Abstract:
A recoverable thrombosis filter that can be implanted and securely positioned within a vein at a desired location, and can be recovered through an endovenous route even after formation of neointima hyperplasia, is disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of application Ser. No. 08/942,531, filed Oct. 2, 1997, which in turn is a continuation-in-part of application Ser. No. 08/757,827, filed Nov. 27, 1996. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to improved thrombosis filters. More particularly, the invention relates to a thrombosis filter that can be percutaneously installed in a selected body lumen at a selected location in the vascular system and is adapted for trapping thrombosis materials or blood clots. Still more particularly, the invention relates to a thrombosis filter that can be percutaneously removed from the vascular system from a single direction.  
         BACKGROUND OF THE INVENTION  
         [0003]    Pulmonary embolism is a recognized medical emergency, and may be caused by venous thrombosis. The venous thrombosis may be caused by blood flow retention, venous intima damage, or coagulation abnormalities. Recognized treatments include administration of anti-coagulant medication therapy, thrombolytic therapy, thrombectomy, and inferior vena cava thrombosis filtering procedures. When an inferior vena cava thrombosis filtering procedure is selected, it can be accomplished using either a laparotomy procedure under general anesthesia, or percutaneously inserting a thrombosis filter under local anesthetic. A laparotomy procedure is a surgical procedure done under general anesthesia, and is susceptible to thrombosis formation due to discontinuance of anti-coagulant therapy prior to such surgery.  
           [0004]    A recognized option is to intravenously insert a thrombosis filter in the vascular system, and in particular into the inferior vena cava, which requires only a local anesthetic. Percutaneous filter insertion has been recognized as an effecateous procedure since only a local anesthetic is required; however, such thrombosis filters have been recognized to become affixed to the inner vena cava wall or vein wall by neointimal hyperplasia within a relatively short time after implantation. This process can occur within two or three weeks, and in prior art filter arrangements renders the filter unremovable by a single percutaneous process without incurring significant vessel trauma.  
           [0005]    There are a number of thrombosis filters which have been developed with the intent of allowing percutaneous removal. Those prior art thrombosis filters that include substantially linear struts tend to distribute forces along the longitudinal axis of the struts. With the struts deployed outwardly to engage the walls of the lumen, asymmetrical compression of the lumen can cause the struts to be forced together in a manner that causes the struts to do damage to the lumen wall.  
           [0006]    As indicated, there are a number of prior art implantable filters. One example is the filter disclosed in U.S. Pat. No. 4,817,600 issued to James Kay Herms, et al., which describes a set of shaped leg portions that are joined at one end and are arranged at an acute angle to the axis of the filter, and form a generally conical arrangement. The shaped legs include hooks at the extremity for hooking into the vein wall and holding the filter in position against the flow of blood within the lumen. Herms, et al. provided for an improved leg structure that would avoid some of the concerns of the filters that used relatively straight struts, and minimized the damage that could occur to the vessel or lumen arising from tipping or tilting of the filter. It did not, however, describe a structure or method for percutaneously removing the filter.  
           [0007]    U.S. Pat. No. 4,990,156 to J. Lefebvre describes a filter that may be percutaneously inserted for temporary use in determining whether or not a more permanent filtering treatment is necessary. The device describes a non-aggressive contact of the filter elements with the vessel and describes a number of elements that each have sharpened and roughened portions contacting the vessel wall and holding the filter in position. A sheath is provided to allow removal should the filter not be required for permanent usage. Once deployed, the filter is positioned for definitive use and may not thereafter be readily removed.  
           [0008]    U.S. Pat. No. 5,324,304 issued to Erik Rasmussen, describes another form of implantable filter that is self-expandable and can be inserted through use of a catheter which encloses the structure. The anchoring legs are designed to have hooks at the ends for engaging the wall of the vein once deployed. The anchoring elements form part of the filtering structure, and once placed would tend to hook firmly into the vein walls. No structure or method is described for percutaneous removal.  
