FR2771921A1 - STAINLESS STEEL STENT TO BE IMPLANTED IN A VASCULAR CONDUIT USING AN INFLATABLE BALLOON - Google Patents

Abstract

The invention concerns a prosthesis (1c) designed to be implanted in a vascular duct being unfolded by the effect of an expanding balloon, and obtained by cutting a stainless steel tube. It has sufficient malleability to be opened by the expansion of a balloon under pressure not more than 3 bars and particularly pressure between 2.5 bars and 3 bars. Preferably, the prosthesis is obtained by carrying out the following operations on a stainless steel tube (1a), having initial hardness and thickness (e1) greater than the hardness and thickness (e1) required for the final prosthesis: cutting the tube (1a) wall and cleaning it by electropolishing; rapid quenching heat treatment (4), so as to reduce the stainless steel hardness, followed by a second electropolishing treatment (5) for reducing the original tube (1a) initial thickness (e1) while improving its surface state.

Description

STAINLESS STEEL ENDOPROSTHESIS FOR USE
IMPLANTED IN A VASCULAR CONDUIT USING A
INFLATABLE BALLOON
The present invention relates to the field of stents, more commonly called "stents", which are used in cardiovascular surgery, being implanted in a vascular duct, such as the coronary artery or peripheral arteries, and act as stakes for the vascular duct. The invention more particularly relates to a stent made from a stainless steel tube whose wall has been cut in particular by laser, and which is intended to be implanted in the vascular duct by means of an inflatable balloon.

 For ten years, it has been known to treat atheromatous vascular diseases, which correspond to narrowing of the arteries, by balloon dilation techniques, called angioplasty. These narrowing of the arteries are caused by an infiltration of the wall and by the formation of an atheroma deposit inside the lumen of the vessel. In angioplasty, the balloon is introduced into the artery, and is inflated as a rule with pressures ranging from 4 to 20 Bars, making it possible to crush the atheromatous material and to swell the arterial wall.

 The commonly used angioplasty techniques unfortunately suffer from a significant recurrence rate of around 30%. These recurrences are due to a double phenomenon: on the one hand an elastic retraction, commonly called "secondary recoil", of the dilated zone and on the other hand an inflammatory reaction, called "intimal proliferation", of the internal part of the wall of the vessel. In addition, a number of angioplasties can be complicated by an intimal tear also called a "flap" of the inner wall of the artery, which can lead to acute obstruction of the artery. This acute thrombosis can be responsible for extremely serious accidents (infarction, death, ...).

 In order to treat and re-stick these intimate tears and reduce restenosis by avoiding secondary recoil, it is currently widely used to implant in the vascular duct, at the area treated by angioplasty, a stent, more commonly called "stent". These stents or stents can be defined as metal supports acting as stakes once they are introduced into the artery at the level of the treated area. In the following description, for the sake of clarity, the stents will be designated by the term "stent".

 To date, there are three types of stents: those made from coiled wire, those made from a mesh and those made from tubes cut in particular by laser. The invention is in the field of stents made from cut tubes.

 The stents produced from cut tubes are designed to be deployed by means of an inflatable net balloon, during their implantation in the vascular duct. For this purpose, the stents must be crimped onto the balloon, in a so-called rest position 4, prior to the introduction of the balloon into the vascular duct.

Once the stent has been delivered to its implantation zone inside the vascular duct, it is deployed by expansion of the balloon, so as to be brought into contact with the wall of the vascular duct.

 The prior operation of crimping the stent onto the balloon must be carried out while ensuring that the stent or the balloon is not damaged, while ensuring that the stent is held perfectly in relation to the balloon. It is indeed essential that keeping the stent in position relative to the balloon is sufficient to avoid the risks of displacement of the stent relative to the balloon, during their passage inside the vascular duct to be treated, and in particular at inside the guiding catheter pre-implanted in the vascular duct. Any displacement of the stent may on the one hand obstruct the expansion of the balloon and on the other hand cause a defective opening and positioning of the stent.

 With regard to the crimping operation, two cases of figures can arise in practice: the stent is sold while already being crimped on a balloon, or the stent is sold separately without being crimped on a balloon. In the first case, the crimping of the stent is carried out on a new balloon, having a perfectly cylindrical surface of revolution, and is carried out by crushing the stent on the balloon, symmetrically and evenly over the entire length of the stent and over its entire length. circumference.

