Abstract:
The stent of the present invention combines a helical strut band interconnected by coil elements. This structure provides a combination of attributes that are desirable in a stent, such as, for example, substantial flexibility, stability in supporting a vessel lumen, cell size and radial strength. The structure of the stent of the present invention provides a predetermined geometric relationship between the helical strut band and interconnected coil.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/103,073, filed Oct. 6, 2008, the entirety of which is hereby incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to a self expanding stent and delivery system for a self expanding stent. The delivery system allows for reconstraining the stent into the delivery catheter simultaneously allowing the stent to change lengths and rotate inside the delivery catheter if required. This invention also pertains to a delivery system for self expanding stent that foreshortens an appreciable amount, for example more than about 10%. 
     2. Description of the Related Art 
     Most commercial self expanding stents are not designed to be recaptured (reconstrained) into the delivery system once the stent has started to expand into the target vessel, artery, duct or body lumen. It would be advantageous for a stent to be able to be recaptured after the stent has started to deploy in the event that the stent is placed in an incorrect or suboptimal location, the stent could be recaptured and redeployed or recaptured and withdrawn. A recapturable stent and delivery system would constitute a major safety advantage over non-recapturable stent and delivery systems. 
     Many conventional self expanding stents are designed to limit the stent foreshortening to an amount that is not appreciable. Stent foreshortening is a measure of change in length of the stent from the crimped or radial compressed state as when the stent is loaded on or in a delivery catheter to the expanded state. Percent foreshortening is typically defined as the change in stent length between the delivery catheter loaded condition (crimped) and the deployed diameter up to the maximum labeled diameter divided by the length of the stent in the delivery catheter loaded condition (crimped). Stents that foreshorten an appreciable amount are subject to more difficulties when being deployed in a body lumen or cavity, such as a vessel, artery, vein, or duct. The distal end of the stent has a tendency to move in a proximal direction as the stent is being deployed in the body lumen or cavity. Foreshortening may lead to a stent being placed in an incorrect or suboptimal location. Delivery systems that can compensate for stent foreshortening would have many advantages over delivery systems that do not. 
     A stent is a tubular structure that, in a radially compressed or crimped state, can be inserted into a confined space in a living body, such as a duct, an artery or other vessel. After insertion, the stent can be expanded radially at the target location. Stents are typically characterized as balloon-expanding (BX) or self-expanding (SX). A balloon-expanding stent requires a balloon, which is usually part of a delivery system, to expand the stent from within and to dilate the vessel. A self expanding stent is designed, through choice of material, geometry, or manufacturing techniques, to expand from the crimped state to an expanded state once it is released into the intended vessel. In certain situations higher forces than the expanding force of the self expanding stent are required to dilate a diseased vessel. In this case, a balloon or similar device might be employed to aid the expansion of a self expanding stent. 
     Stents are typically used in the treatment of vascular and non-vascular diseases. For instance, a crimped stent may be inserted into a clogged artery and then expanded to restore blood flow in the artery. Prior to release, the stent would typically be retained in its crimped state within a catheter and the like. Upon completion of the procedure, the stent is left inside the patient&#39;s artery in its expanded state. The health, and sometimes the life, of the patient depends upon the stent&#39;s ability to remain in its expanded state. 
     Many conventional stents are flexible in their crimped state in order to facilitate the delivery of the stent, for example, within an artery. Few are flexible after being deployed and expanded. Yet, after deployment, in certain applications, a stent may be subjected to substantial flexing or bending, axial compressions and repeated displacements at points along its length, for example, when stenting the superficial femoral artery. This can produce severe strain and fatigue, resulting in failure of the stent. 
