Abstract:
Custom-length self-expanding stent delivery systems and methods enable precise control of prosthesis position during deployment. The stent delivery systems carry multiple stent segments and include a stent bumper for helping control the axial position of the stent segments during deployment. This enables the deployment of multiple prostheses at a target site with precision and predictability, preventing stent segment recoil and ejection from the delivery device and thus eliminating excessive spacing or overlap between prostheses. In particular embodiments, the prostheses of the invention are deployed in stenotic lesions in coronary or peripheral arteries or in other vascular locations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/944,282 (Attorney Docket No. 021629-003000US) now U.S. Pat. No. 7,300,456 which is a continuation-in-part of U.S. patent application Ser. No. 10/879,949 (Attorney Docket No. 021629-002700US), filed Jun. 28, 2004, all of which are hereby incorporated fully by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Stents are tubular prostheses designed for implantation in a vessel to maintain patency of the vessel lumen. Stents are used in various vessels throughout the body, including the coronary arteries, femoral arteries, iliac arteries, renal artery, carotid artery, vascular grafts, biliary ducts, trachea, and urethra. Stents are typically implanted by means of long, flexible delivery catheters that carry the stents in a compact, collapsed shape to the treatment site and then deploy the stents into the vessel. In some applications, balloon expandable stents are used. These stents are made of a malleable metal such as stainless steel or cobalt chromium and are expanded by means of a balloon on the tip of the delivery catheter to plastically deform the stent into contact with the vessel wall. In other applications, self-expanding stents are used. These are made of a resilient material that can be collapsed into a compact shape for delivery via catheter and that will self-expand into contact with the vessel when deployed from the catheter. Materials commonly used for self-expanding stents include stainless steel and elastic or superelastic alloys such as nickel titanium (Nitinol™).  
         [0003]     While self-expanding stents have demonstrated promise in various applications, such stents face a number of challenges. One such challenge is that in some cases the disease in a vessel may be so extensive that a stent of very long length, e.g. 30-200 mm, is called for. Currently available stents are typically less than 30 mm in length, and suffer from excessive stiffness if made longer. Such stiffness is particularly problematic in peripheral vessels such as the femoral arteries, where limb movement requires a high degree of flexibility in any stent implanted in such vessels.  
         [0004]     To overcome the stiffness problem, the idea of deploying multiple shorter stents end-to-end has been proposed. However, this approach has suffered from several drawbacks. First, currently available delivery catheters are capable of delivering only a single stent per catheter. In order to place multiple stents, multiple catheters must be inserted, removed and exchanged, heightening risks, lengthening procedure time, raising costs, and causing excessive material waste. In addition, the deployment of multiple stents end-to-end suffers from the inability to accurately control stent placement and the spacing between stents. This results in overlap of adjacent stents and/or excessive space between stents, which is thought to lead to complications such as restenosis, the renarrowing of a vessel following stent placement. With self-expanding stents the problem is particularly acute, because as the stent is released from the catheter, its resiliency tends to cause it to eject or “watermelon seed” distally from the catheter tip by an unpredictable distance. During such deployment, the stent may displace not only axially but rotationally relative to the delivery catheter resulting in inaccurate, uncontrollable, and unpredictable stent placement.  
         [0005]     Interleaving stents or stent segments such as those disclosed in co-pending U.S. patent application Ser. No. 10/738,666 (Attorney Docket No. 021629-000510US), filed Dec. 16, 2003, which is incorporated herein by reference, present even greater challenges to conventional delivery systems. Interleaving stents have axially extending elements on each end of the stent that interleave with similar structures on an adjacent stent. Such interleaving minimizes the gap between adjacent stents and increases vessel wall coverage to ensure adequate scaffolding and minimize protrusion of plaque from the vessel wall. However, such interleaving requires that the relative rotational as well as axial positions of the adjacent stents be maintained during deployment to avoid metal overlap and excessive gaps between stents. Conventional delivery systems suffer from the inability to control both the axial and rotational positions of self-expanding stents as they are deployed. These issues are addressed, in part, in co-pending U.S. patent application Ser. No. 10/879,949, which was previously incorporated by reference. “Watermelon seeding” of self-expanding stents, where the resiliency of the stents causes them to eject distally from the catheter tip by an unpredictable distance, continues to be a challenge.  
         [0006]     What are needed, therefore, are stents and stent delivery system that overcome the foregoing problems. In particular, the stents and stent delivery systems should facilitate stenting of long vascular regions of various lengths without requiring the use of multiple catheters. Such stents and delivery systems should also provide sufficient flexibility for use in peripheral vessels and other regions where long and highly flexible stents might be required. In addition, the stents and stent delivery systems should enable the delivery of multiple stents of various lengths to one or more treatment sites using a single catheter without requiring catheter exchanges. Further, the stents and stent delivery systems should facilitate accurate and repeatable control of stent placement and inter-stent spacing to enable deployment of multiple self-expanding stents end-to-end in a vessel at generally constant spacing and without overlap. In particular, the stents and delivery systems should enable the deployment of interleaving stents or stent segments with precision and control over the axial spacing of each stent or segment.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention provides prostheses, prosthesis delivery systems, and methods of prosthesis deployment that enable the precise and controllable delivery of multiple prostheses using a single delivery catheter. The prostheses, delivery systems, and methods of the invention provide for the precise control of prosthesis placement so that inter-prosthesis spacing is maintained at a constant and optimum distance. In some embodiments, both axial and rotational displacement of the prostheses relative to the delivery catheter is controlled during deployment, enabling the delivery of multiple prostheses that interleave with one another without overlap. The prostheses, prosthesis delivery systems, and methods of the invention further enable the length of prostheses to be customized in situ to match the length of the site to be treated. The invention is particularly useful for delivery of self-expanding prostheses, but balloon expandable prostheses are also contemplated within the scope of the invention. The invention is well-suited to delivery of stents to the coronary arteries and to peripheral vessels such as the popliteal, femoral, tibial, iliac, renal, and carotid arteries. The invention is further useful for delivery of prostheses to other vessels including biliary, neurologic, urinary, reproductive, intestinal, pulmonary, and others, as well as for delivery of other types of prostheses to various anatomical regions, wherever precise control of prosthesis deployment is desirable.  
         [0008]     In a first aspect of the invention, a catheter system for delivery of a stent to a body lumen includes a stent delivery catheter and a plurality of stent segments. The stent delivery catheter includes a sheath having a first lumen, a shaft extending through the first lumen and slidable relative to the sheath, and a stent bumper mounted to the shaft distally of the sheath and movable from a contracted shape to an expanded shape. The plurality of self-expanding stent segments is carried within the first lumen in a collapsed configuration, and the segments are adapted to resiliently expand from the collapsed configuration to an expanded configuration. The stent segments are deployable from the first lumen so as to expand into the expanded configuration, while the stent bumper is configured to engage a first stent segment during deployment thereof to maintain its position relative to an adjacent stent segment disposed proximal to the first stent segment.  
         [0009]     In a number of embodiments, the stent bumper in the expanded shape has an outer diameter sized to contact an inner wall of the body lumen. The bumper may thus prevent distal migration of the stent segments and be stable in the vessel, without tilting, deflecting or slipping. In some embodiments, at least a portion of the stent bumper includes a lubricious surface for contacting one or more of the stent segments. For example, a proximal surface or portion of the stent bumper may have such a coating.  
         [0010]     The bumper itself may have any of a number of suitable shapes, sizes and configurations, and may be made of any suitable material or combinations of materials. In some embodiments, for example, the stent bumper may comprise an expandable basket, a plurality of expandable blades, rods or petals, an expandable disk, a proximal portion of a nosecone at the distal end of the catheter shaft, or the like. In a preferred embodiment, the stent bumper comprises an inflatable balloon. In such embodiments, the catheter typically further includes an inflation lumen in the shaft (or elsewhere in the catheter), which is in fluid communication with the inflatable balloon. In some embodiments, the balloon is adapted to be deflated, positioned within the deployed first stent segment, and re-inflated to the expanded shape. Optionally, the balloon in the expanded shape may be adapted to further expand the deployed first stent segment. In an alternative embodiment, an elongate balloon is used, the balloon having an axial length at least as long as two stent segments. In one embodiment, for example, the axial length of the elongate balloon is between about 20 mm and about 250 mm. In such embodiments, the catheter may further include an inner sheath disposed over the balloon, with the inner sheath being retractable to expose a portion of the balloon to allow it to be inflated from the contracted shape to the expanded shape. In some embodiments, the balloon is adapted to be expanded within one or more deployed stent segments to further expand the segments.  
         [0011]     Any suitable stents or stent segments may be used. Examples of self-expanding stents are described in U.S. patent application Ser. No. 10/879,949, which was previously incorporated by reference, but any other suitable self-expanding stents or stent segments may be substituted in various embodiment. In some embodiments, balloon expandable stents may be used. In various embodiments, the stent segments may be made of Nitinol, other superelastic alloys, stainless steel, cobalt chromium, other resilient metals, polymers or any other suitable material. Additionally, each stent segment may have any suitable length. In some embodiments, for example, each stent segment has a length of between about 3 mm and about 30 mm, and more preferably between about 4 mm and about 20 mm. Furthermore, any suitable number of stent segments may be loaded onto the catheter in various embodiments. For example, some embodiments may include between 2 and 50 segments. In some embodiments, the catheter may additionally include a pusher slidably disposed over the shaft, proximal to the stent segments, for advancing the stent segments relative to the sheath or holding the stent segments in place while the sheath is retracted.  
         [0012]     In another aspect of the invention, a method of delivering a stent to a body lumen involves: positioning a stent delivery catheter in the body lumen, the delivery catheter carrying at least first and second stent segments; expanding a stent bumper on the delivery catheter; releasing the first stent segment from the delivery catheter into the body lumen proximal to the stent bumper, the first stent segment self-expanding into an expanded configuration in the body lumen, wherein the stent bumper engages the first stent segment during expansion thereof to maintain its position relative to the delivery catheter; and releasing the second stent segment from the delivery catheter into the body lumen adjacent to the first stent segment.  
         [0013]     In a preferred embodiment, expanding the stent bumper involves inflating a balloon. Optionally, such a method may further include, before releasing the second stent segment: deflating the balloon; positioning the deflated balloon within the expanded first stent segment; and inflating the balloon, wherein the balloon engages the second stent segment during expansion thereof to maintain its position relative to the delivery catheter and the first segment. In some embodiments, inflating the balloon within the first segment further expands the first segment. The method may further involve: deflating the balloon; positioning the deflated balloon within the expanded second stent segment; and inflating the balloon, wherein the balloon engages a third stent segment during expansion thereof to maintain its position relative to the delivery catheter and the first and second segments. These steps of deflating, positioning and inflating may be repeated as many times as desired to deploy a desired number of stent segments.  
         [0014]     In an alternative embodiment, before expanding the balloon, a portion of the balloon is exposed from the distal end of an inner sheath that is disposed over the balloon. Such a method may optionally further involve, before releasing the second stent segment: exposing an additional portion of the balloon from the distal end of the inner sheath, the additional portion disposed within the expanded first stent segment; and inflating the balloon, wherein the balloon engages the second stent segment during expansion thereof to maintain its position relative to the delivery catheter and the first segment. In some embodiments, inflating the additional portion of the balloon further expands the expanded first stent segment.  
