Abstract:
An expandable medical device is mounted onto an expansion member so as to ensure reliable and accurate placement of the device within a vessel or passageway. An expandable device such as a polymeric stent is mounted to an expansion member so as to prevent slippage of the polymeric stent relative to the expansion member during delivery while also exhibiting a low profile allowing for passage through the vasculature. A polymeric stent having a latticed structure is placed on an expansion member and a constraining member such as a tube is then placed around the stent. The constraining member retains the stent in proximity to the expansion member. The constraining tube having the expansion member and stent therein is placed in a heating element whereby when the expansion member partially inflates it nests within the lattice of the stent.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to delivery systems for implanting an expandable polymeric stent within a vessel or other passageway of a patient and more particularly to a method for improving the retention of a polymeric stent on a balloon during delivery into the vasculature of a patient. 
         [0003]    2. Discussion of the Related Art 
         [0004]    It is desirable to employ medical devices to improve the patency of anatomical passageways within the human body. For example, stents maintain or restore the patency of an anatomical passageway of a patient and can be implanted without an invasive procedure. Stents can be constructed from metal or polymeric material and generally comprise a lattice structure. In particular, the lattice comprises a series of longitudinal struts each joined together by a curved member in a continuous manner so as to form a cylindrical element. Each cylindrical element is attached to an adjacent cylindrical element by connecting members. The flexibility and strength of the stent can be varied by adding cylindrical elements, connecting members and/or struts. In addition, the thickness and geometry of the stent components can be varied. 
         [0005]    Stents are typically constructed from a metallic material that can be crimped to a first low profile shape for passage through the vasculature of a patient. Upon reaching the desired deployment site, the stent is then expanded to a second profile to restore the patency of the vessel. Metallic stents can be constructed from self-expanding super-elastic material or can be balloon expandable and constructed from a plastically deformable material. Once in place, however, a metallic stent remains in the vessel even after the patency of the vessel has been restored. This has several potential drawbacks, for example, the vessel may be damaged by the stent, and the stent may prevent re-intervention at distal locations within the vessel. In order to overcome these limitations, bioabsorbable stents have been developed. These stents are constructed from polymeric materials that are less stressful to surrounding tissue and are resorbable. 
         [0006]    For endovascular implantation of a stent into a blood vessel, percutaneous deployment is initiated by an incision into the vascular system of the patient, typically via the femoral or carotid artery. A tubular or sheath portion of an introducer is inserted through the incision and into the artery. According to one method for implantation, there is a central lumen through the introducer that provides a passageway through the patient&#39;s skin and artery wall into the interior of the artery. An outwardly tapered hub portion of the introducer remains outside the patient&#39;s body to prevent blood from leaking out of the artery along the outside of the sheath. A valve provided on the introducer is manipulated to block blood flow out of the artery through the introducer passageway. A distal end of a guide wire is passed through the introducer passageway and into the patient&#39;s vasculature. The guide wire is threaded through the vasculature until the inserted distal end of the guide wire extends just beyond the intended treatment site. The proximal end of the guide wire typically extends outside the introducer for manipulation by the medical practitioner. Once the wire is in place, a catheter having an expansion member such as a balloon near its distal end is fed over the wire until its distal end is located at the treatment site. 
         [0007]    A typical balloon catheter comprises an elongate and flexible shaft defining one or more passages or lumens, and an inflatable balloon mounted at one end of the shaft. The balloon is in fluid communication with one of the lumens allowing for inflation of the balloon. The opposite end of the shaft lumen includes a hub for accessing the various lumens of the shaft, for example, providing a means to place the shaft over a guide wire that exits out the distal end of the shaft or places one of the lumens in communication with an inflation source. Some shafts include an access port allowing the wire to enter the shaft just proximal to the distal end of the shaft, exiting at the tip. An expandable medical device such as a stent is placed on the balloon such that inflation of the balloon will expand the stent. 
         [0008]    In order to place the stent in the desired location, the balloon catheter is navigated within the vasculature of the patient. The tortuous nature of the vasculature can complicate the delivery of the stent. For example, the stent may contact the inner walls of the vasculature causing it to slide along the surface of the balloon. A metallic stent can be tightly crimped or compressed onto the balloon to prevent slippage. This does not always prevent slippage, however, as most balloons have a relatively smooth surface and over crimping can lead to puncture or damage of the balloon. One attempt at solving this is disclosed in U.S. Pat. No. 6,942,681 to Johnson. Johnson discloses a balloon having geometric features that cooperate with the structural elements of the stent. One example shows a balloon having bumps that fit within the lattice of the stent preventing lateral movement of the stent during navigation through the vasculature to the targeted site for deployment. 
         [0009]    While a variable geometry balloon such as the one disclosed in Johnson is effective for crimping a metallic stent to a balloon, crimping a polymeric stent is more difficult due to the elastic nature of the material. This prevents the stent from being placed in close proximity with the surface of the balloon. In contrast, metallic stents exhibit high plastic deformation and can be crimped closely to the balloon. This results in a lower profile that is more easily passed through the vasculature. One solution for decreasing the profile of a polymeric stent is to heat the stent so that the crimped profile will be retained. One potential drawback to this approach, however, is that the heat can cause permanent deformation that will not allow the polymeric stent to expand to the desired size and shape or heating can damage the stent lattice. Finally, during storage, the polymeric stent will undergo creep deformation making it difficult to expand the stent without applying heat to the stent at the deployment location. 
         [0010]    In view of the above, a need exists for an apparatus and method that can more reliably and accurately emplace a medical device such as a stent within a vessel or passageway of a patient. 
       SUMMARY OF THE INVENTION 
       [0011]    The method of the present invention overcomes the limitations as briefly described above. 
         [0012]    A method for mounting an expandable medical device onto an expansion member so as to ensure reliable and accurate placement of the device within a vessel or passageway is provided. In particular, an expandable device such as a polymeric stent is mounted to an expansion member so as to prevent slippage of the polymeric stent relative to the expansion member during delivery while also exhibiting a low profile allowing for passage through the vasculature. 
         [0013]    A polymeric stent having a latticed structure is placed on an expansion member and a constraining member such as a tube is then placed around the stent. The constraining member may be constructed from PTFE or any other inert material exhibiting a low coefficient of friction. The constraining member retains the stent in proximity to the expansion member. The constraining tube having the expansion member and stent therein is placed in a heating element. The expansion member is heated and an inert gas is placed in fluid communication with the expansion member. The expansion member partially inflates and nests within the lattice of the stent that has been softened by the heating process. 
         [0014]    Once the supply of inert gas is discontinued and the constraining member removed from the heating block the expansion member having the polymeric stent mounted thereon is allowed to cool within the constraining member. When removed from the nesting tube the expansion member protrudes through, and is bonded to, the stent lattice. For example, a portion of the expansion member will protrude from between struts, flexible connectors, and curved joining members. This crimping arrangement ensures that the stent does not slip during navigation through the vasculature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing and other aspects of the present invention will best be appreciated with reference to the detailed description of the invention in conjunction with the accompanying drawing, wherein: 
           [0016]    The features and advantages of the invention will be apparent to those of ordinary skill in the art from the following detailed description of which: 
           [0017]      FIG. 1  is a perspective view of a delivery system for a stent showing the stent mounted to a distal end thereof. 
           [0018]      FIG. 2  is a longitudinal cross-section view of a delivery system. 
           [0019]      FIG. 3  is a perspective view of a polymeric stent to be mounted on an expansion member of a delivery system pursuant to the method of the present invention. 
           [0020]      FIG. 4  is a schematic view showing a stent placed on an expansion member to be inserted into a tubular member. 
           [0021]      FIG. 5  is a schematic view showing the tubular member having the expansion member and stent therein and placed into a heating element. 
           [0022]      FIG. 6  is a side view showing the stent of  FIG. 1  mounted onto the expansion member such that the expansion member protrudes through the lattice of the stent. 
           [0023]      FIG. 7  is a graphical representation showing the correlation between stent retention and the pressure applied to the expansion member during the method of the present invention. 
           [0024]      FIG. 8  is a perspective view of a balloon contoured in accordance with a method of the present invention. 
           [0025]      FIG. 9  is a schematic view showing the tubular member having a contoured expansion member and stent therein both placed into a heating element 
           [0026]      FIG. 10  is a graphical representation of stent retention for contoured v. non-contoured expansion members prepared in accordance with the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    An example of a delivery system  10  for a medical device is shown in  FIGS. 1-2 . The delivery system  10  has an expansion member  12 , a relatively long and flexible tubular shaft  14 , and a hub  16 . The expansion member  12  is affixed to the shaft  14  near a distal end thereof, and the hub  16  is affixed to the proximal end of the shaft  14 . The shaft  14  defines one or more passages or lumens extending through the shaft, at least one of which is an inflation lumen  18  in fluid communication with the expansion member  12  in order to selectively expand and contract member  12 . A hub inflation port  20  allows for an inflation source to communicate with the lumen  18 . 
         [0028]    In the illustrated embodiment, the shaft  14  comprises an inner and outer body  22  and  24 . The inner body  22  defines a guidewire lumen  26 , while the inflation lumen  18  is defined by the annular space between the inner and outer bodies  22  and  24 . The guidewire lumen  26  is adapted to receive an elongated flexible guidewire  28  in a sliding fashion, such that the guidewire  28  and device  10  may be guided along a path defined by the guidewire  28 . For example, guidewire  28  is typically positioned within a passageway or vessel prior to insertion of device  10  therein. The shaft  14  may have various configurations instead of a coaxial design, including a single extruded tube defining any suitable number of parallel side-by-side lumens, or a proximal shaft portion formed of a metal hypotube connected to a polymer distal shaft portion or other designs. Moreover, the catheter shaft may have a rapid exchange configuration, in which the guidewire exits the shaft at a proximal guidewire port located between the balloon and the hub. 
         [0029]    The proximal hub  16  is affixed to the proximal end of the shaft  14 , and preferably provides an inflation port  20  and a guidewire port  30 , again with a luer-lock fitting or hemostatic valve (not shown in the figures). Such a valve allows the guidewire  28  to traverse and slide within the lumen  26 , yet resist the loss of blood or other fluids through the guidewire lumen  26  or port  30 . 
         [0030]    As shown in the drawings, the inner and outer bodies  22  and  24  are securely received within the hub  16 , and surrounded by a tubular strain relief  32 . The hub  16  provides a fluid communication between the guidewire lumen  26  and a guidewire port  30  as well as between the inflation lumen  18  and the inflation port  20  and coupling. 
         [0031]    A polymeric stent  34  is shown in  FIG. 3 . Stent  34  may be affixed to the expansion member  12  of the system  10  for delivery and deployment at a targeted site within the vasculature of the patient. Stent  34  shown in  FIG. 3  is generally cylindrical having a crimped diameter and a deployed diameter that is obtained when member  12  having the stent  34  mounted thereon is expanded. The stent  34  generally comprises a lattice made up of a series of loops  35   a - f . Each loop  35  comprises a series of longitudinal struts  36  joined together by generally semi-circular joining members  38 . Each adjacent loop  35   a - e  is connected together by flexible links  40 . The loops  35 , struts  36 , members  38  and flexible links  40  can be varied in thickness, number and location to change the mechanical characteristics of the stent  34 . For example, the point of attachment of flexible links  40  varies from one loop  35   a - f  to the next providing added flexibility for the stent  34 . 
         [0032]    Stent  34  is constructed from bioabsorbable polymeric materials. The stent  34  may comprise bioabsorbable or biostable polymers with drugs or other bio-active agents and radiopaque markers incorporated therein. Drugs or other bio-active agents may be incorporated into or coated onto the stent  34  in commonly used amounts or significantly greater amounts. Likewise, radiopaque markers may be provided in or on the stent  34 . The combination of greater amounts of drugs or other agents for delivery from the device and the radiopaque markers improves the treatment of the targeted site, disease or condition and improves the visualization and placement of the device in the patient by the medical practitioner. The bioabsorbable polymeric materials that comprise the stent  34  or other device according to the systems and methods of the invention are chosen based on several factors, including degradation time, retention of the mechanical properties of the stent  34  or other device during the active drug delivery phase of the device, and the ability of the bioabsorbable materials to be processed into different structures and via different methods. Other factors, including cost and availability, may also be considered. Bioabsorbable polymeric materials that comprise the stent  34  may include shape memory polymers, polymer blends and/or composites that contribute to retaining the mechanical integrity until drug delivery is completed. 
         [0033]    Examples of bulk erosion polymers employed for the manufacture of stent  34  include poly (a-hydroxy esters) such as poly (lactic acid), poly (glycolic acid), poly (caprolactone), poly (p-dioxanone), poly (trimethylene carbonate), poly (oxaesters), poly (oxaamides), and their co-polymers and blends. Some commercially readily available bulk erosion polymers and their commonly associated medical applications include poly (dioxanone) [PDS suture], poly {glycolide) [Dexon suture], poly (lactide)-PLLA [bone repair], poly (lactide/glycolide) [Vicryl (10/90) and Panacryl (95/5) sutures], poly (glycolide/caprolactone (75/25) [Monocryl suture], and poly (glycolide/trimethylene carbonate) [Maxon suture]. 
         [0034]    Other bulk erosion polymers employed are tyrosine derived poly amino acid [examples: poly (DTH carbonates), poly (arylates), and poly (imino-carbonates)], phosphorous containing polymers [examples: poly (phosphoesters) and poly (phosphazenes)], poly (ethylene glycol) [PEG] based block co-polymers [PEG-PLA, PEG-poly (propylene glycol), PEG-poly (butylene terphthalate)], poly (α-malic acid), poly (ester amide), and polyalkanoates [examples: poly (hydroxybutyrate (HB) and poly (hydroxyvalerate) (HV) co-polymers]. Other surface erosion polymers include poly (anhydrides) and poly (ortho esters). 
         [0035]    The plastic deformation of metallic stents allows for tight crimping around expansion member  12  so as to provide a low profile and prevent slippage as the stent  34  is navigated through the vasculature of a patient. Even when a metallic stent is crimped closely to the expansion member  12 , however, slippage can occur. In order to further prevent slippage, expansion members can be modified to cooperate with the geometry of the stents. For example, as shown in  FIG. 8 , a balloon  42  can be formed having contours  44 . The contours  44  can extend through the lattice of a stent when mounted on balloon  42 . Of course, this requires that the geometric features of the stent  34  be precisely aligned with the contours  44  of the balloon during crimping. This can prove somewhat difficult with misalignment leading to slippage or improper inflation of the stent. 
         [0036]    It is more difficult to mount polymeric stent  34  onto an expansion member  12 . While balloon expandable metallic stents are plastic in nature, polymeric stent  34  is somewhat elastic. This prevents the stent  34  from being tightly crimped onto the balloon unless elastic recovery is compensated for by, for example heating stent  34 . This can lead, however, to permanent deformation  10  that may adversely impact the structural integrity of polymeric stent  34 . 
         [0037]    According to a method of the present invention a polymeric stent  34  is mounted to an expansion member  12 . As shown in  FIG. 4 , a nesting tube  52  was placed on the stent  34 . The nesting tube  52  can be constructed from PTFE or any other inert material exhibiting a low coefficient of friction. The nesting tube constrains the stent  34  onto the expansion member  12 . As shown in  FIG. 5 , the tube  52  having expansion member  12  and stent  34  therein is placed in slot  56  located within a heating block  54 . After the tube  52  is inserted into the slot  56 , the expansion 12  is heated. An inert gas source  58  via supply line  62  is placed in fluid communication with inflation port  20  and expansion member  12  is partially inflated. A pressure regulator  60  allows for the inflation pressure to be precisely controlled. The expansion member  12  expands and nests within the lattice of stent  34  that has been softened by the heating process. Thereafter, the supply of inert gas is discontinued and nesting tube  52  is removed from the heating block  54 . Expansion member  12  having stent  34  mounted thereon is allowed to cool within the nesting tube  52 . As shown in  FIG. 6 , when removed from the nesting tube  52  the expansion member  12  protrudes from, and is bonded to, the stent lattice. For example, portion  13  of expansion member  12  protrudes from between the struts  36 , flexible connectors  40  and curved joining members  38  of stent  34 . This crimping arrangement ensures that stent  34  does not slip during navigation through the vasculature. 
         [0038]    Nesting conditions depended on the type of material (amorphous, crystalline, etc) used to prepare the stent  34 . In addition, other variables for nesting the stents were temperature of the heating block  54 , time within heating block  54  and the pressure of the inert gas used to inflate expansion member  12  during nesting.  FIG. 7  illustrates the effect of pressure of the inert gas supplied to the expansion member  12  during the nesting process on stent retention. Although the variables can be adjusted, it is desirable that the stent  34  does not become physically deformed or melt during the nesting process. Typical nesting conditions were 50 to 100° C. at 200 to 300 psi of inflation pressure for 1 to 5 minutes. The preferred conditions were 50 to 60° C.; 200 psi and 2 to 5 minutes. This resulted in stent retention values of about 0.2 to 0.6 pounds. 
         [0039]    The method of the present invention may be employed with an expansion member  42  shown in  FIG. 8 . In addition, expansion members can be treated to change the surface roughness or tackiness to improve retention. For example, materials such as PVP and PEG can be coated on the balloon to improve the stent adhesion to the balloon. As shown in  FIG. 9 , the contoured balloon  42  of  FIG. 8  has stent  34  mounted thereon. It is desirable to match the contours  44  of the expansion member  12  to the openings in the lattice of stent  34 . If this is not obtainable, however, the method of the present invention may still be carried out and significant retention is exhibited. For example, inflation of the balloon  42  will allow the non-contoured regions  45  to expand into the lattice of stent  34 .  FIG. 10  illustrates the effects of varying temperature on stent retention during nesting using a bumped expansion member and a regular expansion member. Stents were nested in a range of 50° C. to 60° C. ±0.2° C. with a pressure of 200 psi to 300 psi ±10 psi for 2min to 5min ±5sec. 
         [0040]    Although shown and described is what is believed to be the most practical and preferred embodiments, it is apparent that departures from specific designs and methods described and shown will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention. The present invention is not restricted to the particular constructions described and illustrated, but should be constructed to cohere with all modifications that may fall within the scope for the appended claims.