FR2642301A1 - Composite prosthesis, intended in particular for replacement of the patella, the tibial plates, the cotyles or any other articulation, and providing a shock-absorbing effect - Google Patents

Abstract

Prosthesis, intended in particular for replacement of the patella, the tibial plates or the cotyles, and providing a shock-absorbing effect, closely related to that of the human articular cartilage, against the risks of wrenching, loosening or even fracturing due to the vibrations or pressures to which the said prosthesis is subjected, characterised in that it comprises an attachment base 4 preferably made of porous and biocompatible metal, such as, for example, porous titanium, for binding the implant to the bone, on which base a layer 5 of shock-absorbing material, such as a hard polyurethane resin, adheres with a thickness depending on the prosthesis type, and a shield 3 made of hard and resistant material, for example RCH 1000 polyethylene, of an anatomical shape appropriate for restoring the articular surface, covering the shock-absorbing layer 5 to which the said shield 3 is connected by suitable means.

Description

 The present invention relates to a composite prosthesis particularly intended for replacing the patellar medallion, tibial trays, acetabulum or any other joint providing a damping effect close to that of human articular cartilage against the risks of tearing, loosening, or even fracture due to vibrations or pressures to which said prosthesis is subjected.

 The prostheses concerned by the invention aim to restore to the joint its possibilities for normal operation by recreating the most natural working conditions possible.

 Prostheses of this type are therefore subjected to stresses which can be internal such as friction between prosthetic parts, or external such as for example the vibrations induced by the daily movements of the body; as a result, prostheses are generally subjected to a combination of force and pressure which ends up damaging prosthetic implants. We often observe loosening of prosthesis, even fractures which of course require new and painful interventions.

 It has been known for a long time that human articular systems, when they are healthy, themselves comprise effective means against the pressures to which the joints will be subjected, or even against the vibrations induced by the natural functioning of said joint. In this respect, human articular cartilage is particularly well suited, not only in that it constitutes an element of lubrication of the joint, but also a shock-absorbing element remarkable for its capacities of reversible elasticity.

 We therefore sought joint prostheses whose characteristics of hardness, solidity and above all elasticity are as close as possible to those of human articular cartilage.

 A solution is for example described in French patent FR-8701495, which proposes to reconstruct a femoropatellar joint using a medallion of adequate anatomical shape made of a material in principle elastic such as polyurethane; this anatomical medallion is itself fixed on a metallic circular base comprising suitable means for being arranged and secured for example in the patellar cancellous bone. The solution thus proposed partially answers the problem, in that the elasticity required to absorb the vibrations and other pressures to which the joints are naturally subjected, is well obtained thanks to the use of a material such as polyurethane, the Reversible elasticity is well known. On the other hand, it is also known that polyurethane, and more generally flexible materials, are not very resistant, in particular to friction forces and they generally have very high viscous friction coefficients, incompatible with the requirements of a human joint. It therefore appears that such prostheses are very ill-suited to the criteria of solidity, rigidity or even lubrication which are essential for a joint reconstruction as close as possible to the human joint.

 The present invention overcomes such drawbacks in that it provides prostheses which combine all the criteria of elasticity, hardness and lubrication. In this regard, it is proposed a prosthesis particularly intended for the replacement of the patellar medallion, the tibial trays or even the acetabulum, providing a damping effect close to human articular cartilage, against the risks of tearing, loosening or even fracture due to the vibrations or the pressures to which said prosthesis is subjected, which is characterized in that it comprises a fixing base preferably made of biocompatible porous metal, 'such as advantageously porotitanium, for bonding the implant to the bone, on which a layer of damping material, such as a polyurethane resin for example, adheres according to a thickness depending on the type of prosthesis, a medallion of resistant material, for example of polyethylene RCH 1000, of suitable shape to reconstitute the covering articular surface the damping layer to which said medallion is secured by appropriate means.

 It is therefore understandable that such composite prostheses according to the invention perfectly meet the essential criteria defining the human joint; indeed, the stacking of proposed material not only provides a damping effect in that the upper anatomical medallion comes to rest on a layer of sufficient thickness of a particularly flexible resin, unexpectedly providing the near damping effect human cartilage on the one hand, and the necessary adhesion effect on the bone fixation base on the other hand.

 It will naturally be observed that the medallion which reconstitutes the anatomical element which it is claimed to replace, can be made of a relatively hard material, such as for example RCH 1000 polyethylene, thus making it possible to limit the phenomena of abrasion and rupture linked to normal functioning of any joint, with the added benefit of providing good sliding of the joint surface. This shield can be easily secured to the intermediate layer meeting the criteria of shock absorbers by various means, such as by embedding, which allows easy replacement of said shield, for example to provide for its possible replacement. The intermediate layer, made up as we have seen, of a polyurethane resin, naturally adheres to the base, preferably made of a porous and biocompatible material. This base is advantageously provided with fixing devices in the cortex of the bone or the patellar cancellous bone, namely, for example, a foot with or without a non-return element, with or without spacer elements allowing better securing to the bone receiving the prosthesis.

 Thus constituted, the composite prosthesis according to the invention offers all the required hardness and sliding characteristics, thanks to its medallion, as well as all the flexibility characteristics providing remarkable longevity to said prosthesis, subjected to vibrations and intense forces due in particular orthogonal forces, such as for example the plating forces to which the patellar medallion is subjected.

The invention will be better understood and other advantages will emerge further from the description which follows, of several types of embodiment of damping prosthesis given below by way of nonlimiting example with reference to the drawings in which
FIG. 1 is a view in transverse vertical section of a variant of a patellar prosthesis according to the invention,
- Figure 2 is a transverse vertical sectional view of a second variant of patellar prosthesis according to the invention.

 The patellar medallion 1 constitutes a preferred embodiment of a prosthesis according to the invention, since it is an articular element particularly exposed to numerous mechanical stresses, of which it is advisable to plan all the effects to which the prostheses will inevitably be subjected. intended to replace them.

 In accordance with FIG. 1, a patellar medallion prosthesis 1 has been shown according to a first variant intended to be implanted in the patellar cancellous bone 2, without the use of a fixing cement #.

 In this regard the medallion 1 comprises a shield 3 having externally a shape compatible with the sliding surface which is opposite to it on the femur; the shape of this shield 3 is obtained in a known manner, for example by molding a hard material such as ultra high molecular weight polyethylene of the type for example RCH 1000.

 The mechanical characteristics of this shield gives the medallion 1 its hardness, allowing it to resist all the phenomena of friction of the patellar sphere on the trochlear element, as well as natural or accidental orthogonal plating forces. It will be observed that advantageously the shield 3, which is made of polyethylene, therefore presents a surface condition providing the best coefficient of friction at the prosthetic joint.

 The shield 3, in accordance with the invention, is secured to the fixing base 4 by means of a resin layer 5, advantageously of the type of a hard polyurethane resin, constituting a damping support for the shield 3.

This damping layer 5 is extended over the external surface of the base 4 in a continuous manner and over a thickness of approximately two millimeters. Advantageously, the layer 5 is extended beyond the outer edges of the plate of the base 4 so as to protect the patellar environment from the projecting edges of said base. 4.

 In the variant according to Figure 1, the base 4 consists of a disc 6 made of a porous and biocompatible metal such as porotitane, thus allowing better and faster rehabitation of the bone or cancellous bone.

In an additional way, provision is made in the center of the disc 6 of the base 4, to have a preferably hollow feet, allowing not only the centering of the medallion 1 during its implantation, but also its immediate mechanical fixing in this regard, the hollow feet 7 has a taper to facilitate the introduction of the medallion 1 -in the digging of the patellar cancellous 2, previously carried out for this purpose.

 The feet 7 further comprises at least two lateral slots 71 allowing a transverse spacing of the vertical parts 72, 73 of the feet 7 thus delimited, obtained by the screwing of a screw, without head, cylindrical inside the feet 7 threaded for this; in this way, the lateral parts of the feet 7 compress the walls of the digging to be carried out in the patellar cancellous 2, in order to secure, in a positive manner, the base to the patellar cancellous 2. Furthermore, additional provision is made for non-return elements 9 on the outer walls of the feet 7, so that said elements 9 cooperate in strengthening the general mechanical strength of the medallion 1 embedded in the patellar cancellous 2.

 The shield 3 is secured to the damping layer 5, by embedding at least one protuberance 10 coming from the resin layer 5, coming to be embedded. by force inside a housing of homothetic shape provided inside the shield 3. Said shield 3 is therefore secured to the damping layer 5 by simple pressure, which makes its possible extraction quite possible, thus providing a decisive advantage in the event of subsequent replacement of the prosthesis.

 Passages 11 have been provided in the resin layer 5 for easy access to the screw 8, so that it can be actuated when the base 4 is fixed in the patellar cancellous 2; naturally, it is advantageous to provide that the screw 8 is of the BTR type to facilitate its movement inside the hollow feet 7.

 It goes without saying that there is no need to have the feet 7 in the center of the base 4, and that it can be provided, in addition, to have any number of such feet, identical or not, on the base 4 of the medallion 1, without departing from the scope of the invention.

 Finally, it will be noted that the implantation of such a prosthesis can therefore be carried out without the addition of a # cement cement, which gives an additional advantage to this variant of prosthesis according to the invention.

 In accordance with FIG. 2, a second variant of the invention differs from the variant which has just been explained in that the prosthesis is implanted using a fixing cement, conventionally methyl methacrylate.

 In this variant, there is also provided a shield 3 of anatomical shape adapted in the same way as in the first variant, and produced naturally in a completely similar material, namely polyethylene RCH 1000 preferably.

 The base 4, produced here again in a porous and biocompatible metal, advantageously porotitanium, for reasons already explained, does not include any particular fixing means other than a normally full feet 7 and comprising a non-return element 12, improving the anchoring of said sealed base in the patellar cancellous bone 2 or the cortex of the bone by means of a cement 13, for example methyl methacrylate.

 On the base 4, a layer 5 of polyurethane resin is deposited, continuously and over a thickness of approximately two millimeters; this layer 5 will constitute the damping layer in the same way as in the previous variant. The base 4 has at its upper part an overhanging part 41; in this way, the shield 3 comprising suitable peripheral arrangements can be fitted onto the overhanging parts 41 of said base 4 and the mechanical connection of the shield 3 to the base 4 is thus obtained by simple snap-fastening; it will be observed that, unexpectedly, the protection of the surrounding environment is also ensured, in that the enveloping shape of the shield 3 isolates the biological mass from the possibly projecting edges of the base 4.

 The base 4 is, as in the first variant, consisting of a disc 6 whose thickness is however adapted to create sufficient clearance 14 between the bone or the patellar cancellous bone 2, and the lower peripheral crown 15 of the shield 3 The clearance 14 is normally intended to authorize any travel necessary for the operation of the damper by compression of the layer 5.

 It is quite obvious that it is about a particular form of prosthesis intended for an implantation with cement and that any variant, which would differ only by the mode of fixation of its base in the cortex or the patellar cancellous, would not leave not within the scope of the invention.

 It is understood that the two variants which have just been described have only been given by way of illustration of prostheses in accordance with the invention, and one could likewise produce unicompartmental or bicompartmental prostheses intended for tibial trays for example more generally, the invention is remarkably suitable for any prosthetic part undergoing stresses, vibrations or pressures; This is also the case for hip prostheses where the invention can be usefully adapted to the acetabulum, or to the femoral head.

Claims (6)

 1 - Composite prosthesis particularly intended to replace the patellar medallion, tibial plates, acetabulum, or any other joint providing a damping effect close to that of human articular cartilage, against the risks of tearing, loosening or even fracture due to vibrations or pressures to which said prosthesis is subjected, characterized in that it comprises a fixing base (4) preferably made of porous and biocompatible metal such as porotitane for example, to bond the implant to the bone , on which a layer (5) of damping material such as a polyurethane resin, adheres according to a thickness depending on the type of prosthesis, a shield (3) made of hard and resistant material, for example polyethylene RCH 1000, this shield (3) can have any shape adapted to the opposite articular or prosthetic surface, covering the damping layer (5) to which said shield (3) is secured by appropriate means.  2 - Prosthesis according to claim 1 characterized in that the shield (3) comprises on its internal face, in contact with the damping layer (5), at least one housing (31) of suitable shape in which to be flush-fitted, a protrusion (10) molded in the material of said damping layer (5), providing a reversible fixing of said shield (3) on the base (4) of the prosthesis.  3 - Prosthesis according to any one of the preceding claims, characterized in that the damping layer (5) adhering to the base (4) completely covers the surface of said base (4) opposite the bone, as well as the edges of said base (4) to protect the surrounding biological mass from any injury, with the exception, however, of passages (11) provided for the fixing elements of the prosthesis.  4 - Prosthesis according to any one of the preceding claims, characterized in that the base (4) is provided on its face in contact with the bone with at least one foot (7) centered or not, coming to be embedded in the cortex or in the patellar cancellous bone, to ensure the positioning and fixing of said prosthesis with or without cement (13), in cooperation or not, with other mechanical means such as screws (8), non-return teeth (9 ) on the feet (7) which can be hollow and provided with a conventional screw retractor system.  5 - Prosthesis according to claim 1 characterized in that the base (4) has on its upper face an overhang (41) which is embedded inside the anatomical shield (3), which can thus envelop the base assembly (4) and damping layer (5), fixed in the patellar cancellous (2) by known means of a cement (13) preferably methyl methacrylate.  6 - Prosthesis according to claim 5 characterized in that the anatomical shield (3) is such that it rests exclusively on the shock absorbing layer (5), sufficient clearance (14) being provided for this purpose between the lower peripheral crown (15 ) of the shield (3) and the surface of the patellar bone or cancellous bone (2) on which the implant is placed.