CONFIGURATION OF IMPROVED DOPRÓTESIS
BACKGROUND OF THE INVENTION The present invention generally relates to intravascular stents and more particularly relates to configurations of specialized stents for the treatment of vascular disease, within non-uniform vessels, such as for example a tapered artery or in the ostium or bifurcation of an artery. Endoprostheses or expandable grafts are implanted in a variety of body lumens, in an effort to maintain their opening. These devices are typically implanted by use of a catheter that is inserted into an easily accessible site and then advanced to the deployment site. The stent is initially maintained in a radially compressed or crushed state to allow maneuvering through the lumen. Once in position, the stent is deployed, which depending on its construction is already achieved automatically, for example, by removing a restriction or actively, for example, by inflating a balloon with respect to which the stent is transported in the catheter. Intravascular stents currently in use are typically designed for expansion or to be expanded within a diseased vessel to give a given nominal diameter that is constant over the entire length of the stent. The stent is also typically uniform over the entire length, in terms of its radial strength, longitudinal flexibility and coverage, i.e. the current area of the stent material defining the surface of the stent deployed relative to the area of the stent covered therein. way. Most blood vessels, however, are not of constant diameter, already exhibiting a natural taper or narrowing, particularly near bifurcations. The blood vessels may taper abruptly over short stretches (less than 20 mm) such as in the carotid arteries, or taper gradually over long stretches (more than 20 mm) such as in the iliac arteries. Examples of bifurcation sites in the human circulatory system include the vascular profile where the external and internal carotid arteries branch out from the common carotid artery. The common carotid artery has a diameter of 7-9 mm, while the internal carotid artery is 4-6 mm in diameter. In the event that there is disease in said joint, a deployed endoprosthesis should adjust a diameter change of 3-5 mm over a stretch of approximately 20-30 mm. Another example involves the placement of stents of the renal arteries. In order to cover the entire ostial area, it is necessary for the stent to fit into the interior of the renal artery, and to flare in the aorta significantly larger. Furthermore, the lesions present in the ostium are typically hard and calcified, requiring that the endoprosthesis have greater resistance in that specific region. Similar requirements arise in the treatment of ostial disease in the bifurcation of native coronary arteries or bypass grafts and aorta-ostial disease in the peripheral arteries (eg carotid, renal and iliac). Non-uniformities may also be present, by virtue of a curved or tortuous configuration of the blood vessel at the diseased site. When coupling conventionally shaped stents, ie stents of uniform shape and diameter at these sites, a number of problems arise. In the event that this stent is adapted to a tapered section of artery, either the artery is forced to an unnatural shape or the stent must somehow be distorted during its deployment. By forcing the artery in an unnatural way, certain sections of the tissue extend excessively or other sections will be sub-supported. Stages performed in an effort for non-uniform expansion of a stent uniformly constructed, have the effect of deteriorating non-uniform characteristics undesired to the device such as uneven radial resistance, flexibility and coverage. Another potential side effect associated with the use of a stent of uniform construction is that its deployment in a tortuous segment of the vasculature would cause undesirable straightening of this segment. Therefore, a stent is required with which a non-uniform vessel can be held uniformly, to provide consistency in terms of coverage, radial strength and longitudinal flexibility over its entire length. Alternatively, an endoprosthesis is required that is capable of providing specific variations in terms of coverage, radial strength and longitudinal flexibility at certain sites over its length, in order to adjust various needs over the length of a particular vessel. COMPENDIUM OF THE INVENTION A stent of the present invention can be constructed to provide uniform coverage, uniform radial strength and / or uniform longitudinal flexibility to a non-uniform structure deployment site such as a vessel that tapers or bifurcates. Alternatively, this endoprosthesis can be constructed to adjust non-uniform requirements of a particular site, where specific variations in flexibility, radial strength or coverage are required at specific sites on this stent. The desired functional differentiation is achieved with a stent that structurally differentiates into a preselected shape. Geometric or dimensional differentiation, or both dimensional and geometric differentiation is used to impart the desired variations of functional characteristics to a particular endoprosthesis. This structural differentiation may be gradual or abrupt or may include several different types of differentiations over the length of the stent. A stent of the present invention