Abstract:
Stent designs for use in peripheral vessels, such as the carotid arteries, are disclosed. The stents consist 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 that the stent is flexible in a longitudinal direction. The cylindrical elements are generally serpentine wave pattern transverse to the longitudinal axis between alternating valley portion and peak portions, the valley portion including alternating double-curved portions and U-shaped portions. The interconnecting members are attached to the double-curved portions to connect a cylindrical element to an adjacent cylindrical element and interconnecting members are attached to the U-shaped portions to connect the cylindrical element to the other adjacent cylindrical element. The designs include an eight crown and six crown stent which exhibit flexibility and sufficient radial strength to support the vessel.

Description:
This application is a continuation of Ser. No. 09/475,393 filed on Dec. 30, 1999, now U.S. Pat. No. 6,537,311, which is assigned to the same Assignee as the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient&#39;s body lumen, such as a blood vessel, to maintain the patency thereof. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis. 
     Stents are generally cylindrically-shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through. 
     A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve 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 and accommodate the often tortuous path of the body lumen. 
     Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent. 
     Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maas, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallstent); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference. 
     Further details of prior art self-expanding stents can be found in U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003 (Wolff, et al.), which are hereby incorporated by reference. 
     Expandable stents are delivered to the target site by delivery systems which often use balloon catheters as the means for delivering and expanding the stent in the target area. One such stent delivery system is disclosed in U.S. Pat. No. 5,158,548 to Lau et al. Such a stent delivery system has an expandable stent in a contracted condition placed on an expandable member, such as an inflatable balloon, disposed on the distal portion of an elongated catheter body. A guide wire extends through an inner lumen within the elongated catheter body and out its distal end. A tubular protective sheath is secured by its distal end to the portion of the guide wire which extends out of the distal end of the catheter body and fits over the stent mounted on the expandable member on the distal end of the catheter body. 
     Some prior art stent delivery systems for implanting self-expanding stents include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to “uncover” the compressed stent, allowing the stent to move to its expanded condition into the target area. 
     In many procedures which utilize stents to maintain the patency of the patient&#39;s body lumen, the size of the body lumen can be quite small which prevents the use of some commercial stents which have profiles which are entirely too large to reach the small vessel. In particular, often in PTCA procedures, the stenosis is located in the very distal regions of the coronary arteries which often have small diameters. Many of these distal lesions are located deep within the tortuous vasculature of the patient which requires the stent to not only have a small profile, but also high flexibility to be advanced into these regions. As a result, the stent must be sufficiently flexible along its longitudinal axis, yet be configured to expand radially to provide sufficient strength and stability to maintain the patency of the body lumen. Since many commercial stents lack both the low profile and extreme flexibility needed to reach such distal lesions, they are not available for utilization for such procedures. 
     What has been needed is a stent which has a low profile and a high degree of flexibility so that it can be advanced through tortuous passage ways of the anatomy and can be expanded within the body vessel to maintain the patency of the vessel. Additionally, the expanded stent must have adequate structural strength (hoop strength) to hold the body lumen open once expanded. Such a stent should also have sufficient radiopaque properties to permit it to be sufficiently visualized on external monitoring equipment, such as a fluoroscope, to allow the physician to place the stent in the exact target location. The present invention satisfies these and other needs. 
     SUMMARY OF INVENTION 
     The present invention is directed to stents having low profiles which can be used in body vessels, such as the carotid arteries and other peripheral arteries, along with the coronary arteries. The stents of the present invention are intended, but are not limited, to the effective treatment of diseased vessels having diameters from about 3.0 to 14.0 millimeters. 
     The stents of the present invention can be formed from super elastic nickel titanium alloys, or other shape memory materials, which allow the stent to be self expandable. The expansion occurs when the stress of compression is removed. This allows the phase transformation from martensite to austenite to occur, and as a result the stent expands. The stents of the present invention can be processed to behave superelastically at body temperature. Alternatively, the stent designs of the present invention could be used in conjunction with balloon expandable stents made from stainless steel or other conventional stent materials. 
     In all embodiments, the stents of the present invention have sufficient longitudinal flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens, yet remain stable when expanded radially to maintain the patency of a body lumen, such as an artery or other vessel, when implanted therein. The present invention particularly relates to unique strut patterns which have a high degree of longitudinal flexibility and conformability, while providing sufficient radial-expansibility and strength to hold open the body lumens. The high radial strength possessed by the stents of the present invention allow them to be used in treating calcified lesions. 
