Abstract:
A thrombus filter configured for placement in within a blood vessel lumen defined by a blood vessel wall. Methods and devices for selectively removing the thrombus filter when the presence of a filter in the vascular system is no longer desired. The thrombus filter includes a first strand formation, a second strand formation, and a joined portion.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to filters for use inside blood vessels. More particularly, the present invention relates to thrombus filters which can be securely adjoined at a selected location in the vascular system and selectively removed when the presence of a filter in the vascular system is no longer desired. 
     BACKGROUND OF THE INVENTION 
     There are a number of situations in the practice of medicine when it becomes desirable for a physician to place a filter in the vascular system of a patient. One of the most common applications for vascular filters is the treatment of Deep Venous Thrombosis (DVT). Deep Venous Thrombosis patients experience clotting of blood in the large veins of the lower portions of the body. These patients are constantly at risk of a clot breaking free and traveling via the inferior vena cava to the heart and lungs. This process is known as pulmonary embolization. Pulmonary embolization can frequently be fatal, for example when a large blood clot interferes with the life-sustaining pumping action of the heart. If a blood clot passes through the heart it will be pumped into the lungs and may cause a blockage in the pulmonary arteries. A blockage of this type in the lungs will interfere with the oxygenation of the blood causing shock or death. 
     Pulmonary embolization may be successfully prevented by the appropriate placement of a thrombus filter in the vascular system of a patient&#39;s body. Placement of the filter may be accomplished by performing a laparotomy with the patient under general anesthesia. However, intravenous insertion is often the preferred method of placing a thrombus filter in a patient&#39;s vascular system. 
     Intravenous insertion of a thrombus filter is less invasive and it requires only a local anesthetic. In this procedure, the thrombus filter is collapsed within a delivery catheter. The delivery catheter is introduced into the patients vascular system at a point which is convenient to the physician. The delivery catheter is then fed further into the vascular system until it reaches a desirable location for filter placement. The thrombus filter is then released into the blood vessel from the delivery catheter. 
     In the treatment of Deep Venous Thrombosis, a thrombus filter is placed in the inferior vena cava of a patient. The inferior vena cava is a large vessel which returns blood to the heart from the lower part of the body. The inferior vena cava may be accessed through the patient&#39;s femoral vein. 
     Thrombus filters may be placed in other locations when treating other conditions. For example, if blood clots are expected to approach the heart and lungs from the upper portion of the body, a thrombus filter may be positioned in the superior vena cava. The superior vena cava is a large vessel which returns blood to the heart from the upper part of the body. The superior vena cava may by accessed through the jugular vein, located in the patient&#39;s neck. 
     Once placed inside a blood vessel, a thrombus filter acts to catch and hold blood clots. The flow of blood around the captured clots allows the body&#39;s lysing process to dissolve the clots. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to thrombus filters which may be securely adjoined at a selected location in the vascular system and selectively removed when the presence of a filter in the vasculature systems is no longer required. The present invention also pertains to devices and method for removing a thrombus filter using minimally invasive methods. A thrombus filter in accordance with the present invention includes a first strand formation, a second strand formation and a joined portion. The first strand formation and the second strand formation are both comprised of a plurality of strands, each strand having a joined end and a free end. The joined ends of the strands are joined together proximate the joined portion of the thrombus filter. The strands radiate away from the joined portion of the thrombus filter so that the first strand formation and the second strand formation are both generally conical in shape. The strands of the first strand formation and the strands of the second strand formation radiate in generally opposing directions. 
     When the thrombus filter is disposed in a blood vessel, at least one of the strand formations acts to trap or capture blood clots. The generally conical shape of the strand formation serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
     The free ends of the strands act as opposing wall contacting members and serve to position the thrombus filter in the center of a blood vessel lumen. In a presently preferred embodiment, the strands are biased to spring outward. The radial force applied to the walls of the blood vessel by the strand formations assists in preventing migration of the thrombus filter within the blood vessel lumen. The generally opposed orientation of the first strand formation relative to the second strand formation also makes migration of the filter less likely. Migration of the thrombus filter within a blood vessel lumen may also be made less likely by the inclusion of an anchor member proximate the free end of each strand. 
     In a presently preferred embodiment, a sliding member is disposed about the joined portion of the thrombus filter. One or the other of the strand formations may be collapsed by urging the sliding member toward the free ends of the strands. Urging the sliding member toward the free ends of strands collapses the strand formation from a generally conical shape to a generally cylindrical shape. Once the strand formation is collapsed, it may be urged into the lumen of a retrieval catheter. 
