Endoprosthesis with a ball-and-socket joint

Details Endoprosthesis with a ball-and-socket joint Endoprosthesis with a ball-and-socket joint

1998-02-24

2000-08-01

Yu, Mickey

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

623 2311, 623 1811, A61F 232, A61F 236

Patent

active

060960833

DESCRIPTION:

BRIEF SUMMARY
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

REFERENCES:
patent: 5019015 (1991-05-01), Wiley
patent: 5019105 (1991-05-01), Wiley
patent: 5062853 (1991-11-01), Forte
patent: 5222984 (1993-06-01), Forte
patent: 5507826 (1996-04-01), Besselink et al.
patent: 5549693 (1996-08-01), Roux et al.
patent: 5725589 (1998-03-01), Pfaff et al.

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