MXPA98005124A - Endoprotesis with reinforced columns and deploymentbimo - Google Patents

Abstract

The invention is directed to an expandable stent for implantation in a body lumen, such as an artery, area or peripheral vein. The endoprosthesis consists of a plurality of radially expandable cylindrical elements, generally aligned on a common longitudinal stent axis and interconnected by one or more interconnected limbs positioned so as to limit longitudinal contraction during radial expansion. The radially individual expandable cylindrical elements are formed in a serpentine pattern having alternating bends in peaks and valleys, designed to expand uniformly under radial tension to maximize the overall radial expansion ratio. Each peak and valley includes reinforcing members that extend through and are close to each elbow. The sizing and construction of the columns that form the peaks and valleys can create bi-modal deployment, where the columns bend under increased stresses, to allow the endoprosthesis to expand in larger diameters

Description

ENDOPROTESIS WITH REINFORCED COLUMNS AND BIMODAL DEPLOYMENT BACKGROUND OF THE INVENTION The present invention relates to expandable stent devices, generally called endoprostheses, which are adapted to be implanted in a patient's body lumen, such as a blood vessel, to maintain its opening. These devices are useful in the treatment of repair of atherosclerotic stenoses in blood vessels. Stents in general are cylindrically shaped devices that work to keep open and sometimes expand a segment of a blood vessel or other anatomical lumen. The stents are suitable for use in holding and holding retracted a dissected arterial lining, which if not contained and retained, can occlude the passageway for fluid. A variety of devices for use as stents are known in the art and have included, coiled wires in a set of expanding patterns if they have been intraluminally placed by a balloon catheter; coil-wound springs helically wound from an expandable heat-sensitive metal; and self-expanding stents that are inserted in a compressed state and configured in a zig zag pattern. Additional examples of stents of the art are illustrated in U.S. Pat. No. 4,776,337 granted to Palmaz; the U.S. Patent No. 4,655,771 granted to Wallsten; the U.S. Patent No. 4,800,882 granted to Gianturco; the U.S. Patent No. 4,913,141 granted to Hillstead; and the U.S. Patent. No. 5,292,331 granted to Boneau. Prior art devices include an expandable intraluminal vascular graft that expands within a blood vessel by a balloon that is typically associated with a dilatation catheter. The graft can be a wire mesh tube, a stainless steel tube with rectangular openings or a tube with a honeycomb opening. Another prior art device includes a prosthesis for transluminal implant, comprising a flexible tubular body made of flexible thread elements wound together, each thread having a helical configuration. There are even more conventional endovascular stents. In one design, the wire stent has a generally cylindrical shape, wherein the shape is formed with loops of alternating bent wires. 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 wire-unit structure configured to criss-cross forming a plurality of upper and lower peaks. One of the difficulties encountered using stents in the prior art involved, is to be able to maintain the required radial stiffness, to keep a body lumen open while at the same time maintaining the longitudinal flexibility of the stent, to facilitate its delivery. Another difficulty was the limited range of expansion available. Certain stents of the prior art expand only to a limited extent due to non-uniform stresses created during radial expansion. This makes it necessary to provide stents that have a variety of diameters, thereby increasing the manufacturing cost. Having access to prostheses with various expanded diameters also allows doctors to redirect the stent if the original vessel size was erroneously calculated. Another difficulty with the stent of the prior art was that the stent contracts on its longitudinal axis when the device expands radially. This made it difficult to place the stent within the artery during expansion.

Various means have been designed to supply and implant stents. A frequently described problem for supplying a stent in a desired intraluminal location, involves mounting the expandable stent into an expandable member, such as an inflatable balloon, which is provided at the distal end of an intravascular catheter. The catheter is advanced to the desired site within the patient's body lumen. Inflating the catheter deforms the stent to a permanently expanded condition. The balloon then deflates and the catheter is removed. What has been required and to date is not available is a stent having a high degree of sensitivity, such that it can be advanced through tortuous passages leading to the desired deployment site and can be radially detached over a wide range of diameters. with minimal longitudinal contraction and yet has the resistance to keep open the body lumen in which it expands. In addition, there is a need for an endoprosthesis having high circumferential or "ring" strength to improve crush resistance. The basis of the invention satisfies other needs.

