Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent in combination with graft
Reexamination Certificate
2001-10-24
2004-07-06
Prebilic, Paul B. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent in combination with graft
C623S023700
Reexamination Certificate
active
06758858
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable intraluminal devices, particularly intraluminal stents. Because of the open lattice found in most intraluminal stents, a primary problem with these types of devices is occlusion of the vessel occurring after stent placement. Tissue ingrowth and neointimal hyperplasia significantly reduces the open diameter of the treated lumen over time, requiring additional therapies. The present invention incorporates the use of a biocompatible barrier material that prevents or delays the tissue ingrowth and neointimal hyperplasia, thus maintaining luminal patency for longer periods after initial treatment. The use of expanded polytetrafluoroethylene (ePTFE) as a bio-inert barrier material is well documented. In accordance with certain of its preferred embodiments, the present invention utilizes a radially expandable ePTFE material, such as that described in U.S. Pat. No. 6,039,755, to partially or fully embed the stent lattice, thereby providing a suitable barrier which improves stent patency.
The inventive intraluminal stent-graft device may be implanted either by percutaneous delivery using an appropriate delivery system, a cut-down procedure in which a surgical incision is made and the intraluminal device implanted through the surgical incision, or by laparoscopic or endoscopic or endoscopic delivery. More particularly the present invention relates to shape memory alloy and self-expanding endoluminal stents which are at least partially encapsulated in a substantially monolithic expanded polytetrafluoroethylene (“ePTFE”) covering. In accordance with the present invention, an endoluminal stent, which has a reduced diametric dimension for endoluminal delivery and a larger in vivo final diametric diameter, is encapsulated in an ePTFE covering which circumferentially covers both the luminal and abluminal walls along at least a portion of the longitudinal extent of the endoluminal stent. The endoluminal stent is preferably fabricated from a shape memory alloy which exhibits either shape memory or pseudoelastic properties or from an elastic material having an inherent spring tension.
In a first embodiment of the invention, the endoluminal stent is encapsulated in the ePTFE covering in the stent's reduced diametric dimension and is balloon expanded in vivo to radially deform the ePTFE covering. The endoluminal stent may be either one which exhibits thermal strain recovery, pseudoelastic stress-strain behavior or elastic behavior at mammalian body temperature. While in its reduced diametric dimension the ePTFE encapsulating covering integrally constrains the endoluminal stent from exhibiting either thermal strain recovery, pseudoelastic stress-strain behavior or elastic behavior at mammalian body temperature. Radial deformation of the ePTFE covering releases constraining forces acting on the endoluminal stent by the undeformed ePTFE covering and permits the stent to radially expand.
In a second embodiment of the invention, an endoluminal stent fabricated of a shape memory alloy is encapsulated in its final diametric dimension and the encapsulated intraluminal stent-graft is manipulated into its reduced diametric dimension and radially expanded in vivo under the influence of a martensite to austenite transformation.
In a third embodiment of the present invention, a self-expanding intraluminal stent, fabricated of a material having an inherent spring tension, is encapsulated in its final diametric dimension and manipulated to a reduced diametric dimension and externally constrained for intraluminal delivery. Upon release of the external constraint in vivo the spring tension exerted by the self-expanding stent radially expands both the stent and the ePTFE encapsulating covering to a radially enlarged diameter.
In a fourth embodiment of the invention, the endoluminal stent is fabricated from a material having an inherent elastic spring tension and is encapsulated at a reduced dimension suitable for endoluminal delivery and balloon expanded in vivo to radially deform the ePTFE covering.
Shape memory alloys are a group of metallic materials that demonstrate the ability to return to a defined shape or size when subjected to certain thermal or stress conditions. Shape memory alloys are generally capable of being plastically deformed at a relatively low temperature and, upon exposure to a relatively higher temperature, return to the defined shape or size prior to the deformation. Shape memory alloys may be further defined as one that yields a thermoelastic martensite. A shape memory alloy which yields a thermoelastic martensite undergoes a martensitic transformation of a type that permits the alloy to be deformed by a twinning mechanism below the martensitic transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent austenite phase. The austenite phase occurs when the material is at a low strain state and occurs at a given temperature. The martensite phase may be either temperature-induced martensite (TIM) or stress-induced martensite (SIM).
When a shape memory material is stressed at a temperature above the start of martensite formation, denoted M
s
, where the austenitic state is initially stable, but below the maximum temperature at which martensite formation can occur, denoted M
d
, the material first deforms elastically and when a critical stress is reached, it begins to transform by the formation of stress-induced martensite. Depending upon whether the temperature is above or below the start of austenite formation, denoted A
s
, the behavior when the deforming stress is released differs. If the temperature is below A
s
, the stress-induced martensite is stable; however, if the temperature is above A
s
, the martensite is unstable and transforms back to austenite, with the sample returning to its original shape. U.S. Pat. Nos. 5,597,378, 5,067,957 and 4,665,906 disclose devices, including endoluminal stents, which are delivered in the stress-induced martensite phase of shape memory alloy and return to their pre-programmed shape by removal of the stress and transformation from stress-induced martensite to austenite.
Shape memory characteristics may be imparted to a shape memory alloy by heating the metal at a temperature above which the transformation from the martensite phase to the austenite phase is complete, i.e., a temperature above which the austenite phase is stable. The shape of the metal during this heat treatment is the shape “remembered.” The heat treated metal is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform to the martensite phase. The metal in the martensite phase is then plastically deformed, e.g., to facilitate its delivery into a patient's body. Subsequent heating of the deformed martensite phase to a temperature above the martensite to austenite transformation temperature, e.g., body temperature, causes the deformed martensite phase to transform to the austenite phase and during this phase transformation, the metal reverts back to its original shape.
The term “shape memory” is used in the art to describe the property of a material to recover a pre-programmed shape after deformation of a shape memory alloy in its martensitic phase and exposing the alloy to a temperature excursion through its austenite transformation temperature, at which temperature the alloy begins to revert to the austenite phase and recover its preprogrammed shape. The term “pseudoelasticity” is used to describe a property of shape memory alloys where the alloy is stressed at a temperature above the transformation temperature of the alloy and stress-induced martensite is formed above the normal martensite formation temperature. Because it has been formed above its normal temperature, stress-induced martensite reverts immediately to undeformed austenite as soon as the stress is removed, provided the temperature remains above the transformation temperature.
The martensitic transformation that occurs in
Banas Christopher E.
Edwin Tarun J.
McCrea Brendan J.
Bard Peripheral Vascular, Inc.
Morriso & Foerster, LLP
Prebilic Paul B.
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