         [0015]     Rather than inflating a balloon, in alternative embodiments expanding the stent bumper involves deploying one or more other structures on the catheter device. In one embodiment, for example, a basket of resilient polymer or metal mesh is expanded. In some embodiments, expanding the stent bumper involves releasing one or more shape-memory members from constraint. For example, the shape-memory member(s) may include blades, rods, petals, rings or the like, made of metal, polymer or other resilient material.  
         [0016]     The stents may be released from the delivery catheter via any suitable means. In one embodiment, for example, releasing each stent segment involves maintaining an axial position of the stent segments relative to the delivery catheter using a pusher member of the catheter and retracting an outer sheath disposed over the stent segments. Alternatively, releasing each stent segment may involve maintaining an axial position of an outer sheath disposed over the stent segments relative to the delivery catheter and advancing the stent segments out of a distal end of the sheath using a pusher member of the catheter.  
         [0017]     Further aspects of the nature and advantages of the invention will be apparent from the following detailed description of various embodiments of the invention taken in conjunction with the drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a side, partially cut-away view of a prosthesis delivery catheter according to one embodiment of the present invention.  
         [0019]      FIGS. 2A and 2B  are side cross-sectional views of a distal portion of a prosthesis delivery catheter with a stent bumper in a vessel according to one embodiment of the present invention, showing outer shaft retracted with prosthesis partially deployed, and prosthesis fully deployed, respectively.  
         [0020]      FIGS. 3A-3F  are side cross-sectional views of a distal portion of a prosthesis delivery catheter with a stent bumper in a vessel according to one embodiment of the present invention, demonstrating a method for deploying stents in a vessel.  
         [0021]      FIGS. 4A-4C  are side cross-sectional view of a distal portion of a prosthesis delivery catheter with a stent bumper in a vessel according to another embodiment of the present invention, demonstrating an alternative method for deploying stents in a vessel.  
         [0022]      FIG. 5  is a side cross-sectional view of a distal portion of a prosthesis delivery catheter with a stent bumper in a vessel according to another embodiment of the present invention.  
         [0023]      FIG. 6  is a side cross-sectional view of a distal portion of a prosthesis delivery catheter with a stent bumper in a vessel according to another embodiment of the present invention.  
         [0024]      FIG. 7  is a side cross-sectional view of a distal portion of a prosthesis delivery catheter with a stent bumper in a vessel according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Referring to  FIG. 1 , a first embodiment of a prosthesis delivery catheter  20  according to the invention is illustrated. Delivery catheter  20  may have any of various constructions, including those described in co-pending U.S. patent application Ser. No. 10/637,713, (Attorney Docket No. 021629-000340US), filed Aug. 8, 2003; Ser. No. 10/874,859 (Attorney Docket No. 021629-000350US), filed Jun. 22, 2004; and Ser. No. 10/884,616, (Attorney Docket No. 021629-000360US), filed Jul. 2, 2004, all of which are hereby incorporated by reference. Delivery catheter  20  has a handle assembly  21  and an elongated catheter body  22  that includes three concentric tubular shafts, all axially slidable relative to one another: an outer shaft  24 , a pusher  26 , and an inner shaft  28 . A distal portion of delivery catheter  20  is shown schematically and in partial cutaway view for clarity. The distal portion, as well as other portions of delivery catheter  20  may include additional features not shown. For example, a typical embodiment includes a guidewire tube/lumen for allowing passage of a guidewire. Such features are described in further detail, for example, in the co-pending patent applications described immediately above.  
         [0026]     Outer shaft  24  has a distal extremity  46  defining a first lumen  48 . A plurality of stents  50  (or stent segments) are disposed in a collapsed configuration within first lumen  48 . Stents  50  are preferably composed of a resilient material such as stainless steel or Nitinol so as to self-expand from the collapsed configuration to a radially expanded configuration when deployed from first lumen  48 . While stents  50  as illustrated have a wave-like or undulating pattern in a plurality of interconnected circumferential members, the pattern illustrated is merely exemplary and the stents of the invention may have any of a variety of strut shapes, patterns, and geometries. From 2 up to 10 or more stents may be carried by outer shaft  24 . Optionally, a valve member  49  is mounted within first lumen  48  to facilitate separating those stents  50  to be deployed from those to remain within outer shaft  24 , as described in co-pending U.S. patent application Ser. No. 10/412,714 (Attorney Docket No. 021629-000330US), filed Apr. 10, 2003, which is incorporated herein by reference.  
