Abstract:
A system for delivering a stent having a ratchet mechanism includes a delivery catheter having a movable elastic sleeve. The elastic sleeve covers the ratchet mechanism of the stent and prevents the ratchet mechanism from contacting a guide catheter or the vascular wall, and facilitates retraction of the delivery catheter into a guide catheter. The sleeve is retracted in order to deploy the stent. One embodiment of the invention includes a guide catheter having a curved, flexible distal tip to facilitate delivery of the stent. Another embodiment of the invention includes a method of repositioning and deploying a stent having a ratchet mechanism within a blood vessel.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to catheter deployment of stents. More specifically, the invention relates to a system and method for deploying stents having a ratchet mechanism, while preventing interference between the stent and a guide catheter or the vessel wall.  
       BACKGROUND OF THE INVENTION  
       [0002]     Balloon catheters are used in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during percutaneous transluminal coronary angioplasty (PTCA) 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 due to a process called restenosis.  
         [0003]     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, the balloon is deflated, and the catheter is removed.  
         [0004]     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.  
         [0005]     Stents have been disclosed that are formed by rolling a flat sheet of material into a cylindrical form. When tightly rolled, the stent thus formed has a sufficiently small diameter so that it can be mounted over a balloon on a catheter, obviating the need to crimp the stent to the exterior of the balloon. At the target site, the balloon is inflated, causing the stent to partially unroll and expand to a cylindrical coil having a larger diameter with reduced overlap. In order to maintain the stent at the larger diameter, a locking or ratcheting mechanism is used. Some locking mechanisms comprise teeth on the edge of the sheet inside the coil that engage slots or holes in the adjacent wall of the stent. However, many locking mechanisms include an elongated tongue or belt that is attached to the inner edge of the coil and is drawn along the inner surface of the coil as it expands. In some configurations, the tongue or belt has a series of lateral ridges that engage with corresponding ridges on the inner wall of the stent and form a ratchet mechanism that maintains the stent at the enlarged diameter. Alternatively, the elongated tongue may have a series of holes that engage a corresponding series of projections on the interior wall of the coil and form a locking mechanism that keeps the stent at a fixed diameter. In either case, a portion of the tongue extends beyond the outer surface of the stent when the stent is tightly coiled.  
         [0006]     Many cardiovascular delivery systems include a guide catheter in addition to the stent delivery catheter. In practice, the guide catheter is inserted into the patient&#39;s vascular system and advanced over a guide wire until the distal tip is adjacent to the target site. The stent delivery catheter is then passed through an interior lumen of the guide catheter. The guide catheter facilitates placement of the delivery catheter by providing a conduit having some longitudinal rigidity through the vascular system.  
         [0007]     In order to deliver the stent, the distal portion of the delivery catheter bearing the stent is extended through distal tip of the guide catheter, and the stent is positioned at the target site. If it is necessary to reposition or replace the delivery catheter, the delivery catheter must be retracted into the guide catheter. However, in the tightly coiled configuration, the tongue portion of the ratchet mechanism extends beyond the inner diameter of the guide catheter, preventing its retraction into the guide catheter. A second problem encountered with guide catheters currently in use is that, due to their longitudinal rigidity, the catheters do not readily navigate through the vascular system and may cause an abrasion or dissection where the distal tip of the guide catheter contacts the vessel. It would be desirable, therefore, to provide a method and device for delivering a stent with a ratchet mechanism to a target site that would overcome these problems.  
       SUMMARY OF THE INVENTION  
       [0008]     One aspect of the invention provides a system for delivering a stent, comprising a delivery catheter having a movable elastic sleeve. A stent having a ratchet mechanism is positioned about the distal portion of the catheter and covered by the movable elastic sleeve. In a first position, the elastic sleeve covers the ratchet mechanism of the stent, and in the second position, the ratchet mechanism is uncovered.  
