Abstract:
The disclosure pertains to magnetically retrievable vena cava filters having a low torque associated therewith when subjected to a strong external magnetic field and retrieval devices therefor. The vena cava filter may include a magnetically permeable sphere or a spherical dipole magnet located within the spherical cavity of the apical hub wherein the spherical dipole magnet is free to rotate about any of three mutually orthogonal axes. The retrieval device is capable of substantially containing the vena cava filter in a collapsed state and includes a magnetically active member capable of interacting with the vena cava filter. The retrieval device may optionally include a supplemental mechanical latch.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Serial No. 61/415,109 filed Nov. 18, 2010. 
     
    
     BACKGROUND 
       [0002]    Magnetic positioning and retrieval devices for implantable medical devices may offer advantages with respect to both ease of alignment and engagement; however the presence of magnetically active elements such implantable devices may result in undesirable effects if the patient subsequently requires or would benefit from exposure to strong external magnetic fields such as those employed for Magnetic Resonance Imaging (MRI). In particular, a magnetic element which is not aligned with the external magnetic field may experience a torque tending to twist the magnetic element and the associated medical device. In addition, the misaligned field may tend to demagnetize the magnetic element thereby rendering it less suited for magnetic retrieval. These effects may exclude the patient from certain important diagnostic measures. 
         [0003]    These considerations are particularly important for devices such as, for example vena cava filters which are commonly implanted for extended periods of time. Vena cava filters often are emplaced and never retrieved, remaining effective during their time in place, and remaining permanently in place. However, in some cases it may be desirable to remove or reposition a medical filter after it has been in place for a period of time. Retrieval of medical filters can be challenging. One method of retrieval involves use of a catheter with a loop which is pushed out of the catheter and used to snare a hook on the end of a filter. One of the most difficult aspects of vascular and other medical filter retrievals is locating or aligning the correct part of the filter with the operative end or loop of the retrieval catheter. The surgeon is often working through a long catheter extending through a tortuous anatomy while viewing a shadowy image of the filter and retrieval catheter loop in a two dimensional fluoroscopy image. Fluid flow within the vein or body passage may further increase the challenge. 
         [0004]    In light of these challenges, it would be beneficial to have an improved device and method for removal or repositioning of vascular and other medical filters. It would also be desirable to have a practical and economical system and method which facilitate removal of medical filters after implantation. It would be further desirable to have a retrieval system and method which can reduce the time required for retrieval and which can be used even under view of a shadowy, two dimensional image. Furthermore it would be desirable to have a retrieval system and method which can be used from either a femoral or jugular approach and which can be adapted for use with a wide variety of medical filter designs. 
         [0005]    Further, it would be beneficial to provide implantable medical devices, such as vena cava or other filters, which are both magnetically retrievable from the body and which avoid the difficulties which may be encountered when magnetic elements are subjected to strong external magnetic fields. 
       SUMMARY 
       [0006]    This disclosure pertains to magnetically retrievable medical devices, such as vena cava filters, wherein such magnetically retrievable medical devices experience a low torque when subjected to a strong magnetic field and to retrieval devices adapted for use therewith. 
         [0007]    In some embodiments, the magnetically retrievable vena cava filter comprises an apical hub; a plurality of legs each having a first end fixedly attached to the apical hub and a second free end, said plurality of legs defining a generally conical filter basket in a deployed configuration; a magnetically permeable sphere fixedly attached to the apical hub. 
         [0008]    In other embodiments, the magnetically retrievable vena cava filter comprises an apical hub defining therein a generally spherical cavity; a plurality of legs each having a first end fixedly attached to the apical hub and a second free end, said plurality of legs defining a generally conical filter basket in a deployed configuration; and a spherical dipole magnet located within the spherical cavity of the apical hub wherein the spherical dipole magnet is free to rotate about any axis passing through the center of the sphere. 
         [0009]    Each of the plurality of legs of the vena cava filters may have a tissue anchor associated with its free end. In other embodiments, the filter may include a first plurality of legs having tissue anchors and a second plurality of legs which are free of tissue anchors. 
         [0010]    A retrieval device for a magnetically retrievable vena cava filter may include an elongated shaft having a proximal end and a distal end; a housing having a proximal end, a distal end, and a lumen therebetween wherein the proximal end of the housing is fixedly attached to the distal end of the elongated shaft, said housing being sized and adapted to receive substantially an entire magnetically retrievable vena cava filter in a collapsed configuration; and a magnetically active member actuatable within the housing from a first position in which at least a portion of the magnetically active member extends distally beyond the housing to a second position in which the magnetically active member is located near the proximal end of the housing. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  illustrates a first embodiment of a magnetically retrievable vena cava filter. 
           [0012]      FIG. 1A  illustrates the alignment of the magnetization of a magnetically permeable sphere 
           [0013]      FIG. 2  illustrates another embodiment of a magnetically retrievable vena cava filter. 
           [0014]      FIG. 2A  illustrates a detail of the apical head of the embodiment of  FIG. 2 . 
           [0015]      FIG. 3A  illustrates a first retrieval device for a magnetically retrievable vena cava filter in a first position. 
           [0016]      FIG. 3B  illustrates a first retrieval device for a magnetically retrievable vena cava filter in a second position. 
           [0017]      FIG. 4  illustrates another retrieval device for a magnetically retrievable vena cava filter. 
           [0018]      FIG. 4A  illustrates an exemplary latch in a disengaged position. 
           [0019]      FIG. 4B  illustrates an exemplary latch in an engaged position 
           [0020]      FIG. 5A  illustrates another retrieval device for a magnetically retrievable vena cava filter in a disengaged position. 
           [0021]      FIG. 5B  illustrates another retrieval device for a magnetically retrievable vena cava filter in a disengaged position. 
           [0022]      FIG. 6  illustrates an alternate magnetically active member. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The drawings, which are not necessarily to scale, are not intended to limit the scope of the claimed invention. The detailed description and drawings illustrate example embodiments of the claimed invention. 
         [0024]    All numbers are herein assumed to be modified by the term “about” The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
         [0025]    As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include the plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0026]    It is noted that references in the specification to “an embodiment”, “some embodiments”, “other embodiments”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described in combination. 
         [0027]      FIG. 1  illustrates a magnetically retrievable vena cava filter of the present invention shown and generally indicated by the reference numeral  10 . The vena cava filter  10  includes a plurality of wire or ribbon legs  30  which have a generally conical deployed state. The plurality of legs  30  may have any of the configurations known in the art including various curved, bent, or bifurcated sections. The legs  30  may be self-deploying, thermally activated, or may employ any of various leg deploying means known in the art. Generally, the plurality of legs  30  will each have a first end joined to an apical head  24  and a second free end configured to engage the wall of a vessel in which it may be deployed. The wall engaging portion of the filter legs  30  may include tissue engaging hooks  34  as illustrated and/or other structures (not shown) such as a positioning stent portion. In addition, the filter may include additional legs (not shown) which serve to center the filter within the vessel. 
         [0028]    In one embodiment, attached to the apical head  24  is a solid sphere  20  of magnetically permeable material having low hystresis. Suitable materials include mu metal, soft iron, nickel-iron alloys commonly known as permalloy, nickel-iron-molybdenum commonly known as supermalloy, iron-silicon-aluminum alloys commonly known as Sendust, iron-boron-silicon alloys commonly known as Metglas® available from Metglas, Inc. (Conway, S.C.) or low-hystresis loss ferrites such as DN40 and DN70 available from TDK Corp., (Ichikawa, Japan). 
         [0029]    When the magnetically permeable sphere  20  is placed in a strong external magnetic field, such as that found in an MRI apparatus, its magnetization aligns with the external magnetic field direction as shown in  FIG. 1A  and thus the sphere  20  and associated vena cava filter  10  experience no significant torque as a result of the externally applied field. When a magnet  52 ,  152  or energized electromagnet  252 , associated with a retrieval device to be described below, is positioned near the magnetically permeable sphere  20 , it is attracted to the magnet  52 ,  152 ,  252  to effect or assist in effecting the retrieval of the vena cava filter  10 . 
         [0030]    In an alternate embodiment illustrated in  FIG. 2 , the sphere  20  of magnetically permeable material and apical head  24  may be replaced by an apical head  124  defining an internal spherical cavity  128  or a cage (not illustrated) containing a spherical dipole magnet  126 , said spherical dipole magnet  126  being free to rotate about any axis passing through the center of the sphere. In some embodiments, the spherical dipole magnet  126  may comprise oriented neodymium-iron-boron, samarium-cobalt, or equivalent magnetic materials. In other embodiments, the spherical dipole magnet  126  may include a coating which is capable of reducing friction between the magnet and the walls of the cavity  128  or cage. In yet other embodiments, the spherical dipole magnet  126  may comprise a bar magnet encased in a spherical shell of non-magnetic material. 
         [0031]    Although it is desirable to have the spherical dipole magnet  126  nearly fill the cavity  128  or cage in the apical head  124 , it is useful to include a small gap between the walls of the cavity  128  and the spherical dipole magnet  126  to ensure that the spherical dipole magnet  126  is able to rotate freely within the cavity  128  or cage. To promote free rotation of the spherical dipole magnet  126  within the cavity  128  or cage, one or both of the spherical dipole magnet  126  and the apical head  124  may be fabricated using a material having a low coefficient of friction such as polyethylene or polytetrafluoroethylene. Such materials may also tend to protect the magnetic material of the spherical dipole magnet from corrosion by bodily fluids. In some embodiments, it may be desirable to include a number of small surface irregularities on one or both of the spherical dipole magnet  126  and the cavity  128  to reduce the contact area therebetween. In other embodiments, it may be desirable to include a lubricating fluid, such as a ferrofluid to occupy the space between the spherical dipole magnet  126  and the cavity  128 . In yet other embodiments, bodily fluids may provide a lubricating fluid. In still other embodiments, the spherical dipole magnet  126  may be sealed within the cavity  128 . In general, it is desirable that the wall of the apical head be relatively thin or absent in the direction from which a magnetically active material  152  associated with a retrieval device  70  is expected to approach. In addition to the magnet  52  or energized electromagnet  252 , associated with a retrieval device for the vena cava filter  10  embodiment of  FIG. 1 , a magnetically permeable material having low hystresis, similar to those used to fabricate the magnetically permeable sphere  20 , discussed above, may also be satisfactory to effect or assist in effecting the retrieval of the vena cava filter  10  of the embodiment of  FIG. 2 . 
         [0032]    The vena cava filters  10  of  FIGS. 1 and 2  are well suited for retrieval and/or repositioning using a device which employs an magnetically active material including a magnet  52 , electromagnet  152 , or in some cases a magnetically permeable material having low hystresis to locate and position the retrieval device  70  relative to the apical head  24  and/or magnetically permeable sphere  20 . In addition, the magnetically active material associated with the retrieval device  70  may hold the vena cava filter  10  as a retrieval housing is advanced over the vena cava filter  10  and/or as the vena cava filter  10  is withdrawn into the retrieval device. It will be appreciated that the contacting surface of the magnetically active material  52 ,  152 ,  252  or an equivalent element fabricated from magnetically permeable material may be shaped to enhance attraction between the magnetically active material and the magnetically permeable sphere  20  or apical head  124 . For example, the distal end of the magnetically active material  52  may form a hemispherical cavity which receives the magnetically permeable sphere  20 . Non-limiting exemplary retrieval devices are illustrated somewhat schematically in  FIGS. 3-5 . 
         [0033]    As illustrated in  FIGS. 3A and 3B , retrieval device  70  comprises an elongate shaft  40  having fixedly attached to its distal end a housing  45  sized and adapted to receive substantially an entire magnetically retrievable vena cava filter  10  within a lumen  47  thereof. A magnetically active member  52  is actuatable within the lumen  47  of the housing  45  from a first position in which at least a portion of the magnetically active member  52  extends distally beyond the housing  45  and a second position in which the magnetically active member  52  is located near the proximal end of the housing  45 . The magnetically active member  52  may be actuatable by any of the means known for that purpose such as the illustrated shaft  50 , a tether, pneumatic or hydraulic actuators, and the like. When the magnetically active member  52  in the first position is advanced to the vicinity of the vena cava filter  10 , magnetic attraction between the magnetically active member  52  and either the solid sphere  20  of magnetically permeable material of the embodiment of  FIG. 1  or the spherical dipole magnet  126  of the embodiment of  FIG. 2  will cause them to self-align thereby facilitating the retrieval of the associated vena cava filter  10 . 
         [0034]    In addition to self-alignment of magnetically active components of the vena cava filter  10  and the magnetically active member  52 , their mutual attraction allows the vena cava filter  10  to be drawn proximally into the housing  45 . In the alternative, the attractive force between magnetically active components of the vena cava filter  10  and the magnetically active member  52  may serve to maintain contact as the housing  45  is advanced over the collapsed legs  30  of the vena cava filter  10 . In some embodiments, the attractive force alone will suffice to allow the vena cava filter  10  to be withdrawn and positioned for retrieval within the housing  45 . 
         [0035]    In other embodiments, the attractive force between magnetically active components of the vena cava filter  10  and the magnetically active member  52  may be supplemented by the addition of a retractable filter engaging member or latch  62 ,  162  adapted to engage the solid sphere  20  of magnetically permeable material; the apical head  24 ,  124 ; and/or the legs  30  of the vena cava filter  10 . As in the case of the magnetically active member  52 , it will be appreciated that a filter engaging member or latch, if present, may be actuatable and withdrawn by any of the means known for that purpose such as a shaft, a tether, pneumatic or hydraulic actuators, and the like. The engaging component or latch may include hooks, loops, tapers, barbs, detents, grasping elements, threaded connectors, and the like. In some embodiments, the filter engaging member is operably connected to an actuator located proximate the proximal end of the elongate shaft. 
         [0036]    In the embodiment of  FIGS. 4 ,  4 A, and  4 B, a latch  62  comprises two or more wires  60  having a first position generally along the wall of the housing  45  prior to magnetic engagement as illustrated in  FIG. 4A . The latch  62  is biased toward the longitudinal axis of the housing  45  such that when the magnetically active member  52  and magnetically permeable sphere  20  and/or apical head  24 ,  124  are withdrawn proximally relative to the housing  45 , the latch  62  mechanically engages the magnetically permeable sphere  20  and/or apical head  24 ,  124  as shown in  FIG. 4B  thereby allowing additional retraction force to be applied to the vena cava filter  10  by an associated actuating mechanism. 
         [0037]    In the embodiment of  FIGS. 5A and 5B , latch  162  is formed from a shape memory material  160 , such as Nitinol (nickel-titanium), copper-zinc-aluminum-nickel, copper-aluminum-nickel, certain iron or cobalt-based alloys, certain polymers such as polyethylene oxide-polyethylene terephthalate, and the like, disposed between outer housing  145  and an inner tubular member  149 . Following magnetic alignment and engagement between magnetically active member  152  and apical head  24  several modes of action are possible. The shape memory material  160  may be advanced from between outer housing  145  and an inner tubular member  149  to engage, the apical head  24 , whereupon the outer housing  145  may be advanced to encase the vena cava filter  10  in preparation for removal of the filter. In the alternative, the magnetically active member  152 , inner tubular member  149 , and shape memory material latch  162  may be withdrawn together relative to stationary outer housing  145 . Variations in details of the retrieval process are also possible as in the operation of the retrieval device of  FIGS. 4 ,  4 A, and  4 B. In addition, the retrieval device may cooperate with additional elements, such as catheters and sheaths, which may provide optional elements of a deployment and/or retrieval system. 
         [0038]    Although not illustrated herein, alternate latch configurations which engage one or more filter legs  30  concurrently with or following magnetic engagement between the vena cava filter  10  and a magnetically active member  52 ,  152 ,  252  are also contemplated. 
         [0039]    As noted above, various combinations of mutually magnetically attractive elements may be employed in fabricating a vena cava filter  10  and retrieval device  70  for use as a retrieval system. In some embodiments, a magnetically permeable sphere  20  attached to the apical head  24  of the vena cava filter  10  may be used with a permanent magnet  52 ,  152  or with an electromagnetic active member  252  such as illustrated in  FIG. 6  in which current supplied by wires  254  to coil  256  allows the electromagnet to be activated as desired. In other embodiments, a spherical dipole magnet  126  within the apical head  124  of the vena cava filter  10  may be used with a permanent magnet  52 ,  152 , an electromagnetic active member  252 , or a magnetically active member comprising magnetically permeable material. 
         [0040]    Although the illustrative examples described above relate to vena cava filters and retrieval devices therefor, it is contemplated that the principles and structures described herein may also be applied to other filters and medical devices which may be implanted in a body lumen. 
         [0041]    Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and principles of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth hereinabove. It will be appreciated that the vena cava filters and/or the retrieval devices of this invention may include other elements, such as radiopaque marker bands, which may enhance the utility of the system. Other medical devices may incorporate the magnetically active components described in conjunction with the vena cava filter embodiment and retrieval device therefor. All publications and patents are herein incorporated by reference to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference.