Abstract:
An intravascular stent especially suited for implanting in lumens having variable characteristics such as curvatures, changing diameters as found in ostial regions or variable wall compliance during systolic cycles. The stent can include an end region which is fabricated to have a greater radial strength than the remaining axial length of the stent. Such a stent is particularly suited for use in ostial regions, which require greater support near the end of the stent. The stent alternatively can include sections adjacent the end of the stent with greater bending flexibility than the remaining axial length of the stent. Such a stent is particularly suited for use in curved arteries. The stent can be constructed with an end that has greater radial strength and sections adjacent the end with greater bending flexibility. Such a stent prevents flaring of the stent end during insertion. The stent can also be constructed to have increased longitudinal flexibility when expanded such that it flexes with the vessel wall during systolic cycles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of Ser. No. 09/599,158 filed Jun. 21, 2000, which is a continuation of Ser. No. 09/040,145 filed Mar. 17, 1998 (now U.S. Pat. No. 6,676,697), which is a division of Ser. No. 08/716,039 filed Sep. 16, 1996 (now U.S. Pat. No. 5,807,404). 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to stents for implanting into vessels of a living body. In particular, the present invention relates to intraluminal stents which provide radial support, stability and coverage of the vessel wall when expanded and which may be especially suited for implanting in a variety of lumens having variable characteristics, such as variable curvature, variable diameter, e.g. as found in ostia, and variable wall compliance during systolic cycles.  
         [0004]     2. Description of the Prior Art  
         [0005]     It is well known to use a stent to expand and impart support to different bodily conduits, such as blood vessels, by expanding a tube-like structure inside the vessel requiring support against collapse or closure. U.S. Pat. No. 5,449,373 shows a stent preferably used for vascular implantation as part of a balloon angioplasty procedure. The stent of U.S. Pat. No. 5,449,373 may be delivered through, or implanted in, a curved vessel. One shortcoming of conventional stents is that they may have deficiencies due to “end effects” where the ends of the stent tend to “flare out” during insertion or after expansion or have a decreased radial force at the ends after expansion. Still another shortcoming of conventional stents is they do not have variable properties (e.g., flexibility and rigidity) to accommodate any different characteristics of the vessel (e.g., curvature, diameter and shape) or to comply with the vessel&#39;s natural flexing during systolic cycles.  
       SUMMARY AND OBJECTS OF THE INVENTION  
       [0006]     The present invention provides for various embodiments of an intraluminal stent which includes varied or different mechanical properties along the axial length of the stent in order to improve stent end effects, to accommodate variable vessel features or to comply with the vessel&#39;s natural flexing during systolic cycles. As a result, the various embodiments of the present invention allow for variable properties such as flexibility or radial support between axial regions of the stent. These varied properties can be accomplished in a number of different ways, including decreasing or increasing the thickness or width of elements of one or more of the sections relative to other sections and/or increasing or decreasing the axial length of one or more of the sections and/or changing the cell shape and size and/or changing material properties (e.g., strength, elasticity, etc.) of the material in one section relative to other sections.  
         [0007]     The various embodiments of the stents of the present invention may be adapted to provide more flexibility at the ends to allow the stent to accommodate the curvature of a vessel in which the stent is implanted. The degree of flexibility and the distance from the end of the stent to which the extra flexibility is imparted may be varied as specific applications dictate. This flexibility at the ends reduces the chance of a potential trauma point being created in the vessel by the stent tip pressing on the wall outside of the curve if the stent is not flexible enough along its longitudinal axis. In one embodiment of the present invention, flexibility of the stent ends is increased by reducing the gauge of the material used in a section or sections at the stent ends. In another embodiment the flexibility of the stent ends is increased by changing the dimensions of a section or sections at the stent ends. In yet another embodiment of the invention, the flexibility of the stent ends is increased by changing both the dimensions and the gauge of the material used in a section or sections at the stent ends.  
         [0008]     The various embodiments of the stents of the present invention may also be adapted to insure increased radial strength at the ends. Radial strength is the resistance of a section of the stent, in an expanded state, to radial contraction. Increasing the radial strength of a stent at the ends is particularly advantageous for stents supporting ostia. Because lesions at an ostium tend to be more calcified or hardened, and therefore require more support, the section of the stent supporting the ostium must be relatively strong. It is also the case that a stent with uniform characteristics has a decreased radial force at the end due to the “end effect” whereby the last row has no support on one side. In one embodiment of the present invention, the strength of the stent at the end supporting, e.g., the ostium, is increased by reducing the length of some sections at the stent end.  
