Abstract:
The disclosure relates to retrievable filter devices implantable within a lumen. The retrievable filters include retractable anchoring barbs. Associated apparatus and methods for delivering, retrieving, and/or repositioning the filter are also disclosed herein.

Description:
BACKGROUND 
       [0001]    Blood clot filters are used in combination with other thrombolytic agents to treat pulmonary embolism occurring within a patient. Such devices are generally inserted intravenously into a target location of the body (e.g. an artery or vein), and function by capturing blood clots (emboli) contained in the blood stream before they can reach the heart and/or lungs and cause permanent damage to the body. In the treatment of Deep Vein Thrombosis (DVT), for example, such filters are placed in the inferior vena cava to prevent further blood clotting in the large veins of the lower body. Placement of the filter is typically accomplished percutaneously via the femoral arteries or the jugular vein using a local anesthetic, or by performing a laparotomy with the patient under general anesthesia. 
         [0002]    In certain designs, an introducer sheath may be used to deliver the blood clot filter through the body. Such introducer sheaths are generally tubular in shape, and include an inner lumen configured to transport the blood clot filter in a collapsed position through the body. Once transported to a desired location within the vasculature, the filter can then be removed from within the introducer sheath, allowing the filter to spring open, or to be expanded, and engage the vessel wall. A needle, hook, barb, prong, wedge or other attachment means disposed on the blood clot filter can be used to secure the filter to the vessel wall. 
         [0003]    There are a number of situations in which it may be desirable for a physician to remove the filter once inserted within the body. In certain circumstances, for example, the risk of pulmonary embolism may be relatively short term (e.g. about two weeks), thus requiring insertion of the filter for only a short period of time. Permanent implantation of the filter in such cases may unnecessarily impede the flow of blood within the vessel, and can lead to further thrombosis growth at the filter implantation site. In other circumstances, it may be desirable to reposition the filter within the vessel, or to replace the existing filter with a new filter. 
       SUMMARY 
       [0004]    This disclosure pertains generally to retrievable filter devices implantable within a lumen. Associated deployment/retrieval apparatus and methods for retrieving and/or repositioning the filter device within the body are also disclosed herein. 
         [0005]    In some embodiments, this disclosure relates to an implantable filter having a first generally apical hub, a first plurality of solid legs each having a proximal end, a distal end region, and an intermediate region therebetween, said first plurality of solid legs being fixedly attached to the first generally apical hub and having a first configuration in which the solid legs are substantially parallel and a second configuration in which the distal ends of at least some the first plurality of solid legs are radially expanded relative to a central axis passing through the first generally apical hub; and a second plurality of wires each having distal ends and proximal ends, said wires of the second plurality of wires being fixedly attached at their proximal ends to a second generally apical hub which is slidably moveable relative to the first generally apical hub, wherein each of the second plurality of wires is individually associated with one of the solid legs of the first plurality of solid legs at the intermediate region thereof and distal end region thereof, further wherein the association of each wire with the solid leg at the distal end region thereof comprises the wire passing transversely through the distal end region of the solid leg such that each wire has a first position in which the distal end of the wire protrudes radially outward from the distal end region of the solid leg and a second position in which the wire does not protrude significantly radially outward from the distal end region of the solid leg. 
         [0006]    In other embodiments, this disclosure relates to an implantable filter comprising a generally apical hub; a plurality of legs each having a proximal end, a distal end region, and an intermediate region therebetween, said legs being fixedly attached to the generally apical hub and having a first configuration in which the legs are substantially parallel and a second configuration in which the distal ends of at least some the plurality of legs are radially expanded relative to a central axis passing through the apical hub; a pair of first struts attached to a first leg and a second leg of the plurality of legs within the respective intermediate regions of the first and second legs and extending to a first join proximal to their distal ends; a pair of second struts attached to a first leg and a second leg within the respective intermediate regions of the first and second legs and extending to a second join distal to their proximal ends; and a wire extending from the first join and slidingly received through an aperture associated with the second join, wherein upon radial expansion of the first and second leg to the second configuration the first join and the second join are configured to move generally toward one another whereby the wire protrudes from the second join in a direction radially outward from the central axis passing through the apical hub. 
         [0007]    In another aspect, this disclosure relates to method of deploying a filter comprising positioning the filter within a distal region of a catheter; positioning the distal region of the catheter within a lumen containing a fluid to be filtered; ejecting the filter from the catheter thereby allowing at least some the distal ends of the plurality of solid legs to become radially expanded relative to a central axis passing through the apical hub; and implanting the distal ends of the plurality of wires in a wall of the lumen containing a fluid to be filtered. 
         [0008]    In yet another aspect, this disclosure relates to a method of retrieving a filter comprising positioning a distal region of a catheter proximally adjacent to a first generally apical hub of the filter; moving the first generally apical hub of the filter axially relative to a second generally apical hub, thereby withdrawing a plurality of wires relative to a plurality of solid legs thereby moving the plurality of wires from first position in which the wires protrude significantly radially outward from the distal end region of the solid legs to a second position in which the distal ends of the wires do not significantly protrude radially outward from the distal end region of the solid legs thereby at least partially withdrawing the wire from the wall of the lumen; withdrawing the filter to a position at least partially within a distal region of the catheter; and withdrawing the catheter and the filter of claim  1  from the lumen. 
         [0009]    Although described herein in the context of a temporary vena cava filter, it will be understood that the anchorage system disclosed may be used to provide permanent or temporary anchorage for a variety of endoluminal devices which are not necessarily limited to blood vessels. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  illustrates an embodiment of the implantable filter. 
           [0011]      FIG. 1A  illustrates a detail of the implantable filter rotated 90 degrees. 
           [0012]      FIG. 2  illustrates the implantable filter of  FIG. 1  in a second position. 
           [0013]      FIGS. 3-6 ,  7 ,  7 A,  8  and  8 A illustrate a number of alternate configurations of the distal ends of the legs of the implantable filter of  FIGS. 1 and 2 . 
           [0014]      FIGS. 9-12  illustrate alternate configurations of the intermediate region of the legs of the implantable filter of  FIGS. 1 and 2 . 
           [0015]      FIG. 13  illustrates an alternate configuration of a leg of the implantable filter of  FIGS. 1 and 2 . 
           [0016]      FIG. 14  illustrates an embodiment of the implantable filter. 
           [0017]      FIG. 15A  illustrates a fragmentary representation of an embodiment of the implantable filter in a first configuration. 
           [0018]      FIG. 15B  illustrates the implantable filter of  FIG. 15A  in a second configuration. 
           [0019]      FIG. 16  is a detail of an implantable filter delivery and retrieval system. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The drawings, which are not necessarily to scale, are not intended to limit the scope of the claimed invention. The detailed description and drawings illustrate example embodiments of the claimed invention. 
         [0021]    All numbers are herein assumed to be modified by the term “about.” The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
         [0022]    As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include the plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0023]    It is noted that references in the specification to “an embodiment”, “some embodiments”, “other embodiments”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described, unless cleared stated to the contrary. 
         [0024]      FIG. 1  illustrates an embodiment of an implantable filter  10  having a plurality of filter legs  300  attached to a first generally apical hub  100 . As used herein, the terms “leg” and “legs” are used to indicate a predominantly solid shaft having a high length to width ratio. Although four filter legs  300  are illustrated, it will be appreciated that 3, 5, 6, 7, 8 or even more legs may be employed in each of the filters  10  described. The filter legs  300  have been illustrated as generally straight; however it will be appreciated that the filter legs  300  may be formed in any shape commonly employed for that purpose. For example, they may optionally be curved, include offset portions, and the like. Filter legs  300  may have any of a variety of cross-sectional shapes, such as circular, oval, rectangular, polygonal, and the like. The cross-sectional shape may vary along the filter leg  300  and shape transitions, if present, between cross-sectional shapes may be gradual or step-wise. Although the filter legs  300  have been illustrated as being of substantially equal length, it will be appreciated that filter legs  300  of somewhat different length may provide a benefit related to minimizing the diameter within which the implantable filter  10  may be compressed for delivery. This may be particularly the case if at least some of the filter legs  300  are provided with enlarged distal ends as discussed below. In other embodiments, filter legs  300  of somewhat different lengths may provide a benefit related to positioning the filter asymmetrically within the lumen. 
         [0025]    The filter legs  300  may be fabricated separately and attached to a separate first generally apical hub  100  or the first generally apical hub  100  and filter legs  300  may be formed as an integral unit from a hypotube by laser cutting, etching, and the like. The filter legs  300  and the first apical hub  100  may be formed from materials commonly used for that purpose such as nickel titanium alloy, stainless steel, biocompatible polymers, and the like. The filter legs  300  may optionally be biased to self-expand upon deployment or may be actuated to expand by mechanical or thermal means. 
         [0026]    In some embodiments, the first generally apical hub  100  includes a hook  110 , illustrated in  FIG. 1A , which facilitates deployment and/or retrieval of the implantable filter. In other embodiments, hook  110  is replaced by an alternate structure which may be grasped, pushed, pulled, twisted, or otherwise manipulated during deployment and/or retrieval of the implantable filter. In either event, the first generally apical hub may be moved axially relative to a second generally apical hub  200 . The relative movement of the two generally apical hubs may be limited by suitable structures, as herein illustrated by the non-limiting example of aperture  210  and pin  120 . The second generally apical hub  200  has attached thereto a plurality of wires  400  having distal ends  410  adapted to at least partially penetrate the walls of a vessel in which the implantable filter is deployed. Although the discussion to follow will focus on an implantable filter which may be deployed in a blood vessel, it will be appreciated that such filters may be implanted in lumens of other descriptions, such as catheters, ureters, and the like. 
         [0027]    Each wire  400  is each disposed generally along and somewhat parallel to one of the plurality of filter legs  300  and passes from one side of the leg to the other before passing through a distal opening  320 . In some embodiments, a wire  400  may pass transversely through or alongside its associated leg two or more times between the first generally apical hub and the distal end of the leg. For example, a wire  400  may pass from a position radially outside of the leg through an aperture  310  in an intermediate region of said leg  300  and thence along the leg  300  to and through a second aperture  320  to extend radially outward through the leg  300  forming an anchoring barb  410  as illustrated in  FIG. 2 . In other embodiments, the wire  400  may initially be located radially inward from the associated leg  300  and pass twice through or alongside the leg  300  before passing again radially outward through the leg to form anchoring barb  410 . Alternate embodiments of the anchoring barb  410  will be discussed below. As used herein, the terms “wire” and “wires” are used to indicate a predominantly solid shaft having a high length to width ratio. Wires  400  may have any of a variety of cross-sectional shapes, such as circular, oval, rectangular, polygonal, and the like. The cross-sectional shape may vary along the wire  400  and transitions, if present, between cross-sectional shapes may be gradual or step-wise. In some embodiments, the cross-sectional shape of the wire  400  may be selected to interact with the shape of one or both of the distal opening  320  and aperture(s)  310  of the intermediate region to direct the longitudinal motion of the wire  400  relative to the associated leg  300  in a desired direction such as toward or away from a generally parallel orientation or to impart a torque to the wire  400 . 
         [0028]    The term “wire” is to be interpreted as including structures comprising two or more components which, when joined, provide the features of a wire  400  as described herein. Wires  400  and the second apical hub  200  may comprise materials commonly used for those purposes such as nickel titanium alloy, stainless steel, biocompatible polymers, and the like. In many embodiments the distal tip  410  may be formed from a material which minimizes the adhesion of the wire tip  410  to the lumen wall. The adhesion minimizing material may be either the material from which the wire tip  410  is formed or a coating (not shown) thereon. 
         [0029]    Axial movement of the first generally apical hub  100  relative to the second generally apical hub  200  may cause the wire  400  and distal tip  410  to retract relative to the associated leg  300  thereby withdrawing the distal tip  410  from the wall of the lumen in which the filter is deployed. In some embodiments, the distal tip  410  will be withdrawn from the lumen wall along the axis of the wire tip  410  thereby minimizing the extraction force required. In other embodiments, the distal tip  410  may be adapted to pivot to withdraw from the lumen wall. In yet other embodiments, the distal tip  410  may both withdraw axially while pivoting to a degree. In some embodiments, the leg  300  may include an enlarged region which may serve as a landing pad to limit the penetration of the wire tip  410  and to reduce the likelihood of the distal end of the filter legs  300  becoming engulfed by tissue overgrowth or other deposits. 
         [0030]    It will be appreciated that the filter  10  of these embodiments may be deployed with the wire tip(s)  410  in either the first extended position or the second retracted position. If the filter  10  is deployed with wire tips  410  in the first position, the outward expansion of the filter legs  300  may serve to anchor the wire tip  410  in the lumen wall. In the alternative, the filter  10  may be deployed in the second retracted position and the wire tip(s)  410  may subsequently be extended to engage the lumen wall by moving first generally apical hub  100  relative to second generally apical hub  200 . If or when it becomes desirable to remove the filter  10  from engagement with the lumen wall, either permanently or for repositioning, the wire tip(s)  410  may subsequently be retracted to disengage from the lumen wall by moving first generally apical hub  100  relative to second generally apical hub  200  in the opposite sense. 
         [0031]    A number of non-limiting configurations of distal ends of filter legs  300  are illustrated in  FIGS. 3-6 ,  7 ,  7 A,  8  and  8 A. In some embodiments, the leg ends depicted in  FIGS. 3-6 ,  7 ,  7 A,  8  and  8 A may be formed as linear extensions of the filter legs  300 . In other embodiments, the leg ends may be bent to lie approximately parallel to the vessel wall in a deployed configuration. 
         [0032]      FIG. 3  illustrates a filter leg  300  including simple enlarged landing pad defining a through aperture  320 .  FIG. 4  illustrates an embodiment in which the filter legs  300  are formed from wires and the distal end of leg  300  has been formed into a loop which defines aperture  320 . The loop may be open or closed.  FIG. 5  illustrates a filter leg  300  in which the distal end of the leg  300  includes a slot-like aperture  320  which, depending on the motion imparted by the relative movement of the first and second hubs  100 ,  200  may be open or may be somewhat narrower at the distal end. If, for example, the distal tip  410  of wire  400  includes a curved portion which curves radially outward and somewhat proximally, the resulting forces may tend to hold the wire tip  410  within the slot near its proximal end. Partial closure of the distal end of slot-like aperture  320 , may prevent the distal wire tip  410  from leaving the slot-like aperture  320 , during manipulation. Upon withdrawal of the filter  10  from the lumen in which it is disposed, the wire tip  410  may move somewhat toward the distal end of the slot as it is withdrawn from the lumen wall.  FIG. 6  illustrates a filter leg  300  in which the leg  300  maintains a generally uniform width in the distal region. 
         [0033]      FIGS. 7 and 7A  illustrate a multi-component wire  400  end in which the distal tip  410  assumes the form of a pivoting bar which optionally may include a sharpened edge to facilitate extraction of the distal tip  410  from the tissue in which it is embedded during deployment. 
         [0034]      FIGS. 8 and 8A  illustrate an alternate multi-component wire  400  end in which the distal tip  410  is a generally straight barb which includes one or more elements  420  which serve to prevent the distal wire tip  410  from extending excessively from the filter leg  300 . In some embodiments, the element(s)  420  may resist an outward force provided by the configuration of wire  400  by bearing against the inner surface of the leg  300  while still allowing the distal tip  410  to pass through aperture  320  to engage the lumen wall. 
         [0035]      FIGS. 9-12  illustrate representative non-limiting configurations of the intermediate region of the legs of the implantable filter of  FIGS. 1 and 2 .  FIG. 9  illustrates a simple aperture  310  through filter leg  300 . The aperture  310 , like those described below, may be repeated within the intermediate region of the filter leg  300 . Although the aperture  310  is depicted as round, it may assume any convenient shape. As mentioned above, the shape may be selected to interact with the shape of the wire  400  which passes therethrough.  FIG. 10  represents a portion of a leg  300  which has been twisted into a helical arrangement which allows the wire  400  to pass through the center of the helix. In such embodiments, the wire may be viewed as passing from one side of the leg  300  to the second side of the leg  300  and back again depending on the number of turns of the helix. It will be appreciated that a similar guiding effect may be attained by arranging for the wire  400  to twine around the leg  300  in the intermediate region of the leg  300 . Similarly, when the leg  300  is formed of wire, the wire leg  300  may twine around the wire  400  in the intermediate region.  FIG. 11  illustrates an aperture  310  which penetrates the leg  300  at an acute angle to direct the wire  400  along a path which is more nearly parallel to the surface of the leg  300  in the intermediate region.  FIG. 12  illustrates an alternate arrangement in which the wire  400  passes the filter leg  300  through a short tubular element  312  which is fixedly attached to the side of the leg  300 . Tubular element  312  provides the aperture  310  which controls the passage of wire  400  through the intermediate region of the leg. In some embodiments, the tubular element  312  may lie generally parallel to filter leg  300  and, for the purposes of this description, this arrangement will be considered to provide a transverse passage relative to the filter leg  300 . Such a passage may be viewed as extending from the inside of the filter leg  300  to the outside or from the outside of filter leg  300  to the inside. 
         [0036]      FIG. 13  illustrates a filter leg  300  having a zigzag configuration which is generally believed to have superior filtering capability. In this embodiment, multiple tubular elements  312  have been disposed along the side of the filter leg  300  to provide multiple apertures  310  disposed therealong. In this embodiment, as discussed above, the tubular elements  312  are to be viewed as providing a transverse passage of wire  400  relative to filter leg  300  and a distal portion  460  of leg  400  may be directed inward prior to passing outward through aperture  320  to engage the lumen wall. 
         [0037]    In other embodiments, the alternating diagonal segments of filter leg  300  may direct the intervening longitudinal segments toward and away from the lumen wall and the tubular elements  312  may be disposed along the diagonal segments to allow the generally straight wire  400  to weave alternately between the inner and outer surface of the leg  300  before passing outward through aperture  320  to engage the lumen wall. In yet other embodiments, one or more of the illustrated tubular elements  312  may be omitted and the leg  300  may be viewed as twining about wire  400  to create an extended intermediate region. 
         [0038]      FIG. 14  illustrates a filter of  FIGS. 1 and 2  to which additional centering struts  340  have been added. In some embodiments, centering struts  340  may be provided with barbs or with a set of wires (not shown) which function in the manner of the wires  400  associated with filter legs  300 . 
         [0039]      FIGS. 15A and 15B  illustrate a portion of an embodiment in which the struts  600 ,  602 ,  604 , and  606  provide the guiding function of the apertures  310  described above and the join of struts  604  and  606  optionally are somewhat extended at their join to provide a landing pad. Wire  700  and its distal end  710  is slidingly received at the join between the ends of legs  604  and  606 . In these embodiments, each pair of legs  500  may include therebetween struts  600 ,  602 ,  604 , and  606  or the struts  600 ,  602 ,  604 , and  606  may be disposed between only some pairs of adjacent legs  500 . Only a portion of one pair of legs  500  is shown in the figures to reduce clutter. In these embodiments, wires  700  are attached to the joins between struts  600  and  602  which serve collectively to provide the second apical head of the earlier embodiments. The remaining ends of struts  600  and  604 , as well as struts  602  and  606 , are attached to filter legs  500  in an intermediate region thereof. The paired struts  600  and  604 , as well as struts  602  and  606 , may be joined to legs  500  at a single point within the intermediate region or they may be spaced somewhat apart. The joins between the struts and between the struts and the legs are configured to provide a degree of flexure within the plane of a group of struts  600 ,  602 ,  604 , and  606 . This arrangement causes the join between struts  600 ,  602  and wire  700  to be spaced farther from the join between struts  602  and  606  when legs  500  are in a first configuration in which they are substantially parallel and to be spaced closer together in a second configuration in which at least some of the plurality of legs  500  are radially expanded relative to a central axis passing through the first generally apical hub (not shown). This in turn causes the distal end  710  of wire  700  to extend from the join of struts  604 ,  606  to create an anchoring distal tip  710  which may engage the lumen wall within which the filter is disposed. In some embodiments, the distal tip  710  may curve outward from the plane defined by the adjacent filter legs  500  and their associated struts  600 ,  602 ,  604 , and  606  when the filter legs  500  are radially expanded relative to a central axis passing through the first generally apical hub (not shown). 