           [0009]    U.S. Pat. No. 5,370,657 to Toshiyuki Irie describes a recoverable thrombosis filter having a structure wherein the holding mechanism and the filtering mechanism is comprised of a number of opposed elements that are held in place by an intermediate tension member. It recognizes that removal may be desirable, and has described a series of shaped end portions that cooperate with the wall of the vessel, without piercing it deeply. For removal, it is necessary that dual percutaneous procedures be worked from opposite ends of the filter. A pair of hooking devices are engaged from the opposite ends, and the two halves of the filter are stretched apart until the connecting tension member breaks. While the two halves of the filter are drawn within a pair of sheaths for withdrawal, this removal procedure requires that two opposed removal structures be administered to the opposite ends of the filter, and that manipulation of the two removal devices be coordinated to grasp the opposed hooking elements such that the filter can be broken in half and withdrawn. This removal process doubles the risk to the patient, and due to the small size of the elements, is relatively difficult to accomplish.  
           [0010]    The foregoing described prior art is illustrative of various types of filter structures and handling devices that are known for use in placing and removing thrombosis filters. The prior art structures do not describe filter structures that are readily removable utilizing a single percutaneous removal procedure.  
           [0011]    To address the deficiencies in the prior art, the present invention was developed to provide an improved recoverable thrombosis filter that can be removed through a percutaneous procedure even after having been in place for such time as to have had neointimal hyperplasia to have fully developed. Through the use of a unique holding structure and a filtering portion of the thrombosis filter, the filter is structured such that the holding portion can be collapsed from one end through external manipulation and the entire filter drawn within an enclosing structure for removal. These and other more detailed specific objectives of the invention will become apparent to those skilled in the art from consideration of the drawings and the description of the preferred embodiments.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention comprises a recoverable thrombosis filter that is recoverable by a single recovery procedure. It includes a plurality of thrombosis filtering elements that are shaped in a predetermined manner and which are joined at one end and are deployed about a longitudinal axis to form a generally conical structure. The filtering elements include shaped ends for engaging an inner lumen wall. A plurality of positioning struts are joined at one end and are deployed in an opposite direction around the longitudinal axis. The positioning struts include wall engaging ends that include projections for engaging the inner wall of the lumen to prevent motion of the filter structure in the direction of deployment of the positioning struts. The anchoring device of the present application can be used with other devices such as stents, stent grafts, vaso-occlusive particles, vascular closure devices, filters and the like.  
           [0013]    A recovery mechanism including retracting structure is percutaneously inserted to the vicinity of the filter. The recovery mechanism includes an extensible gripping device, an actuating device, and an outer shield capable of enclosing the filter. The gripping device is manipulated to engage a portion of the retracting structure so that the filter can be held in position. The activating device of the recovery mechanism operates to collapse the plurality of positioning struts to a position where they can be withdrawn by the gripping device into the outer shield. While the outer shield is held firmly in position, the gripping device is further withdrawn and the plurality of thrombosis filtering structures are withdrawn into the shield.  
           [0014]    In one embodiment of the removable structure for the thrombosis filter, a retracting mechanism, in combination with the recovery mechanism, causes the plurality of positioning struts to be withdrawn from contact with the inner lumen wall and to be deflected into a substantially parallel relationship with the struts arranged longitudinally in the direction of their original deployment.  
           [0015]    In another embodiment, the recovery mechanism engages the removable thrombosis filter and holds it in place while the plurality of positioning struts are moved in the direction opposite of their original deployment and are forced into a generally parallel alignment along the longitudinal axis directed toward the direction of deployment of the filtering elements. Once deflected, the outer shield is held in place and the filter is drawn within the outer shield.  
           [0016]    The present invention is thus an improved removable thrombosis filter and method for removal allowing percutaneous removal by a recovery mechanism engaging one end of the filter. Additional features of the invention and the advantages derived therefrom, and the various scopes and aspects of the invention will become apparent from the drawings, the description of the preferred embodiments of the invention, and the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a fragmentary section through a human body from left to right and illustrates a medical procedure of installing the thrombosis filter in the inferior vena cava and removal of the filter by percutaneously entering the venous system at the jugular vein and withdrawing the filter;  
         [0018]    [0018]FIG. 2 is a side cutaway view of a filter in a relaxed position;  
         [0019]    [0019]FIG. 3 is a partial perspective view of the vein engaging end of a filter leg member;  
         [0020]    [0020]FIG. 4 is a side view of a ribbon wire leg member;  
         [0021]    [0021]FIG. 5 is a cross-sectional view taken at line  5 - 5  in FIG. 4;  
         [0022]    [0022]FIG. 6 is a side view of a round leg member;  
         [0023]    [0023]FIG. 7 is a cross-sectional view taken at line  7 - 7  in FIG. 6;  
         [0024]    [0024]FIG. 8 is a plan view of a serpentine shaped leg member;  
         [0025]    [0025]FIG. 9 is a plan view of a spiral shaped leg member;  
         [0026]    [0026]FIG. 10 is a partial perspective view taken at line  10 - 10  in FIG. 9;  
         [0027]    [0027]FIG. 11 is a side cutaway view of a filter positioned in a lumen about to be engaged for removal;  
         [0028]    [0028]FIG. 12 is a side cutaway view of a filter positioned in a lumen having a positioning portion collapsed for removal;  
         [0029]    [0029]FIG. 13 is a side cutaway view of a filter positioned in a lumen having the positioning portion enclosed within a recovery mechanism;  
         [0030]    [0030]FIG. 14 is a side cutaway view of a filter positioned within a lumen having the entire filter enclosed within a recovery mechanism and ready for removal;  
         [0031]    [0031]FIG. 15 is a side cutaway view of another embodiment of a filter in a relaxed position;  
         [0032]    [0032]FIG. 16 is an end view of a joining member taken at line  16 - 16  in FIG. 15;  
         [0033]    [0033]FIG. 17 is an end view of a joining member taken at line  17 - 17  in FIG. 15;  
         [0034]    [0034]FIG. 18 is a side cutaway view of yet another embodiment of a filter in a relaxed position;  
         [0035]    [0035]FIG. 19 is a side cutaway of the filter of FIG. 18 positioned in a lumen;  
         [0036]    [0036]FIG. 20 is a side cutaway of the filter of FIG. 18 engaged for removal;  
         [0037]    [0037]FIG. 21 is a side cutaway of the filter of FIG. 18 with the positioning portion enclosed within a recovery mechanism;  
         [0038]    FIGS.  22 A- 22 E illustrate the deflection and retraction of a flexible anchor member of the type used with the filter of FIG. 18;  
         [0039]    [0039]FIG. 23 is side view of yet another embodiment of the recoverable filter in accordance with the present invention;  
         [0040]    [0040]FIG. 24 is a detailed view of the distal end of an anchoring strut of the filter of FIG. 23;  
         [0041]    [0041]FIG. 25 is a side view of the recoverable filter of FIG. 23 disposed in a vessel lumen;  
         [0042]    [0042]FIG. 26 is a detailed view of the distal end of an anchoring strut as shown in FIG. 25;  
         [0043]    [0043]FIG. 27 is a side view of a recoverable filter of FIG. 23 shown in a vessel lumen during the process of removing the filter from the vessel lumen;  
         [0044]    [0044]FIG. 28 is a detailed view of the distal end of an anchoring strut shown in FIG. 27;  
         [0045]    [0045]FIG. 29 is a side view of yet another embodiment of a recoverable filter in accordance with the present invention;  
         [0046]    [0046]FIG. 30 is an end view of the filter of FIG. 29;  
         [0047]    [0047]FIG. 31 is a view of the filter of FIG. 29 and a removal catheter;  
         [0048]    [0048]FIG. 32 is a view of the filter of FIG. 29 in an early stage of the removal process;  
         [0049]    [0049]FIG. 33 is a view of the filter of FIG. 29 in a stage of the removal process subsequent to that shown in FIG. 32;  
         [0050]    [0050]FIG. 34 is a view of the filter of FIG. 29 in a stage of the removal process subsequent to that shown in FIG. 33; and  
         [0051]    [0051]FIG. 35 is a view of the filter of FIG. 29 shown withdrawn into the removal catheter.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]    [0052]FIG. 1 is a fragmentary section through a human body from left to right and illustrates a medical procedure of installing the thrombosis filter in the inferior vena cava and removal of the filter by percutaneously entering the venous system at the jugular vein and withdrawing the filter. This illustrates the body  10  with a cutaway portion  12  that exposes a portion of the vascular system. The femoral vein  14  leads to the external iliac vein  16 . The common iliac vein  18  leads to the inferior vena cava  20 . As illustrated at the cutaway section  22 , a filter  24  is dispersed within the inferior vena cava and is held in place by the structure that will be described. As shown, a catheter tube is inserted at incision  28  into the venous system. As it extends toward heart  30 , it reaches the inferior vena cava  20  and the filter  24  is deployed. The distal end  32  of the catheter structure  26  is shown after deployment of the filter  24 . For withdrawal or removal of filter  24 , a recovery mechanism ( 36 ) is inserted in the jugular vein  38  at incision  40  and passes through an atrium of heart  30  until its distal end  42  enters the inferior vena cava  20 . The recovery mechanism is not shown in detail in this figure, but will be described in detail below.  
         [0053]    [0053]FIG. 2 is a side cutaway view of a filter in a relaxed position. The filter  44  has a plurality of shaped filtering elements  46 , each having a mounting end  48  and a wall engaging end  50 . Projections  52  have a length sufficient to engage an associated vein wall (not shown) while being short enough so the vein will not be pierced.  
         [0054]    Connecting structure  54  has a frustum shaped end  56  that fixedly attaches the mounting ends  48 . Structure  54  defines a longitudinal cavity  58  within which mounting member  60  is slidably retained.  
         [0055]    A plurality of flexible anchoring struts  62  are mounted on mounting member  60  and project outwardly to wall engaging surfaces  64 . Projections  66  function to position and hold the filter  44  in position when engaged to an inner vein wall. A hooking element  68  is affixed to mandrel  70 . An end  72  of mandrel  70  is affixed within the connecting structure  56 . A tubular member  74  has one end affixed to a pushing structure  76  and a second end  78  mounted in the mounting member  60 . Tubing  74  surrounds a mandrel  70  and is coaxially aligned therewith.  
         [0056]    In the relaxed state, the length L 1  from projection  52  to projection  66  is in the order of 2.0 inches. The length L 2  of the joining member  54  is about 0.4 inch. The tip-to-tip length L 3  is in the order of 1.25 inches, and the projection-to-projection distance L 4  is in the order of 1.2 inches. Various configurations and geometries will be described below, it being understood that when deployed, the filtering portion shown at dashed line  78  will function to restrict the flow of blood clots or thrombosis when blood is flowing in the direction of arrows  80 . At the same time, positioning and holding portion  82  will restrict longitudinal and transverse movement of the filter  44  within the associated lumen or vein. Holding portion  82  preferably centers the device within the lumen.  
         [0057]    [0057]FIG. 3 is a partial perspective view of the vein engaging end of a filter leg member. The end member  50  has a generally curved structure and is flattened to a desired dimension such that the under surface  84  will slidably engage an associated vein wall. The thickness is selected for the desired flexibility. An outward projection  52  is arranged for engaging the vein wall. A similar configuration is utilized for the anchoring elements.  
         [0058]    [0058]FIG. 4 is a side view of a ribbon wire leg member. The filter member  46  has the mounting end  48  positioned at a predetermined angle to the longitudinal arrangement of the member  46 . At its other end, the curved portion  84  deflects in the opposite direction and has projection  52 .  
         [0059]    [0059]FIG. 5 is a cross-sectional view taken at line  5 - 5  in FIG. 4. In one configuration, the flat wire has a thickness T 1  of about 0.006 inch and a width W 1  of about 0.026 inch. It is, of course, understood that differences in thickness relative to width will effect the flexibility of the element.  
         [0060]    [0060]FIG. 6 is a side view of a round leg member. Again, the leg member has a deflection  48 ′ and has an upper thickness T 2  that can be in the order of 0.016 inch. The lower portion has a reduced cross section with a thickness T 3  in the order of about 0.006 inch.  
         [0061]    [0061]FIG. 7 is a cross-sectional view taken at line  7 - 7  in FIG. 6. It illustrates the extent of deflection of portion  48 ′.  
         [0062]    [0062]FIG. 8 is a plan view of a serpentine shaped leg member. As illustrated, serpentine section  86  is provided to yield an improved filtering function when used in combination with other filter members.  
         [0063]    [0063]FIG. 9 is a plan view of a spiral shaped leg member. In this alternative embodiment, a spiral portion  88  is utilized to enhance the filtering operation.  
         [0064]    [0064]FIG. 10 is a partial perspective view taken at line  10 - 10  in FIG. 9. It illustrates the spiral portion  88 . The design of serpentine portion  86  or spiral portion  88  will be selected in accordance with the number of the filter elements used, the overall size of the filter  44 , the attributes of the lumen in which it will be installed, and the flow of blood being filtered.  
         [0065]    [0065]FIG. 11 is a side cutaway view of a filter positioned in a lumen about to be engaged for removal. It will be noted that the positioning portion  82  and the filtering portion  78  have their respective members deflected within the confines of vein  90 . As deflected, the curved engaging portions  50  are brought in contact with the inner wall  90 , as are contact portions  64  of the positioning portion  82 . This arrangement provides for the vein walls  90  to provide adequate tension on the positioning portion  82  and the filtering portion  78  to cause the projections  52  and  66  to engage the inner wall without piercing the inner wall.  
         [0066]    A filter recovery mechanism  92  is inserted in the direction of arrow  94  until a grasping mechanism  96  is brought in proximity to hooking element  68 . Snare  96  is a looped cord or wire that can be externally manipulated to engage mandrel  70  behind the hooking element  68 . In one configuration, the hooking element  68  and the grasping mechanism  96  can be constructed of materials that can be tracked fluoroscopically.  
         [0067]    [0067]FIG. 12 is a side cutaway view of a filter positioned in a lumen having the positioning portion collapsed for removal. When the extensible gripping device  96  engages members  68 , it can be utilized to hold member  68  in a relatively fixed longitudinal position. When thus arranged, actuating device  98  is moved in the direction of arrow  100  to engage element  76  to thereby force tube  74  to move along mandrel  70  and cause the positioning portion  82  to collapse. Mounting element  60  is moved within cavity  58  to effect the collapse, and cause the positioning elements to be substantially parallel aligned along the longitudinal axis.  
         [0068]    [0068]FIG. 13 is a side cutaway view of a filter positioned in a lumen having the positioning portion enclosed within a recovery mechanism. Once the positioning portion  82  has been collapsed, outer shield  102  is moved in the direction of arrow  104  while the gripping device is held steady. When thus positioned, the outer shield  102  is positioned at end  106  to engage the filtering elements  78 .  
         [0069]    [0069]FIG. 14 is a side cutaway view of a filter positioned within a lumen having the entire filter enclosed within a recovery mechanism and ready for removal. Once the positioning portion  82  is enclosed within outer shield  102 , pressure can be applied to the gripping member  96  for moving the gripping member in the direction of arrow  108 . When thus moved, the outer shield  102  is held firm and end  106  functions to collapse filtering portion  78  such that it can be withdrawn within the confines of outer shield  102 . When fully withdrawn within the recovery mechanism  92 , the relatively stiff portion of the recovery mechanism  92  has a length L 5  of about 2.77 inches.  
         [0070]    [0070]FIG. 15 is a side cutaway view of another embodiment of a filter in a relaxed position. Elements having similar functions will have a similar reference numeral designation. In this embodiment, filtering portion  78  is mounted in mounting structure  110  enjoining member  54 . This configuration eliminates the frustum element  56  and provides additional strength at this structure. A hook  112  replaces the button element  68 . Pushing element  76  of FIG. 2 is replaced by a pushing frustum element  114 . This frustum configuration provides an improved blood flow and minimizes turbulence. Further, it gives a larger dimension along mandrel  70  such that there is minimization of the tendency to tilt as it is being moved forward as previously described. In this embodiment, dimensions L 2 , L 3 , and L 4  are similar to those described with regard to FIG. 2. The projection-to-projection length L 6  is in the order of 2.1 inches, while the tip-to-tip length L 7  is in the order of about 2.22 inches. Finally, the over-all relaxed length of the filter is designated L 8 , and is about 2.68 inches.  
         [0071]    [0071]FIG. 16 is an end view of a joining member taken at line  16 - 16  in FIG. 15. It illustrates the housing  54  mounting positioning elements  62 , which number four in this configuration, surrounding mandrel  70  which passes therethrough. This figure is expanded and is not in scale.  
