[Coronary endoprostheses (stents)]

Even before the introduction of transluminal balloon dilatation of coronary arteries, Dotter had conceived intravascular endoprostheses and, in 1969, published a report of his observations after implantation of a spiral-shaped support of surgical steel and nickel-titanium ligation in peripheral arteries of the dog. Interest in intravascular supports developed especially in consideration of frequent unsatisfactory long-term results after coronary angioplasty. Coronary artery occlusion is observed in about 5% of balloon dilatations due to dissection, thrombosis, spasm or a combination of these phenomena. The rate of recurrent stenosis after uncomplicated balloon dilatation is about 30%, that of recanalization of chronic occlusion or dilatation of stenosed bypass grafts about 50%. Stent technology: In order to provide a meaningful solution to unresolved problems of balloon dilatation, the ideal endoprosthesis must be rapidly and reliably implantable as well as sufficiently flexible to conform to vascular curvatures. It should have a low profile, must be biocompatible and not thrombogenic; it should be rapidly endothelialized with no excessive proliferatory stimulus. Rigid stents are limited in length; elastic stents display a better transition to normal coronary arteries. Negatively-charged metallic surfaces appear less thrombogenic than those positively charged, the former, however, are usually not free of corrosion. The generally positive charge of stainless steel, the most commonly used material, is tempered with a special surface treatment. Currently, materials under investigation are nickel-titanium, tantalum and biodegradable plastic. Of importance for endothelialization are the wall thickness of the stent and the relationship of material to pore size. Filaments of stainless steel measuring 0.09 mm in diameter are completely covered by neoendothelium within three weeks. In laboratory animals, within minutes after implantation, all three stents currently in clinical use are covered with a thin layer of thrombocytes and fibrin; after one day, endothelial-like and pseudoendothelial cells develop along the metal structures. After one week, there is a flow-oriented behaviour of the surface cell formation embedding the stent between neoendothelial and lamina elastica interna, a process which is complete in three months. The thickness of the neointimal proliferation may be dependent on the stent geometry and may vary between 0.09 and 0.4 mm.(ABSTRACT TRUNCATED AT 400 WORDS)