Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
1996-06-13
2001-11-06
Prebilic, Paul B. (Department: 3308)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
C623S017120, C606S191000, C606S198000, C604S104000
Reexamination Certificate
active
06312454
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates to a stent assembly which comprises a stent and a delivery device for the stent, to a catheter assembly which includes the stent assembly, and to a method of disposing a stent in a lumen in a human or animal body.
Stents are used in lumens in a human or animal body, including for example blood vessels, bile ducts, urinary tracts and so on. 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. Such stents are often referred to as “self-expanding stents”. 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 characteristic 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 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.
Stents can also be made from materials that do not exhibit the shape memory properties of shape memory alloys. Examples include certain stainless steels.
Self-expanding stents are commonly delivered to a desired location in a lumen using a catheter, in which the stent is constrained in a transversely compressed configuration, from which it can expand when released from the catheter to contact the wall of the lumen. Catheters formed from polymeric material are commonly used, for example because of their flexibility which facilitates steering the catheter through a lumen, and also for reasons of cost. It has been found in certain circumstances that a stent constrained within a catheter formed from a soft polymeric material can become embedded in the internal wall of the catheter due to elastic forces exerted by the stent as it attempts to expand, to an extent which can make it difficult to discharge the stent from the catheter. A stent constraint which is formed from a polymeric material, and which has the physical characteristics appropriate to constrain the stent, will generally have a large wall thickness, making it inflexible and bulky.
SUMMARY OF THE INVENTION
The present invention provides a stent assembly which includes a tubular delivery device formed from a shape memory alloy in which the stent can be located, the wall of the delivery device being configured to facilitate flexing of the tube in bending deformation.
Accordingly, in one aspect, the invention provides a stent assembly which comprises:
(a) a stent whose configuration can change between a transversely compressed state for delivery into a lumen in a human or animal body, and a relaxed state in which in use the stent contacts the lumen to support it, and
(b) a delivery device which can be fitted to or within a catheter for delivery of the stent through a lumen, which comprises a tubular member formed from a shape memory alloy, the stent being positioned in the delivery device and constrained by it in its transversely compressed state.
In another aspect, the invention provides a method of disposing a stent in a lumen in a human or animal body, which comprises:
(a) transversely compressing the stent,
(b) locating the stent in a delivery device which comprises a tubular member formed from a shape memory alloy, the stent being positioned in the delivery device and constrained by it in its transversely compressed state,
(c) moving the delivery device with the stent contained within it to a desired location in the human or animal body by means of an elongate member having distal and proximal ends, the delivery device and stent being fitted to or contained in the elongate member at or towards the distal end thereof, and
(d) discharging the stent from within the delivery device.
The present invention provides a construction in which a stent can be constrained in a transversely compressed configuration for delivery in a lumen by means of an elongate member, especially a hollow member such as a catheter. A suitable catheter might be formed from, for example, a polymeric material, which might be deformed by the stent if located directly in the stent and constrained by contact with the inner wall of the catheter. This facilitates discharge of the stent from the catheter.
The stent can be discharged from the delivery device either by advancing the stent forward with respect to the delivery device, or by withdrawing the delivery device from the site at which the stent is to be deployed and with respect to the stent.
The constraint provided according to the present invention has the advantage of being thin-walled and flexible in bending, while also having sufficient radial stiffness to be able to withstand the forces exerted by the stent as it attempts to recover outwardly, even when these forces are applied over a long period of time at temperatures above body temperature.
Preferably, the stent is formed from a shape memory alloy. Preferably, the shape memory alloy has been treated so that it is superelastic. The superelastic properties are employed by the stent in its change of configuration between compressed and relaxed states. An appropriate treatment can involve a combination of cold working (for example by swaging, drawing or, in particular by mandrel expansion) and heat treatme
Burpee Janet
Duerig Thomas
Stockel Dieter
Garner Dean
Nitinol Devices & Components
Prebilic Paul B.
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