Abstract:
According to one aspect of the preset invention, a fatigue resistant stent includes a flexible tubular structure having an inside diameter, an outside diameter, and a sidewall therebetween and having apertures extending through the sidewall. According to other aspects of the invention, processes for making a fatigue resistant stent are disclosed. According to further aspects of the invention, delivery systems for a fatigue resistant stent and methods of use are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 12/758,687, filed Apr. 12, 2010, which claims the benefit of and priority to U.S. Provisional Patent Application No. 61/168,215, filed on Apr. 10, 2009, the contents of both applications are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an implant having high resistance to fracture when mechanically cycled at a site in a body lumen. More particularly, this invention pertains to a fatigue fracture resistant vascular implant such as a self-expanding stent. 
       BACKGROUND OF THE INVENTION 
       [0003]    Stents are widely used for maintaining an open lumen in a patient&#39;s body. For example, stents may be used to maintain patency of a coronary artery, carotid artery, cerebral artery, popliteal artery, iliac artery, femoral artery, tibial artery, renal artery, other blood vessels including veins, or other body lumens such as the ureter, urethra, bronchus, esophagus, tear duct, fallopian tube, nasal cavity, or other passage. 
         [0004]    Stents are commonly metallic tubular structures made from stainless steel, Nitinol, Elgiloy, cobalt chrome alloys, tantalum, and other metals, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, or metallic, and may elute over time substances such as drugs. Non-bioabsorbable stents may also release drugs over time. In certain designs, stents are open-celled or closed-celled cylindrical structures. Stents are passed through a body lumen in a collapsed state. At the point of an obstruction or other deployment site in the body lumen, the stem is expanded to an expanded diameter to support the luminal wall and maintain an open lumen at the deployment site. 
         [0005]    One type of stent is often referred to as a “balloon expandable” stent. Stent delivery systems for balloon expandable stents are typically comprised of an inflatable balloon mounted on a multi lumen tube. The stent delivery system with stent crimped thereon can be advanced to a treatment site, often over a guidewire, and the balloon inflated to expand and deploy the stent. 
         [0006]    In the case of a shape memory stent the stent is pre-programmed to remember a reduced diameter or shape at one temperature and an expanded diameter or shape at a higher temperature. The stent is compressed onto the distal end of a stent delivery system at a cold temperature and the stent delivery system with stent crimped thereon can be advanced to a treatment site, often over a guidewire, while being maintained below a phase change temperature of the stent. At the deployment site, the stent is warmed or allowed to warm to a higher temperature at which a phase change of the shape memory material results in expansion of the stent from a collapsed state to an expanded state. 
         [0007]    Other stents are so-called “self expanding” stents and do not use balloons to cause the expansion of the stent. An example of a self-expanding stent is a tube (e.g., a coil tube, a mesh tube, or a tube comprised of formed wire with or without welded wire junctions) made of an elastically deformable material (e.g., a superelastic material such a nitinol). Open or closed cell stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods. A very popular type of self expanding stent is made from superelastic nitinol, for example, the EverFlex stent made by ev3, Inc. of Plymouth, Minn. 
         [0008]    Self expanding stents are commonly secured to a stent delivery system under radial compression or under axial tension in a collapsed state. Such a system can be comprised of an outer tubular member and an inner tubular member. The inner and outer tubular members are axially slideable relative to one another. The stent (in the collapsed state) is mounted on the stent delivery system surrounding the distal end of the inner tubular member. The outer tubular member (also called the outer sheath) surrounds the stent at the distal end. 
         [0009]    Prior to advancing the stent delivery system through the body lumen, a guide wire is first passed through the body lumen to the deployment site. The inner tube of the delivery system is hollow throughout at least a portion of its length such that it can be advanced over the guide wire to the deployment site. The combined structure (i.e., stent mounted on stent delivery system) is passed through the patient&#39;s lumen until the distal end of the delivery system arrives at the deployment site without the body lumen. The deployment system and/or the stent may include radiopaque markers to permit a physician to visualize positioning of the stent under fluoroscopy pilot to deployment. At the deployment site, the outer sheath is retracted to expose the stent. The exposed stent is free to self-expand within the body lumen. Following expansion of the stent, the inner tube is free to pass through the stent such that the delivery system can be removed through the body lumen leaving the stent in place at the deployment site. 
         [0010]    Stent deliver systems may be comprised of an over the wire (OTW), rapid exchange (RX), or fixed wire (FW) delivery catheter. OTW delivery catheters allow a guidewire to pass through a lumen that extends over the entire length of the delivery catheter. RX delivery catheters allow a guidewire to pass through a lumen that extends over a partial length (usually 10-30 cm) of the delivery catheter. OTW delivery catheters provide better support than RX delivery systems yet they requite the use of longer guidewires which can be cumbersome to handle. FW delivery systems are very simple in that they do not have a guidewire lumen. The FW system is advanced to a treatment site without the benefit of tracking over a pre-placed guidewire. To assure that a stent can be delivered to the intended treatment site stent delivery catheters with stent mounted thereon must have at least adequate trackability, flexibility, and kink resistance and stents, once implanted, must have at least adequate radial force, kink resistance, and fatigue life. 
         [0011]    Stents implanted in some locations can require different physical attributes than stents implanted in other locations. Stents used in coronary or renal arteries can be fairly short because the stenosed region of the vessel is generally fairly short whereas stents used in the legs often must be very long so that disease typical of leg vessels can be treated without overlapping short stents. 
         [0012]    In other examples, stents implanted in arteries must have high pulsatile fatigue life to withstand the small diameter changes caused by the artery diameter changes that occur with every heart beat whereas stents implanted in non-vascular conduits do not require this attribute. In a further example, stents implanted in limb vessels such as the superficial femoral artery (SFA) must have high fatigue life to withstand vessel length and orientation changes that occur when the leg is bent or straightened whereas stents implanted in coronary vessels need not meet this requirement. Long stents also must be forgiving of having an implanted length quite different (usually longer) from the design length as a result of physician technique during stent implantation and the practical limitations of stent delivery systems. 