         [0009]     Another aspect of the invention provides a method for treating a vascular condition and includes repositioning and deploying a stent having a ratchet mechanism at the treatment site. The distal tip of a guide catheter is advanced to an area adjacent to the treatment site. A delivery catheter carrying a stent having a ratchet mechanism, and covered by an elastic sleeve is advanced through the distal end of the guide catheter. During this procedure, the elastic sleeve prevents the stent from contacting the guide catheter or the vascular wall. Next, the delivery catheter is retracted back into the guide catheter. The elastic sleeve prevents the elongated tongue of the stent from protruding beyond the inner diameter of the guide catheter and preventing its retraction into the guide catheter. The guide catheter is then repositioned adjacent to a final target site. Next, the delivery catheter is advanced through the tip of the guide catheter, and positioned so that the stent is at the final target site. Finally, the elastic sleeve is retracted, and the stent is deployed precisely at the final target site.  
         [0010]     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 understanding. 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 drawings are not to scale. The foregoing 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 FIGURES  
       [0011]      FIG. 1  is an illustration of a stent having a ratchet mechanism, as is known in the prior art;  
         [0012]      FIG. 2  is an illustration of a guide catheter and delivery catheter bearing a stent having a ratchet mechanism, as is known in the prior art;  
         [0013]      FIG. 3A  is an illustration of a delivery system for a stent with a ratchet mechanism, in accordance with one aspect of the invention;  
         [0014]      FIG. 3B  shows the distal portion of the delivery system portrayed in  FIG. 3A , in accordance with one aspect of the invention;  
         [0015]      FIG. 4A  is a side view of a guide catheter including a flexible tubular member adjacent to the distal tip of the guide catheter, in accordance with one aspect of the invention; and  
         [0016]      FIG. 4B  is an illustration of a guide catheter having a flexible tubular member, as the delivery catheter with a stent is advanced through the flexible tubular member, in accordance with one aspect of the invention; and  
         [0017]      FIG. 5  is a flow diagram of a method of repositioning and deploying a stent having a ratchet mechanism within a vessel, in accordance with one aspect of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Throughout this specification like numbers refer to like structures.  
         [0019]     Referring to the drawings,  FIG. 1  is an illustration of a stent  100  having a ratchet mechanism, as is known in the prior art. Such stents are formed by cutting a flat sheet of the stent material and rolling the sheet to form a spiral. The outer surface  102  of the spiral has a cylindrical shape. Stent  100  may be biodegradable or permanent (non-biodegradable), and is composed of a biocompatible material or a combination of biocompatible materials. Appropriate stent materials include metals, metal alloys such as stainless steel, shape memory materials such as nitinol, and biocompatible polymers such as polyetherketone, polymethylmethacrylate, polycarbonate, polyamide, polypropylene, polyethylene, polyethylene terephthalate, polyglycolide, polylactide, copolymers of lactide and glycolide, polyanhydrides, and other medically acceptable polymers, alone or in combination. The stent is deployed at the target site within the vascular system by inflating a balloon inside it, and causing the cylinder to expand and the walls to slide past each other, and form a cylinder with a larger diameter. Depending on the nature of the stent material, a ratcheting or locking mechanism is sometimes needed to prevent recoil and maintain the outer surface  102  of stent  100  in the cylindrical configuration having a larger diameter. Such locking mechanisms include at least one flexible, elongated tongue portion  104  that is attached to the spiral. Elongated tongue  104  passes through a slit or eye  106  on the exterior surface  102  of the stent  100 , and when the stent  100  is tightly coiled, a portion of the tongue  104  extends beyond the outer surface  102  of the stent  100 . As the stent  100  expands, the tongue portion  104  is drawn through the eye  106 . In some configurations, the tongue portion  104  has a series of ridges  108  that engages with a portion of the eye  106  and forms a ratchet mechanism that maintains the stent  100  at the enlarged diameter.  
