fig3 shows only the distal tip of the delivery system , but the remainder of the system is not part of the contribution which the present invention makes to the art and , in any event , is familiar to those skilled in this art . the basic components of a conventional delivery system for a self - expanding stent are an inner catheter and an outer sheath , the purpose of the outer sheath being to confine the self - expanding stent radially , to the small radius delivery configuration , until its release at the site of stenting . the purpose of the inner catheter is to restrain the stent from proximal movement with the sheath , while the sheath is being withdrawn proximally . looking at fig3 of the drawings , the outer sheath 10 of the delivery system has an integral tapered tip 12 which narrows down to an end ring 14 of a diameter appropriate to receive a guidewire ( not shown ). confined within the sheath is a covered stent of which the structural foundation is a stent body 20 which is an apertured tube of nickel titanium shape memory alloy . the stent is covered by an outer layer 22 of eptfe on the abluminal surface of the stent body , and a covering layer 24 of eptfe on the luminal inner surface of the stent body 20 , with the inner and outer layers 24 and 22 being fused together where they can be pressed together within the apertures 26 of the stent body . between the luminal and abluminal surfaces of the stent body 20 is a wall thickness of the metallic stent material annulus . this annulus lies between the luminal and abluminal major surfaces of the stent body and , in the specification , we use the terminology “ envelope ” to indicate the generalised surfaces of the luminal and abluminal major wall surfaces of the stent body . thus , the outer layer 22 lies outside the abluminal envelope stent body 20 , except where it protrudes into the apertures 26 for fusing with the inner layer and , likewise , the inner layer 24 lies radially within the luminal envelope of the stent body 20 except where it protrudes radially outwardly into the stent body apertures 26 . the stent body carries a ring of tantalum radiopaque markers 28 at its distal end and a second ring of radiopaque tantalum markers 30 at its proximal end . it will be appreciated that the presence of these markers may further militate against pushing structures that bear against the end surface of the stent to be deployed . the inner catheter 40 defines a guidewire lumen 42 . conveniently , the inner catheter 40 is based on a stainless steel hypo tube . this of course endows the entire delivery system with substantial pushability , but the hypo tube can also be made remarkably flexible for the desired trackability of the system through particularly tortuous bodily lumens . in any event , if stainless steel is not flexible enough for the distal zone of the delivery system , then it would be feasible to build the inner catheter 40 from other more flexible materials such as particular polymers . the stent delivery system can be arranged as an over the wire system with a full length guidewire lumen , or a rapid exchange system with a guidewire lumen only in a distal zone of the system . the outer sheath 10 can be withdrawn by a full length outer catheter or a pull wire within a shaft lumen . for an example of delivery systems of the present applicant , see wo 03 / 003944 and wo 2004 / 062458 . the inner catheter has an abluminal surface 44 which carries on it a wire 46 arranged as a helix so as to provide a plurality of protrusions ( at least when seen in section as in the drawing ) on the abluminal surface 44 . in the illustrated embodiment , the wire is of stainless steel , fixed to the stainless steel tube 40 by deposits 50 of a bonding material which could be a weld bead or a suitable adhesive . in any event , as can be seen on the drawing , when the stent body is radially inwardly compressed down onto the inner catheter 40 , the inner eptfe layer 24 deforms to accommodate the protrusions 48 , but the protrusions 48 do not reach radially outwardly as far as the luminal envelope of the stent body 20 . in use , when the illustrated distal tip zone has been brought to the site of stenting , the outer catheter 12 is carefully and progressively withdrawn proximally so that the tip stretches and slides over the outer eptfe layer 22 of the stent , progressively releasing the stent , starting at its distal end near the markers 28 . as the stent progressively expands , the inner eptfe layer 24 moves radially outwardly away from the protrusions 48 until , with complete withdrawal of the tip 12 proximally beyond the proximal ring of radiopaque markers 30 , the stent is fully released . it will be appreciated that there is then a substantial annular gap between the lumen of the expanded stent and the envelope containing the protrusions 48 , enabling the inner catheter 40 also to be withdrawn proximally from the lumen of the stent without any snagging of the inner catheter 40 on any part of the stent . it will be appreciated that , for loading a stent into a sheath , an analogous sequence of steps may be performed , with radially inward compression of the stent body down onto the protrusions 48 of a loading tool which has a shape in section analogous to that of the inner catheter 40 . once the stent has been so compressed , a suitable sheath can be offered up to one end of the compressed stent tube , and then the stent can be urged axially into the sheath by imposing an axial force on the line of protrusions 48 through the tube 40 on which they amounted , so that this force is transferred from the protrusions 48 to the inner layer 24 and thence to the stent body 20 and the outer layer 22 , so that the entire covered stent device is urged by the protrusions 48 into the receiving sheath . a particular advantage of the helical structure of protrusions 48 as shown in the drawing is that the pusher within the stent lumen can be removed trouble - free from the lumen of the stent even when it is in a compressed configuration within a sheath as shown in the drawing , simply by “ unscrewing ” the pusher from within the stent lumen . drawing fig1 and 2 show a suitable loading tool 60 , long enough to push the covered stent along the full length of the outer catheter 10 , after being compressed and introduced and advanced into the proximal end of the outer catheter . the tool 60 features at its distal end a radially - outwardly protruding wire spiral 62 with a configuration corresponding to that of the protrusions 48 and the inner catheter 40 ( although non - corresponding configurations are also feasible ). the covered stent is compressed around the protrusions 62 before the tool 60 is used to urge the covered stent by means of the protrusions 62 , from the proximal to the distal end of the outer catheter . the illustrated embodiment shows a system in which the tapered distal tip of the stent delivery system is carried on the distal end of the outer catheter . those skilled in the art are well - aware that many proposed delivery systems feature a tapered tip on the inner catheter instead . the present invention is just as useful in such systems as it is in systems , as illustrated , with the tapered tip on the outer catheter . the stent on which the present device operates can be an covered self - expanding stent . the stent which is the basis of the illustrated embodiment is the one that is the preferred embodiment of wo 2002 / 015820 which is cut from a nickel - titanium tube . however , the invention is equally applicable to other stent design philosophies , such as stents fabricated from wire ( one example is the gianturco “ z ” stent made from zig zag wire rings ) or other metals , such as stainless steel . the invention is particular useful for covered stents in which only the cover connects adjacent ones of a plurality of stenting rings , because the engagement of the pusher over the full length of the stent should avoid any tendency for the stent covering to “ concertina ” between the stenting rings when pushed only from its trailing ( usually proximal ) end . those skilled in the art will be able to recognize from this disclosure many other ways to realise the present invention besides that described with reference to the drawings .
a method of deploying a stent includes providing a delivery system with the stent loaded in a reduced diameter configuration between an inner catheter and an outer sheath , the stent including a covering positioned on a luminal wall surface thereof , the inner catheter including a radially outwardly extending protrusion that extends into the covering but does not intersect a plane along the stent luminal wall surface , advancing the delivery system to a stenting site , and withdrawing the outer sheath to deploy the stent at the stenting site .