
                 Strand 12 
               
               
                 105  
                 Delivery Catheter 
               
               
                 110  
                 First Bundle 
               
               
                 115  
                 Second Bundle 
               
               
                 120  
                 Third Bundle 
               
               
                 125  
                 Fourth Bundle
A radially expandable stent for implantation within a body vessel, comprising one or more continuous, discrete, metal strands. At least three strands repeatedly cross over each other to form a bundle. The strands are joined at the proximal and distal end such that the strands are free to adjust their position relative to each other in response to compression forces. One or more bundles are wound together to form an elongate hollow tube.