         [0013]    Unfortunately, a common problem with stents deployed in some vessels such as the SFA is that the stents fracture over time due to the dynamic loading environment of the vessel. Problems secondary to stent fracture can include intimal hyperplasia, pain, bleeding, vessel occlusion, vessel perforation, high in-stent restenosis rate, non-uniform drug delivery profile, non-uniform vessel coverage and other problems, and re-intervention may be required to resolve the problems. Stent fracture rates in the SFA range from 2% to 28% at 1-3 years after implantation, as reported in various clinical trials (Durability I, Resilient, Scirocco and Absolute) and research papers (for example Prevalence and clinical impact of stent fractures after femoropoliteal stenting; J. Am. Coll. Cardiol. 2005 Jan. 18; 45(2): 312) and Long-Segment SFA stenting—The Dark Sides: In-Stent Restenosis, Clinical Deterioration, and Stent Fractures; J Endovasc Ther 2005; 12:676-684. 
         [0014]    While stents commonly achieve high pulsatile fatigue life, they often have insufficient resistance to fracture under in-patient loading conditions other than pulsatile, such as longitudinal extension, torsion, and flexion, as is appropriate for some implantation sites. Stents implanted in the popliteal artery, iliac artery, femoral artery, tibial artery, carotid, femoral-popliteal and other sites can suffer from large amounts of axial, flexaral, or torsional cyclic loading. Recent data has indicated that some stents, such as those implanted in the SFA, must withstand axial elongation/compression of up to 35% as implanted. While attempts have been made to improve the fatigue resistance of implantable stents, an implant having suitable attributes (especially high fatigue life) for these demanding anatomical locations has yet to be developed. 
         [0015]    What is needed is a stent that can be easily manufactured and that will survive without fracture when implanted in locations that experience high mechanical forces produced by patient activity in addition to those forces produced by the beating heart. Also needed is a stent which will have high fatigue life after implantation and a stent that will survive without fracture despite elongation during implantation. 
       SUMMARY OF THE INVENTION 
       [0016]    According to one aspect of the present invention, a fatigue resistant stent comprises a flexible tubular structure having an inside diameter, an outside diameter, and a sidewall therebetween and having apertures extending through the sidewall. According to other aspects of the invention, processes for making a fatigue resistant stent are disclosed. According to further aspects of the invention, delivery systems for a fatigue resistant stent and methods of use are provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
           [0018]      FIG. 1  illustrates a schematic isometric view of one embodiment of a prior art stent; 
           [0019]      FIG. 1A  schematically illustrates the stent of  FIG. 1  with the stent elongated axially; 
           [0020]      FIGS. 2 and 2A  illustrate an enlarged view of a portion of the stent of  FIG. 1 . The stent is shown diametrically expanded and the stent structure is shown cut longitudinally and laid flat; 
           [0021]      FIG. 3  illustrates the stent of  FIG. 2  with the stent elongated axially. The stent is shown diametrically expanded and the stent structure is shown cut longitudinally and laid flat; 
           [0022]      FIG. 3A  illustrates an isometric view of a portion of the stent in  FIG. 3  with indicia of calculated strain distributions within the stent; 
           [0023]      FIG. 4  illustrates a portion of a fatigue resistant stent in accordance with the principles of the present disclosure. The stent is shown diametrically expanded and the stent structure is shown cut longitudinally and laid flat; 
           [0024]      FIG. 4A  illustrates an isometric view of a portion of the stent in  FIG. 4  with indicia of calculated strain distributions within the stent; 
           [0025]      FIG. 5  illustrates the stent of  FIG. 4  with the stent elongated axially. The stent is shown diametrically expanded and the stent structure is shown cut longitudinally and laid flat; 
           [0026]      FIGS. 6 and 7  illustrate scale drawings of the geometry and dimensions of exemplar stent cells. The stent cells are shown diametrically compressed and the stent structure is shown cut longitudinally and laid flat; 
           [0027]      FIGS. 8A, 8B, 8C and 9  illustrate plan views of exemplar delivery systems for fatigue resistant stents. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    With reference now to the various drawing figures a description is provided of embodiments that are examples of how inventive aspects in accordance with the principles of the present invention way be practiced. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad inventive aspects disclosed herein. It will also be appreciated that the inventive concepts disclosed herein are not limited to the particular stent configurations disclosed herein, but are instead applicable to any number of different stent configurations. 
         [0029]      FIGS. 1 and 2  illustrate prior art stent  10  having a length L, an outer circumference C, and a longitudinal axis A-A.  FIG. 2  shows a portion of prior art stent  10  fully diametrically expanded, cut longitudinally parallel to axis A-A, and laid fiat. Stent  10  is comprised of a tube having outer diameter  12 , inner diameter  14  and wall  16  having a thickness. Openings are cut into wall  16  to form cells  15  (shaded in  FIG. 2 ) and struts  17 . Each individual strut has a length  11  between apices  19  and a thickness  13 . Adjacent struts are joined at apices  19 ,  19 A and have an intersection angle  18 ,  18 A. Adjacent cells are joined by connectors  21 . Connectors  21  on opposite sides of cell  15  are circumferentially offset from one another. Circumferentially offset connectors provide stent  10  with increased axial, bending and torsional flexibility and improved kink resistance when compared to other stent designs having connectors  21  directly opposed along axis A-A. Cell length  22  can be defined as the length in the direction of axis A-A between connectors  21  at either end of a cell. 
         [0030]    Stent  10  can be made from stainless steel, Nitinol, Elgiloy, cobalt chrome alloys, tantalum, and other metals, or polymers, can be a permanent enduring implant, can be bioabsorbable at least in part, and may elute over time substances such as drugs. Cells  15  of stent  10  may be formed by means such as laser cutting followed by processes such as microgrit blasting to remove slag, electropolishing to remove stent material having heat affected zone and other imperfections, and surface passivation to render surface of the stent more resistant to corrosion. 
         [0031]    When diameter  12  of stent  10  is reduced the associated reduction in stent circumference C is accommodated by reduction, in angle  18  between struts with associated increase in bending strain in struts  17  and especially in the vicinity of apices  19 . The amount of strain concentrated near the apices can be calculated using finite element analysis (FEA, discussed below) or by other methods. 
         [0032]    The strain in a single strut in the vicinity of an apex can also be approximated with the help of the following formula (with reference to  FIGS. 2 and 2A ). In  FIG. 2A  stent  10  has been reduced in circumference with no change in length  22  as compared to stent  10  in  FIG. 2 . The strain induced in the strut neat apex  19 A by this change is calculated as follows: 
         [0000]    
       
