WO2013029572A1

WO2013029572A1 - Self-expandable double plastic stent

Info

Publication number: WO2013029572A1
Application number: PCT/CZ2011/000080
Authority: WO
Inventors: Karel Volenec; Petr Kubena
Original assignee: Ella-Cs, S.R.O.
Priority date: 2011-08-26
Filing date: 2011-08-26
Publication date: 2013-03-07
Also published as: EP2747704A1; KR20140068092A

Abstract

A self - expanding non - degradable dual plastic stent being conical in shape is provided with an anti -migration segment (4) and with extracting wires at its proximal end and with an anti-reflux valve (6) at its distal end. The stent is made of two different non - degradable fibres, namely of the PEEK-based carrying fibre (2) and the radiopaque fibre (3), and coated with a silicone foil, the carrying fibre (2) forming the principal skeleton of the stent (1) and the radiopaque fibre (3) enhancing the visibility of the same, the carrying fibre (2) being made of a plastic material selected from the group including PEEK, PEAK, PEK, PEKK, PEKKEK and the other fibre being made of PU+W. The stent (1) is further provided with a pull -tie (5). A method of manufacturing said non - degradable dual conical plastic stent is described, wherein initially the stent is knitted from the first fibre by means of a braiding machine so that it assumes a tubular/ slightly conical shape having the proximal end atraumatic and the distal end traumatic, then the skeleton of the stent is placed oh a mandrel and the other, 1 radiopaque fibre is enlaced into it, both the fibres being laid side by side, afterwards the mandrel with the stent attached to it is removed from the braiding machine and placed into a thermal device where the shape of the stent is thermally fixated by being subjected to a temperature between 120 °C and 250 °C, preferably between 140 °C and 200 °C, during a time interval from 10 to 120 minutes, preferably from 20 to 40 minutes, and finally the stent is provided with the anti -migration segment at its proximal end, said segment comprising one or more fibres forming a ripple at the proximal end of the stent, said ripple adjoining the skeleton of the stent from outside.

Description

Self-expandable double plastic stent

Field of the invention

The invention relates to a self-expanding dual plastic stent prepared from radiopaque fibres and coated with an elastic foil as well as to the method of manufacturing the same. The present invention pertains to the field of medical technology, particularly to the field of elastic reinforcements, such as stents.

Description of the prior art

Stents and stent grafts are tubular formations made from a meshwork using the braiding technique or made from a tubule using the laser trimming technique. The main function of the stents is to ensure the patency of tubular organs inside the human body when the essential patency is restricted in an acute or chronic manner. Such restriction (or constriction) may be caused, e.g., by a proliferating tumor, which oppresses the affected tubular organ; or may develop after a major surgery.

At the present, stents are manufactured from a plurality of metallic materials ranging from stainless steels up to the materials having a shape memory, such as those on the basis of nickel-titanium alloys. Substantially, there are two approaches to manufacturing stents. The first approach is based on the employment of a knitting technique used to form a tubular piece from at least one wire. Such stents may be different in shape, ranging from simple cylindrical tubes, such as those disclosed in the documents U.S. 2010/0249901 and U.S. 2008/111716, up to complex tubular formations which are flared at their individual ends with the aim to eliminate the undesirable migration of the stent, such as those disclosed in the documents U S. 2011/0022 51 and WO 2009/091899. The second method of manufacturing stents is based on the employment of a laser trimming technique. When prepared in the above described manner, the stents always have an original design. Such stents are mostly balloon-expandable and have to be post-dilated in order to assume their nominal diameter. This means that the use of a balloon catheter is inevitable. After having been inserted into the stent, the balloon is inflated. This causes the stent to be expanded and to assume its final diameter. One of the advantages of the above stents, in comparison to classic braided ones, consists in their high rigidity, i.e. high expansion strength. An exemplary embodiment of such stent is disclosed in U.S. 2008033532.

Using polymeric materials for the skeletons of stents is a rare technical solution. One of the commercially available solutions represents the polymer stent Polyflex manufactured by the Boston Scientific company. Another example of contemporary technical solutions is disclosed in the patent document EP 1258229 A1. In the latter case, a self-expanding plastic stent is concerned. With reference to the aforesaid document, the plastic stent is mainly intended for the application in the vascular system. In this case, a radiopaque filler is intermixed with a degradable polymer to form a dispersion. The dispersion is then applied onto the finished stent using a dipping or spraying process. As far as the particular materials are concerned, the stent is prepared from polyethylene-terephtalate and polyethylene.

The stents may be covered, i.e. coated, with a foil. Such foil further enhances, the functionality of the stents, as disclosed, e.g., in the patent document U.S. 2010/0173066 describing a coated stent prepared by laser trimming. The demands imposed on such a foil are considerable, The main parameter is the biological tolerability of the foil by the human body. In addition, the foil must have adequate mechanical properties. Suitable tolerable materials include polymers on the basis of teflon, silicone, or polyurethane. When applied, the foil must be thin enough to enable the stent to be easily compressed when being inserted into the introduction system. This means that the stent must be coated with a thin, impermeable surface layer. The impermeability is on of the most important parameters of coated stents. The main task of the stent is to ensure the patency of a tubular organ and the foil must be able to enclose possible leaks, lesions or bleeding sutures, such as those caused by a resection. Another important parameter of the foul is a certain level of elasticit which must correspond to the behaviour of the stent under deformation, as stated, e.g., in the patent document U.S. 2009220677:

Most of the commercially available stents are provided with diverse mechanisms. The purpose of such mechanisms is to prevent the migration of the stent from the intended location. When a stent is implanted with the aim to dilate an stenotic structure, the migration of the stent is prevented by the structure itself. In case that such stenotic structure, is not present, e.g. when a stent is imp!anted in connection with the treatment of esophageal fistulae, the stent must be forcibly retained in the intended location. As mentioned above, this may be accomplished by diverse retaining mechanisms. One of the applicable principles consists in making both the proximal and the distal end of the stent flared so that the ends assume a tulip-like, dumbbell-like or "dog-bone-like" shape. In case of the solution developed by Boston Scientific, the proximal portion of the stent is flared to assume the tulip-like shape, the distal portion of the same remaining tubular in shape (U.S. 2008/0221670). None of the above principles, however, is capable to completely eliminate any undesirable migration of the stent. Therefore, it is necessary to select an adequate combination of the above mentioned elements so that the risk of undesirable migration could be reduced to* an acceptable level.

In the case that the implantation of a stent is indicated in the lower portion of the esophagus, an undesirable reflux of the acidic content of the stomach into the esophagus may occur, causing damage to the lining of the esophagus and, as the case may be, leading to the formation of pathological structures in the latter. In order to prevent such behaviour from occurring, it is possible to form an elongated sleeve having specific shape and length. Such sleeve will protrude into the stomach without limiting the patient in consuming foot. It will, however, prevent the food from being regurgitated. If the sleeve is long enough, the surrounding moisture causes the individual portion of the foil to adhere to each other and to form a permanent closure preventing regurgitation. Such closure does not obstruct the esophagus in any way and the food may freely pass through, as described in the document U.S. 2003/0060894. Another alternative consists in the creation of overlapping natural obstructions which create a tortuous pathway inside the stent lumen. Overcoming of such closure in the reverse direction is virtually impossible but the stent remains open for supplying food. Another advantage of the above solution consists in that the gas exchange remains unrestricted. This is not possible with the other solutions based on the use of the aforementioned sleeve element.

The stents are placed into the human body by means of endoscopic introducing and monitoring devices. Furthermore, x-ray or fluoroscopic imaging methods are used for the exact evaluation of the correct placement of stents. For this purpose, the stent must be sufficiently visible. In order to achieve this, the stents are provided with radiopaque markers at their proximal and distal ends. Alternatively, the stent itself may be made of a radiopaque material. By the way of example, the material known as Elgiloy may be referred to, where the core of the wire, which is formed from a radiopaque material, is coated with an alloy having shape memory. The respective composition is described in the patent document U.S. 5630840. Nevertheless, stents are mostly made of the materials, which are poorly visible in x- ray images, and so it remains necessary to provide them with radiopaque markers facilitating the corresponding introduction and extraction procedures. The materials used for manufacturing such markers usually have high x-ray absorption coefficients and adequate thicknesses ensuring clear visibility of the markers. Therefore, the radiopaque markers are mostly made of precious metals, such as gold, platinum, iridium of combination thereof. One of the drawbacks of the markers placed on a stent consists in that the stent is not visible along its entire length. The markers only indicate the proximal and distal ends of the stent or several specific portions of the same, as discernible from the patent application U.S. 2004/0044399. Another possibility of enhancing the radiopaque properties of the stent consists in the use of halogen compounds which may be deposited onto the surface of the stent. Owing to the presence of halogen elements, the stents, such as those described in the patent document WO 2006/022754, are sufficiently visible in x-ray images. The aforementioned enhancement techniques, however, have many drawbacks. For example, the conventional radiopaque markers may suffer from galvanic corrosion when coming into contact with other elements having higher positive electric potentials. Such marker may also flake away or cause perforation of the wall of the affected organ inside the human body. The deposited radiopaque layer may get stripped or washed away. All the above drawbacks may be overcome in that the radiopaque matter is directly embedded in the structure of the fibrous material.