     A similar problem exists with respect to stent-like structures. An example would be a stent-like structure used with other components in a catheter-based valve delivery system. Such a stent-like structure holds a valve which is placed in a vessel. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a catheter delivery system for self-expanding stents. The reconstrainable stent delivery system of the present invention comprises a proximal end and distal end, which include inner and outer members, typically shafts or catheter or outer sheath of the catheter, which is crimped onto a slider at the proximal end of the stent. The slider can rotate about and move longitudinally along one of an inner shaft or tube, such as the guide wire tube, such that the proximal end of the stent can move distally as the stent deploys. A pusher can be used on the guide wire tube such that the guide wire tube, pusher, and stent move proximally relative to the outer sheath and reconstrain the stent in the outer sheath. Furthermore, the pusher and guide wire tube could move distally as the outer sheath retracts proximally for stent deployment to accommodate foreshortening. 
     The delivery system can also include a spring element in the catheter delivery system that will react the axial load at the proximal end of the stent during stent deployment. A spring element as described can bias the axial movement of the stent inside the delivery catheter to move distally as the stent is deployed. This biased movement is beneficial for stents that foreshorten an appreciable amount as the biased movement reduce the amount of movement at the distal end of the stent during stent deployment. 
     The catheter delivery system can be used to deploy stents in iliac, femoral, popliteal, carotid, neurovascular or coronary arteries, treating a variety of vascular disease states. 
     The stent of the present invention combines a helical strut member or band interconnected by coil elements. This structure provides a combination of attributes that are desirable in a stent, such as, for example, substantial flexibility, stability in supporting a vessel lumen, cell size and radial strength. However, the addition of the coil elements interconnecting the helical strut band complicates changing the diameter state of the stent. Typically, a stent structure must be able to change the size of the diameter of the stent. For instance, a stent is usually delivered to a target lesion site in an artery while in a small diameter size state, then expanded to a larger diameter size state while inside the artery at the target lesion site. The structure of the stent of the present invention provides a predetermined geometric relationship between the helical strut band and interconnected coil elements in order to maintain connectivity at any diameter size state of the stent. 
     The stent of the present invention is a self-expanding stent made from superelastic nitinol. Stents of this type are manufactured to have a specific structure in the fully expanded or unconstrained state. Additionally, a stent of this type must be able to be radially compressed to a smaller diameter, which is sometimes referred to as the crimped diameter. Radially compressing a stent to a smaller diameter is sometimes referred to as crimping the stent. The difference in diameter of a self-expanding stent between the fully expanded or unconstrained diameter and the crimped diameter can be large. It is not unusual for the fully expanded diameter to be 3 to 4 times larger than the crimped diameter. A self-expanding stent is designed, through choice of material, geometry, and manufacturing techniques, to expand from the crimped diameter to an expanded diameter once it is released into the intended vessel. 
     The stent of the present invention comprises a helical strut band helically wound about an axis of the strut. The helical strut band comprises a wave pattern of strut elements having a plurality of peaks on either side of the wave pattern. A plurality of coil elements are helically wound about an axis of the stent and progress in the same direction as the helical strut band. The coil elements are typically elongated where the length is much longer than the width. The coil elements interconnect at least some of the strut elements of a first winding to at least some of the strut elements of a second winding of the helical strut band at or near the peaks of the wave pattern. In the stent of the present invention, a geometric relationship triangle is constructed having a first side with a leg length L C  being the effective length of the coil element between the interconnected peaks of a first and second winding of the helical strut band, a second side with a leg length being the circumferential distance between the peak of the first winding and the peak of the second winding interconnected by the coil element divided by the sine of an angle A s  of the helical strut band from a longitudinal axis of the stent, a third side with a leg length being the longitudinal distance the helical strut band progresses in 1 circumference winding (Pl) minus the effective strut length L S , a first angle of the first leg being 180 degrees minus the angle A s , a second angle of the second leg being an angle A c  the coil element generally progresses around the axis of the stent measured from the longitudinal axis and a third angle of the third leg being the angle A s  minus the angle A c , wherein a ratio of the first leg length L C  to a length L S  multiplied by the number of adjacent wave pattern of the strut elements forming the helical strut band, N S  is greater than or equal to about 1. This value is defined as the coil-strut ratio and numerically is represented by coil-strut ratio=Lc/Ls*Ns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing description, as well as further objects, features, and advantages of the present invention will be understood more completely from the following detailed description of presently preferred, but nonetheless illustrative embodiments in accordance with the present invention, with reference being had to the accompanying drawings, in which: 
         FIG. 1  is a schematic drawing of the stent delivery system in accordance with the present invention. 
