Abstract:
The invention relates to an implantable medical stent having a low profile for delivery through smaller lumens. The stent is constrained during delivery where the helically wrapped points of a diamond cell structure also include at least one crossed joint of intersecting strands to provide a reduced diameter.

Description:
BACKGROUND OF THE INVENTION 
     Implantable medical prostheses, such as stents, are placed within the body to maintain and/or treat a body lumen that has been impaired or occluded, for example, by a tumor. The stent can be formed of strands of material formed into a tube and are usually delivered into the body lumen using a catheter. The catheter carries the stent to the desired site and the stent is released from the catheter and expands to engage the inner surface of the lumen. 
     A self-expanding stent can be made of elastic materials. These are held in a compressed condition during catheter delivery by, for example, a sheath that covers the compressed stent. Upon reaching the desired site, the sheath constraining the stent is pulled proximally, while the stent is held in the desired position such that the stent expands. 
     There are both self-expanding and non-self-expanding stents. The self-expanding type of device is made with a material having an elastic restoring force, whereas a non-self-expanding stent is often made with elastic, plastically deformable material. It is positioned over a mechanical expander, such as a balloon, which can be inflated to force the prosthesis radially outward once the desired site is reached. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment, the invention features an implantable medical stent having a low profile during delivery. The stent is a tubular body with a body wall structure having a geometric pattern of cells defined by a series of elongated strands extending to regions of intersection. An example of a stent having a cell shape in accordance with the invention can be found in U.S. Ser. No. 08/743,395, filed Nov. 4, 1996, the entire contents of which is incorporated herein by reference. This stent cell structure utilized helically wrapped joints to connect the different strands to form a tubular body. 
     A limitation on the use of the helically joined stent involved the minimum constrained diameter of the stent during delivery. Because of the helically wrapped joints abutting one another along a given circumference, the minimum constrained diameter of the stent was 9 French (3 mm). For example, the length of the helically wrapped joint for a strand having a diameter of 0.006 inches in the constrained position is 0.045 inches. For a five cell structure having five helically twisted abutting joints, this results in a constrained circumference of 0.228 inches with a diameter of 0.072 inches. However, there are many applications in which it is necessary to achieve a smaller constrained diameter to provide delivery, for example, through smaller lumens within the vascular system, to reduce trauma during percutaneous delivery, or to provide endoscopic delivery through small diameter channels of endoscopes. 
     To achieve a smaller constrained diameter of 8 French or less, for example, a preferred embodiment of the invention replaces one or more of the helically wrapped joints along any given circumference with a simple crossed joint in which one strand crosses either above or below a second strand. Thus, the strands at a crossed joint can move more freely relative to each other, but this structure reduces the minimum circumference as the length of one or more helically twisted joints has been removed. This can reduce the constrained diameter by 50%. 
     In another preferred embodiment of the invention, the stent can include a first tubular body made from a first group of strands and a second tubular body surrounding the first tubular body and made from a second group of strands. This type of structure can be used to fabricate a low-profile device having sufficient radial expansion force for a self-expanding stent without a substantial change in foreshortening. This embodiment can include, for example, three or four helically wrapped joints along any circumference of the first and second tubular bodies in which the joints of the two bodies are offset in the constrained state. This embodiment also significantly improves the ratio of the displayed diameter to the constrained diameter. 
     The strands of the first group can have a different shape, diameter or material from the strands of the second group such that the inner body has a larger radial restoring force than the outer body and can thereby impart the outward force to the outer body. 
     In one embodiment, the strands of the inner body can be thicker than the strands of the outer body and can be interleaved with the outer body along the entire length of the stent. In another preferred embodiment, the inner and outer bodies can be interlocked at one or both ends. This can permit the use of a cover between the inner and outer bodies along a certain portion of the stent. The use of the cover can enhance epithialization between the wall of the lumen and the outer body, reduce migration of the stent in certain applications and can prevent tumor in-growth. The cover can also provide a supporting matrix for drug delivery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, 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 in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1A is a flat layout view along the longitudinal axis of a stent; 
     FIG. 1B is an enlarged portion of the stent taken at section  1 B— 1 B in FIG. 1A; 
     FIG. 2A is a perspective view of a stent according to the invention; 
     FIG. 2B is a flat layout view of an expanded low profile stent of FIG. 2A; 
     FIG. 3 is an enlarged cross-sectional view of a delivery tube containing a low profile diamond metal stent; 
     FIGS. 4A and 4B illustrate a mandrel for making a stent of FIGS. 2A,  2 B, and  3 ; 
     FIG. 5A is a distal end view of an endoscope; 
     FIG. 5B is a sectional view of the distal end of the endoscope; 
     FIG. 6A is an “over-the-wire” delivery system; 
     FIG. 6B is an enlarged view of the middle section of the “over-the-wire” delivery system; 
     FIG. 7 is a rapid exchange delivery system; 
     FIGS. 8A-8E illustrate the operation of the delivery of the stent; 
     FIG. 9 is a flat layout view of a double layer stent; 
     FIG. 10 is a flat layout view of an alternative embodiment of a double layer stent; 
     FIG. 11 is an enlarged cross sectional view of the double layer stent of FIG. 10 with an interposed cover in an artery; 
     FIG. 12 is a cross sectional view of the double layer stent with the interposed cover taken along line  12 — 12  of FIG. 11; 
     FIG. 13 illustrates a mandrel for making a stent of FIGS. 9 or  10  and  11 ; 
     FIG. 14A is a perspective view of an alternative stent having six strands; and 
     FIG. 14B is a flat layout view of the stent of FIG.  14 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings in detail, where like numerals indicate like elements, there is illustrated a stent in accordance with the present invention designated generally as  10 . 
     Medical prostheses, such as a stent  10  according to the invention, are placed within the body to treat a body lumen that has been occluded. Stents according to the invention are formed of wire configured into a tube and are usually delivered into the body lumen using a catheter. The catheter carries the stent in a reduced-size form to the desired site. When the desired location is reached, the stent is released from the catheter and expanded so that it engages the lumen wall as explained below. 
     A stent  20  is shown in a flat layout view in FIG.  1 A. The stent  20  is formed of elongated strands  22  such as elastic metal wires. The wires  22  are woven to form a pattern of geometric cells  24 . The sides  26   a ,  26   b ,  26   c , and  26   d  of each of the cells  24  are defined by a series of strand lengths  28   a ,  28   b ,  28   c , and  28   d . Each of the sides  26  are joined to the adjoining side at an intersection where the strands  22  are helically wrapped about each other to form interlocking joints  30 . 
     Referring to FIGS. 1A and 1B, the interlocking joints  30  are loose and spaced from each other in the full expansion position. The cells  24  have a diamond shape. The strand angle is α. When the stent  20  is radially compressed, the interlocking joints  30  are in tight interference such that points  32  and  34  are in close proximity. In addition, the interlocking joints  30  on the same circumference are in close contact, therefore establishing the compressed, reduced size which can be fit within a sleeve for delivery on a catheter. The strand angle α is increased in the compressed or constrained state of the stent. A medical prosthetic stent and method of manufacturing such a stent is described in U.S. patent application Ser. No. 08/743,395 which was previously incorporated herewith by reference. 
     Referring to FIG. 2A, an isometric view of stent  10  according to the invention is shown in an expanded position. The stent  10  is formed from a plurality of strands  42 . In a preferred embodiment, there are five strands  42 , as seen in the layout view of FIG.  2 B. The strands  42  are woven in a pattern starting at a proximal end  44 . The pattern forms a plurality of geometric cells  46 . Each strand  42  forms a pair of sides  48   a  and  48   b  of the most distal cell  46 . Each of the sides, with the exception of at least one as explained below, are joined to the adjoining side at an intersection  52  where the strands  42  are helically wrapped about each other to form interlocking joints  54 . 
     While there are five intersections  52 , at least one of the intersections  52  is formed by strands  42  that just cross forming a cross joint and are not twisted to form a wrap as indicated at point  56  in FIG. 2B. A preferred pattern of where the strands  42  just cross is spaced 1½ cells  46  away, as seen in FIG.  2 B. 
     Referring to FIG. 3, the stent  10  is shown in the contracted position within the sleeve  58 . Similar to the embodiment shown in FIGS. 1A and 1B, the size to which the stent  10  can be constricted is limited by where the interlocking joints  54  engage each other. The elimination of one wrap joint allows for the stent  10  to be compressed to a smaller size. 
