Removable blood clot filter

ES 2 242 425 T3

DESCRIPTION

Removable filter for blood clots.

In recent years, various medical devices have been designated which are adapted for compression to a small size to facilitate introduction into a vascular lumen and which can then be expanded in contact with the lumen walls. These devices, among others, include blood clot filters that expand and are held in position by engagement with the interior wall of a vein. It has been found advantageous to form such devices from a shape memory material to have a first relatively pliable condition at low temperature and a second relatively rigid condition at high temperature. By forming such devices of temperature sensitive material, the device in a flexible and reduced stress state can be compressed and fit within the bore of a delivery catheter when exposed to a temperature lower than 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 from Nitinol, a nickel-titanium alloy that provides the device with thermal memory. The unique characteristic of this alloy is its thermally activated shape memory, which allows a device constructed of the alloy to be cooled below a level of thermal transformation to a martensitic state and therefore soften for loading into a catheter in a relatively compressed state. and elongated, and recover the memory shape in an austenitic state when heated to a selected temperature, above the level of thermal transformation, such as human body temperature. The two interchangeable shapes are possible because of the two different microcrystalline structures that are interchangeable with a small temperature variation. The temperature at which the device assumes its first configuration can be varied within wide limits by changing the composition of the alloy. Thus, although for human use the alloy may be centered in a transition temperature range close to 37 ° C (98.6 ° F), the alloy can be easily modified for use in animals with different body temperatures.

US Patent No. 4,425,908 to Simon describes a very effective blood clot filter made of thermal shape memory material. This filter, like most previously developed vena cava filters, is a permanent filter that, once implanted, is designed to remain in place. Such filters include a structure to anchor the filter in position within the vena cava, such as elongated divergent legs with hooked ends that penetrate the vessel wall and positively prevent migration in any longitudinal direction of the vessel. The hooks on filters of this type are rigid and will not bow, and within two to six weeks after implanting such a filter, the endothelial layer grows over the diverging legs and positively locks the hooks in position. Now any attempt to remove the filter results in a risk of injury or rupture of the vena cava.

Various medical procedures place the patient at a short-term risk of pulmonary embolism that can be mitigated by a filter implant. In such cases, patients are often averse to receiving a permanent implant, because the risk of pulmonary embolism may disappear after a period of several weeks or months. However, most of the existing filters cannot be easily or safely removed after they have been in place for more than two weeks, and consequently longer term temporary filters are not available that do not lead to probability. of vessel wall injury upon removal.

In an attempt to provide a removable filter, two filter baskets have been formed along a central axis which are conical in configuration, each basket formed by spaced spars extending outwardly from a central hub of the basket. The center cubes are kept apart by a compression unit, and the arms of the two baskets overlap so that the baskets face each other. Devices of this type require the use of two extraction devices inserted at each end of the filter to separate the baskets and break the compression unit. The end sections of the arms are formed so that they are in substantially parallel relation to the vessel wall and the tips are inclined inwardly to exclude penetration of the vessel wall. If such a device is removed before the endothelial layer grows over the arms, damage to the vessel wall is minimized. However, after the growth of the endothelial layer, the combined inward and longitudinal movement of the filter sections as they separate can tear said layer. US Patent No. 5,370,657 to Irie is illustrative of a prior art removable filter of this type that requires two extraction devices.

US-A-5 601 595 is recognized in the preamble of claim 1.

A primary object of the present invention is to provide an implantable vessel filter of shape memory material having temperature-induced austenitic and martensitic states, which can be easily removed with a single removable device after a prolonged period of time without injury. the vessel wall.

Another object of the present invention is to provide a Nitinol blood clot filter which operates in a temperature-induced austenitic state to exert a force on the wall of a vessel by means of opposing legs to hold the filter in position, but which it can be easily removed after the endothelial layer has covered the ends of the filter legs without damage to the vessel wall.

Another object of the present invention is to provide a new and improved filter having a group of arms and a group of legs that slope in the same direction from a central axis. The ends of the arms on the group of arms are oriented to engage a cup wall to orient and center the filter in the cup, and the ends of the legs of the group of legs are oriented to engage the cup wall to prevent movement. length of the filter along the vessel. The ends of the legs are provided with hooks configured so as to be more elastic than the legs to allow removal of the endothelial layer without risk of injury to the vessel wall.

