Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent in combination with graft
Reexamination Certificate
1999-10-15
2001-08-07
McDermott, Corrine (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent in combination with graft
C623S901000, C606S191000, C606S194000, C606S198000
Reexamination Certificate
active
06270523
ABSTRACT:
BACKGROUND
The present invention relates to vessels and vascular support structures, such as stays, stents and support rings which are used for maintaining open a biological passage, such as an artery. A great many constructions for both prostheses and stents are known in the art, ranging from simple wire or plastic rings and arrangements of stiff but flexible sheets or shells, to technologically advanced constructions wherein a wire structure of heat memory alloy flips to an enlarged memory configuration, or wherein a solid tubular body is fabricated with microscopically thin laser-cut slots which convert the solid cylinder into an expandable body that opens out to form a mesh-like but reasonably stiff surface support. One commercial embodiment of this latter type of stent, referred to as the Palmaz stent after the surgeon who popularized this construction, is in common use now. Another common form of stent consists of wire crimped into a zig zag pattern which can be expanded to attain a much larger length or diameter. Stents of this form may be formed as individual rings, or serpentine windings, or as pairs of helical windings which act against each other to counterbalance twist while expanding radially. Numerous other constructions are known.
Many if not all of the materials used for stents involve metal or carbon fiber materials which are highly electro-positive and are bio-active. Since stents tend to be used under conditions where they are counteracting disease processes, supporting healing processes, or guarding against stenosis of a passage, bio-activity, which may encourage undesirable or poorly regulated growth processes, or lead to clot formation, should be avoided. Coating of stent can keep the stent from directly contacting surrounding tissue or fluids, and thus can theoretically protect against unwanted electrochemically induced tissue reactions.
In the field of expandable stents, however, a further problem arises due to the fact that many effective or compact stent constructions involve filamentous or wire-like structures which have numerous apertures or spaces between the various stands or structural elements of the stent. With these constructions, tissue may grow through the openings of the stent Furthermore, the stent itself may provoke a foreign body reaction and be both a stimulus for and a framework supporting, proliferative tissue growth, resulting, for example, in scar tissue or restenosis of the very region it is placed to control.
One approach to this drawback is to provide a coating, liner or cover for the stent which prevents the healing or diseased layer of tissue from directly contacting the stent or from passing through the stent in any way. Such liners may be formed, for example, of porous polytetrrafuoroethylene (PTFE) which allows the passage of fluids and vital materials while serving as a barrier to tissue growth. However, when applying such a construction, a further difficulty which may arise is that the layer or sleeve of polymer must be attached to the stent for example, by staples or sutures at one end, or is prone to developing loose pockets or folds which might accumulate organic matter or lead to sepsis or unusual growth. Also, the necessarily thin liner material may detach or degrade. The risk of loose or unattached liner material is particularly great for constructions which utilize poorly adherent polymers, such as PTFE, or structures which seek to combine an expandable stent of stiff material, which changes both its dimension and its shape, with a dissimilar liner or shell.
Accordingly, there remains a need for a covered support construction of enhanced hardiness and implant compatibility.
There is also a need for an expandable vessel support which forms a unitized and non-delaminating tissue barrier.
There is also a need for a need for a vascular liner having atraumatic properties and haemodynamic shape.
SUMMARY OF THE INVENTION
These and other objects are attained in a support according to the present invention wherein a radially expandable support body is enclosed within a solid but expandable polymer body of porous and expanded PTTE material that physically isolates the support body from surrounding blood and tissue.
In one preferred construction, the support body is a stent that is cocooned within a cuffed sheet. In this construction, the sheet is originally a tube of polytetrafluoroethylene (PTFE) material, which passes through the interior of the stent and is cuffed, e.g., is folded back upon itself, over the stent, in a manner similar to the folding of a sock, so that the folded-back end of the tube becomes an outer layer smoothly extending around the end and covering the outside of the stent The assembly is then heated, causing the outer layer to shrink and coalesce with the inner layer so that the stent is enclosed within a folded envelope having a continuous and seamless end portion. Preferably, radial pressure is applied during the heating so that the layers conform tightly to the support body and fill all interstitial spaces thereof In other constructions, support members lie within pockets extending in the direction of expansile deformation.
Preferably, the tube is porous PTFE, having a microstructure of fibrous material interconnecting nodes of solid polymer, and the PTFE forms a soft and pliant surface that cushions the edges of the support body, or stent, and blocks direct contact between the stent and surrounding tissue, so that any fluids or material must penetrate the mat of fibrils to contact the stent environment. By first expanding an end portion of the tube before folding it back over the stent, the end portion, which becomes an exterior surface of the finished product, may be provided with a degree of porosity which is greater than that of the interior surface. In a further embodiment, each end of the central tube is so expanded, and then folded back so that the assembly is closed over at both ends and has a single seam extending circumferentially around the outside where one end meets or overlaps a portion of the other end of the tube part way along the body of the assembly. Alternatively, the outer surface may be covered by a wrap, or by a separate polymer tube; m ibis case the inner tube may have a relatively short end cuff portion, which is preferably folded over the outer cover for a short distance.
In a preferred embodiment, the entire inner and outer portions are formed of a single PTFE tube and are heated to both shrink the tube down into a compact and thin film-like cocoon, and to coalesce the inner and outer layers together at all points where they come in contact so that the polymer cocoon becomes unitary and non-delaminating. Preferably, the stent body itself is of limited axial extent, like a ring, or a series of spaced-apart rings, or else it possesses a number of apertures extending entirely through the stent at short axial spacing, so that the remaining spaces or apertures are covered over or bridged by both the inner and outer polymer layers, which coalesce into a continuous barrier. The apertures, which may comprise five to eighty percent or more of the surface area of the stent, constitute a grid or network of regions or tack points through which the material is coalesced and continuously bonded. When the stent is expanded, its changes in dimension and orientation may locally introduce shear which separates the stent or support body from the polymer. However, the support body is able to shift only within the regions where the inner and outer portions of the tube have not coalesced to each other, and thus it locally distributes strain to the surrounding polymer in a manner generally effective to prevent rupture and prevents the development of extended pockets or voids which could impair performance in use.
In embodiments where a two tube construction is cuffed and assembled to arrive at a similarly unitized and seamless stent This is done as follows: first a tube of polymer is placed through the center of the stent and the ends of the tube are folded back over the stent for a short distan
Gingras Peter
Herweck Steve A.
Karwoski Theodore
Martakos Paul
Atrium Medical Corporation
Koh Choon P.
Lahive & Cockfield LLP
McDermott Corrine
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