Self-expandable metallic stent

A self-expandable metallic stent (or SEMS) is a metallic tube, or stent, used in order to hold open a structure in the gastrointestinal tract in order to allow the passage of food, chyme, stool, or other secretions required for digestion. SEMS are inserted by endoscopy, wherein a fibre optic camera is inserted either through the mouth or retrograde through the colon, in order to reach an area of narrowing. As such, it is termed an endoprosthesis. SEMS can also be inserted using fluoroscopy where an X-ray image is used to guide insertion, or used as an adjunct to endoscopy.

The vast majority of SEMS are used to alleviate symptoms caused by cancers of the gastrointestinal tract that obstruct the interior of the tube-like (or luminal) structures of the bowel — namely the esophagus, duodenum, common bile duct and colon. SEMS are designed to be permanent and, as a result, are often used when the cancer is at an advanced stage and cannot be removed by surgery.

Self-expandable metallic stents are cylindrical in shape, and are devised in a number of diameters and lengths to suit the application in question. They typically consist of cross-hatched, braided or interconnecting rows of metal that are assembled into a tube-like structure. SEMS, when unexpanded, are small enough to fit through the channel of an endoscope, which is meant for delivery of devices for therapeutic endoscopy. They expand through a deployment device which is placed at the end of the SEMS, and are held in place against the wall of the luminal surface by friction.

SEMS may be coated with chemicals designed to prevent tumour ingrowth; these are termed "covered" stents. Nitinol (a shape memory nickel-titanium alloy), polyurethane, and polyethylene are typically used as coatings for SEMS. Covered stents carry the advantage of preventing tumours from growing into the stent, although they run the risk of increased migration after deployment.

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