Patent Publication Number: US-8118828-B2

Title: Retrievable blood clot filter and method of retrieving

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. application Ser. No. 10/991,041, filed Nov. 16, 2004, now U.S. Pat. No. 7,544,202, which claims priority under 35 U.S.C. §119 (e) to U.S. Provisional Applications, Ser. Nos. 60/583,274, filed Jun. 25, 2004, and 60/614,757 filed Sep. 29, 2004, all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a medical device apparatus and method for the capturing of thrombus. More particularly, the present invention relates to a retrievable vena cava filter device for the capture of blood clots and method of manufacture of the device, and the method of deployment and retrieval of the device. 
     BACKGROUND OF THE INVENTION 
     Vena cava filters are used to capture potentially fatal pulmonary emboli at an anatomical location where they may pose less risk of pulmonary emboli for the patient. Since the vast majority of pulmonary emboli originate from the lower body, filters are mainly placed in the inferior vena cava. 
     Vena cava filters have been in use since the 1960s in a variety of configurations. Early filters required open surgical placement (Mobin-Uddin Filter; Kimray-Greenfield filter). Since the late 1970s, improvements in delivery were made and numerous filters were developed for minimally invasive percutaneous placement. These filters included the Greenfield filter, the Gianturco Bird&#39;s Nest Filter, the Vena Tech LGM filter, the Simon Nitinol Filter and others. More recently, filters have been developed and marketed with the capability of retrieval after relatively long terms of implantation, which include the Bard Recovery Filter, the Cordis Optease Filter and the Cook Tulip Filter. 
     Although addressing some desirable characteristics of a filter, the majority of the IVC filters presently on the market do not satisfy other desirable characteristics of an ideal filter. The ideal device should capture blood clots while ensuring continued blood flow through the vessel. Blood flow disruption and turbulence often leads to thrombus formation and buildup at and around the filter. Studies have demonstrated that a conical filter configuration provides the optimal filtering efficiency. Filtering efficiency, for the purposes of this invention can be defined as the capability of the device to capture and retain clots of a pre-determined size, the ability to maintain blood flow through the filter in the presence of captured clots, and the capability of dissolving or lysing the clots caught in the filter. Conical designs force clots toward the center of the filter, allowing blood flow passage around the clot. Continued blood flow through the filter when a clot load is present ensures that captured clots are exposed to the lysing action of the blood flow. 
     Although conical filter configurations currently available on the market provide optimal filtering capabilities, these designs are prone to tilting and misalignment. When not in proper alignment, filtering ability is compromised. The central conical portion of the filter may tilt to the extent that it becomes embedded in the vessel wall. With retrieval designs, the retrieval hook is typically located at the central apex of the cone. If the tilting results in the retrieval hook coming in contact with the vessel wall, retrieval efforts become difficult or may even prevent removal. Laminar blood flow is disturbed, effective lysing of capture clots decreases, and thrombus build-up occurs. 
     To address the misalignment problem, filtering cones have been designed with alignment mechanisms to prevent tilting. For example, stent-like cage constructions have been designed to prevent the conical filter from becoming mis-aligned. The stent-like cage rests up against the vessel wall providing alignment to the filtering conical portion of the filter. This design, while optimizing centering of the filter, cannot be easily retrieved because of the difficulty in snaring and collapsing the cage. An example of this type of filter design is the Vena Tech LP filter which has a conical filtering segment adjoined to a zigzag stent base configuration for centering the cone within the vessel. Although this type of design combines the optimal filtering characteristics of a conical configuration with a non-tilting base, the device is not retrievable. The struts of the non-tilting base become incorporated into the vessel wall and cannot be easily disengaged and removed using standard snare removal techniques. The location of the stabilizing struts prevents the ability to withdraw the device into a sheath for removal. 
     It is possible to build a simple centering cage base/cone filter design that is retrievable by attaching the base to the filter segment in series. This design, while retrievable, is not practical due to the increased length of the device. The desired length of a typical IVC filter is between 3 and 5 centimeters. Longer lengths are undesirable because of the limited implantation space of the vena cava. For example, in some cases it is necessary to deploy a second filter due to malfunction of the initially placed filter. Shortening the filter segment may make the overall device length acceptable, but may result in sub-optimal filter strut angles. Alternatively, shortening the centering cage segment may compromise the alignment function of the device. 
     IVC filters should be capable of remaining in the vessel for long periods of time, and in some cases, indefinitely. The filter should be designed so as not to migrate from its originally deployed position while still allowing for retrieval of the filter. Thus the vessel wall engagement mechanism should be designed so as to maintain position even under a heavy clot load and yet allow easy and a traumatic disengagement from the vessel during retrieval. Longitudinal movement of the filter has traditionally been prevented by configuring filter ends with hooks that embed in the vessel wall. 
     Because of concerns with permanent implantation of filters, including possible migration and structural integrity over long time periods, there is an emerging trend for filters that can optionally be retrieved after a specified period of time. The optimal retrievable filter should have wall-engaging mechanism that is sufficient to ensure that the device does not migrate in either direction while implanted. The wall-engaging mechanism should also be designed to allow percutaneous removal of the device without significant trauma or damage to the vena cava wall even after neointima overgrowth has occurred. These two disparate clinical requirements, long-term fixation and a traumatic removal, are difficult to achieve in a single filter design. Some prior art filter designs have utilized aggressive anchoring mechanisms to ensure fixation, but these designs are difficult to remove. Conversely, designs that limit wall contact are easier and less traumatic to disengage from the vessel, but may be more prone to migration. 
