METHOD FOR MAKING A CONNECTION BETWEEN AT LEAST TWO PARTS USING A CLAMPING PART, CLAMPING PART AND CONNECTION SYSTEM INCLUDING THE CLAMPING PART

This invention relates to the domain of mechanics, and more precisely methods of making a connection between two or more parts.

In mechanics, a connection is frequently made between two or more parts using an additional part called a "clamping part" in the remainder of this description. For example, this part may be a screw, a ring, a staple or a wedge, that keeps the parts to be connected in contact with each other. Thus, all or some of the degrees of freedom of the parts are blocked with respect to each other. This blocking is achieved by friction of the clamping part on the part(s) with which it is in contact.

Friction between two parts is characterised by a limiting sliding value that depends on the applied clamping force. Similarly, blocking is characterised by a limiting value of resistance that depends on the applied clamping force.

In some cases, it is desirable to calibrate the limiting value of the blocking resistance. Since friction is an intrinsic value to the assembly, consequently it is only possible to calibrate the applied clamping force.

There are various known methods of calibrating this limiting value of resistance to a degree of freedom, for example:

- a screw-nut system installed with a controlled tightening torque;

- a system using a cone or an inclined wedge, installed with controlled longitudinal positioning;

- a system using a cam or a crankshaft, installed with a controlled rotation angle. These methods all operate with imposed displacement; therefore, the clamping force generated by the stiffness of the assembly depends on the position of the parts with respect to each other.

The value of the clamping force and its precision directly depend on the stiffness of the assembly: if the stiffness decreases, the precision increases but the value of the clamping force also decreases. A relatively high force and a relatively low stiffness can sometimes be reconciled using an intermediate part such as a spring with low stiffness. But it is usually difficult to make an assembly with a simple design and a high limiting resistance without using a precisely controlled clamping system.

In the field of mechanical devices intended to be implanted in the human or animal body during a surgical procedure, these problems are frequently difficult to over-

come. The surgeon does not always have suitable instruments and the time required to control the intensity of the clamping forces to be imposed on the different parts precisely.

The purpose of the invention is to propose a method of connecting two or more parts using a clamping part, to impose a precise value of a clamping force without necessarily needing very precise control over placement of the clamping part. This method could be used particularly, but not exclusively, in an implantable device for medical or veterinary use.

To achieve this, the object of the invention is a method for making a connection between at least two parts, according to which said parts are blocked in contact with each other using at least one clamping part applying pressure on at least one of said parts, characterised in that a clamping part made from a superelastic metallic shape memory alloy is used, for which the generated force curve (σ) = f(imposed displacement (ε)) has a loading and unloading plateau such that the ratio between the stiffness K and the critical loading force σ _M s is less than or equal to 10, the ratio between the stiffness K and the critical unloading force σ _Af is less than or equal to 10, and the length of the plateau ε _AM is greater than or equal to 3%, and a displacement ε is imposed on the clamping part when the parts are connected, so as to induce a generated force σ corresponding to said plateau, creating a calibrated clamping force.

Said clamping part may display a difference between OM _S and σ _Af less than or equal to 300 MPa and be set on the unloading plateau when the connection is made.

Another object of the invention is a clamping part for making a connection between at least two parts, with said clamping part applying a pressure on at least one of the two parts, characterised in that it is made from a superelastic metallic shape memory alloy for which the generated force curve (σ) = f(imposed displacement (ε)) has a loading and unloading plateau such that the ratio between the stiffness K and the critical loading force σ _Ms is less than or equal to 10, the ratio between the stiffness K and the critical unloading force σ _Af is less than or equal to 10, and the length of the plateau ε _AM is greater than or equal to 3%, and the displacement ε imposed on the clamping part induces a generated force σ corresponding to said plateau and a calibrated clamping force.

The difference between σ _Ms and σ _Af is preferentially less than or equal to 300 MPa.

The clamping part may be made from a Ni-Ti alloy containing 55.6% ± 0.4% by weight of nickel having undergone a martensite plates generation heat treatment by maintaining the part between 480 and 520 ⁰ C for 5 to 60 minutes.

Another object of the invention is a connection system between at least two parts, of the type comprising at least one clamping part applying pressure on at least one of said parts, characterised in that said clamping part is of the type described above.

One of the parts to be connected may comprise a split cup with a shape corresponding to the shape of one end of the other of the parts to be connected that fits into said cup, and said clamping part is a ring surrounding said split cup.

Said split cup may be spherical and the end of the other of the parts to be connected that fits into the cup is a ball pivot.

Said split cup may be cylindrical and the end of the other part to be connected that fits into the cup is cylindrical.

