Endoprosthesis With a Ball-and-Socket Joint

It is known (EP-A 53794) to equip the socket of a ball-and-socket endoprosthesis, for example a hip prosthesis, with a slide surface which is made of ceramic material. If, in the event of so-called subluxation, the ball guided therein moves partially away from the intended bearing position, line contact with the edge of the slide surface may occur, which can lead to damage of both the ball and also the edge of the socket. It has thus been proposed to place a protective ring made of plastically deformable polyethylene in front of the edge of the slide surface (EP-A 53794).

This protective ring is not normally subject to any wear. However, it may happen that in the event of a strongly projecting movement, the neck carrying the ball will strike the polyethylene ring. This can lead to damaging friction. It has also been found that the protective ring may be deformed in the process, so that its function of protecting the ceramic part is impaired.

According to the invention, these disadvantages are avoided by the fact that the front side of the protective ring is covered by a covering ring made of resistant material. This material may be a metal or even a resistant plastic. To ensure that the surface of the ball cannot strike the covering ring and become damaged in the event of subluxation, the protective ring is expediently designed in such a way that it covers the inner edge of the covering ring.

The invention is explained in greater detail hereinbelow, with reference being made to the drawing which depicts an advantageous illustrative embodiment and in which:

FIG. 1 shows an enlarged longitudinal section through a hip-joint endoprosthesis, and

FIG. 2 shows a still further enlarged partial section through the edge of the socket.

The hip socket 1 comprises a holder 2, a polyethylene body 3 and a ceramic insert 4. The holder 2 is externally designed such that it can be anchored easily and permanently in the bone. For this purpose, it is provided, for example, with projections 5. For details of this, reference may be made to the prior art.

It has a frustoconical inner shape 6, 7 to which the outer shape of the polyethylene body 3 corresponds. After implantation of the holder 2, this outer shape can be fitted therein essentially free of play and anchored. This too is known.

The polyethylene body 3 has a frustoconical inner shape 8, 9 to which the outer shape of the ceramic insert 4 corresponds free of play, if appropriate with a slight oversize. This ceramic insert 4 consists of a physiologically compatible ceramic material which exhibits a high degree of resistance to wear, for example high-purity aluminium oxide ceramic. It forms a spherical slide surface 10 which corresponds to the spherical configuration of the ball 11 which is mounted therein. The spherical slide surface 10 extends in each direction over approximately 180.degree.; the equator of the spherical surface forming the slide surface 10 lies somewhere between the start of the edge rounding 12 and the front side 13.

Applied to the front side 13 of the ceramic insert there is a protective ring 14 made of high-density polyethylene or similar resilient and preferably sufficiently elastic material, which ring 14 is in turn covered over a large extent by a covering ring 15 made of suitable material, for example titanium. In the example depicted, this covering ring 15 is designed as a threaded ring which cooperates with the polyethylene body 3 via a thread 16 in order to secure the ceramic insert 4 and the protective ring 14 in position. A securing means (not shown) can be provided in order to prevent undesired loosening of the covering ring 15.

The polyethylene body 3 transmits force in a uniformly distributed manner from the ceramic body 4 to the holder 2 and in this way protects against harmful stress concentrations on the ceramic body 4. Likewise, the protective ring 14 distributes the retaining force, originating from the covering ring 15, to the front side 13 of the ceramic body 4 while avoiding stress peaks.

The protective ring 14 preferably situated on the other side of the equator--as viewed from the slide surface 10 of the ceramic body 4--projects inwards almost to the imaginary spherical surface which continues the slide surface 10 on the opening side. On the one hand, it expediently has a little play with respect to the ball 11, when the latter is lying properly on the slide surface 10, so that frictional wear of the polyethylene is avoided and the joint fluid has free access to the slide gap. On the other hand, it is expedient if its internal diameter is a little smaller than the maximum diameter of the ball 11, in order to form a force barrier preventing the ball from sliding out of the socket. Most subluxations can be prevented in this way. However, the force barrier should be smaller than the force at which the ball 11 can detach from the cone 17 bearing it.

The covering ring 15 protects the protective ring 14 from possibly damaging impacts from the ball neck 18. Because of this protection, it can be produced with greatly reduced dimensions. Its thickness is, for example, of the order of one millimetre. On its inner edge it has a collar 19 which extends axially parallel outwards and which lies in front of the inner edge 20 of the covering ring 15 and so protects against the ball 11 coming into potentially damaging direct contact with the covering ring 15 which is made of harder material. As can be seen in FIG. 2, the collar 19 can even protrude slightly over the edge of the covering ring 15 on the opening side. This admittedly means that the neck 18 can strike against the collar 19, but this does not cause any damage as long as the latter can deflect elastically. Its elastic deflection is possible by virtue of the fact that a wedge-shaped gap opens between the opening-side edge rounding 12 of the ceramic insert and the protective ring 14, into which wedge-shaped gap the inner edge of the protective ring can deflect back elastically in such a