Abstract:
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.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to tubular medical prostheses that are expandable from a radially compact disposition to a radially expanded disposition. In the compact disposition, the prosthesis can by delivered to its operational site in the body, typically trans-luminally and percutaneously, using a delivery system which is a sort of catheter. Exemplary of this class of prosthesis is a nickel titanium shape memory alloy stent for a bodily lumen which is often but not always an arterial lumen. See for example the disclosure of applicant&#39;s earlier WO 01/32102. 
       BACKGROUND 
       [0002]    Bodily soft tissue is remarkably flexible and needs generally to be flexible. It is difficult for a metal stent to match bodily tissue for flexibility. WO 01/32102 is concerned with flexibility of the stent during its journey, from outside the body to the operational site within the body, as for example the catheter delivery system advances along a tortuous path from the point of entry in the body. However, there is also a need for metal stents (and other prostheses) that are destined to be installed at a location in the body where severe bending is to be expected. If the prosthesis could be made more tolerant of severe bending after placement, that would be attractive to doctors and their patients. 
         [0003]    WO 01/32102 shows what could be termed a “classic” self-expanding stent structure of zig-zag stenting rings composed of struts interspersed by points of inflection and with adjacent stenting rings linked axially by connector portions. In the compact delivery disposition of the stent (FIG. 3 of WO 01/32102) the struts of the zig-zag rings are more or less parallel to each other so that each point of inflection represents a change of direction for the material of the stenting ring of more or less 180°. As the stent expands to its deployed configuration (FIG. 4 of WO 01/32102) the radius of the stenting ring expands by movement of the struts away from each other so that gaps open up between adjacent struts, and the adjacent points of inflection move further apart (but nevertheless remain spaced at equal intervals to each other around the circumference of the stenting ring). 
         [0004]    The connectors serve to restrain relative circumferential movement of the zig-zag rings relative to each other. Thus, if the points of inflection of adjacent stenting rings are facing each other in the compact delivery disposition of the stent, as in WO 01/32102, then so will be these points of inflection still facing each other in the expanded disposition of the stent. When an expanded stent is subject, in the body, to extreme bending, so that the longitudinal axis of the stent is no longer a straight line but a pronounced curve (as in a banana), then the facing points of inflection on the inside of the bend approach each other. The more extreme the bending, the closer the facing points of inflection become until, in the end, these facing “peaks” abut each other or rub past each other. 
         [0005]    Any such abutment or rubbing is undesirable. One way to guard against it is to choose a stent design that can be classified as a “peak-to-valley” design rather than a “peak-to-peak” design as seen in applicant&#39;s WO 01/32102. The art is replete with suggestions for peak-to-valley designs in which the peaks of any one zig-zag stenting ring do not face corresponding peaks of the next adjacent stenting ring but, instead, are circumferentially offset to the peaks of the next adjacent stenting ring by half of the gap between two adjacent peaks of the same ring, in the expanded disposition of the stent. Then, under extreme bending, any particular peak on the inside of the bend can advance into the V-shaped space between two peaks of the next adjacent stenting ring, without any abutment or rubbing on any portion of the material of the next adjacent stenting ring. 
         [0006]    The present invention is concerned with the above-explained problem. It seeks to improve the in situ bend capability of stents including those seen in WO 01/32102. However, the invention also seeks to achieve this performance enhancement without sacrificing other attractive qualities of stents such as disclosed in WO 01/32102. For example, simplicity of modelling of stress distributions within the stent is attractive in the management of fatigue performance of metal stents. Manufacturing simplicity of course facilitates management of cost which should improve access to stents, for those people who need them. 
         [0007]    Looking at WO 01/32102, one can quickly see that performance in extreme bending could be enhanced by extending the length of the portions that connect adjacent zig-zag stenting rings. The longer these “bridges” then the more the stent can bend without abutment of peaks on the inside of the bend. However, large axial gaps between adjacent stenting rings are not desirable, for then the tissue of the lumen that is being stented by the prosthesis is relatively unsupported, at least between adjacent stenting rings. 
         [0008]    For the same reason, one seeks as far as possible to distribute the metallic material of the stent as evenly as possible over the length and circumference of the tubular envelope of the stent so that support for the stented tissue is as even and uninterrupted as possible. This of course indicates that all gaps between all adjacent struts and points of inflection of the stent matrix should be as constant as possible. Generally, one does not modulate the strut matrix of a stent, over the length and circumference of the stent, to suit tissue variations within the stenting site. Generally, one does not seek to place a particular point on the circumference of the stent in opposition to a particular zone of tissue within the stenting site. For these reasons alone, one seeks a stent matrix that is everywhere the same over the length and circumference of the prosthesis. 
         [0009]    In principle, the problem addressed in the present invention, and the solution here disclosed, is useful in all classes of stenting prostheses. That is to say, it works with a stent matrix that exhibits the (spiral) turns around the axis of a helical stent, as well as with a stent that features a stack of endless stenting rings. It works with balloon-expandable stents and resilient self-expanding stents as well as with nickel titanium shape memory alloy stents. 
       SUMMARY OF THE INVENTION 
       [0010]    Nevertheless, according to the present invention, it is contemplated to depart from the prior good design practice, and deliberately configure the stent matrix so that it is locally different, at one point at least, around the circumference of at least every second stenting ring within the prosthesis. In short, we envisage within each stenting ring at least one location (we call it a “stagger zone”) where the gap between adjacent points of inflection is smaller (but it could be greater) than the otherwise constant (or near constant) “wavelength” around the circumference of the prosthesis for that particular stenting ring. 
         [0011]    It will be appreciated that, for any particular pair of adjacent stenting rings, even just one such abnormal gap (stagger zone) could be enough to set up a circumferential offset between the otherwise facing “peak-to-peak” points of inflection that we see in, for example, FIG. 4 of WO 01/32102. In practice, however, a design with only one stagger zone on the circumference might not be enough to deliver the desired circumferential offset all around the circumference, especially in designs that feature a large number of struts, in any one turn around the axis of the prosthesis. Four stagger zones are likely to be enough. Designs with two, three or four such zones are presently preferred, but up to 6 stagger zones per turn are readily conceivable, the more so with stent design with a notably small mesh size in their matrix. 
         [0012]    As mentioned above, it is the connectors between adjacent stenting rings that prevent adjacent stenting rings from rotating around the long axis of the prosthesis relative to each other and thereby preserve the designed intended relationship between facing stenting rings and their respective points of inflection. Generally, it will be convenient to incorporate the design variation that creates the abnormal gap between adjacent points of inflection with the design of the connector zone between two adjacent turns. Thus, in perhaps the simplest case, we can envisage a connector from which two struts extend into a stenting ring or turn on one side of the connector, and two other struts extend in the other axial direction, into the next adjacent stenting ring or turn. If the angle between the first two struts is “normal” for that stenting ring, but the angle between the opposite pair of struts, in the other of the two stenting rings, is much smaller than “normal”, then the points of inflection circumferentially on either side of the connector, that would be facing each other in the classic construction of WO 01/32102, will be circumferentially offset from each other, because of the different angle of the struts each side of the connector. Think of the connector as the trunk of a human body, with the legs of the body slightly open and the two arms of the body. Imagine the angle between the upwardly outstretched arms significantly greater than the angle between the legs. 
         [0013]    Rather than set out a string of statements of invention, and then repeat them in a set of claims, we opt for economy of text, refrain from reciting statements of invention, and present various aspects of the above explained inventive concept in claims appended to this description. 
         [0014]    We have stated that setting up a “peak-to-valley” configuration of otherwise facing inflection zones is conveniently incorporated in the design of portions that connect adjacent stenting rings (or helical turns). However, one can readily envisage that the necessary offset can be created in a stagger zone that is circumferentially spaced from connectors. There is no imperative that the stagger zone must be coincident with the connector zone. 
         [0015]    It will be evident that a stagger zone can be created in a number of different ways. One can locally manipulate the mechanical properties of the material of the stagger zone, not just be local modulation of the dimensions of the inflection points or struts within the stagger zone, relative to the rest of the circumference of the stenting ring, but could also contemplate manipulation of the mechanical properties of the material in the stagger zone by local heat treatment or even chemical treatment. It is even possible to envisage adding components to an otherwise “classic” stenting ring structure as in WO 01/32102, such as a flexible “tie” between the two struts that are destined to deliver a smaller than usual gap between adjacent points of inflection. When a painter erects an easel, there is a “piece of string” that runs between the front frame of the easel, and the rear strut of the easel, not far above the ground, where the gap between the front legs of the easel and the single rear leg of the easel rest on the ground. A classic stent such as shown in WO 01/32102 could be modified by inclusion in it of judiciously placed “pieces of string” from biologically compatible material, in order to restrain two adjacent struts of selected stenting rings of the prosthesis from opening to the full extent that occurs for all the other struts of that particular stenting ring. (We mention this possibility not with any expectation that it will be a preferred construction, but in the recognition that competitors stimulated by the present disclosure might be obliged to turn to such constructions in the hope of avoiding the inventive concept of the present application). The motto “keep it simple” is rather powerful and the present inventors are proud of a contribution to the art that is essentially simple but should deliver a powerful enhancement of performance. 
         [0016]    At this juncture, it is worth noting that a shape memory alloy stent is given its “remembered” configuration by a heat treatment. In the case of WO 01/32102, that heat treatment is given with the points of inflection arranged “peak-to-peak”. With shape memory alloy stents embodying the present invention, however, the heat treatment is with the inflection points arranged as intended, therefore, not peak to peak, but in the circumferentially staggered arrangement brought about by the stagger zone(s). 
         [0017]    In the context of the present invention, attention is directed to the disclosure of US/2004/0073290 and, in particular, drawings  FIGS. 4 and 5  and the associated text. We see adjacent zig-zag stenting rings with points of inflection that are more or less “peak-to-peak” in the compact configuration of drawing  FIG. 4 , but not so much peak-to-peak in the expanded configuration of drawing  FIG. 5 . However, the struts of any particular zig-zag stenting ring are not all the same length. Some are relatively long, some are relatively short, and some are of intermediate length. By contrast, it appears that the angle that opens up between any two struts that diverge from any particular point of inflection is always the same, so that any technical effect of circumferential displacement of peak-to-peak points of inflection is accomplished not by local variation of an angle between two adjacent struts but, rather, by modulations of the length of adjacent struts in each of the stenting rings. Putting it another way, each connector portion  24  is the intersection of two straight lines in the form of an “X” shape and this is not what is envisaged with the present invention. 
         [0018]    Thinking about the bend capability of any particular stent design, this might be limited by peak to peak abutment on the inside of the (banana) bend but could also be restricted by incipient buckling of the stent matrix at some point on the matrix. Clearly, to obtain benefit from the present invention, the stent matrix to which it is to be applied must be capable of bending without buckling, to the extent needed to bring the benefits of the present invention into play. In general, the sparser the population of connectors, the more capacity the lattice will have to bend without buckling. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0020]      FIGS. 1 and 2  show, respectively content of FIGS. 6 and 2 of US/2003/0055485, being a tubular stent structure opened out flat, seen in plan view, in respectively the radially compact and radially expanded dispositions; 
           [0021]      FIG. 3  is a reproduction of FIG. 5 of US/2004/0073290, being a portion of a tubular stent structure, opened out flat, and in a radially expanded configuration; 
           [0022]      FIGS. 4 and 5  reproduce drawing FIGS. 3 and 4 of applicant&#39;s WO 01/32102, being respectively side views of a stent structure in the radially compact and radially expanded dispositions; 
           [0023]      FIGS. 6 and 7  are corresponding views of a stent structure in accordance with the present invention; and 
           [0024]      FIGS. 8 and 9  are views corresponding to those of  FIGS. 4 and 5 , or  FIGS. 6 and 7 , showing another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIGS. 1 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. 
         [0026]    Looking first at  FIGS. 1 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  FIG. 2  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  FIG. 1 , 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  FIG. 1 , 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  FIG. 2 , 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. 
         [0027]    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. 
         [0028]    In US/2003/0055485, paragraph 0031, it is stated that the geometry of the stent in the radially compact delivery disposition of  FIG. 1  is “highly flexible” so that it can tolerate axial compression on the inside of a bend as described above. Looking at  FIG. 1  ( FIG. 6  of the US publication) it is not immediately evident how the stated flexibility is provided. 
         [0029]    We turn now to  FIG. 3 , which corresponds to FIG. 5 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 . 
         [0030]    Nevertheless, we see from  FIG. 3 , that there is a circumferential offset between unconnected apex portions of adjacent stenting rings. 
         [0031]    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  FIG. 3  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 . 
         [0032]    In the compact disposition of the stent of  FIG. 3 , 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  FIG. 1  described above. Again, it is not immediately evident how the device delivers bent flexibility in the compact delivery disposition. 
         [0033]    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  FIG. 4 , this corresponding to  FIG. 3  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  FIG. 4 . 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. 
         [0034]      FIG. 5  shows the expanded disposition of the  FIG. 4  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  FIG. 5  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. 
         [0035]    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  FIGS. 6 and 7 . 
         [0036]      FIG. 6  shows a distribution of slits in a tubular workpiece opened out flat, and the similarity with the slit distribution of  FIG. 4  is immediately apparent. Note that  FIG. 4  is a view from the side of the workpiece, and does not show the workpiece opened out flat whereas  FIG. 6  shows the entire circumference of the tubular workpiece, laid out flat on the page. We see from  FIG. 6  that there are two connecting struts  80  connecting any two adjacent stenting rings  82 . We see at the left-hand end of  FIG. 6  the terminal stenting ring  84  which has a longer slit length and is also evident in  FIG. 4 . We note that  FIG. 4  shows the entire length of the stent whereas  FIG. 6  shows only a portion of the length of the stent. Whereas the connectors  80  are interspersed by voids  60  just as in  FIG. 4 , 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 ′. 
         [0037]    Noteworthy in  FIG. 6  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  FIG. 6  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  FIG. 6  with reference  90 . We need to look at  FIG. 7  to appreciate the consequence of short struts  90 . 
         [0038]      FIG. 7  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 . 
         [0039]    Turning our attention to the next adjacent pair of facing circles F and G in  FIG. 7 , 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  FIG. 7  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. 
         [0040]    Nevertheless, the present invention is attractive, when taken in the context of balancing conflicting constraints on the stent designer. Looking at  FIG. 6 , 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. 
         [0041]    Reference is now made to  FIGS. 8 and 9 . These have been annotated with references the same as are used in  FIGS. 6 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.  FIG. 8  shows the left-hand end of the stent whereas  FIG. 9  shows the full length. Actually, the  FIG. 9  stent is not the same as the  FIG. 8  stent because  FIG. 8  shows longer struts in the two end rings but  FIG. 9  does not. 
         [0042]    In  FIG. 9 , 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. 
         [0043]    The embodiments illustrated in  FIGS. 6 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  FIG. 6 . 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. 
         [0044]    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  FIG. 7 , 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. 
         [0045]    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. 
         [0046]    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. 
         [0047]    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.