Flexible stent

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.

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'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'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.

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.RTM.), 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.