Abstract:
A stent has a first loop containing section arranged in a circumferential direction and defining loops therein occurring at a first frequency, a second loop containing section arranged in the circumferential direction and defining loops therein occurring at the first frequency, and a third loop containing section disposed in a generally circumferential space between the first loop containing section and the second loop containing section and coupling the first loop containing section to the second loop containing section for defining cells therebetween, the third loop containing section defining loops therein occurring at a second frequency that is greater than the first frequency.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/516,753, filed Mar. 1, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to stents, which are endoprostheses implanted into vessels within the body, such as blood vessels, to support and hold open the vessels, or to secure and support other endoprostheses in the vessels. In particular, the present invention relates to a stent which is longitudinally flexible before after expansion.  
         BACKGROUND OF THE INVENTION  
         [0003]    Various stents are known in the art. Typically stents are generally tubular in shape, and are expandable from a relatively small, unexpanded diameter to a larger, expanded diameter. For implantation, the stent is typically mounted on the end of a catheter, with the stent being held on the catheter at its relatively small, unexpanded diameter. By the catheter, the unexpanded stent is directed through the lumen to the intended implantation site. Once the stent is at the intended implantation site, it is expanded, typically either by an internal force, for example by inflating a balloon on the inside of the stent, or by allowing the stent to self-expand, for example by removing a sleeve from around a self-expanding stent, allowing the stent to expand outwardly. In either case, the expanded stent resists the tendency of the vessel to narrow, thereby maintaining the vessel&#39;s patency.  
           [0004]    U.S. Pat. No. 5,733,303 to Israel et al. (“&#39;303”), which is expressly incorporated by reference, shows a unique stent formed of a tube having a patterned shape which has first and second meander patterns having axes extending in first and second directions. The second meander patterns are intertwined with the first meander patterns to form flexible cells. Stents such as this one are very flexible in their unexpanded state such that they can be tracked easily down tortuous lumens. Upon expansion, these stents provide excellent radial support, stability, and coverage of the vessel wall. These stents are also conformable, in that they adapt to the shape of the vessel wall during implantation.  
           [0005]    One feature of stents with a cellular mesh design such as this one, however, is that they have limited longitudinal flexibility after expansion, which may be a disadvantage in particular applications. This limited longitudinal flexibility may cause stress points at the end of the stent and along the length of the stent. Conventional mesh stents like that shown in U.S. Pat. No. 4,733,665 may simply lack longitudinal flexibility, which is illustrated by FIG. 1, a schematic diagram of a conventional stent  202  in a curved vessel  204 .  
           [0006]    To implant a stent, it maybe delivered to a desired site by a balloon catheter when the stent is in an unexpanded state. The balloon catheter is then inflated to expand the stent, affixing the stent into place. Due to the high inflation pressures of the balloon—up to 20 atm—the balloon causes the curved vessel  204  and even a longitudinally flexible stent to straighten when it is inflated. If the stent, because of the configuration of its mesh is or becomes relatively rigid after expansion, then the stent remains or tends to remain in the same or substantially the same shape after deflation of the balloon. However, the artery attempts to return to its natural curve (indicated by dashed lines)in FIG. 1 with reference to a conventional mesh stent. The mismatch between the natural curve of the artery and the straightened section of the artery with a stent may cause points of stress concentration  206  at the ends of the stent and stress along the entire stent length. The coronary vasculature can impose additional stress on stents because the coronary vasculature moves relatively significant amounts with each heartbeat. For illustration purposes, the difference between the curve of the vessel and the straightened stent has been exaggerated in FIG. 1.  
           [0007]    U.S. Pat. No. 5,807,404 to Richter, which is expressly incorporated by reference, shows another stent which is especially suited for implantation into curved arterial portions or ostial regions. This stent can include sections adjacent the end of the stent with greater bending flexibility than the remaining axial length of the stent. While this modification at the end of the stent alleviates the stress at the end points, it does not eliminate the stress along the entire length of the stent.  
           [0008]    Various stents are known that retain longitudinal flexibility after expansion. For example, U.S. Pat. Nos. 4,886,062 and 5,133,732 to Wiktor (“the Wiktor &#39;062 and &#39;732 patents”) show various stents formed of wire wherein the wire is initially formed into a band of zig-zags forming a serpentine pattern, and then the zig-zag band is coiled into a helical stent. The stents are expanded by an internal force, for example by inflating a balloon.  
           [0009]    The coiled zig-zag stents that are illustrated in FIGS. 1 through 6 of the Wiktor &#39;062 and &#39;732 patents are longitudinally flexible both in the expanded and unexpanded condition such that they can be tracked easily down tortuous lumens and such that they conform relatively closely to the compliance of the vessel after deployment. While these stents are flexible, they also have relatively unstable support after expansion. Furthermore, these stents leave large portions of the vessel wall uncovered, allowing tissue and plaque prolapse into the lumen of the vessel.  
           [0010]    Thus, it is desired to have a stent which exhibits longitudinal flexibility before expansion such that it can easily be tracked down tortuous lumens and longitudinal flexibility after expansion such that it can comply with the vessel&#39;s natural flexibility and curvature while still providing continuous, stable coverage of a vessel wall that will minimize tissue sag into the lumen.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0011]    Accordingly, an object of the invention is to provide a stent that is longitudinally flexible before expansion so that it can easily be tracked down tortuous vessels and remains longitudinally flexible after expansion such that it will substantially eliminate any stress points by complying with the vessel&#39;s flexibility and assuming the natural curve of the vessel.  
           [0012]    Another object of the present invention is to provide a stent that is longitudinally flexible after delivery such that it flexes during the cycles of the heartbeat to reduce cyclic stress at the ends of the stent and along the stent.  
           [0013]    Another object of the present invention is to provide a stent with a closed cell pattern such that it provides good coverage and support to a vessel wall after expansion.  
           [0014]    Other advantages of the present invention will be apparent to those skilled in the art.  
           [0015]    In accordance with these objects, the stent of the present invention is formed to be a tube having a patterned shape which has first and second meander patterns having axes extending in first and second direction wherein the second meander patterns are intertwined with the first meander patterns.  
           [0016]    In accordance with one embodiment of the invention, the intertwined meander patterns form cells which have three points at which the first and second meander patterns meet each other, and which in this sense could be called triangular cells. These three cornered or triangular cells are flexible about the longitudinal axis of the stent after expansion. These triangular cells provide comparable scaffolding and radial strength to that of cells formed by intertwined meander patterns which have four points at which the first and second patterns meet each other, and which in this sense could be called square cells.  
           [0017]    In another embodiment of the invention, bands of cells are provided along the length of a stent. The bands of cells alternate between cells adapted predominantly to enhance radial support with cells that are adapted predominantly to enhance longitudinal flexibility after expansion.  
           [0018]    In another embodiment of the invention, the first meander patterns are adapted to prevent any “flaring out” of loops of the first meander patterns during delivery of the stent.  
           [0019]    A stent according to the invention retains the longitudinal flexibility associated with the &#39;303 cellular stent in its unexpanded state, and has increased longitudinal flexibility in the expanded state. The stent does so without sacrificing scaffolding—i.e. coverage of the vessel wall—or radial support. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 shows a schematic diagram of a conventional rigid stent deployed in a curved lumen;  
         [0021]    [0021]FIG. 2 shows a schematic diagram of a stent of the present invention deployed in a curved lumen;  
         [0022]    [0022]FIG. 3 shows a pattern for a stent made in accordance with the present invention;  
         [0023]    [0023]FIG. 4 shows an enlarged view of one cell of the pattern of FIG. 3;  
         [0024]    [0024]FIG. 5 shows a pattern for a stent made in accordance with the present invention;  
         [0025]    [0025]FIG. 6 shows an enlarged view of one cell of the pattern of FIG. 5;  
         [0026]    [0026]FIG. 7 shows a pattern for a stent made in accordance with the present invention;  
         [0027]    [0027]FIG. 8 shows an enlarged view of one cell used in the pattern of FIG. 7;  
         [0028]    [0028]FIG. 9 shows an enlarged view of another cell used in FIG. 7;  
         [0029]    [0029]FIG. 10 shows a schematic comparison of a four cornered or “square cell” and a three cornered or “triangular” cell of the present invention.  
         [0030]    [0030]FIG. 11 shows a pattern for a stent constructed according to the principles of the invention which has variable geometry along its length. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    [0031]FIG. 2 shows a schematic diagram of a longitudinally flexible stent  208  of the present invention. The stent  208  may be delivered to a curved vessel  210  by a balloon catheter, and implanted in the artery by inflating the balloon. As described before, the balloon causes the artery to straighten upon inflation of the balloon. However, upon deflation of the balloon, the stent  208  assumes the natural curve of the vessel  210  because it is and remains longitudinally flexible after expansion. This reduces any potential stress points at the ends of the stent and along the length of the stent. Furthermore, because the stent is longitudinally flexible after expansion, the stent will flex longitudinally with the vessel during the cycles caused by a heartbeat. This also reduces any cyclic stress at the ends of the stent and along the length of the stent.  
         [0032]    [0032]FIG. 3 shows a pattern of a stent according to the present invention. This pattern may be constructed of known materials, and for example stainless steel, but it is particularly suitable to be constructed from NiTi. The pattern can be formed by etching a flat sheet of NiTi into the pattern shown. The flat sheet is formed into a stent by rolling the etched sheet into a tubular shape, and welding the edges of the sheet together to form a tubular stent. The details of this method of forming the stent, which has certain advantages, are disclosed in U.S. Pat. Nos. 5,836,964 and 5,997,973, which are hereby expressly incorporated by reference. Other methods known to those of skill in the art such as laser cutting a tube or etching a tube may also be used to construct a stent which uses the present invention. After formation into a tubular shape, an NiTi stent is heat treated, as known by those skilled in the art, to take advantage of the shape memory characteristics of NiTi and its superelasticity.  
         [0033]    The pattern  300  is formed from a plurality of each of two orthogonal meander patterns which patterns are intertwined with each other. The term “meander pattern” is taken herein to describe a periodic pattern about a center line and “orthogonal meander patterns” are patterns whose center lines are orthogonal to each other.  
         [0034]    A meander pattern  301  is a vertical sinusoid having a vertical center line  302 . A meander pattern  301  has two loops  304  and  306  per period wherein loops  304  open to the right while loops  306  open to the left. Loops  304  and  306  share common members  308  and  310 , where member  308  joins one loop  304  to its following loop  306  and member  308  joins one loop  306  to its following loop  304 .  
         [0035]    A meander pattern  312  (two of which have been shaded for reference) is a horizontal pattern having a horizontal center line  314 . A horizontal meander pattern  312  also has loops labeled  316 ,  318 ,  320 ,  322 , and between the loops of a period is a section labeled  324 .  
         [0036]    Vertical meander pattern  301  is provided in odd and even (o and e) versions which are 180° out of phase with each other. Thus, each left opening loop  306  of meander pattern  301   o  faces a right opening loop  304  of meander pattern  301   e  and a right opening loop  304  of meander pattern  301   o  faces a left opening loop  306  of meander pattern  301   e.    
         [0037]    The horizontal meander pattern  312  is also provided in odd and even forms. The straight sections  324  of the horizontal meander pattern  312   e  intersect with every third common member  310  of the even vertical meander pattern  301   e.  The straight sections  324  of the horizontal meander pattern  312   o  also intersect with every third common member  310  of the odd vertical meander pattern  301 .  
         [0038]    Upon expansion of the stent, the loops of the vertical meander patterns  301  open up in the vertical direction. This causes them to shorten in the horizontal direction. The loops in the horizontal meander pattern  312  open up both in the vertical direction and the horizontal direction, compensating for the shortening of the loops of the vertical meander patterns.  
         [0039]    A stent formed from the pattern of FIG. 3 and made of NiTi is particularly well suited for use in the carotid artery or other lumens subject to an outside pressure. One reason is that because the stent is formed of NiTi, it is reboundable, which is a desirable property for stents placed in the carotid artery. The other reason is that the stent of FIG. 3 offers excellent scaffolding, which is particularly important in the carotid artery. Scaffolding is especially important in the carotid artery because dislodged particles in the artery may embolize and cause a stroke.  
