Abstract:
Stents which are inserted into a body lumen preferably are made of materials which are not radiopaque enough, such as S.S. 316L. X-ray visualization of a stent enables an accurate positioning of the stent and also a follow-up of its functioning within the patient&#39;s body. The radiopaque markers described here are rivets made of a material which is more radiopaque than the stent substance so the location of the stent can be identified.

Description:
This is a continuation of application Ser. No. 08/632,739, filed Apr. 15, 1996, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to endoprosthesis devices, generally called stents, and, more particularly, to radiopaque markers for use with endoprosthesis devices. 
     BACKGROUND OF THE INVENTION 
     Stents are generally tubular shaped devices that function to hold open a segment of a blood vessel or other anatomical lumen and are useful in the treatment of atherosclerotic stenoses in blood vessels. Stents are particularly suitable for use in supporting and holding back a dissected arterial lining that can occlude the fluid passageway therethrough. 
     Structures used as stents or intraluminal vascular grafts include coiled stainless steel springs; helical wound spring coil made from shape memory alloy; expanding metal stents formed in a zig-zag pattern; diamond shaped, rectangular shaped, and other mesh and non-mesh designs. Some of the stents currently available employ a self expanding concept, whereby stent expansion is primarily achieved by removing a restraint mechanism holding the stent in a constricted configuration. Other stents in the prior art are delivered to the site by a balloon catheter system, and primarily employ balloon dilation to achieve proper stent expansion. 
     To accomplish precise placement of stents, various means are employed for identifying the position of the stent within a blood vessel. One means used for accomplishing precise placement of a stent is the attachment to the stent of radiopaque markers so that, through the use of fluoroscopy, the position of the stent within a blood vessel can be identified. Once the stent with its radiopaque markers has been implanted, identification of the stent position during subsequent checkups of the treated segment is easily accomplished because the markers remain visible under fluoroscopy. 
     In European patent application No. 95302708, assigned to ACS, Inc., a method of coating the stent edges as markers is described. However, this method has several practical disadvantages. First, heavy coating of radiopaque markers onto a stent is somewhat difficult to accomplish. In addition, the radiopaque material might not be attached properly to the stent material and may detach, leaving no way of identifying the position of the stent within the blood vessel. Furthermore, the radiopaque coating may increase the rigidity of the stent, thereby making proper placement difficult and decreasing the stent&#39;s effectiveness. 
     In another method for enabling the precise identification of a stent location using radiopaque markers, commonly assigned U.S. patent application Ser. No. 08/394,799, filed Feb. 27, 1995, discloses a hollow stent having radiopaque material inserted within the hollow stent wire. Because this method of providing radiopaque marking requires that the stent wire is hollow, this method might not be useful where a hollow stent is not desirable. 
     Another well-known method for enabling the precise identification of a stent location within a blood vessel is producing the stent itself from a radiopaque material such as tantalum. However, a disadvantage of this method is that tantalum is a relatively soft material and it is, therefore, necessary to use more of this metal to achieve sufficient support from the stent. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide improved stents. 
     It is also an object of the invention to provide stents having radiopaque markers. 
     It is a further object of the invention to provide stents where the distal ends of the stents comprise rivets of material that is more radiopaque than the material from which the stents are made. 
     These and other objects of the invention will become more apparent in the discussion below. 
     SUMMARY OF THE INVENTION 
     The radiopaque markers described below are designed for stents produced from a material that is not sufficiently radiopaque to be seen through the use of fluoroscopy, e.g., a material such as Stainless Steel 316L or nitinol. In order to identify the position of the stent during its insertion into the body and after it has been implanted, however, it is enough to mark the stent edges so that they may be seen under X-ray. The location of the stent will thus be evident based upon the pinpoint locations of its two ends. 
     According to this invention the edges of the stents are marked by inserting rivets through the ends or edges of the stents, which rivets are made of a material that is more radiopaque than the stent material. For example, if the stent material is S.S. 316L, the rivets can be made of gold, tantalum or platinum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which the reference characters refer to like parts throughout and in which: 
     FIG. 1 shows a side elevational view of a portion of a balloon-expandable stent structure having a round opening at each distal end or edge in accordance with an embodiment of this invention; 
     FIG. 2 shows a cross-sectional view taken along line A—A of FIG. 1 across the entire stent; 
     FIG. 3 shows a cross-sectional view of a first embodiment of a rivet inserted through the opening of FIG. 1; 
     FIG. 4 shows a cross-sectional view of a second embodiment of a rivet inserted through the opening of FIG. 1; 
     FIG. 5 shows a cross-sectional view of a third embodiment of a rivet inserted through the opening of FIG. 1; 
     FIG. 6 shows a cross-sectional view of a fourth embodiment of a rivet inserted through the opening of FIG. 1; 
     FIG. 7 shows a cross-sectional view of a fifth embodiment of a rivet inserted through the opening of FIG. 
     FIG. 8 shows a side elevational view of a portion of a balloon-expandable stent structure having a second embodiment of an opening at each edge; and 
     FIG. 9 shows a side elevational view of a portion of a balloon expandable stent structure having a third embodiment of an opening and showing a marker inserted through the opening. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Balloon expandable stent structures are well known in the art. In this invention, as schematically depicted in FIG. 1, a balloon-expandable stent structure  1  deliverable by catheter  21  has an opening  2  at each end of the stent end or edge. In one aspect of the invention, stent structure  1  includes a plurality of radially expandable members that include a first radially expandable member  22  and a second radially expandable member  23 , where the first and second radially expandable members  22 ,  23  are curved into peaks  24  and valleys  25 . In one example, the first radially expandable member  22  includes an integral portion that forms a perimeter of the opening  2 . FIG. 1 depicts the first radially expandable member  22  forming a first end  34  of the stent  1 . In addition, the peaks and valleys of the first radially expandable member  22  are arranged with the peaks and valleys of the second radially expandable member  23  such that the first and second radially expandable members have peaks and valleys that are paired with each other in an in-phase relationship. Interconnecting links  27 , each having a curved conformation, extend between first and second radially expandable members  22 ,  23  such that, for each opening  2 , as least one interconnecting link  27  is connected to first radially expandable member  22  at a point on the circumference  28  (perimeter) of the opening  2 . In addition, interconnecting links  27 , having the curved conformation, are connected by a connection at a first end  36  to a point  38  intermediate a peak and a valley on the first radially expandable member  22  and at a second end  40  to a point  42  intermediate a peak and a valley on the radially second expandable member  23 . The interconnecting links  27  illustrated in FIG. 1 have at least one bend  44  adjacent to the connecting point  38  on the first radially expandable member  22  and at least one bend  46  adjacent the connecting point  38  on the second radially expandable member  23  and at least one straight section  48  there between. In a first embodiment of the invention, the opening  2  are round. FIG. 2 shows a cross-section of the stent  1  and shows opening  2  passing radially from the external stent surface into the internal stent surface. 
     According to the invention a marker having radiopaque qualities is inserted through the opening at each end or edge of the stent to mark the ends of the stent so that the position of the stent can be determined by the location of its ends when the markers are seen under X-ray. Because the markers are placed securely into and through holes or openings, they are referred to as “rivets”. 
     FIG. 3 illustrates the placement of a rivet  3  into a stent opening  2 . Rivet  3  comprises a short rod made from a radiopaque material, which is compressed into the opening  2  in the axial direction  4 , thereby compressing the radiopaque material and causing a circumferential force  5  that enables rivet  3  to be held within opening  2  of stent  6 . Because retention of the marker rivet requires an opening whose aperture size does not vary while the rivet is inserted therein, the preferred location of the marker rivet is a region of the stent that is not deformed during expansion of the stent. In particular, for a balloon-expandable stent, this will usually be at an edge of the stent, at the end of the stent&#39;s lattice-like structure. 
     As shown in FIG. 4, an alternative manner of assuring that the rivet remains in the stent opening  2  is to form the rivet edges  7  with a cone-like, outwardly radiating shape, i.e., with beveled edges. Then, after the radiopaque rivet  8  is compressed into opening  2 , the rivet  3  cannot leave the stent due to the friction between the edges  7  of the rivet and the outer edges of the inner walls of opening  2 . 
     Another means of securing the rivet within the lumen is shown in FIG. 5, in which the marker rivet  9  is made with chamfered edges  10 . After rivet  9  is inserted into the opening  2 , the stent  1  is pressed at points  11  so the marker rivet  9  cannot displace from the stent. 
     In a further embodiment of the invention illustrated in FIG. 6, an enlarged head portion  13  of rivet  12  protrudes from opening  2  on one side of the stent in a diameter larger than that of the rivet portion situated within opening  2 . Thus, when the stent is viewed through X-ray, a larger height and greater diameter of the rivets and, therefore, a better visualization of the markers of the ends of the stent, is achieved. FIG. 7 illustrates a variation of this embodiment of the rivet in which the enlarged head portion  13  protrudes from both sides of the stent, achieving even larger height and greater diameter of the rivet and still better visualization of the rivet, and thus the stent ends, under X-ray. 
     FIG. 8 illustrates a second embodiment of the invention. FIG. 8 includes a first radially expandable member  50  with an integral portion that forms a perimeter  54  of opening  14 . The opening  14  of the stent  17  is non-round, such as oval. The non-round opening  14  allows a non-round rivet to be inserted therein. This serves to enlarge the rivet surface without interfering with the fluid flow within the stent. FIG. 8 further includes a second radially expandable member  56  at a second end  58  of the stent  17 . The second radially expandable member  56  can include an integral portion that forms a perimeter of at least one opening. A third radially expandable member can be included intermediate the first and second radially expandable members, where the stent can further include an integral portion of the radially expandable member that forms a perimeter of at least one opening. In one embodiment, the openings formed in the first and second radially expandable members are aligned with at least one opening formed in the third radially expandable member. The openings can also be located in a region of the stent that is not deformed during radial expansion of the stent. 
     In the embodiment of this invention illustrated in FIG.  1  through FIG. 8, marker rivets are completely surrounded by the stent material (e.g., perimeter) so that they either are contained within the stent material or project only from the external and internal surfaces of the stent. Another embodiment of the invention and an additional type of marker rivet is shown in FIG. 9, in which the integral portion of the stent  18  where the marker is placed is not a loop or an opening passing radially through the stent material  18  but is rather a circumferential space defined by a perimeter  60  that includes two circumferential detent projections  16 . The corresponding marker  15  is pressed circumferentially into the space defined by the perimeter  60  and is held there by specially designed projections  16  into adequate shape in the marker. This method enables an insertion of a marker with larger visible surface area by allowing the larger surface area to fit along and be co-extensive with the stent circumference as opposed to projecting radially into and out of the stent shape. 
     The above-described markers are designed for both stent extremities in order to define the ends of the stent during fluoroscopy. Nonetheless these markers can be combined onto the entire stent length and also on several places located along the stent circumference, so that the stent diameter can be detected during fluoroscopy, as well. It is within the scope of the invention that a stent could have, for example, from 2 to 20 marker rivets, located from about 0.5 to 5 cm apart longitudinally and/or from 1 to 4 rivets spaced radially, preferably equidistantly, or a combination thereof. 
     As described above, the rivets are preferably compressed into the stent material. Optionally, in addition to or in place of compression, the surface between the rivets and the stent can be heated to weld or fuse the rivets into position. Preferably such heating would be focused heating, for example, with a laser, where only the rivet and stent material would be present. 
     This invention is intended primarily for use with balloon-expandable stents, although it is envisioned that the technology disclosed herein is applicable to other medical devices, including, but not limited to, self-expanding stents. An important factor is that the material of the rivet be more radiopaque than the primary material used in the device. For example, if a balloon-expandable stent is comprised of stainless steel or nitinol, then rivets comprised of gold, platinum, or titanium would be useful. 
     The invention herein is not limited to a particular lattice-work for a balloon-expandable stent. However, the invention is especially useful with the balloon-expandable stents described in co-pending U.S. patent application Ser. No. 08/543,337, filed Oct. 16, 1995, now U.S. Pat. No. 5,776,161 incorporated herein by reference. 
     It will be further apparent to one skilled in this art that the improvements provided for in the present invention, while described with relation to certain specific physical embodiments also lend themselves to being applied in other physical arrangements not specifically provided for herein, which are nonetheless within the spirit and scope of the invention taught here.