FIELD OF THE INVENTION
The present invention relates generally to medical technology, particularly to expandable cardiovascular stents, which are intended for radical arterial lumen recovery with subsequent restoration of normal blood flow. In the present application the term “stent” refers to a device designed to expand a blood vessel and to maintain the achieved size of a lumen. Traditionally stents are delivered to a target area in the cardiovascular system on an inflatable balloon located on the tip of a transluminal catheter. Then, the balloon is inflated, leading to the expansion of the stent thereby widening the lumen of the vessel. Other less common systems for stent delivery also exist.
BACKGROUND OF THE INVENTION
Most of the existent stents are made from metal. Examples of common designs are set forth in, for example, U.S. Pat. Nos. 4,733,665, 4,969,458, 5,102,417, 5,195,994, 5,513,444, and PCT International Publication No. WO 91/013820, all of which are incorporated herein by reference. Certain properties of any metallic surface lead to thrombogenicity of a stent once it is implanted within the human cardiovascular system. Therefore, one of the important directions in stent development is the improvement of stent thromboresistance because this would reduce the systemic anticoagulation therapy, thereby reducing the complication rate after stent implantation. At present none of the metallic stent designs have achieved the delicate balance between desired durability to sufficiently support the vessel wall and flexibility to reduce the thrombogenicity and intimal hyperplasia. Thus, there is a substantial need for anticoagulation and thrombolitic therapy following stent implantation.
The use of metal in stent design has additional drawbacks. One of the limitations of metallic stents is the presence of a more or less rigid kinematic link between constructive elements of radial strength and flexibility. This factor creates additional difficulties during the delivery of the stent to a target area in the coronary artery, especially in distal segments of the vessel. This factor also plays a major role in the shortening of the stent upon stent expansion, which may lead to the suboptimal implantation of the stent, especially in diseased segments of blood vessels, and also this may activate undesirable post-procedural processes, such as thrombosis and restenosis.
The rigidity of a kinematic link between the constructive elements of radial strength and flexibility in already complicated geometrical forms of the stent structure does not permit the use of thin metal plates in stent manufacture. On the contrary, it requires high inflation pressures upon the deployment of a stent to prevent the stent from collapsing into the vessel lumen. However, ideally a stent structure should combine longitudinal flexibility and radial rigidity, which would correspond optimally to the characteristics of pulsating coronary arteries.
Despite the fact that the descriptions of most conventional stents claim that they are low profile stents, in fact, all known stents have profiles in the range of from about 1.3 to 1.6 mm. This is due to the limitations of the technology of stent manufacture. All stents are placed on balloons with a minimal diameter of 1.6 mm, which already restricts clinical application of stents in small vessels. There is no known stent having parameters that would permit it to be used in vessels of 2 mm or less. Another advantage of stent structure is an ability to perform an adjunctive angioplasty after the deployment of the stent. This also permits the better adjustment of the stent to the arterial wall due to the deeper penetration of the stent outer elements into the media and the atherosclerotic plaque. A disadvantage, on the other hand, is the metallic surface of a stent in general, and especially the texture of the surface, which can attract blood elements and activate the formation of thrombus, as well as initiate an exaggerated healing process, i.e., the proliferation of smooth muscle cells that can result in restenosis.
Therefore, an important part of stent design is the ability to incorporate various bioabsorbable polymers, which can be loaded with antithrombotic and/or antiproliferative pharmacologic agents in high concentrations. These agents, delivered locally into the arterial wall, can prevent thrombosis and neointimal proliferation and also avoid unwanted systemic side effects. However, so far the results of clinical experiments with polymer coated stents show frequent occurrence of inflammatory reactions to the polymers by the vessel wall, which limits their clinical application. Another important limitation of stent use is the expensive technology required for stent manufacture, which involves laser technology in almost all known stents. This lowers the cost-effectiveness of the device and, therefore, its utilization in clinical practice. This technology also leaves the quality of a stent's surface suboptimal, with subsequent higher percentage of thrombus formation on this surface. The “ideal” stent should possess the following high quality properties: flexibility, trackability, non-shortness, ultra-low profile, visibility in X-rays, thromboresistance, biocompatibility, reliable expandability, wide range of available sizes, optional capability of the local drug delivery, and low cost (see, P. Ruygrokand P. Serruys Intracoronary stenting. “Circulation”, 1996, 882-890). These features will widen clinical applications of stenting, enable the reduction of unwanted side effects, and ultimately improve the clinical outcome.