SUMMARY OF THE INVENTION The present invention is directed to an expandable stent having a configuration of the general type described in U.S. Pat. No. 5,569,295 granted to S. Lam and 5,514,154 granted to Lau et al. In a preferred embodiment, the stent of the present invention includes a plurality of adjacent cylindrical members that are expandable in the radial direction and that are arranged in alignment on 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 extending between adjacent cylindrical elements and connecting cylindrical elements adjacent to each other. The interconnecting members ensure 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 through each peak and valley. More precisely, each peak and each valley of a single cylindrical element is formed by the confluence of two straight columns that meet at an elbow. The reinforcing member in this manner extends through the peak or valley bypassing the columns. The reinforcing member provides resistance to alternating peaks and valleys, where the maximum tension area is at or near the elbow. To be sure, the reinforcing member prevents the straight section of the column from buckling or distorting during stent expansion by adding material or a potentially weak area. In addition, the size and geometry of the reinforcement member on the elbow can be adjusted in such a way that the tension is evenly distributed between the two instead of being only supported by the elbow. For safety, the geometry of the reinforcing member in the present invention can acquire many configurations. For example, the reinforcing member may include a loop that curves toward or away from the elbow. The reinforcing member can join the columns at a point farther from or closer to the elbow. The reinforcing member can be formed in the elbow. The resulting stent structure, preferably, is a series of radially expanding cylindrical elements that are spaced longitudinally close enough to each other, such that the elements press back in position, small dissections in the wall of a body lumen, but not so closely as to compromise the longitudinal flexibility of the endoprosthesis. The individual cylindrical elements can rotate slightly with respect to adjacent cylindrical elements, without significant deformation, providing cumulatively a stent that is flexible about its length and about its longitudinal axis, but which is still very stable in the radial direction and thus resistant to crushing. The stent incorporating features of the present invention can be easily delivered to the desired lumen site by mounting it to an expandable member of a delivery catheter, for example a balloon, and then by passing the stent-catheter structure through the body lumen to the implant site. A variety of means for securing the stent to the expandable member in the catheter to deliver to the desired site are available. Currently, it is preferred to compress the stent in the balloon. Other means for securing the stent to the balloon include providing ridges or collars on the inflatable member to restrain lateral movement, or using temporary re-absorbable adhesives. The present invention by the use of reinforcing members characterizes bimodal deployment. That is, when the stent radially expands as described above, it does so in two stages. In the first stage, the columns are bent slightly outward, to receive the increased circumference of each cylindrical element, and the loop portion of the reinforcing member is stretched. The second stage continues from the first stage with the columns continuing to be bent outward, the most severe bending occurs in the reinforcing member, to the point at which the columns come off as widely as possible, to allow the larger one diameter that the endoprosthesis can acquire. A greater dispersion of the endoprostheses is prevented by the presence of the reinforcing member, which limits the maximum circumference of each cylindrical element. By choosing the size and geometry of the reinforcing member and the columns, the amount of force required to absorb the stent to a particular member can be altered. The cylindrical elements of the stent are preferably plastically deformed when expanded (except when nickel-titanium alloys (NiTi), the material from which the elements are formed) are used so that the stent remains in the expanded condition. Therefore, when non-Ni-Ti elements are used, the elements must be sufficiently rigid after expansion to prevent the elements from collapsing after deployment. With superelastic NiTi alloys, expansion occurs when the compression stress is removed as the relief of compression causes the phase transformation of material from the martensite phase back to the expanded austenite phase. After the stent is expanded, some of the peaks and / or valleys may tilt outward and become