biology

Self-expandable metallic stent

Self-expandable metallic stent is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Self-expandable metallic stent rather than just read about it. In short: A self-expandable metallic stent (or SEMS) is a metallic tube, or stent that holds open a structure in the gastrointestinal tract to allow the passage of food, chyme, stool, or other secretions related to digestion. Surgeons insert SEMS by endoscopy, inserting a fibre optic camera—either through the mouth or colon—to reach an area of narrowing.

Key takeaways

Self-expandable metallic stent belongs to biology; place it in that map before memorising details.
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Connect Self-expandable metallic stent to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Self-expandable metallic stent from memory before moving on to harder problems.

Reference excerpt

A self-expandable metallic stent (or SEMS) is a metallic tube, or stent that holds open a structure in the gastrointestinal tract to allow the passage of food, chyme, stool, or other secretions related to digestion. Surgeons insert SEMS by endoscopy, inserting a fibre optic camera—either through the mouth or colon—to reach an area of narrowing. As such, it is termed an endoprosthesis. SEMS can also be inserted using fluoroscopy 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 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.

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 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. A plastic self-expanding stent (Polyflex, Boston Scientific) 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 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, pancreas, bile ducts and colon that are not amenable to surgical therapy. SEMS are used to treat additional complications of cancer, such as tracheoesophageal fistulas from esophageal cancer, and gastric outlet obstruction from stomach, 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 strictures 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 and peripheral vascular system. In the past they have been used for saphenous vein graft and native coronary artery percutaneous coronary interventions.

Deployment

Self-expandable metallic stents are typically inserted at the time of endoscopy, usually with assistance with fluoroscopy or x-ray 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 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. They are inserted at the time of ERCP, a procedure that uses endoscopy and fluoroscopy to access the common bile duct. The bile duct is cannulated with the assistance of a guidewire and the sphincter of 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 stents. 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 medications, which may lead to oversedation, aspiration, or drug reaction. SEMS also expand and can lead to 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 of the tumour tissue in the stent, or through the use of photodynamic therapy. 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

Illustrations

Worked examples

Example 1 — a first encounter with Self-expandable metallic stent

Start with the simplest possible case. Write down what Self-expandable metallic stent claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Self-expandable metallic stent before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Self-expandable metallic stent ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Self-expandable metallic stent

In research: Self-expandable metallic stent appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Self-expandable metallic stent in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Self-expandable metallic stent is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gastroenterology, Implants (medicine), so understanding it makes those chapters shorter.
In everyday life: Look for Self-expandable metallic stent outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Self-expandable metallic stent in 20 minutes

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Frequently asked questions

What is Self-expandable metallic stent in simple terms?

Why does Self-expandable metallic stent matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Self-expandable metallic stent?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Self-expandable metallic stent.