Abstract:
A blood filter includes a hub, a plurality of legs, and a plurality of arms. Each of the legs includes a hook at a distal end thereof, a first linear segment extending substantially parallel to a hub longitudinal axis, a second linear segment extending obliquely with respect to the longitudinal axis at a first angle, and an intermediate linear segment coupled to the first and second segments, the intermediate segment extending obliquely with respect to the longitudinal axis at a second angle less than the first angle. Each of the arms has a free end closer to the longitudinal axis than the hook of each of the legs. The free ends of the arms are spaced radially transverse to the longitudinal axis at substantially the same radial distance.

Description:
PRIORITY 
     This application is a continuation of Ser. No. 13/414,605, filed Mar. 7, 2012, which is a division of U.S. patent application Ser. No. 11/150,661, filed Jun. 10, 2005, now U.S. Pat. No. 8,133,251, which is a continuation of U.S. patent application Ser. No. 09/640,865, filed on Aug. 18, 2000, now U.S. Pat. No. 7,314,477, which is a division of U.S. patent application Ser. No. 09/360,654, filed on Jul. 26, 1999, now U.S. Pat. No. 6,258,026, which is a continuation-in-part of U.S. patent application Ser. No. 09/160,384, filed on Sep. 25, 1998, now U.S. Pat. No. 6,007,558. Each of the previously mentioned applications and patents is incorporated by reference in its entirety into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a vascular passageway and which are subsequently expandable into contact with the walls of the passageway. These devices, among others, include blood clot filters which expand and are held in position by engagement with the inner wall of a vein. It has been found to be advantageous to form such devices of a shape memory material having a first, relatively pliable low temperature condition and a second, relatively rigid high-temperature condition. By forming such devices of temperature responsive material, the device in a flexible and reduced stress state may be compressed and fit within the bore of a delivery catheter when exposed to a temperature below a predetermined transition temperature, but at temperatures at or above the transition temperature, the device expands and becomes relatively rigid. 
     Known self expanding medical devices have been formed of Nitinol, an alloy of titanium and nickel which provides the device with a thermal memory. The unique characteristic of this alloy is its thermally triggered shape memory, which allows a device constructed of the alloy to be cooled below a temperature transformation level to a martensitic state and thereby softened for loading into a catheter in a relatively compressed and elongated state, and to regain the memorized shape in an austenitic state when warmed to a selected temperature above the temperature transformation level, such as human body temperature. The two interchangeable shapes are possible because of the two distinct microcrystalline structures that are interchangeable with a small variation in temperature. The temperature at which the device assumes its first configuration may be varied within wide limits by changing the composition of the alloy. Thus, while for human use the alloy may be focused on a transition temperature range close to 98.6° F., the alloy readily may be modified for use in animals with different body temperatures. 
     U.S. Pat. No. 4,425,908 to Simon discloses a very effective blood clot filter formed of thermal shape memory material. This filter, like most previously developed vena cava filters such as those also shown by U.S. Pat. No. 5,108,418 to Lefebvre, U.S. Pat. No. 5,133,733 to Rasmussen et al., U.S. Pat. No. 5,242,462 to EI-Nounou et al., U.S. Pat. No. 5,800,457 to Gelbfish and U.S. Pat. No. 5,853,420 to Chevillon et al. is a permanent filter which, when once implanted, is designed to remain in place. Such filters include structure to anchor the filter in place within the vena cava, such as elongate diverging legs with hooked ends that penetrate the vessel wall and positively prevent migration in either direction longitudinally of the vessel. The hooks on filters of this type are rigid and will not bend, and within two to six weeks after a filter of this type has been implanted, the endothelium layer grows over the diverging legs and positively locks the hooks in place. Now any attempt to remove the filter results in a risk of injury to or rupture of the vena cava. 
     A number of medical procedures subject the patient to a short term risk of pulmonary embolism which can be alleviated by a filter implant. In such cases, patients are often adverse to receiving a permanent implant, for the risk of pulmonary embolism may disappear after a period of several weeks or months. However, most existing filters are not easily or safely removable after they have remained in place for more than two weeks, and consequently longer term temporary filters which do not result in the likelihood of injury to the vessel wall upon removal are not available. 
     In an attempt to provide a removable filter, two filter baskets have been formed along a central shaft which are conical in configuration, with each basket being formed by spaced struts radiating outwardly from a central hub for the basket. The central hubs are held apart by a compression unit, and the arms of the two baskets overlap so that the baskets face one another. Devices of this type require the use of two removal devices inserted at each end of the filter to draw the baskets apart and fracture the compression unit. The end sections of the arms are formed to lie in substantially parallel relationship to the vessel wall and the tips are inclined inwardly to preclude vessel wall penetration. If a device of this type is withdrawn before the endothelium layer grows over the arms, vessel wall damage is minimized. However, after growth of the endothelium layer the combined inward and longitudinal movement of the filter sections as they are drawn apart can tear this layer. U.S. Pat. No. 5,370,657 to Irie is illustrative of a prior art removable filter of this type which requires two removal devices. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a vessel implantable filter of shape memory material having temperature induced austenitic and martensite states which may be easily removed by a single removal device after an extended period of time without significantly injuring the vessel wall. 
     Another object of the present invention is to provide a blood clot filter of shape memory material which operates in a temperature induced austenitic state to exert a force on the wall of a vessel by means of oppositely disposed legs to maintain the filter in place, but which may easily be removed after the endothelium layer has covered the ends of the filter legs without significant damage to the vessel wall. 
     A further object of the present invention is to provide a novel and improved vessel implantable filter having a group of arms and a group of legs which incline from a central axis. The ends of the arms in the group of arms are oriented to engage a vessel wall to orient and center the filter in the vessel, and the ends of the legs of the group of legs are oriented to engage the vessel wall to prevent longitudinal movement of the filter along the vessel. The ends of at least some of the legs are provided with hooks configured to be more elastic than the legs to permit the hooks to straighten in response to a withdrawal force to facilitate withdrawal from the endothelium layer without risk of significant injury to the vessel wall. In some cases, similar hooks can be formed on the ends of at least some of the arms. 
     Yet another object of the present invention is to provide a novel and improved vessel implantable filter having one or more expandable appendages which engage the wall of the vessel. An elastic hook is formed on the free end of an appendage to pierce the vessel wall and insure that the filter does not migrate in response to normal respiratory functions or in the event of a massive pulmonary embolism. The hook is formed to have a maximum migration force, and when subjected to forces below the maximum migration force, the hook retains its shape. When subjected to forces above the maximum migration force, the hook straightens and can be withdrawn without significant damage to the vessel wall. 
     A further object of the present invention is to provide a novel and improved vessel implantable filter having a plurality of expandable appendages which engage the wall of a vessel. Three to twelve of such appendages are provided which have an elastic hook formed on the free end of the appendage to pierce the vessel wall and insure that the filter does not migrate when subjected to a pressure gradient falling within a range of from 10 mmHg to 120 mmHg in a 28 mm vessel (filter migration resistance). Each hook is formed to have a maximum migration force, and when subjected to forces below the maximum migration force, the hook retains its shape. When subjected to forces above the maximum migration force, the book straightens and can be withdrawn without significant damage to the vessel wall. The maximum migration force for each hook is dependent upon the desired total filter migration resistance and the number of hooks formed on the filter. 
     A still further object of the present invention is to provide a novel and improved removable embolus blood clot filter and filter delivery unit designed to insure delivery of the filter in a centered orientation to a precise location within a vessel. The filter delivery unit includes an elongate pusher wire of shape memory material having temperature induced austenitic and martensite states, with a handle at one end and a filter engaging spline at the opposite end. Spaced inwardly from the spline is a pusher pad which is longitudinally slotted to receive the elongate appendages of the filter. The pusher wire is reduced in diameter between the spline and pusher pad at a point adjacent to the pusher pad to impart a directional hinge to the pusher wire at the reduced portion. 
     According to the invention, a resilient blood clot filter is inwardly radially collapsible toward its longitudinal axis into a collapsed configuration for insertion into a body vessel, but is adapted for automatic radial expansion into contact with the inner wall of the vessel at two longitudinally spaced peripheral locations therein. The filter has leading and trailing ends and comprises a plurality of wires. The wires, in the normal expanded configuration of the filter, are in the form of a plurality of elongated arms and legs with openings between the wires to provide filter baskets opening at the leading end of the filter. The wires have peripheral portions for contact with the inner wall of the vein at two longitudinally spaced peripheral locations. The arms operate to center the filter while the legs terminate in hooks which anchor the filter but which straighten in response to force applied at the trailing end of the filter to facilitate removal of the filter. 
     To provide a filter that is inwardly radially collapsible from its normally expanded configuration toward its longitudinal axis into a collapsed configuration for insertion into a body vessel, the blood clot filter is preferably formed from a plurality of wire portions composed of a thermal shape memory material having a first, low-temperature condition and a second, high-temperature condition. The material in its low-temperature condition is relatively pliable (so that the wire portions may be straightened) and in its high-temperature condition is resiliently deformable and relatively rigid, and takes a pre-determined functional form. 
     In the high-temperature condition of the material, the filter comprises coaxial first and second filter baskets, each filter basket being generally symmetrical about the longitudinal axis of the filter with both filter baskets being concave relative to the filter leading end. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view in side elevation of an expanded blood clot filter of the present invention; 
         FIG. 2  is a view in side elevation of a hook for a leg of the filter of  FIG. 1 ; 
         FIG. 3  is a view in side elevation of a second embodiment of a hook for a leg of the filter of  FIG. 1 ; 
         FIG. 4  is a cross sectional view of the blood clot filter of the present invention in place in a blood vessel; 
         FIG. 5  is a diagrammatic view of a second embodiment of the leg structure for the blood clot filter of the present invention; 
         FIG. 6  is a plan view of the filter delivery unit of the present invention; 
         FIG. 7  is an enlarged view in end elevation of the pusher pad for the filter delivery unit of  FIG. 6 ; and 
         FIG. 8  is an enlarged view of the end section of the filter delivery unit of  FIG. 6  in engagement with a filter. 
     
    
    
     DETAILED DESCRIPTION 
     By forming the body of a blood clot filter of a Nitinol alloy material, such as Nitinol wire, transition between the martensitic and austenitic states of the material can be achieved by temperature transitions above and below a transition temperature or transition temperature range which is at or below body temperature. Such controlled temperature transitions have conventionally been employed to soften and contract the Nitinol filter body to facilitate insertion into a catheter and to subsequently expand and rigidify the body within a vascular or other passageway. Although the filters of the present invention are preferably formed from a temperature responsive shape memory material, such as Nitinol, they can also be formed of a compressible spring metal such as stainless steel or a suitable plastic. 
     Referring now to  FIG. 1 , an expanded blood clot filter  10  is illustrated which is made from sets of elongate metal wires. The wires are held together at the filter trailing end by a hub  12  where they are plasma welded together and to the hub or otherwise joined. In the low temperature martensite phase of wires made of thermal shape memory material, the sets of wires can be straightened and held in a straight form that can pass through a length of fine plastic tubing with an internal diameter of approximately 2 mm (#8 French catheter). In its high temperature austenitic form, the filter  10  recovers a preformed filtering shape as illustrated by  FIG. 1 . Similarly, wires of spring metal can be straightened and compressed within a catheter or tube and will diverge into the filter shape of  FIG. 1  when the tube is removed. 
     In its normal expanded configuration or preformed filtering shape, filter  10  is a double filter, having a first forwardly disposed filter basket section  14  at the forward or leading end of the filter and a second forwardly disposed filter basket section  16 . The two filter basket sections provide peripheral portions which can both engage the inner wall of a body vessel  17  at two longitudinally spaced locations, and the two filter basket sections are generally symmetrical about a longitudinal axis passing through the hub  12 . On the other hand, the second forwardly disposed filter basket section  16 , which is primarily a centering unit, may not always touch the vessel wall on all sides. 
     The second filter basket section  16  is formed from short lengths of wire which form arms  18  that extend angularly, outwardly and then downwardly from the hub  12  toward the forward end of the filter  10 . Each arm  18  has a first arm section  20  which extends angularly outwardly from the hub  12  to a shoulder  22 , and an outer arm section  24  extends angularly from the shoulder toward the forward end of the filter. The outer arm sections  24  are substantially straight lengths with ends which lie on a circle at their maximum divergence and engage the wall of a vessel at a slight angle (preferably within a range of from ten to forty-five degrees) to center the hub  12  within the vessel. For a filter which is to be removed by grasping the hub  12 , it is important for the hub to be centered. Normally, there are six wires  18  of equal length extending radially outward from the hub  12  and circumferentially spaced, such as for example by sixty degrees of arc. 
     The first filter basket section  14  is the primary filter and can include up to twelve circumferentially spaced straight wires  26  forming downwardly extending legs which tilt outwardly of the longitudinal axis of the filter  10  from the hub  12 . Six of the wires  26  are shown in  FIG. 1 , and may be of equal length, but normally they are not so that hooks  28  at the ends of the wires will fit within a catheter without becoming interconnected. The wires  26  are preferably much longer than the wires  18 , and have tip sections which are uniquely formed, outwardly oriented hooks  28  which lie on a circle at the maximum divergence of the wires  26 . There may be from three to twelve of the wires  26  formed with hooks  28 , although in some instances, the wire arms  18  may include similarly formed hooks at the free ends thereof. The wires  26 , in their expanded configuration of  FIG. 1 , are at a slight angle to the vessel wall, preferably within a range of from ten to forty-five degrees, while the hooks  28  penetrate the vessel wall to anchor the filter against movement. The wires  26  are radially offset relative to the wires  18  and may be positioned halfway between the wires  18  and also may be circumferentially spaced by sixty degrees of arc as shown in  FIG. 4 . Thus the combined filter basket sections  14  and  16  can provide a wire positioned at every thirty degrees of arc at the maximum divergence of the filter sections. With reference to the direction of blood flow shown by the arrow in  FIG. 1 , the filter section  14  forms a concave filter basket opening toward the leading end of the filter  10  while the filter section  16  forms a concave filter basket opening toward the leading end of the filter  10  downstream of the filter section  14 . 
     The structure of the hooks  28  is important. As in the case of hooks formed on the legs of previously known permanent vena cava filters, these hooks  28  penetrate the vessel wall when the filter  10  is expanded to anchor the filter in place and prevent filter migration longitudinally of the vessel in either direction. However, when these hooks are implanted and subsequently covered by the endothelium layer, they and the filter can be withdrawn without risk of significant injury or rupture to the vena cava. Minor injury to the vessel wall due to hook withdrawal such as damage to the endothelial layer or local vena cava wall puncture is acceptable. However, previous filters with rigid anchoring hooks could not be withdrawn without causing unacceptable vessel tearing or local hemorrhage. 
     With reference to  FIGS. 1 and 2 , each hook  28  is provided with a juncture section  30  between the curvature of the hook and the leg  26  (or arm  18 ) to which the hook is attached. This juncture section is considerably reduced in cross section relative to the cross section of the leg  26  (or arm  18 ) and the remainder of the hook. The juncture section is sized such that it is of sufficient stiffness when the legs  26  (or arms  18 ) are expanded to permit the hook  28  to penetrate the vena cava wall. However, when the hook is to be withdrawn from the vessel wall, withdrawal force to which the hook is subjected will cause flexure in the juncture section  30  so that the hook moves toward a position parallel with the axis of the leg  26  (or arm  18 ) as shown in broken lines in  FIG. 2 . With the hook so straightened, it can be withdrawn without tearing the vessel wall leaving only a small puncture. 
     With reference to  FIG. 3 , it will be noted that the entire hook  28  can be formed with a cross section throughout its length which is less than that of the leg  26  (or arm  18 ). This results in straightening of the hook over its entire length in response to a withdrawal force. This elasticity in the hook structure prevents the book from tearing the vessel wall during withdrawal. 
     As previously indicated, while it is possible that the filter could be made from ductile metal alloys such as stainless steel, titanium, or elgiloy, it is preferable to make it from nitinol. Nitinol is a low modulus material which allows the arms and legs of the device to be designed to have low contact forces and pressures while still achieving sufficient anchoring strength to resist migration of the device. The force required to cause opening of the hooks  28  can be modulated to the total force required to resist filter migration. This is accomplished by changing the cross sectional area or geometry of the hooks, or by material selection. 
     In addition to temperature sensitivity, nitinol, when in the temperature induced austenitic state, is also subject to stress sensitivity which can cause the material to undergo a phase transformation from the austenitic to the martensitic state while the temperature of the material remains above the transition temperature level. By reducing a portion or all of the cross sectional area of the hooks  28  relative to that of the legs  26  (or arms  18 ), stress is concentrated in the areas of reduced cross section when longitudinal force is applied to the hub  12  in the direction of the trailing end of the filter to remove the filter, and the hooks become elastic and straighten. Thus the hooks, whether formed of nitinol, spring metal or plastic, are designed to bend toward a more straight configuration when a specific hook migration force is applied and spring back to their original shape once the hook migration force has been removed. The force or stress which is required to deform the hook can be correlated to the force applied to each hook of the device when it is fully occluded and the blood pressure in the vessel is allowed to reach 50 mmHg. This force is approximately 70 gms on each leg of a six leg device for 50 mmHg. pressure differential in a 28 mm vessel. The desired total migration resistance force for the filter is desireably 420 gms, and more legs  26  with hooks  28  can be added to lower maximum migration force for each hook. The load on the filter would be correspondingly smaller in vessels of smaller diameter. The object is to have the hook perform as an anchoring mechanism at a predetermined filter migration resistance force within a range of 10 mmHg up to 120 mmHg. Having maintained its geometry at a predetermined filter migration resistance force within this range, the hook should begin to deform in response to a higher force applied in the direction of the filter trailing end and release at a force substantially less than that which would cause damage to the vessel tissue. It is the ability of the hook to straighten somewhat that allows for safe removal of the device from the vessel wall. 
     After the filter  10  has remained in place within a vessel for a period of time in excess of two weeks, the endothelium layer will grow over the hooks  28 . However, since these hooks, when subjected to a withdrawal force become substantially straight sections of wire oriented at a small angle to the vessel wall, the filter can be removed leaving only six pin point lesions in the surface of the endothelium. To accomplish this, a catheter or similar tubular unit is inserted over the hub  12  and into engagement with the arms  18 . While the hub  12  is held stationary, the catheter is moved downwardly forcing the arms  18  downwardly, and subsequently the arms  26  are engaged and forced downwardly thereby withdrawing the hooks  28  from the endothelium layer. Then the hub  12  is drawn into the catheter to collapse the entire filter  10  within the catheter. When the filter is formed from shape memory material, cooling fluid can be passed through the catheter to aid in collapsing the filter. 
     The primary objective of the hooks  28  is to ensure that the filter does not migrate during normal respiratory function or in the event of a massive pulmonary embolism. Normal inferior vena cava (IVC) pressures are between 2-5 mmHg. An occluded IVC can potentially pressurize to 35 mmHg below the occlusion. To ensure filter stability, a 50 mmHg pressure drop across the filter may therefore be chosen as the design criteria for the filter migration resistance force for the removable filter  10 . When a removal pressure is applied to the filter that is greater than 50 mmHg, the hooks  28  will deform and release from the vessel wall. The pressure required to deform the hooks an be converted to force by the following calculations. 
     
