Abstract:
A stent is disclosed which comprises generally of ring having, in the preferred embodiment, crossties that have flexibility by having at least one bend. The rings themselves have predetermined stress-relieving points to predispose, by stress relief, particular segments of each ring to bend upon application of an expansion force such as by a balloon or by other means. In the preferred embodiment, the individual rings have notches, reducing the cross-sectional areas at particular locations adjacent reversing bends such that upon radial expansion, bending occurs at these reduced cross-sectional areas to prevent stress from accumulating at the reversing bends.

Description:
This application is a continuation of copending application Ser. No. 08/582,657, filed on Jan. 4, 1996. 
    
    
     FIELD OF THE INVENTION 
     The field of this invention relates to vascular stents that can be delivered to a predetermined position and allowed to spring outwardly or, in the alternative, which can be expanded in place. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     U.S. Pat. No. 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 stents are required. These designs do not provide any support to hold the wall of the artery, other than the memory of the metal. 
     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. 
     Wiktor, U.S. Pat. No. 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. 
     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. 
     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. 
     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. 
     When we examine the simple coils, such as taught in Scott U.S. Pat. No. 5,383,928 or Gene Samson U.S. Pat. No. 5,370,691 or Rolando Gills U.S. Pat. No. 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. 
     The other types of stents that have been developed are tube stents. Palmer, U.S. Pat. No. 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. 
     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. 
     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. 
     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. 
     Michael Marin, U.S. Pat. No. 5,397,355 also describes a tubular slotted stent with locking prongs. 
     U.S. Pat. No. 5,443,500 illustrates the use of square openings with rectangular prongs that stick therethrough to lock the stent. This design, as well as other locking mechanisms, generally have resulted in very stiff stents because of the use of a tubular-type grid construction. Further, the locking devices have resulted in sharp outwardly oriented tabs which are used for the locking, which could cause vascular damage. 
     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. 
     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 centerline 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. 
     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. 
     Several parameters in design of stents are important. Of the more important parameters is the issue of recoil. Recoil deals with the memory of the stent material which, generally speaking, upon expansion in the blood vessel will want to recoil back to its original shape. This can be problematic because it is desirable for the stent, once expanded, to remain in good contact with the vessel wall to avoid longitudinal shifting. Furthermore, any recoil constricts the flow passage and presents a greater portion of the stent in the blood flowpath, thus creating additional complications due to the turbulence which ensues. 
     Related to the concern regarding recoil is another concern regarding component twist. This phenomenon generally occurs when the cross-sectional area of the components is rectangular, such as when the stent is manufactured from a cylindrical piece which is then cut by lasers or other means to form the particular pattern. Particularly in the honeycombed designs involving the use of square or rectangular element cross-sections, radial expansion of such stents generally results in a twist of the component segments such that they extend into the flowpath in the artery or vein. Again, this causes turbulence which is undesirable. 
     Related to the problem of recoil or constriction after expansion is the ability of the stent to anchor itself in the vascular wall. An anchoring system that does not cause trauma is a desirable feature not found in the prior art. 
     Yet other considerations which are desirable in a stent not found in the prior art is the flexibility to be maneuvered around bends in the vascular system, coupled with the ability to conform to a bend without kinking or leaving large open areas. The stents of the present invention have the objective of addressing the issue of recoil, as well as providing an anchoring mechanism to fixate the stent once set. Several of the designs incorporate flexibility to allow the stent to follow a bend or curve in a vascular flowpath while a the same time providing sufficient radial deformation to ensure proper fixation while minimizing angular twisting movements of the stent components to minimize turbulence through the stent. 
     In a recent article appearing in late 1995, by Dr. Donald S. Baim, entitled “New Stent Designs,” a description is given of the ideal endovascular prosthesis. There, Dr. Baim indicates that the ideal stent should have low implantation profile with enhanced flexibility to facilitate delivery. He goes on to say that the stent should be constructed from a noncorrosive, nonthrombogenic radiopaque alloy and have expanded geometry which maximizes radial strength to resist vascular recoil. The ideal stent described by Baim is further described as having a wide range of diameters and lengths. Dr. Baim concludes that it is unlikely that any current designs satisfy all these requirements. Thus, one of the objectives of the present invention is to go further than the prior designs in satisfying the criteria for the ideal designs as set forth by Dr. Baim in his recent article. 