embedded in the vessel wall. In this way, after expansion, the stent has a smooth outer wall surface, but instead is characterized by projections that are embedded in the vessel wall and thus 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 Figure 1 is an elevation view, partly in section illustrating a stent incorporating features of the invention that is mounted on a delivery catheter and disposed within a body lumen such as a coronary artery. Figure 2 is an elevation view, partially in section similar to that shown in Figure 1, where the stent expands into the artery, pressing the dissected liner against the arterial wall. Figure 3 is an elevation view, partly in section showing the expanded stent within the vessel after removal of the delivery catheter. Figure 4 is an enlarged partial view of the stent in Figure 5 showing a serpentine pattern having peaks and valleys forming the cylindrical elements of the stent. Figure 5 is a plan view of a flattened section of an endoprosthesis of the present invention, illustrating the serpentine pattern of the stent. Figure 6 is an elevation view of the stent in the expanded condition. Figures 7A-L are top plan views of alternating modes of a single spike or reinforced valley. Figure 8 is a plan view of an alternate embodiment of the present invention of reinforced stent. Figure 9 is another alternate embodiment of the present invention of reinforced stent. Figures 10A and B show bimodal deployment of a preferred embodiment stent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 illustrates a stent 10 incorporating characteristics of the invention, which is mounted on a delivery catheter 11. Stent 10 generally comprises a plurality of radially expandable cylindrical elements 12, arranged coaxially and interconnected by members 13 placed between adjacent cylindrical elements. The delivery catheter 11 has an expandable portion or balloon 14 for exposing it to the stent 10 within artery 15 or another vessel. Artery 15 as illustrated in Figure 1 has a dissected liner 16 that has a portion of the arterial passage occluded. The delivery catheter 11 on which the stent 10 is mounted, is essentially the same as the balloon dilatation catheter conventionally used for angioplasty procedures such as percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty. (PTCA = percutaneuos transluminal coronary angioplasty). The balloon 14 can be formed of suitable materials such as polyethylene, polyethylene terephthalate, polyvinyl carbide, nylon and ionomers after as manufactured under the trade marks SURLYN by Polymer Products Division of E.I. Dupont deNemours Company. Other polymers may also be employed, so that the stent 10 remains in place in the balloon 14 during delivery to the side of the damage within the artery 15, the stent 10 is compressed in the balloon. An elastic protective liner is sometimes connected around the balloon 14 such that the stent 10 is folded over the liner, which protects the balloon from the metal stent 10 and ensures uniform expansion of the stent when the balloon and the elastic liner they expand. A retractable protective supply sleeve 20 can also be provided to further ensure that the stent remains in place in the expandable portion of the delivery catheter 11 and to prevent abrasion of the body lumen by the open or exterior surface of the stent 10 during delivery to the arterial site. wanted. Other means for securing the stent 10 on the balloon 14 can also be employed, such as providing collars or ridges at the ends of the working portion, i.e. the cylindrical portion of the balloon. Each radially expandable cylindrical element 12 of the stent 10 can be expanded independently. Therefore, the balloon 14 can be provided with an inflated shape different from the cylindrical one, for example tapered, to facilitate implantation of the stent 10 in a variety of body lumen forms. In a preferred embodiment, the delivery of the stent 10 is achieved in the following manner. The stent 10 is first mounted on the inflatable balloon 14 at the distal end of the delivery catheter 11. The stent can be "folded" down onto the balloon to ensure a low profile. The stent-catheter structure can be introduced into the vasculature of the patient with a conventional Seldinger technique through a guide catheter (not shown). A scrap guidewire is disposed through the arterial section with the detached liner or dissector 16 and then the catheter-stent structure is advanced over the guidewire 18 within artery 15 until the stent-graft is placed within the artery in the detached liner 16. The balloon 14 of the catheter expands, expanding the stent 10 against artery 15, which is illustrated in Figure 2. While not illustrated in the drawing, artery 15 may preferably be slightly expanded by expansion of the stent 10 to seal