Stent plastic

The main advantage of metaUic stents is that they are deployed in a contracted state through a small-cahber tract through the Kver and, once placed, they achieve a much wider diameter than plastic endoprostheses. The larger cahber results in lower rates of migration and bile encrustation. The main drawbacks of metalhc stents are their relatively high cost compared with plastic stents and the inabihty to remove them once deployed. 

Table 1.1. Retrospective outcome analyses of plastic stents in malignant biliary obstruction...

The mechanism of late occlusion of metallic endoprostheses is quite different to that of plastic stents. Plastic endoprostheses occlude because bacterial contamination causes bile decomposition, subsequent deposition, and encrustation. Occluded plastic stents are usually removed and replaced endoscopically (Tibble and... 

Despite intensive efforts to improve plastic endoscopic stents, few modifications have proved advantageous. Plastic stents are simple single lumen devices designed to function as an aqueduct for bile. Conventional plastic Cotton-Leung biliary stents, composed of polyethylene, have single flaps at both ends of the stent The stents are available in varied lengths from 3 to 20 cm ranging in diameters from 7 to 11 F. (Fig. 1.8)... 

The cholangiographic findings during ERCP should be used to aid stent selection. The length of the chosen stent should be based on measurements obtained with a ruled catheter or wire that can be pulled through the stricture (Seibert 1997). Ideally, plastic stents should completely span the stricture and protrude 1-2 cm from the ampulla. Performance of a biliary sphincterotomy prior to stent placement may reduce the incidence of acute pancreatitis in patients who have a proximal stricture of the bile duct (Tamsky et al. 1997). Other reports suggest that the routine use of biliary sphincterotomy increases the duration of the procedure as well as the complication rate (Margulies et al. 1999). 

Serious compHcations of plastic stent placement are rare and most are related to the ERCP. Acute pancreatitis may occur in up to 5% of patients. Stenting may result in occlusion of the pancreatic sphincter due to edema or mechanical obstruction. Long stents, which extend into the intrahepatic ducts, are more likely to cause acute pancreatitis than short stents (Tamsky... 

Fig. 1.11. Rat-toothed forceps are used to retrieve a plastic stent that has migrated from its original position within the bile duct...

Randomized prospective studies have shown that laser therapy produces comparable or better results than plastic stents (Barr et al. 1990 Carter et al. 1992 Loizou et al. 1991). However, the two techniques are probably complementary and are best suited to different tumors. Laser is more effective for fleshy intraluminal tumors, whilst plastic tubes are better in sclerotic mahgnant stenoses. Overall, laser treatment is the more expensive because it has to be repeated. 

The results from three randomized prospective trials, which compared metallic stents with plastic stents in the palliation of esophageal cancer, have shown that metallic stents provide better palliation than plastic stents (Knyrim et al. 1993 De Palma et al. 1996 Sanyika et al. 1999), although the results did not reach statistical significance in any of the trials. 

In summary, the comparative studies performed to date suggest that metallic stents provide better pal-Kation than radiotherapy, chemotherapy, and laser therapy. Two out of the three trials comparing metallic stents with plastic stents showed similar improvement in dysphagia in the two groups. However, both Knyrim s and De Palma s studies are relatively out of date, in that they used previous versions of esophageal metallic stents. Whether results of a trial comparing currently available esophageal metallic stents with plastic stents would produce different results in favor of metallic stents remains to be seen. Finally, all trials have shown that the complication rates of metallic stents are lower than those of plastic stents. 

With the introduction of plastic-covered metallic stents for the palliation of malignant dysphagia, interventionists found that these devices were also very effective at closing fistulas and perforations. Metallic stents are better that plastic stents in this regard because they expand to the diameter of the esophagus and the covering material provides an effective seal over the defect (Watkinson et al. 1995b Do et al. 1993). 

Recent developments of plastic stents have aimed to improve the resistance of plastic stents to external compression forces. Therefore, metal has been incorporated into the plastic material of the stent. One of the latest developments is the dynamic bifurcation stent made of silicone (Freitag et al. 1994). This Dynamic stent (Riisch, Kernen, Germany) is reinforced with horseshoe-shaped steel struts. A posteriorly located flexible membrane allows dynamic compression of the stent during coughing, whereas the steel struts prevent airway compression from external forces. Theoretically, this stent mimics the mechanical dynamics of the normal trachea. The distal end is a Y shape which rides on the carina to prevent distal migration. 

Contraindications to the placement of central airway stents are curable malignant tumors. Due to the difficulty in removing metallic stents, these should not be placed in patients with a curable disease. Metallic stents that have been incorporated into the mucosa are extremely difficult to retrieve at endoscopy and surgery. However, in emergency situations with hyperacute airway compression and the need for rapid restoration of adequate pulmonary ventilation, temporary placement of a (preferably) plastic stent may be indicated. 

Fig. 30.1a-c. Pancreatic adenocarcinoma superior tumor delineation in the pancreatic parenchymal phase, a MRCP reveals stenosis of the distal CBD (arrow) with massive biliary dilatation, suggestive of tumor, b Contrast-enhanced in the CT in the pancreatic phase revels a low-density mass in the uncinate process (arrows), acljacent to the biliary plastic stent in situ, indicative of pancreatic cancer. A hypodense mass is a direct sign of tumor at CT. c In the venous-phase CT image, the lesion is not discernible... 

If the plastic stent is draining, the best course of action is to leave it alone and stent the duodenum. Depending on the level of duodenal obstruction, endoscopic access may be regained through the duodenal stent, if this can be placed with the distal end short of the papilla. Otherwise one has to accept that a transhepatic approach will be necessary for any future biliary intervention. 

If the duodenal stent is placed proximal to the plastic biliary stent, it may be possible to displace this into the duodenum percutaneously, should it occlude (Fotheringham et al. 2002). This is difficult, but gives better results than simply placing a metal stent alongside, as the plastic stent reduces expansion of the metal stent. 

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