         [0040]    In another aspect, the disclosure relates to an implantable filter system, as partially depicted in  FIG. 16 , comprising for example a filter  10  as discussed above, a catheter  800  sized and adapted at its distal end to contain at least a portion of a collapsed configuration of filter  10 , a releasable actuator  810  adapted to engage a portion of filter  10 , such as hook  110  of the embodiment of  FIGS. 1 and 2 , and a manipulation element  820  adapted to eject the implantable filter  10  from the catheter or to withdraw at least a portion of the implantable filter  10  within the distal region of the catheter  800 . For example, releasable actuator  810  may be adapted to engage hook  110  of first generally apical hub  100  while a manipulation element  820 , tubular element  820  or separate elements  820 , engage the second generally apical hub  200 . Following ejection of filter  10  from catheter  800  by distal motion of tubular element  820 , or separate elements  820 , thereby deploying filter  10  with legs  300  and wires  400  in the second position, as illustrated in  FIG. 16 , the legs  300  may be expanded to contact the wall of the lumen in which the implantable filter  10  is to be deployed. Retraction of releasable actuator  810  relative to manipulation element  820  will cause distal tips  410  of wires  400  to extend from distal openings  320  to engage the lumen wall. In the alternative, the filter  10  may be advanced from catheter  800  with actuator  810  retracted relative to manipulation element  820  thereby allowing the extended distal tips  410  of wires  400  to be extended relative to distal openings  320  in filter legs  300  as the filter  10  deploys to allow the extended distal tips  410  to strike the lumen wall as the legs  300  expand. In either event, releasable actuator  810  may be advanced relative to manipulation element  820 , and optionally twisted, to release hook  110  from releasable actuator  810 , whereupon releasable actuator  810 , manipulation element  820 , and catheter  800  may be removed. 
         [0041]    When it is desirable to remove or reposition implantable filter  10 , catheter  800  may be advanced to a position just proximal of the proximal end of implanted filter  10 . In some embodiments, catheter  800  may include alignment elements (not shown) to ensure alignment of catheter  800  with implanted filter  10 . For example, the distal end of catheter  800  may include radially expansible guides which locate the first apical head. In some embodiments, separate elements of manipulation element  820  may provide that function when extended sufficiently from the distal end of catheter  800 . Releasable actuator  810  may then be advanced to engage hook  110  of the embodiment of  FIGS. 1 and 2  or equivalent engagable structure or structures of other embodiments of the invention. The manipulation element  820  may then be retracted until it engages the second generally apical hub  200  of the embodiment of  FIGS. 1 and 2 , if desired, thereby allowing the second generally apical hub  200  of the embodiment of  FIGS. 1 and 2  to be with drawn relative to releasable actuator  810  to retract wires  400 , and distal tips  410  relative to apertures  320  prior to withdrawing implantable filter  10  at least partially within catheter  800  thereby collapsing the filter legs  300  to a first generally parallel configuration for removal or repositioning. In some embodiments, the movement of first generally apical hub  100  relative to second generally apical hub  200  may partially or completely collapse legs  300  prior to withdrawal of filter  10  to a position within the distal end of catheter  800 . If desired, the deployment steps may be repeated in a new location within the lumen containing the liquid to be filtered. 
         [0042]    Although the illustrative examples described above relate to a filter implantable in a body lumen, additional applications are also contemplated. In such an embodiment, the size of the filter may be adjusted to correspond to the size of the lumen of, for example, a piece of tubing or a pipe. Further, although the exemplary filters discussed above have been illustrated as having configurations in which the legs expand symmetrically to position the filter such that its central axis generally coincides with the central axis of the lumen in which it is deployed, it is to be understood that embodiments in which one or more legs of the filter are disposable generally parallel to and potentially in contact with the lumen wall such that the central axis of the filter lies at an angle to the central axis of the lumen are also contemplated. 
         [0043]    In addition to the elements discussed above, the implantable filters may include other elements commonly employed in such filters. For example, the filter and/or the catheter portion of the system may include one or more radiopaque markers. Similarly, in some embodiments, the filter may include a filter membrane, functional coatings, and the like. 
         [0044]    Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and principles of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth hereinabove. All publications and patents are herein incorporated by reference to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference.