Patent Publication Number: US-2010125326-A1

Title: Braided Stent With a Shortenable Tether

Description:
TECHNICAL FIELD 
     The technical field of this disclosure is medical implant devices, particularly, braided stents. 
     BACKGROUND OF THE INVENTION 
     Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents. 
     Stents are used in conjunction with balloon catheters 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. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow. 
     To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen size. The stents acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device. 
     One approach has been to fabricate stents from braided fibers, such as polymer fibers, for making a braided stent with little or no metal. Concern over the long-term effects of stents in the body has led to experimentation with bioabsorbable stents, i.e., stents that are absorbed by the body after deployment. Unfortunately, braided polymer stents often undergo plastic relaxation in the delivery system, leading to a smaller deployment diameter. They also often lack the radial strength to prop open the vessel and maintain a fixed position in the vessel lumen. One approach to alleviate this problem has been to increase the diameter of the fibers forming the braided stent to increase the radial strength. Unfortunately, this increases the crossing profile of the compressed stent, reducing maneuverability and the ability to deploy the stent in smaller vessels. An increased fiber diameter may also increase the time for a bioabsorbable stent to be absorbed and interrupt blood flow dynamics. 
     Another approach to this problem has been to attach elastomeric axial runners to the braided fiber body. Unfortunately, the elastomeric runners can cause problems during storage and deployment. During storage, the elastomeric runners constantly exert force on the braided fiber body, which can permanently distort the braided fiber body. The elastomeric runners can also lose their elasticity with age and the constant loading and become ineffective. During deployment, the elastomeric runners exert force on the braided fiber body as the braided stent leaves the compressing sheath, so placement of the expanding braided stent is difficult. Due to the high degree of foreshortening of braided stents during deployment, an elastomeric material which has sufficient strength to open a stent would likely deform the undeployed stent when stretched to its full length in the delivery system. 
     It would be desirable to have a braided stent that would overcome the above disadvantages. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a stent delivery system including a catheter; and a stent disposed on the catheter. The stent includes a braided stent framework having a first framework end and a second framework end; and a plurality of shortenable tethers, each of the plurality of shortenable tethers having a first tether end and a second tether end, the plurality of shortenable tethers being disposed along a length of the braided stent framework and fixed to the braided stent framework at the first tether end and the second tether end. The plurality of shortenable tethers shorten in response to vessel conditions to urge the first framework end and the second framework end toward each other when the stent is deployed in a vessel to urge a circumference of the braided stent framework toward a vessel wall. 
     Another aspect of the present invention provides a stent for use in a vessel having a vessel wall including a braided stent framework having a first framework end and a second framework end; and a plurality of shortenable tethers, each of the plurality of shortenable tethers having a first tether end and a second tether end, the plurality of shortenable tethers being disposed along a length of the braided stent framework and fixed to the braided stent framework at the first tether end and the second tether end. The plurality of shortenable tethers shorten in response to vessel conditions to urge the first framework end and the second framework end toward each other when the stent is deployed in the vessel to urge a circumference of the braided stent framework toward the vessel wall. 
     Another aspect of the present invention provides a stent for use in a vessel having a vessel wall including a braided stent framework having a first framework end and a second framework end; and means for urging the first framework end and the second framework end toward each other in response to vessel conditions when the stent is deployed in the vessel to urge a circumference of the braided stent framework toward the vessel wall. 
     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a stent delivery system made in accordance with the present invention. 
         FIG. 2  is a side view of a braided stent with a shortenable tether made in accordance with the present invention. 
         FIGS. 3A-3C  are side views of deployment of a braided stent with a shortenable tether made in accordance with the present invention. 
         FIG. 4  is a side view of another embodiment of a braided stent with a shortenable tether made in accordance with the present invention. 
         FIG. 5  is a side view of yet another embodiment of a braided stent with a shortenable tether made in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view of a stent delivery system made in accordance with the present invention. In this example, the stent has been advanced from the sheath as it would be for deployment in a vessel. The stent delivery system  100  includes a catheter  105 , and a stent  120  disposed on the catheter  105 . In one embodiment, a sheath  110  is included in the stent delivery system  100  and the sheath  110  is disposed about the stent  120  to maintain the stent  120  in a compressed state for delivery to the deployment site. In another embodiment, the sheath  110  is omitted and the stent  120  is maintained in the compressed state due to materials or other means of compressing the stent  120 . In one embodiment, the catheter  105  can include a retainer  115 , such as mechanical or adhesive structures, for retaining the stent  120  on the catheter  105  until the stent  120  is deployed. 