         
           
             
               Cha 
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               nge 
                
               
                   
               
                
               in 
                
               
                   
               
                
               Strain 
                
               
                   
               
                
               in 
                
               
                   
               
                
               strut 
             
             = 
             
               
                 SW 
                  
                 
                     
                 
                 [ 
                 
                   
                     sin 
                      
                     
                       ( 
                       θ2 
                       ) 
                     
                   
                   - 
                   
                     sin 
                      
                     
                       ( 
                       θ1 
                       ) 
                     
                   
                 
                 ] 
               
               
                 [ 
                 
                   LS 
                   - 
                   
                     SW 
                      
                     
                         
                     
                      
                     
                       sin 
                        
                       
                         ( 
                         θ2 
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                 ] 
               
             
           
         
       
       
         
           
             where: 
             LS=Length of Strut  11   
             SW=Strut Width  13   
             θ 2 =Final Angle  18   c    
             θ 1 =Initial Angle  18 A 
           
         
       
     
         [0038]    In the case of a stent having no strain in the strut near an apex when the stent is fully expanded (as is usually the case for a self-expanding stem), the initial strain in the stent near the apex is zero, so the strain in the strut near the apex of the stent when deformed equals that calculated using the above formula. Importantly, from the formula we learn that the calculated change in strut strain is lower when the change in angle  18  is lower, lower when the strut width SW is less, and lower when the stent length LS increases. 
         [0039]    The maximum strain in a deformed stent is called the peak strain. The peak stein typically occurs in a single strut in the vicinity of an apex although it may occur elsewhere in the stent depending on the details of stent design. Predominant strains in stents may be tensile (usually defined as positive) or compressive (usually defined as negative). Stents in some implant locations, for example the SFA or popliteal artery, can suffer bending strains and torsional strains in addition to the tensile and compressive strains mentioned above. In some stent embodiments such torsional and bending strains on the stent will resolve primarily into tensile or compressive strains in the vicinity of stent apices. High tensile strains can cause cracks to initiate and propagate through the stent, leading to reduced fatigue life and stent fracture. Compressive strains do not tend to open cracks so generally do not cause reduced stent life unless the magnitude of the strain is extraordinary. Some portions of a deformed stent may be highly strained during use while other portions may not be strained at all, said latter portions having zero strain. A deformed stent can be thought of as a collection of tiny volumetric regions, each region having a strain level; collectively the strain levels of the regions range from a maximum negative value to a maximum positive value. For a stent in service in the body, if stent strains are maintained below the endurance limit of the stent material then high fatigue life may be expected provided the stent material has undergone proper materials processing, surface finishing and has suitable biocompatibility characteristics. However, if a stent in service in the body suffers stent strains above the endurance limit of the stent material then high fatigue life cannot be expected regardless of stent material processing, surface finishing and biocompatibility characteristics. 
         [0040]    Commonly prior art stents such as stent  10  are designed such that the strain in the stent remains at a low level under pulsatile loading conditions, i.e. under oscillating circumferential compressive strains. Stent  10  is also known to accommodate a limited amount of axial elongation (at most about 10-15%) without compromising pulsatile fatigue life. This limited amount of axial elongation is suitable for some stent implant locations. However, it has been determined that stents implanted in other locations, for example the SFA or popliteal arteries, can subject a stent to very large elongations. For example the leg can impart .+−.5% elongation due to normal leg motion and the doctor may impart as much as 30% elongation and possibly more to the stent during stent implantation. In addition the stent may be loaded in torsion, and in bending as well due to normal leg motion. 
         [0041]    Stent percent elongation can be defined as a change in length of a stent cell along axis A-A.  FIG. 1  shows stent  10  at rest and having a length L as previously defined.  FIG. 1A  shows stent  10  elongated by 50% and having a length L′. Stent elongation is defined as the increase in length (stretched length L′ minus unstretched length L) divided by the unstretched length L. If the stent has been uniformly elongated over its entire length then each stent cell will be elongated by the same amount as that of the stent overall.  FIG. 2  shows un-elongated stent  10  having a cell length  22  as previously defined.  FIG. 3  shows stent  10  elongated by 50% and having a cell length  22 ′. Cell elongation is defined as the increase in cell length (cell length  22 ′ minus cell length  22 ) divided by the unstretched cell length  22 . In practice it is found that stent elongation is generally not uniform over the length of the stent, with some cells elongated more than others. It is also sometimes seen that some cells are shortened, i.e., some cells have a negative calculated elongation. It is also possible for stent overall lengths to be shortened. Stent elongation is a useful surrogate model for the prediction of stent fatigue life under more complex loading conditions. Stent elongation can cause high tensile strains in the vicinity of stent apices that can be representative of the strains imparted by more complex loading conditions such as when a stent is subjected to a combination of elongation, torsion, and bending. 