Most stents are made of metallic materials that are, under certain circumstances, exposed to a more or less corrosive environment. Such corrosive environment may exist in the gastrointestinal tract. The bloodstream is also considered to be highly corrosive. In general, the corrosive environments impose considerable demands on medical devices that have to be implanted for an extended period of time. Corrosion is a significant electrochemical process that can cause an extensive damage to the implanted device. Due to mechanical and chemical stresses or to a combination thereof, the stent may be subject to severe local corrosion leading to the destruction of the same. Though rarely occurring, such cases may have disastrous or even fatal consequences with respect to the health status of the affected patient. The risk presented by the corrosion cf stents may be further considerably reduced by using stents made of plastic materials. Such materials may be either degradable or non-degradable. In the former case, the stent, such as that according to the disclosure of WO 2009/099958, controllably decomposes into particles which can be eliminated by the human body. In the latter case, the use of non-degradable plastic stents may be an adequate alternative for certain applications. Such stent must be made of a plastic material that is sufficiently resistant in the environment of the human body and has adequate mechanical properties ensuring good expansion stret gth and overall mechanical behaviour. On¹ of the stents, which are very successful from the commercial point of view, is the stent Polyflex manufactured by the Boston Scientific; company. It is made of a special polyester fibre which is provided with radiopaque strips at its proximal and distal ends as well as in its intermediate portion. The purpose of said radiopaque strips is to enhance the visibility of the stent in x-ray images. Nevertheless, the aforesaid stent has not a sufficient expansion strength and the respective material is strongly susceptible to deformation which might adversely influence the final shape of the stent. Another disadvantage consists in that a sizable introduction device must be used, in which the stent is compressed prior to implantation, which makes the application of the stent arduous.

The objective of the present invention is to provide an alternative solution, wherein the stent would have improved mechanical properties, such as resistance to corrosion or sufficient shape memory.

Disclosure of the invention

The above drawbacks are eliminated by the self-expanding dual polymeric stent, which is prepared from plastic and radiopaque fibres coated by an elastic foil, according to the invention, wherein the stent has conical shape and is provided with an anti-migration segment and with extracting wires at its proximal end and with an anti-reflux valve at its distal end, said stent being made of two different non- degradable fibres, namely of the PEEK-based carrying fibre and the radiopaque fibre, and coated with a silicone foil, the carrying fibre forming the principal skeleton of the stent and the radiopaque fibre enhancing the visibility of the same, the carrying fibre being made of a plastic material selected from the group including PEEK, PEAK, PEK, PEKK, PEKKEK and the other fibre being made of PU+W, the stent further being provided with a pull-tie.

Method of manufacturing the non-degradable dual conical plastic stent according to the invention consists in that initially the stent is knitted from the first fibre by means of a braiding machine so that it assumes a tubular, slightly conical shape having the proximal end atraumatic and the distal end traumatic, then the skeleton of the stent is placed on a mandrel and the other, radiopaque fibre is enlaced into it, both the fibres being laid side by side, afterwards the mandrel with the stent attached to it is removed from the braiding machine and placed into a thermal device where the shape of the stent is thermally fixated by being subjected to a temperature between 120 °C and 250 °C, preferably between 140 °C and 200 °C, during a time interval from 10 to 120 minutes, preferably from 20 to 40 minutes, and finally the stent is provided with the anti-migration segment at its proximal end, said segment comprising one or more fibres forming a ripple at the proximal end of the stent, said ripple adjoining the skeleton of the stent from outside.

Overview of the accompanying drawings

The invention will be further explained by means of the accompanying drawings wherein Fig. 1 shows a schematical view of the stent according to the invention and Fig. 2 shows a detailed view of the arrangement of the fibres.

Exemplifying embodiments of the invention

The present invention discloses the dual stent 1 formed from an interlaced mesh work. The dual stent is composed of two different plastic fibres. According to the invention, either plastic fibre has a different, specific function. The first meshwork comprises the carrying fibre 2 which is made of the material on the basis of PEEK, polyethersulfone or polyamide. The second meshwork comprises the radiopaque fibre 3 which is enlaced into the structure of the stent 1 in order to supplement the carrying fibre and to ensure the clear visibility of the entire stent in x-ray images. The stent is provided with the anti-migration segment 4 at the proximal end. Said segment, also referred to as collar, has corrugated shape and extends around the entire circumference of the proximal portion. I may be situated at the end of the dual stent or underneath that end. Preferably, the collar is situated in the distance of about 10 mm from the proximal end of the stent. The total number, of such collars as well as the locations thereof may be variable. Furthermore, the stent is provided with the pull- tie 5 at the proximal end, said pull-tie being enlaced into the individual meshes of the stent around the circumference of the same. The pull tie may assume either the form of a wire or the form of an extraction lug. The number of such extraction lugs is not subject to any limitation. A single extraction lug, however, is considered' ό be sufficient; In case that the pull-tie of the ste^{^}ht is provided in the form of an extraction wire, the latter may be seized by means of a suitable extraction instrument. Such extraction lugs or wires may be provided both at the proximal end and at the distal end of the stent, the number of those elements, again, being not subject to any limitation. The function of the above elements is to enable the stent to be easily pulled into the respective extraction device. In addition, the stent is provided with the anti-reflux valve 6 at the distal end, said anti-reflux valve having a sleeve-like shape. The anti-reflux valve is perforated in order to enable the free escape of gases. The size of the perforation in the anti-reflux valve 6 must be adequately selected in order to prevent the content of the stomach from passing through. The skeleton of the stent 1 may be conical in shape which in itself further impedes the distal migration of the stent. Furthermore, the skeleton of the stent may be coated with a silicone foil that covers the anti-migration segment at the proximal end of the stent and protrudes beyond the distal end of the same. In this case, the protruding portion of the foil forms the anti-reflux valve in itself. In another embodiment, only the skeleton of the stent is coated with the foil, the anti-migration elements being left blank. The anti-reflux valve and the skeleton of the stent may be prepared from the same material. In case of a coated stent, the valve may be alternatively prepared from a different material. Afterwards, the anti-reflux valve is treated using either a chemical method (etching) or a mechanical method (punching) or an optical method (ablation) to assume the form of the selectively semi-pern eable membrane which is permeable to gases but impermeable to liquids. The skeleton of the stent, the pull-tie forming the extraction wire, the extraction lug and the anti-migration collar are made of a material on the basis of polyetheretherketone (PEEK), polyimide or polyethersulfone. It is desirable that the above materials are enriched with a radiopaque filler on the basis of barium, bismuth or tungsten salts. The proportional content of such a filler should range between 20 and 60%, preferably between 40 and 50%. The above compounds ensure that the stent will be clearly visible in x-ray images. In this particular case, polymeric composite materials are considered. Owing to the composition of the above materials, the whole structure of the stent is visible which in turn makes the correct location, expansion and function of the same to be controllable.

Along the entire length, the stent is covered with an elastic coat on the basis of polyurethane or silicone. The coating material must be adequately elastic and rigid. The material should be moderately cross-linked to form a polymeric grid.

As mentioned above, the present invention provides a self-expanding dual plastic stent, said stent being conical in shape and comprising a composite material on the basis of polyetheretherketone, polyimide or polyethersulfone, said material forming the principal skeleton of the stent. The stent further comprises a radiopaque fibre enhancing the visibility thereof, thus being also referred to as dual stent. The stent is coated with an elastomeric foil extending along the entire length thereof. Furthermore, the stent is provided with an extraction wire and an anti-migration collar that are arranged in the proximal portion of the stent. Besides that, the stent is provided with an anti-reflux valve at its distal end.