     With one strand formation in the lumen of the retrieval catheter, the entire thrombus filter may be urged into the lumen of the retrieval catheter. Pulling the thrombus filter in the lumen of the retrieval catheter causes the strands of the second strand formation to collapse from a generally conical shape to a generally cylindrical shape. With all strands in a collapsed position, the thrombus filter may be pulled completely into the lumen of the retrieval catheter. With the thrombus filter disposed inside the lumen of the retrieval catheter, removing the thrombus filter from the body of the patient may be accomplished by withdrawing the retrieval catheter from the blood vessel lumen. 
     In one embodiment of a retrieval catheter, the retrieval catheter includes an outer tubular member, an inner tubular member and a pulling means. One embodiment of the pulling means includes a plurality of retrieval struts each having a distal end. An engaging member is disposed proximate the distal end of each strut. In one embodiment, each engaging member includes a sharp projection. The distal ends of the struts may be selectively urged inward so that the sharp projections penetrate the sliding member of the thrombus filter. Having thus coupled the struts to the sliding member, a pulling force may be applied to the sliding member by pulling on the proximal ends of the struts. 
     In an additional embodiment of a retrieval catheter, the pulling means may include flanges fixed to the distal end of each strut. The flanges are adapted to be disposed about the fixed portion of the thrombus filter. In a presently preferred embodiment, the joined portion of the thrombus filter has a generally cylindrical outer surface. Also, in a presently preferred embodiment, each flange includes an inner radius which is substantially equal to the outer radius of the joined portion of the thrombus filter. The flanges may be selectively closed around the joined portion of the thrombus filter. When the flanges are closed around the joined portion of the thrombus filter, they butt against each other to form a generally tubular shell around the joined portion of the thrombus filter. 
     The strands of one or the other of the strand formations may be urged into a collapsed position by urging the flanges toward the free ends of the strands. The flanges may be urged toward the free ends of the strands by pulling on the proximal ends of the struts. Once the strands have been moved into a collapsed position, the strand formation of the thrombus filter may be positioned within the lumen of a retrieval catheter. This may be accomplished percutaneously by pulling on the proximal end of the struts. 
     With one of the strand formations positioned within the lumen of the retrieval catheter, the remainder of the thrombus filter may also be urged into the lumen of the retrieval catheter. As described previously, the strands of the second strand formation radiate away from the joined portion of the thrombus filter in a generally opposed direction relative to the strands of the first strand formation. The orientation of the strands allows them to be pulled out of the walls of the blood vessel with minimal force. The strands of the second strand formation may be collapsed by simultaneously pushing on the proximal end of the retrieval catheter and pulling on the proximal ends of the struts. 
     Pulling the thrombus filter into the lumen of the retrieval catheter causes the strands of the second formation to collapse from a generally conical shape to a generally cylindrical shape. With all of the strands in a collapsed position, the thrombus filter may be pulled completely into the lumen of the retrieval catheter. With the thrombus filter disposed inside the lumen of the retrieval catheter, removing the thrombus filter from the body of the patient may be accomplished by withdrawing the retrieval catheter from the blood vessel lumen. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     FIG. 1 is a plan view of a thrombus filter positioned in the lumen of a blood vessel lumen; 
     FIG. 2 is a plan view of a removal catheter disposed in a blood vessel lumen proximate the thrombus filter of FIG. 1; 
     FIG. 3 is a plan view of an alternate embodiment of a thrombus filter and a removal catheter positioned in the lumen of a blood vessel lumen; 
     FIG. 4 is a plan view of the thrombus filter of FIG. 3 illustrating an alternate method of removal; and 
     FIG. 5 is a plan view of an alternate embodiment of a thrombus filter and removal catheter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description should be read with reference to the drawings, in which like elements in different drawings are numbered identically. The drawings which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. 
     Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements. All other elements employ that which is known to those of skill in the field of the invention. Those skilled in the art will recognize that many of the examples provided have suitable alternatives which may be utilized. 
     Reference is now made to the drawings, in which like numbers refer to like elements throughout. FIG. 1 is a plan view of a thrombus filter  20  positioned in a lumen  22  of a blood vessel  24 . Blood vessel  24  includes walls  26  which define lumen  22 . The main components of thrombus filter  20  are a first strand formation  30 , second strand formation  40 , and a joined portion  50 . 