     Generally, the greater the longitudinal flexibility of the stents, the easier and the more safely they can be delivered to the implantation site, particularly where the implantation site is on a curved section of a body lumen, such as a coronary artery or peripheral blood vessel, and especially in saphenous veins and larger vessels. The designs of the present invention have sufficient flexibility to conform to the patient&#39;s vasculature, thus preventing vessel straightening by the stent. Moreover, the stents of the present invention are crush proof, making them particularly suitable for implantation in the carotid arteries. 
     Each of the different embodiments of stents of the present invention include a plurality of adjacent cylindrical elements (often referred to as “rings”) which are generally expandable in the radial direction and arranged in alignment along a longitudinal stent axis. The cylindrical elements are formed in a variety of serpentine wave patterns transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys. At least one interconnecting member (often referred to as a “spine”) extends between adjacent cylindrical elements and connects them to one another. These interconnecting members insure minimal longitudinal contraction during radial expansion of the stent in the body vessel. The serpentine patterns have varying degrees of curvature in the regions of peaks and valleys and are adapted so that radial expansion of the cylindrical elements are generally uniform around their circumferences during expansion of the stent from the collapsed position to the expanded position. 
     The stents of the present invention also have strut patterns which enhance the strength of the ends of the stent and the overall radiopacity of the stent, yet retain high longitudinal flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens and remain stable when expanded radially to maintain the patency of the body lumen. The present invention in particular relates to stents with unique end portions having sufficient hoop strength to maintain a constant inner diameter which prevents the stent from taking on a “cigar” shape when deployed in the body lumen. The end rings used with the present invention are particularly useful on self-expanding stents which may otherwise have end rings that are more susceptible to compressive forces. 
     The resulting stent structures are a series of radially expandable cylindrical elements that are spaced longitudinally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the luminal wall, but not so close as to compromise the longitudinal flexibility of the stent both when negotiating through the body lumens in their unexpanded state and when expanded into position within the vessel. The design of the stents contribute to form small gaps between struts to minimize tissue prolapse. Each of the individual cylindrical elements may rotate slightly relative to their adjacent cylindrical elements without significant deformation, cumulatively providing stents which are flexible along their length and about their longitudinal axis, but which still are very stable in their radial direction in order to resist collapse after expansion. 
     In one embodiment of the present invention, each cylindrical element of the stent includes eight peak regions (often referred to as “crowns”) and eight valley regions which provide sufficient coverage of the vessel when placed in the expanded or deployed position. In this design, each cylindrical element consisting of an alternating pattern of U-shaped portions and double-curved (W) portions connected both axially and circumferentially to eight discontinuous interconnecting members or spines. For example, the U-shaped portion of the cylindrical element can be connected to an adjacent cylindrical element via the interconnecting members. The same cylindrical element can be then connected to another cylindrical element via the interconnecting members connected to the double-curved portions. The cylindrical element can be connected to an adjacent cylindrical element by four interconnecting members. This particular alignment of interconnecting members provides adequate flexibility to the stent and also helps prevent foreshortening of the stent as it expands radially outward. The discontinuing pattern of interconnecting members results in a highly flexible stent that does not kink upon bending. Both the distal and proximal ends of this stent design can be entirely composed of “W” patterns which provide additional strength to the ends of the stent. The resulting stent produces an eight crown, four-cell pattern which has sufficient coverage for vessel scaffolding while maintaining excellent flexibility to reach distal lesions and possessing sufficient radial strength to hold the target vessel open. An alternative pattern using six crowns and six discontinuous interconnecting members also can be utilized and will exhibit these same physical properties. 
     The serpentine pattern of the individual cylindrical elements can optionally be in phase with each other in order to reduce the contraction of the stent along their length when expanded. In these embodiments of the present invention, interconnecting members align behind each other to create a continuous “spine” which extends from one end of the stent to the other. Two or three rows of continuous spines can be used to connect adjacent cylindrical elements. This construction also helps prevent the stent from foreshortening when expanded. 
