Patent Publication Number: US-2006009832-A1

Title: Balloon catheter and method and system for securing a stent to a balloon catheter

Description:
RELATED APPLICATIONS  
      This application claims priority to U.S. Provisional Patent Application Ser. No. 60/586,510, filed Jul. 9, 2004, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND  
      The present invention relates to tissue-supporting medical devices and delivery systems. More particularly the invention relates to a balloon catheter and to a method and system for securing a stent to a balloon catheter.  
      Stents are expandable cylindrical devices that are implanted within a bodily lumen of a living animal or human to support the organ and maintain patency. Stents are typically introduced percutaneously, and transported transluminally until positioned at a desired location. The stent is then expanded either mechanically, generally by the expansion of a balloon positioned inside the stent to support the lumen. Once expanded within the lumen the stents become encapsulated within the body tissue. Stents can be biodegradable or remain a permanent implant.  
      Known stent designs include monofilament wire coil stents (U.S. Pat. No. 4,969,458); welded metal cages (U.S. Pat. Nos. 4,733,665 and 4,776,337); and, most prominently, thin-walled metal cylinders with axial slots formed around the circumference (U.S. Pat. Nos. 4,733,665, 4,739,762, and 4,776,337). Known construction materials for use in stents include polymers, organic fabrics, biocompatible metals, such as, stainless steel, gold, silver, tantalum, cobalt alloys, titanium, and shape memory alloys such as Nitinol, and biodegradable polymers and metal alloys.  
      U.S. Pat. Nos. 4,733,665; 4,739,762; 4,776,337; 6,241,762; and 6,562,065 disclose expandable and deformable stents in the form of thin-walled tubular members with axial slots allowing the members to be expanded radially outwardly by a balloon into contact with a body passageway.  
      Generally stents are delivered after dilation of a lumen by percutaneous transluminal angioplasty (PTA) or atherectomy. When stents are delivered without a predilation process, the process is called direct stenting. With either type of procedure it is important to retain the stent on the balloon catheter while the catheter is advanced through the body lumen to the location where the stent will be implanted. If the stent moves on the balloon or is dislodged from the balloon it may not be accurately delivered to the lumen. It is also important that, after expansion, the stent is no longer adhered to the balloon.  
      The systems for securing stents onto balloons for delivery include systems for crimping or compressing the stent onto the balloon to achieve adherence. Crimping is often used in combination with another technique to increase adherence. For example, stent securing systems including adhesives are described in U.S. Pat. Nos. 6,682,553 and 6,635,078. Other systems for improving adherence involve deformation of the balloon beneath the stent to extend portions of the balloon between cells in the stent creating improved retention. Examples of such balloon deformation systems are described in U.S. Pat. Nos. 6,309,402 and 6,666,880.  
      However, there is a need to retain stents more securely on a balloon catheter without using adhesives that may create difficulties in releasing the stent from the balloon and without adversely affecting any drug on the stent.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a balloon catheter with a pillow and a method and system for securing a stent to a balloon catheter.  
      In accordance with one aspect of the invention, a balloon catheter/stent assembly comprises a balloon catheter having an elongated catheter shaft, a balloon positioned at a distal end of the elongated catheter shaft, and an inflation lumen extending within the elongated catheter shaft for inflation and deflation of the balloon; a stent mounted and crimped onto the balloon, the mounted and crimped stent having an outer diameter; and at least one pillow on the catheter balloon, the at least one pillow formed from the balloon and having an outer diameter larger than the outer diameter of the mounted and crimped stent, wherein the at least one pillow is formed adjacent to an end of the stent.  
      In accordance with another aspect of the invention, a system for securing a stent to a balloon catheter comprises a restrainer having a restrainer lumen configured to receive a balloon catheter with a stent crimped thereon, the restrainer having a first inner diameter which is approximately equal to an outer diameter of the crimped stent and a second inner diameter which is greater than an outer diameter of the crimped stent providing a step within the lumen of the restrainer; a catheter pressuring device for pressurizing the balloon within the restrainer to form at least one pillow adjacent to an end of the stent, the at least one pillow having an outer diameter larger than the outer diameter of the crimped stent; and a heating device for heating the at least one pillow to set the pillow shape.  