     In a preferred embodiment, the strands  42  are formed of nitinol wire. The wires each have a diameter of 0.006 inches. The stent  10  has an outside diameter when fully expanded of 10 millimeters. The stent  10  is capable of compressing into a sleeve  58  of an outside diameter of 8.0 French or less, and preferably 7.0 French (3fr=1 mm). The stent shown in the FIGS. 1A and 1B, of similar material and dimension, is capable of compressing to a diameter of approximately 9 fr. 
     In one preferred embodiment, the length of the legs or sides  48  of the cells  46  is similar to that of the embodiment shown in FIGS. 1A and 1B. The radial force is decreased from the elimination of one of the interlocking or wrap joints. The compressed stent  10  has a length of approximately 120 of the expanded stent. Therefore, for a 10 centimeter stent, the compressed length is 12 centimeters. 
     In one preferred embodiment, the length of the legs or sides  48  of the cells  46  are reduced. The reduced length is to compensate for a decrease in radial force resulting from the elimination of one of the interlocking or wrap joints. The compressed stent  10  has a length of approximately 150 of the expanded stent. Therefore, for a 10 centimeter stent, the compressed length is 15 centimeters. 
     In one preferred embodiment, a plurality of (ten shown) platinum-iridium radiopaque (R.O.) markers  60  are located on the stent  10 . The R.O. markers  60  are threaded onto the terminating cells; five on the proximal end and five on the distal end. 
     A method of making the stent is shown in FIGS. 4A and 4B. A mandrel  62  has a plurality of pins  64  on the outer surface of the mandrel in a pattern that determines the geometric cell  46  pattern. The strands  42  are bent around the top portion  66  of each top anchoring pin  64  to form the proximal end  44  of the stent  10 . The strands  42  are then pulled diagonally downward to an adjacent anchoring pin  64  where the strands  42  are joined. The strands  42  are helically wrapped about each other to form the interlocking joint  54 , with each strand passing through a single 360 degree rotation. The two strands are pulled taught so that the interlocking joint  54  rests firmly against the bottom portion  68  of the anchoring pin  64  such that each strand  42  is maintained in tension. 
     Each level of anchoring pins  64  is missing a pin  64  in a set order, such as to achieve the desired pattern in FIG.  2 B. The stands  42  which pass the missing pin location simply cross to form the cross joint. 
     In a preferred embodiment, the anchoring pins  64  are square. The square pins retain the helically wrap of the strands in a proper position. In a preferred embodiment, the pins have a width of 1 millimeter. The anchoring pins can have a smaller width such as 0.5 mm for use with narrower diameter strands, such as 0.005 inch diameter strands. 
     The free ends of the strands  42  are then pulled downward to the next diagonally adjacent anchoring pin  64 . This process is continued until the desired length of the stent  10  is achieved. 
     The stent  10  is then heat-treated. The strands  42  at the joining end  40  of the stent  10  are attached, for example, by ball welding the ends of the wires. The heat-treating and alternative finishing techniques are described in U.S. patent application Ser. No. 08/236,786 which was filed on Apr. 29, 1994, the entire contents is incorporated herein by reference. 
     Another alternative to the R.O. markers  60  for locating the stent  10  using fluroscopy is to coat the stent with gold. The stent  10  can be either totally or partially coated. In a partially coated stent, only portions of the strands between the joints are coated. Coating of a stent is described in further detail in U.S. Pat. No. 5,201,901 which issued on Apr. 13, 1993, the entire contents is incorporated herein by reference. 
     In one preferred embodiment, the stent  10  is installed using an endoscope  70  as seen in FIGS. 5A and 5B. The endoscope  70  has a channel  72  which is typically used for collecting biopsy samples or for suction. This channel  72  is how the stent  10  is passed into the body as explained below. The endoscope  70  in addition has an air/water nozzle  74  for cleaning the area in front of the endoscope  70 . In addition, the endoscope  70  has a mechanism for the physician to see what is in front of the endoscope  70 ; this mechanism includes an objective lens  76 . A pair of illumination lenses  78  which are used in lighting the site are also shown. 
     FIG. 5B illustrates a cross sectional view of the distal end of the endoscope  70 . An air/water tube  80  extends down to the air/water nozzle  74 . Both the viewing mechanism and the illumination mechanism have optical fiber bundles  82  leading to the respective lens  76  and  78 . 
     Endoscopes come in various sizes and lengths depending on the purpose. The channel  72  likewise has different sizes. It is recognized that it may be desirable to use a smaller diameter scope to be less invasive or that a larger diameter scope will not fit the lumen. The following table is an example of various size endoscopes. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Working Length 
                 Distal Tip 
                 Channel 
               