         [0072]    [0072]FIG. 17 is an end view of a joining member taken at line  17 - 17  in FIG. 15. It illustrates the mounting member  110  which is mounted in the connecting housing  54 , and shows six mounting ends  48  of the filtering members. Mandrel  70  is affixed in the mounting member  110 . Again, this figure is in a different scale to that of FIG. 15.  
         [0073]    [0073]FIG. 18 is a side cutaway view of yet another embodiment of a filter in a relaxed position. In this embodiment, the mounting element  60  is fixedly mounted within channel  58  substantially adjacent to mounting member  110 . The mounting ends  78  of positioning struts  62  are restrained by the outer limits of housing  54 . A hook  112  is fixedly mounted to mandrel  70  which in turn is secured in mounting element  110 . In this embodiment, housing  54  is shorter than the embodiment illustrated in FIG. 2, and has a length L 9  of about 0.265 inch. The overall length L 10  from a projection-to-projection is in the order of 1.85 inches, and the length L 11  from the end of mounting member  110  to the filtering portion  78  projections is in the order of 1.27 inches. The relaxed spacing of the filtering portion L 12  is in the order of 1.2 inches. The releasing and collapsing of the anchoring elements  62  in this embodiment will be described below.  
         [0074]    [0074]FIG. 19 is a side cutaway of the filter of FIG. 18 positioned in a lumen. As shown, positioning portion  82  engages the inner vein wall  90 , as does the filtering portion  78 . When thus contained, the diameter of the lumen or vein L 13  is in the order of 0.866 inch. When installed, the overall length L 14  is in the order of 2.25 inches, while the tip projection-to-projection length L 15  is in the order of 2.11 inches.  
         [0075]    [0075]FIG. 20 is a side cutaway of the filter of FIG. 18 engaged for removal. In this embodiment, a recovery mechanism  120  has a gripping device  122  for engaging hook  112 . When thus engaged, the gripping device can be held firmly externally, and the outer shield  124  extended in the direction of arrow  126  to engage the legs  62  of positioning portion  82 . Outer shield  24  can include a funnel shaped end to assist in directing the filter into the recovery mechanism  120 .  
         [0076]    [0076]FIG. 21 is a side cutaway of the filter of FIG. 18 with the positioning portion enclosed within a recovery mechanism. As shown, the recovery mechanism  120  has had the outer shield  124  moved farther in the direction of arrow  126  such that positioning struts  62  have been bent back upon themselves and are within outer shield  124 . As thus positioned, struts  62  are substantially parallel to each other and aligned along the longitudinal axis of the filter and the recovery mechanism  120 . When the positioning portion is thus collapsed and retracted, the outer shield  124  is held stationary and the gripping device  122  is moved in the direction of arrow  128  for drawing the filtering portion  78  into outer shield  124 . When the filtering portion  78  is fully withdrawn within outer shield  124 , the recovery mechanism  120  can be withdrawn from the body.  
         [0077]    FIGS.  22 A- 22 E illustrate the deflection and retraction of a flexible anchor member of the type used with the filter of FIG. 18. In FIG. 22A, a flexible anchor member  62  is in a holding position on the inner wall of vein  90 . As force is applied in the direction of arrow  130  to the portion of flexible anchor member  62  by the outer shield  124  (see, FIG. 21), the tip  64  and the protrusion  66  are started in a direction to be released from the inner wall  90 . In FIG. 22B, the force has been applied at arrow  130  to start deflection of flexible anchor member  62 . At this juncture, the protrusion  66  has been removed from inner wall  90  and the tip  64  has started to slide along the inner wall. Anchor member  62  can have a predetermined region of greater flexibility to control the location of the deflection caused by the force supplied at arrow  130 .  
         [0078]    In FIG. 22C, the force applied at arrow  130  has deflected flexible anchor member  62  such that the end  64  is merely moving along the inner wall surface  90 . In FIG. 22D, there is an illustration that the force applied in the direction of arrow  130  has proceeded to a point where flexible anchor member  62  has started to bend back upon itself, and the tip  64  is out of contact with the inner wall of vein  90 . Finally, in FIG. 22E, it is shown that force applied in the direction of arrow  130  by the outer shield  124  as bent flexible anchor member  62  back upon itself, such that the outer shield can pass over it.  
         [0079]    [0079]FIG. 23 is a side view of yet another embodiment of a retrievable filter in accordance with the present invention. The filter as shown in FIG. 23 is in a relaxed, uncompressed state. The filter of FIG. 23 is substantially similar to that of FIGS.  15 - 22 , except as the description below may vary from that of the embodiment of FIG. 15 described above.  
         [0080]    The filter of FIG. 23 includes a plurality of filtering elements  146  having wall engaging ends  150 . Wall engaging ends  150  as shown in this embodiment are curved to present a rounded convex face to a vessel wall. Collectively filtering elements  146  form a filter portion or array  178 . Disposed distally of filter array  178  are a plurality of legs or anchoring struts  162 . Anchoring struts  162  have proximal and distal ends. A typical distal end is shown in FIG. 24 in detail. The filter also includes a hook  212  to aid in removal of the filter from a vessel. An enjoining member  154  joins filter array  178 , struts  162  proximate their proximal ends and hook  212  together. Struts  162  preferably are formed in a ribbon shape, wherein the thickness of the ribbon is shown in FIG. 23 and the width is perpendicular to the thickness, i.e., directly into the paper.  
         [0081]    [0081]FIG. 24 is a detailed view of the distal end of a strut  162 . The distal end includes a sharpened portion  166  and a pad portion  164  extending distally beyond sharpened portion  164 . Sharpened portion  164  is sufficiently sharp to penetrate a vessel wall. Pad portion  164  similarly to strut  162 , preferably has a ribbon shape wherein the thickness is shown in FIG. 24 and the width is perpendicular to the thickness, i.e., directly into the paper. As can be seen in FIG. 24, the thickness of pad portion  164  decreases distally. The decrease in pad thickness  164  can create an increase in flexibility of pad portion  164  distally. A similar increase in flexibility distally along pad portion  164  can be created by varying the material characteristics of pad  164 . Pad portion  164  is preferably flexible enough not to puncture the vessel wall, i.e., the pad portion  164  is preferably atraumatic. In a preferred embodiment of the present invention, the length of sharp portion  164  is between about 2 to 6 millimeters and the length of pad portion  162  is between about 4 to 20 millimeters. In a preferred embodiment, pad portion  164  is more than twice as long as sharp portion  164 .  
         [0082]    [0082]FIGS. 25 and 26 show the filter of FIG. 23 disposed in a vessel A during normal use. The direction of blood flow is shown by arrows B. The removal of the filter would be in the direction indicated by arrow C by way of hook  212 . Filter elements  146  are shown moderately compressed in comparison to their relaxed state shown in FIG. 23. As shown in FIG. 26, sharp portion  166  is penetrating the wall of the vessel A and pad portion  164  is generally parallel to the wall of vessel A.  
         [0083]    [0083]FIGS. 27 and 28 show the filter of FIG. 23 in the process of being removed from vessel A in the direction indicated by arrow C. As the filter is withdrawn using the same procedure as that of the filter of FIG. 15, struts  162  will deform generally as shown in FIGS.  22 A- 22 E. Strut  162  as shown in FIG. 27 is in the approximate position of strut  62  in FIG. 22B. As the process continues, strut  162  will assume generally the position shown in FIG. 22E. The ultimate removal of the filter will be accomplished as described with respect to the filter of FIG. 15 above.  
         [0084]    [0084]FIG. 28 is a detailed view of the distal end of strut  162  as shown in FIG. 27. Although the configuration of strut  162  in FIG. 27 is similar to that of strut  62  in FIG. 22B, it can be appreciated that the length of pad portion  164  relative to sharpened portion  166  is substantially greater than element  64  is to element  66  of FIG. 22B, respectively. As can be appreciated the increased length and increasing flexibility of pad portion  164  distally provides an effective cantilever for retracting sharp portion  166  from the wall of vessel A.  