One of the parts to be connected may comprise at least one tab and at least one ring surrounding said tab and the other of the parts to be connected.

One of the parts to be connected may comprise a conical perforation with a shape corresponding to the shape of the clamping part, said clamping part being provided with a perforation into which the other part to be connected is inserted.

Said clamping part may be a wedge inserted between said parts to be connected.

Said wedge may be tapered.

Said wedge may be cam-shaped.

One of the parts to be connected may comprise a fork with two arms between which the other of the parts to be connected fits, and the clamping part applies a force on said arms tending to bring them towards each other.

Said clamping part may be in the shape of a yoke.

Said clamping part may also be annular.

Said clamping part may be a wedge pressed against a first of the parts to be connected by a screw inserted in a threaded hole formed in a second of the parts to be connected, itself comprising an orifice into which said first part is inserted.

Said clamping part may be a U-shaped staple between the arms of which two tabs each forming part of one of the parts to be connected are clamped in contact with each other.

Said at least two parts may be part of an implantable device for medical or veterinary use.

As will have been understood, the invention is based on the use of a shape memory alloy (SMA) of the type with superleasticity properties such that the σ = f(ε) (stress = f(deformation)) curve has a plateau, in other words a range of deformation values for which the stress corresponding to these deformations is practically constant, to form the clamping part.

In this way, if two or more parts are connected using a clamping part for which the position imposes a deformation, the force that the clamping part applies on the parts to be connected is practically independent of this deformation, and therefore practically independent of the precise positioning of the clamping part, provided that the material is working within the area of the plateau on the σ = f(ε) curve.

A precise clamping force is thus obtained without the need for very strict control over the positioning of the clamping part.

This is particularly important in many applications, in which precise positioning of the clamping part is difficult to achieve during the connection and/or to correct when the connected parts are used following a slight displacement or a slight deformation of the clamping part. During placement of these implants (for example implants included in osteosynthesis devices for the spinal column), it is difficult to precisely control the magnitude of clamping forces that they have to impose on the different parts, partly due to lack of suitable equipment for this purpose, and partly due to lack of time.

The use of the method according to the invention can result in the required clamping force in a guaranteed manner, simply by correctly dimensioning the clamping part and putting it into position without any special precautions. The speed and reliability of the assembly are thus very much improved.

Since the alloy is reversibly deformed, all that is necessary to loosen the connection when required is to impose an inverse force on the clamping part with an intensity greater than the clamping force. The clamping part will then return to its initial shape.

In general, the capacity of the clamping part to be significantly displaced without modifying the clamping force generated by it, enables assemblies with relatively low, and therefore precise, stiffness, but which can generate high clamping forces. Reconciling these qualities was not possible in the past with conventional systems using clamping parts made of traditional materials, namely SMAs for which the σ = f(ε) curve did not have a plateau like that defined.

The invention will be better understood after reading the following description with reference to the appended drawings, wherein:

- Figure 1 shows an example of a σ = f(ε) curve for the most frequent type of SMA;

- Figure 2 shows an example of a σ = f(ε) curve for an SMA for use in the context of the invention;

- Figure 3 to 14 diagrammatically show examples of connections in which clamping parts according to the invention can be used.

Figure 1 shows the shape of the σ = f(ε) tension curve for an SMA test piece with the more frequent type of superelastic behaviour, initially in the austenitic domain and at a temperature greater than the temperature Ms of the alloy at which the martensitic transformation is possible.

If an increasing deformation ε is imposed in the tension test and the corresponding stress σ is measured:

- for a stress varying from 0 to OM _S (part 1 of the curve): the behaviour of the test piece is elastic since the structure is purely austenitic;

- between OM _S and a stress σm (part 2 of the curve), a change in the behaviour of the test piece is observed, which is no longer the conventional linear elastic behaviour; this is due to the fact that martensitic transformation has started; the slope of the σ = f(ε) curve present in part 2 is much lower than the slope of part 1 , conveying a much lower stiffness of the material;

- above σ _Mf (part 3 of the curve), the test piece is restored to a conventional linear elastic behaviour which is for martensite, the martensitic transformation having been completed in the deformation corresponding to the stress Ow-

If the tension test is continued by unloading, gradually releasing the stress, the general shape of the resulting curve is similar to the load curve, but with a hysteresis since the start of inverse transformation stress σ _As and the end of inverse transformation stress σ _Af are less than Oiw and σu _s (part 4 of the curve), respectively.

The difference Eσ = σiw - OM _S is called the "transformation spread". The transformation hysteresis is defined by H _σ = OM _S - o ^" Af.

Figure 2 shows the shape of the σ = f(ε) tension curve for an SMA test piece that can be used in the context of the invention, with a particular superelastic behaviour at a temperature exceeding Ms.

