Patent Publication Number: US-2006004438-A1

Title: Prosthesis, delivery system and method for neurovascular aneurysm repair

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. patent application Ser. No. 10/836,909, filed Apr. 30, 2004, and entitled “DELIVERY CATHETER THAT CONTROLS FORESHORTENING OF RIBBON-TYPE PROSTHESES AND METHODS OF MAKING AND USE”. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to prostheses and methods for treating aneurysms in very small vessels, such as the cerebral vessels. More particularly, the present invention is directed to the use of helically wound stent including one or more features for retarding or excluding blood flow into an aneurysm sac.  
     BACKGROUND OF THE INVENTION  
      Today there are a wide range of intravascular prostheses on the market for use in the treatment of aneurysms, stenosis, and other vascular irregularities. Balloon expandable and self-expanding stents are well known for restoring patency in a stenosed vessel, e.g., after an angioplasty procedure, and the use of coils and stents are known techniques for treating aneurysms.  
      Previously-known self-expanding stents generally are retained in a contracted delivery configuration using a sheath, then self-expand when the sheath is retracted. Such stents commonly have several drawbacks, for example, the stents may experience large length changes during expansion (referred to as “foreshortening”) and may shift within the vessel prior to engaging the vessel wall, resulting in improper placement. Additionally, many self-expanding stents have relatively large delivery profiles because the configuration of their struts limits further compression of the stent. Accordingly, such stents may not be suitable for use in smaller vessels, such as cerebral vessels and coronary arteries.  
      Other drawbacks associated with the use of coils or stents in the treatment of aneurysms is that the devices, when deployed, may have a tendency to straighten or otherwise remodel a delicate cerebral vessel, which may cause further adverse consequences. Moreover, such devices may not adequately reduce or exclude blood flow from the vessel into the sac of the aneurysm, and thus may not significantly reduce the risk of rupture.  
      For example, U.S. Pat. No. 6,660,032 to Klumb et al. describes a stent comprising a pair of helical mesh coils interconnected by ladder-like cross members and entirely covered by a graft material. In operation, the stent may be wound into plurality of turns of reduced diameter, and then constrained within a delivery sheath. The delivery sheath is retracted to expose the distal section of the stent and anchor the distal end of the stent. As the delivery sheath is further retracted, subsequent individual turns of the stent unwind to conform to the diameter of the vessel wall.  
      The stent described in the foregoing publication has several drawbacks. For example, the use of graft material along the full length of the stent increases the overall delivery profile of the stent, potentially rendering the device too large and too axially stiff for use in treating aneurysms located in narrow or tortuous neurovascular vessels. In addition, the presence of graft material along the full length of the stent may cause inadvertent closure of perforators—small side vessels. Moreover, due to friction between the turns and the sheath, the individual turns of the stent may bunch up, or overlap atop one another, when the delivery sheath is retracted. This in turn may create gaps in the stent that inadequately limit the flow of blood from the vessel into the sac of an aneurysm.  
      U.S. Pat. No. 4,768,507 to Fischell et al. and U.S. Pat. No. 6,576,006 to Limon et al., each describe the use of a groove disposed on an outer surface of an interior portion of the stent delivery catheter, wherein at least a portion of the stent is disposed within the groove to prevent axial movement during proximal retraction of the sheath. While the delivery catheters disclosed in these patents may reduce axial movement and bunching of the prosthesis during retraction of the sheath of the delivery catheter, those systems do not effectively address the issue of stent foreshortening nor eliminate the creation of gaps that permit blood to circulate into the sac of an aneurysm. For example, once the sheath of the delivery catheter is fully retracted, the turns of the stent may shift relative to one another within the vessel prior to engaging the vessel wall, resulting in inadequate coverage of the stenosis or aneurysm.  
      Aneurysms often arise in smaller vessels at bends, where a change in the direction of blood flow results in high hemodynamic loads being exerted on the vessel wall. Aneurysms thus are often encountered at bifurcations and on the outer bends of tortuous vessels, where flow impinges on the vessel wall and is redirected. Aneurysm repair typically requires surgical intervention, although some efforts to develop percutaneous solutions have been made.  
      One previously-known method of treating aneurysms percutaneously involves deploying platinum coils within the aneurysm sac, thereby causing the blood contained within the sac to clot. In such cases, a microcatheter may be disposed with its tip extending into the aneurysm sac. One or more embolization coils are ejected from the tip of the microcatheter into the sac, precipitating clotting of the blood contained within the aneurysm sac. During the clotting process it is possible for thrombus to enter blood flowing past or through the aneurysm, thereby creating a risk of blocking downstream vessels.  
      In view of the above-identified drawbacks of previously-known methods for percutaneously treating aneurysms of small vessels, it would be desirable to provide prostheses and methods for treating aneurysms that substantially retard or exclude flow into an aneurysm sac.  
      It also would be desirable to provide prostheses and methods for use in treating aneurysms of small or tortuous vessels, wherein prostheses have a small delivery profile that facilitates passage through narrow vessels.  
      It further would be desirable to provide prostheses for use in treating aneurysms of small or tortuous vessels, wherein the prostheses have a high degree of axial flexibility thereby further facilitating delivery through tortuous vessels.  
     SUMMARY OF THE INVENTION  
      In view of the foregoing, it is an object of the present invention to provide percutaneously-deliverable prostheses and methods for treating aneurysms of small vessels, wherein the prostheses substantially retard or exclude flow into an aneurysm sac.  
      It is another object of the present invention to provide prostheses and methods for use in treating aneurysms of small or tortuous vessels, wherein the prostheses have a small delivery profile that facilitates delivery through narrow vessels.  
      It is a further object of this invention to provide prostheses and methods for use in treating aneurysms of small or tortuous vessels, wherein the prostheses have a high degree of axial flexibility, thereby enabling delivery through tortuous vessels.  
      These and other objects of the present invention are accomplished by providing a prosthesis, delivery system and methods wherein the prosthesis includes a self-expanding helical section including a localized feature that retards or excludes blood flow into the sac of an aneurysm. The feature may comprise a segment of graft material disposed only for a discrete portion of the circumference of the prosthesis or a local variation in the pattern of struts making up the prosthesis.  
      In a preferred embodiment, the prosthesis comprises a radially self-expanding distal section coupled to a helically-wound proximal section, wherein the proximal section has a localized feature configured to retard or exclude blood flow into an aneurysm. The feature may comprise an area on the helical section having a locally higher material concentration designed to span the neck of the aneurysm, or graft material disposed on the helical section for a predetermined axial length. Compared to previously known prosthesis designs, such as described in the foregoing patent to Klumb et al., the localized nature of the aneurysm exclusion feature is expected to provide a prosthesis that can be wound to a substantially smaller delivery profile while retaining a high degree of axial flexibility.  
      In accordance with another aspect of the present invention, a specially configured delivery system is provided for use with the inventive prosthesis to assist the clinician in orienting and delivering the prosthesis within a target vessel. The delivery system preferably comprises a catheter having a predetermined non-circular cross-section that cooperates with the tortuosity of the patient&#39;s anatomy to facilitate proper angular orientation of the vascular prosthesis within the vessel. For example, the delivery catheter may comprise a substantially elliptical profile that automatically orients the catheter within the vessel with a known orientation.  
      In accordance with a further aspect of this invention, a method of marking a desired deployed location of a localized feature on the helical section of the prosthesis is provided. The method includes the steps of selecting a reference point on the delivery catheter, determining the axial location of the reference point, determining the axial and angular location of the feature and providing a reference mark on the prosthesis to indicate the desired deployed location of the feature.  
      Methods of using the prosthesis and delivery system of the present invention for treating aneurysms in small vessels, such as the cerebral vessels, also are provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments, in which:  
       FIG. 1  is a perspective view of a prosthesis constructed in accordance with the principles of the present invention;  
       FIG. 2  is a perspective view of an alternative embodiment of a prosthesis of the present invention;  
       FIGS. 3A and 3B  are, respectively, side view and cross-sectional views of a delivery system of the present invention;  
       FIGS. 4A and 4B  are, respectively, side and end views depicting the location of a therapeutic feature in accordance with the principles of the present invention;  
       FIG. 5  is a side view of the prosthesis of  FIGS. 4 , wherein the helical section has been flattened;  
       FIG. 6  is a side view of the vascular prosthesis of  FIG. 4A  disposed around a distal end of the delivery catheter of  FIGS. 3 ;  
       FIGS. 7A and 7B  are side and cross-sectional views, respectively, of the vascular prosthesis and delivery catheter of  FIGS. 3 , wherein the vascular prosthesis is in the deployed configuration; and  
       FIG. 8  is a cross-sectional views showing a method of deploying the prosthesis of the present invention using the delivery system of  FIGS. 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is directed to prostheses, delivery systems and methods for treating aneurysms located within narrow and tortuous vessels, such as in the cerebral vasculature. In accordance with the principles of the present invention, the prosthesis includes a feature disposed on a localized region of the prosthesis to retard or exclude blood flow into the sac of an aneurysm. The prosthesis may be used alone or in conjunction with embolism coils, such as are known in the art.  
      In accordance with the principles of the present invention, the aneurysm exclusion feature comprises a locally-higher density of the strut arrangement of the prosthesis or a portion of graft material disposed only on a discrete portion of the length or circumference of the prosthesis. Due to the localized nature of the feature, the prosthesis of the present invention is expected to provide a smaller delivery profile, and greater flexibility and trackability than previously-known devices.  
      Further in accordance with the invention, a delivery system is provided that facilitates deployment of the prosthesis in the vessel with a specified angular and axial alignment. The delivery catheter also provides a predictable degree of foreshortening of the stent, including substantially zero foreshortening. The catheter also preferably includes a radio-opaque marker arrangement and non-circular cross-section that facilitate delivery of the prosthesis with a desired orientation with a target vessel.  
      Referring to  FIG. 1 , a preferred vascular prosthesis of the present invention is described. As used in this specification, the terms “vascular prosthesis” and “stent” are used interchangeably. Vascular prosthesis  10  is described in copending commonly assigned U.S. patent application Ser. No. 10/342,427, filed Jan. 13, 2003, and comprises helical section  12  and distal section  14 , each capable of assuming contracted and deployed states. In  FIG. 1 , helical section  12  and distal section  14  each are depicted in their respective deployed states.  
      Vascular prosthesis  10  preferably is formed from a solid tubular member comprising a shape memory material, such as nickel-titanium alloy (commonly known as “Nitinol”), using laser cutting techniques that are per se known in the art. The prosthesis is then subjected to an appropriate heat treatment, also known in the art, while the device is held in the desired deployed configuration (e.g., on a mandrel), thus conferring a desired deployed configuration to vascular prosthesis  10  when self-deployed.  
      Distal section  14  is configured to expand radially outward from its contracted position, and comprises a pattern of cells, illustratively having a zig-zag or diamond configuration in the deployed state. Distal section  14  is designed to be deployed from a delivery catheter first to fix the distal end of the stent at a desired location within a vessel. In this manner, subsequent deployment of helical section  12  of the stent may be accomplished with greater accuracy.  
      Helical section  12  comprises mesh  16  having a selected cell pattern formed by multiplicity of struts  18 , wherein the mesh defines a plurality of substantially flat turns  19 . Struts  18  further define a multiplicity of openings  20 . Turns  19  are configured to be wound down onto a delivery system in the contracted delivery configuration, as described in greater detail below, in an overlapping manner. It should be understood that the configuration of helical section  12  depicted in  FIG. 1  is merely illustrative, and other patterns may be advantageously employed. Helical section  12  is coupled to distal section  14  at junction  22 .  
      Still referring to  FIG. 1 , in accordance with one aspect of the present invention, helical section  12  further includes localized feature  24  configured to exclude or reduce flow into an aneurysm sac. Feature  24  may comprise a locally denser arrangement of struts  26 , as depicted in  FIG. 1 , configured to impede blood flow into the sac of an aneurysm. These struts also may be configured to allow for separation or deflection, for example, so that a microcatheter may pass between the struts to deliver coagulation coils. Feature  24  may extend for the several consecutive turns  19 , or only for part of the circumference of a single turn. As will be appreciated by one of skill in the art of helical stent design, the higher the concentration of struts  26  in feature  24 , the greater the axial rigidity of the prosthesis at that axial location. Accordingly, it is desirable to make the length of feature  24  as short as possible to retain axial flexibility and trackability of the stent. In addition, by providing feature  24  on only as much of helical section  12  as required for a particular application, the overall delivery profile of the prosthesis may be kept substantially smaller than previously-known stent designs.  
      Referring now to  FIG. 2 , an alternative embodiment of the vascular prothesis of the present invention is described, in which like parts are identified with like-primed numbers to those used in  FIG. 1  (e.g., prosthesis  10 ′). Prosthesis  10 ′ includes helical section  12 ′ and distal section  14 ′. As in the embodiment of  FIG. 1 , distal section  14 ′ is configured to expand radially outward from its contracted position, and comprises a pattern of cells, illustratively having a zig-zag or diamond configuration in the deployed state.  
      Helical section  12 ′ comprises mesh  16 ′ having a selected cell pattern formed by multiplicity of struts  18 ′ to form plurality of turns  19 ′. Struts  18 ′ define multiplicity of openings  20 ′. Helical section  12 ′ is coupled to distal section  14 ′ at junction  22 ′ and further includes localized feature  24 ′ configured to exclude or reduce flow into an aneurysm sac. Feature  24 ′ comprises a portion of graft material  25 ′, for example, such as expanded PTFE or polyurethane, glued or sintered onto struts  18 ′ for a predetermine number of turns  19 ′ or only for part of the circumference of a single turn.  
      Polyurethane, for example, would provide a thin wall that could be readily pierced by a microcatheter to deliver coils, and would substantially self-seal once the microcather was removed. By providing feature  24 ′ on only as much of helical section  12 ′ as required for a particular application, the overall delivery profile of the prosthesis may be kept substantially smaller than previously-known stent designs, such as the Klumb et al. patent mentioned above.  
      Referring now to  FIGS. 3 , delivery catheter  30  of the present invention is described. Delivery catheter  30  includes inner member  31  having central lumen  32 , distal tip  33  and sheath  34 . Sheath  34  may comprise a polymeric material disposed on a metal braiding and having good flexibility and sufficient radial strength to retain a stent in the contracted delivery configuration on inner member. Sheath  34  may comprise, for example, a stainless steel braid covered by polyurethane or polyethylene material and preferably includes a lubricious inner surface to facilitate retraction of the sheath during stent delivery.  
      Inner member  31  is constructed so as to mitigate or eliminate foreshortening during deployment by imposing on the stent in the contracted delivery configuration the same wrap angle e that the stent will have in the deployed configuration. This is accomplished by forming helical ledge  35  on the outer surface of inner member  31 . Ledge  35  may be formed in a number of ways, such as by gluing, soldering or laminating a helical wire to the outer surface of the inner member, by braiding a helical wire into fibers forming the inner member, or by integrally forming the ledge with the inner member, e.g., using an extrusion or molding process.  
      During wrapping of a stent onto inner member  31 , either a proximal or distal edge of the stent is abutted against helical ledge  35 , so that adjacent turns of the stent overlap one another. Helical ledge  35  also provides linear resistance to stent migration when sheath  34  is retracted during stent deployment. This engagement between the turns of the stent and the inner member maintains the linear stability of the stent, and reduces the risk that overlapping turns of the stent bunch up or seize against the interior surface of the sheath. Moreover, the helical ledge ensures that the stent unwinds on its axis but does not experience significant linear change along the axis. Further details regarding the construction of inner member  31  are provided in co-pending, commonly assigned U.S. patent application Ser. No. 10/836,909, filed Apr. 30, 2004, the entirety of which is incorporated herein by reference.  
      Applicants have observed that aneurysms frequently occur on the outer bends of the smaller vessels due to the hemodynamic loading on the vessel wall associated with redirecting blood flow. Thus, for example, aneurysms frequently occur near bifurcations. In accordance with the foregoing observation, applicants have designed inner member  31  to have a non-circular, and preferably elliptical, cross-section, as shown in  FIG. 3B . Due to the elliptical shape of the inner member, delivery catheter  30  will pass through tortuous anatomy with a known orientation. More particularly, the delivery catheter will automatically orient itself within a vessel so that the major axis of the ellipse faces the outside of the curve.  
      As described in greater detail below, feature  24  or  24 ′ of the prosthesis  10  or  10 ′ may be loaded with a predetermined orientation into the delivery catheter relative to the circumference of inner member  31 . Then, when the delivery catheter and stent are advanced into the target vessel, the non-circular shape of the delivery catheter will ensure that stent is oriented with the vessel so that the feature spans the aneurysm. Embolization coils then may be delivered into the aneurysm sac or attached to the prosthesis to treat the aneurysm.  
      Referring now to  FIGS. 4A and 4B , a method of positioning the vascular prosthesis within the delivery system of FIGS.  3  is described. In order to properly place the vascular prosthesis at a desired location within a vessel with feature  24  or  24 ′ in apposition to the aneurysm neck, it is necessary to accurately determine the location of the feature both axially and circumferentially relative to helical section  12  or  12 ′. This in turn requires determination of the relationship of the feature location between the expanded deployed configuration and the contracted delivery configuration.  
      Referring to  FIGS. 4A and 4B , vascular prosthesis  40  comprises helical section  42  coupled to distal section  44  at junction  46 . Prosthesis  40  includes feature  48 , such as locally higher strut density or graft material, configured to exclude or reduce blood flow into an aneurysm. Radio-opaque mark  50  is disposed on helical section  42  to indicate the location of the distal edge of feature  48  on vascular prosthesis  40 . Feature  48  is defined by two variables: axial distance (x) from the junction  46  and angular distance (r). Junction  46  defines a reference point wherein x j =0 and r j =0.  
      When designing a vascular prosthesis having feature  48  in accordance with the present invention, axial distance x is pre-defined. By way of example, consider a vascular prosthesis design that requires a feature three-quarters of the distance from the distal end of the helical body. Once axial distance x is defined, the angular location may be calculated using the equations set forth in the next paragraph.  
      Referring to  FIG. 5 , vascular prosthesis  40  is shown with helical section  42  flattened out for illustrative purposes. Mark  50  is disposed on helical section  42  to indicate the location of the distal edge of the feature, defined by axial distance x and angular distance r. Angle (θ) and diameter (D 2 ) of the deployed helical body preferably are determined by design. Axial distance x of the vascular prosthesis feature also is known, whereas circumferential distance (y) of the feature is determined as y=Π*D (for exactly one revolution) or y=Π*D*r/360 (for a partial or more than one revolution). Using the known formula for a tangential relationship (tan(θ)=x/y), r is solved in terms of x: tan(θ)=x/y=x/(Π*D*r/360). By solving for angular location r, the following equation is obtained: r=360*x/Π*D*tan(θ).  
      When a feature is present after the first revolution (i.e., r&gt;360), then the number of revolutions to the feature is determined by r/360, thereby resulting in a fractional number. When a feature is disposed at the same angular location as the junction  46 , then r/360 is an integer. Otherwise, there is a fractional portion that is equal to the angular change relative to the last full revolution. By way of example, if r/360=3.25, there are 3 full revolutions and an additional one-quarter revolution (90°) past the angular location of the junction.  
      The relationship between changes in diameter D and changes in angular location r must be determined to accurately wrap the prosthesis onto the delivery catheter for deployment in different size vessels. For a helix, axial distance x does not change (x 1 =x 2 ) when diameter D changes from D 1  to D 2 , as long as angle θ remains constant (θ 1 =θ 2 ).  
      Using the equation r=360*x/Π*D*tan(θ), axial distance x is solved for: x=Π*D*r*tan(θ)/360. Because axial distance x 1  equals axial distance x 2 : Π*D 1 *r 1 *tan(θ)/360=Π*D 2 *r 2 *tan(θ)/360. Solving for r 2 , the following equation is obtained: r 2 =D 1 *r 1 /D 2 . Using this equation, the angular location of one or more features on the vascular prosthesis may be determined at different diameters. In general, angular location r changes proportionally with changes in diameter D.  
       FIG. 6  depicts the distal end of delivery catheter  60  constructed as described above with respect to  FIG. 3A . Delivery catheter  60  includes retractable sheath  61  and inner member  62  having helical ledge  63  disposed thereon. Delivery catheter  60  further comprises distal marker  65  attached to inner member  62  via fillet  66 . During wrapping of a vascular prosthesis onto inner member  62 , the distal turn of helical body  42  is abutted against helical ledge  63 , which acts as a guide for wrapping subsequent turns around the inner member. In the illustrated embodiment, adjacent turns of the stent do not overlap one another in the delivery configuration.  
      Still referring to  FIG. 6 , a method of marking the expected location of a feature on the vascular prosthesis is described. Initially, a reference point on the delivery catheter is selected. Illustratively, the longitudinal edge of the distal turn of the prosthesis is aligned with distal end  67  of helical ledge  63 , which is used as a reference point. Of course, other locations may be selected as the reference point without departing from the scope of the invention.  
      Starting at the proximal edge of distal marker  65 , the axial location (x 1 ) of the distal end of helical ledge  63  is determined by adding: (1) the axial length of the fillet (x f ); (2) the axial length of the distal section (x d ); and (3) the axial length of one turn of the helical body (x b ). Thus, the following equation is obtained for the axial location of the distal end of the helical ledge: x 1 =x f +x d +x b . If junction  46  is aligned with distal end  67  of helical ledge  63 , then r j =r 1 =0. The axial and angular location of the feature now may be calculated using axial location x 1  as the reference point.  
      Referring to  FIGS. 7A and 7B , the helical section  42  of prosthesis  40  is shown in the expanded deployed configuration prior to withdrawal of inner member  62  of delivery catheter  60 . Mark  50  is disposed on helical body  42  so that its axial location is defined by (x m ) and its angular location is defined by (r m ). The location of mark  50  is related to deployed diameter (D dep , x dep , r dep ), such that: (1) x m =x dep −x b ; and (2) r m =r dep =360*x dep /Π* D dep *tan(θ). Using these two equations, the delivery catheter is configured to include a mark that indicates the axial and angular location of a feature on the vascular prosthesis.  
      Proper axial placement of the vascular prosthesis of the invention preferably is achieved using radiopaque markers on the delivery catheter and/or the vascular prosthesis. For example, the markers may be disposed at the center or ends of the feature, thereby allowing the feature to be placed at the desired location with respect to an aneurysm neck.  
      With respect to  FIG. 8 , a preferred method of delivering the prosthesis of the present invention having feature F within vessel V now is described. Initially, prosthesis  40  is loaded onto the inner member of  62  of delivery catheter  60  having mark  50  that identifies an edge or the center of feature F. The vascular prosthesis is oriented radially so that it will open in the vessel in a known orientation. Vascular prosthesis  40  is delivered across aneurysm neck N, where helical section  42  becomes anchored against healthy tissue on either side of aneurysm A.  
      In accordance with the present invention, the delivery catheter preferably has an elliptical cross-section including major axis L 1  and minor axis L 2  that preferentially disposes the feature towards the outer radius of the vessel during transluminal advancement, as illustrated in  FIG. 8 . Once the prosthesis is properly positioned within vessel V, as determined, for example, by using fluoroscopic imaging, the prosthesis is deployed by retracting the outer sheath. Distal section  44  of the prosthesis deploys first by self-expanding into contact with healthy tissue distal to the aneurysm location, and helical section  42  then unwinds from inner member  62  into contact with the wall of the vessel turn-by-turn. Because the prosthesis does not foreshorten during deployment, feature  48  may be accurately place across neck N of the aneurysm. Following placement of prosthesis  40 , a microcatheter may be advanced through the struts of the prosthesis to place embolization coils within the sac of aneurysm A.  
      While preferred illustrative embodiments of the invention are described above, it will be apparent to one skilled in the art that various changes and modifications may be made therein without departing from the invention. The appended claims are intended to cover all such changes and modifications that fall within the true spirit and scope of the invention.