Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2000-08-08
2002-05-14
Milano, Michael J. (Department: 3731)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
Reexamination Certificate
active
06387121
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to stents which are implantable or deployable in a vessel or duct within the body of a patient to maintain the lumen of the duct or vessel open, and more particularly to improvements in stent coatings and in methods for applying such coatings.
When inserted and deployed in a vessel, duct or tract of the body, for example a coronary artery after dilatation of the artery by balloon angioplasty, a stent acts as a prosthesis to maintain the vessel, duct or tract (generally referred to as a vessel for convenience herein) open. The stent has the form of an open-ended tubular element with openings through its sidewall to enable its expansion from a first outside diameter which is sufficiently small to allow the stent to traverse the vessel to reach a site where it is to be deployed, to a second outside diameter sufficiently large to engage the inner lining of the vessel for retention at the site.
An occluded coronary artery, for example, is typically attributable to a buildup of fatty deposits or plaque on the inner lining of the vessel. A balloon angioplasty procedure is the treatment of choice to compress the deposits against the inner lining of the vessel to open the lumen. Alternatively, removal of plaque may be achieved. by laser angioplasty, or by rotationally cutting the material into finely divided particles which are dispersed in the blood stream. The problem with angioplasty for a large segment of cardiac patients is that a new blockage appears within only weeks after the angioplasty procedure, attributable to trauma to the blood vessel wall from the angioplasty. The mechanism responsible for the new blockage is intimal hyperplasia, i.e., a rapid proliferation of smooth muscle cells in the affected region of the wall. Thus, many patients suffer restenosis, or re-occlusion of the vessel lumen.
The customary procedure is to install a stent at the trauma site at the time of or shortly after the angioplasty is performed. The stent is deployed by radial expansion under outwardly directed radial pressure exerted, for example, by active inflation of a balloon of a balloon catheter on which the stent is mounted. In some instances, passive spring characteristics of a pre-formed elastic stent serves the purpose. The stent is thus expanded to engage the inner lining or inwardly facing surface of the vessel wall with sufficient resilience to allow some contraction but also with sufficient stiffness to largely resist the natural recoil of the vessel wall.
The presence of the stent in the vessel, however, tends to promote thrombus formation as blood flows through the vessel, which results in an acute blockage. The thrombosis and clotting can be reduced or even eliminated by localized application of appropriate anti-thrombus or anti-clotting drugs in a biodegradable formulation, which act for a period of time sufficient to achieve this purpose. Some difficulty is encountered in providing a stent surface which is suitable for retention of the necessary drug(s).
At the outward facing surface of the stent in contact or engagement with the inner lining of the vessel, tissue irritation can exacerbate the same type of trauma that occurs during an angioplasty procedure, and possible restenosis. It is desirable to provide a timed release of anti-fibrotic drug(s) from the stent surface to avoid hyperplasia and recurrence of blockage at the stent site.
Another factor affecting the choice of the stent and the stent material is the possibility of allergic reaction of the patient to the stent implant. Biomaterial coatings can be helpful, but a statistically significant percentage of patients are allergic to materials of which some stents are composed, including chrome, nickel, and medical grade 316L stainless steel, which contains about 20% nickel. For such patients, the allergic reaction may be sufficient that stent implant is contraindicated. Wholly biodegradable stents of possibly sufficient radial strength are currently undergoing tests and may prove suitable in such cases.
It is essential that the implanting surgeon be able to see the progress of the stent as it is being inserted into place at the desired target site in the body, and for purposes of examination from time to time thereafter at the implant site, typically by X-ray fluoroscopy. The wall of the stent must be sufficiently thick to withstand the vessel wall recoil after deployment at the target site, but to allow the stent to be seen on the fluoroscope. Various materials, such as 316L stainless steel, possess suitable mechanical strength. Typical stent wall or wire thicknesses have ranged from 70 to 200 microns (or micrometers, &mgr;m). A 70 to 80 &mgr;m 316L steel stent offers sufficient strength to resist recoil so as to maintain a lumen diameter close to the diameter achieved at full deployment by balloon inflation. This relatively thin and tiny metal structure creates little shadow on a fluoroscopic picture, however, since the X-ray absorption of the metal is low. Increasing the wall thickness of the stent to enhance its radiopacity makes the stent less flexible, which makes it more difficult to maneuver the stent through narrow vessels. Greater wall thickness also makes it necessary to apply a larger radial force by balloon inflation during deployment of the stent, with concomitant increased risk of balloon rupture.
It follows that a suitable stent should possess at least the features of flexibility, resistance to vessel recoil, successful interventional placement, good radiopacity, sufficient thinness to minimize obstruction in the vessel being held open, and avoidance of vessel re-occlusion. Stent design plays an important role in influencing these features, together with proper selection or fabrication of the material of which the stent is composed.
Aside from vascular usage, other ducts or tracts of the human body in which a stent might be installed to maintain an open lumen include the tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract. Many of the same requirements are found in these other endoluminal usages of stents.
Despite improvements in the design and construction of coronary stents, restenosis remains a problem. One major contributing factor is the inability of the body to incorporate the implanted foreign material quickly. Basic research with cell cultures and animal experiments have demonstrated that the degree of endothelialization of the foreign body determines the amount of the restenosis. Although an assumption among industry practitioners and researchers has been that a highly polished and smooth surface is beneficial to prevent stent thrombosis and to facilitate endothelialization, experiments have indicated that this is not entirely true.
A significant reason for the lack of a high clinical success rate with electropolished stents is the fact that the smooth muscle cells which seek to envelop a foreign body, such as a stent strut into the vessel wall, require a higher degree of proliferation to cover the foreign body. The continuing flow of blood with a high pressure and high shearing stress prevents the migration of smooth muscle cells, which proliferate from the media and adventitial cells of a stented vessel such as a coronary artery. It has been shown that a slightly rough surface considerably facilitates the coverage by smooth muscle cells, leading to a functional endothelial layer even after 10 to 14 days after stent implantation. A single layer of endothelial cells has been found to seal the neointima and thereby prevent the stimulus which facilitates and enhances the proliferation of cells beyond mere coverage of the foreign body.
The thinner the stent strut, the less the lumen of the stented vessel is obstructed. Moreover, a thin stent is more easily covered by a neoendothelial build-up. Accordingly, it is desirable to make the stent wall as thin as can be practically achieved. But the fluoroscopic visibility of stainless steel in a thickness below 60 &mgr;m is very poor because
Bui Vy Q.
Inflow Dynamics Inc.
Milano Michael J.
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