or otherwise secure the stent to prevent movement within the artery. In some circumstances during treatment of stenotic portions in the artery, the artery may have had to expand considerably in order to facilitate passage of blood or other fluid. The stent serves to keep the artery open 15 after the catheter is removed as illustrated in Figure 3. Due to the formation of the stent 10 from an elongate tubular member, the undulating component of the cylindrical elements of the stent 10 is relatively flat in cross section, such that when the stent is expanded, the cylindrical elements are pressed into the wall of artery 15 and as a result, the risk of developing Thrombosis in artery 15 is minimized. The cylindrical elements 12 of the stent 10 that are pressed into the wall of artery 15, will eventually be covered with endothelial cell growth that also deters thrombosis. The serpentine pattern of cylindrical sections 12 provides good inheritance characteristics to prevent stent movement within the artery. In addition, closely spaced cylindrical elements 12 at regular intervals provide uniform support for the wall of artery 15, and consequently are well adapted to adhere and hold in place small fins or dissections in the wall of artery 15 as illustrated in the Figures 2 and 3.

In the preferred embodiment, as illustrated in Figures 4, 5 and 6, the stresses involved during expansion from a low profile to an expanded profile are much more evenly distributed between the various peaks 36 and valleys 34 than what was experienced with the prior art endoprosthesis. As seen in Figure 4, a portion of a cylindrical member 12 of stent 10 illustrates the serpentine pattern having a plurality of peaks 36 and valleys 34, each characterized by varying radii of curvature 30, 32, 33, these varying radii. of curvature help in the inhomogeneous distribution of expansion forces. The interconnecting members 13 serve to connect adjacent valleys 34 of the cylindrical elements 12 as described above. After expansion, portions of the various elements will bend outwards, forming small projections that will be embedded in the vessel wall. For example, the tip of a peak portion 36 tilts outwardly when it has expanded to a degree that is sufficient to embed the tip within the vessel wall and thereby aids in holding the implanted stent. Upon expansion, the projecting peak 36 provides an exterior wall surface on the stent that is not uniform, but instead has a plurality of projecting peaks 36 all on the surface of the outer wall. While the projections help hold the endoprosthesis in the vessel wall, they are not sharp and thus do not cause trauma or damage to the vessel wall. An important feature of the present invention is the ability of the stent to expand from a low profile delivery diameter to a much larger diameter than has hitherto been achieved, while still maintaining structural integrity of the stent in the expanded state . Due to its novel structure, the stent of the present invention has a total expansion ratio of one to about four, using certain stainless steel compositions. For example, a 316L stainless steel stent of the present invention can be radially expanded from a diameter of one unit to a diameter of approximately 4 units, which deforms the structural members beyond their elastic limits. The endoprosthesis still retains its structural integrity in the expanded state and serves to keep the vessel in which it is implanted open. Materials other than 36 L stainless steel can give higher or lower expansion ratios without sacrificing structural integrity.

Figures 8 and 9 are plan views of further embodiments of the stent according to the invention, each illustrating a flattened section of an endoprosthesis 41 and a stent 42. The Figures illustrate the serpentine patterns of the stent as well as variant configurations of the reinforcing members 40, 46. In the preferred embodiment illustrated in Figure 8, the stent 40 is constituted by a plurality of radially expandable cylindrical elements 48, which are arranged generally coaxially and interconnected by interconnecting members 50. As in the previously described modes, the presently preferred embodiment shown in Figure 8, includes peak portions 52 and alternating valley portions 54. Each peak portion 52 or valley portion 54 is essentially an elbow interconnecting straight columns 58. In this embodiment, each peak portion 52 or valley portion 54 is reinforced by a member 44 extending through the elbow 56 to the legs. interconnecting columns 58. In the preferred embodiment shown in Figure 8, the reinforcing member 44 has an inverted loop 60 that extends in a direction opposite the elbow 56. Optionally, the interconnecting members 98 can be integrated into the loops 60 of the reinforcing members 46 as illustrated in Figure 9.

