Abstract:
The invention is directed to an expandable stent for implantation in a body lumen, such as a coronary artery or peripheral vein. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common longitudinal stent axis and interconnected by one or more interconnecting members placed so as to limit longitudinal contraction during radial expansion. The individual radially expandable cylindrical elements are formed in a serpentine pattern having bends alternating in peaks and valleys designed to expand evenly under radial stress, and to maximize the overall radial expansion ratio. Each peak and valley includes reinforcing members that extend across and proximate to each bend. Sizing and construction of the struts forming the peaks and valleys can create bimodal deployment wherein the struts bend under increasing stresses to enable the stent to expand to larger diameters.

Description:
This application is a continuation of U.S. Ser. No. 08/881,059 filed Jun. 24, 1997, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a body lumen of a patient, such as a blood vessel, to maintain the patency thereof. These devices are useful in the treatment and repair of atherosclerotic stenoses in blood vessels. 
     Stents are generally cylindrically-shaped devices which function to hold open and sometimes to expand a segment of a blood vessel or other anatomical lumen. They are particularly suitable for use to support and hold back a dissected arterial lining which, if not so supported and held, can occlude the fluid passageway therethrough. 
     A variety of devices are known in the art for use as stents and have included: coiled wires in an array of patterns that are expanded after having been placed intraluminally via a balloon catheter; helically-wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted in a compressed state and shaped in a zig-zag pattern. Some more examples are shown in U.S. Pat. No. 4,776,337 to Palmaz; U.S. Pat. No. 4,655,771 to Wallsten; U.S. Pat. No. 4,800,882 to Gianturco; U.S. Pat. No. 4,913,141 to Hillstead; and U.S. Pat. No. 5,292,331 to Boneau. 
     Such prior art devices include an expandable intraluminal vascular graft that is expanded within a blood vessel by balloon associated with, typically, a dilatation catheter. The graft may be a wire mesh tube, a stainless steel tube with rectangular openings, or a tube with honeycomb style openings. Another prior art device includes a prosthesis for transluminal implantation comprising a flexible tubular body made of flexible thread elements wound together, each thread having a helix configuration. 
     There are still more conventional endovascular stents. In one design, the wire stent has a generally cylindrical shape, wherein the shape is formed with alternating bent wire loops. Another conventional stent design comprises a series of continuous corrugations compressed together to form a tube-like mesh. Yet another endovascular stent used for the treatment of restenosis is a unitary wire structure, shaped to criss-cross and form a plurality of upper and lower peaks. 
     One of the difficulties encountered using prior art stents involved maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery. Another problem area was the limiting range of expandability. Certain prior art stents expanded only to a limited degree due to the uneven stresses created upon the stents during radial expansion. This necessitated providing stents having a variety of diameters, thus increasing the cost of manufacture. Additionally, having a stent with a wider range of expandability allowed the physician to re-dilate the stent if the original vessel size was miscalculated. 
     Another problem with the prior art stents was that the stent contracted along its longitudinal axis upon radial expansion of the stent. This caused placement problems within the artery during expansion. 
     Various means have been devised to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location involved mounting the expandable stent on an expandable member, such as an inflatable balloon. The balloon was provided on the distal end of an intravascular catheter. The catheter was advanced to the desired location within the patient&#39;s body lumen. Inflating the balloon on the catheter deformed the stent to a permanently expanded condition. The balloon was then deflated and the catheter removed. 
     What has been needed and heretofore unavailable is a stent which has a high degree of flexibility so that it can be advanced through tortuous passageways and can be radially expanded over a wide range of diameters with minimal longitudinal contraction, and yet have the mechanical strength to hold open the body lumen into which it is expanded. There is further a need for a stent-that has high circumferential or hoop strength to improve crush resistance. The present invention satisfies these needs. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an expandable stent having a configuration generally of the type disclosed in U.S. Pat. Nos. 5,569,295 to S. Lam and 5,514,154 to Lau et al., the entire contents of which are incorporated herein by reference. In a preferred embodiment, the present invention stent includes a plurality of adjacent cylindrical elements which are expandable in the radial direction and which are arranged in alignment along a longitudinal stent axis. The cylindrical elements are formed in a serpentine wave pattern transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys. 
     The present invention also comprises at least one interconnecting member that extends between adjacent cylindrical elements and connects adjacent cylindrical elements to each other. The interconnecting members insure minimal longitudinal contraction of the stent during radial expansion of the cylindrical elements. 
     The present invention further comprises, in each cylindrical element, a reinforcing member that extends across each peak and valley. More precisely, each peak and each valley of a single cylindrical element is formed by the confluence of two straight struts joining at a bend. The reinforcing member thus spans across the peak or valley, bridging the struts. 
     The reinforcing member lends strength to the alternating peaks and valleys, wherein the area of maximum stress is at or near the bend. To be sure, the reinforcing member prevents the straight section of the strut from buckling or distorting during expansion of the stent by adding material to a potentially weak area. Furthermore, the size and geometry of the reinforcing member along with the bend may be adjusted so that stress is evenly distributed between the two instead of just being carried by the bend. 
     Certainly the geometry of the reinforcing member in the present invention can assume many configurations. For example, the reinforcing member could include a loop that curves toward or away from the bend. The reinforcing member could join the struts at a point farther away from or closer to the bend. The reinforcing member can be formed into the bend. 
     The resulting stent structure is preferably a series of radially-expandable cylindrical elements that are spaced longitudinally close enough to each other so that small dissections in the wall of a body lumen may be pressed back into position by the elements against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent. The individual cylindrical elements may rotate slightly relative to adjacent cylindrical elements without significant deformation, cumulatively providing a stent which is flexible along its length and about its longitudinal axis, but which is still very stable in the radial direction in order to resist collapse. 
     The stent embodying features of the present invention can be readily delivered to the desired lumenal location by mounting it on an expandable member of a delivery catheter, for example a balloon, and by then passing the catheter-stent assembly through the body lumen to the implantation site. A variety of means for securing the stent to the expandable member on the catheter for delivery to the desired location are available. It is presently preferred to compress the stent onto the balloon. Other means to secure the stent to the balloon include providing ridges or collars on the inflatable member to restrain lateral movement, or using temporary, bioabsorbable adhesives. 
     The present invention by use of the reinforcing members features bimodal deployment. That is, when the stent is expanded radially as described above, it does so in two stages. The first stage is the type of expansion of the stent radially wherein the struts bend slightly outward to accommodate the increasing circumference of each cylindrical element and the loop portion of the reinforcing member is stretched out. The second stage continues from the first stage with the struts continuing to bend outward and with the most severe bending occurring at the reinforcing member until the struts are pulled wide apart to their limits to accommodate the largest diameter that the stent can assume. Further spreading apart of the struts is prevented by the presence of the reinforcing member, which limits the maximum circumferential size attainable by each cylindrical element. By choosing the size and geometry of the reinforcing member and the struts, the amount of force needed to expand the stent to a particular diameter can be altered. 
     The cylindrical elements of the stent are preferably plastically deformed when expanded (except when nickel-titanium (NiTi) alloys are used as the elements) so that the stent remains in the expanded condition. Therefore, when non-NiTi elements are used, the elements must be sufficiently rigid when expanded to prevent the collapse thereof in use. With super-elastic NiTi alloys, the expansion occurs when the stress of compression is removed which relief causes the phase transformation of the material from the martensite phase back to the expanded austenite phase. 
     After the stent is expanded, some of the peaks and/or valleys may tip outwardly and become embedded in the vessel wall. Thus, after expansion, the stent does not have a smooth outer wall surface, but rather is characterized by projections which embed in the vessel wall and aid in retaining the stent in place in the vessel. 
     Other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view, partially in section, depicting a stent embodying features of the invention which is mounted on a delivery catheter and disposed within a body lumen such as a coronary artery. 
     FIG. 2 is an elevational view, partially in section, similar to that shown in FIG. 1, wherein the stent is expanded within the artery, pressing the dissected lining against the arterial wall. 
     FIG. 3 is an elevational view, partially in section, showing the expanded stent within the vessel after withdrawal of the delivery catheter. 
     FIG. 4 is an enlarged partial view of the stent of FIG. 5 depicting a serpentine pattern having peaks and valleys that form the cylindrical elements of the stent. 
     FIG. 5 is a plan view of a flattened section of a stent of the present invention which illustrates the serpentine pattern of the stent. 
     FIG. 6 is a side elevational view of the stent in the expanded condition. 
     FIGS.  7 (A)-(L) are top plan views of alternative embodiments of a single reinforced peak or valley. 
     FIG. 8 is a plan view of an alternative embodiment of the present invention reinforced stent. 
     FIG. 9 is another alternative embodiment of the present invention reinforced stent. 
     FIGS.  10 (A) and (B) show the bimodal deployment of a preferred embodiment stent. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates stent  10 , incorporating features of the invention, which is mounted onto delivery catheter  11 . The stent  10  generally comprises a plurality of radially expandable cylindrical elements  12  disposed coaxially and interconnected by members  13  disposed between adjacent cylindrical elements. The delivery catheter  11  has an expandable portion or balloon  14  for expanding stent  10  within an artery  15  or other vessel. The artery  15 , as shown in FIG. 1, has a dissected lining  16  which has occluded a portion of the arterial passageway. 
     The delivery catheter  11  onto which stent  10  is mounted is essentially the same as a conventional balloon dilatation catheter for angioplasty procedures such as percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA). The balloon  14  may be formed of suitable materials such as polyethylene, polyethylene terephthalate, polyvinyl chloride, nylon and ionomers such as those manufactured under the trademark SURLYN by the Polymer Products Division of the Du Pont Company. Other polymers also may be used. In order for stent  10  to remain in place on balloon  14  during delivery to the site of the damage within artery  15 , stent  10  is compressed onto the balloon. An elastic protective sheath is sometimes attached around balloon  14  so that stent  10  is crimped onto the sheath, which protects the balloon from the metal stent  10  and insures uniform expansion of the stent when the balloon and elastic sheath are expanded. A retractable protective delivery sleeve  20  also may be provided to further ensure that the stent stays in place on the expandable portion of delivery catheter  11  and to prevent abrasion of the body lumen by the open surface of stent  10  during delivery to the desired arterial location. Other means for securing stent  10  onto balloon  14  also may be used, such as providing collars or ridges on the ends of the working portion, i.e., the cylindrical portion, of the balloon. 
     Each radially expandable cylindrical element  12  of stent  10  may be independently expanded. Therefore, balloon  14  may be provided with an inflated shape other than cylindrical, e.g., tapered, to facilitate implantation of the stent  10  in a variety of body lumen shapes. 
     In a preferred embodiment, the delivery of the stent  10  is accomplished in the following manner. The stent  10  is first mounted onto inflatable balloon  14  on the distal extremity of delivery catheter  11 . The stent may be “crimped” down onto the balloon to ensure a low profile. The catheter-stent assembly can be introduced within the patient&#39;s vasculature in a conventional Seldinger technique through a guiding catheter (not shown). A guidewire  18  is disposed across the arterial section with the detached or dissected lining  16  and then the catheter-stent assembly is advanced over guidewire  18  within artery  15  until stent  10  is positioned within the artery at detached lining  16 . The balloon  14  of the catheter is expanded, expanding stent  10  against artery  15 , which is illustrated in FIG.  2 . While not shown in the drawing, artery  15  preferably can be expanded slightly by the expansion of stent  10  to seat or otherwise fix stent  10  to prevent movement within the artery. In some circumstances during the treatment of stenotic portions of an artery, the artery may have to be expanded considerably in order to facilitate passage of blood or other fluid therethrough. 
     Stent  10  serves to hold open artery  15  after catheter  11  is withdrawn, as illustrated by FIG.  3 . Due to the formation of stent  10  from an elongated tubular member, the undulating component of the cylindrical elements of stent  10  is relatively flat in transverse cross-section, so that when the stent is expanded, the cylindrical elements are pressed into the wall of artery  15  and as a result minimize the development of thrombosis in artery  15 . The cylindrical elements  12  of stent  10  which are pressed into the wall of artery  15  eventually will be covered with endothelial cell growth which further minimizes thrombosis. The serpentine pattern of cylindrical sections  12  provide good tacking characteristics to prevent stent movement within the artery. Furthermore, the closely spaced cylindrical elements  12  at regular intervals provide uniform support for the wall of artery  15 , and consequently are well adapted to tack up and hold in place small flaps or dissections in the wall of artery  15  as illustrated in FIGS. 2 and 3. 
     In the preferred embodiment, as depicted in FIGS. 4,  5  and  6 , the stresses involved during expansion from a low profile to an expanded profile are much more evenly distributed among the various peaks  36  and valleys  34 . As seen in FIG. 4, a portion of cylindrical element  12  of stent  10  illustrates the serpentine pattern having a plurality of peaks and valleys, each having varying radii of curvature, which aids in the even distribution of expansion forces. Interconnecting members  13  serve to connect adjacent valleys of cylindrical element  12  as described above. 
     After expansion, portions of the various elements will turn outwardly, forming small projections which will embed in the vessel wall. For example, the tip of peak portion  36  tips outwardly upon expansion a sufficient amount to embed into the vessel wall and help secure the implanted stent. Upon expansion, projecting peak  36  provides an outer wall surface on the stent that is not smooth, but instead has a plurality of projecting peaks  36  all along the outer wall surface. While the projections assist in securing the stent in the vessel wall, they are not sharp and thus do not cause trauma or damage to the vessel wall. 
     One important feature of the present invention is the capability of the stent to expand from a low-profile diameter to a diameter much greater than heretofore was available, while still maintaining structural integrity of the stent in the expanded state. Due to its novel structure, the stent of the present invention has an overall expansion ratio of 1 up to about 4 using certain compositions of stainless steel. For example, a 316L stainless steel stent of the present invention can be radially expanded from a diameter of 1 unit up to a diameter of about 4 units, which deforms the structural members beyond their elastic limits. The stent still retains its structural integrity in the expanded state and it serves to hold open the vessel in which it is implanted. Materials other than 316L stainless steel may give higher or lower expansion ratios without sacrificing structural integrity. 
     FIGS. 8 and 9 are plan views of a flattened section of stents  40 ,  42  of the present invention, which illustrate the serpentine patterns of the stents as well as varying configurations of reinforcing embers  44 ,  46 . In the preferred embodiment illustrated in FIG. 8, stent  40  is comprised of a plurality of radially expandable cylindrical elements  48  disposed generally coaxially and interconnected by interconnection members  50  which are essentially parallel to the longitudinal stent axis when the stent  40  is in the unexpanded low profile. 
     As in the earlier described embodiments, the present preferred embodiment shown in FIG. 8 includes alternating peak portions  52  and valley portions  54 . Each peak portion  52  or valley portion  54  is essentially a bend  56  interconnecting straight struts  58 . In this embodiment, each peak portion  52  or valley portion  54  each cylindrical element is reinforced by reinforcing member  44  extending across bend  56  to interconnect struts  58 . In the preferred embodiment depicted in FIG. 8, reinforcing member  44  has an inverted loop  60  that extends in a direction opposite to bend  56 . Optionally, interconnecting members  50  may be integrated into loop  60  of reinforcing member  44  as seen in FIG.  9 . 
     The area of peak stress is at or near the apex of bend  56 . The present invention provides apparatus for reinforcing this area with reinforcing member  44 , which is attached to each side of the bend (i.e., strut  58 ) away from the apex of bend  56 . The width of strut  58  along with the width and geometry of reinforcing member  44  as well as the geometry and dimensions of bend  56  forming peak portion  52  or valley portion  54  can be adjusted to distribute the stress between bend  56  and reinforcing member  44 . Furthermore, varying the base material of the stent would affect the design of bend  56  and reinforcing member  44 . 
     In FIG. 7, a variety of alternative embodiments of a peak portion or valley portion of a stent are shown. Specifically, reinforcing members of different constructions are shown in plan views. As seen in FIG.  7 (A), peak portion or valley portion  62  is formed by a bend  64  supported by struts  66 . Reinforcing member  68  has a V shape and is integrated into bend  64 . FIGS.  7 (B) and (C) show varying bend thicknesses. FIG.  7 (D) illustrates a reinforcing member  70  that intersects struts  72  wherein the point of intersection creates sharpened corners  74  that are rounded in FIGS.  7 (B), (C), (E), and (F). In FIGS.  7 (E) and (F), reinforcing member  70  has been moved farther down struts  72  away from bend  64 . FIG.  7 (G) depicts an alternative embodiment wherein reinforcing member  76  has been integrated into bend  78 . In FIG.  7 (H), reinforcing member  80  includes loop  82  that has been pinched together. FIG.  7 (I) is a plan view of an alternative embodiment reinforcing member  84  that has been integrated into bend  86  although slits  88  have been formed in the base material. In FIGS.  7 (J), (K) and (L), the shape of open areas  90 ,  92  have been adjusted to vary the strength at different parts of the stent. Moreover, in FIGS.  7 (J), (K) and (L), reinforcing member  94  has its orientation reversed as compared to the reinforcing members in the previous embodiments. 
     FIG. 9 is a plan view of an alternative embodiment stent  42  wherein the pattern of peaks and valleys have been modified to provide multiple side-by-side valley portions  96 . Furthermore, interconnecting member  98  is attached to bend  100  and transitions into a strut  102  at an opposite end. 
     The present invention further includes a bimodal feature as illustrated in FIGS.  10 (A) and (B). FIG.  10 (A) shows a single cylindrical element  104  having alternating peaks and valleys, wherein each peak and valley is formed by bend  106  joining two struts  108 . In the conditions shown in FIG.  10 (A), struts  108  have been slightly bent, which is the result of a first stage expansion of the stent thereby increasing the circumference of the stent. Thus, struts  108  are no longer parallel and have spread outwards. Reinforcing member  110  helps maintain the angle formed by struts  108 . 
     Also, FIG.  10 (A) shows the first mode in which reinforcing member  110  straightens and locks into position; the loop or kink previously formed in reinforcing member  110  is straightened. Reinforcing member  110  in this configuration provides substantial strength and stiffness to the stent. 
     In FIG.  10 (B), the stent has been expanded to a second stage thereby increasing the circumference of the stent to a greater degree than that shown in FIG.  10 (A). As the stent is expanded further, struts  108  bend at the intersections with reinforcing members  110  until they are aligned with the circumference of the stent as shown in FIG.  10 (B). At this point, the stent is fully deployed to its maximum diameter. Accordingly, struts  108  have been pulled straight and are nearly paralleled with reinforcing member  110 . In this mode, the stent has reached its maximum circumference; further increases in the stent can conceivably be achieved by deformation in struts  108  and reinforcing member  110 . Essentially, the circumference of the stent can be increased by stretching struts  108  and reinforcing members  110  further. 
     It is possible to deploy the stent and reinforcing member with or without two distinct modes. This behavior is controlled by the force required to bend struts  108  at their intersection with reinforcing member  110  as compared to the force required to bend and open the loop in the reinforcing member  110 . The behavior can be controlled by the relative widths and lengths of the various structures. 
     The tubing may be made of suitable biocompatible material such as stainless steel, titanium, tantalum, super-elastic nickel-titanium (NiTi) alloys and even high strength thermoplastic polymers. The stent diameter is very small, so the tubing from which it is made must necessarily also have a small diameter. For PCTA applications, and as an example only, typically the stent has an outer diameter on the order of about 0.065 inches (0.165 cm) in the unexpanded condition, the same outer diameter of the tubing from which it is made, and can be expanded to an outer diameter of about 0.200 inches (0.508 cm) or more. The wall thickness of the tubing is about 0.003 inches (0.008 cm). For stents implanted in other body lumens, such as in non-coronary PTA applications, the dimensions of the tubing forming the stent are correspondingly larger. The dimensions of the stent will vary depending upon the application and body lumen diameter in which the stent will be implanted. 
     In the instance when the stent is made from plastic, it may have to be heated within the arterial site where the stent is expanded to facilitate the expansion of the stent. Once expanded, it would then be cooled to retain its expanded state. The stent may be conveniently heated by heating the fluid within the balloon or the balloon directly by a known method. The stent may also be made of materials such as super-elastic NiTi alloys. In this case the stent would be formed full size but deformed (e.g. compressed) into a smaller diameter onto the balloon of the delivery catheter to facilitate transfer to a desired intraluminal site. The stress induced by the deformation transforms the stent from a austenite phase to martensite phase and upon release of the force, when the stent reaches the desired intraluminal location, the stent expands due to the transformation back to the austenite phase. 
     While the invention has been illustrated and described herein in terms of its use as an intravascular stent, it will be apparent to those skilled in the art that the stent can be used in other instances in all vessels in the body. Since the stent of the present invention has the novel feature of expanding to very large diameters while retaining its structural integrity, it is particularly well suited for implantation in almost any vessel where such devices are used. This feature, coupled with limited longitudinal contraction of the stent when it is radially expanded, provides a highly desirable support member for all vessels in the body. Other modifications and improvements may be made without departing from the scope of the invention.