Flexible expandable sheet stent and technology of its...

Details Flexible expandable sheet stent and technology of its... Flexible expandable sheet stent and technology of its...

2001-04-04

2003-10-07

Milano, Michael J. (Department: 3731)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Arterial prosthesis

Stent structure

Reexamination Certificate

active

06629993

ABSTRACT:

FIELD AND BACKGROUND 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 sub optimal 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 pl

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