     The stent  120  can be any variety of braided implantable prosthetic devices known in the art. In one embodiment, the stent  120  can be capable of carrying a coating, such as a polymer coating carrying one or more therapeutic agents, such as anti-inflammatory agents or anti-proliferative agents. In another embodiment, the stent  120  can include one or more therapeutic agents within the stent material. The stent  120  can be bioabsorbable. 
       FIG. 2  is a side view of a braided stent with a shortenable tether made in accordance with the present invention. The shortenable tethers are made of a material that shortens in response to vessel conditions to urge the braided stent framework toward each other, thus urging the circumference of the braided stent framework toward the vessel wall. In this embodiment, the shortenable tethers are in a linear pattern, which is defined herein as a pattern in which the shortenable tethers are axially aligned with the length of the stent. The stent  120  can be installed in the stent delivery system of  FIG. 1  for implantation in a body lumen. 
     Referring to  FIG. 2 , the stent  120  includes a braided stent framework  122  having a first framework end  124  and a second framework end  126 , and a number of shortenable tethers  130 . In this example, the shortenable tethers  130  are disposed on the outer surface of the braided stent framework  122 . Only a single shortenable tether is shown for clarity of illustration. The number, pattern, and location of the shortenable tethers  130  is selected to balance the tension about the circumference of the framework ends when the stent  120  is deployed. For example, when two shortenable tethers are used, the two shortenable tethers can be located 180 degrees apart on the braided stent framework  122 . The braided stent framework  122  is formed of a number of fibers  128  braided together to form a generally tubular body. Those skilled in the art will appreciate that the particular braid pattern can be selected as desired for a particular application. In one example, the braid is made up of 16 wires braided in a one over-one under pattern, and the wires are wrapped at a 63 degree angle with respect to the longitudinal axis of a 6 mm mandrel. 
     The shortenable tethers  130  are shortenable, which is defined herein as having a first length when not deployed in a vessel and having a second length shorter than the first length in response to vessel conditions when deployed in a vessel. The shortenable tethers  130  can shorten in the vessel due to vessel conditions of temperature, exposure to liquid, a combination thereof, or any other vessel condition that causes the material of the shortenable tether  130  to shorten. Each of the shortenable tethers  130  has a first tether end  132  and a second tether end  134 . The shortenable tethers  130  are disposed along the length of the braided stent framework  122 , and are fixed to the braided stent framework  122  at the first tether end  132  and the second tether end  134 . In operation, the shortenable tethers  130  shorten to urge the first framework end  124  and the second framework end  126  toward each other when the stent  120  is deployed in the vessel to urge the circumference of the braided stent framework  122  toward the vessel wall. 
     The fibers  128  of the braided stent framework  122  are sufficiently flexible and braided in pattern such that urging the framework ends toward each other increases the circumference of the braided stent framework  122 . The fibers  128  of the braided stent framework  122  can be made of a wide variety of medical implantable materials, such as stainless steel (particularly 316-L or 316LS stainless steel), MP35 alloy, nitinol, tantalum, ceramic, nickel, titanium, aluminum, degradable and/or nondegradable polymeric materials, tantalum, MP35N, titanium ASTM F63-83 Grade 1, niobium, high carat gold K 19-22, and combinations thereof. The fibers  128  can be single fibers or can be braided. In one example, the fibers  128  can be made of a nondegradable polymer such as polyethylene naphathalate. In another example, the fibers  128  can be made of a bioabsorbable polymer such as poly(lactide-co-glycolide), poly(L-lactide), poly(L,DL,-lactide), poly(lactide-co-lactide-co-trimethylene carbonate), poly(lactide-co-carprolactone), poly(ε-arprolactone), or blends thereof. Those skilled in the art will appreciate that the fibers  128  at the first framework end  124  and second framework end  126  can be free of each other or connected together as desired for a particular application. 
     The shortenable tethers  130  can be made of any material having a first length when not deployed in a vessel and shortening to a second length shorter than the first length in response to vessel conditions when deployed in a vessel. Exemplary materials include homopolymers and copolymers (including random and block polymers) of D-lactide, L-lactide, DL-lactide, carprolactone, trimethylenecarbonate, glycolide, carprolactone derivatives, P-Dioxanone, and combinations thereof. Polyethylene oxide can be part of the polymer chain. Another exemplary material is degradable polyurethane. In one embodiment, the shortenable tethers  130  can be made of a shape memory polymer that shortens due to temperature change when released from a constraining sheath into physiologic conditions. Exemplary shape memory polymers include block polymers of poly(lactide-b-carprolactone), copolymers of oligo(ε-caprolactone)diol and crystallisable oligo(ρ-dioxanone)diol, and the like. In another embodiment, the shortenable tethers  130  can be made of a shrinkable polymer that shortens when exposed to liquid, heat, or a combination thereof. Exemplary shrinkable polymers include degradable polyurethane, and the like. The shortenable tethers  130  can attain shortenable properties in an initial extrusion or can be subject to a secondary extrusion that softens and draws down the material. The cross section of the shortenable tethers  130  can be circular, rectangular, ellipsoid, or any other cross section as desired for a particular application. 
     The body of the shortenable tether  130  between the first tether end  132  and the second tether end  134  is sufficiently free to move to be able to urge the first framework end  124  and the second framework end  126  toward each other. In one embodiment, intermediate points on the body of the shortenable tether  130  between the first tether end  132  and the second tether end  134  can also the attached to the inside or the outside of the braided stent framework  122 . The shortenable tethers  130  can be attached at the very end of the braided stent framework  122 , i.e., at the edge of the first framework end  124  and the second framework end  126 , or can be attached a few fiber crossings in from the very end. 
     Those skilled in the art will appreciate that the shortenable tethers  130  can be attached to the inside or the outside of the braided stent framework  122 , i.e., along the vessel wall or within the stent lumen. In another embodiment, the shortenable tethers  130  can be woven through the braided stent framework  122  so that the shortenable tether  130  passes back and forth through the braided stent framework  122 . 
     The number and placement of the shortenable tethers  130  can be selected to balance the tension about the circumference of each of the first framework end  124  and the second framework end  126 . In one example, two shortenable tethers  130  can be located 180 degrees apart on the circumference of the braided stent framework  122 . In another example, three shortenable tethers  130  can be located 120 degrees apart on the circumference of the braided stent framework  122 . In yet another example, a number of shortenable tethers  130  wrap around the braided stent framework  122  in a helical shape to form a crossed or net pattern. 
     The first tether end  132  and the second tether end  134  can be attached to the braided stent framework  122  with an adhesive or weld. In one example, the adhesive is an ultraviolet curable adhesive. When the stent  120  is bioabsorbable, the adhesive can also be bioabsorbable. Welding processes include heat welding, laser welding, thermal welding, ultrasonic welding, or the like. 
       FIGS. 3A-3C  are side views of deployment of a braided stent with a shortenable tether made in accordance with the present invention. Referring to  FIG. 3A , the stent  120  is advanced through the vessel  140  to the deployment site. The stent  120  is held in a compressed state by the sheath  110 . Referring to  FIG. 3B , the stent  120  is deployed from the sheath  110  by retracting the sheath  110  or advancing the stent  120  with the catheter (not shown). The braided stent framework  122  expands toward the wall of the vessel  140 . The shortenable tether  130  has a first length when the stent  120  exits the sheath  110 . Referring to  FIG. 3C , the shortenable tether  130  has shortened to a second length, which is shorter than the first length, urging the framework ends toward each other. The circumference of the braided stent framework  122  increases, firmly seating the stent  120  in the vessel  140 . 
       FIG. 4  is a side view of another embodiment of a braided stent with a shortenable tether made in accordance with the present invention. In this embodiment, the shortenable tethers  230  are in a crossed pattern, which is defined herein as a pattern in which the shortenable tethers cross at least once along the length of the stent. 
