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

Stent

In the technical vocabulary of medicine, a stent is an artificial 'tube' inserted into a natural passage/conduit in the body to prevent, or counteract, a disease-induced, localized flow constriction. The term may also refer to a tube used to temporarily hold such a natural conduit open to allow...

, used in order to hold open a structure in the gastrointestinal tract

Gastrointestinal tract

The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....

 in order to allow the passage of food, chyme

Chyme

Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum.Also known as chymus, it is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum...

, stool

Feces

Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

, or other secretions required for digestion

Digestion

Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....

. SEMS are inserted by endoscopy

Endoscopy

Endoscopy means looking inside and typically refers to looking inside the body for medical reasons using an endoscope , an instrument used to examine the interior of a hollow organ or cavity of the body. Unlike most other medical imaging devices, endoscopes are inserted directly into the organ...

, wherein a fibre optic camera is inserted either through the mouth

Mouth

The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 or retrograde through the colon

Colon (anatomy)

The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...

, in order to reach an area of narrowing. As such, it is termed an endoprosthesis. SEMS can also be inserted using fluoroscopy

Fluoroscopy

Fluoroscopy is an imaging technique commonly used by physicians to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope. In its simplest form, a fluoroscope consists of an X-ray source and fluorescent screen between which a patient is placed...

 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 cancer

Cancer

Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

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

Esophagus

The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

, duodenum

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 may be used instead of duodenum...

, common bile duct

Common bile duct

The common bile duct is a tube-like anatomic structure in the human gastrointestinal tract. It is formed by the union of the common hepatic duct and the cystic duct . It is later joined by the pancreatic duct to form the ampulla of Vater...

 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, 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

Shape memory alloy

A shape-memory alloy is an alloy that "remembers" its original, cold-forged shape: returning the pre-deformed shape by heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems...

 nickel

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

-titanium

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 alloy), polyurethane

Polyurethane

A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...

, and polyethylene

Polyethylene

Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

 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

The Boston Scientific Corporation , is a worldwide developer, manufacturer and marketer of medical devices whose products are used in a range of interventional medical specialties, including interventional cardiology, peripheral interventions, neuromodulation, neurovascular intervention,...

) 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

Palliative care is a specialized area of healthcare that focuses on relieving and preventing the suffering of patients...

 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 esophagus

Esophageal cancer

Esophageal cancer is malignancy of the esophagus. There are various subtypes, primarily squamous cell cancer and adenocarcinoma . Squamous cell cancer arises from the cells that line the upper part of the esophagus...

, pancreas

Pancreatic cancer

Pancreatic cancer refers to a malignant neoplasm of the pancreas. The most common type of pancreatic cancer, accounting for 95% of these tumors is adenocarcinoma, which arises within the exocrine component of the pancreas. A minority arises from the islet cells and is classified as a...

, bile ducts

Cholangiocarcinoma

Cholangiocarcinoma is a cancer of the bile ducts which drain bile from the liver into the small intestine. Other biliary tract cancers include pancreatic cancer, gallbladder cancer, and cancer of the ampulla of Vater...

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

Tracheoesophageal fistula

A tracheoesophageal fistula is an abnormal connection between the esophagus and the trachea...

s that may result from esophageal cancer, and gastric outlet obstruction

Gastric outlet obstruction

Gastric outlet obstruction 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 accumulated in the stomach, but which cannot pass into...

 which may result from stomach

Stomach cancer

Gastric cancer, commonly referred to as stomach cancer, can develop in any part of the stomach and may spread throughout the stomach and to other organs; particularly the esophagus, lungs, lymph nodes, and the liver...

, 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

Gastroesophageal reflux disease

Gastroesophageal reflux disease , gastro-oesophageal reflux disease , gastric reflux disease, or acid reflux disease is chronic symptoms or mucosal damage caused by stomach acid coming up from the stomach into the esophagus...

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, usually in the aorta

Aorta

The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...

 and peripheral vascular system. In the past they have been used for saphenous vein graft and native coronary artery percutaneous coronary intervention

Percutaneous coronary intervention

Percutaneous coronary intervention , commonly known as coronary angioplasty or simply angioplasty, is one therapeutic procedure used to treat the stenotic coronary arteries of the heart found in coronary heart disease. These stenotic segments are due to the build up of cholesterol-laden plaques...

s.

Deployment

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

Endoscopy

Endoscopy means looking inside and typically refers to looking inside the body for medical reasons using an endoscope , an instrument used to examine the interior of a hollow organ or cavity of the body. Unlike most other medical imaging devices, endoscopes are inserted directly into the organ...

, usually with assistance with fluoroscopy

Fluoroscopy

Fluoroscopy is an imaging technique commonly used by physicians to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope. In its simplest form, a fluoroscope consists of an X-ray source and fluorescent screen between which a patient is placed...

 or x-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 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 in order 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 in order 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 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 common bile duct

Common bile duct

The common bile duct is a tube-like anatomic structure in the human gastrointestinal tract. It is formed by the union of the common hepatic duct and the cystic duct . It is later joined by the pancreatic duct to form the ampulla of Vater...

. They are inserted at the time of ERCP

Endoscopic retrograde cholangiopancreatography

Endoscopic retrograde cholangiopancreatography is a technique that combines the use of endoscopy and fluoroscopy to diagnose and treat certain problems of the biliary or pancreatic ductal systems...

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

Cannula

A cannula or canula is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of data...

ted with the assistance of a guidewire and the sphincter of Oddi

Sphincter of Oddi

The sphincter of ampulla or sphincter of Oddi is a muscular valve that controls the flow of digestive juices through the ampulla of Vater into the second part of the duodenum. It is named after Ruggero Oddi...

 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

Esophageal stent

An esophageal stent is a stent placed in the esophagus to keep a blocked area open so the patient can swallow soft food and liquids. Esophageal stents may be self-expandable metallic stents, or made of plastic, or silicone, and may be used in the treatment of esophageal cancer.A 2007 study showed...

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

Sedative

A sedative or tranquilizer is a substance that induces sedation by reducing irritability or excitement....

 medications, which may lead to oversedation, aspiration

Aspiration

Aspiration may refer to:In linguistics:*Aspirated consonant, a plosive pronounced with a strong burst of air*Debuccalization, the conversion of a consonant to [h] or [ʔ]*Voiceless glottal fricative In engine technology:...

, or drug reaction

Allergy

An Allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid...

. SEMS also expand and can lead to perforation

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 are called a perforation...

 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 or argon plasma coagulation

Argon plasma coagulation

Argon plasma coagulation or APC is a medical endoscopic procedure used primarily to control bleeding from certain lesions in the gastrointestinal tract, and also sometimes to debulk tumours in the case of patients for whom surgery is not recommended...

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

Photodynamic therapy

Photodynamic therapy is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic...

.

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

Intussusception

Intussusception may refer to:* Intussusception * Intussusception...

after endoscopic intervention.