In the second case, the crimping of the stent is carried out by the surgeon, as a rule on a balloon which has already been previously used to dilate the area of the vascular duct to be treated. the crimping is carried out in this case on a balloon which has already been deployed, and which therefore comprises a membrane having asymmetrical folds and which no longer forms a perfect cylinder.

 A stent must have certain characteristics, the most important of which are radial strength, biocompatibility, flexibility and malleability.

 The radial force of the stent must be sufficient to oppose the elastic retraction of the wall of the vascular duct. A stent made from a cut tube has the advantage of having a greater radial force than the other two types of stents (coiled wire and wire mesh).

 With regard to biocompatibility, the material used to make the stent must obviously be biocompatible and the least thrombogenic possible. Regarding the material used, with the exception of a few rare stents made of Platinum, Tantalum or Nitinol, the vast majority of stents are now made of stainless steel. Biocompatibility, in particular with regard to the thrombogenic aspect, also depends on the surface condition of the stent, that is to say mainly on the roughness of the surface of the stent. The surface condition must be as smooth as possible.

 The flexibility of the stent must be sufficient to allow its routing in the vascular conduit, which can be strongly sinuous, and in addition sometimes calcified. To improve the flexibility of stents, it is widely used today to produce articulated tubular stents, such as those described for example in international patent application WO 96/33671. An articulated stent is in the form of a succession of rings which on the one hand consist of cells having a determined geometric shape and capable of deploying, and which on the other hand are connected in the longitudinal direction of the stent by connections, so as to be articulated with each other.

 The malleability of the stent is linked to various factors, including mainly the hardness of the metal used, the thickness of the stent, and the cutting geometry. The malleability of the stent is today considered to be preponderant for the operation of crimping the stent. The more malleable the stent, the easier it is to deform it by locally crushing it to fix it on the balloon. The malleable nature of the stent is even more preponderant in the case of a stent intended to be fixed on a used balloon, taking into account the irregularity of the surface of the balloon. The malleability of the stent allows it to match the relief of the irregular surface of the balloon, and thereby obtain a more reliable fixation of the stent on the balloon. Finally, it is known that the malleability of the stent also influences the flexible nature of the stent. The more malleable the stent, the greater its flexibility.

 To date, to manufacture a metallic tubular stent, an initial tube, for example made of stainless steel, is selected at the start, the thickness of which is equal to the thickness of the final stent, and which also has the hardness required for the stent. final. On this starting tube, a laser tube wall wall operation is carried out, so as to give it a determined geometric configuration. The manufacturing operation ends with a cleaning operation by electrolytic polishing, which is carried out so as to round the edges of the stent, to remove cutting burrs and carbonization residues due to laser cutting.

 The main object of the invention is a stent intended to be implanted in a vascular duct while being deployed under the effect of the expansion of a balloon, and obtained by cutting a stainless steel tube. The stent of the invention is characterized by a sufficiently high malleability to obtain an opening of the stent by expansion of the balloon under a pressure less than or equal to 3 bars and in particular under a pressure of between 2.5 bars and 3 bars.

 The tubular stents marketed to date are designed in such a way that they exhibit malleability resulting in an opening under an inflation pressure of the balloon which is at least 5 bars. To date, this malleability has been considered sufficient to meet the crimping and flexibility constraints. In addition, it was considered until now, by the manufacturers of tubular stents, that it was not conceivable to increase the malleability of the existing tubular stents, without the stents collapsing under the effect of elastic retraction of the wall of the vascular duct. It is the merit of the invention to have overcome this technical prejudice, by highlighting that by producing a tubular stent cut out of stainless steel which has a greater malleability, resulting in an aptitude of the stent for opening. at a pressure less than or equal to 3 bars and in particular between 2.5 bars and 3 bars, a stent was obtained which could have sufficient radial force to reliably oppose the phenomenon of elastic retraction of a vascular conduit.

 The lower opening pressure of the stent of the invention also provides two important advantages: It makes it possible to obtain better symmetry in the opening of the stent. It contributes to reducing the aggression of the internal wall of the vascular duct by the stent when it is opened, and thereby decreases the risks of restenosis by intimal proliferation.

 More particularly, the endoprosthesis of the invention is obtained by carrying out the operations below on a stainless steel tube, having an initial hardness and thickness (e1) greater than the hardness and thickness (e2) desired for the final stent cutting the wall of the tube and cleaning by electrolytic polishing, - heat treatment of hyperhardening, so as to reduce the hardness of the stainless steel, followed by a second electropolishing treatment allowing the thickness to be reduced initial (e,) of the starting tube while improving its surface condition.

 It is advantageous to start from an initial tube having a high hardness greater than the desired final hardness, because the harder the stainless steel, the lower its roughness. It is therefore easier to obtain a stent having a better surface condition. The reduction in thickness linked to a double electropolishing makes it possible to increase the malleability and the flexibility of the stent, compared to a stent produced according to the aforementioned known method, from a tube of the same initial thickness. The double electrolytic polishing also makes it possible to appreciably improve the final surface condition of the stent.

 Other characteristics and advantages of the invention will appear more clearly on reading the following description of an example of manufacturing a stent of the invention and of several possible geometric configurations of a tubular stent according to the invention, which description is given by way of nonlimiting example and with reference to the appended drawing in which - FIG. 1 illustrates the main steps of a method for manufacturing a stent according to the invention, - and FIGS. 2 to 4 respectively illustrate configurations of a tubular stent according to the invention in the folded state.

 FIG. 1 illustrates the various stages in the manufacture of a tubular stent 1 c according to the invention, that is to say of a stent which is intended to be implanted in a vascular conduit such as a coronary artery or a peripheral artery, after having been crimped onto a balloon, and which is characterized by a malleability sufficiently high to open under an inflation pressure of the balloon of between 2.5 and 3 bars.

 In the particular example which will now be described, this stent 1c is produced from a starting tube la, made of stainless steel having a predetermined initial hardness and a given roughness index Ra. Preferably, it is a seamless stainless steel tube for surgical implants meeting the specifications of ISO 5832-1. The initial hardness of the tube measured in Vickers is between 360HV and 300HV; Vickers hardness measurements were carried out under a load of 300g and obtained on an average of five impressions. The roughness index Ra is approximately 0.8pm. The length of the tube is approximately 17mm. In the case of a stent lc intended to be implanted in a coronary artery, the thickness e and the external diameter d1 of the tube la are preferably substantially equal to 0, 1 mm and 1.34 mm respectively. In the case of a stent lc intended to be implanted in a peripheral artery of larger diameter, the thickness e, and the external diameter d, of the tube la are preferably substantially equal to approximately 0.2mm and 30mm respectively.

In a first step referenced 2 in FIG. 1, a laser cutting of the tube 1a is carried out in the usual manner so as to give it a predetermined geometric configuration such as for example one of the configurations illustrated in FIGS. 2 to 4. The cut tube 1b from step 2 undergoes in a second step 3 cleaning by electrolytic polishing. This step 3 is known per se and consists in removing from the surface of the tube 1 b the cutting burrs and the carbonization residues due to the laser cutting, by electrochemical means, by applying a given electric current density between the tube the and a liquid medium called an electrolyte. It is up to the person skilled in the art to adapt the operating conditions (choice of electrolyte, current density applied, duration of treatment, etc.) to the particular type of tube 1b obtained from step 2 in order to obtain the from
Step 3 a clean tube whose surface no longer has a burr and has been freed of its impurities.

 At the end of the first cleaning step by electrolytic polishing, the tube undergoes a heat treatment of hyper quenching 4 having the function of reducing the hardness of the stainless steel. In a specific exemplary embodiment, this hyper-quenching heat treatment consists of a first step of cooking the tube 1b in a vacuum oven, followed by a second step of rapid cooling of the tube 1b using liquid nitrogen. More particularly, during the cooking step, the tube is brought for a short time (about 1 minute) to a temperature of 1040 C; in the aforementioned preferred embodiment, the Vickers hardness of the stainless steel dropped to a value between 200HV and 240HV.

At the end of the hyper quenching step, the tube undergoes in a step 5, a second electrolytic polishing treatment, similar in its implementation to treatment 3, but with operating conditions, and in particular a treatment duration, calibrated to obtain a reduction in tube thickness within a predetermined range, and preferably between 0.04mm and 0.02mm. In a specific embodiment, for a stent intended to be implanted in a coronary artery, the thickness e2 of the tube lc obtained at the outlet of step 4 was between 0.06mm and 0.08mm, and was worth on average 0.067 mm for a stent intended to be implanted in a peripheral artery,
The thickness e2 of the tube lc obtained at the output of step 4 was between 0.16mm and 0.18mm, and was worth on average 0.17mm. For the two aforementioned types of stents, the roughness index Ra of the stent 1 c obtained at the end of the process, under the effect of the double electrolytic polishing of steps 3 and 4 dropped to a value between 0, lm and 0.4 μm , and was worth on average 0.2 m. The roughness Ra of the stent lc and the initial starting tube la was measured using a TAYLOR-HOBSON surface condition measurement device (TALYSUL F10 model) and a reference standard plate roughness BA5 ETRUG 301, the probe of the measuring device had a radius of 0.0025mm; the measuring force was 1 00mN, the probing length was between 1 mm and 6mm. The measurements were taken on the generatrices of the tube in the longitudinal direction at the level of the median zone of the tubular stent.

