referring to fig1 , we see a slitted tube 10 , opened out flat by parting the slitted tube at interface portions 12 , 14 and 16 to display , opened out flat , a succession of stenting rings i , ii , iii , iv arranged next to each other along the length of the slitted tube parallel to its long axis direction x . each of the four stenting rings exhibits a serial progression of n t struts , here 24 struts , ( 20 ) separated from each other by the slits through the wall thickness of the tubular workpiece , the succeeding struts of each stenting ring being joined by successive cusps 24 . in the unexpanded slitted configuration of fig1 , each cusp is in “ head - to - head ” relationship , along the axis direction x of the slitted tube , with a cusp of the adjacent stenting ring . as can be seen , each stenting ring is connected to the next adjacent stenting ring by four bridges 26 distributed at regular intervals ( 90 °) around the circumference of the slitted tube . the number of bridges per ring is n s and the number of struts between successive bridges is n s so : n t = s • n b . in stent technology , particularly stents made of shape memory alloy ( nitinol ), a strut matrix made by slitting a precursor tube is conventional . turning to fig2 , we see a portion of the fig1 slitted tube radially expanded so that the struts of each stenting ring are inclined to the axial direction x and present themselves as a zig - zag sequence of struts around the circumference of the stent . it will be noted that the cusps 24 of adjacent stenting rings are still in head - to - head disposition skilled readers will appreciate that any gross bending of a deployed stent is liable to bring opposing cusps on the inside of the bend into physical contact with each other . turning to fig3 , we can recognise the same pattern of 24 struts 20 making up 4 adjacent stenting rings i , ii , iii , iv , recognisably equivalent to what is shown in fig1 . further , just as in fig1 , each cusp 24 is in head - to - head relationship with a cusp of the next adjacent stenting ring . just as in fig1 , each stenting ring is connected to the next adjacent stenting ring by four bridges 26 . however , the slits 22 in the tube 10 , that have created the strut matrix , are axially staggered relative to each other , in a way which is not present in drawing fig1 . in consequence of this axial staggering , there is also axial staggering of the gaps 30 between each pair of facing cusps 24 . in fig3 , there is shown a greater axial separation between facing cusps 24 than is apparent from fig1 , but this is not the decisive difference between the fig1 concept and that of fig3 . reverting to fig1 , and concentrating on a pair of struts defining between them an individual gap 22 , one can see that the axial length of the two struts , one each side of the slit 22 1 is the same . however , when we look at fig3 , and a particular slit 22 , we notice that the length of the strut that extends down each side of the slit 22 , from the common cusp 24 at one end of the slit , are different . this has repercussions for the way the struts deform when the slitted tube of fig3 is radially expanded , to the zig - zag pattern shown in fig4 . comparing fig4 with fig2 , it is immediately evident that there are no longer pairs of cusps 24 facing each other , head to head . instead , each cusp points towards a gap between two adjacent cusps of the adjacent zig - zag stenting ring . the skilled reader will appreciate that when the stent of fig4 is bent ( into a banana shape ) each cusp is free to advance axially into the gap between two adjacent cusps of the adjacent stenting ring , rather than striking , head on , the facing cusp of the adjacent stenting ring , as in fig2 . fig5 is a perspective view but shows the same phenomenon as is drawn in drawing fig4 i with the same strut matrix . the skilled reader will grasp that the number of struts in each stenting ring need not be 24 , and the number of bridges between adjacent stenting rings need not be four . another arrangement that shows promise is one in which each stenting ring has 42 struts and adjacent stenting rings are connected by three bridges distributed at 120 ° intervals . such an arrangement is shown in fig9 and is described below . fig6 , 7 and 8 show another attractive design , namely , a slitted tube with 40 struts per ring and four bridges . since other aspects of the design are described above with reference to fig3 and 4 , the same reference numbers are used to identify corresponding features . of the design . again , it can be seen that when the fig6 slitted tube is opened out radially , the cusps 24 automatically move to positions where they are no longer facing head to head any cusp of the adjacent zig - zag stenting ring , with consequential advantages of avoiding cusp to cusp contact when the deployed stent is subjected to bending deformation . in fig6 , in loop iii , three successive bridges are labeled b 1 , b 2 , b 3 . bridges bland b 3 connect loop iii to loop iv . bridge b 2 is one of the four bridges that connect loop iii . between bridges b 1 and b 2 , and between bridge b 2 and b 3 , is a sequence of five struts . three of these struts s 1 , s 2 , s 3 , have the same length . each extends between two free cusps . the other two struts , s 4 and s 5 , have lengths different from each other . this length difference is what takes the free cusps of adjacent loops out of a head - to - head facing relationship in the expanded configuration of the stent , as can be understood from fig7 and 8 , which also reveal that the bridges are correspondingly skewed , relative to the long axis of the stent , in the expanded disposition of the stent . the lengthwise staggering of cusps that characterises the present invention can deliver useful technical effects that include the following . when a self - expanding strut is to be released from its catheter delivery system , the usual way is to withdraw proximally , relative to the stent , a restraining sheath that surrounds its abluminal surface . when all cusps in a loop are at the same point along the axis of the stent , all can spring radially outwardly from the sheath simultaneously . this impulsive release is not ideal for controlled release . axial staggering of cusps can assist in releasing the stent more progressively and steadily , cusps escaping one by one from the inward radial confinement of the proximally retreating sheath . for some stents , the design features non - identical proximal and distal ends , so that it is critically important to load the stent in the delivery system with its distal end nearer the distal end of the delivery system . an advantage of the present invention is that it permits the building of stents with identical distal and proximal ends , that are indifferent to the choice of stent end to lie closer to the distal end of the delivery system . the axial staggering opens up possibilities for “ recesses ” such as recesses 40 in fig3 , where radiopaque marker elements 50 can be located . these elements thus lie snug between circumferentially spaced apart cusps 42 , 44 and axially adjacent to intervening cusps 46 , to which it will be convenient to attach the marker . any axial pushing on the stent , while the confining sleeve is withdrawn is customarily applied to the end surface of the stent . by locating markers in the end recesses and arranging for the end elevation of the stent to comprise both cusps and markers , the stresses on the end elevation are distributed around the circumference as evenly as possible , and over the maximum area of surface of the implant , which is good for fatigue performance , quality control , and efficiency of stent release . finally , with markers recessed into the end zone of a stent , the markers when imaged give a true impression of where the stent matrix is , and where it is not . a short look at us 2006 / 0025847 serves to reveal the advantages of the present proposal over another recent proposal to deal with pushing forces . not to be underestimated is the advantage yielded by this invention , that a “ peak - to - valley ” distribution of cusps in the expanded deployed disposition is automatic , regardless how short are the bridges between adjacent stenting loops . short , strong , robust bridges that connect axially adjacent stenting loops are greatly to be welcomed , for many reasons . in particular / they are less vulnerable to inadvertent straining ( bad for fatigue performance if nothing else ) when stent matrices are being installed in a catheter delivery system , or when being deployed out of one . put another way , the stent with short stubby bridges can be rated for greater loads imposed on it during loading or deployment . since the radial force that a stent can exert on surrounding bodily tissue increases with the number of stenting loops per unit ( axial ) length of the stent , a reduction in the length of the bridges connecting axially adjacent stenting loops will give rise to an increased stenting force . however , short stubby bridges are disadvantageous , to the extent that they prejudice stent flexibility . the more flexible a stent is , the better its resistance to fatigue failure ( other things being equal ). one way to deliver more flexibility , despite an absence of much flexibility in the bridges , is to increase the number of struts in the sequence of struts between each bridge and the next bridge . on that basis , the arrangement of fig9 , with 7 struts between any two bridges b 1 , b 2 or b 2 , b 3 , is superior to the fig6 design with 5 struts , itself superior to that of fig3 , with 3 struts . when it comes to radiopaque markers , it is important to arrange the markers so that they are distributed around the circumference of the stent , in the radially compact delivery disposition of the stent , as evenly as is practicable . in fig3 , the arrangement is even . fig6 shows one possible arrangement of tantalum markers 60 , 62 , 64 , 66 which is not far from an even distribution in the compact form of the stent ( although further from evenly distributed when the stent is expanded ). in the fig9 design it is clear that each end of the stent offers only three recesses for installation of a set of three markers evenly distributed around the circumference of the stent . the markers can be of different shapes , in order to meet these design objectives , as is illustrated in fig6 , as one example . one thing that is striking about the present invention is how it delivers a simple pattern of linear slits in the compact configuration that exhibits in each stenting loop a sequence of stepwise displacements , up and down the axis of the stent , in the positions of the free cusps , yet , in the expanded disposition of the stent , the axial steps are gone . instead , the bridges are skewed , and the free cusps are circumferentially displaced , relative to the free cusps of the adjacent stenting loop that were facing them , head - to - head , in the compact disposition . of significance is that , in the expanded disposition , when the stent must exert radially outward stenting force on the bodily tissue that forms the wall of the stented bodily lumen , the zig - zag struts of each stenting rings march around the circumference of the lumen in a progression in which axial displacement of free cusps , relative to each other , is difficult to discern . instead , the stenting loops deploys in a way that is close to an optimal planar hoop , transverse t the axis , for generating a large mechanical radially outward stenting force . applicant &# 39 ; s wo 2007 / 135090 discloses a stent that is “ bend - capable ” in that cusps move out of a “ head - to - head ” facing relationship in the expanded deployed stent , when the stent tube is bent out of a straight configuration . it will be apparent to the skilled reader that the present invention ( lengthwise staggering of cusps ) can be combined with the invention of wo 2007 / 135090 ( skewed unit cell ) to deliver a stent matrix that avoids a head to head facing relationship of cusps , regardless of the extent to which the stent is bent out of a straight line after deployment . one way to accomplish the result explained in wo 2007 / 135090 is to arrange the strut matrix such that n 8 / 2 is an even number . it hardly needs to be added , that the stents taught in this disclosure can be used in the same way as prior art stents are used . they can carry graft material , or drugs , for example . they can be delivered transluminally , by a suitable catheter delivery system . they can carry radiopaque markers , as is taught in the state of the art . they will find particular application in situations where the stent , after deployment , is subject to a high degree of bending . the present drawings show specific embodiments which are to be assessed as exemplary , not limiting . the stent need not be made from shape memory metal and need not be laser cut . the inventive concept disclosed herein is applicable to a wide range of known stent technologies .
a stent includes a matrix of struts . the matrix of struts may be disposed parallel to one another in a non - expanded disposition . a plurality of loops along the longitudinal axis may be formed by the matrix of struts . in one aspect , a plurality of cusps in each loop of the plurality of loops connects adjacent struts , the plurality of cusps including tied cusps and free cusps . the tied cusps of one loop can be connected to the tied cusps of an adjacent loop by a bridge extending therebetween . in one aspect , the adjacent struts connected by a tied cusp have different lengths . in one aspect , at least one free cusp in each loop joins two struts of approximately equal length .