     First strand formation  30  and second strand formation  40  are both comprised of a plurality of strands  32  and  42  respectively. Each strand  32  of first strand formation  30  has a joined end  34  and a free end  36 . Likewise, each strand  42  of second strand formation  40  has a joined end  44  and a free end  46 . Joined ends  34  of strands  32  and joined ends  44  of strands  42  are joined at joined portion  50  of thrombus filter  20 . 
     Strands  32  radiate away from joined portion  50  of thrombus filter  20  so that first strand formation  30  is generally conical in shape. Likewise, strands  42  radiate away from joined portion  50  of the thrombus filter  20  such that second strand formation  40  is generally conical in shape. As shown in FIG. 1, strands  32  of first strand formation  30  and strands  42  of second strand formation  40  radiate in generally opposing directions. 
     When thrombus filter  20  is disposed in a blood vessel, at least one of the strand formations acts to trap, or capture blood clots. The generally conical shape the strand formation serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
     Strands  32  and  42  act as opposing wall contacting members and serve to position thrombus filter  20  in the center of lumen  22  of blood vessel  24 . In a presently preferred embodiment, strands  32  and  42  are biased to spring outward. The radial force applied to the walls of the blood vessel by first strand formation  30  and second strand formation  40  assists in preventing migration of thrombus filter  20  within blood vessel lumen  22 . The generally opposed orientation of strands  42  relative to strands  32  also makes migration of the filter less likely. 
     Migration of filter  20  within blood vessel lumen  22  may also made less likely by the inclusion of anchors on thrombus filter  20 . An anchor  62  is disposed at free end  46  of each strand  42 . Likewise, each free end  36  of each strand  32  includes an anchor  60 . In FIG. 1, anchors  60  and  62  are pictured as sharp projections or barbs. It should be understood that anchors  60  and  62  may be comprised of other means for anchoring without departing from the spirit or scope of this invention. It should also be understood that embodiments of thrombus filter  20  which include no anchors are possible without departing from the spirit or scope of the present invention. 
     In the embodiment of FIG. 1, joined portion  50  includes a collar  52  disposed about joined ends  34  of strands  32  and joined ends  44  of strands  42 . A sliding member  70  is disposed about joined portion  50 . In the embodiment of FIG. 2 sliding member  70  is preferably comprised of a plastic material. Examples of suitable plastic materials include polyethylene (PE), polypropylene (PP), thermoset polyurethane, thermoplastic polyurethane, and polyether block amide (PEBA). Those with skill in the art will appreciate that sliding member  70  may be comprised of other materials without deviating from the spirit or scope of the present invention. A ring  54  is fixed to joined portion  50  proximate one end of sliding member  70 . Ring  54  may act as a stop to limit the travel of sliding member  70 . 
     Although two strands  32  and two strands  34  are shown in FIG.  1 . Any number of strands  32 ,  34  may be used. In a presently preferred embodiment first strand formation  30  includes between about 3 strands  32  and about 6 strands  32 . In a presently preferred embodiment second strand formation  40  includes between about 3 strands  42  and about 6 strands  42 . 
     In a presently preferred embodiment, stands  32 ,  42  are generally circular in cross section, it should be understood that other cross-sectional shapes are possible without deviating from the spirit or scope of the invention. For example, the cross-sectional shape of stands  32 ,  42  may be circular, rectangular, square, triangular, oval, etc. 
     In a presently preferred embodiment, strands  32 ,  42  are comprised of nickel-titanium alloy. Suitable nickel-titanium alloys are commercially available from Memry Technologies (Brookfield, Conn.), TiNi Alloy Company (San Leandro, Calif.), and Shape Memory Applications (Sunnyvale, Calif.). 
     Those with skill in the art will appreciate that strands  32 ,  42  may be comprised of other metallic or non-metallic materials without departing from the spirit or scope of the present invention. Examples of metallic materials which may be suitable for some applications include stainless steel and titanium. Suitable, non-metallic materials may be selected from the following list, which is not exhaustive: poly(L-lactide) (PLLA), poly(D,L-lactide) (PLA), polyglycolide (PGA), poly(L-lactide-co-D,L-lactide) (PLLA/PLA), poly(L-lactide-co-glycolide) (PLLA/PGA), poly(D, L-lactide-co-glycolide) (PLA/PGA), poly(glycolide-co-trimethylene carbonate) (PGAIPTMC), polyethylene oxide (PEO), polydioxanone (PDS), polycaprolactone (PCL), polyhydroxylbutyrate (PHBT), poly(phosphazene), polyD,L-lactide-co-caprolactone) (PLA/PCL), poly(glycolide-co-caprolactone) (PGAIPCL), polyanhydrides (PAN), poly(ortho esters), poly(phoshate ester), poly(amino acid), poly(hydroxy butyrate), polyacrylate, polyacrylamid, poly(hydroxyethyl methacrylate), polyurethane, polysiloxane and their copolymers. 
     FIG. 2 is a plan view of a removal catheter  90  disposed in blood vessel lumen  22  proximate thrombus filter  20 . Removal catheter  90  includes an outer tubular member  100  having a lumen  102 , a distal end  104 , and a proximal end  106  (not shown). 
     In the embodiment of FIG. 2, an inner tubular member  200  is disposed within lumen  102  of outer tubular member  100 . Inner tubular member  200  includes a lumen  202 , a distal end  204 , and a proximal end  206  (not shown). An elongate shaft  210  and a pulling means  300  are disposed within lumen  202  of inner tubular member  200 . 
     Pulling means  300  includes a plurality of retrieval struts  302  each having a distal end  304 , a proximal end  306  (not shown), and a distal portion  308 . An engaging member  310  is disposed proximate the distal end  304  of each strut  302 . In the embodiment of FIG. 2, each engaging member  310  includes a sharp projection  312 . 
     In the embodiment of FIG. 2, distal portions  308  of retrieval struts  302  are biased to spring outward. Distal portions  308  of retrieval struts  302  may be selectively urged inward by urging distal end  204  of inner tubular member  200  toward distal ends  304  of retrieval struts  302 . During a surgical procedure, this may be accomplished percutaneously by pushing on proximal end  206  (not shown) of inner tubular member  200 , and/or pulling on proximal ends  306  (not shown) of retrieval struts  302 . 
     A method of removing thrombus filter  20  from blood vessel lumen  22  may now be described with reference to FIG.  2 . The retrieval process typically begins by introducing catheter  90  into the patients vascular system. The retrieval catheter typically enters the patients vascular system at a point which is readily accessible to the physician. Once in the vascular system, the retrieval catheter is urged forward until distal end  104  of outer tubular member  100  is proximate thrombus filter  20 . 
     For example, if thrombus filter  20  is located in the inferior vena cava of a patient&#39;s vascular system, removal catheter  90  may enter the vascular system at the femoral vein. Alternately, if thrombus filter  20  is located in the superior vena cava of a patient&#39;s vascular system, removal catheter  90  may enter the vascular system at the jugular vein. In either case, the filter removal procedure is minimally invasive, and generally does not require general anesthesia. 
     Preferably, distal portions  308  of retrieval struts  302  will be in a retracted position while the distal end of retrieval catheter  90  is advanced through the vasculature. Distal portions  308  may be held in a retracted position by inner tubular member  200 . When the distal end of catheter  90  is proximate thrombus filter  20  inner tubular member  200  may be pulled back, allowing distal portions  308  of struts  302  to spring outward. Struts  302  may then be urged forward until distal ends  304  of struts  302  are proximate slide  70  of thrombus filter  20 . 
     Distal portions  308  of struts  302  may then be urged towards slide  70  by urging distal end  204  of inner tubular member  200  toward distal ends  304  of struts  302 . This may be accomplished percutaneously by simultaneously pushing on distal end  206  of inner tubular member  200  and pulling on distal ends  306  of struts  302 . When distal portions  308  of struts  302  are closed onto slide  70 , sharp projections  312  penetrate into slide  70 . 
     A pulling force may then be applied to slide  70 . In a presently preferred embodiment, this pulling force is created by pulling on proximal ends  306  of struts  302 . Also in a presently preferred embodiment, thrombus filter  20  may be held in position by pushing the distal end of elongate shaft  210  against thrombus filter  20 . 
     Strands  32  may be urged into a collapsed position by urging slide  70  toward free ends  36  of strands  32 . Once strands  32  have been moved to a collapsed position, first strand portion  30  of thrombus filter  20  may be positioned within lumen  102  of outer tubular member  100 . This may be accomplished percutaneously by pushing on proximal end  106  of outer tubular member  100  and/or pulling on proximal ends  306  of struts  302 . 
     Once first strand formation  30  is positioned within lumen  102  of outer tubular member  100 , the remainder of thrombus filter  20  may also be urged into lumen  102  of outer tubular member  100 . As described previously, strands  42  of second strand formation  40  radiate away from joined portion  50  of thrombus filter  20  in a generally opposed direction relative to strands  32  of first strand formation  30 . The orientation of strands  42  allows them to be pulled out of walls  26  of blood vessel  22  with minimal force. Strands  42  may be converted to a collapsed position by simultaneously by pushing on proximal end  106  of outer tubular member  100  and pulling on proximal ends  306  of struts  302 . 
     Pulling thrombus filter  20  into lumen  102  of outer tubular member  100  causes strands  42  to collapse causing second strand formation  40  to transform from a generally conical shape to a generally cylindrical shape. With strands  32  and strands  42  in a collapsed position, thrombus filter  20  may be pulled completely into lumen  102  of outer tubular member  100 . With thrombus filter  20  disposed inside lumen  102  of outer tubular member  100 , removing thrombus filter  20  from the body of the patient may be accomplished by withdrawing retrieval catheter  90  from blood vessel lumen  22 . 
     FIG. 3 is a plan view of an alternate embodiment of thrombus filter  20  positioned in a lumen  22  of a blood vessel  24 . Blood vessel  24  includes walls  26  which define lumen  22 . The main components of thrombus filter  20  are a first strand formation  30 , second strand formation  40 , and a joined portion  50 . 
     First strand formation  30  and second strand formation  40  are both comprised of a plurality of strands  32  and  42  respectively. Each strand  32  of first strand formation  30  has a joined end  34  and a free end  36 . Likewise, each strand  42  of second strand formation  40  has a joined end  44  and a free end  46 . Joined ends  34  of strands  32  and joined ends  44  of strands  42  are joined at joined portion  50  of thrombus filter  20 . In the embodiment of FIG. 3, joined portion  50  includes solder  56 . Solder  56  is used to fix joined ends  34  of strands  32  and joined ends  44  of strands  42  together. Other methods may be used to fix joined ends  34 ,  44  of strands  32 ,  42  without departing from the spirit or scope of the present invention. For example, brazing, welding, mechanical fasteners, and the use of adhesives may be suitable for some applications. 
     Strands  32  radiate away from joined portion  50  of thrombus filter  20  so that first strand formation  30  is generally conical in shape. Likewise, strands  42  radiate away from joined portion  50  of the thrombus filter  20  such that second strand formation  40  is generally conical in shape. As shown in FIG. 1, strands  32  of first strand formation  30  and strands  42  of second strand formation  40  radiate in generally opposing directions. 
     When thrombus filter  20  is disposed in a blood vessel, at least one of the strand formations acts to trap, or capture blood clots. The generally conical shape the strand formation serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
     Strands  32  and  42  act as opposing wall contacting members and serve to position thrombus filter  20  in the center of lumen  22  of blood vessel  24 . In a presently preferred embodiment, strands  32  and  42  are biased to spring outward. The radial force applied to the walls of the blood vessel by first strand formation  30  and second strand formation  40  assists in preventing migration of thrombus filter  20  within blood vessel lumen  22 . The generally opposed orientation of strands  42  relative to strands  32  also makes migration of the filter less likely. 
     Migration of filter  20  within blood vessel lumen  22  may also made less likely by the inclusion of anchors on thrombus filter  20 . An anchor  62  is disposed at free end  46  of each strand  42 . Likewise, each free end  36  of each strand  32  includes an anchor  60 . In FIG. 1, anchors  60  and  62  are pictured as sharp projections or barbs. It should be understood that anchors  60  and  62  may be comprised of other means for anchoring without departing from the spirit or scope of this invention. It should also be understood that embodiments of thrombus filter  20  which include no anchors are possible without departing from the spirit or scope of the present invention. 
     Two slides  72  and  74  are disposed about joined portion  50 . In the embodiment of FIG. 3 each sliding member  70 ,  74  is comprised of a helical coil. The strength of slides  72 ,  74  may be increased by soldering or otherwise bonding consecutive turns of the helical coil to each other. In FIG. 3, slide  74  is shown in partial cross section. Slides  72 ,  74  include coupling members  76 ,  78  respectively. 
     In FIG. 3, a removal catheter  400  is disposed in lumen  22  of blood vessel  24 . Removal catheter  400  includes a lumen  402 , a distal end  404 , and a proximal end  406  (not shown). An elongate member  500  is disposed in lumen  402  of removal catheter  400 . Elongate member  500  includes a distal end  504 , a proximal end  506  (not shown), and a coupling member  502  disposed proximate distal end  504 . 
     A method of removing thrombus filter  20  from blood vessel lumen  22  may now be described with reference to FIG.  3 . The retrieval process typically begins by introducing the catheter into the patients vascular system. The retrieval catheter typically enters the patients vascular system at a point which is readily accessible to the physician. Once in the vascular system, the retrieval catheter is urged forward until distal end  404  is proximate thrombus filter  20 . 
     For example, if thrombus filter  20  is located in the inferior vena cava of a patient&#39;s vascular system, removal catheter  400  may enter the vascular system at the femoral vein. Alternately, if thrombus filter  20  is located in the superior vena cava of a patient&#39;s vascular system, removal catheter  400  may enter the vascular system at the jugular vein. In either case, the filter removal procedure is minimally invasive, and generally does not require general anesthesia. 
     The retrieval catheter is advanced through blood vessel lumen  22  until distal end  404  catheter  400  is proximate thrombus filter  20 . Distal end  504  of elongate member  500  is then coupled to sliding member  72 . This may be accomplished by mating coupling member  502  of elongate member  500  with coupling member  76  of the sliding member  72 . A pulling force is then applied to sliding member  72 . In a presently preferred embodiment, this pulling force is created by pulling on proximal end  506  (not shown) of elongate member  500 . 
     Pulling on sliding member  72  urges sliding member  72  toward free ends  36  of strands  32 . As sliding member  72  moves, it causes strands  32  to collapse. Once strands  32  move to a collapsed position, first strand formation  30  of thrombus filter  20  may be urged into lumen  402  of retrieval catheter  400 . 
     Once first strand formation  30  has entered lumen  402  of retrieval catheter  400 , thrombus filter  20  may be urged further into lumen  402  of catheter  400 . As described previously, strands  42  of second strand formation  40  radiate away from joined portion  50  of thrombus filter  20  in a generally opposed direction relative to strands  32  of first strand formation  30 . The orientation of strands  42  allows them to be pulled away from walls  26  of blood vessel  24  with minimal force. 
     Pulling thrombus filter  20  into lumen  402  of retrieval catheter  400  causes strands  42  to collapse causing second strand formation  40  to transform from a generally conical shape to a generally cylindrical shape. The collapse of strands  32  and  42  allows all of thrombus filter  20  to be disposed in lumen  402  of catheter  400 . With thrombus filter  20  disposed inside lumen  402  of retrieval catheter  400 , removing thrombus filter  20  from the body of the patient may be accomplished by withdrawing retrieval catheter  400  from blood vessel lumen  22 . 
     An additional method of removing thrombus filter  20  from blood vessel lumen  22  has been envisioned in which two removal catheters  400 A and  400 B are utilized. This method may be described with reference to FIG.  4 . 
     The retrieval process typically begins by introducing catheters  400 A,  400 B into the patients vascular system. Once in the vascular system, retrieval catheters  400 A,  400 B are urged forward until distal ends  404 A,  404 B are proximate thrombus filter  20 . 
     Retrieval catheters  400 A,  400 B enter the patient&#39;s vascular system at points which allow them to approach thrombus filter  20  from substantially opposing directions. For example, removal catheter  400 A may enter the vascular system at the femoral vein and removal catheter  400 B may enter the vascular system at the patients right internal jugular vein. 
     Retrieval catheter  400 A is advanced through the vasculature of the patient until distal end  404 A of catheter  400 A is proximate first strand formation  30  of thrombus filter  20 . Likewise, retrieval catheter  400 B is advanced through the vasculature of the patient until distal end  404 A of catheter  400 B is proximate second strand formation  40  of thrombus filter  20   
     Distal end  504 A of elongate member  500 A is then coupled to sliding member  72 . This may be accomplished by mating coupling member  502 A of elongate member  500 A with coupling member  76  of the sliding member  72 . Likewise, distal end  504 B of elongate member  500 B is coupled to sliding member  74 . This may be accomplished by mating coupling member  502 B of elongate member  500 B with coupling member  78  of the sliding member  74 . 
