Patent Publication Number: US-2003236563-A1

Title: Stent delivery catheter with retention bands

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates to intraluminal stenting, and in particular, to a stent delivery catheter having at least one elastic band being disposed around a balloon to reduce slippage between the stent and the balloon.  
       BACKGROUND OF THE INVENTION  
       [0002] Intraluminal stenting is useful in treating tubular vessels in the body which are narrowed or blocked and it is an alternative to surgical procedures that intend to bypass such an occlusion. When used in endovascular applications, the procedure involves inserting a prosthesis into an artery and expanding it to prevent collapse of the vessel wall.  
       [0003] Percutaneous transluminal angioplasty (PTCA) is used to open coronary arteries which have been occluded by a build-up of cholesterol fats or atherosclerotic plaque. Typically, a guide catheter is inserted into a major artery in the groin and is passed to the heart, providing a conduit to the ostia of the coronary arteries from outside the body. A balloon catheter and guidewire are advanced through the guiding catheter and steered through the coronary vasculature to the site of therapy. The balloon at the distal end of the catheter is inflated, causing the site of the stenosis to widen. Dilation of the occlusion, however, can form flaps, fissures or dissections which may threaten re-closure of the dilated vessel. Implantation of a stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.  
       [0004] A stent is typically a hollow, generally cylindrical device formed from wire(s) or a tube and the stent is commonly intended to act as a permanent prosthesis. A stent is deployed in a body lumen from a radially contracted configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be either radially self-expanding or expandable by the use of an expansion device. The self expanding stent is made from a resilient material while the device-expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during an angioplasty procedure by using a balloon catheter bearing the compressed stent which has been loaded onto the balloon. The stent radially expands as the balloon is inflated, forcing the stent into contact with the body lumen, thereby forming a support for the vessel wall. Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the balloon catheter.  
       [0005] A balloon of appropriate size is first used to open the lesion using a sufficient inflation pressure. The process can be repeated with a stent loaded onto a balloon. Direct stenting involves simultaneously performing angioplasty and stent implantation using a stent mounted on a dilatation balloon. After the balloon is withdrawn, the stent remains as a scaffold for the injured vessel. In particular, the present invention relates to stents which can be delivered to a body lumen and which can be deployed at a treatment site by expanding the stent radially from a crimped state into an expanded state in which the stent supports the walls of the vessel at the treatment site. As noted above, the radial expansion is achieved by inflating a balloon on which the stent is located. One problem that can arise with this type of stent delivery system is that the stent may accidentally be displaced on the balloon as the delivery system negotiates torturous body vessels along its path to the treatment site. In order to ensure proper placement of the stent at the treatment site, one must avoid relative movement between the stent and the balloon. One means by which this risk of relative movement between the balloon and stent may be lessened is to form pillows on the balloon on either side of the stent to help prevent the stent from slipping off the balloon. Another means of achieving this object is to securely mount the stent onto the balloon through a heat set operation that forms pillows within stent struts, thus encapsulating the stent with the balloon.  
       SUMMARY OF THE INVENTION  
       [0006] The present invention relates to a stent-delivery catheter including a balloon having a generally cylindrical body and at least one narrow elastic band disposed around the cylindrical body. The balloon may have several elastic bands mounted at spaced apart locations thereon. The elastic bands provide improved grip between the balloon and a stent through increased friction and/or mechanical engagement with the edges of the bands. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0007] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended drawings in which:  
     [0008]FIG. 1 is a longitudinal view of a distal portion of a stent delivery catheter in accordance with the invention, shown with the balloon partially inflated;  
     [0009] FIGS.  2 - 4  are enlarged views of modified forms of the invention shown in FIG. 1;  
     [0010]FIG. 5 is a longitudinal view, partially in section, of a distal portion of a stent delivery catheter in accordance with the invention, shown with a stent mounted thereon and the balloon partially inflated;  
     [0011]FIG. 6 is an arrangement of elastic bands mounted to a balloon catheter in accordance with the invention;  
     [0012]FIG. 7 is an alternative arrangement of elastic bands for application to a balloon catheter in accordance with the invention;  
     [0013]FIG. 8 is another alternative arrangement of elastic bands for application to a balloon catheter in accordance with the invention; and  
     [0014]FIG. 9 is yet another alternative arrangement of elastic bands for application to a balloon catheter in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0015] Applicant&#39;s invention is useful with any non self-expanding stent, such as those stents designed for delivery by a balloon. The stent may be generally cylindrical, and it may be mounted on a tubular balloon. FIG. 1 shows catheter  10 , including shaft  15  and balloon  20 , which can retain a stent thereon during delivery. One or more elastic bands  30  are mounted around generally cylindrical body  25  of balloon  20 . Cylindrical body  25  may be centrally located between proximal and distal cones  24 ,  26 , respectively. Proximal and distal cones  24 ,  26  terminate in proximal and distal ends  22 ,  28 , respectively, which are adapted to be sealingly mounted on catheter shaft  15 . Catheter shaft  15  comprises at least one lumen there through to provide fluid communication between balloon  20  and an inflation apparatus (not shown) connected to the proximal end of catheter  10 . Catheter  10  may be fixed wire type, wherein balloon  20  is mounted adjacent the distal end of a shaft comparable to a steerable guide wire. Alternatively, catheter  10  may be of the rapid exchange type, having a short distal guidewire lumen, or catheter  10  may be of the over-the-wire type, having a full length guidewire lumen.  
     [0016] In FIG. 1, balloon  20  is depicted in a partially inflated configuration as it would appear prior to full deflation and mounting a stent thereon, or after deployment of a stent. Balloon  20  can be made according to a variety of stretch blow molding processes that are well known to those skilled in the arts of dilatation and stent delivery balloons. Balloon  20  can be made from single or multiple layers of thermoplastics such as polyolefins, polyurethanes, polyamides, blends or copolymers that include these materials, or other polymers known to be suitable for dilatation and stent delivery balloons. The inelastic properties of such balloons cause wings or flaps to develop when the balloon is deflated. The balloon wings are wrapped compactly around shaft  15  to form as low a deflated profile as possible.  
     [0017] One or more elastic bands  30  are located around balloon  20  to improve retention of stent  40  thereon, as depicted in FIG. 5. Bands  30  provide a high coefficient of friction relative to the adjacent material of balloon body  25 , thus reducing slippage of stent  40  on balloon  20 . Additionally, each band  30  has proximal and distal edges  33 ,  37 , respectively, which can mechanically engage with the inside of stent  40 , further enhancing retention of stent  40  on balloon  20 . In the example shown in FIG. 6, balloon  20  is surrounded by a multiplicity of individual bands  30  disposed with spaces there between, such that numerous band edges  33 ,  37  are provided for gripping stent  40 . If a stent dislodgement force is directed proximally, then distal band edges  37  can arrest proximal slippage of stent  40 , especially by interlocking with proximally facing internal edges of a pattern formed in stent  40 . Conversely, if a stent dislodgement force is directed distally, then proximal band edges  33  can arrest distal slippage of stent  40 , especially by interlocking with distally facing internal edges of a stent pattern. To provide numerous gripping edges  33 ,  37  on catheter  10 , the width of band  30  is substantially narrower than the length of body portion  25  such that body portion  25  can be encircled by a number of bands  30  with spaces there between.  
     [0018] In one example of the invention suitable for stent deployment within a coronary artery, balloon body portion  25  may measure 25 mm in length and be encircled by ten bands  30 , each having a width of about 1.0 mm and spacing there between also measuring about 1.0 mm. The quantity and width of bands  30  selected for a particular stent delivery catheter according to the invention will be determined by the combination of frictional properties of the chosen band material and the number of band edges  33 ,  37  deemed necessary to achieve a desired stent retention force.  
     [0019] Elastic bands  30  have diameters, tensile strength, and elongation properties that have been selected such that bands  30  apply radial compressive force to balloon  20  at all achievable balloon diameters, which range from the deflated profile to the fully inflated, nominal stent deployment size. By having a relaxed diameter smaller than the deflated profile of catheter  10 , elastic bands  30  can grip balloon  20  to retain stent  40  thereon, and to also keep individual bands  30  from slipping off of balloon  20  before, during or after stent deployment. Additionally, bands  30  will retain the balloon wings folded tightly around shaft  15 .  
     [0020] Bands  30  may have tensile strength and elastic characteristics that provide minimal restriction of the inflation of balloon  20  to its nominal, stent deployment diameter. For bands  30  to maintain contact with stent  40  during expansion of stent  40  by balloon  20 , bands  30  stretch radially, without significantly indenting balloon  20 . For elastic band  30 , the maximal stretched diameter that is achievable without plastic yield or structural failure is larger than the nominal inflated diameter of balloon  20 . For example, a stent delivery balloon  20  may have a nominal inflated diameter of 3.0 mm and a deflated profile of about 0.75 mm in diameter. During inflation of balloon  20 , the length, or circumference, of elastic band  30  would stretch 400% without significantly constraining balloon expansion. The minimal restriction of balloon  20  during inflation may be achieved by making band  30  from an elastic material capable of elongation substantially greater than 400%.  
     [0021] FIGS.  2 - 4  illustrate a variety of conditions that may be achieved by selecting elastic bands of different sizes and/or different elongation properties. FIG. 2 shows band  30  not protruding above the outer surface of partially inflated balloon body  25 , as may be accomplished by choosing a relatively tight-fitting elastic band  30 . FIG. 2 can also depict the fully inflated assembly that results from picking an elastic band having a maximal stretched diameter only slightly larger than the nominal inflated diameter of balloon  20 . Despite the flush surfaces of band  30  and balloon body  25 , band edges  33 ,  37  are still exposed for mechanical engagement with stent  40  because balloon body  25  must constrict to fit within the inside diameter of band  30 . Such an example may avoid slippage of stent  40  on balloon  20  by depending more on frictional engagement and less on mechanical engagement.  
