Abstract:
A vena cava filter includes a plurality of filter legs with distal ends. Each distal end is a tip which curves radially outwardly and ends in a spherical or part spherical enlarged head element. A biodegradable barb is attached to the enlarged head element and is made of a biodegrade material, such that after degradation the enlarged head element also is left embedded in the vessel wall. The degradation of the barb will prevent further penetration of the distal end of the filter legs into the vessel wall and avoid any risk of piercing through the vessel wall. The enlarged head element also ensures that the distal ends of the filter legs remain firmly attached into the vessel wall in order to ensure correct long term placement of the filter in the vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(a) to European Patent Application No. EP 15275133.5, filed May 11, 2015, and to Great Britain Patent Application No. GB 1411590.1, filed Jun. 30, 2014, which are hereby incorporated by reference in their entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an implantable medical device and in the preferred embodiments to an improved vascular filter and anchoring arrangement for such a filter. 
       BACKGROUND ART 
       [0003]    There are various types and designs of vena cava filters, each having particular advantages and disadvantages. Generally, it is desirable to have a filter design which is able to be deployed endoluminally into a vessel via the jugular or femoral vein by means of the well-known Seldinger technique. The filter is carried on an introducer assembly in a radially compressed condition and once positioned at the desired location it is released and allowed to expand until it engages the walls of the vessel. It is often also desirable to remove the filter after a period of treatment, in which case the filter is designed such that it can be compressed back to its radially contracted configuration and retrieved endoluminally, normally into a sheath of the retrieval assembly. The filter is typically removed via the jugular or femoral vein. 
         [0004]    It is important for vena cava and other filters to remain reliably in position for the duration of the period in which they are left in the patient. In particular, the filter must remain in the correct orientation and also must not migrate from the site at which is it first placed. For this purpose, filters are generally provided with filter arms or legs which spring radially outwardly to press against the vessel wall. The extremities of the legs are either sharp or have specific anchor elements which pierce into the vessel wall. 
         [0005]    An advantageous design of filter has a conical or part-conical frame provided with a plurality of radially disposed legs coupled to one another at a closed end, or hub, of the frame. Such a filter can provide reliable filtration, can be compressed to a very small diameter for ease of delivery and can minimise the amount of foreign material implanted into the patient. 
         [0006]    A problem which can occur with filter assemblies, and which may also be experienced with other types of implantable medical device such as stents, stent grafts, and occlusion devices, is that as a result of the opening force generated by the device struts or legs and the natural movement of the vessel, the anchoring elements can become progressively further embedded into the vessel wall and eventually may pierce through the entire thickness of the vessel. This effect could be expressed as ratcheting of the filter legs and is described in further detail below. 
         [0007]    Examples of implantable medical devices including filters can be found in US-2012/0283811, US-2009/0306703, US-2008/0027481 and US-2013/0006294. 
       DISCLOSURE OF THE INVENTION 
       [0008]    The present invention seeks to provide an improved implantable medical device and in the preferred embodiments to an improved vascular filter and anchoring arrangement therefor. 
         [0009]    According to an aspect of the present invention, there is provided an implantable medical device including a frame, the frame including at least one leg element having a vessel engaging end and an anchoring element at said end; wherein the anchoring element includes a bulbous element and a barb member extending from the bulbous element; wherein the barb member is made of biodegradable material. 
         [0010]    This structure of leg extremity provides a restraining member, namely the bulbous element, which restrains the barb of the leg from penetrating too deeply into the vessel wall. 
         [0011]    In an embodiment the bulbous element is generally spherical or part-spherical. It has been found that such a shape is optimal for restraining the leg extremity in the vessel wall and in some instances can also facilitate endothelialisation of the leg extremity. This characteristic assists in retaining the leg extremity in position relative to the vessel wall. 
         [0012]    The biodegradable nature of the barb member can provide a significant advantage in that the barb, that is the piercing element, is not a permanent component of the device and once it has degraded the device is preferably left with no other vessel piercing member. The bulbous element, which can become fixed into the vessel wall by the previous piercing action of the anchor element, will act to hold the leg and therefore the device in place in the vessel. For this purpose, it will be appreciated that the bulbous element preferably has a greater diameter than the diameter of the leg, so as to become trapped in the vessel wall both against further progression into the vessel wall and against retraction from the vessel wall. 
         [0013]    In an embodiment, the barb element has an elongate needle shape. In this embodiment, the barb may be biodegradable or non-degradable. 
         [0014]    Advantageously, the barb element may have a tapering shape. 
         [0015]    In an embodiment, the barb element extends in a radially outward direction from the vessel engaging end of the leg. The barb element may extend in a direction rearward relative to a direction of the at least one leg. 
         [0016]    In some embodiments, the barb element extends over the bulbous element. 
         [0017]    The bulbous element may include a barb fixation recess, the barb including a protrusion extending into the fixation recess. 
         [0018]    Preferably, the barb element includes a base portion having a diameter substantially the same as or greater than a maximum diameter of the bulbous element. 
         [0019]    In a practical embodiment, the implantable medical includes a plurality of leg elements disposed in spaced relation to one another and extending to radially outward positions. 
         [0020]    The device may be a vascular filter such as a vena cava filter. 
         [0021]    According to another aspect of the present invention, there is provided an anchoring member for an implantable medical device including a bulbous element and a barb member extending from the bulbous element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: 
           [0023]      FIGS. 1A to 1C  are schematic diagrams showing a leg of a vena cava filter against a vessel wall and depicting how the leg progressively penetrates through the vessel wall during movement thereof; 
           [0024]      FIGS. 2A to 2C  are additional schematic diagrams depicting the ratcheting process which can occur with prior art filter designs; 
           [0025]      FIG. 3  is a schematic diagram of an embodiment of vena cava filter according to the teachings herein; 
           [0026]      FIG. 4  is an enlarged view of a filter leg extremity of the filter of  FIG. 3 , after a degradation period; 
           [0027]      FIG. 5  shows the filter of  FIG. 3  after degradation of barbs elements thereof; 
           [0028]      FIG. 6  shows an embodiment of biodegradable barb structure for the filter of  FIGS. 4 to 6 ; 
           [0029]      FIG. 7  shows another embodiment of biodegradable barb structure for the filter of  FIGS. 4 to 6 ; 
           [0030]      FIGS. 8A to 8C  show another embodiment of biodegradable barb structure for the filter of  FIGS. 4 to 6 ; and 
           [0031]      FIG. 9  shows another embodiment of barb structure for the filter of  FIGS. 4 to 6 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    The accompanying drawings are schematic only. It is to be understood that the dimensions and proportions of the various components of the devices shown in the drawings are not to scale or in proportion relative to one another. It is also to be understood that the drawings depict only the principal components of the device shown therein and that other elements and components of the device which are not central to understanding the teachings herein have been omitted for the sake of clarity. 
         [0033]    Referring first to  FIGS. 1A to 1C , these show an example of conventional filter arrangement and specifically a filter leg  10  of such a filter and a part of the vena cava wall  12 . A person skilled in the art will appreciate that a filter of such a nature will have a plurality of filter legs  10  which extend radially outwardly from a central hub in a generally conical arrangement. The filter legs  10  are typically spaced circumferentially so as to contact the vena cava wall  12  at spaced positions around the vessel wall. 
         [0034]    At the extremity of each filter leg  10  there is provided an anchor element  14 , which is shown schematically as being a section of the leg wire angled by approximately 90° to the main portion of the leg  10 . The anchor element  14  includes a sharp extremity or barb  16  designed to pierce into the vessel wall  12 , and a rearwardly extending hook  18  which in practice ensures that the anchor element  14  cannot be pulled back out of the vessel wall. 
         [0035]    With reference to  FIG. 1A , the vessel wall  12  is shown in continuous lines in what could be described as its normal at rest condition. The anchor element  14  can be seen having pierced into the vessel wall  12  to hold the end of the filter leg  10  to the vena cava wall and therefore to hold the filter in position. The dotted line  20  in  FIG. 1A  depicts the movement of the vessel wall  12 , which can be expected to occur during normal conditions, for example as a result of changes in blood pressure caused by normal heart function. In  FIG. 1A , the vessel wall  12  is shown moving inwardly and thus towards the filter assembly, whereas in  FIG. 1B  the dotted line  20  shows the wall extending outwardly of the centre of the vessel and therefore away from the filter. Again, in  FIG. 1C  the dotted line  20  shows the vessel wall moving inwardly again, towards the centre of the vessel. 
         [0036]    As will be apparent from  FIG. 1A , when the vessel wall  12  moves inwardly as shown by the dotted lines  20 , the anchor element  14  of the strut  10  will tend to be pressed further into the wall  12  by virtue of the sharp point  16 . As depicted in  FIG. 1B , when the vena cava wall  12  expands outwardly, as shown by the dotted lines  20  in  FIG. 1B , the anchor element  14  will be carried with the expanding vessel wall due to the provision of the rearwardly extending hook  18 . Further inward and outward movement of the vena cava wall  12  over time will cause the anchor element to move progressively further into the thickness of the vessel wall  12  and eventually, as shown in  FIG. 1C , it will pierce through the vessel wall. Not only will this pierce the wall  12  itself but can also cause the filter to tilt within the vessel, particularly when only some of the filter struts  10  pierce through the vessel wall, which can occur as the result of differential movement of the vessel wall  12 . 
         [0037]    Referring now to  FIGS. 2A to 2C , these depict in schematic form what occurs in practice with the filter leg arrangement shown in  FIGS. 1A to 1C . As can be seen in  FIG. 2A , the filter leg  10 ′ is attached to the vessel  12 ′ by means of the anchor element  14 ′ in what could be described as the normal position. With reference to  FIG. 2B , with natural outward movement of the vessel  12 ′, the anchor element  14 ′ is pulled with the vessel wall, yet when the vessel wall  12 ′ returns back to its normal or neutral position, the anchor element  14 ′ moves further into the depth of the vessel wall  12 ′ until it will eventually pass all the way through the thickness of the wall  12 ′. This effect could be described as ratcheting of the filter leg into the vessel wall. This ratcheting or progressive movement into the vessel wall  12 ′ is represented by the scale markings  25  in  FIGS. 2A to 2C . The problem arises as a result of the need to have a sharp point to the anchor element and also a mechanism to prevent the anchor element from being withdrawn from the vessel wall. 
         [0038]    Referring now to  FIG. 3 , this shows in schematic form an embodiment of vascular filter  40 , fitted into a vena cava  60 . It is to be understood that although reference is made to the vena cava, the filters and anchoring elements taught herein are suitable for use in all other vessels of the body. 
         [0039]    The filter  40  has a generally conical form with a central hub  42  provided, in this example, with a retrieval hook  43  of known type. The hub  42  may have a lumen passing therethrough to enable the filter  40  to be delivered over the wire. In such a case, the lumen in the hub  40  may be provided with a valve or thrombogenic fibres for closing the lumen to the passage of blood. 
         [0040]    The filter  40  has a plurality of filter legs, or struts,  44  which extend from the hub  42  outwardly in radial manner, giving the filter  40  a generally conical shape as depicted in  FIG. 3 . The filter  40  also includes a second set of shorter legs, or struts,  46 , which similarly extend radially outwardly from the hub  42  and contact the vessel wall at a location longitudinally spaced from the legs  44 , thereby to give the filter  40  two sets of vessel contact points useful in maintaining the orientation of the filter in the vessel  60 . The legs  44 ,  46  are typically wires of round cross-section. 
         [0041]    The conical shape of the filter  40  not only provides good filtering characteristics but also facilitates removal of the filter after use, typically by pulling this into a retrieval sheath via the hook  43 . 
         [0042]    The magnified section in  FIG. 3  shows in better detail the distal, or vessel engaging, end  50  of one of the legs  44 . As can be seen, at the distal end  50  the leg  44  has a radially outwardly curved tip  52 , which in practice will point into the vessel wall. At the tip  52  there is provided a bulbous element  54 , hereinafter referred to as an enlarged head element. The head element  54  is, in this embodiment, spherical and has a diameter greater than the diameter of the leg  44 , thereby providing a surface or shoulder  58  between itself and the leg  44 , best seen in  FIG. 4  onwards. 
         [0043]    In this embodiment, attached to the enlarged head element  54  is a barb  56  which extends generally in a proximal direction, that is towards the hub  42  and substantially parallel to the vessel wall. 
         [0044]    In the example shown in  FIG. 3 , the barb  56  has a sharp tip and a widening base which ends with a diameter generally as great as the diameter of the enlarged head element  54 . The sharp tip of the barb  54  causes the barb, under the pressure of the leg  44 , to pierce into the vessel wall, as shown in  FIG. 3 . 
         [0045]    With reference now to  FIG. 4 , this shows in enlarged form one of the filter legs  44  and enlarged head element  54 . The barb  56  is not shown in this Figure. As will be apparent from  FIG. 5 , the curved portion  52  of the leg  44  causes the enlarged head  54  to be spaced from the longitudinal axis  62  of the leg  44  by a distance d. In practice, this shape of leg end will enable the curved tip  52  to penetrate into the wall of the vessel  60  until the main, straight portion, of the leg  44  comes into abutment with the vessel wall, with thereafter any further penetration of the leg into the vessel wall being limited as a result of the side-on disposition of the major portion of the leg  44 . The distance d is chosen to be equivalent to a part only of the thickness of the vessel wall  60  into which the filter  40  is to be implanted, thereby to ensure that once the outwardly curved tip  52  has become embedded in the vessel wall, this part of the leg  44  will not penetrate through the entire thickness of the vessel wall. 
         [0046]      FIG. 4  can also be said to depict the filter leg  44  after a degradation period in which the barb  50  has degraded to leave the enlarged head  54  fully exposed and with the piercing element effectively removed. 
         [0047]    Each leg  44  of the filter  40  has in this embodiment a distal leg arrangement of the type shown in the drawings and in particular from  FIG. 3  onwards. The shorter filter legs  46  of the second set may in some embodiments also be provided with anchor elements and outwardly curved distal ends of the type shown in connection with the legs  44 . 
         [0048]    Referring now to  FIG. 5 , this shows the filter  40  of  FIG. 3  some time after first deployment and specifically after a specified degradation period. The barbs  56  shown in  FIG. 3  are made of a biodegradable material, for example a biodegradable polymer, such as PLA, PGA or PHA, a magnesium or iron based material, or other suitable material. As a result, and as shown in particular in  FIG. 5 , the barbs will degrade in the vessel over a degradation period determined by the material and structure of the barbs. Once degraded, the distal ends  50  of the of the filter legs  44  will consist solely of the enlarged head elements  54 . As the latter elements  54  have no pointed or piercing characteristic, they will not penetrate further into the vessel wall  60  and therefore will not exhibit the ratcheting effect which can be experienced with prior art filter structures. The rearwardly facing shoulder  58  of the enlarged head  54 , provided by the greater diameter of the enlarged head  54  compared to the diameter of the filter leg  44 , will cause the distal end  50  of the filter leg  44  to remain embedded, or trapped, within the vessel wall, thus ensuring good retention of the filter  14  in the vessel. 
         [0049]      FIGS. 6 to 8C  show different designs of enlarged head element  54  and barb for the filter  40 . Referring first to  FIG. 6 , this shows a leg  44  of a filter  40  having at its distal end  50  a barb  70  which envelopes the entirety of the enlarged head element  54 . The enlarged head element  54  is in this embodiment spherical. As can be seen in  FIG. 6 , the barb  70  has a pointed tip  72  and widens to a base  74  just slightly larger in diameter than the diameter of the spherical head element  54 . The barb  70  will degrade over time to leave the spherical enlarged head element  54  within the vessel wall  60 . 
         [0050]    With reference to  FIG. 7 , this shows a leg  44  of a filter  40  having at its distal end  50  a generally spherical enlarged head element  80  having a tapering recess  82  therein. The recess  82  is preferably round in plan view, but could be of other shapes. The barb  84  includes a tip  86  and a male connector  88  shaped and sized to fit into the recess  82 . The barb  84  widens from its sharp tip  86  to its base. The barb is preferably circular in plan view. In this embodiment, the base of the barb  84  is not as wide as the diameter of the enlarged head element  80  but is sufficiently wide to provide a smooth widening surface from the tip  86  of the barb  84  to the widest portion of the enlarged head element  80 . 
         [0051]    The barb  84  could be attached to the enlarged head  80  by a friction fit or by bonding, for example. 
         [0052]    In both embodiments of  FIGS. 6 and 7  the barb  70 ,  80  extends both in radially outward and in a proximal direction, for example at an angle to around 45 degrees to the longitudinal direction of the filter  40  and in practice of the vessel. 
         [0053]      FIGS. 8A to 8C  show another embodiment of anchor element for a filter  40  of the type shown in  FIGS. 3 ,  4  and  5 . In this embodiment, the enlarged head element  90  is semi-spherical and has a semi-spherical recess  92  therein. A barb member  100  has a pointed tip  102  and widens in conical manner towards a base  104 , which has a diameter substantially the same as the maximum diameter of the part-spherical head element  90 . The barb element  100  also includes a semi-spherical male coupling element  106  at its base, which is sized to fit within the semi-spherical recess  92  of the head element  90 . The barb element  100  is preferably made of a biodegradable material which, after its degradation, will leave the semi spherical head element  90  embedded within the vessel wall. It will be appreciated that in this embodiment, as with the previous embodiments, once the barb element has degraded there will remain in the vessel a blunt retention element  90 . 
         [0054]    In the embodiment of  FIGS. 8A to 8C  the barb  100  extends both in a radially outward, at an angle of around 90 degrees to the longitudinal direction of the filter  40  and in practice of the vessel. 
         [0055]    Referring now to  FIG. 9 , this shows another embodiment of leg  44  for a filter  40  of the type shown in  FIG. 3 . In this embodiment, the leg  44  has a distal end with a spherical enlarged head end  54  as with the embodiments of  FIGS. 3 to 6 , but has a barb  110  which is in the form of a sharp needle extending from the enlarged head  54 . The needle  110  is shown extending in a radial direction of a filter leg  44 , in which case the needle  110 , including the distance d depicted in  FIG. 5  provided by the outwardly curving the distal end  50  of the leg  44  is less than the overall thickness of the vessel wall  60 . In other embodiments, the needle  110 , instead of pointing radially outwardly, may point also in a distal direction, that is towards the hub  42  of the filter  40 , in a manner similar to the barbs shown in  FIGS. 3 ,  6  and  7 . 
         [0056]    In this embodiment, the needle  110  will be sufficiently sharp to pierce into the vessel wall, with the pressure produced by the filter legs  44  and the effect of endothelialisation causing the enlarged head  54  to become embedded into the vessel wall. 
         [0057]    The needle  110  may be formed of a substantially non-biodegradable material, for example of a material similar to that of the major portion of the filter  40 , in which case the needle will not degrade in the manner described above with respect to the embodiments of  FIGS. 3 to 8 . The blunt front end of the head element  54  will reduce the effect of ratcheting of the distal end  50  of the filter leg  44  into the vessel wall. It is not excluded, though, that the needle  110  could be made of a biodegradable material as per the embodiments of  FIGS. 3 to 8 . 
         [0058]    In the embodiments described, the enlarged head elements  54  may have a diameter of around 1 mm and the filter legs  44  a diameter in the region of 0.3 to 0.4 mm. 
         [0059]    Typically, the distal ends  50  of the filter legs  44  will become ingrown within the wall  60  of the vessel within a period of about 4 weeks or so, in which case the barbs are preferably chosen to degrade within a period of 1 to 3 months following deployment of the filter  40  into the patient&#39;s vessel. 
         [0060]    In the preferred embodiments, the anchor elements at the distal ends  50  of the legs  44  have no rearwardly extending hook element of a type equivalent to a fishing hook, which again will reduce the risk of ratcheting of the filter. 
         [0061]    The filter legs  40  may be made of conventional materials, typically spring materials and/or shape memory materials. Examples include elgiloy, nickel titanium alloy such as Nitinol, cobalt chromium and so on. 
         [0062]    Although the filter legs  44 ,  46  are described as being wires, they could equally be laser cut from a tube. 
         [0063]    It is not necessary for the enlarged head elements to be spherical or part-spherical as they could have a variety of other shapes, for instance oval or the like. A person skilled in the art will be able to devise other suitable shapes for these elements. 
         [0064]    The barb elements may be formed as one piece with the filer legs  44  or alternatively may be securely joined to the filter legs, for example by adhesive bonding, welding, friction fit, a mechanical coupling or the like. 
         [0065]    The barbs may point radially outwardly, that is in a direction normal to the direction of the vessel, while in other embodiments may point towards the hub end of the filter. It is to be appreciated that the barbs may be at any angle between these two points, that is to point partially outwardly and partially proximally towards the hub. It is preferred that the barbs do not extend in a forward direction (away from the hub  40 ), although this is not excluded. 
         [0066]    Although in the preferred embodiments described above every filter leg  44  has the form shown, that is with a bulbous element and barb, it is not excluded that in some embodiments only some of the legs  44  may have distal ends of this type, in which case legs without anchor elements may simply have straight distal ends. 
         [0067]    Although the embodiments described above relate to a filter, the teachings herein are applicable to other implantable medical devices including but not limited to: occlusion devices, stents, stent grafts, embolization devices and the like. 
         [0068]    All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another. 
         [0069]    The disclosures in the abstract accompanying this application and in British patent application number 1411590.1, from which this European patent application claims priority, are incorporated herein by reference.