Abstract:
A pusher assembly for a delivery system for a self-expanding stent ( 6 ) which is delivered by proximal withdrawal of a sheath ( 4 ) radially surrounding the stent has a stent pusher element ( 8 ) which defines a lumen for a guidewire ( 2 ), a pusher strand ( 16 ) that extends to the proximal end of the delivery system and bears an end-wise compressive stress during release of the stent. A transfer shaft ( 12 ) links the distal end of the pusher strand ( 16 ) to the pusher element ( 8 ) and lies side-by-side with the guidewire ( 2 ). In a rapid exchange version, an adapter ( 20 ) provides two lumens side-by-side, one ( 22 ) carrying the pusher strand ( 16 ) and the other ( 14 ) defining a proximal guidewire part. To the adapter is mounted the proximal end of the stent sheath ( 4 ). The system allows modular ( 14 ) construction, a tapered tip ( 4 A) on the sheath, and an uncluttered internal configuration which facilitates passage of pulses (F) of liquids from the proximal to the distal end of the system.

Description:
TECHNICAL FIELD 
     This invention relates to a stent pusher assembly, and to a delivery system having a rapid-exchange configuration for deploying a self-expanding stent at a stenting site within a human or animal body. 
     BACKGROUND AND CONTENT OF THE INVENTION 
     EP-A-1 095 634 (EP 634) discloses all features of the preamble of independent claims  1  and  11 . EP 634 discloses a system in which the soft atraumatic distal tip of the system is at the leading end of the inner catheter. The outer sheath of the delivery system has a distal end which stops proximally short of the atraumatic tip. 
     Stents to be deployed at a stenting site within a human or animal body expand radially in the course of delivery, from a radially compact delivery disposition to a radially larger deployed disposition. In self-expanding stents made of stainless steel, the deformation of the stent is below the elastic limit, the stent until its deployment being radially confined and under elastic stress and typically released by proximal withdrawal of a confining sheath while the stent is itself prevented from moving proximally with the confining sheath by abutment with a stop on the distal end of a catheter shaft which suffers axial compressive stress while the surrounding sheath is proximally withdrawn. 
     By contrast, stainless steel stents which are relaxed in a radially compact disposition suffer plastic deformation when expanded into their deployed disposition by inflation of a balloon within the lumen of the stent. 
     An early example of stainless steel self-expanding stents is Gianturco U.S. Pat. No. 4,580,568 and an early example of the balloon expansible stainless stent is Palmaz EP-A-221 570. 
     A third category of stent is the memory metal stent, made of a biologically compatible nickel-titanium shape memory alloy with martensitic and austenitic phases. At body temperature, the stent “seeks” to revert to the austenitic phase. Typically it is confined within a surrounding sheath and again released at the stenting site by proximal withdrawal of this sheath. 
     The present invention offers improvements in systems to deliver those stents which are brought to the stenting site within a confining surrounding sheath. 
     In the technical field of stenting, there is a desire to reduce the transverse dimensions of the stent delivery system. In this field, the widely used measure of transverse cross-section is the unit of “French”, often abbreviated to “F” which is a one third part of a millimeter. Thus, a 6F (six French) delivery system has a diameter of 2 millimeters. 
     For any particular stenting operation, one has to select a particular stent and a particular delivery system. There is a large choice in both of these elements. Accordingly, it would be an advantage for manufacturers of stents and their delivery systems to achieve a degree of modularity in the design and construction of stents and their delivery systems. For example, there is a wide range of stents which could be delivered by a six French delivery system and it would therefore be convenient for the manufacturer of a stent delivery system to be able to tailor a basic six French system to fit any particular stent which would be compatible with a six French delivery system. This would reduce costs, to the advantage of patients, while retaining full flexibility for medical practitioners to optimise their choice of stent for any particular patient. 
     Like many catheter systems, a stent delivery system is often used with a flexible guidewire. The guidewire is preferably made of metal, and is slidably inserted along the desired body passage. The delivery system is then advanced over the thus pre-placed guidewire by “backloading” or inserting the proximal end of the guidewire into a distal guidewire port leading to a guidewire lumen defined by the delivery system. 
     Many conventional delivery systems define guidewire lumens that extend along the entire length of the outer sheath. These delivery systems are described as “over-the-wire” delivery systems, in that the delivery system is guided to the site of the stenosis over the guidewire, the guidewire thereby exiting the delivery system at the proximal end of the delivery system. “Over-the-wire” delivery systems provide several advantages, including improved trackability, the ability to flush the guidewire lumen while the delivery system is inside the patient&#39;s body, and easy removal and exchange of the guidewire while the delivery system remains in a desired position in the patient. 
     In some circumstances, however, it may be desirable to provide a “rapid exchange” delivery system, which offers the ability more easily to remove and exchange the delivery system while retaining the guidewire in a desired position within the patient. In a rapid-exchange delivery system, the guidewire occupies a lumen located only in the distal portion of the delivery system. The guidewire exits the delivery system through a proximal guidewire port, closer to the distal end of the delivery system than to its proximal end, and extends in parallel along the outside of the proximal portion of the delivery system. 
