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 fig1 , 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 fig1 . similarly , wires of spring metal can be straightened and compressed within a catheter or tube and will diverge into the filter shape of fig1 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 fig1 , 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 fig1 , 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 fig4 . 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 fig1 , 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 fig1 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 fig2 . with the hook so straightened , it can be withdrawn without tearing the vessel wall leaving only a small puncture . with reference to fig3 , 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 desirably 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 . 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 : 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 fig5 , 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 fig8 . 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 fig8 . then the legs 26 of the filter , two being shown for purposes of illustration in fig8 , 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 .
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 .