FR2739550A1 - DIGITAL JOINT PROSTHESIS WITH FLOATING AXIS - Google Patents

Abstract

The invention relates to a finger joint prosthesis comprising a first part (10) provided with a female block (13) hinged to a second part (20) provided with a male block (23) by means of a hinged-pin (4). An internal ring (3) and external rings (1, 2) mounted on the blocks provide for movements with reduced frictions with the hinged-pin (4). Clearances, particularly between external rings (1, 2) and the internal ring (3), give the prosthesis possibilities of movements equivalent to those of a normal finger joint.

Description

DIGITAL JOINT PROSTHESIS
WITH FLOATING AXIS
The present invention relates to the field of digital joint prostheses used to replace the metacarpophalangeal and interphalangeal joints.

 Digital joint prostheses or joint implants have been developed in an attempt to restore the use of the fingers of the hand in people who have suffered a trauma or suffering from diseases (for example post-traumatic osteoarthritis, rheumatoid arthritis) leading to progressive deterioration of digital joints, or even their destruction.

 The ideal prosthesis should allow the characteristic movements of the digital joints to be restored. The main movement authorized by the digital joints is the flexion-extension movement of the fingers.

The digital joint has two other degrees of freedom which are used particularly during the flexion-extension movement of the finger: it allows a partial rotational movement of the first phalanx on itself and lateral movement on either side of the axis of the phalanx. Depending on the joint considered, the amplitude of the movement according to the two degrees of freedom of lateral movement and partial rotation is variable: for example the metacarpophalangeal joint allows lateral movement of the fingers of variable amplitude according to the subjects and according to the fingers, while the inter-phalangeal joints give lateral displacement of small amplitude in the proximal joints and zero or very weak in the distal joints.

 The ideal prosthesis should also be strong and wear resistant to have the longest possible life. Its action should not be hampered by tissue regrowth. Its implantation should lead to minimal bone resection in order to ensure as much as possible the conservation of the patient's bone heritage.

 Certain prostheses of the known art make it possible to reproduce all the physiological movements of the digital joints. However, they have certain drawbacks: lack of solidity, blockage of the joint due to tissue regrowth, permanent inflammation due to rejection phenomena.

 The materials used to make this kind of prosthesis are of course of considerable importance.

The silicone implants, which looked very promising, turned out to be disappointing over time. They are the cause of rupture of implants, lateral instabilities, lack of strength, residual pain, pathological reactions of the body to this material.

Other digital joint prostheses are also known which are made from materials of different natures. So, JL CONDAMINE,
JY BENOIT, JJ COMTET, JH AUBRIOT, in an article published in the Annals of Surgery of the Hand, volume 7, nO 4, 1988, pages 283 to 297 and entitled "Proposal for a digital arthroplasty. Critical study of the first results" have proposed prostheses consisting of a proximal piece of stainless steel and a distal piece of polyethylene. The combination of materials such as titanium and polyethylene has also been proposed (see for example
FR-A-2 620 932).

 To remedy certain major defects among those listed above, namely the lack of solidity, insufficient wear resistance and poor biological compatibility, which require a new surgical intervention, a new prosthesis has been proposed by the document FR -A-2 680 967.

This prosthesis comprises two medullary pieces preferably made of titanium alloy joined by an axis made of biocompatible material and with a low coefficient of friction (for example made of pyrocarbon) free to rotate in a bearing also made of this material. This structure makes it possible to avoid any contact or friction other than between materials with a low coefficient of friction. The association of a material such as pyrocarbon with a metal such as titanium makes it possible to obtain a biocompatible prosthesis, of great robustness and great reliability over time.

The prosthesis disclosed by the document
FR-A-2 680 967 only provides the flexion-extension movement of a digital joint which is the main movement of this joint. It does not allow partial rotational movement of the first phalanx on itself or its lateral movement. This implant being constrained, there is a risk of loosening of one of the diaphyseal tails of the prosthesis, this loosening can occur either between the cement and the bone mass, or between the cement and the tail of the implant. In the absence of a stop, there may occur, during an extension movement, an overshoot of the normal amplitude of this movement which the tendons cannot prevent. On the other hand, the tissue regrowth coming to adhere to the surface of the box (i.e. the surface of the unsealed parts of the prosthesis) ends up causing the flexion-extension movement to harden.

 In order to obtain a digital joint prosthesis which is as close as possible to a real digital joint, there is provided, according to the present invention, a prosthesis using the same types of materials as the prosthesis disclosed in document FR-A-2 680 967, but modified to allow the natural movements of a digital joint.

The invention therefore relates to a digital joint prosthesis comprising
a first piece of biocompatible metal formed from a diaphyseal tail extended by a female yoke with two opposite flanges and each pierced with a hole with an axis transverse to the diaphyseal axis,
a second piece of biocompatible metal formed from a diaphyseal tail extended by a male yoke intended to be engaged between the flanges of the female yoke and pierced with a hole with a transverse axis relative to the diaphyseal axis,
- a spindle having a surface made of biocompatible material and with a low coefficient of friction, connecting the two yokes by passing through said holes,
a ring, known as an internal ring, having a surface made of biocompatible material and with a low coefficient of friction, mounted in force in the hole of the male yoke and constituting an intermediate means for the spindle,
- elements having a surface made of biocompatible material and with a low coefficient of friction, provided around the holes of the flanges, characterized in that
- said elements are two rings, called external rings, force-fitted into the holes in the flanges and also constituting an intermediate means for the spindle,
- a clearance is provided between the outer rings and the inner ring,
- The spindle is mounted by force in one of the intermediate means and with a radial clearance in the other intermediate means.

 The spaces (or games) thus created between the moving parts allow all the natural degrees of freedom to be found. The radial clearance with which the spindle is provided ensures a function of advancing or retreating of the distal diaphyseal tail combined with a displacement of the center of rotation of the joint up or down during a flexion-extension movement. This radial clearance, associated with the clearance provided between the inner ring and the outer rings, also allows the axial rotation function of the distal tail as well as the lateral clearance function.

 Chamfers can be provided on the faces of the intermediate means located opposite to form friction surfaces. This has the advantage of reducing the pressures between the inner ring and the outer rings in a position where the lateral movement is associated with a flexion-extension movement.

 According to a preferred variant embodiment of the present invention, the spindle is mounted in force in the outer rings and with a radial clearance in the inner ring.

 The biocompatible metal is preferably based on titanium, for example a biomedical titanium alloy.

 The spindle, the inner ring and the outer rings preferably have pyrocarbon surfaces. Pyrocarbon can be obtained in the state of deposition on a compatible substrate (generally graphite) by thermal decomposition of a gaseous hydrocarbon.

 The axes of the diaphyseal tails may, when they are arranged parallel to one another, be offset by a determined distance in order to improve the resemblance to reality.

 The prosthesis according to the invention may also have means forming a stop to limit the amplitude of the extension movement. These stop-forming means may be constituted by faces of the yokes intended to come into contact with one another for a determined maximum amplitude of the extension movement.

 Each diaphyseal tail may have at least one longitudinal flat. This allows to better immobilize in rotation the diaphyseal tails in the medullary canals.

 In order to prevent tissue regrowth from adhering to the prosthesis casing, the metal parts may advantageously have, with the exception of the diaphyseal tails, surfaces of low roughness (for example polished surfaces) coated with a deposit biocompatible, for example a carbon deposition carried out at low temperature.

The invention will be better understood and other advantages and features will appear on reading the description which follows, given by way of nonlimiting example, accompanied by the appended drawings among which
FIG. 1 is a partially sectioned view from above of a digital joint prosthesis according to the invention,
- Figure 2 is a view of the digital joint prosthesis according to the invention, according to the section
II-II of Figure 3,
FIG. 3 is a partially sectioned side view of a digital joint prosthesis according to the invention,
- Figure 4 is a diagram illustrating the path of the center of rotation on the proximal bone for a natural joint.

 A digital joint prosthesis according to the invention is shown in Figure 1 in top view, that is to say as if it were implanted in a hand whose top would be looked at.

 The prosthesis comprises a first piece 10 of biocompatible metal comprising a diaphyseal tail 11 provided with a succession of radial grooves helping to ensure better sealing of the tail in the medullary canal of the bone concerned. Two opposite flats (only the flat 12 is visible in this figure) are machined on the diaphyseal tail 11. On the articulation side, the tail is extended by a female yoke 13 with two opposite flanges 14 and 15 (see Figure 2) pierced with holes aligned 16 and 17 with a transverse axis relative to the axis of the diaphyseal tail 11.

 The prosthesis comprises a second piece 20 of biocompatible metal comprising a diaphyseal tail 21 provided, like the tail 11, with a succession of radial grooves and two opposite flats (only the flat 22 is visible). On the articulation side, the tail is extended by a male yoke 23 pierced with a hole 24 with a transverse axis relative to the axis of the diaphyseal tail 21.

