there now follows a description of one exemplary embodiment for putting the present invention into effect . fig1 shows a first embodiment of the present invention , being a delivery system for a self - expanding implant to line a bodily lumen . the inner components of the delivery system are essentially conventional , but will be described here to aid the reader in understanding the interaction between the various components of the system . defining an axis of the delivery system is inner catheter 11 , which runs from a distal end of the delivery system ( on the left hand side of the figure ) to a proximal end of the deli very system ( not shown in the figure but some distance beyond the right hand of the figure ). inner catheter 11 defines a lumen through which guide wire 2 runs . guide wire 21 is provided to be inserted percutaneously and guided through the body passages which the stent deliver system is to navigate before the deli very system itself i introduced , in order to more easily guide the proximal end of the stent delivery system to its intended location in the body . coaxial with the inner catheter , and located around it in a compressed configuration is implant 31 , in the present instance being a self - expanding nitinol stent . the stent is held in a radially compressed configuration onto the inner catheter by means of inner sheath layer 41 , which radially surrounds the stent and applies inwardly radial pressure thereto to maintain the stent in its compressed configuration . in the system depicted , inner sheath 41 extends distally and then folds back on itself at a distal turning point to return proximally as outer sheath 42 . this configuration is conventionally known as a roll - back design / as will be explained later in terms of stent deployment . outer sheath 42 extends proximally until a region a , where its radius reduces to that of pull portion 51 , where it attaches . pull portion 51 extends proximally to the proximal end of the deli very system to convey an actuating tensile force from the operator to outer sheath 42 . in contrast , push element 61 is provided to restrain the stent 31 from proximal axial movement relative to inner catheter 11 . accordingly , push element 61 is provided fixed in relation to inner catheter 11 , in some embodiments by means of the inward pressure of inner sheath 41 . atraumatic tip 91 is provided distal of stent 31 to shield the distal end of inner sheath 41 and outer sheath 42 from the body passages through which the stent delivery system travels , and vice versa . what has been described so far is for the most part conventional . however , the embodiment shown in fig1 also provides a confining structure 80 , including rod members 81 a , 81 b , 81 c , 81 d , 81 e and 81 f 1 of which only 81 a and 81 d are shown . the rod members lie essentially parallel to inner catheter 11 at substantially equal circumferential spacings therearound and are confined themselves by sleeve 82 . the radial configuration is shown in fig2 , in which the structures inward of outer sheath 42 have been simplified for clarity . confining element 82 , here being a confining sleeve , provides inward radial pressure on the rods , which themselves provide inward radial pressure on outer sheath 42 at each of the six points of contact of the rods to the outer sheath around the circumference thereof . on the other hand , between the points of contact no pressure is applied . this can be seen more easily in fig2 , including the phenomenon of close compression of the layers 41 , 42 at the contact points of the rods 81 , while voids 71 exist between layers 41 , 42 at regions between the points of rod circumferential contact . by applying this radial compression to outer sheath 42 , the tendency of stent 31 to distort , by virtue of its natural tendency to expand and thus apply radial pressure , inner and outer sheaths 41 and 42 is inhibited . prior to use in surgery , the stent delivery system is provided in the form shown in fig1 in which it may be stored for an extended period . the user , just prior to surgery , removes the confining structure 80 , by , for example , splitting sleeve 82 and discarding the rods 81 . the stent will then be available for use in its design - intended configuration , having dimensions and geometry undistorted over time by the aging process . next , the guide wire is inserted into the body percutaneously and navigated beyond the stent site . the delivery system is then directed along the guide wire to reach a particular body lumen , for example a cardiac artery . in the configuration of stent shown in the present embodiment , the pull element 51 is then retracted by application of tensile force from the proximal end of the system . the outer sheath 42 thus slides proximally over inner sheath 41 such that the folded portion distal of inner sheath 41 and outer sheath 42 progressively rolls back to expose the stent . meanwhile , push element 61 , being coupled to inner catheter 11 , which is held static at the stenting site by compression forces from the proximal end of the system / restrains the stent from proximal movement to ensure accurate deployment at the intended stenting site . as the pull element is retracted , radial pressure is released on the stent and stent 31 assumes its expanded configuration , such that the inner radial void of the stent becomes larger than atraumatic tip 91 , and the stent engages with the walls of the bodily lumen . the stent delivery system may then be swiftly and easily retracted the way it arrived , leaving the stent secured in place . of course , many other configurations of stent delivery system than roll - back systems may be used in conjunction with confining structure 80 . indeed , confining structure 80 provides an effective means of containing any self - expanding implant delivery structure which is otherwise liable to expand over time and therefore potentially exceed its design tolerances . for example , confinement structure 80 can be used with stent delivery systems having pull - back , rather than roll - back , sheaths . the construction of elements within confining structure 80 may be , as has been mentioned , conventional . on the other hand , the innovative confining structure 80 may itself be realized in a number of different forms . considering the arrangement of fig1 and 2 , confining structure 80 is provided as longitudinal rods spaced equidistantly about the circumference of the outer sheath 42 , but other configurations to those shown in fig1 are entirely possible . for example , the rods may instead be formed as hollow cylinders and / or their arrangement and spacing around the circumference of the outer sheath may be varied . for example , four rods or eight rods may be contemplated , and their diameter varied in comparison to the diameter of the outer sheath . in some embodiments , a split - wire 83 , shown schematically in fig2 , may be provided , running the length of the sleeve , to enable the user to easily and swiftly split the sleeve before use , without the use of a separate tool . such a split - wire may run distally ( portion 83 a ) within the sleeve between two of the rods and may then loop at the distal end before returning ( portion 83 b ) to the proximal end on the outside of the sleeve , terminating in a pull - ring 84 . pulling on the pull - ring will then cause the wire to split the outer sleeve longitudinally , distal to proximal . thin steel wire is suitable as a split - wire , in some embodiments . in one embodiment , the rods do not touch but approach each other closely . this permits a high degree of contact with the outer sheath and confinement thereof while preventing variations in confining force or inability to sufficiently compress due to the rods touching one to another . in another embodiment , the rods are configured to touch one another at a desired level of confining pressure or confining diameter , to prevent the inner components of the stent delivery system becoming crushed by overpressure . in the above embodiment , the conventional stent structure lying within the confining structure , namely that lying within the radius of the outer sheath , typically has a diameter of around 2 . 4 mm . in such a configuration , stent diameters themselves of around 2 . 1 mm are conceivable , in their compressed state . of course 1 in their expanded state such stents typically achieve outer diameters of around 7 mm , depending on application . for such applications , rods of the confining structure having a diameter around 2 mm may be appropriate . as to the other components , the atraumatic tip 91 is typically formed from polyurethane , the inner catheter is typically a polyimide tube , while the inner and outer sheaths are typically formed from 80 / pm - thick pet which are respectively cold drawn ( for the inner sheath ) and heat shrunk ( for the outer sheath ) to a reduced thickness during manufacture . the thickness may be reduced from an original thickness of 80 pm down to a reduced thickness of 40 pm , in one exemplary embodiment . further details of the construction of typical roll - back stent delivery systems to which the present invention may be applied may be found in published patent applications , such as wo 2006 / 020028 a1 . the rods are envisaged to be made from steel or polyamide , but other materials , including both metals and polymers , are well within the choice of the skilled designer to select . however , both steel and polyamide are considered to be especially able to give the required resistance to distortion preferred in embodiments of the present invention . indeed / if the rods are sufficiently resistant to deformation , it may not be required to provide a sleeve running the entire length of the confining structure , but to merely provide a number of compressing ligatures spaced along the length of the rods , in the manner of the hoops used to compress a traditional barrel of beer , wine or ale . therefore , another embodiment is possible wherein the outer sleeve is replaced by a series of rings which may be slid along the rods to release them . alternatively , a clamshell clamping arrangement may be provided around the rods , which arrangement may be released by a catch prior to use of the delivery system . another embodiment is contemplated having a configuration of confining structure as shown in fig3 . fig3 does not show the inner stent delivery components or the outer sleeve , but shows how a bundle of six tubes may be arranged to perform the same function as the rods 81 , even though portions of the tubes have been cut out circumferentially , except for certain circumferentially intact portions spaced along the length . these uncut portions , having a complete circumference , transfer the compressive force of the sleeve through the tubes to the confined inner components of the stent delivery system . on the other hand , where the circumference is not complete , sufficient of the circumference remains to provide a line of pressure along the stent delivery components to achieve the effects of the invention . in this embodiment , the characteristics of the material from which the tubes are formed will determine how closely the full - circumference portions need to be spaced and how much of the circumference may be removed in the intervening cut - out portions . however , it is envisaged that the advantages of the present invention may be obtained even when the cut - out portions retain only around 130 ° of circumference each . as to the construction of an embodiment of the complete confined delivery system , starting from a complete conventional stent delivery system , the rods are located in their predetermined positions around the conventional delivery system and heat - shrink tubing applied to the outside . this heat - shrink tubing is typically pet tubing which will shrink radially within around five seconds when a temperature of 200 ° c . is applied . during manufacture of stent delivery systems , it is generally considered highly undesirable to apply heat to a region proximate to a compressed - shape memory stent , in case the memory of the expanded configuration is distorted or destroyed , leading to potential catastrophic deployment failure . however in the described embodiment , heat - shrinking of the outer sleeve is entirely possible , since the intervening rods and air gaps provide sufficient insulation to prevent effective heat transfer to the stent during the period when the heat - shrink tube is heated to cause it to shrink and radially confine the rods . the present invention is not limited to the presently disclosed embodiments , but rather solely by the scope of the appended claims . the skilled reader will easily contemplate how embodiments of the confining structure may be incorporated into other constructions of implant delivery systems where dimensional creep due to aging is undesirable . such embodiments may not be herein explicitly described , but with nevertheless be clearly within the ambit of the skilled reader without undue experimentation and without the exercise of inventive skill .
the present disclosure provides a delivery system for a self - expanding implant which includes a sheath which surrounds and constrains the implant prior to delivery and a confining element which surrounds the sheath during storage . the confining element preferably includes elongate members running axially along the sheath , which compress the sheath and the stent to reduce hoop stress in the system without promoting undesired adhesion between layers of the sheath .