         [0085]    [0085]FIG. 29 is a side view of yet another embodiment of filter  200  in accordance with the present invention disposed within a vessel A. Filter  200  has a longitudinal axis D. Blood or fluid flow in vessel A is shown in the direction indicated by arrows B. Filter  200  includes flexible struts  202  generally extending in the first direction along axis D from a hub  204 , while diverging from axis D. A first end of strut  202  is coupled to hub  204 . Second end  206  of strut  202  is preferably sharpened and barbed to engage with the wall of vessel A.  
         [0086]    Filter  200  also includes a strut retraction member  208 . Strut retraction member  208  includes a plurality of retraction loops  210  extending from retraction member hub  212  generally in the second direction along axis D. Each of loops  210  preferably loops around a strut  202 . A tether connector  214  extends from hub  212  in the first direction along axis D. A telescoping connector  216  connects hub  204  and retraction member hub  212 .  
         [0087]    Various elements of filter  200  such as struts  202  or loops  210  may be formed from nitinol, stainless steel or other biocompatible materials. One skilled in the art would appreciate that the materials described above with respect to the other filter embodiments could advantageously be applied to construct the embodiment  200  as well.  
         [0088]    [0088]FIG. 30 is an end view of filter  200  shown within vessel A. In the illustrated embodiment of filter  200  there are six struts  202  and loops  210 .  
         [0089]    [0089]FIG. 31 is a side view of the filter of FIG. 29 to which a tether  218  having a loop  220  is attached to tether connector  214 . Tether  218  is disposed within a catheter  222  having an outer tubular member  224  and an inner tubular member  226 . Assuming that, for example, filter  220  is disposed in the inferior vena cava, catheter  222  may be advanced to the filter from a femoral vein access point. Outer tube  224  and inner tube  226  are preferably formed from biocompatible materials including polymers known to those skilled in the art. The materials must be of sufficient strength and rigidity or flexibility to accomplish the procedure described below.  
         [0090]    [0090]FIG. 32 is a side view of the filter of FIG. 29 wherein the retraction member  208  is being pulled in the first direction such that loops  210  are advancing along and engaging struts  202 . FIG. 33 is a side view of filter  200  of FIG. 29 wherein inner tube  226  of catheter  222  has been brought into engagement with loops  210 . In dashed lines, inner tube  226  is shown being advanced in a second direction over loops  210  such that struts  202  are brought from a first position engaging the walls of vessel A to a second position adjacent axis D of filter  200 .  
         [0091]    [0091]FIG. 34 is a side view of filter  200  shown in FIG. 29. In FIG. 34 struts  202  are shown disposed in the second position. Second ends  206  of struts  202  have been brought into contact with the distal end of inner tube  226 .  
         [0092]    [0092]FIG. 35 is a side view of filter  200  of FIG. 29 in which filter  200  has been withdrawn into outer tube  224  of catheter  222 . Filter  200  could now be removed from the patient through outer tube  224 , or filter  200  and catheter  222  could be simultaneously removed from the patient.  
         [0093]    It can be appreciated that performing the steps of removal process in reverse would provide a method of placing filter  200  in vessel A as shown in FIG. 29. In a preferred method, however, hubs  204  and  212  would be spaced as shown in FIG. 29 prior to placement in catheter  222 . After deployment of filter  200 , as shown in FIG. 29, tether  218  and catheter  222  can be removed from the patient such that filter  200  may remain in place for an extended period of time.  
         [0094]    From the foregoing, it is understood that various configurations and selection of materials will provide suitable removable filter structures that may be utilized in conformance with the inventive concepts described herein. In general, it is understood that the materials must be suitable for implantation in a human body and will remain intact without adding contaminants to the blood stream. The selection of materials will also determine the flexibility and resiliency of the various members. The various components of the filter can be constructed of a class of elastic materials including nitinol, stainless steel, platinum, tungsten, titanium, and chromium alloys. The shaping and bonding structures are those available in the construction of thrombosis filters of the class described.  
         [0095]    Having described the various embodiments and methods of the invention in conjunction with the drawings, it can be seen that the various stated purposes and objectives have been achieved, and that various modifications and extensions will be apparent to those skilled in the art while remaining within the spirit and scope of the invention. Accordingly, what is intended to be protected by Letters Patent is set forth in the appended claims.