         [0042]      FIG. 3  illustrates stent  10  elongated 50% axially, denoted as stent  10 ′ in the elongated condition. Stent  10 ′ is shown diametrically expanded, cut longitudinally parallel to axis A-A and laid flat. Stent  10 ′ cell geometry is substantially distorted front the geometry of stent  10  ( FIG. 2 ). The distortion can be visualized by comparing angles  18  in the elongated stent  10 ′ to those in the non-elongated stent  10 . It can be seen that angles  18 K,  18 N,  18 P and  18   r  have reduced to angles  18 K′,  18 N′,  18 P′ and  18   r ′ respectively while angles  18 M,  18 Q,  18 S and  18 T have increased to angles  18 M′,  18 Q′,  18 S′ and  18 T′ respectively. The distorted cell geometry seen in  FIG. 3  causes reduced fatigue life of elongated stent  10 ′ by increasing the tensile strain in some stent struts to high levels. 
         [0043]    Specifically, as illustrated  FIG. 3 , the tensile strain in the strut near apices  19 X has increased because of the increased angles  18 M′,  18 Q′,  18 S′ and  18 T′ between some stents, thereby exposing the struts near apices  19 X to lower fatigue life. At other regions of the stent cell  15 , stats near apices  19 Y are loaded in compression due to the decreased angles  18 K,  18 N,  18 P and  18   r  between struts, thereby permitting the struts near apices  19 Y to sustain higher fatigue life. Stated another way, elongation of stent  10  results in concentration of tensile strains in the stent volume in the vicinity of apices  19 X and concentration of compressive strains in the stent volume in the vicinity of apices  19 Y. 
         [0044]      FIG. 4  illustrates one embodiment of an inventive stent that, when elongated, has few or no highly tensioned regions. Stent  40  has a length, an outer circumference, and a longitudinal axis A-A. In  FIG. 4  a portion of stent  40  is shown fully diametrically expanded, cut longitudinally parallel to axis A-A, and laid flat. Stent  40  is comprised of a tube having an outer diameter, an inner diameter and a wall  46  having a thickness. Openings are cut into wall  46  to form cells  45  (shaded in  FIG. 4 ) and struts  47 , and cells  45  have a perimeter comprised of the length of the opening bounded by struts  47 . Each Individual strut has a length  41  between apices  49  and a thickness  43 . Adjacent struts are joined at apices  49  and have an intersection angle  48 . Adjacent cells are joined by connectors  51 . In some embodiments connectors  51  on opposite sides of cell  45  are aligned along axis A-A. In these embodiments the asymmetry of the cell  45  perimeter, which is comprised of relatively thinned struts (discussed below) over a portion of the cell perimeter, provide stent  40  with increased axial, bending and torsional flexibility and improved kink resistance when compared to other stent designs having connectors  51  directly opposed along axis A-A. In other embodiments connectors  51  on opposite sides of cell  45  are circumferentially offset from one another. Circumferentially offset connectors can also provide stent  40  with increased axial, bending and torsional flexibility and improved kink resistance when compared to some stent designs having connectors  51  directly opposed along axis A-A. Cell length  52  can be defined as the length in the direction of axis A-A between connectors  51  at either end of a cell. 
         [0045]    In some embodiments, stent  40  may be comprised of stainless steel, Nitinol, Elgiloy, cobalt chrome alloys, tantalum, and other metals, or polymers, may be a permanent enduring implant, may be bioabsorbable at least in part, and may elute over time substances such as drugs. In some embodiments, stent cells  45  of stent  40  may be formed by means such as laser cutting, followed by processes such as microgrit blasting to remove slag, electropolishing to remove stent material having heat affected zone and other imperfections, and surface passivation to render surface of the stent more resistant to corrosion. Stent  40  may be comprised of self-expanding, shape memory, or balloon expandable characteristics. In various embodiments stent  40  may be prepared from a tube using processes such as laser cutting, chemical etching, punching, grinding, drilling, EDM, or other processes. In other embodiments stent  40  may be prepared by cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, by electroforming the stent onto a suitable pattern or mold or by other methods. In further embodiments stent  40  is prepared by intertwining, joining, overlaying, weaving, braiding, knitting, circular knitting, compressing, or otherwise assembling filaments or wire. Filaments or wire may be joined at one or more regions where they overlap by sintering, bonding, soldering, fusing, welding, or other means. In structures so formed struts  47  of stent  40  are comprised of the length of filament or wire between joined regions and apices  49  of stent  40  are comprised of the joined regions. 
         [0046]    Struts  47  of stent  40  have been modified as compared to struts  17  of stent  10 . Struts  47 B are thickened along the circumferential direction of the stent and struts  47 A are both lengthened along the axial direction of the stent and thinned along the circumferential direction of the stent. In all other dimensional and materials processing aspects stent  40  is similar to stent  10 . As a result of the strut modifications described above, stent  40 , when elongated, has improved fatigue life compared to that of similarly elongated stent  10  (i.e. stent  10 ′). The improved fatigue life is accomplished because the strut modifications described prevent development of high tensile strains in the vicinity of apices  49  when stent  40  or one or more cell  45  is elongated, bent, or twisted. 
         [0047]      FIG. 5  illustrates stent  40  elongated 50% axially, denoted as stent  40 ′ in the elongated condition. Stent  40 ′ is shown fully diametrically expanded, cut longitudinally parallel to axis A-A and laid flat. Stent  40 ′ cell geometry  45 ′ is distorted from the geometry of stent  40 . The distortion can be visualized by comparing angles  48  in the elongated stern  40 ′ to those in the non-elongated stent  40 . It can be seen that angles  48 K,  4 SN,  48 P and  48 R have reduced to angles  48 K′,  48 N′,  48 P′ and  48 R′ respectively while angles  48 M,  48 Q,  48 S and  48 T have increased to angles  48 M′,  48 Q′,  48 S′ and  48 T′ respectively. Unlike stent  10 ′, the tensile strain induced by elongation of stent  40  or of one or more cell  45  in the vicinity of apices  49 X, while increased, has increased to an acceptable level because increased thickness of struts  47 B limits the amount of increase to angles  48 M′,  48 Q′ and the thinner and longer struts  47 A reduce the tensile strain induced in the vicinity of apices  49 X when angles  48 S′,  48 T′ are increased. 
         [0048]    Stated another way, elongation of stent  40  or of one or more cells  45  results in less concentration of tensile strains in the stent volume in the vicinity of apices  49 X and more concentration of compressive strains in the volume in the vicinity of apices  49 Y. The elongated cell geometry seen in  FIG. 5  allows improved fatigue life of elongated stent  40 ′ by limiting the tensile strain to acceptable levels in the vicinity of stent cell apices. 
         [0049]    Stent  40  has another characteristic that improves its fracture resistance and fatigue life when deployed in a treatment location. Within the interior of cell  45 , apices  49  are not directly opposed to one another along axis A-A, instead they are circumferentially offset from one another. Accordingly when stent  40  suffers axial compression (negative calculated elongation) non-opposed apices will not collide with or rub against each other. Such collisions or abrasion of opposed apices can cause stent surface defects to form, which can propagate with cycling over time, leading to stent fracture. 
         [0050]    Another method of calculating material strain in the vicinity of apices involves the use of Finite Element Analysis (FEA). In one method, a commercially available PEA software package (Abaqus) was utilised to calculate the strains. Key modeling parameters were as follows: use of a 20-node three dimensional brick shape second order continuum element; use of high local mesh density in high strain regions and lower mesh densities in low strain regions; and proper mesh transitions between the higher and lower mesh density regions (i.e., brick face aspect ratio&#39;s of 10 or less and included angles of the bricks between 45 and 135 degrees). To balance simulation convergence speed and computing time with simulation accuracy, trial runs were performed for high strain regions and suitable mesh density was chosen at the density where finer mesh did not change results within tolerance. 
         [0051]    Stem material properties were determined by tensile test on coupons having the same composition and similar thermo-mechanical history as the stent being modeled. Stent final geometry was determined by modeling precursor tubing and the effects of the stent manufacturing process (heat expansion, annealing, filing, honing, sand blasting and electrolytic polishing) on the modeled tubing, although other means of determining final stent geometry could be used. The final stent geometry determined by the above modeling was strained in an FEA simulation under tensile loading conditions and the maximum principal strains, mean strains and amplitude of the strains under static conditions were calculated. The final stent geometry determined by the above modeling was also strained in an FEA simulation under loading conditions of a superficial femoral artery implant location, including compaction, extension, uniaxial tensile repetitive load for fatigue, walking, and stair climbing loads. The maximum principal strains, mean strains and amplitude of the strains under cyclic conditions were calculated and fatigue life of the strained stent was predicted. 
         [0052]      FIG. 3A  illustrates partial results of an FEA analysis of strains in stent  10 ′ (stent  10 , elongated by 50%). Only the portion of stent  10 ′ having the highest strain is illustrated. It can be seen that the peak tensile strain of 6.3% is in the vicinity of apex  19 X opposite angle  18 S′. Due to the high tensile strain in the strut near this apex the strut is susceptible to low fatigue life. Also, averaging the highest strain in the vicinity of each of the  16  apices of cell  15  shows that the average strain in the vicinity of the apices is 3.0%.+−.2.6%. 
         [0053]      FIG. 4A  illustrates partial results of an FEA analysis of strains in stent  40 ′ (stem  40 , elongated by 50%). Only the portion of cell  45 ′ having the highest strain is illustrated. It can be seen that the peak tensile strain of 2.97% is in the vicinity of apex  49 X opposite angle  48 T′. Due to the reduced tensile strain compared to prior art the strut near this apex can sustain improved fatigue life compared to stent  10 . Also, averaging the highest strain to the vicinity of each of the  16  apices of cell  45 ′ shows that the average strain in the vicinity of the apices is 2.6%.+−.0.6%, also an improvement over prior an stent  10  due to both the lower average strain, and the reduced variation in strain. The reduced variation in strain causes the radial expansile force of the stent, as determined along the circumference of the stent, to be more uniform. Therefore a stent designed according to the teachings above will have more uniform deployment characteristics and will, more uniformly resist vessel threes tending to reduce the deployed diameter of the stent. 
         [0054]    The average strains in the vicinity of the  16  apices of cell  45 - 45 ′ in stent  40  have been further characterized at various amounts of elongation using FEA analysis. These results are shown in the table below. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Cell 
                 Average 
                 Standard Deviation 
               
