Abstract:
An implantable medical device includes a plurality of legs and, preferably, a circumferential array of support elements. The legs have anchoring elements at their distal ends. The anchoring elements are formed from a turn of the wire forming the legs, while piercing elements are formed by the extremities of the wire. The loops formed in this manner hold the piercing elements in the desired position and orientation during use of the filter assembly and also provide resiliency to counter any effects of movement of the filter assembly caused by movement or changes in the vessel in which the filter assembly is deployed. The anchoring elements can also straighten during removal of the filter assembly, thereby facilitating the removal of the filter even after endothelialisation.

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 15275134.3, filed May 11, 2015, and to Great Britain Patent Application No. GB 1416594.8, filed Sep. 19, 2014, which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to an implantable medical device, for example a vascular filter such as a vena cava filter. The teachings herein may also apply to an occluder or other implantable medical device. 
       BACKGROUND ART 
       [0003]    Various designs of vascular filter are known in the art, many of which can be implanted endoluminally into a patient from a remote percutaneous entry point. Such filters may be left in the patient long term or permanently, for example for the treatment of deep vein thrombosis or for filtering clots in the vena cava. It is also known to use such filters on a short term basis, for instance during a medical procedure or in the course of a temporary ailment. 
         [0004]    It is important to maintain such vascular filters correctly in position in the vessel during their use, and specifically to stop filter migration caused, for example, by blood pressure and/or vessel movement. It is also important to minimise the risk of tilting of the filter, which can cause loss of filtering function and leakage of blood around the filter. In order to minimise the risk of positional migration or tilting, filters often have anchoring elements such as barbs which are arranged in use to pierce into the wall of the vessel, thereby to hold the filter in position. Filters may also include orientation elements, such as a set of secondary filter legs which hold the filter aligned with the vessel. Barbs can cause vessel damage, particularly if they are able to be pulled out of the vessel wall as a result of changes in blood pressure or vessel movement. It is also possible for such barbs to penetrate deeply into the vessel wall, in some instances to pierce all the way through the thickness of the vessel wall. While some filter anchoring systems are intended to pierce through the entire thickness of the vessel wall, in many cases this is not desirable. 
         [0005]    Some examples of vascular filters are disclosed in U.S. Pat. No. 8,409,239, U.S. 2013/0218195, U.S. Pat. No. 7,056,286 and U.S. 2006/0015137. 
       DISCLOSURE OF THE INVENTION 
       [0006]    The present invention seeks to provide an improved implantable medical device and in particular an improved vascular filter or occluder. The device may be a vena cava filter. 
         [0007]    According to an aspect of the present invention, there is provided an implantable vascular device including a hub and a plurality of legs, each leg including a proximal end coupled to the hub and extending distally and radially outwardly of the hub to a distal end of the leg, the legs creating a generally conical form to at least a portion of the device; wherein the distal end of each leg includes an anchoring element, each anchoring element including a loop of resilient material curving so as to cross the leg and having a piercing member at an end of the loop and extending beyond the loop in a direction generally radially of the device, each loop being a sprung element and providing a vessel abutment surface for limiting the penetration of the piercing member into a vessel wall. 
         [0008]    Each anchoring element may be an extension of its respective leg. Preferably, each anchoring element is formed as a bent or curved distal portion of its respective leg. 
         [0009]    In the preferred embodiment, each loop is disposed in a generally radial orientation; such that in use each loop is substantially perpendicular to the vessel wall at the point of contact. 
         [0010]    Advantageously, each loop is resiliently compressible to a smaller loop diameter. The advantage of a loop having such characteristics is that the resiliency of the loop can accommodate changes in the diameter or shape of the vessel and in particular provide for variation to the fixing of the piercing members, enabling better and adaptable fixation of the piercing members to the vessel wall. 
         [0011]    Each piercing element may be substantially straight. They could, in other embodiments, be curved or bent. 
         [0012]    Preferably, each loop has a diameter of at least 3 millimetres. Each piercing member may have a length of between 1 millimetres and 2.54 millimetres. 
         [0013]    Preferably, the device has a longitudinal dimension and each piercing member extends in a direction substantially perpendicular to the longitudinal dimension. In other words, the piercing members may be disposed perpendicularly to the longitudinal axis of the vessel wall, as well as radially outwardly. 
         [0014]    Each anchoring element is preferably sufficiently resilient to be able to be straightened for deployment and filter retrieval. 
         [0015]    The anchoring elements are advantageously unconstrained relative to their associated legs, which means that they will immediately adopt their “at rest”, or non-biased, configurations as soon as released from a delivery catheter or sheath. 
         [0016]    The device may be provided with a plurality of elongate support elements, each support element including a proximal end coupled to the hub and extending distally from and radially outwardly of the hub to a distal end, the support elements creating a generally second conical form to the filter; wherein the support elements have a length less than a length of the legs and having a point of maximum radius spaced from a point of maximum radius of the legs. 
         [0017]    The support elements may be formed from spring steel, although in other embodiments they may be made of other materials such as a shape memory material, typically Nitinol. 
         [0018]    In an embodiment, there is provided a flexible coupling element disposed between the legs and the hub. The flexible coupling element may be a braided or coiled tubular element. 
         [0019]    At least the legs are preferably formed from a shape memory material, an example being a shape memory alloy such as Nitinol. 
         [0020]    The device is preferably a vascular filter or occluder. It may, for instance, be a vena cava filter. 
         [0021]    Other features and advantages will become apparent form the description which follows and in the accompanying drawings. 
     
    
     
       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]      FIG. 1  is a side elevational view in schematic form of an embodiment of vascular filter according to the teachings herein; 
           [0024]      FIG. 2  is an enlarged view of the distal end of a filter leg of the filter of  FIG. 1 ; 
           [0025]      FIG. 3  is an enlarged view of a part of the filter of  FIG. 1 ; 
           [0026]      FIG. 4  is a part cross-sectional view showing a side elevation of the filter assembly of  FIG. 1  disposed in a delivery catheter or sheath; 
           [0027]      FIG. 5  is a part cross-sectional view showing the filter of  FIGS. 1 to 4  deployed in a vessel; 
           [0028]      FIG. 6  is an end view of the filter of  FIG. 5  in the vessel; 
           [0029]      FIG. 7  is a view of a part of another embodiment of vascular filter; and 
           [0030]      FIG. 8  is a side elevational view of another embodiment of vascular filter. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    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 conventional elements and components of the device which are not central to understanding the teachings herein have been omitted for the sake of clarity. 
         [0032]    The embodiments described below with reference to the accompanying drawings are directed to a vascular filter, typically a vena cava filter. It is to be understood, though, that the teachings herein can be used for other implantable medical devices including, for example but not limited to, occlusion devices. An occlusion device could be formed from a structure similar to that shown in the accompanying drawings and described below, for instance by covering the filter legs with an occlusion barrier of an type which will be known to a person skilled in the art. 
         [0033]    Referring first to  FIG. 1 , this shows in schematic form an embodiment of filter assembly  10  which has what could be described as a generally conical form. The filter assembly  10  includes a hub  12 , which is typically a tube of metal or metal alloy, usefully of the same material as the filter legs, although could equally be made of a different metal, metal alloy or of a polymer material. At one end of the hub  12  there is provided an attachment element  14 , which in this embodiment is in the form of a hook, used both in the deployment of the filter assembly  10  and also in the retrieval of filter assembly  10 , in a manner which will be familiar to the skilled person. The attachment element  14  is typically made of metal. 
         [0034]    The filter assembly  10  includes a set of filter legs  16 , each of which has a first end  80  attached to the hub  12  and a second end  20  including an anchoring element  22  described in further detail below. The second ends  20  of the filter legs  16  extend distally from and radially outwardly of the hub  12 , such that the filter legs  16  in combination have a generally conical form widening from the hub  12  to the second ends  20 . The extremities  20  in practice abut against the vessel wall, as is described in further detail below. 
         [0035]    The filter assembly  10  includes a series of support elements  24  which are also, in this embodiment, in the form of legs. Each support element or leg  24  includes a first end  26  which is attached to the hub  12  and a second end  28  spaced distally of the hub  12  and which is radially expanded relative to the hub. Each support element or leg  24  curves outwardly from the hub  12  towards a vessel contact point  30 , before curving inwardly again towards the second ends  28 . 
         [0036]    The filter legs  16  and/or support elements  24  may be made of wire members attached to the hub  12 , for example by fitting into a lumen of a tubular hub  12  and then being welded, bonded or otherwise fixed into the hub  12 . In another embodiment, the legs  16  and/or support elements  24  may be laser cut from a tubing forming the hub  12 , such that the hub and filter legs and/or support elements are formed from a common length of tubing. 
         [0037]    In the preferred embodiment, the filter legs  16  are made of a shape memory material, such as a shape memory alloy. Preferably, the legs  16  are made of Nitinol. The support elements  24  are in a preferred embodiment made of steel, such as spring steel, although they may equally be made of shape memory material such as Nitinol. 
         [0038]    With reference now to  FIG. 2 , this shows the second end  20  of one of the filter legs  16 . It will be appreciated that each of the filter legs  16  has the structure shown in  FIG. 2  in the preferred embodiment. 
         [0039]    The anchoring element  22  includes a loop  32  and a piercing member  34 . In the preferred embodiment, the anchoring element  22  is formed by coiling the end  20  of the filter leg  16  to form a single-turn loop  32  with the extremity of the end  22  being left uncoiled and straight so as to form the piercing member  34 . The wire of the loop  32  is free to slide over itself, that is it is not bonded or fixed to itself. 
         [0040]    Each piercing member  34  extends by a distance D 1  beyond the loop  32  and in particular beyond a vessel abutment surface  36  of the loop  32 . This distance D 1  in practice represents the penetration distance of the piercing element  34  into the vessel wall. 
         [0041]    It is preferred that each loop has a diameter of at least 3 mm and that each piercing member has a length of between 1 mm and 2.54 mm. 
         [0042]    As will become apparent below, in particular with regard to  FIGS. 5 and 6 , the loops  32  are disposed radially from a centre line  40  of the filter assembly  10 , as shown in  FIG. 1 . As a result, described below, the loops  32  will abut the vessel wall substantially perpendicularly to the vessel wall (along radii to the conical form of the device). The piercing elements  34  preferably also extend radially of the centre line  40  and also substantially perpendicularly to that centre line  40  and therefore perpendicular to the longitudinal direction of the filter assembly  10 . 
         [0043]    Referring now to  FIG. 3 , this shows in schematic form a part of the filter assembly  10  and in particular of one of the filter legs  16  and of one of the support elements  24 . The filter legs  16  and support elements  24  of the assembly  10  provide two sets of vessel contact points, one being at the piercing elements  34  and the other at the points  30  of greatest radius of the support elements  24 . These two contact points  30  and  34  are spaced from one another by a distance D 2  in the longitudinal direction  40  of the filter assembly  10 . The provision of these two vessel contact points  30  and  34  assists in giving the filter assembly  10  orientational stability within a vessel. In other words, they ensure that the filter assembly maintains the correct orientation within the vessel and such that the piercing elements  34  are arranged in a plane perpendicular to the longitudinal direction of the vessel, thereby ensuring optimum coupling of the filter assembly  10  to the vessel and optimum patency to the vessel wall to minimise any flow leakage around the outside of the filter assembly  10 . 
         [0044]    The example of filter assembly  10  of  FIG. 1  has six filter legs  16  and six support elements  24 . A person skilled in the art will appreciate that the number of filter legs and support elements can vary from that shown and will generally be dependent upon the size of the vessel and the degree of filtration which is desired to be achieved by the assembly  10 . Where greater filtration is desired, there may be provided a greater number of filter legs  16  and in some embodiments also of the support elements  24 , which will reduce the size of the gaps between adjacent filter legs and support elements, thereby to capture smaller particles in the blood stream. 
         [0045]    When the filter assembly  10  or at least the legs  16  are formed of a shape memory material, the shape of the filter legs  16  shown in  FIGS. 2 and 3  represents the memorised shape of the device, the same applying to the support elements  24  when these are also made of shape memory material. Therefore, when the filter assembly  10  has transitioned through the Af temperature of the shape memory material, this will tend towards the shape shown in  FIGS. 2 and 3 . 
         [0046]    Referring now to  FIG. 4 , this shows the filter assembly  10  disposed in radially compressed form within a delivery catheter or sheath  40  of an introducer assembly. The introducer assembly will include a deployment mechanism located within the catheter  40 , which holds the medical device  10  in position within the catheter  40  and which deploys the medical device into the vessel of a patient. It will also typically include an external manipulation unit for operating the various elements of the introducer assembly. As all of these components and their functions are commonplace in the art, they are not shown in  FIG. 4  and not described further herein. The skilled person will already be able to identify the suitable components and devices from those commonly available. 
         [0047]    It is to be appreciated that in practice the sheath or catheter  42  will have a smaller diameter than the depiction of  FIG. 4 , in order to hold the filter assembly  10  in a tighter contracted state than that shown in  FIG. 4 , thereby to give the introducer assembly a smaller footprint. The schematic depiction of  FIG. 4  better shows the contracted condition of the filter assembly  10 , where it can be seen that the filter legs  16  are straightened, such that the anchoring elements  22  are also substantially straight. In the embodiments in which the legs  16  are made of a shape memory material, the filter assembly  10  is generally maintained below the transition temperature of the material during assembly and transportation, which enables the filter legs  16  and in particular the loop anchoring elements  22  to be readily straightened and held in that straightened configuration. On deployment, the filter assembly  10  will be heated to above the shape memory transition (Af) temperature (typically just below body temperature), which will cause the ends  20  of the legs  16  to regain their memorised shape, that is to regain the looped ends  32  and piercing elements  34 . 
         [0048]    The medical device  10  is typically deployed from the catheter  40  either by pushing or pulling it out from the distal end of the catheter, in dependence upon the chosen deployment mechanism. On deployment of the medical device  10 , that is upon its release from within the catheter  40  and the transition of the legs  16  beyond the Af transition temperature, the legs will coil into the loops  32  shown in  FIGS. 1 ,  2  and  3 . This is typically done only once the filter assembly  10  has been located in the desired position within the patient&#39;s vasculature. 
         [0049]    Referring now to  FIG. 5 , this shows the filter assembly  10  fully deployed within a patient&#39;s vessel  50 . The vessel may be the vena cava or any other vessel within the human or animal body. 
         [0050]    As can be seen in  FIG. 5 , the filter assembly  10  has its longitudinal axis  40  aligned with the longitudinal axis  52  of the vessel  50  and such that the piercing members  34  all lie in a plane substantially perpendicular to the longitudinal axis  52  of the vessel  50 . This is achieved by the provision of the two vessel contact points  30  (of the support elements  24 ) and  36  (of the loops  32  of the anchoring elements  22 ). The piercing members  34  pierce into the thickness of the vessel wall  50 , by the distance D 1  shown in  FIG. 2 . The outermost surface, or vessel abutment surface  36 , of the loops  32  prevents the piercing elements  34  embedding too deeply into the vessel wall  50 , as well as pressing against the vessel wall. With reference also to  FIG. 6 , this shows the filter assembly  10  of  FIG. 5  when viewed into the vessel  50  from the side of the hook  14 , that is from the left of  FIG. 5 .  FIG. 6  shows the support elements  24  interspersed between the filter legs  16  and coming into abutment with the internal surface of the vessel  50 , at the first contact points  30  shown in  FIG. 5 . The filter legs  32 , in particular their second ends  20 , also come into contact with the internal surface of the vessel wall  50  and in a manner in which each loop  32  is substantially perpendicular to the tangent to the vessel wall  50  at the point at which the loops contact the vessel wall, in other words along radii of the vessel. The piercing elements  34  extend radially into the vessel wall, as will be particularly apparent from  FIG. 6 . 
         [0051]    The loops  32  of the anchoring elements  22  have a plurality of advantages over the art. First, they act as stop elements to prevent the piercing members  34  piercing too far into the thickness of the vessel wall  50 . Secondly, they are flexible allowing the loops  32  to flex during movement of the vessel wall and also against the opening force at the distal ends  20  of the filter legs  16  caused by the springiness of the legs. This assists in holding the distal ends  20  of the filter legs  16  in position in the vessel and also in maintaining the piercing members  34  embedded within the vessel wall. This flexibility can in particular retain the filter assembly  10  in place as the vessel wall moves outwardly and inwardly with changes in blood pressure and also as it moves as a result of natural movement of the patient. 
         [0052]    The resiliency of the loops  32  can also assist in maintaining the piercing elements  34  perpendicular to the vessel wall during movement of the vessel, with the result that the filter is better able to stay in place. 
         [0053]    Furthermore, it is not necessary to provide any rearwardly extending hooks on the piercing elements  34 , as is required with some other designs of medical device. 
         [0054]    In practice, once the filter assembly has been implanted within a patient, there can be expected to be tissue growth over the distal ends  20  of the filter legs  16  and in particular over at least a part of the anchoring elements  22 . However, as these are flexible, the filter assembly can still be removed from the vessel. The hook  14  can be attached to a retrieval mechanism which is then used to pull the filter assembly in the direction of the hook  14 . As the assembly  10  is pulled, the loops  32  will straighten and slide out of any ingrown vessel tissue, thereby making the retrieval of the filter assembly readily achievable. This contrasts with some prior art assemblies where the piercing elements are provided with rearwardly extending hooks to hold the piercing elements in the vessel wall, the latter only being removable by tearing from the vessel wall. 
         [0055]    Referring now to  FIG. 7 , this shows a modified form of filter leg  116  which can be used in place of the filter leg  16  of the filter assembly  10  shown in the previous drawings. The filter leg  116  is virtually the same as the filter leg  16  previously disclosed, save for the provision of a section  130  of more flexible material, in one example being a braided or coiled flexible tubing coupled between the hub  12  and the wire forming the distal section of the filter leg  16 . The section  130  adds flexibility to the filter legs, useful in improving the tilting resistance of the filter assembly and in maintaining it centred within the vessel. 
         [0056]      FIG. 8  shows another embodiment of filter assembly  140  having a plurality of filter legs  142 , each terminating in anchoring elements  22  as with the previously described embodiments, but in which there are provided secondary filter struts or legs  152  which extend from an intermediate position just short of the distal ends  20  of the filter legs up to the hub  12 . The struts  152  have the purpose of creating filtering barriers in the spaces between adjacent filter legs  142 , thereby to increase the filtering efficacy of the assembly  140 . 
         [0057]    It is to be appreciated that it is not necessary for the piercing elements  34  to be disposed precisely perpendicularly to the vessel wall as they could be disposed at an angle to the perpendicular. For example, the piercing elements  34  could in some embodiments point towards the hub end of the filter assembly. In another embodiment they could point in the other direction, that is away from the hub  12 , although this is preferably by no more than a modest angle of a few degrees. 
         [0058]    Similarly, it is not necessary for the loops  32 , or for the piercing elements  34 , to be disposed precisely radially relative to the centre line  40  of the filter assembly and similarly precisely in the vessel  50 , as they (loops  32  and/or piercing elements  34 ) could be at a slight angle to this, for instance by a few degrees or so. 
         [0059]    Although in the preferred embodiment the loops  32  are a single turn of the wire forming the legs  22 , in other embodiments the loops  32  could be made from multiple turns of wire. 
         [0060]    As explained above, although the described embodiments are all directed to a filter assembly, the teachings herein are not limited to vascular filters. The teachings could equally apply to occluder elements, in which case there may be provided an impervious or substantially impervious occluding barrier coupled to the filter legs and/or the support elements. 
         [0061]    The device could be configured for deployment over a guide wire, in which case when in the delivery catheter or sheath  40  (shown in  FIG. 4 ) a guide wire may pass alongside the hub  12  and through the mass of filter legs and support elements. In another embodiment a guide wire could pass through a lumen within the hub  12  and through the centreline  40  of the filter assembly  10 . 
         [0062]    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. 
         [0063]    The disclosures in the abstract accompanying this application and in British patent application number 1416594.8, from which this application claims priority, are incorporated herein by reference.