         FIG. 2  is a detailed enlarged view of X-X section shown in  FIG. 1  just prior to stent deployment. 
         FIG. 3  is a detailed enlarged view of X-X section shown in  FIG. 1  just prior to the recapturing. 
         FIG. 4  is a detailed enlarged view of X-X section shown in  FIG. 1  with an alternate embodiment configuration. 
         FIG. 5  is a detailed enlarged view of X-X section shown in  FIG. 1  with an alternate embodiment configuration 
         FIG. 6  is a view of Z-Z section shown in  FIG. 5  with an alternate embodiment configuration. 
         FIG. 7  is a detailed enlarged view of X-X section shown in  FIG. 1  just prior to the start of stent deployment. 
         FIG. 8  is a detailed enlarged view of X-X section shown in  FIG. 1  during stent deployment. 
         FIG. 9  is a schematic drawing of the stent delivery system in accordance with the present invention. 
         FIG. 10  is a plan view of a first embodiment of a stent in accordance with the present invention, the stent being shown in a partially expanded state. 
         FIG. 11  is a detailed enlarged view of portion A shown in  FIG. 1 . 
         FIG. 12  is a plan view of an alternate embodiment of the stent. 
         FIG. 13  is an enlarged detailed view of portion B shown in  FIG. 3 . 
         FIG. 14  is a plan view of an alternate embodiment of the stent. 
         FIG. 15  is a plan view of an alternate embodiment of the stent. 
         FIG. 16  is a plan view of an alternate embodiment of the stent. 
         FIG. 17  is a detailed enlarged view of portion C shown in  FIG. 7 . 
         FIG. 18  is a plan view of an alternate embodiment of the stent. 
         FIG. 19  is a schematic diagram of an alternate embodiment for a coil element of the stent. 
         FIG. 20  is a detailed enlarged view of portion D shown in  FIG. 14 . 
         FIG. 21  is a detailed enlarged view of X-X section shown in  FIG. 1  with an alternate embodiment configuration 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The self expanding stent delivery system  10  of the present invention is shown in  FIG. 1  and is comprised of inner and outer coaxial members, for example a shaft or tube. The outer tube which is also known as outer sheath  11 , constrains stent  12  in a crimped or radially compressed state. The inner members can be comprised of multiple components including distal tip  8 , guide wire tube  14  and pusher  16  to react the axial forces placed on the stent as the outer sheath is retracted to deploy the stent. Pusher  16  can also act as a proximal stop. Other elements of the stent delivery system can include luer lock hub  6  attached to the proximal end of pusher  16 , handle  3  attached to outer sheath  11  that incorporates luer port  4  such that the space between the inner members and outer sheath  11  can be flushed with saline solutions to remove any entrapped air. Pusher  16  is sometimes a composite structure of multiple components, such as a stainless steel tube at the proximal end and a polymer tube inside outer sheath  11 . 