               
                 Elongation(%) 
                 Strain 
                 Of Average Strain 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0.0 
                 0.0 
               
               
                 5 
                 0.2 
                 0.0 
               
               
                 10 
                 0.4 
                 0.1 
               
               
                 15 
                 0.6 
                 0.2 
               
               
                 20 
                 1.0 
                 0.4 
               
               
                 25 
                 1.4 
                 0.4 
               
               
                 30 
                 2.0 
                 0.4 
               
               
                 50 
                 2.6 
                 0.6 
               
               
                   
               
             
          
         
       
     
         [0055]    While stent  40  has the strain and elongation characteristics as described above, stents designed according to the teachings herein having other strain and elongation-characteristics are contemplated. In some embodiments an inventive stent can be elongated 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50% with peak strain remaining below 0.5%. In other embodiments an inventive stent can be elongated 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50% with peak strain remaining below 1%, or below 2%, or below 3%, or below 4%, or below 5%. In other embodiments a stent designed according to the teachings herein can have any peak strain and elongation combination as recited in this paragraph where the standard deviation of the peak strain is one of 0.3 percent strain, 0.5 percent strain, or 1.0 percent strain. 
         [0056]    In other embodiments stents designed according to the teachings herein having various stent cell strain and elongation characteristics are contemplated. In some embodiments an inventive stent cell can be elongated 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50% with peak strain in the vicinity of the apices of the cell, remaining below 0.5%. In other embodiments an inventive stent cell can be elongated 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50% with peak strain in the vicinity of the apices of the cell remaining below 1% or below 2%, or below 3%, or below 4%, or below 5%. In other embodiments a stem cell designed according to the teachings herein can have any peak strain and elongation combination as recited in this paragraph where the standard deviation of the peak strain is one of 0.3 percent strain, 0.5 percent strain, or 1.0 percent strain. 
         [0057]    Stents have been manufactured according to the principles of the invention and have been found to have desirable strain distributions and exceptional fatigue resistance as shown in the examples below. The examplar and competitive stents were tested for fatigue endurance according to the following methods. 
         [0058]    Uni-axial Testing Method: Using a Bose 3300 uni-axial testing machine stent life can be determined from a step fatigue test method. According to the method, step 1 consists of elongating a stent sample to the mean strain tabulated for step 1 followed by oscillating the sample length at 40 Hz for 500,000 cycles along the direction of axis A-A to the cyclic strains tabulated for step 1. if the sample does not fracture during step 1, the sample is additionally subjected to step 2. Step 2 consists of stretching and oscillating the sample as performed in step 1 but at different pre-specified mean and cyclic strains as tabulated for step 2. If the sample does not fracture during step 2, the sample is additionally sequentially stretched and oscillated for 500,000 cycles at each subsequent step (as tabulated below) until the sample fractures. The mean strain of the step at which sample fracture occurs is recorded as the stents mean strain to failure. The fatigue tests were carried out with samples immersed in 37.degree.C. water and specialized fixturing was used to grip the ends of individual stent samples. Strain values are calculated based on the initial length between grips of unstrained sample. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Mean Strain 
                 Cyclic Strain 
               
               
                 Step 
                 (%) 
                 Amplitude (± %) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 11.67 
                 1.30 
               
               
                 2 
                 13.89 
                 1.54 
               
               
                 3 
                 16.11 
                 1.79 
               
               
                 4 
                 18.33 
                 2.04 
               
               
                 5 
                 20.56 
                 2.28 
               
               
                 6 
                 22.78 
                 2.53 
               
               
                 7 
                 25.00 
                 2.78 
               
               
                 8 
                 27.22 
                 3.02 
               
               
                 9 
                 29.44 
                 3.27 
               
               
                 10 
                 31.67 
                 3.52 
               
               
                 11 
                 33.89 
                 3.77 
               
               
                 12 
                 36.11 
                 4.01 
               
               
                 13 
                 38.33 
                 4.26 
               
               
                 14 
                 40.56 
                 4.51 
               
               
                 15 
                 42.78 
                 4.75 
               
               
                 16 
                 45.00 
                 5.00 
               
               
                 17 
                 47.22 
                 5.25 
               
               
                 18 
                 49.44 
                 5.49 
               
               
                 19 
                 51.67 
                 5.74 
               
               
                   
               
             
          
         
       
     
         [0059]    This method is well, suited for both quick and long-term durability studies. It allows adjusting the mean strain and cyclic strain applied on the test sample. These parameters can be finely tuned to mimic the actual mean and cyclic strain values that a particular clinical environment can impose on an implanted stent. 
         [0060]    Multi-axial Peripheral Stent (MAPS) Fatigue Testing Method: This system was developed by Bose Corporation (model 9400 multiaxial peripheral stent test instrument) and it simulates the dynamic loading applied by peripheral arteries on a stent due to bending, twisting, extension, compression and radial distention caused for example by day-to-day activities like walking, siding and stair climbing. Stents were deployed with 8.+−.2% elongation in latex tubes having mechanical properties similar to those of healthy peripheral vessels, the tubes with stents therein were mounted on the tester, and phosphate buffered saline maintained at 37.degree.C. was allowed to flow through the stents at about 64 ml/min and negligible pressure. In one cycle the deployed stents were simultaneously stretched 3.5% along stent axis A-A and twisted around axis A-A by 0.8.degree. per mm of length; then the axes of the deployed stents were bent over a 20 mm radius curve placed at the midpoint of the stent, from 0.degree. (no bending—stent axis A-A is straight) to 69.degree.; then un-bent back to 0.degree,, then simultaneously un-twisted and shortened 3.5% to the initial starting configuration. This stretching, twisting, bending, unbending, untwisting and unstretching cycle occurs at a frequency of 1.5 Hz. The number of cycles sustained by the stent before fracture occurs is recorded as the stent cycles to failure. 
       EXAMPLE 1 
       [0061]    An 6 mm diameter×80 mm long stent having structure as shown in  FIG. 6  was laser cut from binary nitinol alloy tuning, diametrically expanded, heat treated, filed, microgrit blasted, electropolished and passivated (hereafter designated as sample 6). The final (nominal) dimensions (e.g., longitudinal dimensions and strut widths) of each cell of stent sample 6 are shown in  FIG. 6 ; the nominal wall thickness of the stent struts  6 A, apices  6 B, and connectors  6 C was 0.0095 inches. The sample was tested for fatigue life versus competitive stents of the same diameter using the methods described above. 
       EXAMPLE 2 
       [0062]    An 8 mm diameter×80 mm long stem having structure as shown in  FIG. 7  was laser cut from binary nitinol alloy tubing, diametrically expanded, beat treated, filed, microgrit blasted, electropolished and passivated (hereafter designated as sample 7). The final (nominal) dimensions of each cell of stent sample 7 are shown in  FIG. 7 ; the nominal wall thickness of the stent struts  7 A, apices  7 B, and connectors  70  was 0.0095 inches. The sample was tested for fatigue life versus competitive stents of the same diameter using the methods described above. 
         [0063]    Referring to  FIGS. 6 and 7 , the stent comprises a series of stents  6 A,  7 A, apices  6 B,  7 B and connectors  6 C,  7 C, forming undulating row patterns in a circumferential direction of the stent. The stent comprises a plurality of such row patterns consecutively in a longitudinal direction of the stent. Except for proximal and distal end rows, each pair of adjacent rows is connected to one another via connectors  6 C,  7 C to form a cell, for example, as shown in  FIGS. 6C, 7C , the cells may comprise eight-lobe cells. Of course, cells with more or less than eight lobes are contemplated by this disclosure. As shown, the connectors  6 C,  7 C may be positioned at less than all of the apices of each cell. In some aspects, the connectors  6 C,  7 C may be equally spaced circumferentially about the stent. 
         [0064]    In accordance with various aspects of the disclosure, at least one of the stents  6 A extending from the connector  6 C may have a smaller width dimension than the other strut  6 A extending from the connector  6 C. For example, one strut extending from the connector  6 C may comprise a width of 0.0044 inches, while the other strut extending from the connector  6 C may comprise a width of 0.0053 inches. In some aspects, stents  6 A extending from tire connector  6 C may have a longitudinal dimension greater than the next adjacent strut  6 A in either circumferential direction from the struts  6 A that extend from, the connector  6 C. 
         [0065]    According to various aspects of the disclosure, both struts  7 A extending from the connector  7 C may have a smaller width dimension than one or more struts  7 A spaced from the connector  7 C. For example, both struts  7 A extending from the connector  7 C may comprise a width of 0.0045 inches, while a plurality of struts  7 A spaced from the connector  7 C may comprise a width of 0.0051 inches. In some aspects, the next adjacent struts  7 A in either circumferential direction from the struts  7 A that extend from the connector  7 C may also have a reduced width of 0.0045 inches. In some aspects, struts  7 A extending from the connector  7 C may have a longitudinal dimension greater than the next adjacent strut  7 A in either circumferential direction from the struts  7 A that extend from the connector  7 C. In the embodiments illustrated in  FIGS. 6 and 7 , the cell includes serpentine strut patterns which constitute reverse mirror images as to the right and left side of each cell. 
         [0066]    As shown, by the data below, samples 6 and 7 demonstrated exceptional fatigue resistance as compared to that of similarly sized competitive stents. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Uni-axial 
                 Diameter × Length 
                 MAPS 
               
               
                   
                 Mean Strain to 
                 (mm × mm) 
                 Cycles to 
               
               
                   
                 Failure (%) 
                 (for MAPS test) 
                 Failure 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Sample 6 
                 42.78 
                 6 × 80  
                 1,719,875 
               
               
                 Cordis Smart Stent 
                 27.22 
                 6 × 120 
                 320,648 
               
               
                 Bard Life Stent 
                 27.22 
                 6 × 120 
                 820,191 
               
               
                 Sample 7 
                 33.89 
                 Not applicable 
                 Not tested 
               
               
                 Cordis Smart Stent 
                 20.56 
                 Not applicable 
                 Not tested 
               
               
                   
               
             
          
         
       
     
         [0067]      FIGS. 8A, 8B, 8C and 9  illustrate plan views of exemplar delivery systems for fatigue resistant stents.  FIGS. 8A, 8B, and 8C  illustrate RX delivery system  60  comprised of implant delivery catheter  66  having distal region  80  and stent  82 . Implant delivery catheter  66  is comprised of catheter shaft  65 , guidewire lumen  65 A, proximal guidewire exit skive  60 , proximal handle  68 , sheath  84  and distal manifold  67 . Proximal handle  68  is sealingly attached to catheter shaft  65  and may be comprised of polycarbonate. Catheter shaft  65  is relatively flexible, may be comprised of a polymeric material such as nylon or PEBAX, and may mage in length from 60 cm to 300 cm. Catheter outside diameter may range from about 2 Fr to about 10 Fr. Guidewire lumen  65 A diameter may be large enough to allow passage of guidewires ranging in diameter from 0.009″ to 0.038″. Distal manifold  67  is sealingly attached to sheath  84  and may be comprised of polycarbonate. Sheath  84  may be comprised of braid-reinforced polyester, non-reinforced polymers such as nylon, or polyester, or other materials, and adapted to resist kinking and to transmit axial forces along its length. Sheath  84  may be constructed so as to have varying degrees of flexibility along its length. In one embodiment ( FIG. 8C ) sheath  84  is comprised of weep holes  84 B. Weep holes  84 B allow annular space between sheath  84  and catheter shaft  65  to be purged of air. Stent  82  may be comprised of stent  40  or other stents. 
         [0068]      FIG. 9  illustrates OTW delivery system  70  comprised of implant delivery catheter  76  having distal region  80  and stent  82 . Implant delivery catheter  76  is comprised of catheter shaft  75 , guidewire lumen (not shown), proximal guidewire exit port  79 , proximal handle  78 , sheath  84  and distal manifold  77 A. Sheath  84  may optionally be comprised of weep holes  84 B and distal manifold  77 A is comprised of infusion tube with stopcock  77 B. Catheter shall  75 , guidewire lumen, proximal handle  78  and distal manifold have substantially the same construction, dimensions, and function as catheter shaft  65 , guidewire lumen  65 A, proximal handle  68  and distal manifold  67  described above in conjunction with  FIGS. 7A to 7C . Stent  112  may be comprised of stent  40  or other stents. 
         [0069]    Alternate materials for the components of delivery systems  60 ,  70  are generally well known in the art can be substituted for any of the non-limiting examples listed above provided the functional requirements of the component are met. 
         [0070]    An exemplar method of using a stent delivery system to deliver a fatigue resistant stent into a body of a patient is now described. Using techniques well knows in the art, a guidewire is percutaneously inserted into a patient&#39;s blood vessel and advanced to a region of interest the patient&#39;s body. Using imaging techniques such as fluoroscopy the diseased portion of the vessel is identified and a stent having the correct length and diameter for a treatment site is chosen. With reference to  FIGS. 8 and 9  self expanding stent delivery system  60 ,  70  is advanced over the guidewire to the treatment site and by using imaging techniques such as fluoroscopy the stent, for example stent  40 , is positioned at a desired location, relative to the treatment site. 
         [0071]    Catheter shaft  65 ,  75  is held stationary and sheath  84  is withdrawn to expose stent  40 . Stent  40  expands into contact with a luminal wall of the vessel as sheath  84  is withdrawn. In some methods stent  40  is deployed without being elongated. In other methods the physician elongates the stent, during deployment, for example by withdrawing catheter shaft  65 ,  75  while stent is partially deployed, or by withdrawing sheath  84  quickly, or due to friction of sheath  84  against stent  40  in a torturous treatment location, or due to other reasons. In one method the stent is elongated 15%. In other methods the stent may be elongated by 10, 20, 25, 30, 35, 40, 45 or 50%. In further methods the stent may be elongated non-uniformly such that some stent cells, for example cells  45 ′, are elongated more or less than the overall elongation of stent  40 . In one method the stent cell is elongated 15%. In other methods the stent cell, may be elongated by 10, 20, 25, 30, 35, 40, 45 or 50%. 
         [0072]    Following implantation of the fatigue resistant stent into a patient, for example stent  40 , the patient may return to the implanting physician far a follow-up visit. During follow-up the stent may be imaged using imaging techniques such as fluoroscopy, ultrasound, or magnetic resonance imaging to assess whether or not the stent has fractured, or for other reasons. 
         [0073]    The invention contemplated is further suitable for stents in addition to those mentioned above. For example, fatigue resistant stents may comprise tapered stents, flared stents, braided stents, bifurcation stents, and other stents as are known in the art. Tapered stents generally have a proximal end of one diameter and a distal end of a second diameter (typically a smaller diameter). Flared stents generally have a short tapered portion at the proximal end of a cylindrical stent, where the flared section is larger in diameter than the cylindrical portion. Braided stents are typically comprised of a tube manufactured by using a braiding method. An example of a braided stent is the Wallstent, sold by Boston Scientific, Natick, Mass. Bifurcation stents are placed in a patient where a vessel branches. Bifurcation stents are generally comprised of a single stent portion that branches into two stent portions and appears similar to a Y-fitting used to connect one piece of tubing to two pieces of tubing. 
         [0074]    While the various embodiments of the present invention have related to stents and stent delivery systems, the scope of the present invention is not so limited. For example, it will be appreciated that the various aspects of the present invention are also applicable to other types of expandable implants and their delivery systems. By way of non-limiting example, other types of expanding implants that can benefit from application of the invention include, anastomosis devices, blood filters, grafts, stent grafts, support frames for heart valves, vena cava filters, percutaneously implanted valves, aneurism treatment, devices, or other devices. 
         [0075]    Modifications and equivalents of the disclosed concepts are intended to be included within the scope of the claims. Further, while choices for materials and configurations may have been described above with respect to certain embodiments, one of ordinary skill in the art will understand that the materials and configurations described are applicable across the embodiments.