A braided stent of this kind is known from the document DE 197 50 971 A1.
A stent is understood as a radially expandable endoprosthesis representing a typical intravascular implant which is implanted by a transluminal route and which is enlarged radially or expanded after it has been introduced percutaneously. Stents are used to strengthen blood vessels and to prevent restenosis in the vascular system following a previous angioplasty. They can be self-expanding or are expanded by a radial force applied from inside, for example if they are fitted on a balloon.
The stent known from DE 197 50 971 A1 has a hollow cylindrical body whose external diameter corresponds approximately to the internal diameter of the blood vessel in which the stent is to be implanted. The body of the stent is thus open in the longitudinal direction for the passage of blood. The circumferential surface is made up of a number of mutually offset filamentary elements which are braided together to form a braid with a multiplicity of polygonal cells. The braid can be constructed such that two intersecting systems of the filamentary elements are interlaced so that each filamentary element of one system is alternately guided over and under each filamentary element of the other system. Such a pattern of the braid is referred to as a plain weave.
The known stent can be stretched in its longitudinal direction on an applicator, by which means the diameter of the stent can be reduced for the purpose of implantation. The stent is introduced in the stretched or tensioned state into the vessel with the aid of the applicator. After the stent is positioned at the desired location in the vessel, the applicator is removed. Since no longitudinal extension forces now act on the stent, it relaxes elastically into its original length on account of its cell-like structure, expands radially outward and bears snugly on the inside wall of the vessel. This action can be intensified by using materials with a shape memory or by using a balloon catheter to assist or effect the expansion.
While it is known to produce braided stents of this kind continuously in the manner of yardage material, the braided stent known from DE 197 50 971 A1 is in each case produced individually, for which purpose the filamentary elements are reversed at each end of the stent during the braiding procedure. In this way, it is possible to give one end of the stent a crown which widens in a trumpet shape and by which an additional and particularly effective anchoring of the known stent in the blood vessel is achieved.
A disadvantage of braided stents of this kind is generally that they experience a considerable change in length upon stretching, the change in length being all the more pronounced the greater the original diameter and the smaller the original braiding angle. The correspondingly reversed reduction in length upon expansion of the braided stent is, however, often seen as a disadvantage. The positioning of a stent at the desired location in the blood vessel is of course a critical factor that decisively determines the action of the stent and the success of the medical intervention. Since the blood vessel area in which the stent is to be expanded is usually difficult for the practitioner to access, it is important that the diameter and the length of the stent in the expanded state are known exactly, so that it can be positioned with precision.
A further problem associated with braided stents is that the radial force and stiffness decline considerably even upon slight elongation of the stent, with the result that the exact dimensioning and positioning of a braided stent is more critical than in stents which do not shorten upon expansion. Such a stent is described for example in U.S. Pat. No. 6,106,548.
Braided stents are therefore not presently used in cases where a substantial radial force is to be exerted in a very precisely defined area of a blood vessel for the purpose of counteracting a restenosis.
A relatively new field of application for stents is the percutaneous treatment of lesions of the carotid artery following percutaneous angioplasty. The stenoses in the external carotid artery that are treated in this way are caused by arteriosclerosis of the vessel wall. This leads to a hard, brittle inner layer which constricts the blood stream more and more and thereby reduces the supply of blood to the brain. If an occlusion occurs, this results in a far-reaching stroke of the affected half of the brain and irreversible brain damage or even death.
The main complication of percutaneous angioplasty with subsequent stent implantation is caused, however, by the detachment of particles of the brittle inner layer, so-called plaque, which are entrained as emboli into regions of the brain, where they may trigger a local stroke with sometimes severe and irreversible brain damage.
Therefore, if the state of the vessel so permits, a “predilation” of the vessel by a balloon catheter is nowadays dispensed with, and instead a self-expanding stent is inserted directly by a percutaneous and transluminal route into the area of the stenosis. A particularly critical aspect of this, however, is the expansion phase of the stents. After their insertion and release within the stenosis, these stents in many cases do not develop a sufficient radial force to attain the shape in which they were manufactured. Therefore, after the stent has been put in place, a transluminal angioplasty balloon is introduced into the semi-deployed stent and inflated in order to widen the stent and the stenosis. This can lead to the aforementioned iatrogenic detachment of plaque material which, after deflation of the balloon, is entrained in the form of emboli into the brain.