      In accordance with an additional aspect of the invention, a method of forming a catheter/stent assembly comprises the steps of crimping a stent onto a balloon of a balloon catheter; positioning at least a portion of the crimped stent and balloon into a restrainer having a first inner diameter which is approximately equal to an outer diameter of the crimped stent and a second inner diameter which is greater than an outer diameter, wherein the portion of the restrainer with the first inner diameter is placed around the stent and the portion of the restrainer with the second inner diameter is positioned adjacent an end of the stent; pressurizing the balloon within the restrainer to form at least one pillow adjacent to an end of the stent, the at least one pillow having an outer diameter larger than the outer diameter of the crimped stent; and heating the at least one pillow to set the pillow shape. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described in greater detail with reference to the preferred embodiments illustrated in the accompanying drawings, in which like elements bear like reference numerals, and wherein:  
       FIG. 1  is a side cross sectional view of a distal end of a stent delivery system including a balloon catheter and a stent, with the balloon catheter in a deflated condition and the stent in an unexpanded condition.  
       FIG. 2  is a side cross sectional view of the stent delivery system of  FIG. 1  with the balloon catheter in an inflated condition and the stent in an expanded condition.  
       FIG. 3  is a side cross sectional view of a restrainer configured with the stent delivery system received in the lumen of the restrainer for formation of stent retaining pillows.  
       FIG. 4  is a schematic perspective view of a system for securing a stent to a balloon catheter. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  illustrates a catheter/stent apparatus  10  including a balloon catheter  20  with a stent  30  mounted thereon. The balloon catheter  20  has an elongated catheter shaft  22 , only a portion of which is shown. A balloon  24  is positioned at a distal end of the elongated catheter shaft  22  and an inflation lumen  26  extends within the elongated catheter shaft for inflation and deflation of the balloon.  
      The stent  30  which is mounted and crimped onto the balloon  24  may be any known stent, for example, the stent can be a drug delivery stent such as those illustrated in U.S. patent Publication No. 2003/0199970, published on Oct. 23, 2003, which is incorporated herein by reference in its entirety. The stent  30  is positioned on the balloon  24  with a small portion of the balloon extending beyond each end of the stent. This extension of the balloon  24  beyond the end of the stent  30  helps to ensure complete expansion of the stent.  
      The catheter can be any of the known angioplasty or stent delivery catheters. As shown in  FIGS. 1 and 2 , the catheter  20  is preferably a rapid exchange catheter having a short guidewire lumen  28  beneath the balloon  24  and a proximal guidewire opening  29  which is closer to the balloon than to the proximal end of the catheter.  
      As shown in  FIG. 1 , at least one and preferably two pillows  40  are provided on the catheter balloon  24  between the ends of the stent  30  and the ends of the balloon. The pillows  40  are formed from the material of the expandable balloon  24  and are formed by expansion and heat setting the balloon after mounting and crimping the stent on the balloon. The pillows  40  extend radially outward from the outer surface of the uninflated balloon  24  to help to retain the stent  30  onto the balloon during stent insertion. The pillows  40  have an outer diameter which is larger than the outer diameter of the mounted and crimped stent  30 . When the pillows  40  are formed adjacent to the ends of the stent  30  they improve stent retention over crimping alone.  
      As shown in  FIG. 2 , when the balloon  24  is expanded, the pillows  40  expand to essentially the same diameter as the remainder of the balloon  24  and essentially disappear.  
       FIG. 3  shows a cut-away view of a system of sheaths which form a restrainer  50  over the catheter/stent assembly  10  for the pillowing process. The sheaths include a proximal sheath  52 , a center sheath  54 , and a distal sheath  56 . The sheaths are assembled over the catheter/stent assembly  10  to form gaps  58  at the ends of the stent  30 . Heat and pressure of the pillowing process will form the balloon  24  into these gaps to create the pillows  40  or small lumps that prevent the stent from sliding on the balloon  24  during use.  
      The proximal sheath  52  is a simple substantially cylindrical sheath having an inner diameter which is sufficiently large to fit over the catheter shaft and a portion of the end of the balloon  24  without the stent  30 . The center sheath  54  is stepped with a first inner diameter which is approximately equal to an outer diameter of the crimped stent  30  and a second larger inner diameter which is greater than an outer diameter of the stent providing a step within the lumen of the sheath. The distal sheath  56  is also stepped with a first inner diameter which is the same as that of the proximal sheath  52  and a second larger diameter which is the same as the larger diameter of the center sheath  54 . The sheaths are each slid over the catheter  20  and stent  30  and assembled in a manner such that the gaps  58  are formed precisely adjacent the ends of the stent for formation of the pillows  40 .  
      The gaps  58  and the pillows which are formed in these gaps can have a length of about 0.5 to about 5.0 mm, preferably about 1 to about 2.5 mm. The gaps  58  are preferably formed to create a pillow having a height above the maximum crimped outer diameter of the stent which is approximately the same as the thickness of the stent wall. More particularly, the height which the pillow extends beyond the outer diameter of the crimped stent is about ½ to about 3 times the thickness of the stent wall. A difference between an outer diameter of the crimped stent and an outer diameter of the pillow can be about 0.01 to about 1.0 mm, preferably about 0.05 to about 0.3 mm, and more preferably about 0.1 to about 0.2 mm.  
      Each of the sheaths  52 ,  54 ,  56  can be provided with one or more slits  59  or perforations, i.e. two slits on one end of each sheath. The slits  59  allow the sheaths to be peeled off of the catheter/stent assembly  10  after formation of the pillows  40 .  
      The multiple sheath system is used to allow the sheaths to be easily positioned over the catheter. Alternative sheath systems can also be used to form a restrainer, such as a single sheath with two steps or multiple sheaths that have been positioned partially coaxially to form the steps. One example of a multiple sheath system includes three small diameter sheaths placed with two of the sheaths proximal and distal of the pillow locations and a central sheath extending the length of the stent so that gaps occur between the small diameter sheaths at the locations of the pillows. A large diameter sheath can then be placed over the three small diameter sheaths to form the gaps into which the balloon is expanded to form the pillows.  
      As shown in  FIG. 4 , once the sheaths have been positioned over the catheter/stent assembly  10  the pillows can be expanded and set. The catheter is connected to a pressure source  60  for pressurization of the balloon. A pressure source  60  or pressure manifold provides a source of pressurized fluid. The pressurization of the balloon depends on the balloon material, construction, and size. According to one embodiment, the catheter is pressurized to between about 100 and about 200 psi, preferably about 110 to about 160 psi. The pressure is selected to be a pressure at which the balloon is expanded into the gaps  58  and is pressed into contact with the inner diameter of the sheaths to form the pillows  40 .  
      The catheter is maintained pressurized while the pillows are heat set. The heat set process applies heat to the pillow areas of the catheter, without causing significant heating of the stent. Particularly in the mounting of a drug delivery stent, heating of the stent could cause degradation of the drug or melting of any polymer drug carrier. The application of heat to the pillow can be performed in many ways. A hot gas jet  70  can be used as the heating device. The hot gas jet  70  aimed at the area to be heated provides heating of the pillow  40  without overheating the stent. In one example of a heating device, a gas source  72 , such as nitrogen, air, or inert gas, provides gas to a heater  74 , which heats the gas to be delivered by the hot gas jet  70 . The hot gas jet can be integrated into the heater as a hot box. The hot gas jet  70  shown in  FIG. 4  directs the hot gas at the catheter pillow area from two opposite directions. Other configurations of hot gas jets may include a single gas outlet or more than two gas outlets. The hot gas may be applied through a porous holding device, such as a foam pad.  
      The sheath system for retaining the catheter/stent assembly  10  during the pillowing procedure and for providing the gaps  58  into which the balloon expands to form the pillow is only one example of the type of retainer which can be used. Another retainer can include a mold which can be closed around the catheter/stent assembly and provides the gaps  58  for formation of the pillows. An adjustable mold can be used to accommodate different size stents.  
      The heating device including the hot gas jet  70  is only one example of the type of heating devices that can be used. Other heating devices such as a heating block can also be used. As long as the heat is concentrated at the area in which the pillow is to be formed, cooling of the adjacent stent area can be avoided. Shorter heating times also minimize any overheating of the stent. For example, the pillow areas can each be heated for about 20 to about 100 seconds, preferably about 25 to about 45 seconds. The hot gas temperature will be selected depending on the balloon material used. For example, a hot gas temperature of at least 175 degrees F., and preferably about 200 degrees F. is selected for a pillowing temperature for commonly used balloon of nylon or nylon-like materials.  
      Following the heating step, the formed pillows can be cooled by a cold jet or other cooling system while the internal catheter pressure is retained. Upon cooling, the catheter is removed from the pressure source or manifold and the sheaths are removed.  
      The final pillow configuration having a pillow diameter greater than a stent crimped diameter significantly improves retention of the stent  30  on the catheter  20 . This improved retention is due to the fact that the stent  30  would have to expand beyond its crimped diameter to pass over the pillows  40 . It has been found that pillows  40  extending out from the outer diameter of the stent a distance which is approximately equal to the thickness of the stent walls do not have any noticeable affect on the ability of the balloon catheter to deliver the stent to a target site through narrowed vessels. In fact, the balloon pillows  40  are sufficiently pliable that they compress during stent delivery when the catheter passes through tight spaces.  
      The stent according to the present invention can be used for supporting a variety of ducts or lumens within the body, such as coronary or peripheral arteries, lumens of the tracheal or bronchial tree, billiary ducts, and the like.  
      While the invention has been described in detail with reference to the preferred embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made and equivalents employed, without departing from the present invention.