Compared with the curve in Figure 1 , it will be seen that the slopes of parts 2 and 4 of the curve corresponding to the transformation are significantly lower, and Eσ = σuf -

σM _S and H _σ = σwi _s - O _Af also have lower values. It can be said that the σ = f(ε) curve has plateaus, both during loading and during unloading. The length of the plateau EAM = σiw - OA _S is greater, and the transformation stiffness K = E _σ / EAM is lower than in the case of figure 1.

The benefit of superelastic SMAs that can be used within the scope of the invention is that low stiffness along with a high force are obtained, when they are used in the areas of plateaus 2, 4. The force is quantified by the height of the plateau 2, 4 and therefore by the critical force σ _c , which is:

- between σ^s and om when stressed in loading (plateau 2);

- between σ _Af and σ _As when stressed in unloading (plateau 4).

The small difference between OM _S and σiw on one hand and between a^ and σ _As on the other means that the clamping force may be established with very high precision.

The stiffness is quantified by the ratio K = E _σ / E _A M = (o ^" Mf - CJMS) / (σiw - OA _S ). It the same during loading and unloading.

The superelastic SMAs that can be used within the scope of the invention must have:

- a low ratio between the stiffness and critical force, which in concrete terms is conveyed by K/σ _c ≤ 10, i.e.:

^* loading K/OMS ≤ 10;

* unloading K/σAf ≤ 10;

- and a sufficiently long plateau 2, 4 so that the errors and approximations in the positioning of the clamping part can be compensated easily, which in concrete terms is conveyed by EAM ^≥ 3%.

Preferentially, in the case of operation on the unloading plateau 4, it is preferable for the range of the hysteresis is reduced, so as to limit the clamping part assembly forces. In concrete terms, this is conveyed by H _σ < 300 MPa.

In practice, the difference EMS between displacements (deformations) corresponding to σ _s and OA ₅ is often of the order of 5%. This means that for SMAs that can be used within the context of the invention on which a deformation ε corresponding to the plateaus 2, 4 (loading or unloading) is imposed, a difference of a few % in the deformation imposed on the clamping part will only result in a small variation of the clamping stress imposed by said part. Therefore, the exact position of the clamping part in the assembly that it forms with the parts to be connected is much less critical than in the case in which clamping parts made of a conventional material are used, or even in the case in which

the clamping parts made of an SMA with a superelastic behaviour is used but for which the σ = f(ε) curve does not have a "plateau" like that described above.

It will be noted that the σ = f(ε) curve, and in particular the presence, stiffness and length of the plateaus, depends not only on the composition of the alloy, but also on the mechanical and crystalline characteristics of the material: small grain size or monocrystalline material, presence of precipitates, hardening rate, heat treatments undergone. Paradoxically, a material may be suitable either if it is formed from a large number of small grains, or if it is monocrystalline, but intermediate states are not generally desirable.

A non-limitative example of such a material is described particularly in the document EP A 0 864 664. It consists of a nickel and titanium alloy comprising 55.6% + 0.4% by weight of nickel having undergone heat or thermomechanical treatments giving it the desired metallurgical structure. In particular, it underwent a martensite plates generation heat treatment consisting of subjecting the part to a temperature of 480 to 520 ⁰ C for 5 to 60 minutes. Before this treatment, it is also possible to provide for flash annealing at 600-800 ⁰ C for 10 to 30 seconds, and/or recrystallisation annealing at 700-800 ⁰ C for more than 2 minutes, and/or work hardening between 15 and 18%. This document may be referred to in order to obtain more details. Iron, copper or vanadium may partially substitute nickel.

In general, the material that can be used to make the clamping part according to the invention must have a superelastic behaviour as described, at the assembly temperature of the parts to be connected and at their working temperature. For medical applications, the material must be biocompatible.

We will now describe non-limitative examples of connections in which a clamping part according to the invention can advantageously be used.

Figure 3 shows a "blocked ball pivot" system, in which a first part 5 terminates at a ball pivot 6 that fits into a spherical split cut 7 formed at the end of a second part 8 (shown in longitudinal section). The split in the cup 7 is not visible in figure 3. A ring 9 according to the invention clamps the cup 7 and blocks the ball pivot 6 in it. therefore, the ring 9 is stressed in tension.

Figure 4 shows a "blocked sliding pivot" system. A first part 10 with a circular section is inserted in a cylindrical split cup 11 formed at the end of a second part 12 (shown in longitudinal section). A ring 13 according to the invention clamps the cup 11 and blocks the rod 10 in it. Therefore, the ring 13 is stressed in tension.

In general, based on the above two examples, a connection can be made between two parts according to the invention by clamping a split cup with an arbitrary shape formed at the end of a part as described, into a ring, the shape of the cup corresponding to the shape of the end of the other part.

Figure 5 shows a system called a "clamping attachment". A first part 14 shown in longitudinal section is provided with two tabs 15, 16 at its end, against which a spindle 17 is to be kept in contact. To achieve this, each of two rings 18, 19 according to the invention shown in longitudinal section clamps one of the two tabs 15, 16 and the spindle 17. The rings 18, 19 are stressed in tension. Obviously, this type of connection method may be adapted to parts other than a circular spindle 17. It would also be possible to use only one ring 18, 19 and one tab 15, 16.

Figure 6 shows another attachment system onto a spindle. A first part 20 shown in longitudinal section comprises a conical perforation 21 in which a conical clamping part 22 according to the invention is force fitted. This clamping part 22 is itself provided with a cylindrical perforation 23 in which a spindle 24 to be blocked is inserted in the first part 20. The forces applied by the first part 20 on the clamping part 22 are transmitted by the clamping part to the spindle 24 that is thus blocked. The clamping part 22 is stressed in compression. Once again, this connection mode may be adapted to parts other than a circular spindle 24.

Figure 7 shows a connection system using a wedge stressed in bending. The two parts to be connected are a first part 25 in the shape of a C and a rod 26 placed in a groove 27 of the first part 25, opposite the arms of the C. A clamping part in the shape of a grooved wedge 28 according to the invention fills in the available space inside the C between the rod 26 and the arms of the C, such that it is stressed in bending by the arms of the C, the inner faces of the arms being shaped according to the upper face of the clamping part 28.

Figure 8 shows a connection system using a wedge stressed in compression. A first U-shaped part 29 is connected to a second part 30 with a rectangular section. The available space inside the U between the two parts 29, 30 is filled by a clamping part 31 according to the invention in the shape of a wedge that pushes the second part 30 into contact with an arm of the first part 29. In this case, the clamping part 31 is stressed in compression.

Figure 9 shows a system comparable to that in Figure 8 in which that tapered wedge is replaced by a cam 32 with a partially circular and partially elliptical section. It is fitted into a groove with a section following an arc of a circle 33 formed on a first arm of

the U formed by the first part to be connected 34. After inserting the section part to be connected to 35 inside the U, the cam 32 is turned such that its elliptical section part 36 comes into contact with the second part 35 and pushes it into contact with the second arm of the U of the first part 34. The cam 33 is thus stressed in compression.

Figure 10 shows a "fork clamping" system in which the first part 37 is a fork with two arms between which a rod 38 is placed. Clamping is obtained by a clamping part 39 according to the invention in the form of a yoke, in which the inner space is shaped so as to tend to bring the two arms of the first part 37 together so that they clamp the rod 38. The clamping part 39 is stressed in bending.

Figure 11 shows a variant of the system in Figure 10, in which the clamping part 40 according to the invention is an annular part in which the orifice 41 with a rectangular section in the example shown, is shaped so as to clamp the arms of the first part 42 and to make them clamp the rod 43. In this case, the annual part 40 is placed above the rod 43 and is stressed in tension.

Figure 12 shows a variant of the system in figure 11 , in which the clamping part 44 according to the invention, once again in an annular shape with a rectangular orifice, is placed below the rod 45 to be connected to the first part 46 in the shape of a fork, the arms of which are shaped to trap the rod 45 under the effect of the clamping part 44 that is also stressed in tension.

Figure 13 shows a screw clamping system shows a screw clamping system. The first part 47, shown in cross section, includes firstly an orifice 48 in which a rod 49 is blocked using a screw 50 inserted into a threaded hole 51 in the first part 47 opening up into the orifice 48. A housing 51 is provided in the first part 47 so that a clamping part 52 according to the invention can be inserted, composed of a wedge that the screw 50 compresses against the rod 49. Compared with a conventional configuration in which the screw 50 would bear on the rod 49 directly, the configuration according to the invention provides a means of more precisely adjusting the force applied to the rod 49, without it being necessary to monitor and very carefully limit the advance of the screw 50 in the threaded hole 51.

Figure 14 shows a staple clamping system in which two parts 53, 54 each provided with a part 55, 56 are connected by a U-shaped staple stressed in bending, the two tabs 55, 56 being clamped in contact with each other between the arms of the U.

These clamping systems may find applications in various fields, particularly in the field of implantable devices in the human or animal body with the benefits mentioned above. For example, those involving a rod or a spindle can be used to hold a rod of an

osteosynthesis device for the spinal column inside a bone securing part (screw, hook) implanted on a vertebra.