     As with all long-term or permanent implant device, the optimal device design will maintain structural integrity of the device for the duration of implantation. Although rare, filter fractures have potentially fatal complications including filter migration into the right atrium and pulmonary embolism caused by compromised filtering efficiency. The ideal filter device should have minimal connection or attachment points which are more susceptible to fatigue over extended periods of time. In addition to long term performance characteristics, it is desirable to provide an IVC filter that is simple and inexpensive to manufacture without requiring complicated assembly processes that might compromise the long-term integrity of the device or increase the overall cost of the device. 
     Another desirable characteristic of the ideal filter is a small deployment and retrieval system. A design that minimizes the delivery device diameter will result in a smaller insertion site and reduced risks of bleeding, site thrombus and other complications of percutaneous punctures. The ideal vena cava filter should not only be easy to deploy using minimally invasive percutaneous techniques, but also be repositionable during initial deployment. Many filters are designed for ease of deployment but do not allow for repositioning during delivery. 
     SUMMARY OF THE DISCLOSURE 
     A compact retrievable blood clot filter has a primary hub, a set of filter struts having a conical configuration and extending from a primary hub, and a set of alignment struts connected to the filter struts to provide centering of the filter. A set of control struts (connector elements) connected to the alignment struts has a secondary hub which is axially movable relative to the primary hub. Axial movement of the secondary hub relative to the primary hub causes the control struts to pull the alignment struts radially inward into a retractable state for retrieval. 
     In one aspect of the invention, the filter and alignment struts together are constructed from a single tubular element and provide clot capturing and filter centering functions. The control struts are fabricated from a second tube and functions to control deployment and retrieval. The two tubular elements are slidably coupled to each other in a coaxial arrangement. The device can be easily deployed through a small delivery system, repositioned within the vessel after partial deployment and retrieved. The tubular construction provides a fatigue-resistant device due to the absence of weld joints. 
     In another aspect of the invention, a method of retrieving the filter is provided. First, the secondary hub is captured. For example, a snare can be used to hook the secondary hub and a sheath is placed over the secondary hub. Then, the captured secondary hub is axially moved relative to the primary hub to pull the alignment struts radially inward. Once the alignment struts are in a retractable state, the sheath is moved over the filter struts relative to the primary hub to enclose the plurality of filter struts. The sheath is then removed along with the captured filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is plan view of the vena cava filter device of the current invention in a collapsed, non-expanded state. 
         FIG. 2  is an enlarged end view of the vena cava filter device of  FIG. 1 . Hidden lines have been removed for clarity. 
         FIG. 3A  through  FIG. 3C  are plan views of the vena cava filter device in a deployed, expanded position with different functional components highlighted in black. Hidden lines have been removed for clarity.  FIG. 3A  highlights the conical filter functional component  20  and the non-slotted section  19 ,  FIG. 3B  highlights the alignment struts functional component  21 , and  FIG. 3C  highlights the control/connector strut subassembly  3 . 
         FIG. 3D  is an enlarged partial plan view of the vena cava filter device in an expanded position showing the position of the inner cannula component  37 . 
         FIG. 3E  is an enlarged partial plan view of the vena cava filter device in an expanded position showing an alternative embodiment using extensions  13 . 
         FIG. 4  is an end view of the expanded vena cava filter as depicted in  FIG. 3A-3C . 
         FIG. 5  is a plan view of the filter cone/centering strut subassembly  2  of the vena cava filter device of the current invention in a non-expanded state. 
         FIG. 6  is a plan view of the filter cone/centering strut subassembly  2  of a retrievable vena cava filter device in an expanded state. 
         FIG. 7  is a plan view of the control/connector strut subassembly  3  of the vena cava filter device. 
         FIG. 8  is an enlarged plan view of the inner cannula  37  of the vena cava filter device. 
         FIG. 9  is an enlarged plan view of the downstream apex of the alignment strut with a control strut as it is being inserted into the receiving hole. 
         FIG. 10A  is an enlarged plan view of the downstream apex of the alignment strut assembled with the control strut in an expanded position. 
         FIG. 10B  is an enlarged plan view of the downstream apex of the vena cava filter depicting alternative configuration of the anchoring mechanism. 
         FIG. 10C  is an enlarged plan view of the downstream apex of the vena cava filter depicting a further alternative configuration of the anchoring mechanism. 
         FIGS. 11A through 11D  illustrate a method of deploying the vena cava filter within the vessel. 
         FIG. 11A  is a cross-sectional view of the filter within the deployment system prior to deployment in the vein. 
         FIG. 11B  is a plan view of the filter and deployment system with the filtering legs deployed. 
         FIG. 11C  is a plan view of the filter and deployment system with the control struts and the alignment struts deployed. 
         FIG. 11D  is a plan view of the filter and deployment system after the filter has been detached from the deployment system. 
         FIGS. 12A through 12D  illustrate a method of retrieving the vena cava filter from the vessel. 
         FIG. 12A  is a plan view of the method of retrieval of the vena cava filter device depicting capture of the device by a snare sheath. 
