Expandable stents are widely used in gastroenterology. The basic principle of all of these devices is that they can be constrained onto a delivery system of small diameter and then deployed in an area of stenosis without the risk of complications due to excessive dilation. Understanding tissue responses to stents is important both for the design of new stents and for clinicians to balance the benefits and risks of covered and uncovered stents. With biodegradable stents and removable stents, understanding tissue responses provides the basis for timing of removal and assessing treatment response.
Expandable metal stents revolutionized the treatment of malignant stenoses in the gastrointestinal tract. These devices were originally designed for intravascular application, and the earliest studies used vascular stents to demonstrate feasibility of use in the gastrointestinal tract. In the biliary tree, these devices allowed placement of stents that open to a much wider diameter than conventional plastic stents, and the flexible delivery systems and the pliability of the stent materials allowed them to be placed in areas where rigid plastic stents were more difficult to use. In the esophagus, placement of rigid plastic stents often required general anesthesia and was associated with high complication and mortality rates. Expandable stents allow outpatient placement with low complication rates. The flexibility of these stents and the precise positioning allow them to be placed at locations that were traditionally difficult to stent, such as stenoses close to the upper esophageal sphincter. These devices also opened the era of stenting in nontraditional areas, such as the colon and the duodenum, where rigid stents cannot be used. An understanding of the tissue responses to metal stents led to the development of partially covered stents that would resist tumor ingrowth into the stent lumen and development of fully covered stents that could be removed after a period of time because they do not integrate into the wall of the organ. Finally, biodegradable stents are now available that react with tissue in a different way and disintegrate after a period of time. Each of the stent designs has specific applications, risks, and benefits that must be matched to the needs of patients.
Principles of expandable stent placement
The general principle of all expandable stents is to provide a wide lumen without the complications of passing a large introducer system through a stenosis. This prevents complications, such as perforation. Therefore, expandable stents are either provided preloaded on a delivery system with the stent constrained or must be constrained onto a delivery system before being deployed. Stents are most often placed by a combination of endoscopy and fluoroscopy, but in some tumor locations and clinical situations, stents may be placed by fluoroscopy or endoscopy alone.
Stricture Dilation Before Stent Placement
Stricture dilation is occasionally necessary in cases of extremely tight stenosis but it is important to avoid overdilating the stenosis because this can increase the risk of stent migration.
Selecting a Stent of Sufficient Length
In a malignant stenosis, the stent should be long enough to straddle the tumor and long enough to prevent the possibility of tumor overgrowth along the length of the stent. In special locations, precise stent lengths are critical for success. At the gastroesophageal junction, excessive stent length in the body of the stomach increases the risk of migration and impaction against the greater curvature as peristalsis works on the trailing edge of the stent.
Radial Force of the Stent
Stents vary in the degree of radial force exerted. This may be relevant in some circumstances. In patients undergoing chemoradiation for esophageal cancer, stents with high radial force can cause necrosis of the wall of the esophagus and result in serious complications. With high cervical strictures, stents with high radial force can cause stridor due to tracheal compression if a stent has not been previously placed in the trachea.
Stent Shape and Architecture
Flaring the proximal and distal ends of a stent decreases the risk of migration but some stent designs have had widely flared proximal ends that increase tissue hyperplasia. Sharp wires at the ends of the stent can cause tissue reactions by embedding themselves into the mucosa and have generally been replaced by smoother profiles at the ends of the stent.
Uncovered, Covered, and Partially Covered Stents
The choice of stent design depends on the location of the stent, the anticipated life expectancy of the patient, potential treatments, and the biology of the tumor. Only general principles can be offered due to the many variables involved in decision making. In malignant stenoses, tumor ingrowth needs to be balanced against the risk of migration caused by treatment-induced necrosis of the tumor. Tissue responses to the stent are discussed later and additional details are provided about the principles of stent selection.
Tissue responses to stents
Animal Studies of Stent Placement
Animal studies have shown variable tissue responses. These studies provided the initial insight that the stent wires can deeply imbed and move from the lumen into the wall of the organ and beyond. In dogs, an uncovered Wallstent (5-mm diameter) was shown to result in extensive fibrosis in the wall of the bile duct. There was marked hyperplasia of the normal mucosa at the proximal and distal ends of the stent. The stent migrated deep into the muscularis and was close to the serosal surface. In the pig, an 18-mm nitinol stent caused necrosis of the mucosa and submucosa with the stent eroding into the muscular layer of the esophagus.
Human Tissue Responses to Stent Placement
Human tissue responses are difficult to study because tissue samples are hard to obtain after stent placement. Furthermore, histologic examination of the tissues with an implanted stent requires special microtomes that can cut through metal without damaging the delicate tissues that surround the stent. Limited data are, therefore, available from human tissue responses to expandable stents.
Tissue responses to stents
Animal Studies of Stent Placement
Animal studies have shown variable tissue responses. These studies provided the initial insight that the stent wires can deeply imbed and move from the lumen into the wall of the organ and beyond. In dogs, an uncovered Wallstent (5-mm diameter) was shown to result in extensive fibrosis in the wall of the bile duct. There was marked hyperplasia of the normal mucosa at the proximal and distal ends of the stent. The stent migrated deep into the muscularis and was close to the serosal surface. In the pig, an 18-mm nitinol stent caused necrosis of the mucosa and submucosa with the stent eroding into the muscular layer of the esophagus.
Human Tissue Responses to Stent Placement
Human tissue responses are difficult to study because tissue samples are hard to obtain after stent placement. Furthermore, histologic examination of the tissues with an implanted stent requires special microtomes that can cut through metal without damaging the delicate tissues that surround the stent. Limited data are, therefore, available from human tissue responses to expandable stents.
Related posts:
Pancreaticobiliary and Gastrointestinal Stents
Expandable Stents for Benign Esophageal Disease
Expandable Stents for Malignant Esophageal Disease
Expandable Metal Stents for Benign Biliary Disease
Self-Expandable Metal Stents for Malignant Distal Biliary Strictures
Expandable Metal Stents for Malignant Hilar Biliary Obstruction
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