ITTO960374A1 - ANGIOPLASTIC STENT. - Google Patents

Description

DESCRIPTION of the industrial invention entitled: Stent for angioplasty

TEXT OF THE DESCRIPTION

The present invention relates, in general, to the so-called angioplasty stents.

With this denomination we intend to indicate in general those devices intended for an endoluminal application (for example inside a blood vessel), usually implemented through catheterization, with subsequent deployment on site in order to implement a local lumen support action. . All with the main intention of avoiding the reconstitution of a stenotic site in the treated place. It should also be noted that the use of substantially similar structures has already been proposed in the art to achieve the deployment and anchoring in site of vascular grafts: naturally also this possible extension of the field of application is to be understood as falling within the scope of the invention.

For a general review on vascular stents, one can usefully refer to the "Textbook of Interventional Cardiology" edited by Eric J. Topol, W. B. Saunders Company, 1994 and in particular to section IV of the vol. II, entitled "Coronary stenting".

A large number of patent documents have also been dedicated to the subject, as witnessed, for example, by US-A-4776 337, US-A-4 800 882, US-A-4 907 336, US-A-4 886 062, US-A-4 830 003, US-A-4 856 516, US-A-4 768 507, US-A-4 503 569 and EP-A-0 201 466.

Despite the extensive research and experimentation activity, as documented at the patent level, only a very small number of operational solutions have so far found practical use.

This fact is attributable to several factors, among which the following problems or needs can be mentioned:

- ensure that in the phase of progress towards the site to be treated, the stent is able to adapt with sufficient docility to the trajectory traveled, also as regards sections with reduced radii of curvature, such as those that can be found, for example, in some peripheral vessels; all this without negatively influencing the stent's ability to perform, once positioned and deployed, an effective support action,

- avoid, or at least limit, the longitudinal shortening effect that occurs in many stents upon deployment,

- offer to the wall of the lumen that is supported a support surface as extended as possible,

- avoid giving rise to complex geometries and / or possible stagnation sites which, especially in applications to blood vessels, can give rise to negative phenomena such as coagulation processes, thrombization, etc., and

- reconcile the above requirements with simple, reliable methods and implementation criteria within the reach of currently available technologies.

The present invention, having the characteristics referred to specifically in the following claims, has the aim of solving, at least in part, the problems outlined above.

The invention will now be described, purely by way of non-limiting example, with reference to the attached drawings, in which:

- figure 1 is a general perspective view of a first stent for angioplasty made according to the invention,

- figure 2 is a side view, reproduced on a slightly enlarged scale, of the stent of figure 1,

- figure 3 illustrates, in an ideal flat development, the geometric characteristics of the stent wall of figures 1 and 2,

- figure 4, generally similar to figure 3, illustrates a first variant embodiment of a stent generally similar to that illustrated in figures 1 and 2,

- figure 5 and figure 6 are two sectional views, respectively, along the lines V-V and VI-VI of figure 4,

Figure 7 is a perspective view of another angioplasty stent,

figure 8 is a side view of the stent of figure 7, e

- figure 9 illustrates, in ways substantially similar to those of figure 3, the ideal development in plan of the stent wall of figures 7 and 8.

Although referring to different embodiments, the reference numeral 1 is used in Figures 1, 2, 7 and 8 to indicate as a whole a so-called stent for angioplasty.

For a general identification of the methods of use and construction characteristics of such a plant device, please refer to the documentation cited in the introductory part of the description.

By way of synthesis, it will be remembered that the stent 1 is usually made in the form of a body with a tubular envelope with a total length comprised between a few millimeters and a few tens of millimeters, a wall thickness (the wall being usually of opening with meshes or loops, as will be seen better below) of the order, for example, of a few hundredths of a millimeter in view of the possible insertion into a lumen (such as a blood vessel) in a site where it is desired to remedy a phenomenon of stenosis. The stent is normally placed on site by catheterization thus realizing a radial expansion starting from an introduction diameter of the order, for example, of 1.5-1.8 mm to an expanded diameter, for example, of the order of 3-4 mm in such a way that, in this expanded condition, the stent carries out an action to support the lumen avoiding the redetermination of a stenosis. In general, the external diameter in conditions of radial contraction is chosen so as to allow the introduction of the stent into the lumen undergoing treatment, while the expanded diameter corresponds in principle to the diameter to be maintained and established in the lumen once eliminated. the stenosis phenomenon, it should still be noted that, although the main application of the described stents refers to the treatment of blood vessels, the application according to supporting element of any light present in the human or animal body.

As for the methods and criteria that allow the stent to be deployed (i.e. to expand in yestu), the currently most widespread solution is to resort to a so-called balloon catheter, placing the stent around the catheter balloon in contracted conditions and expanding then the balloon once the stent has been brought to the placement site. However, different solutions are also conceivable, such as that of resorting to superelastic materials which, once the containment elements intended to keep the stent in a contracted condition have been removed until the implantation site is reached, lead to the expansion of the stent. In addition or as an alternative, it has also been hypothesized that, for the realization of the stent, recourse to materials having a so-called "shape memory", so as to achieve radial expansion in the implantation position, has been hypothesized.

Usually (for more precise indications, refer to the bibliographic and patent documentation cited in the introduction of the description) the stent is made of metallic material, able to reconcile two fundamental requirements for the application, namely the ability to deform plastically during the of expansion and the ability to resist, while retaining the expanded form, to any stresses that would tend to cause the stent to close again. The material known under the trade name of "Nitinol" has also established itself in view of its qualities of superelasticity and shape memory that may be required in the expansion phase.

In any case, these aspects of a technological nature will not be dealt with in detail in the present description as they are not relevant in themselves for the purposes of understanding and carrying out the invention. This essentially also applies to the stent manufacturing technology according to the invention. As already mentioned, these assume, in general terms, the appearance of a body with a tubular envelope with an open wall. As regards the methods of realization, it is therefore possible to resort, according to known technologies, to at least three basic solutions, namely:

- realization of the stent starting from a continuous tubular blank, destined to be segmented into single stents, with the realization of the open parts through technologies such as laser engraving, photoengraving, electroerosion, etc;

- making the stent starting from a ribbon-like body in which, for example with the technologies mentioned above, the areas opened in view of the subsequent tube closure of the ribbon-like element are made, and

- realization of the stent starting from shaped metal wire with subsequent connection of the wire loops, for example, through operations of micro-welding, brazing, gluing, crimping, etc. ...

The first solution described is the one currently preferred by the Applicant for making stents according to the embodiment examples described below, with the exception of the solution referred to in figure 4, which intrinsically involves the use of a metal wire. In particular, laser beam incision proves to be the most flexible solution as regards the possibility of rapidly modifying the characteristics of the stents in production according to specific application needs.

In any case, it should be emphasized that even this constructive aspect is not relevant, if not in a marginal way, in the terms that will be better recalled below, in particular with reference to Figure 4, for the purposes of implementing the invention. This also applies to the choice of the individual technologies and the order in which the various operations described (realization of the open parts, segmentation, possible bending of the belt element, etc.) are carried out.

In all the embodiments illustrated here, the body of the stent 1 extends along a longitudinal direction generically identified with an axis z. For the sake of clarity, it should in any case be remembered that the stent is intended to be bent, even significantly, during use, easy flexibility being one of the characteristics sought.

In all the embodiments illustrated here, the body of the stent 1 is constituted by a series of successive segments, of a generally annular shape, indicated with 2 in the figures. As can easily be seen, the stent 1 of Figures 1 and 2 comprises seven of these segments, while the stent of Figures 7 and 8 comprises six.

To fix the ideas, without this having to be interpreted in a limiting sense of the scope of the invention, the segments 2 have a length, measured in the longitudinal direction of the stent 1, therefore along the z axis, of the order of about 2 min. In other words, for reasons that will become clearer in the following, the segments 2 are rather "short" in the sense of length.