To prevent the occurrence of strokes, so-called cerebral protection systems are therefore used with which the first emboli are trapped and removed. One such system is the PercuSurge system from PercuSurge Inc., Sunnyvale, Calif., USA. However, these cerebral protection systems entail a further intervention, often with clinical and symptom-related implications for the patients. Moreover, however, particles of plaque that have already loosened may also become detached several days after the stent implantation and may pass through the meshes of a conventional stent into the blood stream and thus trigger serious strokes.
To solve this problem, EP 1 101 456 A1 proposes a stent which, in its central portion, is ensheathed with a biocompatible, elastic material and comes to lie between the endoprosthesis and the wall of the blood vessel after implantation of the stent. In this way, the thrombogenic material is held on the wall, thus preventing plaque from detaching and passing into the blood stream.
However, this “ensheathed” stent has a whole series of disadvantages, in particular for the proposed application. First, the elastic membrane surrounding it prevents anchoring of the stent in the wall of the blood vessel, with the result that there is a risk of its changing position and losing its protective effect. Moreover, the known stent is expensive and complicated to produce, which fact is attributable to the additionally required elastic membrane.
The aforementioned U.S. Pat. No. 6,106,548 is also concerned with the problem of detachment of plaque and its transport as emboli into the brain.
The known stent comprises a large number of rings which each include V-shaped struts. Adjacent rings are interconnected by wave-shaped connecting members which are in each case secured at the apices of V-shaped struts in adjacent rings. These connecting members compensate for the change in length of the rings during expansion, with the result that the stent does not change its length when it is expanded in the blood vessel. Said document further states that the external diameter of the known stent should be slightly greater than the internal diameter of the blood vessel in order to anchor the stent securely at the desired position and prevent it from moving out of its position.
The meshes formed by the V-shaped struts and connecting members can have different mesh widths in different sections, with smaller meshes being used to prevent detachment of plaque.
The known stent can be configured such that, in the expanded state, it has areas with different external diameters, such that it can adapt to vessels or bifurcations where the lumen diameters vary, as is the case for example at the carotid bifurcation.
The known stent is produced from a small tube using a laser cutting technique or is made from prefabricated V-shaped struts and connecting members which are subsequently connected to one another, for example by welding.
Given the demand that this stent should not shorten during expansion, it can be manufactured only by technically complex means, with the result that its production is very cost-intensive.
A stent which likewise shortens only slightly during expansion is known from U.S. Pat. No. 5,938,697. This document is concerned with the problem that, although stents with an equal radial force along their length are able to keep a blood vessel open in the stenosed area, their outer portions press more strongly than is needed against healthy areas of the vessel. Comparable problems are seen in conical areas of vessels, for example at the carotid bifurcation.
The known stent solves this problem by exerting radial forces that vary along its longitudinal extent and by having a differing stiffness.
For this purpose, the stent is made up of annular sections of serpentine-like segments which extend in a zigzag shape around said section. The individual rings are interconnected by means of all or some of the adjacent zigzag segments being connected to one another by struts at their apices. By virtue of the dimensions of these struts and the number of the struts between two rings, a more open or more closed structure is achieved because less or more metal is present between the rings. The rings themselves are identical. This configuration is intended to ensure that the ratio of metal surface to blood vessel surface is constant along the length of the stent.
The stent has the so-called closed structure in those areas in which good stiffness and plaque coverage are to be achieved, whereas the so-called open structure is to be found in areas where greater flexibility is to be achieved.
In one illustrative embodiment, the stent has a central portion in which it exerts a very high radial force, whereas the radial force in the adjoining distal and proximal portions is much less.
A disadvantage of this stent is that it does not provide any effective protection against detachment of plaque, and, in addition, even the excessively high pressure on the vessel wall in the stenosed area cannot prevent extremely small particles of plaque from detaching and passing into the blood stream through the meshes in the individual rings.