Abstract:
A removable filter for capturing thrombi in a blood vessel. The filter comprises a plurality of primary struts having first ends connected to each other to define a central axis of the filter. Each primary strut has a curved member extending from the central axis and terminates at an anchoring hook to engage the blood vessel at a first axial plane. The filter further comprises a plurality of secondary struts connected to the curved members of the primary struts and extending therefrom to a free end at a second axial plane to centralize the filter in the blood vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/776,721, filed Feb. 11, 2004, entitled “REMOVABLE VENA CAVA FILTER”, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/446,711, filed Feb. 11, 2003, entitled, “REMOVABLE VENA CAVA CLOT FILTER,” each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to medical devices. More particularly, the invention relates to a removable vena cava clot filter that can be percutaneously placed in and removed from the vena cava of a patient. 
     Filtering devices that are percutaneously placed in the vena cava have been available for over thirty years. A need for filtering devices arises in trauma patients, orthopedic surgery patients, neurosurgery patients, or in patients having medical conditions requiring bed rest or non-movement. During such medical conditions, the need for filtering devices arises due to the likelihood of thrombosis in the peripheral vasculature of patients wherein thrombi break away from the vessel wall, risking downstream embolism or embolization. For example, depending on the size, such thrombi pose a serious risk of pulmonary embolism wherein blood clots migrate from the peripheral vasculature through the heart and into the lungs. 
     A filtering device can be deployed in the vena cava of a patient when, for example, anticoagulant therapy is contraindicated or has failed. Typically, filtering devices are permanent implants, each of which remains implanted in the patient for life, even though the condition or medical problem that required the device has passed. In more recent years, filters have been used or considered in preoperative patients and in patients predisposed to thrombosis which places the patient at risk for pulmonary embolism. 
     The benefits of a vena cava filter have been well established, but improvements may be made. For example, filters generally have not been considered removable from a patient due to the likelihood of endotheliosis of the filter during treatment. After deployment of a filter in a patient, proliferating intimal cells begin to accumulate around the filter struts which contact the wall of the vessel. After a length of time, such ingrowth prevents removal of the filter without risk of trauma, requiring the filter to remain in the patient. As a result, there has been a need for an effective filter that can be removed after the underlying medical condition has passed. 
     Moreover, conventional filters commonly become off-centered or tilted with respect to the hub of the filter and the longitudinal axis of the vessel in which it has been inserted. As a result, the filter including the hub and the retrieval hook engage the vessel wall along their lengths and potentially become endothelialized therein. This condition is illustrated in prior art  FIG. 1  in which a prior art filter  13  has been delivered through a delivery sheath  25  into a blood vessel  51 . In the event of this occurrence, there is a greater likelihood of endotheliosis of the filter to the blood vessel along a substantial length of the filter wire. As a result, the filter becomes a permanent implant in a shorter time period than otherwise. 
     Some filters have been designed so that the filter has minimal contact with the vessel wall. Ideally, some filters can be removed after several weeks with minimal difficulty and little injury to the vessel wall. One such filter is described in U.S. Pat. No. 5,836,968. The filter is designed so that the filter wires or struts are not positioned parallel to the vessel walls or not in contact with the vessel walls for a substantial portion of the length of the filters. The ends of the struts contact the vessel walls and provide anchoring to reduce the likelihood of filter migration. When the filter is removed, a wire is docked to one end of the device while a sheath or sleeve is passed over the wire. Using counter traction by pulling the wire while pushing the sheath, the sheath is passed over the filter and the filter struts are retracted from the vessel wall. In this way, only small point lesions are created where the filter was attached to the vessel wall. 
     The filter of U.S. Pat. No. 5,836,968 teaches two levels of oppositely expanding filter wires or struts to insure that the filter is properly aligned in the lumen of the vessel. If the filter tilts or becomes misaligned with the central axis of the vessel, the filter wires will contact the wall of the vessel along a greater area, and become endothelialized. As a result of the two levels, removal of the filter from the blood vessel becomes impossible or at least difficult. 
