Patent Document

This application claims benefit of Provisional No. 60/295,128, filed Jun. 1, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a removable stent and an assembly for its delivery or removal from a target site. The present invention particularly relates to a removable stent and system for its use in a medical procedure involving blood vessels, ducts, treatment of enlarged prostate gland, coronary artery disease and the like. 
     BACKGROUND OF THE INVENTION 
     Stents are used in a wide variety of medical procedures where the permanent expansion of an occluded vessel is desired. Usually, stents are constructed of a metal cylinder that is implanted into a patient at the site of obstruction. In a patient suffering from an occluded vessel, balloon catheterization or balloon angioplasty is often the prescribed treatment. However, following such a procedure, restenosis or re-narrowing of the occluded tissue often occurs. Therefore, stents were developed and are used to optimize and improve the initial and long-term outcome in patients treated for an obstructed vessel. In addition to obstructions, stents are also used to provide support for a graft during healing of reconnected vessels. Diseases most often treated using a stent include coronary artery disease, benign prostatic hyperplasia (also referred to as an enlarged prostate gland), and other medical indications where expansion of a lumen, vessel or duct is desired. As such, a variety of stent systems have been developed for medical use. See, for example, U.S. Pat. Nos. 5,100,429; 4,762,128; and 4,800,882. 
     Although the development of stents for use in medical procedures has been a major advance in treating a narrowed lumen, a variety of complications can and do occur in connection with either the delivery of the stent or, at a later time, following deployment of the stent in vivo. Such problems or complications include failure of proper deployment of the stent, misalignment, dislodgement, or damage of the stent after it is deployed, or re-occlusion of the vessel over time once the stent is inserted. In these cases, removal of the stent is desired. Devices and/or assemblies allowing for the extraction of a stent are known and include, for example, U.S. Pat. Nos. 5,474,563; 5,624,450 and 5,411,507. In particular, Hendrik, U.S. Pat. No. 5,624,450 describes an assembly for the removal of an implanted stent. The assembly entails use of an expandable element having an adhesive outer surface. The expandable element is connected to a pulling device. Insertion followed by expansion of the expandable element inside the faulty stent causes its attachment (adhesion) to the inner surface of the stent allowing the user to then “pull” the stent out. A particular disadvantage in this system is that it is unreliable, as attachment of the expandable element to the stent occurs by adhesion. As a result, a more reliable and effective removal system is desired. 
     An example of an additional stent removal system may also be found in U.S. Pat. No. 5,474,563, which describes a system for removal of a cardiovascular stent device from a blood vessel. The system includes a self-expanding elastomeric stent and an extraction catheter for removal of the cardiovascular stent. The extraction catheter is especially designed so as to specifically engage with projections located on the stent. Removal of the stent occurs by engaging the extraction catheter with the projections. One disadvantage of this extraction system and other similar systems is the requirement for complex extraction instrumentation as well as specific and intricate maneuvering by the physician to engage the extraction tool with the stent. 
     The above-described removal systems (and other similar devices not specifically described) offer advantages, including effectiveness and safety to both the user and the patient. However, it has been discovered that an obstacle or disadvantage to such devices is that their use is complicated. Additionally, even with the more simple removal systems, the susceptibility of separation of the removal device from the stent during use result in major limitations to the reliability of these systems. 
     In view of the above, it is apparent that there is a need to provide a removable stent and a system which allows for reliable and minimally traumatic removal of a stent from an in vivo target site. There is also a need to provide a removable stent and system that is efficient, simpler to use for the physician and easy to maneuver in vivo. A reliable and efficient removal system would reduce the overall procedure time required, reduce possible trauma to the lumen wall during use, and therefore reduce patient discomfort during recovery. Such removal systems include properties that reduce the amount of effort required by the physician prior to and during use of the system as well as properties that ensure the system remain intact during removal of the stent. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a removable stent device that addresses the limitations and disadvantages associated with prior removal devices, yet meets the needs of the users. 
     A further object of the invention is to provide a removable stent system that is efficient, requires minimal effort by the user and that is reliable. 
     Still another object of the invention is to provide a removable stent assembly having a collapsing element configured around the circumference of the stent so as to allow retrieval of the stent by collapsing one end of the stent to a smaller diameter. 
     A further object of the invention is to provide a stent removal system where removal of the stent can be achieved by use of a simple removal tool having a grasper attachment. In one aspect of the invention, a stent removal system for removing a stent from a target site in a patient is disclosed. The system can include a removable stent that is collapsible at a proximal end by engagement of a collapsing element with a removal tool. The collapsing element can be a lasso or a discontinuous lasso configured so as to collapse the proximal end of the removable stent for removal from a body site. Alternatively, the collapsing element can be a hook that is movable between an up and a down position. The removal tool is configured so as to allow coupling to the collapsing element of the removable stent. The removal tool can be configured as a tube having a slot at one end allowing for its coupling to the collapsing element. The stent removal system can also include a grasper element designed to aid in the removal of the stent by attachment to a removal tool or a delivery tool. 
     An additional object of the invention is to provide a method of removing a stent in vivo using a stent removal system. 
     These and other objects not specifically enumerated herein are believed to be addressed by the present invention which contemplates a stent removal system for removing a stent from a body site that includes a removable stent having a collapsible proximal end, a collapsing element and a removal tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a preferred embodiment of the present invention in situ including a removable stent having a collapsible proximal end, an inner lumen and a collapsing element; 
     FIGS. 2A and 2B are enlarged views of the area of a removable stent having a hinged hook collapsing element depicting the hinged hook in upright (FIG. 2A) and down (FIG. 2B) positions; 
     FIG. 3 is a cross-sectional view of a removable stent along the  3 — 3  line of FIG. 1; 
     FIGS. 4A and 4B are cross-sectional views through the hinge region of a hook type of collapsing element; 
     FIG. 5 is a side view of a removable stent including a removal tool for grasping a collapsing element of a removable stent; 
     FIG. 6 is a cross-sectional view of a removable stent along line  6 — 6  of FIG. 5; 
     FIG. 7 is a side view of a removable stent in situ with the proximal end in the collapsed position; 
     FIG. 8 is a view of a removable stent having a lasso collapsing element; 
     FIG. 9 is a cross sectional view through the proximal end of a removable stent having a lasso collapsing element; 
     FIG. 10 is a view of a removal tool and a removable stent having a lasso collapsing element; 
     FIG. 11 is a view of a removal tool grasping a removable stent; 
     FIG. 12 is a view of a removable stent having a discontinuous lasso collapsing element; 
     FIG. 13 is a cross sectional view through the proximal end of a removable stent having a discontinuous lasso collapsing element; 
     FIG. 14 is a view of a removal tool grasping a removable stent; 
     FIG. 15 a view of a removal tool grasping a removable stent; 
     FIG. 16 is an enlarged view of an area of an alternate embodiment of a removable stent having eyelets or loops formed near the end of the stent; 
     FIG. 17 is a detailed view of an alternate embodiment of the loop of FIG. 16; 
     FIG. 18 is an enlarged view of an area of an alternate embodiment of the removable stent of FIG. 16; 
     FIG. 19 is a partial view of the removable stent of FIG. 16 having a lasso-type collapsing element; 
     FIG. 20 is a partial side view of a removal tool and the removable stent of FIG. 17; 
     FIG. 21 is a side view of the removable stent of FIG. 17 in a constricted configuration; 
     FIG. 22 is a side view of a removable stent and an alternate embodiment of a collapsing element; 
     FIG. 23 is a side view of a removable stent and an alternate embodiment of a collapsing element; and 
     FIG. 24 is a view of a grasper device for attachment to a removal or a delivery tool. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a representative view of a removable stent  10  in situ within a body vessel  11 . For convenience and ease of comprehension, the medical device referenced in the text and figures of the present disclosure is a stent. However, it should be noted that other medical devices or prosthesis including, but not limited to, balloons, stent coverings, vascular grafts, or other implantable devices, are also within the scope of the claimed invention. 
     The removable stent  10  is an intraluminal prosthesis or device having proximal  22  and distal ends  24  that are open. The removable stent is generally tubular in shape and has an outer surface  30  which contains an inner lumen  32  that extends axially between collapsible proximal  22  and distal ends  24 . A removable stent includes collapsing elements  40 , which aid in collapsing or compressing the stent  10  for its removal from an intralumenal site. The collapsing element  40  of the stent is a feature that aids in its efficient and easy removal from a body lumen. As used herein, the term “proximal” is intended to refer to the end of the stent closest to the physician when deployed at a target site, or the end that will be collapsed for removal of the deployed stent from its target site. The “distal” end is intended to refer to that end which is opposite to the proximal end  22 . 
