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 elements in order to maintain connectivity at any diameter size state of the stent.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is claims the benefit of U.S. Provisional Patent Application No. 60/963,083 filed Aug. 2, 2007 and U.S. Provisional Patent Application No. 61/070,598 filed Mar. 24, 2008 the entirety of both of are hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to expandable tubular structures capable of insertion into small spaces in living bodies and, more particularly, concerns a stent structure having a geometry which is capable of substantial and repeated flexing at points along its length without mechanical failures and with no substantial changes in its geometry. 
         [0004]    2. Description of the Related Art 
         [0005]    A stent is a tubular structure that, in a radially compressed or crimped state, may be inserted into a confined space in a living body, such as a duct, an artery or other vessel. After insertion, the stent may be expanded radially to enlarge the space in which it is located. 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. 
         [0006]    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 depend upon the stent&#39;s ability to remain in its expanded state. 
         [0007]    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. 
         [0008]    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 
       [0009]    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. 
         [0010]    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. 
         [0011]    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 (P 1 ) 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 
         [0012]    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: 
           [0013]      FIG. 1  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. 
           [0014]      FIG. 2  is a detailed enlarged view of portion A shown in  FIG. 1 . 
           [0015]      FIG. 3  is a plan view of an alternate embodiment of the stent. 
           [0016]      FIG. 4  is an enlarged detailed view of portion B shown in  FIG. 3 . 
           [0017]      FIG. 5  is a plan view of an alternate embodiment of the stent. 
           [0018]      FIG. 6  is a plan view of an alternate embodiment of the stent. 
           [0019]      FIG. 7  is a plan view of an alternate embodiment of the stent. 
           [0020]      FIG. 8  is a detailed enlarged view of portion C shown in  FIG. 7 . 
           [0021]      FIG. 9  is a plan view of an alternate embodiment of the stent. 
           [0022]      FIG. 10  is a schematic diagram of an alternate embodiment for a coil element of the stent. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
         [0024]      FIG. 1  with detail shown in  FIG. 2  illustrates stent  500 .  FIG. 1  is a plan view of a first embodiment of stent  500  in accordance with the 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. 1  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. 
         [0025]    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 (P 1 )  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. 10 , 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 . 
         [0026]    Stent  400  shown in  FIGS. 3 and 4  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. 
         [0027]    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. 9  has a similar structure to stent  500 . The coil-strut ratio for stent  200  is about 1.11. 
         [0028]    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. 
         [0029]    Helical strut band  602  of stent  600 , shown in  FIG. 5 , 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 ). 
         [0030]    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.   (P 1 )—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 P 1  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.       
 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Leg Length 
                 Angle 
               
               
                   
               
             
             
               
                   
                 Side 1 
                 Lc 
                 180° minus A s   
               
               
                   
                 Side 2 
                 CCD divided  
                 A c   
               
               
                   
                   
                 by sin(A s ) 
                   
               
               
                   
                 Side 3 
                 SS 
                 A s  minus A c   
               
               
                   
               
             
          
         
       
     
         [0045]    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. 
         [0046]    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. 
         [0047]    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. For the stent of the present invention, the geometric relationship triangle can be used to approximate the expected amount of helical strut band rotation during crimping of the stent. If the geometric relationship triangle can be determined for a given diameter size state of the stent, the geometric relationship triangle can be approximated for any other size state based on the following assumptions; the effective coil length (L c ), effective strut length (L s ), and the longitudinal pitch of the helical strut band (P 1 ) are a constant for any diameter size state. Given the above assumptions and the geometric relationship triangles approximated in the expanded and crimped states, the amount the helical strut band rotates per winding of the helical strut band about the axis of the stent to accommodate the interconnected coil element during crimping can be approximated if the circumferential strut pitch (P s ) of the strut element of the helical strut band is assumed to be equal for all strut elements in the helical strut band. 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 CoronaryStents, Second Edition , Martin Dunitz Ltd., London, 1998.) hereby incorporated by reference in its entirety into this application. 
         [0048]      FIG. 6  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. 
         [0049]      FIG. 7 , with detail shown in  FIG. 8 , 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 . 
         [0050]    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. 8 . 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 . 
         [0051]    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. 
         [0052]    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. 
         [0053]    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. 
         [0054]    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. 
         [0055]    Although presently preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications, and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims. 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.