       
         
           
             
               Since 
               ⁢ 
               
                   
               
               ⁢ 
               51.715 
               ⁢ 
               
                   
               
               ⁢ 
               mm 
               ⁢ 
               
                   
               
               ⁢ 
               Hg 
             
             = 
             
               1.0 
               ⁢ 
               
                   
               
               ⁢ 
               lb 
               ⁢ 
               
                 / 
               
               ⁢ 
               
                 in 
                 2 
               
             
           
         
       
       
         
           
             
               50 
               ⁢ 
               
                   
               
               ⁢ 
               mm 
               ⁢ 
               
                   
               
               ⁢ 
               Hg 
             
             = 
             
               
                 50 
                 51.715 
               
               = 
               
                 0.9668 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 lb 
                 ⁢ 
                 
                   / 
                 
                 ⁢ 
                 
                   in 
                   2 
                 
               
             
           
         
       
     
     For a 28 mm vena cava: 
     
       
         
           
             A 
             = 
             
               
                 
                   π 
                   4 
                 
                 ⁢ 
                 
                   
                     ( 
                     28 
                     ) 
                   
                   2 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   mm 
                   2 
                 
               
               = 
               
                 
                   615.4 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     mm 
                     2 
                   
                 
                 = 
                 
                   0.9539 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     inches 
                     2 
                   
                 
               
             
           
         
       
     
     Migration force is calculated by: 
     
       
         
           
             
               
                 
                   P 
                   = 
                   
                     F 
                     A 
                   
                 
               
               
                 
                   F 
                   = 
                   
                     P 
                     × 
                     A 
                   
                 
               
             
           
         
       
       
         
           
             
               0.9668 
               ⁢ 
               
                   
               
               ⁢ 
               psi 
               × 
               0.9539 
               ⁢ 
               
                   
               
               ⁢ 
               
                 inches 
                 2 
               
             
             = 
             
               
                 0.9223 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 pounds 
               
               = 
               
                 418.7 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 g 
               
             
           
         
       
     
     It is important to recognize that as vena cava diameter increases so does the force required to resist 50 mmHg of pressure. 
     Depending on the number of filter hooks, the strength of each can be calculated. For a device that has six hooks: 
     
       
         
           
               
             
               
                 
                   
                     
                       Hook 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Strength 
                     
                     = 
                       
                     ⁢ 
                     
                       
                         Filter 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Migration 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Resistance 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Force 
                       
                       
                         Number 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Hooks 
                       
                     
                   
                 
               
               
                 
                   
                     = 
                       
                     ⁢ 
                     
                       418.7 
                       6 
                     
                   
                 
               
               
                 
                   
                     = 
                       
                     ⁢ 
                     
                       69.7 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       g 
                     
                   
                 
               
             
           
         
       
     