     A removable stent  10  can be of the type that is self expanding, or of the type that is expandable using a balloon mechanism. Methods for the construction, manufacture or deployment of self-expanding stents are known in the art and are described, for example, in U.S. Pat. No. 5,356,423 as well as in U.S. Pat. No. 4,655,771. Balloon expandable stents are also known in the art and are described, for example, in U.S. Pat. No. 4,893,623. Alternatively, the stent can be expandable by any other means, or can be of any variety of expandable prostheses or intralumenal implantation devices that include an element capable of collapsing or constricting the stent from an end, such as the proximal end. 
     Referring to FIG. 1, a removable stent  10  can be constructed so as to have a mesh structure  26 . The mesh can be made of elongate elements, such as metal wires, that are woven, braided, or stamped. Alternatively, the mesh-work can be formed from any other type of material or structure so long as it is biocompatible and of sufficient rigidity so as to support patency of a body lumen or target site when implanted in vivo. Biocompatible materials suitable for such construction include, for example, stainless steel, alloys, composite materials, or plastics. The removable stent can also be constructed of a flexible or non-metallic material such as an elastic polymer or rubber, medical-grade nylon or polyester, or any material that is either itself collapsible or can be formed or configured to be compressible. 
     In a preferred embodiment, as shown in FIGS. 1 and 7, the removable stent  10  is a braided tubular mesh  26 , constructed of a collection of wires  28  (approximately 24 in number) held together by friction. The wires are formed or woven in such a way so as to facilitate and maintain radial expansion of the stent when deployed in vivo (See FIG.  1 ), while also allowing for radial compression of the stent when pulled along its longitudinal axis (FIG.  7 ). The expansion/compression feature of the removable stent  10  is designed such that application of a pulling force along the stent&#39;s longitudinal (x-x) axis results in radial compression and reduced lumen diameter. Release of the longitudinal force returns the stent to its deployed radially expanded dimension. This feature of the removable stent is similar to the mechanism employed in a child&#39;s Chinese handcuff toy. 
     A collapsing element  40  can be of a variety of structural configurations. The collapsing element  40  is preferably located at or near the proximal end  22  of the removable stent  10  as this is generally the end from which the physician will first encounter the stent during a removal procedure. In a preferred embodiment, a removable stent can include a collapsing element  40  designed as a clasping structure that is located within the inner lumen  32  of the removable stent as shown in FIG. 1 and 3. FIG. 3 is a cross-sectional view of a removable stent taken along the  3 — 3  line at the proximal end  22  of the removable stent  10  of FIG.  1 . Such a collapsing element  40  (also referred to herein as a “hinged hook” or a “hook”e) is movable between a first and a second position. Therefore, as illustrated in FIG. 2 the hook elements  42  can be lifted into an upright or “up” position (FIG. 2A) for engagement with and extraction by a removal tool, or can be pushed into a “down” position (FIG. 2B) where it lays flat against the inner wire surface  42  of the removable stent when the stent is deployed in vivo. As illustrated in FIG. 3, when in the “up” position, the hooks  40  protrude away from the inner surface  30  of the stent into the inner lumen  32 . A removable stent having a hinged hook type of collapsing element  40  will preferably include 2 to 3, or 3 to 4 hooks, spaced equally along the inner circumference or surface  30  of the removable stent. 
     FIGS. 2A and 2B are enlarged views illustrating a hook type of collapsing element  40  of a removable stent when deployed in vivo. When in the first or “down” position (FIG.  2 B), the hook element  40  resides substantially parallel to or flat against the inner surface of a cross wire  42  of a stent. The ability of the hook  40  to lay flat against the inner surface of the removable stent (“down” position) is particularly advantageous in that such a collapsing element  40  will not interfere with the patency, impede the flow, or increase the possibility of clot formation of a fluid through the stent. This is of particular concern when a stent is deployed in a blood vessel. A collapsing element  40  that is movable between an up and a down position allows for placement of the collapsing element  40  into a more accessible position for engagement with a removal tool. This aspect of the present invention provides a more versatile and effective removable stent. 
