Radially expansible stent

Technical field

0001This invention relates to a radially expansible stent made from a tube of stent material, which exhibits a continuous zig-zag spiral of material which wraps around the lumen of the stent in a plurality of turns, advancing from near one end of the stent to near the other end, the zig-zags being composed of struts joined at points of inflection.

Background art

0002Ever since Dotter proposed to use a simple wire spiral as a stent, it has been known to build a stent from a spiral pattern of stenting material. One way to move from a relatively small stent diameter to a relatively large diameter, when the stent is a spiral, is for the number of turns of the spiral to go down, as the diameter goes up. Another way is to build the spiral from a succession of zig-zag struts. In a radially compact (unexpanded) configuration, the struts can all lie more or less parallel with the longitudinal axis of the stent, with the points of inflection at each end of each strut of the zig-zag causing the line of strut material to double-back on itself to extend around an angle of 180° into the respective adjacent struts on either side. A very substantial radial expansion can be achieved by opening up the zig-zags, so that the struts no longer lie parallel to each other and with the long axis of the stent but, rather, in a more open zig-zag, where the angle between two succeeding struts of the zig-zag, at the intervening point of inflection, is more like 120° than the original 180°. One example of such a stent is disclosed in US Patent No. <patcit id="pcit0001" dnum="US6911041B1"><text>US 6, 911, 041 B1</text></patcit>.

0003One big advantage of a spiral pattern for a stent is that it is a great deal more flexible in bending than a simple annular cage of struts such as one finds in the early Palmaz stent. However, the price of such great flexibility is that the stenting force available from a spiral stent, to push bodily tissue away from the long axis of the stent, is liable to be inherently significantly less than with a cage stent.

0004Various spiral pattern stents are known from <patcit id="pcit0002" dnum="WO2007095466A2"><text>WO 2007/095466 A2</text></patcit>, as well as in the prior art citations acknowledged therein.

0005<patcit id="pcit0003" dnum="US2007106369A1"><text>US 2007/106369 A1</text></patcit> discloses a sent design of individual annular elements shaped so as to loosely interfit or interlock together with or without intersection and without cross-over points. The annular elements are multiple independent serpentine rings adjacent to each other having laterally spaced segments 180° out of phase with each other to provide a stepped sequence around each ring, each segment having a wide and a narrow intermediate portion with one set of segment on one side being fitted into an adjacent set of segment on one side of an adjacent serpentine ring to interlock the ring together against longitudinal separation.

0006It would be desirable to be able to retain the flexibility of a spiral stent yet achieve, at the same, enhanced magnitude radially outwardly directed stenting forces.

Summary of the invention

0007According to a first aspect of the present invention, there is provided a radially expansible stent made from a tube of stent material, which exhibits a continuous zig-zag spiral of material which wraps around the lumen of the stent in a plurality of turns, advancing from near one end of the stent to near the other end, the zig-zags being composed of struts joined at points of inflection, wherein each said strut includes a handed double bend located remote from the points of inflection at each end of the strut and between two portions of the strut that are parallel to each other and, when the stent is radially minimized, parallel to the longitudinal axis of the stent tube, succeeding struts of the zig-zag exhibiting double bends that are alternately left-handed and right- handed so that, in a radially unexpanded configuration of the stent, a left-handed double bend faces a right-handed double bend and the length portions of the respective struts to one side of the facing double bends are side by side whereas the length portions of the said two respective struts on the other side of the double bends are spaced apart, and further wherein the turns of the zig-zag spiral overlap, in the sense that each point of inflection of one turn lies between the spaced apart length portions of two struts of the next adjacent turn.

0008According to a second aspect of the present invention, there is provided a method of forming a radially expansible stent, comprising: providing a tube of nitinol material; and cutting the tube with a beam jet, whereby the tube is cut into a pattern of a continuous spiral with a plurality of turns wrapping about a longitudinal axis, the spiral including a plurality of struts in a zig-zag arrangement joined at points of inflection, each strut including a double bend remote from a point of inflection and first and second length portions on opposite sides of the double bend, adjacent struts having handed double bends that are substantially mirror images of one another, the handed double bends facing each other such that in a stent unexpanded configuration the length portions of adjacent struts on a first side of the double bends are spaced closer than the length portions of adjacent struts on a second side of the double bends opposite the first side, the adjacent struts on said first side of the double bends joined at a point of inflection, each point of inflection on said first side of the double bends on one turn of the continuous spiral lying between adjacent struts on said second side of the double bends of the next turn of the continuous spiral, and with the first and second length portions being parallel to each other and to the longitudinal axis of the tube.

0009A stent according to the invention thus may comprise a plurality of struts extending generally in the stent axial direction and arranged adjacent to one another to approximate a cylindrical mesh formed about the stent longitudinal axis. The struts may be approximately the same length as each other, with adjacent struts being alternately longer and shorter than each other to create an axial offset in one axial direction so as to form an axially stepped pattern as one moves successively from one strut to the next around the stent axis, to give the desired helical profile. The spiral helix of the stent material so formed can be a continuous thread of stent material that runs unbroken from one axial end of the spiral to the other axial end, the adjacent struts being joined alternately at one axial end and then at the other axial end to the preceding and successive struts in the series as one moves from strut to strut in one rotational direction about the stent axis. The joints between adjacent struts form points of inflection at the strut ends. These points of inflection are tight curves and may be of a similar width to the struts. In the compact, unexpanded state, for example when first cut from a tube of stent material, the points of inflection curve around an angle of 180° to join together the two adjacent struts that lie substantially parallel to each other. When the stent expands to a larger diameter, the adjacent struts separate laterally, but remain joined at the points of inflection, so that they must rotate relative to each other (like when opening a pair of compasses). The angle around the point of inflection then decreases to less than 180°, to maybe 120° at the expanded diameter.

0010By overlapping the turns of the zig-zag spiral, it is possible to achieve a greater density of stent struts per unit area of surface of the bodily lumen that is to be stented by the stent. Clearly, the more of the resilient stent material that is available to press the bodily tissue outwardly, away from the longitudinal axis of the lumen, the more stenting force can be delivered. The present invention provides a way of overlapping the succeeding turns of the zig-zag spiral, to increase stenting force, without at the same time prejudicing the objective of full bending flexibility.

0011Furthermore, the spiral zig-zag stent of the present invention is remarkably easy to manufacture, using the by now well-known beam jet (normally a laser) cutting techniques successfully used in a range of already commercially successful stent designs.

0012As we can see below in illustrated embodiments, it is convenient to locate the inventive double bend midway along the struts in which they are located. In an archetypal design of the present invention, the as-cut struts in the radially compact disposition of the stent will all lie substantially parallel to each other and to the longitudinal axis of the stent (except for the portion of the length of each strut that constitutes the double bend). As the stent expands to its radially expanded working disposition, the points of inflection will bend to allow the struts to rotate relative to each other about the points of inflection at each end, and relative to the long axis of the stent, to take up their zig-zag dispositions at an oblique angle to the long axis of the stent.

0013The stresses to which the struts are subject, during delivery to the stenting site and during expansion at that site to stent the bodily lumen, will normally not be such as to deform the double bend out of existence, so that inspection of the stent in its finished, radially expanded configuration, will reveal the continuing presence of the double bends.

0014An interesting question arises at the axial ends of a stent when the stenting struts are arranged in a spiral configuration. The spiral will end in a free end of the final strut. Rather than have that free end protruding from the annulus of struts which is the stent, it will generally be desirable to have final strut of the spiral arrangement pointing back towards the middle of the stent spiral, and the other end of the spiral. In that way, there is less likelihood of the cantilevered free end of the final strut puncturing or otherwise damaging bodily tissue on which the stent end impinges. Other arrangements will be known to those skilled in the art, for managing the stresses at the opposed ends of the spiral and for terminating the spiral at each end.

Brief description of the drawings

0015To enable a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:- <dl id="dl0001"><dt>Fig. 1</dt><dd>is a view of a prior published stent design, laid flat</dd><dt>Fig. 2</dt><dd>is a view of a stent matrix design in accordance with the present invention, also laid flat as in <figref idref="f0001">Fig. 1</figref></dd><dt>Fig. 3</dt><dd>is a detail from <figref idref="f0002">Fig. 2</figref></dd><dt>Fig. 4</dt><dd>is an isometric view of a stent matrix. as seen, laid flat, in <figref idref="f0002">Fig. 2</figref>; and</dd><dt>Fig. 5</dt><dd>is an isometric view of the <figref idref="f0003">Fig. 4</figref> stent arrangement, but in its radially expanded configuration.</dd></dl>

Detailed description

0016Referring to <figref idref="f0001">Fig. 1</figref>, there is shown a stent matrix, laid flat, such as is created using the teachings of an earlier patent publication of the present applicant, <patcit id="pcit0004" dnum="WO2001032102A"><text>WO-A-2001/032102</text></patcit>. A laser cuts narrow slits in a tubular workpiece of nickel-titanium shape memory alloy, the multitude of slits all being parallel to the longitudinal axis of the workpiece. Scrap portions are removed from the workpiece to leave apertures 12 in the slitted workpiece. The apertures are flanked by connector portions 14 between successive zig-zag rings 20, 22, 24, 26 and 28 of the stent, succeeding rings being connected to each other by a set of four bridges 14 arranged at 90° intervals around the circumference of the stent workpiece. While each of the stenting rings 20 to 28 can deliver a substantial stenting force to bodily tissue outside the respective ring, the set of four bridges 14 between two adjacent stenting rings can limit the flexibility of the stent matrix, in bending, after deployment.

0017Turning to <figref idref="f0002">Fig. 2</figref>, one immediately recognises similarities and differences between <figref idref="f0001">Fig. 1</figref> and <figref idref="f0002">Fig. 2</figref>. One similarity is that the tubular workpiece can be converted to a functioning state by the use of a laser to cut a multitude of slits in the workpiece, the slits extending through the full wall thickness of the workpiece and all being substantially parallel to the length direction of the workpiece. The main difference is that the overall architecture of the stent matrix is that of a spiral rather than of a succession of stenting rings disposed transverse to the length direction of the workpiece.

0018Careful inspection of <figref idref="f0002">Fig. 2</figref> will reveal that the turns of the spiral are interlaced. Thus, a first turn of the spiral can be recognised as corresponding to reference 30, in which case the second turn of the spiral is embraced by reference 32, with almost half the length of the struts of these two spirals overlapping, in the overlap region 34. The ability of the struts of each turn of the spiral, to accommodate in overlap the succeeding zig-zags of the next adjacent turn of the spiral, is accomplished by a "double bend" or "slalom" feature midway along the length of each strut that makes up the zig-zag. To see this better, we turn to <figref idref="f0002">Fig. 3</figref>, which shows a small detail of the complete matrix shown in <figref idref="f0002">Fig. 2</figref>.

0019Looking at <figref idref="f0002">Fig. 3</figref>, we can see part of a first strut 40 and a second strut 42, that are connected at a point of inflection which is outside the field of <figref idref="f0002">Fig. 3</figref>, beyond the right-hand margin, indicated by reference 44. Midway along the length of strut 40 is a portion 46 of the length of strut 40 which is not parallel to the long axis of the tubular stent. It lies between a first bend 48 and a second bend 50 which curve in opposite directions and together make up the slalom or double bend portion midway along the length of strut 40. The double bend or slalom is characterized by the first and second bends 48 and 50 being closely spaced, rather than separated by any substantial straight section of strut.

0020There is a similar double bend feature 52 midway along the length of strut 42, between corresponding bend portions 54 and 56. The respective double bend portions 46 and 52 are "handed" (left-handed and right-handed) in the sense that they are in mirror image to each other rather than being parallel to each other. The consequence is that length portions 58 and 60 of struts 40 and 42 lie side by side to each other, separated only by the width of the beam that cut the slit between them. Conversely, length portions 62 and 64 of the adjacent struts 40 and 42 lie spaced apart from each other, by double the distance by which a double bend portion displaces the length of its strut laterally, i.e. circumferentially, with respect to the longitudinal axis of the stent.

0021As the reader will already have appreciated, the gap between length portions 62 and 64 accommodates the point of inflection 66 and side by side length portions 68 and 70 of a zig-zag of the next adjacent turn around the circumference of the stent of the spiral stent matrix. This pattern is repeated throughout the spiral stent matrix and one can see even from the <figref idref="f0002">Figure 3</figref> detail that there is an absence of bridges between succeeding turns of the spiral and that the zig-zags of the successive turns of the spiral are densely packed. The arrangement thereby can accomplish the objectives of the invention, namely, to preserve flexibility in bending but also to offer a densely enough packed arrangement of struts as to provide substantial stenting force to bodily tissue outside the annulus of the stent. The bending flexibility is enjoyed not only during trans-luminal catheter delivery of the stent but also after release at the stenting site, with the stent in its finished, radially expanded configuration. Suitable delivery devices and methods are disclosed and discussed, for example, in <patcit id="pcit0005" dnum="EP1447057A1"><text>EP 1447 057 A1</text></patcit>, US Patent Application No. <patcit id="pcit0006" dnum="US2007032860A1"><text>US 2007/032860 A1</text></patcit>, US Patent Application No. <patcit id="pcit0007" dnum="US2003060869A1"><text>US 2003/060869 A1</text></patcit>, or <patcit id="pcit0008" dnum="US20070112408A1"><text>US Patent Application No. 2007,0112408 A1</text></patcit>.

0022Turning now to <figref idref="f0003">Figs. 4</figref> and <figref idref="f0004">5</figref>, we find no new information but simply three dimensional representations of the stent of <figref idref="f0002">Figs. 2 and 3</figref>, first (<figref idref="f0003">Fig. 4</figref>) in the "as cut" radially compact configuration in which the tubular workpiece is cut with a laser and in which it is transluminally delivered on a catheter to the stenting site within the body of the patient. By contrast, <figref idref="f0004">Fig. 5</figref> shows the <figref idref="f0003">Fig. 4</figref> stent duly expanded radially at the stenting site, released from its catheter delivery system, with the zig-zags opened out to achieve a much greater diameter than is evident in <figref idref="f0003">Fig. 4</figref>. The reader will notice that the struts of the opened out zig-zag pattern still display the "double bend" feature midway along their length. This is not an essential part of the invention but, equally, it is not a disadvantage of the relatively simple manufacturing process of the illustrated embodiment of the present invention that double bends remain after the stent has expanded.

0023The inventor can envisage a system in which shape memory alloy is heat-treated to remember an expanded configuration that does not display double bends, yet can be compressed into a radially compact disposition with double bends to accommodate the overlaps of the spiral. However, the inventor does not see that such a prosthesis would be so straightforward to design and build, and does not see enough advantages from eliminating the double bends in the deployed configuration, that would make the additional complications of design and manufacture worthwhile. Nevertheless, included within the scope of the present invention are devices which no longer display double bends in the radially expanded configuration. In particular, the form of the spiral stent of the present invention is not facile to replicate except by the laser beam jet method proposed herein, other stent-formation techniques being found to be more complicated and expensive or time-consuming. The benefits of beam jet cutting are thus realised to a high degree with the spiral stent of the present invention, which speaks against the introduction of largely unnecessary additional complications (although the possibility to do so is, of course, recognised).

0024Although the illustrated embodiment is described in terms of a nickel-titanium shape memory alloy, such a material is not a necessary feature of the present invention, which can equally well be embodied in balloon expandable stents of stainless steel or indeed any stent whether it undergoes plastic deformation in being deployed, or elastic deformation, or the quasi-elastic deformation of nickel-titanium shape memory alloys. Suitable materials for stents and stent coverings are discussed in <patcit id="pcit0009" dnum="WO2007095466A2"><text>WO2007/095 466 A2</text></patcit>, and in the prior art citations referenced therein.

0025Although the specific description has been in terms of a bare uncovered stent, the invention is likely to have a particular application in the field of covered stents and stent grafts. The present applicant has particular expertise in covering nickel-titanium shape memory alloy stents with expanded PTFE coverings, for example as disclosed in US Patent Application No. <patcit id="pcit0010" dnum="US2001039446A1"><text>US 2001/039 446 A1</text></patcit>, US Patent No. <patcit id="pcit0011" dnum="US6214039B1"><text>US 6, 214,039 B1</text></patcit>, <patcit id="pcit0012" dnum="US6579314B1"><text>US Patent No. 6,579,314 B1</text></patcit>, or US Patent No. <patcit id="pcit0013" dnum="US6383214B1"><text>US 6,383,214 B1</text></patcit>, but coverings of other materials are known and are feasible with the present stent matrix. Coverings may be particularly useful for cladding the opposite end portions of the spiral, shielding bodily tissue from possible damage by those spiral ends.

0026Increasingly, stents are drug-eluting or coated with an active agent or drug. It is contemplated that stents in accordance with the present invention will include drug-eluting or coated stents. Such a stent is disclosed, for example, in <patcit id="pcit0014" dnum="US6712846B1"><text>US Patent No. 6,712,846 B1</text></patcit>.

0027Although stents are mostly used in vascular applications, whether coronary or peripheral, there are many other bodily lumens that can benefit from stenting, for example biliary applications. These are also contemplated with the stent matrix of the present invention, which will find particular application whenever a high degree of flexibility after deployment is required.

0028Stents often carry radiopaque markers, usually at each end of the stent annulus. In stents of the present invention, an attractive location for such markers is the stent spiral ends. The markers could be placed as wide area paddles or spoons, again to blunt the otherwise pointed end and so shield bodily tissue from damage.

0029In other embodiments, it may be attractive to provide at each end of the zig-zag spiral a loop at material (such as an endless zig-zag stenting ring) around the lumen of the stent, and attach each spiral end to one of these end loops . In that case, it may well be desirable to connect the spiral to the end loop at a small number (2 to 5, maybe 6) locations around the circumference of the lumen, to preserve the relative orientations and locations of the three components of the loop-spiral-loop assembly.

0030Other embodiments will be evident to those skilled in the art.