Abstract:
A composite intraluminal prostheses for treating a stenotic region in a blood vessel. The intraluminal prostheses including a thermally expandable stent and an open cell stent cover where the cell size is selected to be small enough to prevent plaque prolapse and the ingrowth of diseased tissue through the openings in the stent, while allowing for the re-endothelialization of the blood vessel wall with healthy tissue. The stent cover is further able to expand and contract with the stent without becoming loose upon contraction and without exerting significant resistence to radial expansion.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed generally to the field of expandable intraluminal prostheses, commonly referred to as stents, and more particularly to a composite device including a thermally expandable stent and an open mesh stent cover. 
     Stents are typically used as adjuncts to percutaneous transluminal balloon angioplasty procedures, in the treatment of occluded or partially occluded arteries and other blood vessels. In a typical balloon angioplasty procedure, a guiding catheter or sheath is percutaneously introduced into the cardiovascular system of a patient through the femoral arteries and advanced through the vasculature until the distal end of the guiding catheter is positioned at a point proximal to the lesion site. A guidewire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guidewire sliding within the dilatation catheter. The guidewire is first advanced out of the guiding catheter into the patient&#39;s vasculature and is directed across the arterial lesion. The dilatation catheter is subsequently advanced over the previously advanced guidewire until the dilatation balloon is properly positioned across the arterial lesion. Once in position across the lesion, the expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressure to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter can be withdrawn from the patient&#39;s vasculature and blood flow resumed through the dilated artery. As should be appreciated by those skilled in the art, while the above-described procedure is typical, it is not the only method used in angioplasty. 
     Balloon angioplasty sometimes results in short or long term failure. That is, vessels may abruptly close shortly after the procedure or gradual restenosis may occur for months thereafter. To counter the tendency of recurrent vessel occlusion following angioplasty, implantable intravascular prostheses, commonly referred to as stents, have emerged as a means by which to achieve long term vessel patency. Stated simply, a stent functions as permanent scaffolding to structurally support the vessel wall and thereby maintain luminal patency. Stents are typically small expandable metallic tubes having interconnecting spans and struts which form a generally open cellular construction. Stents are transported to a lesion site by means of a delivery catheter. 
     There are several types of stents mainly, balloon expandable stents, spring-like self-expandable stents, or thermally expandable stents. Balloon expandable stents are delivered by means of a dilitation catheter and are plastically deformed by means of an expandable member, such as an inflation balloon, from a small initial diameter to a larger expanded diameter. Self-expanding stents are formed as spring elements which are radially compressible about a delivery catheter. A compressed self-expanding stent is typically held in the compressed state by a delivery sheath. Upon delivery to a lesion site, the delivery sheath is retracted allowing the stent to expand. Thermally expandable stents are formed from shape memory alloys which posses the ability to expand from a small initial diameter to a second larger diameter upon the application of heat to the alloy. Although each method of stent expansion is effective, self-expanding stents tend to be difficult to deploy accurately, and balloon expandable stents may, in rare circumstances, inflict undesirable trauma on particularly fragile vessels. 
     Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference. 
     While stents alone perform adequately for the purpose of holding open otherwise occluded, partially occluded, or weakened blood vessels, due to the open structure of a stent there is a tendency for a stent to permit the passage of material through the stent body. Such material may include excessive cell or tissue growth, thrombus formations, and plaque. These materials may have a tendency to block or otherwise re-occlude the open vessel. 
     One technique to reduce the susceptibility for materials to pass through the wall of a deployed stent includes providing the stent with an outer covering formed from a bio-compatible polymer surrounding the open stent construction. One such material commonly used for this purpose is GORE-TEX Vascular Graft (W. L. Gore &amp; Associates, Inc, Flagstaff, Ariz.), which is a micro porous polymer film. The stent cover, however, presents somewhat of a problem in that some transfer of cellular material through the cover is generally desirable. If the cover material is porous, then cells, tissue, and capillaries can penetrate the pores, allowing the blood vessel to be re-endothelialized with new healthy tissue. However, if the covering is too porous, there may be a tendency for diseased tissue to transfer itself to the newly created intima and damage healthy tissue. Micro porous materials like GORE-TEX Vascular Graft have pore sizes on the order of 10-100 microns and are effective at preventing diseased tissue ingrowth. However, due to the very small pore size, re-endothelialization with new healthy tissue may be somewhat compromised. Also, the stent cover material must be sufficiently flexible and expandable to permit deployment of the stent from its initial diameter to its deployed diameter. Many non-porous and micro-porous films, when formed as tubular covers, do not readily expand in the radial direction. For this reason most prior art stent covers are either foldable or have an overlapping slidable design to permit stent expansion. Typically, the covers are attached to the stent along a single seam running along the length of the stent. Thus, the covers may slip or deploy non-uniformly during stent expansion. 