As can be better appreciated in the side view of Figure 2, the various segments of the stent 1 illustrated therein are connected to each other by pairs of jumpers 3, 4 (in fact constituting integral parts of the stent wall, as will be better illustrated below. , for example with reference to Figure 3) whose essential characteristic (this applies both to the stent of Figures 1 and 2 and to the stent of Figures 7 and 8) is that of providing.

according to an alternating sequence, a hinged connection of the segments 2 respectively connected by them according to bending or folding axes orthogonal to each other.

This type of solution achieves two advantages.

In the first place, the longitudinal flexibility characteristics of the stent 1, necessary to facilitate its placement on the implant site, are essentially delegated to the bridges 3, 4, while the characteristics of structural resistance, therefore supporting the lumen, are delegated to the structures actual segments 2; all with the possibility of achieving, through an exact adaptation of the sections of the various component elements, the optimization of the desired characteristics.

secondly, the ordering in sequence (usually, but not necessarily, alternating) of the jumpers, together with the fact that the segments 2 are, as mentioned, rather short, allows to easily realize, practically at any point of the extension longitudinal of the stent 1, a bending in any direction of space, even with rather small radii of curvature.

This concept can be more easily understood by referring to the solution of figures 1 and 2 (as will be seen, the same also applies to the solution according to figures 7 and 8) noting how, thanks to their arrangement at 180 ° in a diametrically opposite position on the wall of the stent 1, the bridges 3 allow the stent 1 to be flexed locally in correspondence with a respective axis x generically transversal with respect to the longitudinal axis z. The bridges 4, also arranged at 180 ° from each other in an orthogonal plane with respect to the bridges 3, allow the stent 1 to be flexed locally in correspondence with a second axis y transverse with respect to the longitudinal axis z and, in the example illustrated here, orthogonal to the x axis mentioned above.

Since, as already mentioned, the segments 2 are rather short, the aforesaid x and y axes are in close proximity to each other, arranged in alternating sequence along the longitudinal development of the stent 1, whatever the number of segments 2.

From this it follows the fact that, practically in correspondence with any longitudinal position of the stent 1, the stent 1 itself can be easily curved around a generic axis d, which can be defined on the basis of a relationship of the type

(1) that is, as a linear combination of the bending movements around the axes identified by the vectors

Referring to the general theory of vector spaces, it can also be easily understood that having respective bending capacities in place along two orthogonal axes in sequence, preferably alternating, constitutes the simplest solution to achieve the desired end. At least in principle, however, solutions would be conceivable in which successive segments 2 of the stent 1 are connected by jumpers such as jumpers 3 and 4 (or elements that allow similar bending capacities, as will be better illustrated below with reference to figures 7 and 8) in correspondence of axes not orthogonal to each other. By way of example, a solution can be mentioned in which, for example, pairs of diametrically opposite bridges ordered in sequence and offset by angles of 60 ° from each other are provided.

Again, the alternating sequence described above, that is: x axis, y axis, x axis, y axis can be, at least in principle, replaced by a sequence of a different type, for example: x axis, x axis, y axis, y axis, x axis, x axis, etc. Indeed, the fact of having, as in this last example cited, a bending capacity around the x axis in correspondence with two adjacent segments 2, with subsequent bending capacity along the y axis, repeated for two adjacent segments 2 , can be advantageous in those applications in which it is intended to privilege the possibility of obtaining very small radii of curvature.

In the solution according to Figures 7 and 8, the same conceptual solution is achieved in slightly different ways.

In the solution illustrated in Figures 7 and 8, in fact, the various segments 2 are connected to each other by means of bridges constituting respective parts of two "backbones" of the stent consisting of integral elements of the stent 1 which extend according to a generally sinuous trajectory along two generatrices of the stent. ideal cylindrical surface of the stent in diametrically opposite positions. The related construction details will become clearer from the description given below.

Observing in particular figure 8, and adopting the same formalism adopted with reference to figure 2, it can be seen that the possibility, foreseen for the backbones 30, to flex in correspondence with respective loops extending between successive segments 2, realizes the local flexion capacity along the y axis with respect to the general direction of extension defined by the z axis.

