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Methods of minimizing stent contraction following deploymentMethods of minimizing stent contraction following deployment description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20090105800, Methods of minimizing stent contraction following deployment. Brief Patent Description - Full Patent Description - Patent Application Claims
The use of stents in various surgical, interventional cardiology, and radiology procedures has quickly become accepted as experience with stent devices accumulates and as the advantages of stents become more widely recognized. Stents are often used in body lumens to maintain open passageways such as the prostatic urethra, the esophagus, the biliary tract, intestines, and various coronary arteries and veins, as well as more remote cardiovascular vessels such as the femoral artery. Stents are often used to treat atherosclerosis, a disease in which vascular lesions or plaques consisting of cholesterol crystals, necrotic cells, lipid pools, excess fiber elements and calcium deposits accumulate in the walls of an individual\'s arteries. One of the most successful procedures for treating atherosclerosis is to insert a deflated balloon within the lumen, adjacent the site of the plaque or atherosclerotic lesion. The balloon is then inflated to put pressure on and “crack” the plaque. This procedure increases the cross-sectional area of the lumen of the artery. Unfortunately, the pressure exerted also traumatizes the artery, and in 30-40% of the cases, the vessel either gradually renarrows or recloses at the locus of the original stenotic lesion. This renarrowing is known as restenosis A common approach to prevent restenosis is to deploy a metallic stent to the site of the stenotic lesion. Although metallic stents have the mechanical strength necessary to prevent the retractile form of restenosis, their presence in the artery can lead to biological problems including vasospasm, compliance mismatch, and even occlusion. Moreover, there are inherent, significant risks from having a metal stent permanently implanted in the artery, including erosion of the vessel wall. The stents may also migrate on occasion from their initial insertion location raising the potential for stent induced blockage. Metal stents, especially if migration occurs, cause irritation to the surrounding tissues in a lumen. Also, since metals are typically much harder and stiffer than the surrounding tissues in a lumen, this may result in an anatomical or physiological compliance mismatch, thereby damaging tissue or eliciting unwanted biologic responses. In addition, the constant exposure of the stent to the blood can lead to thrombus formation within the blood vessel. Stents also allow the cellular proliferation associated with the injured arterial wall to migrate through the stent mesh, where the cells continue to proliferate and eventually lead to the narrowing of the vessel. Further, metal stents typically have some degree of negative recoil. Finally, metallic stents actually prevent or inhibit the natural vascular remodeling that can occur in the organism by rigidly tethering the vessel to a fixed, maximum diameter. Because of the problems of using a metallic stent, others have recently explored use of bioabsorbable and biodegradable materials stents. The conventional bioabsorbable or bioresorbable materials from which such stents are made are selected to absorb or degrade over time. This degradation enables subsequent interventional procedures such as restenting or arterial surgery to be performed. It is also known that some bioabsorbable and biodegradable materials tend to have excellent biocompatibility characteristics, especially in comparison to most conventionally used biocompatible metals. Another advantage of bioabsorbable and biodegradable stents is that the mechanical properties can be designed to substantially eliminate or reduce the stiffness and hardness that is often associated with metal stents. This is beneficial because the metal stent stiffness and hardness can contribute to the propensity of a stent to damage a vessel or lumen. Examples of novel biodegradable stents include those found in U.S. Pat. No. 5,957,975, which is incorporated by reference in its entirety. There are, however, still problems with many biodegradable stents. For example, testing in animals has shown that biodegradable stents still suffer from multiple complications, including relaxation-related negative recoil, lack of sufficient radial strength, difficulty in deployment and distal migration of the entire stent or portions thereof and formation of an occlusive thrombus within the lumen of the stent. Accordingly, it is desirable to have a new stent that overcomes the disadvantages of the current stent designs. A polymer-based stent that exhibits little to no relaxation-related negative recoil when implanted in the blood vessel or duct of a mammalian subject is desirable. Indeed, it is preferred that the stent have a positive recoil. It is also desirable to have a polymer-based stent assembly that does not require a mechanical restraint to prevent the stent from expanding when stored at room temperature. To achieve these goals, the stent is fabricated using several heating steps. For instance, in a typical fabrication there is at least one preheating stage performed prior to the cutting procedure. Crimping step is performed as a two-step process at a temperature of 65° C. This method, however, has the drawback in that the multiple heating steps alter the stent “memory” of the ideal final diameter. The inventors have found a novel method of stent fabrication that decreases the time the stent is exposed to adverse temperature condition, thereby enabling greater memory retention of the polymers diameter. The present invention provides methods for fabricating a stent using a preheating stage. As opposed to using a two or more step heating process, the inventors have found a preferable embodiment using new fabrication methods that result in the same and/or better product quality stent using a single step process performed at a temperature of below 65° C., more preferably below 60° C., most preferably below 55° C. In certain embodiments, a temperature below about 50° C. is most preferred. Stent fabrication under such reduced temperature conditions permits technicians to avoid burning their hands but more importantly, results in a reducing the exposure of the stent to adverse temperature conditions, thereby enabling the greater retention of the polymer\'s memory. Maintaining the stent at a temperature of fabrication as described also provides a beneficial result by being below the glass transition temperature of the polymeric material. Under the previously employed procedure(s), the stent would be expanded to balloon nominal diameter of: coronary balloon from typically 2.0 mm to 4.5 mm; vascular peripheral balloon (PTA) from typically from 3 mm to more than 20 mm depending on balloon diameter. For example, for a 3 mm balloon, the stent would be expanded to 3 mm and when the balloon was removed, it would contract to a diameter of 2.7, followed by a slow expansion to the final desired diameter of 3.2 mm. Under the presently employed procedure at T=zero, the stent is expanded to 3 mm when deployed. Following initial deployment the stent does not contract, but instead remains at a diameter of 3 mm and gradually over time expands to the desired diameter of 3.2 mm. Furthermore, this deployment to a final desired diameter is essentially balloon inflation independent. “Bioresorbable polymer” as used herein refers to a polymer whose degradation by-products can be bio-assimilated or excreted via natural pathways in a human body.
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