     What is needed, therefore, is a composite intraluminal prosthesis including a stent and a macro-scale outer mesh covering which fits over the stent and presents little or no resistance to radial expansion. In addition, the cell size of the mesh cover should be large enough to allow re-endothelialization of the diseased blood vessel wall with healthy tissue and should be small enough to prevent plaque prolapse or the growth of diseased tissue within the open structure of the stent. Further, it maybe desirable for the underlying stent to be of the thermally expandable type in order to minimize vessel trauma which may be caused by balloon expandable stents or other self-expanding stents. The present invention satisfies these and other needs. 
     SUMMARY OF THE INVENTION 
     The present invention is an intraluminal prosthesis or covered stent comprising a tubular expandable stent having an open cell mesh stent cover. The stent has an exterior surface, a luminal surface, and plurality of openings through the wall to provide scaffolding support for the stent cover. The stent can be formed from a two-way shape memory alloy. Particular use is made of the thermal expansion properties of the shape memory alloy to provide a stent which expands and implants itself within a blood vessel without inflicting trauma to the vessel lumen. The stent cover can be formed from a mesh having an open, square or diamond cell pattern. The particular cell size of the mesh is selected so as to prevent plaque prolapse and the ingrowth of diseased tissue through the openings in the stent, thereby inhibiting possible re-occlusion of the vessel. The mesh cell size is further selected to allow for the re-endothelialization of the vessel wall with healthy tissue. The stent cover is able to expand and contract with the underlying stent without becoming loose if contracted and without exerting significant resistance to radial expansion. 
     Other features and advantages of the present invention will become apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view, partially in section, of an intraluminal prosthesis embodying features of the invention which is mounted on a delivery catheter and disposed within a diseased artery. 
     FIG. 2 is an elevational view, partially in section, showing the expanded intraluminal prosthesis within the artery after withdrawal of the delivery catheter. 
     FIG. 3 is a side view of a stent embodying features of the invention as shown in an expanded or deployed position. 
     FIG. 4 is a side view of a stent cover embodying features of the invention shown in an expanded position, and of the stent of FIG. 3, showing the relationship between the stent and the stent cover. 
     FIG. 5 is an another embodiment of a stent embodying features of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1, illustrates a composite intraluminal prosthesis  10  in accordance with the present invention which is mounted on a delivery catheter  11 . The delivery catheter  11  includes a balloon  12  or other member capable of holding a volume of liquid. Referring now to FIGS. 3 and 4, the composite intraluminal prosthesis  10  includes a thermally expandable stent  20  formed from a shape memory alloy and having an open mesh stent cover  30  which presents little resistence to radial expansion. 
     The stent  10  of the present invention can be made of a wide variety of two-way shape memory alloys. A particularly suitable material is Nitinol a nickel titanium binary alloy. Other alloys such as Ni—Ti—X (X being V, Co, Cu, Fe) ternary alloys, Cu—Al—Ni ternary alloys and Cu—Zn—Al ternary alloys are also suitable. A two-way shape memory material is characterized by having a low temperature martensitic phase and a high temperature austenitic phase and by the ability to transit between the two phases whenever the material&#39;s temperature passes below the martensitic phase transition temperature (“T 1 ”) or above the austenitic phase transition temperature (“T 2 ”). These two transition temperatures are intrinsic characteristics of the material. Typically shape memory alloys are soft and flexible in their martensitic phase and are hard and ridged in their austenitic phase. 
     In particular, if shape memory alloys are cooled below the transition temperature T 1  such that they are in the martensitic phase, and are then stressed sufficiently to physically deform them into what is an apparently permanent new shape, upon heating above the transition temperature T 2 , all of the deformation which occurred in the martensitic phase will be reversed and the original shape will be recovered. That is the deformed object will simply revert to the shape in which it existed prior to the cycle of cooling, deformation and reheating. 
     In accordance with the present invention particularly good use can be made of the characteristics of shape memory alloys by fabricating the stent in a desired expanded shape while the material is in the high temperature austenitic phase and subsequently cooling the stent to the low temperature martensitic phase and deforming the stent to a low profile about a delivery catheter. Upon delivery to a lesion site, the stent may be reheated and will thereby expand to its original diameter. 
