Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Method of implanting
Reexamination Certificate
1998-10-14
2001-01-30
Yu, Mickey (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Method of implanting
C623S001130, C623S001180, C623S001200, C623S001120
Reexamination Certificate
active
06179878
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a composite stent, to a stent assembly which includes a composite stent device, and to a method of making 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 enable 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.
Non-linear superelastic properties can be introduced in a shape memory alloy by a process which involves cold working the alloy for example by a process that involves pressing, swaging or drawing. The cold working step is followed by an annealing step while the component is restrained in the configuration, resulting from the cold working step at a temperature that is sufficiently high to cause dislocations introduced by the cold working to combine and dislocations to align. This can ensure that the deformation introduced by the cold work is retained.
The technique for introducing superelastic properties can be varied from that described above. For example, instead of subjecting the alloy to a heat treatment while restrained in the deformed configuration, the alloy could be deformed beyond a particular desired configuration and then heat treated such that there is a thermally induced change in configuration of the kind discussed below, the change taking the configuration towards the particular desired configuration. Introduction of the superelastic properties might also involve annealing at high temperature (for example towards the recrystallisation temperature of the alloy), followed by rapid cooling and then a heat treatment at a lower temperature.
The properties of shape memory alloys can also involve thermally induced changes in configuration in which an article is first deformed from a heat-stable configuration to a heat-unstable configuration while the alloy is in its martensite phase. Subsequent exposure to increased temperature results in a change in configuration from the heat-unstable configuration towards the original heat-stable configuration as the alloy reverts from its martensite phase to its austenite phase. It is known from U.S. Pat. No. 5,197,978 to make use of the thermally induced change in configuration of an article made from a shape memory alloy in a stent.
The use of a stent which is formed from a shape memory alloy is attractive because it can exert an outward force on the lumen in which it is to be used continuously after it has been deployed in the desired location. This allows the lumen to be maintained open. It can also mean that the stent remains in the desired location. The enhanced elastic properties of shape memory alloys also allow a stent to move and flex with a lumen after installation. This can be particularly important when a stent is positioned in an exposed lumen, such as a femoral or carotid artery. Forces applied externally to these vessels can cause them to flatten substantially from their normally round cross-section.
It is important that the configuration of a shape memory alloy stent towards which it attempts to recover while in the lumen is properly selected. If that configuration is too small, the stent will be loose in the lumen; this can result in the lumen not being properly supported by the stent and in the stent becoming dislodged from the desired location. If the configuration towards which the stent attempts to recover is too big, the residual force exerted by the stent on the lumen can be too high; it is thought that this could be undesirable in some situations because of a risk of damage to the lumen.
SUMMARY OF THE INVENTION
The present invention provides a composite stent device which includes a restraint element which can be deformed plastically and which can restrict the maximum transverse dimension to which the shape memory stent sleeve can expand outwardly.
Accordingly, in one aspect, the invention provides a composite stent device which comprises (a) a shape memory alloy stent sleeve which is treated so that it can exert an outward force on a lumen in which the stent device is to be deployed, and (b) a restraint element which restricts the maximum transverse dimension to which the stent sleeve can expand outwardly.
The stent device of the invention can be used in a lumen where the size of the lumen is not known accurately. The transverse dimension to which the stent device expands in the lumen can be adjusted by deformation of the restraint element, until the transverse dimension is large enough to ensure that an appropriate force is exerted on the lumen. The deformation of the stent device will generally take place while it is located in the lumen. The deformation involves deformation of the restrained element, allowing the stent sleeve to recover further towards its relaxed configuration in which recovery forces are resolved. This can be achieved by means of an expansion device which, when positioned within the stent device, can expand the device by plastic deformation of the restraint element. Accordingly, in another aspect, the invention provides a stent assembly which comprises (a) a stent device of the type discussed above, and (b) an expansion device which, when positioned within the stent device, can increase the maximum transverse dimension to which the device can expand by plastic deformation of the restraint element. An example of a suitable expansion device is an inflatable balloon.
As well as restricting the maximum transverse dimension to which the stent sleeve can expand outwardly, the restraint element should preferably be capable of being deformed transversely with the stent sleeve, for example under force applied externally to the lumen in which the stent device is located. It is possible for the restraint element to tolerate such transverse deformation and to reform elastically under the restoring force provided by the stent sleeve, while restricting the maximum transverse dimension of the stent device.
The presence of the shape memory alloy stent sleeve in the device of the invention has the advantage that the device continues to exert an outward force against
Duerig Thomas
Stockel Dieter
Garner Dean
Nguyen Tram A.
Yu Mickey
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