         [0009]     The various embodiments of the stent of the present invention also reduce the chance of “flare” at the end of the stent while the stent is being fed into a vessel. During insertion of the catheter delivery system into a curved vessel, the delivery system, including the stent crimped on it, bend along the curvature of the vessel. This bending of the stent can cause a “flaring out” of the leading edge of the stent. This flaring could cause the stent to catch on the surface of the vessel which could result in trauma to the vessel, could inhibit further insertion and proper positioning in the target area, and could cause plaque to break off, which could embolize and clog the vessel. In one embodiment of the present invention, flare is minimized by making the section at the stent end stronger by reducing its length, and by making sections adjacent to the stent end more flexible by reducing their widths, thus, decreasing the bending strength of those sections. Bending strength is the resistance of a section of the stent to axial bending. As a result, the end of the stent remains tightly crimped on the balloon, and the bending moment is taken up by the deformation of the more flexible sections. Upon expansion, the reduced bending strength allows the end of the stent to curve and fit better the curvature of the vessel, thereby, reducing the pressure of the tip of the stent on the internal wall of the vessel being treated.  
         [0010]     It is an object of this invention to provide a stent which does not have sharp points or protrusions at its end concentrating pressure on the vessel&#39;s wall upon expansion of the stent in a curved portion of a vessel.  
         [0011]     It is another object of this invention to provide a stent having a radial force at its distal end that is greater than the radial force in the portion of the stent proximal to the distal end.  
         [0012]     It is yet another object of this invention to provide an expandable stent, comprising: a plurality of interconnected flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, wherein the cells disposed in the distal row of the stent are adapted to exert greater radial force and are further adapted to be more flexible than the cells disposed in the rows disposed between the distal row and the proximal end of the stent.  
         [0013]     It is still another object of this invention to provide an expandable stent, comprising: a plurality of interconnected flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of said stent and a proximal row disposed at the proximal end of the stent, wherein the cells in the distal row of the stent and the cells disposed in the proximal row of the stent are adapted to exert greater radial force and are further adapted to be more flexible than the cells disposed in the rows disposed between the distal row and the proximal row.  
         [0014]     It is another object of this invention to provide an expandable stent, comprising: a plurality of interconnected flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, each of the flexible cells comprising a first member, a second member, a third member, and a fourth member; b) a first C-shaped loop disposed between the first member and the third member; c) a second C-shaped loop disposed between the second member and the fourth member; d) a first flexible connector disposed between the first member and the second member; and e) a second flexible connector disposed between the third member and the fourth member, wherein the cells of the distal row are provided with first and third members that are shorter than the second and fourth members in the distal row, and wherein the distal row is provided with first and second flexible connectors that are more flexible than the flexible connectors in the cells in the other rows of the stent.  
         [0015]     It is yet another object of this invention to provide an expandable stent, comprising: a) a plurality of interconnected flexible cells defining a longitudinal stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of interconnected flexible rows disposed along the longitudinal axis of the stent with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, each of the flexible cells comprising a first member, a second member, a third member, and a fourth member; b) a first C-shaped loop disposed between the first member and the third member; c) a second C-shaped loop disposed between the second member and the fourth member; d) a first flexible connector disposed between the first member and the second member; and e) a second flexible connector disposed between the third member and the fourth member, wherein the cells of the distal row are provided with first and third members that are shorter than the second and fourth members in the distal row, and wherein the distal row, and the row proximal to the distal row, are provided with first and second flexible connectors that are more flexible than the flexible connectors in the other rows of the stent.  
         [0016]     It is a further aspect of this invention to provide an expandable stent comprising: a) a plurality of flexible cells defining a stent having a proximal end and a distal end and a longitudinal axis, the cells arranged in a plurality of flexible rows along the longitudinal axis with a distal row disposed at the distal end of the stent and a proximal row disposed at the proximal end of the stent, each of the flexible cells comprising a first member, a second member, a third member, and a fourth member; b) a first C-shaped loop disposed between the first member and the third member; c) a second C-shaped loop disposed between the second member and the fourth member; d) a first flexible connector disposed between the first member and the second member; and e) a second flexible connector disposed between the third member and the fourth member, wherein the cells of the distal row are provided with first and third members that are shorter than the second and fourth members in the distal row, and wherein the cells of the proximal row are provided with second and fourth members that are shorter than the first and third members in the proximal row, and wherein the distal row, and the row proximal to the distal row, and the proximal row and the row distal to the proximal row are provided with first and second flexible connectors that are more flexible than the flexible connectors in the other rows of the stent.  