An effective technical stent design executed from slotted tubes simultaneously combines flexibility and sufficient radial strength, as is shown, for example, in PCT International Publication No. WO 98/20927, incorporated herein by reference. A more progressive stent design is disclosed in the PCT patent application No. PCT/IL 98/00189, filed Apr. 21, 1998, incorporated herein by reference. In this prototype design (FIGS. 1, 2) the constructive elements, preliminary shaped as a stencil on a thin sheet metallic blank surface, form flexible twisting loops (1), closed on two bands (2) and (3) as consecutively united pockets. Before the installation of the stent, branches of loops (1) are in turn oppositely moved apart in such a way that each pair of loops is transformed into a shape close to that of a circle (ring). Then, after the calibration, the stent is located on an inflatable balloon (4) of a delivery catheter for its subsequent introduction into an afflicted vessel. However, this known stent has a substantial disadvantage: the presence of a critical plane on which the appositively located bands (2,3) in a shape of the consecutively united pockets are located. This plane has proven to be very rigid and, upon the deformation for bending, can hamper overcoming a vessel's anatomic curvature. This characteristic hinders location of this known stent in curved vessels as well as creation of stents of a required length. In practice several stents have to be implanted in a row, which prolongs the time of intravascular intervention and causes additional vessel trauma.
In other axial planes at the known stent bending rigidity is minimized in the plane perpendicular to a critical one. However, in all cases, excluding the last one, the bands (2, 3) with the chains of the united pockets change their length due to the bending deformation. The band length increases on the outward radius and decreases on the inward one upon the bending of a stent in a vessel. This prohibits accurately determining the length of a polymer thread loaded with medicinal preparations for local drug delivery. The thread's length should not be less than that of an extent of the united pocket chain on the stent bending outward radius, corresponding to its maximal tension. This could lead to the sag of the polymer loaded thread on the stent bending inward radius and to the jamming of it among the loops (1).
A shift from the critical plane in such a stent design could be partially done by twisting the bands (2,3) in relation to the longitudinal axis in such a way that the chain bands of consecutively united pockets locate in the spirals. However, it does not fully solve the rigidity problems, and, in addition, the twisting (and a possible untwisting) of the stent leads to the changing of its axial and radial sizes, as well as to the changing in the distance among loops (1). The restriction of the vessel wall natural movements could promote the development of stenosis. An attempt to prevent the vascular stenosis with a help of a stent will be more successful the more flexible the stent and the less it restricts the possible natural local vessel wall movements. The presence in a prototype-stent of the two comparatively rigid bands (2,3) with consecutively united pockets chains upon the close contact with a vessel wall greatly limits the degrees of freedom of its wall. This may become the cause for restenosis. The practical work shows that after the installation of the sufficiently rigid stent in a vessel of a developed length, restenosis occurs in more than 25% of the clinical cases.
SUMMARY OF THE INVENTION
According to the invention a stent has increased flexibility with a container for a polymer loaded thread of a fixed length, whereas there is support for the favorable dynamic action on a vessel wall (of a massage type). Also, the consequences of edge effects from blood flow action on the stent face end surface sections in a vessel are avoided. Further, a stent configuration has diameters differentiated in length for a simultaneous deployment in a main vessel and in one of its bifurcations with an increased rigidity for a better fixation of the stent in the place of vessel diameters transition. This is achieved by the fact that in a flexible expandable sheet stent design constructive elements, preliminarily formed as a stencil on a thin sheet metallic blank surface, in their regular form represent one relatively rigid band consisting of consecutively united pockets, the branches of which form periodically repeating winding closed outlines, whereas the components of the longest sides of each outline are appositively located in a form of a closed free loop with a configuration that approximates a circle, forming an independent ring with a fastening point on the relatively rigid band. In the preliminarily formed stencil, the stent constructive elements occupy the primary part of the area of the sheet, excluding that which falls at the stencil slots, at the radii of the formed pockets rounding off and the radii of free loops short closed sides. The width of the slots is executed as minimally possible technologically.