The local capacity of the aforementioned bridges to extend, and in particular the possibility of one of the bridges to extend while the diametrically opposite bridge maintains approximately corresponding longitudinal dimensions or extends to a more limited extent or, possibly, contracts slightly in the longitudinal direction, allows to realize the bending movement around the x axis, as schematically indicated with dashed line for the segment 2 which is located furthest to the left in Figure 8.

Also in this case, therefore, and in correspondence with each position of connection between adjacent segments 2, it is possible to obtain the flexion of the stent 2 around a generic axis d defined by a relationship such as the relationship (I) previously introduced.

All this will be appreciated once again, keeping the structure of the individual segments 2 substantially unchanged, thus making the longitudinal flexion movement of the stent 1 essentially attributable to the flexion and / or in general to the local deformation of the bridges that connect segments 2 adjacent to each other.

Returning to figure 9, it can be noted that it constitutes, as already indicated above, an ideal flat development, reproduced on a magnified scale, of the stent wall of figures 1 and 2.

In Figure 3, in fact, the seven segments 2 connected to each other in alternating sequence can be recognized by the pairs of jumpers 3 and 4 arranged in an alternating sequence at 90 ° of pairs of elements diametrically opposite to each other. As already mentioned, this is an ideal flat development which can correspond to the development of a strip blank starting from which the stent 1 is then obtained by folding this blank into a tube.

From the observation of the same figure 3 it can also be seen that the body of each of the segments 2, of an overall annular shape, is constituted, in the illustrated embodiments, by a complex of approximately sinusoidal loops with extension (measured circumferentially with respect to the element 2) substantially constant and doubled in correspondence with the loops from which the bridges 3, 4 extend, according to the methods better illustrated below.

Within each segment 2 it is ideally recognizable a respective median plane X2, extending, in the illustrated examples, in a direction generally orthogonal to the longitudinal axis z Two of these planes, indicated by X2, have been represented in the view of Figure 3 ( and of figure 9): naturally, since it is a flat development, the planes in question are represented in the figure by straight lines.

It can then be noted that each segment 2 is made up of a sequence of loops, each loop (approximately comparable to a half wave of a sine wave) defining a respective concave part 5, with concavity facing the median plane X2, connecting with two arms About -6 straights.

To fix the ideas, only two of these loops, connected to each other by a bridge 3, have been specifically characterized in Figure 3. These are in particular the two loops whose concave parts are indicated with the references 5 and whose side branches are identified by references 6.

It can easily be understood that the radial expansion movement of the stent 1 is substantially achieved as a result of a spreading movement of the aforementioned loops: to fix the ideas, with reference to the plane development of Figure 3, the radial expansion movement of the stent corresponds to a stretching of the plane development represented in Figure 3 in the direction of an increase in height, therefore to an expansion in the vertical direction of Figure 3.

In practice, this radial expansion movement corresponds to a spreading of the concave parts 5 while the lateral branches 6 of each loop remain substantially straight.

The localization of the plastic deformation of the stent 2 in correspondence with the concavities of the loops 5 can be favored (as will be better seen in the following with reference to Figure 4) by intervening on the sections and / or the cross-sectional areas of the parts of each loop.

In any case, the aforementioned radial expansion movement (vertical stretching of the plane extension of Figure 31 substantially affects the concavities 5 of the loops of the elements 2 and does not in any way affect the jumpers 3, 4 extending in the longitudinal direction (axis z).

It will be appreciated that the same also applies to the solution illustrated in Figure 4 (which will be better explained below) in which one of the median planes X2 has been represented, highlighting only one of the loops and identifying in a specific way its concave part 5 and its lateral branches 6. The same criterion also applies to the solution according to figures 7 and 8: in this regard, observe the plane development of figure 9. Here too, as in figure 3, two median planes X2 of two segments 2 have been indicated, highlighting then the concave part 5 and the lateral branches 6 of two opposing loops between which a section of one of the two backbones 30 with a sinusoidal development extends like a bridge.

As can be seen better from the comparison of Figures 3, 4 and 9, a common characteristic of all the solutions described is given by the fact that the radial expansion movement of the segments 2 corresponds, within each segment 2 itself, to an ideal approach of the concave part 5 of each loop to the median plane X2 of the segment 2 of which this loop is part.

Whoever reads this description can simply realize this fact by thinking of extending vertically: for example, segment 2 corresponding to the X2 plane furthest to the right in Figure 3. As a result of this stretching, carried out precisely along the line X2 which identifies the said plane, the concavity 5 of the indicated loop precisely approaches the line X2, the same behavior being followed, in opposite directions, according to the different location with respect to the line X2, by the concave parts of all the other loops.

If, with reference to the jumpers 3 {and the same is also valid for the jumpers 4 as well as for the single sections of the backbones 30 which define the parts equivalent to the jumpers 3 and 4 in figure 9) it is considered that the connection to the relative segments 2 is made in correspondence with the concave part (or intrados) of a respective loop it can easily be appreciated that the radial expansion movement of the segments 2 is accompanied, so to speak, with a thrust movement exerted on the jumpers 3, 4 (and on the respective sections of dorsal 30). This pushing movement corresponds, so to speak, to an expulsion movement of the jumpers or of the dorsal sections in question with respect to the corresponding segment 2.

Returning once again to focus attention on segment 2 whose median plane X2 appears furthest to the right in figure 3, if one thinks of stretching the segment 2 vertically, it is noted that. due to the approach of the concave parts to the X2 plane (such as for example the concave part indicated with 5 in the segment 2 in question) the respective bridges 3 tend to move to the left with respect to the median plane X2 of the corresponding segment 2.

This ejection effect of the jumpers 3 is beneficial in eliminating the tendency, demonstrated by many stents according to the known art, to contract longitudinally during radial expansion.

By adopting a geometry as illustrated in Figures 3 and 4, for example, the axial contraction movement of the segments 2 deriving from their radial expansion is in fact compensated (and possibly even overcome) by the "expulsion" movement of the jumpers 3 and 4 described above. The experiments carried out by the Applicant show in this regard that, for example, a geometry such as that illustrated in Figures 3 and 4 causes the stent 1 not only not to shorten, but rather tends to lengthen slightly during the radial expansion movement.

The explanation of this mechanism is relatively simple. In this regard, it is sufficient to think, again with reference to Figure 3, what would happen if, ideally, the jumpers 4, instead of being placed where illustrated (thus connecting the respective concave parts of loops of adjacent segments 2), were arranged as follows indicated with broken lines and with the reference 4 'of two aligned loops of two adjacent segments 2, therefore not connecting concave parts (intrados) of loops, but connecting convex parts (i.e. the extrados) of loops.

The jumpers 4 'indicated above are of extremely reduced length (remember, by way of reference, that the axial length of the segments 2 can be of the order of 2 mm). In any case, even during a radial expansion movement the concave parts (and consequently the convex parts) of all the loops of each segment retain, at one end and at the other end of each segment 2, the alignment with a plane parallel to the plane by X2. This alignment is therefore preserved also by the concave or convex parts connected, between two adjacent segments 2, by the same bridge 3, the length of which is not altered during the radial expansion movement.

Consequently, a stent in which the jumpers 3 were arranged as illustrated in Figure 3 and the jumpers 4 as schematically indicated with 4 'in the same Figure 3 (naturally with reference to all the pairs of jumpers 4 present in the stent) would practically retain its axial length unchanged during radial expansion.

As already said, however, by adopting the geometry shown in Figure 3, due to the effect of the overlapping of the various deformation movements, not only is the axial length of the stent 1 kept constant but also tends to increase slightly. the location of the jumpers indicated with 4 ', even if cited ideally for an explanation, could actually be adopted according to specific needs. the movement of radial expansion does not necessarily require that all the bridge elements (not affected by the deformation resulting from the radial expansion) belong to concave or intrados parts of the respective loops of the segments 2. For this purpose it is in fact sufficient that such a connection is provided for each longitudinal section of the Stent 1 target to contract longitudinally due to the radial expansion movement.

