Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Having marker
Reexamination Certificate
2000-08-29
2002-05-07
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Having marker
C600S003000
Reexamination Certificate
active
06383217
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to implants for prevention and treatment of vascular restrictions. More specifically, the present invention relates to stents made of metal or alloy to which is admixed at least one naturally occurring or enriched stable radioactivatable isotope that when activated safely and uniformly emits desired dosages of radiation for prevention and treatment of various types of vascular restrictions when implanted in blood vessels.
2. Description of the Prior Art
According to the American Heart Association, in 1995, almost one million Americans lost their lives because of heart disease, more than any other single illness, and approximately seventeen million persons are at risk of a first heart attack. An estimated one hundred million people in the United States, Japan and the five leading countries of the European Union have clinical or subclinical atherosclerosis. New estimates suggest that carotid artery disease, another vascular disorder, is responsible for half of the 700,000 strokes that occur in the United States annually. Each year, more than 150,000 people die from stroke. Peripheral artery disease restricts blood flow to the leg, the kidney (frequently requiring dialysis), and to other organs. These devastating ailments are the leading cause of disability and the cost to the United States health care system was $274.2 billion last year, a figure that is expected to increase dramatically as the baby boom generation ages.
A common treatment for vascular restrictions is angioplasty, also known as percutaneous transluminal angioplasty, which involves threading a flexible shaft into an artery and briefly inflating a balloon catheter that stretches the blood vessel open and squeezes away the obstruction. It is a non-surgical procedure and therefore is much less expensive and much safer than the typical alternative-bypass surgery. In 1995, the average cost of this procedure was $20,370. Approximately 6.9 million diagnostic and interventional catheterizations of this type are performed annually.
Although angioplasty is used to restore blood flow, the technique is only a partial solution to find a cure for vascular disease and in particular, to treat restenosis, a fairly common complication following angioplasty. Restenosis is a reclosing of arteries as a result of injury to the arterial wall during the angioplasty procedure, and can necessitate repeat angioplasty procedures or bypass surgery, with substantially higher cost and risk to the patient. The condition affects up to forty percent of patients undergoing an angioplasty procedure, usually within six months. Long term restenosis may cause symptoms such as chest pain and fatigue, and an increased danger of heart attack, stroke or kidney failure. Patients also continue to be at risk of thrombogenesis (blood clotting), and atherosclerosis (hardening of the arteries). It can lead to recoil which is the mechanical collapse of dilated vessel segment in response to vascular injury. Plaque is also generated within blood vessels after angioplasty, which restricts blood flow.
Restenosis is believed to be caused by smooth muscle cell proliferation or neointimal proliferation in the vessel wall, a repair response of the body prompted by the arterial trauma resulting from angioplasty. This hyperplasia of smooth muscle cells narrows the lumen opened by angioplasty. Restenosis is also believed to be caused by elastic recoil, which is contraction of the vessel wall to its previous position after having been stretched by balloon angioplasty, and by vessel wall remodeling, which is the formation of scar tissue where balloon angioplasty caused trauma. Thus, methods for treating restenosis have focused on inhibiting such remodeling and hyperplasia of smooth muscle cells and on implants to prevent recoil closure of arteries.
Methods for inhibiting hyperplasia of smooth muscle cells have employed intravascular radiotherapy (“IRT”). Radiation is commonly used to treat catastrophic diseases such as cancer because of its effectiveness in reducing the hyperproliferation of cancerous cells. Localized radiation inhibits cellular proliferation, including smooth muscle proliferation, and has been shown to inhibit the typical wound healing process. It is believed that radiation breaks down genetic material in the vascular endothelium causing cell death, known as apoptosis, thus preventing cellular division. There is a dose dependent hyperplastic response and a marked reduction in smooth muscle cell build-up. Which component of the arterial wall serves as the target tissue for radiation still needs to be determined. What has been determined is that intraarterial radiation effectively shuts down the neointimal proliferative response process.
In early intravascular clinical trials for the prevention and treatment of restenosis, high-dose rate ionizing radiation delivery systems containing wire or seeds of radioisotopes such as iridium-192, a highly penetrating gamma ray emitter, and long-lived strontium-90 have demonstrated efficacy. However, iridium-192, and the devices utilized for endovascular afterloading-irradiation treatment with iridium-192, have shortcomings. As noted above, iridium-192 is principally a gamma ray emitter, and given the high dose utilized, the gamma ray emissions can travel a considerable distance in the surgical suite, and thus can irradiate the patient's healthy tissue and cells en route to the target site at the distal end of an afterloading probe. Iridium-192 based therapies can have exposure times ranging from 200 seconds to twenty minutes. Should the distal end of the delivery catheter containing the iridium-192 wire or seed be blocked or delayed in the tortuous journey to the target site, endothelial membrane could be overexposed to the radiation source, resulting in weakening of healthy tissue as well as the stenosed artery wall, resulting in cellular damage. Physicians and technicians may also receive an excessive radiation dose during a procedure involving the use of iridium-192. Also, there are disposal problems given the 73.83 day half-life of iridium-192.
Furthermore, the vascular surgeon cannot adjust the actual dose other than by extending dwell time in the artery. Dosimetry must be carefully calculated taking into account decay time and other factors. The approach of using a short-term, high-dose application can present other potentially acute problems, including maintenance of sterility. Given the high doses of iridium-192 utilized, even with lead shielding, the radioactive emissions travel a considerable distance in a surgical suite.
Similarly, strontium-90 has a 29.1 year long half-life, which also presents a number of other problems. This radioisotope is a beta emitter and only travels a short distance. However, strontium-90 also has inherent risks of patient contamination and device sterility problems arising from multiple patient use. Radioactive waste disposal difficulties arise because of its long half-life and unrestricted disposal requires 10 half lives, or 291 years. Similarly, should the distal end of the delivery catheter containing the strontium-90 source be stuck in the tortuous route to the target site, healthy tissue and cells will be irradiated. Overexposure to radiation from the high-dose strontium-90 applicator irradiation may weaken healthy or stenosed artery walls. The application of short-term high-dose irradiation requires the presence of a radiation therapist or oncologist as well as a cardiovascular surgeon or interventional radiologist, resulting in increased procedural cost and time.
Attempts have been made to deliver radiation doses by coating an implant with other pure beta emitting radioisotopes such as phosphorus-32 and yttrium-90. A fundamental problem with pure beta emitting radioactive coatings such as phosphorus-32 or activated wire made from monoisotopic yttrium is that the radioactivity cannot be precisely calibrated in the microcurie range in a typical catheterization laboratory setting
Frayne Clifford G.
Jackson Suzette J.
Marn Louis E.
Willse David H.
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