CA2381083A1

CA2381083A1 - Method for storing a shape memory alloy

Info

Publication number: CA2381083A1
Application number: CA002381083A
Authority: CA
Inventors: Rene Suire
Original assignee: Individual
Current assignee: MAT INOV SARL
Priority date: 1999-08-04
Filing date: 2000-08-03
Publication date: 2001-02-15
Also published as: EP1204779A1; RU2262550C2; WO2001011097A1; FR2797275B1; FR2797275A1; CN1183271C; CN1373816A

Abstract

Process for memorizing two geometric states of a product made from a shape memory alloy, and applications of this process to products in medical, dental, veterinary fields or the like.
The present invention relates to a process for memorizing two geometric states of a product made in a shape memory alloy, said process being characterized in that it consists of:
a) educating said alloy for the first geometric state by acting on said alloy to bring it to and leave it in said first state at a first temperature, then b) educating said alloy for the second geometric state by acting on said product to bring it to and leave it in said second state at a second temperature.
The present invention also relates to applications of this process to products in medical, dental, veterinary or other fields.

Figure 1

Description

PROCESS FOR MEMORIZING 'l~f,~ GE_,__OMETRICAL STATES IN A_ PRODUCT MADE FR M A SHAPE MEMORY ALLOY AND APPLICATIONS
The present invention relates to a process for memorizing two geometric states of a product made from a shape memory alloy. It relates to applications of this process to products intended to be fixed to another element and later to be removed therefrom.
It also relates to applications of this process to products in the medical, dental, veterinary fields or the like.
Shape memory alloys, also called SMAs, are already known. With this memory effect, a material which has been deformed at a first temperature will revert to its initial shape when it returns to a second temperature. This effect is due to thermoelastic transformation between an austenite solid phase and a martensite phase which is also solid.
Different known alloys may show this effect.
The use of these alloys for this effect in the field of medicine is also known, in particular to produce connecting parts for bone elements or clips- Reference may be made in particular to FR-A-2 700 464 which describes the uee of such memory material.
It will be noted that the appliearion of the present invention to the medical sector is given here by way of illustration.
The use of shape memory alloys in the medical sector is currently limited to the foxing of a product to an element, and does not allow a subsequent removal phase of the said product from the element. For txample, in respect of human implants, the use of memory alloys today facilitates their insertion but in no way contributes to their removal which is moreover not considered.
The object of the present invention is to put forward a process of treating a memory effect product so that with this effect it is possible, firstly, to apply the product concerned to an element, and secondly to remove said product from said element. The purpose of the present imrention is therefore to propose a process for memorising not one geometric state as described in the prior art, but two geornetric states of a product made in a shape memory alloy.
For this purpose, the process of the present invention therefore consists of a) educating said alloy for the first geometric state by acting on said product to bring it to and leave it in said first state at a first temperature, then b) edueati»g said alloy for the second geometric state by acting on said product to bring it to and leave it in said second state at a second temperature.
If the product under consideratiow is an implant intended to be placed in an organism, the insertion of this implant is made by bringing it to one of the two geometric states by providing the necessary heat so that it reaches the temperature corresponding to this state. Its removal is made by bringing it to the other geometric state by providing the necessary heat so that it acquires the temperature corresponding to this other state.
It will be noted that the geometric state for inxrtion corresponds to the state in which said implant acts on an element, such as another implant, another component or pan of the body so as to fix itself thereto. As for the geometric state for removal, this corresponds to the state in which said implant reverts to its initial state before insertion or in a similar state.
Therefore, the fact that the implant is made in an educated alloy to have two distinct geometric states enables its insertion in one of the states and its removal in the ather state.
The alloy used is for example a nickel or titanium alloy, or a composite material reinforced by said alloy.
For exstmple the highest of the two temperatures under consideration is in the range of +37°C and +55°C while the lowest temperature lies between +30°C and -30°C.
If the present invention is given application in the medical field, the highest temperature may be provided by diathermy knife and the lowest temperature by a physiological serum, a cryode, etc.
According to another characteristic of the invention, said process consists of finishing said product after education of said alloy for the first geometric state but before education of the said alloy for the second geometric state. This finishing stage may consist of polishing said product, for example by sanding.
In order to prevent the nickel, which may be contained in said memory alloy, from entering into contact with the human body, this finishing stage may consist of applying a thin protective layer, in the region of 1 to 5 microns for example.
This layer may be made in one of the following materials:
nickel carbide, aluminium oxide, molybdenum bisulphide, titanium carbonitride, nickel, titanium and aluminium ternary materiel, chromium nitride, carbon diamond in the amorphous state, titanium carbide, titanium nitride, etc.
This protective layer is for example a layer obtained by nitriding the surface of said product at low temperature. It may be conducted by ion bombardment or by gaseous phase deposit.
it has been noted that the effect of this nitriding stage at tow temperature, besides protection of the human body, is to amplify the alloy memories in the hot and cold state.
Moreover, it shortens and therefore simplifies the polishing stage of the unfinished product since a very good surface condition is obtained more quickly- It also prepares parts exposed to friction or load.
The present invention may be applied to any sector insofar as two geometric states of a product are required. In particular, it relates to applications to products intended to be fixed by tightening onto an element and later to be removed therefrom.
In the remainder of this disclosure, applications in the medical field will be considered for illustrative purposes, for example human implants.
Examples of application of the process of the invention are described below with reference to the appended drawings, in which:
Figure l is a view illustrating the present invention taking the exaarple of a clip, Figures 2a to 2d are views illustrating the process of the present invention taking the example of a pin, Figure 3 is a perspective view showing a socket to which the presem invention may be applied, Figures 4a to 4c are views illustrating the process of the present invention taking the example of an anchor peg, and Figure 5 is a view of a femoral stem system using the memorizing process of the present invention.

Firstly, an application to human implants will be considered in relation to Figure 1, these implants being clips for orthopaedic use or injury repair and which are used either to maintain ligaments together or to join bone (epiphyseal clips for example). They may also be clips used for spinal column treatment. They may also be suture staples for medical or surgical use.
The process of obtaining a clip according to the present invention is as follows.
This clip is obtained from a wire 10, for example made of an alloy of nickel and titanium, advantageously in a stoicheiometric ratio close to one, which is then mechanically conformed so as to give it an "n" shape 11 with two limbs 11 a and 11 b connected together by a core l lc. This is the so-called initial shape of the clip.
Limbs 11a and l 1b of the conformed clip are then respectively subjected to forces so that they draw close to one another. The first geometric state 12 is then obtained. This operation is conducted at a temperature T1, for example in the range of 37°C to 55°C, for example 40°C. This operation is conducted for example in a vice or wing a die 20 of complementary shape to the required shape. It can then be said that the clip has been educated for its first memory state.
A finishing stage may then be applied to the clip in its state 12. This may be polishing, for example by sanding, or the low temperature deposit of a coating, for example by gas phase deposit or nitriding. This stage is referenced N in Figure 1.
~ After this finishing stage, the limbs of the clip are subjected to a force so to give there s second geometric state different to the first geometric state 12.
Depending upon the intended application, this may be a state 13 in which the clip returns to its initial state 11 with limbs ila and llb parallel or, further, state 13' in which the clip limbs are draw» away from each other. This operation is conducted at a temperature T2, for example between -30°C and +30°C, for example 5°C. It is for example conducted using a pulley system or using a die 21 or 21' of appropriate complementary shape to the desired shape for the second geometric state.
In practice, in respect of clips, state 13 is preferred to state 13' to allow its easy removal.
The process of use of this type of clip is as follows. At the time of insertion, the clip is positioned in place and heat is imparted to it so that it reaches a temperature T1. Since this is a high temperature a diathermy knife for example may be used. It then acquires its geometric state 12 in which the two limbs are drawn close to one another. The clip then maintains this state. In this state, the clip is secured to its support, a bone for example, and can no longer be dismounted without further action being taken.
For its removal, heat is imparted to the clip under consideration such that it reaches a temperature in the region of temperature T2 so that it then acduires its geometric state 13 in which its limbs are again parallel. The clip can then be removed by withdrawing it parallel to its limbs. This temperature being a low temperature, physiological serum may be used.
The shapes corresponding to geometric states 12 and 13 or 13' are determined at the time they are placed in memory and can therefore be chosen so that they are suitable for the clip under consideration.
It will be noted that, in the example of the clip given above, and according to the variants of the present invention, temperature Tl could be the lowest, for example between 30°C and +30°C, and temperature T2 could be the highest, for example between +37°C and 55°C.
A fbrther application of tha process of the present invention is described in relation to Figures 2a and Zd and relates to a pin 30 which is mainly used to join bone.
Pin 30 is essentially made up of a tube opened on the side by a longitudinal slit 31. Its lower end 32 is slightly troear-tipped to facilitate insertion in a cavity drilled for this purpose.
Figure 2b shows an arc of a circle section of pin 30 in its initial state 30A
before education. In Figure 2c, pin 30 is in its first geometric state 30B
educated for temperature Tl. The arc of a circle which represents the section of pin 30 is open in relation to initial state 30A. In Figure 2d, pin 30 is in its second geometric state 30C
educated for temperature T2. The arc of a circle which represents the section of pin 30 is slightly closed in relation to the preceding geometric state 30B but open in relation to initial state 30A, The section in the form of a simple arc of a circle is given by way of example.
It will be understood that it could have more sophisticated shapes.
The last stage previously described may be followed by a further stage in which pin 30 is brought back to its initial state. This stage is conducted mechanically at room temperature.
The method of use of this type of pin 30 is as follows. The surgeon makes a hole in the bone, for example using a reamer, so that it can house pin 30 in its initial state. The diameter of the hole is slightly larger than the outer diameter of pin 30 so that it can be inserted.
Once inserted, pin 30 is subjected to heat so that it reaches temperature T1.
Pin 30 then positions itself in its first geometric state 308 in which it wedges itself against the side wall of the hole made in the bone. It then keeps this shape during the time of treatment and cannot be dismounted without further action being taken.
On the day of its removal, it is subjected to heat so that it reaches temperature T2. It then contracts and places itself in its second geometric state 30C.
It will be noted that the fact that its degree of closure in its second geometric state 30C is slightly greater than in its first geometric state 30B but smaller than in its initial state 30A means that it does not damage cancellous tissue which has grown inside the pin during treatment.
In addition to the fact that with the process of the invention it is possible to dismount an implanted pin 30, the advantage arising through the use of a material with dual memory is, in this case, that it facilitates the insertion of a pin in relation to the state of the art in which it was fixed by means of a screw whose insertion required the use of a fluorescent monitoring screen which generally is a source of harmful even dangerous radiation. Also, the insertion and removal operations are made easier.
In the same manner, that is to say in the shape of a tube which is slit longitudinally, it is also possible to make rivets normally used to fix plates, ligaments etc. Their use is substantially the same as the description given above for pin 30 in Figure 2a.
Also in the same manner, it is also possible to make femoral stem components.
It will be noted that only one part of the pin or rivet may be provided with a side slit, or even several side slits.
Figure 3 shows a socket 40 normally intended firstly to house the spherical head of a prosthesis, for example a femoral pact, and secondly to fix itself into the corresponding joint cavity.
This socket 40 is made up of a hollow cup that is substantially hemispherical opened on the side by a slit 41 whose width is e. During manufacture, width a has an initial value e~,~. During the first education to temperature T1, the socket is caused to take on its first geometric state in which width a has the value e1. For example, this value ei is greater than the value emu.

During the second education to temperature T2, the socket is caused to take on its second geometric state in which width a has the value ei, which value may for example be slightly smaller than value e, but greater than initial value e~,;, for the same reason as for pin 30 described above.
The method of use of said socket is as follows. As previously, the socket is placed in the joint cavity and then subjected to heat so that it reaches temperature T1.
It then assumes its first geometric state in which the width of slit 41 has the value e1, so that it secures itself against the walls of the joint cavity. If temperature T1 is high, the heat may be provided by a diathermy knife.
Once in place, the socket remains secure.
For its removal, it is subjected to heat so that it reaches temperature T2 in which it assumes it second geometric state with a width es of slit 41. This width value of a is slightly lower than value e1 of the first geometric state which, when the socket is withdrawn, avoids damaging any ingrown tissue.
Said socket in a material with dual memory, in addition to the advantages arising through the process of the invention, provides for easier, quicker insertion than with sockets of the prior art which required screws or cementing. Also, if screws were used, their friction against the polyethylene core of the socket was a source of considerable wear.
It will be noted that socket 40 could be provided with one or more slits such as slit 41.
The present invention can also be applied to implants for a metatarso-phalangeal prosthesis.
It is also applicable to lumbar wiring to realign vertebrae that is shaped in a dual-memory material and educated to have two separate geometric states: one for application of the wiring between vertebrae and the other for its removal.
The present invention may also be applied to an anchor peg of a prosthesis, a knee prosthesis for example, which is shown in Figures 4a to 4c. This peg 50 has a substantially cylindrical shape and is surmounted by a hemispherical cup 51 intended to house the end of the prosthesis. It will be noted that this cup 51 could be equipped with a prosthesis fixture system, for example a bayonet or other fitting.
In the cylindrical part of peg 50, tabs 52 are cut out which are integral with the body 53 of the peg via their flexible bases 54. Therefore each one can be bent outwards in relation to its base 54.

Figure 4b shows a top view of a peg 50 whose tabs 52 are bent autwards. This state of peg 50 is the first geometric state and corresponds to the state which will be given at the time the peg is inserted by heating the peg so that it reaches a temperature close to temperature T1.
In Figure 4c, peg 50 has its tabs 52 retracted alongside the shaft of peg 50.
This state of peg 50 is its second geometric state and corresponds to the state which will be given at the time is peg is removed by applying the necessary heat for it to reach a temperature clox to temperature T2.
A further application of the present invention will now be described in relation to Figure 5 which relates to a femoral stem component system_ This system is made up of a flat shaft 60 of which one end is fitted with a centring cylinder 61 intended, in a manner known in itself, either directly or by means of a sleeve, to fit into a sphere intended to form the head of the femoral prosthesis. This system also comprises a module 62 provided to adapt to and cover shaft b0. This module 62 in the embodiment 1 S shown comprises two side walls 62a and 62b connected to each other by a core 62c.
Module 62 is made of a memory material and undergoes dual education according to the process of the present invention. In the initial state, walls 62n and b2b are parallel to each other and the distance d which separates them has a value da,;
slightly greater than the thickness of shaft 60 so that said module 62 may be mounted over said shaft 60. In a first geometric state the side walls 62a and 62b are slightly drawn towards each other and distance d is less than value d;"; and slightly less than the thickness of shaft 60. This geometric state corresponds to the insertion and fixation of module 62 by simple tightening onto shaft 60, which insertion is made at temperature T1. In a second geometric state, side welts 62a and 62b are substantially parallel to each other and distance d is therefore substantially equal to the iru'tial value d;,; and corresponds to the removal of module 62 from shag 60 which is conducted at temperature T2.

Claims

1. Process for memorizing two geometric states of a product made from a shape memory alloy, characterized in that it consists of:
a) educating said alloy for the first geometric state by acting on said product to bring it to and leave it in said first state at a first temperature, then b) educating said alloy for the second geometric state by acting on said product to bring it to and leave it in said second state at a second temperature.

2. Memorizing process according to claim 1, characterized in that it consists of using as alloy a nickel and titanium alloy in a stoicheiometric ratio that is close to 1, or a composite material reinforced with said alloy.

3. Memorizing process according to claim 1 or 2, characterized in that the highest temperature of the two temperatures under consideration is in the range of +37°C and +55°C while the lowest temperature lies between +30°C and -30°C.

4. Memorizing process according to any of the preceding claims, characterized in that said second geometric state is the initial state of said product before its first education.

5. Memorizing process according to any of the preceding claims, characterized in that it consists of finishing said product after the education of said alloy for the first geometric state but before the education of said alloy for the second geometric state.

6. Memorizing process according to claim 5, characterized in that said finishing stage consists of polishing said product, for example by sanding.

7. Memorizing methoed according to claim 5 or 6 characterized in that said finishing stage consists of depositing at low temperature a thin protective layer made of one of the following materials:
nickel carbide titanium carbide titanium nitride aluminium oxide molybdenum bisulphide titanium carbonitride nickel, titanium and aluminium ternary material chromium nitride carbon diamond in the amorphous state

8. Application of the memorizing process according to any of claims 1 to 7 to products intended to be fixed by tightening onto an element and to be removed therefrom.

9. Application of the memorizing process according to claim 8, characterized in that said products are human implants.

10. Application of the memorizing process according to claim 9, characterized in that said human implants are clips comprising two limbs connected together by a core, said two limbs in the first geometric state being drawn towards each other and, in said second geometric state, being substantially parallel to one another.

11. Application of the memorizing process of claim 9, characterized in that said implants are made up of a tube opened on the side by at least one longitudinal slit, said tube being more open in the said first geometric state than in an initial state, and which, in the said second geometric state, is in a state close to said first geometric state that is also more open in relation to its initial state.

12. Application of the memorizing process of claim 9, characterized in that said implants are sockets made up of a hemispherical hollow cup provided with at least one slit, said slit being more open in said first geometric state than in the second geometric state.

13. Application of the memorizing process of claim 9, characterized in that said implants are anchor pegs essentially made up of a cylinder whose side wall is provided with longitudinal tabs able to be bent outwards in relation to their base, said tabs being in the bent position in said first geometric state and are placed alongside said cylinder in said second geometric state.

14. Application of the memorizing process of claim 8, characterized in that said products are modules with a "U" shaped section able to be mounted on a flat shaft so as to form a femoral stem, the side walls of said module being drawn towards each other in said first geometric state and being substantially parallel to each other in said second geometric state, said side walls tightening said flat shaft in said first geometric state so that it can be secured by tightening.