According to the invention the pockets of the stent are formed by the bending of a saw-shaped profile that is a component of the closed free loops foundation, whereas the bending of the pockets is executed into one or alternatively into different sides for an angle of the order of 120°. The polymer loaded thread of a fixed length is placed in the consecutively united pockets.
In the stent of the invention the short sides of the closed free loops on the stent end side surfaces are fastened by the fragments of the relatively rigid band in a shape of the pockets, whereas in case of stent diameter differentiated in its length according to the different diameters and extent of the afflicted vessel by the said fragments of the relatively rigid band in a shape of the pockets are fastened the short sides of the closed free loops in the place of their transition from one stent diameter to the other. Single closed free loops are uniformly distributed along the stent length or in places, where the increased flexibility is most desirable, including a construction variant with one closed free loop, placed in the stent middle part, whereas the other short sides of the loops are executed in a shape of a relatively rigid band consisting of the consecutively united pockets.
The technological manufacturing process of the proposed stent design includes the following steps:
separation of the thin sheet metallic blank with a multiple unwasted quantity of the stent designs;
execution of a calculated geometrical profile stencil of stent constructive elements on the surface of the thin sheet metallic blank;
shaping of consecutively united pockets by bending;
deployment of the stencil into a step-by-step gauge fixing the distances among the closed free loops;
introduction of cylindrical gauges into all said loops,
positioning appositively the long sides of the each loop and trying to achieve the stent minimal diameter necessary for an uninflated balloon;
fixation of the stent on an uninflated balloon of a delivery catheter;
positioning and fixation of a polymer loaded thread in a passage formed by the chain of the said consecutively united pockets; and
packing of a ready device.
In the case of using a variant of the stent design with single free loops, uniformly distributed along the stent length or in the places where the increased flexibility is most desirable, the technological process includes the set of prototype stent steps exclusively.
Implantation of the stent in a blood vessel, preferably controlled by use of X-ray, is executed in such a way as to provide the location of the relatively rigid band of the consecutively united pockets on a vessel wall adjoining the cardiac muscle. As a result, the stent, with one relatively rigid band of the consecutively united pockets, preserves all the positive properties of the prototype stent while possessing an increased flexibility, making it possible to carry out successively the complex intravascular angioplasty and, if necessary, to install stents of enlarged length. The maximal flexible rigidity of the stent is at least half or less as the minimal rigidity of the prototype stent. Since the band with consecutively united pockets (a container for the polymer loaded thread) can bend in all the spatial directions but does not share in the stent bending, determined by its cross-sections, then the size of the axial band, and consequently of the polymer loaded thread, remains invariable. In case of the band with the consecutively united pockets adjoining the cardiac muscle, the flexibility of this band in the direction of the cardiac muscle functioning is minimal. Therefore, any pulsation of the cardiac muscle (dynamic action) is taken by the band effectively enough.
The dynamic actions from this band are transmitted to the loops, independently to some extent, and the loops, resting against the opposite vessel wall, exert a massaging action on it, without practically limiting the pulsating vessel degrees of freedom.
The stent can have a partially increased rigidity at any place of its linear length at the expense of fastening the loop short sides by the fragments of the relatively rigid band in the shape of the pockets. For example, by fastening in this way the stent end loops, the rigidity of end surface sections increases, and the possibility of their deformation from the blood flow action decreases. By fastening the joining loops of the stent, differentiated in diameter, the rigidity of the design middle part increases, thus increasing the fixation reliability of a stent part with a greater diameter, preventing its penetration into a vessel with a lesser diameter.
At the same time the relatively increased stent rigidity in a transitional section of a vessel promotes a more efficient destruction of the pathological formations in the place of implantation.
The stent sections with fragments, increasing its rigidity, resemble the prototype stent design. Since the relative rise in stent rigidity takes place on the longitudinal extent of small length fragments only, then the general design flexibility does not decrease and remains as intended.
It is possible to fragmentarily insert into the stent not only the increased rigidity but also an increased flexibility with the help of separate single free loops arranged in the stent along the stent's length. The increased stent flexibility takes place in the zones of separate single free loops location.