Being made of a plastic material, the stent provides good mechanical properties, excellent resistance to corrosion and good shape memory. Another advantage of the stent according to the invention consists in the possibility of using a compression device having a low diameter profile between 28F and 20F. Although the above diameter values may appear to be high with respect to classic metallic stents or national stents, they still remain relatively low when compared with the commercially available catheters that are used for the introduction of plastic stents. The intended field of application of the present, invention primarily comprises the gastrointestinal tract. Herein, the esophagus, the transitional portions of the esophagus and stomach and those of the stomach and small intestine respectively, the small intestine, the colon and the rectum are mainly concerned. The possible secondary applications involve the biliary, urethral, renal and pancreatic tracts. Another preferred field of application of the invention involves the indications of the stents in respiratory tract, including both the trachea and the bronchi / bronchioles. The stent according to the invention may also be applicable in the lacrimal pathways. When adequately adapted, the stent might be applicable in the vascular system. Although the latter field of application is feasible, it does not primarily fall into the scope of the present invention.

The foreseen clinical indications include benign and, malignant stenoses, and anastomotic leaks. Furthermore, the clinical indications may include implantations of stents having the purpose to cover up fistulae. In general, the primary field of application of the present invention consists in ensuring the patency of the above mentioned tubular organs in the human body. Another foreseen field of application of the present invention involves the use of the stent as a mechanical supporting element for transplanted organs, such us the trachea, in which case the stent is left inside the recipient's body until the adoption of the donated organ by the latter, in tne latter manner, the stent may be implanted into any part of the human body when a tubular organ is to be substituted resected.

Preparation of the dual skeleton of the stent

The stent is manufactured using the following method: The individual steps are listed in the exact, consecutive order. First, the stent is knitted from the first fibre by means of a braiding machine so that it assumes a tubular, slightly conical shape having the proximal end atraumatic (the fibre is bent) and, the distal end traumatic (the end is not formed from bent fibres but is terminated by a sharp edge of one of the fibres). Then, the skeleton of the stent is placed on a mandrel and the other, radiopaque fibre is enlaced into it, the technique being the same as that described in the preceding paragraph. Both the fibres are arranged side by side. The radiopaque fibre may be less in diameter than that forming the skeleton of the stent. Afterwards, he mandrel with the stent attached to it is removed from the braiding machine, which does not fall into the scope of the present invention, and placed into a thermal device where the shape of the stent is thermally fixated. The position of the stent is fixated by attaching the same to the mandrel. When being attached to the mandrel, the stent must have the final shape to be thermally fixated. After having undergone the heat treatment, the stent cannot shrink or expand any more because its dimensions are stabilized according to the size of the mandrel. In this heat treatment stage, the stent subjected to a temperature between 120 °C and 250 °C, preferably between 140 °C and 200 °C, during a time interval from 10 to 120 minutes, preferably from 20 to 40 minutes. Thus, the desirable shape memory is imparted to the material of the stent. Afterwards, the stent is provided with the anti-migration segment at its proximal end, said segment comprising one or more fibres forming a' ipple at the proximal end of the stent, said ripple adjoining the skeleton of the stent from outside (when being introduced into the human body, the ripple will be dilating). The anti-migration collar considerable reduces the risk of migration of the stent into the stomach but keeps both the shape and the exact positioning of the stent unchanged. The anti-migration segment may be made of the same fibre or of a different one. Finally, the stent is provided with a pull-tie facilitating the extraction of the same. The pull-tie is attached to the stent in that the fibre forming the same is alternately interlaced between the individual links in the inward and outward directions over the entire circumference of the stent and terminated by a loop once the circumference is closed. A plurality of such loops may be optionally provided at either end of the stent. Thus, the loops form a lug which may be handled in that its tab is seized and pulled, whether with an extractor or with pincers, causing the stent to be separated from a hollow organ lumen, such as esophagus. In a case of need, the stent may be displaced upwards. If the stent is provided with the pull-tie instead of the extraction wire, it may be handled in a similar manner.

Coating of the stent and the anti-reflux valve

After having been stabilized in shape, the stent is undamped from the mandrel and made ready for being coated. Afterwards, the stent is attached to another mandrel, which is suitable for the subsequent coating process, and placed into a coating device. Neither the coating mandrel nor the coating device, however, fall into the scope of the present invention. Using a pouring technique, the meshwork of the stent is covered by a polymeric solution. Then, the stent is placed into a dryer where it undergoes the second heat treatment stage. During the latter stage, the shape memory of the material will be established and the coating of the stent will assume the cross-linked form. The above mentioned second heat treatment is accomplished in the temperature range from 120 to 70 °C, preferably from 140 to 160 °C, during a time interval from 1 to 4 hours. After having undergone the second heat treatment, the stent is undamped from the mandrel, thus being ready for the next processing step.

