Abstract:
A delivery system for a stent includes a catheter having an elongated inner member and at least one flexible, longitudinally oriented outer member that is operably attached to the distal portion of the inner member. A stent is placed concentrically around the flexible outer member, near the distal end of the catheter. When the elongated inner member is retracted, the flexible outer member folds longitudinally adjacent to the lumen of the stent causing the stent to be deployed. Another aspect according to the invention provides a method for delivering a stent to a treatment site within the vascular system.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to catheter deployment of stents. More specifically, the invention relates to a stent deployment system and method that uses a catheter but does not require a balloon to expand or place the stent at the target site. 
   BACKGROUND OF THE INVENTION 
   Balloon catheters are used in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel and improving blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow. 
   To prevent restenosis, short flexible mesh cylinders known as stents, constructed of metal or various polymers, are implanted within the vessel to maintain lumen size. Balloon-expandable stents are mounted on the periphery of the collapsed balloon portion of a balloon catheter at a diameter smaller than when deployed. During angioplasty, the balloon catheter carrying the stent is advanced through a network of tortuous blood vessels to the desired site. The balloon is inflated and expands the stent to a final diameter. After deployment, the stent remains in the vessel and the balloon is deflated, and the catheter is removed. 
   Although widely used, balloon catheters have significant limitations as stent delivery devices. The stent must be firmly attached to the exterior of the balloon, so that it does not become dislodged as the catheter passes through the vascular system to the target site. For this purpose, the stent is crimped to a sufficiently small diameter so that it grips the balloon. The shape of the balloon may be used to help secure the stent. Some catheter designs include sleeves that cover the ends of the stent, and stabilize it during passage through the vascular system. 
   The characteristics of the balloon including strength, flexibility and compliance are optimized to provide the desired performance. Nevertheless, problems may be encountered during expansion of the balloon and deployment of the stent. Frequently the stent does not cover the entire surface of the balloon. Consequently, as the balloon is pressurized, the areas of the balloon not covered by the stent expand first, causing uneven expansion and possibly deformation of the stent. In addition, after the balloon is deflated, it assumes an irregular shape, causing a relatively large effective diameter, and making retraction of the catheter from the vascular system difficult. Because of these limitations, it would be desirable to devise a mechanism for deploying a stent from a catheter that does not require a balloon. 
   To avoid the need for a balloon, various mechanical means to expand and deploy stents have been disclosed. U.S. Pat. No. 6,217,585 discloses a deployment device that consists of an expansion framework or cage near the distal end of the catheter. The cage consists of elongated strands coupled at opposite ends to the catheter and to a control means at the proximal end of the strands. Axial movement of the control device relative to the catheter either elongates the strands to radially collapse the cage, or axially reduces the distance between the strand ends for radial enlargement. U.S. Pat. No. 6,364,887 discloses a stent deployment device that comprises a catheter with an introducing head near the distal end of the catheter that transports the stent. On the circumference of the introducing head are radially expanding elements such as springs that, when released, press against the inside of the stent and cause it to expand. Both of these inventions have the disadvantage of requiring a fairly complex mechanism to enable the operator to control and engage the expansion mechanism without putting undue force on surrounding tissues. 
   U.S. Pat. No. 5,902,333 discloses a delivery system that includes a catheter that transports a radially compacted stent or other prosthesis near the distal end of the catheter. The catheter has a dilating tip that is distal to the stent mounting area. Both the proximal and distal portions of the dilating tip are gradually tapered so that the mid-portion of the tip has the largest diameter, and is slightly larger than the diameter of the radially compacted stent. As the catheter is advanced to the treatment site, the distally tapered tip may be used to gently widen the vessel lumen. After the stent is released from the catheter, the dilating tip may be passed through the lumen of the stent as the catheter is retracted, causing the tip to expand the stent to an internal diameter approximately equal to the largest diameter of the tip. Although simple to use, the utility of this device is limited because the diameter of the dilating tip must be smaller than the diameter of the blood vessel, and therefore, smaller than the optimal, final diameter of the deployed stent. 
   It would be desirable, therefore to provide a device and method delivering and deploying a stent or other prosthesis that would overcome these and other limitations. 
   SUMMARY OF THE INVENTION 
   One aspect according to the invention provides a delivery system for a stent that includes a catheter having an elongated inner member and at least one flexible, longitudinally oriented outer member that is operably attached to the distal portion of the inner member. A stent is placed concentrically around the flexible outer member, near the distal end of the catheter. When the elongated inner member is retracted, the flexible outer member folds longitudinally and the stent is deployed to the target site. 
   Another aspect according to the invention provides a method for delivering a stent to a treatment site within the vascular system. The method includes providing a delivery system that comprises a catheter having an elongated inner member and a flexible, longitudinally oriented outer member that is operably attached to the distal portion of the elongated inner member, and a stent on a distal portion of the flexible outer member of the catheter. The method further includes advancing the delivery device through the vasculature until the distal portion of the device is adjacent to the treatment site, retracting the inner member and thereby causing the flexible outer member to fold longitudinally and the stent to be deployed as a result of the folding of the outer member. 
   Another aspect of the invention provides a stent delivery device comprising a catheter having an elongated inner member and at least one flexible, longitudinally oriented, outer member that is operably attached to a distal portion of the elongated inner member. Any of a variety of stent designs having a selected diameter may be coupled to the distal portion of the flexible outer sheath of the catheter. When the elongated inner member is retracted, the flexible outer member folds longitudinally adjacent to the lumen of the stent causing the stent to be deployed. 
   The present invention is illustrated by the accompanying drawings of various embodiments and the detailed description given below. The drawings should not be taken to limit the invention to the specific embodiments, but are for explanation and clarity. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. The forgoing aspects and other attendant advantages of the present invention will become more readily appreciated by the detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a cross sectional side view of a delivery system for stents or other tubular prostheses in which both the inner and outer members are fully extended, in accordance with one aspect of the invention; 
       FIG. 2  is a cross sectional side view of a delivery system for stents and other tubular prostheses in which the elongated inner member is partially retracted and the flexible member is partially folded adjacent to the inner side of the stent, in accordance with one aspect of the invention; and 
       FIG. 3  is a flow diagram of a method of delivering a stent to a treatment site within the vascular system, in accordance with one aspect of the invention. 
   

   DETAILED DESCRIPTION 
   Referring to the drawings,  FIG. 1  is a side view of a medical device delivery system  100  for treating vascular lesions, according to the present invention. The medical device delivery system comprises a catheter having a distal end cap  10 , with an elongated inner member  14  attached to end cap  10 . Also attached to distal end cap  10  is flexible outer member  12 . In one embodiment, both inner member  14  and outer member  12  are attached to end cap  10  using adhesives. In another embodiment, both inner member  14  and outer member  12  are attached to end cap  10  using fusion bonding. In one embodiment to the invention, a radiopaque marker  18  is embedded in the distal end portion  10  to facilitate placement of the distal tip of the catheter at the desired location in the vascular system. 
   The distal end portion  10  is comprised of silicone, or a biocompatible polymeric material such as polyurethane, polyethylene or polytetrafluroethylene (PTFE). The use of silicone in certain embodiments may reduce the incidence of vascular tissue damage due to the lubricious surface provided by silicone. The distal portion of end cap  10  is shaped to facilitate passage of the catheter through the vascular system. In various embodiments of the invention, end cap  10  is rounded, tapered or bullet-shaped, among other appropriate shapes. In one embodiment of the invention, the end cap  10  has a lumen  28  that can accommodate a guide wire running longitudinally through the end cap  10 . The guide wire is, in one embodiment, of metallic construction, and is inserted into the femoral artery and threaded through the vascular system to the target site. The end cap  10  is then slipped over the guide wire and the catheter is guided along the vascular route, until both the guide wire and catheter are at their desired target locations. The guide wire is then withdrawn from the body. Using fluoroscopy, the radiopaque marker  18  can be observed during the placement procedure, and thereby facilitate placement of the distal tip of the catheter at a desired location. 
   The inner member  14  comprises a rod or shaft attached at its distal end to end cap  10  and extends the entire length of the catheter. Inner member  14  must be sufficiently flexible so that the catheter can be threaded through the vascular system, but must also have sufficient longitudinal stiffness so that it does not kink during placement of the catheter. Inner member  14  may be made of a metallic material such as stainless steel, titanium or nitinol, or a biocompatible polymeric material such as polyurethane, polyethylene, nylon, PTFE or combinations of these or similar materials. The diameter of inner member  14  is selected to give it the needed longitudinal stiffness and lateral flexibility, and will depend on the properties of the material(s) of which it is composed. Generally, the diameter of inner member  14  will be in the range of 0.2 to 2.0 mm. 
   Attached to end cap  10  is at least one outer member  12 . The primary purpose of outer member  12  is to transport the stent during transit through the vascular system and to deploy the stent at the target site. In one embodiment, outer member  12  is a flexible sheath. Outer member  12  may comprise a flexible, biocompatible, polymeric material such as polyurethane, polyethylene, nylon, or PTFE. 
   In other embodiments, outer member  12  is a mesh of flexible wires comprising nylon, polyether-block co-polyamide polymers such as Pebax® Resins, a metallic material such as braided stainless steel or polymer-coated, braided stainless steel. The polymer coating comprises nylon, polyether-block co-polyamide polymers such as Pebax® Resins, or any other appropriate polymeric material. A distal portion  26  of outer member  12  holds the stent in place as it is transported through the vascular system. In one embodiment, a stent or other tubular prosthesis  16  is slipped over the distal end of the catheter, positioned over a distal portion  26  of outer member  12  and crimped to a reduced diameter so that the stent  16  is held firmly in place. The diameter of the distal portion  26  of outer member  12  is selected so that the outer diameter of the crimped stent is no greater than the diameter of end cap  10 , giving the catheter a smooth outer surface and facilitating its passage through the vascular system. Two radiopaque circumferential bands  15  and  17  are located on inner member  14  at each end of the stent  16 , and serve as radiopaque markers so that the precise location of the stent within the vascular system can be observed using fluoroscopy. The radiopaque markers  15 ,  17 , and  18  comprise gold or any other appropriate material. In other embodiments, a plurality of radiopaque circumferential bands  15  are located on the inner member  14 . 
   A hub  20  is attached to the proximal end of outer member  12 , and a second hub  22  is attached to the proximal end of inner member  14 . In one embodiment, the two hubs are locked to each other as shown in  FIG. 1 . When the two hubs are locked, the outer member  12  is maintained in a fully extended configuration and held taut between the distal end cap  20  and the locked hubs ( 20  and  22 ). This configuration prevents outer member  12  from flexing inward toward the center of the catheter during its passage through the vascular system and releasing the stent prematurely. 
   When the distal portion of the catheter is placed adjacent to the target site, the hubs  20  and  22  are unlocked from each other, allowing inner member  14  and outer member  12  to move in relation to each other. Inner member  14  is partially retracted as shown in  FIG. 2 . As inner member  14  is retracted, it draws end cap  10  in a proximal direction, causing the distal portion  26  of outer member  12  to pass over end cap  10 , and fold longitudinally ( FIG. 2 ), allowing end cap  10  to move in a proximal direction and pass through the lumen of the stent  16 , surrounded by a double layer of outer member  12 . The diameter of the combination of end cap  10  and the folded outer member  12  causes the stent  16  to expand and to be deployed from the catheter to the target site. The diameter of end cap  10  and the thickness of the folded outer member  12  are selected so that the combination causes the stent  16  to expand to a predetermined diameter, beginning at the distal end of the stent  16 , and progressing to its proximal end. The rate of stent deployment is controlled by the rate at which the operator retracts inner member  14 ; consequently, the stent may be deployed slowly if so desired. After the stent is deployed, inner member  14  is advanced in a distal direction causing outer member  12  to unfold. When outer member  12  is fully extended, the hubs  20  and  22  will be aligned with each other, and can once again be locked to each other. The catheter will have regained its smooth outer surface and low profile, and may be removed from the patient. 
   In one embodiment of the invention, the distal portion of outer member  12  is coated with a gel that includes or encapsulates a drug or therapeutic agent. The therapeutic agent or agents may be dispersed within or encased by a polymeric coating, and are eluted at the target site within the vascular system as the stent is deployed. A therapeutic agent is capable of producing a beneficial effect against one or more conditions including coronary restenosis, cardiovascular restenosis, angiographic restenosis, arteriosclerosis, hyperplasia, and other diseases and conditions. The therapeutic agent may comprise, for example an antirestenotic drug, an antisense agent, an antineoplastic agent, an antiproliferative agent, an antithrombogenic agent, an anticoagulant, an antiplatelet agent, an antibiotic, an anti-inflammatory agent, a steroid, a gene therapy agent, an organic drug, a pharmaceutical compound, a recombinant DNA product, a recombinant RNA product, a collagen a collagenic derivative, a protein, a protein analog, a saccharide, a saccharide derivative, a bioactive agent, a pharmaceutical drug, a therapeutic substance, or combinations thereof. 
   In one embodiment of the invention, the device can be used to deliver a drug or therapeutic agent into the vascular system. In this embodiment, inner hub  22  includes a port and a means to accommodate a syringe such as a luer fitting, or any other appropriate fitting. Inner member  14  includes a lumen connected to the port at hub  22  and to lumen  28  connecting to an orifice in the distal end cap  10  for delivery of the drug or therapeutic agent. The drug or therapeutic agent may be any of those described above. 
     FIG. 3  is a flowchart illustrating a method  200  for treating a target site within the vascular system by delivering a stent or other tubular prosthesis in accordance with the present invention. The method begins at block  30  wherein a target site is selected. Next, as indicated at block  32 , a guide wire may be inserted into the femoral vein or at another site, advanced to the target site, and positioned so that the distal tip of the guide wire is adjacent to the target site. The end cap of the delivery system may be slipped over the guide wire and the catheter threaded through the vascular system. The distal tip of the catheter is placed adjacent to the target site, as indicated in block  34 . Next, the hubs at the proximal ends of the inner member and the outer member are unlocked from each other as indicated in block  36 . With the hubs unlocked, the inner member and the outer member can be moved in relation to each other. 
   As indicated in block  38 , the inner member is retracted while the outer member remains where it was placed. This causes the end cap to be retracted and drawn into the lumen of the stent, and the distal portion  28  of the outer member to fold longitudinally and pass over the exterior of the end cap  10 , as shown in  FIG. 2 . The diameter of the end cap  10  with the folded outer member surrounding it is larger than the inner diameter of the stent  16 , and therefore, causes the stent to expand and be deployed from the catheter. Once the stent is deployed, the inner member may be advanced to its original, distal position, so that the outer member is fully extended, as indicated in block  40 . The hubs may then be locked to each other, as in block  42 , so that the catheter will have a smooth exterior surface and a constant diameter. Finally, as indicated in block  44 , the catheter may be safely removed from the body. 
   Because the system of the present invention does not require a balloon or other complex apparatus to deploy the stent, it is simple and inexpensive to manufacture compared to previously disclosed stent delivery systems. Further, its low profile makes it comparatively easy to maneuver through the vascular system, and reduces the potential for damage to the vascular tissues. In addition, the delivery system of the present invention can be used with a variety of stent designs and other tubular prostheses, and the rate of delivery may be controlled by the operator. 
   While the invention has been described with reference to particular embodiments, it will be understood by one skilled in the art that variations and modifications may be made in form and detail without departing from the spirit and scope of the invention.