         [0027]     Coupled with inner shaft  28  is an expandable stent bumper  60 . In various embodiments, stent bumper  60  may comprise an expandable wire or mesh basket, an expandable ring, shape-memory members such as petals, blades, prongs or other protrusions, or any of a number of other configurations. In a preferred embodiment, as shown, stent bumper  60  is an inflatable balloon. In some embodiments, stent bumper  60  is inflatable via an inflation lumen disposed within inner shaft  28 . Such an inflation lumen may alternatively be disposed on an outer surface of inner lumen  28  or the like. In some embodiments, stent bumper  60  may be attached to, or a proximal extension of, a nosecone  36  of delivery catheter  20 . When expanded, stent bumper  60  helps control the deployment of stents  50 . For example, if stent bumper  60  is expanded and a stent  50 ′ is deployed out of the distal end of catheter body  46 , stent bumper  60  has a diameter large enough, in its expanded configuration, to stop deployed stent  50 ′ from moving distally, thus preventing “watermelon seeding” of stent  50 ′. The operation of stent bumper  60  will be described further below with reference to subsequent drawing figures.  
         [0028]     Handle assembly  21  has a rotatable retraction knob  52  coupled to a shaft housing  53 , to which outer shaft  24  is fixed. By rotating retraction knob  52 , outer shaft  24  may be retracted proximally relative to pusher  26  and inner shaft  28 . A switch  56  engages and disengages pusher  26  with outer shaft  28 , so that pusher  26  either moves with outer shaft  24  or remains stationary as outer shaft  24  is retracted. Indicia  58  on shaft housing  53  indicate the extent of retraction of outer shaft  28  by distance, number of stents, or other suitable measure. Other aspects of handle assembly  21  are described in co-pending application Ser. No. 10/746,466 (Attorney Docket No. 021629-002200US), filed Dec. 23, 2003, which is hereby incorporated by reference. Except as stated otherwise, any of the embodiments of the stent delivery catheter described below may incorporate the features, and be otherwise constructed as, just described.  
         [0029]     With reference now to  FIGS. 2A and 2B , in some embodiments, a stent delivery catheter  180  includes an stent bumper  160  comprising an inflatable balloon. Delivery catheter  180  has a plurality of stents  182  disposed in an outer shaft  184 . An inner shaft  186 , with a distal nosecone  190 , extends through outer shaft  184  and stents  182  and is axially movable relative thereto. A pusher shaft (not shown) is slidably disposed over inner shaft  186  and engages stents  182  for purposes of deploying stents  182  from outer shaft  186  and repositioning the remaining stents  182  within outer shaft  186 , as in earlier embodiments. In this embodiment, stents  182  comprise a plurality of struts  191  forming a series of rings  192  interconnected at joints  193 . Each ring  192  has a series of closed cells  194  interconnected circumferentially and having an “I” shape in the unexpanded configuration. Other aspects of stents  182  are described in co-pending U.S. application Ser. No. 10/738,666, which was previously incorporated by reference.  
         [0030]     As outer shaft  184  is retracted to deploy one or more stents  182 , at least a distal ring  192 ′ is configured to expand into engagement with stent bumper  160  before the entire length of stent  182  is deployed from outer shaft  184  ( FIG. 2A ). Once distal ring  192 ′ is engaged with stent bumper  160 , the remainder of stent  182  is deployed ( FIG. 2B ), stent bumper  160  thus preventing “watermelon seeding” of stent  182  from catheter  180 . Each stent  182  has at least two, and preferably four or more rings  192 , each ring being about 2-5 mm in length, giving stent  182  an overall length of at least about 8-20 mm. Of course, stents of shorter or longer length are also contemplated within the scope of the invention. Lesions longer than each stent  182  may be treated by deploying multiple stents  182  end-to-end. Advantageously, each stent  182  can be deployed precisely at a desired spacing from a previously-deployed stent  182  because stent bumper  160  prevents unwanted overlapping of, or gaps between, stents  182  caused by watermelon seeding.  
         [0031]     Rings  192  are preferably formed from a common piece of material and are integrally interconnected at joints  193 , making joints  193  relatively rigid. In this embodiment, the majority of flexibility between rings  192  is provided by struts  191  rather than by joints  193 . Alternatively, joints  193  may comprise welded connections between rings  192  which are also fairly rigid. As a further alternative, joints  193  may comprise hinge or spring structures to allow greater deflection between adjacent rings  192 .  
         [0032]     In various alternative embodiments, any of a number of alternative stents with alternative designs, shapes, sizes, materials and/or the like may be used. For example, a number of exemplary self-expanding stents that may be used with delivery catheter  180  are described in co-pending U.S. patent application Ser. Nos. 10/879,949 and 10/738,666, which were previously incorporated by reference. Stents  192  may be made of any suitable material, such as but not limited to Nitinol™, a superelastic alloy, stainless steel, cobalt chromium, other resilient metals, resilient polymers or the like. In some embodiments, stent  192  may be balloon expandable, rather than self-expanding, although this description focuses on the preferred self-expanding embodiments.  
         [0033]     Various alternative types of interconnecting structures between adjacent stents and between the stents and the pusher shaft are also possible within the scope of the invention, including those described in co-pending application Ser. No. 10/738,666, previously incorporated by reference. Such interconnecting structures may also be breakable or frangible to facilitate separation as the stent expands. In addition, a mechanism such as an expandable balloon or cutting device may be disposed at the distal end of delivery catheter  180  to assist in separating stents  192  upon deployment. Further, the interconnections between stents may be different than the interconnection between the proximal-most stent and the pusher shaft. For example, the pusher shaft may have hooks, magnets, or other mechanisms suitable for releasably holding and maintaining traction on the proximal end of a stent until it is deployed.  
         [0034]     Referring to  FIGS. 3A-3F , a method for deploying stents in a vessel is shown schematically. In  FIG. 3A , a stent delivery catheter  210  is positioned within a vessel V, such that a nosecone  212  attached to the distal end of an inner shaft  216  of catheter  210  is distal to a lesion L. A stent bumper  260  coupled with inner shaft  216  is expanded (in this case an inflated balloon) to contact the vessel wall. Multiple stents  250  (or stent segments) are housed within an outer shaft  220  or sheath of catheter  210 , and a pusher  214  is used to maintain the axial position of stents  250  relative to outer shaft  220 .  
         [0035]     In  FIG. 3B , outer shaft  220  is retracted relative to stents  250  and inner shaft  216 , while pusher  214  maintains the relative axial position of stents  250 . As outer shaft  220  is retracted, a distal stent  250 ′ begins to be deployed out of its distal end. Distal stent  250 ′ contacts stent bumper  260 , which prevents stent  250 ′ from ejecting (“watermelon seeding”) distally.  FIG. 3C  shows distal stent  250 ′ fully deployed within the vessel V.  
         [0036]     As shown in  FIG. 3D , after a first distal stent  250 ′ has been fully deployed, stent bumper  260  may be deflated, repositioned with distal stent  250 ′, and re-expanded. In some cases, this re-expansion helps further expand distal stent  250 ′, thus enhancing its ability to prop open the vessel V. Turning to  FIG. 3E , after re-expansion of stent bumper  260 , outer shaft  220  may again be retracted relative to stents  250  and inner shaft  216 , thus deploying a second distal stent  250 ″. Second distal stent  250 ″ contacts stent bumper  260 , thus again avoiding watermelon seeding, which might cause stents  250 ″ and  250 ′ to overlap.  FIG. 3F  shows first distal stent  250 ′ and second distal stent  250 ″ fully deployed within the vessel. This process may be repeated as many times as desired, to deploy as many stents  250  (or stent segments) as desired.  