     A hinged hook type of collapsing element  40  includes an upper region that is curved or shaped as a hook  44 . The lower or bottom portion  46  of the collapsing element  40  is configured so as to enclose or wrap around a wire  43  of the stent so as to function as a hinge. The collapsing element  40  can be attached to a stent wire  43  at the inner surface of the removable stent. A hinged hook  44  can be designed or configured so as to snap onto a stent wire  43  with a portion  46  of the hinged hook disposed toward the outer surface of the stent. Once snapped into place, the hinged hook  44  can be pushed or slid along the stent wire  43 , passing underneath a crossing wire  41 , so as to position the hook  44  underneath a cross wire  42  as shown in FIG.  2 B. 
     In addition, a hook type of collapsing element can be configured so as to include tab components, which allow it to be easily snapped or clipped onto a stent wire. Referring to FIGS. 4A and 4B detailed cross-sectional views through the hinge portion  46  of a hook  44  illustrate two exemplary designs of tab components of a hook hinge region. In the first design (FIG.  4 A), the tab gap element  8  is an extension of the hinge portion  46  that, when pressed, will become captured in the hinge latch  47  which is an extension of the hook  44 . In the second design (FIG.  4 B), the hinge latch  47  is pressed such that it becomes captured by the tab gap element  8 . Both of these hinge designs allow rotational movement of the cross wire  43  relative to the hook  44 . That is, the hinge  46  portion of a hook type  44  of collapsing element  40  allows the collapsing element to be movable between its deployed, “down” position and a removable “up” position. A hinged hook  44  type of collapsing element as described herein is also advantageous in that the tab gap design reduces undesired movement of the hook element  44  between its two positions (upright or down) by virtue of how the tab gap design is comprised around and in frictional contact with the cross wire  43 . Therefore, a hinged hook is preferably manufactured of a material that maintains a certain degree of resistance so as to ensure the collapsing element remains in either the up or the down position, as is desired by the physician during use. A hinge material can be manufactured of a same material as the stent wire, such as stainless steel or plastic. An optimal method of fabricating and handling a hinged hook is by continuous metal stamping on a band or a ribbon. 
     A stent of the present invention can be removed from an intralumenal site by collapsing one of its ends, such as the proximal end, with the aid of a removal tool. Several types of removal tools  62  can be used to remove a stent  10  of the present invention, as will be apparent to one skilled in the art. Referring to FIG. 5, a removal tool  62  suitable for use with a hook type of collapsing element  40  can be designed to have coupling structure  64  for engaging the hook elements  40 . The stent  10  can then be collapsed beginning at its proximal end  22  using the removal tool  62  to grasp the hooks  40  and to apply a pulling force. FIG. 6 illustrates a cross-sectional view of a removable stent having hook elements  40  engaged with a removal tool. As indicated in FIG. 6, application of a rotational force on the constricting elements  40  via the coupling means  64  collapses the stent  10  radially inward. 
     In a preferred embodiment, a removal tool  62  can take the form of a standard stone basket tool. One such tool includes a stone basket as manufactured by Cook Urologic. In this embodiment, the strands forming the basket of the tool engage the hooks  40  as shown in FIG.  6 . 
     FIG. 7 illustrates a stent  10  of the present invention during removal from an intralumenal site  11 . Radial constriction begins at the proximal end  22  of the stent  10  extending toward the distal end  24  longitudinally, as the stent  10  is pulled by a removal tool  62  coupled to the collapsing elements  40 . Radial compression and collapse of the stent  10  beginning with the proximal end  22  allows the physician to easily withdraw or ensnare the stent  10  into an endoscope sheath for its removal from an in vivo intralumenal site  11 . This aspect of the removable stent  10  is particularly advantageous to achieving the goals of the invention in that a stent  10  having a collapsible end allows for easy manipulation of the stent  10  into a catheter or endoscope sheath to completely remove the stent  10  from the patient&#39;s body. 
     Referring to FIG. 8, a collapsing element can also be configured as a lasso  80  situated around the circumference of the proximal end  22  of a stent  10 . The lasso  80  is constructed of a wire which functions as a noose capable of constricting or tightening itself around the outside surface  82  of the stent  10 . The lasso  80  can include a loop region  84 , which allows for engagement of a removal tool  101  to grasp and remove the stent  10  (see FIG.  10 ). The loop region  84  of the lasso  80  is disposed internally within the stent  10  with the remainder of the lasso wire  86  wrapped externally around the stent  10 . Alternatively, the lasso  80  can be woven into and out of the stent surface  82 . Therefore, the lasso  80  is preferably incorporated substantially around the circumference on the outside surface  30  of the stent  10 . 
