A self-expandable metallic stent (or SEMS) is a metallic tube, or

stent In medicine, a stent is a metal or plastic tube inserted into the lumen of an anatomic vessel or duct to keep the passageway open, and stenting is the placement of a stent. A wide variety of stents are used for different purposes, from expandab ...

that holds open a structure in the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans a ...

to allow the passage of food,

chyme Chyme or chymus (; from Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumstool, or other secretions related to

digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

. Surgeons insert SEMS by

endoscopy An endoscopy is a procedure used in medicine to look inside the body. The endoscopy procedure uses an endoscope to examine the interior of a hollow organ or cavity of the body. Unlike many other medical imaging techniques, endoscopes are inse ...

, inserting a

fibre optic An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...

camera—either through the

mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on t ...

or colon—to reach an area of narrowing. As such, it is termed an ''endoprosthesis''. SEMS can also be inserted using

fluoroscopy Fluoroscopy () is an imaging technique that uses X-rays to obtain real-time moving images of the interior of an object. In its primary application of medical imaging, a fluoroscope () allows a physician to see the internal structure and functi ...

where the surgeon uses an X-ray image to guide insertion, or as an adjunct to endoscopy. The vast majority of SEMS are used to alleviate symptoms caused by

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal bl ...

s of the gastrointestinal tract that obstruct the interior of the tube-like (or luminal) structures of the bowel — namely the

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine ...

common bile duct The common bile duct, sometimes abbreviated as CBD, is a duct in the gastrointestinal tract of organisms that have a gallbladder. It is formed by the confluence of the common hepatic duct and cystic duct and terminates by uniting with pancr ...

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.

Composition and structure

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 An endoscope is an inspection instrument composed of image sensor, optical lens, light source and mechanical device, which is used to look deep into the body by way of openings such as the mouth or anus. A typical endoscope applies several modern t ...

, which is meant for delivery of devices for therapeutic endoscopy. They expand through a deployment device 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 Nickel titanium, also known as Nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages. Different alloys are named according to the weight percentage of nickel; e.g., Nitinol 55 and ...

shape memory In metallurgy, a shape-memory alloy (SMA) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. It may also be called memory metal, memory alloy, smart metal, smart alloy, or muscle wire. P ...

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

alloy),

polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...

, and

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including ...

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. A plastic self-expanding stent (Polyflex,

Boston Scientific Boston Scientific Corporation ("BSC"), incorporated in Delaware, is a biomedical/biotechnology engineering firm and multinational manufacturer of medical devices used in interventional medical specialties, including interventional radiology, i ...

) has also been developed for similar applications. It confers an additional advantage as it is designed to be removable, and may have a less traumatic insertion than metal stents. The Polyflex stent has shown benefit in palliation of esophageal malignancies.

Applications

The primary application of SEMS is in the

palliation Palliative care (derived from the Latin root , or 'to cloak') is an interdisciplinary medical caregiving approach aimed at optimizing quality of life and mitigating suffering among people with serious, complex, and often terminal illnesses. Wit ...

of tumours that obstruct the gastrointestinal tract. When they expand within the lumen, they are able to hold open the structure and allow passage of material, such as food, stool, or other secretions. The usual applications are for cancers of the

pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an en ...

bile ducts A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates. Bile is required for the digestion of food and is secreted by the liver into passages that carry bile toward the hepatic duct. It ...

and colon that are not amenable to surgical therapy. SEMS are used to treat additional complications of cancer, such as

tracheoesophageal fistula A tracheoesophageal fistula (TEF, or TOF; see spelling differences) is an abnormal connection (fistula) between the esophagus and the trachea. TEF is a common congenital abnormality, but when occurring late in life is usually the sequela of surgic ...

s from esophageal cancer, and

gastric outlet obstruction Gastric outlet obstruction (GOO) is a medical condition where there is an obstruction at the level of the pylorus, which is the outlet of the stomach. Individuals with gastric outlet obstruction will often have recurrent vomiting of food that has ...

from

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

, duodenal, or pancreatic cancer. SEMS and self-expanding plastic stents have also been used for non-malignant conditions that cause narrowing or leaks of the esophagus or colon. These include

peptic stricture A benign esophageal stricture, or peptic stricture, is a narrowing or tightening of the esophagus that causes swallowing difficulties. Signs and symptoms Symptoms of esophageal strictures include heartburn, bitter or acid taste in the mouth, chok ...

s caused by esophageal reflux and perforations of the esophagus. SEMS may also be placed in tandem fashion to treat ingrowth or overgrowth tumours, and fractures or migration of other SEMS. For the latter, the second SEMS in usually deployed within the lumen of the first. SEMS are also sometimes used in the

vascular system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

, usually in the

aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...

and

peripheral vascular system The peripheral vascular system is the part of the circulatory system that consists of the veins and arteries not in the chest or abdomen (i.e. in the arms, hands, legs and feet). The peripheral arteries supply oxygenated blood to the body, ...