     SUMMARY OF THE INVENTION 
     A stent is disclosed which comprises generally of ring having, in the preferred embodiment, crossties that have flexibility by having at least one bend. The rings themselves have predetermined stress-relieving points to predispose, by stress relief, particular segments of each ring to bend upon application of an expansion force such as by a balloon or by other means. In the preferred embodiment, the individual rings have notches, reducing the cross-sectional areas at particular locations adjacent reversing bends such that upon radial expansion, bending occurs at these reduced cross-sectional areas to prevent stress from accumulating at the reversing bends. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the stent of the present invention in an unrolled condition prior to expansion. 
     FIG. 2 is the stent shown in FIG. 1 in an unrolled condition after expansion. 
     FIG. 3 is a section along lines A—A of FIG.  1  and illustrates several different cross-sectional shapes that can be used for the stent illustrated in FIG.  1 . 
     FIG. 4 is a detailed view of the stent in FIG. 1, shown without any cross-sectional changes to the undulating design of the ring structure illustrated in FIG.  1 . 
     FIG. 5 is similar to FIG. 4 except that it employs singular notches adjacent reversing bends. 
     FIG. 6 employs a change in the cross-sectional shape taking place adjacent each reversing bend. 
     FIG. 7 illustrates a joint involving a transverse tab adjacent the reversing bends. 
     FIG. 8 involves opposed notches on each side of the wire adjacent a reversing bend. 
     FIG. 9 illustrates what occurs on radial expansion of each of the rings without the use of a stress-relief mechanism such as a notch or a cut-out. 
     FIG. 10 illustrates the action upon radial expansion using a notch and its effect on the reversing bend. 
     FIG. 11 is a perspective view of the stent shown in FIG. 2 in the expanded position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows, in flattened out form, a stent S which is unrolled along its longitudinal axis. The stent S has a series of rings  10  which are preferably of a wire material (preferably stainless steel, nickel-titanium alloys, tantalum alloys) bent in a series of reversing undulations  12  and  14 . The wire can be coated with polymer such as polyethylene, polytetrafluoroethylene (Teflon®), or polylactates containing heparin or drugs or radioactive material. The bends  12  may have a similar radius or may vary as among bends  12  or as among bends  14 . In other words, each of the bends  12  may be identical to each other. Each of the bends  14  may be identical to each other. Each bend  12  may be identical to each bend  14 . One bend  12  can be different from another bend  12 , which is in turn also different from another bend  14 , or any combinations of the above. While rounded bends are shown as  12  and  14 , other shapes can be used to create a generally undulating pattern, such as sharp bends which generally form a V-shape. Connecting each row  10  is one or more crossties  16 . In the preferred embodiment, the crossties  16  have flexibility in that they have at least one bend  18 , while a double bend, such as including  18  and  20 , is preferred for the construction of the crossties  16 . One or more crossties can be used which connect a bend  14  to its opposing bend  12 . Thus, as shown in FIG. 1, the crossties  16 , looking from bottom to top, make a bend to the left and a bend to the right on their way from reverse bend  12  to a reverse bend  14 . One or more crossties  16  can be used between rings  10  up to a maximum where every reversing bend, such as  14 , is connected to an adjacent but offset circumferentially reversing bend  12 . 
     FIG. 2 illustrates the stent S in a radially expanded form, illustrating that the crossties  16  continue to retain flexibility because of the reversing bends  18  and  20 . Thus, the longitudinal flexibility of the stent S is retained, even in the expanded position. The use of the crossties with, at minimum, a single bend gives them flexibility. The design involving rings  10  connected by crossties  16  prevents stiffness experienced in some prior designs that had a particular longitudinal segment with undue stiffness giving the stent S a “backbone,” thus making it unduly stiff longitudinally. Use of the flexible crossties  16  also provides flexibility for relative rotation between rings  10  while the expansion is taking place. Flexibility is also provided in the longitudinal direction as the crossties  16  may elongate in that direction without putting the stent S into a kink or a longitudinal bind. 