     A stent made in accordance with the present invention can be readily delivered to the desired target location by mounting it on a stent delivery catheter which includes a retractable sheath, or other means, to hold the stent in its collapsed position prior to deployment. 
     These and 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 the stent embodying features of the present invention mounted on a delivery catheter disposed within a vessel. 
     FIG. 2 is an elevational view, partially in section, similar to that shown in FIG. 1, wherein the stent is expanded within a vessel, pressing the lining against the vessel 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 a plan view of one preferred embodiment of a flattened stent of the present invention, which illustrates the serpentine pattern including peaks and valleys which form the cylindrical elements of the stent and permit the stent to achieve a small crimp profile, yet is expandable to a larger diameter to maintain the patency of a small vessel. 
     FIG. 5 is an enlarged partial view of the stent of FIG. 4 depicting the serpentine pattern along with the peaks and valleys which form one preferred embodiment of a cylindrical element made in accordance with the present invention. 
     FIG. 6 is a plan view of an alternative embodiment of a flattened stent of the present invention, which illustrates the serpentine pattern along with the peaks and valleys which form the cylindrical elements of the stent and permit the stent to achieve a small crimp profile, yet is expandable to a larger diameter to maintain the patency of a small vessel. 
     FIG. 7 is an enlarged partial view of the stent of FIG. 6 depicting the serpentine pattern along with the peaks and valleys which form another preferred embodiment of a cylindrical element made in accordance with the present invention. 
     FIG. 8 is a plan view of an alternative embodiment of a flattened stent of the present invention, which illustrates the serpentine pattern along with the peaks and valleys which form the cylindrical elements of the stent and permit the stent to achieve a small crimp profile, yet is expandable to a larger diameter to maintain the patency of a small vessel. 
     FIG. 9 is an enlarged partial view of the stent of FIG. 8 depicting the serpentine pattern along with the peaks and valleys which form another preferred embodiment of a cylindrical element made in accordance with the present invention. 
     FIG. 10 is a plan view of an alternative embodiment of a flattened stent of the present invention, which illustrates the serpentine pattern along with the peaks and valleys which form the cylindrical elements of the stent and permit the stent to achieve a small crimp profile, yet is expandable to a larger diameter to maintain the patency of a small vessel. 
     FIG. 11 is an enlarged partial view of the stent of FIG. 10 depicting the serpentine pattern along with the peaks and valleys which form another preferred embodiment of a cylindrical element made in accordance with the present invention. 
     FIG. 12 is an enlarged view of a double-curved portion (w) which has a sweep cut which helps the stent to be crimped to a low diameter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior art stent designs, such as the MultiLink Stent™ manufactured by Advanced Cardiovascular Systems, Inc., Santa Clara, Calif., include a plurality of cylindrical rings that are connected by three connecting members between adjacent cylindrical rings. Each of the cylindrical rings is formed of a repeating pattern of U-, Y-, and W-shaped members, typically having three repeating patterns forming each cylindrical element or ring. A more detailed discussion of the configuration of the MultiLink Stent™ can be found in U.S. Pat. No. 5,569,295 (Lam) and U.S. Pat. No. 5,514,154 (Lau et al.), whose contents are hereby incorporated by reference. 
     Beyond those prior art stents, FIG. 1 illustrates an exemplary embodiment of stent  10  incorporating features of the present invention, which stent is mounted onto delivery catheter  11 . FIG. 4 is a plan view of this exemplary embodiment stent  10  with the structure flattened out into two dimensions to facilitate explanation. Stent  10  generally comprises a plurality of radially expandable cylindrical elements  12  disposed generally coaxially and interconnected by interconnecting members  13  disposed between adjacent cylindrical elements  12 . The delivery catheter  11  has an inner tubular member  14  upon which the collapsed stent  10  is mounted. A restraining sheath  15  extends over both the inner tubular member  14  and stent  10  in a co-axial relationship. The stent delivery catheter  11  is used to position the stent  10  within an artery  16  or other vessel. The artery  16 , as shown in FIG. 1, has a dissected or detached lining  17  which has occluded a portion of the arterial passageway. 