     Stent delivery system  10  of the present invention, shown in the detail view of X-X section,  FIG. 2 , is comprised of outer sheath  11 , a delivery catheter in which stent  12  is constrained in a crimped, or radially compressed state. Stent delivery system  10  can be referred to as catheter delivery system as a delivery catheter. Slider  13  is positioned to interface with the inside diameter of crimped stent  12 . Slider  13  is coaxial with guide wire tube  14  and slider  13  is free to rotate and slide relative to guide wire tube  14 . Distal stop  15  is fixed to guide wire tube  14  at a position distal to slider  13 . Pusher  16  is positioned proximal to stent  12  and slider  13  and reacts the axial forces transmitted to stent  12  as outer sheath  11  is retracted to deploy the stent and provides a proximal stop. Stent  12  and slider  13  are free to move, translate or rotate, within outer sheath  11  and relative to guide wire tube  14  as outer sheath  11  is retracted and stent  12  is deployed. This is advantageous when the stent design is such that stent  12  shortens in length and/or rotates as it expands from the crimped state to a larger diameter expanded state. The delivery system of the present invention allows the stent movement to occur inside outer sheath  11  instead of inside the body lumen. Before outer sheath  11  is fully retracted, thereby releasing stent  12 , the stent can be recaptured by moving guide wire tube  14  and attached distal stop  15  proximally relative to stent  12  and slider  13  until distal stop  15  contacts slider  13 , as shown in detail view of X-X section,  FIG. 3 . Because stent  12  and slider  13  are intimate contact with each other, outer sheath  11  can be moved distally relative to stent  12 , slider  13 , guide wire tube  14  and distal stop  15 , there by recapturing stent  12  inside outer sheath  11 . In this embodiment, pusher  16  is in contact with stent  12  as outer sheath  11  is retracted to deploy stent  12 . 
     In an alternate embodiment, slider  13  is designed to interface with the inside diameter of stent  12  and contact pusher  16  as outer sheath  11  is retracted, as shown in  FIG. 4 . This embodiment reduces the axially load directly placed on stent  12  during stent deployment. 
     In the embodiment described above, slider  13  is coaxial with guide wire tube  14  and slider  13  is free to rotate and slide relative to guide wire tube  14 . Guide wire tube  14  can be hollow, forming a lumen that runs the length of the stent delivery system to accommodate a guide wire which is often used to facilitate locating the stent delivery system in the target vessel, artery, duct or body lumen. Alternatively, guide wire tube  14  can be a non-hollow solid shaft  18 , as shown in  FIG. 5 . 
     In an alternate embodiment axial force at the proximal end of the stent is reacted by a proximal stop  19 , attached to non-hollow shaft  18 , such that proximal stop  19  and non-hollow shaft are a unitary member as shown in  FIG. 21 . Proximal stop  19  and non-hollow shaft  18  could be made from different materials that are affixed together or made from the same material. 
     In an alternate embodiment shown in Z-Z section view,  FIG. 6 , slider  13  is formed of a structure where a portion of slider  13  is a polymer that is molded or formed to inside diameter  21  of stent  12  and/or sidewall  22  of stent  12 . Slider  13  can be a composite or laminated structure comprising polymer portion  23  interfacing with stent  12  and rigid portion  24  near the inside diameter of slider  13 . 
     In another embodiment as shown in detail view of X-X section,  FIG. 7  and  FIG. 8 , spring element  25  is incorporated into pusher  16  such that spring element  25  is compressed as the axial force at the proximal end of stent  12  increases until outer sheath  11  starts to move in a proximal direction relative to stent  12 . As stent  12  deploys, spring element  25  continues to react the axial load at the proximal end of stent  12  and simultaneously pushes the proximal end of stent  12  distally as stent  12  foreshortens coming out of outer sheath  11 .  FIG. 7  shows spring element  25  in an uncompressed state prior to the start of stent  12  deployment where stent  12  is not under an axial load.  FIG. 8  shows spring element  25  in a compressed state after the start of deployment where stent  12  is under an axial load, where X 2 &lt;X 1 . As stent  12  expands out of outer sheath  11  the axial load on stent  12  will typically decrease from a peak load near the beginning of the deployment. As the axial load decreases, the spring force will push the proximal end of stent  12  forward to bias any movement of stent  12  due to foreshortening occurring at the proximal end of stent  12 , such that the proximal end of stent  12  moves distally instead of the distal end of stent  12  moving proximally. 