According to the invention, a longitudinally extending elastic blood clot filter can collapse radially inward towards its lon axis.

ES 2 242 425 T3 longitudinal to a flattened configuration for introduction into a vein, but is adapted for automatic radial expansion into contact with the inner wall of the vein at two longitudinally spaced peripheral positions. The filter has front and rear ends and includes a plurality of threads. The threads, in the normal expanded filter configuration, are in the form of a plurality of elongated arms and legs with holes between the threads that provide filter baskets that open to the front end of the filter. The threads have peripheral portions for contact with the inner wall of the vein at two longitudinally spaced peripheral positions. The arms are intended to center the filter while the legs end in hooks that anchor the filter, but straighten in response to force to facilitate removal of the filter.

To provide a filter that can be collapsed radially inward from its normally expanded configuration towards its longitudinal axis to a collapsed configuration for introduction into a vein, the blood clot filter is preferably formed of a plurality of composite wire portions 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 can be straightened) and in its high temperature condition it is elastically deformable as well as relatively rigid, and assumes a predetermined functional shape.

In the high temperature condition of the material, the filter includes first and second coaxial filter baskets, each filter basket being generally symmetrical about the longitudinal axis of the filter, both filter baskets being concave relative to the front end of the filter.

Figure 1 is a side elevational view of an expanded blood clot filter of the present invention.

Figure 2 is a side elevation view of a hook for a leg of the filter of Figure 1.

Figure 3 is a side elevation view of a second embodiment of a hook for a leg of the filter of Figure 1.

Figure 4 is a side view of a second embodiment of the blood clot filter of the present invention.

Figure 5 is a sectional view of a portion of a leg for the filter of Figure 4.

Figure 6 is a sectional view of a portion of the leg for the filter of Figure 4 with the hook removed.

Figure 7 is a side elevational view of a hook extraction unit for a blood clot filter of Figure 4.

Figure 8 is a side elevational view of the hook extraction unit of Figure 7 in an extraction configuration.

Figure 9 is a cross-sectional view of the blood clot filter of the present invention in position in a blood vessel.

And Figure 10 is a side elevation view of a third embodiment of a filter with a hook extraction unit.

By forming the body of a blood clot filter of a Nitinol alloy material, such as Nitinol wire, the transition between the martensitic and austenitic states of the material can be achieved by temperature transitions above and below a temperature of transition or transition temperature range that is at or below body temperature. Such temperature controlled transitions have been conventionally employed to soften and contract the Nitinol filter to facilitate insertion into a catheter and then expand and stiffen the body within a vascular passage or other passage. Although the filters of the present invention are preferably formed of a shape memory temperature sensitive material, such as Nitinol, they can also be made of a compressible elastic metal such as stainless steel or a suitable plastic.

Referring now to Figure 1, there is illustrated an expanded blood clot filter 10 that is made of elongated wire assemblies. The wires are held together at one end in a hub 12 where they are plasma and hub welded or otherwise joined. In the low temperature martensitic phase of yarns made of thermal shape memory material, the sets of yarns can be straightened and held in a straight shape that can pass through a length of thin plastic tube with an internal diameter of about 2mm (8 French catheter). In its austenitic high temperature form, filter 10 recovers a preformed filter formation as illustrated in Figure 1. Similarly, the elastic metal wires can be straightened and compressed within a catheter or tube and will diverge to the filter shape of Figure 1 when the tube is removed.

In its normal expanded configuration or preformed filtration form, filter 10 is a dual filter, having a forwardly disposed first filter basket section 14 at the front end of the filter and a forwardly disposed second filter basket section 16. The two filter basket sections provide peripheral portions that can engage the inner wall of vein 17 at two longitudinally spaced positions and the two filter basket sections are generally symmetrical about a longitudinal axis passing through hub 12. On the other hand, the forwardly disposed second filter basket section 16, which is primarily a centering unit, may not touch the vessel wall on all sides.