     In a preferred embodiment, the delivery of the stent  10  is accomplished in the following manner. Stent  10  is first mounted onto the delivery catheter  11  with the restraining sheath placed over the collapsed stent. 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 guide wire  18  is disposed through the damaged arterial section with the detached or dissected lining  17 . The catheter-stent assembly is then advanced over guide wire  18  within artery  16  until the stent  10  is directly under the detached lining  17 . The restraining sheath  15  is retracted exposing the stent  10  and allowing it to expand against the inside of artery  16 , which is illustrated in FIG.  2 . While not shown in the drawing, artery  16  is preferably expanded slightly by the expansion of stent  10  to seat or otherwise embed stent  10  to prevent movement. Indeed, 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 there through. 
     While FIGS. 1-3 depict a vessel having detached lining  17 , stent  10  can be used for purposes other than repairing the lining. Those other purposes include, for example, supporting the vessel, reducing the likelihood of restenosis, or assisting in the attachment of a vascular graft (not shown) when repairing an aortic abdominal aneurysm. 
     In general, stent  10  serves to hold open the artery  16  after catheter  11  is withdrawn, as illustrated in FIG.  3 . Due to the formation of stent  10 , the undulating component of the cylindrical elements of stent  10  is relatively flat in a transverse cross-section so that when stent  10  is expanded, cylindrical elements  12  are pressed into the wall of artery  16  and as a result do not interfere with the blood flow through artery  16 . Cylindrical elements  12  of stent  10  that are pressed into the wall of artery  16  will eventually be covered with endothelial cell growth that further minimizes blood flow turbulence. 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  16 , and consequently are well adapted to tack up and hold in place small flaps or dissections in the wall of artery  16  as illustrated in FIGS. 2 and 3. 
     The stresses involved during expansion from a low profile to an expanded profile are generally evenly distributed among the various peaks and valleys of stent  10 . Referring now to FIGS. 4-5, one preferred embodiment of the present invention as depicted in FIGS. 1-3 is shown wherein each expanded cylindrical element  12  embodies a serpentine pattern having a plurality of peaks and valleys that aid in the even distribution of expansion forces. In this exemplary embodiment, interconnecting members  13  serve to connect adjacent valleys of each adjacent cylindrical element  12  as described above. The various peaks and valleys generally have U, W and inverted-U shapes, in a repeating pattern to form each cylindrical element  12 . It should be appreciated that the cylindrical element  12  can be formed in different shapes without departing from the spirit and scope of the present invention. 
     The cylindrical element  12  of this stent  10  includes double-curved portions (W)  21  located in the region of the valley where each interconnecting member  13  is connected to an adjacent cylindrical element  12 . The peak portions (inverted-U)  22  and the valley portions (U)  23  also form the cylindrical element  12  of the stent  10 . A shoulder region  24  extending from each valley portion to peak portion (inverted U)  22  allows the peak portion to be nested in a tight formation next to an adjacent cylindrical element  12 . This shoulder region  24  provides a transition region between the peak portions (inverted U)  22  and the valley portions (U)  23  and double-curved portion (W)  21  to allow adjacent cylindrical elements to nest within one another and thereby better support the artery walls with smaller gaps between stent struts. In this manner, the shoulder region  24  provides more dense coverage of the serpentine pattern of the cylindrical element to create a fairly uniform strut pattern which fully supports the walls of the diseased artery. For this reason, there are no or few areas of the stent wall which do not have struts for supporting the wall of the artery. Each of the valley portions (U)  23  forms a Y-shaped member when connected to an interconnecting member  13 . As can be seen in this particular design, each of the valley portions (W&#39;s and U&#39;s)  21  and  23  have an interconnecting member which connects that cylindrical element  12  to an adjacent cylindrical element. As a result, each cylindrical element  12  is connected to an adjacent cylindrical element by at least four interconnecting members  13 . The peak portions (inverted “U”)  22  are not directly connected to any adjacent cylindrical element to allow for radial expansion. The eight interconnecting members  13  which are connected to each cylindrical element  12  are discontinuous with each other to produce a highly flexible stent that does not kink upon bending. This particular design allows the stent  10  to be placed in tortuous anatomy, where the stent  10  will conform to the particular anatomy of the patient. For example, if the stent  10  is placed in a curved portion of a artery, then the flexibility of the stent will allow it to take on the same curved shape without kinking and will still be capable of fully supporting the artery. Additionally, the stent&#39;s resistence to kinking helps prevent occlusion of the vessel lumen by the stent struts. Even though the stent  10  is flexible, it is still rigid when collapsed so that it can be placed on the delivery catheter and moved into the desired location in the patient&#39;s vasculature. 