         FIG. 12B  is a plan view of the method of retrieval of the vena cava filter device depicting the retraction of the non-slotted section  18  of the control strut subassembly  3  into the sheath. 
         FIG. 12C  is a plan view of the method of disengagement of the control struts from the vessel wall in which the control struts are completely collapsed in the center of the vessel. 
         FIG. 12D  is a plan view of the method of retrieval depicting the filter completely collapsed and within the retrieval sheath. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purposes of the present application, the terms upstream and downstream refer to the direction of blood flow. Accordingly, blood flows from an upstream direction towards a downstream direction. Referring to  FIG. 1 , there is shown an embodiment of the present invention in a plan view of a non-expanded state of the vena cava filter. Although not shown in  FIG. 1 , the filter  1  is held in a collapsed, non-expanded state by a sheath, catheter or other tubular construct. The filter  1  is comprised of two coaxially arranged tubular subassemblies, a filter cone/alignment strut subassembly  2  and a control strut subassembly  3  which are collapsed to form a slender tubular construct  1  that can be percutaneously inserted through a catheter or sheath into a patient. The filter also includes an inner cannula  37  which is positioned coaxially between subassembly  2  and subassembly  3 . 
     In one aspect of the invention, the filter cone/alignment strut subassembly  2  is formed from one tube, which is of a larger diameter than the second tube forming the control strut subassembly  3 . The filter cone/alignment strut subassembly  2  is coaxially positioned around a portion of control strut subassembly  3  in a slideable arrangement. The two tubular subassemblies are interconnected by barbs  6  of the control strut subassembly  3  which are positioned through the barb receiving holes  7  of the outer, filter cone/alignment strut subassembly  2 . The control strut subassembly  3  includes a retrieval hook  4  as is well known in the art for facilitating removal of the filter from the body. The inner cannula  37  is fixedly connected to subassembly  2  through a friction fit or other connection means. 
     The tubes are preferably of a material with shape-memory characteristics such as nitinol to allow expansion from the collapsed state shown in  FIG. 1  to a deployed filter configuration as shown  FIG. 3A . Nitinol is an alloy well suited for vena cava filters because of its superelastic characteristics, which enables it to return to a pre-determined expanded shape upon release from a constrained position. Other shape memory metals such as stainless steel may be used to form filter  1 . 
     To manufacture the device, the tube subassemblies are first cut into the desired configuration using laser-machining techniques commonly known in the art. Other cutting techniques such as photo or acid etching may be used to form the desired cut patterns for subassembly  2  and subassembly  3 . Material is cut away from each tube in a predetermined pattern to form the desired configuration, as shown in  FIG. 5  and  FIG. 7 . The outer cut tube is then annealed or heat-treated to form the expanded deployment shape of  FIG. 3A . The barb portion of second inner tube  3  may also be annealed into a curved configuration. To assemble the filter, the inner cannula  37  is placed within the lumen of subassembly  2  and abuts up against the upstream edge of a hub/non-slotted section  18 . The cannula  37  may either be press fit or connected in other ways commonly known in the art. The two tube subassemblies  2  and  3  are then coaxially assembled together and then constrained within a sheath or catheter to the non-expanded profile as shown in  FIG. 1 . 
     In one novel aspect of the invention, the method of manufacture of each tube subassembly  2  and  3 , and cannula  37 , provides unitary components that have no welded joints. The singular tube construction of each subassembly avoids the necessity of welding or otherwise connecting individual wire elements. In addition, the assembly of the tubular subassemblies  2  and  3 , and the inner cannula  37  into the final finished vena cava filter device  1  may be achieved without requiring any welding steps. 
     As is well known in the art, welding or otherwise connecting shape-memory materials is difficult, time-consuming and results in connection points that are more susceptible to long-term fatigue stress and loss of structural integrity than non-jointed segments. By contrast, the lack of weld joints in the present invention: (1) minimizes the possible complication of filter fracture caused from the loss of structural integrity of the filter over time; (2) simplifies assembly whereby decreasing manufacturing costs; and (3) provides a lower unexpanded profile of the filter allowing the use of smaller delivery and retrieval systems. 
       FIG. 2  is an enlarged end view from the downstream end of the vena cava filter  1  depicted in  FIG. 1  in a collapsed state. The filter cone/alignment strut subassembly  2  coaxially surrounds the control strut subassembly  3 . The outer diameter of the subassembly  2  is preferably 0.071″ for use with a small delivery sheath such as a 6 French size. The inner diameter of the filter cone/alignment strut subassembly  2  is approximately 0.051″, providing a wall thickness of approximately 0.010″. The inner wall of the control strut subassembly  3  defines a through channel  14  which is dimensioned at approximately 0.038″ to allow the passage of a 0.035″ guidewire or other device (not shown) through the filter  1  to facilitate deployment or removal from the vena cava. 
     The outside diameter of the control strut subassembly  3  is preferably 0.048″ with a corresponding wall thickness of 0.005″. Although barbs  6  which extend primarily in a radial direction which is perpendicular to the plane of  FIG. 2 , they also extend beyond the longitudinal profile of the filter cone/centering strut subassembly  2  by approximately 1 millimeter when assembled. When constrained within the delivery catheter or sheath, the barbs  6  are forced down onto and become aligned with the outer surface of the filter cone/centering strut subassembly  2 . Other alignment strut subassembly  2  and control strut subassembly  3  dimensions are possible. 