constructed for deployment in a tapered vessel has a gradually increased expansion ratio over its length. This differentiation can be achieved with a structure of axially aligned rings, each with a serpentine structure, wherein each repetitive pattern of streamers defines a single unit cell. By selecting the size of the unit cells in successive rings to be incrementally wider, an increased amount of material becomes available for expansion. Upon expansion, that stent acquires the tapered shape of a truncated cone to correspond to the shape of the tapered artery. Despite its tapered shape, the stent however provides uniform coverage of the walls of the tapered container, as well as exhibiting uniform radial strength and longitudinal flexibility over its entire length. The stent may be constructed alternately for expansion within any of a variety of shapes or profiles to fit a particular application. Geometric or dimensional variations as well as both geometric and dimensional unitary cell can be used to achieve this functional variation. As a further alternative, a stent of the present invention can be constructed non-uniformly in order to achieve a uniform diameter over its entire length while exhibiting pre-select variations in coverage, radial strength or longitudinal flexibility over its length. This is achieved, for example, by varying the thickness of the serpentine elements while the width of each serpentine element remains constant, or by varying the number of unit cells in certain serpentine elements. A similar result can be achieved by simultaneously varying the dimensions (ie, column width and / or unit cell thickness) or geometry, or both the dimensions and the geometry of the individual unit cells. These and other features and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment which, when taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a greatly amplified plan view of a longitudinally crushed, cut stent of the present invention, which is illustrated in a pre-deployment state. Figure 2 is a perspective view of the stent that is illustrated in Figure 1, also in the pre-deployment state. Figure 3 is a greatly enlarged plan view of the longitudinally cut and crushed stent of Figure 1, shown in the deployed state. Figure 4 is a perspective view of the stent shown in Figure 3, also in the unfolded state. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The stents of the present invention are constructed specifically for particular deployment sites and thus overcome the inherent disadvantages of trying to fit a uniform stent at a non-uniform site. The non-uniformity of the site may include a taper, a bifurcation, a ostium or any other variation in terms of dimensions or support requirements. The stent is maneuvered on site in the conventional manner, such as by a catheter with respect to which it is transported while in its crushed or collapsed state. Once in place, the stent is expanded such as by inflation of one or more balloons, or in the case of self-expanding stents, a confinement liner is removed to allow the stent to expand automatically. Subsequent removal of the catheter and associated deployment devices leave the stent in place to maintain the vessel opening. By providing a tapered stent for deployment within a tapered artery, uniform coverage, uniform radial strength and uniform stiffness can be achieved, however, over the entire length of the stent. Alternatively, the versatility of the system of the present invention allows a non-uniform stent to be constructed that imparts improved coverage, strength or stiffness at predetermined sites, so that, for example, it allows the required support requirements in a diseased ostium. . Figure 1 illustrates the stent 12 incorporating features of the present invention and more specifically a stent for deployment in a tapered artery. The stent is typically tubular in its overall form, however the drawing shows the stent in a longitudinally cut and crushed state in order to clearly exhibit its structure. The stent structure generally consists of a series of circumferentially extending serpentine elements (14, 16, 30) that are interconnected by articulations 32 extending between adjacent serpentine elements. Each of the serpentine elements can be characterized as consisting of a number of individual unit cells 34, wherein each cell consists of a joint 32 connected to two adjacent U-shaped or V-shaped ribs 36, 38. In the illustrated embodiment, a total of four unit cells defines each serpentine element. Adjacent serpentine elements are arranged in such a way that the respective series of cusps are in phase, and in longitudinal alignment with each other. All the joints extend from the same side of the serpentine elements. In the illustrated embodiment, all the joints extend between the left edges of the individual serpentine element. In the embodiment illustrated in Figure 1, each serpentine element differs from the adjacent serpentine elements in terms of the serpentine pattern width, i.e. the length of each rib element as well as of each articulation element. In the particular embodiment shown, each successive serpentine element is progressively wider than the previous element, consequently the rib and the articulation elements of the respective unit cells are longer. The number of