Note: Descriptions are shown in the official language in which they were submitted.
1
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
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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,
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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.
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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
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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
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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.
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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.
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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.
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