     [0022]FIG. 3 shows band  30  protruding slightly above the outer surface of partially inflated balloon body  25 , as may be accomplished by choosing an elastic band  30  that does not fit as tightly as the example of FIG. 2. FIG. 3 can also depict the fully inflated assembly that results from picking an elastic band having a maximal stretched diameter larger than the example illustrated in FIG. 2. In this example, band edges  33 ,  37  are further exposed for better mechanical engagement with stent  40 , as compared to the previous example.  
     [0023]FIG. 4 shows band  30  protruding above the outer surface of partially inflated balloon body  25  by approximately the full wall thickness of band  30 . This result may be accomplished by choosing an elastic band  30  that does not fit as tightly as the example of FIG. 3. FIG. 4 can also depict the fully inflated assembly that results from picking an elastic band having a maximal stretched diameter even larger than the example illustrated in FIG. 3. In this example, band edges  33 ,  37  are even further exposed for better mechanical engagement with stent  40 . Such an example may avoid slippage of stent  40  on balloon  20  by depending more on mechanical engagement and less on frictional engagement.  
     [0024] Bands  30  may be made of biocompatible elastic or elastomeric materials. Suitable elastic materials may include natural or synthetic rubbers, such as latex, silicone or other compounds. Suitable elastomeric materials may include thermoplastic elastomers (TPE&#39;s) such as Texin®, a polyether or polyester based polyurethane resin available from Bayer Corporation, Pittsburgh, Pa., USA or block copolymers, such as C-Flex®, a styrenic TPE comprising styrene ethylene/butylene styrene (SEBS) sold by Consolidated Polymer Technologies, Clearwater, Fla., USA. Bands  30  may be individually molded or cut from a molded or extruded tube. Bands  30  may also be cut from flat sheets of material, then rolled to form a circular band with its ends joined.  
     [0025]FIG. 7 illustrates an alternative configuration of bands  30  according to the invention, wherein spaced-apart bands  30  are joined by longitudinal connectors  50 . One or more connectors  50  join sequential bands  30 , and may help maintain the desired position of bands  30  on balloon  20 . Connectors  50  may be useful to prevent band  30  from slipping off of balloon  20 . Longitudinal connectors  50  also provide additional surface area for frictional retention of stent  40 . Longitudinal connectors  50  may be incorporated into a unitary molded configuration of bands, or connectors  50  may be made separately and subsequently joined to bands  30 . In the latter example, connectors  50  need not be the same material as bands  30 , especially since longitudinal connectors  50  do not have the same elongation requirements as bands  30 .  
     [0026]FIG. 8 illustrates another alternative configuration of bands  30  according to the invention, wherein two separate pairs of spaced-apart bands  30  are joined by longitudinal connectors  50 . FIG. 9 illustrates yet another alternative configuration of bands  30  according to the invention, wherein two spaced-apart bands  30  are joined by helical connector  60 , which can encircle balloon body  25 . Helical connector  60  provides the same function as longitudinal connectors  50 . In addition, depending on the pitch angle of the helix, the edges of helical connector  60  may assist in stent retention in the same manner as band edges  33 ,  37 , described supra. Because of its circumferential component, helical connector  60  needs some degree of radial stretchability, as is required of bands  30 . Helical connector  60  may also be used with different combinations of elastic bands  30 , similar to the alternative configurations shown in FIGS. 7 and 8.  
     [0027] Elastic bands  30  and connectors  50 ,  60  may be mounted over balloon body  25  by any of several methods. Bands  30  may be stretched and loaded onto a generally tubular mounting tool (not shown). The mounting tool may then be slipped over deflated, wrapped balloon  20 , and bands  30  may be slid or rolled off of the tool into the desired compressive position on balloon  20 . Alternatively, bands  30  made from TPE may be forced into the desired location around balloon  20  by use of heat shrink tubing in a method well known to those of skill in the arts of making medical catheters. In this example, TPE bands  30  are formed slightly oversized by molding or extrusion. After sliding bands  30  into position around balloon  20 , heat shrink tubing is slipped over bands  30  and heat is applied to shrink the tubing and thermally reform bands  30  to a smaller diameter. Finally, the heat shrink tubing is removed.  
     [0028] Another alternative mounting process specific to silicone bands is to temporarily enlarge bands  30  for fitting over balloon  20 . The temporary enlargement can be achieved by immersing silicone bands  30  in a silicone tube swelling fluid such as Lenium® TS, which is a proprietary blend of 3M™ HFE-7100 and octamethyltrisiloxane. Lenium TS is available from Petroferm Inc., Fernandina Beach, Fla., USA. After positioning enlarged bands  30  around balloon  20 , the swelling fluid is permitted to evaporate, allowing bands  30  to shrink into engagement with balloon  20 .  
     [0029] While the invention has been particularly shown and described with reference to the embodiments and methods described, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.