Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1999-04-01
2000-09-19
Smith, Jeffrey A.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
623 1811, A61F 232
Patent
active
061205451
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a joint prosthesis comprising two metal parts which are movable with respect to one another, the metal parts being coated with a ceramic surface layer.
In known joint prostheses, for example hip prostheses, a spherical head with a diameter of, for example, 28 millimeters is attached to the thighbone. This head may be formed, for example, of CrCoMo or CrCoTi. The hip-joint socket which is attached to the pelvis and in which the head is held rotatably is produced from ultra-high molecular weight polyethylene.
It is also known to form femoral heads from ceramic material, such as aluminium oxide. The disadvantage of using polyethylene in the hip-joint socket is that the polyethylene is worn down by the surface of the head rubbing along the socket. This wear may amount to 100 to 200 microns per annum. This results in the formation of polyethylene particles which accumulate in the tissue cells in the vicinity of the prosthesis and which may cause a reaction, which might lead to the stem of the femoral head giving way.
The problem caused by the formation of polyethylene particles as a result of wear can be avoided by the use of a metal hip-joint socket and femoral head. In this case, both the head and the socket may be formed, for example, from chromium cobalt molybdenum alloys. The disadvantage of metal-on-metal prostheses is that during the casting process compounds are formed with carbon, i.e. the so-called carbides. In the case of the chromium cobalt molybdenum prostheses, these are so-called M7C3 carbides. The dimensions of these compounds is between 20 and 30 micrometers. The carbides may project above the surface of the head and the socket and, after being worn away, may pass into the joint cavity between the moving parts of the prosthesis. Improved casting methods and accurate surface treatments have allowed the dimensions of the carbides to be limited to 3-10 micrometers.
From FR-A-2088764 a hip protheses is known comprising a head and hip-joint socket which are coated with a wear resistant material, according to the preamble of claim 1. The wear resistant material is contacted with the surfaces at supersonic velocities. The head and socket may be comprised of a CrCoMo alloy, the wear resistant material being a ceramic material. When the ceramic material wears away upon use, undesired metal to metal contact or contact of metal on ceramic material may cause release of metal or ceramic particles into the gap of the prostheses.
It is an object of the present invention to provide a joint prosthesis in which wear is counteracted in an effective manner and which is compatible with the human body without causing reactions. It is a further object of the present invention to provide a joint prosthesis which has a long useful service life.
To this end, the joint prosthesis according to the invention is characterized in that the surfaces are covered with an abrasion layer of ceramic material which has a hardness greater than that of the carbides, the thickness of the abrasion layer being less than 0.5 micrometer, preferably virtually equal to 0.3 micrometer. It has been found that, by applying an abrasion layer made of ceramic material which is harder than the carbides, these carbides can be worn away by the abrasion layer. Since the ceramic layer is relatively thin, the particles which are formed during the wear process of the carbides are smaller than 0.1 micrometer. The particles which are formed by the wear process and have such a dimension can be discharged by the synovial fluid which is present between the movable parts of the prosthesis and removed via the lymphatic system. As a result, these released particles do not contribute further to wear to the parts of the prosthesis.
The metal parts of the prosthesis are preferably spaced apart from one another at a distance of at most 0.4 millimeter, preferably at most 0.2 millimeter. This small degree of play between the two movable components results in a low frictional resistance and leads to optimum initial wearing-away of the carbides.
REFERENCES:
patent: 5365359 (1994-11-01), Raab
patent: 5702448 (1997-12-01), Buechel et al.
Hamelijnck Karel
Woering Roelandt Gustaaf
Accis BV
Smith Jeffrey A.
van Doorn Johan A. J.
van Straten Jaap
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