Abstract:
A flexible stent is disclosed which reduced openings between rings by two alternative techniques. In the first instance, adjacent sinusoidally bending rings are nested to compact them closer together to reduce the opening sizes therebetween. In another embodiment, adjacent sinusoidal rings overlap each other to achieve the same effect. The nested design can be fabricated from a long continuous sinusoidal band having alternating high and low peaks wrapped spirally around the mandrel and welded at crossties. The assembly can be covered with a graft as a support therefor.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of this invention relates to vascular stents.  
         BACKGROUND OF THE INVENTION  
         [0002]    Vascular stents are structures that are designed to maintain the patency of a vessel in the body. The stent provides internal support to allow the circulation to proceed therethrough. Stents can be used in the vascular system in ureters, bile ducts, esophagus, and in many other tubular structures in the human body.  
           [0003]    Stents can be tubular or can be made from wire. Stents are typically made from a metal or polymeric substance or a metal coated with polymers which are biocompatible or contain heparin to reduce blood clotting or other tissue reactions. Many prior designs have used a coil approach where a wire is helically wound on a mandrel. Yet other designs have evolved-braided wire mesh and angulated wire forms wrapped on a spindle to form a coil.  
           [0004]    U.S. Pat. Nos. 5,292,331 by Boneau and U.S. Pat. No. 5,403,341 describe such wire forms. These devices have very poor radial support to withstand the hoop strengths of the artery or vein and further are not suitable for arteries that are bent or curved or for long lesions; multiple stent are required. These designs do not provide any support to hold the wall of the artery, other than the memory of the metal.  
           [0005]    Wall Stent, produced by Pfizer Inc., is a braided wire tube. Although this stent is flexible so as to be placed in curved arteries or veins and other body cavities, it does not have any radial strength imparted to it by design.  
           [0006]    Wiktor, U.S. Pat. Nos. 4,649,922; 4,886,062; 4,969,458; and 5,133,732 describe a wire form stent. He describes stents made of wire helix made of a preformed wire which is in the sinusoidal form, in which either all or some of the adjacent strands are connected.  
           [0007]    Arthus Fontaine, U.S. Pat. No. 5,370,683, also describes a similar device where a flat wire form of sinusoidal shape is wound on a mandrel to form a helical coil. The wire bends are “U” shaped and are connected to alternate “U”-shaped bands.  
           [0008]    Allen Tower, U.S. Pat. Nos. 5,217,483 and 5,389,106 describes a similar device where the wire is preformed to a sinusoidal shape and subsequently wound on a mandrel to form a helical coil.  
           [0009]    All of the above-described art fails to provide radial support. The pre-shaped wire form (sinusoidal in most of the prior art) is wrapped on a mandrel to form a coil. However, the forces imported by the vessel wall&#39;s hoop strength are radially inward. In other words, the force is acting perpendicular to the plane of the U-shaped wire form. This means that the bends that are in the wire add no structural strength to the wire form to support the force produced by the wall, which is radially inward.  
           [0010]    When we examine the simple coils, such as taught in U.S. Pat. Nos. 5,383,928 to Scott or Gene Samson 5,370,691 or Rolando Gills 5,222,969, it is apparent that the spring coil will withstand substantial radial forces due to the vessel wall; however, all these stents are bulky in their pre-expanded form and are hard to place in small and curved arteries or veins of the body. Also, a major disadvantage of this design is that when the coil stent is placed in a curved artery or vein, it forms an “accordion” shape whereby some strands in the outer radius are spread and those of the inner radius are gathered. Spring coils can also “flip” to form a flat structure when a longitudinal force is applied on one side of the stent.  
           [0011]    The other types of stents that have been developed are tube stents. Palmer, U.S. Pat. Nos. 4,733,665; 4,739,762; 7,776,337; and 4,793,348 describe such a tube stent of slotted metal tube. The slotted metal tube is expanded by a high-pressure balloon to implant the stent into the inside wall of the artery or vein.  
           [0012]    Joseph Weinstein, U.S. Pat. No. 5,213,561 describes a similar stent made of tubular materials with slots cut into it. On expansion using a balloon, it forms a structure with diamond-shaped slots.  
           [0013]    Henry Wall, U.S. Pat. No. 5,266,073 also describes a stent, tubular, that has slots machined into it. When expanded, the edges of the stent lock to form a cylinder. Not only is this device stiff and can only be used for short lesions, but also the diameter cannot be adjusted to meet the exact needs of the particular vessel but it is fixed to the predetermined sizes.  