. In the past they have been used for

saphenous vein graft The great saphenous vein (GSV, alternately "long saphenous vein"; ) is a large, subcutaneous, superficial vein of the leg. It is the longest vein in the body, running along the length of the lower limb, returning blood from the foot, leg and thi ...

and native

coronary artery The coronary arteries are the arterial blood vessels of coronary circulation, which transport oxygenated blood to the heart muscle. The heart requires a continuous supply of oxygen to function and survive, much like any other tissue or organ o ...

percutaneous coronary intervention Percutaneous coronary intervention (PCI) is a non-surgical procedure used to treat narrowing of the coronary arteries of the heart found in coronary artery disease. The process involves combining coronary angioplasty with stenting, which is th ...

Deployment

Self-expandable metallic stents are typically inserted at the time of

, usually with assistance with

x-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

images taken to guide placement. Prior to the development of SEMS small enough to pass through the channel of the endoscopy, SEMS were deployed using fluoroscopy alone. Esophageal SEMS are placed after a gastroscopy is performed to identify the area of narrowing. The area may need to be dilated to allow the gastroscope to pass. The tumour is usually better seen with the direct vision of endoscopy than on a fluoroscopic image. As a result, radio-opaque markers are usually placed on the surface of the patient to mark the area of narrowing on fluoroscopy. The SEMS is placed through the channel of the endoscope into the esophagus over a guidewire, marked on fluoroscopy, and mechanically deployed (using a device that sits outside of the endoscope) such that it expands when in position.

Hypaque Diatrizoate, also known as amidotrizoate, is a contrast agent used during X-ray imaging. This includes visualizing veins, the urinary system, spleen, and joints, as well as computer tomography (CT scan). It is given by mouth, injection into a vein ...

or other water-soluble dye may be placed through the passage to ensure patency of the stent on fluoroscopy. Enteric and colonic SEMS are inserted in a similar fashion, but in the duodenum and colon respectively. Biliary SEMS are used to palliatively treat tumours of the pancreas or bile duct that obstruct the

. They are inserted at the time of

ERCP Endoscopic retrograde cholangiopancreatography (ERCP) is a technique that combines the use of endoscopy and fluoroscopy to diagnose and treat certain problems of the biliary or pancreatic ductal systems. It is primarily performed by highly skille ...

, a procedure that uses endoscopy and fluoroscopy to access the common bile duct. The bile duct is

cannula A cannula (; Latin meaning 'little reed'; plural or ) is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of samples. In simple terms, a cannula can surround the inner or outer surfaces ...

ted with the assistance of a guidewire and the

sphincter of Oddi The sphincter of Oddi (also hepatopancreatic sphincter or Glisson's sphincter), abbreviated as SO, is a muscular valve that in some animals, including humans, controls the flow of digestive juices (bile and pancreatic juice) out of the pancreas t ...

that is located at its base is typically cut. A wire is kept in the bile duct, and the SEMS is deployed over the wire in a similar fashion as

esophageal stent An esophageal stent is a stent (tube) placed in the esophagus to keep a blocked area open so the patient can swallowing, swallow soft food and liquids. They are effective in the treatment of conditions causing intrinsic esophageal obstruction or ...

s. The location of the SEMS is confirmed by fluoroscopy.

Complications

The complications of SEMS are related to a number of factors. The first is that the endoscopic procedure used to insert a SEMS involves the use of

sedative A sedative or tranquilliser is a substance that induces sedation by reducing irritability or excitement. They are CNS depressants and interact with brain activity causing its deceleration. Various kinds of sedatives can be distinguished, but ...

medications, which may lead to oversedation, aspiration, or

drug reaction An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this term ...

. SEMS also expand and can lead to

perforation A perforation is a small hole in a thin material or web. There is usually more than one perforation in an organized fashion, where all of the holes collectively are called a ''perforation''. The process of creating perforations is called perfor ...

of the bowel or compression of structures adjacent to the bowel. Long-term complications of SEMS may be related to the underlying tumour being treated: the tumour may grow into the stent wall (tumour ingrowth) or over the end of the stent (tumour overgrowth), leading to obstruction. These complications may be limited by the use of coated stents. Tumour ingrowth or overgrowth can be additionally palliated by the placement of a second stent through the lumen of the first, through

electrocautery Cauterization (or cauterisation, or cautery) is a medical practice or technique of burning a part of a body to remove or close off a part of it. It destroys some tissue in an attempt to mitigate bleeding and damage, remove an undesired growth, ...

argon plasma coagulation Argon plasma coagulation (APC) is a medical endoscopic procedure used to control bleeding from certain lesions in the gastrointestinal tract. It is administered during esophagogastroduodenoscopy or colonoscopy. Medical use APC involves the use o ...

of the tumour tissue in the stent, or through the use of

photodynamic therapy Photodynamic therapy (PDT) is a form of phototherapy involving light and a photosensitizing chemical substance, used in conjunction with molecular oxygen to elicit cell death ( phototoxicity). PDT is popularly used in treating acne. It is used ...

. Over time, SEMS may also migrate to a different position that does not help with treatment of the obstructed area. This may be treated with placement of a second SEMS, or endoscopic attempts to reposition or remove the first. Rarely, SEMS may fracture or intussescept after endoscopic intervention.

References

{{reflist, 30em Gastroenterology Implants (medicine)