fig1 to 5 give only the most cursory impression of the wealth of stent strut matrix proposals contained within the state of the art . however , they provide enough disclosure to set the advantages of the present invention in the context of relevant prior art . looking first at fig1 and 2 , we see an example of a stent 10 which utilises circumferential support structures 12 in the form of zig - zag stenting rings . these are spaced apart along the long axis of the stent . they are made up of struts 12 interspersed by points of inflection 18 which us / 2003 / 0055485 designates “ apex portions ”. these apex portions are mostly free to take up positions , in the expanded fig2 disposition of the stent , which are governed only by the stresses transmitted through the struts of that particular stenting ring , or transmitted to that stenting ring by the bodily tissue that presses against it . however , there are also connecting struts 16 each of which joins a selected one of the apex portions of one stenting ring with a selected apex portion of an adjacent stenting ring . from fig1 , we see that the connecting struts have length direction that is in the circumferential direction of the stent tube , so that the opposite ends of each particular connecting strut 16 are circumferentially spaced from each other . in the radially compact disposition of fig1 , the circumferentially extending connecting strut 16 spans an intervening apex portion between its two ends . however , when the stent expands into its radially expanded configuration represented by fig2 , the circumferential length of the connecting strut 16 is not enough to span across an intervening apex portion , because the apex portions arranged around the circumference of any particular stenting ring have moved away from each other by an amount greater than the length of the connecting strut . by selecting a connecting strut length that is approximately half the distance between two adjacent apex portions of the same stenting ring , in the expanded configuration of the stent , one can achieve a “ staggering ” of the evenly spaced apex portions of one stenting ring , relative to the equal spacing of the apex portions of the next adjacent stenting ring that “ faces ” the stenting ring of the other end of the connecting strut . in consequence , when the stent in its expanded configuration is subject to longitudinal compression , or when it is bent sharply ( so that its longitudinal axis is not longer straight but arcuate ) the facing apex portions on the inside of the bend , or that approach each other as the stent is longitudinally compressed do not butt up against each other but , instead , move into the free gap between two spaced apart apex portions of the other of the two facing stenting rings . in us / 2003 / 0055485 , paragraph 0031 , it is stated that the geometry of the stent in the radially compact delivery disposition of fig1 is “ highly flexible ” so that it can tolerate axial compression on the inside of a bend as described above . looking at fig1 ( fig6 of the us publication ) it is not immediately evident how the stated flexibility is provided . we turn now to fig3 , which corresponds to fig5 of us / 2004 / 0073290 . in this case , one can see that the connector portions between adjacent zig - zag stenting rings have a very simple construction . they are remarkably short in length but , to the extent that they have a length direction , it is parallel to the longitudinal axis of the stent , rather than in a circumferential direction . accordingly , there is no circumferential offset between the apex portions facing each other and connected by the connector strut 24 . nevertheless , we see from fig3 , that there is a circumferential offset between unconnected apex portions of adjacent stenting rings . the offset is accomplished by providing a range of different strut lengths in any particular stenting ring . in particular , each connector 24 is at the apex of a first pair of relatively long struts and a second pair of relatively short struts . each stenting ring features struts of three different lengths , namely a ) short , b ) intermediate length and c ) long . and we see from fig3 how the strut length progresses around the circumference of each stenting ring in a sequence abccba . we can see how the apex portions at the open end of a bifurcation between two a - struts both fall within a single gap between two c - struts of the adjacent stenting ring . this overlap occurs periodically around the circumference of the prosthesis , on each occasion midway between two adjacent connector portions 24 . in the compact disposition of the stent of fig3 , apex portions are “ head - to - head ” or “ peak - to - peak ” all the way around the circumference of the stenting cylinder , not just at those locations where a connector portion 24 lies between the head - to - head apex portions . in this respect , there is reduced flexibility in the compact delivery disposition of the stent device , comparable with that of the device of fig1 described above . again , it is not immediately evident how the device delivers bent flexibility in the compact delivery disposition . the present applicant specialises in self - expanding stent devices that are formed from a tubular workpiece of nickel titanium shape memory alloy (“ nitinol ”™). the tube is formed into a stent precursor by forming in it a multiplicity of slits ( cut by a laser ) that it is convenient to provide all mutually parallel to each other and to the long axis of the tubular workpiece . one such construction can be seen in fig4 , this corresponding to fig3 of applicant &# 39 ; s earlier publication wo 01 / 32102 , the contents of which are hereby incorporated by reference . reference to the wo document will reveal how , following laser cutting , portions of the tubular workpiece are removed to leave “ holes ” in the slitted tube , indicated by reference 60 in fig4 . it will be grasped by the skilled reader that provision of these voids in the slitted tube endows the tube , in its delivery system , confined by a sheath that prevents premature self - expansion , greater flexibility for the sheath to advance along a tortuous delivery path to a site of stenting in the body . fig5 shows the expanded disposition of the fig4 stent construction . a pattern of zig - zag stenting rings connected by short connecting portions 62 can be readily recognised and it can also be seen that the apex portions ( here in designated “ points of inflection ”) of adjacent zig - zag stenting rings are facing each other , not only across the connected 62 but also elsewhere around the circumference of the stenting device . thus , when the expanded stenting device of fig5 is subject to severe lengthwise compressive stress , or severe bending , there is a possibility for facing points of inflection of axially adjacent zig - zag stenting rings to approach each other closely , or even touch on the inside of the bend . the problem being tackled when the present invention was made was how to reduce the likelihood of this adverse event occurring . one embodiment of the present invention , which does succeed in setting the facing points of inflection of adjacent stenting rings circumferentially offset from each other , will now be described be reference to drawings fig6 and 7 . fig6 shows a distribution of slits in a tubular workpiece opened out flat , and the similarity with the slit distribution of fig4 is immediately apparent . note that fig4 is a view from the side of the workpiece , and does not show the workpiece opened out flat whereas fig6 shows the entire circumference of the tubular workpiece , laid out flat on the page . we see from fig6 that there are two connecting struts 80 connecting any two adjacent stenting rings 82 . we see at the left - hand end of fig6 the terminal stenting ring 84 which has a longer slit length and is also evident in fig4 . we note that fig4 shows the entire length of the stent whereas fig6 shows only a portion of the length of the stent . whereas the connectors 80 are interspersed by voids 60 just as in fig4 , there are no voids between the increased length end stenting ring 84 and its next adjacent normal length stenting ring 82 ′. whereas there are two connector portions 80 between the normal length stenting rings , there are 14 connectors 86 between the end stenting ring 84 and its neighbour 82 ′. noteworthy in fig6 is the pattern of length of the individual slits cut by the laser . in general , the slits have a single length , but there are two exceptions . the first exception is that the slit length is longer at the end of the tubular workpiece , to form the end stenting ring 84 . one such slit is marked in fig6 with reference 88 . the second exception is the length of the slit that terminates at one end of each connector portion 80 . one such slit , of shorter length then the others , is designated in fig6 with reference 90 . we need to look at fig7 to appreciate the consequence of short struts 90 . fig7 reveals two of the many connector portions 80 . each connector 80 sits between two adjacent stenting rings at the points of inflection 92 of any one stenting ring are all to be found on a notional circular locus that is transverse to the longitudinal axis of the stent . a gap “ t ” exists between any particular such circular loci d and e facing each other at periodical intervals down the length of the stent . within these two circles , we find a circumferential offset between the spaced apart points of inflection 92 in circle e and those of the facing circle d . the offset is found everywhere except at the periodically spaced connectors ( in this example there are four ) 80 . turning our attention to the next adjacent pair of facing circles f and g in fig7 , the origin of the circumferential offset can be seen in the opposing relationship of the points of inflection 92 a , b , c and d . the gap between apex 92 b and 92 c on the stenting ring that includes circle g is the regular gap between two struts of “ normal ” length . however , because of the reduced slit length 90 , the circumferential gap between points of inflection 92 a and 92 d is abnormally small so that both points of inflection 92 a and 92 d “ fit ” in the normal sized gap between apex 92 b and 92 c of circle g . in the terminology adopted in the present specification , the zone that includes points of inflection 92 a , b , c and d is designated a stagger zone . from fig7 it is immediately evident that providing such a stagger zone by using an abnormally short slit length has a disadvantage , namely , that the gap between points of inflection 92 a and 92 d is shorter than the regular gaps between other points of inflection spaced around circle f . it hardly need be stated that optimal use of material within a stent matrix calls for a regular matrix of struts , with a minimal amount of material in the radially compact delivery disposition , and a uniformed distribution of that material in the expanded configuration ( all gaps the same size ) so as to achieve a maximum ratio of expanded diameter to radially compress delivery diameter . deliberately accepting a plurality of unnecessarily small gaps around the circumference of each stenting ring will have an adverse effect on this ratio of diameters and is therefore not something of itself desirable to stent designers . nevertheless , the present invention is attractive , when taken in the context of balancing conflicting constraints on the stent designer . looking at fig6 , one can see the evident simplicity of the stent strut and slit arrangement . the government regulatory authorities impose stringent quality requirements on stent manufacturers . for example , stents must meet stringent metal fatigue requirements . finite element analysis of stent designs is of crucial importance . a design that is inherently simple should lend itself to reliable prediction of its properties in service . being able to predict how a stent will perform after it has been installed in a human body is a significant advantage for stent manufacturers that compete to provide the stents most attractive to doctors and medical services . reference is now made to fig8 and 9 . these have been annotated with references the same as are used in fig6 and 7 , to identify corresponding features . noteworthy is that there are four connectors rather than two , connecting adjacent stenting rings . clearly , each stent ring comprises pairs of struts which are unconnected to any pairs of struts of an adjacent stent ring . further is clear that a number of adjacent stent rings are spaced from each other . fig8 shows the left - hand end of the stent whereas fig9 shows the full length . actually , the fig9 stent is not the same as the fig8 stent because fig8 shows longer struts in the two end rings but fig9 does not . in fig9 , it is helpful to consider connectors 80 a and 80 b . the former has the asymmetric shape of all the other connectors of zig - zag rings 82 . the latter has an x shape because it is part of the transition from the stagger zone rings 82 to the end ring 84 that lacks any stagger zone . shown is that the closest end points of a pair of struts of one stent ring and a pair of struts of another stent ring are connected . the embodiments illustrated in fig6 to 9 represent only one of a multitude of ways to bring about an angle between struts that is different from the otherwise regular angle between struts around the remainder of the circumference of any particular stenting ring . for example , one could locally modify the material of the stenting ring , either in the points of inflection at one or both ends of the struts that are to form the abnormal size gap , or by judicious modification of the dimensions of those points of inflection or the two struts running between them . above - mentioned wo 01 / 32102 contemplates manual removal of individual scrap portions to create voids 60 . there are voids 60 in fig6 . if manual intervention is to be relied upon to create the voids 60 in an embodiment of the present invention , then it would not be beyond the bounds of imagination to intervene locally , and manually , at portions of the abluminal surface of the workpiece where the properties of the material are to be modified locally in order to deliver a gap of different size , or angle of different size , between two adjacent struts of any particular stenting ring . one envisages that the material could be modified in its composition , by local application of a substance to cause a chemical reaction , or by local application of a substance to modify the microstructure of the material at that point , or by local application of heat or cooling to give the material at that location a different thermal history of that of the material of the remainder of the stenting ring . in this context , we incorporate , by reference to it , the disclosure of wo2001 / 076508 , from the present applicant , which explains how particular strut configurations can be created by using a jig to hold the workpiece in a particular desired configuration during the heat treatment which “ sets ” the “ remembered ” configuration of the struts in the shape memory alloy . thinking along these lines , one could use instead of struts of different lengths a jig that holds the struts in a configuration such as is shown in fig7 , when giving the workpiece its “ remembered ” configuration , so that it should open up at the stenting site to the remembered disposition even if the length of slit 90 is just the same as the length of all the other slits in the stenting ring . when thinking of workpieces of every day household dimensions , such as how to prevent a door opening too far , one would use a strut between the door and the frame that has a set length corresponding to the maximum opening that one wishes to impose on the door . in the same way , one could envisage some sort of collapsing tie to impose a maximum size on the gap between points of inflection 92 a and 92 d , that extends between the respective struts between the slit 90 . of course , stents are very small , but by no means as small as the nanometer dimensions that are in the minds of designers of medical devices , so there seems no justification for dismissing such tie pieces as impracticable . the reality is that , as stent designs become ever more sophisticated , so the range of applications for stents becomes ever greater and , with that , the demand for stents to mimic ever more closely the flexible behaviour of the original bodily tissue , especially when called upon , from time to time , to bend tightly along its length . the challenge is to build a stent that is strong enough to perform the stenting function which is after all the reason for its surgical implantation in the body while , at the same time , rendering the prosthesis as soft and bendy as possible in all other aspects . the present invention makes a valuable contribution to this objective . stents need not be made of nickel titanium alloy . another biologically compatible material familiar to stent designers is stainless steel . great efforts are currently being made to use other materials such as biologically compatible polymers . all such stents can benefit from the present invention regardless how they are formed . the illustrated embodiments are not limiting . stents need not display the same strut matrix over their entire length . we envisage embodiments in which only part of the length of the prosthesis is given the high flexibility of the present invention . thus , there may be some turns of the stent matrix that include stagger zones , to deliver flexibility , and other parts of the length of the stent ( e . g . end zones , or a mid - length portion ) where high flexibility is contra - indicated , and so no stagger zones need be provided in these parts of the stent matrix .
stent exhibiting a succession of turns around the axis , adjacent turns being connected by connector portions distributed around the circumference of the prosthesis , each of the turns being made up of struts that extend the length of the turn and are interspersed by inflection zones located at the axial ends of each of the turns . when the prosthesis expands radially , gaps open up between adjacent struts of each of the turns , the inflection zones being distributed regularly around the circumference so that the gaps are substantially the same size as each other around the circumference of the turn , characterised in that at least every second turn exhibits a stagger zone within which the said gap between adjacent said struts is of an individual size different from that common to the other gaps of that second turn .