           [0014]    Lau and Hastigan, U.S. Pat. No. 5,344,426 describes a slotted tubular stent that has a structure similar to Henry Wall&#39;s but has provided prongs that will lock in as the stent is expanded.  
           [0015]    Michael Marin, U.S. Pat. No. 5,397,355 also describes a tubular slotted stent with locking prongs.  
           [0016]    All the above-described tube stents, although typically providing substantial radial support when expanded, are not flexible enough to be placed in curved vessels. Arteries and veins in the human body are mostly curved and are tapered. As such, these tube stents suffer from this main disadvantage.  
           [0017]    European patent document 042172982 employs wires that are doubled up and whose ends are snipped off to make a given joint. Such doubling up at the junction of two elements with snipped off free ends creates a potential puncture problem upon radial expansion. The sheer bulk of the doubled up wires makes them rotate radially outwardly away from the longitudinal center-line of the stent, while the plain ends on such an arrangement which are snipped off offer the potential of sharp points which can puncture or damage the intima. On the other hand, the apparatus of the present invention, employing sharp angles, as defined, avoids this problem in an embodiment which illustrates a continuous wire or wire-like member bent into a sharp angle. This type of structure alleviates the concerns of sharp edges, as well as the tendency of a doubled up heavy joint to rotate outwardly toward the intima upon radial expansion of the stem, as would be expected in the EPO reference 042172982.  
           [0018]    Often these stents are layered with polymeric sheaths that are impregnated with biocompatible substances or can be coated with heparin or hydrogel. Most sheath-type coatings reduce endothelial cell growth through the stent, which is a major requirement in successful stenting of body cavities such as arteries and veins.  
           [0019]    One of the problems with prior designs of slotted tube and wire stents is that in their expanded state, the openings in them become fairly large. This allows tissue to protrude through these openings or windows. When there are protrusions into the body cavity through the stent, it causes disturbances to the blood flow, causing activation of platelets causing blood clotting. This phenomenon can also enhance the process of restenosis due to the large area exposed for neointimal formation.  
           [0020]    [0020]FIG. 1 depicts two rings of a stent of a design known in the prior art. Rings  10  and  12  are each sinusoidal, having respective peaks  14  and  16  joined together by crossties such as  18 . Respective valleys  20  and  22  are deposed opposite each other to create a lengthy elongated opening  24 , which has a length  26  nearly as long as the distance from opposing valleys  20  and  22 . FIG. 2 illustrates what happens to the oblong openings  24  when the rings  10  and  12  are expanded radially to set the stent of the prior art shown in FIG. 1. As shown in FIG. 2, each of the openings  24  is quite large, allowing tissue growth to enter therethrough, as shown in FIG. 3, which shows more rings than the rings  10  and  12  illustrated in FIG. 1. The tissue growth  28  significantly constricts the blood flow passage through the stent of the prior art shown in FIGS.  1 - 3 .  
           [0021]    One of the objectives of the present invention is to provide a stent which overcomes this problem. Alternative solutions are illustrated to achieve the objective of making the opening smaller to provide better resistance to tissue growth into the blood flow passage through the stent. Thus, in one embodiment, the objective is accomplished by nesting adjacent rings which have sinusoidal bending so as to more closely pack them to reduce the opening sizes between them. In yet another embodiment, adjacent sinusoidal rings are made to be overlapping to again accomplish the objective of deceasing opening sizes in the expanded state for a stent of a given diameter and length. These and other objectives will be readily apparent to those skilled in the art from a description of the preferred embodiments of the invention below.  
         SUMMARY OF THE INVENTION  
         [0022]    A flexible stent is disclosed which reduced openings between rings by two alternative techniques. In the first instance, adjacent sinusoidally bending rings are nested to compact them closer together to reduce the opening sizes therebetween. In another embodiment, adjacent sinusoidal rings overlap each other to achieve the same effect. The nested design can be fabricated from a long continuous sinusoidal band having alternating high and low peaks wrapped spirally around the mandrel and welded at crossties. The assembly can be covered with a graft as a support therefor. 
       
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a partial flattened view of a stent of the prior art, showing sinusoidal rings with crossties connecting adjacent peaks.  
         [0024]    [0024]FIG. 2 is a view of one of the openings formed between adjacent rings in the prior art stent shown in FIG. 1.  