         [0040]    [0040]FIG. 4 is an expanded view of one flexible cell  500  of the pattern of FIG. 3. Each flexible cell  500  includes: a first member  501  having a first end  502  and a second end  503 ; a second member  504  having a first end  505  and a second end  506 ; a third member  507  having a first end  508  and a second end  509 ; and a fourth member  510  having a first end  511  and a second end  512 . The first end  502  of the first member  501  is joined to the first end  505  of the second member  504  by a first curved member  535  to form a first loop  550 , the second end  506  of the second member  504  is joined to the second end  509  of the third member  508  by a second curved member  536 , and the first end  508  of the third member  507  is joined to the first end  511  of the fourth member  510  by a third curved member  537  to form a second loop  531 . The first loop  530  defines a first angle  543 . The second loop  531  defines a second angle  544 . Each cell  500  also includes a fifth member  513  having a first end  514  and a second end  515 ; a sixth member  516  having a first end  517  and a second end  518 ; a seventh member  519  having a first end  520  and a second end  521 ; an eighth member  522  having a first end  523  and a second end  524 ; a ninth member  525  having a first end  526  and a second end  527 ; and a tenth member having a first end  529  and a second end  530 . The first end  514  of the fifth member  513  is joined to the second end  503  of the first member  501  at second junction point  542 , the second end  515  of the fifth member  513  is joined to the second end  518  of the sixth member by a curved member  539  to form a third loop  532 , the first end  517  of the sixth member  516  is joined to the first end  520  of the seventh member  519  by a fifth curved member  548 , the second end  521  of the seventh member  519  is joined to the second end  524  of the eighth member  522  at third junction point  540  to form a fourth loop  533 , the first end  523  of the eighth member  522  is joined to the first end  526  of the ninth member  525  by a sixth curved member  549 , the second end  526  of the ninth member  525  is joined to the second end  530  of the tenth member  528  by a seventh curved member  541  to form a fifth loop  534 , and the first end  529  of the tenth member  528  is joined to the second end  512  of the fourth member  510 . The third loop  532  defines a third angle  545 . The fourth loop  533  defines a fourth angle  546 . The fifth loop  534  defines a fifth angle  547 .  
         [0041]    In the embodiment shown in FIG. 4, the first member  501 , the third member  507 , the sixth member  516 , the eighth member  522 , and the tenth member  528  have substantially the same angular orientation to the longitudinal axis of the stent and the second member  504 , the fourth member  510 , the fifth member  513 , the seventh member  519 , and the ninth member  512  have substantially the same angular orientation to the longitudinal axis of the stent. In the embodiment shown in FIG. 4, the lengths of the first, second, third and fourth members  501 ,  504 ,  507 ,  510  are substantially equal. The lengths of the fifth, sixth, seventh, eighth, ninth and tenth members  513 ,  516 ,  519 ,  522 ,  525 ,  528  are also substantially equal. Other embodiments where lengths of individual members are tailored for specific applications, materials of construction or methods of delivery are also possible, and may be preferable for them.  
         [0042]    Preferably, the first, second, third, and fourth members  501 ,  504 ,  507 ,  510  have a width that is greater than the width of the fifth, sixth, seventh, eighth, ninth, and tenth members  513 ,  516 ,  519 ,  522 ,  525 ,  528  in that cell. The differing widths of the first, second, third, and fourth members and the fifth, sixth, seventh, eighth, ninth, and tenth members with respect to each other contribute to the overall flexibility and resistance to radial compression of the cell. The widths of the various members can be tailored for specific applications. Preferably, the fifth, sixth, seventh, eighth, ninth, and tenth members are optimized predominantly to enable longitudinal flexibility, both before and after expansion, while the first, second, third, and fourth members are optimized predominantly to enable sufficient resistance to radial compression to hold a vessel open. Although specific members are optimized to predominantly enable a desired characteristic, all the portions of the cell interactively cooperate and contribute to the characteristics of the stent.  
         [0043]    [0043]FIGS. 5 and 6 show a pattern and an expanded view of one cell of an embodiment of the present invention which is specially adapted for a stent made of stainless steel. The pattern is similar to the pattern of FIGS. 3 and 4, and the same reference numerals are used to indicate the generally corresponding parts.  
         [0044]    In this embodiment of the invention, for example, the second loops  531  are made stronger by shortening the third and fourth members  507 ,  510 . This helps assure that the second loops do not “flare out” during delivery of the stent through tortuous anatomy. This “flaring out” is not a concern with NiTi stents which are covered by a sheath during delivery.  
         [0045]    Furthermore, the length of the members in this embodiment may be shorter than the length of the corresponding members in the embodiment illustrated in FIGS. 3 and 4. Typically, the amount of strain allowed in a self-expanding NiTi stent may be around 10%. In a stainless steel stent, the amount of strain allowed typically may be 20% or greater. Therefore, to facilitate stents made of NiTi and stents made of stainless steel expanding to comparable diameters, the members of the NiTi stent may be longer than the members of a stainless steel stent.  
