Abstract:
An endoluminal prosthesis is described and comprises a stent ( 40 ), At least one barb ( 42 ) secured to the stent, and a degradable barb guard ( 52 ) covering at least a portion of the barb and protecting the distal tip when the stent is in a compressed configuration. The barb guard ( 52 ) comprises a degradable structure that degrades to expose the distal tip of the barb when the stent is placed in a vessel. An endoluminal prosthesis deployment system comprising a sheath having a lumen and a prostesis within the sheath lumen is also described

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to medical devices and, in particular, to devices, systems, and methods for anchoring a prosthesis such as an implantable medical device in a body lumen. 
       BACKGROUND ART 
       [0002]    Deploying an endoluminal prosthesis into the vessel of a patient from a remote location is generally known. An endoluminal prosthesis, such as a stent or stent graft, is introduced into a vessel in a radially constrained state. Once the prosthesis is positioned, it can then be expanded within the vessel. The prosthesis may employ one or more barbs that can engage the vessel wall to limit movement of the prosthesis within the vessel. 
         [0003]    An endoluminal prosthesis is typically introduced into a vessel via a delivery and deployment device. Such a device may include a cover or sheath that is placed within the vessel. The prosthesis is manipulated within the sheath into the vessel. The sheath is retracted from the prosthesis to allow the prosthesis to expand within the vessel. If the prosthesis is self-expanding, it will expand within the vessel when it is no longer constrained by the sheath. If the prosthesis is balloon expanded, a balloon will be required to expand the device after the sheath is removed. U.S. Pub. Pat. App. Nos. 2004/0098079, 2004/0106974, 2005/0085890, and 2005/0060018, which are herein incorporated by reference, disclose various examples of prostheses and devices for delivering and deploying a prosthesis. 
         [0004]    When a prosthesis is disposed within a sheath, at least a portion of the prosthesis engages the sheath in frictional contact. Such frictional engagement may be sufficiently high so as to create significant interference between the stent and the sheath. This is particularly the case when the prosthesis is self-expanding or when the prosthesis has one or more exposed barbs that engage the inner surface of the sheath. Such frictional engagement may result in increased operating effort to remove the sheath from the device. 
       DISCLOSURE OF THE INVENTION 
       [0005]    The present invention seeks to provide an improved implantable medical device provided with barbs, an improved barb arrangement for implantable medical devices and an improved method and system for deploying implantable medical devices. 
         [0006]    According to an aspect of the present invention, there is provided an endoluminal prosthesis as specified in claim  1 . 
         [0007]    According to another aspect of the present invention, there is provided an endoluminal prosthesis deployment system as specified in claim  8 . 
         [0008]    Various devices, systems, and methods are disclosed throughout the specification and in the drawings. In one example, an endoluminal prosthesis is described and comprises a stent having a compressed configuration and an expanded configuration, a barb secured to the stent and having a distal tip, and a degradable barb guard covering at least a portion of the barb and protecting the distal tip when the stent is in the compressed configuration. The barb guard comprises a degradable structure that degrades to expose the distal tip of the barb when the stent is placed in a vessel. The exposed distal tip will thus be able to engage a surrounding vessel where the unexposed tip would not. 
         [0009]    The guard may comprise any degradable structure, for example, a dissolvable structure or a frangible structure. Thus, in some examples, the barb guard degrades by dissolving to expose the distal tip of the barb when the stent is placed in a vessel and, in some examples, the barb guard degrades by breaking to expose the distal tip of the barb when the stent is placed in a vessel. The barb guard may be attached directly to the stent and, in some cases, directly to the barb. 
         [0010]    In another example, an endoluminal prosthesis system comprises a sheath having a sheath lumen and a prosthesis disposed within the sheath lumen. The prosthesis comprises a stent and a barb secured to the stent. The barb comprises a distal tip and a degradable barb guard covering at least a portion of the barb and preventing direct contact between the distal tip of the barb and the sheath. The barb guard comprises a degradable structure, such as a dissolvable or frangible structure, that degrades to expose the distal tip of the barb after the stent is released from the sheath. 
