GB2369062A

GB2369062A - Extendable stent

Info

Publication number: GB2369062A
Application number: GB0121381A
Authority: GB
Inventors: Erik Andersen; Der Leest Machiel Van
Original assignee: CATHNET SCIENCE HOLDING AS
Current assignee: CATHNET SCIENCE HOLDING AS
Priority date: 2000-09-08
Filing date: 2001-09-04
Publication date: 2002-05-22
Anticipated expiration: 2021-09-04
Also published as: ITMI20011869A1; GB0121381D0; FR2813785A1; GB2369062B; ES2212872A1; GB0022097D0; ES2212872B2; FR2813785B1; DE20114667U1; ITMI20011869A0

Abstract

A stent of the type comprising a plurality of tubular elements 1 joined together along a common axis by linking members (4). Each tubular element 1 consists of a strip of corrugated shape, each corrugation being defined by a bent portion (2A,2B) which connects together rectilinear intermediate portions (3). One or more linking members (4) are used to join adjacent tubular members (1), and each linking member extends at an angle to the axis. In addition, at least some of the linking members (4) are at an opposite angle, with respect to the longitudinal axis (10) of the stent to the remaining linking members. The stent is constructed such that, in its expanded state, the corrugations of all of the tubular members are approximately spatially in phase with one another so as to give reasonably uniform support to the vessel being treated.

Description

EXPANDABLE STENT

This invention relates to an expandable tubular stent for implantation in the lumen of a body duct in order to ensure a passage therein.

5 Such stents are used mainly in the treatment of blood vessels exhibiting stenoses, and more generally in the treatment of diseases of various anatomical ducts of the human or animal body, such as, for example, the urinary ducts, especially the urethra, or the digestive ducts, especially the oesophagus.

to The percutaneous implantation of an expandable tubular stent in a stenotic blood vessel is generally recommended, for example after a conventional angioplasty, for preventing the dilated vessel from closing up again spontaneously or for preventing its occlusion by the formation of a new atheromatous plaque and the possible recurrence of stenosis.

15 International patent application WO 98/58600, discloses an expandable tubular stent consisting of an assembly of radially expandable, tubular elements aligned along a common longitudinal axis and successively joined together in pairs by respective sets of linking members.

Each of the tubular elements consists of a strip forming a zigzag do corrugation defining bent extreme portions which are successively connected together in pairs in opposite directions by rectilinear intermediate portions. By virtue of this zigzag corrugation, the stent is expandable between a first, unexpanded state, enabling it to be implanted percutaneously by means of an insertion device of reduced diameter, and a 25 second, expanded state, in which the stent makes it possible to ensure a passage in the lumen of the body duct. To install the stent, it is placed in the unexpanded state on an angioplasty balloon catheter. Once in place, the balloon is inflated in order to cause the stent to expand. Alternatively, the stent may be made from a material which has a recovery capacity, so 30 that the stent may automatically expand, once in place.

International patent applications WO 96/26689 and WO 98/20810 disclose stents of the same general type as that disclosed in the

aforementioned WO 98/58600 but in which the linking members are directed at an angle to the longitudinal axis of the stent. In addition, EP 0928606 discloses a stent in which the linking members are not only angled with respect to the longitudinal axis, but in which those angles s change in polarity along the length of the stent.

The primary purpose of the stent is to support the interior of the body duct to maintain a passage through the duct. It will be appreciated that, as the stent expands, the corrugations forming the tubular elements open out from their position unexpanded so that, in the fully expanded stent, quite to large gaps exist between individual portions of the strip forming the corrugations. The existence of excessively large gaps can lead to a deterioration in the support characteristics of the stent in the area concerned. The opening out of the corrugations is of course an inevitable consequence of the expansion of the stent, but one of the objects of the 15 present invention is to provide a stent in which excessively large gaps are avoided so that, when expanded, the stent provides as uniform a supporting surface as possible.

It has also been found that different and potentially useful characteristics can be realised by making the sign of at least some of the do angles of the linking members different. It will be understood that the sign of the angle of each linking member with respect to the longitudinal axis may either be positive, or negative, or neither positive or negative, in which latter case the linking member is at zero angle to the longitudinal axis - i.e. parallel with the axis.

25 This is illustrated diagrammatically in Figure 1 of the accompanying drawings. In Figure 1 just a single linking member 4 is shown in isolation, and its three potential angular positions with respect to the longitudinal axis 10 of The stent are illustrated diagrammatically. Reference 4A shows the linking member at a positive angle to the axis 10, reference 4B shows the so linking member at a negative angle and reference 4C shows the linking member at an angle which is neither positive or negative with respect to the axis - in other words is parallel to the axis. It will be appreciated that the

terms "positive" and "negative" are used here in a relative sense, and could just as easily be reversed - in other words, there is no particular significance in the fact that the linking member having a positive angle to the axis is on the left of the axis 10 in Figure 1; this could just as well be on 5 the right. What is significant, however, is that the sign of the angles which the respective linking members 4A, 4B, C4 make with the axis 10 is different and, in particular the linking members 4A and 4B make equal but opposite angles to the axis.

According to the invention there is provided a stent for implantation To in the lumen of a body duct in order to provide a passage therein, said stent consisting of an assembly of tubular elements aligned along a common longitudinal axis and successively joined together by respective sets of linking members, each of which extends generally at an angle to the longitudinal axis, wherein each tubular element consists of a continuous 15 strip forming a generally zigzag corrugation defined by a first set of bent portions facing in one direction and a second set of bent portions, axially spaced from the first, facing in the opposite direction, said bent portions being successively connected together in pairs in opposite directions by rectilinear intermediate portions, said stent being characterized in that: o 1) in the unexpanded condition of the stent, the facing bent portions belonging to adjacent tubular elements are circumferentially spaced from one another by a distance approximately equal to half the distance between the bent portions of each set; 2) each of said linking members extends from a bent portion 25 belonging to one tubular element to a facing bent portion belonging to the next adjacent tubular element, the bent portions so joined being circumferentially spaced from one another by a distance approximately equal to half the distance between the bent portions of each set; and 3) at least some of the angled linking members are at an opposite so angle, with respect to the longitudinal axis, to the remaining angled linking members. It will thus be understood that the structure of the stent is generally

cylindrical, with each adjacent pair of tubular elements being joined to the next by a respective set of linking members. Each set of linking members may consist of one or more linking members, but preferably all the sets contain the same number of linking members.

5 In the present invention the angled linking members of at least some of the sets are at an opposite angle, with respect to the longitudinal axis of the stent, to the angled linking members of the remaining sets. In addition, the stent may have non-angled linking members, the latter being understood to be linking members whose angle to the longitudinal axis is to zero. The magnitude of the angles are preferably all the same typically about 50 to the longitudinal axis - but the invention also encompasses the possibility that the magnitude of the angles is different as between different linking members in order to achieve different mechanical characteristics in different parts of the stent.

15 Preferably the relative numbers of linking members making a positive angle to the longitudinal axis compared to those making a negative angle to the longitudinal axis is approximately balanced so as to give the stent an overall structural stability; however, small sections of the stent may be specifically engineered to have a non-stable distribution to give special JO characteristics. In a particular preferred embodiment, each set of one or more linking members at a particular angle to the longitudinal axis is balanced by a corresponding set of one or more linking members at an equal but opposite angle to the axis. Preferably such equal and opposite sets of linking members are situated physically fairly close to one another in 25 order to give a reasonably regular distribution of the two types of linking members (negative and positive) about the structure of the stent. For example, the structure may be such that the sign of the angle of the linking members alternates along the length of the stent: positive, negative, positive, negative etc. so The linking members may be situated in any angular position about the circumference of the stent; however it is desirable not to give the structure a bias which could result in uneven bending. To this end, it is

preferable to position the linking members of individual sets at different spaced radial positions about the circumference so that the linking members are spread approximately evenly about the 360 of the circumference of the stent. This does not mean to say that no two linking 5 members can occupy the same angular position; simply that there should preferably be a balanced distribution of the linking members about the 360 . It is also preferred that, when the stent as a whole is considered, the various linking members are arranged approximately evenly about the Jo circumference so that, for example, there is not a concentration of linking members at a particular angular position such as might lead to a mechanical bias to the stent. In the preferred embodiment of the invention, the sets of linking members are arranged in a regular pattern along the stent, being spaced circumferentially as between each pair of 15 tubular elements in an approximately helical manner. The circumferential spacing between adjacent sets of linking members may be set according to the circumstances, but is preferably equal along the stent. In practice adjacent sets of linking members will be circumferentially spaced from one another by a distance approximately equal to the distance between the so bent portions of each set or a multiple thereof, for example twice or 3 times, this distance.

Preferably the linking members are rectilinear, by which is meant that, between their attachment point with the bent portion of one tubular element and their attachment point with the bent portion of the next as adjacent tubular element, they are substantially straight. Preferably also they are rectangular in cross section, having a depth in the radial direction of the stent greater than their width. However, they may also have other cross sections, such as square.

Although it was mentioned above that the linking members are so rectilinear, it will be noted that, since they extend at an angle to the longitudinal axis of the stent, they will in practice, unless they are very rigid, bow outwards slightly with a radius of curvature approximately equal to that

of the stent. Thus, strictly speaking, such linking members could more accurately be said to have a part-helical shape. The use of the word rectilinear in this specification should be understood in this context.

As mentioned above each set of linking members may comprise one s or more linking members. If there are a plurality of linking members in each set, these may be such as to provide a balanced set- i.e. the same number of positive linking members as negative linking members - or an unbalanced set in which there are a smaller number of one sign than the other. Possibly, all of the angled linking members in each set may be of to one sign. In the case of such an unbalanced set of linking members, this is preferably compensated for by the fact that other sets along the length of the stent are also unbalanced, but in the opposite sense. Thus, for example, alternate sets of linking members may be unbalanced in opposite directions. ts In the preferred embodiment of the present invention, each set of linking members comprises only a single linking member, and this will be assumed throughout the remainder of the description. The result of using

just a single linking ember is that each linking member can act on its own without the constraint imposed by the other linking members so interconnecting the same pair of tubular elements. In particular the use of a single linking member between adjacent tubular elements allows the maximum freedom of movement between adjacent elements, providing the stent as a whole with exceptional flexibility. Thus adjacent tubular elements can freely move with respect to one another about an axis which 25 iS approximately radial with respect to the stent and about an axis through the circumference which is approximately at right angles to the radial direction and indeed about all axes in between. It will also be seen that adjacent tubular elements can freely move about an axis through the circumference which is approximately parallel to the longitudinal axis of the so stent, resulting in twisting or torsion in the single linking member.

The linking members may be attached at their ends by adhesive or welding. Preferably however, the linking members are formed integrally

with the tubular members.

The invention is applicable to any type of stent having the general structure described above, for example that described in our application WO 98/58600. In the stent described in this application the thickness of 5 the strip forming each of the tubular elements, measured radially relative to the tubular element, is greater than the width of the strip in the bent portions. In this way the distribution of the deformation forces during the expansion of the stent is optimised by adjusting, at least in certain portions constituting each tubular element of the stent, the thickness/width ratio as a Jo function of the forces exerted thereon.

Advantageously the geometry of the rectilinear portions should favour bending in the radial direction of the stent over the circumferential direction. The invention will be better understood, and other objects, 15 characteristics and advantages thereof will become more clearly apparent, from the following explanatory description referring to the attached

schematic drawings, which are given solely by way of a non-limiting example illustrating a currently preferred embodiment of the invention, and in which: o Figure 1 is a diagram to explain the three possible orientations of the linking member with respect to the longitudinal axis of the stent; Figure 2 is a two-dimensional view of the evolute of the surface of a stent according to the invention, in its unexpanded state; Figure 3 is an enlarged view of a portion of Figure 2; and 25 Figure 4 is a plan view of a stent according to the invention, in its expanded state.

The stent shown in Figure 2 consists of an elongate, approximately tubular body or frame defined by a plurality of tubular elements 1 aligned along a common longitudinal axis and successively joined together by a so plurality of linking members 4, which will be described in greater detail below. A stent of six elements is shown in the drawings, however typical

stents can have as few as 3 elements, or as many as 20 elements, or even more, depending upon the circumstances.

Each tubular element 1 consists of a strip forming a zigzag corrugation defined by bent portions 2 which are successively connected 5 together in pairs in opposite directions by rectilinear intermediate portions 3. For each tubular element, there can thus be defined two sets of bent portions: bent portions 2A connecting the rectilinear portions 3 on one end of the tubular element; and bent portions 2B connecting the rectilinear portions 3 on the opposite end of the tubular element. In Figure 2 the bent to portions 2A and 2B are arbitrarily shown on the left and right hand ends respectively of each tubular element. It will be noted that Figure 2 shows a two-dimensional evolute of the stent, showing the stent as it would look if cut longitudinally and flattened. Thus, it will be understood that the corrugation forming each tubular element 1 is in the form of a closed loop 15 in other words the cut ends 12,13 are in fact joined.

