Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
1999-12-30
2002-03-12
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S001340, C427S002250
Reexamination Certificate
active
06355058
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to endoprosthesis devices, most often referred to as stents, and more particularly pertains to increasing the radiopacity of such devices.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency and are especially well-suited for the treatment of atherosclerotic stenosis in blood vessels. Intercoronary stents have become a standard adjunct to percutaneous coronary angioplasty in the treatment of arterial atherosclerotic disease. Although commercial stents vary in design and materials, they share similar structural features. Most current stents in clinical use are metallic and are either self-expanding or are expanded by the force of an expandable member, such as an angioplasty dilatation balloon. These devices are typically implanted via a delivery catheter which is inserted at an easily accessible location on the patient and then advanced through the patient's 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 and into the stenosis. Once in position, the stent is deployed which, depending upon its construction, is achieved either automatically by the removal of a restraint, or actively by the inflation of a balloon about which the stent is carried on the 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 that are imposed thereon as it supports the lumen walls. In addition to having adequate radial strength or more accurately, hoop strength, the stent should nonetheless 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 of 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 upon it, including the cyclic loading induced by the pulsatile character of arterial blood flow. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
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 the stent allows for its direct visualization. Unfortunately, no single material to date has been identified that simultaneously satisfies all requirements inherent in a stent application. Those materials that do satisfy the mechanical requirements are either insufficiently or excessively radiopaque and/or have not been adequately proven to be biocompatible in a vascular setting. Thus, with current stent materials, constructing a radiopaque stent wholly out of a single material has not provided an optimal solution. A number of different approaches, however, have been employed wherein different materials are combined in an effort to render a mechanically sound and biocompatible stent to be visible by a fluoroscope system.
Several metals, such as stainless steel, nickel titanium alloys, tantalum and platinum alloys have been used to construct stents. These materials vary widely in their mechanical properties and radiopacity. All these materials can, by varying the design, be used to create the sent. However, the mechanical and radiopacity characteristics are not independent, but linked. Strength requirements dictate, for example, the strut thickness, geometry and percentage of the arterial wall which is to be covered by the stent structure. The resulting radiopacity is largely fixed and can only be adjusted with an alteration of the mechanical characteristics of the stent. For some materials, such as tantalum, the resulting stents can be too radiopaque, which results in obscured images of the anatomy in the stent lumen. This makes, for example, visualization of any possible restenosis within the stent very difficult to visualize on a fluoroscope. Other stent designs comprising of less radiopaque materials, such as stainless steel or nitinol, can have excellent mechanical functionability, but offer sub-optimal radiopacity except in cases where the stent struts can be very thick, as in an aortic stent-graft. In addition, the short-term hemocompatability and long term biocompatability of stents could be improved. In a short time-frame, the issue of stent thrombogenicity may be critical since modern coronary stents have a low, but measurable, rate of short term (one to seven days) thrombotic occlusion. This is true even if the patient is provided with systemic anticoagulation therapy. Metals such as tantalum and stainless steel, although inert, are actually coated with serum proteins and, to the extent that they are still activated, platelets after insertion into the bloodstream. In long-term implantation, stents become endothelialized. Therefore, biocompatability, particularly the foreign body response, can be of great concern. Growth of smooth muscle cells with extra cellular matrix production may lead to the restenotic closing of the arterial lumen. Platelet derived growth factor from thrombus-bound platelets can stimulate smooth cell muscle cells to proliferate. Metal ions that leech from the stent may catalytically oxidized low density lipo-proteins which exacerbate the original atherosclerotic condition.
One means frequently described for accomplishing fluoroscopic 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 somewhat traumatize the walls of the blood vessel. Additionally, galvanic corrosion that might 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 could corrode, and in the worst case, cause the marker to become separated from the stent which could be problematic should the marker be swept downstream. 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. Finally, markers also give an incomplete picture of the stent expansion and orientation. Usually there are two markers, one at each end. By making the entire stent radiopaque and visible, its degree of expansion and curvature for its full length can be assessed.
Stents also have been previously marked by coating selected portions thereof with radiopaque material. Radiopaque metals, such as gold, platinum and tantalum can be coated by sputtering, evaporation or electroplating processes. It is important that these coated layers have good adhesion and conform to the stent during deformation. The deformation is typically greatest during stent expansion. 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 coated, can amount to a sizeable exposure. Unfortunately, cracking, flaking and delamination can be a problem with this approach. When the stent is expanded and certain portions thereof are caused to undergo substantial deformation, there is a risk that cracks would form in the plating and that sections thereof would become separated from the underlying substrate. This has the potential for causing turbu
Mroz Joanna
Pacetti Stephen
Advanced Cardiovascular Systems Inc.
Fulwider Patton Lee & Utecht LLP
Jackson Suzette J.
Willse David H.
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