         [0020]     The tongue portion  104  of the stent  100  comprises a biocompatible material that gives tongue portion  104  sufficient flexibility to enable it to slide through opening or eye portion  106  of the stent  100 , but also sufficient rigidity to lock into place and support the stent  100  in the expanded configuration in the presence of the pressure exerted by the vessel wall. Consequently, when the stent  100  is in the tightly rolled configuration, the tongue portion  104  protrudes through the eye  106 , and beyond the stent surface  102  and gives the stent  100  a larger effective diameter than it would otherwise have.  
         [0021]      FIG. 2  is an illustration of delivery system  200 , as is known in the art. The delivery catheter  201  includes a catheter shaft  202  having a tapered or rounded distal tip  204 . Proximal to distal tip  204  is an inflatable balloon  206  shown in  FIG. 2  in a collapsed configuration. A stent  100  is tightly rolled into a cylinder having a sufficiently small diameter so that when the stent  100  is placed over the balloon  206 , it adheres firmly to the exterior surface of the collapsed balloon  206 . As shown in  FIG. 2 , the distal portion of the delivery catheter  201  bearing the stent  100  is extended through distal tip of the guide catheter  208 . Once the stent  100  is outside the guide catheter  208 , the elongated tongue  104  of the stent ratchet mechanism protrudes beyond the inner diameter of the guide catheter  208 , making it impossible to retract the delivery catheter into the guide catheter  208 .  
         [0022]      FIG. 3A  shows a side view of delivery system  300  for stents having a ratchet mechanism, in accordance with one aspect of the invention. The delivery catheter  301  includes a catheter shaft  302  having a tapered distal tip  304 . The catheter shaft  302  comprises a flexible, biocompatible polymeric material such as polyurethane, polyethylene, nylon, or polytetrafluroethylene (PTFE). In some embodiments, the delivery catheter  301  has a lumen that can accommodate a guide wire  316 . The lumen runs longitudinally through the catheter  301 , so that the delivery catheter  301  may be slipped over the guidewire, and, when no longer needed, the guide wire  316  may be withdrawn through the lumen of the catheter  301 .  
         [0023]     Proximal to distal tip  304  is an inflatable balloon  306  shown in  FIG. 3  in a collapsed configuration. The balloon  306  comprises biocompatible, compliant, semi-compliant or non-compliant materials such as polyamides, polyurethanes, low density polyethylene, polyethylene terephthalate (PET), polyamide copolymers, polyurethane copolymers, and thermoplastic elastomers, as is presently known in the art. Balloon  306  is attached to the catheter body at the proximal and distal ends of the balloon by heat bonding, fusion bonding, adhesives or any other suitable means. The deflated balloon  306  is folded into longitudinal pleats, and wrapped around the catheter shaft. Balloon  306  is connected to a lumen  312  that extends through the delivery catheter  301  to the proximal end of delivery catheter  301 . Balloon  306  is inflated by pumping a fluid through lumen  312  into the balloon, and thereby causing the longitudinal pleats to open, and the balloon  306  to expand. A stent  100  is tightly rolled into a cylinder having a sufficiently small diameter so that when the stent  100  is placed over the balloon  306 , it adheres firmly to the exterior surface of the collapsed balloon  306 .  
         [0024]     In one embodiment of the invention, balloon  306  includes a proximal end portion  308  that has a diameter that is larger than the inner diameter of the stent  100  in the tightly rolled configuration. The enlarged proximal end portion  308  of the balloon  306  may be a ring around the catheter body or a pillow of a flexible polymeric material. The enlarged portion  308  maintains the stent  100  in position over the wrapped balloon  306  and prevents the stent  100  from sliding in a proximal direction along the catheter shaft  302 .  