     FIG. 3 illustrates alternative cross-sectional shapes for the wire cross-section which makes up each of the rings  10  and/or the crossties  16 . Thus, FIG. 3 illustrates squares, rectangles, circles, ovals, and composite shapes. 
     One of the concerns with an undulating structure, such as illustrated in FIG. 1, is the reversing bends  12  or  14 , unless some provisions are made, experience undue stress and are even prone to bending out of their plane when the stent is radially expanded. This phenomenon is illustrated in FIG.  9 . There, a pair of straight segments  22  and  24  are joined together by a reversing bend  26 . As illustrated in FIG. 9, the cross-sectional area of the segments  22  and  24  are rectangular, one of the shapes shown in FIG.  3 . It should be noted that other cross-sections, apart those illustrated in FIG. 3, can be used without departing from the spirit of the invention. 
     With no significant cross-sectional change occurring at the transition or near the transition  28  between the reverse bend  26  and the segments  24  or  22 , the stress is transferred to the reverse bend  26  when an expansion force F tries to radially expand the stent S by moving segments  22  and  24  apart. Depending on the amount of stress induced, a bending occurs, as shown in FIG. 9, where the reverse bend  26  bends out of plane so that it is no longer in alignment with the segments  22  and  24 , which was its condition prior to the application of force F. 
     FIG. 10 shows the contrast of the behavior of the reverse bend  26  when a notch  30  is placed adjacent the transition  28  between the reverse bend  26  and the segment  22  and a similar notch  32  is placed near transition  34  between the reverse bend  26  and the segment  24 . What results is a reduced cross-sectional area at transitions  28  and  34 . Thus, when force F is applied to the segments  22  and  24 , there is a permanent bending occurring at the zone of least cross-sectional area, i.e., transitions  28  and  34 , with their respective notches  30  and  32 . Accordingly, the stress from radial expansion of a ring  10  as illustrated in FIG. 1 is absorbed by a bending or deformation at the transitions  28  and  32 , thus minimizing if not eliminating the applied stress to the reverse bend  26  after radial expansion of the stent S by expanding all of the rings  10 . This type of structure illustrated in FIG. 10 can be employed in the unrolled stent shown in FIGS. 1 and 2. 
     Other alternative mechanisms for reducing the stress at the reverse bend are illustrated in FIGS. 5-8. It should be noted that the features illustrated in FIGS. 5-8 are to be found in the stent shown in FIGS. 1 and 2; however, in order to show the overall layout of the stent S, FIGS. 1 and 2 are not sufficiently magnified so that these details can be seen. However, FIGS. 5-8 represent a greater magnification of adjacent reverse bends, such as  12  and  14 . 
     In FIG. 6, the connecting segments  36  and  38  have a smaller cross-sectional area than the cross-sectional area at the reverse bends  12  and  14 , thus creating zones of transition of cross-section  40  adjacent reverse bend  14  and  42  adjacent reverse bend  12 . This construction is typical for each of the rings  10  of a particular stent. It should be noted that the various features illustrated in FIGS. 5-8 can be used uniformly throughout the stent or mixed and matched for a desired effect. 
     The detail in FIG. 7 illustrates a cross-sectional area transition point  44  and  46 , respectively adjacent reverse bends  12  and  14 . Here, there is not only a transition cross-sectional area but transverse tabs  48  are used to secure the joint between segments  50  and  52 , which have a smaller cross-sectional area than the cross-sectional area of reverse bends  12  and  14 . 
     FIG. 8 illustrates the use of opposed notches  54  and  56  adjacent the entrance and exit to each reverse bend  12  and  14 . FIG. 5 illustrates the use of similar notches  58  and  60  at the entrance and exit of each reverse bend  12  and  14 . The difference between FIG.  5  and FIG. 8 is that in FIG. 8, the notches  54  and  56  oppose each other at the entrance and exit of each reverse bend  12  or  14 , while in FIG. 5 the notches can be interiorly located, as shown in FIG. 5, or in the alternative, exteriorly located at the entrance and exit to each reverse bend  12  and  14 . It should be noted that the changes in cross-sectional area do not need to be literally at the point of transition between the rounded portion of a reverse bend  12  or  14  and the straight segment which adjoins the reverse bends. However, the preferred location is at that transition. Locating the cross-sectional area change before entering the transition from the straight segment to the curved segment is also possible, depending on the degree of stress relief desired. 
     FIG. 11 illustrates the stent S shown in unrolled form in FIGS. 1 and 2 in a perspective view after radial expansion. It should be noted that the crossties  16  retain their flexibility, even after expansion, and that the reverse bends  12  and  14  have not buckled out of the cylindrical surface defined by the expanded stent S shown in FIG.  11 . The buckling feature, which can occur in prior designs without the stress relief mechanism, is illustrated in FIG.  9 . 
     FIG. 4 illustrates that it is within the purview of the invention to use a plurality of rings  10  connected by flexible crossties  16  without the change in cross-sectional area occurring at the reverse bends  12  and  14 . While the embodiments in FIGS. 5-8 are preferred, it is within the purview of the invention to provide a stent with a multiplicity of rows  10  of undulating wire components which are connected by one or more crossties  16 , each of which have at least one bend so that upon radial expansion into the position shown in FIGS. 2 and 11, the crossties  16  continue to retain flexibility in at least one but preferably more directions. Thus, the individual rings  10  have longitudinal flexibility and may rotate to some degree with respect to each other, all to conform to the tortuous path in which the stent S may be placed. By adding the change in the cross-sectional area feature, as shown in FIGS. 5-8, by using one or more of those features in a single stent, a stent is produced that is flexible, yet when expanded, retains its flexibility and is not subjected to stress to a significant degree at reversing bends after complete radial expansion. By focusing the stress occurring during radial expansion to a particular point outside the reversing bend, a simple-to-make construction occurs which addresses the concerns of some of the prior art designs which have tackled this problem by using varying degrees of curvature, such as European application No. 0662307, assigned to Advanced Cardiovascular Systems. This design, with the flexible crossties  16 , represents a considerably more flexible design than rolled up coil springs such as that illustrated in U.S. Pat. No. 4,969,458. Crossties which are essentially straight, such as those illustrated in U.S. Pat. No. 5,421,955, do not afford the flexibility realized by the stent S of the present invention. It should be noted that as more bulk is presented at the transition between segments such as  22  and  24  in FIG. 9, the more likely is the bending to occur when subjected to radial expansion, as illustrated schematically by force F. Thus, designs that use doubled up wires at the apex, such as European application No. 0421729, assigned to Medtronic, exacerbate the bending results shown in FIG. 9, as well as increasing the stiffness of the stent and the force necessary for radial expansion of each of its individual rings. Additionally, by use of crossties which are coiled springs which protrude out of the cylindrical surface defined by the stent S, additional complications are created since the crossties will intrude into the vascular wall, creating additional irritation to the patient or worse damage if there is penetration of the vascular wall. 
     Accordingly, the above-described stent S of the present invention has the advantages of flexibility in view of the unique crossties which are used. The crossties remain in the cylindrical surface defined by the shape of the stent S, even upon radial expansion. The crossties  16  retain their flexibility, even after full radial expansion occurs. By use of the cross-sectional area changes, the applied stresses from radial expansion are focused to this transition zone as opposed to other places, such as the return bends. By focusing the deformation to the transition zone, stress is minimized or reduced in the reverse bend section, such as  12  or  14 , and further the tendency of the reverse bends such as  12  or  14  to protrude out of the cylindrical surface defined by the stent S is greatly reduced, if not eliminated. 
     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.