Abstract:
The inventive stent delivery system includes a catheter having a retractable outer sheath near its distal end. A shape memory contraction member having a memorized contracted shape is connected to the retractable outer sheath. A heat generating device connected to the shape memory contraction member causes the shape memory contraction member to heat up to its transition temperature and assume its contracted position, retracting the retractable outer sheath. Another embodiment utilizes 2 springs, a “normal” spring and a shape memory alloy (SMA) spring, the two springs selected and designed so that the “normal” has an expansion force which is less than SMA spring when the SMA spring is austenitic, but greater than the SMA spring when the SMA spring is martensitic. Yet another embodiment utilizes a shape memory latch which in its martensitic state abuts a stop to prevent a spring from moving the sheath proximally, but in its austenitic state releases the stop, allowing the spring to retract the sheath to release the stent for deployment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application from application Ser. No. 09/283,444 filed on Apr. 1, 1999 now U.S. Pat. No. 6,206,888, which is a continuation-in-part of U.S. application Ser. No. 09/204,644, filed Dec. 2, 1998 now ABN, which is a continuation of U.S. application Ser. No. 08/947,619, filed Oct. 9, 1997 now ABN, which is a continuation-in-part of U.S. application Ser. No. 08/941,978, filed Oct. 1, 1997 now ABN, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved wire pull back delivery system. More specifically, the invention relates to a wire pull-back stent delivery system which utilizes a shape memory contraction member to retract the retractable outer sheath and deploy a medical implant for a minimally invasive application, such as an endovascular stent graft, vena cava filter, aneurysm repair particles, self-expanding stent, balloon expandable stent, or the like. 
     Delivery systems for deploying medical implants, such as an endovascular stent graft, vena cava filter, self-expanding stent, balloon expandable stent or the like, are a highly developed and well known field of medical technology. These medical devices have many well known uses and applications. In particular, a stent is a prosthesis which is generally tubular and which is expanded radially in a vessel or lumen to maintain its patency. Stents are widely used in body vessels, body canals, ducts or other body lumens. Balloon expandable stents are mounted on a balloon which when expanded delivers the stent, exerting radial force on the constricted portion of the body lumen to re-establish patency. A self-expanding stent is a stent which expands from a compressed delivery position to its original diameter when released from the delivery device, exerting radial force on the constricted portion of the body lumen to re-establish patency. One common self-expanding stent is manufactured of Nitinol, a nickel-titanium shape memory alloy, which can be formed and annealed, deformed at a low temperature, and recalled to its original shape with heating, such as when deployed at body temperature in the body. A common material for balloon expandable stents is stainless steel. 
     Wire pull-back stent delivery systems commonly assigned with this application include U.S. Pat. No. 5,571,135, U.S. Ser. No. 08/753,641 filed Sep. 27, 1996 and U.S. Pat. No. 5,733,267, the entire contents of which are hereby incorporated by reference. Another wire pull-back stent delivery system is shown in U.S. Pat. No. 5,360,401. One important factor in delivering the stent is a controlled precise retraction of the retractable outer sheath. What is needed is a wire pull-back stent delivery system which provides for a controlled and precise retraction of the retractable outer sheath and enables the physician to accurately determine proper positioning of the stent. 
     SUMMARY OF THE INVENTION 
     The inventive stent delivery system includes a catheter having a retractable outer sheath near its distal end. A shape memory contraction member having a memorized contracted shape is connected to the retractable outer sheath. A heat generating device connected to the shape memory contraction member causes the shape memory contraction member to heat up to its transition temperature and assume its contracted position, retracting the retractable outer sheath. 
     Another embodiment utilizes 2 springs, a “normal” spring and a shape memory alloy (SMA) spring, the two springs selected and designed so that the “normal” spring has an expansion force which is less than SMA spring when the SMA spring is austenitic, but greater than the SMA spring when the SMA spring is martensitic. 
     Yet another embodiment utilizes a shape memory latch which in its martensitic state abuts a stop to prevent a spring from moving the sheath proximally, but in its austenitic state releases the stop, allowing the spring to retract the sheath to release the stent for deployment. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a cross-sectional view of a first embodiment of the inventive catheter with a single coiled wire for its shape memory contraction member; 
     FIG. 2 shows a cross-sectional view of a second embodiment of the inventive catheter with a balloon beneath the stent and with a coil and/or twisted wire contraction member; 
     FIG. 3 shows a cross-sectional view of a third embodiment of the inventive catheter with a multiple wire contraction member with the wires coiled in parallel; 
     FIG. 4 shows a cross-sectional view of a fourth embodiment of the inventive catheter with a braided wire tube contraction member; 
     FIG. 5 shows a schematic cross-sectional view of a fifth embodiment of a shape memory retraction catheter, shown in the undeployed position; 
     FIG. 6 shows a schematic cross-sectional view of a fifth embodiment of a shape memory retraction catheter, shown in the deployed position; 
     FIG. 7 shows a schematic cross-sectional view of a sixth embodiment of a shape memory retraction catheter, showing in the undeployed position; 
     FIG. 8 shows a schematic cross-sectional view of a sixth embodiment of a shape memory retraction catheter, shown in the deployed position; 
     FIG. 9 shows the shape memory latch of the sixth embodiment in its martensitic state, and 
     FIG. 10 shows the shape memory latch of the sixth embodiment in its austenitic state. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the inventive catheter is shown generally at  10  and is of well known construction with an inner shaft  12  and an outer shaft  14 . Connected to the outer shaft  14  is a retraction assembly shown generally at  16 , which is comprised of a collapsible accordian section  18  and a stent sheath section  20 . For more information on the collapsible retractable sheath please refer to U.S. Pat. No. 5,534,007 and PCT/US96/07143 filed May 17, 1996, both of which are commonly owned with this application and the entire contents of which are hereby incorporated by reference. 
     A medical device such as stent  22  is carried on inner shaft  12  under retraction assembly  16 , as is well known in the art. Stent  22  can be self-expanding or balloon expandable. The inventive catheter may be used to delivery endovascular stent grafts, vena cava filters, aneurysm repair particles, self-expanding stents, balloon expandable stents, or the like. 
     An annular collar  26  is attached to the proximal portion of stent sheath  20  and a shape memory contraction member  28  is connected to annular collar  26 . In this embodiment the shape memory contraction member  28  is a one-way Nitinol coiled wire spring, which after martensitic to austenitic transition has a shortened longitudinal length, causing annular collar  26  to be retracted proximally, collapsing accordian section  18  of the retractable outer sheath  16  and retracting stent sheath  20  so the medical device such as stent  22  can be delivered. 
     As is well known in the art Nitinol can be made with an austenitic final (A f ) temperature above body temperature. At room temperature the Nitinol wire is in its martensite phase and can be easily deformed. In the first embodiment the contraction member  28  is made from Nitinol wire, formed into a coil and heat set into a spring shape. After the spring is made, the spring is deformed at room temperature to elongate the spring. One end of the spring is attached to the annular collar  26  and the other end is fixedly attached to the inner shaft  12 , at bumper  37 . 
     In the first embodiment the shape memory contraction member  28  takes the form of a spring, however it should be understood that any geometry which resulted in a reduced longitudinal length, causing retraction could be utilized. The length of the spring would determine the amount of retraction and can be selected for various size stents. An alternate embodiment is an elongate Nitinol wire which shortens up longitudinally upon transition (muscle wire). With a one meter long wire which contracts 8% for example, a retraction of 80 mm could be provided, which is adequate for the various stent lengths in common use. Other shape memory alloys can provide various longitudinal contraction as a percent of length and could be utilized as well, if desired. Contraction member  28  could also take a zig-zag shape. The single wire  28  could also be replaced with a plurality of smaller diameter wires which could be braided, intertwined or the like, discussed below in more detail in connection with FIGS. 2 and 3. 