     As illustrated again in FIG. 9, the inner lumen  90  of the removable stent  10  has a cross-sectional diameter, along its x-x, or its y-y axis, which dimensions can vary depending on use. FIG. 9 shows a cross-sectional view of a removable stent  10  when expanded. The diameter of the inner lumen  90  can vary between a relatively larger diameter such as when the stent is deployed, and a significantly smaller diameter. Therefore, when the proximal end  22  of the stent is collapsed, through constriction of the lasso  80 , the inner lumen will have a reduced cross-sectional diameter allowing for its extraction from an intralumenal site (see FIG.  11 ). This aids the physician in removing the stent from its target site with minimal damage or trauma to the surrounding tissue. 
     FIGS. 10 &amp; 11 shows views from internal perspectives (inside viewing outward) of a removable stent  10  during collapse and removal of the stent  10 . The lasso  80  type of collapsing element functions by radial contraction, which collapses a removable stent  10  from an expanded diameter (FIG. 10) to a reduced diameter (FIG.  11 ). The loop region  84  of the lasso  80  protrudes into the inner lumen of the removable stent  10 , thereby allowing the physician to grasp the lasso  80  by a hook element  102  disposed at the end of the removal tool  101  (FIG.  10 ). Constriction of the proximal end  22  of the stent  10  is achieved by rotating the removal tool  101 , which causes the lasso to twist upon itself, thus also collapsing the stent  10 . Alternatively, the lasso can be designed to include internal elements such as a ball, a hook, or a loop that assists in locating as well as operating the lasso. The lasso  80 , therefore, constricts or collapses the proximal end  22  of the removable stent  10  by radial tightening  88  of the lasso  80  upon itself. A lasso type of collapsing element can be constructed from permanent materials (stainless steel, or metal wire, for example) or from temporary degradable materials. 
     Referring to FIGS. 12-15, in a further embodiment of the present invention, the collapsing element of a removable stent  10  need not extend over the entire circumference of the removable stent  10  as a lasso or a noose, but can be a collapsing element that is a discontinuous type of lasso. Referring to FIG. 12, the collapsing element includes a discontinuous lasso  120  having a first end  122  and a second end  124 . The first end  122  is attached or anchored to the proximal end  22  of the removable stent  10 . This attachment can be permanent if desired. The second end  124  of the collapsing element or discontinuous lasso  120  is not attached to the stent, but is freely associated with the stent  10 . This allows for the free expansion of the stent as needed in vivo. At least a portion of the discontinuous lasso between its first and second ends is juxtaposed, adjacent or interwoven to the outer surface  30  of the stent  10 . 
     A discontinuous lasso  120  can be a wire  128  that is woven in between the wires  26 ,  28  forming the removable stent  10 . The discontinuous lasso  120  is connected to the removable stent  10  at a diamond  140  that is formed by the weaving of the wires  142  comprising the removable stent  10 . The second end  124  of the discontinuous lasso  120  includes a ball  126  which protrudes into the lumen  32  of the stent  10 , thereby allowing the user access to the discontinuous lasso  120  by insertion of a removal tool  150  into the lumen of the stent  10 . The diameter of the ball  124  is slightly larger than the diagonal diameter of the diamond  140 , thereby maintaining the ball  126  within the inner lumen  32  of the stent  10 . When the removable stent  10  is expanded, the ball  126  is seated partly within a diamond  140 . The removable stent  10  can, therefore, freely expand to any given cross-sectional diameter without being restricted by its collapsing element. The design of a discontinuous lasso type of constricting element allows for a freely associated second end  124  which does not restrict expansion of the removable stent  10 . 
     Referring to FIG. 14 &amp; 15, in order to collapse and remove a stent having a discontinuous lasso type of collapsing element  120 , a removal tool  150  is used. An example of such a tool is shown in FIG.  14 . The removal tool  150  can be configured as a cylindrically shaped tube having a slot  152  disposed at its end. The removal tool should be sufficiently rigid so as to allow the user to easily maneuver and engage the ball  126  of the collapsing element  122  into the slot  152 . The slot  152  is slid underneath the ball  126  of the collapsing element  120  (FIG.  14 ), thereby lifting the ball  126  out of its seating within the stent  10 . Once the ball  126  is lifted, the removal tool  150  is twisted, wrapping the wire of the discontinuous lasso  128  around the outer surface of the removal tool  150 . Because the pulling force of the discontinuous lasso  120  and the holding force of the removable stent  10  are at the tip  154  of the removal tool  150  (FIG. 14) the proximal end  22  of the stent  10  is easily collapsed (FIG.  15 ), enabling the physician to pull or withdraw the collapsed stent away from the implanted site in vivo. 