Advantageously, for a given tubular element, the rectilinear p -

3 are all of the same length and the bent portions are all identical and approximately semi-circular. Thus the corrugation advantageously has uniform shape. The tubular elements 1 are joined in such a way that each 20 bent portion 2A is in approximately axial alignment with a corresponding bent portion 2A on each of the remaining tubular elements 1. Likewise, each bent portion 2B is in approximately axial alignment with a corresponding bent portion 2B on each of the remaining tubular elements 1. In addition, each of the bent portions 2A is situated approximately mid s way between two adjacent bent portions 2B, when seen in the circumferential direction of the stent. In other words, each corrugation of each tubular element 1 iles at approximately the same angular position about the circumference of the stent as a corresponding corrugation of the next adjacent stent. The corrugations of adjacent tubular elements, and 30 indeed all of the tubular elements, can thus be said to be spatially in phase with one another, resulting in the expanded stent providing uniform support

for the body duct being treated. This will be discussed in more detail below. As can be seen in Figure 2, the thickness of the strip forming each tubular element 1 in the bent portions 2 is greater than the width I of this 5 strip in the bent portions 2. The thickness of the strip in the rectilinear portions 3 is approximately equal to the thickness of the strip in the bent portions 2. The thickness in this context is as measured radially relative to the respective tubular element.

The above-described profile (thickness greater than width) ensures to that the bent portions behave well when they are subjected to the radial forces exerted during the expansion of the tubular elements.

Advantageously, the width 10 of the strip in the rectilinear portions 3 is greater than the width I of the strip in the bent portions 2.

By way of example, the thickness in the bent portions will typically Is be of the order of 0.135 mm, the width I in the bent portions will typically be of the order of 0.116 mm and the width lo in the rectilinear portions will typically be of the order of 0.165 mm. The length of each tubular element 1, measured in the direction of the longitudinal axis will typically be about 2 mm. so Preferably, the thickness and width transitions between the rectilinear portions 3 and the bent portions 2 is gradual in order to avoid the formation of an incipient fracture.

Extending between each tubular element 1 and its next adjacent tubular element 1, is a single linking member 4 which is the sole 25 mechanical interconnection between the successive tubular elements. As already mentioned, the present invention encompasses the possibility that adjacent tubular elements are joined by multiple linking members, which members are preferably evenly spaced in the circumferential direction of the stent. However, the present description describes only a stent in which

so a single linking member is used to join each adjacent pair of tubular elements, since this feature itself has some advantages. Each linking member 4 is rectilinear and extends from the bent portion 2A of one tubular

element 1 to the next adjacent bent portion 2B of the next adjacent tubular element 1. Since the bent portions 2A and 2B are angularly spaced about the circumference of the stent, the linking members are necessarily angled with respect to the longitudinal axis of the stent, the circumferential s distance between the connection points at opposite ends of the linking member 4 being approximately equal to half the distance between adjacent bent portions 2A, which is of course the same as the corresponding distance between adjacent bent portions 2B. This distance is represented in Figure 2 by the reference L. to It will be seen from Figure 2 that the angles of the linking members 4 are not all the same. In particular, it will be seen that the leftmost linking member 4 extends from a bent portion 2A of one tubular element 1 to the next adjacent higher bent portion 2B of the next adjacent tubular element 1.

The next linking member 4 in the rightwards direction, however, extends is from a bent portion 2A of a tubular element 1 to the next adjacent lower bent portion 2B of the next adjacent tubular element 1. It will be understood that the terms lower and higher are here used in reference to the orientation of Figure 2 and not to any physical difference in height as between the bent portions concerned.

so As will be seen in Figure 2, this pattern continues throughout the length of the stent and means that the angle of the linking member 4 alternates positively and negatively about the longitudinal axis of the stent.

It can thus be considered that the linking members fall into one of two groups: one group whose angle is positive with respect to the longitudi-

s axis and the other group whose angle is negative with respect to the longitudinal axis. Preferably the magnitude of the angles for each group is approximately the same; it is the sign of the angle that is different. The two groups can be arranged in various ways along the length of the stent; the preferred way is to alternate one or more members from one group with so one or more members from the other group along the length of the stent.

