WO2004084769A2

WO2004084769A2 - Stents

Info

Publication number: WO2004084769A2
Application number: PCT/ZA2004/000035
Authority: WO
Inventors: Damian John Conway; Gregory Richard Starke; Markus Lehmann; Kenneth Stuart Park
Original assignee: Damian John Conway; Gregory Richard Starke; Markus Lehmann; Kenneth Stuart Park
Priority date: 2003-03-28
Filing date: 2004-03-26
Publication date: 2004-10-07
Also published as: WO2004084769A3

Abstract

A stent for implantation into a blood vessel of a patient to maintain the patency of the blood vessel is disclosed. The stent comprises a series of rings (12.1) each composed of bars (16.1, 18.1) arranged zig-zag fashion and joined at their ends by hairpin bends (20.1). The rings are joined by longitudinals. The configuration of the bars and hairpin bends is such that, in the circumferential direction, dimension W3 is less than W2, and dimension W2 is less than W1, which latter dimension is measured in the axial direction. From the larger dimension portion at W2 to the smaller dimension portion at W3, each bar (16.1, 18.1) tapers smoothly without steps or other discontinuities. Recesses or holes (22, 24) can be provided in which drugs can be placed for release into the patient's blood stream.

Description

STENTS

FIELD OF THE INVENTION

THIS INVENTION relates to stents

BACKGROUND TO THE INVENTION

A stent is used medically in any one of a multiplicity of vessels of the human body to maintain its patency.

Applicant is aware of European patent specifications EP 0 669 114 B1 , EP 0 821 920 B1 and EP 0 821 921 A1 and US specifications 6503272, 6342067, 5733303, 5195984, 5102417, 4733665, 6179867, 5514154, 6068656, 6190403, 5913895 and 5643312 which disclose stents of various types. A further stent is disclosed in WO03/007842A1.

Certain studies (such as the ISAR STEREO I and ISAR STEREO II clinical trials) have demonstrated that the thickness of stent struts has a significant influence on clinical outcomes. These studies have shown that reductions in stent strut width and thickness result in statistically significant reductions in neointimal hyperplasia and in-stent restenosis. Thinning down all stent strut dimensions to reduce neointimal hyperplasia also reduces the radial strength of the stent and thus reduces the stents ability to provide sufficient radial support to a diseased, narrowed artery after stent expansion. Furthermore, reducing stent strut thickness has the effect of reducing the radiopacity or visibility of the stent making delivery and angiographic visualisation post-procedurally (for subsequent angiographic investigations) more difficult.

The studies mentioned above have been published as follows: Kastrati A, Mehilli J, Dirschinger J, et al. Intracoronary stenting and angiographic results: strut thickness effect on restenosis outcome (ISAR- STEREO) trial. Circulation. 2001 ; 03: 2816-2821. Pache J, Kastrati A, Mehilli J, Schuhlen H, Dotzer F, Hausleiter J, Fleckenstein M, Neumann FJ, Sattelberger U, Schmitt C, Muller M, Dirschinger J, Schomig A. Related Articles, Links Intracoronary stenting and angiographic results: strut thickness effect on restenosis outcome (ISAR- STEREO-2) trial. J Am Coll Cardiol. 2003 Apr 16;41 (8):1283-8.

The object of the present invention is to provide a stent which reduces neointimal hyperplasia and restenosis without compromising the radial support provided by the stent to the wall of the vessel that it is in or the visibility of the stent under fluoroscopy.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the present invention there is provided a stent of cylindrical form which comprises a plurality of rings extending around the longitudinal axis of the stent, the rings being spaced apart along the axis and being joined to one another, each ring being of zig-zag form and comprising a plurality of first bars all of which are parallel to one another and a plurality of second bars all of which are parallel to one another and at an angle to the bars of the first set of bars, first bars alternating with second bars and each first bar being joined at each end thereof to the adjacent second bars by way of hairpin bends, each bar having end portions contiguous with the hairpin bends and a centre portion which joins said end portions to one another, the bars and hairpin bends having their thickness dimension in the radial direction, characterized in that the width of the centre portion of each bar is less than that of its end portions, and the width of each end portion is less than that of the hairpin bends.

The stent can comprise a metallic substrate and a coating on the metallic substrate, the coating consisting of one or more drugs.

The drug or drugs in the coating can be selected from 2- ethoxyestridiol or its derivatives or prodrugs or Trapidil.

The drug coating is preferably itself coated with a barrier layer of biocompatible polymer. The thickness of the barrier layer can be between 0.01 and 20 microns.

In one form the stent has recesses in which there are drugs for release after implantation. The stent can additionally or alternatively have through holes in which there are drugs for release after implantation. The stent is preferably of chromium cobalt alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:-

Figure 1 illustrates a known stent in layflat form; and Figure 2 illustrates, to a larger scale, a detail of a stent according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Stents are cut from a metal tube using a techniques such as laser cutting. Use of such a technique is necessary in view of the small dimensions involved and the fineness of the detail required.

The known stent illustrated in Figure 1 comprises a plurality of sections 12 which, in the cylindrical stent, constitute rings which extend around the longitudinal axis of the stent. The stent is shown flat in Figure 1 so that its construction can be seen.

The sections 12 are spaced apart along the longitudinal axis of the stent and are joined to one another by longitudinals 14 which span between one section 12 and an adjacent section 12. Each section 12 is a zig-zag form and comprises a plurality of first bars 16 all of which are parallel to one another and a plurality of second bars 18 all of which are parallel to one another and at an angle to the bars 16. Bars 16 alternate with bars 18.

The bars are joined to one another by way of hairpin bends 20 which form the connections between adjacent sections 12.

Each longitudinal 14 joins a hairpin bend 20 of one section 12 to a hairpin bend 20 of the adjacent section. In the illustrated example, each longitudinal 14 is of a generally sinusoidal or "S" shaped configuration. As will be seen from Figure 1 , some hairpin bends are not joined to opposing hairpin bends.

In Figure 2 two bars 16.1 and 18.1 of the stent are shown to a larger scale. The width of the material of the hairpin bend 20.1 is shown as W1. Adjacent the hairpin bend 20.1 the bars 16.1 , 18.1 have a width W2 and at their centres the bars have a width W3. It will be understood that when the cut material is formed into a tube, the dimensions W2 and W3 become the dimensions of the bars in the circumferential direction and W1 a dimension in the axial direction. The relationship between W1 , W2 and W3 is such that W1 is greater than W2 which is in turn greater than W3. The change in dimensions from W1 to W2 to W3 is gradual, the change in dimension from W2 to W3 particularly being achieved by tapering the bars from both ends towards their centres. The configuration shown in Figure 2 minimises stress and/or strain concentrations at the hairpin bends whilst maintaining the requisite high strength in the radial direction and minimising strut width in areas which bear low loads. The ratios of W1 to W2 and W2 to W3 are optimised by a parametric method such as the finite element method in order to minimise peak stresses and strains whilst obtaining adequate strength in the radial direction with minimum bar widths.

Bars and hairpin bends as described reduce the stimuli for restenosis whilst providing adequate vessel support in the radial direction. Adequate visibility under a fluoroscope can be achieved by using radiopaque materials such as chromium cobalt alloy. This alloy is available commercially under the brand names L605, Hayes Alloy 25, Eligloy, Phynox and MP35N.

To provide for delivery of drugs to the implantation site, the bars 16.1 , 18.1 are provided with through slots 22 or with recesses 24. If slots 22 are provided these are open at both their radially inner and radially outer ends. If recesses are provided then these are only open at one end and can be in the radially inner surfaces of the bars of the cylindrical stent or in the radially outer surfaces of the bars.

The slots and/or recesses provide reservoirs in which slow release drugs can be placed. The drugs can be in a polymer or other binder or in the form of the pure drug in crystalline form. More, or larger, slots and recesses can be provided where there is a need for greater drug elution. Generally greater drug elution is required at the axial ends of the stent.

Suitable drugs are Methoxyestridiol or any of its derivatives or pro- drugs and Trapidil.

It is also possible to coat the stent with a drug layer by use of a method such as plasma deposition, vapour deposition, ion deposition, ion beam splutter, spluttering or thermal spraying. The drug coating can itself be covered by a biocompatible polymer. Such polymers are commercially available under the names Pursil, ElastEon or Parylene and provide a diffusion barrier. The thickness of the barrier can be from 0.01 microns and 20 microns to provide for a desired rate of drug release.

Claims

CLAIMS:

1. A stent of cylindrical form which comprises a plurality of rings extending around the longitudinal axis of the stent, the rings being spaced apart along the axis and being joined to one another, each ring being of zig-zag form and comprising a plurality of first bars all of which are parallel to one another and a plurality of second bars all of which are parallel to one another and at an angle to the bars of the first set of bars, first bars alternating with second bars and each first bar being joined at each end thereof to the adjacent second bars by way of hairpin bends, each bar having end portions contiguous with the hairpin bends and a centre portion which joins said end portions to one another, the bars and hairpin bends having their thickness dimension in the radial direction, characterized in that the width of the centre portion of each bar is less than that of its end portions, and the width of each end portion is less than that of the hairpin bends.

2. A stent as claimed in claim 1 , and which comprises a metallic substrate and a coating on the metallic substrate, the coating consisting of one or more drugs.

3. A stent as claimed in claim 2, wherein the drug or drugs in said coating are selected from 2-Methoxyestridiol or its derivatives or prodrugs or Trapidil.

4. A stent as claimed in claim 3, wherein said drug coating is itself coated with a barrier layer of biocompatible polymer.

5. A stent as claimed in claim 4, wherein the thickness of the barrier layer is between 0.01 and 20 microns.

6. A stent as claimed in claim 1 and which has recesses in which there are drugs for release after implantation.

7. A stent as claimed in claim 1 or in claim 6 and which has through holes in which there are drugs for release after implantation.

8. A stent as claimed in any preceding claim and which is of chromium cobalt alloy.