Abstract:
A blood clot filter ( 10 ) comprises a number of diverging legs ( 26 ) extending from a common central hub ( 16 ) to define a blood clot reservoir. Adjacent legs ( 26 ) share a common root ( 18 ) from which branches off at least two main branches ( 24 ). Each main branch ( 24 ) connects at a distal end to an adjacent main branch ( 24 ) to form a plurality of interconnected anchoring members ( 30   a   , 30   b ) about an open upstream end of the blood clot reservoir.

Description:
RELATED APPLICATIONS  
       [0001]     This is a continuation of International Patent Application No. PCT/CA2003/001864 filed Nov. 28, 2003, which claims benefit of U.S. Provisional Patent Application No. 60/429,545 filed on Nov. 29, 2002. The contents of both of which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to filters and, more particularly, to a blood clot filter which is particularly suited for filtering emboli from blood circulating through a blood vessel.  
         [0004]     2. Description of the Prior Art  
         [0005]     It is well known to introduce a filter in the inferior vena cava of a patient to prevent pulmonary embolism. Most presently available blood clot filters are permanently implanted in the inferior vena cava and remain there for the duration of the patient&#39;s life. One drawback associated with permanent filters resides in the fact that permanent filters can become clogged and cause obstruction of the inferior vena cava, thereby resulting in chronic swelling of the lower extremities. Efforts have thus been made to develop removable or retrievable vena cava filters.  
         [0006]     In an attempt to provide retrievable filters, it has been proposed to provide radially collapsible filters with retrieving aids, such as hooks, by which the filters can be caught and then percutaneously withdrawn from the patient&#39;s blood vessel through a catheter. However, in practice, particularly where a filter of this type is engaged on a relatively long portion of the length thereof with the inner wall of a blood vessel, such as in the case of the filter disclosed in U.S. Pat. No. 6,059,825 issued on May 9, 2002 to Hobbs et al., it has been found that such “retrievable” filters often become securely embedded in the wall of the blood vessel as their wall engaging portions become endothelialized by the vessel wall, whereby any subsequent attempt to percutaneously remove the filters results in serious risks of injury to the blood vessel or inability to remove these filters. On the other hand, where the filter is of a generally conical shape and anchored to the vessel wall only at a distal end thereof, as for instance disclosed in U.S. Pat. No. 5,324,304 issued on Jun. 28, 1994 to Rasmussen, it becomes difficult to centrally position and maintain the filter in axial alignment with the blood vessel. This might impede the filter&#39;s ability to catch clots and renders the recovery of the filter more difficult.  
         [0007]     The need for centrally positioning and maintaining the filter in axial alignment with the blood vessel and the need for minimizing the integration of the filter with the wall of the blood vessel to permit easy retrieval of the filter have been heretofore difficult to satisfy simultaneously.  
         [0008]     In view of the foregoing, there is thus a need for a new blood clot filter having self-centering and anti-tilting features while still being not prone to rapid integration. It has also been found that there is a need for a new blood clot filter having added clots holding capacity and which is of a more sturdy construction.  
       SUMMARY OF THE INVENTION  
       [0009]     It is therefore an aim of the present invention to provide a new blood clot filter having self-centering feature while still being easily retrievable for a relatively long period of time after having been implanted into a patient&#39;s blood vessel.  
         [0010]     It is also an aim of the present invention to provide a blood clot filter that can still be easily removed from a blood vessel without important risk of injury to the vessel wall even after an extended period of implantation.  
         [0011]     It is a further aim of the present invention to provide a blood clot filter having improved stability once implanted into a blood vessel of a patient.  
         [0012]     It is a still further aim of the present invention to provide a blood clot filter that is of a more sturdy construction.  
         [0013]     It is a still further aim of the present invention to provide a blood clot filter having added clot holding capacity.  
         [0014]     Therefore, in accordance with the present invention, there is provided a filter positionable into a blood vessel for filtering blood clots from a stream of blood, the filter comprising a central hub, a plurality of interconnected legs extending from said central hub and arranged about a central longitudinal axis of the filter to form a filter basket having an open end, wherein each leg is provided at a distal end thereof opposite said hub with an anchoring limb adapted to anchor the filter basket to the blood vessel, wherein said anchoring limbs are distributed about said open end of said filter basket and extend at different distances from said central hub to provide at least two anchoring levels.  
         [0015]     In accordance with another general aspect of the present invention, there is provided a blood clot filter comprising a set of diverging primary branches extending from a common central hub, each primary branch bifurcating into a pair of main branches, wherein each main branch connects at a distal end to an adjacent main branch originating from a different primary branch to form a series of interconnected diverging anchoring members.  
         [0016]     In accordance with a further general aspect of the present invention, there is provided a filter positionable into a blood vessel for trapping blood clots, comprising a number of legs extending from a common central hub to define a blood clot reservoir, wherein adjacent legs share a common root from which branches off at least two main branches, each main branches connecting at a distal end to an adjacent main branch to form a plurality of interconnected anchoring members about an open upstream end of said blood clot reservoir. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which:  
         [0018]      FIG. 1  is a schematic side elevation view, in section, of a blood vessel in which a self-expending, retrievable blood clot filter has been implanted, the filter being shown in a partly expanded state, in accordance with a first embodiment of the present invention;  
         [0019]      FIG. 2  is a perspective view of the filter shown in a fully expanded state; and  
         [0020]      FIG. 3  is a top plan view of the filter shown in  FIG. 2 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     Referring now to  FIG. 1 , a self-centering, self-expandable blood clot filter  10  is shown anchored at two longitudinally spaced-apart locations in a blood vessel  12  (e.g. the inferior vena cava) for filtering blood clots from a stream of blood flowing in the direction indicated by arrow  14 . The filter  10  is preferably placed in the vessel  12  via a delivery catheter (e.g. a 6 French Catheter) inserted through a puncture in the vessel  12 . When in the catheter, the filter  10  is in a radially contracted state. Upon release from the distal end of the catheter, the filter  10  radially expands to its deployed state for securely engaging the inner wall of the vessel  12 , thereby ensuring centering of the filter  10  in the vessel  12 , as shown in  FIG. 1 . The filter  10  is preferably made out of a shape memory material or a temperature responsive material. According to a preferred embodiment of the present invention, the filter  10  is formed of Nitinol, an alloy of titanium and nickel. In this way, the filter  10  can be easily compressed and inserted within a catheter when exposed to temperature below a predetermined transition temperature, but at temperatures at or above the transition temperature, the filter  10  expands and becomes relatively rigid.  
         [0022]     As best shown in  FIG. 2 , the filter  10  is provided at a proximal or downstream end thereof with a retrieval hook  15  extending from a central hub  16  from which radiates a set of primary branches  18  (6 branches in the illustrated embodiment). The primary branches  18  are provided in the form of fine wires or struts and are uniformly distributed in a wide opening cone configuration about a central longitudinal axis  20  of the filter  10  to define a relatively wide shoulder  22  immediately upstream of the central hub  16 . According to an embodiment of the present invention, the shoulder has a diameter “a” of 15 mm (see  FIG. 3 ). However, the diameter “a” could range from about 10 to 20 mm. The opening angle θ of the branches  18  is preferably of at least about 50 degrees when the filter  10  is in a fully expanded state. The opening angle “θ” will normally range from 50 to 70 degrees. As will be seen hereinafter, the fact that the primary branches  18  open rapidly close to the full width of the filter advantageously contributes to increase the amount of blood clots that can be trapped by the filter  10  in that it provides for the formation of a filter basket having added volume holding capacity as compared to fully conical filters. The relatively wide shoulder  22  also acts as an anti-tilting member by limiting the angular freedom of movement of the filter  10  in the blood vessel  12 . This contributes to ensure that the filter  10  remains in axial alignment with the blood vessel  12  in which it has been implanted, thereby increasing the efficiency of the filter  10 . This also facilitates retrieval of the filter by centering the retrieval hook  15 .  