 Each hole 16 and 17 of the female yoke 13 receives, in tight fit, a ring (called external ring) 1 and 2 respectively. The hole 24 of the male yoke 23 receives, in close fit, a ring 3 called the inner ring. A pin 4 provides the connection between the male and female yokes. The spindle 4 is mounted in tight fit in the rings 1 and 2 and is therefore immobilized in rotation and in translation. A judicious choice of dimensions allows the correct distribution of the tightening of the outer rings on the female yoke and of the spindle on the outer rings. This choice can be made and verified without problem by a person skilled in the art.

 The outer rings 1 and 2, the inner ring 3 and the spindle 4 offer biocompatible surfaces with a low coefficient of friction. These surfaces are advantageously made of pyrocarbon. The pyrocarbon can be obtained in the state of deposition on a graphite substrate as shown in FIG. 2 where the pin 4 has a deposition of pyrocarbon 41 on a cylindrical substrate 42 in graphite.

 The bore of the inner ring 3 inserted in the hole 24 of the male yoke is of a diameter greater than the diameter of the spindle 4. The radial clearance thus created can vary between 0.5 and 2 mm. As can be seen in Figures 1 to 3, lateral play is also provided between the inner ring and the outer rings. This lateral play can be between 0.5 and 2 mm. Thanks to the radial clearance and the lateral clearance, the spindle 4 can move with a certain angular freedom in the bore of the internal ring, without preferential position. In the longitudinal direction, we see that there is a possible piston effect between the parts 10 and 20 of the prosthesis thanks to the radial clearance existing between the pin 4 and the bore of the internal ring 3. The pin can be called axis floating in its housing.

 It can also be seen that the outer rings serve as stops for the lateral movement of the inner ring. Compared to the digital joint prosthesis described in document FR-A-2 680 967, the lateral support of the male yoke is now carried out by two elements (the external rings) instead of four (two support cheeks and two flanges reported).

The present prosthesis has the additional advantage of requiring fewer components.

 All possible movements causing friction are carried out between elements having pyrocarbon surfaces.

 As can be seen in FIG. 1, the outer rings 1 and 2 have chamfers on their faces directed towards the inside of the joint and of inclination such that in extreme positions the friction of the inner ring on the outer rings or surface in order to reduce the pressures. The chamfer angle can be chosen from a range from about 5 to about 150. To get closer to the natural behavior of a digital joint, this chamfer can be made in only one sector to limit the freedom of lateral movement in function of the amplitude of the flexion-extension movement. The upper limit indicated for the chamfering angle can be increased if it is desired to increase the amplitude of the lateral and axial deflections.

 Figure 1 shows the prosthesis in a lateral deflection position. Figure 2 shows, seen in cross section along the axis of the spindle, the joint in the rotational position around the axis of the diaphyseal tail of the male yoke. This axial rotation can be provided to take place at a maximum angle of between approximately 5 and 150 on either side of the neutral position.

 Figure 3 shows the prosthesis casing in section. It allows to realize the amplitude of the piston movement that can perform the part 20, between a first position sketched in phantom for which the part 20 is stopped in the retracted position by one side of the spindle 4 (position not shown in the figure), and a second position effectively represented in the figure for which the part 20 is stopped in the stretched position by an opposite side of the spindle 4.

 FIG. 3 shows that a stop has been provided to limit the flexion-extension movement to an amplitude deemed to be maximum. This stop is constituted by faces 18 and 28, respectively 10 and 20 yokes for the extension movement, and by faces 19 and 29 of the same yokes for the bending movement. As an indication, we can limit this movement to an angle of - 50 for extension and + 1050 for flexion.

 As shown in Figure 3, the axis of the holes 16 and 17 of the flanges of the female yoke, and therefore the axis of the pin 4, is offset from the axis of the diaphyseal tail 11 by a distance between 0.5 and 3 mm. Similarly, when the diaphyseal tails 11 and 21 have their axes placed parallel to each other, these axes are not confused but are offset by a height (the joint being viewed from the side in FIG. 3) of between 0.5 and 3 mm.

 The offset of the axis of the spindle and the offset of the axes of the diaphyseal tails improve the resemblance between the joint prosthesis and the real joint.

 The prosthesis according to the invention adapts particularly well to all morphologies. Indeed, the anatomical study of the geometric location of the displacement of the center of rotation of a real digital joint shows that this displacement is not identical from one individual to another. As it is not possible to make personalized prostheses for each patient, the fact of making the spindle free in the internal ring allows the flexion-extension movement to be carried out freely according to the morphology of each individual, therefore not not subject the prosthesis to additional mechanical stresses. The lower and upper limits of the values given above can be varied to produce, for example, prostheses voluntarily avoiding certain movements depending on the type of disease from which the patient suffers.