               
                   
                 (cm) 
                 O.D. (mm) 
                 Diameter (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 55 
                 4.8 
                 2.0 
               
               
                   
                 55 
                 6.0 
                 2.6 
               
               
                   
                 63 
                 12.2 
                 3.2 
               
               
                   
                 102 
                 9.8 
                 2.8 
               
               
                   
                 102 
                 12.6 
                 3.7 
               
               
                   
                 124 
                 11.0 
                 2.8 
               
               
                   
                 124 
                 11.0 
                 3.2 
               
               
                   
                 125 
                 11.3 
                 4.2 
               
               
                   
                 173 
                 13.0 
                 3.2 
               
               
                   
                   
               
             
          
         
       
     
     The stent  10  as described in relation to FIGS. 2A-4B can be used with channels of 3.2 mm or greater as described below. 
     In addition, the stent  10  can be introduced using a percutaneous insertion. In both the method using the endoscope  70  and the percutaneous procedure, an over the wire delivery system  86  as seen in FIG. 6A can be used. The over-the-wire delivery system  86  has an elongated catheter  88  over which the stent  10  is positioned. The catheter  88  extends from a proximal handle  90  to a distal tip end  92 . The catheter  88  extends through a shaft  94  at the proximal end. 
     An outer sheath  98  is located at the distal end of the over the wire delivery system  86 . The outer sheath  98  is moved towards the handle  90  using a pull wire  102  and a pull ring  104  as seen in FIG. 6B. A guidewire  108  extends through the catheter to the distal end tip  92 , as best seen in FIG.  6 A. 
     In a preferred embodiment, the outer sheath  98  has an outer diameter in the range of between 0.072 inches and 0.094 inches. The inner diameter of the outer sheath  98  has a range of between 0.066 inches and 0.086 inches. The outer sheath tends to the lower portion of the range when the stent can contract to the 6 French size and towards the upper portion of the range when the stent can contract to the 7 French size. 
     In one preferred embodiment, the outer sheath  98  is formed having several layers of material. The nominal outer diameter is 0.093 inches and a nominal inner diameter of between 0.078 and 0.081 inches. The inner layer is composed of polyethylene or TFE and has a nominal thickness of 0.001 inches. A layer of EVA or polyurethane of a nominal thickness of 0.0005 inches forms the second layer. A braid metal spring stainless or liquid crystal polymer fiber having a thickness of 0.0015 to 0.0025 inches overlies the second layer and forms the core of the outer sheath  98 . 
     In a preferred embodiment, the fourth layer varies in material composition as it extends from the proximal end to the distal end. The proximal end of the sheath is formed of Pebax or polyamide and the material varies to a polyamide or cristamid at the distal end. This layer has a nominal thickness of 0.002 inches. This varying of the material is for increased flexibility at the distal end to move through tortures easier and increased rigidity at the proximal end to give the catheter better push. 
     The sheath  98  has a finish layer of a hydrophlic coating having a thickness of between 0.0005 and 0.001 inches. The coating is for increase lubricativity. 
     The shaft has an outer diameter of 0.074 inches (1.88 mm). The shaft is formed of nylon  12  cristamid or cristamid. 
     In a preferred embodiment, the tip extrusion has an outer diameter in the range of between 0.042 and 0.055 inches. The inner diameter of the tip extrusion has a range of between 0.036 and 0.040 inches. 
     In one preferred embodiment, the tip extrusion or catheter has a nominal outer diameter of 0.047 inches and an inner diameter of 0.037 inches. The inner diameter defines the passage for the guidewire. In a preferred embodiment, the catheter is formed of Peek (Polyether ether ether Keetone) Peek Braid Peek, Polyimide or Polyimide Braid Polyimide. In a preferred embodiment, the guide wire  108  has a diameter of 0.035 inches. 
     An alternative method to the over-the-wire delivery system  86  shown in FIGS. 6A and 6B is a rapid exchange delivery system  112  shown in FIG.  7 . The rapid exchange delivery system  112  has a shaft  114  that extends from a proximal handle  116 . A guidewire  118  extends from a two lumen transition zone  120  through an outer sheath  122  to a distal tip end  124 . In contrast to the over the wire delivery system  86 , the guide wire  118  does not extend all the way back to the proximal handle  116 . Similar to the over the wire delivery system  86 , the outer sheath  122  of the rapid exchange delivery system  112  is moved towards the handle  116  using a pull wire  128  and a pull ring  130 . 