For example, in the embodiment illustrated with solid line in Figure 3 (the same also applies to the embodiment of Figure 4 and Figure 9) each of the segments 2 constitutes a section intended to contract longitudinally due to the radial expansion movement Therefore, in correspondence with each of these segments a corresponding connection of jumpers 3, 4 is provided according to the criteria described above.

If, on the other hand, reference is made to the connection diagram of the jumpers 4 'indicated, with a primarily didactic intent, in Figure 3, it can be seen that each complex of two segments 2 connected to each other by respective jumpers 3 constitutes, precisely for the reasons described above , a section of stent which during the radial expansion movement does not contract substantially longitudinally. For this reason, the connection between these sections can take place through jumpers such as the jumpers indicated with 4 'illustrated with dashed lines in the figure, which are not connected to a concave part (or intrados) of a loop,

By examining the diagram of Figure 9, it can be seen that all the segments 2 illustrated therein are capable of contracting axially as a result of the radial expansion movement. For this reason the bridges defined, for connecting adjacent segments 2, by the sinusoidal backbones 30 satisfy precisely the condition described above, namely the connection to the concave part of a respective loop.

As regards the general geometry, the variant embodiment of Figure 4 repeats the connection diagram described previously with reference to Figure 3.

Figure 4 shows how a stent wall structure having the geometry described with reference to Figure 3 can also be made starting from one or more pieces of bent wire so as to form a loop assembly substantially similar to that illustrated in Figure 3, in which the jumpers 3 and 4 are constituted by coupled sections of wire (ie side by side parallel to each other) and connected for example by welding or other consolidation processes (for example brazing, gluing, crimping, etc.).

The fact of working with a wire allows to attribute (for example through a mechanical shaping operation of the wire), sections and / or areas of different sections to the concave parts 5 of the loops and to the straight branches 6 that branch off from them. For example, it is immediate to appreciate that the section of figure 5 is precisely the section of a concave part taken in its vertex part, while the section of figure 6 corresponds to the connection area of two straight branches extending in the general longitudinal direction (axis z ) of the stent.

In particular, in correspondence with the concave parts of the loops, the wire constituting the stent wall can retain a round section and instead assume, in correspondence with the rectilinear sections 6, a generically flattened section in the general extension plane of the wall (therefore along the ideal cylindrical development) of the stent 1.

This different shape allows to achieve different results.

The rectilinear sections 6 are intrinsically more resistant to bending in their general flattening plane, so that the spreading stress of the two branches 6 connecting to a common concave part 5 causes a deformation of the loop in correspondence with the concave part 5. The branches 6 , although spread apart, they instead retain an overall rectilinear course: in this regard it will be noted that the branches combined to form the bridges 3 and 4 still maintain a rectilinear orientation along the longitudinal axis z of the stent 1.

Thanks to their flattened conformation, the branches 6 expose a more extended surface to the lumen wall supported by the stent in its radially expanded position. The lumen wall is therefore subjected to a distributed stress, avoiding the formation of areas of concentrated stress.

The dimensions of the wire can be optimized in correspondence with the concave parts 5, in order to achieve optimal characteristics of plastic deformability at the time of the radial expansion of the stent and, simultaneously, of resistance to subsequent stresses which may tend to close the stent 1.

however, it should be specified, for clarity, that the solution of differentiating the sections and / or the sectional areas of the various parts of the stent wall, in the terms illustrated by way of example with reference to Figure 4, is feasible (albeit with different technological solutions with respect to the mechanical squeezing of the wire referred to with reference to figure 4) also in the solutions illustrated with specific reference to figure 3 and figure 9.

Moving on to examine the latter solution, also with reference to the perspective and elevation views of Figures 7 and 8, it can be noted that, in principle, the structure and conformation of the loops constituting the various segments 2 is on the whole similar to that described above with reference to Figures 3 and 4. In particular, as indicated in correspondence with the segment 2 located furthest to the left, also in the loops represented in Figure 9 a concave part (or intrados) 5 can generally be distinguished from which extend two straight branches 6 intended to be spread apart when the stent 1 is radially expanded.