         [0023]     As shown in FIGS.  1  to  3 , each primary branch  18  bifurcates at a Y-shaped junction into a pair of diverging main branches  24  that extend angularly outwardly and then linearly downwardly away from the downstream end of the filter  10 . The main branches  24  are collectively arranged in a generally cylindrical or slightly conical geometric configuration about the central axis  20  of the filter  10 . The filter  10  is less likely to cause occlusion due to its cylindrical shape which can accommodate larger amounts of thrombus as compared to cone-shaped filters. According to one aspect of the present invention, each main branch  24  connects at a distal end thereof to an adjacent main branch  24  originating from a different primary branch  18 , thereby providing a sturdy framework of branches interconnected to one another so that a force applied to a single branch will be automatically distributed to the other branches. The main branches  24  are grouped in pairs so as to form a set of circumferentially-spaced legs  26  (6 in the illustrated embodiment), each leg  26  being partly formed of two adjacent main branches  24  which are united together at their distal ends through a V-shaped junction but which are connected at their respective proximal ends to different primary branches  18 . Each leg  26  defines an open cell  28  having opposed triangular ends and an intermediate rectangular section.  
         [0024]     Each leg  26  terminates into an anchoring limb  30  extending in continuity from the lower V-shaped junction of two adjacent main branches  24 . Each anchoring limb  30  has a hooked distal end  32  for penetrating the wall of the blood vessel  12  to anchor the filter  10  against movement. As shown in  FIG. 3 , the limbs  30  are preferably angled outwardly by an angle β ranging from 120 to 160°. The bend in each limb  30  contributes to minimize the surface of contact between the legs  26  and the wall of the blood vessel  12 . By so limiting the surface of contact between the body of the filter  10  and the wall of the blood vessel  12 , the growth of tissue over the filter  10  is significantly reduced and, thus, the filter  10  can remain implanted in the vessel  12  for a longer period of time without becoming securely embedded in the vessel wall. This advantageously enables easy retrieval of the filter  10  for extend periods of time.  
         [0025]     To maintain the filter  10  centered in the vessel with minimal contact therewith, the limbs  30  are not all equal in length but rather includes a set of long limbs  30   a  and a set of short limbs  30   b  so as to provide two anchoring levels. By so anchoring the filter  10  at two longitudinally spaced-apart locations in the blood vessel  12 , the filter  10  remains more stable even though it only has discrete points of contact with the vessel wall. This is also advantageous in that the pressure exerted by the filter  10  on the blood vessel are more uniformly distributed, thereby reducing the risk of local injury to the vessel wall.  
         [0026]     As best seen from  FIG. 3 , the legs  26  are alternately provided with long and short anchoring limbs  30   a  and  30   b  to provide a succession of short and long anchoring limbs  30  about the open end of the filter basket. The distal hooked ends of the short anchoring limbs  30   b  lie on a circle having a diameter “b” ranging from 30 to 45 mm when the filter is fully expanded. Likewise, the distal hooked ends of the long anchoring limbs  30   a  lie on a circle having a diameter “c” ranging from 30 to 45 mm when the filter is fully expanded.  
         [0027]     According to an embodiment of the present invention, the struts forming the primary branches  18  and the main branches  24  are rectangular in cross-section. For instance, each strut could have a cross-section of 0.25 mm by 0.35 mm. The anchoring limbs  30  could also have a rectangular cross-section but of 0.7 mm by 0.25 mm.  
         [0028]     According to another feature of the present invention, there is provided a rapid transition in width from the limbs  30  to the fixation hooks  32  to prevent the limbs  30  from penetrating completely through the wall of the blood vessel.