     In an alternate embodiment, spring element  26  can be incorporated at the proximal end of stent delivery system  10 , where distal end  27  of spring element  26  effectively interfaces with pusher  16 , and proximal end  28  of spring element  26  is fixed, such that pusher  16  compresses spring element  26  as the axial force at the proximal end of stent  12  increases until outer sheath  11  starts to move in a proximal direction relative to stent  12 . As stent  12  deploys, spring element  26  moves pusher  16  proximally as stent  12  foreshortens coming out of outer sheath  11 . 
       FIG. 10  with detail shown in  FIG. 11  illustrates stent  500  which can be used in stent delivery system  10 .  FIG. 10  is a plan view of a first embodiment of stent  500  in accordance with the teachings present invention shown in a partially expanded state. As used herein, the term “plan view” will be understood to describe an unwrapped plan view. This could be thought of as slicing open a tubular stent along a line parallel to its axis and laying it out flat. It should therefore be appreciated that, in the actual stent, the top edge of  FIG. 10  will be joined to the lower edge. Stent  500  is comprised of helical strut band  502  interconnected by coil elements  507 . Side-by-side coil elements  507  form coil band  510 . Coil band  510  is formed as a double helix with helical strut band  502  and progresses from one end of the stent to the other. Helical strut band  502  comprises a wave pattern of strut elements  503  that have peaks  508  on either side of the wave pattern and legs  509  between peaks  508 . Coil elements  507  interconnect strut elements  503  of helical strut band  502  through or near peaks  508 . NSC portion  505  of helical strut band  502  is defined by the number of strut elements  503  (NSC) of helical strut band  502  between coil element  507  as helical strut band  502  progresses around stent  500 . The number of strut elements  503  (NSC) in NSC portion  505  of helical strut band  502  is more than the number of strut elements  503  (N) in one circumference winding of helical strut band  502 . The number of strut elements  503  (NSC) in NSC portion  505  is constant. 
     In this embodiment, stent  500  has N=12.728 helical strut elements  503  in one circumference winding of helical strut band  502  and has NSC=16.5 helical strut elements  503  in NSC portion  505 . CCDn portion  512  of NSC portion  505  of helical strut band  502  is defined by the number of strut elements  503  (CCDn) equal to NSC minus N. The number of strut elements  503  (CCDn) in CCDn portion  512  and the number of strut elements  503  (N) in one circumference winding of helical strut band  502  does not need to be constant at different diameter size states of stent  500 . Stent  500  has CCDn=3.772 helical strut elements  503  in CCDn portion  512 . Because this connectivity needs to be maintained at any diameter size state a geometric relationship between the helical strut band  502  and coil element  507  can be described by geometric relationship triangle  511 . Geometric relationship triangle  511  has a first side  516  with a leg length equal to the effective length (Lc)  530  of coil element  507 , a second side  513  with a leg length equal to circumferential coil distance (CCD)  531  of CCDn portion  512  of helical strut band  502  divided by the sine of an angle A s    535  of helical strut band  502  from the longitudinal axis of stent  500 , a third side  514  with a leg length (SS)  532  equal to the longitudinal distance (Pl)  534  helical strut band  502  progresses in 1 circumference winding minus the effective strut length L S    533 , a first angle  537  of first side  516  is equal to 180 degrees minus angle A s    535 , a second angle  536  of second side  513  is equal to the angle A c    536  of coil element  507  from the longitudinal axis of stent  500  and a third angle  538  of third side  514  equal to angle A s    535  minus angle A c    536 . If the circumferential strut distance (P s )  539  of helical strut element  503  is the same for all helical strut elements  503  in CCDn portion  512 , circumferential coil distance CCD  531  is equal to the number of helical strut elements  503  in the CCDn portion  512  multiplied by the circumferential strut distance (P s )  539 . The distances in any figure that shows a flat pattern view of a stent represent distances on the surface of the stent, for example vertical distances are circumferential distances and angled distances are helical distances. First side  516  of geometric relationship triangle  511  is drawn parallel to the linear portion of coil element  507  such that the coil angle Ac  536  is equal to the angle of the linear portion of coil element  507 . If coil element  507  does not have a substantially linear portion, but progresses about the stent in a helical manner, an equivalent coil angle  536  could be used to construct the geometric relationship triangle  511 . For instance if coil element  507  is a wavy coil element  907 , as shown in  FIG. 19 , line  901  could be drawn fitted through the curves of the wavy coil element  907  and line  901  can be used to define coil angle  536 . 
