Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1998-10-01
2000-10-03
Milano, Michael J.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
623 1811, 623 234, A61F 232
Patent
active
061266957
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an artificial joint, in particular to an artificial hip joint, comprising a bearing shell which has a concave spherical bearing surface A with center M.sub.S, and with a joint ball which has a convex spherical bearing surface B with center M.sub.K, which as a rule is rotationally symmetrically arranged with respect to a mounting axis D in the direction of the femur neck of an artificial hip joint shaft.
Artificial joints require pairings of the materials of the bearing bodies which move relative to one another which have good emergency running properties. The classical starting point in the combination of materials thus consists in pairing dissimilar partners. Thus relatively soft bearing shells of polyethylene are combined with hard joint heads of metal or ceramics and, in the early days of the artificial hip joints, metallic materials of differing hardness and wear resistance were combined with one another. In spite of all efforts, it was never possible to completely eliminate the wear of the partners with these material combinations. With polyethylene, for instance, an abrasion takes place in the hip joint through which the bearing surface is recessed by approx. 0.2 mm yearly in the direction of the principal force. Even with metallic surfaces wear arises due to point loads and micro-weldings at the surfaces which--once they have begun--very rapidly affect the entire surface of engagement.
SUMMARY OF THE INVENTION
It is an object of the invention to attain a low-wear arrangement. This is satisfied in that the bearing shell and the joint head consist of a wear-resistant metallic material; in that the surface A has an average radius R.sub.m and the surface B has an average radius r.sub.m, with their difference amounting to 35 .mu.m<R.sub.m -r.sub.m <85 .mu.m; in that the shape error of the surface A amounts to less than .+-.7.5 .mu.m over an angle 90.degree.<.alpha.<180.degree.; in that the shape error of the surface B amounts to less than .+-.2 .mu.m over an angle .beta.>140.degree.; and in that the joint ball is continued outside the area B by a set-back surface C which has a distance to the center M.sub.K which is less than that of surface B, while the roughness of the surface A corresponds to a value R.sub.a <0.08 .mu.m and the roughness of the surface B corresponds to a value R.sub.a <0.08 .mu.m. By manufacturing, measuring and pairing of bearing surfaces of the same wear-resistant metallic material, a geometry between the bearing surfaces is achieved which, together with the capillary action of the body fluid and the buoyancy in bearing surfaces gliding past one another, largely eliminates micro-weldings and wear. Through the suppression of micro-weldings for identical wear-resistant metallic materials, the positive properties of these materials such as toughness, stability of shape and elasticity can be exploited. Surfaces with a homogeneous structure arise which neither break through due to hardness differences between the surface and base body under high stress nor separate from the base body. At the same time, the surfaces are matched so well to one another that no unallowable surface pressure arises when standing still. Especially suitable materials are cobalt, chromium and nickel alloys such as for instance the material PROTASUL 21 WF of the SULZER AG corporation-in accordance with ISO 5832/4 when manufactured in accordance with a method in which a circular generator for the spherical form likewise rotates, but with its axis of rotation swivelled with respect to the axis of rotation of the workpiece in order to grind, hone and polish a spherical shape until the prespecified tolerances for the diameter, accuracy of shape and surface quality have been attained.
This effect will be still better if the roughness of the surfaces A and B corresponds to a value R.sub.a <0.05 .mu.m.
Advantageous further developments of the invention relate to releasable connections for inner shells which are known from the use of inner sh
REFERENCES:
Streicher, R.M., et al., "Investigation of the Tribological Behaviour of Metal-on-Metal Combinations for Artificial Hip Joints" in: Biomedizinische Technik 35 (1990) May, No. 5, Berlin (in German).
Milano Michael J.
Sulzer Orthopaedie AG
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