The wall structure of Figure 9 differs from that illustrated in Figures 3 and 4 essentially in that the bridges which connect the various segments 2 to each other consist of the two backbones 30 extending with a general sinusoidal course along two diametrically opposite generatrices of the structure stent 1.

Of course, the presence of two days of these backbones is not an imperative choice. For example, instead of having two backbones 30 diametrically opposed to each other (therefore angularly spaced 180 ° from each other), it is possible to use a single backbone of this type or three backbones angularly spaced 120 °, etc. .

In any case, even a backbone structure of the type described is capable of implementing, for the purposes of longitudinal flexion of the stent 1, a relationship of the type of relationship (I) mentioned above. The difference with respect to the embodiment illustrated in Figures 1 to 4 lies in the fact that in this first solution the x and y axes actually correspond to the axes around which the bending of the pairs of jumpers 3, 4 can be achieved. On the contrary, in the form implementation of figures 7 to 9 (see in this regard also the elevation view of figure 8) each stretch of backbone 30 extending to connect two adjacent segments 2 is capable of expressing two possibilities of relative orientation between the two connected segments 2 , that means:

- a torsion or, more correctly, a flexion in the general plane of lying of the dorsal section 30, e

- an extension or, in general, a variation of the length in this extension plane.

This concept can be clearer to mechanics experts noting that in practice both the solution illustrated in Figures 1 to 4 and the solution illustrated in Figures 7 to 9 provide a structure that is generally similar to that of a cardan joint.

The overall sinusoidal trend of the two backbones 30 allows to exploit the longitudinal extensibility of the backbones themselves for the purpose of flexion without giving rise to stresses oriented in a tangential direction with respect to the stent wall, therefore with the risk of triggering unwanted torsion phenomena. In any case, it will be appreciated that the stent of Figures 7 to 9 is able to vary its length (ie the extension according to the z direction) in a completely independent way from the radial expansion movement of the segments 2. This fact it can be easily noticed by noting how the backbones 30 have a sinusoidal trend in their overall development and also where they are connected to concave parts of respective loops (note in particular the dorsal section 30 which appears in a lower position in figure 9 to connection between the two elements 2 of which the median planes X2) have been indicated the connection with these concave parts 5 does not alter the general sinusoidal development of the affected ridge. In other words, the ridge 30 joins the outer or extrados edge of the concave part 5 at one side or flank thereof and starts again on the intrados side of the concave part itself at the opposite side or flank.

Naturally, the principle of the invention remaining the same, the details of construction and the embodiments may be widely varied with respect to what has been described and illustrated without thereby departing from the scope of the present invention.

Claims (4)

CLAIMS 1. Stent for angioplasty comprising a body (1) with an overall tubular envelope capable of being dilated in use starting from a radially contracted position towards a radially expanded position, characterized in that: - said body (1) comprises a plurality of successive segments (2) having an overall annular development; the wall of each segment (2) being defined by a plurality of loops (5, 6); And - at least some of said segments (2) are connected to each other by bridge elements (3, 4; 30) extending in the general direction (z) of extension of the stent and having at least one end connected to the concave or intrados part (5) of a respective loop (6), whereby, in the radial expansion movement of the stent, the axial contraction of said segments (2) deriving from the spreading of the respective loops is compensated by an axial projection effect of said bridge elements (3, 4; 30) starting from the respective concave part (5). 2. Stent according to claim 1, characterized by the fact that said bridge elements (3, 4) have opposite ends aligned with the general direction of extension (z) of the stent and by the fact that both said opposite ends are connected to respective concave parts (5) of respective loops of the two segments (2) connected by each bridge element (3, 4). 3. Stent according to claim 1 or claim 2, characterized in that all the bridge elements (3, 4; 30) connecting respective pairs of said segments (2) lead, at least one end, to the concave part ( 5) of a respective loop. 4. Stent according to claim 2 and claim 3, characterized in that all the bridge elements (3, 4) are connected to both their opposite ends, to concave parts (5) of respective loops.