Abstract:
Disclosed is a stent introducer apparatus comprising a introducer catheter, usually comprising clear polytetrafluoroethylene, and a pusher assembly that is configured to be able to deliver a stent, such as a self-expanding stent, within a tortuous duct or vessel, even if the introducer catheter becomes kinked during the procedure. In an embodiment for use in the biliary system, the pusher assembly includes a first tubular portion, comprising a material with high column strength, such as polyetheretherketone, and a shorter second tubular portion, which is made of a highly flexible material such as metal-braided polyimide or nititnol tubing, that is divided into a distal, stent-carrying section and a proximal, flexible section. The second tubular portion may be made of a smaller diameter that the first tubular portion to reduce possible impingement by the introducer catheter is the latter kinks during a procedure. At the junction between the stent-carrying and flexible sections is a pusher member to urge the stent from the distal end of the introducer catheter. In one aspect of the invention, the distal tip and pusher member tightly hold the stent to eliminate gaps so that the likelihood of the introducer catheter kinking at the contact point between the pusher member and stent is greatly reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority of provisional application Serial No. 60/180,453, filed Feb. 4, 2000. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to medical devices, more particularly to an apparatus for delivering an implantable prosthesis.  
         BACKGROUND OF THE INVENTION  
         [0003]    Placement of a stent within the biliary tree can be problematic in that the catheter delivery system must make a severe turn from the duodenum into the ostium in order to access the common bile duct. Current biliary and pancreatic stent delivery systems comprise an introducer catheter with the stent loaded at the distal end. A pusher catheter is used to deploy the stent from introducer catheter. Physicians strongly prefer that the delivery catheter be made of a clear material in order that they can see the stent within the catheter. This usually requires that the catheter be made of polytetrafluoroethylene (PTFE) which by the nature of the material, makes the catheter predisposed to kinking. When the introducer catheter kinks, it can impinge on the pusher catheter, preventing it from being able to advance the stent from the outer catheter. While the stent and pusher catheter serve to fill the lumen of the introducer catheter, making kinking within these portions less of a problem, the junction between stent and pusher is vulnerable point on the catheter where a severe kink can occur. If so, the pusher may not be able to traverse the catheter stricture to advance the stent. Some manufacturers avoid this problem because they use an axially contracting stent which overlaps with the distal end of the pusher, resulting in the most likely kinking point being reinforced by the stent and pusher from within. However, this system has other disadvantages in that stents that shorten are less desirable than non-contracting stents because of difficulty in placement. Non-shortening biliary stents, such as the ZA-STENT™ or SPIRAL Z™ Biliary Stents (Wilson-Cook Medical, Inc., Winston-Salem, N.C.), can be placed more accurately and provide superior coverage; however, the point on the catheter most susceptible to kinking is not protected by the stent, making kinking more of serious concern when PTFE is used for the introducer catheter. Another common problem with current biliary stent delivery systems is diminished recapture capability—the inability to retrieve the introducer system following stent delivery without having it become entangled within the stent or upon the introducer catheter itself. What is needed is a biliary and pancreatic stent introducer system that can still be deployed when the outer catheter kinks and that can be easily removed once the stent is deployed.  
         SUMMARY OF THE INVENTION  
         [0004]    The foregoing problems are solved and a technical advance is achieved in a stent introducer apparatus having a two-part pusher assembly with a lumen therethrough for introduction of a wire guide. The pusher assembly can be used to deploy a preloaded self-expanding stent from the distal end of an introducer catheter, such as a PTFE introducer sheath used to delivery a biliary or pancreatic stent. The pusher assembly comprises a first or proximal tubular portion that substantially fills the introducer catheter lumen and is made of a material with superior column strength, such as polyetheretherketone (PEEK), and a second or distal tubular portion which has a combination of good column strength and superior flexural properties, such as braided polyimide or nitinol, to distribute the severe bending force more evenly along the introducer catheter and help reduce the severity of kinking. Located at a point along the second tubular portion of the pusher assembly is a pusher member designed to urge the stent forward. The pusher member can comprise one or more separate elements attached to the second tubular portion or it can be an integral modification thereof than provides a mechanism for advancing or deploying the stent. In one embodiment, the pusher member comprises a pusher head made of metal or an insert-molded polymer that provides a broad surface for applying force to advance the stent. Typically, the stent is loaded while applying pressure against the pusher head to reduce any gap therebetween and help force any kinks experienced during the procedure to occur proximal to the pusher member, thereby not interfering with the ability of the pusher assembly to advance the stent from the introducer catheter.  
           [0005]    In another aspect of the invention, the pusher member is configured such that the proximal portion of the pusher member can more easily negotiate a kink in the introducer catheter during withdrawal of the pusher assembly following delivery. This can be accomplished by tapering the distal tubular portion. In the illustrative embodiment, a similar proximal taper occurs on the distal tip of the pusher assembly, located distal to the stent. The face of the pusher member contains a chamfer to help prevent it from digging into the inner wall of the introducer catheter. In one embodiment, there is a second member at the junction between the second tubular portion and the first tubular portion. This second member is tapered distally to help facilitate its advancement through any kink that might occur along the section of the introducer catheter that is distal to that point. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 depicts a partially sectioned side view of an illustrative embodiment of the present invention;  
         [0007]    [0007]FIG. 2 depicts a enlarged cross-sectional view of the embodiment of FIG. 1;  
         [0008]    [0008]FIG. 3 depicts a partially sectioned view of the embodiment of FIG. 1 in a kinked introducer catheter;  
         [0009]    [0009]FIG. 4 depicts a partially sectioned view of a second embodiment of a pusher member of the present invention;  
         [0010]    [0010]FIG. 5 depicts a cross-sectional view of an embodiment of the present invention in which the second tubular portion extends at least substantially the length of the first tubular portion; and  
         [0011]    FIGS.  6 - 7  depict cross-sectional views two embodiment of the present invention in which the first and second tubular portions or the pusher assembly comprise a single member. 
     
    
     DETAILED DESCRIPTION  
       [0012]    The present invention comprises a stent introducer apparatus  10 , an illustrative embodiment of which is depicted in FIGS.  1 - 2 . The stent introducer apparatus  10  comprises a pusher assembly  30  for advancing a stent  17  for deployment within a duct or vessel. In embodiment depicted in FIG. 1, the stent is a self-expanding biliary stent such as the COOK SPIRAL Z™ Stent; however, the type of stent is not considered important to the understanding of the invention. In the example in FIG. 1, the minimum size of the introducer catheter typically ranges from 8.0 to 8.5 FR (2.67 to 2.83 mm), depending on the stent used. The SPIRAL Z™ Biliary Stent, being somewhat larger than the ZA-STENT™ Biliary Stent, requires the larger introducer, while the smaller stent can be deployed from either sized introducer.  
         [0013]    As depicted in FIGS.  1 - 2 , the stent introducer apparatus  10  may further include an introducer catheter  11 , which in the illustrative embodiment, is made primarily of a substantially clear polymer such as PTFE. The pusher assembly  30  and the preloaded stent  17  are coaxially disposed within passageway  27  of the introducer catheter  11  with the stent  17  residing in the distal portion  34  of the introducer catheter until it is expelled from the distal end  21  thereof by advancement of the pusher assembly  30  or withdrawal of the introducer catheter  11 .  
         [0014]    The pusher assembly of FIGS.  1 - 2  comprises a first or proximal tubular portion  13  and a second or distal tubular portion  12 . The first and second tubular portions  12 , 13  can be formed as separate members and attached, or represent different portions of a single member, each having different physical properties. Each portion  12 , 13  has a lumen extending therethrough that is sufficiently large for accommodating an ancillary device such as a 0.035″ (0.89 mm) wire guide. The first tubular portion  13  can comprise a rigid or non-rigid member or portion thereof, depending on the application. In the illustrative embodiment, the first tubular portion  13  comprises a non-rigid polymer tube made of a material with superior column strength. Possible materials include, but are not limited to PEEK, polyvinyl chloride (PVC), polyimide, and polyurethane. The O.D. of the first tubular portion  13 , approximately 0.07″ (1.78 mm) in the illustrative example, is such that it takes up most of the I.D. of the passageway  27  of the introducer catheter  11 , thereby providing support thereto and reducing the likelihood and severity of kinking in the introducer catheter  11 . Maximizing the pusher catheter O.D. also adds column strength for pushing the stent from the catheter. The second tubular portion  12  extends distally from the first tubular portion  13 , to which it is joined, and comprises a tube made of a flexible material, also with sufficient column strength to allow the pusher assembly  30  to advance the stent from the introducer catheter  11 . In the illustrative embodiment, the second tubular portion  12  comprises a polyimide tube reinforced with a stainless steel braid. Other possible materials include PEEK or metal tubing such as nitinol or stainless steel, depending on the degree of bending that the introducer must undergo. Nitinol tubing exhibits good laterally flexibility and kink-resistance, but is generally stiffer than braided polyimide tubing. Both the pusher assembly  30  and the introducer catheter  11  are connected at their proximal ends to a well-known coaxial medical device handle (not illustrated) that permits the pusher assembly  30  to be advanced relative to the introducer catheter  11  for deployment of the stent  17 . An example of a suitable slider-type handle can be found on the previous-generation delivery systems for the Wilson-Cook SPIRAL Z™ and ZA-STENT™ Biliary Stents.  
         [0015]    As a means to push the stent  17  out of the introducer catheter, a pusher member  14  is affixed to, integrally formed with the second tubular portion  12 . In the illustrative embodiment, the pusher member  14  comprises a pusher head that includes a broad face  24  to contact the proximal end  31  of the stent and urge the stent forward until deployment has been achieved. The illustrative pusher member  14  can be made of metal such as 303 or 304 stainless steel, or it can comprise a polymer that is insert molded, bonded, or otherwise attached to the second tubular portion. The O.D. of the pusher member generally depends on the type of stent to be delivered. In the illustrative example, a SPIRAL Z™ Biliary Stent, which is deliverable through a 8.5 Fr (2.83 mm) introducer catheter, would have a 0.088″ (2.24 mm) O.D. pusher member  14 . The ZA-STENT™ Biliary Stent, which can be introduced through either a 8.0 or 8.5 Fr (2.67 or 2.83 mm) introducer, could have a 0.077″ O.D. (1.96 mm) pusher member  14  if the 8.0 Fr (2.67 mm) introducer is used. The dimensions of the pusher member  14  could vary further, depending on a number of factors, particularly the I.D. of the introducer catheter lumen  27 . Because of the desirability of having the pusher member  14  diameter be as close to the I.D. of the introducer catheter lumen  27  as possible, an optional chamfer  25  is included at the outside edge of the face  24  to help prevent the pusher member  14  from digging into the inner wall  28  of the introducer catheter  11  during advancement. In the illustrative embodiment, the pusher member  14  is placed over and glued to the second tubular portion  12  such that the contact point  22  between the two lies at an intermediate point along the second tubular portion  12 . In the illustrative embodiment, the pusher member  14  represents a junction  38  between two sections of the second tubular portion  12 . Proximal to the pusher member  14 , lies the flexible section  36  of the second tubular portion  12 , while distal to the contact point  22  lies the stent loading section of the second tubular portion  12 . While these two sections  35 , 36  comprise a single piece of reinforced polyimide tubing in the illustrative embodiment, it is also possible that they be constructed with different materials or properties insomuch that each section  35 , 36  is likely to experience bend stresses during introduction due to the presence of the preloaded stent  17  over the stent loading section  35 . The length of the stent loading section  35  corresponds to the length of the stent  17 . A distal tip  16 , made of PEBAX® (Atofina Chemicals, Philadelphia, Pa.) or a similar soft polymer with good bonding properties, is bonded to the distal end  37  of the second tubular portion  12  after the stent  17  has been preloaded thereon. The distal tip  16  may include barium sulfate or some other agent or marker to provide radiopacity. Both the distal tip  16  and distal end  21  of the catheter are rounded for atraumatic entry into the bile duct.  
         [0016]    The two-part pusher assembly  30  provides an advantageous combination of both strength and flexibility that is desirable for biliary access. The section of the second tubular portion  12  proximal to the contact point  22  provides the stent introducer apparatus  10  with the ability to make a tortuous bend, such as into the ostium of the common bile duct, by distributing the bending stresses over a large area (approximately 20 cm in the illustrative embodiment). In the illustrative embodiment, the second tubular portion  12  is made to have a smaller O.D., approximately 0.045″ (1.14 mm), to increase laterally flexibility. The first tubular portion  13  comprises the majority of the pusher assembly  30  because of the increased column strength and protection to the introducer catheter  11  it provides. For example, a pusher assembly  30  might measure  190  cm from the proximal end of the catheter (distal end of the handle) to the proximal end  31  of the stent  17 , wherein 160 cm of this length might comprise the first tubular portion  12  with only 30 cm comprising the flexible section  36  of the second tubular portion  12 . Generally, the flexible section should comprise about 10-20% of the pusher assembly  30  in biliary applications. For other applications, the actual length of the flexible section can be vary, depending on the application. For example, the entire stent introducer apparatus  10  could be made smaller for deploying vascular stents, or it could have utility in placing colonic stents where the anatomy can also produce severe angle that can be of concern. For biliary applications, the distance from the junction between the handle and catheter to the distal end  20  of the introducer apparatus should generally measure at least 200 cm for a typical adult patient. As shown in FIG. 2, the second tubular portion  12  is attached to the first tubular portion  13 , by a well-known bonding method, such as gluing. In the illustrative embodiment, a second member  15 , such as a band similar to pusher member  14 , and which is made of metal or plastic, is placed at the junction  29  between the distal and first tubular portions  12 , 13  and glued in place with the two portions overlapping each other by approximately 3-5 mm. FIG. 5 depicts an embodiment in which the second tubular portion  12  extends the entire length (or nearly the entire length) of the first tubular portion  13  such that the latter portion is essentially providing column strength and kink resistance (especially because of the increased diameter) to the proximal or remaining portion of the pusher assembly  12  proximal to initial junction  29  point. The second tubular portion  12  can be bonded along the length of the first tubular portion  13  or affixed at one or more points, such as junction  29 .  
         [0017]    FIGS.  6 - 7  depicts additional embodiments of the pusher assembly  30 . that comprise a single continuous piece of tubing in which is modified to produce a more flexible second tubular portion  12  and a more kink-resistant first tubular portion  13 . The embodiment of FIG. 6 depicts a single-piece tube in which the first tubular portion  13  is bumped down in diameter to form a thinner wall and therefore, more flexible first tubular portion  12 . Extrusion techniques to vary the diameter of thermoplastic tubing are well know in the catheter arts. In the illustrative embodiment, an optional braid  23  is added to the second tubular portion  12  to allow it to be more flexible and less prone to kinking. An optional second member  15 , such as that of FIG. 1, can be affixed over the transition zone  41  (or junction  29 ) between the two tubular portions  12 , 13  to facilitate negotiation of any kinks in the introducer catheter  11  that might form distal to that point. A thin layer  42  of polymer such as a shrink wrap or other type of polymer film, can be added to secure the braided portion  42  to the outer surface of the second tubular portion  12 . In another embodiment, FIG. 7 depicts a pusher assembly  30  that has been extruded as two materials having different physical properties such as different degrees of column strength and/or flexibility. The first material, comprising the first tubular portion  13 , blends with a second material comprising the second tubular portion  12  over a transition zone  41  from which the second tubular portion  12  extends distally, the second tubular portion  12  being generally more flexible than the proximal first tubular portion  13 . The two materials must be compatible for co-extrusion and can include different polymers or two different compounds (e.g., different durometers) of the same polymer. Methods of co-extruding different polymers to form a single length of tubing are well known in the catheter arts.  
         [0018]    In assembling the illustrative stent introducer apparatus  10 , the stent is loaded over the distal end  37  of the second tubular portion  12 , and then distal tip  16  is placed thereover and bonded thereto, thereby holding the stent  17  in place. While the distal tip  16  is being affixed to the pusher assembly  30 , pressure is applied such that the proximal end  31  of the stent  17  is forced tightly against the face  24  of the pusher member  14 . This virtually eliminates any gap at the contact point  22 , a gap which otherwise becomes a likely point of kinking when the introducer catheter is navigated through a severe bend, such as the common bile duct. The kink  39  generally occurs at that point along the introducer catheter  11  which experiences the greatest lateral bending forces during severe bending, this being largely determined by the degree of support provided by indwelling devices such as the pusher assembly  30  and the stent  17  itself. By reducing the weakness found at the contact  22  point between the pusher member  14  and the stent  17 , the most likely location of any kink  39  (FIG. 3) in the introducer catheter  11  will be the flexible section  36  of the second tubular portion  12  which lies between junction  29  and the proximal end  31  of the stent  17 . If a kink  39  develops within that section, it generally does not interfere with the ability of the pusher assembly  30  to slide within the introducer catheter  11  and expel the stent  17  therefrom. This is due to the pusher member  14  being distal to the kink  39  and in the case of the illustrative embodiment, the second tubular portion  12  is of a sufficiently small diameter such that the restriction of the introducer catheter lumen  27  still permits movement therethrough. Because this particular section of the introducer catheter  30  is flexible over an extended portion, any kink  39  that might occur is usually less severe than would be experienced in delivery systems of designs where the pusher system is stiff in comparison, and most of the bending force would be thus concentrated at the vulnerable contact point between the stent and the pusher member.  
         [0019]    The stent introducer apparatus  10  of FIGS.  1 - 2  is designed to facilitate recapture, i.e., removal of the pusher assembly  30  back through the deployed stent. A number of points on a typical introducer apparatus have the potential of snagging and catching a strut, or otherwise becoming ensnared in the stent after delivery. To reduce the possibility of this occurring in the present invention, the proximal surface  18  includes a taper  18  that has been added to the distal tip  16  of the stent pusher assembly  30 . In addition, proximal surface  19  of the pusher member  14  is also tapered as well. These tapers not only reduce the likelihood of an edge catching the stent during withdrawal, in the normal situation where the introducer catheter  11  is advanced by the physician after deployment to “recapture” the pusher assembly  30 , but the tapers  18 , 19  also help guide the introducer catheter  11  over the distal tip  16  and pusher member  14  rather than having the distal end  21  of the introducer catheter  11  becoming temporarily caught up. In addition, the proximal tapers  16 , 18 , especially that of the pusher member  14 , help provide a guide to traverse any strictures during withdrawal of the pusher assembly  30  if the introducer catheter  11  becomes kinked. It should be understood that the invention includes other shapes or modifications of the proximal surfaces  18 , 19  of the distal tip and pusher member, other than a simple taper, that would produce a surface or edge that has a reduced likelihood or catching on the stent.  
         [0020]    While the illustrative embodiment includes an expandable stent such as the SPIRAL Z™ Biliary Stent, knowledge of the type of stent to be used with the present invention, or how it is delivered is not essential for an understanding of the invention. Although the illustrative embodiment depicts a pusher member  14  to urge the stent  17  from the introducer catheter  11 , alternative embodiments of the present invention could include a modified pusher assembly  30  that engages with the stent in another manner rather than pushing against the proximal end  31  of the stent  17 . For example, the second tubular portion could extend into the lumen of the loaded stent and be frictionally engaged therewith. For example, FIG. 4 depicts a second embodiment of pusher member  14  that urges the stent  17  forward by engaging the struts or coils of the stent  17  from inside the stent lumen  45  via one or more engagement members  44  affixed over the shaft of the second tubular member  12 . These engagement members can be made of plastic or metal and vary in shape, number, and distribution along the stent loading portion  35  of the second tubular portion  12 . When the stent  17  is deployed and expands, the engagement members  44  no longer engage the stent  17 , permitting withdrawal of the pusher member  30 . Other embodiments could include a releasable engagement mechanism between the pusher assembly  30  and stent  17 . Because of the variety of medical procedures for which this invention can be used, as well as the wide variety of stents that can be deployed, further modifications of the stent introducer apparatus of the present invention additional to the embodiments described herein are within the spirit of the invention and the scope of the claims. The invention contemplates embodiments comprising and consisting of the disclosed examples.