Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2001-09-07
2004-07-20
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S001150
Reexamination Certificate
active
06764507
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to tissue-supporting medical devices, and more particularly to expandable, non-removable devices that are implanted within a bodily lumen of a living animal or human to support the organ and maintain patency, and that have improved spatial distribution for delivery of a beneficial agent to the intervention site.
2. Summary of the Related Art
In the past, permanent or biodegradable devices have been developed for implantation within a body passageway to maintain patency of the passageway. These devices are typically introduced percutaneously, and transported transluminally until positioned at a desired location. These devices are then expanded either mechanically, such as by the expansion of a mandrel or balloon positioned inside the device, or expand themselves by releasing stored energy upon actuation within the body. Once expanded within the lumen, these devices, called stents, become encapsulated within the body tissue and remain a permanent implant.
Known stent designs include monofilament wire coil stents (U.S. Pat. No. 4,969,458); welded metal cages (U.S. Pat. Nos. 4,733,665 and 4,776,337); and, most prominently, thin-walled metal cylinders with axial slots formed around the circumference (U.S. Pat. Nos. 4,733,665; 4,739,762; and 4,776,337). Known construction materials for use in stents include polymers, organic fabrics and biocompatible metals, such as, stainless steel, gold, silver, tantalum, titanium, and shape memory alloys such as Nitinol.
U.S. Pat. Nos. 4,733,665; 4,739,762; and 4,776,337 disclose expandable and deformable interluminal vascular grafts in the form of thin-walled tubular members with axial slots allowing the members to be expanded radially outwardly into contact with a body passageway. After insertion, the tubular members are mechanically expanded beyond their elastic limit and thus permanently fixed within the body. U.S. Pat. No. 5,545,210 discloses a thin-walled tubular stent geometrically similar to those discussed above, but constructed of a nickel-titanium shape memory alloy (“Nitinol”), which can be permanently fixed within the body without exceeding its elastic limit. All of these stents share a critical design property: in each design, the features that undergo permanent deformation during stent expansion are prismatic, i.e., the cross sections of these features remain constant or change very gradually along their entire active length. These prismatic structures are ideally suited to providing large amounts of elastic deformation before permanent deformation commences, which in turn leads to sub-optimal device performance in important properties including stent expansion force, stent recoil, strut element stability, stent securement on delivery catheters and radiopacity.
U.S. Pat. No. 6,241,762 which is incorporated herein by reference in its entirety, discloses a non-prismatic stent design which remedies the above mentioned performance deficiencies of previous stents. In addition, preferred embodiments of this patent provide a stent with large, non-deforming strut and link elements, which can contain holes without compromising the mechanical properties of the strut or link elements, or the device as a whole. Further, these holes may serve as large, protected reservoirs for delivering various beneficial agents to the device implantation site.
Of the many problems that may be addressed through stent-based local delivery of beneficial agents, one of the most important is restenosis. Restenosis is a major complication that can arise following vascular interventions such as angioplasty and the implantation of stents. Simply defined, restenosis is a wound healing process that reduces the vessel lumen diameter by extracellular matrix deposition and vascular smooth muscle cell proliferation and which may ultimately result in renarrowing or even reocclusion of the lumen. Despite the introduction of improved surgical techniques, devices and pharmaceutical agents, the overall restenosis rate is still reported in the range of 25% to 50% within six to twelve months after an angioplasty procedure. To treat this condition, additional revascularization procedures are frequently required, thereby increasing trauma and risk to the patient.
Several techniques under development to address the problem of restenosis are irradiation of the injury site and the use of conventional stents to deliver a variety of beneficial or pharmaceutical agents to the traumatized vessel lumen. In the latter case, a conventional stent is frequently surface-coated with a beneficial agent (often a drug-impregnated polymer) and implanted at the angioplasty site. Alternatively, an external drug-impregnated polymer sheath is mounted over the stent and co-deployed in the vessel.
While acute outcomes from radiation therapies appeared promising initially, long term beneficial outcomes have been limited to restenosis occurring within a previously implanted stent, so-called ‘in-stent’ restenosis. Radiation therapies have not been effective for preventing restenosis in de novo lesions. Polymer sheaths that span stent struts have also proven problematic in human clinical trials due to the danger of blocking flow to branch arteries, incomplete apposition of stent struts to arterial walls and other problems. Unacceptably high levels of MACE (Major Adverse Cardiac Events that include death, heart attack, or the need for a repeat angioplasty or coronary artery bypass surgery) have resulted in early termination of clinical trials for sheath covered stents.
Conventional stents with surface coatings of varius beneficial agents, by contrast, have shown promising early results. U.S. Pat. No. 5,716,981, for example, discloses a stent that is surface-coated with a composition comprising a polymer carrier and paclitaxel (a well-known compound that is commonly used in the treatment of cancerous tumors). The patent offers detailed descriptions of methods for coating stent surfaces, such as spraying and dipping, as well as the desired character of the coating itself: it should “coat the stent smoothly and evenly” and “provide a uniform, predictable, prolonged release of the anti-angiogenic factor.” Surface coatings, however, can provide little actual control over the release kinetics of beneficial agents. These coatings are necessarily very thin, typically 5 to 8 microns deep. The surface area of the stent, by comparison is very large, so that the entire volume of the beneficial agent has a very short diffusion path to discharge into the surrounding tissue. The resulting cumulative drug release profile is characterized by a large initial burst, followed by a rapid approach to an asymptote, rather than the desired “uniform, prolonged release,” or linear release.
Increasing the thickness of the surface coating has the beneficial effects of improving drug release kinetics including the ability to control drug release and to allow increased drug loading. However, the increased coating thickness results in increased overall thickness of the stent wall. This is undesirable for a number of reasons, including increased trauma to the vessel lumen during implantation, reduced flow cross-section of the lumen after implantation and increased vulnerability of the coating to mechanical failure or damage during expansion and implantation. Coating thickness is one of several factors that affect the release kinetics of the beneficial agent, and limitations on thickness thereby limit the range of release rates, durations, and the like that can be achieved.
Recent research described in a paper titled “Physiological Transport Forces Govern Drug Distribution for Stent-Based Delivery” by Chao-Wei Hwang et al. has revealed an important interrelationship between the spatial and temporal drug distribution properties of drug eluting stents, and cellular drug transport mechanisms. In pursuit of enhanced mechanical performance and structural properties stent designs have evolved to more complex geometries with inherent inhomogeneity in the circumferen
Edelman Elazer R.
Eigler Neal L.
Shanley John F.
Blanco Javier G.
Conor Medsystems, Inc.
Lynch Cindy A.
Willse David H.
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