         [0011]    In some examples, the barb guard may contact the sheath when the stent is disposed within the sheath. The barb may be biased towards an extended configuration and have a bias force that is greater than the strength of the guard. In these examples, the guard will break when the stent is released from the sheath. The guard may comprise any degradable structure, such as a dissolvable and/or frangible structure. 
         [0012]    The term prosthesis as used herein is intended to be understood in a broad manner and to include any implantable medical device such as stents, stent grafts, vena cava filters, occluders, prosthetic valves and so on. 
         [0013]    According to another aspect of the present invention, there is provided a method of deploying an implantable medical device provided with one or more barbs, including the steps of covering or otherwise protecting said barb or barbs by a removable covering provided on the medical device, said covering being degradable or frangible upon deployment of the device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  depicts a portion of a stent and a barb; 
           [0016]      FIG. 2A  depicts an example of a stent and a degradable barb guard, with the stent disposed in a contracted state within the lumen of a sheath; 
           [0017]      FIG. 2B  depicts the stent of  FIG. 2A  in an expanded state; 
           [0018]      FIG. 3  depicts another example of a degradable barb guard; 
           [0019]      FIG. 4  depicts an example of a barb guard, and the stent of  FIG. 3 , in cross-section; and 
           [0020]      FIG. 5  depicts another example of a degradable barb guard. 
       
    
    
     TERMINOLOGY 
       [0021]    Throughout the specification, when referring to a barb or a portion thereof, the terms “distal” and “distally” shall denote a position, direction, or orientation along the barb that is generally towards, or in the direction of, the anchor. The terms “proximal” and “proximally” shall denote a position, direction, or orientation along the barb that is generally away from, or in the opposite direction of, the anchor. 
         [0022]    The term “prosthesis” means any device, object, or structure that supports, repairs, or replaces, or is configured to support, repair, or replace a body part or a function of that body part. It can also mean a device that enhances or adds functionality to a physiological system. Examples of prostheses include stents, stent grafts, vessel occlusion devices, vena cava filters, and the like. 
         [0023]    The term “stent” means any device or structure that provides, or is configured to provide, rigidity, expansion force, or support to a body part (e.g., a diseased, damaged, or otherwise compromised body lumen.). A stent may comprise any suitable material, including, but not limited to, biocompatible metals and plastics. Examples of suitable materials include metals such as stainless steel and NITINOL, and plastics such as polyethylene terephthalate (“PET”), polytetrafluoroethylene (“PTFE”) and polyurethane. 
         [0024]    A stent may be “expandable,” that is, it may be capable of being expanded from a constricted configuration to an expanded configuration. A stent may be self-expanding and expand by virtue of its own resilience. Alternatively, a stent may be pressure-expandable and expand only upon the application of an external force. In addition, a stent may be expandable upon application of heat, such as when exposed to body temperature. An example of a self-expanding stent is the Z-STENT®, which is available from Cook Incorporated, Bloomington, Ind., USA. 
         [0025]    A stent may comprise one or more barbs. A barb may comprise a suitable biocompatible material, such as a biocompatible metal or plastic. Suitable biocompatible materials include the stent materials described above. Where the stent comprises a metal, the barb and the stent may comprise the same or a similar material. For example, the materials may have similar electromotive forces. Such a construction may minimize corrosion at the junction between the barb and the stent. 
         [0026]    The term “secured” refers to a structure that is “connected,” “joined,” “attached,” and the like. “Secured” is not limited to a particular method or manner of connecting, joining, or attaching. Thus, a barb “secured” to a stent refers to barb-stent structures formed by connecting, joining, or attaching independently formed structures (such as by welding, soldering, or the like), as well as monolithic barb-stent structures. 
         [0027]    The term “degradable” refers to a structure that is capable of degrading within a medically effective timeframe. “Degrading” includes tearing, breaking, severing, fracturing, dissolving, dissociating, and the like. Terms such as “tearable,” “breakable,” “severable,” “fracturable,” “dissolvable,” “dissociable,” and the like, may be used and refer to structures that are capable of degrading, as described, within a medically effective timeframe. 