     Stent  400  shown in  FIGS. 12 and 13  is similar to stent  500  in that it is comprised of helical strut band  402  interconnected by coil elements  507 . Stent  400  is different in that helical strut band  402  is comprised of two adjacent wave patterns of strut elements  403   a  and  403   b  that have peaks  508  on either side of the wave pattern. Strut element  403   a  being connected to strut element  403   b . Similar to helical strut band  502 , helical strut band  402  also has a NSC portion  405  and a CCDn portion  412 . Helical strut band  402  can be defined as having a number Ns of wave patterns of strut elements equal to 2. Helical strut band  502  can be defined as having a number Ns of wave patterns of strut elements equal to 1. In an alternate embodiment, the stent of the present invention can have a helical strut band with a number Ns of wave patterns of strut elements equal to 3, which would be a triple strut band. In an alternate embodiment, the stent of the present invention could have a helical strut band with a number Ns of wave patterns of strut elements equal to any integer. Stents with helical strut bands having a number Ns of wave patterns of strut elements equal to or greater than 2 provide an advantage in that the helical strut band would form a closed cell structure with smaller cell size which is desired when there is additional risk of embolism. Stents with smaller cell sizes tend to trap plaque or other potential embolic debris better than stents with larger cell sizes. 
     Stent structures described provides the combination of attributes desirable in a stent when the coil-strut ratio, ratio of Lc to Ls multiplied by the number of wave patterns of strut elements Ns in the helical strut band (Lc multiplied by Ns divided by Ls), is greater than or equal to 1. For example the coil-strut ratio for stent  500  is 2.06 and for stent  400  is 2.02. Stent  200  shown in  FIG. 18  has a similar structure to stent  500 . The coil-strut ratio for stent  200  is about 1.11. 
     In order for the stent of the present invention to crimped to a smaller diameter, the geometry of the structure undergoes several changes. Because of the helical nature of the helical strut band, strut angle A s  must get smaller as the stent diameter decreases. Because of the interconnectivity between a first winding of the helical strut band and a second winding of the helical strut band created by the coil element, the angle of the element A c  must also get smaller, or become shallower, to accommodate the smaller strut angle A s . If the angle of coil element A c  can not become shallower or is difficult to become shallower as the stent crimps and stent angle A s  gets smaller, the coil elements will tend to interfere with each other and prohibit crimping or require more force to crimp. The changing of the angle of the coil element during crimping is facilitated if the coil-strut ratio is greater than 1. Coil-strut ratios less than 1 tend to stiffen the coil element such that more force is required to bend the coil element to a shallower angle during the crimping process, which is not desirable. 
     Helical strut band  602  of stent  600 , shown in  FIG. 14 , transitions to and continues as an end strut portion  622  where the angle of the winding AT 1  of the wave pattern of strut elements  624   a  forming end strut portion  622  is larger than the angle of the helical strut band A s . End strut portion  622  includes a second winding of the wave pattern of strut elements  624   b  where the angle AT 2  of the second winding is larger than the angle of the first winding AT 1 . Strut elements  603  of helical strut band  602  are interconnected to strut elements  624   a  of the first winding of end strut portion  622  by a series of transitional coil elements  623  that define transition coil portion  621 . All strut elements  624   a  of the first winding of end portion  622  are connected by coil elements  623  to the helical strut band  602 . Peaks  620  of helical strut band  602  are not connected to end strut portion  622 . Transitional coil portion  621  allows end strut portion  622  to have a substantially flat end  625 . Helical strut band  402  of stent  400  transitions to and continues as an end portion where the angle of the first winding AT 1  of the wave pattern of strut elements forming of the end portion is larger than the angle of the helical strut band As. The angle of the second winding AT 2  is larger than AT 1 , and the angle of subsequent windings of the end portion are also increasing (i.e. AT 1 &lt;AT 2 &lt;AT 3 &lt;AT 4 ). As shown in  FIG. 20 , stent  600  includes one peak  626  of end strut portion  622  connected to two peaks  620  of helical strut band  602  by transitional coil elements  623 . 
