EP1267754A1

EP1267754A1 - Cotyloid insert for total hip prosthesis

Info

Publication number: EP1267754A1
Application number: EP01921477A
Authority: EP
Inventors: Jean-Paul Carret; Jean-Paul Cisterne; Michel-Henri Fessy; Emmanuel Baulot; Frédéric FARIZON; André Rambert
Original assignee: Fessy Michel Henry; Societe dEtudes de Recherches et de Fabrication SAS SERF
Current assignee: Fessy Michel Henry; Societe dEtudes de Recherches et de Fabrication SAS SERF
Priority date: 2000-04-06
Filing date: 2001-04-04
Publication date: 2003-01-02
Also published as: US20030171817A1; FR2807315A1; WO2001076511A1; FR2807315B1

Abstract

The invention concerns a cotyloid insert comprising a spherical head (2) arranged at one end of a medullary rod, and mounted articulated inside a core, itself mounted articulated inside an acetabular component. The spherical head (2) is made of zircon and the core comprises an inner cup (7) consisting of an alumina cup whereon is locked a polyethylene cup (8) overmoulded on the alumina cup (7). Furthermore, the spherical head (2) is designed to be capable of being freely engaged into the cup (7) and directly and solely retained thereby.

Description

COTYLOID INSERT FOR TOTAL HIP PROSTHESIS

The present invention relates to an acetabular insert for a total hip prosthesis. The deterioration of the cotyioid cavity frequently involves the installation of a total hip prosthesis. Such a prosthesis comprises, on the one hand, a femoral element having a medullary rod intended to be fixed to the bone inside the femur, and the free end of which is provided with a spherical head and, on the other hand hand, an acetabular element intended to be engaged and fixed in the acetabular cavity. The acetabular element delimits a spherical cavity receiving, with the possibility of pivoting, the spherical head of the femoral element.

A known acetabular element, in particular from document FR 2 710 836, consists of a core of material promoting sliding, most often of high density polyethylene, which delimits the cavity in which the spherical head is pivotally mounted. This polyethylene core is inserted into a metal cup fixed in the cotyioid cavity. Most of the movements are obtained by pivoting the spherical head inside the core. However, in certain large amplitude movements, the polyethylene cup can also pivot inside the metal cup. It is thus possible to obtain a double mobility of the spherical head relative to the cotyioid cavity.

Patent application FR 99 08 1 94 describes a total hip prosthesis, in which the surfaces in contact with the head and the core are made of materials with hard friction.

The object of the invention is to provide a total hip prosthesis allowing dual mobility, on the one hand, of the spherical head relative to the nucleus and, on the other hand, of the nucleus relative to the cotyioid cavity, which limits the risks of dislocation and which has a significant longevity.

To this end, in the prosthesis that it relates to, of the type comprising a spherical head disposed at one end of a medullary rod intended to be fixed to the bone inside the femur, and mounted articulated inside '' a core, itself mounted articulated inside an acetabular component constituted for example by a metal cup fixed in the cotyioid cavity, the spherical head is made of zirconia and the core includes an inner cup which, intended for mounting the spherical head, consists of an alumina cup on which is immobilized a polyethylene cup overmolded on the cup in alumina, and in that the spherical head is arranged to be able to be engaged without constraint in the inner cup and to be able to be retained directly in this cup, without action of the outer polyethylene cup.

The retention of the head in the cup is ensured by the inner cup without the use of any elastic retention by the outer polyethylene cup.

In order to ensure a maximum bearing surface of the spherical head and to ensure its retention, the alumina cup has, from its bottom and in the direction of its opening, a portion of spherical surface with the same radius as that of the head of the femoral element, extending over more than a half-sphere, and extended below its equatorial plane by a cylindrical portion of diameter less than that of the head of the femoral element.

In order to allow the introduction of the head inside the alumina cup, the head comprises, on a part of its surface, at least one zone of material withdrawal relative to the spherical surface, corresponding to a reduction in diameter to a value less than the diameter of the inlet of the inner cup, to allow effortless passage of the head into the inlet thereof, in a position in which the mounting axis of the head on the neck of a femoral stem is inclined relative to the axis of the entry.

According to another characteristic of the invention, the polyethylene cup has a return covering at least partially the edge of the alumina cup delimiting the opening thereof. This return of the polyethylene cup serves as a stop for the neck of the femoral stem during significant inclinations thereof.

In order not to limit the movements of the neck of the femoral stem excessively, the return of the polyethylene cup includes an inlet of generally conical shape which widens outwards. In order to ensure an excellent hold of the polyethylene cup on the alumina cup, the latter has a certain number of flats formed on its periphery and comprises at least one annular and peripheral groove perpendicular to its axis.

According to another characteristic of the invention, the zirconia head has a frustoconical recess with a taper of the order of 4 °, allowing it to be mounted on a neck of corresponding taper belonging to a femoral stem. This low taper makes it possible to generate significant frictional forces opposing the expulsion of the zirconia head from the scope of the femoral neck, without needing for this a significant axial impaction force, which would be detrimental to the good mechanical strength of the zirconia head.

This insert can be mounted in a metal cup fixed in the cotyioid cavity by lugs or studs, or forcefully engaged in this cavity. Such a prosthesis has the advantages of dual mobility, as well as the advantages of hard friction, zirconia-alumina, having a low wear rate, and avoiding the release of polyethylene, as is the case when the head of the prosthesis is engaged in a polyethylene cup. This results in a considerably increased lifetime of the prosthesis compared to a prosthesis comprising metal-polyethylene or alumina-polyethylene friction, while avoiding the drawbacks resulting from the release of polyethylene particles inside the organism.

In any case, the invention will be clearly understood with the aid of the description which follows, with reference to the appended diagrammatic drawing representing, by way of nonlimiting example, an embodiment of this insert:

Figure 1 is an exploded perspective view of the various parts of this insert;

Figure 2 is a sectional view of the core along the line II-II of Figure 1; Figure 3 is a sectional view during the introduction phase of the prosthesis head inside the core;

Figure 4 is a sectional view of the prosthesis in the implanted position.

The prosthesis shown in the drawing comprises a spherical head 2 mounted at one end of a medullary rod, not shown in the drawing, intended to be fixed to the bone inside the femur. Head 2 is mounted on the medullary rod via a conical neck 3 secured to the rod, with a cone cone of the order of 4 °, engaged in a recess 4 in the head 2. The head 2 has, on its outer face , two zones 5 of material withdrawal arranged symmetrically, and centered on a plane perpendicular to the entry of the recess 4. The head 2 is intended to be mounted articulated inside a core 6 comprising an alumina cup 7 on which is molded a cup of high density polyethylene 8. Figure 1 is a very schematic and theoretical view, insofar as the cup 8 does not exist as such and independently, since it is molded on the cup 7. The cup 7, which is made of alumina, comprises, on its outer face, several flats 9 as well as a peripheral annular groove 10. The flats 9 and the groove 10 aim to ensure excellent blocking of the cup in polyethylene 8 on the cup at a lumine 7. The polyethylene cup has a return 12 partially covering the edge of the cup 7 delimiting the opening thereof. The return 1 2 has a conical entry 1 3 flared outwards.

The alumina cup has, from its bottom and in the direction of its entry or opening, a spherical surface portion 14 of the same radius as that of the head 2 of the femoral element extending over more than half a sphere and extended by a cylindrical portion 1 5 of diameter smaller than that of the head of the femoral element.

However, it is possible to introduce the head 2 of zirconia inside the cup 7 of alumina by virtue of the areas of material withdrawal that the head comprises. These material withdrawal zones in fact provide, at their level, a section which is smaller than the section of the inlet of the cup 7 in its cylindrical portion 1 5. After engagement in a position in which the opening of the frustoconical recess 4 is substantially perpendicular to the inlet of the cup 7, the head can be tilted before mounting the conical neck 3. Once this mounting is done, the head of the prosthesis can no longer be removed from the alumina cup 7.

This insert can be mounted in a metal cup 1 6, for example a cup engaged in force and maintained by notches 1 7 in the bone 1 8 in which is formed the cotyioid cavity. It can be noted that, as shown in FIG. 4, the return 12 of the cup 8 in polyethylene stop form for the neck 3 of the medullary rod, when movements of large amplitude are carried out.

As is apparent from the above, the invention brings a great improvement to the existing technique by providing an acetabular insert for total hip prosthesis, the lifespan of which is very important, insofar as friction takes place between the zirconia of the spherical head and the alumina of a cup, the mobility being increased thanks to the possibility of pivoting of the polyethylene cup inside the metal cup 16 fixed in the basin. As is obvious, the invention is not limited to the sole embodiment of this acetabular insert, described above by way of example, on the contrary it embraces all variants. Thus, in particular that the means for blocking the polyethylene cup on the alumina cup could be different and constituted not by flats and by a groove but by hollow cavities, that the return of the polyethylene cup could have a different shape, that the alumina cup could have a chamfer at its entry, or that the polyethylene cup could be mounted in a metal cup fixed otherwise than by force in the cotyioid cavity, without being so far from the scope of the invention.

Claims

1. Cotyloid insert for total hip prosthesis of the type comprising a spherical head (2) disposed at one end of a medullary rod intended to be fixed to the bone inside the femur, and mounted articulated inside a core, itself mounted articulated inside an acetabular component constituted for example by a metal cup fixed in the cotyioid cavity, characterized in that the spherical head (2) is made of zirconia and the core comprises an inner cup (7) which, intended for mounting the spherical head (2), is constituted by an alumina cup on which is immobilized a polyethylene cup (8) overmolded on the alumina cup (7), and in that the head spherical is arranged to be able to be engaged without constraint in the inner cup (7) and to be able to be retained directly in this cup (7), without action of the outer cup (8) in polyethylene.

2. acetabular insert according to claim 1, characterized in that the alumina cup (7) comprises, from its bottom and in the direction of its opening, a spherical surface portion (14) of the same radius as that of the head (2 ) of the femoral element, extending over more than half a sphere, and extended below its equatorial plane by a cylindrical portion (1 5) of diameter smaller than that of the head of the femoral element.

3. acetabular insert according to one of claims 1 t 2, characterized in that the head (2) comprises, on a part of its surface, at least one zone (5) for withdrawal of material with respect to the spherical surface, corresponding to a reduction in diameter to a value less than the diameter of the inlet of the inner cup (7), in order to allow the head to pass effortlessly through the inlet thereof, in a position in which the mounting axis of the head on the neck of a femoral stem is inclined relative to the axis of the entry.

4. acetabular insert according to one of claims 1 to 3, characterized in that the polyethylene cup (8) has a return (1 2) covering at least partially the edge of the cup (7) in alumina delimiting the opening of it.

5. acetabular insert according to claim 4, characterized in that the return (1 2) of the cup (8) in polyethylene comprises an inlet (1 3) of generally conical shape flaring outwards.

6. acetabular insert according to one of claims 1 to 5, characterized in that the alumina cup (7) has a number of flats (9) formed on its periphery.

7. acetabular insert according to one of claims 1 to 6, characterized in that the alumina cup (7) has at least one annular and peripheral groove (10) perpendicular to its axis.

8. acetabular insert according to one of claims 1 to 7, characterized in that the zirconia head (2) has a frustoconical recess (4) with a taper of the order of 4 °, allowing it to be mounted on a neck ( 3) of corresponding conicity belonging to a femoral stem.