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What is a coronary stent? During the treatment of coronary heart disease the arteries must be kept open for the proper functioning of the heart. Coronary stents find their application here as tubes placed in coronary arteries to supply blood to heart. Coronary heart disease is the narrowing(stenosis) of coronary arteries which is caused due to accumulation of atherosclerotic plaque in the artery.As a result of this oxygen supply to the heart is affected.Arteries with different diameters may be affected due to CHD and the narrowing may be total or partial.Exertion will be restricted or prevented as a result of severe chest pain (angina) in coronary artery stenosis.myocardial infarction or even death may occur if there's a critical reduction in supply of blood to heart. In procedures involving percutaneous revascularization the implantation of coronary stents is very important. After the treatment of coronary artery the vessel must be kept open and coronary stent is an artificial support that serves this purpose. The size of the stent varies with the size of the artery and the material used to construct these stents in most cases is stainless steel. They are used to keep coronary arteries open mostly following balloon angioplasty or other interventional procedure. Stents provide a rigid support and prevent the treated vessel from reclosing and thus reduce chances for bypass surgery.
The process begins with the insertion of a catheter that has a balloon at its tip which is directed to the narrowed point in the coronary artery. The narrowed artery is viewed on a special monitor by the doctor with the injection of a contrast material through the catheter. Balloon is then expanded at the narrowed site so that the blockage is forced into artery walls and site for blood flow is increased. This is followed by the insertion of the stent into the artery along the blood tip catheter. After the correct positioning of the coronary stent in the artery the balloon is made to inflate as a result of which the steel expands along walls of artery after which the balloon is deflated and removed. The coronary stent stays in position holding coronary artery open. Drug coated stents decrease narrowing and clotting of the site and have significantly reduced restenosis rates (formation of new blocks after surgery).Cordis CYPHER became the first coated stent to be approved by FDA in April 2003(http://www.surgeryencyclopedia.com/Ce-Fi/Coronary-Stenting.html). The coated drug must have an anti-proliferative and anti-migratory effect on smooth muscle cells and it must also enhance re-endothelialization,to prevent delayed restenosis. Sirolimus and paclitaxel are some of the commonly used drugs for coating in stents. Stents are of different types according to their structure and function. A wide range of biomaterials are used for the manufacture of stents, in which stainless steel are the most common biomaterial in seven of the eight FDA approved stents. The others being in the list are Titanium, Platinum, Iridium, Tantalum, Magnesium alloy and Nitinol.
Reference: Types of Coronary Artery Stents | eHow.com http://www.ehow.com/list_6954768_types-coronary-artery-stents.html#ixzz13qC3gUyD
Catheters and balloons are the basis for implanting a coronary stent to keep an artery open. The first known used of a catheter was in ancient Egypt. The catheter uses later progressed and are now inserted into a vein to clear and open a constrained artery and implant a stent. The FDA approved the first device to clear and hold open an artery on Aug.3, 1994.Commonly Stainless steel device was used in coronary artery to keep it open.( http://www.ehow.com/facts_7171801_history-coronary-stents.html) .Later on metallic alloys were used and had a direct impact on the extent of neointima formation. Metallic stents differ with respect to mechanical features and in biocompatible properties, which are of major importance in the induction of vessel wall injury, inflammatory processes and cell properties. A few years ago drug eluting stents were introduced and is widely used nowadays.A drug eluting stent is same to metallic stent with drugs coated to it and helps fight blockage of blood vessels.Stenosis is caused by the proliferation of cells,and the drug helps to prevent the proliferation.
The development and expansion of drug eluting stents is one of the major revolutions in the field of Interventional cardiology. There has been a significant reduction of restenosis by drug eluting stents comparing with metallic stents.drug eluting refers to the capability of the stent to be treated with medications. These medications are released in times and allow frequent medication delivery to the area of artery where stent is positioned. This delivery method helps in preventing blood clotting and reclosure of blood vessel. Stents represent an ideal platform for permanent scaffolding properties and prevents shortening of blood vessels and harmful remodelling.
evolution of main types of devices currently used in clinical practice.
From the time the concept of coronary stents was introduced, there has been a significant development in the materials and properties used for the purpose. Stents mainly differ in their properties and functions. It can be of plastic or metal, coiled or slotted, bare or coated and self expandable or not. Slotted stent tubes are very strong stents and they cover a high surface area whereas coil stents are very flexible. Mesh stents has both properties of slotted stents and coil stents. Drug eluting stents are medication coated stents that help to prevent restenosis for a long time according to the drug used and the properties of drugs. Stent grafts otherwise called as covered stents are fabric covered stents that stops blood from flowing into a weakened area of the coronary artery and are mostly used by the people who are too sick to undergo a surgery. Radioactive stents are those stents capable of emitting a small amount of radiations to the blood vessel walls to reduce restenosis and is not applicable in ends or edges.
Drug eluting stents are a part of interventional cardiology from the time Andreas gruentzig introduced balloon angioplasty in 1977.The drug eluting stents are capable of releasing distinct or various bioactive agents to bloodstream and surrounding tissues. these stents have polymer coating for drug delivery. The major factors in a drug eluting stents are the type of the stent that carries the drug coating, the way in which drugs are delivered and the type of drug used in it. Some of the other considerable things are the sizing of the stent that should cover the blocked area, its diameter and its size after inflammation.
CONSIDERATIONS FOR DESIGN IMPROVEMENT
General designs of intravascular stents are now going on for permanent implant.Stents made of polyacitic and polyglycolic acid and other absorbable materials have been proposed to avoid the permanent scaffolding. Several efforts have been made to coat the metal surface of the stent to reduce its thrombogenic properties.
Stent coating can be classified as prosthetic or biological. Prosthetic coatings can be divided to active and passive ones. Passive coatings are composed of materials such as pyrolytic carbon and urethanes, which have peculiar surface characterisations that make their thrombogenecity lower than that of metals. Active coatings materials that have lower molecular weight heparin released slowly for sustained local action. Biological coatings are coating of endothelial cells over the stent surface before scaffolding.
The majority of medical device classifications used worldwide are based mostly on one among the two systems,the US FDA system or EUROPEAN MEDICAL DEVICE DIRECTIVES.
The FDA defines a medical device as an instrument,apparatus,implement,machine contrivance,implant,in vitro reagent,or other similar or related article,including a component part,or accessory which is recognised in the official National formulary,or the united states pharmacopoeia,or any supplement too them.It must be intended for use in the diagnosis of disease or other dconditions,or in the cure,mitigation,treatment,or prevention of disease,in man or other animals,or intended to affect the structure or any function of the body of man or other animals. And which does not achieve any of its primary intended purpose through chemical action within or on the body of man or other animals and which is not dependent upon being metabolized for the achievement of any of its primary intended purposes.
In Europe the MDD's define a medical device as any instrument apparatus,appliance,material or other article, whether used alone or in combination, including the software necessary for its proper application intended by the manufacturer to be used for human beings for the purpose of;diagnosis,prevention,monitoring,treatment or alleviation of disease, and diagnosis,monitoring,treatment,alleviation for one compensation for an injury or handicap.control of conception and investigation,replacement or modification of the anatomy or of a physiological process and which does not achieve its principal intended action in or on the human body by pharmacological,immunological or metabolic means,but which may be assisted in its function by such means.
The FDA system medical devices are classified into class I, II and III.Regulatory control increases from class I to class III.classification is risk based,class I includes devices with lower risks and class III includes those with greater risks.
MATERIAL CONSIDERATIONS IN CORONARY STENTS.
An ideal coronary stent should not cause any reaction in human body. The materials used in a coronary stent must be flexible, supportive,capable of expansion and also biocompatible. Generally when a foreign body enters into human body, it results in trauma,inflammation,immune response and eventual healing or scarring. The implanted material must be biocompatible, if not so can bring many complications such as cycotoxic chemical build up and chronic inflammation. Stainless steel is the most cheap and economic stent material available but is not fully compatible to human body following restenosis and thrombosis. They also pose difficulties in magnetic resonance imaging also. Alternate possibilities such as gold,titanium,cobalt-chromium alloy, tantalum alloy,nitinol and several other polymers are also been considered.
Metallic characters, size and surface properties, design and chemistry of the material, etc are all the essential factors to be considered in the development of an ideal stent. Materials to be used as stents strength of character must accomplish inflexible physical, mechanical and chemical properties.Metal of an expandable stent should have enough plasticity and elasticity to remain in the place stemmed in proper deployed size,shape and strength to prevent blood vessel shrinking and closure.
Biodegradable and bioabsorbable stents have a property of degrading quickly making them efficient for short term use. Bioeradible stents have water insoluble polymers that are converted to water soluble materials. Biodegradable materials degrades within a short time.it releases drug into the walls of blood vessels, delivers drugs efficiently and also can be mixed with antirestinotic drug, thus decreases the scare of restenosis up to an extent effectively. Biocompatibility of a material is the ability of a material to achieve appropriate response in a particular environment. The material should match and act with the surface environment to prevent early hazards such as thrombosis, inflammation and neointima formation.The challenge of hemocompatibility is another major problem in all types of stent implantations. Late problems of stents can be divided as mechanical failure due to material fatigue due to considerable stress imposed to stents by cardiac contractions and chemical failure where depolymerisation can release potential toxic substances such as nickel,degradation products or contaminants.
Corrosion properties are the vital thing in chemical characteristics of coronary stents.Configurating a surface metal oxide film retards corrosion and for metals like chromium and titanium, this process is highly effective.
Within the biodegradable DES camp, there are polymers and metallics. Polymers have been utilized for their ability to dissolve promptly.