         [0046]    [0046]FIG. 7 illustrates another aspect of the present invention. The stent of FIG. 7 is also constructed from orthogonal meander patterns  301 ,  302 . The meander patterns form a series of interlocking cells  50 ,  700  of two types. The first type of cell  50  is taught by U.S. Pat. No. 5,733,303. These cells are arranged so that they form alternating bands  704  of first type of cells  50  and bands  706  of the second type of cells  700 .  
         [0047]    As seen in FIG. 8 and particularly with respect to the cell labeled for ease of description, each of the &#39;303 cells  50  has a first longitudinal apex  100  and a second longitudinal end  78 . Each cell  50  also is provided with a first longitudinal end  77  and a second longitudinal apex  104  disposed at the second longitudinal end  78 . Each cell  50  also includes a first member  51  having a longitudinal component having a first end  52  and a second end  53 ; a second member  54  having a longitudinal component having a first end  55  and a second end  56 ; a third member  57  having a longitudinal component having a first end  58  and a second end  59 ; and a fourth member  60  having a longitudinal component having a first end  61  and a second end  62 . The stent also includes a first loop or curved member  63  defining a first angle  64  disposed between the first end  52  of the first member  51  and the first end  55  of the second member  54 . A second loop or curved member  65  defining a second angle  66  is disposed between the second end  59  of the third member  57  and the second end  62  of the fourth member  60  and is disposed generally opposite to the first loop  63 . A first flexible compensating member (or a section of a longitudinal meander pattern)  67  having curved portion and two legs with a first end  68  and a second end  69  is disposed between the first member  51  and the third member  57  with the first end  68  of the first flexible compensating member  67  joined to and communicating with the second end  53  of the first member  51  and the second end  69  of the first flexible compensating member  67  joined to and communicating with the first end  58  of the third member  57 . The first end  68  and the second end  69  are disposed a variable longitudinal distance  70  from each other. A second flexible compensating member (or, a section of a longitudinal meander pattern)  71  having a first end  72  and a second end  73  is disposed between the second member  54  and the fourth member  60 . The first end  72  of the second flexible compensating member  71  is joined to and communicates with the second end  56  of the second member  54  and the second end  73  of the second flexible compensating member  71  is joined to and communicates with the first end  61  of the fourth member  60 . The first end  72  and the second end  73  are disposed a variable longitudinal distance  74  from each other. In this embodiment, the first and second flexible compensating members, and particularly the curved portion thereof,  67  and  71  are arcuate.  
         [0048]    The second type of cell  700  is illustrated in FIG. 9 and the same reference numerals are used to indicate generally corresponding areas of the cell. The apices  100 ,  104  of the second type of cell  700  are offset circumferentially. Also, each flexible compensating member  67 ,  71  includes: a first portion or leg  79  with a first end  80  and a second end  81 ; a second portion or leg  82  with a first end  83  and a second end  84 ; and a third portion or leg  85  with the first end  86  and a second end  87 , with the second end  81  and the second end  84  being joined by a curved member and the first end  83  and the first end  86  being joined by a curved member. The first end of a flexible compensating member  67 ,  71  is the same as the first end  80  of the first portion  79 , and the second end of a flexible compensating member  67 ,  71  is the same as the second end  87  of the third portion  85 . A first area of inflection  88  is disposed between the second end  81  of the first portion  79  and the second end  84  of the second portion  82  where the curved portion joining them lies. A second area of inflection  89  is disposed between the first end  83  of the second portion  82  and the first end  86  of the third portion  85  where the curved portion joining them lies.  
         [0049]    While FIG. 7 illustrates a pattern of alternating bands of cells, the stent may be optimized for a particular usage by tailoring the configuration of the bands. For example, the middle band of the second type of cells  700  may instead be formed of cells  50 , or vice versa. The second type of cells in FIG. 7 may also utilize the cell configurations described with respect to FIGS. 4 and 6. The cell configurations of FIGS. 4 and 6 provide the advantage that they will not cause any torque of one portion of the cell relative to another portion of the cell about the longitudinal axis of the stent upon expansion, which may happen when the second type of cells  700  expand, a torque which could cause a stent to deform, and stick out.  
         [0050]    As illustrated in FIG. 7, all of the flexible compensating members are arranged so that the path of the flexible compensating members, from left to right, travels in a generally downward direction. The cells  700  can also be arranged so that the flexible compensating members in one band are arranged in a generally upward direction, and the flexible compensating members in an adjacent band are arranged in a generally downward direction. One skilled in the art can easily make these modifications.  