     Sliding member  72  may now be urged towards distal ends  36  of strands  32  by applying a pulling force to proximal end  506 A (not shown) of elongate member  500 A. Simultaneously, sliding member  74  may now be urged towards distal ends  46  of strands  42  by applying a pulling force to proximal end  506 B (not shown) of elongate member  500 B. 
     Urging sliding member  72  toward free ends  36  of strands  32  causes first strand formation  30  to collapse. Likewise, urging sliding member  74  toward free ends  46  of strands  42  causes second strand formation  40  to collapse. 
     Once strands  32 ,  42  move to a collapsed position, thrombus filter  20  may be urged into lumen  402 A of retrieval catheter  400 A. Alternately, thrombus filter  20  may be pulled into lumen  402 B of retrieval catheter  400 B. With thrombus filter  20  disposed inside the lumen of a of retrieval catheter, removing thrombus filter  20  from the body of the patient may be accomplished by withdrawing the retrieval catheter from blood vessel lumen  22 . 
     Other embodiments have been envisioned. For example, sliding member  72  and sliding member  74  may be replaced with one sliding member comprised of a continuous helical coil. In this embodiment, pulling in opposing directions on coupling members  76 ,  78  would cause the helical coil to expand in length. 
     FIG. 5 is a plan view of an alternate embodiment of thrombus filter  20  positioned in a lumen  22  of a blood vessel  24 . Blood vessel  24  includes walls  26  which define lumen  22 . The main components of thrombus filter  20  are a first strand formation  30 , second strand formation  40 , and a joined portion  50 . 
     First strand formation  30  and second strand formation  40  are both comprised of a plurality of strands  32  and  42  respectively. Each strand  32  of first strand formation  30  has a joined end  34  and a free end  36 . Likewise, each strand  42  of second strand formation  40  has a joined end  44  and a free end  46 . Joined ends  34  of strands  32  and joined ends  44  of strands  42  are joined at joined portion  50  of thrombus filter  20 . In the embodiment of FIG. 5, joined portion  50  includes a collar  52  disposed about joined ends  34 ,  44  of strands  32 ,  42 . 
     Strands  32  radiate away from joined portion  50  of thrombus filter  20  so that first strand formation  30  is generally conical in shape. Likewise, strands  42  radiate away from joined portion  50  of the thrombus filter  20  such that second strand formation  40  is generally conical in shape. As shown in FIG. 5, strands  32  of first strand formation  30  and strands  42  of second strand formation  40  radiate in generally opposing directions. 
     When thrombus filter  20  is disposed in blood vessel lumen  22 , at least one of the strand formations acts to trap, or capture blood clots. The generally conical shape the strand formation serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
     Strands  32  and  42  act as opposing wall contacting members and serve to position thrombus filter  20  in the center of lumen  22  of blood vessel  24 . In a presently preferred embodiment, strands  32  and  42  are biased to spring outward. The radial force applied to the walls of the blood vessel by first strand formation  30  and second strand formation  40  assists in preventing migration of thrombus filter  20  within blood vessel lumen  22 . The generally opposed orientation of strands  42  relative to strands  32  also makes migration of the filter less likely. 
     Migration of filter  20  within blood vessel lumen  22  may also made less likely by the inclusion of anchors on thrombus filter  20 . An anchor  62  is disposed at free end  46  of each strand  42 . Likewise, each free end  36  of each strand  32  includes an anchor  60 . In FIG. 5, anchors  60  and  62  are pictured as sharp projections or barbs. It should be understood that anchors  60  and  62  may be comprised of other means for anchoring without departing from the spirit or scope of this invention. It should also be understood that embodiments of thrombus filter  20  which include no anchors are possible without departing from the spirit or scope of the present invention. 
     FIG. 5 includes a removal catheter  590  disposed in blood vessel lumen  22  proximate thrombus filter  20 . Removal catheter  590  includes an outer tubular member  600  having a lumen  602 , a distal end  604 , and a proximal end  606  (not shown). 
     In the embodiment of FIG. 5, an inner tubular member  700  is disposed within lumen  602  of outer tubular member  600 . Inner tubular member  700  includes a lumen  702 , a distal end  704 , and a proximal end  706  (not shown). An elongate shaft  710  and a pulling means  800  are disposed within lumen  702  of inner tubular member  700 . 
     Pulling means  800  includes a plurality of struts  802  each having a distal end  804 , a proximal end  806  (not shown), and a distal portion  808 . An engaging member  810  is disposed proximate the distal end  804  of each strut  802 . In the embodiment of FIG. 5, each engaging member  810  includes a flange  812 . Flanges  812  are adapted to be disposed about collar  52  of thrombus filter  20 . In a presently preferred embodiment, collar  52  has a generally cylindrical outer surface. Also in a presently preferred embodiment each flange  812  includes an inner radius which is substantially equal to the outer radius of collar  52 . When flanges  812  are closed around collar  52  they butt against each other to form a generally tubular shell around collar  52 . 
     In the embodiment of FIG. 5, distal portions  808  of retrieval struts  802  are biased to spring outward. Distal portions  808  of retrieval struts  802  may be selectively urged inward by urging distal end  704  of inner tubular member  700  toward distal ends  804  of retrieval struts  802 . During a surgical procedure, this may be accomplished percutaneously by pushing on proximal end  706  (not shown) of inner tubular member  700 , and/or pulling on proximal ends  806  (not shown) of retrieval struts  802 . 
     A method of removing thrombus filter  20  from blood vessel lumen  22  may now be described with reference to FIG.  5 . The retrieval process typically begins by introducing catheter  590  into the patients vascular system. Once in the vascular system, the retrieval catheter is urged forward until distal end  604  of outer tubular member  600  is proximate thrombus filter  20 . 
     Preferably, distal portions  808  of retrieval struts  802  will be in a retracted position while the distal end of retrieval catheter  590  is advanced through the vasculature. Distal portions  808  may be held in a retracted position by inner tubular member  700 . 
     When the distal end of catheter  590  is proximate thrombus filter  20  inner tubular member  700  may be pulled back, allowing distal portions  808  of struts  802  to spring outward. Struts  802  may then be urged forward until distal ends  804  of struts  802  are proximate joined portion  50  of thrombus filter  20 . 
     Distal portions  808  of struts  802  may then be urged towards joined portion  50  by urging distal end  604  of inner tubular member  700  toward distal ends  804  of struts  802 . This may be accomplished percutaneously by simultaneously pushing on distal end  706  (not shown) of inner tubular member  700  and pulling on distal ends  806  of struts  802 . When distal portions  808  of struts  802  are closed onto joined portion  50 , flanges  812  close around collar  52 . Flanges  812  are adapted so that they butt together and form a substantially tubular shell around collar  52 . 
     Strands  32  may be urged into a collapsed position by urging flanges  812  toward free ends  36  of strands  32 . Flanges  812  may be urged toward free ends  36  of strands  32  by pulling on proximal ends  806  of struts  802 . In a presently preferred method, thrombus filter  20  may be held in position by pushing the distal end of elongate shaft  710  against thrombus filter  20 . Once strands  32  have been moved to a collapsed position, first strand portion  30  of thrombus filter  20  may be positioned within lumen  602  of outer tubular member  600 . This may be accomplished percutaneously by pushing on proximal end  606  of outer tubular member  600  and/or pulling on proximal ends  806  of struts  802 . 
     Once first strand formation  30  is positioned within lumen  602  of outer tubular member  600 , the remainder of thrombus filter  20  may also be urged into lumen  602  of outer tubular member  600 . As described previously, strands  42  of second strand formation  40  radiate away from joined portion  50  of thrombus filter  20  in a generally opposed direction relative to strands  32  of first strand formation  30 . The orientation of strands  42  allows them to be pulled out of walls  26  of blood vessel  22  with minimal force. Strands  42  may be converted to a collapsed position by simultaneously by pushing on proximal end  606  of outer tubular member  600  and pulling on proximal ends  806  of struts  802 . 
     Pulling thrombus filter  20  into lumen  602  of outer tubular member  600  causes strands  42  to collapse causing second strand formation  40  to transform from a generally conical shape to a generally cylindrical shape. With strands  32  and strands  42  in a collapsed position, thrombus filter  20  may be pulled completely into lumen  602  of outer tubular member  600 . With thrombus filter  20  disposed inside lumen  602  of outer tubular member  600 , removing thrombus filter  20  from the body of the patient may be accomplished by withdrawing retrieval catheter  590  from blood vessel lumen  22 . 
     Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The inventions&#39;s scope is, of course, defined in the language in which the appended claims are expressed.