     The stent  10  also includes end rings  25  and  26  which comprise all “W” shaped portions  27  to provide additional strength to the ends of the stent  10 . This “W” pattern also helps to increase the overall radiopacity of the stent by virtue of the additional material needed to create such a “W” pattern. As a result, the stent  10  should be easily observable by a physician using imaging instrumentation, such as a fluoroscope. 
     In another embodiment of the present invention, as is shown in FIGS. 6 and 7, the stent  10  made with six crowns or peak portions (inverted U)  22 , rather than the eight crowns shown in the previous embodiment. Otherwise, the strut pattern is virtually identical. The stent shown in FIGS. 6 and 7 include six valley portions, namely three valley portions (W)  21  and three valley portions (U)  23 . This particular design also has six discontinuous interconnecting members  13  which connect each cylindrical element  12  to an adjacent cylindrical element. Again, the interconnecting member  13  are connected to each of the valley portions (W)  21  and valley portion (U)  23  to help prevent shortening of the stent during radio expansion. This pattern also helps increase the flexibility of the strut. End rings  25  and  26  which comprise of all “W” shaped portions  27  provide additional strength to the ends of the stent  10 , while increasing the radiopacity of the stent as well. 
     In another embodiment of the invention, as shown in FIGS. 8 and 9, the stent  30  is made with cylindrical elements  32  which include six crowns or peak portions (inverted U&#39;s)  29  and six valley portions, namely three valley portions (W)  31  and three valley portions (U)  34 . This particular design differs from the previous two embodiments by utilizing three continuous interconnecting members  33  which are utilized to connect each of the cylindrical elements  32  to an adjacent cylindrical element. Each interconnecting member  33  is connected to the valley portion (W)  31  which creates a continuous spine  35  which extends from one end  36  to the other end  37  of the stent  30 . In this manner, the serpentine pattern of each individual cylindrical element  30  are in phase with each other in order to help reduce the contraction of the stent along their lengths when expanded. These continuous spines  35  help prevent the stent  30  from shortening when each of the cylindrical elements  30  are radially expanded. 
     The cylindrical element  32  also differs from the previous embodiments since a valley portion (U)  34  is not utilized to interconnect adjacent cylindrical elements to each other. However, the cylindrical element  32  includes a shoulder region  38  which extends between each of the valley portions and peak portions to provide a transition region which allows the peak portion (inverted U)  29  to be crimped in close proximity to an adjacent cylindrical element. In this manner, the stent  30  can be crimped down to a low profile which helps reduce the overall profile of the stent and delivery catheter when placing the stent  30  through the tortuous anatomy of the patient&#39;s vasculature. 
     In still another embodiment of the present invention, as is shown in FIGS. 10 and 11, a stent  40  is shown having a plurality of cylindrical elements  42  which are connected together by interconnecting members  43 . Each of the cylindrical elements  42  include a peak portion (inverted U)  39  and valley portions (W)  41  and valley portions (U)  44  which form the composite ring. In this particular design, five valley portions (W)  41  are utilized and each of the cylindrical element  42  is connected to an adjacent cylindrical element  42  by an interconnecting member  43  which is connected to the valley portion (W)  41 . As with the previous embodiment, each interconnecting member  43  extends directly behind one another to form a continuous spine  45  which extends from one end  46  to the other end  47  of the stent  40 . In this particular embodiment, five continuous spines  45  are created on the composite stent  40 . The peak portions (inverted U)  39  and the valley portion (U)  44  are not connected by any interconnecting members. The end ring  48  of this particular stent  40  includes five double curved portions (W)  41  which helps increase the radial strength of this end while enhancing the radiopacity as well. As can be seen from the single cylindrical element  42  shown in FIG. 11, the double curved portion (W)  41  include a “sweep cut”  49  which helps to reduce the collapsed profile of the stent  40  when it is placed on a delivery catheter. This reduced portion of the double curved portion (W)  21  enables the peak portion (inverted U)  39  to be collapsed closer to the double curved portion (W)  41  without hitting the double-curved portion (W)  41  when the stent  40  is crimped onto the delivery catheter. As a result, there should be no metal to metal contact when the stent is crimped and the stent  40  should be crimped on to an even smaller profile which again helps in reducing the over profile of the stent and delivery catheter and in reaching tight distal vessels. While this sweep cut  49  is shown only in conjunction with the embodiment shown in FIGS. 10 and 11, this sweep cut could be created on any of the other embodiments disclosed herein to help and reduce the overall diameter of the stents when they are being crimped on to the stent delivery catheters. 
     It should be appreciated that the present design can be made with a number of peaks and valleys ranging from 4 to 16. The number of peaks and valleys will depend upon the particular physical characteristics desired, along with the particular application to which the stent will be used. 
     In many of the drawing figures, the present invention stent is depicted flat, in a plan view for ease of illustration. All of the embodiments depicted herein are cylindrically-shaped stents that are generally formed from tubing by laser cutting as described below. 
     One important feature of all of the embodiments of the present invention is the capability of the stents to expand from a low-profile diameter to a larger diameter, while still maintaining structural integrity in the expanded state and remaining highly flexible. Stents of the present invention each have an overall expansion ratio of about 1.0 up to about 5.0 times the original diameter, or more, using certain compositions of materials. The stents still retain structural integrity in the expanded state and will serve to hold open the vessel in which they are implanted. Some materials may afford higher or lower expansion ratios without sacrificing structural integrity. 
     While the stent design of the present invention has very practical applications for procedures involving vessel diameters from about 3.0 to 14.0 millimeters, it should be appreciated that the stent pattern could also be successfully used in procedures involving larger lumens of the body, without departure from the spirit and scope of the present invention. Due to the increase of the longitudinal flexibility provided by the present stent design, such applications could include larger diameter vessels where added flexibility in reaching the vessel is needed. 
     The stents of the present invention can be made in many ways. However, the preferred method of making the stent is to cut a thin-walled tubular member, such as Nitinol tubing to remove portions of the tubing in the desired pattern for the stent, leaving relatively untouched the portions of the metallic tubing which are to form the stent. It is preferred to cut the tubing in the desired pattern by means of a machine-controlled laser. 
     A suitable composition of Nitinol used in the manufacture of a self expanding stent of the present invention is approximately 55% nickel and 44.5% titanium (by weight) with trace amounts of other elements making up about 0.5% of the composition. The austenite transformation temperature is between about −15° C. and 30° C. in order to achieve superelasticity. The austenite temperature is measured by the bend and free recovery tangent method. The upper plateau strength is about a minimum of 60,000 psi with an ultimate tensile strength of a minimum of about 155,000 psi. The permanent set (after applying 8% strain and unloading), is approximately 0.5%. The breaking elongation is a minimum of 10%. It should be appreciated that other compositions of Nitinol can be utilized, as can other self-expanding alloys, to obtain the same features of a self-expanding stent made in accordance with the present invention. 
     The stent of the present invention can be laser cut from a tube of super-elastic (sometimes called pseudo-elastic) nickel titanium (Nitinol) whose transformation temperature is below body temperature. All of the stent diameters can be cut with the same stent pattern, and the stent is expanded and heat treated to be stable at the desired final diameter. The heat treatment also controls the transformation temperature of the Nitinol such that the stent is super elastic at body temperature. The transformation temperature is at or below body temperature so that the stent will be superelastic at body temperature. The stent can be electro polished to obtain a smooth finish with a thin layer of titanium oxide placed on the surface. The stent is usually implanted into the target vessel which is smaller than the stent diameter so that the stent applies a force to the vessel wall to keep it open. 
     The stent tubing of a self expanding stent made in accordance with the present invention may be made of suitable biocompatible material besides super-elastic nickel-titanium (NiTi) alloys. In this case the stent would be formed full size but deformed (e.g. compressed) to a smaller diameter onto the balloon of the delivery catheter to facilitate intra luminal delivery to a desired intra luminal site. The stress induced by the deformation transforms the stent from an austenite phase to a martensite phase, and upon release of the force when the stent reaches the desired intra luminal location, allows the stent to expand due to the transformation back to the more stable austenite phase. Further details of how NiTi super-elastic alloys operate can be found in U.S. Pat. No. 4,665,906 (Jervis) and U.S. Pat. No. 5,067,957 (Jervis). 