 By way of example, the diagram not shown in FIG. 3 represents the path from the center of rotation on the proximal bone of a natural joint for three individuals I1, I2 and I3.

 The box is preferably polished to give it low roughness and is coated with a biocompatible deposit, for example a low temperature carbon deposit of PVD (Physical Vapor Deposition) type.

This prevents adhesion to the tissue regrowth box.

 The simplification of the method of immobilizing the outer rings compared to the known technique allows the production of a less bulky box and the reduction of the resection length.

Still by way of example, a small articular implant was produced according to the invention. The characteristics of the movements resulting from the different games were as follows
- a flexion-extension movement at an angle of - 50 to + 1050,
- lateral clearance of 5 around the neutral position,
- rotation of the diaphyseal axis by 50 on either side of the neutral position,
- longitudinal displacement (from the retracted position to the stretched position, corresponding to the radial clearance) 0.7 mm.

 For this implant, the resected length is 8 mm.

Claims (13)

 1. Digital joint prosthesis including  - A first piece (10) of biocompatible metal formed by a diaphyseal tail (11) extended by a female yoke (13) with two opposite flanges (14, 15) and each pierced with a hole (16, 17) of transverse axis with respect to the diaphyseal axis,  - A second piece (20) of biocompatible metal formed from a diaphyseal tail (21) extended by a male yoke (23) intended to be engaged between the flanges (14, 15) of the female yoke (13) and pierced with a hole (24) with a transverse axis relative to the diaphyseal axis,  - a spindle (4) having a surface made of biocompatible material and with a low coefficient of friction, connecting the two yokes (13, 23) by passing through said holes,  - A ring (3), called internal ring, having a surface in biocompatible material and with low coefficient of friction, forcibly mounted in the hole (24) of the male yoke (23) and constituting an intermediate means for the spindle (14 ),  - elements having a surface of biocompatible material and with a low coefficient of friction, provided around the holes (16, 17) of the flanges (14, 15), characterized in that  - said elements are two rings (1, 2), called external rings, force-fitted into the holes (16, 17) of the flanges (14, 15) and also constituting an intermediate means for the spindle (4),  - a clearance is provided between the outer rings (1, 2) and the inner ring (3),  - The spindle (4) is force fitted in one of the intermediate means and with a radial clearance in the other intermediate means.  2. Digital joint prosthesis according to claim 1, characterized in that chamfers are provided on the faces of the intermediate means located opposite to form friction surfaces.  3. Digital joint prosthesis according to one of claims 1 or 2, characterized in that the spindle (4) is force-fitted in the outer rings (1, 2) and with a radial clearance in the inner ring (3).  4. Digital joint prosthesis according to claim 3, characterized in that the axis of the spindle (4) is at a determined distance from the axis of the diaphyseal tail (11) of the first part (10).  5. Digital joint prosthesis according to any one of claims 1 to 4, characterized in that the biocompatible metal is based on titanium.  6. Digital joint prosthesis according to any one of claims 1 to 5, characterized in that the spindle (4), the inner ring (3) and the outer rings (1, 2) have pyrocarbon surfaces.  7. Digital joint prosthesis according to any one of claims 1 to 6, characterized in that the axes of the diaphyseal tails (11, 21) have, when they are arranged parallel to one another, a shift by a determined distance.  8. Digital joint prosthesis according to any one of claims 1 to 7, characterized in that it also has means forming a stop to limit the amplitude of the flexion-extension movement.  9. Digital joint prosthesis according to claim 8, characterized in that the means forming a stop consist of faces (18, 28; 19, 29) yokes (13, 23) which are intended to come into contact with one of the 'other for a determined maximum amplitude of the flexion-extension movement.  10. Digital joint prosthesis according to any one of claims 1 to 9, characterized in that each diaphyseal tail (11, 21) has at least one longitudinal flat (12, 22).  11. Digital joint prosthesis according to any one of claims 1 to 10, characterized in that said metal parts (10, 20) have, with the exception of the diaphyseal tails (11, 21), surfaces of low roughness coated of a biocompatible deposit.  12. Digital joint prosthesis according to claim 11, characterized in that the surfaces of low roughness are polished surfaces.  13. Digital joint prosthesis according to one of claims 11 or 12, characterized in that the biocompatible deposit is a carbon deposit carried out at low temperature.