Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2000-04-21
2001-12-04
McDermott, Corrine (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S022230, C623S022320, C623S023530
Reexamination Certificate
active
06325829
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a metal socket for a joint endoprosthesis, which has the form of a bowl with a convex outer surface and concave inner surface. Such sockets are generally known and serve to support so-called inlays of polyethylene, ceramic or similar human-compatible material. A socket of this kind is often used as a component of an artificial hip joint. As a rule such sockets are made of titanium or a titanium alloy. When they are implanted without the use of cement, it is advantageous for the outer surface to be roughened or porous, in order to promote adhesion of the bone and growth of the bone into the pores of the socket.
The patent EP 0 380 055 B1 discloses a hemispherical socket that is flattened so as to depart from the circular or spherical shape in the region of its pole.
In the patent WO 94/23670 it is proposed that the outer surface of a hip-joint socket comprises an ellipsoidal region.
These two previously known constructions are intended to improve retention in the bone in comparison to a purely hemispherical shell. However, trials have shown that this goal is only inadequately achieved with a geometry according to EP 0 380 055 B1. The object of this known construction is therefore merely to increase slightly the separation of the polar surface from the surface of the natural socket seating into which it has been force-fitted, so that pressure exerted on the polar surface of the socket when the bone in the natural socket seating is placed under load will not push the socket out of position. This retraction of the polar surface is intended to reliably prevent contact between it and the bone even when the bone is heavily loaded. In this case, retention of the socket in the natural socket seating depends entirely on a force-fitting in the equatorial region of the socket. Very often, however, this is insufficient. It is in acknowledgement of this problem that the proposal to make the outer surface of a hip-joint socket ellipsoidal is made in WO 94/23670. One result is that the polar surface is separated further from the bone, with the advantages described above; another is that the surface over which the socket is force-fitted into the natural seating is increased. However, a disadvantage is that it is more expensive to construct an ellipsoidal surface, because the manufacturing technology for such a three-dimensional shape is complex.
SUMMARY OF THE INVENTION
The object of the present invention is thus to create a socket of the kind cited above that, firstly, is characterized by an improved force-fitting within the natural socket seating and, secondly, is considerably simpler to manufacture than the construction according to WO 94/23670.
This object is achieved in accordance with the invention by configuring the outer surface of the socket such that it is specified by at least three spherical-segment surfaces, each with a different radius. Surfaces of spherical segments are considerably simpler to construct than an ellipsoidal surface, because of their rotational symmetry. With regard to its anchoring properties, the construction in accordance with the invention provides about the same advantages as the construction with ellipsoidal surface regions.
The greater the number of spherical-segment surfaces that define the outer surface of the socket, the less abrupt are the transitions between adjacent areas of the socket surface, with the consequence that stress peaks in the bone caused by such transitions are correspondingly reduced. Because these stress peaks have a detrimental effect on the stability with which the socket is anchored in the bone, it is important that they (and hence abrupt surface transitions) are avoided. This is achieved in accordance with the invention.
Advantageous structural details of the invention are described. In one embodiment, the centers of the spherical-segment surfaces lie on a common axis of rotation.
In order to flatten the socket in the polar region, the center of the spherical segment that forms the peripheral or equatorial surface is closer to the pole of the socket than the center of the spherical segment forming the surface near the pole, in particular closer than that forming the polar surface itself.
In order to fix the socket in the natural socket seating so that it does not rotate, it can be advantageous to provide the equatorial spherical-segment surface with ribs or similar elevations that are directed towards the pole.
It is also conceivable to replace the equatorial spherical-segment surface by a peripheral conical surface with a cone angle between 3 and 6 degrees. This embodiment is mainly advantageous when the natural socket seating must be specially shaped in the peripheral region. Then all that is needed is to mill-cut a complementary conical seating surface in this region.
It is also conceivable for the pole of the socket to be additionally flattened by forming a surface that extends perpendicular to the axis of rotation.
Finally, it is further conceivable for the axes of rotation of the individual spherical-segment surfaces to be displaced from one another in a direction perpendicular to the surfaces, so as to extend the socket or outer surface thereof in this direction. This embodiment is particularly suitable for revision sockets.
In principle it is also conceivable for the axes of rotation of the individual spherical-segment surfaces to be tilted with respect to one another. In all cases, however, the crucial consideration is that the outer surface of the socket is defined by at least three spherical-segment surfaces of differing radius.
REFERENCES:
patent: 4666448 (1987-05-01), Ganz
patent: 4892549 (1990-01-01), Figgie, III et al.
patent: 5370704 (1994-12-01), DeCarlo, Jr.
patent: 5549695 (1996-08-01), Spotorno et al.
patent: 5782928 (1998-07-01), Ries et al.
patent: 5972032 (1999-10-01), Lopez et al.
patent: 86 23 855.8 (1986-11-01), None
patent: 40 21 677 (1992-01-01), None
patent: 44 08 527 (1995-09-01), None
patent: 0 380 055 (1990-01-01), None
patent: 2 715 556 (1994-02-01), None
patent: 2 159 416 (1985-03-01), None
patent: WO 94/23670 (1994-10-01), None
patent: WO 97/16138 (1997-05-01), None
patent: WO 97/19656 (1997-06-01), None
Chattopadhyay Urmi
Knobbe Martens Olson & Bear LLP
McDermott Corrine
Plus Endoprothetik AG
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