     In another embodiment of the invention, shown in FIGS. 16 to  21 , the removable stent  10  includes one or more eyelets or loops  170  formed at or near the ends of the elongate elements or wires  172  of the stent  10 . The loops  170  can be shaped either before or after the wires  172  are assembled into the mesh-structure that forms the stent  10 . The shape of the loops  170  can include “p”-shaped, helical, twisted, oval, circular, diamond, square or any other similar configuration that forms a hole capable of receiving and/or capturing a cord-like member, such as a lasso  80 . The design and alignment of the loops  170  at the ends of the wire elements  172  are configured to prevent the loops  170  and/or lasso  80  from projecting or extending into the interior of the stent  10 . This, in turn, reduces the potential for encrustation or clot formation within the lumen of the implanted stent  10 . 
     In one embodiment, at least one end of the wire element  172  is twisted or wound into a loop  170 . Although the material characteristics of the wire element  172  maintain the end of the wire element  172  in a loop configuration, a resistance laser weld, crimp or other connection can be made at the location on the loop  170  where the wire element  172  crosses over itself. The resistance weld can be used to further secure the end of the wire element  172  in a permanent loop arrangement. 
     In an alternate embodiment, the elongate element  172  can be laser-cut, stamped or punched from a sheet of material. As shown in FIG. 17, at least one loop  170  is formed at an end of the stamped element  172 . Other embodiments of forming the loop  170 , though not specifically described herein, are also included within the scope of the claimed invention. 
     Referring to FIG. 16, the mesh-structure of the stent  10  is formed from two sets or groups of parallel wires  172 . The first set of parallel wires  174  is placed at approximately a 90 degree angle with respect to the second set of parallel wires  176 , forming a diamond-shaped pattern  178  of wire elements  172 . However, the actual placement of the two sets of parallel wires  174 ,  176  may vary within the range of 10 degrees to 170 degrees. As shown in FIG. 16, a loop  170  is formed at or near an end of every other wire  172  of the first set of parallel wires  174 . The configuration of the first set of wires  174 , in particular the placement of the wires  172  so that each wire end rests on the external surface of a wire element  172  (whereby the external surface of the wire element  172  corresponds to the outside surface  82  of the stent  10 , not shown) from the second set of wires  176 , further enhances the constriction characteristics of the stent  10 . In addition, this configuration also prevents potential flaring of the distal end of the stent  10  as the proximal end  22  of the stent  10  is constricted or collapsed during stent removal. Flaring of the distal end of the stent  10  not only impedes stent removal, but also causes the wire ends to anchor or embed into the wall tissue of the lumen. Thus, the loop design at the ends of the wire elements  172  also mitigates potential tissue trauma associated with the stent removal procedure. 
     In another embodiment of the invention, shown in FIG. 18, a total of at least three loops  170  are formed at the ends of the wires  172  near the proximal end  22  the stent  10 . In general, the loops  170  are formed at the ends of the wires  172  and a lasso-type element is woven through the loops  170 . Placement of the loops  170  at the ends of the wires  172  improves user accessibility to the lasso for facilitated stent  10  removal. In another embodiment, the loops  170  can be formed anywhere within the region approximately two diamonds  178  distal to or 10 mm or 5 mm (0.39 inches or 0.020 inches) from the ends of the wire elements  172 . 
     As shown in FIGS. 19 and 20, the lasso  80  type collapsing element is woven or threaded through the loops  170  of the stent  10 . In one embodiment, the lasso  80  type collapsing element is a flexible closed-loop or ring-like structure. When the stent  10  is in an expanded configuration, portions of the lasso  80  extending between each loop  170  are easily accessible for engagement from the end of the stent  10  in vivo using a standard alligator gripper or custom removal tool  178 . As such, the stent  10  is removed from the lumen of the patient by twisting the removal tool  178  so that the lasso  80  wraps around the outer surface of the removal tool  178  or pulling the lasso axially. Because the pulling force of the lasso  80  and the holding force of the removable stent  10  are at the tip of the removal tool  178 , the proximal end  22  of the stent  10  is easily collapsed, enabling the physician to pull or withdraw the collapsed stent  10  away from the implanted site in vivo. Referring to FIG. 21, during constriction of the proximal end  22  of the stent  10 , the lasso  80  is configured so that it acquires a folded profile. The folded configuration of the lasso  80  prevents portions of the lasso  80  from hanging into the lumen and causing associated blockages or clot formations. As such, any slack or folds in the lasso  80  is generally held or captured between the external surface of the stent  10  and the tissue wall of the patient&#39;s lumen. 