     The outside diameter of the cannula  37  is preferably 0.053″ with a corresponding inner diameter of 0.041″. Cannula  37  terminates at upstream cannula end  38  and downstream cannula end  39  (see  FIG. 3D ), with a length of approximately 0.13″. Downstream cannula end  39  is sized to be friction fit into the inner diameter of the non-slotted section  19  of the filter cone/alignment strut subassembly. The lumen of the inner cannula  37  accepts control struts/arms  5  as shown in  FIG. 2 . As stated, subassembly  3 , is cut from a cannula with an outside diameter of 0.048″. This creates an interference fit with the inside diameter of cannula  37 . The slots cut in subassembly  3  allow the control struts  5  to compress centrally and pass through cannula  37 . 
     Referring now to  FIG. 3A  through  FIG. 3C , plan views of the device  1  of the current invention are depicted in an expanded shape highlighting different functional components.  FIG. 3A  highlights the conical filtering configuration/section  20  (bolded for clarity) of the filter cone/alignment strut subassembly  2 . The conical filtering configuration  20  performs the primary clot capturing and lysing function of the device as well as a secondary anchoring function to prevent migration of the filter in an upstream or retrograde direction. The configuration  20  is comprised of individual filtering struts/legs  9  and a hub  19  which is a non-slotted tubular section. Each filtering leg  9  extends radially and outwardly in an upstream direction from the hub  19  of the outer tube. In the preferred embodiment, the filtering legs  9  are four in number, although other configurations are possible. Leg  9  terminates in a wall-engaging portion  10 . 
     In the embodiment shown, the device  1  contacts the vessel wall on a first plane at apices  17  which are the downstream ends of the alignment struts  8  as shown in  FIG. 3B  and on a second more upstream plane at wall-engaging portion  10  of the filter legs  9  which are the upstream ends of the filter legs. The wall-engaging portion  10  of leg  9  is approximately 2 to 5 mm in length and is comprised of a foot  11  and barb  12 . The barb  12  functions to positively anchor the legs  9  in the vena cava wall to prevent retrograde migration of the device. The barbs are dimensioned at about 0.5 to 1.5 mm in length to ensure stable securement to the wall without migration of the legs  9  through the vena cava into the surrounding tissue. The angle of the barb relative to the longitudinal axis of the device is between 15 and 40 degrees, allowing for disengagement during retrieval in a direction that minimizes tearing of the vessel wall. The foot  11  rests parallel to and on the surface of the vessel wall, providing a platform that limits the depth of barb  12  penetration during implantation. 
       FIG. 3B  highlights the alignment structure/section  21  (bolded for clarity) of the filter  1 . Although highlighted separately in  FIG. 3A  and  FIG. 3B  for clarity purposes, the alignment structure  21  and conical filter  20  are formed of a single tube  2  with no welded connection points. The alignment structure  21  provides central alignment of the conical filter  20  within the vessel and ensures symmetrical deployment of the filter legs  9 . The alignment structure  21  shown in  FIG. 3B  is formed of a series of individual alignment struts  8 . In the preferred embodiment, the alignment struts  8  are eight in number, although the number of struts  8  is dependent on the number of filtering legs  9 . Each strut  8  is approximately 0.014″ wide with a thickness of 0.010″ corresponding to the wall of the outer tube subassembly  2 . 
     Each alignment strut  8  terminates at a downstream apex  17  which contains a barb-receiving hole  7 , which accepts barb  6  of control strut  5 . The other end of each alignment strut  8  connects to the leg  9  at a juncture/connecting point  15 . The juncture as shown is at an intermediate zone, preferably a mid-zone, which is located away from the upstream end  10  and the hub/downstream end  19 . The downstream apex  17  of each alignment strut  8  forms a juncture with the downstream end of an adjacent alignment strut. The series of alignment struts  8  together form an alignment structure  21  which when expanded angles radially and outwardly from the longitudinal axis of the filter in a downstream direction at an angle to contact the vessel wall at apex  17 . Thus, in one novel aspect of the invention, centering of the filtering legs  9  within in the vessel is achieved with only point contact to the vessel wall and without extending the overall length of the device  1 . 
     The interconnecting arrangement of the alignment struts  8  to each other ensures that each strut and filter leg  9  are symmetrically deployed around the inner vessel wall. Leg crossing or entanglement during or after deployment is avoided by the interconnecting design of the alignment struts  8 , which ensure equal spacing is maintained between each apex at both the upstream and downstream ends of the filter. The symmetrical deployment, anchoring and alignment features of this design allow the filter to be consistently placed in vena cavas of varying dimensions and shapes. 
       FIG. 3C  highlights control strut subassembly  3  (bolded for clarity), which is formed from the inner tube  3 . The control strut subassembly  3  functions to control deployment of the device, maintain central alignment during implantation, provide a secondary filtering mechanism and facilitate the collapse of the device  1  during retrieval. The control strut subassembly  3  is comprised of a series of individual control/connector struts  5  connected to a hub which is a non-slotted section  18  at the downstream end and terminating in barbs  6  at the other end. The non-slotted section  18  of the control strut subassembly  3  is approximately 0.50″ in length. Within the non-slotted section  18  is a retrieval hook  4  for the capture and removal of the device from the vena cava. The struts are preferably 25 to 31 nm in length to accommodate a wide range of vena cava sizes. 
