Stent having varying thickness along its length

[0002] 1. Field of the Invention

[0003] This invention generally relates to a stent. More specifically, the present invention relates to a stent comprising a stent body which is in particular used for expanding narrowed hollow vessels.

[0004] 2. Background Information

[0005] Different types of stents are known from the prior art. These stents form a vascular prosthesis made from a physically compatible material. Stents are in general used for expanding hollow vessels, such as blood vessels or body orifices, and for keeping the vessels or orifices in an expanded state. To this end, the stent is normally positioned in its non-expanded state within a patient's body inside a narrowed hollow vessel and is then expanded by suitable means, for instance a balloon catheter. The stent body normally consists of a web structure, which comprises a plurality of neighboring cells, each cell being defined by webs. During expansion the individual web portions of the stent are deformed, so that the portions permanently remain in the expanded form.

[0006] The problem of restenosis often arises in such expanded hollow vessels. After some time the portion of the hollow vessel which has been expanded by the stent is narrowed again. Such a restenosis may, inter alia, be caused by the inherent stiffness of the stent. As schematically shown in FIG. 6, the hollow vessel 10 is stiffened by the stent 1′. Strong irritations R of the vessel wall which result in a restenosis of the hollow vessel can in particular be observed on the two end portions of the stent due to the inherent stiffness of the stent body 2′.

[0007] In view of the above, there exists a need for a stent which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.

[0008] One object of the present invention is to provide a stent that reduces the risk of restenosis in a hollow vessel in the area of an implanted stent.

[0009] Hence, according to the invention, there is provided a stent the wall thickness of which has been changed. This results in a stent having portions that due to the different wall thicknesses exhibit different inherent stiffnesses. Hence, the stent body within the portions of a reduced wall thickness has an increased flexibility, whereby irritations of the vessel wall at these places can be reduced or even be prevented altogether. Hence, the risk of restenosis in these portions can be reduced considerably.

[0010] Preferably, the wall thickness of the two end portions of the stent body is smaller than the wall thickness of a main portion of the stent body. As a consequence, the inherent stiffness of the two end portions of the stent body can in particular be reduced, so that the end portions exhibit an increased flexibility, whereby an abrupt “directional change” or a bending of the hollow vessel due to a different stiffness degree between the portion of the hollow vessel without stent and the portion of the hollow vessel with stent can be prevented. As a result, one obtains a more gentle transition between the hollow vessel and the stent, whereby a strong irritation on the end portions of the stent can be avoided.

[0011] According to a preferred embodiment of the present invention, the stent body comprises exactly one end portion having a smaller wall thickness than the remaining portion of the stent body. Such a stent can preferably be used in branches or rami of hollow vessels. The stent is arranged in the branched portion such that the end portion of a reduced wall thickness is oriented away from the branched portion. Hence, the stent portion is provided in the area of the branched portion or directly next to the branched portion with an increased wall thickness because the risk of irritations created by the stent is small on account of the natural extension of the branched portion in said area. In the area that is more remote from the branched portion, the stent, however, has a smaller wall thickness so that an increased flexibility of the stent is observed in said area and the stent can follow the natural course of the hollow vessel. Hence, the risk of irritations is reduced thanks to the high flexibility of the end portion of the stent.

[0012] To achieve a defined transition between the main portion and the end portion, which has a reduced wall thickness, the transition between said two portions is stepped or graded. As a result, the stiffness of the stent can be changed abruptly and the desired flexibility of the stent can be obtained in defined portions of the stent body.

[0013] To attain a gradual change in the stiffness or flexibility, respectively, of the stent body, the transition between the portion of an increased wall thickness to the portion of a reduced wall thickness is made continuous. The transition between the two wall thicknesses may e.g. be configured such that it is linear or exhibits a gradually changing increase.

[0014] To provide different inherent stiffnesses of the two end portions of the stent body, the wall thicknesses of the two end portions may be different. To provide a symmetrical stent, the end portions of the stent body may also have identical wall thicknesses. Furthermore, flexible end portions may also be achieved by identical or different lengths of the end portions in the axial direction or, in the case of gradually changing wall thicknesses in the transitional portion, by different axial lengths of the transitional portions.

[0015] These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention.

[0016] Referring now to the attached drawings which form a part of this original disclosure:

[0017] FIG. 1 is a schematic cross-sectional view of a stent in the longitudinal direction according to a first embodiment of the present invention;

[0018] FIG. 1A is a schematic cross-sectional view of a stent in the longitudinal direction according to a second embodiment of the present invention;

[0019] FIG. 2 is a schematic cross-sectional view of a stent in the longitudinal direction according to a third embodiment of the present invention;

[0020] FIG. 2A is a schematic cross-sectional view of a stent in the longitudinal direction according to a fourth embodiment of the present invention;

[0021] FIG. 3 is a schematic cross-sectional view of a stent in the longitudinal direction according to a fifth embodiment of the present invention;

[0022] FIG. 3A is a schematic cross-sectional view of a stent in the longitudinal direction according to a sixth embodiment of the present invention;

[0023] FIG. 4 is a schematic cross-sectional view showing an arrangement of the stent according to the fifth embodiment of the present invention in a branched portion of a hollow vessel;

[0024] FIG. 5 is a schematic cross-sectional view showing the arrangement of a stent according to the first embodiment in a hollow vessel; and

[0025] FIG. 6 is a schematic cross-sectional view of a prior art stent located within a hollow vessel.

[0026] Referring initially to FIGS. 1, 1A and 5, a stent 1 in accordance with two embodiments of the present invention is illustrated. Since the second embodiment of FIG. 1A is substantially identical to the first embodiment, similar parts of the stents are designated by the same reference numerals. Preferably, the stent 1 consists of a substantially cylindrical stent body, which is made hollow in its interior to enable fluid to pass therethrough. The stent body 2 consists of a flexible web structure, which is not shown for the sake of clarity. Rather, the stent body 2 will be illustrated as a solid tube for the sake of clarity.

[0027] Basically, the flexible web structure of the stent body 2 is used for expanding hollow vessels, such as blood vessels or body orifices, and for keeping the vessels or orifices in an expanded state. To this end, the stent body 2 is normally positioned in its non-expanded state within a patient's body inside a narrowed hollow vessel. Then the stent body 2 is expanded by suitable means, for instance by a balloon catheter. The stent body 2 normally consists of a web structure, which comprises a plurality of neighboring cells, each cell being defined by webs. For example, the web structure of the stent body 2 can be constructed in a manner that is similar to the web structure disclosed in U.S. patent application Ser. No. 09/171,293, filed on Oct. 16, 1998. The entire disclosure of U.S. patent application Ser. No. 09/171,293 is hereby incorporated herein by reference. Of course, other types of known web structures can be used, and thus, modified in accordance with the present invention. During expansion the individual web portions of the stent body 2 are deformed, so that the web portions permanently remain in the expanded form. The flexible web structure of the stent body 2 is well known in the art, and thus, it will not be discussed in detail herein.

[0028] As seen in FIG. 1, the stent body 2 of the first embodiment is subdivided into three portions, namely a main or center portion 5, a first end portion 3 and a second end portion 4.

[0029] In the first and second embodiments of the present invention, the wall thicknesses W_E1and W_E2of the first and second end portions 3 and 4 are smaller than the wall thickness W_Hof the main portion 5. Also in the first and second embodiments of the present invention, the wall thicknesses W_E1and W_E2of the end portions 3 and 4 are about half the wall thickness W_Hof the main portion 5. However, any desired wall thickness ratios W_E/W_Hare possible between end portions and main portion. The wall thicknesses W_E1and W_E2of the two end portions 3 and 4 can also be chosen such that they differ from each other. Moreover, as seen in FIG. 1A, it is possible to vary the flexibility of the stent 1 through different lengths of the end portions 3 and 4 having the reduced wall thickness W_E1or W_E2.

[0030] The main portion 5 and the first and second end portions 3 and 4 have arranged therebetween a transition or transitional portion 6 which connects the main portion 5 to the respective end portions 3 and 4. As shown in FIG. 1, the transition 6 from the main portion 5 to the end portions 3 and 4 is stepped or graded. This means that different flexibilities of the respective stent portions are obtained because of the different wall thicknesses of main portion 5 and end portions 3 and 4, resulting in an improved flexibility because of the smaller wall thickness W_E1or W_E2in the area. Since the transition 6 is configured to be perpendicular to the surface of the end portions 3 and 4 and the main portion 5, respectively, a sudden change in the flexibility of the stent is obtained in this area.

[0031] As shown in FIG. 5, a stent of such a design can very easily be adapted to the respective conditions or natural extension of the vessel 10. As becomes also apparent from FIG. 5, the end portions 3 and 4 can be adapted to a relatively strong curvature of the hollow vessel 10. As a consequence, the vessel 10 extends in a uniform manner without any sudden directional changes, i.e. also in the area of the implanted stent. In particular, strong irritations, which are observed in conventional stents, can thus be prevented on the end portions of the stent, whereby the risk of restenosis is considerably reduced.

[0032] FIG. 6 shows, by way of comparison, a standard or prior art stent 1′, which has been implanted into a hollow vessel 10. Since the stent body 2′ of the stent has a uniform wall thickness, points or places R which are subjected to strong irritations are observed on the end portions of the stent 1′, in particular in the case of the hollow vessel 10 extending in bent fashion. Such irritations R may be responsible for restenosis, and thus for a narrowing of the hollow vessel 10, which means that a so-called restenosis is observed in the area of the implanted stent.

[0033] Thanks to the end portions 3 and 4 of the stent 1, which are made flexible according to the invention, such points or places R of considerable irritation can be avoided, since the whole stent 1 can be adapted to the natural bent course of the hollow vessel 10 on account of the increased flexibility in the end portion. To be more specific, a sudden change in stiffness which is normally observed between the hollow vessel 10 and the portion of the hollow vessel that is provided with the stent is consequently weakened or attenuated by the flexible ends. Hence, restenosis caused by irritations on account of the implanted stent can be reduced in an efficient manner.

[0034] FIGS. 2 and 2A show third and fourth embodiments of a stent 1 according to the present invention. Corresponding or identical parts of these stents are designated by the same reference numerals as used in the first embodiment. The stent 1 has a tubular stent body 2, which consists of a main portion 5 and two end portions 3 and 4. The wall thickness W_Eof the end portions 3 and 4 is about half the size of the wall thickness W_Hof the main portion 5. The main portion 5 is connected to the end portions 3 and 4 via a respective transition or transitional portion 6. The transition 6 is designed such that a continuously extending transition is obtained between main portion 5 and end portions 3 and 4 (cf. FIG. 2). Starting from the end portions 3 and 4, the wall thickness of the stent body 2 increases towards the main portion 5. As a result of such a gradual increase in the wall thickness from the end portion to the main portion, the flexibility of the stent 1 is also changed in this portion. This means that the flexibility of the transitional portion 6 is reduced from the end portion towards the main portion. In contrast to the first embodiment, there is no abrupt change in flexibility and no sudden change in the inherent stiffness of the stent in the transitional portion, but the stiffness changes slowly and continuously in the transition 6.

[0035] Hence, the stent 1 of this embodiment exhibits an increased flexibility in the end portion of the stent 1, whereby restenosis caused by irritations due to the implanted stent is prevented in an efficient manner. In addition, the transition 6 between the end portions 3 and 4 and the main portion 5 can be designed in response to the desired requirements in such a manner that the flexibility of the transition 6 is changed progressively or stepwise. The transition 6 may be designed in any desired manner. For instance, a linear or parabolic transition 6 may be provided for, or a transition 6 including a plurality of small steps. In the illustrated embodiments of FIGS. 2 and 2A, transition 6 forms a conical outer surface between the main portion and each of the end portions 3 and 4.

[0036] FIGS. 3 and 4 show a fifth embodiment of a stent according to the present invention, while FIG. 3A shows a sixth embodiment of the present invention. Corresponding or identical parts of the stent have again been designated by the same reference numerals as used in the prior embodiments. Each of the stents 1 of these embodiments comprises a tubular stent body 2, which has a main portion 5. In contrast to the two preceding embodiments, the stent body 2 of these embodiments have only arranged thereon one end portion 3′ with a smaller wall thickness W_Ethan the main portion 5. The other end of the stent body 2 has the same wall thickness W_Has the main portion 5 (cf. FIG. 3). A transition 6 between the main portion 5 and the end portion 3′ can be stepped as in the first embodiment or gradually sloped as in the third and fourth embodiments. Hence, there is a sudden change in the flexibility of the stent body 2 on the transitional portion 6 due to the different wall thicknesses. Of course, the transition 6 may be configured in any desired manner that will still carry out the principles of the present invention.

[0037] As shown in FIG. 4, the stent 1 of the fifth embodiment is primarily used on branches or ramifications of hollow vessels. FIG. 4 shows, by way of example, a branch or ramus 12 extending from the aorta 11 to the kidney 13. Since the ramus 12 directly next to the place branching from the aorta 11 extends substantially along the branching direction and is devoid of any torsions or curve-like or curved extensions, or the like, the stent body 2 may be given a standard flexibility in said area. As a consequence, the end of the stent body 2 in the area may have the same wall thickness as the main portion 5 (cf. FIG. 4). The other end portion 3′ of the stent body 2 which is oriented towards the kidney 13 is again provided with a smaller wall thickness W_Ethan the main portion 5, by analogy with the first two embodiments. Hence, the stent 1 can be adapted in an ideal manner to the natural course of the ramus 12 of the hollow vessel, and irritations of the vessel wall can be reduced on the respective end portions of the stent. Since hollow vessels, such as the kidney, are moving on account of inhalation and exhalation processes, the vessel 12 which connects the kidney 13 and the aorta 11 to each other is also moved considerably. In comparison with a conventional stent, the flexibility of the vessel is thus increased considerably by the inventive stent of the third embodiment, and irritations of the wall of the vessel are attenuated. As a consequence, the risk of restenosis in the area of the implanted stent is reduced considerably.

[0038] While several embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.