     Each hook must be capable of resisting approximately 70 grams of force for the filter  10  to resist 50 mmHg pressure gradient in a 28 mm vessel. 
     To prevent excessive vessel trauma the individual hook needs to be relatively weak. By balancing the number hooks and the individual hook strength, minimal vessel injury can be achieved while still maintaining the 50 mmHg pressure gradient criteria, or some other predetermined pressure gradient criteria within a range of from 10 mmHg to 120 mmHg. 
     Referring to  FIG. 5 , the legs  26  may be angled outwardly from a shoulder  30  adjacent to but spaced from the outer end of each leg. When the legs are released from compression in a catheter or other tube into a body vessel, this bend in each leg insures that the hooks  28  are, in effect, spring loaded in the tube and that they will not cross as they are deployed from the tube. Since the legs angle outwardly from the shoulders  30 , the hooks  28  are rapidly deployed outwardly as the insertion tube is withdrawn. 
     The filter delivery unit  32  is adapted to deliver the filter  10  through a catheter or delivery tube  34  to a precise, centered position within a body vessel. The filter delivery unit includes a handle  36  at one end, and an elongate pusher wire  38  extends outwardly from the handle  36 . At the free end of the pusher wire is an enlarged filter engaging pusher pad  40 . 
     The elongate pusher wire  38  is preferably formed of superelastic material and may be formed of thermally responsive shape memory material, such as nitinol. The pusher wire includes sections  42 ,  44  and  46  which progressively decrease in cross section beginning at the handle  36 . The temperature transformation level of the pusher wire is such that when the wire is encased in a catheter or delivery tube, it remains in a martensitic state and is therefore somewhat pliable and flexible so that it can conform to curvatures in a catheter or delivery tube which passes through a body vessel. As the delivery tube is withdrawn, body temperature causes the exposed portions of the pusher wire to assume the move rigid austenitic state for filter positioning. 
     A slotted spline  48  is secured to the pusher wire  38  between the sections  44  and  46 . The pusher pad is provided with a plurality of spaced, peripherally arranged, longitudinally extending grooves  50  of sufficient number to individually receive the legs  26  of a filter  10 . The spline is spaced from the pusher pad  40  for a distance less than the length of the filter legs  26  so that the legs can be received in the grooves  50  when the pusher pad engages the filter hub  12  as shown in  FIG. 8 . It will be noted that the pusher wire section  46  is reduced in cross section at  52  adjacent to the spline  48 . 
     To load the filter delivery unit  32  to insert a filter  10  into a body vessel, the pusher wire section  46  is inserted from the leading end of the filter  10  under the arms  18  and legs  26  until the pusher pad  40  engages the underside of the hub  12  at the apex of the filter as shown in  FIG. 8 . Then the legs  26  of the filter, two being shown for purposes of illustration in  FIG. 8 , are inserted into the grooves  50  in the spline, and the arms  18  are spirally wrapped around the spline. 
     The pusher wire, with the filter in place, is inserted into a catheter or delivery tube  34 . When the catheter or delivery tube with the filter  10  is at a desired location within a body vessel, it is removed from around the delivery unit and filter to expose the filter. First the hub  12  of the filter is exposed and then the pusher wire section  46  emerges. When the pusher wire is formed of thermal shape memory material, the emergence of wire section  46  causes this section, with the exception of the portion of reduced cross section  52 , to transform to the austenitic state and to become more rigid. As the filter pad  48  emerges, the centering arms  18  of the filter  10  are exposed and released and transform to the austenitic state to achieve radial expansion outwardly toward the vessel wall. If the filter is not centered in the vessel, some of the arms  18  will engage the vessel wall and apply stress to the reduced cross section portion  52  of the pusher wire section  46 . Stress causes this portion  52  to remain in the flexible martensitic state, and the pusher wire section  46  will pivot at the portion  52  to permit radial movement of the spline  40  in all directions to aid the arms  18  in centering the filter  10  within the vessel. Thus the portion  52  provides a directional hinge for centering the filter. 
     With the filter centered, the legs  26  are exposed and expand radially to engage the vessel wall and anchor the filter against migration. The pusher wire and catheter or delivery tube are now withdrawn from the body vessel. 
     When the pusher wire is formed of flexible material which is not a thermal, shape memory material, the reduced cross sectional portion  52  to the pusher wire section  46  has greater flexibility than the remainder of the pusher wire and thus forms a flexible, directional hinge to aid in centering the filter in the manner previously described.