The peak tension area is at or near the apex of each elbow 56. The present invention provides an apparatus for reinforcing this area with reinforcing members 44, each of which is connected to each side of an elbow (ie say to the columns 58) away from the apex of the elbows 56. The width of the columns 58, together with the width and geometry of the reinforcing members 44 and the geometry and dimensions of the elbows 56 (which already form peak portion) 52 or valley portions 54) may be adjusted to distribute the tension between the elbows 56 and the reinforcing members 44. In addition, varying the base material from which the stent is formed will affect the design of the elbows 56 and the reinforcing members. 44. Figure 7 illustrates a variety of alternative embodiments of what may be comprised of either the peak portions or the valley portions of a stent according to the invention. Specifically, reinforcing members of different constructions are illustrated in plan views. As seen in Figure 7A, the peak or valley portion 62 is formed by an elbow 64 which is supported by a column 66. The reinforcing member 68 is V-shaped and integrated into the elbow 64. Figures 7B and 7C show peak or valley portions 62, with different thicknesses for the elbows 64. Figure 7D illustrates a reinforcing member 70 intersecting the columns 72, wherein the point of intersection creates tapered corners 74 that are rounded off in Figures 7B, 7C , 7D, 7E and 7F. In Figures 7E and 7F, the reinforcing member 70 has moved further down the columns 72, ie farther from the elbow 64. Figure 7G illustrates an alternate embodiment wherein the reinforcing member 76 has been integrated into the elbow 78. In Figure 7H, the reinforcing member 80 includes a loop 82 that has been clamped. Figure 71 is a plan view of an alternate embodiment, a reinforcing member 84 that has been integrated into the elbow 86 although the grooves 88 have been deformed in the base material. In Figures 7J, 7K and 7L, the shape of the open areas 90, 92 in the peak or valley positions 62, have been adjusted to vary the resistance in various parts of the stent. Still further, in Figures 7J, 7K and 7L the reinforcing member 94 has its orientation inverted as compared to the reinforcing members of the peak and valley portions 62 illustrated in the other Figures of Figure 7. Figure 9 is a plan view of an alternate embodiment of a stent 42 wherein the pattern of peaks and valleys has been modified to provide multiple side-by-side valley portions 96. In addition, interconnecting members 98 are connected at one end to the elbows 100 and at the other end they pass to columns 102.

The present invention also includes a bimodal feature as illustrated in Figures 10A and 1QB. Figure 10A shows a single cylindrical element 104 having alternating peaks and valleys, wherein each peak and valley are formed by a bend 106 and joined by a column 108. Under the conditions shown in Figure 10A, and as a result of an expansion of First stage of the stent, the columns 108 have been slightly bent, thereby increasing the circumference of the stent. In this way, the columns 108 are no longer parallel and have been spread outward. The reinforcing member 110 helps maintain the angle formed by the columns 108. Also, Figure 10A shows the first mode in which the reinforcing member 110 is straightened and locked in position; the loop or bend previously present in the reinforcing member 110, is straightened. The reinforcing member 110 in this configuration provides substantial strength and rigidity to the stent. In Figure 10B, the stent has expanded to a second stage, thereby increasing the circumference of the stent to a greater degree than that illustrated in Figure 10A. As the stent is further expanded, the columns 108 are bent at the intersections with the reinforcing members 110., until the columns align with the circumference of the stent as shown in Figure 10B. At this point, the stent is deployed completely over its maximum diameter. Accordingly, the columns 106 have been pulled straight and are almost parallel with the reinforcing member 110. In this way, the stent has reached its maximum circumference, although further increases in the stent conceivably can be achieved by deformation of the stent. columns 108 and reinforcing member 110. Essentially, the circumference of the stent may be increased by stretching the columns 108 and the reinforcing members 110+. It is possible to deploy the stent and reinforcement member with or without two different modes. This behavior is controlled by the force required to bend the columns 108 at the intersection of each column with a 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 pipe can be made of suitable biocompatible material, such as stainless steel, titanium, tantalum, super elastic nickel-titanium alloys (NiTi) and even high strength thermoplastic polymers. The diameter of the elbow is very small, so that the pipeline from which it is necessarily made must also have a small diameter. For PCTA applications, and as an example only, typically the stent has an outside diameter in the order of approximately .165 cm (.075") in the expanded condition, the same outer diameter characterizes the pipeline from which it is made, and can expand to an outside diameter of approximately .508 cm (.200") or more. The wall thickness of the tubing is approximately .20008 cm (.003"). For stents implanted in other body lumens such as in non-coronary PTA applications, the dimensions of the tubing forming the endoprosthesis are correspondingly larger. The dimensions of the stent will vary depending on the application and diameter of the body lumen, where the stent will be implanted.In the case where the stent is made from plastic, it may have been heated within the arterial site where the stent is intended to be deployed In order to facilitate the expansion of the endoprosthesis, once expanded, it will then cool to retain its expanded state.The endoprosthesis can conveniently be heated by heating the fluid within the balloon or in the balloon directly by a known method. It can be made of materials such as super elastic NiTi alloys. The endoprosthesis will be formed at full size but deformed (eg, compressed) into a smaller diameter over the balloon of the delivery catheter to facilitate transfer to a desired intraluminal site. The effort induced by the deformation, transforms the endoprosthesis from a obtained phase to a martensite phase and when releasing the force, when the endoprosthesis reaches the desired intraluminal location, the endoprosthesis expands due to the transformation back to the austenite phase. While the invention has been illustrated and described in terms of its use as an intravascular stent, it will be apparent to those skilled in the art that the stent may be employed in other instances in all vessels in the body. Because the stent of the present invention has the novel feature of being able to expand in very large diameters while retaining its structural integrity, it is particularly well suited for implantation in almost any vessel where these devices are used. This feature, coupled with limited longitudinal contraction of the stent when it expands radially, provides a highly convenient support member for all vessels in the body. Other modifications and improvements can be made without departing from the scope of the invention.

Claims (23)

1. CLAIMS l. - A flexible endoprosthesis for implantation in a body and expandable lumen from a condition contracted to an expanded condition, characterized in that it comprises: a plurality of adjacent cylindrical elements that expand in the radial direction and arrange in alignment on a longitudinal stent axis; the cylindrical elements formed in a serpentine wave pattern run along the longitudinal axis and contain a plurality of alternating peaks and valleys; at least one interconnecting member extends between adjacent cylindrical elements and connects them together; at least one reinforcing member extends across a width of alternate peaks and valleys; The serpentine pattern contains varying degrees of curvature in regions of the peaks and valleys adapted such that the radial expansion of the adjacent cylindrical elements is substantially uniform around their circumferences during expansion of the stent from its contracted condition to its expanded condition.
2. 2. - The stent according to claim 1, characterized in that the stent further comprises at least one reinforcing member that extends across a width of each of the alternate peaks and valleys.
3. 3. The stent according to claim 1, characterized in that the interconnecting member connects a valley of a cylindrical element with a valley of an adjacent cylindrical element.
4. 4. - The stent according to claim 1, characterized in that the interconnecting member connects a reinforcing member of a valley of a cylindrical element with a valley of an adjacent cylindrical element.
5. 5. The stent according to claim 3, characterized in that the interconnecting member is unitary with the valley of a cylindrical element and the valley of the adjacent cylindrical element.
6. 6. - The stent according to claim 1, characterized in that the reinforcing member curves opposite the respective peaks and valleys.
7. 7. The stent according to claim 1, characterized in that the alternating peaks and valleys are also constituted by columns of straight length intersecting at an angle, and wherein the reinforcing member engages the intersecting columns at points of elbow.
8. 8. The stent according to claim 7, characterized in that each elbow point is a portion of the column that has reduced material to facilitate bending.
9. 9. - The endoprosthesis according to claim 1, characterized in that the alternating peaks and valleys are also constituted by columns of elongated straight length intersecting an angle, and wherein the reinforcing member couples the intersecting columns at elbow points of the elongated columns.
10. 10. - The stent according to claim 1, characterized in that the reinforcing member is constituted by a rotation of a first room that transits half a turn, which transits a turn of a second quarter.
11. 11. The stent according to claim 1, characterized in that an intersection of the reinforcing member and the peaks and valleys are rounded.
12. 12. - The stent according to claim 1, characterized in that an intersection of the reinforcing member and the peaks and valleys are angular.
13. 13. The stent according to claim 1, characterized in that the reinforcing member is also constituted by an elongated area integrated to the peak and valley.
14. 14. - The stent according to claim 1, characterized in that the reinforcing member is also constituted by an enlarged area that is integrated into the peak and valley having through grooves.
15. 15. - The stent according to claim 1, characterized in that the stent is formed of a biocompatible material selected from the group consisting of stainless steel, tungsten, tantalum, NiTi super elastic alloys and thermoplastic polymers.
16. 16. - The stent according to claim 1, characterized in that the endoprosthesis is formed from a single section of pipe.
17. 17. The endoprosthesis according to claim 1, characterized in that the endoprosthesis is covered with a biocompatible coating.
18. 18. - A longitudinally flexible endoprosthesis for implanting in a body lumen and expanding from a contracted condition to an expanded condition, characterized in that it comprises: a plurality of adjacent cylindrical elements that expand independently in the radial direction and arrange in alignment on a longitudinal stent shaft; the cylindrical elements formed in a serpentine wave pattern run along the longitudinal axis and contain alternating peaks and valleys; at least one interconnecting member extends between adjacent cylindrical elements and connects them together; a reinforcing member that extends through each peak and valley; and the serpentine wave pattern is configured in size and shape such that the cylindrical elements generally expand uniformly around their circumferences during expansion of the stent from its contracted condition to its expanded condition.
19. 19. - The stent according to claim 18, characterized in that within a single cylindrical element, the serpentine wave pattern includes a sequence containing a peak, a valley, a peak, a valley, a valley and a peak.
20. 20. The stent according to claim 18, characterized in that the at least one interconnecting member connects a valley of a cylindrical element with a valley of an adjacent cylindrical element.
21. 21. The stent according to claim 18, characterized in that the endoprosthesis is formed of a biocompatible material selected from the group consisting of stainless steel, tungsten, tantalum, super elastic NiTi alloys and thermoplastic polymers.
22. 22. A method for constructing a flexible endoprosthesis for implantation in a body lumen, wherein the endoprosthesis is expanded from a contracted condition to an expanded condition, characterized in that it comprises the steps of: providing a plurality of adjacent cylindrical elements that expand independently in the radial direction and arranged in alignment on a longitudinal stent axis; forming the cylindrical elements formed in a serpentine wave pattern transverse to the longitudinal axis and containing a plurality of alternating peaks and valleys; providing at least one interconnecting member extending between adjacent cylindrical elements and connecting them together; providing at least one reinforcing member that extends across a width of alternate peaks and valleys; and wherein the irregular serpentine pattern contains varying degrees of curvature in regions of the peaks and valleys, adapted such that radial expansion of the adjacent cylindrical elements is substantially uniform around its circumferences during expansion of the stent from its contracted condition to its expanded condition.
23. 23. The method according to claim 22, characterized in that the process further comprises the step of connecting the interconnecting member between a valley of a cylindrical element and a valley of an adjacent cylindrical element.