referring to the drawings , fig1 is a detailed view of the distal end of a typical balloon catheter stent delivery system 2 . as used herein , the terms distal and proximal refer to the balloon end and luer fitting end respectively . the device includes an inner catheter shaft 25 and an outer catheter shaft 26 . a balloon 24 is welded at its distal end to the outside of the inner shaft 25 and at its proximal end to the outside of the outer shaft 26 , and a stent 23 is assembled onto the balloon 24 . the balloon 24 can be any balloon suitable for angioplasty procedures or any other stent delivery systems . a preferred type of balloon 24 is one with a plurality of folds ( not shown ) such that on inflation of the balloon , the stent is caused to expand radially in an even manner . the stent 23 shown is of the type having a hollow cylindrical body composed of a plurality of linked circumferentially extending rings . other forms of stents and delivery systems for other intravascular procedural or diagnostic devices will equally be suitable for use with the device of the invention . [ 0025 ] fig2 is an elevational view of the distal portion of a stent delivery system 40 according to the invention , with the stent omitted for clarity . the distal end includes a balloon 44 bonded at distal balloon bond or weld 47 to the outer surface of the inner shaft 45 and at proximal balloon bond or weld 48 to the outer surface of the outer shaft 43 . in use , a guidewire ( not shown ) is extended through the lumen l of the inner shaft 45 and projects beyond its distal terminus 45 a to help guide the delivery system to the desired treatment site . located on the outer surface of the inner shaft 45 is a pair of marker bands 46 which underlie the balloon 44 . the inner shaft 45 also includes a pair of transition sleeves 41 fabricated of a material to provide increased stiffness relative to the shafts 43 , 45 and balloon 44 and of diminished stiffness relative to a stent mounted on the balloon . one sleeve 41 is positioned over the inner shaft 45 beneath the balloon adjacent the distal balloon weld 47 and a second sleeve 41 is positioned beneath the balloon 44 and adjacent the proximal balloon weld 48 . in an ideal arrangement , a stent positioned on the balloon 44 will be located to overlie the space between the transition sleeves 41 and will partially overlap a transition sleeve at each of the ends of the stent . the marker bands 46 are a typical feature of stent delivery systems . they are usually formed of a radiopaque material and are used by the physician to enable the position of the stent in a body vessel to be visualised . whilst the stent has been omitted from this diagram for clarity , if it were in position it would be crimped over the balloon 44 between the marker bands 46 . in this embodiment of the present invention , the sleeves 41 comprise lengths of tubing fixed in position on the inner shaft 45 above and overlapping the marker bands 46 . the material selected is of a stiffness intermediate those of the relatively stiffer stent and the relatively flexible catheter shafts 45 , 43 . once a stent is assembled onto the balloon 44 , it rests between the pair of marker bands 46 so that the bands are clearly visible to the eye and are not rendered obscured to a non - visual imaging system . the visibility of the marker bands assists the assembly person in locating the stent at a correct position on the balloon intermediate the two marker bands and for this reason , it is desirable that the sleeve material should be transparent or translucent so that the bands remain visible beneath the sleeves . a first portion 41 a of each sleeve is provided to underlie the stent and a second portion 41 b is disposed clear of the stent so as to ensure as even as possible a transition in stiffness from catheter shaft to balloon to stent . the purpose of these stiffness transitions is to smooth out the stiffness profile of the combined balloon catheter and stent thus reducing the risk of tracking and efficiency discontinuities resulting in the catheter getting hung - up or hinging . whilst different methods of assembling the stent delivery system 40 will suggest themselves to the skilled person , one convenient method is to make a first sub - assembly by necking the distal tip of the inner shaft 45 to decrease its outer diameter dimension , then sliding onto it and positioning in place the pair of marker bands 46 . the necking of the distal tip has the advantage of rendering it easier for the operator to slide the marker bands onto the shaft 45 and this feature also provides a contribution to the ease of movement of the inner shaft through bodily lumens when the delivery system is being deployed . once past the neck , the marker bands 46 are preferably swaged onto their desired positions on the inner shaft 45 . next , the pair of transition sleeves 41 are slid onto the inner shaft 45 and over the necked area . one sleeve 41 is placed over each marker band 46 and fixed or bonded into position by suitable means such as laser welding , heat welding or use of an adhesive . the fixation of the sleeve 41 has the secondary advantage of also fixing the marker band 46 in place , preventing its accidental dislocation during subsequent assembly steps . this completes the first subassembly . a second sub - assembly is made independently by cutting a length of balloon from a tube of balloon material , sliding the proximal end of the balloon over the distal end 43 a of outer shaft 43 and bonding it into position at proximal balloon bond 48 . the final assembly is then made by sliding the first sub - assembly into the second sub - assembly , then bonding the distal end of the balloon to the distal outer surface of the inner shaft to form the distal balloon bonding 47 . referring now to fig3 a , there is shown a view of a stent 51 mounted on a balloon catheter 52 traversing through an acute bend , there being no transition sleeve provided . the balloon catheter 52 is exhibiting what is known by those skilled in the art as hinging at 54 . fig3 b shows a view of a stent 51 mounted on a balloon catheter 56 with a transition sleeve 416 as described above traversing through the same acute bend . the balloon catheter 56 is not exhibiting the hinging effect . track testing on catheters with and without transition sleeves located proximal and distal to the stent was undertaken and profiles were derived from the test results . the test involved pushing distal sections of the catheters around a fixed wire tube with a radius of 3 mm and a 90 ° bend . the resistance felt by the attached load cell as the various sections of the catheter track around the bend radius were recorded . fig4 shows a typical track profile observed . this track profile of the catheter without transition sleeves has a series of peaks and valleys , fig5 and 6 are track profiles for two different sized stent delivery systems . each figure features a track profile for a same sized stent delivery system with and without transition sleeves provided . there is noticeable smoothing of the track profile for the catheters with transition sleeves . it is important to note that the system track performance does not deteriorate significantly due to the addition of the transition sleeves . the track profile is broken up in sections a to g along the displacement axis . these sections define the reactive force experienced by the load cell as specific sections of the device track around the bend and are as follows : a : wire movement value , i . e . the resistance force experienced as the distal section travels along the straight section of wire . this value is used to normalize all the specimens in the test series . b : tip seal force ; the resistance encountered as the portion of the inner shaft 45 which extends distally of the balloon welds , the weld itself and finally the balloon neck travel around the bend . c : distal trough ; this part of the profile is where the flexible section of the device between the rigid tip seal weld / balloon neck and the stent tracks around the bend . d : mid - stent force ; approximation of the mean resistance force encountered as the stent tracks around the bend . e : proximal trough ; the part of the profile where the flexible section of the device between the rigid stent and proximal balloon weld and balloon neck tracks around the bend . f : balloon bond force ; the resistance encountered as the balloon neck and the balloon weld travel around the bend . g : distal shaft force ; the resistance encountered as the outer and inner distal shafts negotiate the bend , the main resistance contribution coming from the outer shaft . h : distal trough amplitude = max . tip seal force value minus the min . distal trough force value . i : proximal trough amplitude = max . balloon bond force value minus the min . proximal trough force value . in fig5 the stent used is a 2 . 5 mm diameter , 9 mm length one and in fig6 the stent used is a 4 . 0 mm diameter , 18 mm length one . in each case , the transition sleeve were comprised of 100 % 7233 0 . 0255 ″× 0 . 029 ″ ( 0 . 6477 mm × 0 . 7366 mm ) clear pebax ™ extrusion cut to length . the effect of the transition sleeves can be seen on the graphed track profiles , and the two distinctive peaks in the distal and proximal troughs produced by the areas described . the important contribution of the transition sleeves is effectively to reduce the width and amplitude of the two troughs , thereby smoothing out the track profile between the relatively stiff and flexible regions of the device . the device effectively tracks much smoother to the touch , as opposed to the jumpy track effect experienced with the device without the transition sleeve . the effect of the sleeves on performance characteristics appears negligible . the system track performance does not deteriorate significantly due to the addition of the transition sleeves . we have found that the track forces in general are slightly higher on shorter , small diameter balloons , while there seems to be improved track with larger longer balloons . this is expected , as the shorter , smaller diameter devices may not have the dimensional capacity to effectively articulate the increased stiffness in a beneficial manner . various materials may be selected for the transition sleeve and indeed each “ sleeve ” may be formed in any way which serves the desired purpose of providing a region of material flanking the distal and proximal ends of the stent which has a stiffness which is intermediate the relatively stiff stent and relatively flexible shaft or shafts and the term “ sleeve ” as used therein is to be construed accordingly . thus , whilst the invention has been described particularly above in terms of a pair of transition sleeves located on an inner shaft , it is to be understood that the desired effect may be achieved in many ways including but not limited to : providing a single transition sleeve , either at the distal or proximal side of the stent or both ; providing a single transition sleeve extending from proximal to distal the stent ; providing the or each transition sleeve as an area of the inner shaft and / or the balloon and formed integrally therewith , for example , as an area of increased shaft diameter or balloon thickness . providing the or each transition sleeve in such a way that it combines the roles of providing the desired stiffness transition with the role of providing a visualisation marker , either of a visible or radiopaque nature or preferably both , the sleeve and marker being formed integrally or separately . it will of course be understood that the invention is not limited to the specific details herein described , which are given by way of example only and that various alterations and modifications may be made without departing from the scope of the invention .
the present invention provides a medical stent delivery device for delivering a stent to a treatment site . the device has an elongated catheter with an inflatable balloon attached to its distal end for receiving a stent which is expandable radially upon inflation of the balloon . the elongated catheter includes a transitional area of material proximal and / or distal to the stent .