     The stent  220  includes a braided stent framework  222  having a first framework end  224  and a second framework end  226 , and shortenable tethers  230 ,  231 . In this example, the shortenable tethers  230 ,  231  wrap around the outside of the braided stent framework  222  in a generally helical arc. Only a single pair of shortenable tethers is shown for clarity of illustration. Typically, another pair of shortenable tethers would be disposed on the opposite side of the braided stent framework  222  to balance the tension about the circumference of framework ends when the stent  220  is deployed. The braided stent framework  222  is formed of a number of fibers  228  braided together to form a generally tubular body. 
     The shortenable tether  230  has a first tether end  232  and a second tether end  234 , and the shortenable tether  231  has a first tether end  233  and a second tether end  235 . The shortenable tethers  230 ,  231  cross at a crossing point  236 . The shortenable tethers  230 ,  231  are disposed along the length of the braided stent framework  222 , and are fixed to the braided stent framework  222  at the tether ends. In operation, the shortenable tethers  230 ,  231  shorten to urge the first framework end  224  and the second framework end  226  toward each other when the stent  220  is deployed in the vessel to urge the circumference of the braided stent framework  222  toward the vessel wall. 
     In one embodiment, the crossed pattern can be extended into a net pattern encircling the braided stent framework  222 . Adjacent tether ends at one framework end can be joined or affixed to the braided stent framework  222  so that the shortenable tethers form a continuous mesh. In one embodiment, a particular shortenable tether can cross more than one other shortenable tether in forming the net pattern, which is defined herein as a crossed pattern that passes around the circumference of the braided stent framework  222 . The crossed pattern can be symmetric and at a lower braid angle with respect to the longitudinal axis than fibers making up the braid. Shortenable tethers that arc around the stent framework are subject to less strain during crimping than axial aligned shortenable tethers, and as such, are less likely to plastically deform the stent framework during crimping and storage. 
       FIG. 5  is a side view of yet another embodiment of a braided stent with a shortenable tether made in accordance with the present invention. In this embodiment, the shortenable tethers are in a staggered pattern, which is defined herein as a pattern in which the shortenable tethers are shorter than the length of the stent and the axial position of at least one shortenable tether overlaps the axial position of another shortenable tether along the length of the stent. 
     The stent  320  includes a braided stent framework  322  having a first framework end  324  and a second framework end  326 , first shortenable tethers  330 , second shortenable tethers  331 , and third shortenable tethers  336 . The braided stent framework  322  is formed of a number of fibers  328  braided together to form a generally tubular body. 
     The first shortenable tethers  330  have a first tether end  332  and a second tether end  334 , the second shortenable tethers  331  have a first tether end  333  and a second tether end  335 , and the third shortenable tethers  336  have a first tether end  337  and a second tether end  338 . The shortenable tethers  330 ,  331 ,  336  are disposed along the length of the braided stent framework  322 , and are fixed to the braided stent framework  322  at the tether ends. 
     The shortenable tethers  330 ,  331 ,  336  are staggered, i.e., the axial position of the second tether ends  334  of the first shortenable tethers  330  overlaps the axial position of the first tether ends  333  of the second shortenable tethers  331  and the axial position of the second tether end  335  overlaps the axial position of the first tether ends  337  of the third shortenable tethers  336 . In operation, the shortenable tethers  330 ,  331 ,  336  shorten to shorten the axial portion of the braided stent framework  322  adjacent the shortenable tethers, urging the circumference of the braided stent framework  322  in that axial portion toward the vessel wall. Those skilled in the art will appreciate that the shortenable tethers can be provided over a single axial portion of the braided stent framework to expand the circumference of just that single axial portion, or can be provided over a number of axial portions of the braided stent framework to expand the circumference of a number of axial portions. 
     It is important to note that  FIGS. 1-5  illustrate specific applications and embodiments of the present invention, and are not intended to limit the scope of the present disclosure or claims to that which is presented therein. Upon reading the specification and reviewing the drawings hereof, it will become immediately obvious to those skilled in the art that myriad other embodiments of the present invention are possible, and that such embodiments are contemplated and fall within the scope of the presently claimed invention. 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.