         [0017]     It is yet another object of this invention to provide an expandable stent, comprising: a plurality of flexible cells defining a stent having a proximal end and a distal end, the stent provided with means for imparting a radial force at its distal end that is greater than the radial force in the portion of the stent proximal to the distal end.  
         [0018]     It is yet a further object of this invention to provide an expandable stent, comprising: a plurality of flexible cells defining a stent having a proximal end and a distal end, the stent provided with means for imparting a radial force at its proximal and distal ends that is greater than the radial force of that portion of the stent disposed between the proximal and distal ends.  
         [0019]     It is another object of this invention to provide an expandable stent for treating a lumen having a unique characteristic along a portion of the lumen, comprising: a plurality of interconnected flexible cells, the cells arranged in a plurality of interconnected flexible rows defining a stent having a proximal end and a distal end and a longitudinal axis, wherein at least one of the rows is adapted to accommodate the unique characteristic of that portion of the lumen in contact with the adapted row or rows.  
         [0020]     It is yet another object of this invention to provide a single flexible stent with a unibody or one-piece construction which is capable of imparting support to a lumen or vessel along the entire length of the stent and in which portions of the stent are adapted or modified so as to have characteristics, e.g., bending strength or radial strength, that are different than the characteristics or features in the rest of the stent along it&#39;s longitudinal axis or about its circumference. The change in stent features will either accommodate non-uniformity in the treated lumen or may create different environmental conditions in different areas in the lumen. Non-uniformity in a treated vessel can be of many different types such as an ostium, change in diameter, change in curvature, non-continuous cross-section such as triangular or square, or non-uniformity in surface nature, etc. To accommodate such non-uniformity, portions of the stent may be adapted to provide changing dimension, flexibility, rigidity, size of cells, shape of cells, and response to pressure as dictated by specific applications. Specific applications may dictate, e.g., a desired higher radial force at one end while the other portions of the stent provide a substantially continuous support to the vessel wall with the gaps in the stent sized small enough to reduce the likelihood of tissue prolapse. Other applications may dictate a desired degree of stiffness in the center to reduce the likelihood of breakage and impart the desired degree of softness at the end to allow for the best fit with the anatomy of the target area. Other applications may dictate that one or more of the rows be provided with cells that are sized larger than the cells in the remaining rows of the stent so as to provide access to a side branch in the lumen, e.g., for introducing a second stent through one of the larger sized cells so as to permit construction of a bifurcated stent within the lumen. Still another application may dictate that one or more of the rows be provided with cells which are adapted or modified so that upon expansion of the stent the portion of the stent defined by the adapted or modified row or rows has a diameter that is either larger or smaller than the remaining portions of the stent to accommodate lumens with non-uniform diameters. One or more rows of cells may also be adapted or modified so as to have varying radial force, or varying longitudinal flexibility, or to correct for a change in properties at the end of the stent.  