         [0051]    [0051]FIG. 10 is a schematic representation comparing the cells  804  of the present invention, which have three points where the intertwined first and second meander patterns meet and are in that sense three cornered or triangular cells, with cells  802  of the &#39;303 stent which have four points where the intertwined first and second meander patterns meet and are in that sense four cornered or square cells. More particularly, on the left side of FIG. 10, a pair of vertical meander patterns  806 ,  826  are joined by members  808 ,  810 ,  812  (which are sections of longitudinal meander patterns) to form a plurality of three cornered or triangular cells  804 . By triangular cell, it is meant that there are three sections  810 ,  812 ,  814 , each having loop portions and three associated points  816 ,  818 ,  820  of their joining, forming each cell.  
         [0052]    On the right side of FIG. 10, a pair of vertical meander patterns  822 ,  824  are joined together compensating members  828 ,  830 ,  832 ,  834  (which are sections of a longitudinal meander) to form a plurality of square cells  804 . By square cell, it is meant that there are four sections, each having loop portions, and four associated points of their joining, forming each cell. For example, the shaded cell  802  is formed from four sections  832 ,  836 ,  830 ,  838 , with four associated points of their joining  840 ,  842 ,  844 ,  846 .  
         [0053]    Both the square cell and the triangular cell have two kinds of sections with loops. The first kind of loop containing section is formed from a vertical meander pattern and is optimized predominantly to enable radial support. The second kind of loop containing section is optimized predominantly to enable flexibility along the longitudinal axis of the stent. Although each loop containing section is optimized predominantly to enable a desired characteristic of the stent, the sections are interconnected and cooperate to define the characteristics of the stent. Therefore, the first kind of loop containing section contributes to the longitudinal flexibility of the stent, and the second kind of loop containing section contributes to the radial support of the stent.  
         [0054]    In the square cell  802 , it can be seen that the second kind of loop containing sections  830 ,  832  each have one inflection point  848 ,  850 . In the triangular cell, the loop containing sections  810 ,  812  each have two inflection point areas  852 ,  854 ,  856 ,  858 . The higher number of inflection points allows more freedom to deform after expansion of the stent and distributes the deformation over a longer section, thus, reducing the maximal strain along these loop containing sections.  
         [0055]    Furthermore, it can be seen that a square cell  802  is generally more elongated along the longitudinal axis of the stent than a triangular cell  804 , which is generally more elongated along the circumference of the stent. This also contributes to higher flexibility after expansion.  
         [0056]    If the first meander patterns  806 ,  822 ,  824 ,  826  of both types of cells are constructed identically and spaced apart by the same amount, the area of a triangular cell  804  is the same as a square cell  802 . This can be more readily understood with reference to a band of cells around the circumference of a stent. Each band will encompass the same area, and each band will have the same number of cells. Accordingly, the area of each cell in one band formed of square cells will be the same as the area of each cell in another band formed of triangular cells.  
         [0057]    Although the areas of the cells are equal, the perimeter of the triangular cell is larger than the perimeter of the square cell. Therefore, in comparison to a square cell, a triangular cell offers increased coverage of a vessel wall.  
         [0058]    In the particular embodiments described above, the stent is substantially uniform over its entire length. However, other applications where portions of the stent are adapted to provide different characteristics are also possible. For example, as shown in FIG. 11, a band of cells  850  may be designed to provide different flexibility characteristics or different radial compression characteristics than the remaining bands of cells by altering the widths and lengths of the members making up that band. Or, the stent may be adapted to provide increased access to a side branch lumen by providing at least one cell  852  which is larger in size then the remaining cells, or by providing an entire band of cells  854  which are larger in size than the other bands of cells. Or, the stent may be designed to expand to different diameters along the length of the stent. The stent may also be treated after formation of the stent by coating the stent with a medicine, plating the stent with a protective material, plating the stent with a radiopaque material, or covering the stent with a material.  
         [0059]    Thus, what is described is a longitudinally flexible stent that utilizes a closed cell structure to provide excellent coverage of the vessel wall. The general concepts described herein can be utilized to form stents with different configurations than the particular embodiments described herein. For example, the general concepts can be used to form bifurcated stents. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention is defined by the claims which follow.