         [0028]    A structure is not “degradable” merely because it is capable of degrading over an indeterminate period of time. As defined, the structure must be capable of degrading over a medically effective timeframe. A medically effective timeframe depends on many factors, including the type of device and procedure. In some examples, the timeframe may be less than a second. In other examples, the timeframe may be minutes or more. A structure that is capable of degrading in a period of an hour is not “degradable,” as that term is used herein, if the medical procedure warrants that the structure degrade in less than a second. On the other hand, such a structure may be “degradable,” as that term is used herein, if the medical procedure allows for the structure to degrade within, for example, 24 hours. 
         [0029]    The term “frangible” means readily or easily torn, broken, severed, fractured, or the like. 
         [0030]    The term “lumen” describes a cavity or channel within a tube or a tubular body, such as body vessel. The term “endoluminal” means within a lumen, and can refer to objects that are found or that can be placed within a lumen, or methods or processes that occur within a lumen. An “endoluminal prosthesis” is thus a prosthesis that is found, or that can be placed, within a lumen. Examples of endoluminal prostheses include, but are not limited to, stents, grafts, stent grafts, venous or aortal valves, and vena cava filters. An endoluminal prosthesis may be generally tubular and comprise one or more lumens. Examples of tubular prostheses include straight, branched, and bifurcated prostheses. 
         [0031]    The term “score” refers to a channel, depression, cut, score, notch, line, perforation, aperture, or the like, and includes both recessed structures as well as projected structures. The term “score” may also refer to a series of channels, depressions, cuts, scores, notches, lines, perforations, apertures, or the like. A score may be provided by any mechanical, thermal, or chemical means known in the art, such as cutting by knife or carbide tip, by sanding, by chemical etching, by laser scoring, or by molding. 
         [0032]    The term “monolithic” refers to structures that are formed as a single piece, rather than being separately formed and attached. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]      FIG. 1  depicts a prior art stent  10  with a barb  12 . The barb  12  has a barb body  14  that extends distally towards a barb tip  16 . The tip  16  is preferably sharp and, when the stent is inserted into a vessel, can engage the vessel to limit movement of the stent within the vessel. In this example, the barb  12  is fixedly attached to a strut  18  of the stent  10  via weld, solder, adhesive or the like. In other examples, the barb  12  and the stent  10  may be monolithic. In other examples, the barb  12  may be variably, rather than fixedly, attached to the stent. Examples of barbs and attachment techniques are disclosed, for example, in U.S. Pub. Pat. App. Nos. 2003/0236570 A1 and 2005/0240259 A1. Each of these references is herein incorporated by reference. 
         [0034]    The barb of  FIG. 1  is shown in an extended configuration. The distal tip  16  extends outwardly from the stent  10  in this configuration. The distal tip  16  is unprotected and may engage tissue of a vessel in which the stent  10  is placed. The barb  12  may have a retracted configuration (not shown) whereby the distal tip  16  is retracted towards the stent  10 , away from the extended configuration. This may be advantageous, for example, when the stent is compressed within a sheath. In typical devices, the distal tip  16  is unprotected in the retracted configuration and, therefore, is still able to engage the sheath during delivery and deployment. This engagement may be minimal if the barb is oriented in the direction of sheath withdrawal. If, however, the barb is oriented against the direction of sheath withdrawal, then the unprotected tip  16  may dig into the sheath, increasing the deployment effort. 
         [0035]      FIG. 2A  depicts an example of a stent  20  in a constricted configuration, disposed within the lumen of a sheath  30 . The stent  20  has a barb  22 , shown in a retracted configuration. The stent may comprise additional barbs as necessary or desired. A barb guard  32  covers a portion of the barb  22  and protects the distal tip  26 . As shown, the barb guard  32  is disposed between the barb  22  and the sheath  30  and prevents direct contact between the tip  26  and the sheath. Thus, the barb guard  32  prevents the tip  26  from engaging the sheath during delivery. 
         [0036]      FIG. 2B  depicts stent  20  after it has been released from the sheath  30 . The stent  20  expands to an expanded configuration and the barb  22  extends to an extended configuration. The barb guard  32  no longer covers barb  22  and the distal tip  26  is unprotected. Accordingly, the distal tip  26  is free to engage a surrounding vessel. 
         [0037]    The barb guard  32  has a degradable structure that degrades to release the barb  22  from the barb guard  32 . For example, the barb guard  32  may comprise a material that readily dissolves or dissociates after the stent  20  is released into a vessel. In these examples, the barb  22  is released, and the distal tip  26  exposed, by virtue of the dissolution or dissociation of the barb guard  32 . 
         [0038]    Examples of suitable dissolvable materials include, but are not limited to, synthetic materials such as polylactides, polyglycolides, and polyvinyl acetate, or natural materials such as gelatin. The material is preferably formulated to dissolve over a narrow temperature range at or below body temperature, for example between 25-35° C., or 30-35° C. The material is preferably stable at temperatures below this range. In particular, the material is preferably stable in the presence of fluids, such as saline or heparin, that are exposed to the stent during pre-procedural preparation. 
         [0039]    In the example shown in  FIG. 2A , the barb guard  32  comprises a dissolvable strip or band  34  that is disposed about the periphery of the stent  20 . The band  34  may be disposed about the entire periphery of the stent and retain the entire stent in a constricted configuration. Alternatively, a band may be provided that is disposed about only a portion of the stent. For example, the band may be disposed about fewer than all of the stent struts. If the stent  20  is self-expanding, it will exert an expansion force against the barb guard  32 . In some examples, the barb guard  32  may have a tensile strength that is greater than the force exerted by the stent  20 , and the stent will not be able to expand. In these examples, the band  34  preferably comprises a material that dissolves relatively quickly (e.g., in a matter of seconds), so that the tensile strength of the band  34  becomes less than the force exerted by the stent  20 . At that point, the band  34  will break under the pressure exerted by the stent and allow the barb  22  to extend and the distal tip  26  to engage the vessel. 
         [0040]    In other examples, the dissolvable band  34  may have an initial tensile strength that is less than the force exerted by the stent  20 . In these examples, the band  34  can break with little or no dissolution. Accordingly, the band may comprise a material that dissolves more slowly, for example, in a matter of minutes or hours. 
         [0041]      FIG. 3  depicts a stent  40  with a degradable barb guard  52  disposed about a single strut  48  and barb  42 . In this example, the barb  42  is monolithic with the stent  40  and may be formed, for example, by cutting from a cannula. The barb  42  is preferably biased towards an extended configuration where the distal tip  46  extends away from the strut  48 . In this example, the barb guard  52  holds the barb  42  in a retracted configuration and protects the distal tip  46 . The guard  52  prevents the barb  42  from springing towards the extended configuration and prevents direct contact between the tip  46  and a sheath (not shown) when the stent  40  is placed in the sheath. 
         [0042]    The barb guard  52  may comprise a material that is dissolvable, as described above with reference to  FIG. 2A . Additionally, or alternatively, the barb guard may comprise a frangible material. In a preferred example, the biasing force of the barb  42  is greater than the initial strength of the barb guard  52 . When the stent  40  is compressed in a sheath (not shown), the sheath provides a counterbalance to the biasing force of the barb, preventing the barb from extending. When the stent  40  is released from the sheath, however, the barb  42  presses against the barb guard  52  with a force that is sufficient to degrade the guard. The guard breaks, tears, severs, or the like, allowing the distal tip  46  to extend outwardly and to engage a surrounding vessel. 
         [0043]    The barb guard  52  may comprise any suitable biocompatible material. Examples of suitable materials include synthetic materials, such as polylactides, polyglycolides, polyvinyl acetate, polyester, polytetrafluoroethylene (PTFE), and polyurethane, and natural materials such as gelatin, extracellular matrix (ECM), and small intestinal submucosa (SIS). The guard  52  may be attached to the barb  42  and/or the stent  40  by any suitable process. For example, the guard may be formed as an independent strip or band and folded, wrapped, stretched, or otherwise placed about the barb and strut. A suitable biocompatible adhesive may be used to fix the guard to the stent, if necessary. In some examples, the guard may be molded or cast with the stent and barb. In other examples, the guard may be formed independently of the stent (e.g., by molding, extruding, casting, or the like) and attached to the stent in a separate step. 
         [0044]      FIG. 4  depicts a cross-sectional view of the stent  40  shown in  FIG. 3  and an exemplary degradable barb guard  52 . In this example, the barb guard retains the barb  42  and comprises a frangible structure that breaks to release the barb and expose the distal tip  46 . The guard  52  has a three-point attachment system  61  that attaches to the stent  40  and retains the barb  42  in a retracted configuration. The attachment system  61  here includes a retaining structure  60  and strut attachments  62 A,  62 B. The barb  42  is positioned within the retaining structure  60 , which, in the example shown, engages and retains the barb about its entire periphery. Attachments  62 A,  62 B are configured to form a “snap-on” attachment with strut  48 . The barb  42  may be placed within the retaining structure  60  by sliding the structure onto the barb, for example, when the barb is in its extended configuration. The barb can then be retracted to its retracted configuration and the attachments  62 A,  62 B snapped into place at respective portions of the strut  48 . Bridge structures  64 A,  64 B connect the retaining structure  60  and attachments  62 A,  62 B and form a relatively linear connection therebetween. 
         [0045]    The stent  40  may then be placed within a sheath with the barb guard  52  retaining the barb  42  in a retracted configuration. The barb  42  is biased towards the extended configuration and, therefore, exerts a biasing force when it is in the retracted configuration. The sheath counterbalances the biasing force of the barb, preventing the barb from acting on the barb guard  52 . The barb guard  52  protects the distal tip  46  and prevents direct contact between the distal tip and the sheath. When the stent  40  is released from the sheath, the barb  42 , no longer constrained by the sheath, will exert its biasing force against the guard  52 . The frangible retaining structure  60  will break to release the barb  42  and expose the distal tip  46 . 
         [0046]    As shown in  FIG. 4 , the barb guard  52  may comprise a score  54 . The score  54  acts as a stress-riser and facilitates degradation of the guard. The biasing force of the barb  42  is preferably greater than the strength of the barb guard  52  so that the guard breaks at the score  54  when the stent is released from the sheath. When the guard  52  breaks, the barb  42  becomes unprotected and the distal tip  46  can extend outwardly. At this point, the barb guard is still secured to the stent via attachments  62 A,  62 B and bridge structures  64 A,  64 B. The barb guard may remain attached to the structure throughout the life of the prosthesis. Alternatively, the barb guard may comprise a dissolvable material, as described above, that dissolves over a period of time, for example over days, weeks, or months. 
         [0047]      FIG. 5  depicts another example of a stent  80  with a degradable barb guard  92 . In this example, the guard comprises a degradable blunt cap  94 , hereinafter referred to as a blunt, that covers the distal tip  86  of the barb  82 . The blunt  94  may be provided, for example, by applying a degradable material over the distal tip  86 . Suitable applications include dipping, molding, and the like. The degradable blunt  94  covers and protects the distal tip  86  and, when the stent is disposed within a sheath, prevents direct contact between the distal tip and the sheath. The blunt  94  is preferably made of a material that dissolves or dissociates as the stent is released into a vessel, as described above with reference to  FIG. 2A . The material is preferably formulated so that the blunt dissolves quickly (e.g., in seconds), to allow the distal tip  86  to engage the vessel immediately after the stent  80  is expanded. 
         [0048]    Throughout this specification various indications have been given as to preferred and alternative embodiments of the invention. However, it should be understood that the invention is not limited to any one of these. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting. 
         [0049]    The disclosures in U.S. patent application No. 61/172,987, from which this patent application claims priority, and in the abstract accompanying this application, are incorporated herein by reference.