Abstract:
A renal stent includes a balloon expandable segment intended for deployment in the renal vessel and a self expanding segment intended for deployment in the aortic segment. One or both of the balloon expandable and self expanding segments can be deployed in the ostial region of the renal vessel, typically the renal artery. The balloon expandable segment provides superior radial strength for maintaining dilated diameter of the renal vessel. The self expanding segment expands to conform to the flared ostial and aortic regions of the vessel. The self expanding segment can be balloon dilated to enhance conformance of the self expanding stented segment to the ostial and aortic regions.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to luminal implants, and, more particularly, to stents for use in treating vascular disease.  
       BACKGROUND OF THE INVENTION  
       [0002]     Stents are widely used for supporting a lumen structure in a patient&#39;s body. For example, a stent may be used to maintain patency of a coronary artery, other blood vessel or other body lumen such as the ureter, urethra, bronchus, esophagus, or other passage. A stent is typically a metal, tubular structure, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, or metallic, and may elute over time substances such as drugs.  
         [0003]     In certain stent designs, the stent is an open-celled tube that is expanded by an inflatable balloon at the deployment site. Another type of stent is of a “self-expanding” type. A self-expanding stent does not use a balloon or other source of force to move from a collapsed state to an expanded state. A self-expanding stent is passed through the body lumen in a collapsed state. At the point of an obstruction, or other deployment site in the body lumen, the stent is expanded to its expanded diameter for its intended purpose. An example of a self-expanding stent is a coil structure that is secured to a stent delivery device under tension in a collapsed state. At the deployment site, the coil is released so that the coil can expand to its enlarged diameter. Coil stents can be manufactured using a variety of methods, such as winding of wire, ribbon, or sheet on a mandrel or by laser cutting from a tube, followed by the appropriate heat treatments. Another type of self expanding stent is an open-celled tube made from a self-expanding material, for example, the Protégé GPS stent from ev3, Inc. of Plymouth, Minn. Open cell tube stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods.  
         [0004]     The shape, length and other characteristics of a stent are typically chosen based on the location in which the stent will be deployed. However, selected segments of the human vasculature present specific challenges due to their shape and configuration. One such situation involves the ostium of short renal arteries within the human body.  
         [0005]     Conventional stents are generally designed for segments of long cylindrical vessels. When such stents are deployed at the ostium of short renal arteries, in an attempt to prevent further progression of arteriosclerosis disease from aorta into renals, they may extend into the aorta and disrupt the normally laminar blood flow. This result further compounds an existing need to minimize disruption of the flow pattern at the ostium. In addition, stents are hard to position on a consistent basis at the precise ostial location desired, and placement of renal stents can release arteriosclerotic debris from the treatment area. Such debris will flow distally into the kidney and embolize, causing impaired renal function.  
         [0006]     Accordingly, it is desirable to flare the end of a stent to minimize disruption to flow pattern at ostium and to simplify re-access in the future. However, existing stents are hard to flare with existing expansion means. Stents suitable for expansion of renal arteries must have high radial strength when expanded to resist vessel forces tending to radially collapse the stent. This need for high stent strength makes suitable stents difficult to flare. A stent configuration designed to address these concerns is disclosed in commonly assigned U.S. patent application Ser. No. 10/816,784, filed Apr. 2, 2004, by Paul J. Thompson and Roy K. Greenberg, US Publication Number US 2004/0254627 A1. However, the stent, when flared, has a lower percentage of coverage of vessel wall at flared regions than is desirable. It is known that stent struts, when expanded into contact with the vessel wall, should cover a certain percentage of the internal vessel wall area in order to prevent prolapse of tissue through the open spaces between stent struts.  
         [0007]     Accordingly, a need exists for a stent that can be placed at the renal ostium which is both strong and provides a high percentage of vessel wall coverage.  
         [0008]     Further need exists for a stent that will minimize disruption of the flow pattern at the ostium and which will lower the risk of embolization during deployment.  
       SUMMARY OF THE INVENTION  
       [0009]     According to one aspect of the present invention, a renal stent comprises a balloon expandable segment which is deployed in the renal vessel and a self expanding segment which is deployed in the aortic segment. Either or both of the balloon expandable and self expanding segments can be deployed in the ostial region of the renal vessel, typically the renal artery. The balloon expandable segment provides superior radial strength for maintaining dilated diameter of the renal vessel. The self expanding segment expands to conform to the flared ostial and aortic regions of the vessel. The self expanding segment can be balloon dilated to enhance conformance of the self expanding stented segment to the ostial and aortic regions.  
         [0010]     According to one aspect of the present invention, a stent for insertion into a body lumen comprises a first tubular, self-expanding section; a second tubular, balloon-expandable section; and a mechanism for limiting axial movement of the first and second tubular sections relative to each other.  
         [0011]     According to a second aspect of the present invention, a stent for insertion into a body lumen comprises a tube formed of a uniform material having i) a first section to which a first process is applied, and ii) a second section to which a second process is applied; and wherein the expansion characteristics of the first and second sections within the body lumen are different.  
         [0012]     According to a third aspect of the present invention, a stent for insertion into a body lumen comprising a plurality of sections, each section defining a plurality of cells, each cell at least partially defined by a plurality of struts, selected of the struts in each section connecting to struts of an adjacent stent section, wherein the number of connecting struts between adjacent segments increases proximally.  
         [0013]     According to a fourth aspect of the present invention, system for delivering a medical device within a body lumen comprises: a tubular catheter having proximal and distal ends and comprising an outer shaft member slidably disposed about an inner shaft member; first and second balloons carried at a distal end of inner shaft member; a medical device comprising i) a first tubular, balloon-expandable section; and ii) a second tubular, self-expanding section; and wherein the first section of the medical device is disposed intermediate the first balloon and the outer shaft member, and wherein the second section of the medical device is disposed intermediate the second balloon and outer shaft member.  
         [0014]     According to a fifth aspect of the present invention, a method for placement of a medical device within a body lumen comprises: disposing a delivery system within the body lumen, the delivery system comprising: a tubular catheter having an outer shaft member slidably disposed about an inner shaft member; first and second balloons carried at a distal end of inner shaft member; a medical device comprising: i) a first balloon-expandable section disposed intermediate the first balloon and the outer shaft member, and a second self-expanding section disposed intermediate the second balloon and outer shaft member. The method further comprises withdrawing the outer shaft member proximally to expose the first balloon-expandable section of the medical device to the body lumen; inflating the first balloon to expand the first section of the medical device against the body lumen; withdrawing the outer shaft member proximally to expose the second self-expanding section of the medical device to the body lumen; and inflating the second balloon to expand the second section of the medical device at least partially against the body lumen. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:  
         [0016]      FIGS. 1-4  illustrate conceptually a partial cross-sectional diagram of a stent and a plan view of a stent delivery system in accordance with the present invention;  
         [0017]      FIG. 5  illustrates conceptually an alternate embodiment of the delivery system of  FIGS. 1-4 ;  
         [0018]      FIG. 6  illustrates conceptually a cross-sectional diagram of another embodiment of a stent in accordance with the present invention;  
         [0019]      FIG. 7  illustrates conceptually a plan view of an alternative embodiment of a stent in accordance with the present invention;  
         [0020]      FIG. 8  is a schematic diagram of a stent in accordance with the present invention;  
         [0021]      FIG. 9  illustrates conceptually a plan view of an alternate embodiment of a stent in accordance with the present invention;  
         [0022]      FIG. 10  illustrates conceptually the profile of a flared stent in accordance with the present invention;  
         [0023]      FIGS. 11 and 11 A are schematic diagrams of an alternate embodiment of a stent in accordance with the present invention;  
         [0024]      FIGS. 12 and 13  illustrate conceptually a plan view of another embodiment of a stent in accordance with the present invention;  
         [0025]      FIG. 14  is a schematic diagrams of a film material used in the stent embodiment of  FIG. 17 ;  
         [0026]      FIGS. 15 and 16  illustrate conceptually exploded and compressed cross-sectional diagram, respectively, of a film suitable for use with the stent embodiment of  FIG. 17 ; and  