In case that the same material is selected both for the anti-reflux valve and for the coating of the stent, the preparation of the anti-reflux valve is accomplished in the following way. The distal portion of the stent is provided with the anti-reflux valve which is composed of a silicone or polyethylene foil having half the length of the stent. The purpose of the anti-reflux valve is to prevent the gastric juice from penetrating into the esophagus.

In case that different materials are selected for the anti-reflux valve and for the coating of the stent, the material of the former being e.g. a PE foil, the preparation is accomplished in the following way. The coated stent is released from the coating mandrel and attached to another mandrel which serves as a supporting device. Then, a piece of PE foil having the desired shape is cut away from a reel. The piece of foil is then trimmed to assume the required size, i.e. diameter and length. After having been prepared in the above manner, the sleeve is attached to the stent (using an adhesive or by means of a thermal method).

The present invention may be exploited to ensure the patency of the hollow organs in the human body. Primarily, the stent is intended for tubular human organs, particularly for those of the gastrointestinal and respiratory tracts. In the gastrointestinal tract, the may fields of application include the esophagus, the transitional area between the esophagus and stomach, the stomach, the duodenum sections, the small intestine, the colon and the rectum. Furthermore, the stent may be implanted into the hollow organs which are directly connected to the gastrointestinal tract, such as into the biliary and pancreatic tracts. The stent may also be applied in the urethra or in the ureters. As far as the respiratory tract is concerned, the main areas of application of the stents are the trachea and the bronchi / bronchioles.

Claims

PATENT: CLAIMS

1 . Se!f-expanding dual plastic stent, characterized by being conical in shape and provided with the anti-migration segment (4) and with extracting wires at its proximal end and with the anti-reflux valve (6) at its distal end, wherein the stent is made of two different non-degradable fibres, namely of the PEEK- based carrying fibre (2) and the radiopaque fibre (3), and coated with a silicone foil, the carrying fibre (2) forming the principal skeleton of the stent (1 ) and the radiopaque fibre (3) enhancing the visibility of the same, the carrying fibre (2) being made of a plastic material selected from the group including PEEK, PEAK, PEK, PEKK, PEKKEK and the other fibre being made of PU+W, the stent (1 ) further being provided with the pull-tie (5).

2. Non-degradable dual conical plastic stent, characterized Sin that the radiopaque fibre (3) is thinner than the other fibre.

3. Method of manufacturing the non-degradable dual conical plastic stent according to claim 1 , characterized in that initially the stent is knitted from the first fibre by means of a braiding machine so that it assumes a tubular, slightly conical shape having the proximal end atraumatic and the distal end traumatic, then the skeleton of the stent is placed on a mandrel and the other, radiopaque fibre is enlaced into it, both the fibres being laid side by side, afterwards the mandrel with the stent attached to it is removed from the braiding machine and placed into a thermal device where the shape of the stent is thermally fixated by being subjected to a temperature between 120 °C and 250 °C, preferably between 140 °C and 200 °C, during a time interval from 10 to 120 minutes, preferably from 20 to 40 minutes, and finally the stent is provided with the anti-migration segment at its proximal end, said segment comprising one or more fibres forming a ripple at the proximal end of the stent, said ripple adjoining the skeleton of the stent from outside.

4. Method of manufacturing the non-degradable dual conical plastic stent according to claim 3, characterized n that the anti-migration segment is made of the same fibre or of a different one.

5. Method of manufacturing the non-degradable dual conical plastic stent according to claim 3, characterized in that the stent is further provided with a puil-tie in order to facilitate the extraction of the stent, said pull-tie being attached to the stent in that the fibre forming the same is alternately interlaced between the individual links in the inward and outward directions over the entire circumference of the stent and terminated by a loop once the circumference is closed, a plurality of such loops being optionally provided at either end of the stent, said loops forming a lug which may be handled in that its tab is seized and pulled, whether with an extractor or with pincers, thus causing the stent to be separated from a hollow organ lumen, such as esophagus.