     Referring to FIGS. 8A-8F, the over-the-wire delivery system  86  of FIGS. 6A and 6B is shown for positioning a stent  10  in a bile duct. Stents are used in many uses including for treatment of an obstruction  134 , such as a tumor in the bile duct. The delivery system can position a prosthesis, such as a stent  10 , to move the obstruction out of the lumen  136 . 
     Typically, the occlusion substantially closes off a lumen, such as a bile duct which has a healthy diameter of about 8-10 mm. The obstruction may be several centimeters in length. After the obstruction is located using one of several diagnostic techniques, the physician gains access to the lumen. Using ultrasound or fluoroscopy, the guidewire  108  such as seen in FIG. 8C, is positioned through the outer access sheath  98  so that it extends past the obstruction. 
     Referring to FIG. 6A, the delivery system  86  is advanced axially and distally until the distal radiopaque marker  60  is positioned axially at a location at least about 1 cm distal of the occlusion  134 . This location substantially corresponds to the position at which the distal end  47  of the stent  10 , when expanded, will engage the lumen wall  136 . The location is selected so the stent  10  is positioned beyond the occlusion  134  but not too close to the end of the bile duct, for example. The marker  138  indicates the position of the proximal end  40  of the stent  10  in the expanded position and is such that the proximal end  40  of the prosthesis will engage healthy tissue over a length of at least 1 cm. Where possible the stent  10  is centered about the obstruction, based on the fully expanded length indicated by markers  138  and  140 . The marker  139  indicates the proximal end of the stent when the stent is in the fully compact form, which has an overall length of approximately 20 percent longer than in its expanded state. Therefore for a stent of 7.5 centimeters, the compressed state has a length of approximately 9 centimeters. 
     The sheath  98  is retracted in one continuous motion as illustrated in FIG.  8 B. With the sheath  98  partially withdrawn, (arrow  144 ), portions of the stent  10  expand (arrow  146 ). The lengthening of the stent  10  has a simultaneous effect of reducing the radial force the stent exerts on the wall of the sheath  98  and, therefore, reducing the frictional force between the inner wall of the sheath and the stent  10 , allowing a smoother retraction of the sheath  98  with less axial force. 
     After sheath retraction continues but usually to a point less than the marker  138 , the proximal end  40  of the expanding and contracting prosthesis  10  exits the sheath  98  and engages the lumen wall  136 , forcing open the lumen  136  to its normal diameter and firmly anchoring the stent so that it resists axial motion, as illustrated in FIG.  8 C. 
     The stent is released entirely from the catheter body  88  by drawing the catheter body  88  proximally (arrow  152 ) as seen in FIG. 8D, which causes the end loops to be positioned at more distal positions along the members, until the radial force of the stent  10  causes the members to deflect outwardly (arrows  154 ). 
     The catheter  88  is then removed from the body, leaving the prosthesis  10  properly positioned as illustrated in FIG.  8 E. 
     An alternative embodiment of the low profile diamond stent is shown as a flat layout view in FIG.  9 . The stent  160  has two separate layers  162  and  164 ; an inner layer  162  shown in hidden line and an outer layer  164 . Each layer  162  and  164  of the stent  160  has a plurality of strands  166 . In a preferred embodiment, each layer has four strands; this is in contrast to the five strands in the previous embodiment. 
     The strands are woven in a pattern of geometric cells  169  starting at the distal end  170 . Each strand  166  forms a pair of legs  144  of the most distal opening on the cell  168 . The inner layer  162  and the outer layer  164  are intertwined at both the distal end  170  and the proximal end  172 . 
     The sides  176   a ,  176   b ,  176   c , and  176   d  of each of the cells  168  are defined by a series of strand lengths  178   a ,  178   b ,  178   c , and  178   d . Each of the sides  176  are joined to this adjoining side at an intersection where the strands are helically wrapped about each other to form interlocking joints  180 . 
     Similar to the embodiment shown in FIGS. 1A and 1B and in contrast to the previous embodiment, every intersection has an interlocking joint  180 . Without the fifth strand  166 , the stent  160  can be contracted into a smaller diameter than that of the stent  20  shown in FIGS. 1A and 1B. 
     In a preferred embodiment for use in a colon, both layers are formed of identical materials. Each strand is composed of nitinol and has a diameter of 0.010 inches. 
     The two separate layers  162  and  164  in the constricted position are off-set from each other so the interlocking joints of one layer do not engage with the interlocking joints of the other layer. The off-set between layers is created by an off-set created when formed as described below or created by the related motion of the layers as the layers are constricted. The related motion can be the result of the constraints of the strands or the material properties. One property difference can be the thickness of the strands as described in the next embodiment. 
     The stent can be coated with a silicon lubricant or suitable lubricant to ease the self-expanding of the stent. 
     An alternative embodiment of the double layer stent  160  of FIG. 9 is shown in FIGS. 10-12. In contrast to the double layer stent  160  of FIG. 9, the double layer stent  188  has a cover layer  190  interposed between an outer layer  192  and an inner layer  194 . The outer layer  192  is shown in hidden line and the cover layer  190  is shown in hidden line in FIG.  10 . 
     Similar to the previous embodiment, the inner layer  194  and the outer layer  192  are intertwined at both the proximal end  170  and the distal end  172 . The intertwining of the layers  192  and  194  retains the cover layer  190  in position. 
     In a preferred embodiment, each layer has four strands and are woven similar to the embodiment shown in FIG. 8 to define the geometric cells  198 . The strands of the two layers are formed of two different thickness wires in a preferred embodiment. The inner layer has a thicker wire. 
     FIG. 11 shows the stent in an artery. The stent is moving an obstacle out of the passage. The cover prevents tumor in-growth, will seal fistulas and block aneurysms. 
     One technique for placing a stent into the circulation system of a patient is to enter from the brachial artery located in the arm. This point of entry can be used for insertion into the vascular system including for example, peripheral locations such as the knee which require the flexibility of the diamond stent. 
     A cross-sectional view of the stent  188  is shown in FIG.  12 . The inner layer  194  having the thicker strands forces the cover  190  and the outer layer  192  outward. The over  190  is in engagement with both the inner layer  194  nd the outer layer  192 . 
     In a preferred embodiment, the strands are formed of nitinol. The inner layer has strands having a diameter of 0.006 inches. The strands of the outer layer have a diameter of 0.005 inches. The radial expansion force of the thicker wire inner layer is transmitted to the outer layer. The radial expansion force can be altered by varying one or both layers. 
     In another preferred embodiment, the stent has three strands on each layer. The inner layer has a diameter of 0.008 inches. The strands of the outer layer have a diameter of 0.005 inches. 
     The outer layer can be formed from a non self-expanding material. The outer layer can be chosen for its radiopaque characteristics. Materials that can be chosen for their radiopacity characteristics include tantalum, platinum, gold or other heavy atomic metal. 
     In a preferred embodiment, a cover is interposed between the layers. The cover can be made of several types of material which allow the stent to be compressed to a small diameter and also be self-expanding. A preferred material is a woven carbon fiber, a metal mesh, a polymer such as a polyurethane, or a material treated with a drug for time release. Different agents can be employed on the inside and the outside. An electrical current can be applied to tissue using the stent. Different materials for the layers can be used than the interposed cover depending on the treatment site and the desired method of treatment. 
     In one preferred embodiment, the layers  192  and  194  are interwoven for the entire stent without an interposed cover. Referring to FIG. 13, a mandrel  262  has a plurality of anchoring pins  164 . For a stent having two layers of four strands each, each row has eight (8) anchoring pins  164  at the same height. The top row, however, has the anchoring pins  164  for one strand positioned ½ millimeter higher than the other set. After the stent is woven, the distal end of each stent is pulled to the same position, therein resulting in the rest of the interlocking joints being offset. 
     If there is no cover between the two layers, the two layers can be interwoven from the distal end to the proximal end. 
     FIGS. 14A and 14B illustrate a single layer stent  210  having six strands. The stent  210  has four wrap joints  254  a pair of cross joints  256 . 
     In one preferred embodiment, the stent  210  has a diameter of  14  millimeters in the expanded state. The stent has foreshortening in the range of 12 to 18 percent. With the strands having a diameter of 0.006 inches, the stent with only four wrap joints  254  per row can compress to fit within a 7 French system. 
     EQUIVALENTS 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the claims.