         [0037]     Referring now to  FIGS. 4A-4C , in an alternative embodiment, a stent delivery catheter  310  may include a stent bumper  360  that comprises an elongate inflatable balloon. In such an embodiment, first distal stent  250 ′ is deployed the same way as shown in  FIGS. 3A-3C . As shown in  FIG. 4A , after deployment of first distal stent  250 ′, a sheath  322  disposed over a proximal portion of stent bumper  360  is retracted proximally to expose an additional portion of stent bumper  360 , and the newly exposed portion of the inflatable balloon stent bumper  360  is inflated within first distal stent  250 ′. As described above, stent bumper  360  in some embodiments may be used to further expand an already-expanded distal stent  250 ′.  
         [0038]     As shown in  FIG. 4B , second distal stent  250 ″ is then deployed from the distal end of outer shaft  220  to contact stent bumper  360 . In  FIG. 4C , second distal stent  250 ″ is fully deployed, sheath  322  has been retracted farther proximally, and an additional portion of stent bumper  360  has been inflated within second distal stent  250 ″. This process may be repeated as many times as desired to deploy as many stents  250  as desired.  
         [0039]     Referring now to  FIG. 5 , an alternative embodiment of a stent delivery catheter  410  includes an expandable wire structure  460  that acts as a stent bumper. Wire structure  460  acts analogously to the stent bumpers described above. In the embodiment shown, wire structure  460  is made of shape memory, super-elastic or other resilient material and assumes its expanded shape when exposed from the distal end of a sheath  422 . Sheath  422  may be retracted farther proximally to expose additional portions of wire structure  460  to help deploy additional stents  250 . In other embodiments, a wire ring or tube, expandable wire basket, mesh basket or the like may be pushed by a proximal pusher member or pulled by a puller coupled to its distal end to force the expandable stent bumper to buckle or otherwise expand.  
         [0040]     In another embodiment, and with reference now to  FIG. 6 , a stent delivery catheter  510  include a stent bumper  560  comprising multiple self-expanding petals  562  (or alternatively prongs, blades, bristles or the like) coupled to an outer shaft  522  slidable over inner shaft  216 . Petals  562  are normally disposed within nosecone  212  and deploy/expand when inner shaft  216 , to which nosecone  212  is attached, is advanced distally relative to outer shaft  522 , thereby exposing petals  562 . Nosecone  212  may be advanced further to allow greater expansion of petals  562  or to expose additional sets of petals  562 . Petals  562  may be made of metal, polymer or any other suitable resilient material(s).  
         [0041]     Referring now to  FIG. 7 , in another embodiment, a stent delivery catheter  610  includes a stent bumper  660  comprising multiple self-expanding prongs  662  coupled with inner shaft  216 . Before deployment, prongs  662  are disposed with a sheath  632  slidably disposed over inner shaft  216 . When sheath  632  is retracted proximally relative to inner shaft  216  and/or inner shaft  216  is advanced relative to sheath  632 , prongs  662  are exposed, thus allowing them to assume their expanded configuration, as shown. After deploying first stent  250 ′, which is prevented from watermelon seeding by stent bumper  660 , sheath  632  may be retracted farther proximally and/or inner shaft  216  may be advanced farther distally to expose a second set of prongs  662  (not shown). A second stent  250  may then be deployed to contact the second set of prongs  662 . In various embodiments, any number of stent bumpers  660 /sets of prongs  662  may be included, for promoting deployment of any number of stents  250 . Prongs  662  may be made of any resilient material, such as Nitinol, spring stainless steel, or other shape-memory or super-elastic materials.  
         [0042]     While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, improvements and additions are possible without departing from the scope thereof, which is defined by the claims.