The second filter basket section 16 is formed from short lengths of wire that form arms 18 that extend angularly, outward and then downward from hub 12 toward the front end of filter 10. Each arm 18 has a first section of arm 20 extending angularly out of hub 12 to a projection 22, and an outer arm section 24 extending angularly from the projection toward the front end of the filter. The outer arm sections 24 are substantially straight lengths with ends that are circled at their maximum divergence and engage the wall of a vessel at a small angle (preferably within a range of ten to forty-five degrees) to center the cube. 12 inside the glass. For a filter to be removed by grasping the hub 12, it is important that the hub is centered. Typically, there are six wires 18 of equal length extending radially out of hub 12 and spaced circumferentially, such as for example by sixty degrees of arc.

The first filter basket section 14 is the filter

ES 2 242 425 T3 primary and typically includes six circumferentially spaced straight threads 26 that form downwardly extending legs that slope away from the longitudinal axis of filter 10 from hub 12. Threads 26 may be of equal length, but they are typically not such that the hooks 28 at the ends of the wires fit into a catheter without interconnecting. Strands 26 are preferably much longer than strands 18, and have tip sections that are uniquely shaped, outwardly facing hooks 28 that are circled at the maximum divergence of strands 26. The wires 26, in their expanded configuration of Figure 1, are at a small angle to the vessel wall 17, preferably within a range of 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 can be positioned midway between the wires 18 and can also be circumferentially spaced sixty arc degrees as shown in Figure 9. Thus, the combined filter basket sections 14 and 16 can provide a wire placed every thirty degrees of arc at the maximum divergence of the filter sections. Referring to the direction of blood flow in Figure 1, filter section 14 forms a concave filter basket that opens toward the front end of filter 10 while filter section 16 forms a concave filter basket that opens toward the front end of filter 10 downward of filter section 14.

The structure of the hooks 28 is important. As in the case of the hooks formed on the legs of previously known permanent vena cava filters, these hooks 28 penetrate the vessel wall as the filter 10 expands to anchor the filter in position and prevent migration of the filter longitudinally of the vessel. In any direction. However, when these hooks are implanted and then covered by the endothelial layer, they and the filter can be removed without risk of injury or rupture of the vena cava.

With reference to Figures 1 and 2, each hook 28 is provided with a connecting section 30 between the hook and the leg 26 to which the hook is attached. This joint section is considerably reduced in cross section relative to the cross section of leg 26 and the rest of the hook. The attachment section is dimensioned such that it is of sufficient rigidity when the legs 26 expand so that the hook 28 can penetrate the wall of the vena cava. However, when the hook is to be withdrawn from the vessel wall, the pulling force to which the hook is subjected will flex the joint section 30 such that the hook moves to a position parallel to the axis. of leg 26 as shown in dotted lines in Figure 2. With the hook thus straightened, it can be removed without tearing the vessel wall.

With reference to Figure 3, it will be noted that the entire hook 28 can be formed over its entire length with a cross section that is less than that of the leg 26. This results in straightening of the hook over its entire length in response to a extraction force. This elasticity in the hook structure prevents the hook from tearing the vessel wall during extraction.

As noted above, although it is possible that the filter can 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 that allows the device arms and legs to be designed to have low contact forces and pressures, while still achieving sufficient anchoring strength to resist device migration. The load necessary for the opening of the hooks 28 can be modulated to the forces necessary to resist migration. This is done by changing the cross-sectional area or the geometry of the hooks, or by material selection.

In addition to temperature sensitivity, nitinol, when in the temperature-induced austenitic state, is also subjected to stress sensitivity that can cause the material to undergo a phase transformation from the austenitic to martensitic state while the material's temperature 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, stress is concentrated in the areas of reduced cross-section when force is applied to remove the hooks from a vessel wall and the hooks. they become elastic and straighten. Thus, the hooks, whether made of nitinol, elastic metal, or plastic, are designed to bend into a straighter configuration when a specific load is applied and return to their original shape once the load has been removed. The load or effort required to deform the hook can be correlated to the load applied to each hook of the device when it is completely occluded and the blood pressure in the vessel can reach 50 mm Hg. This load is approximately 68.6 N (70 gms) on each leg in a six-leg device for 50 mm Hg differential pressure in a 28 mm vessel. Since fabric tears at a load of approximately 78.5 N (80 gms), the load on each leg required to straighten a hook should be less than 78.5 N (80 gms). The desired total load for the filter is desirably 412 N (420 gms), and more legs 26 can be added with hooks 28 to decrease the load on each leg. The load on the filter would be correspondingly less for smaller diameter vessels. The object is to have the hook preform as an anchoring mechanism at a predetermined load that is consistent with a maximum pressure of 50mm Hg. Having maintained its geometry at said load, the hook should begin to deform above the load and loosen at a load substantially less than that which would damage the vessel tissue. It is the ability of the hook to straighten somewhat that allows the safe removal of the device from the vessel wall.