     Additionally, the configuration of the second level of filter wires in the device of U.S. Pat. No. 5,836,968 provides a filter which may be too long for the segment of the vessel that the filter would normally be placed. The normal placement segment of a vena cava filter is between the femoral veins and the renal veins. If the lower part of the filter extends into the femoral veins, filtering effectiveness will be compromised. Moreover, it is not desirable to have filter wires crossing the origin of the renal veins, since the filter wires may interfere with the flow of blood from the kidneys. In the device disclosed in U.S. Pat. No. 5,836,968, both levels of filter wires are attached at one point as a bundle at the central axis of the filter. The resulting diameter of this bundle of filter wires results in a filter that may be too large for easy placement and becomes an obstacle to blood flow in the vena cava. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a vena cava filter comprising struts configured to align the filter about the center axis of a blood vessel and minimize engagement with the blood vessel. The filter comprises a plurality of primary struts, each of which having a first end. A hub axially connects the first ends of the struts to define a central axis of the filter. Each primary strut has a curved member extending from the central axis. Each curved member terminates at an anchoring hook to engage the blood vessel at a first axial plane and secure the filter in the blood vessel. Each anchoring hook includes a barb formed at an angle relative to the strut to allow a removal sheath to be advanced over the filter and allow the hooks to be removed straight away from the vessel wall, resulting in minimal vessel damage. The filter further comprises a plurality of secondary struts. Each secondary strut is connected to one of the curved members and extends therefrom to a free end for engaging the blood vessel at a second axial plane, aligning the filter in the blood vessel. 
     In one embodiment, a set of at least two secondary struts are connected to the curved member of one primary strut. The set of secondary struts extend radially from each side of the primary strut, forming a netting configuration of the filter. In another embodiment, one secondary strut is connected to the curved member of one primary strut. The secondary strut extends from the primary strut and is in radial alignment with the primary strut, avoiding interference with blood flow. 
     In a collapsed configuration, the vena cava filter occupies a reduced diameter, since the hub is the origin to only primary struts. In an expanded configuration, the hub occupies a reduced cross-sectional area. As a result, interference with blood flow is lessened in the vena cava. 
     In an expanded configuration, the vena cava filter occupies a reduced length, since the secondary struts merely extend within the axial length of the primary struts. As a result, the filter can more easily be placed in the vena cava of a patient, lessening the risk of interference in the femoral and renal veins. 
     Further aspects, features, and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a prior art filter deployed in a blood vessel; 
         FIG. 2  is an illustration of the anatomy of the renal veins, the femoral veins, and the vena cava in which one embodiment of a vena cava filter of the present invention is deployed; 
         FIG. 3  is a side perspective view of one embodiment of the vena cava filter of the present invention; 
         FIG. 4  is a cross-sectional view of a blood vessel showing the filter of the present invention partially deployed; 
         FIG. 5  is a cross-sectional view of a blood vessel showing the filter of the present invention fully deployed; 
         FIG. 6  is a cross-sectional view of a blood vessel in which the filter of  FIG. 3  has been deployed; 
         FIG. 7  is a cross-sectional view of the blood vessel of  FIG. 6  taken along line  7 - 7 ; 
         FIG. 8  is a cross-sectional view of a blood vessel showing a portion of a retrieval device for the filter in  FIG. 3 ; 
         FIG. 9  is a side perspective view of a vena cava filter in accordance with another embodiment of the present invention; and 
         FIG. 10  is a cross-sectional view of a blood vessel in which the filter in  FIG. 9  is disposed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with a first embodiment of the present invention,  FIG. 2  illustrates a vena cava filter  20  implanted in the vena cava  50  for the purpose of lysing or capturing thrombi carried by the blood flowing through the femoral veins  54 ,  56  toward the heart and into the pulmonary arteries. As shown, the femoral veins from the legs merge at juncture  58  into the vena cava  50 . The renal veins  60  from the kidneys  62  join the vena cava  50  downstream of juncture  58 . The portion of the vena cava  50 , between the juncture  58  and the renal veins  60 , defines the inferior vena cava  52  in which the vena cava filter  20  has been percutaneously deployed through one of the femoral veins  54 . Preferably, the vena cava filter  20  has a length smaller than the length of the inferior vena cava  52 . If the lower part of the filter extends into the femoral veins, filtering effectiveness will be compromised and if the filter wires cross over the origin of the renal veins the filter wires might interfere with the flow of blood from the kidneys. 
