A stent is a generally tubular medical implement used to address various diseases causing stenosis or occlusion of a lumen such as a blood vessel. The stent is implanted at the stenosis portion or occluded portion of a blood vessel or other biological lumen so as to dilate the stenosis portion or occluded portion and help secure the lumen.
The stent is inserted from the outside into the inside of an organism, and implanted therein. Therefore, the stent is smaller in diameter at the time of insertion for indwelling, and is enlarged in diameter through dilation (or expansion) at the desired stenosis portion or occluded portion. By virtue of a dilation holding force of the enlarged stent, the lumen at the desired portion is dilated and held in the dilated state.
Stents are classified, by function and indwelling method, into self-expandable stents and balloon-expandable stents. A balloon-expandable stent is a stent which itself does not have a dilating function or capability. The balloon-expandable stent is inserted into a desired portion and is then dilated (plastically deformed) to be fixed in close contact with the inside surface of the desired lumen under a dilating force of a balloon located inside the stent. In using this type of stent, the above-mentioned stent dilating operation is required.
A commonly practiced procedure of implanting a coronary artery stent is as follows. To carry out a stent embedding procedure, it is necessary to introduce various catheters into a blood vessel. First, the blood vessel is secured by indwelling a sheath in a blood vessel (for example, mainly the femoral artery, the elbow artery or the radial artery) having an inside diameter permitting insertion of the catheters. In general, the sheath has a thin-walled plastic tube body, and a seal valve provided at the proximal end of the tube body so as to prevent leakage of blood and to permit the catheters to be inserted and removed in a substantially liquid-tight condition.
Then, a catheter called a guiding catheter is inserted in the sheath, and its tip is fixed to the desired coronary artery orifice (the right or left coronary artery orifice). As a result, a passage between the exterior and the coronary artery is secured or achieved.
Thereafter, a guide wire having a small diameter of about 0.014 inch is inserted, and is passed through a stenosis portion (desired portion of treatment) of the coronary artery. Then, a dilating catheter fitted with a balloon at its tip is inserted along the guide wire, the balloon is dilated at the stenosis portion to dilate the stenosis portion, and the dilating catheter is removed. The balloon dilation of the stenosis portion is thus completed. Thereafter, a contrast medium is injected through the guiding catheter, and the dilated condition of the stenosis portion is checked. If the stenosis portion is found to be sufficiently dilated and no trouble is found, the procedure is finished.
However, if the dilation is deemed to be unsatisfactory or an inner membrane is found to be abraded, a stent indwelling procedure is carried out successively. In recent years, the implantation of stents has become more frequent. A stent embedding process is conducted by a method involving a biological organ dilating implement equipped with a balloon-expandable stent that is inserted along the guide wire to the stenosis portion treated as above. The balloon is dilated to cause the stent to make close contact with and expand the inside wall of the stenosis portion. This thus causes the stent to be implanted in the stenosis portion. Then, the balloon is shrunk (contracted), and the biological organ dilating implement is removed.
This stent implantation procedure is quite widely used as a generalized procedure, and many kinds of stents have been used clinically. The stent, in general, is fabricated by hollowing a single metallic tube into one of various shapes.
The basic functions required of a stent are the delivery performance and the restenosis preventive function. The delivery performance refers to the ability of the stent to be relatively easily delivered to the desired blood vessel portion. Factors related to the delivery performance include the diameter of the stent in the state of being mounted on the balloon of the biological organ dilating implement, the degree of close contact between the balloon and the stent in the mounted state, and the flexibility of the stent part in the mounted state. When the blood vessel is sharply bent or calcified, a strut (filamentous portion) constituting a part of the stent may become caught by the blood vessel portion, thereby hindering the progress or implantation of the stent.
In addition, it is known that restenosis is generated to a certain extent at the portion where the stent is implanted. The restenosis preventive function thus refers to the ability of the stent to prevent or suppress the generation of such a restenosis. Since the possible occurrence of restenosis is checked fluoroscopically, the implanted stent is required to have good radiopacity. In addition, where the stent has high radiopacity, it is easy to confirm the indwelling conditions of the stent at the stenosis portion, for example, the indwelling position of the stent and the dilated condition of the stent.
In addition, once a stent is implanted in an organism, it is difficult to remove other than by a surgical operation and so the stent should also possess high biocompatibility.
Several proposals have been made for the blank material for stents. For example, JP-A-2003-527931 (corresponding to International Application Publication No. WO01/72349) discloses stents formed from a blank material containing cobalt, chromium and other metals and having a wall thickness of not less than 25 μm. However, the stents disclosed in this document have the drawback of being low in radiopacity.
Japanese Patent Laid-open No. 2003-260142 (corresponding to U.S. Patent Application Publication No. 2005/125052) discloses stents formed of a single material having a radiopacity higher than stainless steels, such as gold and platinum, and having a wall thickness of 50 to 100 μm. Since these stent are formed of a single metal, however, they are low in mechanical strength and possess a relatively large wall thickness.
JP-A-2004-505651 (corresponding to International Application Publication No. WO00/61203) discloses noble metal alloy stents which, among various mechanical properties, are restricted with respect to their yielding point and elongation. However, the alloys used in this patent contain large amounts of palladium, which is pointed out to have the problem of acting as an allergen, so that the alloys have a high possibility of causing allergy. The noble metal alloys used in the dental field will sometimes cause a metallic allergy. As a therapeutic method to cope with the allergy, simple removal of the relevant metal (device) may be satisfactory in the dental field. However, stents cannot be removed other than by surgery. Therefore, it is dangerous to apply the noble metal alloys used in the dental field to biological organ dilating stents.
Japanese Patent No. 2746755 (corresponding to International Application Publication No. WO94/16646) and Japanese Patent No. 3493195 (corresponding to International Application Publication No. WO93/19804) disclose clad wires which are fabricated by use of two metals or alloys for the purpose of enhancing radiopacity.