     Because a substantial length of the guidewire is outside the delivery system, it may be manually held in place close to the point where it passes the entry point on the body of the patient, as the delivery system is removed. This facilitates handling, removal and exchange of the delivery system for the practitioner for the following reasons. 
     With a guidewire lumen that is much shorter than the full catheter length a single physician can insert and remove a stent delivery system into and from the patient&#39;s body. Whereas over-the-wire delivery systems require a guidewire having a length at least double the length of the outer catheter, the rapid-exchange configuration allows the use of much shorter guidewires which enable a single physician to handle the proximal end of the guidewire at the same time as the catheter at the point of its entry into the body of the patient. 
     Accordingly, the present invention advantageously provides a stent delivery system having a rapid-exchange configuration for delivering and deploying a self-expanding stent. 
     Stents themselves cannot be directly seen during their journey to the stenting site, nor can one directly see whether the stent is exactly located as desired within the stenting site. Rather, indirect means have to be used to follow the progress of the stent through the body and make sure that it is correctly located before it is deployed. Thus, a stent delivery system is used during deployment to carry radiopaque contrast or marker fluid to the stenting site so that the target stenosis can be seen through the reduced amount of radiopaque fluid in the bodily lumen at the stenosis. This radiopaque fluid is generally injected through an injection port at the proximal end of the delivery system and through an annular space between an outer sheath of the delivery system and a proximal portion of an inner catheter shaft. The visibility of the stenosis is adversely affected when the lumen, through which radiopaque contrast fluid is injected, is too small to deliver a strong pulse of contrast fluid. As pulses of fluid are used for visualisation, the effectiveness of visualisation depends on the volume flow in each pulse. This in turn depends on the ease of flow of the fluid along the full length of the delivery system, from the point of injection at the proximal end, to the stenosis beyond the distal end of the delivery system. 
     Thus, delivery systems which offer a large cross-section and unimpeded lumen for contrast fluid will be favoured by radiologists, other things being equal. The visibility can additionally be increased by further reducing the resistance of the system to pulses of contrast fluid. It is therefore an object of the present invention to provide good visualisation with contrast fluid, without sacrifice of other important performance aspects of the delivery system, including pushability and low overall diameter. By increasing “pushability” we mean the capability to be advanced longer distances along narrower and more tortuous bodily lumens. 
     Furthermore, the delivery system invariably carries at least one radiopaque marker at a known location relative to the length of the stent, so that radiologists can be sure of the location of the ends of the stent, on the basis of their knowledge of the location of the radiopaque marker. Even if the stent is rendered sufficiently radiopaque for it to be seen, it is still useful to have a radiopaque marker on the distal end of the delivery system, to reveal successful separation of the stent from the delivery system. 
     Thus, in our example of a six French delivery system, to be used for delivering stents of various lengths, there will be a wish to provide radiopaque markers within the delivery system at two spaced-apart locations on the axis of the delivery system, corresponding to the opposite ends of the stent (until the stent is deployed out of the system). One object of the present invention is to offer a degree of modularity in this design aspect. 
     With delivery systems having a rapid-exchange configuration, just as with over-the-wire systems, the stent delivery system is advanced over the guidewire, itself normally within a guide catheter, in order to bring the distal tip and stent to the stenting site. Depending on the application, different diameter guidewires are specified. Two commonly used guidewire diameters are 0.46 mm/0.018 inches and 0.89 mm/0.035 inches (commonly known as 18 thou or 35 thou guidewires). Thus, a further degree of modularity can be achieved by offering a delivery system which is compatible with a range of guidewire diameters, specifically, both 18 thou and 35 thou guidewires. 
     It would be an advantage for any new stent delivery system to be able straightforwardly to take the place of those previous delivery systems which individual practitioners have grown to be comfortable using. One such system uses in its proximal portion a metallic rod, which can be either solid or hollow, made of stainless steel. 
     Good design for stent delivery systems is indicated by manufacturing steps which can be performed with high precision and reliability, yet with acceptable cost levels. This is yet another objective of the present invention. 
     Finally, for any system which is extremely long in proportion to its diameter, and features at least three co-axial elements, the cylindrical surfaces of these co-axial elements need to be so composed and conformed that friction between them is low enough that the co-axial elements can be moved tolerably easily axially relative to each other. It is yet another object of the present invention to provide systems which enable bringing these friction levels down to advantageously low levels. 
     Another consideration when a self-expanding stent is released progressively by successive proximal stepwise movements of the outer confining sheath results from the delivery system typically being extremely long in proportion to its cross-sectional dimensions, and constructed predominantly or wholly from synthetic polymeric materials which have substantial elasticity and marked kinetic aspects to their deformation characteristics. In such a case, any particular rate of strain imposed on the proximal end of the outer sheath is likely to be experienced at the distal end of the same sheath in a somewhat different strain rate. For example, a fast squeeze of the trigger of a deployment system at the proximal end of the sheath will likely result in a somewhat slower resulting proximal advancement of the distal end of the same sheath. Furthermore, a pull on the sheath will impose compressive stresses along the length of the inner shaft, likely leading to a proximal movement of the stent which then relaxes back to the original, more distal, position of the stent as the tensile stress in the outer sheath eases back towards zero. In its own delivery systems, present applicant has observed what happens at the distal end of a stent delivery; system during successive squeezes of the trigger of a delivery system which pulls the outer sheath proximally in a series of steps. The appearance at the stent end of the system is as if the system were “breathing” in that it, and the stent, moves axially first proximally, then distally, with each squeeze of the trigger. 