     The tubing also may be made of suitable biocompatible material such as stainless steel. The stainless steel tube may be alloy-type: 316L SS, Special Chemistry per ASTM F138-92 or ASTM F139-92 grade 2. 
     The stent diameters are very small, so the tubing from which it is made must necessarily also have a small diameter. For PTCA applications, typically the stent has an outer diameter on the order of about 1 mm (0.04-0.09 inches) in the unexpanded condition, the same outer diameter of the hypotubing from which it is made, and can be expanded to an outer diameter of 4.0 mm or more. The wall thickness of the tubing is about 0.076-0.381 mm (0.003-0.015 inches). For stents implanted in other body lumens, such as PTA applications, the dimensions of the tubing are correspondingly larger. While it is preferred that the stents be made from laser cut tubing, those skilled in the art will realize that the stent can be laser cut from a flat sheet and then rolled up in a cylindrical configuration with the longitudinal edges welded to form a cylindrical member. 
     Referring now to FIG. 5, the width of the strut of the cylindrical element, indicated by arrows  50 , can be about from 0.003 to 0.009 inches. The width of the strut of the interconnecting member, indicated by arrows  51 , can be from about 0.003 to 0.009 inches. The length from the double-curved portion to the shoulder region, indicated by arrow  52 , can be from about 0.05 to 0.10 inches. The length from the shoulder region to the top of the peak portion, indicated by arrow  53 , can be from about 0.05 to 0.10 inches. The width of the peak portions (unexpanded) indicated by arrows  54 , can be from about 0.012 to 0.040 inches. These same dimensions would apply specifically to the embodiments of the present invention shown in FIGS. 6 and 7 and the embodiment of FIGS. 8 and 9. 
     Referring now to FIG. 12 the width of the strut of the cylindrical element, indicated by arrows  50 , can be about from 0.003 to 0.009 inches. The width of the strut of the interconnecting member, indicated by arrows  51 , can be from about 0.003 to 0.009 inches. The length from the double-curved portion to the peak portion, indicated by arrow  52 , can be from about 0.070 to 0.150 inches. The width of the peak portions indicated by arrow  54 , can be from about 0.03 to 0.06 inches. 
     Due to the thin wall and the small geometry of the stent pattern, it is necessary to have very precise control of the laser, its power level, the focus spot size, and the precise positioning of the laser cutting path. In cutting the strut widths of the embodiment shown in FIGS. 1-5, it is preferable to have a very focused laser spot size which will allow the precise strut pattern to be created on the tubing. For this reason, additional instrumentation which includes a series of lenses may be necessary to be utilized with the laser in order to create the fine focused laser spot necessary to cut that particular pattern. 
     Generally, the tubing is put in a rotatable collet fixture of a machine-controlled apparatus for positioning the tubing relative to a laser. According to machine-encoded instructions, the tubing is then rotated and moved longitudinally relative to the laser which is also machine-controlled. The laser selectively removes the material from the tubing by ablation and a pattern is cut into the tube. The tube is therefore cut into the discrete pattern of the finished stent. Further details on how the tubing can be cut by a laser are found in U.S. Pat. No. 5,759,192 (Saunders) and U.S. Pat. No. 5,780,807 (Saunders), which have been assigned to Advanced Cardiovascular Systems, Inc. and are incorporated herein by reference in their entirety. 
     The process of cutting a pattern for the stent into the tubing generally is automated except for loading and unloading the length of tubing. For example, a pattern can be cut in tubing using a CNC-opposing collet fixture for axial rotation of the length of tubing, in conjunction with CNC X/Y table to move the length of tubing axially relative to a machine-controlled laser as described. The entire space between collets can be patterned using the CO 2  or Nd:YAG laser set-up. The program for control of the apparatus is dependent on the particular configuration used and the pattern to be ablated in the coding. 
     After the stent has been cut by the laser, electrical chemical polishing, using various techniques known in the art, should be employed in order to create the desired final polished finish for the stent. The electropolishing will also be able to take off protruding edges and rough surfaces which were created during the laser cutting procedure. 
     While the invention has been illustrated and described herein in terms of its use as intra vascular stents, it will be apparent to those skilled in the art that the stents can be used in other instances in all conduits in the body, such as, but not limited to, the urethra and esophagus. Other modifications and improvements may be made without departing from the scope of the invention.