     In an alternate embodiment, the collapsing element of the removable stent  10  need not extend over the entire circumference of the removable stent  10  as a lasso or a noose does. As shown in FIGS. 22 and 23, the collapsing element can be a discontinuous spiral  180  or c-shaped  182  clip member. In general, the spiral  180  or c-shaped  182  clip member can be fabricated from a rigid or semi-rigid material. As with the elongate elements  172  of the stent  10 , the clip member  180 ,  182  can be fabricated from a variety of materials including, but not limited to, laser cut, woven, braided, or stamped. Various other material types and configurations may also be used provided that the type of material or structure is biocompatible and of sufficient rigidity so as to support constriction of the stent  10 . Examples of suitable biocompatible materials include, but are not limited to, stainless steel, alloys, composite materials, plastics, or other non-metallic materials such as an elastic polymer or rubber, medical-grade nylon or polyester. 
     In one embodiment, the spiral or c-shaped member  180 ,  182  is attached or clipped onto the external surface of the stent  10 . To constrict the stent  10 , a removal tool is maneuvered between the external surface of the stent  10  and the lumen of the patient and engages/grasps the ends  184  of the clip member  180 ,  182 . Using the tool, the ends  184  of the clip member  180 ,  182  are then moved toward each other and/or, depending on the desired degree of constriction of the stent  10 , past each other. Radial constriction of the clip member  180 ,  182  causes inward collapse or radial contraction the stent  10 . In an alternate embodiment, one end of the clip member  180 ,  182  is attached or secured to the stent  10  (not shown). As such, the removal tool need only grasp or engage the unattached end of the clip member  180 ,  182 , moving the free end of the clip member  180 ,  182  toward and/or past the secured end. As with the previous embodiment, constriction of the clip member produces radial contraction of the stent  10 , without causing the stent  10  to rotate within the lumen of the patient. 
     Due to the dynamic nature of living tissue, ingrowth can occur around an implanted stent. As a result, it is sometimes necessary for the physician to resect or cut the implanted stent away from the surrounding tissue. As such, heated methods of resection are often used to cut the stent out. Therefore, a lasso type collapsing element  120  can be a wire constructed of a material that is resistant to heat during resection, such as a flexible carbon fiber substance. In addition, the discontinuous lasso  120  can be coated with an anti-adhesive substance such as heparin, or other pharmaceutical or chemical agent which aids in preventing adherence of tissue to the collapsing element. 
     As disclosed above, a collapsing element can be configured in a number of ways and is preferably designed so as to allow the user to grasp the stent and collapse the proximal end of the stent. It should be noted, however, that also included in the present invention is a removable stent having a collapsing element whose location is not at or restricted to the proximal end of the stent. For example, a collapsing element can be disposed anywhere along the length of the stent so long as the collapsing element is designed so as to be capable of collapsing an end of the stent. This allows for removal of the stent from an intraluminal site with minimal damage to adjacent tissue or intraluminal wall of the target site. 
     A variety of tools or devices can be used to grasp the collapsing element of a removable stent. In addition to the removal tool as described above, stone or basket extractors or grasping forceps known in the art can be used with the present invention. For example, with a removable stent having a hinged hook type of collapsing element, a physician can use a basket extractor to engage and grasp the hinged hooks. Using a stone extractor, for example, the physician will first position the hooks into their upright position (while viewing the deployed stent through a cystoscope). Once upright, the physician then engages the wires of the extractor with the hooks, thereby grasping the stent by the collapsing elements. By collapsing the wires of the extractor, the physician also pulls on the collapsing elements, thereby collapsing the proximal end of the stent. Once collapsed, the stent can then be pulled into the sheath of the cytoscope. 
     An additional aspect of the present invention comprises a removal attachment or grasper for use with a delivery tool to engage or grasp the removable stent. FIG. 24 shows a view of a removal attachment  200  that can be used with a removal tool. Alternatively, such an attachment can be attached or used with a stent delivery tool. Removal tools suitable for use with a removal attachment include those described herein as well as others known in the art, such as the Urolume Delivery System. Stent delivery tools suitable for use with a removal attachment include the Urolume Delivery System. 
     The removal attachment includes a base portion  202  that is tubular in shape. The base portion  200  has a front end  208  and a back end  210 . The base portion  202  includes an element  204  disposed at the back end  210  which locks the removal attachment  200  to a delivery tool such as a grapple. The front end  208  of the removal attachment  200  includes prong(s)  212  that are configured or shaped as curved finger-like projections that extend outward or away from the removal attachment  200 . The prong(s)  212  can be manipulated so as to grasp or hook a collapsing element of a removable stent. One aspect of the removal attachment that is particularly advantageous to achieving the goals of the invention is that it is designed to be adaptable with a wide variety of tools used in the art for delivery and/or removal of a stent. 
     Method for Extraction of a removable Stent 
     The methods and devices of the present invention provide simple, accurate and stable removal of a stent or prosthesis from an intraluminal or other, site in vivo. The features of the invention, as described herein, provide a removable stent and system that is reliable and less awkward or cumbersome for the physician to use. 
     The present invention can be used for a variety of medical treatments where removal of a stent from a patient is or may be desirable. For example, in the treatment of an enlarged prostate gland, a stent is often placed in the patient&#39;s urethra, intraluminally at the site of compression by the enlarged prostate. The deployed stent is often intended as a permanent means of providing support and radial expansion to the constricted urethra so that urine flow from the bladder through the compressed site is remedied. However, failure of the implanted stent to function properly can occur. For example, over time, tissue ingrowth from the urethral wall through the openings of the stent wall can occur, resulting in reobstruction or restenosis of the lumen. The present invention is particularly advantageous in that it has elements allowing for its easy access, and removal in vivo should the need arise to remove or explant the stent after deployment. Therefore, the present invention can function as a permanently implanted stent or as a temporary or removable stent in vivo. Use of the present invention in medical indications requiring stent treatment adds a precautionary measure not provided in permanent stent implants or prostheses. 
     Depending upon the location of the deployed stent and/or the physician&#39;s preference, a removable stent can be extracted using a removal tool and withdrawn into a catheter or a sheath of a viewing instrument. For example, as shown in FIG. 10, with a removable stent  10  having a lasso type of collapsing element  80 , a removal tool  101  having a hooked end  102  is first inserted into a patient&#39;s urethra through a viewing instrument, up to the site of the deployed stent. Once at the stent, the physician engages the hooked end of the removal tool  101  with the loop  84  portion of the collapsing element  80 . The physician maneuvers by twisting or rotating the removal tool so as to cause the lasso  80  to constrict upon itself Tightening of the lasso  80  around the stent  10  collapses the proximal end  22  of the removable stent  10  (FIG.  11 ). The collapsed end of the removable stent is then easily aimed into the lumen of an extraction catheter or the sheath of a cystoscope. Once inside the lumen, the stent  10  is extracted by further pulling on the lasso  80 , which is coupled to the stent. The remainder of the stent  10  is then pulled away from and out of its in vivo site. 
     Alternatively, as described previously, where a stent having an alternative collapsing element configuration is to be removed, there are a variety of commercially available devices that can be used as the removal tool. If the deployed stent comprises a swinging hook type of collapsing element, a removal tool such as a loop snare, wire basket stone extractor, stone forceps, or other device can also be used to grasp and withdraw the deployed stent. These types of devices are known in the art, available commercially, and described, for example, in U.S. Pat. No. 5,330,482. A basket retrieval device can be used to catch or engage the stent from its internal lumen. As described previously, when deployed in a target site, a removable stent can have a hinged hook collapsing element in a down position so as not to impede the fluid flow. In order to collapse the proximal end of the stent, the physician can position the hook(s) into an upright or up position. The resistive characteristics of the hinge portion of the hook maintains the collapsing element in the up position, allowing the physician to manipulate the wires of the basket retrieval device until they engage with the hooks. Retraction of the basket pulls on the collapsing element, which collapses the proximal end of the stent. The stent is withdrawn and removed from the patient. 
     The systems and methods of the present invention provide accurate, easy to use and stable grasping of a stent allowing for its safe removal from a target site in vivo. The features of the invention, as described herein, also provide a removable stent that is less awkward or cumbersome for the physician to use. 
     Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof. 
     All publications and patent applications in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated by reference.

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