     This “breathing” phenomenon is of course a complicating factor when it comes to precision of placement of the stent within any particular stenting site. It is yet another object of the present invention to ameliorate this problem. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a pusher assembly for a delivery system for a self-expanding stent, the pusher assembly constituting a catheter shaft with a proximal pusher end to receive an end-wise compressive force and a distal pusher end to deliver said force to a stent to be delivered, the pusher assembly comprising a pusher strand extending from the proximal pusher end to a distal strand end which is nearer the distal pusher end than the proximal pusher end; a pusher element which abuts the stent in use to deliver said force to the stent; and a transfer shaft having a proximal and a distal end, the proximal end being connected to the distal tube end and the distal end being connected to the pusher element and characterised in that the pusher element defines a guidewire path, and the transfer shaft lies to one side of said path. 
     By contrast, in earlier systems such as that of EP 634 in which the atraumatic tip is carried on the inner catheter, the pusher element is mounted on a tube which has a guidewire lumen and extends distally all the way to the tip. 
     According to another aspect of the present invention, there is provided a stent delivery system having a rapid exchange configuration for a self-expanding stent which provides improved visualisation through an increased volume flow in each pulse. The volume flow in each pulse is increased in the present invention due to a simplified and reduced internal structure of the delivery system. 
     The scheme of a simplified delivery system is represented in  FIG. 1  which shows the essential features of a basic delivery system including an outer sheath  4  confining the stent  6  in a radially compressed state and a pusher element  8  preventing proximal movement of the stent when the outer sheath  4  is proximally withdrawn. The pusher element is carried on an inner catheter shaft  3 . Here, the delivery system is inserted over a guidewire  2  into a lumen of a human or animal body. 
     In one of its aspects, the present invention employs a short inner catheter shaft so that its distal end is relatively close to the proximal guidewire lumen exit port. In conventional delivery systems, the inner catheter shaft  3  extends beyond the distal end of the stent  6  to provide a tapered tip, for ease of insertion of the delivery system into the patient&#39;s body and for reducing trauma whenever the catheter is advanced distally. Above-mentioned EPO 634 discloses a stent delivery system which conforms to this conventional model. 
     In the present invention, using the pusher element to define at least a short distal guidewire lumen, and providing the system tip taper on the distal end of the outer sheath, renders redundant an inner catheter within the stent and distal of the stent. Therefore, the internal structure of the delivery system is more open, which consequently enhances ease of flow and the volume of contrast fluid that can be ejected from the distal end of the delivery system with each successive pulse imposed from the proximal end of the delivery system. Hence, visualisation is improved. 
     In another aspect of the invention the manufacturing and assembling steps required to get the delivery system of the present invention ready for use are minimised due to the simplified internal structure. There exists no longer the need for keeping the stent at a fixed position on the inner catheter shaft while the outer sheath is fitted over the stent. Also, the risk of advancing the stent too far distally and out of the distal opening of the outer sheath during assembly of the delivery system is minimised, since the outer sheath in the present invention comprises the tapered tip which acts as a distal stopper for the stent during assembly. 
     The introduction of a stent using the stent delivery system of the present invention, and subsequent removal of the delivery system, is facilitated especially in tortuous vessels and other body lumens having a relatively narrow diameter because, once the stent has been placed at a desired site inside the patient&#39;s body, there need be no component of the delivery system which is radially inwardly located from the stent and which has to be proximally withdrawn through the stent lumen. Especially in narrow and sharply curved body vessels, this might introduce a risk that the distal tip being withdrawn through the stent lumen interferes with bodily tissue protruding radially inwardly through the interstices of the stent and into the stent lumen. The delivery system of the present invention avoids this problem by providing the tapered tip on the distal end of the outer sheath so that, during removal of the delivery system out of the patient&#39;s body, there need be no system components which travel proximally within the stent lumen and are likely to engage with the inner surface of the stent. 
     In one preferred embodiment, the pusher element is a cylinder which has a distal-facing end face at the distal end of the cylinder to push on the proximal end of the stent. Thus, the end face will likely be flat and transverse to the axis of the cylinder. The pusher element can serve as, and preferably does serve as, a radiopaque marker. 
     If desired, the pusher element can also serve as a mount for a distal marker carrier tube cantilevered distally forward from the pusher element to lie within the space that will correspond to the lumen of the stent to be deployed by the system. This is useful when it is required to have on the delivery system a radiopaque marker for the distal end of the stent. This radiopaque marker can be placed on the carrier tube at a position at or towards the distal end of the carrier tube and corresponding to the distal end of the stent. For stents of different lengths, the length of the carrier tube can easily be varied to correspond to the stent length, prior to fixing the distal marker on the carrier tube. 
     It will be appreciated that the carrier tube requires relatively little strength, so can be made thin and flexible, thereby reducing the risk of its interfering with tissue protruding through the stent during its withdrawal from the stenting site. 
     As the carrier tube is a relatively simple and isolated part of the delivery system, and conveniently made of a synthetic polymeric material, it will be a relatively simple matter to change the length of the carrier tube to suit any particular stent destined to be carried on the system. If desired, the carrier tube can be extended backwardly proximally from the pusher element and given a bell end or flared end outwardly proximally. This flared end provides security against the possibility of unwanted distal slippage of the carrier tube distally through the pusher element and of being left behind in the body when the delivery system is withdrawn. It may also be useful to guide the guidewire through the system whenever there is need to introduce the distal end of the guidewire from the proximal end of the system. 
     In yet another aspect of the invention, the modular construction of the delivery device results in fewer steps during manufacturing and assembly of the stent delivery system. The device may be modularized by using a transfer shaft connecting the rod or inner catheter with the pusher element. This can be set to any desired length, to accommodate stents of different length in a delivery system which features standard length catheter components such as the sheath, rod or inner catheter and pusher tube. It may be convenient to use a welded joint to fasten one or both of the two ends of the transfer shaft to the pusher element and rod, respectively. 
     For a better understanding of the various aspects of the present invention, and to show more clearly how its several features can be carried into effect, individually or in selected combinations reference will now be made, by way of example, to the accompanying drawings of embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows in longitudinal axial section the distal portion of a prior art delivery system; 
         FIG. 2  is a cross-section of the distal portion of a delivery system having a rapid-exchange configuration in accordance with a preferred embodiment of the present invention; 
         FIG. 3  shows an isometric view of the adapter having two lumens effecting the rapid exchange configuration; 
         FIG. 4  shows a cross-section of the proximal portion of the delivery system, the pull-back device used to proximally retract the outer sheath, in accordance with a preferred embodiment of the present invention; 
         FIG. 5  shows a cross-sectional view of the distal portion of an over-the-wire pusher assembly according to a second embodiment of the invention; 
         FIG. 6  shows a cross-sectional view of the distal portion of another over-the-wire pusher assembly according to a third embodiment of the invention; 
         FIG. 7  shows a cross-sectional view of the distal portion of yet another over-the-wire assembly, being a fourth embodiment of the invention; 
         FIG. 8  shows at larger scale the distal tip portion of the  FIG. 7  embodiment; and 
         FIG. 9  shows at the scale of  FIG. 8  a part of the  FIG. 7  distal portion which is proximal of the tip shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments of the present invention is merely illustrative in nature, and as such does not limit in any way the present invention, its application, or uses. 
       FIG. 2  shows a cross-section of the distal portion of a delivery system having a rapid-exchange configuration in accordance with a preferred embodiment of the present invention. 
     In  FIG. 2 , a guidewire  2  extends beyond the distal end of the distal portion of the delivery system along which the stent delivery system is advanced to the site of the stenosis inside the patient&#39;s body. The stent  6  is held in a radially compressed state by means of an outer sheath  4  the distal end of which constitutes the distal end of the stent delivery system. The distal tip  4 A of the outer sheath  4 , as shown in  FIG. 2 , is tapered in order to facilitate advance of the stent delivery system along a bodily lumen. Furthermore, the outer sheath  4  comprises a radiopaque marker  27  the position of which is indicative for the distal end of the stent until deployment of the stent. The guidewire  2  extends all the way through the outer sheath lumen and exits the distal portion of the stent delivery system at a proximal guidewire port  24 . 
     A pusher element  8  abutting the stent  6  in use of the delivery system prevents proximal movement of the stent  6  when the outer sheath  4  is withdrawn proximally to release the stent. The pusher element  8 , which at the same time serves as a proximal radiopaque marker, is connected to a transfer shaft  12 . The pusher element  8  is preferably laser-welded to the distal end of the transfer shaft  12 . For ease of connection the distal end of the transfer shaft  12  is tapered and embedded in a respective slot provided in the proximal end of the pusher element  8 . The distal end of the transfer shaft  12  is tapered, and the transfer shaft  12  is corresponding oblate at its distal end, so that the distal end of the transfer shaft  12  can be fitted into a respective slot of the adjacent pusher element  8 , with the circumferential surface over a specific arc length of the oblated end being flush with the circumferential surface of the pusher element  8 . The slot provided in the proximal end of the pusher element  8  has an axial length which extends from the proximal end of the pusher element  8  beyond midway along the axial length of the pusher element  8 . This ensures a sufficiently rigid connection of the transfer shaft  12  with the pusher element  8 . Such shaping of the distal end of the transfer shaft  12  and the pusher element  8  optimises the flow of injected contrast fluid F, since the fluid does not meet any unnecessary barrier when travelling along the length of the transfer shaft  12 . In this way, the flow resistance of the injected contrast fluid F is minimised. 
     The transfer shaft  12  is capable of receiving an endwise compressive force C and transmitting the force C to the proximal end of the stent  6 , thereby preventing proximal movement of the stent  6  when the outer sheath  4  is withdrawn proximally by imposition of tensile force T on the sheath  4 . The arrows in  FIG. 2  are indicative for the direction of the respective forces T and C. 
     A connection piece  14 , such as a tube, at the proximal end of the transfer shaft  12 , as shown in  FIG. 2 , enables the accommodation of different stent lengths in an unchanged sheath  4  by an appropriate adjustment in the length of the transfer shaft  12  in accordance with the length of the respective stent  6 . 
     The cut-to-length transfer shaft end within the connection tube  14  is glued or soldered to the connection tube  14 . The proximal end of the transfer shaft  12  is directly connected to the distal end of the rod  16  by means of a solder joint or glue. Otherwise, the connection tube  14  can be no more than a collar into which two adjacent ends of separate transfer shaft portions are inserted end-to-end and approximated, such that both abutting ends of the transfer shaft  12  portions are in physical contact with each other inside the collar. Therefore, there is no relative axial movement of the two adjacent ends of the transfer shaft  12  portions within the collar. Thus, the longitudinal force transmission between the proximal end of the tube  16  receiving the endwise compressive force C to the proximal end of the stent  6  is optimised. 
     The proximal end of the distal portion of the stent delivery system, as shown in  FIG. 2 , comprises an adaptor  20  having two lumens  22 ,  24  for effecting the rapid-exchange configuration. The guidewire  2  exits the distal portion of the stent delivery system through a guidewire port  24  of the adaptor  20 , so as to be exposed outside the stent confining sheath  4  to enable the rapid exchange. The guidewire port  24  is preferably off-centre of the adapter  20 . The orifice of the second lumen  22  is defined by a pipe  18 . 
     Referring to  FIG. 2 , a rod  16  being part of the pusher assembly and preferably made of metal abuts at a distal end thereof the proximal end of the transfer shaft  12  inside the connection piece  14 . Its proximal end extends beyond the proximal end of the pipe  18 . The rod  16  extends distally from the distal portion of the delivery system through the second lumen  22  of the adapter. At its proximal end it receives the endwise compressive force C. 
     In a further embodiment of the present invention, not shown, the rod  16  can be provided as a tube with a lumen running from the proximal end of the system to the lumen of the pipe  18 . 
     In both embodiments, the pipe  18  is connected to the adaptor  20  and furthermore, the adaptor  20  is connected to the outer sheath  4 . The integrity of this connection is somewhat crucial for the proper functioning of the delivery system, since the outer sheath  4  is usually made of a polymeric material whereas the adaptor  20 , the rod  16  (or tube), and the transfer shaft  12  are preferably made of metal, such as stainless steel. Metal-to-polymer connections are normally made by means of an adhesive. 
     To permit sufficient rigidity and to provide a rupture-resistant connection of the pipe  18  through the adaptor  20  to the outer sheath  4 , the pipe  18  is advantageously welded into a recess of the adaptor  20 . Tension studs  20 A, as shown in  FIG. 3 , are provided in the proximity of the distal end of the adapter  20  to engage along the entire circumference of the adapter  20  with individual strands of a braid  43  encapsulated by the polymeric material of the outer sheath  4 . The tension studs  20 A protrude radially outwardly into the interstices of the braid  43  to reduce the dependence on glue to prevent rupture of the connection between the adapter  20  and the outer sheath  4 . The stud to braid link between the pipe  18  and the outer sheath  4  via the adapter  20  feature metal all the way from one end of the system to the other so that the risk that the adhesive joint between the adapter  20  and the outer sheath  4  may break is reduced and the strain suffered by the system in releasing a stent is also kept small. Other type of connections will be apparent to those skilled in the art and an explicit explanation thereof is therefore omitted. 
     When using the stent delivery system, a tensile force T acts on the pipe  18 , thereby proximally displacing the outer sheath  4  to release the stent  6 , and at the same time a compressive force C is received by the tube or the rod  16  at its proximal end and is transmitted to the transfer shaft  12  in order to prevent proximal displacement of the stent  6  during stent deployment. 
     Since the pusher element  8  provides a lumen for the guidewire, abuts the stent  6  in use and is supported axially by the transfer shaft  12 , and since the stent  6  is self-expanding and so is pressing radially outwardly on the sheath  4 , there is no need for an inner catheter to extend beyond the proximal end of the stent  6 . The tapered tip  4 A of the sheath  4  facilitates advance of the catheter system through a tortuous lumen of the patient&#39;s body. The tapered tip  4 A also resists inadvertent or premature distal movement of the stent  6  relative to the sheath  4 , such as when the delivery system is introduced into a narrow vessel inside the patient&#39;s body. In this way, the tapered tip  4 A of the outer sheath  4  can act a distal stopper for the stent. 
     For a detailed description of such tapered tips and their use, see Applicant&#39;s WO 01/34061. 
     A distal marker carrier  10 , itself carried on the pusher element  8 , exhibits a length sufficient to project distally beyond the stent  6  and defines a lumen for the guidewire  2 . In use, the guidewire  2  extends along an axial path which lies side by side with the transfer shaft  12 , which shaft  12  is off the axis of the outer sheath  4 . The proximal end of the distal marker carrier  10  is attached, conveniently by glue, to the inner surface of the pusher element  8  to fix its axial position. The proximal end of the distal marker carrier  10  has a flared end or shows some sort of tulip-shape which facilitates distal advancement of the guidewire  2  through the pusher assembly of the delivery system. The fixing established by the glue and the flared ends also reduces the likelihood of separation of the carrier tube  10  from the pusher element  8 . 
     The distal marker carrier  10  carries a distal marker  26 , such as a radiopaque marker, indicating the position of the distal end of the stent  6 . The inner surface of the distal marker  26  is flush with the inner surface of the distal marker carrier  10  for undisturbed elative axial movement of the guidewire  2 . Preferably, a particular heat treatment is employed to attach the distal marker  26  to the distal marker carrier  10 , so that the distal marker is partially fused together with the distal marker carrier  10 . It is also conceivable to embed or swage the distal marker  26  into the distal marker carrier  10  because the material used for the distal marker carrier  10  is relatively soft, preferably a resin tube. 
     The distal marker carrier  10  is a polymeric tube whereas the pusher element  8 , the transfer shaft  12 , and rod  16  or tube  16  are made of metal, conveniently stainless steel. It is also conceivable to use other material combinations for these parts, such as nickel titanium shape memory alloy for the transfer shaft  12  and a composition of platinum/iridium (90/10) for the pusher element  8 . 
     The outer sheath  4  may also carry a marker band such as one  27  on its inner luminal surface just proximal of its tapered tip  4 A for marking the distal end of the outer sheath  4 . 
     Some applications require a thicker guidewire  2 , such as a 35 thou guidewire. In such cases, one may choose to omit the distal marker carrier  10 . Otherwise, one may choose to locate the marker  26  distal of the distal end of the stent in the free volume  40  between the stent and the tip  4 A, thereby minimising the consumption of lumen cross-section inside the stent lumen. The remaining structure of the pusher assembly can remain the same. Hence the versatility of the pusher assembly is increased because of its usefulness with guidewires of different diameters. 
       FIG. 3  shows an isometric view of the adapter  20 , preferably made of metal, such as stainless steel, effecting the rapid exchange configuration. The adapter comprises two lumens  22 ,  24  one of which is a guidewire lumen  22  and the other one of which permits the rod  16  or tube to exit the adapter. Lumen  24  of the adapter is defined by two opposing arcuate segments  23 A and  23 B. The pipe  18  is introduced into lumen  24  of the adapter  20  from the proximal end of the adapter which has the shape of a mushroom until it abuts the distal end of a recess (not shown). In this manner, the adapter does not need to have a circumferential side wall which encloses lumen  24  by 360°. Hence, the lateral dimensions are minimised. Furthermore, as shown in  FIG. 3 , tension pins (studs)  20 A are provided on the outer circumferential surface of the distal portion of the adapter  20  engaging with the braid  43  which is encapsulated by the polymeric material of the outer sheath  4 . Lumen  24  which is a guidewire lumen is located off-centre of the adapter  20  and allows the guidewire  2  to exit the delivery system to effect the rapid-exchange configuration. The adapter is preferably made of metal, such as stainless steel, but the use of other alloys is conceivable. 
     Referring now to  FIG. 4 , a cross-sectional view of the proximal portion of the stent delivery system is shown. The proximal portion is part of a pull-back device used for proximally retracting the outer sheath  4  to release the stent  6 . The pipe  18  which is connected to the sheath  4  via the adapter  20  is linked to an adapter ring  36 . A welded joint is preferably be used for the link but other types of joints may be used, such as glue or an interference fit etc. The adapter ring  36  is joint to a polymeric sleeve  38  fitted into the distal portion of a distal hub  40 . As the distal hub is successively pulled back proximally with every squeeze on the trigger of the pull-back device (not shown), a proximal hub  46  at the proximal end of the rod  16  or tube is held stationary at the same time by a compressive force being transmitted from the proximal hub  46  via rod  16  and transfer shaft  12  to the pusher element  8 . In this way, controlled release of the stent at a desired position inside the patient&#39;s body is achieved. 
     The proximal portion of the stent delivery system further provides the possibility to insert contrast fluid through the Luer-adapter  42  into the annulus between the distal hub  40  and a supporting member  44  being sealed by an O-ring  48  and connected to rod  16 . The contrast fluid passes beyond the distal end of the distal hub  40 , creeps through the gap between the adapter ring  36  and the rod  16  and emerges from the distal end of the pipe  18  finally to reach the distal end of the outer sheath  4  to get squirted out into the vessel of the patient&#39;s body. 
     The Luer-valve assembly  42  also comprises a safety lock for locking the axial movement of rod  16 , (the subject of Applicant&#39;s PCT/EP02/06782 and earlier British Patent Application No. 0114939.2), which ensures safe transport of the packaged delivery system without the risk of inadvertent release of the stent and to enable the physician to interrupt the stent deployment process, when needed, without having to be concerned with the displacement of the stent whilst the physician is not holding the delivery system in his/her hands. 
     The pusher assembly, as shown in  FIG. 5 , is destined to be used for an  18  thou guidewire  20 . The entire pusher assembly is enclosed by an outer catheter  4  of an over-the-wire stent delivery system prior to deployment of the stent  6 . In this condition the stent  6  is held in a radially compressed configuration by the same outer catheter  4 . For deployment of the stent  6 , the outer catheter  4  is withdrawn until the distal tip  63  is proximal of the proximal end of the stent  6 . 
     The pusher assembly incorporates a catheter shaft  66 , the distal end of which is connected to a transfer shaft  64 . A pusher element  68  is connected to the distal end of the transfer shaft  64 . During the course of stent deployment the distal end  69  of the pusher element  68  abuts the proximal end of the stent  6 . Thus, the pusher element  68  serves as a stop for the stent  6  during stent deployment, to prevent proximal movement of the stent as the outer catheter  4  is withdrawn proximally. 
     The proximal end of the pusher element  68  is laser-welded to the distal end of the transfer shaft  64  and the same manner of connection is used for connecting the proximal end of the transfer shaft  64  to the distal end of the catheter shaft  66 . 
     For ease of connection, both the distal and the proximal ends of the transfer shaft are tapered and embedded in respective slots provided in the proximal end of the pusher element  6  and the distal end of the catheter shaft  66 . The ends of the transfer shaft  64  are tapered such that the circular cross-section of the transfer shaft  64  between its ends is oblate at its ends, so that both ends can be fitted into respective slots of the adjacent pusher element  68  and catheter shaft  66 , with the circumferential surfaces over a specific arc length of both oblated ends being flush with the circumferential surface of the pusher element  68  and the catheter shaft. The slot provided in the proximal end of the pusher element  68  has an axial length which extends from the proximal end of the pusher element beyond mid-way along the axial length of the pusher element  68 . The length of the slot in the distal end of the catheter shaft  66  is much the same length, and long enough to ensure that a sufficient connection between the transfer shaft  64  and the catheter shaft  66  is obtained. Such shaping of the two ends of the transfer shaft and the pusher element  68  and the catheter shaft  66  maximises the flow of injected contrast fluid, since the fluid does not meet any unnecessary barrier when travelling along the length of the transfer shaft. In other words, the resistance to the flow of the injected contrast fluid is minimised. 
     A connection piece such as a tube  78  at an intermediate position of the transfer shaft  64  enables the accommodation of different stent lengths in an unchanged sheath  4  and catheter shaft  66 , by an appropriate adjustment in the length of the transfer shaft portions in accordance with the length of the respective stent. The two cut-to-length transfer shaft portion ends bridged by the connection tube  78  are either glued or soldered to the connection tube  78 . The connection tube  78  can be no more than a collar into which the two adjacent ends of the separate transfer shaft portions are inserted and approximated, such that both ends of the transfer shaft are in physical contact with each other inside the collar. Therefore, there is no relative axial movement of the two adjacent ends of the transfer shaft portions within the collar. 
     A distal marker carrier  74 , itself carried on the pusher element  68 , exhibits a length sufficient to project distally beyond the stent  6  and defines a lumen for the guidewire  20 . In use, the guidewire  20  extends along an axial path which lies side-by-side with the transfer shaft  64  which is off the axis of the outer sheath  4 . The proximal end of the distal marker carrier  74  is attached, conveniently by glue, to the inner surface of the pusher element  68  to fix its axial position. The proximal end of the distal marker carrier  74  has a flared end or shows some sort of tulip-shape for undisturbed distal advancement of the guidewire  20  through the pusher assembly of the delivery system. The fixing established by the glue and the flared end also reduces the likelihood of separation of the carrier tube  74  from the pusher element  68 . The distal marker carrier  74  carries a distal marker, such as a radiopaque marker  72 , indicating the position of the distal end of the stent. 
     The distal marker carrier  74  is a polymeric tube whereas the pusher element  68 , the transfer shaft  64 , the catheter shaft  66  and the connection tube  78  are made of metal, conveniently stainless steel. It is also conceivable to use other material combinations for these parts, such as nickel titanium shape memory alloy for the transfer shaft and a composition of platinum/iridium (90/10) for the pusher element  68 . 
     The distal marker  72  can be embedded or swaged into the distal marker carrier  74  because the material used for the distal marker carrier  74  is relatively soft, preferably a resin tube. 
     Some applications require a thicker guidewire  20 , such as a 35 thou guidewire. In such cases, the distal marker carrier  74  may need to be omitted, as shown in  FIG. 6 . The remaining structure of the pusher assembly can remain the same. Hence, the versatility of the pusher assembly is increased because of its usefulness with guidewires of different diameters. 
     Reverting to the embodiment shown in  FIG. 5 , however, a thicker guidewire can be accommodated if the distal marker  72  is moved to a position just distal of the distal end of the compressed stent  6 . To resist bowing of the pushing wire  64 , it can be bonded to an additional short length of tube mounted distally to the catheter shaft  66 . The bonding could be with glue. The mounting could be a telescopic mounting within the distal open end of the shaft  66 , the tube length glued to the said distal end and extending, cantilevered, distal of the distal end with the pushing wire glued to its outside cylindrical surface. Denial of bowing of the pushing wire within the lumen of the outer catheter should eliminate any substantial “lost motion” when the outer catheter is initially pulled back proximally, and the pushing wire  64  goes into compression, in the initial stages of stent release. 
     Drawing  FIGS. 7 ,  8  and  8  show another embodiment of the invention which is, in some respects, a hybrid of the embodiments of  FIGS. 5 and 6 . 
     In  FIG. 7  there is an inner catheter  140  of polymeric material, glued inside the stainless steel shaft  116  and extending distally to a distal tip zone  142  which lies distal of the stent  6 . Swaged around this distal tip zone is a distal marker  112 , lying just distal of a distal end of the stent  6 . For the remaining distal tip portion  142  of the inner catheter  140 , lying distal of the distal marker  112 , the diameter is slightly increased, as can best be seen in 
       FIG. 8 , which increases the security with which the marker  112  is retained on the inner catheter shaft  140 , with corresponding reduced likelihood of loss of the marker  112  by slipping off the distal end of the catheter  140 . As can be seen, the guidewire  20  extends through the shaft  116  and inner catheter  140 , being a relatively snug fit within this lumen. 
     Lying on the outside cylindrical surface of the inner catheter  140  is a transfer shaft  114  and connector  118 . With a sequence of glue spots  144 , the transfer shaft  114  is bonded to the inner catheter shaft  140 , thereby preventing any tendency for the transfer shaft  114  to bow when it is put in longitudinal compressive tension for release of the stent  6 . 
     As shown in  FIG. 8 , at the distal end of the transfer shaft  14  is the pusher  108  and this carries, on its outside cylindrical surface, an additional thin platinum/iridium radiopaque marker band  146 . A further marker  148  is integrated in the thickness of the outer catheter wall  4 , just distal of the stent  6 , overlying the marker  112  on the inner catheter  140 . During progressive deployment of the stent, by proximal withdrawal of the outer catheter  4 , the radiologist will be able to observe the progressive movement of the outer catheter marker  48 , proximally away from the distal stent marker  112  and towards through and beyond the proximal marker  146 . 
     In the following, some of the advantages of the subject pusher assembly are elucidated. 
     Since the catheter shaft tube  116 , the pusher element  108  and the guidewire  20  are all of metal, friction between the guidewire and the stent delivery system is low, and so PTFE or other special low-friction coatings can be omitted, thereby saving manufacturing costs. 
     During release of the stent, the transfer shaft remains under a more or less constant compressive strain once it has undergone a certain amount of bowing within the lumen of the outer catheter sheath  4  as a result of the proximal withdrawal of the outer sheath. This bowing typically reduces the distance between the pusher element  108  and the catheter shaft  114  by approximately 5 mm. The compressive strain suffered by the transfer shaft  14  remains constant throughout the deployment of the stent for as long as the outer catheter  4  is in axial tension. Hence, a precise placement of the stent with respect to the stenting site can be achieved and no significant “breathing”, as mentioned above, to be observed. 
     The simplified internal structure of the distal portion of the delivery system enables improved visualisation of the stenosis due an increased volume flow of contrast fluid with each pulse. 
     During release of the stent, virtually no proximal movement of the stent is seen, while the outer sheath is being withdrawn proximally. The present invention provides a metal structure all the way from the proximal end of the pull back unit receiving the endwise compressive force to the pusher element to keep the stent in place during stent deployment. Therefore, no adverse bowing of the force transmitting components is caused during stent release. Furthermore, the component that is withdrawn proximally, including the outer sheath  4 , can also exhibit metal-to-metal connections end-to-end. 
     As shown in the illustrated embodiments, the length of the transfer shaft, which preferably amounts to a maximum of 3 cm, is relatively short compared to its diameter, so that appreciable bowing is suppressed. In addition, the transfer shaft confined by the outer sheath and lying side-by-side to the guidewire inside the lumen of the outer sheath has nowhere to go when it seeks to bend under compression during stent release, thereby preventing shortening of the distance between the pusher element and the distal end of the rod. Hence, more precise placement of the stent with respect to the stenting site can be achieved. Furthermore, assembly of the system is facilitated and manufacturing cost are reduced. 
     The system is further adaptable to guidewires of different diameters, which enhances the versatility of the system and its acceptability to the practitioner. 
     The delivery system may be used in connection with a guiding catheter. The physician attempting to bring a stent to a stenosis site inside the patient&#39;s body uses an outer guide catheter to be first introduced in the patient&#39;s body. Once the guide catheter has been properly placed, a guidewire is introduced through the guide catheter lumen along which the delivery system is advanced to the site of the stenosis in a next step. Here, the contrast fluid to be used to visualise the stenosis can be injected, if the physician prefers to do so, through the gap between the internal surface of the guide catheter and the external surface of the delivery system. Hence, the annulus between the pipe  18  and the rod  16  or tube, shown in  FIG. 2 , can be further reduced in order to minimise the transverse dimension of the delivery system, which is advantageous in terms of both, the recovery of the patient and the handling comfort for the physician. 
     Prior to use of the delivery system, as is the case for any devices used to inject fluids into the human body, the delivery system needs to be vented and primed, i.e. the system is flushed with a biocompatible solution, such as a sodium chloride solution, until all the air confined inside the system has been driven out of the system. The delivery system of the present invention may be flushed with such a solution from the distal tip of the delivery system prior to use. This may enhance the practical usefulness of the delivery system, since the guidewire is also inserted into the delivery system from the distal end of the system, so that the physician can carry out the flushing and the guidewire insertion almost in one go. This allows the physician to choose the alternative with which he/she has grown most comfortable and which is best suited for the specific circumstances.