     In an alternative embodiment, the width of each control arm  5  tapers from about 0.010″ wide at the non-slotted section  18  to approximately 0.024″ at the downstream apex  17 . This design is advantageous in that the struts have a reduced profile at the center of the vessel to minimize blood flow turbulence with increased width, strength and stiffness at the anchoring portion of the strut  5  for secure attachment and fatigue resistance as the vessel wall moves. 
     When fully deployed, the control struts  5  are drawn outwardly from the non-slotted section  18  at an angle of almost 90 degrees relative to the longitudinal axis of the vessel. The control struts  5  bisect the cross-sectional area between two adjacent filter legs  9  providing a secondary level of cava filtration (see  FIG. 4 ). The drawn out control struts  5  provide a secondary filtering mechanism by capturing thrombus that might otherwise pass through the area between two adjacent filtering legs  9 . 
     The barbs  6  are moveably connected to apices  17  of the alignment struts  8  through the barb-receiving holes  7 . The barbs  6  penetrate the vessel wall at an angle substantially perpendicular to the vessel wall. This angle is advantageous when the filter is removed because the barbs will pull out of the wall in a manner that creates only minimal vessel wall damage rather than slicing or tearing the neointimal layer. Other barb configurations are also possible, including barbs of differing profiles and locations along the alignment struts  8 . 
       FIG. 3D  illustrates the inner cannula/tube  37  component of filter  1  and its position within the assembled filter. Similar to the other subassemblies, the cannula  37  is formed from a single tube with no welded connections. The inner cannula  37  is not laser cut, although the upstream cannula end  38  is preferably radiused to facilitate smooth deployment and retraction of the control struts  5 . The cannula  37  is positioned coaxially between the non-slotted section  18  of the control strut subassembly  3  and the non-slotted section  19  of the outer tube  2 , press fit into assembly  2  and slidable with assembly  3 . Thus positioned, the inner cannula  37  functions to facilitate retrieval by directing the collapsing force of the alignment struts  8  inwardly instead of longitudinally. The inner edge  38  of the cannula  37  is chamfered to further facilitate retraction of the control struts  5  during retrieval. 
       FIG. 3E  illustrates an alternative design for directing the expansion and collapsing force of the alignment struts which does not require a separate inner cannula component. The filter cone/alignment strut subassembly  2  is cut in a pattern that includes the filter legs  9  and extensions  13 . Extensions  13  are arranged between each filter leg  9  and act to direct the expansion of the control struts  8  outwardly in a controlled manner during deployment and inwardly during retrieval. The inner upstream edge of extension  13  may be beveled to facilitate smooth expansion and retrieval of the filter. 
     Referring now to  FIG. 4 , an end view of the filter device  1  in an assembled and expanded state is shown from a downstream view. The alignment struts  8  self-center the device  1 . Primary filtering legs  9  extend radially outward from the non-slotted outer tube section  19 . The alignment struts  8  intersect filtering legs  9  at connecting point  15  and act to prevent the legs from crossing each other during deployment and implantation. The control struts  5  bisect the cross-sectional area between two adjacent filter legs  9  providing a secondary level of cava filtration. Thus, control struts  5  provide a secondary filtering mechanism by capturing thrombus that might otherwise pass through the area between two adjacent filtering legs  9 . The control struts  5  extend outward from the non-slotted section  18  through the end of cannula  37  (not shown). Barbs  6  at apexes  17  provide primary fixation against downstream migration. Wall-engaging portion  10 , comprised of barb  12  and foot  11 , provides a secondary fixation against upstream migration of the device  1 . The through channel  14  provides a lumen through which a guidewire or other medical device accessories can be advanced to assist in placement or retrieval. 
     Turning now to a detailed description of filter cone/alignment strut subassembly  2 , plan views of the subassembly are shown in a non-expanded and expanded state in  FIG. 5  and  FIG. 6  respectively.  FIG. 5  illustrates the preferred pattern to which the tube is laser-cut to form the non-slotted hub section  19 , alignment struts  8 , filtering cone legs  9 , downstream apices  17 , barb-receiving hole  7 , and the wall engaging portion  10  of filtering legs  9  which comprise the filter cone/alignment strut subassembly  2 . 
     After subassembly  2  is laser cut into the pattern shown in  FIG. 5 , it is annealed into the expanded shape shown in  FIG. 6 . When expanded, the interconnecting alignment struts  8  provide superior centering of the conical shape within the vessel. Centering is achieved by providing two sets of point contact with the vessel wall along two separated planes. Specifically, each alignment strut  8  contacts with the vessel wall at the apex  17 , providing a first downstream plane of contact. The wall-engaging portion (upstream end)  10  of the conical filtering legs  9  contact the vessel wall at a second upstream plane. Thus, the filter legs/struts  9  and alignment struts  8  together function to provide a stable, self-centering device with minimal vessel wall contact. 
     Thus, another advantage of the current device  1  is that the filter is deployed symmetrically within the vessel. Problems with tilting, leg-crossing and asymmetrical axial alignment of struts are eliminated by the interconnecting strut design. Each strut  8 , because it is connected at the apex  17  to the adjacent strut  8 , will automatically align symmetrically against the vessel wall, regardless of the individual anatomical profile of the vessel. The legs  9  also deploy symmetrically due to the interconnections of the struts  8 . 
     The wall-engaging portion  10  of the conical filtering legs  9  provide vessel wall fixation to prevent migration of the device  1  in a upstream direction. Wall-engaging portion  10  is comprised of a foot  11  and a barb  12 . Barb  12  engages the vessel wall and is dimensioned to ensure optimum penetration depth. Foot  11  rests against the vessel wall in a longitudinal direction and prevents over penetration of barb  12 . It is well known in the art that endothelialization of any portion of a filter strut in contact with the vessel wall occurs as early as twelve days after filter placement. The wall-engaging portions  10  of the legs  9  and apices  17  of the alignment struts provide secure engagement with only minimal contact with the vessel wall, thereby reducing the extent of endothelialization at the device site. Thus, this design facilities retrieval because the wall-engaging portions will disengage from the vessel wall without significantly disrupting the endothelial layer of the vessel wall. 
     Thus, the filter cone/centering strut subassembly  2  incorporates a stabilizing structure that ensures the centering alignment of the conical filter  20  is maintained throughout the implantation period of the device. This unique design overcomes prior art problems with cone misalignment and tilting and reduces the resulting complications of blood flow turbulence, compromised lysing capability and filter-induced thrombus buildup. The overlapping design of the alignment struts  8  and filter legs  9  represents another advantageous aspect of the current invention. This overlapping design minimizes the overall length of the device when deployed, provide optimal filtering angle and also provides a mechanism for accurate, reliable and easy placement during deployment. 
     Turning now to the control strut subassembly  3 , a plan view of the subassembly is shown in  FIG. 7 .  FIG. 7  illustrates the preferred pattern to which the tube is laser-cut to form the control struts  5  which terminate at the upstream end in barbs  6  and the downstream end with the non-slotted section  18  of the control strut subassembly  3 . The non-slotted section  18  includes a cut out section that forms the retrieval hook  4 . Each strut includes a curved transition segment  23  of reduced width between the straight portions of control strut  5  and barb  6 . 
     Prior to assembly with the filtering cone/alignment strut subassembly  2  and inner cannula  37 , the series of control struts  5  are substantially straight and uncurved. Only the curved transition segment  23  portion of the control struts is pre-shaped using standard nitinol heat forming processes. When barb  6  of control strut  5  is assembled through the barb receiving hole  7  of downstream apex  17  and subassembly  2  is allowed to expand, the interlocking connection of the two components at apex  17  draws the control struts  5  outwardly from the non-slotted section  18  curving to a generally perpendicular orientation relative to the vessel wall. 
     Referring now to  FIG. 8 , a plan view of the inner cannula  37  component of the filter device  1  is shown. Inner cannula  37  is formed of a metallic material and terminates in cannula end  38  at the upstream end and cannula end  39  at the downstream end. Cannula end  38  may include an internal bevel or chamfer which provides a smooth, curved inner surface to facilitate the expansion and retraction of the control struts  8  during deployment and retrieval. Cannula end  39  includes an external bevel or chamfer which facilitates the insertion of the cannula  37  into the inner diameter of non-slotted section  19  of outer tubular subassembly  2  during the assembly. 
     Referring now to  FIG. 9 , an enlarged plan view of the downstream apex  17  of the alignment strut  8  is shown with an end portion of the unassembled control strut  5 . The downstream apex  17  contains a barb-receiving hole  7  through which the barb  6 , located at the end of control strut  5 , is received. The barb-receiving hole  7  is sized to pass barb  6  when oriented in one direction, allows rotation during assembly and prevents disconnection when the barb  6  is oriented 90 degrees in the other direction. 
     In the preferred embodiment, the barb-receiving hole  7  has a longitudinal slot length of approximately 0.016″ for slideably accepting the barb  6 , which has a base  24 , of approximately 0.015″ in width. The barb-receiving hole  7  is dimensioned with a slot width of approximately 0.008″ so as to accept the barb thickness of 0.005″. The control strut  5  width is approximately 0.010″ with segment  23  having a reduced width of approximately 0.005″ to allow rotation within the barb-receiving hole  7 . Curved segment  23  also functions to limit movement of the control strut  5  once positioned within the slot to the length of the reduced dimension segment  23 . In addition, segment  23  is designed to achieve a lower overall profile when in the constrained, undeployed position, thus allowing use of a smaller device delivery system. 
     The barb  6  end of the control strut  5  is pre-shaped to the engaged orientation such that the barb base  24  abuts the lateral sides of the hole  7 . When assembling the control strut subassembly  3  with the filtering cone/alignment strut subassembly  2 , barb  6  is oriented to allow passage through the barb-receiving hole  7 . Once threaded through the barb-receiving hole  7 , the barb  6  will be returned to its preformed orientation by turning 90 degrees. In this position, the barb  6  dimensions are wider than the barb-receiving hole  7  and accordingly it cannot slide back through the barb-receiving hole  7 . 
     When the device is assembled with the control strut subassembly  3 , the barb  6  and the end section of the control strut  5  extend through the barb-receiving hole  7 .  FIG. 10A  illustrates the relationship between the control strut  5  with barb  6  and the downstream apex  17  with barb receiving hole  7  when deployed within the vena cava. Barb  6  embeds into the vessel wall, providing a securement mechanism to prevent migration of the filter  1 . Specifically, the control strut  5  orientation of the barb  6  is perpendicular to the vessel wall and blood flow and thus provide increased resistance to filter movement when under clot load. As will be discussed in further detail below, the control arm/barb orientation is in line with the withdrawal direction and thus allows the barbs to pull straight out of the vessel wall whereby minimizing wall damage during retrieval. 
     Alternative embodiments of the wall-engaging portion of the control arm  5  and downstream apex  17  is shown in  FIG. 10B  and  FIG. 10C . The barb-receiving hole  7  has a longitudinal slot length of approximately 0.016″ for slideably accepting the button end  49 , which has a barbless curved outer profile. Fixation is provided by the downstream apex  17  terminating in a barb  16  which penetrates the vessel wall at a downstream angle. The barb orientation relative to the vessel wall prevents downstream migration, even under heavy clot load. Specifically, increased force against the filter caused by captured thrombus will produce and increase the force against the barbs, driving them deeper into the vessel wall. The depth of penetration is limited by the length of control strut  5  which prevents barb portion  16  of apex  17  from driving too far into the vessel. 
       FIG. 10C  depicts a wall-engaging portion of the filter in which anchoring barbs are present at the termination of both the apex  17  and the control strut  5 . The interconnecting arrangement of the control strut  5  and the apex  17  together limit the depth of penetration. The advantage of this embodiment is superior fixation from a dual-anchoring system where the clot load against the filter is the heaviest. In all three embodiments, the design limits the amount of penetration by the barbs. 
     The method of filter deployment will now be described with reference to  FIG. 11A-11D . For the purposes of clarity,  FIG. 11A-11D  depict the filter in a slightly different rotation than  FIG. 3A-3D . To place the filter percutaneously, a deployment device  25  containing the filter  1  in a collapsed state is introduced through a standard introducer sheath (not shown) through the jugular vein into the vena cava  32 .  FIG. 11A  depicts the filter  1  collapsed within the deployment device  25  prior to deployment. The deployment device  25  is preferably a three-component coaxial tube system comprising an inner deployment member  26 , intermediate deployment member  27  and outer deployment tube  28 . 
     The outer deployment tube  28  coaxially surrounds the intermediate member  27 . Outer deployment tube  28  provides an outer housing to retain the filter  1  in a constrained/collapsed position prior to deployment and also functions to deploy the filter legs  9  when the outer tube  28  is retracted. 
     Intermediate member  27  abuts up against and is releaseably connected to the non-slotted section  19  of the filter cone/alignment strut subassembly. Member  27  may be in a coaxial arrangement with inner member  26  or may be longitudinally juxtaposed with inner member  26  within the outer deployment tube  28 . Member  27  functions to prevent filter  1  from moving in a longitudinal direction within the deployment device  25 , when the outer tube  28  is retracted or advanced. 
     The inner deployment member  26  controls the deployment and retraction of the control struts  5  and the interconnected alignment struts  8 . Inner member  26  may include a through lumen  31  for guidewire passage. Inner member  26  abuts up against and is releaseably connected to the non-slotted section  18  of the control strut assembly  3 . Advancement of member  26  forces the control strut assembly forward into an expanded state. 
     The tubular construction of the filter  1  minimizes the deployment device  25  diameter. In the preferred embodiment, the deployment device  25  will fit within a 6 French delivery system, providing a small insertion site and less trauma to the patient. 
     Once correct positioning within the vena cava has been confirmed, the filter  1  is controllably deployed in the vessel  32 . In one embodiment, the filter  1  is deployed through a series of three steps. Each step controls the deployment of a separate part of the filter. In this staged deployment, the filter legs  9  are first released, followed by release and expansion of the alignment  8  and control struts  5 . After the filter is fully expanded and engaging the vessel on two planes, a third step separates the deployment system  25  from the filter  1 . The three steps are sequentially performed in a smooth motion. 
       FIG. 11B  illustrates the first deployment step in which the filter legs  9  are deployed. The outer deployment tube  28  is retracted while maintaining the position of the inner member  26  and intermediate member  27 . As the outer tube  28  is retracted, the upstream section of the filter  1  is uncovered and expands to its pre-formed shape, causing the filter legs  9  to engage the vena cava wall  32 . Specifically, barbs  12  of the wall-engaging portion  10  of the filter legs  9  will contact and advance into the vessel wall  32 . Advancement of the barbs  12  is stopped when the foot  11  contacts the surface of the vessel wall  32 . 
       FIG. 11C  illustrates the second deployment step in which the remaining portions of the filter  1  are deployed. Although not apparent in  FIG. 11C  due to the spatial orientation of the device  1 , the barbs  6  are in contact with the vessel wall  32 . In the second deployment step, the inner member  26  is advanced causing the control strut subassembly  3  to also advance and expand outwardly to contact vessel  32 . This movement allows the alignment struts  8  to expand radially outward until the barbs  6  engage the vessel wall at an angle perpendicular to the longitudinal axis of the vessel. 
       FIG. 11D  depicts the filter  1  in its fully deployed configuration after detachment from the deployment device  25 . Various detachment mechanisms may be used to release the filter from the deployment system  25 . Once disconnected, the device  1  may not be repositioned but may be retrieved using a retrieval system, as will be explained below. 
     Advantageously, this staged deployment method allows the device to be repositioned after either steps one or two until the deployment system is physically detached from the filter. If the physician needs to reposition the device  1  after the barbs  12  have engaged but prior to full filter deployment, the outer deployment tube  28  may be advanced to enclose and re-collapse the filter legs  9 . The barbs  12  disengage from the wall  32  as the device  1  is drawn back into the deployment device  25 . In one aspect of the invention, barb  12  disengagement is achieved at an angle equivalent to the engagement angle, whereby minimizing vessel wall trauma. 
     The outer deployment tube  25  thus control both the deployment and if desired the recapture of the filter legs  9 . The filter may be repositioned before or after the wall-engaging portion  10  have engaged the vena cava wall  32 . In addition, the filter legs  9  do not spring open suddenly, but rather may be gradually and controllably deployed. 
     If repositioning of the device  1  is necessary after complete deployment but prior to disengagement from the delivery system, the inner member  26  may be retracted causing the barbs  6  of control struts  5  to be drawn out of the wall  32 . Further retraction will result in control strut  5  collapse and withdrawal into device  25 . Then the outer deployment tube  28  may be advanced to enclose and re-collapse the filter legs  9  if desired. 
       FIG. 11A-11D  depict a deployment of the filter using a jugular vein approach. A femoral vein approach for deployment may sometimes be preferred. The vena cava filter of the current invention may be deployed from either approach. With the femoral approach, the filter is positioned within the delivery sheath in the opposite longitudinal orientation so that the hub end of the filter will be deployed first. Specifically, the control struts and interconnecting alignment struts are deployed into the vessel first. The filter legs are then deployed. The delivery device is then detached as previously described. 
     Thus, in a novel aspect of the current invention, a vena cava filter device is provided that is easy to deploy through a small delivery system using either a femoral or jugular approach and can be repositioned within the vessel after partial or even fill deployment. Thus, the staged deployment design of this invention provides the user with not only a small, simple system that features controlled deployment from different approaches but provides the option of repositionability at each deployment step. 
     Methods of device retrieval will now be discussed with reference to  FIG. 12A through 12D .  FIG. 12A  illustrates the first step in retrieval of the fully deployed filter  1  from within a vena cava. A standard sheath retrieval system  33  is comprised of a snare wire or loop  34 , an inner retrieval tube  35  and an outer retrieval tube  36 . In  FIG. 12A , the snare wire  34  has been positioned within the slot of the retrieval hook  4  of the non-slotted section  18  of the control strut subassembly  3 . 
     Referring now to  FIG. 12B , the inner retrieval tube  35  is advanced over the snare  34  and the non-slotted section  18  of the control strut subassembly  3  until the end of the inner retrieval tube  35  abuts up against the downstream edge of the non-slotted section  19  of the filtering cone/alignment strut subassembly  2 . 
       FIG. 12C  depicts the retrieval step of collapsing the control struts  5  and alignment struts  8 . To activate the collapse of these filter components, the snare wire  34  is retracted, causing retraction of the inner tubular subassembly  3 . Retraction of the subassembly  3  in turn causes the control struts  5  to be drawn toward the center of the vessel  32 , disengaging the barbs  6  from the vessel wall at an angle perpendicular to the longitudinal axis of the vessel. The inner cannula  37  facilitates retraction by directing the collapsing force of the alignment struts  8  inwardly instead of longitudinally. Because the barbs  6  of control struts  5  are interconnected to the alignment struts  8  through barb-receiving holes  7 , the inward radial movement of the control struts causes a corresponding radial movement of the alignment struts  8 . In other words, in this embodiment, the retrieval step includes axially moving the captured secondary hub in a downstream direction relative to the primary hub thereby causing the plurality of connector elements  5  to be pulled through the lumen of the primary hub such that the alignment struts  8  are pulled radially inward. 
     Further retraction of the snare wire  34  results in the complete collapse of the control strut subassembly  3 , as shown in  FIG. 12C . As the control struts  5  are drawn inwardly to the center of the vessel, the interconnected alignment struts  8  are also drawn inwardly by the retraction force of the control struts  5 . The apices  17  of alignment struts  8  will be drawn inward until they are in completely aligned with the center longitudinal axis of the device as shown in  FIG. 12C . 
       FIG. 12D  depicts the collapsed filter device  1  totally enclosed within the retrieval sheath device  33 . To complete the collapse the device  1 , the outer tube  36  is independently advanced over the downstream portion of the filter legs  9 . Further advancement of the outer sheath causes disengagement of wall-engaging portion  10  and complete collapse of the legs  9 . Alternatively, retraction of the snare wire will cause collapse of the filter legs  9 . 
     Thus, the current invention provides a conical filter design incorporating stabilizing and centering alignment struts with a control strut mechanism for disengaging the alignment struts from the vessel wall and inwardly radially collapsing them into a profile that can easily be deployed and removed from the body using standard interventional devices. 
     Other configurations and methods of creating a retrievable vena cava filter are also within the scope of this invention. Modifications to the details illustrated in this disclosure, including filter and component shapes, barb designs, dimensions, materials, methods of construction and methods of use are within the scope of this invention. Accordingly, the scope of the invention is not limited to the foregoing specification, but instead is given by the appended claims along with their full range of equivalents.