unit cells in each serpentine element however, remains unchanged as well as the thickness and width of all rib and spine elements. Figure 2 is a perspective view of the stent 12 as it currently appears in use before deployment. The total tubular structure is of uniform diameter and each of the serpentine elements extends with respect to the entire circumference of the device. The individual serpentine elements are recognized as rings while the individual joints 32 extend only between adjacent rings. The diameter of the stent is chosen to be small enough to allow passage through the vasculature of a patient to the deployment site. Figure 3 illustrates the stent shown in Figures 1 and 2, in its deployed state. The device is again illustrated longitudinally cut and crushed in order to more clearly exhibit its structure. As is clearly visible, the expansion of the device results in a tapered shape, wherein the wider serpentine elements shown towards the right side of the device, expand in a greater proportion than the narrower serpentine elements shown towards the left side of the device. This expansion results in a truncated cone shape, when illustrated in perspective as in Figure 4. Each of the ribs in U or V 36, 38, acquires a wider, more open angle while the joints 32, remain essentially stationary and aligned. The deformation and expansion of the ribs allows the increase in the diameter of the device, however the overall length of the endoprosthesis remains essentially unchanged during deployment of the endoprosthesis, since the joints connect only adjacent rings to each other. This is a highly desirable feature of a stent, since any trimming would not only reduce the total area supported by the device, but may also cause trauma to the surrounding tissue during deployment. Additionally, because more stent material is present in the larger diameter rings by virtue of the presence of the same number of longer ribs, the radial strength, coverage and stiffness of the stent remain substantially constant over its entire length. While the figures illustrate the structure of a single embodiment of the present invention and the distortion to which they are subjected during deployment, it will be understood that vast numbers of variations are possible to allow a stent to be tailored to the specific requirements. of a particular application. By varying or differentiating the geometry of the endoprosthesis structure or individual unit cells, a proportional variation or differentiation in function is achieved. Functional differentiation is also achieved by varying the thickness or width of the individual ribs. Desired functional differences are also achieved by varying the number of unit cells, either gradually over the length of the stent or in isolated sites. Both the number of unit cells, the geometry of these cells and the dimensions of these cells can be varied in any combination to achieve a particular functional effect. The variables described above can be chosen in such a way that the resulting endoprosthesis is specifically tailored to a very specific anatomical requirement, be it the taper, bifurcation twist or ostium of a vessel. In addition to adjusting the dimensional requirements of a particular non-uniformity in anatomy, the same variables may be selected such that the resulting stent is specifically tailored to provide desired radial strength, longitudinal flexibility or coverage differentiation. A stent of the present invention can be constructed using any number of well-known techniques. Preferably, a stainless steel tube is laser cut wthe desired stent pattern as known in the art. Digital angiography and advanced computation algors are valuable tools that are easily used to create a structurally differentiated endoprosthesis that adapts to the natural profile of a vessel before deployment. Techniques of electropolishing or chemical etching, which are well known, can be used subsequently to selectively vary the wall thickness of this stent. Deployment can be achieved by shaped globes for globe-expanding stents, whereby a tapered balloon is used to expand a tapered stent wn a tapered vessel. Alternatively, multiple balloons of different sizes can be used to achieve a similar effect, as can the tapered section of an oversized balloon. As a further alternative, the stent may be constructed for self-expansion by any of several techniques well known in the art. Deployment of a self-expanding stent can be achieved by subjecting the crushed or collapsed stent that is constructed of a shape memory alloy at certain temperatures that cause it to expand. A stent made of elastic material can be forcedly folded and restrained wn a liner. Removal of the liner allows the endoprosthesis to expand automatically. Endoprostheses can be manufactured for balloon expansion from any number of ductile metals and alloys including stainless steel, tantalum, and platinum-iridium alloys, er coated or uncoated. Self-expanding endoprostheses are constructed of superelastic or shape memory materials or alloys such as NiTi alloys, including Nitinol, and Cu-Zn alloys. 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 may be made wut departing from the spirit and scope of the invention. Accordingly, the invention is not intended to be limited except by the appended claims.