Abstract:
A configuration for expandable stents wherein a series of circumferentially disposed serpentine elements are spaced such that apexes defined by adjacent serpentine elements are nested within one another and only apexes extending along the same side of each serpentine element are linked. Such configurations impart enhanced coverage areas, expansion ratios and radial strength to the stent.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to intravascular stents and more particularly pertains to improvements thereto that provide for increased coverage and greater expansion ratios without a compromise in strength. 
     Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency. These devices are typically intraluminally implanted by use of a catheter which is inserted at an easily accessible location and then advanced to the deployment site. The stent is initially in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed which, depending upon its configuration, is achieved either automatically or actively by the inflation of a balloon about which the stent is carried on the catheter. 
     As stents are normally employed to hold open an otherwise blocked, constricted or occluded lumen, a stent must exhibit sufficient radial or hoop strength in its expanded state to effectively counter the anticipated forces. Not only is it advantageous to distribute such loads over as much of the stent as possible but it also is most beneficial to distribute the load over as much lumen wall as possible. This will help minimize injury to the vessel wall. Also by minimizing the gaps between stent struts it is possible to prevent prolapse of the plaque between the struts into the lumen. As a consequence, it is desirable to maximize the coverage of the lumen wall by creating uniform, small gaps between the stent struts. It is, however, simultaneously necessary for the stent to be as small and compact as possible in its collapsed state in order to facilitate its advancement through the lumen. As large an expansion ratio as possible is therefore most desirable. 
     A number of very different approaches have been previously devised in an effort to address these various requirements. A popular approach calls for the stent to be constructed wholly of wire. The wire is bent, woven and/or coiled to define a generally cylindrical structure in a configuration that has the ability to undergo radial expansion. The use of wire has a number of disadvantages associated therewith including for example, a substantially constant cross-section which may cause greater or lesser than an ideal amount of material to be concentrated at certain locations along the stent. Additionally, wire has limitations with respect to the shapes it can be formed into thus limiting the expansion ratio, coverage area and strength that can ultimately be attained therewith. The welding of adjoining sections of wire together has also been previously employed to increase strength albeit with a substantial increase in manufacturing costs. 
     As an alternative to wire-based structures, stents have been constructed from tube stock. By selectively removing material from such tubular starting material, a desired degree of flexibility and expandability can be imparted to the structure. Chemical etching techniques as well as laser-cutting processes are utilized to remove material from the tube. Laser etching provides for a high degree of precision and accuracy with which very well defined patterns of material can be removed from the tube to conversely leave very precisely and accurately defined patterns of material in tact. The performance parameters of a function of the pattern in which material is removed form the tube stock. The selection of a particular pattern has a profound effect on the coverage area, expansion ratio and strength of the resulting stent. 
     While the tube-based stents offer many advantages over the wire-based designs, it is nonetheless desirable to improve upon such designs in an effort to further increase coverage area and expansion ratios while maintaining strength. 
     SUMMARY OF THE INVENTION 
     The present invention provides for an improved tube-based stent having an increased coverage area, expansion ratio and strength. The improvements arise with the selection of a precisely defined pattern of voids that are cut or etched into the tube stock. The pattern of material that remains to define the stent comprises a series of nested serpentine elements wherein selected apexes are interconnected to one another by bridging members. More particularly, each serpentine element extends circumferentially about the stent such that successive apexes of each element alternatively extend distally and proximally along the stent&#39;s surface. Adjacent serpentine elements are spaced about the stent wherein each element is nested in the adjacent elements such that the apexes of one element extend into the space between the apexes of the adjacent element. The serpentine elements are joined to one another along either all of their distal or all of their proximal apexes. Each of a particular element&#39;s apexes extending along its interconnected edge is alternatingly joined to the corresponding apexes of proximally and distally adjoining serpentine elements by bridging elements. By properly shaping the junction area between each bridging element and apex, stress risers are reduced or eliminated with a corresponding decrease in the potential for fracturing during expansion. By curving the transverse sections that extend between each of the apexes of a given clement, adjacent elements become more tightly packed. Coverage exceeding 20% with expansion ratios of 3.0 are thereby achievable, and expansion ratios exceeding 6.0 are also achievable. More importantly, this dense coverage is uniformly distributed throughout the stent. 
     These and other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1, is a flattened plan view of a section of stent of the present invention in its collapsed state. 
     FIG. 2 is a flattened plan view of a section of an alternative embodiment stent in its collapsed state. 
     FIG. 3 is flattened plan view of a section of another alternative embodiment stent in its collapsed state. 
     FIG. 4 is flattened plan view of a section of another alternative embodiment stent in its collapsed state. 
     FIG. 5 is view of the stent shown in FIG. 4, in its partially expanded state. 
     FIG. 6 is view of the stent shown in FIG. 5 in a further expanded state. 
     FIG. 7 is a flattened plan view of a section of yet another alternative embodiment stent of the present invention in its collapsed state. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is directed to a stent formed from tube stock having a configuration that imparts greater coverage and expansion ratios than have heretofore been possible without compromising strength. Each of the figures illustrates a section of such stents in a flattened plan view for enhanced clarity. Typically the stents of the present invention are formed from tubular members utilizing a chemical etching process or by a laser cutting process. A typical chemical etching process is disclosed in U.S. Pat. No. 5,421,955 (Lau et al.), which is commonly assigned and commonly owned by Advanced Cardiovascular Systems, Inc., which is incorporated herein by reference thereto. Preferably, the stents of the present invention are formed by a laser cutting process which is disclosed in U.S. patent application Ser. No. 08/783,698, which is commonly assigned and commonly owned by Advanced Cardiovascular Systems, Inc., and is incorporated herein by reference thereto. 
     FIG. 1 illustrates the general concept of the present invention. Shown is an assembly  12  of serpentine elements  14  wherein apexes  16  of adjacent elements are nested between one another. In its three dimensional form, each serpentine element extends circumferentially about the stent such that for example, end  20  is attached to end  22 . The total number of elements assembled in such fashion determine the overall length of the stent. Each of the parallel serpentine elements is joined to the adjacent elements by bridging members  18  that extend from the apexes located along one edge of the serpentine element. Such apexes are alternatingly joined to the corresponding apexes of the proximally and distally adjoining serpentine elements. 
     FIG. 2 illustrates an alternative embodiment nested design  24  wherein further improvement in terms of coverage area is realized with the modification of the bridging elements  26 . Each bridging element is widened to achieve a commensurate increase in surface area. Sufficient clearance  28 , 30  is maintained between the bridging element and the adjoining sections of the serpentine elements so as to preclude contact and interference during expansion. 
     FIG. 3 illustrates a further alternative embodiment  32  wherein the stresses generated during expansion of the stent near the junction  34  of the bridging element  36  with the concave side  38  of the apex  40  are reduced when compared to the embodiments shown in FIGS. 1 and 2. The configurations shown in FIGS. 1 and 2 define acute angles at such junctures along with an increased concentration of mass at such point. During expansion, the bending of the material would therefore be concentrated adjacent the acute corners and greatly diminished at the center of the apex and thereby promote fractures and the potential for failure. By eliminating the acute angles and the concentration of mass at the apex with its formation into a “W” shape, stress is more evenly distributed along the apex during expansion. 
     FIG. 4 illustrates a further alternative embodiment  42  which incorporates the advantageous features of the embodiment shown in FIG. 3 along with additional features that serve to increase the stent&#39;s coverage area as well as its expansion ratio. As is visible in the figure, the transverse section  44  of each serpentine element that links successive apexes  46 ,  48  is curved at  50  to accommodate the apex of the adjacent serpentine element and to thereby achieve denser packing. The sections are sufficiently curved to come as close as possible to the bridge member at  52  prior to curving around the adjacent apex. Alternating sections are sufficiently curved to substantially close off the apex at  54 . The configuration imparts a substantially enhanced coverage area and expansion ratio while nonetheless allowing for the stent to undergo unimpeded expansion. FIG. 5 illustrates such stent configuration partially expanded to show the lack of interference as all elements are caused to move away from one another. FIG. 6 illustrates the stent in a substantially more expanded state. The configuration shown in FIG. 6 yields a theoretical expansion ratio of up to 6.7. 
     FIG. 7 illustrates the most preferred embodiment of the present invention which incorporates many of the features of the embodiments shown in the other Figures, including the nested configuration of parallel serpentine elements, bridging members joining corresponding apexes of adjacent serpentine elements, the stress-reliefed “W” patterned apex and the curved linking segments. The wider serpentine elements and bridging members along with the tighter radii yield a stent with an exceptionally high coverage area and expansion ratio along with substantial strength. 
     The stents of the present invention are preferably formed utilizing laser cutting techniques well known in the art. The material used in the manufacture of such stents may be substantially worked stainless steel of low ductility materials such as titanium for balloon expandable versions. Alternatively, NiTi may be used for self expanding embodiments. 
     While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. The present invention is not limited to stents for any particular body lumen. The size of the stent can vary in terms of inner diameter, outer diameter, wall thickness and length. Accordingly, it is not intended that the invention be limited except by the appended claims.