         [0027]      FIG. 17  illustrates conceptually a plan view of another embodiment of a stent in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0028]      FIG. 1  illustrates a stent  15  in accordance with the present invention. Stent  15  comprises a balloon expandable stent segment  12  and a self expanding stent segment  13 . Balloon expandable stent segment  12  may comprise stainless steel alloys, cobalt chrome alloys, titanium, tantalum, platinum, gold, or other materials or their alloys as are known in the art. The self expanding stent segment  13  may be comprise high elastic limit materials such as Elgiloy, cobalt chrome alloys, or other materials as are known in the art. The self expanding stent may comprise so-called shape-memory metals such as nitinol. Shape-memory metal stents can self-expand when thermo mechanically processed to exhibit superelastic material properties. Such shape-memory stents can also self-expand through use of a pre-programmed shape memory effect. Stents processed to exhibit a shape memory effect experience a phase change at the elevated temperature of the human body. The phase change results in expansion of the stent from a collapsed state to an enlarged state  
         [0029]      FIGS. 1-4  show delivery of the inventive stent  15  to a treatment site. In use, the renal stent  15  is delivered to the treatment site, typically a renal artery, on a catheter  18  with both an outer sheath  11  coaxially retractable from inner balloons  10 ,  16 , as illustrated in  FIG. 1 . The outer sheath  11  constrains at least the self expandable segment  13  of the stent  15 . Distal balloon  10  is used to expand the balloon expandable segment  12  of the stent  15 . Optional proximal balloon  16  may be used to dilate the self expanding segment  13  of stent  15 . At the treatment site, distal balloon  10  is inflated to expand the balloon expandable segment  12  and dilate the artery RA, thereby fixing the stent at the treatment site, as illustrated in  FIG. 2 . Outer sheath  11  is then withdrawn, preferably before deflation of distal balloon  10 , and self expanding segment  13  diametrically enlarges and conforms to the typically flared ostial O and/or aortic Ao regions near the vessel, as illustrated in  FIG. 3 . Optionally, the flared portion of the stent  15 , that is the portion of the stent  15  opposite the ostial O and aortic Ao regions, can be further dilated with proximal balloon  16 . Inflated distal balloon  10  helps to axially anchor catheter  18  in vessel RA so that proximal balloon  16  can deliver force to self expandable stent segment  13  and thereby enhance stent segment  13  apposition to ostial and aortic regions of the vessel. Balloons  10  and  16  are then deflated. Optionally the outer sheath  11  is advanced relative to balloons  10  and  16  to cover the balloons in whole or in part, and the stent delivery catheter  18  is withdrawn from the treatment site, as illustrated in  FIG. 4 . It is not necessary to position balloon expandable stent segment  12  in the vessel RA with precision because subsequent expansion of the self expanding stent segment  13  will assure continuous coverage of the vessel wall in regions RA, O, and Ao. Stenting of the vessel RA, ostium O, and aorta Ao is accomplished by delivery of one stent to the region thereby reducing the amount of debris generated during stenting as compared to procedures involving delivery of multiple devices.  
         [0030]      FIG. 5  shows an alternate embodiment of a delivery system  18 ′ shown in  FIGS. 1-4 . In most respects the delivery system shown in  FIG. 5  is used in a manner similar to that described above in conjunction with  FIGS. 1-4 . In this embodiment, balloon  17  provides the function of both balloons  10  and  16  of delivery system  18 ′. With sheath  11  covering self expanding stent segment  13  and balloon portion  17   b , balloon portion  17   a  is used to dilate balloon expandable stent segment  12 . Subsequently, balloon  17  is deflated to a pressure low enough to proximally withdraw sheath  11  until it no longer covers self expanding stent segment  13  and balloon portion  17   b . Next, balloon  17  is inflated, causing balloon portion  17   a  to anchor catheter  18  in the vessel and causing balloon portion  17   b  to further dilate self expanding stent portion  13 . In this embodiment, balloon  17  may be constructed from appropriate materials so as to have differing compliance characteristics between sections  17   a  and  17   b  at the same inflation pressure, or, balloon  17  may be formed to assume a profile similar to that illustrated in  FIG. 5 .  
         [0031]      FIG. 6  illustrates another embodiment of the inventive stent  15  in which the stent segments are at least partially coextensive. In an illustrative embodiment, the balloon expandable segment  1  disposed within the self expanding segment  2 , as shown. Alternatively, the balloon expandable segment  1  can be arranged outside of the self expanding segment  2  (not shown). The balloon expandable and self expandable segments may be attached to each other so as to limit axial motion of one segment relative to the other. The stent segments can be attached to one another using means  3  known in the art, including, but not limited to, mechanical interlock, welding, adhesive bonding, over molding, sintering, diffusion bonding, cladding, explosive bonding, ultrasonic welding. If more than one attachment site is required, stent segments  1  and  2  may have matched axial shortening or lengthening during expansion to prevent detachment of attachment sites of means  3 . Alternatively, stent segments  1  and  2  can have mis-matched axial shortening or lengthening during expansion provided the attachment sites of means  3  are designed to accommodate such mis-match without becoming detached.  
         [0032]      FIG. 7  illustrates another alternative embodiment of the inventive stent  15 . In this embodiment, stent  15  comprises a balloon expandable segment  1  and a self expanding segment  2 . The balloon expandable segment  1  is attached to the self expanding segment  2  at attachment region  20 . The balloon and self expandable stent segments may be attached to each other so as to limit axial motion of one segment relative to the other. The stent segments can be attached to one another using means known in the art, including, but not limited to, mechanical interlock, welding, adhesive bonding, over molding, sintering, diffusion bonding, cladding, explosive bonding, ultrasonic welding. For example, a stent segment  1  comprising a titanium alloy can be welded to a stent segment  2  comprising Nitinol.  FIG. 8  illustrates an example of implementation of the stent of  FIG. 7 . In  FIG. 8 , stent  15  comprises laser cut tubular stent segments  1  and  2  welded at attachment points  5  in attachment region  20 . In  FIG. 8 , the longitudinal axis of stent  15  is indicated in the figure by dashed line “X.” 
         [0033]      FIG. 9  illustrates another alternative embodiment of a stent  15  having a singular piece of material to which different processes have been applied. As shown, stent  15  comprises stent segments  1  and  2  having different overall processing histories. For example, a body temperature superelastic nitinol stent may be heated in the region of stent segment  1  to raise the transition temperature in segment  1  to above body temperature, resulting in a balloon expandable stent segment  1  and a self expanding stent segment  2 . Similarly an Elgiloy or cobalt-chrome alloy stent, initially heavily work hardened and self expanding, may be preferentially annealed to render a portion of the stent balloon expandable.  
         [0034]      FIG. 10  illustrates a side cut-away profile of another alternative embodiment of a stent  15  having cell designs with the same number of struts along its length. For example, in non-flared region  41  having stent diameter D, the stent struts have approximately 14-18% metal coverage area. In flared region  42  having stent diameter  2 D, the stent struts have approximately 7-9% metal coverage area. In flared region  43  having stent diameter  4 D, the stent struts have approximately 3.5-4.5% metal coverage area. It is known that stents having metal coverage area less than approximately 14-18% do not perform well due to tissue prolapse through the cells of the stent into the stented lumen, and due to potential extrusion of atheromatous material through the stent metal coverage area into the stented vessel lumen. To avoid sub-par stent performance in the stented and flared region, it is desirable to increase the metal coverage area to the approximately 15-18% range. One way to do this is by increasing the number of stent struts in region of the stent that will be flared as compared to the number of struts in the region of the stent that will not be flared.  
         [0035]      FIGS. 11 and 11 A illustrate a stent embodiment for increasing the percent metal coverage area in the flared region of the deployed stent. In  FIG. 11  the number of struts progressively increases in regions  41 ,  42 , and  43  respectively. In  FIG. 11  the compressed stent diameter is held constant while the compressed strut spacing varies in segments  41 ,  42 , and  43 . Attachment points between regions in this embodiment may comprise a laser cut tube segment, as illustrated in detail in  FIG. 11A , with the number of connecting struts increasing between adjacent segments as the density of the cell/strut configuration increases between adjacent segments. In one embodiment, segment  41  comprises 6 struts, segment  42  comprises 12 struts, and segment  43  comprises 24 struts. It is recognized that any number of struts and segments may be used to produce an overall stented region metal coverage area of a target value appropriate to the anatomy. For arteries the illustrative target metal coverage area is thought to be approximately 15-18%.  
         [0036]     Another embodiment of stent  15  is illustrated in  FIGS. 12 and 13 . Stent  15  comprises balloon expandable segment  1  and self expanding segment  2 . Balloon expandable segment  1  may have the construction and function similar any of the embodiments described herein. Self expandable segment  2  comprises leafs  50  and film  52 . Leafs  50  comprise materials such as those described above for self expanding stent segment  2 . Leafs  50  are attached to balloon expandable stent segment  1  using techniques similar to those described above regarding the embodiment of  FIG. 7 .  
         [0037]     Film  52  is attached to leafs  50  by bonding. Film  52  may comprise any of a variety of membranous materials including those which facilitate cellular in-growth, such as ePTFE. The suitability of alternate materials for film  52  can be determined through routine experimentation. The film  52  may be provided on one or both radially facing sides of the leafs  50 . In one embodiment, the film  52  comprises two layers, with one layer on each side of leafs  50 . The two layers may be bonded to each other around the leafs using any of a variety of techniques, for example by heat bonding, with or without an intermediate bonding layer such as polyethylene or FEP, adhesives, sutures, or other techniques which will be apparent to those of recently skill in the arts in view of the disclosure herein. The film  52  preferably has a thickness of no more than about 0.006″ and a pore size within the range of from approximately 5 μm to approximately 60 μm.  
         [0038]     Film  52  in one embodiment preferably is securely attached to leafs  50  and retains a sufficient porosity to facilitate cellular ingrowth and/or attachment. One method of manufacturing a suitable composite membrane film  52  is illustrated in  FIGS. 14-17 . As illustrated schematically in  FIG. 14 , a bonding layer  254  preferably comprises a mesh or other porous structure having an open surface area within the range of from about 10% to about 90%. In one embodiment, the open surface area of the mesh is within the range of from about 30% to about 60%. The opening or pore size of the bonding layer  254  may be within the range of from about 0.005 inches to about 0.050 inches, and, in one embodiment, is about 0.020 inches. The thickness of the bonding layer  254  can be varied widely, and is generally within the range of from about 0.0005 inches to about 0.005 inches. In a illustrative embodiment, the bonding layer  254  has a thickness of about 0.001 to about 0.002 inches. One suitable polyethylene bonding mesh is commercially available from Smith &amp; Nephew Inc., Memphis, Tenn. under the code SN9.  
         [0039]      FIG. 15  is an exploded view illustrating the relationship between first membrane  250 , second membrane  252 , bonding layer  254  and leafs  50 . Bonding layer  254  is disposed adjacent one or both sides of leafs  50 . The bonding layer  254  and leafs  50  are then positioned in-between a first membrane  250  and a second membrane  252  to provide a composite membrane stack. The first membrane  250  and second membrane  252  may comprise any of a variety of materials and thicknesses, depending upon the desired functional result. Generally, the membrane has a thickness within the range of from about 0.0005 inches to about 0.010 inches. In one embodiment, the membranes  250  and  252  each have a thickness on the order of from about 0.001 inches to about 0.002 inches, and comprise porous ePTFE, having a pore size within the range of from about 10 microns to about 100 microns.  
         [0040]     The composite stack is heated to a temperature of from about 200° F. to about 300° F., for about 1 minute to about 5 minutes under pressure to provide a finished composite membrane assembly with embedded leafs  50 , as illustrated schematically in  FIG. 16 . The final composite membrane has a thickness within the range of from about 0.001 inches to about 0.010 inches, and, preferably, is about 0.002 to about 0.003 inches in thickness. However, the thicknesses and process parameters of the foregoing may be varied considerably, depending upon the materials of the bonding layer  254 , first membrane  250 , and second membrane  252 .  
         [0041]     As illustrated in top plan view in  FIG. 17 , the resulting finished composite membrane film  52  has a plurality of “unbonded” windows or areas  256  suitable for cellular attachment and/or ingrowth. The attachment areas  256  are bounded by leafs  50 , and the cross-hatch or other wall pattern formed by the bonding layer  254 . Preferably, a regular window  256  pattern is produced in the bonding layer  254 .  
         [0042]     The foregoing procedure allows the bonding mesh to flow into the first and second membranes  250  and  252  and gives the composite membrane film  52  greater strength (both tensile and tear strength) than the components without the bonding mesh. The composite membrane allows uniform bonding while maintaining porosity of the membrane film  52 , to facilitate tissue attachment. By flowing the thermoplastic bonding layer into the pores of the outer mesh layers  250  and  252 , the composite flexibility is preserved and the overall composite layer thickness can be minimized. In another embodiment film  52  may be non-porous and comprise a polymer such as polyurethane or silicone.  
         [0043]     A composite membrane film  52 , when used in the stent embodiment illustrated in  FIGS. 12-13 , provides a barrier and prevents emboli from being shed from the stented region, when the self expanded segment  2  apposes ostial O and aortic Ao regions of a vessel.  
         [0044]     While this document has described an invention mainly in relation to renal artery stenting, it is envisioned that the invention can be applied to other conduits in the body as well including arteries, veins, bronchi, ducts, ureters, urethra, and other lumens intended for the passage of air, fluids, or solids. The invention can be applied to any site of branching of an artery, vein, bronchus, duct, ureter, urethra, and other lumen including but not limited to the junction of the common, internal, and external carotid arteries, the junction of the main, left anterior descending, and circumflex coronary arteries, the junction of the left main or right coronary artery with the aorta, the junction of the aorta with the subclavian artery, and the junction of the aorta with the carotid artery.  
         [0045]     While the various embodiments of the present invention have related to stents and stent delivery systems, the scope of the present invention is not so limited. Further, while choices for materials and configurations may have been described above with respect to certain embodiments, one of ordinary skill in the art will understand that the materials described and configurations are applicable across the embodiments.