     The accompanying definitions are described below.
         (N)—Number of helical strut elements in one circumference winding of the helical strut member.   (A s )—Angle of helical strut band winding measured from the longitudinal axis of the stent.   (A c )—Effective angle of coil element measured from the longitudinal axis of the stent.   (Pl)—Longitudinal distance (pitch) the strut member progresses in 1 circumference winding. Equal to the circumference of the stent divided by the arctangent of A s .   (P s )—Circumferential distance (pitch) between strut legs of a helical strut element of the helical strut band. Assuming the circumferential strut pitch is equal for all strut elements of the helical strut band, the circumferential strut pitch is equal to the circumference of the stent divided by N.   (NSC)—Number of strut elements of the strut band between a helical element as the strut member progresses   (CCDn)—Number of strut elements of the strut band between interconnected strut elements, equal to NSC minus N   (CCD)—Circumferential Coil Distance is the circumferential distance between interconnected strut elements, equal to the CCDn times the P s  if the Ps is equal for all strut elements in the CCDn portion.   (Lc)—Effective length of the helical element as defined by the geometric relationship triangle described in table 1.   (SS)—Strut Separation as defined in the geometric relationship triangle described in table 1.   (Ls)—Effective Strut Length. Equal to Pl minus SS.   (Ns)—Number of adjacent wave patterns of the strut elements forming the helical strut band.   Coil-Strut ratio—Ratio of L C  to a length L S  multiplied by the number of adjacent wave pattern of the strut elements forming the helical strut band, N s . Numerically equal to Lc/Ls*Ns.   Strut length-Strut Separation ratio—Ratio of the effective strut length (Ls) to the Strut Separation (SS), numerically equal to Ls/SS.       

     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Leg Length 
                 Angle 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Side 1 
                 Lc 
                 180° minus A s   
               
               
                   
                 Side 2 
                 CCD divided by sin(A s ) 
                 A c   
               
               
                   
                 Side 3 
                 SS 
                 A s  minus A c   
               
               
                   
                   
               
             
          
         
       
     
     In one embodiment, the difference between the strut angle, A s , and coil angle, A c , is more than about 20 degrees. Because of the necessity of the coil angle to become shallower as the stent is crimped, if the coil angle and the strut angle in the expanded state are too close to each other there is increased difficulty in crimping the stent. 
     For the stent of the present invention the Strut length-Strut Separation ratio is a measure of the relative angle of the strut angle and coil angle. Stents with Strut length-Strut Separation ratios less than about 2.5 have improved crimping behavior. Stent attributes can further be improved if the angle of the strut member is between 55 degrees and 80 degrees and the coil angle is between 45 degrees and 60 degrees in the expanded state. Additionally, steeper coil angles A c  in the expanded state make crimping the stent of the present invention more difficult. Coil angles of less than 60 degrees in the expanded state facilitate crimping the stent of the present invention. 
     For the stent of the present invention, in addition to the coil angle changing during crimping, the helical strut band rotates about the longitudinal axis of the stent to accommodate the connectivity between subsequent windings of helical strut bands during crimping resulting in more windings of the helical strut band along the length of the stent when the stent is crimped. In one embodiment, the longitudinal pitch of the helical strut band (Pl) is approximately the same in both the expanded state and crimped state. Considering that an increase of helical strut band windings along the length of the stent when the stent is crimped contributes to stent foreshortening it is advantageous for the stent of the present invention to have an approximated increase in the amount of helical strut band windings of less than about 30% when crimped, preferably less than about 26%. A 26% increase in helical strut band winding corresponds to about 20% foreshortening which is considered the maximum clinically useful amount of foreshortening (Serruys, Patrick, W., and Kutryk, Michael, J. B., Eds.,  Handbook of Coronary Stents, Second Edition , Martin Dunitz Ltd., London, 1998.) hereby incorporated by reference in its entirety into this application. 
       FIG. 15  is a plan view of another embodiment of stent  700  in accordance with the teachings of the present invention. Helical strut band  702  progresses helically from one end of stent  700  to the other. Each strut element  703  is connected to a strut in a subsequent winding of helical strut band  702  by coil element  707 . Strut element  703  includes leg portions  709 . Each of leg portions  709  has an equal length. 
       FIG. 16 , with detail shown in  FIG. 17 , is a plan view of another embodiment of stent  800 . In this embodiment, coil element  807  includes curved transition portion  852  at ends  853  and  854 . Curved transition portion  852  connects to strut element  803 . 
     Stent  800  includes transitional helical portions  859  and end strut portions  858  at either end  861  of stent  800 . End strut portions  858  are formed of a pair of connected strut windings  860 . Coil element  807  is comprised of two coil portions  807   a  and  807   b  which are separated by gap  808 , as shown in  FIG. 17 . Gap  808  can have a size equal to zero where coil portions  807   a  and  807   b  are touching. Gap  808  terminates near ends  853  and  854 . Gap  808  can terminate anywhere along the length of coil  807  or at multiple points along coil  807  such that the gap would have interruptions along coil  807 . 
     Stents  400 ,  500 ,  600 ,  700  and  800  are made from a common material for self expanding stents, such as Nitinol nickel-titanium alloy (Ni/Ti), as is well known in the art. 
     In an alternate embodiment, stent  12  can be a stent as described in U.S. Pat. No. 7,556,644 hereby incorporated by reference into this application. 
     The stents of the present invention may be placed within vessels using procedures well known in the art. The stents may be loaded into the proximal end of a catheter and advanced through the catheter and released at the desired site. Alternatively, the stents may be carried about the distal end of the catheter in a compressed state and released at the desired site. The stents may either be self-expanding or expanded by means such as an inflatable balloon segment of the catheter. After the stent(s) have been deposited at the desired intralumenal site, the catheter is withdrawn. 
     The stents of the present invention may be placed within body lumen such as vascular vessels or ducts of any mammal species including humans, without damaging the lumenal wall. For example, the stent can be placed within a lesion or an aneurysm for treating the aneurysm. In one embodiment, the flexible stent is placed in a super femoral artery upon insertion into the vessel. In a method of treating a diseased vessel or duct a catheter is guided to a target site of a diseased vessel or duct. The stent is advanced through the catheter to the target site. For example, the vessel can be a vascular vessel, femoropopliteal artery, tibial artery, carotid artery, iliac artery, renal artery, coronary artery, neurovascular artery or vein. 
     Stents of the present invention may be well suited to treating vessels in the human body that are exposed to significant biomechanical forces. Stents that are implanted in vessels in the human body that are exposed to significant biomechanical forces must pass rigorous fatigue tests to be legally marketed for sale. These tests typically simulate loading in a human body for a number of cycles equivalent to 10 years of use. Depending on the simulated loading condition, the number of test cycles may range from 1 to 400 million cycles. For example, stents that are intended to be used in the femorpopliteal arteries may be required to pass a bending test where the stent is bent to a radius of about 20 mm 1 to 10 million times or axially compressed about 10% 1 to 10 million times. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. For example, a stent could be made with only right-handed or only left-handed helical portions, or the helical strut band could have multiple reversals in winding direction rather than just one. Also, the helical strut band could have any number of turns per unit length or a variable pitch, and the strut bands and/or coil bands could be of unequal length along the stent. 
     The stent delivery system of the present invention can be used with any stent that allows recapturing after partial deployment.