 More particularly, the laser cutting (step 2) of the starting tube la is carried out so as to give the wall of the tube a geometrical configuration in accordance with one of the three variants of FIGS. 2 to 4 which will now be successively described and which represent the geometric structure of the stent in the resting state, that is to say of the unexpanded stent.

 If we refer to the embodiment of Figure 2, the tubular stent lc is an articulated stent, consisting of a succession of annular segments 6, of width 11, connected in pairs in the direction of the longitudinal axis 8 of the stent, by short connections 7 of length 12. In FIG. 2, these connections 7 have the shape substantially of an S with a central inflection point 7a. Each annular segment 6 fdrme a coil capable of deploying, and consists of successions of arms 6a connected two by two by an articulation zone 6b. In the particular case of FIG. 2, when the ballast lc is at rest, the arms 6a of the segment 6 are substantially parallel and oriented transversely to the longitudinal axis 8 of the stent, at an angle α equal to approximately 30. Each articulation zone 6b is constituted by a segment oriented substantially perpendicular to the longitudinal axis 8 of the stent.

The connections 7 connect two adjacent annular segments 6 by means of their respective articulation zones 6b. More particularly, in FIG. 2, the annular segments 6 are oriented successively at opposite angles, and the articulation zones 6b of two adjacent segments 6 are aligned in the direction of the longitudinal axis 8 of the stent 1c. Two adjacent annular segments 6 and their connections 7 form a plurality of deployable C cells, one of which is shown in hatched lines in FIG. 2.

 Each articulation zone 6b of an annular segment 6 is connected to two arms 6a and to a connection 7 via respectively three pivots P1, P2, and P3 allowing the deformation of the stent 1c during the expansion of the balloon on which the stent is mounted. In FIG. 2, the pivot P2 corresponds to the articulation of an arm 6a making an angle A, with respect to the articulation zone 6b; in the example illustrated, the angle A is greater than 90 and is more particularly about 120; the pivot P1 corresponds to the articulation of an arm 6a making an angle B with the articulation zone 6b; this angle B is less than 90 and is more particularly around 30; the pivot P3 corresponds to the junction of an articulation zone 6b with a connection 7.

 When the stent lc passes from its rest state of FIG. 2, to its deployed state under the effect of expansion of a balloon, the cells C open, so that each arm 6a makes a substantially angle of 45 "relative to the longitudinal axis 8 of the stent. Each arm 6a of angle A opens at an angle a1 of approximately 15, while each arm 6a of angle B opens at an angle a2 about 75.

 In a preferred embodiment, the width l1 of each segment 6 is preferably between 1mm and 1.5mm. More particularly, for a stent intended to be implanted in a coronary artery, this width!, Is preferably between 1 mm and 1.2 mm; for a stent intended to be implanted in a peripheral artery, this width l1 is preferably of the order of 1.4 mm. The length 12 of the connections 7 is substantially 0.5 mm. The thickness Ep of the annular segments 6 is equal to the thickness of the connections 7 and is substantially equal to 0.1 mm.

 The embodiment of FIG. 3 differs from that of FIG. 2 by the geometry of the articulation zones 6b of the annular segments 6. Each articulation zone 6b is constituted by two curved segments S1 and S2 forming between them a angle B being substantially 9O. The two angles A and B of the pivots P1 and P2 are advantageously greater than or equal to 90c, which facilitates the opening of the stent, compared to the stent of FIG. 2 for which only one of the two angles, in this case l angle A, constitutes an obtuse angle.

In the particular example in Figure 3, the angles A and B are worth about 135.

 The example of FIG. 4 differs from that of FIG. 3 on the one hand in that the arms 6a of the annular segments 6 are oriented substantially parallel to the longitudinal axis 8 of the stent, and on the other hand that the connections 7 have a different shape and are oriented in a transverse direction with an angle y relative to the longitudinal axis 8 of the stent.