In this way, the number of linking members from one group is the same or nearly the same as the number of linking members from the other groups.

The configuration of the linking members 4 is such that, during bending of the whole stent in its expanded state, the primary force exerted on each of the linking members 4 is a torsional force, causing the linking member to tend to twist in the manner of a corkscrew.

5 The cross sectional shape of the linking members 4 is rectangular having a thickness in the radial direction which is greater than its width lo.

By way of example, the thickness of the linking members 4 is of the order of 0.135 mm, while the width lo is of the order of 0.086 mm. It will also be noted that the width of the linking members 4 is less than that of the bent to portions 2A,2B.

The geometry of a single linking member 4 is shown in enlarged detail in Figure 3. It will be seen that the linking member 4 extends at a tangent to the circular bent portion 2A and at a tangent to the circular bent portion 2B. In particular the linking member 4 is arranged such that its 15 centre line makes a tangent with the centre lines of the two bent portions 2A,2B to which it is joined at each end.

As already explained, in order to allow even flexure in all directions, the various linking members 4 should be evenly angularly distributed about the circumference of the stent. The circumferential spacing between go adjacent linking members in the embodiment of Figure 2 is equal to 4L; however, other multiples of the distance 2L are possible, including 2L itself.

It will be appreciated that 2L corresponds to the circumferential distance between adjacent bent portions 2A (or 2B) in each tubular element 1.

Preferably this link spacing continues for all linking members, and in the 25 same direction: thus link 4.2 is spaced from link 4.1 by a circumferential distance 4L in a particular direction, likewise link 4.3 is spaced from link 4.2 by the same distance 4L, and in the same direction. This pattern continues along the length of the stent, giving an approximately helical distribution of linking members. This ensures that the linking members are So regularly spaced about the 360 circumference of the stent for the reasons discussed above.

Reference is now made to Figure 4 which shows the expanded condition of the stent. Although the stent shown is the same as that of Figure 2, only 4 elements 1 are shown in order to enable more detail to be represented. This drawing is also unlike Figure 2 in that it attempts to s represent the 3-D structure of the stent, rather than the artificially flattened form of Figure 2.

As the stent is expanded the corrugations forming the tubular members 1 open out from the position illustrated in Figure 2 so that, in the fully expanded stent, quite large gaps exist between individual elements of JO the strip forming the corrugations. The opening out of the corrugations during expansion cannot, of course, be prevented, but excessively large gaps have been avoided by designing the stent described herein in such a way that, when in the expanded state, the corrugations as between the individual tubular elements 1 remain, as far as physically possible, in phase us with one another. This provides as uniform a supporting surface as possible. It will be seen from Figure 4 that this goal has not been achieved perfectly but, to a reasonable approximation, the corrugations are in phase.

The fact that there is only a single linking member between the tubular elements is largely responsible for this advantageous arrangement when in JO the expanded stent; the presence of any linking member from one tubular element to another will act to restrain the tubular element from expanding naturally and therefore the fewer the number of linking members between the tubular elements, the better.

The circumferential distance between adjacent linking members 4.1, 25 4.2 etc., also has an influence on the manner in which the stent expands: as discussed above, in the illustrated stent, this distance is equal to 2 x the distance between adjacent bent portions 2A (or 2B). This gives rise to the same number (2) of phase relationships between the corrugations of the various tubular elements making up the stent. If Figure 4 is studied 30 carefully, it will be seen that the corrugations of every 2 tubular elements are fairly accurately in phase, an exception being made only in the region of linking member itself which distorts the local picture. If the

circumferential distance between adjacent linking members 4.1, 4.2 etc., had been just 1 x the distance between adjacent bent portions 2A (or 2B) then the corrugations of every tubular element would have been approximately in phase subject, as before, to a distortion in the area of the 5 linking member.

The stent which has been described is therefore expandable betvveen an unexpanded state, enabling it to be guided inside the lumen through a body duct, such as a blood vessel, for example, and an expanded state, in which the stent, after a uniform expansion, comes into to contact with the inner wall of the body duct, defining a passage of approximately constant diameter inside said duct.

The stent will generally be forcibly expanded mechanically under the action of a force exerted radially outwards, for example under the effect of the inflation of a balloon.

15 Alternatively, the stent may be of the "auto-expandable" type, i.e. capable of changing by itself from a first, unexpanded condition under stress, enabling it to be guided through the body duct, to a second, expanded, working condition.