         [0021]     It is yet another object of this invention to provide an expandable stent having interconnected flexible cells which provide good radial support, stability and coverage of the vessel wall when it is expanded and implanted in the vessel and which flexes with the vessel during the systolic cycles.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  shows an illustration of the basic pattern of an embodiment of the stent of the present invention, shown in an unexpanded state;  
         [0023]      FIG. 2  shows an illustration of the pattern of the stent of  FIG. 1 , in a partially expanded state;  
         [0024]      FIG. 3  is a side view showing a conventional stent and a stent manufactured in accordance with one embodiment of the invention;  
         [0025]      FIG. 4  shows the stents of  FIG. 3  crimped on a balloon catheter and bent prior to expansion;  
         [0026]      FIG. 5  shows the stents of  FIG. 4  after they have been expanded in a curve;  
         [0027]      FIG. 6  shows the stents of  FIG. 3  partially expanded on a substantially straight balloon catheter;  
         [0028]      FIG. 7  shows an alternative embodiment of the invention provided with a shortened C-shaped loop and in which two rows of cells are provided with thinner gauge U-shaped loops;  
         [0029]      FIG. 8  shows the stent of  FIG. 7  partially expanded on a substantially straight balloon catheter;  
         [0030]      FIG. 9  shows the stent of  FIG. 7  after it has been expanded on a curved catheter as it would be when inserted around a bend in a vessel;  
         [0031]      FIG. 10  shows an alternative embodiment of a stent constructed in accordance with the invention;  
         [0032]      FIG. 11  shows the “S” or “Z” shaped loops constructed in accordance with the invention;  
         [0033]      FIG. 12  shows an alternative embodiment of a stent constructed in accordance with the invention;  
         [0034]      FIG. 12   a  shows a stent pattern of the alternative embodiment illustrated in  FIG. 12 ; and  
         [0035]      FIG. 13  shows an alternative embodiment of a stent constructed in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]      FIG. 1  shows the general configuration of one embodiment of a stent  1  fabricated in accordance with the present invention. The stent  1  may be fabricated of bio-compatible materials such as stainless steel 316L, gold, tantalum, nitinol or other materials well known to those skilled in the art as suitable for this purpose. The dimensions and gauge of material utilized may be varied as specific applications dictate. The stents of the present invention generally may be constructed in a manner in accordance with the stent described in U.S. patent application Ser. No. 08/457,354, filed Jun. 1, 1995, the disclosure of which is incorporated herein by reference.  
         [0037]      FIG. 1  is a side view of the distal end  2  of stent  1  of the present invention, showing the general pattern of the stent. As shown in  FIGS. 1 and 2  the pattern may be described as a plurality of cells  3  and  3 ′. Each cell  3  is provided with a first member  4 , a second member  5 , a third member  6 , and a fourth member  7 . A first C-shaped loop  10  is disposed between the first member  4  and the third member  6  and a second C-shaped loop  11  is disposed between the second member  5  and the fourth member  7 . In each of the cells  3 , first member  4 , second member  5 , third member  6 , and fourth member  7  are substantially equal. Thus, first C-shaped loop  10  is displaced a distance D 1  and second C-shaped loop  11  is displaced a distance D 2  from the center of cell  3 . In a preferred embodiment, D 1  is substantially equal to D 2 . A first flexible connector  8  is disposed between the first member  4  and the second member  5  and a second flexible connector  9  is disposed between third member  6  and fourth member  7 . The flexible connectors  8  and  9  may be made in a variety of shapes, e.g., an “S” or a “Z” shape as shown in  FIG. 11 . In a preferred embodiment, a “U” shape is utilized as shown in FIGS.  1  to  10 .  
         [0038]      FIG. 1  shows the pattern of stent  1  in an unexpanded state, i.e., that state in which the stent  1  is first inserted in a particular vessel in which a balloon angioplasty procedure is to be performed, but before balloon inflation.  FIG. 2  shows the pattern of stent  1  in a partially expanded state, i.e., that state after the balloon has been expanded, e.g. by a balloon, and the state in which the stent  1  remains in the vessel which it supports. The plurality of interconnected cells  3  and  3 ′ form a plurality of interconnected rows  25 ,  26 ,  27 , and  28  of cells disposed along the longitudinal axis of the stent  1 .  FIGS. 1 and 2  show a distal row  25  disposed at the distal end  2 , a row  26  adjacent to and proximal to distal row  25 , a row  27  adjacent to and proximal to row  26 , and a row  28  adjacent to and proximal to row  27 . It will be appreciated that the number of rows, and the number of cells per row, and the shape of each cell, may be varied as specific applications require.  
         [0039]     As shown in  FIGS. 1 and 2 , the cells  3 ′ in distal row  25  differ from the cells  3  in rows  26 ,  27 , and  28 . The first member  4 ′ and the third member  6 ′ of the cells  3 ′ in row  25  are shorter than the first member  4  and the third member  6  of the cells  3  in rows  26 ,  27  and  28 . In cell  3 ′, first member  4 ′ is substantially equal to third member  6 ′, however, first member  4 ′ and third member  6 ′ are shorter than second member  5 ′ and fourth member  7 ′. The shorter members  4 ′ and  6 ′ result in a first C-shaped loop  10 ′ that is not disposed as far away from the center of the cell  3 ′ as second C-shaped loop  11 ′. Thus, first C-shaped loop  10 ′ may be thought of as being “shorter” than second C-shaped loop  11 ′. As shown in  FIG. 2 , first C-shaped loop  10 ′ is disposed a distance D 1 ′ that is less than the distance D 2 ′ that second C-shaped loop  11 ′ is disposed from the center of the cell  3 ′. In an especially preferred embodiment, D 1 ′ is about 15% less than D 2 ′.  
         [0040]      FIGS. 1 and 2  also show that the distal row  25  of the stent  1  is provided with a first U-shaped loop  8 ′ and a second U-shaped loop  9 ′ that are more flexible than the first U-shaped loop  8  and second U-shaped loop  9  of cells  3  in rows  26 ,  27 , and  28  of the stent  1 . This greater flexibility in the U-shaped loops  8 ′ and  9 ′ may be accomplished in a variety of ways, for example, by utilizing a different material, by treating the material e.g., by utilizing stainless steel annealing to impart selective degrees of hardness to the different portions of the stent. Alternatively, if, e.g., NiTi (Nitinol) is utilized, selected portions of the stent may be selectively thermo-mechanically treated so that portions of the stent, e.g., the U-shaped members, will remain in a martensitic phase while other portions of the stent will be transformed into austenitic phase in this section to yield different properties. Greater flexibility may also be achieved by changing the shape of the “U”, for example to a “Z” or an “S” (as shown in  FIG. 11 ), or by reducing the amount of material utilized to make the U-shaped loops  8 ′ and  9 ′. In the embodiment shown in  FIGS. 1 and 2 , the U-shaped loops  8 ′ and  9 ′ of row  25  are provided with the same thickness of material as the U-shaped loops  8  and  9  of the cells  3  in rows  26 ,  27 , and  28 , however, U-shaped loops  8 ′ and  9 ′ are not as wide. As shown in  FIGS. 1 and 2 , U-shaped loops  8 ′ and  9 ′ have a width W 1  that is less than the width W 2  of U-shaped loops  8  and  9  in the cells  3  of rows  26 ,  27 , and  28 . In a preferred embodiment, W 1  is about 50% narrower than W 2 . In an especially preferred embodiment, W 1  is about 40% narrower than W 2 .  
         [0041]      FIG. 3  is a side-by-side comparison of two stent sections and shows a conventional stent  12  compared to the stent  1 , shown in  FIGS. 1 and 2 .  FIG. 4  shows stents  1  and  12  shown in  FIG. 3  as they appear when they are crimped on a balloon and bent as they would be during insertion around a curve in a vessel. As shown in  FIG. 4 , conventional stent  12  flares at its leading edge  13  in contrast to stent  1  which does not.  FIG. 5  shows the stents of  FIG. 4  after the stents have been expanded in a curve. The tip of conventional stent  12  produces a protrusion or sharp point  13  which could cause local pressure and possible trauma to the vessel wall. In contrast, the stent  1  constructed in accordance with the invention bends gently at its end  2  without forming a protrusion or sharp point because the deformation of the of U-shaped loops  8 ′ and  9 ′ in distal row  25  make the end  2  softer.  
         [0042]      FIG. 6  shows the stents  1  and  12  of  FIG. 3  at partial expansion (before reaching maximum pressure) disposed on a substantially straight catheter. As shown, although the two stents  1  and  12  are subjected to the same outward force, the end  2  of stent  1  is less expanded than the end  13  of conventional stent  12  demonstrating the increased radial force of the end  2  of stent  1  constructed in accordance with the invention. At full pressure the radii of the stents  1  and  12  will be equal, however, the end  2  of stent  1  will have greater radial resistance to collapse than the end  13  of stent  12 .  
         [0043]      FIG. 7  shows an alternative embodiment of the invention. As shown in  FIG. 7 , the cells  3 ′ in row  25  are provided with a first member  4 ′ and third member  6 ′ that are shorter than second member  5 ′ and fourth member  7 ′. The cells  3 ′ in row  25  are provided with a first U-shaped loop  8 ′ and a second U-shaped loop  9 ′ that are thinner than the U-shaped loops  8  and  9  in the cells  3  in rows  27  and  28 . The cells  3 ″ in row  26  are provided with first U-shaped loops  8 ″ and second U-shaped loops  9 ″ that are narrower than the U-shaped loops  8  and  9  in the cells  3  in rows  27  and  28 .  
         [0044]      FIG. 8  shows the stent  20  of  FIG. 7  during partial expansion of the stent showing the decreased expansion of row  25  at partial expansion because of the higher radial force of the end  2  of the stent which results from construction with shorter C-shaped loops  10 ′ in row  25 , construction with narrower, i.e., more flexible, U-shaped loops  8 ′ and  9 ′ in row  25 , and  8 ″ and  9 ″ in row  26 .  
         [0045]      FIG. 9  shows the stent  20  of  FIGS. 7 and 8  after it has been expanded in a curved vessel and shows the bends of the U-shaped loops  8 ′ and  9 ′ in row  25  and  8 ″ and  9 ″ in row  26  which allows the end portion  2  of the stent  20  to more readily conform to the curve of the vessel, creating smooth ends with no sharp points or projections projecting into the vessel wall.  
         [0046]     The changes can be made on one side only or on both sides of the stent as specific applications dictate. Additionally, different combinations of embodiments of the invention may be mixed such as using thinner U-shaped loops, longer U-shaped loops or different shaped loops, e.g., “Z” or “S”.  
         [0047]     One example of how this may be achieved is shown in  FIG. 10 .  FIG. 10  shows how the stent shown in  FIG. 7  may be modified, if additional flexibility is desired. As shown in  FIG. 10 , the distal row  25 , and the proximal row  29  of stent  30  are provided with first and second U-shaped loops that are more flexible than the U-shaped loops in the other rows of the stent disposed between the distal and proximal rows  25  and  29 . In the embodiment of the invention shown in  FIG. 10 , the distal row  25  is provided with shortened members  4 ′ and  6 ′ and more flexible U-shaped loops  8 ′ and  9 ′, as previously discussed, and the proximal row  29  is provided with shortened second and fourth members  5 ″ and  7 ″ and more flexible U-shaped loops  8 ′″ and  9 ′″. This arrangement imparts greater radial strength and greater flexibility to both ends of the stent.  
         [0048]     If even greater flexibility at the ends of the stent is desired, the stent shown in  FIG. 10  may be modified by replacing the U-shaped loops in rows  26  and  28  with more flexible loops. Thus, the distal row, the row proximal to the distal row, the proximal row, and the row distal to the proximal row are provided with U-shaped loops that are more flexible than the U-shaped loops in the cells in the remaining rows of the stent.  
         [0049]      FIG. 12  shows an alternative embodiment of the invention. In this embodiment, the stent is adapted to provide radial support and uniform coverage of the vessel wall when expanded and implanted into the vessel wall, as well as increased flexibility to comply with changes in the vessel wall, particularly during systolic cycles.  
         [0050]      FIG. 12  shows a stent pattern having a plurality of circumferential rows  115 ,  116 ,  117 ,  118 ,  119  and  120  of alternating interconnected cells  103  and  103 ′ disposed along the longitudinal axis of the stent. As shown in  FIG. 12 , cells  103  and  103 ′ are provided with a first C-shaped loop  110  having a first end  121  and a second end  122  and a second C-shaped loop  111  having a first end  123  and a second end  124 . Cells  103  and  103 ′ further include a first flexible connector  108  disposed between the first end  121  of first C-shaped loop  110  and the first end of second C-shaped loop  111  and a second flexible connector  109  disposed between the second end  122  of first C-shaped loop  110  and the second end  124  of second C-shaped loop  111 .  
         [0051]     To increase the flexibility of the stent while maintaining good radial support, stability and coverage when the stent is expanded, cells  103  and  103 ′ are provided with second C-shaped loops  111 , first flexible connectors  108  and second flexible connectors  109  that are more flexible than first C-shaped loops  110 . The increased flexibility of second C-shaped loops  111 , first flexible connectors  108  and second flexible connectors  109  may be achieved in a variety of ways, including reducing the gauge of the material used in these sections of the stent. In the embodiment shown in  FIG. 12 , the entire stent has the same radial thickness, however, the second C-shaped loops  111 , first flexible connectors  108  and second flexible connectors  109  are not as wide as the first C-shaped loops  110 . As shown in  FIG. 12 , second C-shaped loop  111  and first and second flexible connectors  108 ,  109  have a width W 1  that it less than the width W 2  of first C-shaped loop  110 . In a preferred embodiment, W 1  is about 50% less than W 2 . In a particularly preferred embodiment, W 1  is about 40% less than W 2 . It will also be understood that the gauge of the material used to form the second C-shaped loop  111  and first and second flexible connectors  108  and  109  relative to that the of the first C-shaped loop can be varied by reducing the thickness of the material. Alternatively, the increased flexibility in the second C-shaped loops  111 , first flexible connectors  108  and second flexible connectors  109  may be accomplished by using a more flexible material or altering the properties of the material to make it more flexible than the material of the first C-shaped loops.  
         [0052]     As shown in  FIG. 12 , the first and second flexible connectors  108 ,  109  are generally U-shaped loops. These U-shaped loops can be described as having two generally straight portions having an area of inflection therebetween. It will be understood that the increased flexibility of the first and second flexible connectors  108 ,  109  may also be achieved by changing the shape, for example, to a “Z” or an “S” (as shown in  FIG. 11 ) or by varying the lengths of the generally straight portions of the loops. It will be further understood that the closed ends of the U-shaped flexible connectors may extend downwardly in a circumferential direction as shown in  FIG. 12 , extend upwardly in a circumferential direction or be alternately oriented in upward and downward circumferential directions along the longitudinal axis of the stent.  
         [0053]     As further shown in  FIG. 12 , adjacent circumferential rows of interconnected flexible cells  103  and  103 ′ share either the same first C-shaped loop  110  or second C-shaped loop  111 . For example, the cells  103  in circumferential row  115  share the same second C-shaped loop  111  as cells  103 ′ in circumferential row  116 . Similarly, cells  103 ′ in circumferential row  116  share the same first C-shaped loops  110  as cells  103  in circumferential row  117 .  
         [0054]     Referring now to  FIG. 12   a , the stent pattern of the embodiment shown in  FIG. 12  can also be described as having alternating even and odd circumferential bands of loops  131   e  and  131   o  which are 180° out of phase. The stent pattern further includes a plurality of longitudinal bands of loops  132  that are coupled to the loops of adjacent even and odd circumferential bands of loops  131   e  and  131   o . As shown in  FIG. 12   a , the even and odd circumferential bands of loops  131   e  and  131   o  are interconnected with the longitudinal bands of loops  132  to form a stent comprising a plurality of cells  103  and  103 ′ defining a uniform cellular structure. Further, at least one loop  133  of the longitudinal bands of loops  132  is disposed between each adjacent even and odd circumferential bands of loops  131   e  and  131   o  to provide a stent which minimally shrinks in the longitudinal direction during expansion.  
         [0055]     As further shown in  FIG. 12   a , each cell  103  and  103 ′ includes a loop  142  of the even circumferential band of loops  131   e  having a first end  143  and a second end  144 , a loop  145  of the odd circumferential band of loops  131   o  having a first end  146  and a second end  147 . A first flexible connector  134  having a first end  135  and a second end  136  is disposed between loops  142  and  145  with the first end  135  of the first flexible connector  134  coupled to the first end  143  of loop  142  and the second end  136  of the first flexible connector  134  coupled to the first end  146  of loop  145 . A second flexible connector  137  having a first end  138  and a second end  139  is also disposed between loops  142  and  145  with the first end  138  of the second flexible connector  137  coupled to the second end  144  of loop  142  and the second end  139  of the second flexible connector  137  coupled to the second end  147  of loop  145 . As shown in  FIG. 12   a , loop  145  of the odd circumferential bands of loops  131   o  and flexible connectors  134  and  137  are provided with widths that are smaller than the width of loop  142  of the even circumferential bands of loops  131   e.    
         [0056]     The particular embodiment shown in  FIG. 12   a  includes alternating even and odd circumferential bands of loops  131   e  and  131   o  where each odd circumferential band of loops  131   o  has a smaller width than the even circumferential bands of loops  131   e . Depending on the embodiment, other patterns of circumferential bands of loops having smaller widths may be utilized. For example, the stent design according to the present invention may have two or more consecutive circumferential bands of loops having smaller widths or longer lengths at the ends of the stents to provide for flexibility at the ends of the stent. Also depending on the embodiment, the stent may have two or more circumferential bands of loops having greater widths or shorter lengths at the ends of the stent for increased rigidity or radial support. It will be understood that the present invention is not limited to any specific stent design and can be utilized in any stent design that includes contiguous cell structures having loops and flexible connectors.  
         [0057]      FIG. 13  illustrates another embodiment of the present invention having one kind of circumferential bands of loops  162  which are generally in phase with each other, rather than being 180° out of phase like the even and odd circumferential bands of loops  131   e ,  131   o  as in  FIGS. 12 and 12   a.    
         [0058]      FIG. 13  shows a stent pattern having a plurality of interconnected cells  160 . Each cell  160  includes a loop  164  of one of the circumferential bands of loops  162  having a first end  165  and a second end  166 , a loop  167  of a neighboring circumferential band of loops  162  having a first end  168  and a second end  169 , a first flexible connector  170  having a first loop  171  with a first end  172  and a second end  173 , a generally straight member  177  and a second loop  174  with a first end  175  and a second end  176 , and a second flexible connector  180  having a first loop  181  with a first end  182  and a second end  183 , a generally straight member  187  and a second loop  184  with a first end  185  and a second end  186 . The first flexible connector  170  is disposed between loops  164  and  167  such that first end  172  of first loop  171  is coupled to the first end  165  of loop  164  and second end  176  of second loop  174  is coupled to the first end  166  of loop  165 . The second flexible connector  180  is also disposed between loops  164  and  167  such that the first end  182  of first loop  181  is coupled to the second end  166  of loop  164  and the second end  186  of second loop  184  is coupled to the second end  169  of loop  167 .  
         [0059]     As further shown in  FIG. 13 , the first flexible connector  170  and the second flexible connector  180  are provided with widths smaller than the widths of loops  164  and  167  of the adjacent circumferential bands of loops  162 . It will be understood that the flexibility of the first and second flexible connectors  170 ,  180  can be increased or decreased by varying the lengths of the generally straight members  177 ,  187  and the lengths of the generally straight portions of the loops  171 ,  174 ,  181  and  184 . It will be further understood that the first and second flexible connectors may include additional alternating loops and generally straight members. For example, the flexible connectors may comprise three loops and two generally straight members forming a loop/straight member/loop/straight member/loop configuration or three generally straight members and two loops forming a straight member/loop/straight member/loop/straight member configuration. In addition, the orientation of the loops may also be varied such that each of the closed ends of the loops extend downwardly in a circumferential direction as shown in  FIG. 13 , extend upwardly in a circumferential direction or be alternately oriented in upward and downward circumferential directions.  
         [0060]     The stent pattern shown in  FIG. 13  can also be described as a plurality of circumferential bands of loops  162  coupled by a plurality of flexible connectors  170  and  180 . As shown in  FIG. 13 , the circumferential bands of loops  162  are in phase with each other and the flexible connectors  170  and  180  connect neighboring loops of adjacent circumferential bands of loops  162 . Because the circumferential bands of loops  162  are in phase, the flexible connectors  170  and  180  are offset from the longitudinal direction such that they couple apices of closed ends of the loops in the adjacent circumferential bands of loops  162 .  
         [0061]     The stent shown in  FIG. 13  can also be described as a modified version of the stent shown in  FIGS. 12 and 12   a . As can be seen from comparison of  FIGS. 12, 12   a  and  13 , the stent of  FIG. 13  is generally the same as the stent of  FIGS. 12 and 12   a  with exception that every second generally straight portion of each odd circumferential band of loops  1310  has been removed. This provides further flexibility along the longitudinal axis of the stent. Further, cells  160  of the stent shown in  FIG. 13  are larger than the cells  130 ,  103 ′ of the stent shown in  FIGS. 12 and 12   a . The increased cell size in the embodiment shown in  FIG. 13  may be beneficial for side branch accessing.  
         [0062]     The present invention contemplates a number of different variations and changes in different properties to achieve other non uniform features such as, but not limited to, cell size, cell shape, radio-opacity, etc. on the above-described preferred embodiments. The specified changes are brought only as an example for the application of the general concept, which is the basis for the present invention that stents with varying mechanical properties between sections along the stent may correct undesired effects at singular points such as stent ends and provide for a better fit to a vessel with properties changing along its axis. It is to be understood that the above description is only of one preferred embodiment, and that the scope of the invention is to be measured by the claims as set forth below.