 The orientation of the arms 6a parallel to the longitudinal axis 8, combined with the structure with two segments S1 and S2 of the articulation zones 6b, advantageously allows to have a better symmetry of opening of the stent, each arm 6a s' opening at substantially the same angle, i.e. approximately 45.

 The orientation of the connections 7 transversely to the longitudinal axis 8 of the stent makes it possible to produce connections 7 whose length is greater, which allows better compensation for the reduction in the length of the stent when it is opened. More particularly, in the example of FIG. 4, the connections 7 consist of a wavy segment with at least four inflection points 7a for their elongation. This particular structure makes it possible to have a greater length reserve compared to the connections 7 of FIGS. 2 and 3.

Claims (15)

CLAIMS 1. Endoprosthesis (1c), more commonly known as a "stent", intended to be implanted in a vascular conduit while being deployed under the effect of expansion of a balloon, and obtained by cutting a stainless steel tube, characterized in that it has a sufficiently large malleability to allow its opening by expansion of a balloon under a pressure less than or equal to 3 bars and in particular under a pressure between 2.5 bars and 3 bars. 2. Endoprosthesis according to claim 1 characterized in that it is obtained by carrying out the operations below on a stainless steel tube (1a), having an initial hardness and thickness (e1) greater than the hardness and thickness (e2) sought for the final stent cutting the wall of the tube (la) and cleaning by electrolytic polishing, - heat treatment of hyper quenching (4), so as to reduce the hardness of the stainless steel, followed by a second electropolishing treatment (5) making it possible to reduce the initial thickness (e1) of the starting tube (1a) while improving its surface condition. 3. Endoprosthesis according to claim 1 characterized in that it has a Vickers hardness of between 200HV and 240HV. 4. Endoprosthesis according to claims 2 and 3 characterized in that it is obtained from a starting stainless steel tube (la) having an initial Vickers hardness between 360HV and 300HV. 5. Endoprosthesis according to one of claims 1 to 3 intended to be implanted in a coronary artery characterized in that it has a thickness <e2i between 0.06 and 0.08 mm. 6. Endoprosthesis according to claims 2 and 5 characterized in that it is obtained from a starting stainless steel tube (la) having an initial thickness of the order of 0.1 mm. 7. Endoprosthesis according to one of claims 1 to 3 intended to be implanted in a peripheral artery, characterized in that it has a thickness (e21 of between 0.16 mm and 0.18 mm. 8. Endoprosthesis according to claims 2 and 7 characterized in that it is obtained from a starting stainless steel tube (la) having an initial thickness of the order of 0.2mm. 9. Endoprosthesis according to one of claims 1 to 8 characterized in that it has a roughness index R, between 0, ltjm and 0.4 m, and more particularly of the order of 0.2 jim. 10. Endoprosthesis according to claims 2 and 9 characterized in that it is obtained from a starting stainless steel tube (1a) having a roughness index Ra of about 0.8pm.  1 1. Endoprosthesis according to one of claims 1 to 10 characterized in that it is constituted by a succession of annular segments (6) articulated two by two by connections (7). 12. Endoprosthesis according to claim 11 characterized in that each annular segment (6) forms a serpentine formed by a succession of arms (6a) connected two by two by a zone of articulation (6b), in that each zone of articulation (6b) is constituted by two short segments (S,) and (S2) forming between them an angle (fi), and in that each segment (S,) and (S2) is extended by an arm (6a) according to an angle (A) or (B) greater than or equal to 90. 13. Endoprosthesis according to claim 1 1 characterized in that the angle (ss) is worth approximately 90 and the angles A and B are worth substantially 135. 14. Endoprosthesis according to one of claims 12 or 13 characterized in that the arms (6a) of each annular segment (6) are oriented substantially parallel to the longitudinal axis (8) of the endoprosthesis. 15. Endoprosthesis according to one of claims 1 1 to 14 characterized in that each connection (7) has substantially the shape of an S with a central inflection point (7a). 16. Endoprosthesis according to one of claims 1 1 to 14 characterized in that each connection (7) is oriented transversely at an angle (y) relative to the longitudinal axis (8) of the endoprosthesis, and is constituted by a wavy segment comprising at least four points of inflection (7a). 17. Endoprosthesis according to one of claims 1 1 to 16 characterized in that the width (l1) of the annular segments (6) is between 1 mm and 1.5 mm.