     Power supply  30  supplies power to rheostat  32  which supplies current to the Nitinol spring  28  via lead wire  34 . The Nitinol spring  28  acts as a resistor and heats up, which causes the Nitinol to go through its transition temperature and assume its memorized shape. The transition temperature must be above body temperature. When the current flow is stopped, the spring  28  will stop contracting. Depending on the medium surrounding the spring  28  heat loss will vary and hence the time to stop contraction will vary as well. By replacing a manually operated pull wire with the inventive shape memory contraction member greater control of the retraction is achieved by using the rheostat to control the electrical input into the system. This will eliminate the jerking which can result from manual retraction of a pull wire, which can be caused by excessive force being used to overcome the high frictional and compressive forces created with larger stents. 
     Positive lead wire  34  is connected to contraction member  28  through contraction chamber  39 . The negative lead wire is shown at  35 . Contraction member  28  extends through contraction chamber  39  and is attached to annular collar  26 . To protect the body from electrical and thermal conduction, either the contraction member  28  or contraction chamber  39  or both may be thermally and/or electrically insulated. 
     Although in the first embodiment the section of the contraction member  28  between contraction chamber  39  and annular collar  26  is Nitinol, contraction member  28  could be made of a different material such as stainless steel if desired. The geometry of the spring coil provides the contraction which retracts the outer sheath  16 , so only the portions of contraction member  28  in the contraction chamber  39  needs to be manufactured of shape memory alloy. 
     Referring now to FIG. 2, stent  22  is shown with balloon  24  beneath it for dilation of a balloon expandable stent. Stent sheath  20  acts as a protective sheath for the stent and is withdrawn using shape memory actuator  28 . The actuator or contraction member  28  is shown as multiple wires twisted and/or braided together. 
     Referring now to FIG. 3, a third preferred embodiment of the inventive catheter is shown in which actuator  28  is comprised of multiple wires coiled in parallel. If the wires are insulated, the distal ends of the wires can be connected and the wire leads are then both at the proximal end of the contraction member  28 . Using smaller wires coiled in parallel enables the profile of the actuator  28  to be reduced while maintaining the ability to generate the same retraction force as a single larger wire. In this embodiment the accordian section  18  is replaced with a sliding sleeve design where stent sheath  20  moves proximally over the contraction chamber lumen  40  during retraction of stent sheath  20  to expose stent  22 . The sliding sleeve section could also be designed to slid under lumen  40  if desired. 
     Referring now to FIG. 4, a fourth embodiment of the inventive catheter is shown in which the contraction member  41  is a shape memory braided wire tube. Shape memory contraction member  41  is connected to stent sheath  20  via annular collar  26 . Upon heating, shape memory contraction member  41 , moves proximally, hence moving stent sheath  20  proximally. 
     It should be understood from the above description of the different embodiments that the contraction member may consist of single wires, parallel wires, braided wires, twisted wires, or combinations thereof shaped into a coil. Also, the contraction member could consist of a braided tube comprised of single wires, parallel wires, braided wires, twisted wires, or combinations thereof. 
     It should also be understood that contraction member  28  or  41  could be heated using current, as in FIGS. 1-4, or could be heated conductively, either by being conductively connected to a heat source or by being bathed in a warm fluid bath. 
     It should also be understood that the shape memory contraction member  28  or  41  could be manufactured of one-way or two-way shape memory alloy. As is well known in the art two-way shape memory alloy takes two different shapes with different temperatures. Therefore, with two-way shape memory alloy contraction member  28  could contract at a first temperature selected during manufacture and expand at a second selected temperature. This would allow the retractable outer sheath  16  to be closed if the user changed their mind about delivery or during delivery. 
     The inventive device can deliver other medical devices other than stents and can be used in connection with fixed wire, single operator exchange (SOE)/rapid exchange (RX) or over the wire (OTW) catheter configurations. 
     A fifth embodiment of a shape memory retraction catheter is shown in FIGS. 5 and 6, which shows a schematic view of a distal end of a catheter in both an undeployed and deployed position, shown respectively in FIGS. 5 and 6. In this embodiment a shape memory alloy retraction device is utilized to retract sheath  20  to release stent  22  for deployment. The shape memory alloy retraction device consists of first compressed spring  50 , which is fixedly attached to the distal end of the catheter at  52  and is attached to the annular at  54 , and second spring  56  which is attached to the annular collar at  58  and fixedly attached to the catheter at  60 . Second compressed spring  56  is made of a shape memory alloy (SMA) which is formulated to be austenite at body temperature, which is approximately 37° C., and is designed to exert a distal force which is greater than the proximal force of first spring  50  at body temperature. First spring  50  is not made of shape memory alloy in the preferred embodiment, but could be made of SMA with a very low A f  temperature, so that it did not change states with the cold water flush. Cold water flushing causes second spring  56  to transform to a martensite state, in which the proximal force exerted by spring  50 , which is not affected by the cold water flushing, is greater than the distal force of second spring  56 . The greater force exerted by spring  50  when spring  56  is martensitic moves the sheath  20  proximally to release the stent  22  for deployment. Although cold water flushing is preferred, it should be understood that any known medium cooling device could be utilized to cause second spring  50  to transform. With suitable design changes and if desired, second spring  56  could be heat actuated, if the transformation temperature is above body temperature. 
     An important feature of the fifth embodiment is that springs  50  and  56  are designed so that spring  50  has a proximal force which is less than the distal force of second spring  56  when spring  56  is austenitic, but greater than the distal force of spring  56  when spring  56  is martensitic. However, it should be understood that the positions of first spring  50  and second spring  56  could be switched and by suitable and opposite selection and design, second spring  56  could exert a proximal force on the sheath  20  which is less than the distal force exerted on the sheath  20  by the first spring  50 , when the second spring  56  is in its martensitic state. However, when spring  56  is in its austenitic state it could be selected and designed to exert a proximal force on sheath  20  which is greater than the distal force exerted on the sheath  20  by the first spring  50 . 
     An alternate embodiment for the device of FIGS. 5 and 6 would be to make both spring  50  and spring  56  of nitinol with equal A f &gt;37° C. (i.e. martensite at body temperature). With each of spring  50  and  56  connected to its own separate electric resistance heating (not shown), sheath  20  could be cycled back and forth as alternate springs  50  and  56  change from martensite to austenite by resistance heating. 
     A sixth embodiment of a shape memory retraction catheter is shown in FIGS. 7 and 8, which shows a schematic view of a distal end of a catheter in both an undeployed and deployed position, shown respectively in FIGS. 7 and 8. In this embodiment the retractable sheath  20  includes a stop  60  extending from the proximal end of sheath  20 . Stop  60  is engaged by a shape memory latch  62 , which is fixedly attached to the catheter. A compressed spring  64  is arranged to exert a proximal force on sheath  20 , which is held in the undeployed state by shape memory latch  62  as shown in FIG.  7 . In FIG. 8, latch  62  is shown in a released position, which allows spring  64  to move sheath  20  proximally to release the stent  22  for deployment. Latch  62  may be actuated by either cooling or heating as discussed above, with suitable material selection. It should also be understood that latch  62  could be designed to soften to permit retraction. 
     The shape memory latch  62  is shown in both its engaged and released states in FIGS. 9 and 10, respectively. As seen in FIG. 9, when the latch tip  66  is in its martensitic state, it angles downwardly to abut stop  60 . As seen in FIG. 10, when the latch tip  66  is in its austenitic state, it straightens to release stop  60 , allowing spring  64  to move sheath  20  proximally. 
     The above Examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto.