Niti-type medical guide wire and method of producing the same

Details Niti-type medical guide wire and method of producing the same Niti-type medical guide wire and method of producing the same

2000-07-06

2003-01-21

Nguyen, Anhtuan T. (Department: 3763)

Surgery

Means for introducing or removing material from body for...

Treating material introduced into or removed from body...

C604S164130, C600S434000, C600S585000

Reexamination Certificate

active

06508803

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a medical guidewire comprised of an NiTi-based alloy having a high elasticity over a wide range of strain and a method of producing the same, more particularly relates to a catheter guidewire etc. made of an NiTi-based alloy wire having a straightness, and shape and characteristics of a stress-strain curve preferable for a medical guidewire.
BACKGROUND ART
Medical guidewires include catheter guidewires and endoscope guidewires. Here, the explanation will be given taking as an example a catheter guidewire.
A catheter guidewire is used for guiding a catheter (thin tube) for treatment or examination into a blood vessel and leaving it in the affected area.
Therefore, a catheter guidewire is required to have enough flexibility and shape recovery for insertion into branched and meandering blood vessels without damaging them by conforming to the shape of the blood vessels. The demand for this characteristic has become much stronger in recent years as catheters are now being introduced close to the end of blood vessels.
In the past, stainless steel wire has been used for the catheter guidewire, however it suffers from the disadvantage that stainless steel wire permanently deforms when passed through a sharply curved blood vessel. The wire ends up remaining curved and can no longer proceed further or cannot be reinserted.
Therefore, three types of wire have been recently proposed: (1) a superelastic wire using the superelasticity of an NiTi-based alloy (Japanese Examined Patent Publication (Kokoku) No. 2-24550, Japanese Examined Patent Publication (Kokoku) No. 2-24548, and Japanese Examined Patent Publication (Kokoku) No. 2-24549), (2) a wire obtained by cold drawing an NiTi-based alloy and then heat treating it at a low temperature (hereinafter referred to as “cold drawn, low temperature heat-treated wire”) (Japanese Examined Patent Publication (Kokoku) No. 6-83726 and U.S. Pat. No. 5,230,348), and (3) a cold drawn wire obtained by only cold drawing (WO97/18478).
The above superelastic wire of type (1) utilizes the characteristic that deformation caused by stress-induced martensitic transformation is recovered by reverse transformation when unloaded, so it is much more flexible compared with conventional stainless steel wire and has a strong shape recovery, i.e., so-called superelasticity.
Note that superelasticity is given by heat treatment (shape memory heat treatment, for example, 400 to 500° C.) for giving superelasticity after cold drawing.
Superelastic wire, however, has a yield point F on the stress-strain curve as shown in FIG.
1
D. Stress is not increased even if more strain is given when exceeding the point, so there are the disadvantages that the wire is poor in pushability, cannot be inserted close to an end of the blood vessel, does not allow rotation by the holder to be easily conveyed to the tip of the wire, and therefore is poor in operability (torque transmission).
Also, cold drawn, low temperature heat-treated wire of type (2) is produced by shaping an NiTi-based alloy wire having a cold working rate of 35 to 50% to make it straight (for example, holding it at 350 to 450° C. for 10 to 30 minutes). As shown in the stress-strain curve of
FIG. 1C
, almost no stress-induced martensitic transformation or reverse transformation occurs and the apparent modulus of elasticity is large, so the pushability is excellent.
However, there is a large stress difference H, between loading and unloading, at a strain of 2% after applying strain up to 4%, then unloading. Simultaneously, sufficient straightness cannot be obtained by shaping. Therefore, the torque transmission declines.
The cold drawn wire of type (3) has the advantage, as shown in
FIG. 1B
, that the apparent modulus of elasticity is larger than that of the (2) cold drawn, low temperature heat-treated wire, however, since there is a large residual strain after deformation, the wire ends up permanently deforming when passed through a curved blood vessel.
Further, just cold working is not enough to obtain a wire having a high straightness, so the torque transmission is poor. Also, in spite of the high modulus of elasticity, since the straightness is low, the pushability is inferior for the high modulus of elasticity. Further, there is a large residual strain and therefore a problem in reusability. Especially, in wires of the types (2) and (3), the low straightness is a problem. Improvement of this point has been desired.
As explained above, the conventional superelastic type wire, low temperature heat treated type wire after cold drawing cold drawn type wire do not show the sufficient characteristics for use as a medical guidewire. Development of a wire superior in all of the pushability, torque transmission, and reusability required for use as a medical guidewire has therefore been desired.
SUMMARY OF THE INVENTION
In view of this background art, the present inventors closely analyzed the relationship between characteristics in use required for a medical guidewire and mechanical characteristics of the guidewire and discovered that the required characteristics of the above guidewire can be secured by giving the guidewire specific mechanical characteristics and therefore attempted to give a guidewire a specific shape and characteristics of a stress-strain curve as determined by tensile tests.
Namely, the present inventors read a variety of characteristics from the stress-strain curve as determined by the above tensile tests and found the relationship between the characteristics of the wire and the characteristics in use required for medical guidewire.
Then, they found that the straightness of the wire and the following four characteristics, that is, a total of five characteristics, are closely related with a medical guidewire.
Among the five characteristics, the (1) straightness has to be good from both the viewpoints of reusability (insertability) and torque transmission when used as a medical guidewire.
The other four characteristics are the shape and characteristics of the stress-strain curve as found by tensile tests of the wire, that is, (2) the shape, (3) the apparent modulus of elasticity, (4) the stress difference (that is one of the parameters with reference to stress hysteresis) at a constant strain (strain of 2%) between loading and unloading of a load, and (5) strain recovery after removing strain (residual strain).
Here, the maximum strain at the tensile tests was made 4%. The stress-strain curve was obtained by giving 4% strain, then reducing the load to 0.
The reason why the maximum strain was made 4% in this way was that by conducting a tensile strength test up to a strain of 4%, it is possible to obtain sufficient information enabling evaluation of the material characteristics under the standard conditions of use of medical guidewire.
The inventors conducted further studies and as a result learned that the relationship between the characteristics (1) to (5) of wire and characteristics in use of medical guidewire becomes as follows:
Namely, the pushability is affected by the above (2) and (3). The (2) shape of the stress-strain curve should be one of a monotonous increase in stress v.s. strain without any yield points or inflection points. A guidewire like the conventional superelastic wire which has a yield point and where the gradient of the stress with respect to the strain decreases when the yield point is exceeded easily buckles in a blood vessel and is difficult to push further in. A guidewire which has a high (3) apparent modulus of elasticity has stiffness as a wire and can be easily inserted further.
Further, the torque transmission is affected by the characteristics (1) and (4). A guidewire which has a low (1) straightness has a larger frictional force with the inner wall of the blood vessel. The torque is no longer transmitted accurately and the guidewire cannot be inserted close to the end of the blood vessel with good operability. Also, a guidewire which has a large (4) stress difference is slow to turn at its tip portion in response to rotation

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