Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
1997-04-25
2001-11-06
Thaler, Michael H. (Department: 3731)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S017120, C606S198000
Reexamination Certificate
active
06312455
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a stent. Stents are used in lumens in a human or animal body. When properly positioned in a lumen, a stent can contact the wall of the lumen to support it or to force the wall outwardly.
Stents can be made from a material which enables the stent to be compressed transversely elastically so that they can then recover outwardly when the compressing force is removed, into contact with the wall of the lumen. The enhanced elastic properties available from shape memory alloys as a result of a transformation between martensite and austenite phases of the alloys make them particularly well suited to this application. The nature of the superelastic transformations of shape memory alloys is discussed in “Engineering Aspects of Shape Memory Alloys”, T. W. Duerig et al, on page 370, Butterworth-Heinemann (1990). Subject matter disclosed in that document is incorporated in this specification by this reference to the document.
A principal transformation of shape memory alloys involves an initial increase in strain, approximately linearly with stress. This behaviour is reversible, and corresponds to conventional elastic deformation. Subsequent increases in strain are accompanied by little or no increase in stress, over a limited range of strain to the end of the “loading plateau”. The loading plateau stress is defined by the inflection point on the stress/strain graph. Subsequent increases in strain are accompanied by increases in stress. On unloading, there is a decline in stress with reducing strain to the start of the “unloading plateau” evidenced by the existence of an inflection point along which stress changes little with reducing strain. At the end of the unloading plateau, stress reduces with reducing strain. The unloading plateau stress is also defined by the inflection point on the stress/strain graph. Any residual strain after unloading to zero stress is the permanent set of the sample. Characteristics of this deformation, the loading plateau, the unloading plateau, the elastic modulus, the plateau length and the permanent set (defined with respect to a specific total deformation) are established, and are defined in, for example, “Engineering Aspects of Shape Memory Alloys,” on page 376.
SUMMARY OF THE INVENTION
The stress strain behaviour of a shape memory alloy component which exhibits enhanced elastic properties can exhibit hysteresis, where the stress that is applied at a given strain during loading is greater than the stress exerted at that strain during unloading. It is generally desirable when exploiting the enhanced elastic properties of a shape memory alloy component to minimise the difference between the stresses on the loading and unloading curves in a deformation cycle (that is to minimise the hysteresis). However, according to the present invention, it has been found that it can be advantageous in a stent to make use of an alloy which is capable of exhibiting a large hysteresis in a loading and unloading cycle. This can be obtained by using certain nickel titanium based alloys, with ternary additions of at least one of niobium, hafnium, tantalum, tungsten and gold.
Accordingly, in one aspect, the invention provides a stent for use in a lumen in a human or animal body, which has a generally tubular body formed from a shape memory alloy which has been treated so that it exhibits enhanced elastic properties with a point of inflection in the stress-strain curve on loading, enabling the body to be deformed inwardly to a transversely compressed configuration for insertion into the lumen and then revert towards its initial configuration, into contact with and to support the lumen, the shape memory alloy comprising nickel, titanium and from about 3 atomic percent (hereinafter at. %) to about 20 at. %, based on the weight of the total weight of the alloy composition, of at least one additional element selected from the group consisting of niobium, hafnium, tantalum, tungsten and gold.
The use of the specified ternary elements in a nickel titanium alloy has the advantage that the resulting stent is able to exhibit a wider hysteresis in the stress-strain behaviour in a loading and unloading cycle. This is particularly advantageous in a stent for use in a lumen in a human or animal body, which is moved through the stent while in a transversely compressed configuration from which it can expand elastically into contact with and to support the lumen. The wide hysteresis means that the inward force required to compress the stent transversely once in place in the lumen is relatively high, while the outward force that the stent exerts on the lumen as it attempts to revert to its original undeformed configuration is relatively low. This can also mean that the lumen will be resistant to being crushed by externally applied forces which can be a problem in the case of lumens close to the surface such as arteries in the thigh and neck. It can also mean that the lumen does not tend to be distorted undesirably by a large outward force exerted by the stent on the lumen.
The use of the alloy specified above can enable the ratio of the stress on loading to the stress on unloading at the respective inflection points on the stress-strain curve to be at least about 2.5:1, preferably at least about 3:1, more preferably at least about 3.5:1, for example at least about 4:1, measured at body temperature. This relationship between the loading and unloading stresses in the loading-unloading cycle provides the combination of resistance to crushing of a stent-supported lumen and low outward force tending to deform the lumen, discussed above.
Accordingly, in another aspect, the invention provides a stent for use in a lumen in a human or animal body, which has a generally tubular body formed from a shape memory alloy which has been treated so that it exhibits enhanced elastic properties with a point of inflection in the stress-strain curve on unloading, enabling the body to be deformed inwardly to a transversely compressed configuration for insertion into the lumen and then revert towards its initial configuration, into contact with and to support the lumen, the ratio of the stress on loading to the stress on unloading at the respective inflection points on the stress-strain curve being at least about 2.5:1, preferably at least about 3:1, measured at body temperature.
The use of the alloy specified above can enable the difference between the stress on loading and the stress on unloading at the respective inflection points on the stress-strain curve, after deformation to a strain of 10%, to be at least about 250 MPa, preferably at least about 300 MPa, more preferably at least about 350 MPa, for example at least about 400 MPa. This relationship between the loading and unloading stresses in the loading-unloading cycle can also provide the combination of resistance to crushing of a stent-supported lumen and low outward force tending to deform the lumen, discussed above.
Accordingly, in a further aspect, the invention provides a stent for use in a lumen in a human or animal body, which has a generally tubular body formed from a shape memory alloy which has been treated so that it exhibits enhanced elastic properties with a point of inflection in the stress-strain curve on loading, enabling the body to be deformed inwardly to a transversely compressed configuration for insertion into the lumen and then revert towards its initial configuration, into contact with and to support the lumen, the difference between the stress on loading and the stress on unloading at the respective inflection points on the stress-strain curve, after deformation to a strain of 10%, being at least about 250 MPa, preferably at least about 300 MPa, more preferably at least about 350 MPa, for example at least about 400 MPa.
A further significant advantage of the use of at least some of the alloys referred to above in the stent of the invention is that their radio-opacity is enhanced compared with that of nickel-titanium shape memory alloys conventionally used for stents, greatly facilita
Burpee Janet
Duerig Thomas
Stockel Dieter
Nitinol Devices & Components
Thaler Michael H.
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