         [0025]    [0025]FIG. 3 is a section view through the stent shown in FIG. 1.  
         [0026]    [0026]FIG. 4 illustrates two rings of a stent of one of the embodiments of the present invention shown in a flattened form.  
         [0027]    [0027]FIG. 5 illustrates one of the elongated openings of the stent of FIG. 4 after expansion.  
         [0028]    [0028]FIG. 6 is a section view of the stent of FIG. 4.  
         [0029]    [0029]FIG. 7 shows how adjacent rings are connected to each other to form the stent of FIG. 4.  
         [0030]    [0030]FIG. 8 shows an alternative embodiment to the stent of FIG. 4 and the technique for connecting adjacent rings.  
         [0031]    [0031]FIG. 9 shows the stent of FIG. 8 with two overlapping rings connected.  
         [0032]    [0032]FIG. 10 is the stent of FIGS. 8 and 9 showing four overlapping rings.  
         [0033]    [0033]FIG. 11 is the stent of FIG. 4 with a graft around the outside.  
         [0034]    [0034]FIG. 12 illustrates a technique for making the stent of FIG. 4 which is an alternative to the technique of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    Referring to FIG. 4, the preferred embodiment is illustrated. A portion of a stent S is shown in FIG. 4. More specifically, adjacent undulating rings  30  and  32  are illustrated. Each of these rings has an undulating, preferably sinusoidal shape with alternating high and low peaks. For example, in ring  30  alternating high peaks  34  exist between low peaks  36 . Between peaks  34  and  36  are valleys  35 . The height is defined as the distance from valleys  35  to peaks  34 . The crossties  38  connect ring  30  to ring  32 . Crossties  38  are optional as ring  30  can be connected directly to ring  32  without them. Crossties can connect peaks to peaks, valley to valley, or at least one peak or one valley to another location on the next ring. Ring  32  has alternating high peaks  40  and low peaks  42 . The high peaks  40  of ring  32  are juxtaposed against the low peaks  36  of ring  30 . Similarly, the low peaks  42  of ring  32  are juxtaposed against the high peaks  34  of ring  30 . The result of this construction is that the oblong openings  44  get smaller as adjacent rings such as  30  and  32  in a typical stent S get nested into each other. The minimum length of opening  44  is less than the combined height of peaks  34  and  42 , including the material. FIG. 5 illustrates the resultant shape of the opening  44  after radial expansion of the rings such as  30  and  32  which make up the stent S. FIG. 5 can be compared to FIG. 2 to illustrate that the typical opening in the stent S of the preferred embodiment of the present invention is smaller than the large oblong openings  24 , which necessarily arise when rings such as  10  and  12  are aligned adjacent peak  14  to peak  16  with a crosstie  18  in between. In a sense, the rings  10  and  12  of the prior art get no closer to each other then the length of the crosstie  18  and there is no nesting or overlapping between rings  10  and  12  of the design in the prior art. Referring to FIG. 11, additional rings are shown besides rings  30  and  32 . These rings  46  and  48  reflect a continuation of a pattern. As can be seen from FIG. 11, the peaks  50  of ring  46  extend in alignment but in the opposite direction from the low peaks  42  of ring  32  to create an opening  44 ′ which is the same size as opening  44 . In the embodiment shown in FIG. 11, a graft  52  is mounted over the stent S. Those skilled in the art will appreciate that, in the preferred embodiment, the openings  44  and  44 ′ are identical and form a spiral pattern around the periphery of the stent S. The spiral pattern is continued with openings  44 ″ which exist between rings  46  and  48 . Those skilled in the art will appreciate that each ring does not need to be identical to its adjacent ring. It is within the scope of the invention that the greatest peak-to-valley height is varied from one ring to the next. The preferred embodiment is to make such height longer at ends of the stent and shorter in between. Also, the degree of nesting of adjacent rings such as  30  and  32  can be varied along with the width of openings such as  44  by adjusting the heights of the corresponding peaks and valleys. The prior art FIG. 1 depicts an extreme in the spacing between adjacent rings which provides the maximum width of openings  24 . At the other extreme, adjacent rings such as  30  and  32  in effect become a single ring. The present invention is directed to the range of designs in between the two stated extremes which result in narrowing the longitudinal gap such as  44 ,  44 ′, and  44 ″, etc., while maintaining the rings such as  30 ,  32 ,  46 , and  48  distinct and connected with crossties such as  38 . Referring to FIG. 12, an alternative technique to making individual rings such as  30 ,  32 ,  46 , and  48 , and connecting such as by welding the crossties such as  38  in between is illustrated. Here in FIG. 12, a single elongated band is produced having alternating peaks and valleys as described previously, with at least some but preferably each of the high peaks and low peaks such as  34  and  36  having a crosstie  38  thereon. The assembly shown in the top part of FIG. 12 is then wrapped around a mandrel  54  in a spiral fashion such that the crossties  38  bridge between a low peak  36  and a high peak  34 , as illustrated in the lower part of FIG. 12. In essence, the same stent S is produced as is shown in FIGS. 4 and 11, with the difference being that the elongated openings such as  44  are skewed with respect to the longitudinal axis  56  but are still disposed in a generally spiral pattern akin to that shown in FIG. 11, despite the skew shown in FIG. 12 due to the method of assembly. The stent S of FIG. 11 is assembled in a technique shown in FIG. 7, which involves taking adjacent rings such as  30  and  32 , aligning them as previously described, and welding the crossties  38  to join ring  30  to ring  32  in alternating locations. Each opening  44  is identified by a welded crosstie  38  above and below.  
         [0036]    Accordingly, those skilled in the art can see that when the stent S of the preferred embodiment illustrated in FIG. 4 is inserted into a vessel as shown in FIG. 6, the gaps, such as between rings  30  and  32  which define the width of openings  44 , are significantly smaller than the oblong openings  24  between rings  10  and  12  of the prior art as shown in FIG. 3. Thus, the central passage  58  is not obstructed by an invasion of tissue  28  in the design of FIG. 4, illustrated in section in FIG. 6. This should be contrasted to the constriction and internal roughness of the passage  60  as illustrated in FIG. 3.  
         [0037]    Yet another embodiment of the present invention is shown in FIGS. 8 through 10. Here, adjacent rings  62  and  64  are built identically with an undulating, preferably sinusoidal shape, with each bend preferably having the same height  65  as the adjacent band akin to the individual rings  10  and  12  illustrated in the prior art. Varying heights can also be used. However, the method of connection of adjacent rings  62  and  64  is substantially different wherein the crossties  66  are connected to an opposing valley  68 . Thus, for example, as shown in FIG. 8, the crossties  66  from a peak  70  connect to valley  68  of ring  62 . The crossties can be connected to other locations. FIG. 9 illustrates the appearance of rings  62  and  64  after the crossties  66  are positioned for welding. As can readily be seen, the ring  64  overlaps ring  62 . The amount of overlap can be varied with a variety of techniques, such as variation of the length of the crossties  66  or the peak-to-valley heights of either of the rings  62  or  64 . FIG. 10 illustrates rings  62  and  64  with additional rings  70  and  72 . As seen in FIG. 10, crossties  74  extend from ring  62  at its various peaks  76  to be connected to peaks  78  of ring  70 . The crossties  74  literally extend between the undulations of ring  64  to reach the peak  78  of ring  70 . Ring  72  is connected to ring  70  by crossties  80 , putting ring  72  in an overlapping relationship with ring  70 , while ring  70  overlaps ring  64  and, in turn, ring  64  overlaps ring  62 . As shown in FIG. 10, it can be seen that a series of oblong openings of different sizes are provided. Openings  82  are the widest in this design and their width is affected by the configuration of the individual rings as well as the length of the crossties connecting them so that the width of opening  82  can be lengthened or shortened as desired. Those skilled in the art will appreciate that the smaller the width of opening  82 , the stiffer the stent and the more difficult the stent can become to maneuver. The stent of FIG. 10 is generally stiffer than the stent of FIG. 4 in view of the fact that the rings  62 ,  64 ,  70 , and  72  overlap each other, generally increasing the thickness of the stent being formed and somewhat decreasing its central passage when compared to a comparable design using the nesting technique shown in FIG. 4. The advantage of the design in FIG. 10 is that the size of the openings, particularly their width, can be more carefully controlled and reduced to present the stent with a smaller opening area so as to take maximum advantage of the smaller openings to obtain the desired effect shown in FIG. 6. Thus, either of the two designs can be used alternatively, depending on the application and the accessability to the location for setting the particular stent. The desirable advantage of either design is that the intrusion of tissue due to overly large openings, which present themselves after expansion in the designs of the prior art such as shown in FIG. 1, are dramatically reduced with either of the alternative designs illustrated.  
         [0038]    The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.