Radiopaque stent

Details Radiopaque stent Radiopaque stent

2000-06-22

2003-11-25

Willse, David H. (Department: 3738)

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

Arterial prosthesis

Having marker

C623S001150, C623S001160

Reexamination Certificate

active

06652579

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to endoprosthesis devices, which are commonly referred to as stents, and more particularly to radiopaque stents.
Stents are generally thin walled tubular-shaped devices composed of complex patterns of interconnecting struts which function to hold open a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for supporting a dissected arterial lining or intimal flap that can occlude a vessel lumen. At present, there are numerous commercial stents being marketed throughout the world. These devices are typically implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed. In the case of self-expanding stents, deployment is achieved by the removal of a restraint, such as the retraction of a delivery sheath. In the case of balloon expandable stents, deployment is achieved by inflation of a dilation balloon about which the stent is carried on a stent-delivery catheter.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First and foremost, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. In addition to having adequate radial strength or more accurately, hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
In addition to meeting the mechanical requirements described above, it is preferable that a stent also be fluoroscopically visible. Fluoroscopy has typically been relied upon to facilitate the precise placement of a stent as well as to verify the position of a stent within a patient throughout its service life. The use of radiopaque materials in the construction of a stent allows for its direct visualization. The most common materials used to fabricate stents are stainless steel and nickel-titanium alloys. These materials are known to be bio-compatible and satisfy the mechanical requirements for stents. However, neither of the materials is particularly radiopaque. This factor, in combination with the minimal surface area and thin wall thickness of typical stent patterns, renders stents produced from these materials insufficiently radiopaque to be adequately visualized with fluoroscopy procedures. Alternative structural materials are either excessively radiopaque or have not been proven to be sufficiently biocompatible for long term use in a vascular setting. For these reasons, simply constructing a radiopaque stent wholly out of a single material has heretofore not been considered a viable option. Thus, the art has moved in the direction of combining different materials to produce a mechanically sound, biocompatible and fluoroscopically visible stent. A number of such approaches have been developed.
One means frequently described for accomplishing flouroscopic visibility is the physical attachment of radiopaque markers to the stent. Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, such markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Depending on their specific location, the marker may either project inwardly to disrupt blood flow or outwardly to traumatize the walls of the blood vessel. Additionally, galvanic corrosion may result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker. Such corrosion could eventually cause the marker to become separated from the stent which may be problematic should the marker be swept downstream. Finally, although such markers are typically fairly small, this approach does cause the radiopaque material to come into direct contact with living tissue which may be problematic should there be any biocompatibility issues.
Stents also have been previously marked by plating selected portions thereof with radiopaque material. However, a number of disadvantages are associated with this approach as well. This again causes the radiopaque material to come into direct contact with living tissue which, depending on the total area that is plated, can amount to a sizeable exposure. Additionally, when the stent is expanded certain portions undergo substantial deformation, creating a risk that cracks may form in the plating causing portions of the plating to separate from the underlying substrate. This has the potential for creating jagged edges that may inflict physical trauma on the lumen wall tissue or cause turbulence in the blood flowing past the stent, thereby inducing thrombogenesis. Moreover, once the underlying structural material becomes exposed, interfaces between the two disparate metals become subject to galvanic corrosion. Further, should the plating pattern cover less than all of the stent's surfaces, the margins between the plated and unplated regions are also subject to galvanic corrosion.
As a further alternative, a stent structure has been described that is formed from a sandwich of structural and radiopaque materials. Three tubes of the materials are codrawn and heat treated to create a structural/radiopaque/structural composite. Struts and spines are then formed in the tube by cutting an appropriate pattern of voids into the tube as is well known in the art. While this approach does provide a stent that is radiopaque and that fulfills the necessary mechanical requirements, the thin cross section of the radiopaque material is nonetheless exposed along the edges of all cut lines. The biocompatiblity of the radiopaque material therefore remains an issue and more significantly, a sizeable area is thereby created that is subject to galvanic corrosion. Any cuts in the sandwich structure cause two disparate metal interfaces, i.e., the juncture between the outer structural layer and the central radiopaque layer as well the juncture between the central radiopaque layer and the inner structural layer, to become exposed to blood, an electrolytic solution.
As can be seen, composite stents, whether of the plated or coated type, sandwich type, or simply those equipped with markers, have several disadvantages; namely, potential flaking of the radiopaque coating or plating, galvanic corrosion, or poor biocompatibililty. Thus, a stent configuration is required that overcomes the shortcomings inherent in previously known devices. Preferably, such a stent would be biocompatible, possess the required mechanical characteristics, would be sufficiently radiopaque to be readily visible using fluoroscopy procedures, and would be formed from a single material.
SUMMARY OF THE INVENTION
The present invention provides a stent made from a single material that overcomes the shortcomings of previously known stent devices. The stent fulfills all of the mechanical and structural requirements attendant to its function as a stent. Moreover, in contrast to the prior art, at least a portion of the stent is fluoroscopically visible without the addition of an extra layer of radiopaque material.
The stent of the present invention includes generally a plurality of cylindrical rings that are interconnected by a plurality of links. The

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