     The first embodiment of the present invention will be discussed with reference to  FIGS. 3-8  in which filter  20  is shown.  FIG. 3  illustrates filter  20  comprising four primary struts  12  each having first ends that emanate from a hub  10 . Hub  10  secures the first ends of primary struts  12  together in a compact bundle to define a central or longitudinal axis of the filter. The hub  10  has a minimal diameter for the size of wire used to form the struts. Preferably, the primary struts  12  are formed from stainless steel wire, MP35N, Nitinol, or any other suitable superelastic material that will result in a self-opening or self-expanding filter. In this embodiment, the primary struts  12  are formed from wire having a round cross-section with a diameter of about 0.015 inches. Of course, it is not necessary that the primary struts have a round cross-section. For example, the primary struts could have a square shaped or other suitable shaped cross section without falling beyond the scope or spirit of the present invention. 
     Each primary strut  12  is formed with a first curved portion  13  that is configured to bend away from the longitudinal or central axis of the filter  20  and a second curved portion  15  that is configured to bend toward the longitudinal axis of the filter  20 . Each primary strut  12  maintains a non-parallel relationship with the longitudinal axis of the filter  20 . The primary struts  12  terminate at anchoring hooks  18  that will anchor in the vessel wall when the filter  20  is deployed at a delivery location in the blood vessel. When the filter is deployed, the anchoring hooks define a first axial plane to secure the filter in the blood vessel. The anchoring hooks  18  prevent the filter  20  from migrating from the delivery location in the blood vessel where it has been deposited. The primary struts  12  are shaped and dimensioned such that, when the filter  20  is deployed and expanded, the filter  20  has a diameter of about 35 mm and a length of about 5 cm. For example, when expanded, the filter  20  may have a diameter of between about 30 mm and 40 mm, and a length of between about 3 cm and 7 cm. The primary struts  12  have sufficient spring strength that when the filter is deployed the anchoring hooks  18  will anchor into the vessel wall. 
     In this embodiment, each primary strut  12  has two secondary struts  14  secured thereto by laser welding, brazing, crimping or any suitable process that will avoid damaging the material or adding to the thickness of the filter and thus the size of the delivery system. The secondary struts  14  may be made from the same type of material as the primary struts. However, the secondary struts may have a smaller diameter, e.g., about 0.012 inches, than the primary struts. Each of the secondary struts  14  is formed of a single curve and is secured to one of the primary struts  12  on its first curved portion  13  such that the secondary strut  14  becomes a continuation or an extension of the first curved portion  13  of the primary strut  12 . In this embodiment, two secondary struts  14  flare away from each side of one primary strut  12  to form a part of a netting configuration of the filter  20 . 
     When opened, free ends  17  of the secondary struts  14  will expand radially outwardly to a diameter of about 35 mm to engage the vessel wall. For example, the secondary struts  14  may expand radially outwardly to a diameter of between about 30 mm and 40 mm. The free ends  17  define a second axial plane where the vessel wall is engaged. The secondary struts  14  function to stabilize the position of the filter  10  about the center of the blood vessel in which it is deployed. As a result, the filter  20  has two layers or planes of struts longitudinally engaging the vessel wall of the filter. The length of the filter is preferably defined by the length of a single set of primary struts. Furthermore, the diameter of the hub  10  is defined by the size of a bundle containing the primary struts  12 . In this embodiment, the eight secondary struts, although maintaining the filter in a centered attitude relative to the vessel wall and formed as a part of the netting configuration of the filter, minimally add to the diameter of the hub or the overall length of the filter. 
       FIG. 4  illustrates the filter  20  partially deployed in inferior vena cava  52 . For deployment of the filter  20 , a delivery tube  24  is percutaneously inserted through the patient&#39;s vessels such that the distal end of the delivery tube is at the location of deployment. In this embodiment, a wire guide is preferably used to guide the delivery tube to the location of deployment. The filter is preferably inserted through the proximal end of the delivery tube  24  with the removal hook  16  leading and free ends of the primary struts  12  held by a filter retainer member. The filter retainer member may be connected to a pusher wire (not shown) that is fed through the proximal end of the delivery tube  24  until the filter reaches the distal end of the delivery tube  24 . For a more complete disclosure of a filter delivery system that may be used to deliver the filter  20  to a desired location, reference may be made to U.S. Pat. No. 5,324,304 which is incorporated herein by reference. 
     As shown in  FIG. 4 , filter  20  is deployed leading with removal hook  16  from the delivery tube  24 . The secondary struts expand first. When the free ends of the secondary struts emerge from the distal end of delivery tube  24 , the secondary struts expand to an expanded position shown in  FIG. 4 . The free ends engage the inner wall of the vessel in which the filter is being deployed. The free ends of the secondary struts function to stabilize the attitude of filter  20  about the center of the blood vessel. The filter is then pushed further by the pusher wire (not shown) until it is fully deployed as shown in  FIG. 5 . 
     As shown in  FIG. 5 , the ends of the primary struts  12  and the secondary struts  14  are in engagement with the vessel wall. The anchoring hooks of the primary struts have anchored the filter at the location of deployment in the vessel, preventing the filter  20  from moving with the blood flow through the vessel. As a result, the filter  20  is supported by two sets of struts that are spaced axially along the length of the filter. The struts avoid engaging the vessel wall along their lengths and thus avoid becoming endothelialized in the vessel wall. 
       FIGS. 6 and 7  show the filter  20  fully expanded after being deployed in inferior vena cava  52 . In  FIG. 6 , the inferior vena cava  52  has been broken away so that the filter  20  can be seen. The direction of the blood flow BF is indicated in  FIG. 6  by the arrow that is labeled BF. The anchoring hooks  18  at the ends of the primary struts  12  are shown as being anchored in the inner lining of the inferior vena cava  52 . The anchoring hooks  18  include barbs  19  that, in one embodiment, project toward the hub  10  of the filter. The barbs  19  function to retain the filter  20  in the location of deployment. 
     In this embodiment, the filter  20  is pushed in a direction BF of the blood flow by the pusher wire (not shown) during deployment. The pusher wire pushes the filter  20  from the delivery tube, causing the barbs  19  to move in the direction BF of the blood flow and secure anchoring hooks  18  in the inferior vena cava  52 . The spring biased configuration of the primary struts  12  causes the anchoring hooks  18  to puncture the vessel wall and anchor the filter at the location of deployment. After initial deployment, the pressure of the blood flow on the filter  20  contributes in maintaining the barbs  19  anchored in the inner lining of the inferior vena cava  52 . As seen in  FIG. 6 , the free ends  17  of secondary struts  14  also have a spring biased configuration to engage with the vessel wall. In this embodiment, the free ends  17  of secondary struts  14  are not provided with anchoring hooks, minimizing the trauma of retrieving the filter  20 . 
       FIG. 7  illustrates a netting configuration formed by the primary struts  12 , secondary struts  14 , and the hub  10 . The netting configuration shown in  FIG. 7  functions to catch thrombi carried in the blood stream prior to reaching the heart and lungs to prevent the possibility of a pulmonary embolism. The netting configuration is sized to catch and stop thrombi that are of a size that are undesirable to be carried in the vasculature of the patient. As shown, the hub  10  houses a bundle of first ends of the four primary struts  14 . Due to its compacted size, the hub minimally resists blood flow. 
     As seen in  FIG. 6 , the hub  10  and removal hook  16  are positioned downstream from the location at which the anchoring hooks  18  are anchored in the vessel. When captured by the struts, thrombi remains lodged in the filter. The filter along with the thrombi may then be percutaneously removed from the vena cava. When the filter  20  is to be removed, the removal hook  16  is preferably grasped by a retrieval instrument that is percutaneously introduced in the vena cava in the direction opposite to the direction in which the filter was deployed. 
       FIG. 8  illustrates part of a retrieval device  65  being used in a procedure for removing the filter  20  from the inferior vena cava  52 . The retrieval device  65  is percutaneously introduced into the superior vena cava via the jugular vein. In this procedure, a removal catheter or sheath  68  of the retrieval device  65  is inserted into the superior vena cava. A wire  70  having a loop snare  72  at its distal end is threaded through the removal sheath  68  and is exited through the distal end of the sheath  68 . The wire is then manipulated by any suitable means from the proximal end of the retrieval device such that the loop snare  72  captures the removal hook  16  of the filter  20 . Using counter traction by pulling the wire  70  while pushing the sheath  68 , the sheath  68  is passed over the filter. As the sheath  68  passes over the filter  20 , the secondary struts  14  and then the primary struts  12  engage the edge of the sheath  68  and are caused to pivot at the hub  10  toward the longitudinal axis of the filter. The pivoting toward the longitudinal axis causes the ends of the struts  14  and  12  to be retracted from the vessel wall. In this way, only surface lesions  74  and small point lesions  76  on the vessel wall are created in the removal procedure. As shown, the surface lesions  74  are created by the ends of the secondary struts  14  and the small point legions  76  are created by the anchoring hooks  18  of the primary struts  12 . However, it is to be noted that any other suitable procedure may be implemented to remove the filter from the patient. 
     A second embodiment of the present invention will be discussed with reference to  FIGS. 9 and 10  in which a filter  28  is shown.  FIG. 9  illustrates filter  28  comprising six primary struts  32  each having first ends that emanate from a hub  30 . Hub  30  secures the first ends of primary struts  32  together in a compact bundle to define a central axis of the filter. Similar to the hub  10  in the first embodiment discussed above, the hub  30  in this embodiment has a minimal diameter for the size of wire used to form the struts. 
     The primary struts  32  in this embodiment are similar in structure to the primary struts  12  in the first embodiment above. For example, in the second embodiment, each primary strut  32  of the filter  28  includes first and second curved portions  33  and  35 , removal hook  36 , free ends  37 , an anchoring hook  38 , and a barb  39  which are respectively similar to the first and second curved portions  13  and  15 , removal hook  16 , free ends  17 , the anchoring hook  18 , and the barb  19  of the filter  28  in the first embodiment. Preferably, the primary struts  32  are shaped and dimensioned such that, when the filter  28  is deployed and expanded, the filter  28  has a diameter of about 35 mm and a length of about 5 cm. For example, when expanded, the filter  28  may have a diameter of between about 30 mm and 40 mm, and a length of between about 3 cm and 7 cm. The primary struts  32  have sufficient spring strength such that when the filter is deployed the anchoring hooks  38  will anchor into the vessel wall. 
     Preferably, the primary struts  32  are formed of the same material as the primary struts  12  mentioned above, e.g., stainless steel wire, MP35N, Nitinol, or any other suitable material. In this embodiment, the primary struts  32  are formed from wire having a round cross-section with a diameter of about 0.015 inches. As stated above, it is not necessary that the primary struts have a round cross-section. 
     In this embodiment, each primary strut  32  has one secondary strut  34  secured thereto by laser welding, brazing, crimping or any suitable process that will not damage the material or add to the thickness of the filter and thus the size of the delivery system. The secondary struts  34  may be made from the same type of material as the primary struts. Preferably, the secondary struts may have a smaller diameter, e.g., about 0.012 inches, than the primary struts. As in the first embodiment, each of the secondary struts  34  in this embodiment is formed of a single curve and is secured to one of the primary struts  32  on the first curved portion such that the secondary strut  34  becomes a continuation or extension of the first curved portion of the primary strut  32 . As shown, each of the secondary struts  34  flares away from one primary strut  32  and is in radial alignment therewith. 
     When opened, the free ends of the secondary struts  34  will expand outwardly to a diameter of about 35 mm to engage the vessel wall. For example, the secondary struts  34  may expand outwardly to a diameter of between about 30 mm and 40 mm. Similar to the secondary struts  14  in the first embodiment, the secondary struts  34  in this embodiment function to stabilize the position of the filter  28  about the center of the blood vessel in which it is deployed. As a result, the filter  28  has two layers or planes of struts longitudinally engaging the vessel wall of the filter. The length of the filter is preferably defined by the length of a single set of primary struts. Furthermore, the diameter of the hub  30  is defined by the size of a bundle containing the primary struts  32 . As in the first embodiment, the secondary struts in this embodiment, although maintaining the filter in a centered attitude relative to the vessel wall and formed as a part of a netting configuration of the filter, minimally add to the diameter of the hub or the overall length of the filter. 
       FIG. 10  illustrates the netting configuration of the filter  28  formed by the primary struts  32  and the hub  30 . As shown, the secondary struts  34  are positioned behind and in alignment with the primary struts  32  and, thus, avoid substantially affecting blood flow. The netting configuration functions to catch thrombi carried in the blood stream prior to reaching the heart and lungs to prevent the possibility of a pulmonary embolism. The netting configuration is sized to catch and stop thrombi that are of a size that are undesirable to be carried in the vasculature of a patient. As shown, the hub  30  houses a bundle of ends of the six primary struts  34 . Due to its compacted size, the hub minimally resists blood flow. 
     It is to be noted that the filter  28  may be deployed in the vena cava in the same manner previously discussed for filter  20  with reference to  FIGS. 2 ,  4 , and  5 . Additionally, the filter  28  may be removed from the vena cava with the removal procedure previously discussed for filter  20  with reference to  FIG. 8 . 
     Although the embodiments of this device have been disclosed as being constructed from wire having a round cross section, it could also be cut from a tube of suitable material by laser cutting, electrical discharge machining or any other suitable process. 
     While the present invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made to those skilled in the art, particularly in light of the foregoing teachings.