After the filter 10 has remained in position within a vessel for a period of time greater than two weeks, the endothelial layer will grow over the hooks 28. However, since these hooks, when subjected to extraction force are substantially straight wire sections oriented at a small angle to the vessel wall, the filter can be removed leaving only lesions at six leg points on the surface of the endothelium. To do this, a catheter or similar tubular unit is introduced over the hub 12 already engaged with the arms 18. While the hub 12 is kept stationary, the catheter is moved downward pushing

ES 2 242 425 T3 pulling the arms 18 downwards, and then the arms 26 are hooked and pushed downwards thereby removing the hooks 28 from the endothelial layer. The hub 12 is then drawn onto the catheter to crush 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 facilitate collapse of the filter.

Referring now to Figures 4, 5 and 6, the filter legs 10 are formed as small tubes that open at one end to the hub 12. These legs can be formed of tubular plastic, elastic metal, or shape memory material. thermal. Hooks 28 are formed at the ends of a long shaft 32, depicted in dashed lines in Figure 4, which extends through a tubular leg 26 and into hub 12 where it connects with a ring 34 at the end of a rod. traction 36. The hook and its shaft can be formed of wire or thermal memory material, and the cross-sectional area of ​​the hook is such that the hook will straighten and enter the tubular leg 26 as depicted in Figure 6 when pulled. of the shaft 32 upward in Figure 4 with the pull rod 36. Thus, for removal of the filter, while the filter is still in position, the pull rod is grasped and the hooks are pushed to the tubular legs 26. Then, an extraction tube is moved over arms 20 and 26 to crush the filter.

Various elastic devices or similar structures may be provided adjacent to hub 12 to bring hooks 28 to tubular legs 26 via shafts 32. As shown in Figures 7 and 8, hub 12 may be spaced from sleeve 38 by a plurality of metal washers 40. Sleeve 38 receives and supports the ends of the tubular legs 26 and, if arms are provided, the ends of the arms 20.

Thus, the sleeve 38 is fixed in position, and the shafts 32 extend through the open centers of the washers and are connected to the hub 12. The washers 40 are formed of thermal shape memory material, and below a level of thermal transformation for the material, are flat against sleeve 38 as depicted in Figure 7. However, when the washers are subjected to temperatures above their level of thermal transformation, they curve upward along the longitudinal axis of filter 10 as shown in Figure 8 moving hub 12 away from sleeve 38 so that the hub push shafts 32 up to push hooks 28 to tubular legs 26.

When legs 26 or legs 26 and arms 20 are formed of thermal shape memory material, the level of thermal transformation of this material will normally be body temperature or a lower temperature, but close to body temperature. In this case, the level of thermal transformation for washers 40 will be higher than for arms 20 and legs 26 so that the washers will be flat when the filter is used. Heated saline or other known means may be applied to heat the washers 40 to temperatures above their thermal transformation level when the filter 10 is to be removed.

With this tubular leg design, the need to load tissue from a support vessel to straighten and remove hooks 28 is eliminated. Here, the load required to straighten a hook is created by tubular leg 26.

Referring to Figure 10, the tubular legs 26 can be angled outward from a boss 42 adjacent to, but spaced from, the outer end of each leg. When the legs are released from compression on a catheter or other tube to a body vessel, this bend in each leg ensures that the hooks 28 are, in effect, biased into the tube and that they do not intersect when deployed from the tube. Since the legs slope outward from the projections 42, the hooks 28 rapidly unfold outward when the introduction tube is withdrawn towards the rear of the filter 10.