     In accordance with the present invention, the shape memory alloy should have an austenitic transition temperature T 2  above the mean human body temperature of about 37 degrees Celsius. Austenitic transition temperatures in the range of about 40 to 80 degrees Celsius are suitable. In addition, the material should have a martensitic transition temperature T 1  below mean human body temperature. Suitable martensitic transition temperatures are in the range of about −10 to 30 degrees Celsius. 
     Referring now to FIG. 3, the stent  20  comprises a tubular body  21  having a longitudinal axis, a proximal edge  22 , and a distal edge  23 . The tubular body  21  is formed from a plurality of spaced apart rings  24  which are composed of a plurality of adjacent deformable oval scaffolding elements  25 , which may be compressed radially inwardly about the catheter balloon  12 , when the stent  20  is chilled below its martensitic transition temperature T 1 . The rings  24  of the scaffolding elements  25  are interconnected by a plurality of connecting links  26 . When viewed in isolation, each continuous oval element  25  includes a pair of opposing straight parallel longitudinal struts  27  and a pair of opposing semicircular sectors  28 . Each of the semicircular sectors  28  has a midpoint  29 . The struts  27  are connected at each end to the semicircular sectors  28  to form the continuous oval element  25 . 
     As formed on the stent  20 , each longitudinal strut  27  is shared by each adjacent oval element  25 . Thus, each oval element  25  is connected to each adjacent oval element  25  by the struts  27 . To form the rings  24 , the oval elements  25  are evenly spaced angularly at a constant radius about the longitudinal axis of the stent. Each oval element  25  in each ring  24 , is coaxially aligned with the other oval elements  25  in the spaced apart rings  24 , so as to form rows of oval elements  25  along the length of the tubular body  21 . Each oval element  25  is connected to the next coaxially aligned oval element  25  in the adjacent rings  24  by the longitudinal connecting elements  26 . Each individual connecting element  26  is connected at each end to the midpoints  29  of each opposing semicircular sector  28  of the coaxially aligned oval elements  25 . 
     Generally, each oval element  25  possesses an aspect ratio of about 2:1. That is, the length of the struts  27  are about twice as long the diameter of the semicircular sectors  28 . An aspect ratio of 2:1 provides for good shape recovery upon reheating to the material&#39;s austenitic phase after cooling to the martensitic phase and subsequent deformation to a low profile delivery diameter. In addition, an aspect ratio of 2:1 provides good scaffolding support for the mesh cover  30  and provides sufficient resistence against blood vessel recoil to prevent radial collapse of the stent  20 . The thickness of the struts  27  and the semicircular elements  28  vary dependant on the degree of radial strength required for the particular application. 
     The length of the interconnecting links  26  varies dependant on the degree of scaffolding support required. Generally, a link length approximately equal to the diameter of the semicircular elements  28  provides good support. 
     In one particulaly preferred embodiment of the stent  20 , the overall length of the oval elements  25  is about 2 mm and the corresponding width of the elements is about 1 mm. The length of the interconnecting links  26  is also about 1 mm. 
     An alternative embodiment of the stent  20   b  is shown in FIG.  5 . In this embodiment the interconnecting links  26  are removed and each ring  24  of oval elements  25  is connected to each adjacent ring  24  at the midpoints  29  of each opposing semicircular sector  28 . 
     The above-described embodiments of the stent  20  are meant to be exemplary and should not be construed as limiting. The elements  25  may be ellipsoidal rather than oval, with the major and minor axes of the ellipse possessing an aspect ratio of about 2:1. Further, in an embodiment where the interconnecting links  26  are used, the links need not be connected between each adjacent element. In some circumstances, particularly where a high degree of stent flexibility is required it may be preferable to have as few as one interconnecting link  26  between each ring of elements  24 . 
     The stent  20  of the present invention can be made by a number of methods known to those skilled in the art. One method is to laser cut or electro-discharge machine the stent pattern from drawn tubing made of shape memory material. Stents made by these methods are typically finished by electro-polishing which is also known to those skilled in the art. Other methods of making stents are also suitable such as chemical etching. 
     Referring now to FIG. 4, the mesh cover  30  comprises an elongated open mesh tube  38  with a proximal edge  34 , a distal edge  36 , and a repeating pattern of square shaped cells  32 . Other cell shapes such as a repeating diamond pattern are also suitable. The cells  32  should be small enough to prevent plaque prolapse and the ingrowth of diseased tissue, yet should be large enough to allow for re-endothelialization of the vessel wall with healthy tissue. A cell size within a range of about 0.2 mm to 1.5 mm square is suitable, with 1 mm square being preferred. 
     The mesh cover  30  must be able to expand and contract along with the stent  10  to which the cover is attached. The mesh cover  30  should not exert significant resistence to radial expansion and should not become loose or “bunch up” when the stent is compressed to its low profile delivery diameter. One preferred material which has these properties is expanded polytetrafluorothylene (PTFE). Other materials such as polyurethane are also suitable. Typically, expanded PTFE is made by pulling or expanding the PTFE in the longitudinal direction after extrusion. Expanded PTFE is capable of being pushed or pulled between a shorter and longer length without stretching or exerting significant resistence against expansion when going from the shorter to the longer length. To best take advantage of the properties of expanded PTFE, it is preferred, though not required, that the PTFE be spirally wrapped, as shown in FIG. 3, so that each filament of the mesh is angularly offset from the longitudinal axis of the stent  20 . 
     The expanded PTFE mesh may be made by weaving, braiding, or knitting PTFE filaments about a mandrel. These methods and others are known to those skilled in the art. The mesh cover  30  is affixed to the stent  20  with an adhesive. The adhesive may be a thermoplastic adhesive and preferably a thermoplastic fluoropolymer adhesive such as fluorinated ethylene propylene (“FEP”). The mesh cover  30  may be affixed to each individual cell  25  and strut  26  of the stent  20 . Alternatively, the cover  30  may be fixed along its proximal and distal edges  34  and  36  to the corresponding proximal and distal edges  22  and  23  of the stent  20 . 
     Referring now to FIGS. 1-2, the covered stent  10  of the present invention is used as follows. A balloon angioplasty procedure is performed at the site of a lesion  17  within an interior lumen  15  of a blood vessel  16 . Subsequently, the covered stent  10  is chilled below its martensitic transformation temperature (T 1 ), causing the shape memory alloy to transition to its soft martensitic state. Upon being chilled to below T 1 , the covered stent  10  is compressed from its first diameter corresponding to its austenitic phase to a second smaller or low profile delivery diameter. The stent may then be crimped into place. Typically, the covered stent  10  will be compressed to its low profile diameter after being slipped over the catheter balloon  12 . The covered stent  10  will retain its second small or low profile diameter as long as it is in its martensitic phase. Preferably, the catheter  11  and the balloon  12  are initially filled with chilled saline solution to keep the covered stent  10  in its martensitic state during delivery. 
     Generally, prior to preparing the covered stent  10  for delivery, a guiding catheter  4  has been placed in the patient&#39;s vasculature and advanced through the body lumen  5  to a point proximal of the lesion  17 . A guidewire  14  has also been advanced through the guiding catheter  13  and is advanced out of the guiding catheter across the lesion  17  to a point distal of the lesion. Often, the guiding catheter and guidewire that were used during the angioplasty procedure are left in place in the patient and reused during the stent placement procedure. The delivery catheter  11  and covered stent  10  are subsequently advanced over the guidewire  14  until the covered stent  10  is positioned across the lesion  17 . Once positioned across the lesion  17 , the catheter  11  is flushed with warm saline solution to heat the covered stent  10  above its austenitic transformation temperature (T 2 ). Upon reaching T 2 , the covered stent  10  expands to its as machined (first) diameter, or austenitic state diameter and conforms with and supports the blood vessel wall. The mesh  30  expands with the stent and engages the inner surface of the vessel lumen  15 , thereby preventing plaque prolapse and the ingrowth of diseased tissue. After expansion, the delivery catheter  11  is withdrawn leaving the expanded covered stent  10  in place within the blood vessel  16  as shown in FIG.  2 . 
     It should be noted that the catheter  11  is not used to expand the covered stent  10  as with balloon expandable stents. Here, the balloon catheter  11  serves to deliver the covered stent  10 , to cool the stent prior to delivery, and to heat the stent upon delivery to the lesion  17 . As will be appreciated by those skilled in the art, catheter-like devices built specifically for delivering a thermally expandable stent may be used in place of the balloon catheter  1 . 
     It will be appreciated that a new form of covered stent has been presented. While only the presently preferred embodiment has been described in detail, as will be apparent to those skilled in the art, modifications and improvements may be made to the device disclosed herein without departing from the scope of the invention. Accordingly, it is not intended that the invention be limited except as by the appended claims