Atherosclerosis is a disease in which vascular lesions or plaques consisting of cholesterol crystals, necrotic cells, lipid pools, excess fiber elements and calcium deposits accumulate on the interior walls of an individual's arteries. The presence of such plaques in the artery leads to thickening of the arterial wall and narrowing of the lumen. Eventually the enlargement of such plaques can lead to an occlusion of the lumen of the artery at the site of the lesion. One of the most successful procedures for treating atherosclerosis of the coronary arteries is percutaneous transluminal coronary angioplasty, hereinafter referred to as “PTC angioplasty”. PTC angioplasty consists of introducing a deflated balloon into the lumen of the atherosclerotic artery, placing the balloon adjacent the site of the plaque or atherosclerotic lesion, inflating the balloon to a pressure of approximately 6 to 20 atmospheres thereby “cracking” the plaque and increasing the cross-sectional area of the lumen of the artery.
Unfortunately, the pressure that is exerted on the plaque during PTC angioplasty also traumatizes the artery. Accordingly, in 30-40% of the cases the vessel either gradually renarrows or recloses at the locus of the original stenotic lesion. This gradual renarrowing or reclosure, which is hereinafter referred to as “chronic restenosis,” is a phenomenon that occurs almost exclusively during the first three to six months following angioplasty. Studies of the mechanism of chronic restenosis have shown that it is due in large part to a chronic constriction of the artery at the site of the barotraumatization, hereinafter referred to as the “retractile form of restenosis”, and to a lesser extent to a proliferation of smooth muscle cells, hereinafter referred to as the “proliferative form of restenosis”. Lafont et al. (1995) Restenosis After Experimental Angioplasty, Circulation Res. 76:996-1002.
A number of approaches for preventing restenosis are currently being used or tested. One approach involves the use of bioactive agents to prevent proliferation of the smooth muscle cells. To date, the use of bioactive agents alone has proven to be unsuccessful. Another approach employs a metallic stent which is deployed at the site of the stenotic lesion following PTC angioplasty. 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. Occasionally, technical difficulties, including distal migration and incomplete expansion, have also been observed with metallic stents. Moreover, there are inherent, significant risks from having a metal stent permanently implanted in the artery, including erosion of the vessel wall. In addition, the constant exposure of the stent to the blood can lead to thrombus formation within the blood vessel.
Stents made from degradable polymers have also been suggested for preventing restenosis. Although, generally an attractive alternative to metallic stents, testing in animals has shown that degradable stents still suffer from multiple complications, including relaxation-related negative recoil 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. It is also desirable to have a stent assembly comprising an inflatable balloon catheter, and a degradable polymeric stent that is stably and snugly disposed thereon. A polymer-based stent assembly that does not require a mechanical restraint to prevent the stent from expanding when stored at room temperature or when exposed to the physiological conditions found in the bloodstream of a human patient are especially desirable. Methods of preparing such stents and stent assemblies are also desirable.