Abstract:
Methods for making a loaded catheter assembly for delivering a self-expanding stent where the self-expanding stent is carried in a compressed state and the compressed stent has an inside diameter smaller than the outside diameter of the catheter distal tip. The methods can utilize catheter sub-assemblies lacking already attached tips or having partially formed distal tips. A stent can be proximally and co-axially slid over the distal end of the catheter shaft and constrained by a retractable sheath disposed co-axially about the compressed stent. The catheter distal tip can be added or more fully formed after the loading of the stent. Some catheters include a preformed distal conical tip held in position by a heat-shrink film. Other catheters have an elastomeric distal tip waist for slipping over and engaging an outward projection on the catheter shaft distal region. Some catheters are adapted to engage catheter shaft distal threaded regions.

Description:
This application is a divisional of U.S. application Ser. No. 09/480,438, filed on Jan. 10, 2000 now U.S. Pat. No. 6,322,586. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related generally to medical devices. More specifically, the present invention is related to catheters. The present invention includes apparatus and methods for affixing catheter tips to stent delivery catheters after a self-expanding stent has been loaded onto the catheter. 
     BACKGROUND OF THE INVENTION 
     Heart attacks are a leading cause of death in the industrialized world, particularly in the United States. Many heart attacks are caused in part by a narrowed, stenosed coronary blood vessel. A medical procedure commonly used to deal with coronary vessel stenoses is angioplasty. Angioplasty, in particular Percutaneous Transluminal Coronary Angioplasty (PTCA), includes inserting a balloon catheter into the femoral artery near the groin, and advancing the catheter over the aortic arch and into a coronary artery. The balloon can be advanced through the coronary artery to the stenosis and inflated to widen or dilate the narrowed region. The balloon catheter can then be withdrawn. In some cases, the widened coronary vessel rebounds or re-closes, narrowing the vessel over a period of time. 
     Stents have come into increasing use to prevent the widened vessel regions from narrowing after angioplasty. A stent, typically having a tubular shape, can be put in place in the widened vessel region to hold the vessel walls apart and the lumen open in the event the vessel attempts to narrow again. One class of stents requires that the stent be forcibly outwardly expanded to put the stent into position against the vessel walls. Another class of stents, self-expanding stents, can be delivered to a site in a compressed or constrained configuration and released in the vessel region to be supported. The self-expanding stent then expands in place to a configuration having a wide lumen, typically pressing firmly against the vessel walls where released. The stent is commonly placed at a recently dilated, stenosed vessel region. 
     Self-expanding stents can be delivered to a target site mounted over an inner tube or shaft and constrained within the distal end of an enclosing retractable tube or sleeve. The self-expanding stent can be freed from the restraint of the outer sheath by either distally pushing the inner shaft against the stent or proximally pulling the retractable outer sheath from over the stent. Once free of the outer restraint, the self-expanding stent can expand to force itself against the vessel inner walls. Self-expanding stents are often elastically biased to assume an original larger shape after being temporarily compressed into a smaller size to more easily be transported through blood vessels to the target site. 
     Preferably, the stent is only temporarily compressed within a retractable sheath and compressed for a limited time. The exact size of the stent to be delivered may not be known until the patient is in the operating or treatment room of a hospital. In general, a catheter should have a maximum radial extent or profile no larger than necessary, in part to enable the catheter to reach further into narrower vessel regions. A self-expanding stent is most easily loaded in a proximal direction onto a catheter by compressing the stent and sliding the stent co-axially over the inner shaft distal end and within the retractable outer sheath. The stent must thus typically be slid over the catheter distal tip. The distal tip is optimally tapered, having a proximal width about the same as the width of the outer sheath, to provide a smooth transition from the distal tip to the outer sheath. This can present a situation where the compressed stent has an inner diameter too small to be advanced over the larger outer diameter distal tip of the catheter. 
     What would be desirable is a delivery catheter and method which would ease loading of a self-expanding stent by not having the tip on the delivery system at the time of loading. This would allow the self-expanding stent to be slid under the sheath with subsequent tip attachment. 
     SUMMARY OF THE INVENTION 
     The present invention includes a loaded, self-expanding stent delivery catheter assembly having an inner shaft or tube, a retractable sheath disposed over the inner shaft, and a compressed, self-expanding stent disposed co-axially between the inner shaft and retractable sheath. The inner shaft is a solid shaft in some embodiments, and a hollow shaft or tube having at least one lumen therethrough in other embodiments. The inner shaft terminates distally in a tip, preferably tapered, which can have a profile sufficiently large, such that the compressed stent cannot be passed proximally over the distal tip when mounted on the catheter distal end. The catheter assembly includes a distal tip adapted to be easily secured to the inner shaft after the stent has been proximally loaded over the catheter distal end. The present invention also allows attachment of catheter tips to tubes even after the tubes have been coated with material which are incompatible with later adhesive use. 
     One catheter includes a shrinkable film secured to the catheter inner shaft distal end and having a sufficiently small profile when collapsed or pressed against the inner shaft to allow the stent to be passed over the film, and onto the catheter distal region. After the stent is loaded, the tip is added by co-axially sliding a preformed conical tip over the inner shaft and under the shrinkable film and securing the tip to the inner shaft by shrinking the film over the tip and inner shaft. One embodiment utilizes heat-shrinkable film. One embodiment secures the film proximally to the inner shaft with outwardly projecting barbs or ridges. Another embodiment bonds the shrinkable film to the inner shaft. 
     In one set of embodiments, the distal tip has a proximal region allowing outward projections from the inner shaft to extend and cut into the region wall and secure the tip to the inner shaft. In some embodiments, the proximal region wall is sufficiently elastic to allow the proximal region to expand and contract when passed over the projections. In some embodiments, the proximal region is swollen with solvent to pass over the projections and the solvent later flashed off. In some embodiments, the proximal region wall has internal recesses or channels partially through the wall to receive outward projections from the inner shaft. In other embodiments the wall has holes or slots entirely through the wall. The proximal region holes or slots can be used to secure the tip to the inner shaft by allowing outward projections to fit through the openings, inhibiting the distal tip from moving relative to the inner shaft. 
     One catheter distal region according the present invention has a first configuration as a distal sleeve or tube, allowing a compressed stent to be passed over the tube. The tube can then be formed into a catheter distal tip by heating and reforming the tube material into a tip such as a conical tip having a lumen therethrough. One distal tip includes a proximal threaded region adapted to be threadably secured to a corresponding distal threaded region on the inner shaft. Another distal tip includes a proximal coil adapted to secure the tip to a corresponding distal threaded region on the inner shaft. 
     One group of distal tips according to the present invention is already attached to the inner shaft, but in a compressed or low profile state, and expands radially after a compressed stent has been passed proximally over the tip. One distal tip is formed of a reversibly compressible material which can be constricted by a compression tube while the stent is passed over the tube and allowed to expand to a larger profile by removing the compression tube. Another distal tip is formed of a swellable material which is chemically swollen to a larger outside diameter after a stent is allowed to pass over the tip and onto the distal region of the inner shaft. One suitable tip material is water-swellable and the tip is hydrated after the stent is in place. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, side, cross-sectional view of a stent delivery catheter in assembly, having an inner shaft, a retractable sheath thereover, a distal sleeve of heat-shrinkable material bonded thereto, and a preformed distal cone; 
     FIG. 2 is a fragmentary, side, cross-sectional view of the catheter of FIG. 1, having an expandable stent loaded between the inner shaft and retractable sheath and the distal cone affixed distally to the inner shaft with the heat-shrink distal sleeve; 
     FIG. 3 is a fragmentary, side, cross-sectional view of a stent delivery catheter in assembly, having an inner shaft with distal barbs, a constrained stent and retractable sheath thereover, and a preformed distal cone; 
     FIG. 4 is a fragmentary, side, cross-sectional view of the catheter of FIG. 3, having the distal cone fixed in position over the inner shaft, and a heat-shrinkable sleeve disposed over the barbs and cone; 
     FIG. 5 is a fragmentary, side, cross-sectional view of a catheter in assembly including an inner shaft having distal barbs, and a distal tip; 
     FIG. 6 is a fragmentary, side, cross-sectional view of the catheter of FIG. 5, having the distal tip slipped over and secured to the distal barbs; 
     FIG. 7 is a fragmentary, side, cross-sectional view of a catheter in assembly having an inner shaft with a distal annular ridge; 
     FIG. 8 is a fragmentary, side, cross-sectional view of the catheter of FIG. 7, having a matching distal tip with an annular channel fit over the inner shaft annular ridge; 
     FIG. 9 is a fragmentary, side, cross-sectional view of a catheter in assembly having an inner shaft with distal fins and a distal tip with receiving slots for securing the tip to the inner shaft fins; 
     FIG. 10 is a fragmentary, perspective view of the inner shaft and distal fins of FIG. 9; 
     FIG. 11 is a fragmentary, side, cross-sectional view of the catheter of FIG. 9, after the distal tip slots have been fit over the inner shaft fins; 
     FIG. 12 is a fragmentary, side, cross-sectional view of a catheter in assembly having an inner shaft, a constrained stent and retractable sheath thereover, and a proximally bonded distal polymer sleeve; 
     FIG. 13 is a fragmentary, side, cross-sectional view of the catheter of FIG. 12, having the distal sleeve formed into a distal tip; 
     FIG. 14 is a fragmentary, side, cross-sectional view of a catheter assembly having an externally threaded inner shaft and an internally threaded catheter tip; 
     FIG. 15 is a fragmentary, side, cross-sectional view of the catheter assembly of FIG. 14, having the tip threaded onto the inner shaft; 
     FIG. 16 is a fragmentary, side, cross-sectional view of a catheter assembly having an externally threaded inner shaft and a catheter tip, comprised of a coil, adapted to secure the tip to the inner shaft threads; 
     FIG. 17 is a fragmentary, side, cross-sectional view of a coiled catheter tip adapted to be secured to the inner shaft threads of FIG. 15; 
     FIG. 18 is a fragmentary, side, cross-sectional view of a catheter distal region having an unconstrained, compressible distal tip; 
     FIG. 19 is a fragmentary, side, cross-sectional view of the catheter of FIG. 18, having the tip compressed with a compression tube and an expandable stent fit between the inner shaft and a retractable sleeve; 
     FIG. 20 is a fragmentary, side, cross-sectional view of a catheter in assembly having an inner shaft, an expandable stent and retractable sleeve slid thereover, and an expandable or swellable distal tip; and 
     FIG. 21 is a fragmentary, side, cross-sectional view of the catheter of FIG. 20, after the distal tip has been expanded. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a stent delivery catheter assembly  30  in the process of manufacture. Catheter assembly  30  includes an inner tube  32  having a lumen  33  therethrough, a distal region  40  and a distal end  42 . Inner tube  32  is preferably formed of a polymeric material suitable for delivering a stent through tortuous vessel passages and suitable for receiving a guidewire therethrough. One suitable material includes braided polyimide tubing. A retractable sheath  34  having a distal region  44  and a distal end  46  is slidably disposed over inner tube  32 , having an annular space  66  sufficient in size to receive a compressed stent between inner tube  32  and retractable sheath  34 . Retractable sheath  34  can be formed of a polymeric material preferably sufficiently lubricious to ease in advancing catheter assembly  30  through increasingly smaller blood vessels. Sheath  34  can be formed of materials such as high density polyethylene or nylon. A stop  35  is affixed to the inner tube  32  proximal of its distal end, about the length of a stent  50  (shown in FIG.  2 ). The stop functions to hold the stent axially during deployment relative to the outer tube  34 . 
     A distally positioned film or sleeve  36  is disposed over inner tube distal region  40  and is affixed to inner tube  32  as indicated at  58 . Sleeve  36  is preferably formed of a shrinkable film material, for example a heat-shrinkable material such as polyolefin copolymer, nylon, or polytetrafluoroethylene. Sleeve  36  can be secured to inner tube  32  using heat bonding, adhesives, or solvent welding, using methods well known to those skilled in the art. Sleeve  36  can include a waist portion  64  and a free portion  60 . Sleeve  36  is illustrated having an open distal end  70  and an annular space  68  for receiving a distal cone, described later. While film  36  is illustrated in a tubular sleeve configuration, other film geometries are also within the scope of the invention. 
     Distal tip  38  is illustrated having a lumen  52 , a proximal end  54 , and a distal end  56 . Tip  38  is preferably tapered to aid in tracking catheter  30  through vessel passages and turns. Tip  38  is conically tapered in one embodiment and has a curved or arcuate taper in another embodiment. 
     FIG. 2 illustrates catheter assembly  30  in a later stage of assembly, having a self-expanding stent  50  inserted into annular space  66  between retractable sheath  34  and inner tube  32 . Stent  50  can be placed over inner tube  32  by sliding stent  50  proximally over inner tube distal end  42 , and over distal sleeve  36 . Sleeve  36  can be drawn distally or otherwise decreased in outer diameter, such as by compressing, to allow stent  50  to pass proximally over the sleeve. Stent  50  is preferably compressed using a suitable tool or jig, to decrease the outer diameter of the stent. With the stent preferably compressed, the stent can be axially and proximally slid over inner tube  32  and within sleeve distal region  44 , to reside in annular space  66 . With the stent constrained by retractable sleeve  34 , any restraining tool or jig can be removed from the catheter. 
     With stent  50  in position, tip  38  can be affixed to inner tube  32 . Tip lumen  52  can be slid axially and proximally within annular space  68  over inner tube  32  and within distal sleeve  36 . Tip  38  has a maximum radial extent indicated as “D 1 ” in FIG.  2 . D 1  is preferably dimensioned such that tip  38  can be slid within annular space  68 . Retractable sheath  34  has a distal radial extent indicated as “D 2 ” in FIG.  2 . In one embodiment, D 1  and D 2  are approximately equal, such that when tip  38  abuts retractable sleeve  34 , the outer diameter is approximately constant over the transition. In another embodiment, tip outer diameter D 1  is larger than sleeve distal diameter D 2 , such that tip  38  can sufficiently dilate narrow vessels to pass over tip and over retractable sleeve distal end  46 . 
     With tip  38  slid within shrinkable film or sleeve  36 , the film material can be tucked over the tip and within inner tube  32 , as indicated in a tucked or draped portion  62 . Alternatively, the film material may extend distally and have a distal portion inner surface in contact with the outer surface of the inner tube, both of which extend distally beyond the distal end of the tip. This alternative is depicted in the embodiment of FIG.  4 . The film material can be shrunk over tip  38 , thereby securing tip  38  in place over inner tube distal region  40 . As illustrated in the example of FIG. 2, stent  50  is dimensioned such that the stent could not have been slid over inner tube  32  and within sheath  34  if tip  38  had been in place. The present invention allows the stent to be positioned proximally of the ultimate location of the distal tip by allowing the stent to be slid proximally over the shrinkable film material rather than over the tip. The present invention allows the tip to be affixed to the inner tube after the stent has been mounted over the tube. 
     Referring now to FIG. 3, another catheter assembly  80  is illustrated. Catheter assembly  80  is similar is some respects to catheter assembly  30  of FIGS. 1 and 2, with catheter assembly  80  having an inner tube  82  having a distal end  83  and having outward projections or barbs  84  disposed in an inner tube distal region  81 . Barbs  84  provide one method for securing a shrinkable film to inner tube  82 , as an alternative to bonding. FIG. 3 illustrates stent  50  already positioned over inner tube  82  and constrained by retractable sleeve  34 . Distal tip  38  is also illustrated, in position for further assembly. 
     Referring now to FIG. 4, distal tip  38  is illustrated as secured in position by shrinkable film  36  draped over barbs  84 . To manufacture the embodiment illustrated in FIG. 4, distal tip  38  can be slid axially over inner tube  82 . Shrinkable film or sleeve  36  can be slid over or wrapped around both tip  38  and inner tube barbs  84 . As shown in FIG. 4, the distal portion of the film  36  and a distal portion of the inner tube may extend beyond distal end  56  of tip  38  to bond to one another. Shrinking the film can bond the film to both tip and inner tube, with barbs  84  engaging film  36 . In one embodiment, film  36  is draped over distal tip distal end  56  and shrunk to further secure tip  38  to inner tube  82 . In one embodiment, inner tube distal end  83  is softened and splayed outward about distal tip distal end  56  to further secure tip  38  to inner tube  82 . In one method inner tube distal end  83  is heated and pressed into a concave die to form a slightly mushroomed tip. Inner tube  82  can also be further bent back around tip distal end  56  as illustrated in FIG.  4 . Barbs  84  can thus provide a device for securing film  36  to inner tube  82 . 
     Referring now to FIGS. 5 and 6, a catheter assembly  88  is illustrated in the process of assembly, having inner tube  82  with barbs  84  and a distal tip  86 . The self-expanding stent and retractable sleeve have been omitted for simplicity. Distal tip  86  has a proximal sleeve or waist portion  92 , a distal tip portion  90 , and is preferably formed of a polymeric material adapted to slip over barbs  84 . In one embodiment, the polymeric material elastically expands to accommodate barbs  84 , then contracts again proximal of barbs  84 . FIG. 6 illustrates proximal waist  92  in place over barbs  84 . In one embodiment, the polymeric material is swollen with solvent, waist  92  forced over barbs  84 , and the solvent flashed off, shrinking the waist over the barbs. Using solvent to swell film sleeves followed by flashing off solvent can be used in other embodiments as well. Barbs  84  can bite into proximal sleeve portion  92  to secure tip  86  to inner tube  82  or alternatively, sleeve portion  92  can form around barbs  84  to provide a mechanical lock. 
     The embodiment of FIGS. 5 and 6 allow a stent and retractable sleeve to be axially slid over the distal end of an inner tube prior to securing the distal tip to the inner tube. In one embodiment, a discrete number of barbs are distributed around the inner tube outer wall. In one embodiment two barbs are present while other embodiments have three and four barbs. In one embodiment, the barbs are formed as sharp, proximally inclined annular ridges surrounding the inner tube rather than as a discrete barbs. The embodiment of FIGS. 5 and 6 allow a distal tip to be secured by sliding the tip over the inner tube distal end after the expandable stent has been slid into place. 
     Referring now to FIGS. 7 and 8, another catheter assembly  100  is illustrated in the process of assembly, having an inner tube  102  and a distal tip  106 . Inner tube  102  includes outward protrusions  104 . Outwardly directed protrusions  104  can be formed as a series of discrete outward protrusions, as a single continuous annular ring, or as a series of discrete annular ring segments. Distal tip  106  includes a distal end  114 , a proximal sleeve or waist portion  112 , a lumen  108 , and an inner detent or recess  110 . Inner recess  110  can include a series of discrete recesses, a single annular channel, or a series of annular channel segments. Inner recess  110  is preferably matched to receive outward protrusions  104  on inner tube  102 . In FIG. 8, inner tube  102  has been inserted into distal tip  106 , causing outward protrusions  104  to fit into inner recess  110 . The self-expanding stent and retractable sleeve have been omitted for clarity. Distal tip waist  112  is preferably formed of a sufficiently elastic material to expand and slide over outward protrusions  104  until the protrusions are fit within the inner recess of the distal tip. The embodiment of FIGS. 7 and 8 thus allows a stent to be slid over the inner tube and constrained by a retractable sheath, followed by attachment of the distal tip to the inner tube. 
     Referring now to FIGS. 9,  10 , and  11 , another catheter assembly  120  in the process of assembly is illustrated, having an inner tube  122  and a distal tip  126 . Inner tube  122  has a lumen  123  therethrough and has outwardly directed protrusions or fins  124  mounted on the external inner tube walls. Distal tip  126  includes a distal end  128 , a proximal end  129 , a proximal waist  130 , a lumen  138  therethrough, and openings or slots  132  through the wall of distal tip waist region  130 . FIG. 10 further illustrates one embodiment of fins  124 , illustrating two fins  124  having a distal ramp region  134  and a more proximal land region  136 . More or less than two fins can be utilized. Fin ramps  134  can serve to force a sufficiently elastic tip sleeve or waist region to expand over the fins. FIG. 11 illustrates distal tip  126  forced proximally over fins  124  until the fins protrude through slots  132 , securing distal tip  126  to inner tube  122 . During assembly, a self-expanding stent can be slid proximally of fins  124  and constrained by a surrounding retractable sheath. After placement of the stent, the distal tip can be slid proximally over the fins, securing the tip to the inner tube. 
     Referring now to FIG. 12, another catheter assembly  140  is illustrated in the process of being assembled. Catheter assembly  140  is illustrated having stent  50  and retractable sheath  34  already in place over an inner tube  142  having a distal end  144 . A distal sleeve or tube  148  having a distal end  150  and a proximal end  152  is bonded at a bonding region  146  to inner tube  142 . In one embodiment, distal tube  148  is flexible while in another embodiment distal tube  148  is rigid. Distal tube  148  is preferably formed of a thermosetable polymeric material such as polyurethane or polyamide. Stent  50  can be proximally inserted over distal tube  148  and inner tube  142  while the distal tube is in the configuration illustrated in FIG. 13, in which distal tube  148  can have an outer diameter approximately equal to that of inner tube  142 . 
     After stent  50  has been positioned over inner tube  142  and constrained by retractable sleeve  34 , distal tube  148  can be formed into a distal tip  149  as indicated in FIG.  13 . In the embodiment illustrated in FIG. 13, the distal sleeve has been pushed back to form a proximal wall portion  154  substantially transverse to the catheter longitudinal axis. In this embodiment, the sleeve has been tapered distally, ending at a distal tip portion  156  where the sleeve can be bonded to the inner tube. The distal sleeve can be heated to reform the sleeve, and the sleeve allowed to cool and set in the new, conically tapered shape. In some methods, the sleeve is formed into the desired shape with a heated mold or die. In one method, the distal end of distal sleeve  148  is tucked back inside itself and the proximal end of distal sleeve  148  bunched back and expanded in diameter such that the sleeve assumes a conically tapered shape with the sleeve forming both an internal and an external wall. In some embodiments, the distal tip is formed from a substantially remelted polymeric sleeve such that the distal tip has a substantially solid configuration. In embodiments allowing for use of a guide wire, the tip can be shaped to allow for passage of a guide wire lumen therethrough. The distal sleeve thus allows for slipping a stent proximally over the sleeve while the sleeve is in the small diameter configuration. The distal sleeve then allows for reformation of the sleeve into a distal tip for use in delivering the stent. 
     Referring now to FIGS. 14 and 15, FIG. 14 illustrates a catheter assembly  160 , having an inner tube  162  and a not yet attached distal tip  168 . Inner tube  162  has a lumen  164  therethrough and a distal exterior threaded region  166 . Distal tip  168  has a proximal waist region  172  including a proximal interior threaded region  170 . In the embodiment illustrated, the tube distally threaded region is externally threaded, for later attachment of the internally threaded tip. FIG. 15 illustrates inner tube  162  threadably secured to distal tube  168 . In another embodiment, not requiring illustration, the inner tube is internally threaded for mating to the externally threaded proximal waist of a distal tip adapted to be received within the inner tube distal end. The threaded regions of the inner tube and the distal tip allow the distal tip to be secured to the inner tube after a stent has been loaded over the inner tube distal end. 
     In one method, threads are cut on the outside of a hypotube section which can be bonded to the outside of a tube requiring outer threads. Threads can be cut on the inside of another hypotube section which can be bonded to the inside of the tube to receive inner threads. The hypotube sections can be insert molded into the catheter tubes and distal tip. Threads could also be molded or cut into the inside diameter of a polymer tip. 
     Referring now to FIG. 16, another catheter assembly  180  prior to final assembly is illustrated. An inner tube  182  has a distally threaded or scored region  184 . A distal tip  186  includes a proximal waist region  188  and an internally mounted coil  190 . Coil  190  is dimensioned so as to securably engage inner tube threaded region  184  when distal tip  186  is rotatably advanced over inner tube  182 . The threaded region on inner tube  182  includes a series of concentric annular grooves in one embodiment and a single helical groove in another embodiment. The distal tip internal coil allows securing the distal tip to the inner tube after a stent has been proximally slipped over the inner tube distal end. 
     Referring now to FIG. 17, inner tube  182  having distally threaded or scored region  184  is again shown, in the process of being secured to a distal tip  192  having a waist region  196  and formed of a coil  194  having a plurality of helically wound strands  198 . Tip  192  can be formed entirely of a coil. Coil proximal waist region  196  is adapted to be secured to inner tube threaded region  184  by rotatably advancing the tip of the distal end over the inner tube. In the embodiment illustrated in FIG. 17, the inner tube is externally threaded and the tip is adapted to be mounted externally over the inner tube. In another embodiment, not requiring illustration, the inner tube is internally threaded and the distal tip is dimensioned to be received within the inner tube distal end. 
     The inner diameter of the coil can be less than the minor diameter of the threads. The coils on the tip can be coiled so that when the coil is torqued to put the tip onto the shaft, the coil grows in diameter and the torque required to put the tip onto the inner tube is reduced. The coil can then cinch down onto the threads when the coil is torqued to remove it from the inner tube. The coils on the tip can be coiled so that the space between the coils is less than the pitch of the threads on the inner tube. This creates friction between the coil and the teeth of the threads, which can also prevent the distal tip from coming off the inner tube. Threads can be cut into a section of hypotube bonded or insert molded into the inner tube. Threads can also be formed of a coiled wire bonded to the inner tube wall. 
     Referring now to FIGS. 18 and 19, a catheter assembly  200  having an inner tube  202  and a compressible distal tip  204  is illustrated in an assembly process. Compressible distal tip  204  is shown in an expanded, unconstrained configuration. Distal tip  204  can be made of a compressible polymeric material such as silicone (i.e., polydimethylsiloxane) or polystyrene. FIG. 19 illustrates catheter assembly  200  having a compression tube  206  forced proximally over distal tip  204 , thereby compressing the tip within the compression tube, causing the tip to have a reduced outer diameter. With the compression tube in place, stent  50  can be slid proximally over compressed tip  204  and compression tube  206 , to be restrained within retractable sheath  34 . Once stent  50  is in position, compression tube  206  can be retracted distally, allowing distal tip  204  to expand such that the radial outer extent of the distal tip is at least about as large as that of the retractable sheath distal end. 
     Referring now to FIGS. 20 and 21, a catheter assembly  210  having an inner tube  212  and a swellable distal tip  214  is illustrated in the process of assembly. In FIG. 20, stent  50  and retractable sheath  34  have already been placed in position. In the compressed configuration, distal tip  214  has a sufficiently small profile to allow stent  50  to pass proximally over the tip and within the retractable sheath. Once stent  50  is in position, swellable tip  214  can be expanded in profile, or swollen, by adding water or another chemical suitable to swell the distal tip material in size. Examples of material suitable for making a swellable tip include hydrogels and polystyrene. Examples of solvents or compounds suitable for swelling the distal tip include isopropyl alcohol, heptane, water, xylene, and toluene. FIGS. 21 and 22 thus illustrate devices and methods for providing a catheter which can be loaded with an expandable stent while the distal tip is small in profile. The distal tip can later be increased in size by swelling the catheter tip to provide a distal tip having a profile at least as large as that of the retractable sheath distal end. 
     Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.