The stent may be made of any material compatible with the body to duct and the body fluids with which it may come into contact.

In the case of an auto-expandable stent, it will be preferable to use a material with a recovery capacity, for example, stainless steel, Phynox or nitinol. In the case of a stent utilising a forced expansion, a material with a 25 tow elastic recovery capacity may be used to advantage. Examples are metallic materials such as tungsten, platinum, tantalum, gold, or stainless steel. The stent may be manufactured from a hollow tube with an approximately constant thickness corresponding to the desired thickness.

so The pattern of tubular members and linking members may be formed either by laser cutting followed by electrochemical polishing, or by chemical or electrochemical treatment.

The stent may alternatively be manufactured from a sheet of approximately constant thickness corresponding to the desired thickness of the stent. The geometric configuration of the stent can be obtained either by lasercutting followed by electrochemical polishing, or by chemical or s electrochemical treatment. The sheet cut in this way is then rolled up to form a cylinder and welded to give the desired final structure. The stent which has been described can be inserted in a manner known per se. In the case of a stent utilising mechanically forced expansion, the insertion system will preferably comprise a balloon-tip catheter on which the device JO will be positioned in the unexpanded state before being introduced into an insertion tube for guiding it to the site to be treated.

Claims

1. A stent for implantation in the lumen of a body duct in order to provide a passage therein, said stent consisting of an assembly of tubular 5 elements aligned along a common longitudinal axis and successively joined together by respective sets of linking members, each of which extends generally at an angle to the longitudinal axis, wherein each tubular element consists of a continuous strip forming a generally zigzag corrugation defined by a first set of bent portions facing in one direction and To a second set of bent portions, axially spaced from the first, facing in the opposite direction, said bent portions being successively connected together in pairs in opposite directions by rectilinear intermediate portions, said stent being characterized in that: 1) in the unexpanded condition of the stent, the facing bent portions Is belonging to adjacent tubular elements are circumferentially spaced from one another by a distance approximately equal to half the distance between the bent portions of each set; 2) each of said linking members extends from a bent portion belonging to one tubular element to a facing bent portion belonging to the To next adjacent tubular element, the bent portions so joined being circumferentially spaced from one another by a distance approximately equal to half the distance between the bent portions of each set; and 3) at least some of the angled linking members are at an opposite angle, with respect to the longitudinal axis, to the remaining angled linking 2, members.

2. A stent as claimed in claim 1 wherein each set of linking members consists of just a single linking member.

3. A stent as claimed in either one of claims 1 or 2 wherein the angle of a first group of said angled linking members is positive with respect to said so longitudinal axis whereas the angle of a second group of said angled linking members is negative with respect to said longitudinal axis.

4. A stent as claimed in claim 3 wherein the relative numbers of linking members in the first and second groups are approximately balanced so as to give the stent an overall structural stability.

5. A stent as claimed in either one of claims 3 or 4 wherein one or 5 more members from said first and second groups of linking members alternate when considered in the direction of the longitudinal axis.

6. A stent as claimed in any one of the preceding claims wherein all of the angled linking members make substantially the same angle with the longitudinal axis.

to

7. A stent as claimed in any one of the preceding claims in which each linking member at a particular angle to the longitudinal axis is balanced by a corresponding linking member at an approximately equal but opposite angle to the longitudinal axis.

8. A stent as claimed in any one of the preceding claims wherein each 15 linking member is of rectangular cross section, having a thickness, in the radial direction, greater than its width.

9. A stent as claimed in any one of the preceding claims wherein the linking members as a group are distributed approximately evenly about the circumference of the stent.

so

10. A stent as claimed in any one of the preceding claims wherein each set of linking members is circumferentially spaced from the adjacent set or sets of linking members by an amount equal to a multiple of the circumferential distance 2L between adjacent bent portions belonging to one of said sets of bent portions.

25

11. A stent as claimed in claim 10 wherein the circumferential spacing between all of the sets of linking members is the same along the length of the stent, thus giving an approximately helical pattern of linking members along the stent.

12. A stent as claimed in any one of the preceding claims wherein each so linking member joins its respective bent portions at a tangent.

13. A stent as claimed in claim 12 wherein the bent portions are approximately semi-circular in shape, and wherein the centre line of the

linking members tangential to the centre lines of the respective bent