         [0025]     In one embodiment of the invention, a cylindrical elastic sleeve  310  extends from the proximal end of the tapered distal portion  304  of the delivery catheter  301  and surrounds the stent mounting portion of delivery catheter  301  including at least a portion of the stent  100  and wrapped balloon  306 . The elastic sleeve  310  comprises thermoplastic elastomers having an optimal elongation index and flexibility, latex, and natural or synthetic rubber, or any other suitable material. Elastic sleeve  310  is sized and positioned so that it is slightly stretched over the exterior of stent  100  including the tongue portion, and holds the tongue portion against the exterior surface of the stent  100 . Consequently, the elastic sleeve  310  prevents the tongue portion from extending away from the stent  100 , and preventing retraction of the delivery catheter  301  into the guide catheter  318 . However, catheter  301  may also be used to deliver stents that do not have a ratchet mechanism. For example, in one embodiment of the invention, the external surface of elastic sleeve  310  is coated with a lubricious substance such as silicone, polytetrafluroethylene (PTFE), or a hydrophilic coating. In this embodiment, elastic sleeve  310  provides the delivery catheter  301  with a low profile and a uniform, smooth lubricious exterior surface, providing an advantageous delivery system for stents of various designs.  
         [0026]     In one embodiment of the invention, tubular sleeve  310  extends to the proximal end of delivery catheter  301 . In this embodiment, the sleeve is retracted by pulling the proximal end  314  of the sleeve  310  so that the stent  100  is exposed to the interior of the blood vessel. As the elastic sleeve  310  is withdrawn, enlarged portion  308  prevents the stent  100  from being drawn by the elastic sleeve  310  in a proximal direction along the catheter shaft  302 . In one embodiment of the invention, the interior surface of the elastic sleeve  310  is coated with a lubricious material such as silicone, polytetrafluroethylene (PTFE), or a hydrophilic coating. The lubricious interior surface of the elastic sleeve facilitates retraction of the sleeve  310  so that it readily slides over the stent  100  without dislodging it from the catheter shaft  302 .  
         [0027]      FIG. 3B  is an external side view of the distal portion of delivery system  300 . The distal portion of delivery catheter  301  including the tip  304  and the adjacent stent mounting area have been advanced through the distal end of the guide catheter  318 . Portions of the wrapped balloon  306  and of the stent  100 , including the ratchet mechanism, are covered by elastic sleeve  310 . The elongated tongue portion  104  of the stent  100  is retained within the elastic sleeve  310 , and does not extend beyond the inner diameter of the guide catheter  318 . In this embodiment of the invention, the delivery catheter  301  can easily be retracted into the guide catheter  318 .  
         [0028]      FIG. 4A  is a side view of a guide catheter  400  that, in one embodiment of the invention, is used in conjunction with delivery catheter  300  illustrated in  FIG. 3 . The body  402  of catheter  400  is a hollow tubular structure comprising a flexible, biocompatible polymeric material such as polyurethane, polyethylene, nylon, or polytetrafluroethylene (PTFE), or any other suitable material. In one embodiment, guide catheter  400  has a lumen that runs longitudinally through catheter body  402  and can accommodate a guide wire. The guide catheter  400  is slipped over the guide wire, and guided along the vascular route, until the distal portion of the guide wire and guide catheter  400  are at their desired target locations. The stent delivery catheter  300  is then advanced through the interior lumen of the guide catheter  400  to the treatment site. The guide catheter body  402  has sufficient flexibility to accommodate sharp bends in the vascular system, but also has sufficient longitudinal rigidity to enable it to pass through narrow stenotic lesions.  
         [0029]     In one embodiment of the invention, a flexible tubular member  406  is attached to the distal end of catheter body  402 . Tubular member  406  comprises a flexible, pliable material such as a deformable elastomer, silicone rubber, polyester fabric, or other suitable materials. In one embodiment of the invention, the external surface of tubular member  406  is coated with a lubricious material such as silicone, polytetrafluroethylene (PTFE), or a hydrophilic coating. Tubular member  406  facilitates passage of guide catheter  400  through sharp bends, branch points, and ostia of the vascular system. As guide catheter  400  is pushed forward through the vascular system, if the curved tubular member  406  engages an impediment within the vessel, it will slide over the impediment, or bend and allow the guide catheter to by-pass the impediment. In one embodiment of the invention, the interior surface of the tubular member  406  is coated with a lubricious material such as silicone, polytetrafluroethylene (PTFE), or a hydrophilic coating. In combination with the lubricious exterior surface of the elastic sleeve  310  on the delivery catheter, the lubricious surface of tubular member  406  facilitates retraction of the distal portion of the delivery catheter  300  through tubular member  406 .  FIG. 4A  shows the delivery catheter  300  placed in the distal portion of the guide catheter  400  prior to deployment.  
         [0030]      FIG. 4B  presents an external view of guide catheter  400  as the delivery catheter is advanced through tubular member  406 . Tubular member  406  has sufficient elasticity to expand over the balloon portion of the delivery catheter during its passage through tubular member  406 , and allow deployment of the distal portion of the delivery catheter to the delivery site in the blood vessel.  
         [0031]      FIG. 5  is a flow diagram illustrating a method  500  for delivering a stent having a ratchet mechanism to a target site in the vascular system. The method begins wherein the distal end of the guide catheter is advanced through the vascular system and placed adjacent to the target site (Block  502 ). A guide wire may be used to guide the catheter through the vascular system. The guide wire is inserted into the femoral vein, the jugular vein, subclavian vein, or other point of access, depending upon the location of the lesion to be treated. Guide catheter  400 , shown in  FIG. 4 , is then slipped over the guide wire and guided through the vascular system until the distal tip of the catheter arrives at the target site. The flexible tubular member  406  at the distal tip of guide catheter  400  facilitates its passage through the vascular system. The procedure may be visualized using fluoroscopy, echocardiography, intravascular ultrasound, angioscopy, or other means of visualization.  
         [0032]     Next, the delivery catheter  301 , shown in  FIG. 3 , is passed through the guide catheter  400  until the distal tip of the delivery catheter is adjacent to the distal end of the guide catheter. Near the distal tip of the delivery catheter, a tightly rolled stent of the type shown in  FIG. 1  is mounted on the exterior surface of a balloon. The stent is covered by an elastic sleeve that is stretched over the exterior surface of the stent, including the tongue of the ratchet mechanism. The distal portion of the delivery catheter is advanced through the distal end of the guide catheter (Block  504 ). Because the stent is covered by the elastic sleeve, the tongue of the ratchet mechanism is held close to the stent, and the tongue cannot contact the vessel wall.  
         [0033]     It is sometimes impossible to place the stent precisely at the target site, making it necessary to retract the delivery catheter into the guide catheter (Block  506 ) and then reposition the delivery catheter (Block  508 ). The elongated tongue portion of the stent is retained within the elastic sleeve so that the tongue portion of the ratchet mechanism does not extend beyond the inner diameter of the guide catheter, preventing its retraction into the guide catheter. Additionally, the retraction process is facilitated by the smooth lubricious exterior surface of the elastic sleeve covering the stent, and the lubricious interior surface of the guide catheter.  
         [0034]     It is sometimes discovered during the course of the procedure that the stent is not the correct size, or of optimal design to treat the lesion. In such circumstance, the delivery catheter may be withdrawn and replaced with a delivery catheter (Block  510 ) and, if appropriate, a stent more suitable for the patient.  
         [0035]     Next, the delivery catheter is once again advanced through the distal end of the guide catheter (Block  512 ). The delivery catheter is manipulated so that the stent is placed precisely at the target site. The manipulation of the delivery catheter is facilitated by the low profile and smooth lubricious exterior surface of the distal portion of the delivery catheter provided by the elastic sleeve that fits tightly over the balloon and stent, and covers the ratchet mechanism of the stent.  
         [0036]     With the stent placed precisely at the target site, the elastic sleeve is retracted by pulling the proximal end of the elastic sleeve (Block  514 ). The retraction process is facilitated by the lubricious interior surface to the elastic sleeve. The stent is held in place by the enlarged pillow at the proximal end of the balloon as the elastic sleeve slides over the stent. Next, the stent is deployed at the final target site by expanding the balloon on the delivery catheter (Block  516 ). The stent is expanded by the balloon, and is held in the expanded configuration by the ratchet mechanism. Finally, both the delivery catheter and the guide catheter are withdrawn from the body.  
         [0037]     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.